Water electrolysis for hydrogen production: from hybrid systems to self-powered/catalyzed devices

Abstract

The electrocatalytic splitting of water holds great promise as a sustainable and environmentally friendly technology for hydrogen production. However, the sluggish kinetics of the oxygen evolution reaction (OER) at the anode significantly hampers the efficiency of this process. In this comprehensive perspective, we outline recent advancements in innovative strategies aimed at improving the energy and economic efficiency of conventional water electrolysis, thereby facilitating efficient hydrogen generation. These novel strategies mainly include: (i) sacrificial-agent-assisted water electrolysis, which integrates thermodynamically favorable small molecules to replace the OER while simultaneously degrading pollutants; (ii) organic upgrading-assisted water electrolysis, wherein thermodynamically and kinetically favorable organic oxidation reactions replace the OER, leading to the production of high-value chemicals alongside hydrogen; (iii) self-powered electrolysis systems, achieved by coupling water splitting with metal-based batteries or fuel cells, enabling hydrogen production without the need for additional electricity input; and (iv) self-catalyzed electrolysis systems driven by the spontaneous metal oxidation at the anode, which provides electrons for hydrogen evolution at the cathode. In particular, we emphasize the design of electrocatalysts using non-noble metal elements, elucidate the underlying reaction mechanisms, and explore the construction of efficient electrolyzers. Additionally, we discuss the prevailing challenges and future prospects, aiming to foster the development of electrocatalytic systems for highly efficient hydrogen production from water in the future.

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Jin-Tao Ren

Jin-Tao Ren received his PhD degree from Nankai University in 2020 under the supervision of Prof. Zhong-Yong Yuan. He is currently a postdoctoral fellow at Nankai University. His research interests focus on advanced nanomaterials for application in electrocatalysis, metal–air batteries, fuel cells, etc.

Lei Chen

Lei Chen received her BE degree from Northeast Forestry University in 2017 and obtained her ME degree from Nankai University in 2020. She is currently a PhD candidate under the supervision of Prof. Zhong-Yong Yuan at Nankai University. Her current research focuses on the fabrication of nanostructured materials for energy-related applications.

Hao-Yu Wang

Hao-Yu Wang received his BE degree in 2019 from Nankai University. He is currently a PhD candidate in Nankai University under the supervision of Prof. Zhong-Yong Yuan. His research interest focuses on the rational design and related applications of advanced electrocatalysts.

Wen-Wen Tian

Wen-Wen Tian is currently pursuing her PhD under the supervision of Prof. Zhong-Yong Yuan at Nankai University. She received her BE degree from Northeast Forestry University in 2018. Her current research focuses on the design, fabrication, and application of advanced electrocatalysts for energy conversion reactions.

Zhong-Yong Yuan

Zhong-Yong Yuan received his PhD degree in Physical Chemistry from Nankai University in 1999. He worked as a postdoctoral fellow at the Institute of Physics, Chinese Academy of Sciences from 1999 to 2001. He then moved to Belgium, working as a research fellow at the University of Namur from 2001 to 2005, prior to joining Nankai University as a full professor. In 2016, he was elected as a fellow of the Royal Society of Chemistry (FRSC). His research interests are mainly focused on the self-assembly of hierarchically nanoporous and nanostructured materials for energy and environmental applications.

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Broader context

Exploring cutting-edge technologies for the efficient production of green hydrogen energy is imperative to alleviate the energy crisis and environmental pollution. Conventional overall water electrolysis is largely restricted by the sluggish kinetics of the anodic oxygen evolution reaction. In this original perspective, we present a comprehensive summary and discussion of the recent notable advancements in energy-efficient hydrogen production through electrolysis, employing four distinct strategies categorized based on the electrode reactions or products of the entire system. These strategies encompass (i) sacrificial-agent-assisted water electrolysis, (ii) organic upgrading-assisted water electrolysis, (iii) self-powered electrolysis systems, and (iv) self-catalyzed electrolysis systems. We will mainly focus on the development of various systems as well as the corresponding electrocatalysts and reaction mechanisms. Additionally, we thoroughly discuss and compare the advantages, differences, and critical issues of each system. The current challenges and related prospects will also be discussed, hopefully to benefit further development of energy-saving hydrogen production from renewable resources and waste products.

1. Introduction

Because of the escalating concerns surrounding environmental pollution and the energy crisis, there is a pressing need to supplant conventional fossil fuels with sustainable energy sources.^1,2 Among the potential alternatives, molecular hydrogen (H₂) has garnered significant attention as a promising energy carrier due to its remarkable energy density and environmentally benign characteristics.³ Electrochemical water splitting, specifically through the utilization of a water electrolyzer, has emerged as a prominent avenue for hydrogen production owing to its affordability and zero-carbon emissions when coupled with intermittent renewable energy sources.^4,5 In a conventional overall water splitting (OWS) system, the cathode drives the hydrogen evolution reaction (HER), while the anode drives the oxygen evolution reaction (OER). Thermodynamically, the HER (2H⁺/H₂) and OER (2H₂O/O₂) possess equilibrium potentials of 0 and 1.23 V (vs. the reversible hydrogen electrode (RHE)), respectively.⁶ Thus, a theoretical minimum voltage of 1.23 V is necessary to initiate electrochemical water splitting. As such, high-performance electrocatalysts, such as Pt for the HER and IrO₂/RuO₂ for the OER, are highly essential to accelerate the apparent sluggish reaction kinetics, consequently, resulting in decreased overpotentials and enabling efficient electrochemical processes.

Due to its potential to simplify electrolyzer configurations and reduce overall costs, overall water electrolysis has entered a prosperous and critical period in the twenty-first century. Nevertheless, despite these advantages, several inherent challenges continue to impede progress in electrocatalytic water splitting for hydrogen production.⁷ (i) The OER is a multi-electron coupled proton transfer process that demands significantly higher overpotentials compared to the HER to achieve equivalent current densities. (ii) Currently, with the simplest configuration of water electrolyzers, both the HER and OER take place simultaneously within a single-compartment setup. This design raises concerns regarding the formation of explosive H₂/O₂ gas mixtures. (iii) The coexistence of gas mixtures and electrocatalysts may generate reactive oxygen species, leading to membrane degradation and premature device failure. (iv) Gas crossover not only presents safety hazards but also reduces energy efficiency, as O₂ may undergo reduction back to water at the cathode side. (v) Efficient water electrolysis necessitates a substantial power supply to drive both the HER and OER processes, resulting in a considerable energy consumption burden. While harnessing or converting energy from renewable sources like wind, thermal, solar, tidal, and self-powered energy holds promise, these approaches are still in the early stages of technological development.

Previously, a considerable emphasis has been placed on the development of advanced strategies to design high-performance electrocatalysts, including nanostructure construction, defect introduction, atomic dispersion, and heterointerface engineering.^8–10 These approaches have successfully reduced the input voltage required for OWS, consequently lowering the energy cost. However, the economic viability of water splitting for commercial applications remains a considerable challenge. The primary objective of water oxidation is to extract electrons and transport them to the cathode for the production of hydrogen, which serves as the desired product of water splitting. Therefore, one potential strategy to overcome the aforementioned obstacles in electrolytic hydrogen production is to replace the OER with organic oxidation reactions that are thermodynamically more favorable.¹¹ For instance, it has been demonstrated that electrooxidation of renewable alcohols can achieve a remarkable 67% energy saving compared to traditional water electrolysis while generating equivalent amounts of hydrogen.¹²

These small molecule oxidation reactions typically encompass sacrificial-agent oxidation reactions (such as urea oxidation, hydrazine oxidation, polysulfide oxidation, etc.) and electrochemical synthesis reactions (such as alcohol oxidation, aldehyde oxidation, carbohydrate oxidation, and primary amine oxidation, etc.),^13–15 which possess lower theoretical equilibrium potentials (<1.23 V vs. RHE), as depicted in Fig. 1a. The former reactions are often employed for the treatment of environmental pollutants due to the toxic nature of substances like urea and hydrazine, which find widespread use as industrial raw materials and can cause significant harm to the environment. Specifically, the electrocatalytic oxidation of these small molecules offers the potential to remove toxic materials from industrial wastewater without the need for additional oxidants or complex separation methods, while also significantly reduces the overall voltage required during hydrogen production via water electrolysis. The latter reactions provide a sustainable pathway that utilizes electricity from renewable energy sources for chemical upgrading, obviating the use of organic solvents, homogeneous catalysts, and hazardous/poisonous strong oxidants (e.g., hydrogen peroxide, chloroperbenzoic acid, peroxy acids, oxone, or iodine), as well as the elevated temperatures and pressures typically associated with traditional organic oxidation reactions.^16–19 Consequently, such hybrid water electrolysis offers distinct advantages when compared to overall water electrolysis (Fig. 1b). (i) By harnessing the flexible thermodynamic oxidation reactions of organic reactants, this method significantly reduces the electric energy consumption required for hydrogen production, leading to the enhanced efficiency of energy conversion. (ii) Through sacrificial-agent-assisted water electrolysis, it becomes feasible to efficiently decompose undesirable components present in wastewater, such as urea from feed/fertilizer wastewater, hydrazine from pharmaceutical/petrochemical wastewater, and polysulfide from industrial wastewater, which will contribute to the mitigation of environmental pollution. (iii) Organic upgrading-assisted water electrolysis allows for the concurrent production of value-added compounds at the anode. These compounds can find applications in various domains, including chemical synthesis, polymer production, and pharmaceutical manufacturing. (iv) The absence of oxygen production at the anode eliminates the risk associated with H₂/O₂ explosion and the generation of harmful reactive oxygen species. (v) Given that the anodic products in organic upgrading-assisted water electrolysis are typically non-gaseous, this method can be executed without the need for a membrane, simplifying the overall electrolysis setup. The past decade has witnessed significant advancements in the coupling of organic oxidation reactions with the HER in hybrid water electrolyzers, particularly in recent years. Nevertheless, the catalytic activities and cell configurations are still far from reaching the level required for practical applications.

Fig. 1 (a) Schematic diagram of polarization curves for the HER and other available/potential oxidation reactions with low-onset potential, such as the HzOR (hydrazine oxidation reaction), the SOR (polysulfide oxidation reaction), the AOR (alcohol oxidation reaction), and the UOR (urea oxidation reaction). (b) The merits of small molecule oxidation-assisted H₂ production.

Electrocatalytic systems conventionally rely on electricity derived from fossil fuels, incurring substantial costs and wastage of electrons during operation. Additionally, the treatment of discharged water, gases, and other low or no-value chemicals results in additional and often unforeseen expenses. In reality, the overall profitability and cost of an electrocatalytic system, whether apparent or hidden, can be primarily attributed to the efficient utilization of electrons and atoms in value-added conversions.^20,21 Enhancing the atom and electron utilization, which translates to increased product profitability and decreased energy costs, lies at the core of advancing electrocatalytic systems. Consequently, the invention and development of innovative electrocatalytic systems are essential for achieving this goal. To enhance the electron economy of electrolysis, self-powered electrocatalytic systems have been devised, utilizing low-value chemical energy sourced from metal-based batteries, fuel cells, and similar technologies.^22,23 These systems inherently generate electricity inside the electrocatalytic systems to drive the electrocatalytic reactions at adjacent or opposite electrodes. In contrast to traditional water electrolyzers or organic oxidation reaction-based electrolyzers, self-powered electrocatalytic systems can operate without an external energy supply. Notably, in these self-powered electrocatalytic systems, if sustainable energy derived from sources like wind and solar power is exclusively used to charge rechargeable metal-based batteries (such as zinc–air batteries), the overall economic effectiveness of these systems can be significantly enhanced.²⁴ Similarly, due to the low equilibrium potential of metal oxidation, the electrochemical hydrogen evolution can be harnessed as the cathodic reaction in metal-based batteries, as observed in zinc–H₂O batteries.^25,26 In these batteries, hydrogen is generated at the cathode during the discharge process, simultaneously providing the necessary electricity. These systems are referred to as self-catalyzed electrolysis systems, as they can occur spontaneously upon the connection of the anode and the cathode. The aforementioned systems exhibit vastly improved economy throughout the reaction system, benefitting from their unique system configurations. Regrettably, comprehensive and in-depth discussions concerning approaches to maximize energy utilization in electrocatalytic systems remain notably absent in current literature.

This perspective aims to provide a concise overview of the latest advancements and notable achievements in the innovative strategies employed to enhance the energy efficiency of hydrogen generation through electrochemical approaches. From an economic standpoint, we will commence with a brief introduction elucidating the fundamentals of selecting appropriate anodic reactions and designing efficient electrocatalysts. Subsequently, we will provide a systematic summary of the current progress in different small molecule oxidation reactions at the anode side, categorizing them into sacrificial-agent reactions and organic upgrading reactions. Furthermore, we will explore self-powered hydrogen production systems and self-catalyzed hydrogen production systems, which exhibit enhanced energy utilization efficiency. Fig. 2 schematically illustrates these novel electrolyzers, facilitating clear visualization and comparison. Table 1 summarizes the relationship and distinct advantages of these system configurations. Each section will delve into key aspects of modulated catalyst strategies, catalytic activity, structure–performance relationships, related catalytic reaction mechanisms, and electrolyzer configurations. Additionally, we will discuss the remaining challenges and offer perspectives for the future development of these electrocatalytic systems. Our ultimate objective in this perspective is to provide an up-to-date account of the advancements in efficient electrocatalytic systems and underscore the potential of these captivating technologies for energy-saving H₂ production in the future.

Fig. 2 Schematic representation showing the electrolyzer/devices for energy-saving H₂ production, (a) the conventional OWS system, (b) sacrificial-agent-assisted water electrolysis, (c) organic upgrading-assisted water electrolysis, (d) self-powered electrocatalytic system, and (e) self-catalyzed electrolysis system.

Table 1 Feature comparison among the configurations of electrocatalytic systems for hydrogen production

Types	Products	Electricity input	Electricity output	Economy efficiency	Range of current density (A cm^−2)	Hydrogen production rate^b (mmol cm^−2 h^−1)	Electron utilization	Recyclability
a The applied cell voltage at 2.0 V on the basis of reported literature (measured in a H-type cell). b The hydrogen production rate calculated at the largest current density.
Alkaline water electrolyzer	H2 and O2	Yes	No	Average	0–0.4^a ^27,28	7.46	50%	No
Sacrificial-agent-assisted water electrolysis	H2 and N2(CO2)	Yes	No	High, if the organics are obtained from wastewater	0–1.0^a ^29–31	18.66	50%	No
Organic upgrading-assisted water electrolysis	H2 and valued-added compounds	Yes	No	High	0–0.6^a ^32–34	11.19	100%	No
Self-powered electrolysis system	H2 + O2 or H2 + N2	No	No	High	<0.1^35–37	1.87	50%	Yes
Self-catalyzed electrolysis system	H2	No	Yes	Low	<0.2^38–40	3.73	100%	Yes

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2. Electrochemical fundamentals and electrocatalyst/device design

2.1 Fundamentals of the anodic oxidation reaction

Indeed, anodic small molecule oxidation reactions offer promising advantages for expediting hydrogen production and the generation of value-added products. The integration of these reactions into water splitting systems allows for lower cell voltages compared to the OER, thereby improving energy efficiency. Additionally, the byproducts generated during these organic oxidation reactions, such as carboxylic acids, esters, and other valuable compounds, have widespread applications in various industries, including biofuel, fine chemical, and pharmaceutical. To maximize the energy return on investment and optimize the small molecule oxidation assisted electrocatalytic water splitting process, the careful selection of suitable small molecules is crucial. Here are some considerations for this aspect: (i) Appropriate redox potentials: the chosen small molecules should have theoretical potentials lower than that of the OER. This ensures that they do not interfere or compete with the OER, thus maintaining the efficiency advantages of the electrocatalytic process. (ii) High selectivity and conversion efficiency: it is crucial to maximize the selectivity and conversion efficiency of the oxidation reactions. This enables simultaneous cathodic hydrogen production and anodic degradation of pollutants or generation of value-added products, leading to a more sustainable and efficient water splitting process. (iii) Interference with the HER: neither the chosen small molecules nor their resulting products should significantly interfere with the HER at the cathode. This ensures that hydrogen production is not compromised and maintains the overall efficiency of the water splitting system. (iv) Availability and natural resources: the selected small molecules should be readily obtainable or derived from abundant natural resources or wastewater sources. This ensures their availability and scalability for large-scale implementation. Utilizing abundant resources also contributes to the economic viability and sustainability of the process. By considering these factors, small molecules that fulfill the necessary requirements for efficient small molecule oxidation assisted electrocatalytic water splitting can be identified, allowing for simultaneous hydrogen production, pollutant degradation, and generation of value-added products. In recent years, numerous small molecules, including urea, hydrazine, polysulfide, amines, glucose, alcohols, and aldehydes, have garnered increasing attention and have been successfully employed in electrochemical hydrogen production.

Various small molecule oxidation reactions increasingly studied can be broadly categorized into sacrificial-agent oxidation reactions and electrochemical upgrading reactions. To gain a deeper understanding of the reaction pathways and mechanisms involved in these small molecule oxidation reactions, researchers have extensively employed a combination of experimental techniques and theoretical calculations. By employing methods such as in situ Raman spectroscopy, in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), in situ X-ray photoelectron spectroscopy (XPS), in situ X-ray absorption spectroscopy (XAS), in situ X-ray diffraction (XRD), operando transmission electron microscopy (TEM), in situ differential electrochemical mass spectrometry (DEMS), high-performance liquid chromatography (HPLC), inductively coupled plasma (ICP) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy, researchers have been able to elucidate the structure, morphology, content, and dynamic changes of catalysts and reaction products during the reaction process.^41,42 To illustrate the distinctions among the characterization techniques discussed in this perspective, we have compiled a summary in Table 2. This table provides insights into the capabilities, advantages, limitations, probing regions, and typical references of these techniques for a better understanding. Computational methods, including density functional theory (DFT) calculations and molecular dynamics simulations, play a crucial role in elucidating reaction mechanisms and understanding the underlying principles governing small molecule oxidation. These calculations provide insights into the electronic structure, reaction energetics, and kinetics, aiding in the interpretation and prediction of experimental observations. By combining these experimental and theoretical approaches, researchers can unravel the complex reaction pathways and mechanisms involved in small molecule oxidation reactions, which will contribute to the development of efficient catalysts, reaction optimization, and the design of sustainable processes for hydrogen production, pollutant degradation, and value-added product generation.

Table 2 Representative characterization techniques with corresponding capability, advantages, limitations, probing regions, and typical references

Technique	Capability	Advantage	Limitation	Probing region	Typical references
In situ Raman	Analysing interfacial water structures	Non-invasive for samples	Insensitive for pure metals and unable to detect reactions occurring deeper within the electrode	Bulk	43 and 44
	Detecting surface active species	High spatial resolution	Low Raman peak intensity	Interface
	Probing phase reconstruction	Used in a liquid environment	Limited spectral range
In situ ATR-SEIRAS	Comparing the bonding strength of reaction intermediate adsorption	Non-destructive towards samples	Sensitivity to water	Interface	45 and 46
	Identifying reaction intermediate species	Applied to various electrochemical reactions	Poor signal intensity
		Relatively cheap and easy to implement	Limited depth profiling
In situ XPS	Providing information about chemical composition, oxidation states, and electronic structure changes on the electrode surface	High sensitivity	Only surface sensitivity	Bulk	47 and 48
		Non-destructive technique	Ultrahigh vacuum	Interface
		Identify and quantify the chemical elements
In situ XAS	Analysing local atomic structure	High energy resolution for precise determination of bond distances and coordination environments	Ultrahigh vacuum	Bulk	49–52
	Monitoring oxidation states	No requirement for the crystallinity	Possible radiation damage to samples	Interface
		Used in liquid phase to hard XAS	Complexity about operation
In situ XRD	Probing lattice parameter and the kinetics of phase transitions	Precisely probe the crystal phase structure and lattice parameters in bulk	High requirement for crystallization	Bulk	53–55
	Characterization of phase transformations	Quantitatively analyzed to determine phase compositions, and crystallite sizes	Limited spatial resolution
		Relatively cheap and easy to implement	Unable to reflect local defects in samples
Operando TEM	Insights into structural changes, growth mechanisms, and electrochemical behavior of samples	Directly observe structural reconstruction on the nanoscale	Low image resolution in liquid electrolyte	Bulk	56 and 57
	Provide crystallographic information, including lattice fringes, crystal orientations, and defects	Identification of elemental distribution during reactions	Limited field of view
		High spatial and temporal resolution	Damaged by electron beam
			Difficult to maintain a stable environment under radiation
In situ DEMS	Insights into the activity and selectivity during reactions	High temporal resolution	Complex instrumentation	Interface	58 and 59
	Suitable for gas phase analysis	High sensitivity	Limited mass range
		Molecular information
		Obtaining quantitative information
HPLC	Insights into the products, intermediates, and kinetics	High resolution	Time-consuming and requiring additional steps	−	60 and 61
		Wide applicability	Lack of temporal resolution
		Separation of mixtures	Sensitivity to mobile phase
		High sensitivity for trace species
ICP	Simultaneously detecting multiple elements	High sensitivity	Limited molecular information	−	62
		Low interference	Complex instrumentation
		Wide applicability	Reduced sensitivity in complex samples
		Isotopic analysis
NMR	Providing detailed structural information about molecules	Molecular-level insights	Long acquisition times	−	63 and 64
		Versatility	High purity for samples
		Isotopic labelling	Complex instrumentation and high cost

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2.2 Rational design of electrocatalysts

Small molecule oxidation reactions often involve the transfer of multiple electrons, making them kinetically sluggish and complex. To address this issue and improve reaction rates while reducing overpotential, the development of efficient catalysts is crucial. Initially, catalysts based on precious metals such as Pt, Pd, Ru, and Au were commonly used for these reactions.^65,66 However, the limited availability and high cost of these metals hinder their widespread application. As a result, researchers have been motivated to explore alternative, cost-effective electrocatalysts derived from nonprecious-metal elements that offer both high performance and stability.⁶⁷ These nonprecious-metal catalysts encompass a wide range of materials, including transition-metal oxides/oxyhydroxides, nitrides, phosphides, sulfides, and alloy systems. Each material exhibits unique properties and catalytic mechanisms that can be tailored to specific reaction requirements. By carefully designing the composition, structure, and morphology of these catalysts, researchers aim to enhance their electrocatalytic activity, selectivity, and durability. In recent years, various design strategies have been investigated to enhance the electrocatalytic activities of catalysts based on nonprecious metals for small molecule oxidation reactions. Fig. 3 depicts recent progress in the rational design of these electrocatalysts, categorized into three principal dimensions: electronic structure engineering, surface/interface design, and morphology regulation.

Fig. 3 Schematic illustration of the general strategies for the rational design of electrocatalysts.

Indeed, over the last decade, significant advancements have been made in the field of the HER. Through a comprehensive understanding of the fundamental mechanisms driving the HER and strategic engineering active sites, a variety of electrocatalysts with remarkable performance have been developed, rivaling that of benchmark Pt/C catalysts. Numerous insightful reviews have been published, covering various aspects of electrocatalyst design for the HER as well as their potential applicability in practical water electrolyzers.^68–72 Hence, in this section, our primary focus is to summarize and discuss the design strategies employed for electrocatalysts targeting anodic oxidation reactions.

2.2.1 Electronic structure engineering. Electronic structure engineering has emerged as a powerful strategy for enhancing the intrinsic activity of electrocatalysts. This approach involves various aspects aimed at modifying the electronic structure of catalysts, including heteroatom doping, vacancy creation, surface strain, and edge site generation. Heteroatom doping is a commonly used strategy in which foreign atoms or ions, such as N, P, O, B, and S, are introduced into the catalyst's lattice.⁷³ The presence of these heteroatoms induces local redistribution of electrons, activating both the dopants and neighboring atoms as catalytically active sites.⁷⁴ The introduction of heteroatoms can also facilitate efficient charge transfer from the electrode interface to the adsorbed species, improving the overall electrocatalytic performance.^75,76 However, compared to metal element doping, the homogeneous introduction of non-metal dopants into catalysts often pose challenges. Non-metal dopants often have lower solubility and tend to segregate, limiting their high-level doping in the catalyst's lattice.⁷⁷ Achieving precise control over the dopant distribution and concentration is crucial to maximize the benefits of heteroatom doping. For instance, Wang et al. recently reported the synthesis of a unique 2D/3D hierarchical structure comprising Fe, F co-doped Ni₂P encapsulated by a N-doped carbon shell (Fe/F-Ni₂P@NC) for the HzOR.⁷⁸ To achieve F doping, the FeNiPBA precursor was immersed in a solution of 5.0 M NH₄F. After undergoing low-temperature phosphorization under inert gas, Fe/F-Ni₂P@NC was formed, featuring both Fe and F dopants. The Fe, F co-doping resulted in an intermediate d-band center, facilitating balanced adsorption and desorption processes and thereby improving bifunctional catalytic performance. Furthermore, DFT calculations indicated that Fe/F doping enhances the thermodynamic behavior of the dehydrogenation process in the HzOR compared to samples without Fe/F doping.

The introduction of vacancies into catalysts can have a significant impact on their electronic structure and catalytic performance. Vacancies refer to the absence of atoms at specific lattice sites within the catalysts. These vacancies can lead to localized fluctuations in electron density and atomic relaxation at the defect boundaries, influencing the catalyst's properties.⁷⁹ It is worth noting that the specific effects of vacancies on catalytic performance depend on the materials, the type of reaction, and the specific conditions. The concentration, distribution, and interaction of vacancies within the catalyst are critical factors that determine their influence on catalytic activity.⁸⁰ Specifically regarding the UOR, the amino group (–NH₂) in the initial urea molecule possesses lone pair electrons and acts as an electron-donating moiety during the UOR, making it prone to adsorb on anion vacancies or cation sites. Fu et al. recently reported a novel and efficient electrocatalyst comprising N vacancies-enriched, Ce-doped Ni₃N hierarchical nanosheets supported on carbon cloth (Ce–Ni₃N@CC), aiming to optimize the UOR kinetics, particularly the rate-determining CO₂ desorption step.⁸¹ DFT calculations revealed that the incorporation of Ce into Ni₃N reduces the formation energy of N vacancies (Fig. 4a), resulting in a high density of N vacancies in Ce–Ni₃N@CC. Furthermore, the N vacancies, in combination with Ce doping, optimize the local charge distribution around the Ni sites, thereby balancing the adsorption energy of CO₂ during the rate-determining step and influencing the initial adsorption structure of urea. These factors contribute to the superior catalytic performance of Ce–Ni₃N@CC for the UOR.

Fig. 4 (a) Different kinds of formation energies.⁸¹ Reproduced with permission. Copyright 2022, Elsevier. (b) The Gibbs free energy profiles of the proposed UOR pathway at O_vac-V-γ-NiOOH.⁹⁸ Reproduced with permission. Copyright 2022, Wiley-VCH. (c) The illustration graph of Pt on V_O rich CeO₂. (d) CVs of methanol oxidation. (e) Stability testing.¹⁰⁸ Reproduced with permission. Copyright 2018, Elsevier.

Strain has garnered significant attention in recent years and has been extensively investigated and reported.⁸² In the context of electrocatalysts, strain primarily manifests as changes in bond lengths.^83,84 The strain imposed on the lattice of an electrocatalyst can alter the coordination numbers and electron densities of surface atoms, thereby modifying the surface energy and enhancing specific adsorption of target molecules. Strain defects can be intentionally introduced through various fabrication methods, such as core–shell structures, morphology modulation, alloying, and particle size regulation.^85,86 By tuning the width of the surface d-band, the average energy can be adjusted, thus controlling chemical properties such as the dissociative adsorption energy of hydrogen. Lattice strain, whether compressive or tensile, can significantly modify the surface electronic structure by regulating the distances between surface atoms.^87–89 For instance, previous studies have shown that a mere 1% lattice strain in Pt can shift the d-band center by approximately 0.1 eV, resulting in more than a 300% improvement in oxygen reduction reaction (ORR) activity through optimized adsorption energy.^90–92 However, the conventional approach of using a metal overlayer can introduce both ligand and synergistic effects, complicating the correlation between reactivity and lattice strain. In a recent study, porous rhodium nanosheets (Rh-NSs) with tunable lattice strain were successfully synthesized by direct annealing of metastable trigonal rhodium oxide (Tri-RhO₂) nanosheets in a hydrogen atmosphere.⁹³ The magnitude of compressive strain in the Rh-NS electrocatalysts could be controlled by adjusting the annealing temperature. In stark contrast, metallic rhodium obtained by annealing rutile-RhO₂ under the same conditions exhibited almost no compressive strain. Electrochemical results demonstrated that the optimized Rh-NSs-300 exhibited an ultralow working potential of −103 mV (vs. RHE) at a current density of 10 mA cm⁻², with a Tafel slope of 15.4 mV dec⁻¹ for the HzOR. DFT calculations indicated that Rh-NSs with tunable compressive strain could reduce the Gibbs free energy of the potential-determining step in the HzOR.

Recent investigations have provided compelling evidence that atoms located at the edges of 2D materials exhibit distinct properties and higher catalytic activity compared to their basal planes.^94–96 Nickel hydroxide (Ni(OH)₂), a well-known 3d transition metal-based material with a layered structure, has been extensively studied and considered a promising electrocatalyst for the UOR due to its favorable activity and impressive stability in alkaline environments. Luo et al. reported the synthesis of ultrathin Ni(OH)₂ nanosheets with small lateral sizes through a simple liquid-phase reaction.⁹⁷ The combination of theoretical calculations and experimental investigations revealed that the abundant active edges in Ni(OH)₂ nanoflakes not only facilitate an accelerated phase transformation from Ni(OH)₂ to the electroactive NiOOH but also enhance the kinetics of urea adsorption during the electrochemical process, resulting in significantly improved electrocatalytic activity for the UOR. Similarly, Jiao et al. reported the synthesis of O-vacancy-rich Ni(OH)₂ through facile V doping, leading to the formation of O_vac–V–Ni(OH)₂.⁹⁸ V doping not only generates more exposed edge active sites but also introduces oxygen vacancies, shifting the rate-determining step of the UOR from *COOH deprotonation to the N–H bond cleavage process. This modification lowers the thermodynamic barrier by approximately 1.13 eV compared to pristine γ-NiOOH (Fig. 4b), thereby significantly improving the reaction performance.

2.2.2 Surface/interface design. Surface/interface design has emerged as a highly promising strategy for achieving enhanced electrocatalytic performance and exploiting synergistic effects in nanomaterials.^99–101 In particular, the construction of heterostructures, which involve combining different materials at the nanoscale, has garnered significant attention in the field of electrocatalysis. Heterostructures offer unique opportunities to regulate electronic configurations and exploit synergistic effects at the interfaces between different components.¹⁰² The interfaces within heterostructures facilitate interfacial electron transfer, enabling the regulation of the electronic structure.^103,104 This controlled electronic structure strongly influences the electrocatalytic performance of the catalysts. Furthermore, the increase in the number of interfaces in heterostructures leads to a higher density of accessible active sites. This higher density of active sites provides more opportunities for reactant adsorption and reaction, thus facilitating improved catalytic activity.^105,106 Additionally, the rational design and control of interface structures enable the optimization of electrolyte–catalyst contact, which can enhance mass transport and reaction kinetics. By controlling the interface structures, we can harness synergistic effects, increase the density of active sites, and optimize electrolyte–catalyst interactions, ultimately leading to improved catalytic activity and efficiency.

The catalytic performance of the methanol oxidation reaction (MOR) is highly surface and interface sensitive. Therefore, the presence of interfaces can have a profound impact on the catalytic performance in the MOR, as it influences the optimization of electronic configurations through interfacial electron transfer in heterostructures. The construction of Pt-based heterogeneous catalysts is particularly important for optimizing the binding energies of reaction intermediates during the MOR, thereby influencing the electrocatalytic performance and stability.¹⁰⁷ Wang et al. used the CeO₂ nanorods with rich oxygen vacancies (V_O) and a rough surface as the Pt support to engineer the metal-oxide interface.¹⁰⁸ This unique property of the CeO₂ substrate allows for effective modification of the particle size and dispersion of Pt nanoparticles, as well as the regulation of the Pt–ceria interaction (Fig. 4c). Notably, the abundant V_O on the surface of CeO₂ provides surplus electrons that transfer to the sub-surface, leading to a decrease in the Ce valence in the sub-surface of CeO₂, as well as alters the electron transfer between Pt and ceria, resulting in an increased electron density in Pt. Due to the distinctive electron structure of Pt and CeO₂, the catalytic activity and durability of the electrocatalyst for the MOR are significantly enhanced (Fig. 4d and e).

Mott–Schottky heterojunctions, characterized by the presence of semiconducting phases, exhibit unique properties arising from the differences in Fermi energy and charge density between the two sides of the interface. This leads to the transfer of electrons across the interface, creating a built-in electric field, a space charge region, and local charge accumulation or depletion. The band bending at the heterointerfaces induces a charge distribution, which facilitates the activation of small molecules such as urea.¹⁰⁹ In the context of bifunctional electrocatalysis for both the HER and UOR, a bimetal heterostructure consisting of CoMn and CoMn₂O₄ was designed as a bifunctional electrocatalyst.¹¹⁰ The CoMn/CoMn₂O₄ Schottky synergistic effects were proposed as the basis for the UOR catalytic mechanism, in which a self-driven charge transfer occurs at the interface (Fig. 5a). This charge transfer process facilitates the adsorption of reactants and the breaking of chemical bonds, resulting in optimal adsorption energy (Fig. 5b). Consequently, both the HER and UOR are triggered. Remarkably, ultralow cell voltages of −0.069, 1.32, and 1.51 V were required to achieve a current density of 10 mA cm⁻² for the HER, the OER, and urea-assisted hybrid water electrolysis, respectively.

Fig. 5 (a) The schematic UOR catalytic mechanism based on CoMn/CoMn₂O₄ Schottky synergistic effects. (b) Calculated adsorption energies of CO(NH₂)₂, H₂O, CO₂, H₂, and N₂ on CoMn₂O₄ and CoMn/CoMn₂O₄ surfaces.¹¹⁰ Reproduced with permission. Copyright 2020, Wiley-VCH. In situ FTIR spectra of n-valeraldehyde adsorption on (c) NiMn₂O₄ and (d) Au_0.3 SACs-NiMn₂O₄.¹¹⁶ Reproduced with permission. Copyright 2022, Wiley-VCH. (e) Linear sweep voltammetry (LSV) curves of NiO, Ru₁–NiO, and RuO₂ in 1.0 M PBS with 50 mM HMF. (f) Proposed HMF oxidation reaction mechanism over Ru₁–NiO in the neutral medium.¹¹⁷ Reproduced with permission. Copyright 2022, Wiley-VCH. (g) The contact angels of water (CA_water) and bubbles (CA_bubble) on NiCo@C/MXene/CF or NiCo@C/CF, and the optical images of gas bubbles released from both electrodes during the HER. Scale bars: 500 μm. (h) Chronopotentiometric curves of NiCo@C/MXene/CF and NiCo@C/CF for hybrid seawater electrolysis.¹²⁹ Reproduced with permission. Copyright 2021, Nature Publishing Group.

In recent years, there has been a lot of interest in the field of single-atom electrocatalysts (SACs). SACs are characterized by discrete metal atoms that are anchored onto a suitable substrate, offering unique advantages such as near-unity atomic utilization and enhanced catalytic performance.^111,112 By preventing agglomeration and dissolution phenomena, the anchored metal atoms can maintain their isolated and well-defined structures, leading to enhanced catalytic performance. The interplay between the individual metal atoms and their surrounding counterparts derived from the support materials or other metal atoms, plays a pivotal role in governing the catalytic activity of SACs. Therefore, selecting a suitable support system involves careful consideration of various factors. The support materials should possess a suitable structure, high surface area, and appropriate surface chemistry to promote strong interactions with the metal atoms. It should also exhibit good chemical stability under the harsh reaction conditions to ensure the durability of the SACs. To date, various classes of SACs with different supports have been successfully synthesized.^113–115 Carbon-based SACs, including graphene, nitrogen-doped graphene, and other porous carbon materials, have been extensively explored. SACs supported by two-dimensional materials such as transition metal dichalcogenides, MXene, black phosphorus, and layered double hydroxides (LDH), have been investigated. Furthermore, support-confined SACs, such as metal alloys and carbides, have also been explored. The inherent high reactivity and stability of SACs make them promising candidates for various catalytic reactions.

Each support material offers unique properties and interactions that can be tailored to specific catalytic applications. For example, Li et al. developed Au SACs–NiMn₂O₄ spinel synergetic composites for the electrocatalytic upgrading of n-valeraldehyde to octane.¹¹⁶ Experimental and theoretical studies revealed that the presence of Au SACs on the spinel surface lowers the adsorption energy of the initial n-valeraldehyde molecule, thereby accelerating the dimerization process of alkyl radicals originating from Mn–C₄H₉ and Au–C₄H₉, leading to the formation of octane. In situ Fourier transform infrared spectroscopy (FTIR) further confirmed the enhanced adsorption performance of n-valeraldehyde on Au SACs–NiMn₂O₄. As shown in Fig. 5c and d, the characteristic peaks of n-valeraldehyde are observed at 1751, 2712, and 2970 cm⁻¹, which are associated with C=O and C–H stretching vibrations. For Au_0.3 SACs–NiMn₂O₄, the intensity of n-valeraldehyde peaks gradually increases with prolonged reaction time. Additionally, new species are formed on the surface over time. The appearance of a peak at 1712 cm⁻¹ indicates the vibrational mode of the carbonyl group in fatty acids, specifically n-valeric acid formation. The intensity of the n-valeric acid peak initially increases and subsequently decreases as time goes on, indicating its continuous involvement in the reaction on the surface of Au_0.3 SACs–NiMn₂O₄. These in situ FTIR results provide evidence that the presence of Au single atoms on NiMn₂O₄ enhances the adsorption of n-valeraldehyde, thereby facilitating its conversion. In another example, Duan et al. reported the utilization of single-atom Ru on nickel oxide (Ru₁–NiO) as a catalyst for the electrooxidation of alcohols derived from biomass into aldehyde products.¹¹⁷ For the oxidation reaction of 5-hydroxymethyl furfural (HMF) in a 1.0 M phosphate buffer solution (PBS), Ru₁–NiO shows a low potential of 1.283 V at 10 mA cm⁻² (Fig. 5e). Through a combination of cyclic voltammetry (CV), Raman spectroscopy, and operando electrochemical impedance spectroscopy analyses, the authors revealed that the remarkable activity of Ru₁–NiO ascribed to the significantly improved water dissociation ability occurring at the single-atom Ru sites. As depicted in Fig. 5f, water molecules adsorbed on Ru atoms dissociate into OH* species, accompanied by simultaneous electron and proton transfers. And the resulting OH* species react with the nucleophile, HMF, yielding 2,5-diformylfuran and water.

In contrast to SACs, dual single-atom catalysts (DSACs) possess multiple active centers with distinct electronic states within a localized domain.¹¹⁸ This characteristic enables DSACs to fulfill the requirements for intermediate absorption in multi-reaction steps, thereby exhibiting enhanced activity and selectivity as atomic dispersion catalysts.^119,120 Pt-based catalysts have been recognized for their efficiency in the HER. The synergistic effect between metal atoms in DSACs facilitates the maintenance of a near-zero hydrogen adsorption energy, promoting efficient hydrogen production. For instance, Sun et al. employed an atomic layer deposition process to fabricate a Pt–Ru dimer catalyst for the HER.¹²¹ To assess the HER activity under varying hydrogen adsorption scenarios, the Gibbs free energy for hydrogen adsorption (ΔG_H) of the Pt–Ru dimer was calculated. When one hydrogen atom is adsorbed on both sides of the Ru and Pt atoms in the Pt–Ru dimer (designated as Pt(1H)Ru(1H)), ΔG_H is approximately −1 eV, indicating that the hydrogen atoms are preferentially adsorbed on the Ru atoms. Upon further adsorption of a second hydrogen atom on the dimer, ΔG_H for Pt(2H)Ru(1H) reaches a high value of 0.6 eV. Remarkably, ΔG_H of the Ru atom exhibits an exceptionally low value of 0.01 eV when considering the pathway from Pt(3H)Ru(3H) to Pt(3H)Ru(2H), even lower than that of a single Pt atom. This clearly demonstrates that hydrogen can easily dissociate from the Ru atom when the coverage reaches its maximum. Furthermore, the presence of Pt atoms strongly influences the electronic structure of the Ru atoms. DFT calculations reveal that the Pt–Ru dimer can be readily converted between metallic and semiconducting states upon adsorption, leaving vacant orbitals. The synergistic effect induced by Pt modulates the interaction between H and Ru, thereby enhancing the HER activity of the Pt–Ru dimer catalyst.

2.2.3 Morphology regulation. Nanomaterials possess unique properties due to their size and morphology. These properties include ultrahigh catalytic activity, elevated surface energy, substantial specific surface area, directional electron/photon/heat conduction, and efficient mass transport.^122–124 These distinctive features allow nanomaterials to fulfill various requirements for catalyzing target chemical reactions. The different dimensional attributes of nanomaterials offer distinct advantages. For instance, 0D nanoparticles and quantum dots exhibit high surface area-to-volume ratios, allowing for a large number of catalytic sites and efficient reactant adsorption.¹²⁵ One-dimensional nanowires, nanoneedles, nanotubes, and nanoarrays provide extended one-dimensional pathways for fast mass transport and electron conduction. Two-dimensional nanosheets offer large exposed surface areas and facilitate charge transfer at the surface. Three-dimensional nanostructures, such as composites and hierarchical structures, combine multiple dimensionalities, creating complex architectures that can synergistically enhance catalytic performance. By tailoring the nanostructure, we can optimize the catalytic activity, selectivity, and stability of electrocatalysts.¹²⁶ The ability to design and control the nanostructures of electrocatalysts is crucial for advancing electrocatalysis. Previous studies have employed a variety of synthetic techniques, such as solvothermal, hydrothermal, chemical vapor deposition, and self-assembly methods, to precisely control the size, shape, composition, and architecture of nanomaterials.¹²⁷ These techniques enable the fine-tuning of catalytic properties to meet the specific requirements of different reactions and applications.

To facilitate efficient water splitting at high current densities, it is crucial to ensure the rapid desorption of generated gas bubbles. This is essential because the adhesion and accumulation of gas bubbles can introduce significant ionic diffusion resistance and hinder access to active sites within the system.¹²⁸ Furthermore, the growth, collapse, and detachment of bubbles from the electrode surface can impose substantial mechanical strain and stretching forces, potentially leading to catalyst detachment and an overall decline in cell performance. It is evident that a hydrophilic surface plays a pivotal role in enhancing the adsorption of water while repelling the generated gas bubbles. Such characteristics are highly favorable for water splitting, especially under conditions of high current density. To achieve this, the engineering of super-wettability surfaces with special interactions at the solid–liquid–gas interfaces holds great promise. This approach can expedite the mass transfer process and reduce the ohmic resistance at the catalyst surface. Importantly, the wettability of a catalyst can be effectively adjusted by tuning its geometric structures. For instance, Sun et al. have developed a novel NiCo/MXene-based electrode by assembling NiCo-metal organic framework (MOF) nanosheets on MXene-wrapped Cu foam (MXene/CF), followed by annealing in NH₃.¹²⁹ The resulting NiCo@C/MXene/CF electrode exhibits a mesoporous array of NiCo-decorated carbon nanosheets (NiCo@C) with an average size of 400–800 nm and a thickness below 50 nm on the MXene/CF substrate. Importantly, this electrode possesses a remarkably high contact angle for gas bubbles (CA_bubble = 153°), which effectively facilitates the rapid release of small gas bubbles (<60–80 μm) during the electrolysis process (Fig. 5g). This improvement is critical for maintaining a sufficient triple-phase interface between the electrode, electrolyte, and gas phase, thereby ensuring stable electrolysis operation under conditions of vigorous gas evolution. In contrast, when MXene is absent, the surface of NiCo@C/CF exhibits irregular microstructures due to poor chemical coupling between NiCo@C and CF. Such structural degradation not only diminishes the availability of active sites but also compromises the electrode's resistance to gas accumulation. Consequently, the NiCo@C/CF electrode experiences significant voltage fluctuations caused by the vigorous accumulation and detachment of large gas bubbles (Fig. 5h), leading to a reduction in hydrogen production efficiency. Therefore, the incorporation of macroporous gas transport channels and a nanoarray-based superaerophobic surface into the NiCo@C/MXene/CF electrode represents a substantial advancement towards optimizing the gas evolution process, particularly at higher current densities.

2.2.4 Design steps towards electrocatalysts. Electrocatalysts are pivotal components in driving a wide array of electrochemical reactions, encompassing processes such as the OER and the oxidation of small molecules. While both categories of reactions rely on electrocatalysts, it is important to acknowledge the notable differences in the catalysts employed for these purposes, primarily stemming from the distinct mechanisms and specific reaction prerequisites associated with each. OER catalysts focus on promoting the efficient evolution of oxygen gas from water, while small molecule oxidation catalysts are designed for diverse reactions, each with its own unique requirements.

On the basis of the current research's achievements, the key steps and considerations for designing active catalysts towards small molecule oxidation are outlined in Fig. 6. (i) Identify the target reaction: different reactions have different requirements and mechanisms, so understanding the fundamentals of the reaction is crucial. (ii) Choose suitable catalyst materials: according to the reaction requirements, the appropriate catalyst materials are selected. (iii) Optimization of catalyst composition: adjust the catalyst composition by alloying or mixing with other materials to improve catalytic activity. Bimetallic catalysts, for example, can exhibit enhanced activity due to synergistic effects between different metal components. (iv) Surface modification: modify the surface of the chosen catalyst material to enhance its catalytic activity. (v) Electrocatalyst characterization: utilize various characterization techniques to thoroughly analyze the catalysts’ structure and properties, gaining insights into their suitability for the target reaction. (vi) Electrocatalyst testing: conduct electrochemical tests to evaluate the catalyst's performance. Compare the catalyst's activity and selectivity to benchmarks and evaluate its performance under realistic conditions. (vii) Understanding reaction mechanisms: investigate the detailed reaction mechanisms through experimental techniques and computational modeling. A deep understanding of the reaction pathways and intermediates is critical for catalyst design. (viii) Electrocatalyst stability: assess the stability and durability of the catalyst under electrochemical conditions. Catalyst degradation and poisoning should be minimized for long-term performance. (ix) Iterative design: employ the knowledge gained from experiments and modeling to refine the catalyst's design. Iterative testing and modification is necessary to optimize its performance continually. (x) Scale-up and integration: if the catalyst exhibits promise in laboratory settings, scale up its production and seamlessly integrate it into practical electrochemical devices. (xi) Environmental and economic considerations: consider the environmental and economic aspects of the catalyst design, including the use of cost-effective and sustainable materials.

Fig. 6 The design flow diagram towards efficient electrocatalysts.

Designing active electrocatalysts for small molecule oxidation reactions is a multidisciplinary process that requires a combination of material synthesis, electrochemical characterization, and a deep understanding of the underlying reaction mechanisms. Collaboration between chemists, material scientists, and electrochemists is often crucial to succeed in this field.

2.3 Reaction devices

Besides catalyst design, the development of efficient devices plays another crucial role in enhancing the efficiency and performance of chemical reactions.^130,131 For water splitting, the configuration of the electrolysis cell can significantly impact the overall performance and product selectivity. The conventional single-pool cell configuration is commonly used in lab-scale experiments to screen catalysts due to its simplicity in assembly.^132,133 In this configuration, the working electrode, usually the anode for the OER and the cathode for the HER, is placed in the same electrolyte pool (Fig. 7a). However, one drawback of this configuration is the occurrence of undesired reactions at the counter electrode, where the liquid-phase products generated at the working electrode can undergo electrochemical reconversion. To address this issue, H-type cells, also referred to as two-compartment or divided cells, have been widely employed in various electrochemical reactions by incorporating an ion-exchange membrane (Fig. 7b).¹³⁴ The H-type cell consists of two compartments, typically separated by a membrane, such as an anion exchange membrane (AEM), a proton exchange membrane (PEM), and a bipolar membrane (BPM), which allows for the selective transport of ions into distinct chambers while preventing direct contact between the working and counter electrodes.^135–137 AEMs, acting as ionic conductors and separators, ensure the operation of the cell in mildly alkaline electrolytes. The use of a PEM as a solid polymer electrolyte enables the conduction of H⁺ ions from the anode to the cathode. The BPM combines the advantages of both the AEM and PEM by facilitating the anodic reaction in the basic electrode and promoting the preferential occurrence of the cathodic reaction in an acidic solution, thus reducing the cell's overpotential.¹³⁸ Given that the HRE is kinetically favorable under acidic conditions and the OER under alkaline conditions, it would be preferable to realize these half reactions simultaneously in different electrolytes with a BPM.^139,140 For instance, with the catalysis of bifunctional cobalt–nickel phosphide nanowire (Co–Ni–P/NF) catalysts, a water splitting electrolyzer with a BPM delivers 10 mA cm⁻² at 1.567 V, while an electrolyzer with an AEM requires a voltage of 1.623 V (Fig. 7c).¹⁴¹ Moreover, the FEs for both the HER and OER in this BPM-assembled electrolyzer also reach 100% (Fig. 7d). Benefiting from these advantages of the BPM compared to conventional AEM and PEM, the BPM has been widely used in various systems for hydrogen production, such as photoelectrochemical water splitting,^142,143 electrochemical water splitting,^144,145 hybrid water splitting,^146,147 alkali–acid hydrazine fuel cell,^148–150 and zinc–H₂ batteries.¹⁵¹ Nonetheless, it is worth noting that the electrochemical resistance is directly influenced by the distance between two electrodes. Due to the increased separation between the two electrodes and the presence of an ion-exchange membrane, a H-type cell exhibits inherently higher electrochemical resistance compared to a single-pool cell configuration. Consequently, the productivity of H-type cells is significantly reduced, leading to increased energy costs.

Fig. 7 Schematic of (a) the single-pool cell configuration, and (b) H-type cell configuration for performance evaluation. Water electrolysis tests performed in a two-compartment Teflon cell separated by a polymer membrane, (c) BPM-based asymmetric water electrolysis and AEM/PEM-based water electrolysis, (d) FE of the BPM-based asymmetric water electrolysis.¹⁴¹ Reproduced with permission. Copyright 2020, Wiley-VCH. Schematic of (e) AEM electrolyzer, (f) PEM electrolyzer, and (g) BPM electrolyzer for performance evaluation. (h) Crucial factors to be considered for designing electrolyzers with superior activity and long-term stability. (i) Schematic illustration of the flow cell configuration. (j) LSV curves for the water electrolysis with and without glycerol addition using the flow cell. (k) LSV curves for the water electrolysis using a single-chamber electrolyzer.³³ Reproduced with permission. Copyright 2023, Wiley-VCH.

To address these challenges, innovative flow cells, for instance membrane reactors, have been developed to establish direct contact between the electrodes and ion-exchange membrane through the assembly of membrane electrode assemblies (MEAs). According to the type of the used membrane, these membrane reactors include an AEM electrolyzer (Fig. 7e), a PEM electrolyzer (Fig. 7f), and a BPM electrolyzer (Fig. 7g). MEAs typically consist of a five-layer structure, wherein a catalyst-coated membrane (CCM) is sandwiched between two gas diffusion layers (GDLs) or an ion-exchange membrane is placed between two gas diffusion electrodes (GDEs).^152,153 Notably, the continuous circulation of the electrolyte over the electrode surface in MEAs helps overcome the mass-transport limitations encountered in conventional H-type cells or single cells.^154–156 Furthermore, the zero-gap configuration achieved by pressing the GDL, catalyst layer, and ion-exchange membrane together in an MEA greatly reduces system impedance, thereby improving the reaction rate and overall energy efficiency. As an example, in a GDE-type electrolyzer operating with 1.0 M KOH electrolyte and a thickness of 3 mm in the cathode chamber, the protons/ions traverse the electrolyte with an internal resistance of approximately 1.875 Ω cm², resulting in a voltage loss of approximately 800–900 mV at an operating current density of 500 mA cm⁻².¹⁵⁷ Recently, a series of novel flow cells have been employed for large-scale H₂ production at high current densities. For instance, by utilizing highly efficient NiVRu-LDHs NAs/NF as both the cathode and anode for the HER and the glycerol oxidation reaction (GOR), respectively, a flow electrolyzer with an AEM (Fig. 7i) achieves a low cell voltage of 1.933 V to deliver a current density of 1 A cm⁻² (Fig. 7j), whereas a single-chamber electrolyzer can only achieve 273.3 mA cm⁻² (Fig. 7k).³³ Furthermore, it demonstrated considerable formate and H₂ productivities of 12.5 and 17.9 mmol cm⁻² h⁻¹, respectively, at an industry-level current density, with high faradaic efficiency (FE) of nearly 80%.

The latest advancements in electrochemical reactor and system design underscore the need for unconventional approaches to water electrolysis, as well as the optimization of more efficient and durable system designs. In Table 3, we have presented the characteristics, advantages, and limitations of various electrolytic reactors to facilitate a straightforward comparison between them. However, it is important to note that apart from the exploration of high-performance electrocatalysts and devices, the activity, long-term stability, and product yields of membrane reactors are also influenced by several critical factors. These factors encompass considerations such as the choice of membrane, support materials, binder/ionomer, gas diffusion characteristics, operational conditions, local pH values, and the presence of impurities, as depicted in Fig. 7h.

Table 3 Comparison of the main characteristics displayed by various electrolyzer towards hydrogen production

Reactor	Separator	Electrolyte	Advantages	Drawbacks
Single-pool cell	None	Acidic/neutral/alkaline	Low costs	Inevitable reconversion of electrode products
			Easy to assemble and test
H-Type cell	Various ion exchange membrane	Depending on the selected membrane	Mature technology	Inherent high ohmic loss
			Easy to achieve large-scale applications	Limited current density
AEM-based electrolyzer	Anion exchange membrane	Typically KOH	Fast response, long life and low price	Low OH^− conductivity in polymeric membranes
			Nonprecious metal-based catalysts
			High current density
PEM-based electrolyzer	Proton exchange membrane (e.g., Nafion 117)	Typically pure water	Small floor space	Noble metal-based catalysts
			Fast response	Poor durability
			High current density	High capital cost
			High-purity hydrogen production
BPM-based electrolyzer	Bipolar membrane	Typically an extreme pH gradient	Without restriction of operation to electrolytes	Ion leakage, resulting in pH changes and energy loss
			Less pH gradient	High cost and short service life

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3. Sacrificial-agent-assisted H₂ production

Sacrificial-agent-assisted hydrogen production systems have indeed shown great potential for highly efficient hydrogen generation, thanks to their ultralow theoretical voltages. These systems involve the consumption of sacrificial agents such as urea and hydrazine, which provide a source of electrons and protons for the HER. However, a drawback of these systems is the generation of low-value-added byproducts during the sacrificial agent consumption process. Interestingly, these sacrificial-agent-assisted systems also hold promise for water treatment applications, as the sacrificial agents themselves, such as urea and hydrazine, are recognized as typical pollutants in wastewater (Fig. 8). Thus, the use of these sacrificial agents not only enables efficient hydrogen production but also provides a means of removing these pollutants from wastewater streams. In recent years, significant advancements have been made in sacrificial-agent-assisted hydrogen production systems, and there is ongoing research to elucidate the underlying catalytic mechanisms involved. In this section, we aim to present a comprehensive overview of the recent advancements in sacrificial-agent-assisted hydrogen production systems, for instance, urea oxidation reaction, hydrazine oxidation reaction, and polysulfide oxidation reaction, and elucidate the underlying catalytic mechanisms involved.

Fig. 8 Schematic illustration of cost-effective and sustainable H₂ production from a renewable-powered and sacrificial-agent-assisted electrolyzer with cost-free seawater and industrial sewage as the feeds.

3.1 Urea oxidation reaction

A significant amount of wastewater, both from industrial and domestic sources, contains urea, and approximately 80% of this wastewater is discharged directly into the environment each year.^30,158 If left untreated, urea naturally decomposes, leading to the release of ammonia and nitrate. This decomposition process will result in severe environmental pollution and water resource eutrophication. To address this issue, the urea oxidation reaction (UOR, CO(NH₂)₂ + 6OH⁻ → CO₂ + N₂ + 5H₂O + 6e⁻, 0.37 V vs. RHE) can be employed to utilize urea from wastewater, making it an attractive approach for wastewater treatment due to its low theoretical voltage.¹⁵⁹ By coupling the UOR with the HER, it becomes possible to simultaneously achieve wastewater treatment and highly efficient hydrogen generation. This approach offers the advantage of utilizing urea as a renewable resource for hydrogen production while addressing the environmental issue of urea-containing wastewater. However, the UOR is a complex process involving the transfer of 6e⁻ and includes intricate adsorption and desorption steps for reactants and products. As a result, the UOR suffers from inherently sluggish kinetics, which hinders its efficiency. Thus, the development of highly efficient and active catalysts for the UOR remains a challenging task.

Extensive research has been conducted on Ni-based catalysts for the UOR due to their outstanding performance. Huang's group synthesized coupled NiSe₂ nanowrinkles with a Ni₅P₄ nanorod heterogeneous structure onto nickel foam (NiSe₂@Ni₅P₄/NF) via the successive phosphorization and selenization processes.¹⁶⁰ Theoretical simulations confirmed that the closely contacted interface led to fast charge transfer from Ni₅P₄ to NiSe₂, thus constructing high-flux electron transfer pathways, enhancing carrier concentration and facilitating electron transfer. This significant electron interaction optimizes the adsorption energy of urea molecules and results in the decreased reaction energy barriers during the UOR. Experimental results revealed that this designed NiSe₂@Ni₅P₄/NF demonstrated a higher current density of 500 mA cm⁻² at a low potential of 1.402 V vs. RHE, with a small Tafel slope of 27.6 mV dec⁻¹, as well as the excellent stability of 950 h at 100 mA cm⁻². Zhang's group prepared Ce-doped Ni₂P nanosheets through a simple hydrothermal and vapor-phase phosphorization method, which demonstrated excellent catalytic activities for both the HER and the UOR.¹⁶¹ A two-electrode urea-assisted electrolyzer utilizing the Ce-doped Ni₂P nanosheets achieved a current density of 10 mA cm⁻² at a cell voltage of 1.51 V, which is lower than the voltage required for overall water electrolysis (∼1.62 V). Other Ni-based catalysts, such as r-NiMoO₄,¹⁶² Ni₂P/CC,¹⁶³ Ni–Mo nanotubes,¹⁶⁴ Ni₃Se₂/MoO₂@Ni₁₂P₅,¹⁶⁵ NiO-CrO_x,¹⁶⁶ NiF₃/Ni₂P,¹⁶⁷ Ni₂P/NiMoP heterostructure,¹⁶⁸ FeP₄ nanotube@Ni–Co–P nanocage,¹⁶⁹ and Ni₃N/Ni_0.2Mo_0.8N,¹⁷⁰ have also demonstrated compelling performance in the UOR.

The high-valence Ni³⁺ is commonly believed to show higher intrinsic activity towards the UOR. With respect to the UOR, the engineering of high-valence active sites can be achieved by hetero-element doping, alloying, and heterostructures. For example, the recently reported catalyst systems, such as Rh-doped NiFe-LDH,¹⁷¹ Ru-Co₂P/N-C/NF,¹⁷² Fe, V doped NiS arrays,¹⁷³ V doped FeNi₃N/Ni₃N,¹⁷⁴ NiMoV LDH/NF with Mo and V dopants,¹⁷⁵ V-Ni₃N/NF,¹⁷⁶ S-incorporated CoNiFe(oxy)hydroxides nanosheets,¹⁷⁷ and F-doped Ni(OH)₂,¹⁷⁸ are all exhibit improved UOR performance. Wang et al. developed a W-doped NiO (Ni-WO_x) catalyst, which demonstrated remarkably fast reaction kinetics and a high conversion frequency in urea oxidation, surpassing the performance of the catalysts without W elements.¹⁷⁹ XPS and in situ X-ray absorption near edge spectra (XANES) analysis revealed that the presence of W dopants induced changes in the charge distribution around Ni atoms (Fig. 9a–c), facilitating the formation of Ni³⁺ active centers with excellent electrocatalytic activity during the reaction. Additionally, the electron transfer from Ni to W atoms resulted in the formation of electron-deficient and electron-rich regions, as depicted in Fig. 9d. These localized regions were beneficial for the adsorption of –NH₂ and C=O groups, respectively, thereby contributing to the intriguing UOR performance and faster reaction kinetics. Feng et al. synthesized NiS₂–MoS₂ hetero-nanorods through the sulfidation of NiMoO₄ nanorods, aiming to achieve a high-valence Ni/Mo synergism for urea-assisted water electrolysis.¹⁸⁰ The presence of Mo⁶⁺ induced the formation of Ni³⁺ species and facilitated electronic synergism between Ni and Mo. The resulting NiS₂–MoS₂ hetero-nanorods exhibited exceptional catalytic activity, stability, charge transfer ability, and catalytic kinetics compared to pure NiS₂ and MoS₂ phases. Through XPS analysis (Fig. 9e–g) after a stability test, it was observed that the aged sample showed an increased amount of Mo⁶⁺ and Ni³⁺, indicating the involvement of high-valence Mo⁶⁺ and Ni³⁺ species during the catalytic reaction. Besides, Ce-incorporated Ni₂P nanosheets,¹⁶¹ N-doped carbon nanorod supported Ni₂P,¹⁸¹ core–shell Ni(OH)₂/NiO-C/WO₃ hierarchical arrays,¹⁸² and Fe-doped Ni-based MOF nanosheet arrays183 were also recently reported to promote the Ni²⁺/Ni³⁺ conversion, thus accelerating the UOR kinetics. High-valence Co species are also favorable for the electrooxidation of urea.¹⁸⁴ Jiao et al. synthesized Mo-doped cobalt carbonate hydroxide nanoarrays (Co_xMo_yCH) as catalysts for the UOR.¹⁸⁵ Post-UOR XPS characterization and DFT calculations provided compelling evidence for the formation of high-valence Co³⁺ species within CoOOH because Co electrons transfer to Mo with the higher valence state, enhancing the conversion of Co²⁺ to Co³⁺. DFT calculations further confirmed that the incorporation of Mo sites promoted the activation and adsorption of urea on Co sites, owing to the changed electron density.

Fig. 9 (a) Ni K-edge XANES of Ni-WO_x-1.6 V and NiO_x-1.6 V, respectively. (b) and (c) High-resolution Ni 2p XPS spectra of Ni–WO_x and NiO_x after the reaction, respectively. (d) Charge density difference for Ni–WO_x from DFT calculations.¹⁷⁹ Reproduced with permission. Copyright 2021, Wiley-VCH. XPS spectra of (e) Ni 2p, (f) Mo 3d, and (g) S 2p for the NiS₂–MoS₂ catalyst before and after the stability test.¹⁸⁰ Reproduced with permission. Copyright 2022, Elsevier.

The intricate nature of urea, encompassing both electron-donating amino (–NH₂) and electron-withdrawing carbonyl (C=O) groups, necessitates a comprehensive understanding of the interplay between these groups and the adsorption strength of urea. Such an understanding serves as a valuable foundation for designing and developing highly efficient UOR catalysts. To address this, a CoS₂/MoS₂ Schottky heterojunction was fabricated as a model catalyst to modulate urea molecule adsorption strength and activate its chemical bonds.¹⁰⁹Fig. 10a–c show the experimental and theoretical findings, which reveal that the spontaneous electron transfer from CoS₂ to MoS₂ induces the formation of electrophilic and nucleophilic regions at the heterointerfaces. This intricate interplay facilitates the adsorption of electron-donating and electron-withdrawing groups, triggering the decomposition of urea and consequently enhancing UOR activity. Likewise, bimetallic CoMn/CoMn₂O₄ with a significant Mott–Schottky heterostructure exhibits analogous reaction mechanisms.¹¹⁰ In a study by Liu et al., a Co and V co-doped NiS₂ (NCVS) catalyst was prepared and evaluated for UOR performance.¹⁸⁶ The XPS results demonstrated a synergistic effect among the Ni, Co, and V elements. In situ electrochemistry Raman spectra (Fig. 10d) provided insights that the introduction of heteroatoms does not entirely alter the UOR pathway. Furthermore, in situ electrochemistry mass spectrometry isotope tracing experiments (Fig. 10e and f) confirmed the formation of N₂ through urea intermolecular N–N coupling under the catalysis of metal sulfides. Anti-CO poisoning experiments highlighted the role of Co in expediting the oxidation of carbonaceous intermediate products and enhancing catalyst stability. Combining these findings with DFT investigations, the authors established that Ni, Co, and V elements perform as active sites, stabilizers, and catalytic promoters, respectively.

Fig. 10 (a) The charge density difference in the heterostructure of CoS₂ and MoS₂. (b) XPS spectral comparison of CoS₂–MoS₂ and CoS₂. (c) The schematic UOR catalytic mechanism using the CoS₂–MoS₂ Schottky catalyst.¹⁰⁹ Reproduced with permission. Copyright 2018, Wiley-VCH. (d) In situ electrochemistry Raman spectra of the UOR by the NCVS-3 catalyst at various potentials from 1.40 to 1.80 V (vs. RHE). The in situ electrochemistry mass spectrometry isotope tracing experiment results for periodic measurement of the UOR in 1.0 M KOH with (e) 0.33 M urea [CO(¹⁴NH₂)₂] and (f) 0.33 M urea [CO(¹⁴NH₂)₂/CO(¹⁵NH₂)₂ = 4 : 1] under the catalysis of NCVS-3.¹⁸⁶ Reproduced with permission. Copyright 2021, American Chemical Society. (g) XRD pattern with indexed peaks of the synthesized Ni₂Fe(CN)₆ catalyst. Inset: Schematic illustration of the catalyst. (h) The Gibbs free energy diagrams of the reaction from urea to NH₃. (i) Comparison of both thermodynamically limiting and reaction determining step (RDS) of the first stage (reaction from urea to NH₃) on the Ni site and on the Fe site. Inset: Bond lengths of the transition states for the CO₂ dissociation reaction. (j) The comparison of both thermodynamically limiting and RDS of the second stage (from NH₃ to N₂) on the Ni site and on the Fe site.¹⁸⁹ Reproduced with permission. Copyright 2021, Nature Publishing Group.

The electrocatalytic oxidation of urea poses a complex mechanism involving a 6e⁻-transfer process, as well as multiple intermediate transfers and gas-desorption steps. Accurately understanding this reaction mechanism remains a prominent research area. Peng's group proposed a lattice-oxygen-involved pathway for the UOR by combining DFT, ¹⁸O isotope labeling mass spectrometry, and in situ FTIR.¹⁸⁷ Their findings demonstrated the involvement of lattice oxygen in the conversion of *CO to CO₂, thereby enhancing the reaction rate of the UOR. DFT calculations further revealed that lattice oxygen exhibits a preference for the Ni⁴⁺ active site in the UOR, exhibiting significantly faster reaction kinetics compared to the conventional pathway on the Ni³⁺ active site. Furthermore, Wang et al. investigated the electrooxidation behavior of urea using β-Ni(OH)₂ as a model catalyst, unraveling the intramolecular coupling of the N–N bond during the UOR.¹⁸⁸ Through isotope-labeled in situ DEMS, they provided compelling evidence that each N₂ molecule originates from one urea molecule, indicating the intramolecular coupling process. And Co-doping was found to modulate the charge density distribution at the Ni atoms, lowering the energy barrier for the formation of the β-Ni(OH)O intermediate. This intermediate can undergo spontaneous nucleophilic dehydrogenative oxidation with a urea molecule, thereby accelerating the UOR reaction rate. Impressively, Qiao's group identified a two-stage reaction pathway for the UOR using a Ni₂Fe(CN)₆ catalyst (Fig. 10g), distinguishing it from conventional Ni-based materials that generate NiOOH as the active phase.¹⁸⁹ The urea molecule is oxidized at the Ni site, forming intermediate CO₂ and NH₃ species, with NH₃ subsequently decomposing to N₂ at the Fe site. They employed an ion ammonia-selective electrode to confirm the generation of the NH₃ intermediate during the UOR, and found that the concentration of NH₃ in the electrolyte reached up to 0.30 ppm as the reaction time increased at an oxidation potential of 1.34 V. DFT calculations (Fig. 10h–j) further demonstrated the cooperative action of Ni and Fe sites enhanced the UOR rate. As a result, benefiting from the distinct rate-determining steps with more favorable thermal/kinetic energetics, Ni₂Fe(CN)₆ achieved an anodic current density of 100 mA cm⁻² at a potential of 1.35 V.

3.2 Hydrazine oxidation reaction

Hydrazine (N₂H₄) is an important industrial raw material widely used in various applications. The hydrazine oxidation reaction (HzOR, N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻, −0.33 V vs. RHE) plays a significant role in hybrid water electrolysis due to its lower theoretical voltage compared to many other anodic oxidation reactions., associated with the production of environmentally friendly products, such as H₂O and N₂, without producing toxic byproducts, catalyst-poisoning species, or greenhouse gases.^190,191 Furthermore, the HzOR can be employed as an effective electrochemical approach to remove hydrazine from wastewater, addressing the environmental concern associated with hydrazine pollution. These advantageous features have stimulated extensive research on the HzOR for highly efficient hydrogen production in hybrid water electrolysis systems and wastewater treatment. However, the HzOR suffers from inherently sluggish kinetics due to intricate adsorption and desorption steps for intermediates. The design and optimization of catalysts aiming to improve the kinetics and efficiency of the HzOR will lead to more efficient and sustainable hydrogen production.

In the field of hydrazine-assisted hydrogen production, various electrocatalysts have been developed, including both precious metals (such as Ru, Pd, and Pt) and non-precious metals (such as Fe, Co, Ni, and Cu), demonstrating their promising performance. Zhang et al. reported the use of partially exposed RuP₂ nanoparticle-decorated N,P dual-doped carbon porous microsheets for both the HzOR and the HER in an alkaline environment.¹⁹² Remarkably, this catalyst achieved an ultrasmall working potential of 70 mV to reach 10 mA cm⁻² for the HzOR (compared to 131 mV for 20 wt% Pt/C) in 1.0 M KOH + 0.3 M N₂H₄, and an extremely low overpotential of 24 mV at 10 mA cm⁻² for the HER (compared to 35 mV for 20 wt% Pt/C) in 1.0 M KOH. Furthermore, the catalyst demonstrated a record-low cell voltage of 23 mV to achieve 10 mA cm⁻² in a two-electrode system for overall hydrazine splitting (OHzS) (compared to 166 mV for 20 wt% Pt/C). It also exhibited an ultrahigh current density of 522 mA cm⁻² at a low cell voltage of 1.0 V, significantly lower than that of conventional water splitting. Xia's group proposed a tubular CoSe₂ nanosheet electrode as a bifunctional catalyst for efficient hydrogen production through hydrazine-assisted oxidation.¹⁹³ The study revealed that the activity of the HzOR on CoSe₂ nanosheets increased with increasing N₂H₄ concentration. Specifically, the as-prepared CoSe₂ nanosheets demonstrated appreciable catalytic activity and strong durability in the HER and HzOR, with a current density of 10 mA cm⁻² at −84 mV for the HER and −17 mV for the HzOR. Consequently, using the two-electrode hydrazine-assisted water electrolyzer utilizing CoSe₂ nanosheets as a bifunctional catalyst, a current density of 10 mA cm⁻² can be achieved at an ultralow cell voltage of 164 mV, much lower than that required for OWS. Moreover, the FE for H₂ production reached 98.3%, demonstrating the complete utilization of electrons in the hydrazine-assisted HER process.

The strategies employed to enhance the electrochemical HzOR performance of metal-based electrocatalysts are similar to those used for the UOR. These efficient strategies including the utilization of single-atom configurations, doping with heteroatoms, constructing heterostructures, forming bifunctional active sites, and engineering hierarchical porous 3D structures have been widely reported for catalyst systems, for instance, Ru SAs on tungsten disulphide (WS₂),¹⁹⁴ Ru SAs on WO₃,¹⁹⁵ B-doped Co(OH)₂ nanosheets on a Cu dendrite surface,¹⁹⁶ CoP and N-doped Ni₅P₄,¹⁹⁷ Mo doped Ni₃N and Ni heterostructures,¹⁹⁸ Cu₄N/Ni₃N heterostructures,¹⁹⁹ N doped Ni₁Co₃Mn_0.4O hybrids,²⁰⁰ porous Fe-doped Ni₂P nanosheets,²⁰¹ Ni_0.6Co_0.4Se/NF,²⁰² P/Fe-NiSe₂,²⁰³ and porous Ni₂P hollow nanotubes.²⁰⁴

In the pursuit of efficient hydrazine-assisted water electrocatalysis, the development of bifunctional electrocatalysts capable of facilitating both the HzOR and HER has garnered significant attention. Zhang et al. demonstrated the construction of a P,W dual-doped Co₃N nanowire array electrode, where DFT calculations revealed that P and W co-doping optimized the adsorption/desorption of hydrogen intermediates (H*) for the HER and the dehydrogenation kinetics for the HzOR.²⁰⁵ This work highlighted the importance of metal/nonmetal co-doping and the design of hierarchical structures in achieving enhanced bifunctional activity. Inspired by these groundbreaking findings, Wang's group developed a unique 2D/3D hierarchical structure comprising Fe, F co-doped Ni₂P encapsulated by a N-doped carbon shell (Fe/F-Ni₂P@NC).⁷⁸ DFT calculations demonstrated that Fe and F co-doping optimized the electronic distribution and charge transfer behavior within the catalyst. Consequently, the Fe/F-Ni₂P@NC catalyst exhibited remarkable bifunctional activity, achieving working potentials of 122 mV and 323 mV to attain a current density of 1000 mA cm⁻² for the HzOR and HER, respectively, in an alkaline electrolyte. The OHzS test showed that Fe/F-Ni₂P@NC requires a cell voltage of only 568 mV to reach 1000 mA cm⁻², with excellent overall stability for 100 h above 100 mA cm⁻² at the constant potential. This OHzS system saved 3.35 kW h for generating 1.0 m³ of H₂ in comparison with a N₂H₄-free system. Additionally, Wang's group has successfully fabricated other doped catalyst systems such as Cu-doped CoFe/Co encapsulated by N-doped carbon,²⁰⁶ Fe/Co dual-doped Ni₂P and MIL-FeCoNi heterostructure arrays,²⁰⁷ self-supporting Ru-doped FeP₄ nanosheets,²⁰⁸ and Fe-doped Ni₂P/CoP encapsulated by nitrogen-doped carbon layers.²⁰⁹ These catalysts have demonstrated promising HzOR and HER activity, further showing the potential of doped catalyst systems for efficient hydrazine-assisted water electrocatalysis.

At present, the production of H₂ through water electrolysis is predominantly carried out using freshwater, which poses limitations in arid regions due to the scarcity of freshwater resources.²¹⁰ To address this, electrolysis of seawater for H₂ production has emerged as a promising alternative. However, the complex ionic environment of seawater presents challenges such as interference from anion side reactions and the corrosion of electrolyzers and catalysts. The competition between the chlorine electrooxidation reaction (ClOR) and the OER is a significant problem, as ClOR can lead to the release of toxic chlorine gas (Cl₂) and corrosive hypochlorite (ClO⁻), reducing electrolysis efficiency and the lifespan of the electrolyzer. To resolve this tricky challenge, some unique tactics, such as designing selective OER catalysts and constructing Cl⁻ blocking layers, have been proposed.^211,212 Meanwhile the thermodynamically favorable oxidation reaction provides another approach to realize seawater electrolysis for hydrogen production. In the context of seawater electrolysis, the oxidation potential of the HzOR is much lower than that of the ClOR by 2.05 V (Fig. 11a), providing an advantage in avoiding chlorine-related issues without compromising the electrolysis current and H₂ yield efficiency. Sun et al. explored a chlorine-free H₂ production by employing hybrid seawater electrolysis (HSE) coupled with N₂H₄ degradation (Fig. 11b).¹²⁹ They developed a self-supported NiCo/MXene-based catalyst (NiCo@C/MXene/CF) and investigated its performance in hydrazine-assisted seawater splitting in seawater electrolytes. The electrolyzer using the NiCo@C/MXene/CF catalyst achieved high activity, requiring ultralow voltages of 1.05 V to attain a current density of 500 mA cm⁻² (Fig. 11c) in neutral seawater, which is 48% lower in terms of the energy input compared to commercial alkaline water electrolyzers. Importantly, this system avoided the generation of Cl₂/ClO⁻, effectively mitigating the influence of the ClOR and enabling industrial-scale H₂ production through seawater electrolysis. Furthermore, the HSE system can be integrated with photovoltaic cells powered by clean and readily available solar energy. The hybrid seawater electrolyzer, equipped with the NiCo@C/MXene/CF catalyst, could operate at a current density of ∼310 mA cm⁻² and an average photovoltage of ∼0.876 V when connected to a single commercial solar cell (1.0 W). Under simulated solar illumination (AM 1.5G) with a power density of 100 mW cm⁻², the system yielded hydrogen at a decent rate of 6.0 mol h⁻¹ g_cat⁻¹ from seawater.

Fig. 11 (a) The Pourbaix diagram of the HzOR, HER, OER, and ClOR in artificial seawater with 0.5 M Cl⁻ in pH 7–14. (b) The merits of HSE over ASE for energy-saving and chlorine-free hydrogen production. (c) The LSV curves of HSE using neutral or alkaline seawater as the catholyte, compared with ASE.¹²⁹ Reproduced with permission. Copyright 2021, Nature Publishing Group. (d) Chronoamperometry curves of FeSA/CNT for HzOR under 0.77 V at 0.1 M N₂H₄ + 1.0 M KOH. DEMS analysis of N₂ (m/z = 28) and NH₃ (m/z = 15) (e). (f) The ratio of NH₃ charge, Q(NH₃) to N₂ charge, Q(N₂) against the electron transfer number of MSA/CNT for the HzOR. (g) Scheme for the two pathways of the HzOR by MSA/CNT.⁵⁹ Reproduced with permission. Copyright 2021, Nature Publishing Group. (h) CVs on the Ni(Cu) CNP electrode during successive addition of different concentrations of hydrazine at a scan rate of 10 mV s⁻¹ in a 1.0 M KOH solution; the inset shows the calibration curve of peak current versus the concentration of hydrazine. (i) CVs of the HzOR obtained with 10 mM hydrazine in 1.0 M KOH solution at different CV scan rates over Ni(Cu) CNPs. (j) Plot of HzOR peak potential E_pversus log(v).²¹⁷ Reproduced with permission. Copyright 2020, Royal Society of Chemistry. (k) CV curves of the HzOR obtained over G(CN)–Co (1.2 wt%), G-(CN)–Co(1.5 wt%), G(CN)–Co (3.4 wt%), and G(CN) using 50 mmol L⁻¹ hydrazine in PBS (pH 7.4) at a scan rate of 10 mV s⁻¹ and (l) the corresponding current density normalized to the total mass of Co atoms.²¹⁸ Reproduced with permission. Copyright 2021, Wiley-VCH.

Generally, in the oxidation of N₂H₄ in alkaline solution, several typical pathways (reactions (1)-(5)) can be followed to produce H₂ and NH₃.²¹³

N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻ (1)

N₂H₄ + xOH⁻ → N₂ + (4 − x)/2H₂ + xH₂O + xe⁻ 1 ≤ x ≤ 3 (2)

N₂H₄ + OH⁻ → 1/2N₂ + NH₃ + H₂O + e⁻ (3)

N₂H₄ → N₂ + 2H₂ (4)

3N₂H₄ → N₂ + 4NH₃ (5)

For some polycrystalline metal catalysts like a ZrNi alloy,²¹⁴ N₂H₄ can undergo complete electrochemical oxidation to N₂ and H₂O through a 4e⁻ reaction (reaction 1), or it can be incompletely oxidized to N₂ and H₂ (reaction 2). Alternatively, the combination of N₂ and H₂ can be obtained via a fully chemical decomposition pathway (reaction 4), as observed with Pt catalysts.^215,216 In addition, minor amounts of NH₃ can be generated through incomplete dehydrogenation of N₂H₄via a 1e⁻ electrochemical oxidation (reaction 3) or dissociative reaction (reaction 5). Wang et al. used on-line DEMS to analyze the gaseous products during HzOR catalyzed by transition metal SACs including FeSA/CNT, CoSA/CNT, and NiSA/CNT.⁵⁹ Under open circuit voltage (OCV), the absence of ionic current density indicated the absence of gaseous products from the HzOR. However, upon applying an oxidation potential of 0.77 V, the ionic current density associated with N₂ (m/z = 28) increased significantly, indicating the generation of N₂ (Fig. 11d). Only trace amounts of NH₃ (approximately 0.5 at%) were detected by DEMS during the HzOR. The content of N₂ and NH₃ increased with applied potential or reaction time under a fixed potential. Conversely, hydrogen was not detected under both OCV and applied potentials. Moreover, FeSA/CNT showed no activity for the electrochemical oxidation of NH₃ and H₂. Based on these findings, it can be concluded that the predominant formation of N₂ and minor amounts of NH₃ occurs during the electrochemical oxidation of N₂H₄, rather than through chemical decomposition reactions under ambient conditions. Similar reaction pathways were observed for CoSA/CNT and NiSA/CNT, along with dominant N₂ and trace amounts of NH₃ during the HzOR under applied potential, indicating similar behavior to FeSA/CNT. The calculated charge ratios of NH₃ to N₂ [Q(NH₃)/Q(N₂)] (Fig. 10e) were 0.126%, 0.141%, and 0.131% for FeSA/CNT, CoSA/CNT, and NiSA/CNT, respectively, showing a reverse correlation with the electron transfer number for the HzOR. In other words, a higher charge ratio of NH₃ to N₂ corresponds to a lower electron transfer number for the catalyst. The absence of mass peaks corresponding to intermediate oxygen species (m/z = 30, 44, and 46) in the DEMS detection (Fig. 11f) indicates the absence of these species during the HzOR. The absence of H₂ during the HzOR rules out reactions (2) and (4), as the MSA/CNT catalysts showed no activity for hydrogen oxidation. Additionally, the increasing ammonia content with applied potential and reaction time under a fixed applied potential excludes the possibility of chemical decomposition of N₂H₄ to NH₃via reaction (5). Therefore, the results suggest that the electrochemical oxidation of N₂H₄ on MSA/CNT primarily follows pathways involving reactions (1) with minor extent through reaction (3), as illustrated in Fig. 11g.

In addition to the previously mentioned pathways, evidence has also been found supporting continuous 2e⁻ + 2e⁻ reaction pathways in the HzOR.²⁰³ For example, Zhao et al. developed a Cu-doped Ni cubic nanopore (Ni(Cu) CNP) catalyst for the HzOR through an electrodeposition process and in situ electrochemical etching.²¹⁷ They investigated the 2e⁻ transfer mechanism of the HzOR using a double potential step chronopotentiometry test (Fig. 11h–j). The CV curves showed only oxidation peaks and no cathodic peaks during the reverse scans, confirming the irreversibility of the oxidation (or HzOR) on the Ni(Cu) CNP catalyst. Furthermore, kinetic studies revealed that increasing the electrochemical scan rates caused a slight shift towards positive peak potential in the catalytic HzOR. The number of electrons involved in the rate-determining step was estimated to be approximately 2.2, suggesting that the HzOR on the Ni(Cu) CNP electrode proceeds through a slow 2e⁻ transfer process in the initial rate-determining step to form the diazene intermediate, followed by a fast 2e⁻ process to generate the final product of N₂. Similarly, Gawande et al. performed CV measurements (Fig. 11k and l) and determined the total number of electrons involved in the HzOR for tested single-atom Co-based catalysts to be close to 4.²¹⁸ This indicates that the reaction proceeds through a 4e⁻ process to fully oxidize hydrazine. These findings highlight the existence of both 2e⁻ and 4e⁻ transfer pathways in the HzOR, suggesting the involvement of different mechanisms and catalytic active sites in the electrochemical oxidation of hydrazine.

The hydrazine splitting process is typically considered a stepwise dehydrogenation process from N₂H₄ to N₂, where the rate-determining step is often the dehydrogenation from N₂H₃* to N₂H₂* on transition metal-based catalysts.²¹⁹ Impressively, Shi's group discovered a new N–N single bond breakage pathway during the HzOR using NiCoP–CoP heterostructures (Ni–Co–P/NF) as catalysts.²²⁰ They also found that the presence of hydrazine enables the instantaneous recovery of the metal phosphide active sites, ensuring robust HzOR activity during long-term operation. CV measurements on Ni–Co–P/NF in both KOH and 1.0 M KOH + 0.1 M N₂H₄ (Fig. 12a) showed that the in situ electrochemically oxidized Ni–Co–P/NF in the anodic potential range can be recovered back to active metal phosphide (MP) species again by N₂H₄. In situ Raman measurements were conducted to investigate the compositional and structural changes of the catalyst during the HzOR. The addition of 0.1 M N₂H₄ in the electrolyte led to the appearance of three strong N₂H₄-related peaks at 677, 1116, and 1598 cm⁻¹ (Fig. 12b). These peaks correspond to N–H, N–N stretching modes of adsorbed *NH₂NH₂, and N–H bending modes of intermediate *NH₂ on the Ni–Co–P/NF surface. Varying the applied voltages (Fig. 12c) resulted in a gradual weakening of the M-P peak density (400 cm⁻¹) and the appearance of a weak peak near 1020 cm⁻¹ attributed to the stretching bond of P–O in MPO_x. This indicates that some MP species were gradually oxidized to MPO_x during the HzOR. However, at a constant potential of 0.2 V and varying reaction time (Fig. 12d), the intensity of the M–P bond peak (400 cm⁻¹) increased again, while the peak of the P–O bond (broad band at about 1020 cm⁻¹) disappeared. This confirmed that the slightly oxidized species (MPO_x) could be reduced and recovered back to active MP species in the presence of N₂H₄. Importantly, the peak of *NH₂ became significantly stronger, indicating the accumulation of reaction intermediate *NH₂. This suggests that the adsorbed *NH₂NH₂ breaks the N–N bond to form two *NH₂ groups, which then gradually dehydrogenate to generate N₂. DFT calculations (Fig. 12e) also supported this finding, showing that the N–N bond breaking in hydrazine molecules on the NiCoP–CoP heterostructures has a favorable reaction energy compared to the traditional stepwise dehydrogenation process.

Fig. 12 (a) CV curves in 1.0 M KOH (blue curves) or 1.0 M KOH + 0.005 M N₂H₄ obtained under different cycles of scanning of the Ni–Co–P/NF catalyst (red: first cycle, yellow: second cycle and green: third cycle). (b) Raman spectra of Ni–Co–P/NF in 1 M KOH or 1.0 M KOH + 0.1 M N₂H₄. In situ electrochemical Raman spectra of Ni–Co–P/NF in 1.0 M KOH + 0.1 M N₂H₄ (c) at varied applied potentials and (d) at different reaction time intervals under constant 0.2 V. (e) Free energy changes of the HzOR at the NiCoP(111)/CoP(011) interface with the most stable adsorption configuration of the intermediate.²²⁰ Reproduced with permission. Copyright 2023, Nature Publishing Group.

3.3 Polysulfide oxidation reaction

Hydrogen sulfide (H₂S) is a highly toxic pollutant released in large quantities from industries such as oil refineries and natural gas extraction.²²¹ Electrochemical methods offer the potential for simultaneous H₂ production and sulfur recovery at the cathode and anode, respectively, through the removal and utilization of this hazardous waste. Since H₂S is a diprotic acid, a two-step ionization process of H₂S to HS⁻ and S²⁻ should occur in an aqueous electrolyte. By replacing the OER with the sulfide oxidation reaction (SOR, S_x²⁻ + S²⁻–2e⁻ → S_x+1²⁻), the theoretical voltage required for hydrogen production can be significantly reduced from 1.23 V to 0.17 V vs. RHE, and hence 86% electricity energy could be saved.²²² However, the development of efficient hydrogen production systems based on the SOR is limited, primarily due to the strong poisoning effect of sulfur species on metallic electrocatalysts.²²³ Sulfur chemisorption and etching on the catalyst surface result in decreased activity and stability, posing challenges for industrial-scale applications. Therefore, it is crucial to develop low-cost, highly efficient, and robust electrocatalysts to enable sustainable electrocatalytic H₂S decomposition and effective H₂ production.

Deng's group has developed a template-assisted method to fabricate an efficient electrocatalyst for simultaneous H₂ and S production from H₂S by coupling the HER and the SOR.²²³ The catalyst of CoNi@NGs consists of a non-precious CoNi nanoalloy enclosed in N-doped graphene. The developed CoNi@NGs exhibit an anode reaction onset potential of 0.25 V vs. RHE, which is 1.24 V lower than the onset potential required for the OER (Fig. 13a). This significant reduction in the required voltage demonstrates the effectiveness of the catalyst in catalysis of the SOR. Moreover, after a durability test lasting 500 h, the structure of the CoNi nanoparticles encapsulated in graphene shells remains well-maintained, indicating the catalyst's robustness in harsh environments. In situ electrochemical ultraviolet and visible (UV-Vis) spectrophotometry provides evidence for the formation of short-chain polysulfides (S₂²⁻–S₄²⁻) in the electrolyte during galvanostatic testing (Fig. 13b). Furthermore, after subjecting the electrolyte to acid treatment following long-term SOR testing, solid elemental sulfur powder can be obtained (Fig. 13c). This result confirms the successful conversion of H₂S into sulfur through the electrocatalytic process. DFT calculations suggest that the high activity for the SOR can be attributed to the modulation of the graphene's electronic structure by the encapsulated metal alloy and nitrogen doping. This modulation facilitates the adsorption of S* species and the formation of polysulfide intermediates on the graphene's surface, leading to enhanced catalytic performance.

Fig. 13 (a) Comparison of SOR and OER polarization curves for CoNi@NGs. (b) UV-Vis spectra of 250 times diluted electrolyte samples corresponding to the electrolytes in the inset photograph with the reaction proceeding in a galvanostatic test at 100 mA cm⁻². (c) XRD characterization of product S. The inset photograph is the collected product of S powder.²²³ Reproduced with permission. Copyright 2020, Royal Society of Chemistry. Schematic illustration of sulfophobic design for desulfurization and hydrogen evolution. (d) Strong interaction between sulfides and electrodes leads to sulfur passivation whereas (e) weak interaction enables self-cleaning electrolysis. (f) Electrolytic sulfur passivates electrodes during long-term operation. (g) Sulfophobic NiS₂ repels electrolytic sulfur, leading to a self-cleaning process for sulfur removal and hydrogen evolution. (h) Contact angle measurements of sulfur droplets (120 °C) on various substrates.²²⁶ Reproduced with permission. Copyright 2021, Wiley-VCH. (i) Pourbaix diagram of the SOR, HER, OER, and ClOR under alkaline conditions. (j) Schematic illustration of hydrogen production at cell voltages below 1.0 V by coupling seawater reduction with the SOR on Co-based electrocatalysts. (k) The voltage differences (ΔV) between the HER on CoO@C/MXene/NF and the SOR or the OER on CoS₂@C/MXene/NF in different electrolytes.²⁹ Reproduced with permission. Copyright 2022, Wiley-VCH.

In accordance with the principles of the hard and soft acid and base theory, it is postulated that the lattice Cu(I) within the Cu₂S electrode can be classified as a soft Lewis acid site.^224,225 Consequently, it is expected to exhibit a higher affinity for binding with the soft-base species of HS⁻ in the electrolyte rather than other hard or borderline acid sites. Inspired by this idea, Zhong et al. introduced a straightforward technique for the synthesis of Cu₂S micro-flakes supported on nickel foam (Cu₂S/NF).²²² Benefiting from the synergistic effect of porous morphology, high electronic conductivity, and the enhanced sulfur adsorption on Cu(I), an anodic potential as low as 0.44 V vs. RHE was obtained to deliver 100 mA cm⁻² for the SOR with high FE of exceeding 97%. Furthermore, a simple two-electrode system by utilizing Cu₂S/NF as both the cathode and anode can enable a cell voltage of merely 0.64 V at 100 mA cm⁻², which also exhibits a remarkable 74% reduction in energy consumption compared to conventional water splitting while generating the same quantity of hydrogen. This energy-saving efficiency is in close proximity to the theoretical energy-saving limit of 86%. The DFT calculations demonstrated that the strong adsorption of S on Cu₂S can stabilize S* for chain growth. The impressive performance of Cu₂S/NF can be attributed to the collective influence of several factors: (i) the inherent preference of Cu(I) for the HS⁻ species in the electrolyte, (ii) the excellent electrical conductivity of the Cu₂S/NF electrode, and (iii) the compatibility of Cu₂S with sulfide electrolytes.

During the process of SOR, the formation of elemental sulfur as an electro-oxidized product leads to the passivation of the electrode surface, resulting in increased energy consumption and rendering continuous operation impractical. Moreover, the unresolved issues pertaining to interfacial chemistry contribute to the accumulation of sulfur particles within the system, inevitably diminishing the long-term activity of the electrodes. Zhang et al. recently made a significant discovery by observing a sulfophobic phenomenon (low affinity to elemental sulfur) having a weak interaction between transition metal disulfides and elemental sulfur (Fig. 13d–g).²²⁶ Utilizing sulfophobic properties, they designed a self-cleaning NiS₂ electrode for highly efficient electrochemical desulfurization. Experimental analyses confirmed that NiS₂ exhibits considerably lower chemical affinity to sulfur compared to conventional desulfurization electrodes such as Pt, Ni, Ir/MMO, and carbon. This was intuitively demonstrated through contact angle measurements, where NiS₂ displayed a significantly higher contact angle (108°) compared to the other electrodes (Fig. 13h). Theoretical studies further revealed that the presence of more reducing Ni–S bonds facilitates electron transfer from sulfur species to catalysts, thereby enabling self-cleaning electrolysis and preventing voltage fluctuations during desulfurization. Thanks to the de-wetting properties of NiS₂, passivation caused by solid sulfur deposition is circumvented, allowing for long-term self-cleaning desulfurization. When coupled with sulfur vacancy engineered NiS₂ (v-NiS₂) as the HER catalyst, the hybrid electrolyzer exhibited a low cell voltage of only 0.65 V to achieve 20 mA cm⁻², showing efficient desulfurization and simultaneous hydrogen production. Impressively, the system demonstrated excellent voltage stability over 100 h.

The utilization of redox polysulfides (S_x+1²⁻/S_x²⁻) as effective mediators in certain devices enables the maintenance of excellent redox cyclability to storage/release electrons through appropriate electrode catalysis. Taking advantage of this phenomenon, Zhang et al. developed a high-performance device by employing polysulfides as mediators and utilizing graphene-encapsulated CoNi nanoparticles (CoNi@NGs) as catalysts, achieving highly efficient conversion of surplus electricity into hydrogen energy.²²⁷ The operation mechanism of this decoupled electrolyzer involves two distinct steps. During Step 1, referred to as the “valley time” typically occurring at night, a cathodic polysulfide reduction reaction (SRR, S_x+1²⁻ + 2e⁻ → S_x²⁻ + S²⁻) takes place, accompanied by the anodic oxidation of OH⁻ for the OER leading to O₂ production. In the subsequent Step 2, known as the “peak time” typically occurring during the daytime, the HER occurs at the cathode through the reduction of H₂O, while simultaneously, the SOR occurred simultaneously at the anode. Consequently, this device provides a promising strategy for peak load regulation in electricity. By utilizing the efficient catalysis of CoNi@NGs, the developed device demonstrated significant hydrogen production with low energy consumption. Furthermore, the device exhibited remarkable cyclability in the extensive recycle tests in 15 day without any performance decay.

Specifically, compared to the ClOR, the oxidation potential of the SOR can be dramatically reduced by 1.3–1.4 V (Fig. 13i). This reduction in potential offers the opportunity to completely circumvent the hazardous chlorine chemistry in seawater electrolysis, while simultaneously greatly reducing energy expenses. Hence, integrating the SOR with seawater electrolysis holds the potential for hydrogen production with added economic and environmental benefits, as shown in Fig. 13j. To address this, Wang's group developed CoS₂ nanoparticles in a carbon matrix on MXene/NF (CoS₂@C/MXene/NF) for the SOR(Fig. 13k).²⁹ The utilization of an asymmetric hybrid seawater electrolyzer employing CoS₂@C/MXene/NF catalysts enables operation at a low cell voltage ranging from 0.68 to 0.83 V to achieve the current densities of 200–400 mA cm⁻², cutting by 56–64% compared to alkaline seawater electrolyzers (1.85–1.9 V). Importantly, the reduced oxidation potential eliminates the occurrence of the ClOR and the subsequent release of ClO⁻, fully avoiding the anode corrosion regardless of Cl⁻ crossover to the anode side. Consequently, stable electrolysis can be sustained for over 180 h below 0.97 V (without iR correction) at 300 mA cm⁻², in stark contrast to the rapid failure of overall water splitting observed within 30 h with a huge cell voltage of 2.45 V. DFT calculations revealed that the CoS₂ (100) facet exhibits the lowest energy barrier for the rate-limiting step in the SOR, compared to the (111) and (110) facets. Furthermore, the presence of MXene coating contributes to a large contact angle for sulfur molecules (110.6°) and high conductivity (∼5600 S cm⁻¹), effectively accelerating the reaction kinetics in the SOR.

The oxidation of polysulfides offers a promising approach for both the reutilization of toxic sulfion waste and the highly efficient production of hydrogen. However, the catalytic performance for the low-potential electrooxidation of polysulfides still faces challenges and remains inadequate. Despite the potential benefits of polysulfide oxidation, research on its practical application is currently limited. Further investigations are necessary to overcome the reaction barriers and develop efficient and practical strategies for polysulfide oxidation. One of the key challenges in polysulfide oxidation is the lack of efficient electrocatalysts, particularly bifunctional electrocatalysts that can simultaneously promote the oxidation of polysulfides and the HER. In addition to catalyst development, a comprehensive understanding of the underlying reaction mechanisms is essential for optimizing the performance of polysulfide oxidation processes. Elucidating the complex reaction pathways and identifying the key intermediates and reaction kinetics will enable the rational design and optimization of catalysts and reaction conditions. By addressing these challenges, it will be possible to enhance the catalytic performance, improve energy efficiency, and enable the practical application of polysulfide oxidation for various energy-related processes, including the reutilization of sulfion waste and the production of hydrogen.

In summary, benefitting from the ultralow thermodynamic equilibrium potential and the production of harmless by-products, as reported in the available literature shown in Table 4, the urea/hydrazine/polysulfide-assisted hybrid water electrolysis holds promise for replacing conventional water electrolysis. Remarkably, due to their notably lower reaction potentials, these hybrid electrolysis processes can also be carried out using seawater as the electrolyte, offering an attractive avenue for low-cost hydrogen production. Although the oxidation products of the UOR, HzOR, and SOR may have limited commercial value, they offer an alternative to alleviate the significant environmental pollution and eutrophication of water resources. For instance, Wang's group demonstrated that direct electrolysis of human urine yielded only slightly lower performance compared to when urea molecules were added to an alkaline electrolyte.¹⁷¹ Qiu's group discovered that rapid hydrazine degradation to around 3 ppb can be achieved through the HzOR.¹²⁹ Zhang et al. achieved the electrocatalytic selective removal of H₂S from simulated industrial syngas (49% CO, 49% H₂ and 2% H₂S).²²³ These typical examples illustrate that the UOR or the HzOR or the SOR provides viable approaches for treating urine and hydrazine-containing wastewater, even at trace amounts, resulting in both high-efficiency hydrogen production and waste degradation.

Table 4 Performance comparison of urea/hydrazine/polysulfide-assisted H₂ production systems in recently reported literature

Catalyst	Coupling reactions	Electrolyte	Overpotential at 10 mA cm^−2 for cathode (mV)	Tafel slope for cathode (mV dec^−1)	Potential at 10 mA cm^−2 for anode (vs. RHE)	Tafel slope for anode (mV dec^−1)	Cell voltage at 10 mA cm^−2	FE	Stability	Ref.
O-NiMoP/NF	HER + UOR	1.0 M KOH + 0.5 M urea	54	49	1.41 V at 100 mA cm^−2	34	1.55 V at 50 mA cm^−2	Cathode: 96.8%	10 h at 20 mA cm^−2	30
20% Pt/C(−)//Ce–Ni3N@CC(+)	HER + UOR	1.0 M KOH + 0.5 M urea	—	—	1.31 V	31.1	1.34 V	Cathode: 100%	15 h at 40 mA cm^−2	81
CoS2–MoS2	HER + UOR	1.0 M KOH + 0.5 M urea	—	—	1.29 V	32	1.29 V	—	60 h at 10 mA cm^−2	109
CoMn/CoMn2O4	HER + UOR	1.0 M KOH + 0.5 M urea	69	90	1.32 V	38	1.51 V	—	60 000 s at 100 mA cm^−2	110
P-CoNi2S4	HER + UOR	1.0 M KOH + 0.5 M urea	135	65	1.306 V	—	1.402 V	—	100 h	159
Ni–Mo nanotube	HER + UOR	1.0 M KOH + 0.1 M urea	44	55	1.36 V	22	1.43 V	Cathode: 98.7%	24 h at 10 mA cm^−2	164
NiF3/Ni2P@CC	HER + UOR	1.0 M KOH + 0.33 M urea	121	75	1.36 V	33	1.83 V at 50 mA cm^−2	—	10 h at 10 mA cm^−2	167
Ni3N/Ni0.2Mo0.8N/NF	HER + UOR	1.0 M KOH + 0.5 M urea	55	54	1.328 V	17	1.348 V	Cathode: 100%; anode: 100%	500 h at 10 mA cm^−2	170
NiFeRh-LDH	HER + UOR	1.0 M KOH + 0.33 M urea	24	27	1.346 V	35	1.344 V	Cathode: 100%;	62 h at 1.47 V	171
Ru–Co2P/N–C/NF	HER + UOR	1.0 M KOH + 0.5 M urea	65	65	1.351 V at 50 mA cm^−2	60	1.58 V at 100 mA cm^−2	—	20 h at 1.60 V	172
V-FeNi3N/Ni3N	HER + UOR	1.0 M KOH + 0.33 M urea	62	108.2	1.382 V	29.6	1.46 V	—	25 h at 50 mA cm^−2	174
NiMoV LDH/NF	HER + UOR	—	—	—	1.40 V at 100 mA cm^−2	24.29	∼1.63 V at 100 mA cm^−2	—	—	175
V-Ni3N/NF	HER + UOR	1.0 M KOH + 0.5 M urea	83	45	1.361 V	—	1.416 V	—	200 h at 10 mA cm^−2	176
Ni(OH)2/NiO-C/WO3 HAs	HER + UOR	1.0 M KOH + 0.5 M urea	53	92	1.340 V	19	1.370 V	—	60 h at 20 mA cm^−2	182
FeNi-MOF NSs	HER + UOR	1.0 M KOH + 0.33 M urea	288 mV at 100 mA cm^−2	96.9	1.361 V	28.0	1.431 V	—	10 h at 10 mA cm^−2	183
Co2Mo0.2CH	HER + UOR	1.0 M KOH + 0.33 M urea	82	133	1.33 V	32	1.40 V	—	40 h at 10 mA cm^−2	185
Ni3N–Co3N	HER + HzOR	1.0 M KOH + 0.1 M N2H4	43	35.1	−88 mV	21.6	71 mV	Cathode: 100%	20 h at 0.14 V	37
Fe/F–Ni2P@NC	HER + HzOR	1.0 M KOH + 0.5 M N2H4	212 mV at 100 mA cm^−2	105	100 mA cm^−2 at 12 mV	64.4	0.568 V at 1000 mA cm^−2	Cathode: 100%; anode: 100%	100 h at about 100 mA cm^−2	78
CoSe2 nanosheets	HER + HzOR	1.0 M KOH + 0.5 M N2H4	84	84	−17 mV	—	164 mV	Cathode: 98.3%	14 h at 10 mA cm^−2	193
WS2/Ru SAs	HER + HzOR	1.0 M KOH + 0.5 M N2H4	32.1	53.2	−74 mV	42.2	15.4 mV	Cathode: 100%	100 h at 10 mA cm^−2	194
N-Ni5P4@CoP/CFP	HER + HzOR	1.0 M KOH + 0.1 M N2H4	56	63	−32 mV	24.9	0.037 V	—	10 h at 10 mA cm^−2	197
Mo–Ni3N/Ni/NF	HER + HzOR	1.0 M KOH + 0.1 M N2H4	45	45	−0.3 mV	48	55 mV	Cathode: 100%; anode: 100%	10 h at 50 mA cm^−2	198
Cu1Ni2–N	HER + HzOR	1.0 M KOH + 0.5 M N2H4	71.4	106.5	0.5 mV	44.1	0.24 V	Cathode: 95%	75 h at 10 mA cm^−2	199
PW–Co3N NWA/NF	HER + HzOR	1.0 M KOH + 0.1 M N2H4	41	40	−55 mV	14	28 mV	Cathode: 96%; anode: 96%	20 h at 0.098 V	205
Ru–FeP4/IF	HER + HzOR	1.0 M KOH + 0.5 M N2H4	110 mV at 100 mA cm^−2	51.58	27.0 mV at 100 mA cm^−2	115.94	0.90 V at 1000 mA cm^−2	Cathode: 100%; anode: 100%	80 h at 100 mA cm^−2	208
Ru–Cu2O/CF	HER + HzOR	—	31	50	−41 mV	34	17.4 mV	—	18 h at 0.03 V	228
Ni–C HNSA	HER + HzOR	1.0 M KOH + 0.1 M N2H4	37	28.7	−20 mV	16.2	0.14 V at 50 mA cm^−2	Cathode: 100%	30 h at 0.14 V	229
Ni2P/NF	HER + HzOR	1.0 M KOH + 0.5 M N2H4	290 mV at 200 mA cm^−2	—	−25 mV at 50 mA cm^−2	55	1.0 V at 500 mA cm^−2	Cathode: 100%	10 h at 100 mA cm^−2	230
Ni-SN@C	HER + HzOR	1.0 M KOH + 0.1 M N2H4	28	39	16.8 mV	21	0.366 V	Cathode: 100%	24 h	231
CoO@C/MXene/NF(−)//CoS2@C/MXene/NF(+)	HER + SOR	1.0 M NaOH + 1.0 M Na2S	232 mV at 500 mA cm^−2	64.5	389 mV at 100 mA cm^−2	60.9	0.97 V at 300 mA cm^−2	Cathode: 96%; anode: 80%	180 h at 300 mA cm^−2	29
Cu2S/NF	HER + SOR	1.0 M NaOH + 1.0 M Na2S	180	—	260 mV	68	0.64 V at 100 mA cm^−2	Cathode: 97%	140 h at 10 mA cm^−2	222
v-NiS2//NiS2	HER + SOR	0.1 M NaOH + 50 mM Na2S	150 mV at 145.4 mA cm^−2	83	410 mV	104	0.64 V at 20 mA cm^−2	Cathode: 96.5%	100 h at 20 mA cm^−2	226
CoNi@NGs	HER + SOR	1.0 M NaOH + 0.5 M Na2S2	—	—	—	—	0.82 V at 100 mA cm^−2	Cathode: >98%	15 h at 100 mA cm^−2	227
WS2 NSs	HER + SOR	1.0 M NaOH + 1.0 M Na2S	214	64.9	480 mV	—	—	Cathode: 99.22%	192 h at 1.3 V	232
Co3S4	HER + SOR	1.0 M NaOH + 1.0 M Na2S	193 mV at 100 mA cm^−2	86.4	262 mV at 100 mA cm^−2	47.9	0.496 V at 100 mA cm^−2	Cathode: 95%	25 h at 100 mA cm^−2	233

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While sacrificial-agent-assisted water electrolysis presents a promising avenue for hydrogen production, several challenges still need to be addressed in this field. (i) Maintaining a constant supply of sacrificial agents at an industrial scale can be challenging, potentially limiting the scalability of the process, especially for large-scale hydrogen production applications. (ii) The anodic products of certain sacrificial agents, such as urea and hydrazine, are gases, which can affect the purity of the hydrogen gas produced. This may necessitate the use of membrane-contained electrolyzers, increasing the overall cost of hydrogen production. (iii) Some reaction intermediates, such as CO, may be generated during these anodic reactions, potentially leading to catalyst deactivation to varying degrees. (iv) Harsh testing conditions (strong acid, strong base or high concentration salt) may result in the shortened lifetime of equipment and an increased cost for the entire system. (v) The diversity of possible intermediate products during these anodic reactions can make it challenging to fully elucidate the reaction mechanisms. Unwanted side reactions or incomplete conversion of sacrificial agents can reduce the overall efficiency of the process. To further advance the field, it is essential to investigate the structure–activity relationships of designed electrocatalysts, with a specific focus on understanding the catalytic mechanisms of the UOR/HzOR/SOR in relation to transition metal-based catalyst orientations of high-valence metal species. Such investigations will contribute to the ongoing development of improved electrocatalytic materials and enhance our understanding of the underlying processes involved in sacrificial-agent-assisted water electrolysis.

4. Organic upgrading-assisted H₂ production

Currently, various small molecule compounds, such as alcohols, glucose/xylose, aldehydes, carbohydrates, and primary amines, have been extensively investigated as anodic substrates for electrochemical synthesis reactions. These compounds can undergo oxidation at the anode, where hydroxyl or aldehyde groups are converted to carboxyl groups, without generating molecular oxygen. Simultaneously, the donated electrons participate in water reduction at the cathode, producing molecular hydrogen. The oxidation reactions of these small organic molecules offer several advantages.^234–236 Firstly, they exhibit significantly reduced anodic potentials compared to the OER, making them more favorable for practical electrochemical applications with lower required electrolysis voltage. This reduction in the required voltage enables more energy-efficient electrochemical processes and contributes to overall system efficiency. Secondly, the absence of molecular oxygen generation eliminates the risk of potential explosions, improving the safety of the electrochemical system. Thirdly, electrooxidation reactions at the anode can generate valuable intermediates that serve as building blocks for further chemical transformations or synthesis routes (Fig. 14). Additionally, anode electrooxidation reactions can offer high selectivity for specific products, allowing for tailored synthesis and controlled transformations of organic molecules. Hence, these electrooxidation reactions hold great promise for practical applications and have attracted considerable interest among researchers. In this section, we primarily focus on elucidating the anode electrooxidation reactions, encompassing reaction types, representative non-noble metal catalysts, catalytic performance evaluations, reaction mechanisms, and the design and construction of electrolyzers.

Fig. 14 The blueprint of hydrogen production coupled with the electrochemical upgrading of organics.

4.1 Alcohol oxidation reaction

The exploration of alcohol oxidation reactions has gained significant attention due to their potential applications in direct fuel cells and synthetic chemistry. In recent years, several studies have reported various alcohol oxidation reactions as alternatives to the anodic OER in water splitting.^237,238 These alternative reactions not only lower the electrolysis voltage required for hydrogen production but also enable the simultaneous generation of value-added chemicals at the anode. In the past few years, the coupling of these alcohol oxidation reactions with the HER has demonstrated successful conversions, including the generation of formate from methanol,²³⁹ acetate²⁴⁰/ethyl acetate²⁴¹ from ethanol, benzaldehyde²⁴²/benzoic acid^243,244 from benzyl alcohol, glycerate/oxalate from glycerol,²⁴⁵ acetone from 2-propanol,²⁴⁶ acrylate from 1,3-propanediol,²⁴⁷ glycolic acid from ethylene glycol,²⁴⁸ gluconic acid/gluconolactone from glucose,²⁴⁹ and cyclohexanone from cyclohexanol.²⁵⁰ These studies have achieved notable efficiency, showing the potential of alcohol oxidation reactions for practical and sustainable applications in direct fuel cells, energy storage, and synthetic chemistry.

4.1.1 Methanol oxidation. Methanol (CH₃OH), as a small-chain alcohol, has several advantageous properties, including its low cost, wide availability, and low oxidation potential (0.103 V vs. RHE), making it an attractive candidate for the methanol oxidation reaction (MOR) as an attractive alternative to the OER for hydrogen production.^251,252 Additionally, methanol exhibits a relatively high hydrogen content, allowing for the transfer of a maximum number of electrons during the complete oxidation process, thus enhancing the overall efficiency of hydrogen production.²⁵³ Although Pt group noble metal catalysts are known for their exceptional electrocatalytic activity in promoting the MOR, their practical applications are limited due to factors such as their high cost, limited availability, and susceptibility to carbon monoxide (CO) poisoning.²⁵⁴ CO, which is the byproduct of methanol oxidation, can adsorb onto the catalyst surface, reducing its activity and efficiency during the MOR.²⁵⁵ Therefore, the exploration and investigation of non-Pt-based catalysts for the MOR are of significant importance in advancing the widespread utilization of methanol.

Non-Pt-based catalysts offer the potential for low-cost alternatives and improved stability and resistance to CO poisoning. The typical transition-metal-based electrocatalysts for the MOR are Co/Ni-based materials, for instance, Co_xP@NiCo-LDH,²⁵⁶ Mo-modified Co₄N nanoarrsys,²⁵⁷ NiCo/N-CNFs nanofibers,²⁵⁸ Co₃O₄–Ni₃S₄–rGO,²⁵⁹ CNFs@NiSe,²⁶⁰ h-NiSe/CNTs,²⁶¹ NR-Ni(OH)₂,²⁶² defects-rich Ni₃S₂-CNFs,²⁶³ Ni₉₇Bi₃ aerogel,²⁶⁴ bimetallic NiSn nanoparticles,²⁶⁵ Fe–Ni nanoparticles,²⁶⁶ Ni(OH)₂ nanosheets,²⁶⁷ V_O-rich NiO nanosheets,²⁶⁸ NiCu@Cu,²⁶⁹ Pd/Ni(OH)₂/N-rGO,²⁷⁰ Ni–Cu alloys,²⁷¹ and Cu/NiCu NWs.²⁷² Xiang et al. prepared cobalt hydroxide@hydroxy sulfide nanosheets on carbon paper (Co(OH)₂@HOS/CP) as one of the promising catalysts for the electrooxidation of CH₃OH into formate.²⁷³ Benefiting from the optimization of the composition, surface properties, electronic structure, and chemistry of Co(OH)₂, Co(OH)₂@HOS/CP exhibited impressive performance, delivering a current density of 10 mA cm⁻² at potentials of 1.461, 1.397, 1.385, and 1.361 V vs. RHE with increasing methanol concentrations from 0.4 to 6.0 M (Fig. 15a). Notably, when Co(OH)₂@HOS/CP was employed as both the anode and cathode catalysts for the concurrent production of hydrogen and formate, an ultralow cell voltage of 1.497 V was achieved to reach 10 mA cm⁻² (Fig. 15b), which is 134 mV lower than that required for OWS. And this hybrid system demonstrated a remarkable FE of 100% and remarkable durability for continuous 20 h of operation without obvious decay. In another study, Wang et al. fabricated NiIr-based MOF nanosheet arrays on Ni foam (NiIr-MOF/NF) with abundant active sites for the methanol-assisted production of hydrogen.²⁷⁴ The synergistic effects of electronic modulation and unique morphology characteristics endow NiIr-MOF/NF with exceptional electrochemical activity. Remarkably, the two-electrode system employing NiIr-MOF/NF as a bifunctional catalyst achieved a remarkably low cell voltage of 1.39 V to obtain 10 mA cm⁻², which is 170 mV lower than that required for the traditional OWS, as well as a high FE approaching 100% at the cathode for formate conversion and at the anode for H₂ production in 1.0 M KOH + 4.0 M methanol.

Fig. 15 (a) The polarization curves of Co(OH)₂@HOS/CP with different concentrations of methanol in 1.0 M KOH. (b) LSV curves of the as-obtained electrocatalysts in 1.0 M KOH with and without methanol in a two-electrode system.²⁷³ Reproduced with permission. Copyright 2020, Wiley-VCH. (c) LSV curves of Fe₂O₃/NiO/NF and MoNi₄ catalysts in 1.0 M KOH with and without 1.0 M methanol electrolyte. (d) In situ infrared reflection absorption spectroscopy (IRRAS) of the MOR on the catalyst surface in the electrolyte of 1.0 M KOH with 1.0 M methanol at different potentials. (e) In situ IRRAS under a p-polarized and s-polarized IR beam in the wave number range from 1250 to 1650 cm⁻¹. (f) The detailed variations of HCO₃⁻¹ and HCOO⁻¹ with potential. (g) The pivotal potential gaps of the RDS during the MOR process for NiO, Fe₂O₃, and Fe₂O₃/NiO.³² Reproduced with permission. Copyright 2023, Royal Society of Chemistry. MS analyses of the produced gases from (h) solution 0 (H₂O solution of 1.0 M KOH and 3.0 M Me–OH), (i) solution 1 (1.0 M KOH and 3.0 M Me–OH dissolved in D₂O), (j) solution 2 (1.0 M KOH and 3.0 M D₃COD dissolved in H₂O) in the first hour, and (k) solution 2 in 12 h of chronoamperometry measurement at 1.4 V vs. RHE.²⁸³ Reproduced with permission. Copyright 2018, American Chemical Society.

Among the non-precious electrocatalysts employed for the MOR, Ni-based materials have attracted significant attention. However, their performance in terms of current density at moderate potentials is still a challenge, primarily due to the difficulties in the dissociation of hydrogen protons.²⁷¹ Furthermore, the excessive adsorption of reaction intermediates hinders the availability of catalytic active sites, thus limiting the activity and long-term stability of catalysts.²⁷⁵ Consequently, the selective conversion of methanol to formate at higher voltages becomes difficult, impeding the generation of hydrogen at large current densities. Previous investigations have primarily focused on the role of higher valent Ni³⁺ species as critical active sites in the selective oxidation of methanol to formate.^276,277 Various strategies, including surface engineering, structural modification, alloying with other metals, and elemental doping, have been explored to control the density of Ni³⁺ active sites, aiming to enhance the performance of the MOR at larger current densities.^278,279 Recently, Peng et al. proposed a novel approach by developing highly dispersed heterojunctions of FeNi oxide, wherein high valence state Ni³⁺ species and abundant Ni–O–Fe interfaces were anchored onto nickel foam (Fe₂O₃/NiO/NF).³² This innovative catalyst exhibited exceptional performance in the MOR, manifesting an onset potential of 1.328 V vs. RHE. Impressively, Fe₂O₃/NiO/NF achieved an absolute current density of 500 mA cm⁻² at 1.654 V (Fig. 15c) with a remarkable FE > 98% in a practical dual-electrode, membrane-free electrolyzer. In-depth investigation through in situ infrared spectroscopy (Fig. 15d–f) and theoretical calculations (Fig. 15g) unveiled that the heterostructure of Fe₂O₃/NiO/NF effectively modulates the electronic state of NiO via robust electronic interactions, thus creating synergistic active sites that facilitate the desirable dynamic conversion of methanol to formate while impeding further oxidation. Moreover, the interface confinement effect plays a crucial role in stabilizing the metastable nickel active site, ensuring the structural stability of the catalyst during reversible redox cycling. As a result, a consistent and dynamically enhanced catalytic process is achieved, exemplifying the superior performance of this catalyst.

The selective electrooxidation of methanol to formate intermediates remains a challenging aspect of the MOR, as the typical oxidative products are CO₂ and H₂O.^280–282 Thus, a thorough investigation into the MOR reaction mechanism is imperative. To clarify the MOR reaction mechanism, Shi's group used mass spectroscopy (MS) to analyze the produced gas.²⁸³ As depicted in Fig. 15h–k, D₂ and HD can only be observed in solution 1 containing 3 M CH₃OH dissolved in deuterated water (D₂O), while no D₂ and HD are produced in solution 2 containing methanol-4D (D₃COD) in non-deuterated H₂O. This observation provides evidence supporting the methanol-assisted water splitting mechanism, rather than methanol reformation, for hydrogen production. More recently, Wang et al. explored the impact of different oxyanions (TO_x: T = P, S, and Se) on the coordination environments of Ni sites, aiming to optimize the electrocatalytic performance of the MOR.²⁸⁴ The Ni metalloids (NiT_x, T = P, S, and Se) were firstly prepared through surface anionization of nickel foams. Subsequently, active amorphous NiOOH coordinated with residual oxyanions (NiOOH-TO_x) were constructed via in situ anodic electrochemical oxidation, resulting in the formation of distinct coordination environments of Ni sites. Based on various in situ and ex situ experiments, they confirmed that the optimized local coordination environment of NiOOH with oxyanions effectively modulates the adsorption of OH* intermediates and methanol molecules, thus favoring the formation of CH₃O* intermediates. Among the various samples, NiOOH-PO_x exhibited the most favorable local coordination environment and significantly enhanced the electrocatalytic activity of Ni sites towards the selective oxidation of methanol to formate.

4.1.2 Ethanol oxidation. Ethanol (CH₃CH₂OH) oxidation reaction (EOR) represents another promising alternative to the anodic OER for energy-saving hydrogen production. However, different to the oxidation process involved in the MOR, the efficient activation of C–C bonds in ethanol requires the presence of active precious metals such as Pt and Pd (Fig. 16a). The activation energies associated with the different bond activations in the EOR on Pt(111) are illustrated in Fig. 16b and c.²⁸⁵ The results show that initially, at 0 V vs. RHE, the activation of ethanol predominantly occurs on the C–H bonds. This is followed by the activation of O–H bonds in hydroxylethyl intermediates. As the potential is increased to 0.68 V vs. RHE, the activation of C–C bonds becomes more energetically favorable. However, it is observed that the dehydrogenation reactions in the EOR exhibit increased exothermicity with increasing potentials, indicating that higher potentials are required for these dehydrogenation processes to occur, which may compromise the overall oxidation efficiency. This result also suggests that higher potentials are necessary for the complete oxidation of ethanol and the efficient generation of hydrogen.

Fig. 16 (a) Proposed reaction pathways for the electrooxidation of ethanol on a platinum surface in acidic electrolytes. Reaction energies of elementary steps (referenced to adsorbed ethanol) for dehydrogenation and C–C cleavage reactions (dashed lines) for (b) Pt(111) at 0 V and (c) 0.68 V vs. RHE.²⁸⁵ Reproduced with permission. Copyright 2013, Elsevier. (d) EOR polarization curves of pristine and aged Ru₁–Pt₃Ni/NiF. (e) Adsorption energies of CH₃COOH on different catalysts facets.²⁸⁹ Reproduced with permission. Copyright 2023, Elsevier. (f) Polarization curves for ethanol oxidation of F-modified β-FeOOH.²⁹⁰ Reproduced with permission. Copyright 2018, American Chemical Society. (g) LSV curves of PdAg/NF in 0.5 M KOH with and without addition of 1.0 M ethylene glycol.²⁴⁸ Reproduced with permission. Copyright 2021, Elsevier. (h) LSV curves for NiS₂/CFC in the absence and presence of alcohol precursors. (i) The conversions and FEs of NiS₂/CFC for the electrocatalytic conversion of selected alcohols to the corresponding ketones.³²⁴ Reproduced with permission. Copyright 2017, Nature Publishing Group.

During the EOR, the cleavage of C–C bonds and the prevention of CO poisoning play pivotal roles in promoting reaction kinetics. Prior investigations have focused on incorporating non-noble metals such as Co, Ni, and In into Pt/Pd nanostructures to modulate the electronic structure and weaken CO adsorption, thereby enhancing EOR performance.^286–288 Huang et al. successfully developed subnanometer-sized, single-atom In-doped Pt nanowires (SA In–Pt NWs) as electrocatalysts with remarkable performance for both the HER and EOR.²⁸⁷ The integration of the HER and the EOR using SA In–Pt NWs/C allows circumvention of the significant overpotential associated with the sluggish OER, enabling achievement of a lower voltage of 0.62 V (compared to 2.07 V for water splitting) to attain a current density of 10 mA cm⁻² for H₂ production. Moreover, the anodic cell utilizing SA In–Pt NWs exhibited a high FE exceeding 93% in the conversion of ethanol to valuable acetate. Mechanistic investigations revealed that the ultrathin 1D morphology of the nanowires, combined with the presence of single-atom In species, provides a maximized number of active sites and effectively activates Pt atoms for catalytic purposes. DFT calculations ascertained that the incorporation of single-atom In efficiently reduces the limiting potential for the HER, facilitating hydrogen release. Simultaneously, it also decreased the energy barrier associated with acetate conversion and desorption, leading to the impressive selectivity and activity of the EOR in the anodic cells.

Noble metals usually show high activity for the EOR, however, an unfortunate drawback is their propensity to induce C–C bond cleavage, leading to the generation of commercially less valuable CO₂ (or CO₃²⁻ under alkaline conditions) during the EOR. In contrast, the partial oxidation products of ethanol, such as acetaldehyde and acetic acid, possess higher economic benefits as value-added fine chemical products. To overcome this challenge and achieve simultaneous high selectivity, activity, and stability during the EOR, Zhou et al. synthesized a unique electrocatalyst comprised of single dispersed Ru-anchored porous Pt₃Ni alloy on nickel foam (Ru₁–Pt₃Ni/NF).²⁸⁹ The resulting Ru₁–Pt₃Ni/NF exhibited remarkable activity and selectivity for the EOR in alkaline media (Fig. 16d), with acetate being the sole detected product. DFT calculations revealed that the incorporation of Ru significantly reduced the Gibbs formation energy of adsorbed hydroxyl species while weakening the adsorption of acetic acid on the catalyst surface (Fig. 16e). These effects collectively enhanced the activity of ethanol oxidation and the selectivity for acetic acid production. When utilized as an anodic electrocatalyst for the EOR in an ethanol oxidation membrane-free cell (consisting of 2.0 M KOH + 2.0 M ethanol), Pt₃Ni/NiF(−)//Ru₁–Pt₃Ni/NF(+) required only 0.7 V of electrolysis voltage to achieve a current density of 125 mA cm⁻² for cathodic hydrogen generation.

The substitution of noble-metal catalysts with earth-abundant transition metal elements for the EOR provides several advantages such as low cost, abundant redox reactions, improved corrosion resistance, tunable properties, and environmental sustainability. For instance, by introducing electronegative F dopants into FeOOH, the binding energies between active Fe sites and reactants such as C₂H₅O⁻ and OH⁻ can be effectively moderated, leading to increased positive charge densities on the Fe sites and enhanced performance for the EOR.²⁹⁰ Notably, F-modified β-FeOOH showed remarkable activity, achieving a current density of 10 mA cm⁻² at a potential of 1.207 V vs. RHE (Fig. 16f), with a Tafel slope of 30.1 mV dec⁻¹. DFT calculations revealed that F dopants can weaken the adsorption of intermediates or CH₃COO⁻ on β-FeOOH, facilitating the desorption of products and promoting the generation of acetic acid as the main oxidation product. Consequently, a low cell voltage of 1.43 V was sufficient to achieve a current density of 10 mA cm⁻² in a two-electrode electrolyzer. Another notable example involves the utilization of a NiOOH–CuO nano-heterostructure on Cu foam (NiOOH–CuO/CF), which exhibited abundant heterointerfaces and changed electronic structure.²⁹¹ Remarkably, an ultralow potential of 1.347 V vs. RHE was attained at 200 mA cm⁻², accompanied by an acetate FE of 79.1% at a high current density of 200 mA cm⁻². DFT calculations indicated that the coupling between CuO and NiOOH by charge redistribution decreases the energy barrier at the rate-limiting step during the EOR. Finally, in a hybrid electrolyzer with NiOOH–CuO/CF as the anode and Pt/C/NF as the cathode, it only required a cell voltage of 1.430 V at 50 mA cm⁻², 181 mV lower than that required for traditional OWS. And the acetic acid is the only liquid product, while the FE of H₂ at the cathode reaches 100%. Non-noble metal-based catalysts, such as NiSn/C,²⁹² Co–MnO/NCNTs,²⁹³ NiFeOOH,²⁹⁴ Cu–Ni–Fe₂O₃,²⁹⁵ NiFe-LDH,²⁹⁶ NiAl-LDH,²⁹⁷ Cu₁Ni₂-S/G,²⁹⁸ have also been developed for the EOR.

4.1.3 Ethylene glycol oxidation. The ethylene glycol oxidation reaction (EGOR) is an important electrochemical reaction that has garnered significant attention due to its potential application in energy conversion and chemical synthesis. The oxidation of ethylene glycol involves the removal of electrons from the hydroxyl groups, resulting in the formation of oxidation products including glycolaldehyde, glyoxal, glycolic acid, glyoxylic acid, carbonate and oxalic acid, or their mixtures.^299,300 These products have various applications in the synthesis of fine chemicals, polymers, and pharmaceuticals. During the ethylene glycol oxidation reaction, the hydroxyl groups are converted to carbonyl groups (C=O), leading to the formation of various oxidation products.³⁰¹ The specific products obtained depend on the reaction conditions, catalysts used, and applied potential. Noble metals such as Pt, Pd, Au, and their alloys always exhibit high activity for the EGOR, such as PtRh_0.02@Rh NWs,³⁰² Cu₁Pd₁/Ir_0.03 NSs/NPG,³⁰³ Pd–Bi₂Te₃,³⁰⁴ Pd–PdSe,³⁰⁵ PtMo/C,³⁰⁶ RhCu nanoboxes,³⁰⁷ 2D AuCu triangular nanoprisms,³⁰⁸ and 3D PdCu nanoflowers.³⁰⁹ Shi et al. developed a PdAg electrocatalyst grown in situ on nickel foam (PdAg/NF) for the EGOR in alkaline solution.²⁴⁸ PdAg/NF enables efficient EGOR with a low potential of 0.57 V vs. RHE at 10 mA cm⁻² (Fig. 16g), which is 980 mV lower than that required for the OER in the same electrolyte without ethylene glycol. And a high FE of up to 92% can be achieved for the conversion from ethylene glycol to glycolic acid. Furthermore, glycolic acid can be electrocatalytically produced with high efficiency even at an extremely high current density of 300 mA cm⁻² without the occurrence of oxygen evolution. When coupled with a Pt cathodic catalyst, the overall electrochemical cell requires a potential of 1.02 V to achieve 20 mA cm⁻², for cathodic hydrogen production with a FE of 100%.

Notably, Ni and Co-based electrocatalysts have shown high activity for the EGOR. For instance, Wang et al. fabricated ultra-thin CoNi_0.2P nanosheets on nickel foam (CoNi_0.2P-uNS/NF).³¹⁰ CoNi_0.2P-uNS/NF revealed prominent electroactivity for the EGOR due to the surface electrochemical reconstruction in an alkaline environment. The assembled hybrid electrolyzer with the CoNi_0.2P-uNS/NF catalyst only required 1.35 V to achieve 10 mA cm⁻² for coproduction of formate and hydrogen. Chen's group replaced the OER with the EGOR over Ni₃N–Ni_0.2Mo_0.8N nanowires, which generated formate selectively at the anode.³¹¹ Specially, ys-ZIF@CoFe-LDH NCs,³¹² and CoNi-MOF,³¹³ also exhibit high performance for the EGOR.

4.1.4 Propanol oxidation. Propanol-category organics, such as 2-propanol, have emerged as promising alternatives to the OER for low-cost hydrogen production in hybrid electrolysis due to their lower oxidation potential.³¹⁴ The efficient and selective conversion of propanol to valuable products, such as acetone or propionic acid, is essential for achieving high-performance electrocatalysis.³¹⁵ Several catalysts have been investigated for the electrocatalytic oxidation of propanol, aiming to improve the reaction kinetics, selectivity, and stability. Noble metal-based catalysts, such as Pt, Pd, Rh, and their alloys, have demonstrated remarkable electrocatalytic activity for propanol oxidation.^316,317 For example, Tong et al. shown that that Pd exhibits higher current density and more negative onset potential for 2-propanol oxidation compared to Pt and Au in an alkaline electrolyte.³¹⁸ In addition, dealloyed Cu@Pt core–shell electrocatalysts have been fabricated for 2-propanol electrooxidation in acidic solution.³¹⁹ This core–shell catalyst consists of a Cu-rich core and a Pt-rich shell. The lattice mismatch between the core and shell regions leads to electronic structure modification, higher compressive lattice strain (approximately 0.85%), and a larger electrochemical active surface area. As a result, the Cu@Pt catalyst has shown higher catalytic activity and greater stability towards 2-propanol electrooxidation compared to Pt-only catalysts.

Although noble metal catalysts demonstrate high catalytic efficiency for propanol oxidation, their scarcity and high cost pose limitations for practical applications.^320,321 Therefore, there is growing interest in developing non-noble metal catalysts or earth-abundant alternatives to mitigate these challenges. Non-noble metal catalysts, including transition metal oxides, carbides, nitrides, and sulfides, have shown promising electrocatalytic performance.^277,322 In noticeable example, a series of LaFe_1−xCo_xO₃ perovskite catalysts were developed to enable electrocatalytic OER, isopropanol oxidation reaction, and glycerol oxidation reaction.³²³ The investigation focused on establishing a structure–composition–activity relationship, revealing distinct trends for anodic oxidation reactions arising from variations in active sites and involved reaction intermediates. Interestingly, no correlation was found between the electrochemical surface areas and the activities of electrochemical oxidation reactions. However, the phase/metal composition of the LaFe_1−xCo_xO₃ catalysts influenced the selectivity towards different oxidation products, highlighting the trade-off between achieving high current densities and obtaining high-value chemical products. In another study, a facile vapor-phase hydrothermal method was employed to prepare a carbon fiber cloth supported single-crystalline NiS₂ nanostructure (NiS₂/CFC).³²⁴ This catalyst exhibited remarkable activity, selectivity, and durability in the HER, OER, and the oxidation of various alcohols including 2-propanol, 2-butanol, 2-pentanol, and cyclohexanol. In the presence of 0.45 M 2-propanol, the NiS₂/CFC catalyst demonstrated a 1.2 times higher H₂ production rate and a 280 mV lower cell voltage at a current density of 20 mA cm⁻² compared to that required for conventional water electrolysis, accompanied by high-value-added acetone production with near-unity FEs for H₂ (100%) and acetone (98%) (Fig. 16h and i).

4.1.5 Glycerol oxidation. Glycerol, a low-value byproduct generated during biodiesel production, presents an intriguing opportunity for selective electrooxidation to produce various chemicals such as glyceric acid, dihydroxyacetone, glycolic acid, hydroxypyruvic acid, and formate/formic acid.³²⁵ Furthermore, the glycerol oxidation reaction (GOR) holds significant potential as an alternative to the OER for hydrogen production due to its low theoretical oxidation potential (0.003 V vs. RHE). The control of selectivity is critical in glycerol-assisted water electrolysis, as different reaction pathways can lead to the formation of different products.³²⁶ Noble metals like Pt, Pd, Au, and their alloys are recognized as highly active materials for the cleavage of C–C and C–O bonds, which are considered as rate-determining steps in the GOR.^245,327 Shao et al. have reported on a catalyst consisting of CoPt nanoparticles uniformly distributed on a carbon nanosheet array (CNs@CoPt).³²⁸ CNs@CoPt demonstrated an exceptionally low onset potential of 1.32 V vs. RHE for the GOR, and an overpotential of 19.1 mV to achieve a current density of 10 mA cm⁻² for the HER. The glycerol-assisted water electrolysis only required a cell voltage of 1.71 V at 100 mA cm⁻², surpassing that of traditional water splitting systems (1.91 V). Importantly, the hydrogen yield in this coupled system reached 48 L h⁻¹ m⁻² at a cell voltage of 1.70 V, which was 1.7 times greater than that required for OWS. Simultaneously, a high production rate of the more valuable formate (113 g h⁻¹ m⁻²) could be readily achieved, effectively reducing the overall cost of this hydrogen production system.

Fortunately, nonprecious metal catalysts, particularly Ni- and Co-based materials, for example, NiOOH,³²⁹ NC/Ni–Mo–N/NF,³³⁰ Ni_xBi_1−x,³³¹ NiO_x/MWCNTs,³³² Co₃O₄ nanosheets,³³³ amorphous CoO_x,³³⁴ Bi-doped Co₃O₄,³³⁵ ZnFe_xCo_2−xO₄,³³⁶ CoO_xH_y,³³⁷ CuCo-oxide,³³⁸ CuCo₂O₄,³³⁹ CuO,³⁴⁰ and Cu-CuS/BM,³⁴¹ have also exhibited promising glycerol oxidation activities. Li et al. fabricated a cost-effective MnO₂ electrode and employed it in the glycerol-assisted hydrogen production under acidic conditions.³⁴² Remarkably, this MnO₂ catalyst exhibited exceptional activity with an anodic potential of 1.36 V vs. RHE at 10 mA cm⁻² (Fig. 17a) and extremely high stability over 865 h for the GOR, which was significantly longer than the 10 h stability observed for the OER. In situ Raman spectroscopy (Fig. 17b) and DFT calculations (Fig. 17c) provided valuable insights into the underlying mechanism. The results demonstrated that glycerol molecules possess a robust electron-donating ability towards the positively charged MnO₂ catalyst during the electrolysis process. The adsorption of glycerol molecules on the surface of MnO₂ not only hindered the oxidation of the catalyst, preventing the formation of dissolved MnO₄⁻, but also impeded the formation of chemisorbed oxygen species, contributing to the impressive stability observed. Recently, Wen's group developed a novel high-entropy alloy (CoNiCuMnMo HEA) self-supported electrode for the GOR and utilized self-developed machine learning-based Monte Carlo simulation to investigate its surface atomic configuration.³⁴³ To demonstrate the potential industrial prospects of this high-entropy alloy, a hybrid alkali/acid flow electrolytic cell was designed with the HEA as the anode for the alkaline GOR and the commercial RhIr/Ti as the anode for acidic HER. This system required only an applied voltage of 0.55 V to achieve a current density of 10 mA cm⁻² and exhibited exceptional long-term stability over 12 days continuous running at 50 mA cm⁻².

Fig. 17 (a) Polarization curves over a MnO₂/CP//Pt/C/CP electrolyzer with and without the addition of 0.2 M glycerol addition. (b) In situ Raman spectra of MnO₂/CP collected with and without glycerol at an applied potential of 1.58 V. (c) Reaction free-energy diagram of the OER (LOM) and glycerol oxidation to glyceraldehyde.³⁴² Reproduced with permission. Copyright 2021, Wiley-VCH. (d) LSV curves for the water electrolysis with and without glycerol addition using the flow cell. (e) The amount of generated formate and H₂ with corresponding FEs.³⁴ Reproduced with permission. Copyright 2023, Wiley-VCH. (f) Electrocatalytic H₂ generation combined with the production of high-purity Ph-COOH at large current densities with no interference of the OER. (g) The in situ Raman spectra of the A-Ni–Co-H/NF electrode and the corresponding structural conversion under the changing potentials vs. Ag/AgCl.³⁴⁹ Reproduced with permission. Copyright 2020, Royal Society of Chemistry. (h) The LSV curves of Au/CoOOH in 3.0 M KOH with or without 0.2 M Ph-CH₂OH at 70 °C in the membrane-free flow electrolyzer.²³⁸ Reproduced with permission. Copyright 2022, Nature Publishing Group.

Despite recent advancements in biomass electrooxidation, there is still a lack of research focusing on investigating energy conversion efficiency at industrial-level current densities (∼1 A cm⁻²) and developing subsequent separation methods, which are crucial for practical applications. Recently, Zhang et al. reported a biphasic transition metal nitride electrode by in situ growing Ni₃N/Co₃N heterostructure nanowires with abundant heterointerfaces on Ni foam.³⁴ This electrode exhibited remarkable activity for nucleophilic reaction electrocatalysis, achieving an ultrasmall work potential of 1.26 V to reach a current density of 50 mA cm⁻² for the GOR and an excellent FE of 94.6% for formate production. Importantly, under a two-electrode configuration, the electrode enabled the concurrent production of formate and H₂, delivering current densities of 50 and 400 mA cm⁻² at 1.47 and 2.04 V, respectively, which surpass those required forthe OWS for O₂ and H₂ generation (1.72 and 2.22 V at 50 and 400 mA cm⁻², respectively). As a proof-of-concept demonstration for practicality, a glycerol hybrid electrolysis based on a flow electrolyzer achieved an industry-level current density of 1 A cm⁻² at 2.01 V (Fig. 17d) with impressive stability over 200 h for continuous operation, realizing efficient energy-saving efficiency of 9.7% compared to OWS, and outstanding productivities of 11 and 21.4 mmol cm⁻² h⁻¹ for formate and H₂ at a current density of 1 A cm⁻², respectively (Fig. 17e).

4.1.6 Benzyl alcohol oxidation. The oxidation products obtained from benzyl alcohols (Ph-CH₂OH) have significant potential for application in various industries, including pharmaceutical, polymer, and commodity.³⁴⁴ The ability to selectively oxidize Ph-CH₂OH and generate these important intermediates is of great significance in the synthesis of diverse products across multiple industries. In addition, the low oxidation potential of Ph-CH₂OH also allows for more favorable hydrogen production by replacing the OER in water electrolysis. Noble metals such as Pt and Pd have been commonly used as catalysts for Ph-CH₂OH oxidation due to their high catalytic activity and selectivity.^237,345 However, the use of noble metals is limited by their high cost and availability, prompting the exploration of alternative, earth-abundant electrocatalysts.³⁴⁶ In 2017, Sun et al. synthesized a 3D hierarchical porous nickel catalyst (hp-Ni) for hybrid water electrolysis, utilizing the oxidation of various organic substrates such as Ph-CH₂OH, 4-nitrobenzyl alcohol (NBA), 4-methylbenzyl alcohol (MBA), ethanol, and HMF.²⁴⁴ Intriguingly, the integrated electrolyzer, operating with a concentration of 1.0 M KOH + 10 mM Ph-CH₂OH, only requires a cell voltage of 1.50 V to afford 10 mA cm⁻², 190 mV lower than that required for OWS. Notably, the hp-Ni catalyst exhibited exceptional FE for the HER (∼100%) and benzoic acid (Ph-COOH) formation (∼97%). Through post-characterization analyses, it was deduced that the in situ formation of high-valent nickel species on the hp-Ni catalyst likely serves as the true catalytic active sites responsible for the anodic alcohol oxidation. Furthermore, when employed in the oxidation of diverse alcohols, the hp-Ni catalyst displayed similar onset potentials, regardless of the distinct intrinsic thermodynamics governing these oxidation reactions, implying the significant effect of catalysts on electrocatalytic organic oxidation.

Li et al. fabricated N-doped nickel–molybdenum oxide (N–Mo–Ni/NF) loaded on Ni foam for the selective electrooxidation of Ph-CH₂OH to benzoic acid in alkaline electrolytes.³⁴⁷ The N–Mo–Ni/NF electrode shows excellent electrocatalytic activity and stability at the anode and only requires a low potential of 1.338 V vs. RHE to afford 100 mA cm⁻² in a 0.1 M Ph-CH₂OH solution, which is 252 mV lower than that required for the OER. In addition, a yield of 98.2% of benzoic acid has been obtained with a high FE of 98.7%. Yu's group prepared 2D nickel-based nanoarrays directly grown on a carbon cloth substrate (CC@NiO/Ni₃S₂) using a facile one-step electrodeposition technique, enabling selective catalysis of Ph-CH₂OH to Ph-COOH under alkaline conditions.³⁴⁸ CC@NiO/Ni₃S₂ demonstrated exceptional catalytic properties for the oxidation of Ph-CH₂OH, exhibiting a low working potential of 1.38 V vs. RHE at 10 mA cm⁻² and a remarkable selectivity of 98% towards Ph-COOH. Moreover, in the presence of 0.2 M Ph-CH₂OH + 1.0 M KOH, a current density of 10 mA cm⁻² was achieved at 1.458 V in the hybrid water electrolysis, 0.151 V lower than that for conventional water electrolysis. Qiu et al. fabricated amorphous nanosheets of Ni, Co hydroxide supported on Ni foam (A-Ni-Co-H/NF), enabling operation at high current densities (>400 mA cm⁻²) for the upgrading of Ph-CH₂OH to Ph-COOH without interference from the OER (Fig. 17f).³⁴⁹ Notably, compared to the OER, the oxidation of Ph-CH₂OH using A-Ni–Co-H/NF yielded a Ph-COOH conversion approaching 100% while significantly reducing the electric energy consumption by 0.024 W h after working for 1 h at a current density of 100 mA cm⁻². The reversible structural evolution and recovery of the catalyst were observed through in situ Raman spectroscopy (Fig. 17g), confirming the formation of Co-containing nickel oxyhydroxide (Co–NiOOH) as the real active species. Co/Ni-based catalysts are commonly used for Ph-CH₂OH oxidation, such as Mo–Ni alloy nanoparticle-modified MoO₂,²⁴³ h-Ni(OH)₂,³⁴⁴ Ni₂P/NF,³⁵⁰ NiCo₂O₄ nanosheets,³⁵¹ β-NiOOH,³⁵² 2D Ni-based MOFs,³⁵³ plasma modified nickel foam,³⁵⁴ Ni(OH)₂ nanosheet/Ni foam,³⁵⁵ NiCo(OOH)_x nanosheets,³⁵⁶ Ni–Fe thin films,³⁵⁷ Co–Ni LDH,³⁵⁸ and Co₃O₄ NPs/Ti.³⁵⁹

The electrochemical synthesis reactions discussed earlier exclusively yield hydrogen at the cathode, while no gas is generated at the anode. As a result, the implementation of a membrane-less electrolyzer becomes viable, simplifying operation under practical conditions and reducing reactor costs associated with membranes and maintenance. Duan et al. successfully demonstrated the application of a membrane-free flow electrolyzer operating under industrially relevant conditions.²³⁸ They developed cobalt oxyhydroxide nanosheet-supported gold nanoparticles (Au/CoOOH) for Ph-CH₂OH-assisted hydrogen production. An impressive current density of 540 mA cm⁻² was achieved at 1.5 V vs. RHE for the electrooxidation process. In a more realistic two-electrode membrane-free flow electrolyzer setup (Fig. 17h), the absolute current could be further increased to 4.8 A at 2.0 V. Experimental and theoretical findings indicate that Ph-CH₂OH can be selectively enriched at the interface of Au/CoOOH and oxidized by the electrophilic oxygen species (OH*) generated on CoOOH, leading to higher activity compared to pure Au catalysts. Notably, the catalyst exhibits reversible oxidation/reduction behavior under anodic potential/open circuit conditions. Building on this observation, an intermittent potential strategy was designed for long-term alcohol electrooxidation, achieving a high current density (>250 mA cm⁻²) over 24 h with enhanced productivity and reduced energy consumption.

4.2 Aldehyde oxidation reactions

Aldehydes, such as 5-hydroxymethyl furfural (HMF) and furfural (FUR), are important raw materials in the synthesis of fine chemicals, biofuels, and medicines. Especially, the conversion of biomass-derived compounds containing –CH=O groups, like HMF and FUR, to higher-value chemicals has attracted significant attention in recent decades.³⁶⁰ Conventional thermochemical oxidation methods for HMF and FUR typically require the use of precious-metal catalysts and harsh reaction conditions, including high temperatures, strong basicity, high oxygen pressures, and toxic or expensive reagents. These conditions often pose challenges in terms of cost, energy consumption, and environmental impact. In contrast, electrooxidative conversion provides a more environmentally friendly pathway for upgrading HMF and FUR. This approach operates in aqueous solution, without the need for organic solvents or oxidizing agents, and can be conducted at ambient temperature and pressure, offering several advantages, including high selectivity, mild reaction conditions, and the potential to use non-noble metal electrocatalysts.^361–363 Moreover, by coupling the electrooxidation of HMF or FUR with the HER in hybrid water electrolysis, it becomes possible to simultaneously produce hydrogen and valuable chemicals, which provides a promising strategy for achieving efficient and sustainable energy conversion processes.

4.2.1 5-Hydroxymethyl furfural oxidation. The oxidation of HMF presents a promising route for the production of valuable chemicals such as dimethyl formamide (DMF), diformylfuran (DFF), and 2,5-furandicarboxylic acid (FDCA).^364,365 Among these products, FDCA is particularly important as a precursor for the synthesis of important fine chemicals and polymers. When combined with the HER in hybrid water electrolysis, the anode facilitates FDCA production while the cathode simultaneously generates hydrogen.^366,367 For instance, the utilization of bifunctional Ni₂P/Ni foam as an electrocatalyst allows for achieving low voltages of 1.44 and 1.58 V, respectively, to attain current densities of 10 and 50 mA cm⁻² in the presence of HMF (1.0 M KOH + 10 mM HMF) (Fig. 18a), compared with 1.65 and 1.80 V for conventional water electrolysis, respectively.³⁶³ Typically, noble metal electrocatalysts such as Ru, Pd, Ag, Pt, and Au are known for their high activity in the oxidation of HMF.^368,369 However, their high cost and partially unfavorable electrocatalytic properties have motivated the exploration of non-noble metal-based catalysts.^370,371 Consequently, diverse electrocatalysts have been developed, including porous MoO₂–FeP@C,³⁷² CoO–CoSe₂,³⁷³ CoP,³⁷⁴ nanocrystalline Cu,³⁷⁵ Ni₃S₂/NiF,³⁷⁶ Ni/Co/Fe-OOH,³⁷⁷ mesostructured NiO,³⁷⁸ Ni NSs,³⁷⁹ NiFe-LDH,³⁸⁰ NiCoFe-LDH,³⁸¹ NiCoMn-LDHs,³⁸² coralline-like CoP/Ni₂P–NiCoP@NC,³⁸³ Co–CoS_x heterojunctions,³⁸⁴ CoOOH,³⁸⁵ Ni₃N–V₂O₃,³⁸⁶ NiO_x/NF,³⁸⁷ NiMoO–Ni,³⁸⁸ Cu doped Co₃O₄ nanowires,³⁸⁹ branched Ni NPs,³⁹⁰ Ni_xB,³⁹¹ NiS_x/Ni₂P nanotube arrays,³⁹² NiCo₃O₄ NWs,³⁹³ NiCo₂@MoO₂/NF,³⁹⁴ CuNi(OH)₂,³⁹⁵ and (FeCrCoNiCu)₃O₄ NSs.³⁹⁶

Fig. 18 (a) LSV curves for a Ni₂P NPA/NF catalyst couple in 1.0 M KOH with and without 10 mM HMF.³⁶³ Reproduced with permission. Copyright 2016, Wiley-VCH. (b) Two possible pathways of HMF oxidation to FDCA.⁶ Reproduced with permission. Copyright 2018, American Chemical Society. Adsorption evaluation for Ir–Co₃O₄ and Co₃O₄. TPD spectra of Ir–Co₃O₄ and Co₃O₄ at HMF/He (c), ethylene (d), and CO (e) atmospheres.⁴⁰⁷ Reproduced with permission. Copyright 2021, Wiley-VCH. LSV curves of a Cu-modified glass carbon electrode in 1.0 M KOH with and without 50 mM HMF (f) and in 1.0 M KOH with and without 50 mM furfural (g). (h) The bipolar hydrogen production system of furfural oxidation in in the electrolyser using Cu foam as the anode and Pt/C as the cathode.⁴¹⁷ Reproduced with permission. Copyright 2021, Nature Publishing Group.

The catalytic conversion of HMF to FDCA requires the oxidation of both the alcohol and aldehyde groups in HMF. Fig. 18b illustrates two potential routes (route I and route II) for the synthesis of FDCA.⁶ In route I, the oxidation begins with the alcohol group and proceeds via DFF as the initial intermediate. Subsequent transformations lead to the conversion of DFF into 5-formyl-2-furancarboxylic acid (FFCA), which further undergoes oxidation to form FDCA. In route II, the aldehyde group in HMF is oxidized, resulting in the formation of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) as the primary intermediate. Then HMFCA undergoes further conversion into FFCA and subsequently transforms into FDCA.^397,398 Experimental techniques such as in situ SFG vibrational spectroscopy,³⁹⁹operando electrochemistry coupled with attenuated total reflection infrared spectroscopy,^391,400 and operando surface-enhanced Raman spectroscopy^401–403 have provided insights into the electrooxidation process of HMF. These studies have supported that route II is the most plausible pathway for the electrooxidation of HMF under strongly alkaline conditions. In addition to experimental techniques, DFT calculations have been employed to simulate the reaction pathway and elucidate the reaction mechanism.⁴⁰⁴ These computational studies help in understanding the transition states, solvation effects, and other factors influencing the electrooxidation process. Despite these efforts, the precise reaction mechanism and pathway for the electrooxidation of HMF to FDCA are still not fully understood. Further exploration using in situ/operando techniques and comprehensive theoretical investigations is needed to gain a detailed understanding of the intricacies of the electrooxidation process.

Among various candidates, transition metal Co-based materials have emerged as prominent catalysts for HMF oxidation. During the electrooxidation process, these Co-based catalysts generate high-valence Co species, including Co³⁺ and Co⁴⁺, which play specific roles in the oxidation of different functional groups in HMF. Deng et al. revealed that Co³⁺ primarily facilitates the oxidation of the formyl group to a carboxylate, while Co⁴⁺ exerts a pivotal effect on the initial oxidation of the hydroxyl group and significantly influences the overall reaction rate.⁴⁰⁵ On the basis of this understanding, they achieved selective synthesis of HMFCA and FDCA by applying distinct oxidation potentials. Wang's group conducted a comprehensive investigation by synthesizing a series of cobalt spinel oxides (Co₃O₄, ZnCo₂O₄, CoAl₂O₄) to occupy different geometric sites.⁴⁰⁶ They found that the tetrahedral (Co²⁺_Td) site in Co₃O₄ enhances the chemical adsorption of HMF, while the octahedral (Co³⁺_Oh) site facilitates HMF oxidation. Thereafter, Cu²⁺ was introduced into Co₃O₄ to improve the exposure degree of Co³⁺ and to boost adsorption and thus enhancing the performance for HMF oxidation. In addition to spinel oxides, this group also found that the introduction of atomically dispersed Ir sites (Ir-Co₃O₄) also enhances HMF adsorption by interacting with the C=C group, thereby promoting HMF oxidation activity, which could be clearly reflected by the temperature-programmed desorption (TPD) results (Fig. 18c–e).⁴⁰⁷ Furthermore, Co₃O₄ nanosheets with oxygen vacancies (V_O-Co₃O₄) have also been investigated by this group to understand the adsorption behavior between HMF and OH⁻ during HMF oxidation.⁴⁰⁸In situ XAS and DFT calculations demonstrated that oxygen vacancies in Co₃O₄ preferentially adsorb OH⁻, enabling efficient coupling with HMF and enhancing the rate of HMF oxidation. These studies highlight the importance of understanding the intricate interplay between the catalyst composition, adsorption behavior, and specific reaction pathways in advancing the field of HMF oxidation. By elucidating the roles of different Co species and optimizing the catalyst composition and structure, we expediently enhance the electrocatalytic performance and selectivity for the oxidation of HMF, enabling the efficient conversion of this biomass-derived compound into valuable chemicals.

In the development of bifunctional catalysts with exceptional activities for both HER and HMF oxidation, various transition-metal-based catalysts have been synthesized and investigated for their performance in these reactions. Strategies such as surface modification, alloying, and heteroatom doping have been employed to further enhance the catalytic performance of these materials. For instance, Fu's group prepared a carbon-encapsulated MoO₂–FeP heterostructure (MoO₂–FeP@C) that exhibited excellent HER and HMF oxidation activity due to its unique interfacial electronic structure.³⁷² The overpotential for the HER at 10 mA cm⁻² was measured at 103 mV. In the presence of 1.0 M KOH with 10 mM HMF, MoO₂–FeP@C demonstrated a potential of 1.359 V vs. RHE at 10 mA cm⁻², which is lower than that required for the OER (1.474 V). Notably, when employed in an electrolyzer for cathodic H₂ production and anodic FDCA production, MoO₂–FeP@C achieved a low voltage of 1.486 V at 10 mA cm⁻² and exhibited high selectivity for FDCA (98.6%). Moreover, MoO₂–FeP@C displayed excellent catalytic performance in the electrooxidation of various biomass substrates, including benzyl alcohol, furfural, furfuryl alcohol, 4-nitrobenzyl alcohol, and 4-methoxybenzyl alcohol, coupled with the cathodic HER. Zhang et al. reported a Ni₃N/carbon nanosheet (Ni₃N@C) catalyst for HER and HMF oxidation.³⁹⁹ The strong interaction between Ni₃N and carbon effectively modulates the electronic structure of Ni⁺, which is a crucial factor for optimizing electrochemical activity. In the presence of HMF in 1.0 M KOH, the overpotential required to achieve a current density of 50 mA cm⁻² was reduced to 1.38 V vs. RHE, which is 0.22 V lower than the overpotential required for the OER. Moreover, the HER overpotential of Ni₃N@C was measured at 113 mV to reach 50 mA cm⁻², and the LSV curves exhibited negligible differences with or without HMF. Utilizing HMF oxidation instead of the OER as the anode reaction in a two-electrode configuration, the required potential to achieve a current density of 50 mA cm⁻² was only 1.55 V, which is lower than 1.79 V required for the traditional OWS. Similarly, other bifunctional electrocatalysts, such as, Ni₂P,³⁶³ and CoP,³⁷⁴ Ni₃S₂,³⁷⁶ CoNW/NF,⁴⁰⁹ NiCo-LDH NiCo_NSs/Cu_NWs,⁴¹⁰ NF@Mo–Ni_0.85Se,⁴¹¹ Cu_xS@NiCo-LDHs,⁴¹² and NiSe@NiO_x NWs,⁴¹³ were also reported, opening up possibilities for efficient hybrid electrocatalysis in the conversion of biomass substrates.

4.2.2 Furfural oxidation. Furfural, with its aldehyde functional group, can undergo various oxidation reactions to yield different products, such as 2-furoic acid, FDCA, furfuryl alcohol, and furan derivatives.⁴¹⁴ These products have potential applications in the production of biofuels, polymers, and specialty chemicals. Furfural oxidation can be carried out through different methods, including electrochemical oxidation, catalytic oxidation, and chemical oxidation using oxidizing agents. Electrochemical methods offer advantages such as milder reaction conditions, precise control over the reaction, and the possibility of integrating with other electrochemical processes, such as the HER for simultaneous hydrogen production.⁴¹⁵ In a significant breakthrough in 2017, Sun et al. achieved the electrocatalytic conversion of furfural oxidation coupled with the HER utilizing a cost-effective bifunctional catalyst of Ni₂P/Ni/NF.⁴¹⁶ In 1.0 M KOH electrolyte with furfural, the hybrid electrolyzer demonstrated nearly 100% FE and exhibited remarkable stability in the generation of 2-furoic acid and hydrogen. Notably, this device required a cell voltage of 1.48 V to achieve 10 mA cm⁻², representing a significant reduction of approximately 110 mV compared to that of a traditional OWS (1.59 V). Post-electrolysis analysis involving XPS confirmed the formation of nickel oxides as the active species for furfural oxidation.

In 2021, Wang et al. successfully realized the low-potential electrooxidation of HMF and furfural at approximately 0.1 V vs. RHE using a metallic Cu catalyst.⁴¹⁷ Unlike conventional aldehyde electrooxidation, wherein the hydrogen atom in the –CHO group is oxidized to form H₂O at high potentials (>1.0 V vs. RHE, 3OH⁻ + R − CHO ⇌ 2H₂O + R–COO⁻ + 2e⁻), low-potential aldehyde oxidation involves the combination of hydrogen atoms to generate H₂ (2OH⁻ + R–CHO ⇌ 1/2H₂ + R–COO⁻ + H₂O + e⁻). In the presence of both HMF and furfural in an electrolyte, a distinct oxidation current is observed (Fig. 18f and g), and the onset potential reaches an exceptionally low value of 0.05 V vs. RHE. Conversely, in the presence of furfuryl alcohol, which lacks an aldehyde group and only possesses a hydroxyl group, no discernible difference from pure KOH solution is observed, emphasizing the significance of the aldehyde group in this design. Consequently, in the novel bipolar H₂ production system, which incorporates cathodic HER and low-potential anodic aldehyde oxidation on the developed metallic Cu catalysts, an incredibly low voltage of 0.1 V is sufficient to drive H₂ generation (Fig. 18h). Notably, the assembled electrolyzer produces H₂ at both the cathode and the anode, while 2-furoic acid or HMFCA is generated at the anode, with an apparent FE of about 200%. The energy input for 1 m³ H₂ production is approximately 0.35 kW h, significantly lower than 5 kW h required for 1 m³ H₂ generation through conventional water electrolysis. This approach presents a promising pathway for efficient hydrogen production with reduced electricity consumption. However, the catalytic performance of the low-potential electrooxidation of furfural and HMF still requires further exploration of advanced catalysts to achieve enhanced efficiency and effectiveness. Recently, this group investigated the correlation between the valence state and the adsorption behavior of the Cu-based electrocatalyst in furfural oxidation.⁴¹⁸ Combined with the characterization of the valence state evolution and the absorption behavior on the designed mixed-valence Cu-based electrocatalyst, they found that Cu⁰, in its metallic form, acted as an adsorption site with low intrinsic activity for furfural oxidation. In addition, Cu⁺, existing in the form of Cu(OH)_ads in an alkaline electrolyte, had no adsorption ability but played a crucial role in improving the performance of Cu⁰ by promoting the adsorption of furfural. Based on these findings, they proposed that the design principle for stable Cu-based catalysts for furfural oxidation is maintaining the stability of the valence state or the adsorption behavior of copper species.

4.2.3 Formaldehyde oxidation. Despite notable advancements in organic oxidation-assisted water splitting, the production of H₂ is typically limited to the cathode, and most electrocatalytic systems require a cell voltage exceeding 1 V to achieve industrially relevant current densities (>500 mA cm⁻²). Consequently, a significant challenge remains in developing alternative strategies for ultra-low voltage H₂ production from water, with the added desirability of generating H₂ at both the cathode and anode. One potential avenue is the partial oxidation of formaldehyde (HCHO) through the formaldehyde oxidation reaction (FOR), leading to the formation of formate and the release of H₂ at the anode with a minimal thermodynamic potential (HCHO + 2OH⁻ → HCOO⁻ + 1/2H₂ + H₂O + e⁻, E⁰ = −0.22 V vs. RHE). Furthermore, the adsorbed hydrogen (*H) resulting from C–H bond cleavage during the FOR can be oxidized to generate H₂O at this potential. Therefore, the coupling of the FOR with the HER presents the opportunity for environmentally beneficial H₂ production, particularly if toxic formaldehyde residues in wastewater can be utilized as a valuable feedstock.

Although H₂ production through the partial FOR has been demonstrated on a limited number of metallic electrodes, the coupling of the FOR with the HER for simultaneous H₂ production at both the anode and cathode remains relatively unexplored. In a recent study, Sun et al. reported a novel and cost-effective electrocatalytic system utilizing Cu₃Ag₇ as the anode catalyst and Ni₃N/Ni as the cathode catalyst to drive the FOR and HER, respectively, under alkaline conditions.⁴¹⁹ This system achieved H₂ production with an apparent FE of 200% and demonstrated industrially relevant current densities of 100 and 500 mA cm⁻² at remarkably low cell voltages of only 0.22 V and 0.60 V, respectively (Fig. 19a). Importantly, the energy consumption of this two-electrode electrolyzer for H₂ production is merely 0.30 and 0.70 kW h m⁻³ H₂ at current densities of 100 and 500 mA cm⁻², respectively (Fig. 19b), which are much lower than the theoretical energy demand for OWS (4.10 and 4.70 kW h m⁻³ H₂). To date, the reported FOR catalysts have been concentrated on Cu-based materials, such as Cu₂O catalyst,⁴²⁰ ZrO₂–CuO/Au,⁴²¹ Ni doped Cu,⁴²² Cu₂O,⁴²³ hollow PdCu alloy,⁴²⁴ and Cu nanosheet arrays.⁴²⁵ Some other transition metal-based catalysts, such as Ni nanowires,⁴²⁶ S-Ni@Ni(OH₎₂/NF,⁴²⁷ NiCo–NiCoP@PCT,⁴²⁸ NiMn phosphates,⁴²⁹ S-doped MnO₂,⁴³⁰ and Co–N_x–C@Co,⁴³¹ have also been used for the FOR.

Fig. 19 (a) The two-electrode CV curves of HER/FOR (red) and HER/OER (blue) in which Cu₃Ag₇/CF and Ni₃N/Ni/NF were employed as the anode and cathode for the former while Ni/NF and Ni₃N/Ni/NF for the latter. (b) Comparative analysis of the calculated electricity consumption for H₂ production between the formaldehyde (red) and paraformaldehyde (black) oxidation-integrated strategy using the Ni₃N/Ni/NF(−)//Cu₃Ag₇/CF(+) electrode couple and traditional water electrolysis (HER/OER) using the Ni₃N/Ni/NF(−)//Ni/NF(+) electrode couple.⁴¹⁹ Reproduced with permission. Copyright 2023, Nature Publishing Group. (c) Schematic diagram of an asymmetric electrolyte electrolyzer. (d) Polarization curve for the GOR. (e) LSV curves for the asymmetric-electrolyte electrolyzer.⁴⁵³ Reproduced with permission. Copyright 2020, Elsevier. (f) Electrocatalytic conversion of primary amines into nitriles integrated with H₂ production in water. (g) LSV curves of the NiSe anode in 1.0 M KOH with and without 1 mmol BA. (h) LSV curves of a CoP(−)//NiSe(+) electrolyzer with and without 1 mmol BA.⁴⁵⁶ Reproduced with permission. Copyright 2018, Wiley-VCH.

The origin of catalytic activity and the underlying reaction mechanism for the FOR remains elusive. Wang et al. employed a novel approach by utilizing partially reduced CuO on Cu foam (Cu_xO@CF) catalysts to investigate the FOR mechanism.⁴³² The Cu_xO@CF electrocatalyst, comprising a mixture of Cu and Cu₂O phases, was synthesized through electrodeposition followed by an electrochemical activation process. To gain insights into the catalytic processes, in situ characterization studies such as in situ XAS, in situ ATR-SEIRAS, in situ DEMS, and DFT calculations were performed to identify and explore the presence of Cu⁰ and Cu⁺ species, as well as the synergistic effects governing the FOR, thereby validating the proposed reaction pathway. The obtained results unveiled that Cu⁰ species play a crucial role in lowering the reaction energy barrier for the HOCH₂O* + HO* process, while Cu⁺ species are found to be more favorable for the C–H bond cleavage. Additionally, comprehensive analysis confirmed that the generated H₂ molecules solely originate from HCHO decomposition.

4.2.4 Glucose oxidation. Electrochemical glucose oxidation has gained significant attention as an environmentally friendly and efficient method for glucose conversion. When glucose undergoes complete oxidation, it releases 24e⁻, making it highly suitable for the coupled HER in glucose-assisted hybrid electrolysis. The oxidation of glucose involves several steps, including the adsorption of glucose onto the catalyst surface, the cleavage of C–C bonds, the formation of intermediates such as gluconic acid or glucuronic acid, and the final conversion to desired products such as formic acid, carbon dioxide, or other organic acids.^433–435 Electrocatalysts based on Pt, Pd, and Au have been extensively investigated for the GOR.^436–438 For instance, focusing on the selectivity towards high-value glucaric acid, the electrocatalytic GOR was explored on Cu, Pt, and Au electrodes.⁴³⁹ Experimental investigations were conducted using three distinct electrolytes, including 0.04 M gluconic acid, glucuronic acid, and glucaric acid in 0.1 M NaOH. On the Cu electrode, C–C bond cleavage leading to formic acid formation was favored at higher potentials, while the oxidation of the aldehyde group at C₁ and the hydroxymethyl group at C₆ resulted in the production of gluconic and glucaric acids, respectively, at lower potentials. At higher potentials, the Pt electrode exhibited a preference for the oxidation of the aldehyde group at C₁, similar to the behavior observed on the Cu electrode at lower potentials. Among the investigated catalysts, Au displayed the highest activity and selectivity, favoring the oxidation of the aldehyde group at C₁ to produce gluconic acid at higher potentials. Moreover, the oxidation of the hydroxymethyl group at C₆ also occurred, facilitating further oxidation to glucaric acid.

In situ formed high-valence metal species, such as CoO_x/CoOOH and NiOOH, under electrochemical reaction conditions, are always pointed out as the catalytic active centers during the GOR. Recently, Duan et al. identified two types of reducible Co³⁺–oxo active sites, namely adsorbed hydroxyl on Co³⁺ ions (μ₁-OH-Co³⁺) and di-Co³⁺-bridged lattice oxygen (μ₂-O-Co³⁺), which play crucial roles in the GOR.⁴⁴⁰ By employing operando Raman, operando/ex situ XAS and isotope-labeling experiments, they found that the μ₁-OH-Co³⁺ and μ₂-O-Co³⁺ species participate in different steps of the GOR. The μ₁-OH-Co³⁺ species catalyze the oxygenation process, while the μ₂-O-Co³⁺ species primarily catalyze the dehydrogenation step. It was also observed that the μ₂-O-Co³⁺ species exist in a protonated form (μ₂-OH-Co^2+/3+) at an equilibrium state during the GOR, suggesting the fast kinetics of these species in the dehydrogenation of glucose. Additionally, DFT calculations confirmed the pivotal roles of the Co³⁺-oxo species, with μ₁-OH-Co³⁺ facilitating oxygenation and μ₂-O-Co³⁺ predominantly driving the dehydrogenation process. These oxidative steps are essential for the electrooxidation of glucose to formate.

In recent years, there has been growing interest in the coupling of the HER and the GOR on non-noble metal electrocatalysts, particularly Fe, Co, Ni, and Cu-based catalysts, such as Co@CoO heterojunctions,⁴⁴¹ CoNi foam,⁴⁴² NiCoSe_x,⁴⁴³ Ru@NiB,⁴⁴⁴ amorphous NiFe-LDH,⁴⁴⁵ NiFeO_x,⁴⁴⁶ Co/N-codoped carbon,⁴⁴⁷ Co/CoP cluster,⁴⁴⁸ 2D NiCo phosphate nanosheets,⁴⁴⁹ Cu/Cu₂O,⁴⁵⁰ and Fe₂P/SSM.⁴⁵¹ Yu et al. fabricated NiV LDH by electrochemical and N₂/H₂ plasma regulations for boosting the activity of the GOR and HER.⁴⁵² Specially, the electrochemically regulated NiV LDH with highly active Ni³⁺ exhibited a low potential of 1.23 V at 10 mA cm⁻² and a FE of 94% during the GOR. While the plasma-regulated NiV LDH with a lower valence state of Ni species exhibited high HER activity, requiring an overpotential of 45 mV to deliver 10 mA cm⁻². When integrated into a glucose electrolytic cell, the assembled electrolyzer just required 1.25 V to achieve 10 mA cm⁻² for both production of formate and hydrogen, which is 320 mV lower than that required for water electrolysis. Additionally, Wen's group constructed a triprofitable alkaline-acidic asymmetric electrolytic cell (3A-EC), coupling the GOR under alkaline conditions with the HER under acidic conditions (Fig. 19c).⁴⁵³ They designed and fabricated a bifunctional electrode comprising iron-doped cobalt diselenide nanowires on conductive carbon cloth (Fe_0.1-CoSe₂/CC), which exhibited highly attractive electrocatalytic activity and stability for the GOR under alkaline conditions (Fig. 19d), as well as for the HER in acidic media. The assembled 3A-EC achieved an electrolytic current density of 10 mA cm⁻² at an applied voltage of only 0.72 V (Fig. 19e) for H₂ and gluconate generation. This remarkable performance was attributed to the successful harnessing of electrochemical energy from both glucose oxidation and neutralization processes.

4.3 Electrooxidation of amines

The electrocatalytic oxidation of primary amines provides an attractive approach to produce a variety of valuable compounds, such as imines, nitriles, amides, amine oxides, and azo compounds, which have diverse applications in pharmaceutical and agrochemical syntheses.⁴⁵⁴ Compared to the oxidation reactions of alcohols and aldehydes, the amine oxidation reaction exhibits a significantly lower thermodynamic equilibrium potential of −0.77 V vs. RHE, demonstrating a substantial thermodynamic advantage even before considering the specific reaction kinetics.⁴⁵⁵ In addition, coupling the electrooxidation of amines with the HER in a hybrid water electrolysis system not only enables the simultaneous production of H₂ and valuable chemicals but also presents the potential for wastewater remediation due to amine-associated water pollution derived from the feed and fertilizer wastewater.

In 2018, Zhang et al. presented a pioneering study on the coupling of electrooxidation of primary amines (–CH₂–NH₂) with the HER in 1.0 M KOH solution (Fig. 19f).⁴⁵⁶ They successfully electrooxidized various aromatic and aliphatic primary amines on a NiSe nanorod array anode, yielding the corresponding nitriles with excellent yields (>93%) and selectivity (>94%). Taking the conversion of benzylamine (BA) as an example, the electrooxidation of BA to benzonitrile (BN) occurred at approximately 1.34 V vs. RHE, which was significantly lower than the onset potential for the OER (∼1.55 V vs. RHE) (Fig. 19g). By utilizing NiSe as the anode for BA conversion and CoP as the cathode for the HER, a two-electrode electrolyzer was assembled, which achieved a remarkable current density of 20 mA cm⁻² at only 1.59 V, much lower than 1.70 V required for the OWS (Fig. 19h). At 1.5 V, the FE for BN and H₂ reached as high as 98% and ∼100%, respectively. In situ Raman spectroscopy revealed that the generated Ni^II/Ni^III sites on the NiSe acted as redox-active species, facilitating the conversion of primary amines to nitriles. Fu et al. constructed a biphasic Mo_0.8Ni_0.2N–Ni₃N heterojunction for enhanced BA electrooxidation by tailoring the d-band centers.⁴⁵⁷ The Mo_0.8Ni_0.2N–Ni₃N heterojunction requires a low potential of 1.54 V to achieve 240 mA cm⁻² in 0.1 M KOH/0.5 M Na₂SO₄/25 mM BA, along with excellent BA conversion, high BN yield, and FE > 99%. Quasi-in situ XPS characterization revealed the Ni–OOH species generate at a lower potential (1.54 V) for Mo_0.8Ni_0.2N–Ni₃N during BA oxidation and the Ni³⁺/Ni²⁺ ratio is higher than that of Ni₃N under the same conditions. Theoretical calculations indicate that charge transfer in the Mo_0.8Ni_0.2N–Ni₃N heterojunction causes an upshift of the d-band centers, which facilitates the production and adsorption of OH* from water, leading to the easy formation of NiOOH on Ni₃N and the optimized adsorption energy for BA, thus further enhancing the catalytic efficiency of BA oxidation. Propylamine electrooxidation was investigated by Zhai et al. by using vacancy-rich Ni(OH)₂ atomic layers (VR-Ni(OH)₂) as the catalyst.⁴⁵⁸ DFT calculations indicated that defect-induced local lean electron of the VR-Ni(OH)₂ surface facilitated the conversion of amino C–N bonds to nitrile C≡N bonds. In situ FTIR spectroscopy confirmed the transformation of amino C–N bonds into nitrile C≡N bonds. Consequently, the VR-Ni(OH)₂ catalyst demonstrated a low operating potential of 1.36 V vs. RHE to achieve a current density of 10 mA cm⁻² and simultaneously exhibited an impressive FE of 96.5% at a potential of 1.38 V for the electrooxidation of propylamine to propionitrile.

The bifunctional electrocatalysts used for anodic amine oxidation and cathodic HER are also widely investigated. For instance, Guo et al. reported an atomically thin CoSe₂ subnanometer belt (SB) with anion vacancies and cation substitutions, demonstrating their efficacy in enhancing the electrooxidation of butylamine to produce high-value-added butyronitrile while simultaneously generating hydrogen.⁴⁵⁹ The synthesized CoSe₂/Ni-SVs SBs, specifically engineered with Se vacancies and Ni substitutions, displayed an exceptionally low onset potential of 1.3 V vs. RHE and achieved an impressive FE of about 98.5% for butyronitrile production, surpassing the performance of all previously reported Co- and Ni-based catalysts. In-depth investigations using in situ electrochemical FTIR spectroscopy and electrochemical impedance spectroscopy uncovered the underlying mechanisms behind the significantly enhanced electrooxidation performance. It was found that the optimized adsorption behavior and accelerated dehydrogenation kinetics were key contributors to the improved performance. Theoretical studies further elucidated that the Se vacancies served as robust Lewis acid sites, promoting effective N atom adsorption, while Ni substitutions played a crucial role in enhancing dehydrogenation thermodynamics by optimizing the sequence of dehydrogenation steps. Importantly, the CoSe₂/Ni-SVs SBs exhibited high versatility as efficient catalysts for the electrosynthesis of propylamine, BA, and cyclohexane methylamine into nitriles, coupled with hydrogen generation. Notably, a two-electrode electrolyzer based on CoSe₂/Ni-SVs SBs achieved a voltage of 1.37 V at a current density of 10 mA cm⁻², which led to a remarkable reduction in cell voltage by up to 320 mV compared to that required for the conventional OWS. Other transition metal-based catalysts, including Ni–Ni₃N heterostructures,⁴⁶⁰ Ni/Co MOF derivative,⁴⁶¹ ordered Ni₂Si NPs,⁴⁶² V-doped Ni(OH)₂,⁴⁶³ cobalt cyclotetraphosphate (Co₂P₄O₁₂) nanorods,⁴⁶⁴ have also been reported for amine upgrading and hydrogen production.

4.4 Other electrochemical synthesis reactions

In addition to the above small organic molecules, there have been some reports on the electrooxidation of other molecules for energy-saving H₂ production and organic upgrading, such as cyclohexanol,²⁵⁰ sterol intermediates,⁴⁶⁵ nitroalkanes,⁴⁶⁶ tetrahydroisoquinolines (THIQs),⁴⁶⁷ styrene,⁴⁶⁸ alkene,⁴⁶⁹ and C–C coupling reactions.⁴⁷⁰ For instance, Wang et al. reported Co₂(OH)₃Cl/FeOOH nanosheets on nickel foam synthesized by the impregnation method.²⁵⁰ These nanosheets exhibited dual functionality as both the cathode and anode under alkaline conditions, facilitating the H₂ production and high value-added cyclohexanone production from cyclohexanol. The remarkable synergy between Co₂(OH)₃Cl and FeOOH obtained a substantial reduction in the anodic potential required for cyclohexanol oxidation by 120 mV, compared to the OER at a current density of 10 mA cm⁻². Notably, the integration of bifunctional Co₂(OH)₃Cl/FeOOH catalysts into a two-electrode electrolyzer demonstrated a voltage of 1.46 V at 10 mA cm⁻² to drive cathodic hydrogen generation and anodic cyclohexanone production. Upon scaling up the configuration to a 10 × 10 cm² membrane electrode assembly reactor, an impressive cyclohexanone production rate of 3.44 g h⁻¹ and energy savings of approximately 0.24 kW h m⁻³ (H₂) were achieved. Zhang et al. realized the synthesis of E-nitroethene through the electrooxidation of α-nitrotoluene on NiSe nanorod arrays in an alkaline electrolyte.⁴⁷¹ They assembled a hybrid electrolyzer with NiSe nanorod arrays serving as both the anode and cathode, which enabled simultaneous production of E-nitroethene and H₂. At 1.8 V, this setup achieved a FE of 100% for H₂ and a selectivity of 88% for E-nitroethene. In situ and ex situ experiments revealed that the in situ-formed NiOOH species played a crucial role in the electrocatalytic process. Furthermore, Zhang et al. reported the electrocatalytic semi-dehydrogenation of THIQs to synthesize dihydroisoquinolines (DHIQs) using a Ni₂P anode in alkaline solution.⁴⁷² In 1.0 M KOH, the electrooxidation of THIQs exhibited faster kinetics than the OER. In a hybrid electrolyzer employing Ni₂P as both the anode and cathode, DHIQs and H₂ were simultaneously produced with high activity and stability. The in situ formation of Ni^II/Ni^III redox-active species at the anode played a vital role in the semi-dehydrogenation of THIQs to DHIQs.

Table 5 provides a comprehensive summary of the electrochemical performances exhibited by anodic catalysts employed in small organic-assisted hybrid water electrolysis. Notably, the reduction in applied voltages was observed, indicating enhanced electrooxidation capabilities of various organic compounds. In addition, these reactions yielded diverse value-added products, achieving simultaneous organic upgrading and hydrogen production. However, there are several challenges that need to be addressed. (i) Although selective electrooxidation of organic compounds replaces the anodic OER in theory, practical systems often operate at voltages exceeding the theoretical voltage input for water electrolysis due to inherent limitations. Hence, developing efficient and stable catalysts, optimizing reaction conditions, and improving system designs are urgently required to reduce the operational voltages and enhance overall system efficiency. (ii) There remain diverse oxidation reactions and value-added products to select, necessitating the identification of the reaction mechanism and optimal strategies considering industrial demand, technological feasibility, and environmental implications. (iii) The corrosiveness of the organic molecules used and the radicals generated in situ can lead to a shortened lifetime of equipment, complicate the operation procedure, and increase the overall cost of the system. The aggressive nature of these chemicals can affect the durability of materials and components. (iv) In cases where carbonaceous chemicals are employed as the organic substrate, conditions such as elevated reaction temperatures, higher substrate concentrations, and the presence of aprotic electrolytes at the electrocatalyst surface can promote oxidative polymerization. This phenomenon can lead to catalyst deactivation to varying degrees, further complicating the efficiency and stability of the process. (v) Exploring alternative oxidation reactions within a hybrid water electrolysis system can enhance its versatility and potential for industrial-scale implementation. Furthermore, to realize the three-fold benefits of high-efficiency H₂ production, value-added chemical synthesis, and biowaste treatment, it is crucial to explore the utilization of diversified biowaste in this electrochemical strategy. Addressing these challenges will promote the advancement of efficient and sustainable strategies in small organic-assisted hybrid water electrolysis, unlocking the potential for simultaneous hydrogen production, valuable chemical synthesis, and biowaste utilization.

Table 5 Performance comparison of small organic molecule-assisted H₂ production systems in recently reported literature (some values of overpotential for anode/cathode are cautiously read from the figures in their publication)

Catalysts (cathode//anode)	Electrolyte of anode	Overpotential for cathode	Tafel slope for cathode (mV dec^−1)	Potential for anode	Tafel slope for anode (mV dec^−1)	Cell voltage at 10 mA cm^−2	FE (H2)	FE (oxidation product)	Products	Ref.
MoNi4//Fe2O3/NiO	1.0 M KOH + 1.0 M methanol	−	—	1.328 V at 10 mA cm^−2	—	1.381 V	∼100%	98%	H2 + formate	32
CoxP@NiCo-LDH/NF//CoxP@NiCo-LDH/NF	1.0 M KOH + 0.5 M methanol	100 mV at 10 mA cm^−2	—	1.13 V at 10 mA cm^−2	58	1.43 V	∼100%	∼100%	H2 + formate	256
Mo–Co4N//Mo–Co4N	1.0 M KOH + 3.0 M methanol	45 mV at 10 mA cm^−2	42	1.356 V at 10 mA cm^−2	—	1.427 V	∼100%	∼100%	H2 + formate	257
CNFs@NiSe//CNFs@NiSe	1.0 M KOH + 1.0 M methanol	—	—	1.32 V at 100 mA cm^−2	24	—	98.17%	97.90%	H2 + formate	260
Co(OH)2@HOS/CP//Co(OH)2@HOS/CP	1.0 M KOH + 3.0 M methanol	148 mV at 10 mA cm^−2	—	1.385 V at 10 mA cm^−2	71	1.497 V	∼100%	∼100%	H2 + formate	273
Ni–Mo nanopowders/NF//Co3O4 nanosheets/CP	1.0 M KOH + 1.0 M ethanol	—	—	1.489 V at 10 mA cm^−2	138	1.5 V	—	98%	H2 + ethyl acetate	241
Pt/C//F-modified β-FeOOH	15 : 5 volume ratio of ethanol to water	∼0.3 V at 10 mA cm^−2	—	1.207 V at 10 mA cm^−2	30.1	1.43 V	91.66%	72.28%	H2 + acetate	290
Cu1Ni2-N//Cu1Ni2-S/G	1.0 M KOH + 1.0 M ethanol	—	—	1.37 V at 10 mA cm^−2	15.2	∼1.495 V at 10 mA cm^−2	—	96%	H2 + ethyl acetate	298
CoS2–MoS2//CoNi-PHNs	1.0 M KOH + 1.0 M ethanol	—	—	1.39 V at 10 mA cm^−2	47	1.80 V at ∼50 mA cm^−2	90.50%	94.10%	H2 + acetate	473
NiS2/CFC//NiS2/CFC	1.0 M KOH + 0.45 M 2-Propanol	67 mV at 10 mA cm^−2	114	1.348 V at 10 mA cm^−2	—	1.41 V at 20 mA cm^−2	100%	98%	H2 + acetone	324
Ni3N/Co3N-NWs//Ni3N/Co3N-NWs	1.0 M KOH + 0.1 M glycerol	69 mV at 10 mA cm^−2	91	1.18 V at 20 mA cm^−2	131.4	1.47 V at 50 mA cm^−2	∼100%	94.6%	H2 + formate	34
CNs@CoPt//CNs@CoPt	1.0 M KOH + 10 mM glycerol	19.1 mV at 10 mA cm^−2	88.7	1.52 V at 100 mA cm^−2	85.4	1.71 V at 100 mA cm^−2	97%	79%	H2 + formate	328
Pt/C/CP//MnO2/CP	0.005 M	—	—	1.36 V at 10 mA cm^−2	242.1	1.38 V	100%	100%	H2 + formic acid	342
	H2SO4 + 0.2 M glycerol
RhIr/Ti//HEA-CoNiCuMnMo	1.0 M KOH + 0.1 M glycerol	—	—	1.25 V at 10 mA cm^−2	53.4	0.55 V	∼100%	∼99%	H2 + formate	343
Ni–Mo–N/CFC//Ni–Mo–N/CFC	1.0 M KOH + 0.1 M glycerol	40 mV at 10 mA cm^−2	70	1.30 V at 10 mA cm^−2	87	1.36 V	99.7%	95.0%	H2 + formate	474
NF//Au/CoOOH	3.0 M KOH + 0.2 M benzyl alcohol at 70 °C	—	—	1.30 V at 340 mA cm^−2	—	2.0 V at 4.8 A	99.99%	—	H2 + benzoic acid	238
Mo–Ni alloy//Mo–Ni alloy	1.0 M KOH + 10 mM benzyl alcohol	48 mV at 50 mA cm^−2	53.4	1.35 V at 15 mA cm^−2	23.2	1.40 V at 20 mA cm^−2	∼100%	—	H2 + benzoic acid	243
hp-Ni//hp-Ni	1.0 M KOH + 10 mM benzyl alcohol	219 mV at 50 mA cm^−2	—	—	—	1.50 V at 50 mA cm^−2	∼100%	∼97%	H2 + benzoic acid	244
PtO2/h-Ni(OH)2//Ni(OH)2	1.0 M KOH + 40 × 10^−3 M benzyl alcohol	61 mV at 10 mA cm^−2	67	—	—	1.48 V	—	—	H2 + benzoic acid	344
CC@NiO/Ni3S2//CC@NiO/Ni3S2	1.0 M KOH + 0.2 M benzyl alcohol	91 mV at 10 mA cm^−2	117.4	1.391 V at 50 mA cm^−2	60.1	1.458 V	—	—	H2 + benzoic acid	348
NF//A-Ni–Co-H/NF//A-Ni–Co-H/NF	1.0 M KOH + 0.1 M benzyl alcohol	—	—	1.35 V at 100 mA cm^−2	—	1.30 V at ∼20 mA cm^−2	—	∼90–95%	H2 + benzoic acid	349
Ni2P NPA/NF//Ni2P NPA/NF	1.0 M KOH + 10 mM HMF	9 mV at 10 mA cm^−2	86	1.40 V at ∼200 mA cm^−2	—	1.44 V	100%	98%	H2 + FDCA	363
MoO2–FeP@C//MoO2–FeP@C	1.0 M KOH + 10 mM HMF	103 mV at 10 mA cm^−2	48	1.359 V at 10 mA cm^−2	38	1.486 V	—	—	H2 + FDCA	372
Ni3N@C//Ni3N@C	1.0 M KOH + 10 mM HMF	113 mV at 50 mA cm^−2	—	1.38 V at 50 mA cm^−2	48.9	1.55 V at 50 mA cm^−2	—	—	H2 + FDCA	399
NiCoNSs/CuNWs//NiCoNSs/CuNWs	1.0 M KOH + 10 mM HMF	50 mV at 10 mA cm^−2	97.2	1.45 V at 50 mA cm^−2	—	1.44 V	—	—	H2 + FDCA	410
NF@Mo–Ni0.85Se//NF@Mo–Ni0.85Se	1.0 M KOH + 10 mM HMF	130 mV at 10 mA cm^−2	98.98	—	—	1.50 V at 50 mA cm^−2	100%	—	H2 + FDCA	411
CuxS@NiCo-LDH//CuxS@NiCo-LDH	1.0 M KOH + 10 mM HMF	107 mV at 10 mA cm^−2	35	1.30 V at 87 mA cm^−2	—	1.34 V	∼100%	∼100%	H2 + FDCA	412
Ni2P/Ni/NF//Ni2P/Ni/NF	1.0 M KOH + 30 mM furfural	—	—	∼1.41 V at 200 mA cm^−2	—	1.48 V	—	—	H2 + 2-furoicacid	416
Pt/C//metallic Cu	1.0 M KOH + 100 mM HMF	—	—	0.4 V at ∼1.0 mA cm^−2	—	0.27 V at 100 mA cm^−2	100%	100%	H2 + FDCA	417
Pt/C//metallic Cu	1.0 M KOH + 200 mM furfural	—	—	0.45 V at ∼3.5 mA cm^−2	—	0.31 V at 100 mA cm^−2	100%	100%	H2 + 2-furoicacid	417
Ni3N/Ni//Cu3Ag7/CF	1.0 M KOH + 0.6 M HCHO	—	—	0.10 V at 100 mA cm^−2	—	0.60 V at 500 mA cm^−2	100%	100%	H2 + formate	419
NiFeNx//NiFeOx	1.0 M KOH + 0.5 M glucose	40.6 mV at 10 mA cm^−2	39	1.30 at 87.6 mA cm^−2	19	1.39 V at 100 mA cm^−2	—	80%	H2 + glucaric acid	446
Pt/C//Fe2P/SSM	1.0 M KOH + 0.5 M glucose	—	—	1.35 V ∼10 mA cm^−2	71	1.22 V	100%	—	—	451
Fe0.1–CoSe2/CC//Fe0.1–CoSe2/CC	1.0 M KOH + 0.5 M glucose	270 mV at 100 mA cm^−2	40.0	∼1.10 V at 10 mA cm^−2	—	0.72 V	∼99%	—	H2 + gluconate	453
CoS2–MoS2//VR-Ni(OH)2	1.0 M KOH + 10 mM propylamine	—	—	1.36 V at 10 mA cm^−2	31.5	1.36 V	—	96.5%	H2 + propionitrile	458
CoP//CoSe2/Ni-SVs SBs	1.0 M KOH + 20 mM butylamine	—	—	1.37 V at 10 mA cm^−2	54.8	1.37 V	98.9%	96.7%	H2 + butyronitrile	459

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5. Self-powered H₂ production

One significant limitation of current water splitting technology is its reliance on an external power supply to drive the electrochemical process. This constraint renders it impractical for use in mobile devices, vehicles, and field activities where access to a continuous power source may be limited or unavailable. Therefore, there is a pressing need for the development of self-powered electrocatalytic hydrogen generation systems that can operate autonomously without the need for an external power supply. Such systems would greatly expand the applicability of hydrogen production technology and enable its use in a wider range of real-world scenarios. The concept of self-powered systems was pioneered by Wang's group in 2013 with the development of a cell-electrolyzer coupling system.⁴⁷⁵ This system combined a hybrid energy cell capable of harnessing mechanical, thermal, and solar energies to generate electricity with an electrolyzer for water splitting and chemical conversions. These self-powered electrolysis approaches offer potential advantages in terms of energy efficiency, sustainability, and decentralized hydrogen production. They can enable hydrogen generation in remote areas or off-grid locations where a constant power supply may not be readily available.⁴⁷⁶

In self-powered electrolysis systems, the internally generated electricity is crucial for driving electrocatalytic reactions. These systems rely on the thermodynamic feasibility of internal redox reactions to provide the necessary electrical potential for driving the desired electrochemical reactions.⁴⁷⁷ To ensure efficient operation, the equilibrium potential difference between the redox reactions at the two electrodes must be sufficiently large to overcome the internal resistance of the system and the energy barriers associated with the catalytic reactions. The incorporation of highly active electrocatalysts is essential for promoting the internal reactions and enhancing the overall efficiency of a self-powered system.

In self-powered systems, zinc–air batteries (ZAB) and direct hydrazine fuel cells (DHzFCs) are commonly used as power suppliers. These energy sources provide the necessary electricity for driving the electrocatalytic reactions in a water splitting electrolyzer for hydrogen production. Current research in the field of self-powered systems is focused on the development of multifunctional catalysts that can enhance the kinetics of internal reactions in both chemical cells and electrolyzers. These catalysts play a crucial role in improving the overall performance and efficiency of self-powered electrolysis systems.

5.1 Aqueous zinc–air battery assisted OWS

An aqueous zinc–air battery is a type of rechargeable battery that utilizes the oxidation of zinc and the reduction of oxygen from the air to generate electricity.⁴⁷⁸ During the discharge process, zinc metal at the anode undergoes oxidation, releasing Zn²⁺ ions into the electrolyte solution. Meanwhile, at the air cathode, oxygen from the air combines with water and electrons from the external circuit to form hydroxide ions through the ORR process. The flow of electrons through the external circuit from the zinc anode to the air cathode provides electrical energy that can be utilized to power devices or systems. In the charging process, when an external power source is connected to the battery, the zinc oxide formed during discharge is converted back into metallic zinc, and oxygen is released back into the air through the OER at the cathode. The ORR and OER, typically governed by a 4e⁻ process, exhibit relatively sluggish kinetics compared to the HER. Consequently, the development of efficient bifunctional catalysts for the ORR and OER at the air cathode is of paramount importance in enhancing energy efficiency.⁴⁷⁹ To realize a self-powered system based on ZABs, it is imperative to design a multifunctional electrocatalyst that can simultaneously enhance the kinetics of the ORR, OER, and HER, as depicted in Fig. 20a. Generally, due to the overpotential associated with the HER and OER, the voltage required for water splitting is at 1.5–2.3 V, exceeding the thermodynamic potential of 1.23 V. In contrast, aqueous zinc–air batteries typically possess an OCV of approximately 1.4 V. Consequently, two zinc–air batteries connected in series are necessary to drive the overall process of water splitting.

Fig. 20 (a) Illustration of the self-driven water splitting powered by zinc–air batteries. (b) Photovoltage charging and following discharging curves of the fabricated FeNiP/NPCS-based zinc–air battery, and the inset is the schematic diagram of the photovoltage charging and following discharge process.⁵⁰³ Reproduced with permission. Copyright 2020, Elsevier. (c) Digital images of the uninterrupted H₂ production system by water splitting powered by solar photovoltaic and aqueous zinc–air batteries. (d) Current density versus time curves of this hydrogen production system powered by solar energy in day and aqueous zinc–air batteries in night.⁵⁰⁴ Reproduced with permission. Copyright 2023, Royal Society of Chemistry.

In 2016, Dai's group synthesized a series of metal-free bifunctional catalysts (N and P co-doped carbon networks⁴⁸⁰ and N, P, and F tri-doped graphene⁴⁸¹) for the ORR and the HER in a ZAB coupled OWS system. To further elevate the power densities of ZABs and decrease the applied voltage of OWS, metal-based electrocatalysts, particularly Co-based catalysts, garnered significant attention. Several catalysts were investigated, including CoP@P,N co-doped carbon (CoP@PNC),⁴⁸² CoMn_2−xCr_xO₄,⁴⁸³ FeCo/Co₂P nanoparticles,⁴⁸⁴ hierarchical CuCoNC nanowire arrays,⁴⁸⁵ Co–N-carbon monolithic electrode,⁴⁸⁶ Co,O-doped carbon,⁴⁸⁷ Co₂P/CoNPC,⁴⁸⁸ CoFeN-NCNTs//CCM,⁴⁸⁹ CoO_x/CoN_y@CN_z,⁴⁹⁰ Co₄N@NC,⁴⁹¹ Co@NCL,⁴⁹² Co₃W₃C/CoP/NPC,⁴⁹³ Co/MoC@PC,⁴⁹⁴ Co@NC-CNTs@NiFe-LDH,⁴⁹⁵ FeCo/NPC,⁴⁹⁶ Co/CoFeNC@N-CNF,⁴⁹⁷ CoFe-N-CNTs/CNFs,⁴⁹⁸ and CoFe₂O₄/CoFe.⁴⁹⁹ For instance, Yoo et al. fabricated a hierarchical FeCoMoS nanoflower encapsulated in nitrogen doped graphene (FeCoMoS@NG) for zinc–air batteries and overall water splitting.⁵⁰⁰ This FeCoMoS@NG catalyst exhibited excellent trifunctional activity, with a half-water potential (E_1/2) for the ORR of 0.83 V vs. RHE, an overpotential at 10 mA cm⁻² (η₁₀) for the OER of 238 mV, and η₁₀ of 137 mV for the HER. When employed as the cathode catalyst in a ZAB, FeCoMoS@NG demonstrated a significantly low charging–discharging voltage gap of 0.77 V at 2 mA cm⁻², allowing for a stable working period over 70 h. Moreover, when utilized as both the cathode and anode for overall water splitting, this catalyst only required a low cell voltage of 1.58 V to achieve a current density of 10 mA cm⁻² and with remarkable stability. Notably, a series connection of three zinc–air batteries effectively facilitated the operation of the OWS system for self-powered H₂ production with a H₂ evolution rate of 0.7 mL min⁻¹.

To enhance the kinetics of the ORR and improve the ionic conductivity of the electrolyte, current research on ZABs primarily focuses on alkaline electrolytes. However, it is widely acknowledged that alkaline electrolytes suffer from corrosion and carbonation issues. To ensure safety and durability, there is growing interest in utilizing neutral electrolytes as an alternative to alkaline electrolytes, which would help mitigate these concerns. In the context of neutral electrolytes, the development of high-performance electrocatalysts becomes crucial to meet the demands of ZABs. Zhu et al. fabricated a single-atom Ir-embedded N-doped hierarchically porous carbon catalyst (SA-Ir/NC).⁵⁰¹ This catalyst demonstrated exceptional performance when applied to ZABs in neutral media (0.1 M PBS with 0.02 M zinc acetate), delivering a high OCV of 1.42 V and an impressive power density of 90.4 mW cm⁻². DFT calculations revealed that the chelation of single-atom Ir sites with N atoms enables the export of a stronger positive charge compared to other transition metal sites, thus facilitating O₂ absorption. In addition, the long-term operation of ZABs is essential for the development of self-powered systems based on ZABs, which necessitates the use of robust electrocatalysts. Our group fabricated the FeNiP nanoparticles dispersed on heteroatom-doped graphene as an efficient electrocatalyst for ZABs.⁵⁰² This catalyst exhibited high electrocatalytic activity with an overpotential of 290 mV at 10 mA cm⁻² for the OER, as well as a E_1/2 of 0.83 V for the ORR. When used for ZABs as air cathode, the assembled battery demonstrated a peak power density of 118 mW cm⁻² and excellent cycling stability over 600 h at 10 mA cm⁻². The favorable surface reconstruction properties and the robust graphene substrate are considered primary factors contributing to the outstanding stability of the ZAB system.

In recent years, significant progress has been made in electrocatalytic water splitting driven directly by electricity or solar cells. However, these approaches still present certain drawbacks. For instance, the use of electricity from a power grid is energy-consuming and economically inefficient. The intermittence of sunlight usually requires connection of energy storage devices, which suffer from complicated structures and external energy loss. zinc–air batteries offer a unique advantage since they can serve as both energy storage devices and high-discharge voltage conversion technologies. This makes them ideal for storing solar energy during light reactions and releasing it as electricity during dark reactions to drive water splitting, thereby maximizing the utilization of intermittent sunlight and eliminating its restrictions. Consequently, a sustainable and uninterrupted hydrogen production system can be realized. To achieve this goal, our group developed FeNiP nanoparticles loaded onto N,P-modified carbon nanosheets (FeNiP/NPCS) as trifunctional electrocatalysts for a zinc–air battery-powered water electrolyzer for hydrogen production.⁵⁰³ Solar cells were used as the energy source to charge the zinc–air battery under sunlight, effectively storing transient solar energy. Upon charging with a commercial silicon photovoltaic system for 10 minutes (as illustrated in Fig. 20b), the discharge platform with a current density of 10 mA cm⁻² of the FeNiP/NPCS-catalyzed battery operated stably for 1300 minutes without any significant variations, demonstrating the significant potential for assembling a self-driven energy conversion and storage system for green H₂ production. Expanding on this concept, a self-powered system based on FeNiP-loaded carbon nanofibers as trifunctional electrocatalysts achieved uninterrupted 24 h H₂ production from the two-electrode water splitting electrolyzer.⁵⁰⁴ This system was powered by a silicon photovoltaic cell during the day and an aqueous zinc–air battery during the night, as depicted in Fig. 20c. The corresponding current density versus time curves are presented in Fig. 20d, and such an integrated hydrogen production device demonstrated continuous operation throughout a 15 day test period. In another example, Wang et al. proposed and assembled a self-powered energy system based on a Ni–P/Fe–P collaborative electrocatalyst for the HER, OER, and ORR.⁵⁰⁵ This system achieved a solar-to-hydrogen efficiency of 4.6% and a solar-to-water splitting device efficiency of 5.9%. Additionally, various catalysts, including core–shell Co₂P@NC electrocatalysts,⁵⁰⁶ N,S-codoped 3D carbon matrix with Co₉S₈/CoO heterojunction (Co–S–O/NSCN),⁵⁰⁷ Co_0.85Se/NC,⁵⁰⁸ and CoNi alloys coupled N-doped carbon nanotube arrays (CoNi@NCNTs/CC),⁵⁰⁹ have demonstrated the feasibility of self-powered energy systems utilizing solar energy. In summary, these solar energy-assisted systems fully utilize solar energy to generate renewable electricity and produce clean H₂ without relying on external nonsolar energy sources or causing environmental pollution. They realize a closed-loop energy and substance cycle, offering a novel approach to energy conversion, storage, and the production of clean and renewable energy.

5.2 Hydrazine fuel cell-assisted OHzS

Although significant achievements have been made in the hydrogen production by hydrazine-assisted water electrolysis, most of the current research requires the utilization of external electrical energy, which poses challenges for portable devices and outdoor operations. To overcome this limitation, the concept of self-power supply has emerged, leading to innovative breakthroughs. Direct hydrazine fuel cells (DHzFCs) have garnered significant attention because they utilize hydrazine as the fuel and an oxidant, typically oxygen or air, to generate electricity through an electrochemical reaction.^66,510 These fuel cells offer high theoretical power density and produce carbon-free by-products (N₂ and H₂O). Given the high reactivity of N₂H₄ with H₂O and H⁺ to form N₂H₅⁺, alkaline solutions are commonly employed as the electrolyte in DHzFCs, and therefore transition metal-based catalysts can be used for DHzFCs.⁵¹¹ Additionally, hydrazine is a liquid at room temperature, which simplifies the storage and transportation compared to other gaseous or cryogenic fuels, ensuring convenience and safety in the development of DHzFCs. Generally, the OCV of DHzFCs is approximately 0.8 to 1.0 V.⁵¹² The favorable kinetics of the HzOR in the OHzS unit necessitates a voltage of about 0.3–0.5 V to achieve H₂ production. As a result, one DHzFC can effectively drive the OHzS for H₂ production, offering a significant advantage compared to zinc–air battery assisted overall water splitting.

In 2018, inspired by the previous advancements in the fields of the HER and fuel cells, Ding's group developed a highly efficient and stable bifunctional catalyst composed of thin Fe-doped CoS₂ (Fe–CoS₂) nanosheets for both the HER and the HzOR.⁵¹³ The Fe–CoS₂ nanosheets demonstrated Pt-like activity for the HER, exhibiting low overpotentials at 10 mA cm⁻² (40 mV in 1.0 M KOH, 31 mV in 0.5 M H₂SO₄, and 49 mV in 1.0 M PBS), high stability and 93–98% FE at all pH values. Furthermore, the Fe–CoS₂ nanosheets exhibited a working potential of only 129 mV to achieve a current density of 100 mA cm⁻² for the HzOR in 1.0 M KOH. Considering these excellent properties, the Fe–CoS₂ nanosheets were utilized as the anode material to construct a DHzFC using H₂O₂ or O₂ as an oxidizing agent. The assembled DHzFCs demonstrated remarkable maximum power density values of 246 mW cm⁻² (H₂O₂) and 125 mW cm⁻² (O₂) (Fig. 21a), among the best reported values for DHzFCs employing Co-based electrocatalysts. To further explore the ability of the Fe–CoS₂ nanosheets, they integrated a DHzFC with an electrolyzer for OHzS, forming a self-powered system for H₂ production. In this configuration, hydrazine served both as the fuel for the DHzFC and as the splitting target (Fig. 21b). Notably, the Fe–CoS₂ nanosheets were employed as both the anode and cathode catalysts for OHzS. The self-powered system exhibited good stability at 0.7 V for 20 h and achieved a hydrogen evolution rate of 9.95 mmol h⁻¹ (Fig. 21c). Theoretical calculations demonstrated that the remarkable performance of the Fe–CoS₂ nanosheets was attributed to the introduction of Fe dopants, which decreased the free-energy changes of H adsorption and the dehydrogenation of adsorbed NH₂NH₂ on CoS₂.

Fig. 21 (a) Current density–voltage and current density–power density plots for the DHzFC with the oxidizing agent of O₂. (b) Schematic illustration of a self-powered H₂ production system integrating a DHzFC and an OHzS unit. (c) Generated amounts of H₂ and N₂ in the system with the hydrazine concentration of 5.3 M at 0.7 V and room temperature.⁵¹³ Reproduced with permission. Copyright 2018, Nature Publishing Group. (d) Electrocatalytic coproduction of furoic acid and hydrogen in the electricity output mode. Left: Configuration of a single cell; right: polarization curves of the corresponding half-reactions. (e) Polarization and power-density curves for the assembled device using 0.1 M furfural as the substrate undergoing oxidation.⁵¹⁷ Reproduced with permission. Copyright 2022, Wiley-VCH. (f) Illustration of the structure and working principle of the Mg/seawater battery. (g) Illustration of the Mg/seawater battery driven self-powered seawater electrolysis system.⁴⁰ Reproduced with permission. Copyright 2022, Elsevier.

Zhang et al. fabricated a Ru₂P catalyst with a partially exposed surface (RP-CPM).¹⁹² The DHzFC utilizing the RP-CPM catalyst achieved a high power density of 64.77 mW cm⁻² at an output potential of 0.36 V. Additionally, the RP-CPM catalyst exhibited excellent HER performance, delivering 10 mA cm⁻² at a low overpotential of 24 mV, surpassing that of commercial Pt/C catalysts (35 mV). By integrating the DHzFC in series with a water splitting electrolyzer, a self-powered system was assembled, which is capable of supplying a high output voltage of 1.0 V and generating hydrogen with a productivity of 0.72 mmol h⁻¹. To address the cost of electrocatalysts, they also prepared a hierarchical porous nanosheet arrays with rich Ni₃N–Co₃N heterointerfaces on nickel foam (Ni₃N–Co₃N/NF).³⁷ This Ni₃N–Co₃N/NF exhibited excellent HER and HzOR performance, achieving 10 mA cm⁻² at working potentials of −43 mV and −88 mV, respectively. In a constructed OHzS with Ni₃N–Co₃N/NF, low cell voltages of 0.071 and 0.76 V were sufficient to reach 10 and 400 mA cm⁻², respectively. Additionally, to demonstrate the potential feasibility in mobile devices, a self-powered hydrogen production system was fabricated by utilizing Ni₃N–Co₃N/NF, which exhibited an impressive H₂ production rate of 0.65 mmol h⁻¹. Through experimental characterization and DFT calculations, they confirmed that interfacial engineering effectively regulated the electronic structure and optimized the adsorption strength of active species, thereby enhancing the kinetics of the catalytic reactions. Furthermore, the authors also explored other electrocatalysts for DHzFC and hydrazine-assisted H₂ production, including Mo-doped Ni₃N and Ni heterostructures,¹⁹⁸ P, W codoped Co₃N nanowires,²⁰⁵ and dual nanoislands on a Ni/C hybrid nanosheet array,²²⁹ all of which showed an extraordinary performance. However, it is important to note that the anodic catalysts in these DHzFCs still relied on commercial Pt/C catalysts to catalyze the ORR.⁵¹⁴ Therefore, the development of trifunctional electrocatalysts for the HzOR, HER, and ORR remains a prominent research direction in DHzFC-powered OHzS.

Simultaneously, the utilization of hydrazine-assisted self-powered hydrogen generation under seawater conditions presents a promising avenue for promoting chlorine-free hydrogen production. In 2021, Sun et al. integrated a hybrid seawater electrolyzer with a DHzFC, with NiCo@C/MXene/CF as the anode and 20% Pt/C as the cathode in a DHzFC.¹²⁹ This self-powered system demonstrated the capability to produce hydrogen from seawater at a remarkable rate of 1.6 mol h⁻¹ g_cat⁻¹, solely relying on hydrazine as the consumable energy. Subsequently, Wang's group fabricated FeP/FeNi₂P encapsulated in N, P co-doped hierarchical carbon (FeNiP-NPHC) in situ grown on nickel foam via a hydrothermal-pyrolysis-phosphidation procedure.⁵¹⁵ Benefiting from the strong coupling effect among FeP, FeNi₂P, and N, P co-doped carbon at the three-phase heterojunction interface, as well as the unique 1D/3D hierarchical structure, the prepared FeNiP-NPHC showed excellent ORR (E_1/2 = 0.83 V), HzOR (E₁₀₀ = 7 mV), and HER (η₁₀₀ = 180 mV) performance in alkaline seawater. DFT calculations indicated that the construction of this unique interface in FeNiP-NPHC effectively modulates the d-band center and electronic structure, facilitating the fine-tuning and optimization of its trifunctional electrocatalytic behavior. As a result, a DHzFC assembled using FeNiP-NPHC displayed an OCV of 0.98 V and a peak power density of 31 mW cm⁻² at ambient temperature. Furthermore, a two-electrode OHzS system using FeNiP-NPHC demonstrated the intuitive activity improvement compared to OWS in a seawater system. Finally, a self-powered system by integrating DHzFC using FeNiP-NPHC as both the anode and cathode to drive OHzS in alkaline seawater for H₂ production was constructed, exhibiting nearly 100% FE for H₂ production in OHzS. This group also fabricated Ru, Fe dual-doped Ni₂P nanosheets as trifunctional catalysts for DHzFC powered OHzS to realize the self-powered H₂ production in seawater.⁵¹⁶ This integrated self-powered system achieved an impressive hydrogen production rate of 10.8 mmol h⁻¹ without any other power supplies.

5.3 Others

Wang et al. found that the aldehyde groups of HMF and furfural can be selectively oxidized to their corresponding carboxylates together with hydrogen production at remarkably low potentials (onset potential as low as 0.05 V vs. RHE).⁴¹⁷ Therefore, the combination of the HER and low-potential aldehyde oxidation can enable hydrogen production at both the anode and cathode. In contrast, the conventional hydrogen fuel cells operate through the coupling of the hydrogen oxidation reaction (HOR) and the ORR to generate electrical energy. Notably, the potential for low-potential aldehyde oxidation is comparable to that of the HOR, as depicted in Fig. 21d. Hence, it is possible to construct new types of fuel cells by combining low-potential aldehyde oxidation with the ORR. By adopting this innovative approach, simultaneous hydrogen production, biomass upgrading to carboxylates, and electricity generation can be achieved simultaneously. Based on this strategy, they coupled anodic low-potential furfural oxidation and the cathodic ORR to construct a novel electrochemical cell that generates electricity together with H₂ production and furfural oxidation.⁵¹⁷ Employing Cu as anodic and Pt/C as cathodic catalysts, an OCV of 0. 95 V was achieved, and the maximum power density reached about 200 mW cm⁻² at a current density of approximately 450 mA cm⁻² (Fig. 2e). Furthermore, this design concept exhibits versatility and suitability for other substrates containing similar types of aldehyde groups, including HMF, 5-methyl furfural, and 4-carboxybenzaldehyde, indicating its broad applicability.

The selective electrooxidation of formaldehyde to formic acid presents an alternative pathway for hydrogen production. Considering the lower onset oxidation potential of formaldehyde (∼0.1 V vs. RHE), Fu et al. developed a direct formaldehyde fuel cell, which enabled the generation of electricity while simultaneously producing H₂ and valuable formate at the anode.⁴²⁵ They fabricated a highly active and selective electrocatalytic anode of Cu nanosheets arrays on Cu foam, which ensures the efficient conversion of formaldehyde to formic acid and hydrogen at the anode. Upon assembling a direct formaldehyde fuel cell, the Cu nanosheet array electrode demonstrated an impressive OCV of up to 1 V and a peak power density of 350 mW cm⁻². This distinctive configuration enabled the production of 1 kW h of electricity, 0.62 N m³ of hydrogen, and 53 mol of formate consuming 53 mol of formaldehyde fuel.

Mg/seawater batteries, which utilize seawater as both the electrolyte and the cathodic reactant, offer a dual functionality of electricity generation and hydrogen production at the cathode, as depicted in Fig. 21f. These batteries hold significant promise as power sources for long-term operation in deep-sea apparatuses due to their open-structured design and the use of seawater as the electrolyte. By integrating a seawater battery pack with a seawater electrolyzer, as illustrated in Fig. 21g, a self-powered seawater electrolysis system driven by Mg/seawater batteries can be realized. Notably, since hydrogen can be generated from both the cathodes of the seawater electrolyzer and the Mg/seawater batteries, the total hydrogen production rate is expected to be substantially high in this self-powered seawater electrolysis system. In 2022, Jiang's group demonstrated a self-powered direct seawater electrolysis system driven by Mg/seawater batteries, which realized continuous hydrogen production and held potential for mobile and undersea apparatus applications.⁴⁰ To achieve this goal, they employed a MoNi/NiMoO₄ heterostructure as the catalyst for the HER occurring at the cathodes of both the Mg/seawater batteries and the seawater electrolysis unit. Notably, the self-powered seawater electrolysis system with a MoNi/NiMoO₄ catalyst achieved a high hydrogen production rate of 12.11 mL cm⁻² h⁻¹, and the conversion efficiency of Mg-to-hydrogen reaches up to 83.97%. These results surpass the performance of most of the state-of-the-art self-powered hydrogen production systems.

In order to harness solar energy for continuous self-powered hydrogen production, various energy storage systems have been integrated with photovoltaic devices. For instance, Sun et al. utilized a Li-ion battery to power a conventional water electrolyzer, which can be conveniently charged by commercial solar cells.⁵¹⁸ Zhang et al. developed aqueous Ni–Zn batteries with an output voltage of 1.75 V to connect solar cells and water splitting devices.⁵¹⁹ Sun et al. employed a photovoltaic cell to drive a micro zinc-ion battery array, providing a stable voltage to continuously power the seawater electrolyzer.⁵²⁰ These hybrid systems demonstrated the ability to sustain uninterrupted water splitting for 24 h. Besides, the supercapacitor powered OWS was also reported.^521,522 Despite these innovative approaches, several key challenges persist in this field. These challenges include addressing safety concerns associated with the energy storage component, enhancing the stability and durability of the water electrolyzer, and overcoming the complexity and bulkiness of the overall system configuration.

Significant progress has been achieved in the self-powered systems for the enhanced electron utilization and H₂ production without the need for an external power supply, as summarized in Table 6. However, it is important to note that these systems still have certain limitations. Zinc–air batteries, for example, are not ideal for high-current applications due to their relatively high internal resistance, which restricts their ability to deliver high power outputs. This characteristic makes them less suitable for H₂ production at large current densities. Moreover, the high cost of hydrazine presents a significant constraint for the widespread adoption of hydrazine-contained energy systems for large-scale H₂ production. As a result, it is crucial to continue exploring and investing in novel research and development efforts to overcome the limitations associated with current self-powered H₂ production systems, ultimately facilitating the widespread adoption of self-powered H₂ production technologies for a sustainable energy future.

Table 6 The performance of multifunctional electrocatalysts in zinc–air battery/fuel cells and water splitting

Catalysts	Reaction types	η 10 (mV) for HER	E 1/2 (V) for ORR	η 10 (mV) for OER or Potential (mV) for HzOR	Peak power density (mW cm^−2)	Stability for zinc–air battery/fuel cells	Potential at 10 mA cm^−2 for electrolyzer	Stability for electrolyzer	Ref.
NiS2/CoS2–O NWs	ZAB assisted OWS	174	0.70	235	—	30 h at 5 mA cm^−2	1.768	20 h at 1.70 V	35
GO-PANi31-FP	ZAB assisted OWS	520	—	—	38	50 h	—	—	481
CoP@PNC-DoS	ZAB assisted OWS	173	0.803	316	138.57	150 h at 30 mA cm^−2	1.740	30 h at 10 mA cm^−2	482
CoMn2−xCrxO4	ZAB assisted OWS	180	0.82	390	140.26	43 h at 10 mA cm^−2	—	20 h at 10 mA cm^−2	483
FeCo/Co2P@NPCF	ZAB assisted OWS	∼260	0.79	330	154	107 h at 10 mA cm^−2	1.68	43 200 s at 1.75 V	484
Cu-Foam@CuCoNC-500	ZAB assisted OWS	59.2	0.84	245	140	360 h at 10 mA cm^−2	1.52	20 h at 1.6 V	485
CoNC	ZAB assisted OWS	36	0.858	229	181.3	110 h at 5 mA cm^−2	1.51	300 h at 30 mA cm^−2	486
CoFeN-NCNTs//CCM	ZAB assisted OWS	151	0.84	325	145	445 h at 10 mA cm^−2	1.63	55 h retaining 85.2%	489
FeZn4Co@CNFs	ZAB assisted OWS	200	0.83	360	107.6	118 h at 10 mA cm^−2	—	—	499
FeNiP/NPCS	ZAB assisted OWS	181	0.84	318	163	110 h at 10 mA cm^−2	1.71	20 h	503
FeNiP@p-NPCF/CC	ZAB assisted OWS	89	0.82	317	117	500 h at 10 mA cm^−2	1.67 V at 20 mA cm^−2	20 h	504
NiFeP	ZAB assisted OWS	—	—	370 mV at 50 mA cm^−2	138	>600 h at 10 mA cm^−2	1.9 V at 20 mA cm^−2	—	505
Fe–Co2P@Fe-N-C	ZAB assisted OWS	77	0.88	300	81.3	∼280 h at 10 mA cm^−2	1.58	10 h	506
Co0.85Se/NC	ZAB assisted OWS	216	0.797	275	108	160 h at 20 mA cm^−2	1.70	20 000 s	508
Fe2P/Co@NPC	ZAB assisted OWS	235	0.876	331	233.56	180 h	1.73	60 h	523
PPy/FeTCPP/Co	ZAB assisted OWS	240 mV (0.1 M KOH)	0.86	380 mV (0.1 M KOH)	—	24 h	—	12 h at 10 mA cm^−2	524
NiCoOS hollow polyhedron	ZAB assisted OWS	300	0.79	470 mV	90	170 h at 5 mA cm^−2	1.52	15 h	525
Fe0.5Ni0.5@N-GR	ZAB assisted OWS	350	0.83	210	85	40 h at 20 mA cm^−2	—	—	526
Pt@CoS2-NrGO	ZAB assisted OWS	39	0.85	235	114	55 h at 10 mA cm^−2	1.48	50 h at 200 mA cm^−2	527
Ni3N–Co3N PNAs/NF	DHzFC-assisted OHzS	43	—	−88 mV at 10 mA cm^−2	60.3 mW cm^−2 with commercial Pt/C cathode	—	71 mV at 10 mA cm^−2	20 h at 0.14 V	37
NiCo@C/MXene/CF	DHzFC-assisted OHzS	49	—	−25 mV at 100 mA cm^−2	53.5 mW cm^−2 with commercial Pt/C cathode	—	0.31 V at 500 mA cm^−2	150 h at 500 mA cm^−2	129
RP-CPM	DHzFC-assisted OHzS	24	—	−27 mV at 10 mA cm^−2	64.77 mW cm^−2 with commercial Pt/C cathode	—	23 mV at 10 mA cm^−2	20 h at 10 mA cm^−2	192
Mo-Ni3N/Ni/NF	DHzFC-assisted OHzS	45	—	−0.3 mV at 10 mA cm^−2	37.8 mW cm^−2 with commercial Pt/C cathode	—	55 mV at 10 mA cm^−2	10 h at 50 mA cm^−2	198
PW-Co3N NWA/NF	DHzFC-assisted OHzS	41	—	−55 mV at 10 mA cm^−2	46.3 mW cm^−2 with commercial Pt/C cathode	—	28 mV at 10 mA cm^−2	20 h at 50 mA cm^−2	205
Fe–CoS2	DHzFC-assisted OHzS	40	—	129 mV at 100 mA cm^−2	125 mW cm^−2 with commercial Pt/C cathode	—	0.95 V at 500 mA cm^−2	40 h at 100 mA cm^−2	513
FeNiP-NPHC	DHzFC-assisted OHzS	180 mV at 100 mA cm^−2	0.83	7 mV at 100 mA cm^−2	31	—	0.25 V at 100 mA cm^−2	150 h at 1.35 V	515
RuFe-Ni2P@NF	DHzFC-assisted OHzS	54	—	140 mV at 100 mA cm^−2	60.1 mW cm^−2 with commercial Pt/C cathode	—	0.41 V at 200 mA cm^−2	100 h at 100 mA cm^−2	516
Cu/Cu foam (anode)	—	—	—	—	200 mW cm^−2 with commercial Pt/C cathode	—	—	—	517
CF@Cu-NS (anode)	Direct formaldehyde fuel cell	—	—	—	350 mW cm^−2 with commercial Pt/C cathode	—	—	—	425
MoNi/NiMoO4	Mg/seawater batteries powered seawater electrolysis	29	—	—	21.08	100 h at 3 mA cm^−2	—	—	40
Ni/V2O3	Mg/seawater batteries powered seawater electrolysis	34	—	—	17.81	24 h at 3 mA cm^−2	—	—	528

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6. Self-catalyzed H₂ production

Self-catalyzed H₂ electrolysis involves driving electrocatalytic processes using self-generated electricity for hydrogen production, eliminating the need for external power sources and making the electrolysis system more efficient and sustainable.⁵²⁹ In self-catalyzed hydrogen production systems, metallic anodes are commonly used to obtain self-generated electricity through chemical reactions at the anode. This self-generated electricity, which can flow through an external circuit, is then utilized to drive the electrocatalytic reactions at the cathode, where hydrogen gas is evolved. By integrating the electrochemical reactions at the anode and cathode, self-catalyzed hydrogen production systems allow for simultaneous H₂ production and electricity generation, making them more efficient and self-sustained. This approach reduces the dependence on external power sources, making the hydrogen production process more energy-efficient and potentially more economically viable. Exploiting this phenomenon offers tremendous potential for the development of self-catalyzed metal–H₂O batteries. It is worth noting that various materials have been explored as anode materials in self-catalyzed batteries, including metals such as Zn, Al, and Mg, which exhibit spontaneous reactivity with water and thus forms the corresponding metal oxides during the discharge process.⁵³⁰ During the charging process, the coordinated metal ions in the electrolyte are reduced back to their original metal electrode, allowing for the regeneration of the metal anode. This cyclic process of oxidation and reduction enables the self-sustainability of the metal–H₂O battery and provides a continuous source of hydrogen and electricity. Of course, each metal anode has its own advantages and limitations in terms of electrochemical reactivity, cost, and stability. Given the challenges associated with the large-scale deployment of hydrogen, including risks of explosion and high storage and transportation costs, the self-powered, functional, and portable metal–H₂O batteries hold promise for on-demand H₂ generation and energy supply.

6.1 Metal–H₂O battery

Lithium (Li), being the most prevalent anode metal, exhibits a sufficiently low equilibrium potential (−3.040 V vs. standard hydrogen electrode (SHE)), providing ample driving force for water splitting reactions. However, a direct reaction between the pure Li metal and water is hindered due to the explosive nature of Li in an aqueous electrolyte. To circumvent this limitation, Guo et al. designed an asymmetric dual cell employing an organic/aqueous electrolyte with a Li_1.35T_1.75Al_0.25P_2.7Si_0.3O₁₂ (LTAP) film serving as the separator, wherein the oxidation of the Li anode provides electrons for the cathodic hydrogen production, as depicted in Fig. 22a.⁵³¹ Benefiting from the specific operational conditions, this Li-based battery can be adapted as either a Li–O₂ battery (O₂-rich environments) or a Li–H₂O fuel cell (O₂-free environments). Although this Li-based fuel cell demonstrates remarkable reactivity and a wide working voltage range, it still exhibits some drawbacks such as low hydrogen production efficiency, limited availability of lithium resources, safety concerns associated with the organic electrolyte, and high cost of the separator, and a suboptimal electrical energy output persists. To address these challenges, alternative alkaline-earth metals have been explored for constructing functional batteries. In 2018, Wen et al. reported a Zn–H₂O battery operating in an alkaline-acid hybrid system, enabling cathodic H₂ generation and electricity production.⁵³² This battery configuration involved a Zn anode and a commercial Pt/C cathode, as illustrated in Fig. 22b. To enhance the kinetics of Zn oxidation and the HER at the cathode, NaOH (4.0 M) and H₂SO₄ (2.0 M) were employed as dual electrolytes in the anode and cathode, respectively, separated by a bipolar membrane to prevent direct neutralization of the anolyte and catholyte with the release of heat energy. The hybrid alkaline–acid Zn–H₂O fuel cell theoretically exhibited an OCV of 1.32 V and an energy density of 1082.4 W h kg⁻¹ with hydrogen production at the cathode according to the Nernst equation. Experimentally, the cell demonstrated impressive performance, with an OCV of 1.249 V, an energy density of up to 934 W h kg⁻¹, and a power density of 80 mW cm⁻² (Fig. 22c). Furthermore, it exhibited a hydrogen production rate of 0.166 mL s⁻¹ at a voltage of 0.6 V, along with excellent long-term stability at a current density of 10 mA cm⁻² with an output voltage of 1.16 V.

Fig. 22 (a) Schematic illustration of a Li–water fuel cell which consists of a Li anode in the organic electrolyte and a cathode electrode in the alkaline electrolyte, separated by a LTAP film.⁵³¹ Reproduced with permission. Copyright 2017, American Chemical Society. (b) Schematic illustration of the as-proposed Zn–H₂O fuel cell, separated by a bipolar membrane. (c) Polarization curves (left-hand y axis) and power density (right-hand y axis) for Zn–H₂O fuel cells using Pt/CNTs, Pt/C, and CNTs.⁵³² Reproduced with permission. Copyright 2018, Wiley-VCH. (d) Schematic illustration of aqueous Zn– or Al–CO₂ systems and their reaction mechanism.⁵³⁵ Reproduced with permission. Copyright 2019, Wiley-VCH. (e) Self-co-electrolysis system assembled using a Zn plate as the anode, N,Cu-CoP/CC as the cathode, and neutral 1.0 M PBS as the electrolyte that can generate H₂ gas, NaZnPO₄, and electric energy concurrently.⁵⁴⁶ Reproduced with permission. Copyright 2022, Wiley-VCH. (f) Concept of the coupled systems of electrolytic hydrogen evolution and sulfur production with the assistance of the Fe²⁺/Fe³⁺ redox cycle and H₂S absorption reaction.⁵⁴⁷ Reproduced with permission. Copyright 2018, Wiley-VCH. (g) The Fe(II) electrooxidation coupled with HER, and the Fe(III) can be chemically reduced to Fe(II) by SO₂. (h) Schematic diagram of the cycle of iron ion redox by adding SO₂. (i) LSV curves using the flow cell electrolyzer in 0.1 M FeCl₃ and 0.5 M H₂SO₄ electrolyte with continuous inflow SO₂.⁵⁵⁰ Reproduced with permission. Copyright 2023, Wiley-VCH.

Although Zn–H₂O batteries exhibit significant potential for future applications without the use of organic electrolytes, there are concerns regarding their high cost of bipolar membranes and potential risks associated with dual-electrolyte systems employing strong acid and strong alkaline solutions. To address these challenges, Hou et al. presented an alkaline Zn–H₂O battery utilizing 1.0 M KOH as the anolyte and catholyte.⁵³³ The HER performance in this battery was enhanced by the strong coupling effect of the constructed interface between Ni₂P nanoparticles and Ni-MOF nanorod arrays, leading to a reduced overpotential of 132 mV at 10 mA cm⁻². Although the alkaline Zn–H₂O battery achieved a notable power density of 4.1 mW cm⁻², its current density remained constrained by the relatively low ionic conductivity resulting from the implementation of the ion exchange membrane. In contrast to alkaline–acid Zn–H₂O batteries, this alkaline device demonstrated stable discharge–charge cycling behavior at 5 mA cm⁻² over 90 cycles, highlighting its promising potential for long-term operation. Nadeema et al. developed an Al–H₂O battery in a single-pool electrolyzer, employing a low concentration of alkaline solution (0.1 M KOH) as the electrolyte.⁵³⁴ To enhance the kinetics of the HER, they engineered a novel Co@CoAl-LDH/N-doped graphene catalyst. This innovative catalyst exhibited excellent performance for the HER in the Al–H₂O battery. The assembled Al–H₂O battery demonstrated a relatively high cell voltage of approximately 1.0 V at a current density of 5 mA cm⁻². The specific capacity of the battery was estimated to be approximately 538 mA h g⁻¹. Notably, this configuration eliminated the risks associated with a dual-electrolyte system and minimized the additional costs associated with the use of an ion exchange membrane.

Kim et al. have designed a noteworthy aqueous Zn- or Al-based CO₂ system for both hydrogen production and electrical energy generation.⁵³⁵ In contrast to the aforementioned metal–H₂O batteries, these Zn– or Al–CO₂ devices do not utilize strong alkaline or acid solutions in the cathode pool. Instead, CO₂ introduced into the electrolyte would react with water to form H₂CO₃. Upon the oxidation of the metal at the anode, the H⁺ ions dissociated from H₂CO₃ undergo reduction to yield H₂, as depicted in Fig. 22d. Simultaneously, K⁺ ions migrate from the anode pool to the cathode pool through a glass membrane to maintain charge balance. The Al–CO₂ system exhibited a high OCV of 1.3 V and achieved a maximum power density of 125 mW cm⁻². Furthermore, the actual H₂ generation rate was determined to be 0.681 mL min⁻¹ at a current of 100 mA, which closely aligns with the theoretically predicted generation rate of 0.696 mL min⁻¹. This measurement highlights a remarkable FE of 97.9% for H₂ production, emphasizing the efficiency of the system in converting electrical energy to hydrogen gas.

The Mg/seawater battery represents another type of metal–H₂O battery that not only enables electricity generation but also facilitates the production of clean hydrogen using abundant seawater as a resource.^536–538 In comparison to the traditional Mg/dissolved oxygen battery,^539,540 which relies on the oxygen concentration present in seawater, the Mg/seawater battery exhibits significant advantages. Notably, the cathodic reaction of the Mg/seawater battery involves the HER directly from seawater, eliminating the dependency on dissolved oxygen and thereby enhancing the potential for improved discharging performance.^540–543 The performance of the Mg/seawater battery is primarily governed by the cathode, and the sluggish kinetics of the HER in neutral seawater necessitates the development of ideal cathode catalysts to minimize the polarization of the cathodic HER. Early works on the Mg/seawater batteries employed Pt-based cathodes.⁵³⁶ To address this challenge, Jiang et al. developed a unique porous CoP/Co₂P heterostructure electrode for the pH-universal HER.⁵⁴⁴ The optimized porous CoP/Co₂P heterostructure demonstrates remarkable HER performance, with low overpotentials of 87, 133, and 454 mV at 10 mA cm⁻² in acidic, alkaline, and seawater media, respectively. The assembled Mg/seawater battery with the porous CoP/Co₂P heterostructure cathode exhibited promising performance, achieving a peak power density of 6.28 mW cm⁻², maintaining satisfactory stability over 24 h. Subsequently, this group also developed heterostructured MoNi/NiMoO₄ catalysts for Mg/seawater batteries, which yielded a remarkable peak power density of up to 21.08 mW cm⁻².⁴⁰

The metal–H₂O batteries discussed herein fully meet the principles of green chemistry, demonstrating both non-toxicity and high efficiency. However, the generation of electrical energy and value-added chemicals in these systems relies on the consumption of pure metals, which increases the cost of these systems. Kékedy-Nagy et al. found that MgNH₄PO₃ could be synthesised by the spontaneous combination of Mg²⁺, NH⁴⁺ and PO₄³⁻.⁵⁴⁵ Notably, Mg as an active alkaline–earth metal element possesses a low equilibrium potential (−2.372 V vs. SHE) and readily undergoes oxidation to form Mg²⁺ ions. By employing NH₄H₂PO₄ solution as the electrolyte, the Mg–H₂O battery can simultaneously produce MgNH₄PO₃ at the anode and hydrogen gas at the cathode.

Due to the high cost and oxidation activity, Mg is not an ideal material for application in aqueous solutions for safety and cost concerns. As an alternative, Zn has emerged as a promising anode material for metal–H₂O batteries due to its moderate oxidation kinetics. While alkaline electrolytes have been commonly used in Zn–H₂O batteries to enhance reaction kinetics, the utilization of neutral electrolytes has been rarely reported. Shi's group firstly reported a neutral Zn–H₂O battery by utilizing PBS as the electrolyte, as depicted in Fig. 22e.⁵⁴⁶ To accelerate the reaction kinetics of the Zn–H₂O battery, a N and Cu dual-doped CoP (N,Cu–CoP) catalyst was synthesized as the cathode material, which showed excellent activity for the HER with a low overpotential of 68 V to reach 10 mA cm⁻² in 1.0 M PBS electrolyte. The N,Cu–CoP-catalyzed Zn–H₂O battery exhibited an OCV of 0.79 V, a maximum power density of 1.83 mW cm⁻² and simultaneously generated a hydrogen production rate of 13.7 mL cm⁻² h⁻¹ without requiring an external energy supply. Furthermore, by adjusting the pH values (6, 7, and 8) and concentrations (0.01, 0.1, and 1.0 M) of the PBS electrolyte, various valuable solid-state chemicals, such as NaZnPO₄ (an important raw material for heat-reflective materials, Ni–Zn battery anode materials, and light-emitting diodes) and Zn₃(PO₄)₂ can be controllably synthesized at the anode during the discharge process.

Besides the electrooxidation of pure metal, the oxidation of multivalent metals with the low potential (e.g., 0.69 V vs. RHE for Cu⁺/Cu²⁺, and 0.75 V vs. RHE for Fe²⁺/Fe³⁺) has been considered as a suitable substitution reaction of the OER for energy-saving H₂ production. For instance, with the electrodes of CC@N-CoP (cathode)//CC(anode) in 0.5 M H₂SO₄ (catholyte)//0.5 M H₂SO₄ with 0.96 M FeSO₄/0.74 M Fe₃(SO₄)₂ (anolyte), the fabricated electrolyzer only requires 0.89 V to drive a current density of 10 mA cm⁻² due to the lower redox potential of Fe²⁺/Fe³⁺, saving 53% energy consumption compared to the traditional OWS.⁵⁴⁷ Wang et al. used a Cu⁺/Cu²⁺ redox cycle to replace the OER process, and the assembled hybrid system could deliver 100 mA cm⁻² at 0.94 V (2.02 V in the case of OWS), correspondingly electricity consumption is 2.23 kW h N m⁻³ H₂ at 100 mA cm⁻² (4.80 kW h N m⁻³ H₂ for OWS).⁵⁴⁸ However, the anodic reaction of metal ions also faces a challenging obstacle of how to improve the continuity of the electrooxidation for metal ions. To address this issue, researchers have proposed the introduction of a reducing agent, which facilitates the spontaneous conversion of high-valent cations into low-valence components. Fig. 22f provides an illustrative example, wherein a coupled H₂S absorption and Fe²⁺/Fe³⁺ redox cycle is employed. This approach ensures the uninterrupted electrooxidation of metal ions while simultaneously enabling low-potential H₂ production when coupled with the HER. Notably, glucose⁴⁷ and ascorbate⁵⁴⁸ were introduced into the Cu⁺/Cu²⁺ redox cycle, glucose/starch/cellulose⁵⁴⁹ and H₂S⁵⁴⁷ were coupled with the Fe²⁺/Fe³⁺ redox cycle. However, the utilization of these reducing agents in redox systems may not be economically or environmentally sustainable due to the consumption of organic resources or chemicals. In a recent study inspired by the reducing ability of SO₂, Wang's group proposed a continuous Fe²⁺/Fe³⁺ redox assisted by the SO₂ waste gas under ambient conditions for hydrogen production (Fig. 22g).⁵⁵⁰ In this system, SO₂ chemically reduces Fe³⁺ to Fe²⁺, and the resulting Fe²⁺ is subsequently electrochemically oxidized to Fe³⁺, providing electrons for cathodic H₂ production. Meanwhile, SO₂ was converted to sulfuric acid (Fig. 22h). Due to the low oxidation potential of Fe²⁺, the assembled electrolyzer achieved a significantly lower electrolytic voltage of 0.97 V at 10 mA cm⁻² compared to traditional water electrolysis (1.85 V) (Fig. 22i).

6.2 Others

In contrast to water, sulfide ions (S²⁻) exhibit a greater propensity to donate electrons through the SOR (for instance, S²⁻ → S + 2e⁻, E⁰ = −0.48 V vs. SHE), as the SOR is thermodynamically more favorable than the OER. Weng et al. developed WS₂ nanosheets (NSs) using a low-temperature molten-salt-assisted method, which demonstrated remarkable electrocatalytic properties for the alkaline SOR and the acidic HER.²³² The researchers assembled a hybrid alkali–acid electrochemical cell, with WS₂ NSs serving as the alkaline anode and the acidic cathode. The anolyte and catholyte were separated by a cation exchange membrane (CEM) and an anion exchange membrane (AEM), respectively, with deionized water flowing through the middle chamber, as depicted in Fig. 23a. In this cell, the SOR occurs at the alkaline anode, releasing electrons that flow through the external circuit and are consumed at the cathode through the HER. Na⁺ and SO₄²⁻ ions are simultaneously transported into the middle chamber via the CEM and AEM, respectively, completing the circuit. For comparison, a conventional cell was also established by employing 1.0 M NaOH as the anolyte and catholyte for reference purposes. The LSV curves of the hybrid alkali–acid SOR/HER cell, alkali–alkali SOR/HER cell, and alkali–alkali OER/HER cell are presented in Fig. 23b. The hybrid alkali–acid cell demonstrated the ability to achieve oxidation conversion of S²⁻ and hydrogen generation under bias-free voltage conditions. In contrast, the alkali–alkali SOR/HER cell required a higher voltage of 0.73 V to drive the electrolysis and an applied voltage of 1.17 V to achieve a current density of 10 mA cm⁻². Notably, the alkali–alkali OER/HER cell demanded a significantly higher voltage (>2.0 V) to initiate water splitting due to the high overpotential of WS₂ NSs for the OER. The alkali–acid electrochemical cell operated as a self-powered cell exhibits an OCV of about 0.45 V, slightly lower than the theoretical output voltage (0.53 V), and can deliver a current density of ∼2.93 mA cm⁻² without an applied voltage. These results highlight that the asymmetric alkali-acid electrolyte substantially reduces the electricity required for electrochemical H₂ generation, attributable to the pH gradient between the anode and cathode chambers. Moreover, these results demonstrate that the optimized SOR/HER electrolysis system (acid: 2.5 M H₂SO₄, alkaline: 1.0 M NaOH/2.0 M Na₂S) can provide a maximum current of 8.54 mA cm⁻² at a bias-free voltage while simultaneously producing hydrogen and degrading sulfide.

Fig. 23 (a) Illustration of the home made H₂ production flow-cell system coupled with the SOR (a peristaltic pump is used to flow deionized water through the intermediate transition chamber, so that the ion concentration can be maintained relatively stable in the middle chamber). (b) Comparison of LSV curves for the alkali–alkali OER/HER cell, alkali–alkali SOR/HER cell, and alkali–acid SOR/HER cell.²³² Reproduced with permission. Copyright 2021, Wiley-VCH. Electrochemical performance of the Zn–Hz battery. (c) Schematic illustration of the Zn–Hz battery. (d) Discharge and charge voltage profiles. (e) Galvanostatic discharge–charge cycling curves at 5 mA cm⁻².³⁸ Reproduced with permission. Copyright 2022, Wiley-VCH.

Inspired by the simultaneous resource utilization and energy storage characterization of Zn–CO₂ batteries, Wang's group has developed a rechargeable alkaline Zn–hydrazine (Zn–Hz) battery that enables efficient and separate hydrogen generation through decoupled hydrazine splitting.³⁸ Unlike typical decoupled electrolysis approaches, the decoupled hydrazine splitting in the Zn–Hz battery is achieved using bifunctional electrocatalysts in a two-electrode system, enabling hydrogen generation without the need for purification through temporal separation. The proposed Zn–Hz battery consists of bifunctional electrocatalysts as the cathode and Zn foil as the anode, aiming to the separate electrochemical reactions of the HER during discharge process and the HzOR during the charging process (Fig. 23c). During the discharge process, hydrogen is generated at the cathode through the electrochemical HER from H₂O, while hydrazine oxidation occurs at the cathode during the charging process, enabling separate hydrogen generation through temporally decoupled electrochemical hydrazine splitting. To drive this device, a 3D hierarchical Mo₂C/Ni@C/CS catalyst is fabricated, wherein Mo₂C and Ni nanoparticles are encapsulated in porous carbon and uniformly decorated on a 3D carbon sphere. This catalyst demonstrates bifunctional activity for both the HER and the HzOR, achieving small potentials of −76 and 42 mV to achieve a current density of 10 10 mA cm⁻², respectively. When employed as the cathode catalyst in the Zn–Hz battery, an OCV of 0.366 V is obtained. Notably, the discharge voltage is 0.364 V at 0.4 mA cm⁻², while the charge voltage is 0.379 V at 0.4 mA cm⁻² (Fig. 23d), indicating that the Zn–Hz battery can achieve an ultrahigh energy efficiency of over 96%. Moreover, this novel battery has the ability to simultaneously produce hydrogen and generate electric energy, eliminating the continuous energy consumption associated with conventional decoupled electrolysis and achieving efficient hydrogen evolution. As revealed in Fig. 23e, the battery exhibits excellent durability for 600 cycles (200 h) at 5 mA cm⁻² with a slight voltage change.

Significant achievements have been made in the development of self-catalyzed systems for much enhanced electron utilization and without the need for an external power supply (Table 7). Even though these systems can reduce the consumption of electrical energy through additional reactions, they often require large quantities of sacrificial anode metal, leading to a significant reduction in the overall economic effectiveness. Therefore, it is imperative to develop electrocatalytic systems that fully utilize atoms and electrons, achieving an internal power supply while minimizing the waste of electrode materials. To accomplish those objectives, two strategies are proposed. (i) Assembling rechargeable functional batteries for electrode material recycling and high-value chemical production: by designing and implementing rechargeable batteries with functional electrodes, the electrode materials can be effectively recycled and utilized for the production of high-value chemicals. This approach not only promotes sustainability by reducing waste but also enables the recovery of valuable resources from the electrode materials. (ii) Converting electrode materials into high-value-added chemicals during the battery discharge process: rather than considering the electrode materials as consumables, this strategy aims to transform them into valuable chemicals during the discharge process of the battery. By harnessing the electrochemical reactions within the battery, the electrode materials can undergo controlled transformations, leading to the production of high-value-added chemicals. This approach maximizes the utilization of electrode materials, minimizing waste and enhancing the overall efficiency of the system. These strategies provide promising approaches for the development of atom- and electron-optimized electrocatalytic systems, which not only achieve internal power supply but also enable the conversion of electrode materials into valuable products. By adopting these approaches, researchers can advance the self-catalyzed systems towards more sustainable and efficient electrochemical processes.

Table 7 The performance of self-catalyzed H₂ production systems

Catalysts	Reaction types	Electrolyte	Anode	Open circuit voltage	Peak power density (mW cm^−2)	Stability	Recyclability	Ref.
Pt/CNTs	Zn–H2O battery	4.0 M NaOH (anode)/2.0 M H2SO4 (cathode)	Zn	1.25 V	80	18 h at 5, 10, and 20 mA cm^−2	—	532
Ni-MOF/Ni2P@EG	Zn–H2O battery	1.0 M KOH (anode + cathode)	Zn	—	4.1	—	90 cycles (∼35 h) at 5 mA cm^−2	533
N,Cu-CoP/CC	Zn–H2O battery	1.0 M PBS (anode + cathode)	Zn	0.79 V	1.83	—	—	546
Mo-WC@NCS	Zn–H2O battery	1.0 M KOH (anode)/0.5 M H2SO4 (cathode)	Zn	1.08 V	41.4	10 h at 10 mA cm^−2	—	551
Mo–Co0.85SeVSe/NC	Zn–H2O battery	1.0 M KOH (anode + cathode)	Zn	—	3.9	12 h at 10 mA cm^−2	—	552
Co@CoAl/NG	Al–H2O battery	0.1 M KOH (anode + cathode)	Al	∼1.24 V	—	—	—	534
PrBa0.5Sr0.5Co1.5Fe0.5O5+δ	Al–CO2 battery	4.0 M NaOH (anode + cathode)	Al	1.3 V	125.4	—	—	535
MoNi/NiMoO4 heterostructure	Mg/seawater battery	Simulated seawater (0.5 M NaCl) (anode + cathode)	Mg	1.18–1.56 V	21.08	100 h at 3 mA cm^−2	—	40
Ni/V2O3	Mg/seawater battery	Simulated seawater (3.5% NaCl) (anode + cathode)	Mg	1.16–1.39 V	17.81	24 h at 3 mA cm^−2	—	528
CoP/Co2P heterostructure	Mg/seawater battery	Simulated seawater (3.5% NaCl) (anode + cathode)	Mg	1.2–1.4 V	6.28	24 h at 3 mA cm^−2	—	544
Ni-doped MoO3	Mg/seawater battery	Simulated seawater (3.5% NaCl solution) (anode + cathode)	Mg	1.23 V	6.54	24 h at 3 mA cm^−2	—	553
Mo2C/Ni@C/CS	Zn–Hz battery	1.0 M KOH + 0.2 M N2H4 (cathode)	Zn	0.366 V	—	—	240 cycles (200 h) at 5 mA cm^−2	38
NiCoP/NF	Zn–Hz battery	1.0 M KOH + 0.2 M N2H4 (cathode)	Zn	0.315 V	—	—	240 cycles (80 h) at 5 mA cm^−2	554

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7. Conclusion and perspectives

This perspective highlights recent advancements in four innovative strategies for energy-saving hydrogen production, aiming to focus on the discussion of the strategies to synthesize the catalysts, select suitable organic molecules, discuss reaction mechanisms, overcome challenges in traditional water electrolysis, and show various advantages in hybrid electrolysis. Water splitting assisted by the oxidation of sacrificial-agent species (e.g., urea, hydrazine, and polysulfide) provides an elegant approach for highly efficient electrochemical hydrogen production at lower electrolysis voltages, simultaneously enabling wastewater treatment at the anode. The typical electrooxidation reactions of small organic molecules, including alcohols, aldehydes, amines, and biomass, open a sustainable strategy for the upgrading of organic compounds at the anode while achieving energy-saving hydrogen production at the cathode, which also mitigates the production of explosive H₂/O₂ mixtures and reactive oxygen species. Self-powered electrocatalytic systems provide a promising approach for uninterrupted hydrogen production without the need for an external power supply, making them suitable for mobile devices, vehicles, and field activities. Self-catalyzed electrolysis systems can achieve 100% electron and atom utilization for hydrogen production in the absence of external energy input. These coupling processes significantly reduce the cell voltage for energy-saving hydrogen production and enhance the atom/electron utilization of electrolyzers. Despite the superior advantages and significant achievement in these coupling approaches, these technologies are still in the early stages of development, and the transformation efficiency remains unsatisfactory. However, through further advancements in electrode development and electrolyzer design, it is anticipated that these appealing technologies will find practical applications in chemical manufacturing. Continued research and innovation in this field hold promise for achieving efficient and sustainable hydrogen production.

Theoretically, there are several important issues that are needed to be addressed for further development and enhanced energy utilization for the energy-saving hydrogen production. These aspects include the rational design of high-performance electrodes, identification of the reaction mechanism, exploration of novel reactions, design of electrolyzer systems, and comprehensive economic cost analysis, which are crucial for a sustainable and low-carbon energy future. Regarding the future direction in this burgeoning field, we provide some perspectives as follows (Fig. 24).

Fig. 24 Prospects for developing energy-saving H₂ production.

7.1 Catalyst preparation

Electrode materials play a crucial role in determining the electrochemical reaction kinetics. Exploring effective electrode catalysts through morphology engineering and active-site design is essential for improving reaction kinetics and facilitating the desired reactions. At present, the catalyst design strategies primarily originate from traditional water electrolysis, and there is still a need for targeted catalyst design specifically tailored for small molecule oxidation reactions. It is necessary to gain a deep understanding of the catalytic reaction mechanisms involved and realize targeted designs for each small molecule oxidation reaction to fully exploit the potential of catalysts. Additionally, since small molecules often contain diverse functional groups, it is important to establish a fundamental understanding of the relationships between these groups and catalysts. This understanding can greatly benefit the design of highly efficient catalysts by optimizing the specific interactions and reactivity of different functional groups. Furthermore, with respect to biomass-assisted hybrid water electrolysis systems, especially for alcohols and aldehydes, these are opportunities to lower the thermodynamic equilibrium potentials, produce value-added chemicals, and utilize a wide range of raw materials. However, the complete oxidation of these compounds often leads to the generation of worthless CO₂ and H₂O. It is crucial to conduct in-depth research to facilitate specific reaction kinetics and selectively control the production of particular valuable products, which will enable the fabrication of efficient and highly applicable electrocatalysts for the hybrid water electrolysis. Moreover, the utilization of bifunctional catalysts based on earth-abundant elements for hybrid water electrolyzers is highly desirable in terms of cost reduction and practical applications, making the technology more economically viable and scalable.

7.2 Mechanism investigation

Currently, the mechanisms underlying small molecule oxidation reactions remain a huge debate. A comprehensive understanding of the principal electrocatalytic and interfacial processes holds great potential for precisely controlling the reaction pathway and fine-tuning the selectivity of desirable products. A reasonable investigation of the intricate reaction mechanism is of paramount importance for exploring efficient catalysts. It is worth noting that many non-precious metal-based electrocatalysts often undergo a reconstruction process during anodic oxidation reactions, during which high-valence metal sites are formed and subsequently serve as the true active sites. Consequently, it is imperative to develop advanced in situ characterization techniques to analyze pivotal intermediates and catalytic species. To attain this objective, in situ/operando technologies must be employed in conjunction with existing techniques, such as in situ XAS, in situ XRD, in situ Raman spectroscopy, and in situ FTIR, to monitor the behavior of key intermediates and identify the actual active sites at the atomic scale. The profound identification of active sites and a comprehensive understating of structure–property–activity relationships will undoubtedly provide valuable insights for the intelligent design of effective materials. Furthermore, simultaneous theoretical investigation using DFT and machine learning provides another powerful approach to identify the reaction mechanism and facilitate the design of advanced catalyst systems. By combining the presently available operando techniques with advanced DFT simulations, a more reasonable understanding of structural evolution and the reaction mechanism during catalysis can be achieved. The new mechanistic insights thus obtained will serve as invaluable feedback for knowledge-driven reaction engineering and catalyst optimization endeavors.

7.3 Reaction exploration

The development of novel electrooxidation reactions holds significant importance for a range of applications, including biomass upgrading, industrial organic pollutant removal, and fine chemical production. In particular, the efficient electrooxidation of organic pollutants by coupling the HER provides a sustainable approach for simultaneously producing valuable chemicals and purifying water contaminated by industrial wastes. In contrast to the electrolysis of pure water, the electrolysis of seawater for hydrogen production presents a more cost-effective alternative that obviates the need for desalination or purification units. However, the practical implementation of seawater electrolysis is severely hindered by the occurrence of the ClOR at the anode. This competing reaction interferes with the desired OER, resulting in the generation of undesirable chlorine and/or hypochlorite species that gradually corrode catalysts, membranes, and other crucial components during long-term tests. It is noteworthy that the oxidation of small molecules occurs at lower potentials compared to both the OER and ClOR. Consequently, coupling the electrooxidation of small molecules with the HER holds potential to enable high-current-density seawater electrolysis without the detrimental interference of the ClOR. This advancement offers a promising avenue for large-scale, environmentally friendly hydrogen production from seawater.

7.4 Cell configuration design

The successful presentation of the feasibility of using the prepared electrocatalysts under industrial-scale current densities, reaching up to 1 A cm⁻², represents a critical milestone for their potential applications. However, achieving both high selectivity and efficiency at such high current densities poses a significant challenge. Overcoming this challenge necessitates close collaboration to simultaneously improve catalytic performance and optimize electrolyzer design. To address these requirements, the utilization of a flow electrolyzer assembly proves advantageous. Such an assembly facilitates the efficient transport of reactants and ensures the effective removal of products from the catalyst surface. This design strategy maximizes the contact between reactants and active sites, allowing for the attainment of a substantial current density at low voltage. Importantly, this configuration also impedes the occurrence of the competitive OER that tends to arise at high working voltages. The advantages of the flow electrolyzer assembly extend beyond improved performance, as it also offers scalability to larger modules and holds significant commercial potential.

7.5 Product separation

The downstream separation and purification of valuable products generated at the anode play a crucial role in industrial applications, necessitating further considerations and improvements. Many electrocatalytic systems yield gases or liquids as products, which require subsequent separation and purification steps. While the separation of gas-phase products can be conveniently achieved using dual-pool reactors, the cost associated with separating soluble products from electrolytes, particularly at low concentrations, is often high. Therefore, the production of insoluble or easily separable chemicals in or from electrolytes is highly desirable for the advancement of electrocatalytic systems. Urgent efforts are required to develop efficient methods for the separation of target products. Furthermore, it is essential to explore novel molecules that can be oxidized to yield hydrophobic, rather than hydrophilic, valuable products. This approach would facilitate easier separation and purification processes. Additionally, to maximize the utilization of the product/precursor mixed electrolyte obtained after the electrocatalytic reaction, other chemicals can be introduced into the electrolyte mixture to generate novel high-value compounds through subsequent reactions such as precipitation and phase separation. Simultaneously, improving the selectivity of ion-exchange membranes is of significance to prevent the crossover of small molecules during real-world applications. Enhancing the membrane's selectivity will enable more efficient separation and purification processes, contributing to the overall performance and practicality of electrocatalytic systems.

7.6 Economic evaluation

In evaluating the feasibility and profitability of any coupled process, careful consideration must be given to the economics, particularly energy costs. The profitability of a reaction system is determined by the balance between consumption costs and the value of products generated. When considering consumption costs, both visible and invisible factors should be taken into account. Visible consumption encompasses the costs of raw materials, electrocatalysts, and all components of devices, which are typically considered in economic analyses. However, it is equally important to consider the often overlooked invisible consumption, which includes electricity, treatment costs for products or pollutants, and electrolytes. The cost of electricity plays a pivotal role in the economic analysis, and exploring strategies to minimize energy consumption is crucial. Renewable energy sources such as solar or wind can be harnessed to reduce the overall energy cost and enhance the economic feasibility of the electrocatalytic systems. In this context, self-powered electrocatalytic systems, particularly those stored chemical energy derived from sustainable energy sources, hold promise for future development. Therefore, a combination of a clean power source, moderate operating conditions, aqueous electrolytes, high selectivity/faradaic efficiency, and nonprecious metal-based electrocatalysts emerges as a key research field for maximizing the profitability of a whole electrocatalytic system. By addressing these factors, the overall energy costs can be minimized, making the hydrogen production more economically viable.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22105108 and 22179065) and China Postdoctoral Science Foundation (2020M680860).

Notes and references

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