Hydrazine-assisted water electrolysis system: performance enhancement and application expansion

Abstract

Powered by renewable energy sources, water electrolysis has emerged as a highly promising technology for energy conversion, attracting significant attention in recent years, but it faces severe challenges, especially at the anode. Accordingly, hydrazine-assisted water electrolysis, incorporating the electro-oxidation of hydrazine at the anode, holds great promise for greatly reducing the input voltage and optimizing the system by application expansion. In this review, we present an in-depth overview of hydrazine-assisted water electrolysis, introducing its reaction mechanisms, basic parameters, specific advantages compared with conventional water electrolysis and other hybrid water electrolysis systems, strategies for developing efficient electrocatalysts with enhanced electrocatalytic performances, and especially its potential application expansion. An analysis of its technical and economic aspects, feasibility studies, mechanistic investigations, and relevant comparisons are also presented for providing a deeper insight into hydrazine-assisted water electrolysis. Finally, the potential avenues and opportunities for future research on hydrazine-assisted water electrolysis are discussed.

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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.

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. Subsequently, he 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 mainly focus on the self-assembly of hierarchically nanoporous and nanostructured materials for energy and environmental applications.

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Wider impact

As fossil fuel resources continue to dwindle, there is an increasing urgency to replace conventional hydrogen production methods, which are reliant on fossil energy, with clean and environmentally friendly water electrolysis technology. Owing to the thermodynamically/kinetically unfavorable oxygen evolution reaction at the anode, the practical application of conventional water electrolysis is hindered by various challenges for practical application. In this case, hydrazine-assisted water electrolysis, incorporating the electro-oxidation of hydrazine at the anode, holds great promise for reducing the input voltage and optimizing the system by application expansion, exhibiting various advantages compared with conventional water electrolysis and other hybrid water electrolysis systems. In this review, from the perspective of developing bifunctional electrocatalysts with enhanced electrocatalytic performances and to further explore application expansion related to hydrazine-assisted water electrolysis, we provide a comprehensive overview of this novel energy conversion technology, introducing its reaction mechanisms, basic parameters, specific advantages compared with conventional water electrolysis and other hybrid water electrolysis systems, and strategies for developing efficient electrocatalysts with enhanced electrocatalytic performance, especially for potential application expansion.

1. Introduction

Considering the escalating environmental pollution and global energy demand, it is essential to reduce the dependence on dwindling fossil fuels by developing renewable energy systems with advanced energy conversion technologies.^1–6 Among them, hydrogen (H₂) holds great promise as an ideal alternative considering its high gravimetric energy density, earth abundance and environmental friendliness.^7,8 It can also be an ideal energy barrier in clean energy production for meeting future energy needs. Despite its widespread use, steam reforming of fossil fuels such as natural gas accounts for over 95% of global hydrogen production, creating a paradox, where carbon emissions continue to rise alongside efforts to adopt renewable energy solutions.^9–11 In this case, hydrogen production through water electrolysis offers a sustainable and highly efficient approach, resulting in the generation of hydrogen gas with exceptional purity.¹²

Water electrolysis has entered a crucial and promising phase in the 21st century, driven by its ability to streamline electrolyzer designs at significantly lower costs.¹³ Basically, a conventional water electrolysis system involves hydrogen production through the hydrogen evolution reaction (HER) at the cathode and oxygen production through the oxygen evolution reaction (OER) at the anode with thermodynamical equilibrium potentials of 0 and 1.23 V vs. the reversible hydrogen electrode (RHE), respectively.^14–16 A high theoretical voltage of 1.23 V with an energy input of ΔG = 237.1 kJ mol⁻¹ is required for the activation of conventional electrocatalytic water splitting systems. However, the sluggish reaction kinetics contribute to a much higher input voltage in practical water electrolyzers, even when using commercial RuO₂/IrO₂ for the OER at the anode and Pt/C for the HER at the cathode to lower the overpotentials.¹⁷ Furthermore, although the reaction mechanisms of the HER and OER have been intensively investigated and a great deal of efficient electrocatalysts have been developed in recent studies, the development of water electrolysis is still in its infancy, accounting for only 4% of global hydrogen production.¹⁸ Numerous fundamental obstacles still hinder the advancement of electrocatalytic water splitting for hydrogen production,^19–21 as follows: (1) the four-electron transfer process involved in the OER requires large overpotentials, especially for high current densities in industrial applications. Obviously increased input voltages are undesirably required for the production of oxygen, which is a useless byproduct. (2) The most basic design of the HER/OER in a single compartment raises safety concerns owing to the potential for the generation of explosive mixtures of H₂ and O₂ gases, although the gas tightness has been improved in some modern electrolyzer designs to ensure their safety. (3) The interaction between these gases and electrocatalysts can produce reactive oxygen species, which may cause damage to the membrane and shorten the lifespan of the water electrolyzer. (4) Gas crossover can decrease the overall energy efficiency, given that oxygen may be reduced back to water at the cathode. (5) Hydrogen is produced as the only valuable chemical in the conventional water electrolysis system without added value. Considering these challenges, novel strategies for the advancement of conventional water electrolysis system are desired.

Hybrid water electrolysis, employing alternative electrocatalytic oxidation reactions at the anode, is promising to circumvent the existing challenges in conventional water electrolysis and create added value.^22–24 The OER can be replaced by thermodynamically more favorable oxidation reactions, such as hydrazine,^25–28 organic,^29–31 and urea^32–36 oxidation reactions, at the anode to realize significantly lower input voltages. Furthermore, by preventing the creation of hazardous hydrogen/oxygen mixtures, it becomes feasible to develop high-efficiency water electrolysis devices without membranes. Among the alternative oxidation reactions, the hydrazine oxidation reaction (HzOR) possesses the lowest theoretical oxidation potential of −0.33 V vs. RHE, making hydrazine-assisted water electrolysis systems significantly promising for future application. Considering this excellent feature, various efficient electrocatalysts have been developed for monofunctional/bifunctional electrocatalytic hydrogen evolution/hydrazine oxidation in hydrazine-assisted water electrolysis systems. The composition and structure of these catalysts have been optimized by heteroatom doping,^37–39 heterostructure construction,^40–42 defect engineering^43–45 and 3D microstructure design,^46–48 delivering the same current densities at much lower device voltages compared with the conventional water electrolysis systems and other hybrid water electrolysis systems. For example, we reported a multifunctional electrocatalyst of interfacial nickel/cobalt phosphide heterostructured microspheres supported on Ni foam (NiCoP/NF).²¹ When applied as the both electrode of a conventional water electrolysis system and hybrid water electrolysis system, an input voltage of only 107 mV was required for a current density of 100 mA cm⁻² in the hydrazine-assisted water electrolysis system, which is 1.38, 1.45, 1.41, 1.43 V lower compared with that of the glucose, urea, ethanol and methanol-assisted water electrolysis systems, respectively, and especially 1.65 V lower than the conventional water electrolysis system. The integration of HzOR into the water electrolyzer substantially reduced the energy consumption, making it much more efficient compared to the electro-oxidation of other alternative chemical compounds in hybrid water electrolysis systems.

Beyond simply decreasing the cell voltage in electrolyzers, the integration of HER and HzOR provides more advantages in the application expansion of strategy combination and device construction, as follows: (1) a theoretical potential coincidence region of 0.33 V exists between the HER (0 V vs. RHE) and the HzOR (−0.33 V vs. RHE), making self-activated/propelled hydrazine-assisted water electrolysis realistic.^21,49,50 The co-production of hydrogen and electricity can also be accomplished.^51–53 (2) The HzOR can be integrated into aqueous metal-redox bifunctional catalyst batteries^54–57 and fuel cells,^58–60 constructing self-powered hydrogen production systems with the combination of hydrazine-assisted water electrolysis. (3) In terms of the biggest challenge in direct seawater splitting, the integration of electrocatalytic hydrazine oxidation at the anode can efficiently decrease the cell voltage to a chlorine chemistry-forbidden area, even at industrial-level current densities, ensuring the long-term stable operation of hydrazine-assisted seawater electrolysis under natural seawater conditions.^61–63 (4) Conventional methods for wastewater treatment, such as multi-effect evaporation crystallization and biological processes, typically involve complex separation stages, significant energy demands and use of additional chemicals, making hydrazine-assisted water electrolysis efficient technology for the treatment of hydrazine-containing industrial wastewater.^64–66 However, the recent progress on the application expansion of hydrazine-assisted water electrolysis has not been discussed in previous reviews.

This review offers a comprehensive and detailed exploration of hydrazine-assisted water electrolysis systems, providing perspectives on both the development of high-activity HER/HzOR electrocatalysts and their recently reported novel application expansion (Fig. 1). Firstly, we present an in-depth overview of hydrazine-assisted water electrolysis, introducing its reaction mechanisms and basic parameters. Furthermore, some advantages compared with conventional water electrolysis systems and other hybrid water electrolysis systems are discussed. Then, we delve into the key recent developments in bifunctional electrocatalysts for hydrazine-assisted water electrolysis, focusing on effective approaches to improve their activity and durability under specific reaction conditions. Subsequently, the recent progress on the application expansion of self-propelled operation, battery development/combination and hybrid seawater electrolysis are discussed. Also, an analysis of the technical and economic aspects, feasibility studies, mechanistic investigations, and relevant comparisons is provided for deeper insight into hydrazine-assisted water electrolysis. Additionally, we outline the challenges and opportunities related to catalyst development, device design, and potential for the industrial-scale implementation of hydrazine-assisted water electrolysis, hopefully offering directions for future developments.

Fig. 1 Schematic of hydrazine-assisted water electrolysis system.

2. Overview of hydrazine-assisted water electrolysis

2.1. Electrocatalytic reaction mechanisms

Basically, the cathodic HER and the anodic HzOR are involved in hydrazine-assisted water electrolysis systems. To further highlight the advantage of hydrazine-assisted water electrolysis systems compared with conventional water electrolysis systems and other hybrid water electrolysis systems, the electrocatalytic reaction mechanisms of the OER and some other alternative oxidation reactions are also briefly introduced.

2.1.1. Electrocatalytic hydrogen evolution at the cathode. In terms of the HER at the cathode, hydrazine-assisted water electrolysis is almost identical to conventional water electrolysis. A two-electron transfer process is involved in the electrocatalytic hydrogen evolution. Eqn (1) and (2) show the overall reaction mechanism of the HER under acidic and alkaline condition, respectively.

Acidic condition: 2H⁺ + 2e⁻ → H₂ (1)

Alkaline condition: 2H₂O + 2e⁻ → H₂ + 2OH⁻ (2)

Typically, the Volmer–Tafel mechanism is predominant for noble metal-based electrocatalysts, while the Volmer–Heyrovsky mechanism is more favorable for electrocatalysts based on non-noble metals,^67–69 as follows:

Acidic conditions:

Volmer step: H⁺ + M + e⁻ → M–H_ad (3)

Tafel step: 2M–H_ad → 2H₂ (4)

Heyrovsky step: M–H_ad + H⁺ + e⁻ → M + H₂ (5)

Alkaline conditions:

Volmer step: H₂O + M + e⁻ → M–H_ad + OH⁻ (6)

Tafel step: 2M–H_ad → 2H₂ (7)

Heyrovsky step: M–H_ad + H₂O + e⁻ → M + H₂ + OH⁻ (8)

where M represents the active surface sites and H_ad denotes the absorbed hydrogen. The Volmer step is commonly considered the rate-determining step (RDS) of electrocatalytic hydrogen evolution under alkaline conditions due to the slow dissociation of water. Thus, an efficient HER electrocatalyst must exhibit strong capabilities for both water molecule dissociation and hydrogen adsorption on its active surface sites.

2.1.2. Electrocatalytic hydrazine oxidation at the anode. In hydrazine-assisted water electrolysis, the HzOR at the anode is a four-step dehydrogenation process from hydrazine to nitrogen with four-electron transfer, which is commonly investigated under alkaline conditions,^70–72 as follows:

N₂H₄ + OH⁻ → N₂H₃* + H₂O + e⁻ (9)

N₂H₃* + OH⁻ → N₂H₂* + H₂O + e⁻ (10)

N₂H₂* + OH⁻ → N₂H* + H₂O + e⁻ (11)

N₂H* + OH⁻ → N₂ + H₂O + e⁻ (12)

The adsorption of hydrazine and the four consecutive deprotonation steps should be efficiently accelerated by the developed electrocatalytic hydrazine oxidation catalysts.

Additionally, an accelerated two-step HzOR mechanism was also suggested.^73,74 The transfer of two electrons with the desorption of two hydrogen atoms is involved in each step. We demonstrated this accelerated “2 + 2” HzOR mechanism on an Ni foam-supported ultrathin phosphide/iron co-doped nickel selenide catalyst (P/Fe–NiSe₂) (Fig. 2).⁶⁶ The peak potentials and current densities of the hydrazine oxidation/nickel hydroxide-relevant peaks observed in the cyclic voltammograms (CVs) measured in 1.0 M KOH with the addition of 10 mM hydrazine were applied to calculate the electron transfer number in the electrocatalytic oxidation process on P/Fe–NiSe₂ (Fig. 2a–c).^74,75 Thus, a two-electron process with the production of a diazene intermediate is involved in the RDS of HzOR on P/Fe–NiSe₂. Fig. 2d and e show the different intensity ratios of N–H stretching at around 3280 cm⁻¹, H–N–H bending at around 1450 cm⁻¹ and –NH₂ wagging at around 1280 cm⁻¹ in the in situ Fourier transform infrared (FTIR) spectra of P/Fe–NiSe₂ and Fe–NiSe₂, indicating different reaction mechanisms.^76–78 Additionally, density functional theory (DFT) calculations (Fig. 2f–h) also demonstrated a lower energy barrier for the RDS of the two-step hydrazine oxidation process from N₂H₄ to N₂H₂* (the dotted line) on P/Fe–NiSe₂ compared with that of the four-step process (the solid line), indicating the theoretical tendency of the accelerated “2 + 2” HzOR mechanism, as shown in Fig. 2i.

Ni* + N₂H₄ + 2OH⁻ → Ni–N₂H₂* + H₂O + 2e⁻ (13)

Ni–N₂H₂* + 2OH⁻ → Ni* + N₂ + 2H₂O + 2e⁻ (14)

where Ni* is the active nickel sites in P/Fe–NiSe₂ for the adsorption of the HzOR intermediates.

Fig. 2 CVs over (a) P/Fe–NiSe₂ and (b) Fe–NiSe₂. (c) HzOR peak potential plots versus log(v). (d) and (e) Electrochemical in situ FTIR spectra of P/Fe–NiSe₂ and Fe–NiSe₂ for HzOR. (f) Gibbs free energy diagram for H adsorption. (g) H₂O adsorption energy. (h) Free energy diagram of the HzOR intermediates for Fe–NiSe₂ and P/Fe–NiSe₂. (i) Schematic of the proposed HzOR processes over P/Fe–NiSe₂. Reproduced with permission.⁶⁶ Copyright 2023, Springer Nature.

2.1.3. Other electrocatalytic oxidation reactions at the anode. Besides the much higher thermodynamic theoretical equilibrium potentials of electrocatalytic oxygen evolution and other alternative oxidation reactions, their sluggish kinetics is another obstacle preventing them from competing with the input voltage of hydrazine-assisted water electrolysis.^79–81 Herein, some reaction mechanisms are introduced and discussed for comparison.

The challenging four-electron transfer process makes the OER a major source of energy loss in conventional electrocatalytic water splitting.^82–84

Acidic conditions: 2H₂O → 4H⁺ + 4e⁻ + O₂ (15)

Alkaline conditions: 4OH⁻ → 2H₂O + 4e⁻ + 2O₂ (16)

Under alkaline conditions, the adsorption of hydroxide ions and the formation of M–O/M–OOH species are involved in different pathways, i.e., direct and indirect OER pathways, leading to sluggish kinetics,^85–87 as follows:

Direct pathway:

M + OH⁻ → M–OH + e⁻ (17)

M–OH + OH⁻ → M–O + H₂O + e⁻ (18)

2M–O → 2M + O₂ (19)

Indirect pathway:

M + OH⁻ → M–OH + e⁻ (20)

M–OH + OH⁻ → M–O + H₂O + e⁻ (21)

M–O + OH⁻ → M–OOH + e⁻ (22)

M–OOH + OH⁻ → M + H₂O + O₂ + e⁻ (23)

The OER proceeds either through the direct coupling of two M–O species or through the formation of an M–OOH intermediate, which subsequently decomposes to release both water and oxygen.

As another alternative in hybrid water electrolysis, electrocatalytic urea oxidation under alkaline conditions can be described as follows:^88–90

CO(NH₂)₂ + 6OH⁻ → N₂ + 5H₂O + CO₂ + 6e⁻ (24)

The electro-oxidation of urea, involving six electrons, results in slow reaction kinetics, thus necessitating the use of highly efficient electrocatalysts at the anode to overcome this limitation. Various pathways involving C–N cleavage/oxidation, intramolecular N–N coupling, intermolecular N–N coupling, N₂O formation and NO₂⁻ overoxidation exist in urea electrooxidation, further impeding its rapid progress.⁹¹

Although value-added products are derived from the electrocatalytic oxidation of biomass-derived organics, its multi-electron transfer, high breaking energy of some carbon bonds and some strongly adsorbed reaction intermediates induce large overpotentials and complex reaction pathways, together with the generation of various by-products.^92–95 Taking methanol, one of the simplest organics, as an example, its electro-oxidation involves six-electron transfer.^96–98 Also, various reaction intermediates such as CO, HCOOH and COH are involved in its complex mechanisms. Generally, a dual-pathway mechanism is observed, consisting of a direct and an indirect route. The direct pathway involves weakly adsorbed or partially dissolved intermediates, such as HCOOH, whereas the indirect pathway is associated with more strongly adsorbed intermediates of CO. The removal of the strongly adsorbed CO intermediate, which induces a poisoning effect on noble metal-based electrocatalysts, is commonly the RDS in the electro-oxidation of methanol.

Based on the above discussion, electrocatalytic hydrazine oxidation not only shows an advantage in terms of thermodynamic equilibrium potential but also possesses facile kinetics with low electron transfer number and simple reaction pathway compared with conventional water electrolysis systems and other hybrid water electrolysis systems.

2.2. Considerations in electrocatalysts

For the efficient electrocatalysis of the HER at the cathode and the HzOR at the anode in hydrazine-assisted water electrolysis, the development of active, selective, stable and low-cost electrocatalysts is highly desirable. Catalysts with various compositions and structures have been reported in recent works for application in conventional and hybrid water electrolysis systems. Different from other electrolysis systems, in terms of the strongly oxidizing reaction environment, some specific considerations are involved in hydrazine-assisted water electrolysis.

2.2.1. Activity. For the evaluation of electrocatalytic activity of the fabricated catalysts, the overpotential (η) is the primary index.^99,100 This is the difference between the thermodynamic equilibrium potential (E₀) and the actual applied potential (E_a) with iR-correction, which can be expressed as follows:¹⁰¹

η = E_a − E₀ − iR (25)

where i is the specific current density and R is the series resistance of the catalyst, substrate, solution and wiring. The overpotential in electrolysis processes is influenced by several factors that reduce the overall efficiency of electrochemical reactions.⁸⁵ Firstly, the activation overpotential arises from the kinetic barriers that hinder the onset of reactions at the electrodes, requiring energy in excess of the thermodynamic potential. Secondly, the ohmic overpotential occurs due to the resistive properties of the electrolyte and electrode materials, causing voltage drops as current flows through the system. In addition, the concentration overpotential results from the development of concentration gradients of reactants and products near the electrode surface, which can impede the reaction rate. Lastly, at higher current densities, mass transport limitations exacerbate the overpotential, given that the transport of ions and gases fails to match the demands of the reaction. These combined factors contribute to the overall overpotential in electrolysis systems.

Besides, the cell voltage of water electrolysis can be commonly described using the overpotential (26), as follows:

V = ΔE_eq + η_a + η_c + iR (26)

where V is the applied cell voltage, ΔE_eq is the thermodynamic equilibrium voltage, and η_a and η_c are the overpotential at the anode and cathode, respectively. Notably, ΔE_eq is the difference between the thermodynamic equilibrium potentials of the anode and the cathode. Apparently, this value for hydrazine-assisted water electrolysis is negative (−0.33 V), indicating that the practical input cell voltage in hydrazine-assisted water electrolysis systems cannot be simply derived using the thermodynamic equilibrium voltage and overpotentials through this equation.

Other parameters can also be applied for the evaluation of the electrocatalytic performance. The Tafel slope is an index related to the intrinsic properties of catalysts, especially the reaction kinetics, according to:^102–104

η = b·log(J/J₀) (27)

where η, b, J and J₀ represent the overpotential, Tafel slope, current density, and exchange current density, respectively. In the case of the HER, the Tafel slope of the three basic steps of Volmer, Tafel and Heyrovsky steps is 118, 30 and 40 mV dec⁻¹, respectively. The calculation of the Tafel slope can not only qualitatively evaluate the reaction kinetics but also determine the specific reaction pathway for the HER.

It has been reported that the double layer capacitance (C_dl) is correlated with the electrochemically active surface area (ECSA), providing a comparative assessment of the structural quality and intrinsic catalytic activity of synthesized electrocatalysts.¹⁰⁵ A large ECSA indicates more electrochemically accessible active sites, which are beneficial for the enhancement of electrocatalytic activity. The C_dl can be determined using cyclic voltammograms recorded at different scan rates in a narrow non-faradaic potential range based on:

C_dl = (j_a − j_c)/(2 × v) = (j_a + |j_c|)/(2 × v) = Δj/(2 × v) (28)

where j_a and j_c are the anodic and cathodic current density, respectively, and v is the scan rate. The C_dl can be further converted into the ECSA using the roughness factor (r_f), as follows:

r_f = C_dl (mF cm⁻²)/C_dl,_ideal (mF cm⁻²) (29)

j_ECSA = j/r_f (30)

where C_dl,_ideal is the double layer capacitance of an ideally flat electrode, which is usually taken as C_dl,_ideal = 0.04 mF cm⁻² for alkaline media, and j_ECSA is the current density normalized by the ECSA.

Moreover, the turnover number (TON) and turnover frequency (TOF) are essential metrics for evaluating the intrinsic catalytic performance. TON reflects the number of electron moles transferred per active site, whereas TOF measures the rate at which this catalytic activity occurs over time.^106–108 The TOF can be calculated as follows:

TOF = jA/αnF (31)

where A is the electrode area, F is Faraday's constant, n is the amount of active species, and α is the charge transfer coefficient.

2.2.2. Selectivity. Despite the branching approaches such as the formation of ammonia through hydrolytic dissociation, no competing reaction is involved in hydrazine-assisted water electrolysis in previously reported works. This means that the electro-oxidation of hydrazine for nitrogen production is dominant in hydrazine-assisted water electrolysis and regulation of the electrocatalytic selectivity of HzOR catalysts is relatively unnecessary, which is the same for the HER. Considering the significantly lower theoretical equilibrium potential of HzOR, an HzOR electrocatalyst with high activity toward other electro-oxidation reactions can operate selectively and stably in a mixed electrolyte containing various substances. This makes it particularly suitable for practical applications such as seawater and industrial wastewater treatment. Also, it greatly facilitates the design of electrocatalysts.

2.2.3. Stability and cost. Given the strongly oxidizing environment and possibly complex composition of the electrolyte (seawater or industrial wastewater) in hydrazine-assisted water electrolysis compared to conventional water electrolysis, it is crucial to consider the long-term stability of electrocatalysts. Emphasis should be placed on maintaining the integrity of their active sites, as well as their structure and surface morphology. It is important to prevent electrode surface blockage and the degradation of electrocatalysts on both electrodes, which may result from the presence of contaminants such as stray ions, microbes, and solid impurities. Also, it is worth noting that the stability test of fabricated electrocatalysts should be as close to practical application as possible, considering weather conditions, industrial requirement and geographical position.

Furthermore, reducing the cost of electrocatalyst production is essential to enhance the feasibility of industrial applications. This not only includes the cost of oxidation substances but also expenses associated with their preparation processes, transportation, and environmental impact.

2.3. Considerations in terms of hydrazine

In terms of hydrazine-assisted water electrolysis system, concerns about the source of hydrazine are important. Actually, it is uneconomic to produce hydrazine on a large scale to serve as a substrate in hydrazine-assisted water electrolysis to produce hydrogen. Besides, hydrazine is a toxic and flammable chemical, which is another disadvantage in the usual consideration on energy conversion technologies. Overall, the extensive study of hydrazine-assisted water electrolysis is basically due to the favorable thermodynamics of the hydrazine oxidation reaction, and consequently the origin of hydrazine has been somewhat neglected. The current idea for the practical application of this system is to use hydrazine in industrial wastewater. Actually, compared with the hydrazine concentration of about 0.2 M used in most recent works, industrial wastewater typically contains higher hydrazine concentrations, ranging from 5% to 10%, making it a realistic assumption. The toxicity and flammability of hydrazine also make this idea feasible. Based on this consideration, hydrazine-assisted water electrolysis is a specific-application system, and thus the practical operating conditions are based on the plant specific requirements. Although significant progress has been made, it is important to recognize that the hydrazine-assisted water electrolysis approach remains in the early stages of development, even within laboratory settings, and is far from being ready for practical application. Several key challenges must be overcome to refine the current laboratory experiments and pave the way for commercialization, highlighting potential avenues for future research.

3. Strategies for electrocatalyst development

Since the pioneering work on an Ni foam-supported nickel phosphide array (Ni₂P/NF) was reported by Sun and coworkers,²⁵ numerous electrocatalysts have been designed for hydrazine-assisted water electrolysis. Strategies of composition optimization, heteroatom doping, heterostructure construction, alloy formation, defect regulation, and lattice strain and micromorphology control have been widely employed for enhancing the performance of bifunctional electrocatalytic HER/HzOR electrocatalysts. Herein, the latest advancements and notable achievements in electrocatalyst design for hydrazine-assisted water electrolysis are introduced and discussed in terms of different strategies for electrocatalyst development.

The selection of active species plays a fundamental and crucial role in determining the overall electrocatalytic performance of catalysts. During the design of electrocatalysts, the composition optimization, especially the selection of optimal active metal sites should be considered prior to other strategies.

Noble metal-based electrocatalysts, including Pt,^109–111 Ru,^112–114 Rh,^45,115–117 Pd,^48,118,119 Ir,^47,61,120 Au^121,122 and Ag,¹²³ show exceptional intrinsic electrocatalytic activity in hydrazine-assisted water electrolysis. Mesoporous N-doped carbon-supported Ru nanoparticles (Ru/MPNC) were synthesized through a mixing, pyrolysis and etching process (Fig. 3a and b),¹²⁴ and the greatly exposed Ru particles were stabilized in mesoporous N-doped carbon. The strong combination of Ru with NC allowed the modification of the electronic properties of the exposed Ru sites, leading to a reduced water dissociation energy barrier. This integration also optimized the adsorption energies of H* and the dehydrogenation intermediates of the HzOR. The assembled two-electrode electrolyzer showed a current density of 50 mA cm⁻² with a low cell voltage of 0.149 V. Alternatively, a defect-rich low-crystalline Rh metallene (l-Rh metallene) was reported by Deng et al.¹²⁵ An amorphous/crystalline hetero-phase structure was constructed, providing the unique electronic structure of abundant Rh sites for highly efficient hydrazine-assisted water electrolysis. Potentials of −38 mV and −2 mV were required at a current density 10 mA cm⁻² for the HER and HzOR, respectively. In a two-electrode hydrazine-assisted seawater electrolyzer, the l-Rh metallene showed an ultra-low input voltage of 28 mV to achieve 10 mA cm⁻².

Fig. 3 (a) Schematic of the synthesis of Ru/MPNC and (b) corresponding free energies for the adsorption of reaction intermediate in the HzOR. Reproduced with permission.¹²⁴ Copyright 2022, Elsevier. Three representative models of (c) Ag (100), (d) Ru (1013) and (e) Ru@Ag (100). Stability diagram for (f) Ag (100), (g) Ru (1013) and (h) Ru@Ag (100). (i) HzOR free energy profile. (j) Potential-dependent reaction energy profiles at pH = 14. (k) N–N bond cleavage barriers. (l) N–N bond cleavage structure on two interfaces of Ru (1013) and Ru@Ag (100). (m) Simulated exchange currents. Reproduced with permission.¹²³ Copyright 2024, The Royal Society of Chemistry. (n) Charge density difference of Ru₁–NiCoP. (o) Density of d-band states and (p) H* energy profiles of NiCoP and Ru₁–NiCoP. (q) Reaction pathways with structural configurations. (r) HzOR free energy profiles. (s) Charge density difference with the adsorption of HNNH*. Reproduced with permission.¹²⁶ John Wiley and Sons.

However, despite the numerous advantages exhibited by noble metal-based electrocatalysts in electrocatalytic HER and HzOR, these monometallic catalysts still have some shortcomings. Typically, the introduction of a secondary metal active site to construct a hybrid bimetal structure can induce a synergistic effect, enhancing their electrocatalytic performance. For example, although Ru-based electrocatalysts exhibit the lowest overpotentials, the undesirable cleavage of N–N bonds on Ru sites hinders their further development.^127–129 Duan and coworkers decorated Ag nanoparticles with Ru atoms (Ru@Ag) to construct Ag–Ru interfaces to solve this problem.¹²³ A high N–N bond cleavage barrier and easier nitrogen desorption were obtained on the Ag–Ru interfaces, indicating their high electrocatalytic HzOR activity and selectivity (Fig. 3c–m). The Ru@Ag-assembled hydrazine-assisted water electrolyzer could deliver high current densities of 100 and 983 ± 30 mA cm⁻² at low cell voltages of 16 and 450 mV, respectively. Based on a similar strategy, Mao et al. constructed a low-content Pt-doped metallene (Pt-Rhene) to optimize the electronic structure of Rh by the synergistic effect between Pt and Rh.¹¹⁰ The incorporation of Pt effectively tuned the d-band center of the Rh atoms, optimizing the adsorption of different reaction intermediates during the HER and HzOR processes. When assembled in both electrodes of the hydrazine-assisted water electrolysis system, low voltages of 0.06, 0.19 and 0.28 V were required to achieve the current densities of 10, 50, 100 mA cm⁻², respectively.

The high cost and limited durability of the noble metal-based catalysts discussed above have somewhat restricted their practical application. The combination of non-noble metal species and noble metal-based catalysts has become an efficient strategy for composition optimization, and thus overall performance enhancement.^43,44,130 Besides, the introduction of non-noble metal species also has some merits such as optimized electronic structure and 3D microstructure. The proportion of noble/non-noble metal species has been constantly regulated to enhance the electrocatalytic activity of the fabricated electrocatalysts, where a reduction in the content of noble metal is favorable. Chen et al. introduced trace Co doping in an Ru matrix, constructing oxygenated carbon black-supported dispersed RuCo alloy particles (RuCo/C).¹³¹ By incorporating small amounts of Co in the alloy, both the water dissociation energy barrier and the hydrogen adsorption/desorption kinetics for the HER were improved on the Ru active sites. Additionally, this modification significantly lowered the rate-determining energy barrier of the HzOR process. Energy-saving hydrogen production at 100 mA cm⁻² was achieved by applying a low cell voltage of 215.4 mV with the assistance of RuCo/C. Alternatively, the amount of Ru species was decreased by Li et al., constructing an Ni foam-supported Ru-doping Ni(OH)₂ nanowire network electrode (Ru–Ni(OH)₂ NW²/NF).¹³² The doping of Ru heteroatoms enhanced the electrocatalytic activity of Ni(OH)₂ by shifting its d-band centers to higher energies. This modification increased the anti-bonding states, thereby intensifying the interaction between the catalyst and the adsorbed reaction intermediates. Consequently, Ru–Ni(OH)₂ NW²/NF showed excellent bifunctional HER and HzOR activity, reaching 1 A cm⁻² in a flow electrolyzer at a record-low working potential of 1.051 V. Moreover, Hu et al. developed Ru single atoms on a twisted NiCoP nanowire array (Ru₁–NiCoP).¹²⁶ Cooperative Ni(Co)–Ru–P sites were constructed in Ru₁–NiCoP, which could optimize the adsorption of *H in the HER process and *N₂H₂ in the HzOR process (Fig. 3n–s), contributing to an enhanced bifunctional electrocatalytic performance. Desirable potentials of −32 and −60 mV were required to deliver a current density of 10 mA cm⁻² for the HER and the HzOR, respectively.

Furthermore, transition metal-based electrocatalysts without the addition of noble metals can also work efficiently in hydrazine-assisted water electrolysis. Ni-,^133–135 Co-,^58,136,137 Fe-,^39,40,138 Mn-,^38,139,140 Mo-,^135,141,142 Zn-^46,143,144 and Cu-based^42,145,146 electrocatalysts have been intensively investigated as highly efficient bifunctional HER and HzOR electrocatalysts in the last two years. Based on the composition optimization of these catalysts, further improvement in their electrocatalytic performance in hydrazine-assisted water electrolysis system can be achieved through the following strategies.

3.1. Heteroatom doping

The incorporation of heteroatoms has been proven to be highly effective in adjusting the electronic structure and increasing the density of metallic active sites. Different metal- and nonmetal-doping strategies have emerged as a versatile and powerful method for enhancing the electrocatalytic performance of various catalysts. Mo doping in P vacancy-rich Ni₂P nanosheets supported on Mo–Ni foam (Mo–Ni₂P_v@MNF) was demonstrated,¹⁴⁷ indicating that the introduction of Mo heteroatoms could not only optimize the adsorption energy for H* to a thermoneutral value for the HER but also accelerate the dehydrogenation process from *N₂H₄ to *NHNH₂ intermediate during the electrocatalytic hydrazine oxidation process (Fig. 4a–d). Assembling Mo–Ni₂P_v@MNF as the both electrodes in hydrazine-assisted water electrolysis, an ultralow cell voltage of only 571 mV was required to deliver a high current density of 1 A cm⁻². Mo–Ni₂P_v@MNF could also work efficiently in a self-assembled hydrazine-fuel cell and decrease the concentration of hydrazine in industrial hydrazine sewage to about 5 ppm. Also, an Ni foam-supported Al-doped Ni₂P nanoflower architecture (Al–Ni₂P/NF) was fabricated through the combination of hydrothermal and phosphorization processes.²² The incorporation of Al element could induce the electro redistribution of the Ni–P bond and optimize the adsorption of different reaction intermediates in the HER and HzOR processes. Consequently, Al–Ni₂P/NF showed an excellent bifunctional electrocatalytic HER and HzOR performance, achieving a high current density of 500 mA cm⁻² at potentials of −205 and 300 mV, respectively. A cell voltage of 0.717 V was required for 500 mA cm⁻² in the assembled hydrazine-assisted water electrolyzer.

Fig. 4 (a) Structural model of Mo–Ni₂P_v. (b) Density of states. (c) Free energies for the adsorption of H*. (d) HzOR free energy diagrams. Reproduced with permission.¹⁴⁷ Copyright 2023, John Wiley and Sons. Structural models of (e) CoP and (f) WO–CoP. (g) Calculated charge density difference of WO–CoP. (h) Density of states. (i) Adsorption energies for water and hydrazine. (j) HzOR free energy diagrams. (k) Free energy diagrams for water dissociation. (l) Gibbs free energies for the adsorption of *H. Reproduced with permission.¹⁴⁸ Copyright 2023, Springer Nature.

Besides optimizing the electronic structure of the original metallic active sites, the introduction of heteroatoms can also offer new adsorption sites to accelerate the HER and HzOR process. Meng et al. introduced tungsten-bridged oxygen species in CoP nanoflakes (6W–O–CoP/NF) through a hydrolysis etching process.¹⁴⁸ The incorporation of [W–O] groups, which served as adsorption sites for both water dissociation and hydrazine dehydrogenation, facilitated the creation of a porous structure on the CoP nanoflakes (Fig. 4e–l). Additionally, the [W–O] groups modified the electronic structure of Co, ultimately enhancing the electrocatalytic hydrogen evolution and hydrazine oxidation performance. Potentials of −185.60 mV and 78.99 mV were required for a current density of 1000 mA cm⁻² for the HER and HzOR, respectively.

The employment of heteroatom doping in noble/nonnoble metal- and nonmetal-based electrocatalysts for the HER and HzOR has also been reported such as Pt-doping in MnCo₂O₄ nanosheet arrays (Pt–MnCo₂O₄/NF),¹⁴⁰ Ru-doping in FeP₄ nanosheets (Ru–FeP₄/IF),¹⁴⁹ Ru-incorporating in NiRh-MOF,¹⁵⁰ Fe-doping in Co₃N (Fe–Co₃N),¹³⁶ Fe-doping in Co₂P with heterostructured CeO₂ (Fe–Co₂P/CeO₂),¹³⁷ Fe-doping in CoF₂ nanorod (Fe–CoF₂/NF),¹⁵¹ Co-doping in bimetallic hydroxysulfide of NiFeSOH nanosheet arrays (CoeFeNiSOH/NFF),¹⁵² Mn-doping in 2D hexagonal NiCo hydroxide nanosheets with 1D carbon nanotubes (Mn–NiCo HNS/CNT),¹⁵³ Mn-doping in Ni₂P (Ni_2−xMn_xP),³⁸ Mo-doping in Ni₃S₄ lattice (Mo–Ni₃S₄/CW),¹⁵⁴ Mo-doping in CoP with heterostructured Yb₂O₃ (Mo–CoP/Yb₂O₃),¹⁵⁵ Zn-doping in oxygen-deficient nickel cobalt oxide alloy array (Zn–NiCoO_x−z),¹⁵⁶ Cu-doping in N-doped carbon-encapsulated CoFe/Co (Cu–CoFe/Co/NC),¹⁵⁷ Ce-doping in Ni₃N nanosheet arrays (Ce–Ni₃N/NF),¹⁵⁸ S-doping tri-metal phosphide (S-TMP),³⁹ N-doping in mesoporous Ru film (N-mRu/NF),¹⁵⁹ N-doping in NiZn–Cu-layered double oxides with reduced graphene oxide (N–NiZnCu LDH/rGO),¹⁶⁰ P-doping in Co/Ni₃P hybrid ((P–Co/Ni₃P)_A3/NF),¹⁶¹ F-doping CoP (F–CoP/CF),¹⁶² B- and P-doping in Cu dendrite surface-supported Co(OH)₂ nanosheets (B–Co_NS@Cu_PD and P–Co_NS@Cu_PD, respectively),¹⁶³ Mn- and N-doping in NiCo oxide (N–Ni₁Co₃Mn_0.4O/NF)¹⁶⁴ and Fe- and P-doping in nickel phosphide (P–NiFeP/Ni)¹⁶⁵ (Table 1). These samples not only prove the effectiveness of the heteroatom doping strategy, but also show its potential for further development.

Table 1 Electrocatalytic performance of the heteroatom-doped electrocatalysts under alkaline conditions. η(x)_j=x represents the overpotential at a current density of x mA cm⁻². E(x)_j=x represents the applied voltage at a current density of x mA cm⁻²

Catalyst	η(x)j=x/mV (mA cm^−2) (vs. RHE) for the HER	η(x)j=x/mV (mA cm^−2) (vs. RHE) for the HzOR	E(x)j=x/mV (mA cm^−2)	Ref.
Mo–Ni2Pv@MNF	259(3000)	392(500)		147
		456(1000)	571(1000)
Al–Ni2P/NF	205(500)	630(500)	717(500)	22
6W–O–CoP/NF	186(1000)	409(1000)	9(10)	148
			166(100)
			277(300)
Pt–MnCo2O4/NF	195(1000)	680(1000)	160(100)	140
			620(1000)
Ru–FeP4/IF	318(1000)	665(1000)	900(1000)	149
NiRh-MOF	49(10)	347(10)	60(10)	150
Fe–Co3N	∼270(400)	510(1000)	650(1000)	136
Fe–Co2P/CeO2	52(10)	379(10)	14(10)	137
	204(400)	463(200)	287(200)
			438(300)
Fe–CoF2/NF	—	1000(10)	—	151
CoeFeNiSOH/NFF	266(100)	685(100)	260(10)	152
			540(1000)
Mn–NiCo HNS/CNT	—	331(10)	—	153
Ni2−xMnxP	192(50)	385(10)	59(10)	38
			500(50)
Mo–Ni3S4/CW	17(10)	—	190(10)	154
Mo–CoP/Yb2O3	33(10)			155
	100(100)	482(100)	253(100)
Zn–NiCoOx−z	—	446(50)	700(50)	156
Cu–CoFe/Co/NC	217(10)	281(10)	663(10)	157
			872(50)
			1025(100)
Ce–Ni3N/NF	112(10)	256(10)	156(10)	158
			671(400)
S-TMP	—	588(10)	350(10)	39
N-mRu/NF	60(100)	62(100)	23(10)	159
			184(100)
N–NiZnCu LDH/rGO	183(100)	—	10(10)	160
(P–Co/Ni3P)A3/NF	10(10)	409(10)	50(300)	161
F–CoP/CF	90(1000)	371(1000)	490(1000)	162
B–CoNS@CuPD	—	476(10)	60(10)	163
P–CoNS@CuPD	70(10)	—	60(10)	163
N–Ni1Co3Mn0.4O/NF	177(100)	260(100)	272(100)	164
P–NiFeP/Ni	18(10)	407(10)	162(10)	165

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3.2. Heterostructure construction

Besides providing extra active species, the construction of heterogeneous nanostructures is a highly effective strategy for promoting electron redistribution and fine-tuning the adsorption/desorption energies of the reaction intermediates.¹⁶⁶ It can optimize the electrocatalytic performance of catalysts by precisely adjusting their electronic structure and shifting their d-band center.¹⁶⁷ The obtained abundant heterojunction interfaces can be highly active for various electrocatalytic reactions.^168–170 Liu et al. reported the preparation of an Ni foam-supported Mo-doped Ni₃N and Ni heterostructure (Mo–Ni₃N/Ni/NF), constructing rich Ni₃N/Ni heterojunction interfaces.¹⁷¹ The synergy of interface engineering of Ni₃N/Ni (Fig. 5a) and chemical substitution of Mo, Mo–Ni₃N/Ni/NF resulted in the optimized adsorption of H* in the HER process (Fig. 5b) and the accelerated hydrogenation process for the HzOR. In the case of Mo–Ni₃N/Ni/NF, a current density of 10 mA cm⁻² could be achieved at potentials of −45 mV and −0.3 mV for the HER and the HzOR, respectively. The Mo–Ni₃N/Ni/NF-assembled hydrazine-assisted water electrolysis yielded a current density of 10 mA cm⁻² at an ultralow cell voltage of 55 mV, which could be powered by a homemade direct hydrazine fuel cell, a commercial solar cell and a waste AAA battery.

Fig. 5 (a) Structural models of Mo–Ni₃N/Ni, Ni₃N/Ni and pure Ni₃N. (b) H* free energy diagrams. Reproduced with permission.¹⁷¹ Copyright 2021, John Wiley and Sons. (c) Schematic of effect on n- and p-type semiconductor heterostructure for the HER. Reproduced with permission.¹⁷² Copyright 2024, John Wiley and Sons. (d) Charge density difference of FeOOH/Ni₁₂P₅/Ni₂P. (e) Work function values. (f) Schematic of the charge transfer process in FeOOH/Ni₁₂P₅/Ni₂P heterostructure. (g) and (h) Density of states. (i) Electron number of 3d states. Reproduced with permission.⁴⁰ Copyright 2024, John Wiley and Sons. (j) Density of states of FeP, FeNi₂P and FeNiP-NP/C d band. (k) Charge density difference. Reproduced with permission.⁷¹ Copyright 2022, John Wiley and Sons.

N-type semiconductor heterostructures combining Ru nanoclusters and a range of metal oxides (M–O) of Fe₂O₃, ZnCo₂O₄, CuCo₂O₄, NiCo₂O₄, Co₃O₄ and NiO (Fig. 5c) were synthesized,¹⁷² in which the p-type M–Os could be transformed to n-type M–O/Ru semiconductors with the assistance of Ru. The design of n-type semiconductor heterostructures can effectively minimize the space-charge regions and enhance the charge carrier density, leading to a substantial improvement in the electrical conductivity of these electrocatalysts. Employing the NiO/Ru heterostructure in hydrazine-assisted water electrolysis, cell voltages of 0.021 and 0.22 were required for current densities of 10 and 100 mA cm⁻², respectively.

The built-in electric field on the constructed interfaces can efficiently regulate the d-band center of the active metal sites in heterostructured electrocatalysts for hydrazine-assisted water electrolysis.¹⁷³ Considering that the intensity of the built-in field significantly affects the electronic structure of the active metal sites, multiple heterostructures are promising to further improve the electrocatalytic performance of these electrocatalysts. An Ni foam-supported tri-phase FeOOH/Ni₁₂P₅/Ni₂P structure was constructed.⁴⁰ An outward built-in electric field was generated at the FeOOH/Ni₁₂P₅ interface, whereas an inward built-in electric field was formed at the Ni₁₂P₅/Ni₂P interface. The combination of the dual built-in field could manipulate the d-band electron configuration of the Fe sites, thus obtaining a moderate Fe–*N₂H₄ binding strength for the acceleration of the dehydrogenation processes in the HzOR process (Fig. 5d–i). Thus, the power consumption for hydrogen production could be lowered to 16.4 W h L⁻¹ H₂ by FeOOH/Ni₁₂P₅/Ni₂P in a hybrid seawater electrolyzer. Another example is the N,P co-doped hierarchical carbon-encapsulated FeP/FeNi₂P heterostructure (FeNiP-NPHC).⁷¹ The construction of a three-phase heterojunction of FeP, FeNi₂P and N,P co-doped carbon could effectively tune the d-band center of the active metal sites, thus enhancing the electrocatalytic performance of FeNiP-NPHC (Fig. 5j and k). The potentials at a current density of 100 mA cm⁻² for the HER and HzOR on FeNiP-NPHC were −180 and 7 mV, respectively.

Other designs of Fe-doped Ni₂P–Co₂P–Zn₃P₂ heterostructure (Fe–NiCoZnP/NF),¹⁴³ Co_0.52Cu_0.48/Cu heterostructure (Co_0.52Cu_0.48/Cu@S–C),¹⁴⁵ hierarchical Ohmic contact NiMo alloy and Ni₂P heterostructure (NiMo/Ni₂P),⁴¹ cobalt–phosphorous–boron (CoPB) and NiFe hydroxide heterostructure (CoPB@NiFe–OH/NF),¹⁷⁴ Co₄S₃ and WC heterostructure (Co₄S₃/WC/Nb₄C₃T_x@NC),¹⁷⁵ LDH-modified Cu₂O_S_Co_CoFe heterostructure,¹⁴⁶ CoSe–Ni_0.95Se heterostructure (CoSe–Ni_0.95Se/MXene),¹⁷⁶ Ir/I₂P hybrids (Ir/PNPC),¹²⁰ triphasic crystalline Co(OH)F, amorphous Co–S and crystalline CeO₂ heterostructure (Co(OH)F/Co–S/CeO₂/NF),¹⁷⁷ Co₃O₄/CoS₂ heterostructure,¹⁷⁸ Co(OH)₂/MoS₂ nanosheet array (Co(OH)₂/MoS₂/CC),¹⁷⁹ Co–Co₂P heterostructure (Co–Co₂P@CC-300),¹⁸⁰ Ni₂P/Zn–Ni–P heterostructure,⁷⁰ Ni/β-Ni(OH)₂ nanosheet array (Ni/β-Ni(OH)₂ NSAs)¹⁸¹ and Fe/Co dual-doped Ni₂P and MIL-FeCoNi heterostructure (FeCo–Ni₂P@MIL-FeCoNi)¹⁸² have also been reported (Table 2).

Table 2 Electrocatalytic performance of heterostructured electrocatalysts under alkaline conditions. η(x)_j=x represents the overpotential at a current density of x mA cm⁻². E(x)_j=x represents the applied voltage at a current density of x mA cm⁻²

Catalyst	η(x)j=x/mV (mA cm^−2) (vs. RHE) for the HER	η(x)j=x/mV (mA cm^−2) (vs. RHE) for the HzOR	E(x)j=x/mV (mA cm^−2)	Ref.
Mo–Ni3N/Ni/NF	45(10)	30(10)	55(10)	171
NiO/Ru	29(10)	251(10)	21(10)
	88(100)	282(100)	220(100)	172
	129(200)	327(300)
	215(500)	379(500)
FeOOH/Ni12P5/Ni2P	—	322(10)	220(10)	40
		374(100)	330(50)
			380(100)
FeNiP-NPHC	180(100)	338(100)	250(100)	71
Fe–NiCoZnP/NF	121(1000)	343(1000)	150(50)	143
Co0.52Cu0.48/Cu@S–C	118(10)	377(10)	36(10)	145
			210(100)
			282(200)
NiMo/Ni2P	15(10)	313(10)	181(100)	41
	81(100)	362(100)	343(500)
	195(500)	444(500)
CoPB@NiFe–OH/NF	32(10)	195(10)	33(10)	174
Co4S3/WC/Nb4C3Tx@NC	84(10)	264(10)	211(10)	175
LDH-modified Cu2O_S_Co_CoFe	280(100)	673(100)	—	146
CoSe–Ni0.95Se/MXene	161(400)	446(400)	350(100)	176
Ir/PNPC	22(100)	376(10)	167(100)	120
Co(OH)F/Co–S/CeO2/NF	152(10)	410(10)	200(10)	177
Co3O4/CoS2	—	570(10)	490(10)	178
		670(100)
Co(OH)2/MoS2/CC	134(100)	508(100)	1429(10)	179
			197(50)
			271(100)
Co–Co2P@CC	61(10)	89(10)	37(10)	180
Ni2P/Zn–Ni–P	63(10)	27(10)	165(10)	70
			358(100)
			532(400)
Ni/β-Ni(OH)2 NSAs	58(10)	315(10)	160(10)	181
FeCo–Ni2P@MIL–FeCoNi	210(100)	278(100)	400(1000)	182
	310(1000)	372(1000)

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3.3. Alloy formation

The formation of alloys can combine the advantages of multiple metals, provide additional active sites and improve the electrocatalytic properties of the original metal active sites.^183–185 Alloying Ru with Rh has shown effectiveness for the HER, OER and other electrocatalytic reactions.^186–188 Duan and coworkers synthesized a bifunctional electrocatalyst of ultrathin RhRu_0.5-alloy wavy nanowires¹⁸⁹ based on their experience in designing efficient electrocatalysts for practical applications and proving the advantage of developing efficient HzOR electrocatalysts without complex considerations about selectivity. The formed RhRu alloy shows a lower d-band center and ultrahigh ECSA, thus decreasing the overpotentials for the HER and HzOR. A high current density of 853 mA cm⁻² could be achieved at a cell voltage of 0.6 V in a hydrazine-assisted water electrolyzer with RhRu_0.5-alloy as a bifunctional electrocatalyst.

High-entropy alloys (HEAs) have garnered considerable attention in energy catalysis research.^190–192 Their unique properties, such as the ability to tailor electronic structures, optimize the d-band centers and benefit from high configurational entropy, make HEAs promising candidates as catalysts that offer both high activity and long-term stability in real-world applications.^193,194 When employing hydrazine-assisted water electrolysis, Xia and coworkers constructed high-entropy alloy nanoclusters with an average size of only seven atomic layers (1.48 nm).¹⁹⁵ The 72 active sites in the constructed high-entropy alloy nanoclusters were evaluated, revealing that all the active sites were effective for hydrazine-assisted water electrolysis, with some especially suitable for the HER and/or the HzOR (Fig. 6). The constructed high-entropy alloy nanoclusters showed a record mass activity of 250.2 mA mg_catalysts⁻¹ and an ultralow cell voltage of 0.181 V to achieve a current density of 100 mA cm⁻² for hydrazine-assisted water electrolysis.

Fig. 6 Electrocatalytic mechanisms for the active sites in high-entropy alloy nanoclusters. (a) H* adsorption energies. (b) Energy barriers for the RDS of the HzOR. (c) Summary diagram of the electrocatalytic active sites for the HER and HzOR. (d) Schematic of full-active-site catalytic mechanism for hydrazine-assisted water electrolysis system. Reproduced with permission.¹⁹⁵ Copyright 2024, John Wiley and Sons.

Other alloys of CuPd nanoalloy,¹⁹⁶ P-modified amorphous high-entropy CoFeNiCrMn compound (CoFeNiCrMnP/NF),¹⁹⁷ Ru-decorated MoNi/MoO₂ micropillar (Ru–MoNi/MoO₂),¹⁹⁸ amorphous RhPb nanoflowers,¹¹⁵ carbon-supported RuNi alloy,¹⁹⁹ RuPd alloy nanoparticles on pretreated activated carbon (RuPd/C),¹¹⁸ CuPd alloy (Cu1Pd3/C),²⁰⁰ MoNi alloys supported on MoO₂ nanorods (MoNi@NF)²⁰¹ and AuPt alloys¹²¹ have also been reported, indicating the effectiveness of forming alloys (Table 3).

Table 3 Electrocatalytic performance of alloy electrocatalysts under alkaline conditions. η(x)_j=x represents the overpotential at a current density of x mA cm⁻². E(x)_j=x represents the applied voltage at a current density of x mA cm⁻²

Catalyst	η(x)j=x/mV (mA cm^−2) (vs. RHE) for the HER	η(x)j=x/mV (mA cm^−2) (vs. RHE) for the HzOR	E(x)j=x/mV (mA cm^−2)	Ref.
RhRu0.5-alloy wavy nanowires	—	—	54(100)	189
			600(853)
CuPd nanoalloy	611(100)	631(10)	648(10)	196
CoFeNiCrMnP/NF	51(100)	268(100)	91(100)	197
Ru–MoNi/MoO2	13(10)	296(10)	570(50)	198
Amorphous RhPb nanoflowers	36(10)	348(10)	95(10)	115
			321(100)
RuNi alloy	24(10)	265(10)	58(10)	199
			327(100)
RuPd/C	15(10)	252(10)	18(10)	118
			148(100)
Cu1Pd3/C	315(10)	560(10)	505(10)	200
MoNi@NF	56(100)	690(100)	540(1000)
	219(1000)	800(1000)
AuPt alloy	26(10)	501(10)	172(10)	121

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3.4. Defect regulation

Structural defects such as edges and vacancies can be introduced and regulated for the creation of additional active sites and optimized electron configurations in electrocatalysts.²⁰² Thus, the regulation of defects can be an effective strategy for the development of electrocatalysts in hydrazine-assisted water electrolysis. For example, the surface oxygen vacancy content was elaborately regulated in Rh–Rh₂O₃ nanoclusters through the post-treatment of reducing the self-adaptively oxidized Rh nanoclusters (Rh–Rh₂O₃/C).⁴⁵ The chemical states of Rh were controlled by the regulation of the O vacancy content and the binding energies for H* were linearly dependent on it. Thus, Rh–Rh₂O₃/C showed low potentials of 12 and 31 mV to enable a current density of 10 mA cm⁻² for the HER and HzOR, respectively. Thus, in a hydrazine-assisted water electrolyzer, Rh–Rh₂O₃/C required a low cell voltage of 0.161 V at 100 mA cm⁻².

Additionally, abundant metal defects were fabricated in an Ni foam-supported defect-enriched 2D heterogeneous nickel copper selenide (D/Ni–Cu–Se) electrocatalyst through a hydrothermal approach and a metal-defect engineering route of alkaline etching on the precursor.⁴² The purposeful incorporation of metal defects was shown to effectively enhance energy-efficient H₂ production with the assistance of electrocatalytic hydrazine oxidation. The role of Cu is pivotal in promoting the formation of NiOOH catalytic sites and optimizing the adsorption of intermediate species. Moreover, the creation of a hierarchical, porous, and fluffy network architecture ensures ample active site exposure, efficient bubble release and facilitates the swift migration of ions and charges. Consequently, D/Ni–Cu–Se revealed a cell voltage of 0.268 V at 10 mA cm⁻² for hydrazine-assisted water electrolysis, which is 1.304 V lower than that for conventional water electrolysis. Defect regulation has been also employed in the integration of P vacant CoP and amorphous Co(OH)₂ (CoH–CoP_v@CFP),²⁰³ Co substrate-supported oxygen-vacancy-rich Ru-doped Co₃O₄ nanosheet array (Ru–Co₃O₄|V_O/CF)⁴³ and RuO₂_CuO/Al₂O₃ ternary composite.²⁰⁴

3.5. Lattice strain

Lattice strain engineering has emerged as a powerful and flexible technique for optimizing the electrocatalytic performance of various catalysts, including both precious and nonprecious metals.²⁰⁵ Its primary advantage is its ability to fine-tune the interaction energies of the reaction intermediates, thereby enhancing the overall catalytic efficiency. The continuous modulation of the lattice strain in electrocatalysts is promising to tune the adsorption strength with the reaction intermediates in the HER and HzOR processes on their active sites to the vertex of the adsorption “volcano plot”. Commonly, heteroatom doping is an efficient approach for modulating the lattice strain in electrocatalysts. A mild sulfurization approach was applied to induce a phase transformation and lattice-disordered microstructure in high-entropy septenary NiCrFeCoCuMnZn sulfide (HE-LH-S).¹³⁹ The disordered lattice structure and high-entropy characteristics of the pre-oxidation-optimized sulfide catalyst worked together to enhance the availability of active sites and boost the intrinsic activity for the HzOR. The accumulation of high-valence active species, the increase in surface roughness, and the in situ formation of SO₄²⁻ ions adsorbed on the catalyst surface collectively contributed to the improved catalytic kinetics, leading to a significant enhancement in HzOR performance, with no noticeable degradation during prolonged catalytic cycles. The optimized HE-LH-S-7 exhibited a potential of 0.49 mV to achieve a current density of 200 mA cm⁻² in hydrazine-containing electrolyte, which is 1.13 V lower than that for the OER.

A dual-cation co-doping strategy was used to continuously tune the strain of Ni₂P through the incorporation of Co and Cu atoms (Cu₁Co₂–Ni₂P/NF).²⁰⁶ Tension strain could be achieved by single-doping of Co or Cu heteroatoms, while the co-doping of Co and Cu heteroatoms could induce a compression strain of about 3.62%, enabling enhanced electrocatalytic activity for the HER and HzOR. It was demonstrated by density functional theory that the compressive strain could promote the dissociation of water and optimize the adsorption energy for *H in the HER process. Regarding the HzOR process, the energy barrier of the RDS of *N₂H₄ to N₂H₃ could be reduced, thus accelerating the dehydrogenation process. The optimized Cu₁Co₂–Ni₂P/NF could deliver current densities of 10 and 100 mA cm⁻² with cell voltages of 0.16 and 0.39 V in hydrazine-assisted water electrolysis, respectively.

Based on the discussion in Sections 3.2 and 3.6, the incorporation of heteroatoms can simultaneously induce electron structure and lattice strain engineering. Thus, we further investigated the respective and synergistic effect of these two aspects on the HER and HzOR in Ni foam-supported urchin-like Fe,Ni-codoped CoP (FeNi–CoP/NF).¹⁹ To distinguish the effects of these two factors, theoretical models of heteroatom-doping CoP with a fixed lattice size and undoped Co₂P with difference lattice strains in the range of −1.59% to 4.76% were constructed (Fig. 7). Thus, the heteroatom-induced electron redistribution and tension strain in FeNi–CoP/NF were proven to separately and synergistically optimize the adsorption of hydrogen during the HER process and accelerate the dehydrogenation of hydrazine during the HzOR process. Benefitting from this, FeNi–CoP/NF could deliver a current density of 10 mA cm⁻² with low overpotentials of only 36 and 236 mV for the HER and HzOR, respectively. Besides, a record voltage of only 163 mV was required for an industrial-level current density of 1.5 A cm⁻² at 70 °C for hydrazine-assisted seawater electrolysis.

Fig. 7 DFT calculations. (a) Density of states, (b) d band centers, (c) charge density difference analysis, (d) free energies for H* adsorption and (e) free energies of HzOR intermediates for FeNi–CoP/NF, Fe–CoP/NF, Ni–CoP/NF, and CoP/NF. (f) Density of states, (g) d band centers, (h) free energies for H* adsorption, and (i) free energies of HzOR intermediates for CoP models with different lattice strains. (j) Schematic of the synergy of electronic effect and lattice strain in FeNi–CoP/NF. Reproduced with permission.¹⁹ Copyright 2024, John Wiley and Sons.

3.6. Micromorphology control

The construction of a controlled micromorphology such as nanosheets, nanorods and nanospheres is highly desirable to provide a large ECSA with more accessible active sites for electrocatalytic reactions.^207–209 Besides, the optimization of surface properties can be achieved by various surface treatments such as etching, annealing and electrodeposition, further improving the electron/mass transfer on the two/three-phase interface, exposing more active sites, and thus enhancing the performance for various electrocatalytic applications. For example, a core–shell nanorod heterostructure of crystalline/amorphous cobalt phosphosulfide (q-CoPS) was fabricated through a “gas-phase phosphorus vulcanization-quenching” treatment (Fig. 8a).²¹⁰ The crystalline/amorphous ratio of a-CoPS could be altered by regulating the initial quenching temperature to obtain abundant active Co sites at the crystalline/amorphous heterointerface, which could efficiently accelerate the reaction kinetics for electrocatalytic hydrogen evolution and hydrazine oxidation. A high ECSA could also be obtained owing to the ample exposure of these Co sites. Only overpotentials of 90 and 390 mV were required to reach a current density of 1 A cm⁻² for the HER and HzOR on q-CoPS, respectively. Furthermore, we constructed Ni foam-supported N-doped carbon layer-encapsulated N-species-incorporated Co₂P nanosheets arrays (Ni–CoP@NC) through electrodeposition, soaking, ions exchange and phosphorization processes.²⁴ A pronouncedly twisted and coarse structure on high-specific-surface microspheres was constructed in Ni–CoP@NC, providing a 30-fold ECSA compared with the original Ni foam. Benefitting from the highly exposed Co active sites with the synergistic effect of heteroatom incorporation and Ni-doped carbon encapsulation, Ni–CoP@NC could deliver a high current density of 1 A cm⁻² at low potentials of −143 and 51 mV for the HER and HzOR, respectively. When assembling Ni–CoP@NC in a hydrazine-assisted water electrolyzer, only 0.49 V was required to achieve 1 A cm⁻², which is a 5-fold reduction in energy consumption compared with conventional water electrolyzer.

Fig. 8 (a) Schematic of the synthesis process of q-CoPS/CF. Reproduced with permission.²¹⁰ Copyright 2024, Elsevier. (b) Static contact angles. (c) Adhesive force measurements of the gas bubbles on the surface of Ni–Co–Se. Reproduced with permission.²¹² Copyright 2020, The Royal Society of Chemistry.

As gas production reactions, the management of gas products on the surface of electrocatalysts in the HER and HzOR processes is of great significance as the mass transport of hydrazine and hydroxide ions can be hindered by the accumulation of gas products of hydrogen and nitrogen in the flow field.²¹¹ Thus, it is necessary to reasonably regulate the hydrophilicity of the catalyst surface for hydrazine-assisted water electrolysis. Employing a bubble template method, Feng et al. fabricated a superaerophobic surface in Ni foam-supported porous-structure Ni_0.6Co_0.4Se nanosheet arrays (Ni–Co–Se).²¹² The superaerophobic nature of Ni–Co–Se induced by the porous structure could efficiently weaken the gas product adhesion and accelerate the bubble leaving for oxygen during the OER process and nitrogen during the HzOR process (Fig. 8b and c). A low potential of 0.4 V was required to achieve a current density of 300 mA cm⁻² for the HzOR with the assistance of Ni–Co–Se.

Given the progress in the development of efficient electrocatalysts for hydrazine-assisted water electrolysis, further improvement in electrocatalytic performance can be expected with the assistance of the above-mentioned strategies (Table 4). The combination of different strategies in the design of new-type electrocatalysts is desirable, which may promote the field of hydrazine-assisted water electrolysis to a new level.

Table 4 Electrocatalytic performance of the electrocatalysts with optimized defect/lattice/micromorphology under alkaline conditions. η(x)_j=x represents the overpotential at a current density of x mA cm⁻². E(x)_j=x represents the applied voltage at a current density of x mA cm⁻²

Catalyst	η(x)j=x/mV (mA cm^−2) (vs. RHE) for the HER	η(x)j=x/mV (mA cm^−2) (vs. RHE) for the HzOR	E(x)j=x/mV (mA cm^−2)	Ref.
Rh–Rh2O3/C	12(10)	361(10)	161(100)	45
D/Ni–Cu–Se	88(10)	—	268(10)	42
CoH–CoPv@CFP	77(10)	269(10)	230(500)	203
Ru–Co3O4|VO/CF	49(100)	234(100)	700(100)	43
HE-LH-S	—	570(50)	—	139
Cu1Co2–Ni2P/NF	51(10)	278(10)	160(10)	206
			390(100)
FeNi–CoP/NF	36(10)	236(10)	163(1500)	19
q-CoPS	90(1000)	270(1000)	390(500)	210
Ni–CoP@NC	143(1000)	381(1000)	490(1000)	24
Ni–Co–Se	—	730(300)	—	212

---

Design strategies of composition optimization, heteroatom doping, heterostructure construction, alloy forming, defect regulation, lattice strain and micromorphology control are discussed in this section. The electronic structure of the active sites can be tuned by heteroatom doping, heterostructure construction, alloy forming and defect regulation to improve the intrinsic activity of electrocatalysts. The sufficient exposure of constructed active sites and mass transfer can be obtained by micromorphology control. In the case of material composition optimization, it is fundamental and necessary to be used in combination with other strategies. Actually, advanced electrocatalysts are usually developed using the combination of several strategies to obtain optimized active sites with sufficient accessibility.

4. Application expansion

Beyond simply decreasing the cell voltage in electrolyzers, the incorporation of electrocatalytic hydrazine oxidation in water electrolysis presents specific advantages in application expansion of strategy combination and device construction, benefitting from the qualitative change caused by the substantial reduction of the required cell voltage and flexible electrochemical properties. Herein, we introduce three significant and unique application expansions for hydrazine-assisted water electrolysis systems, including a potential coincidence region, battery-combined self-powered hydrogen production systems and direct seawater splitting.

4.1. Potential coincidence region

As discussed above, the thermodynamic equilibrium potentials of the HER and HzOR are 0 and −0.33 V vs. RHE. A potential coincidence region exists between the HER and HzOR, which can bring some qualitative change for hydrazine-assisted water electrolysis combing these two electrocatalytic reactions. However, highly efficient HER/HzOR bifunctional electrocatalysts with combined overpotentials of less than 0.33 V are required to make this potential coincidence region significant. Thus, we developed an HER/HzOR bifunctional electrocatalyst of Ni foam-supported interfacial heterogeneous Ni₂P/Co₂P microspheres (NiCoP/NF) to prove the intrinsic advantage of the potential coincidence region and fabricate a self-activated/propelled hydrazine-assisted water electrolysis system.²¹ Benefitting from the strong coupling effect between Ni₂P and Co₂P with unique 3D microsphere structure, NiCoP/NF exhibited excellent HER and HzOR activity, requiring overpotentials of only 70 and 230 mV at 10 mA cm⁻², respectively. When employed in hydrazine-assisted water electrolysis, NiCoP/NF could deliver current densities of 100 and 200 mA cm⁻² with cell voltages of 107 and 212 mV, respectively. As shown in Fig. 9a, there is a potential coincidence region of about 0.1 V between the HER and HzOR polarization curves on NiCoP/NF, while the absence of a potential coincidence region on Ni foam-supported interfacial heterogeneous Ni₂P microspheres (NiP/NF). Benefitting from the potential coincidence region, a self-activated/propelled water electrolysis could be achieved on NiCoP/NF, observing the generation of bubbles on its surface without external electricity input (Fig. 9b). Considering the intermittent nature of renewable energy resources such as solar, tidal and wind, the ability to operate intermittently is necessary for energy conversion technologies. As shown in Fig. 9c and d, the self-activated electrolysis on NiCoP/NF with potential coincidence region could operate stably for 10 intermittent periods without an activation process compared with the energy loss of approximately 40% for the non-self-activated electrolysis on NiP/NF.

Fig. 9 (a) Potential coincidence region between the HER and HzOR polarization curves of NiCoP/NF. (b) Photographs of NiCoP/NF and NiP/NF in hydrazine-assisted alkaline electrolyte. Comparison of (c) non-self-activated electrolysis on NiP/NF and (d) self-activated electrolysis on NiCoP/NF. Reproduced with permission.²¹ Copyright 2023, the American Chemical Society. (e) Photographs of gas production behavior and (f) corresponding volume of non-self-activated and self-activated electrolysis. (g) Schematic of the gas production behavior on both electrodes with the increasing current density. Reproduced with permission.²⁰ Copyright 2024, the American Chemical Society. (h) Pourbaix diagram of different electrocatalytic reactions. (i) Potential coincidence region between the HER and HzOR. (j) I–t curve without external energy input. (k) I–t curves of the HER/OER and HER/HzOR systems. Hydrogen production rate (l) with and (m) without the assistance of hydrazine. Reproduced with permission.⁵⁰ Copyright 2024, John Wiley and Sons. (n) Polarization curves of the HER, OER and HzOR in different electrolytes. (o) Electrocatalytic performance and (p) energy consumption for 1 N m³ hydrogen of different water electrolysis systems. Reproduced with permission.⁴⁹ Copyright 2024, John Wiley and Sons.

Furthermore, we further investigated the gas production behaviors in this unique self-activated/propelled water electrolysis system.²⁰ Prior to this investigation, an optimized bifunctional electrocatalyst of Ni foam-supported interfacial heterogeneous Fe₂P/Co₂P microspheres (FeCoP/NF) was designed, exhibiting overpotentials of 10 and 203 mV at 10 mA cm⁻² for the HER and HzOR, respectively. The gas production behavior and volume of hydrazine-assisted water electrolysis systems with and without potential coincidence region at different current densities were recorded (Fig. 9e and f), respectively. Through the difference analysis between these two systems, we proposed the illustration of the gas production behavior on the electrode surface together with different current densities in self-activated water electrolysis system (Fig. 9g). In hydrazine-assisted self-activated water electrolysis, the catalyst surface supports both electrochemical and potentially non-electrochemical hydrazine splitting at low current densities. This dual interaction between the HER and HzOR in a non-cross-linked manner helps explain the observed gas production behavior. This process can be divided into three stages including the self-activated phase, the conventional electrochemical phase, and their combined operation. Initially, at low current densities, the system produces nitrogen and hydrogen in equal proportions at neighboring active sites, a process that can be considered a non-electrochemical mechanism or the functioning of many small fuel cells on the catalyst surface. As current density increases, electrochemical gas production, driven by an external electrical input, begins to dominate, and gradually takes over. At high current densities, the self-activated phase disappears, leaving a conventional hydrazine-assisted seawater electrolysis system as the sole mechanism for gas production.

Similarly, Yao et al. reported excess hydrogen output in hydrazine-assisted water electrolysis (Fig. 9h–m).⁵⁰ Interestingly, when different P-doping Co-based electrocatalysts of P-Co/Co₃O₄ and P-Co/Co₃O₄/Co(OH)₂ were applied for the HER at the cathode and the HzOR at the anode in hydrazine-assisted water electrolysis, respectively, the system could operate spontaneously with an external circuit current at the input cell voltage of 0 V. The open circuit potential was about 0.22 V, indicating the potential co-generation of hydrogen and electricity in a small voltage range with the maximum power density of 2.2 mW cm⁻². Excess hydrogen production with a faradaic efficiency of 104.1% was also demonstrated.

By utilizing a pH gradient across a membrane to harvest electrochemical neutralization energy, Yu et al. further demonstrated a bipolar membrane-enabled hydrazine-assisted acid–alkaline dual–electrolyte seawater electrolysis system for the co-generation of hydrogen and electricity.⁴⁹ A trifunctional electrocatalyst of 2D layered PtTe nanosheets (e-PtTe NSs) for the HER, oxygen reduction reaction (ORR) and HzOR was fabricated to support this system, requiring overpotentials of 136 and 610 mV at a current density of 10 mA cm⁻² for the HER and HzOR, respectively. As shown in Fig. 9n, the potential coincidence region could be enlarged by applying different electrolytes at the anodic and cathodic compartments separated by a bipolar membrane. Specifically, the alkaline HzOR could be combined with the acidic HER, providing a potential coincidence region of 0.21 V at 50 mA cm⁻², which can be the voltage output in this hydrogen/electricity co-production system. Fig. 9o and p show the polarization curves and electricity consumption for different hydrogen production systems, respectively, further proving the advantage of potential coincidence region in hydrazine-assisted water electrolysis for electricity economization, hydrogen production enhancement and hydrogen/electricity co-generation. Other hydrazine-assisted water electrolysis systems for the co-generation of hydrogen and electricity have also been explored on electrocatalysts such as Co-based zeolitic imidazolate framework (p-ZIF-67),⁵¹ carbon nanofiber-embedded heterostructured RuTe₂–Ru nanoparticles (RuTe₂–Ru@CNFs)⁵² and S,N-codoped carbon nanofiber-anchored Ru nanoparticles (Ru@SNCNFs).⁵³ To further take advantage of the potential coincidence region in hydrazine-assisted water electrolysis, more highly efficient bifunctional HER/HzOR electrocatalysts are required and the related investigations on application expansion are desirable.

4.2. Battery-combined self-powered hydrogen production

For the better combination of renewable resources such as solar energy and hydrazine-assisted water electrolysis, a relay station is required to store excess energy during the daytime and support the operation of hydrogen production at night to constitute a self-powered hydrogen production system. To further utilize developed bifunctional HER/HzOR electrocatalysts for hydrazine-assisted water electrolysis, it is desirable to incorporate the electro-oxidation of hydrazine in different batteries to propose new-type battery-combined self-powered hydrogen production systems for the improvement of the integrity of system and the generation of added value.

Direct hydrazine fuel cells (DHzFC) commonly consist of an ORR electrocatalyst on the cathode and HzOR electrocatalyst on the cathode.²¹³

Overall reaction: N₂H₄ + O₂ → N₂ + 2H₂O (32)

Anode reaction: N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻ (33)

Cathode reaction: O₂ + 2H₂O + 4e⁻ → 4OH⁻ (34)

Qian et al. investigated their application to power a hydrazine-assisted water electrolysis system with a bifunctional electrocatalyst of Ni foam-supported Ni₃N–Co₃N heterointerface-abundant hierarchical porous nanosheet arrays (Ni₃N–Co₃N PNAs/NF).²¹³ These heterointerfaces were proven to optimize the hydrogen adsorption energy for the HER and promote the hydrazine dehydrogenation processes for the HzOR, thus endowing Ni₃N–Co₃N PNAs/NF with an excellent bifunctional electrocatalytic performance. In the assembled DHzFC, a stable open-circuit voltage of about 1.0 V and a maximum power density of 60.3 mW cm⁻² were obtained, which could be used to power the hydrazine-assisted water electrolyzer employing Ni₃N–Co₃N PNAs/NF as both electrodes.

Furthermore, a 2D Ru-enriched metal–organic framework (NiRu-ABDC) was fabricated and applied in a hydrazine-assisted hydrogen production system powered by a hydrazine/hydrogen peroxide fuel cell.⁶⁰

Overall reaction: N₂H₄ + 2H₂O₂ + 4OH⁻ + 4H⁺ → N₂ + 8H₂O (35)

Anode reaction: N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻ (36)

Cathode reaction: 2H₂O₂ + 4H⁺ + 4e⁻ → 4H₂O (37)

By replacing the ORR with the electro-reduction of H₂O₂, a higher theoretical open circuit voltage of 2.09 V was obtained compared with that of 1.56 V for DHzFC. Employing NiRu-ABDC as the anode and Pt net as the cathode, the hydrazine/hydrogen peroxide fuel cell achieved a high open circuit voltage of 1.74 V and a maximum power density of 386.90 mW cm⁻², which could power hydrazine-assisted water electrolysis with a hydrogen production rate of 14.3 mol h⁻¹ m⁻² (Fig. 10a–e).

Fig. 10 (a) Photograph of the open circuit voltage. (b) Open circuit voltages. (c) Discharging polarization curves and the corresponding power densities. (d) Hydrogen production rate. (e) Schematic of the self-powered hydrogen production system. Reproduced with permission.⁶⁰ Copyright 2024, the American Chemical Society. (f) Schematic of a Zn–Hz battery. (g) Discharge/charge curves at different current densities. (h) Charge and discharge curves. (i) Charge and discharge voltage profiles. (j) Cycling stability test at 5 mA cm⁻². Reproduced with permission.⁵⁴ Copyright 2022, John Wiley and Sons.

Combining the HER and HzOR in an aqueous metal-redox bifunctional electrocatalyst battery, Wang and coworkers designed a new-type aqueous Zn–hydrazine (Zn–Hz) battery with as-fabricated 3D carbon decorated with core–shell Mo₂C@C and Ni@C (Mo₂C/Ni@C/CS) as the cathode and Zn foil as the anode.⁵⁴

Discharge process:

Anode:

Zn → Zn²⁺ + 2e⁻ (38)

Zn²⁺ + 4OH⁻ → Zn(OH)₄²⁻ (39)

Cathode:

2H₂O + 2e⁻ → H₂ + 2OH⁻ (40)

Overall discharge reaction:

Zn + 2H₂O + 2OH⁻ → H₂ + Zn(OH)₄²⁻ (41)

Charge process:

Anode:

Zn(OH)₄²⁻ → Zn²⁺ + 4OH⁻ (42)

Cathode:

1/2N₂H₄ + 2OH⁻ → 1/2N₂ + 2H₂O + 2e⁻ (43)

Overall charge reaction:

Zn(OH)₄²⁻ + 1/2N₂H₄ → 1/2N₂ + Zn + 2OH⁻ + 2H₂O (44)

Overall reaction:

1/2N₂H₄ → 1/2N₂ + H₂ (45)

The Zn–Hz battery could generate hydrogen through electrocatalytic hydrogen evolution during the discharging process and enable the electro-oxidation of hydrazine during the charging process (Fig. 10f–j). The Mo₂C/Ni@C/CS-assembled Zn–Hz battery demonstrated an ultrahigh energy efficiency of more than 96% and a stable operation at 50 mA cm⁻² over 600 cycles. Other Zn–Hz battery-powered hydrogen production systems using electrocatalysts of N-doped carbon decorated with cubic α-MoC nanoparticles merging Ru nanoclusters and single atoms (α-MoC/N–C/Ru_NSA),²¹⁴ CoP/C with Co vacancies⁵⁶ and NiCoPt-10 alloy²¹⁵ have also been explored.

4.3. Direct seawater splitting

In conventional seawater electrolysis, the efficiency of electrocatalysts in selectively promoting the OER at the anode is critical to the success of the process. Unlike pure water, seawater is a complex mixture containing various ions (such as Cl⁻, Na⁺, SO₄²⁻, Mg²⁺, and Ca²⁺), solid particles and microbial contaminants. A key challenge is the presence of Cl⁻ in seawater, with concentrations exceeding 0.5 mol kg⁻¹, which introduces complex chlorine-related chemistry in the electrolysis process. Factors such as temperature, pH, and applied voltage collectively determine which chloride-based reactions will compete with the OER at the anode. Dionigi et al.²¹⁶ developed a Pourbaix diagram (Fig. 11a), assuming a chloride concentration of 0.5 mol L⁻¹, to depict the chloride chemistry involved in direct seawater splitting. Under acidic conditions, the chloride electrolysis reaction (ClER) becomes the dominant competing process, as follows:

2Cl⁻ → Cl₂ + 2e⁻ (46)

Fig. 11 (a) Pourbaix diagram for different reaction mechanisms in direct seawater electrolysis. (b) Overpotential range where only the OER is thermodynamically possible. Reproduced with permission.²¹⁶ Copyright 2016, John Wiley and Sons. (c) Pourbaix diagram for different reaction mechanisms in direct seawater electrolysis. (d) Energy saving and chlorine-free hydrogen production. Reproduced with permission.⁶³ Copyright 2021, Springer Nature.

Alternatively, under alkaline conditions, the primary reaction of chlorine chemistry is the hypochlorite formation reaction, as follows:

Cl⁻ + 2OH⁻ → ClO⁻ + H₂O + 2e⁻ (47)

The thermodynamic equilibrium potential difference between the hypochlorite formation reaction and OER under alkaline conditions was significantly larger, about 0.48 V. This means that direct seawater electrolysis can operate effectively in an alkaline environment at a maximum overpotential of 0.48 V without interference from chlorine chemistry. Consequently, an “alkaline design criterion” was proposed for optimizing selective OER catalysis in direct seawater electrolysis (Fig. 11b), as follows:

η_OER ≤ 480 mV (pH > 7.5) (48)

However, this overpotential is commonly not sufficient to achieve high current densities, especially for industrial application. The incorporation of the HzOR in direct seawater electrolysis offers a solution to achieve energy-saving and chlorine-free seawater electrolysis (Fig. 11c and d).⁶³ An MXene-wrapped Cu foam-supported NiCo-decorated carbon mesoporous nanosheet array (NiCo@C/MXene/CF) was applied in hydrazine-assisted seawater electrolysis. The electrocatalytic chlorine-related reactions were avoided due to the low input cell voltages, which were thermodynamically unfavorable for chlorine electrochemistry even at large current densities. A high hydrogen production rate of 9.2 mol h⁻¹ g_cat⁻¹ at 500 mA cm⁻² was obtained. The much lower electricity expense of 2.75 kW h m⁻³ H₂ at a current density of 500 mA cm⁻² and the enhanced stability indicate the effectiveness of this combined application expansion of hydrazine-assisted seawater electrolysis.

5. Conclusions and outlook

In this review, we discussed the recent advances in hydrazine-assisted water electrolysis systems, focusing on the development of multi-functional electrocatalysts with enhanced electrocatalytic performance and application expansion arising from the intrinsic advantages of hydrazine electro-oxidation with its incorporation in water electrolysis systems. We provided a macroscopic understanding of hydrazine-assisted water electrolysis with the emphasis on its advantages compared with conventional water electrolysis system and other alternative hybrid water electrolysis systems, including thermodynamic equilibrium potential, simple reaction pathways and fast reaction dynamics. The latest advancements and notable achievements in the development of highly efficient bifunctional HER/HzOR electrocatalysts were introduced and discussed, focusing on the design strategies of composition optimization, heteroatom doping, heterostructure construction, alloy forming, defect regulation, lattice strain and micromorphology control. The specific application expansions for hydrazine-assisted water electrolysis were summarized, explaining the existence and merit of the potential coincidence region, introducing different battery-combined self-powered hydrogen production systems and illustrating the applicability of hydrazine electro-oxidation in direct seawater electrolysis.

On the one hand, in terms of the development of electrocatalysts with enhanced electrocatalytic performance, the cost-effective non-noble metal-based electrocatalysts still show relatively unsatisfactory activity compared with noble metal-based electrocatalysts. The composition, electronic structure and micromorphology should be optimized and the structure–activity relationship should be obtained for the further development of electrocatalysts. Computational studies and in situ characterization techniques are required to further investigate the reaction mechanism on specific active sites and indicate the direction of future research. The reconstruction process should be noted and the multi-dimensional long-term stability of electrocatalysts should be highlighted. Besides, as mentioned above, hydrazine-assisted water electrolysis is a specific-application system and its practical operating conditions are based on the plant specific requirements. Thus, the electrocatalytic activity and stability of these catalysts at low/high current densities in specific reaction environments should be highlighted.

On the other hand, regarding the application conditions and expansion, standard test conditions should be assigned to better evaluate the electrocatalytic performance, and the conditions should be formulated as close to practical application, including reaction temperature, electrode distance, analyte feed and flow dynamics and the cost of construction and maintenance. Especially, in hydrazine-assisted water electrolysis, the source of hydrazine needs serious consideration. Besides, new approaches of utilizing the potential coincidence region and new-type batteries for self-powered hydrogen production systems are highly desirable.

Considering the advancements, obstacles, and potential in this rapidly evolving field, we are confident that this innovative approach offers a promising solution to overcome the limitations of conventional water electrolysis, positioning it as a crucial component in the hydrogen economy. This review aimed to offer valuable insights for the advancement of hydrazine-assisted water electrolysis and contribute to future breakthroughs in this field.

Data availability

The datasets used and/or analyzed during the present study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22179065).

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