Paired electrolysis by regulated electronic distribution and lowering of overpotential with enhanced current density

Abstract

This review highlights the pairing of an oxidation half-reaction with a reduction half-reaction with compatibility at a lower potential for simultaneously generating desirable products at the electrode with current–voltage characterization. The cathodic hydrogen evolution (HER) or CO₂ electroreduction (eCO₂R) is accelerated by pairing with anodic oxidation, which facilitates an electrified simultaneous reaction with maximized atomic and energy efficiencies. First, we emphasize the anodic oxidation for obtaining value-added products to highlight its significance for pairing with the cathodic process. We also highlight by comparing the materials for anodic oxidation in the paired electrolysis process. The unique features of cell design include (a) an H-type electrochemical cell and (b) flow cell electrolyzers that are evaluated before discussing their interfacial electron distribution, free-standing electrodes, and significance, focusing on effective electrodes for H₂ production. Furthermore, the role of single atoms of transition elements, nanostructures and their effects in lowering the overpotential are included. A special emphasis was given to the electrode kinetics of simultaneous electrolysis with extension to prototypes for continuous paired electrolysis with a focus on critical factors in the fabrication process, device performance, and long-term production challenges. In light of the limitations and potentials, we discuss the challenges and solutions through electronic structure modification, device modification, and cell simplification as conclusive remarks and outlook.

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1 Introduction

A hybrid electrolysis system capable of coupling an alternative oxidation process at the anode with a reduction process at the cathode is of greater interest for lowering energy consumption and thermodynamic potentials to produce high-value products at both electrodes.¹ Electrolysis systems are recognized as comprising two half-cell reactions: anodic oxidation and cathodic reduction.² Nevertheless, most electrochemical transformations employ a single half-cell process, rendering the other ineffective. Conversely, paired electrolysis (Fig. 1), which utilizes both half-cell processes, has garnered far less interest.³ Paired electrolysis can be classified into four separate categories: parallel, sequential (or linear), convergent, and divergent electrolysis (Fig. 2).⁴ In paired electrolysis, the anodic process provides protons and electrons without the need for high operating potentials. Similarly, the cathodic process (such as hydrogen evolution reaction, CO₂ reduction reaction and nitrogen reduction reaction) enhances faradaic efficiency for producing the desired product using renewable electricity at a low full-cell voltage. This can be achieved using a cathode gas electrode in a traditional H-cell.⁵ The oxygen evolution reaction (OER) exhibits a high equilibrium potential of 1.23 V vs. RHE and sluggish kinetics, requiring substantial overpotentials that significantly increase the total cell voltage and overall energy consumption. To reduce the overpotential, alternative anodic oxidation reactions such as glycerol, urea, hydrazine,⁶ and other small-molecule oxidations have been explored. These reactions can have much lower equilibrium potentials (e.g., hydrazine oxidation: ∼ −0.33 V vs. RHE in alkaline media; urea oxidation: ∼0.37 V vs. RHE), enabling the CO₂RR at reduced cell voltages with the co-production of value-added chemicals.^5–7 Conventional anodic oxidation of organic substrates often yields low faradaic efficiencies for a single product due to multiple competing reaction pathways. Coupling cathodic electro-reduction with alternative anodic oxidation reactions can improve the overall charge utilization and enable the co-production of value-added chemicals, thereby reducing cell voltage and enhancing overall energy and material efficiency. Anodic oxidation reaction (AOR) is a thermodynamically favorable alternative to the OER because it operates at a lower potential.⁸ The AOR involves the direct oxidation of organic compounds (such as HMF,⁹ formaldehyde,¹⁰ alcohols,^11–13 hydrazine,¹⁴ ammonia,¹⁵ biomass¹⁶ and urea oxidation^17,18) and other chemical species at the anode surface. Similarly, electrocatalytic CO₂ reduction at the cathode has been a prevalent reaction in paired electrolysis, facilitating the transformation of this ubiquitous greenhouse gas into valuable chemicals such as CO, formate, methanol, and ethylene. Moreover, various carbon functionalization processes such as the arylation of aromatic substrates and aromatization of non-aromatic equivalents have been investigated in paired electrolysis. The potential of coupling such eCO₂R with suitable anodic oxidation can be promising in terms of the production of essential fuels at the anode in place of oxygen in an OER despite advanced techniques such as the usage of bipolar membrane (BPM) systems, featuring junction bonding contact at the interface of the electrode, suppressing the degradation of cells.¹⁹ This approach adheres to green chemistry principles by optimizing reactant usage and reducing by-products, satisfying atom economy standards. Consequently, the necessity for concurrent electrolysis at the anode to produce value-added products, in conjunction with cathodic H₂ generation, offers novel options. Multiple experiments have established the viability of co-producing high-value-added compounds in conjunction with the hydrogen fuel.^20,21 The urea electrooxidation reaction (UOR) offers a thermodynamically advantageous route with a reduced energy barrier, effectively delivering electrons for the associated cathodic hydrogen evolution reaction (HER).²² Coupling water oxidation at the anode with oxygen reduction at the cathode can enable the simultaneous electrosynthesis of hydrogen peroxide at both electrodes. In certain systems, catalyst reconstruction under electrochemical conditions promotes high selectivity, leading to elevated faradaic efficiencies for concurrent H₂O₂ production.²³ In practical operation, both H-type and flow cell electrolyzers facilitate anodic oxidation and cathodic reduction, providing controlled environments that influence intermediate adsorption and enhance product selectivity at each electrode.^24,25 Flow cell electrolyzers circulate electrolytes through continuous channels between anodic and cathodic compartments, using ion-exchange membranes to enable ionic conduction while preventing product crossover.²⁶ This architecture has been applied to paired electrolysis, such as coupling hydrogen evolution at the cathode with electrooxidation of biomass-derived compounds like furfural or formic acid at the anode, enabling simultaneous generation of H₂ and value-added chemicals.²⁷ The catalysts can comprise different structures such as single atoms, nanoparticles, bimetallic arrangements or polymetallic alloys and similar materials that interact within various atomic configurations.^28,29

Fig. 1 The paired electrolysis for the water oxidation or anodic oxidation reaction to produce valuable chemicals.

Fig. 2 Four categories of paired electrolysis. Schematic of the four types of paired electrolysis. (a) Parallel: two different substrates are independently oxidized and reduced at the anode and cathode to yield two distinct products. (b) Sequential (linear): both electrodes contribute to forming a single product via a shared intermediate generated at one electrode and consumed at the other. (c) Convergent: two different substrates are converted into distinct intermediates at separate electrodes, which subsequently combine to form one product. (d) Divergent: a single substrate is transformed into two distinct products, one produced at each electrode.

Given the distinct advantages of coupling two electrolysis processes to accelerate reactions in a hybrid configuration, this review aims to comprehensively summarize the current trends and achievements in paired electrolysis across a broad range of systems, including hydrogen generation, biomass electroreforming, and CO₂ fixation, with a focus on producing liquid and gaseous products at high concentrations. The review also seeks to identify mechanistic contributions to cathodic and anodic overpotentials, with the goal of minimizing energy consumption in coupled electrolysis. We introduce the fundamental concepts of H-cells and flow cells as electrolyzers for paired anodic and cathodic processes and discuss self-supported electrodes, highlighting recent strategies for anodic oxidation-based paired electrolysis using both noble and non-precious metal materials. The general approach and role of interfacial electron distribution and free-standing electrodes in coupling cathodic and anodic reactions are examined, followed by an exploration of the effects of single-atom catalysts, nanosheet atomic arrangements, and nanowire arrays, revealing advanced strategies for improving the electrode performance in simultaneous electrolysis. By consolidating these developments, this review aims to provide a foundation for designing next-generation paired electrolysis systems with superior efficiency, selectivity, and scalability.

2 Classifications of paired electrolysis

The different classification of paired electrolysis is summarized in Fig. 3. In the subsections, we will discuss the detailed explanation of each type of paired electrolysis.

Fig. 3 Parallel paired electrolysis. Top right: examples of possible anodic and cathodic counter reactions in electrosynthesis. Bottom: borohydride oxidation illustrated as an anodic counter reaction. Reprinted with permission,³⁰ copyright 2025, Wiley-VCH.

2.1 Parallel paired electrolysis

Parallel paired electrolysis exemplifies the principle of full-cell productivity, where both electrodes play an active role in chemical transformation instead of one functioning solely as a counter electrode. This system employs the anode to promote the oxidation of a specific substrate, resulting in the formation of a desired product, while the cathode enables the reduction of a different substrate, leading to the generation of another valuable compound. For example, the oxidation of borohydride is illustrated as an anodic counter reaction at the bottom of Fig. 3. This setup facilitates the simultaneous synthesis of two distinct value-added products, thereby optimizing the chemical yield for each unit of energy utilized. By carefully aligning anodic and cathodic reactions within similar kinetic and potential ranges, energy losses can be minimized, leading to enhanced overall process efficiency and sustainability. The practical potential of parallel paired electrolysis spans a wide array of applications, encompassing industrial redox processes, the production of green oxidants, and efforts in environmental remediation. Prominent instances encompass the concurrent production of peracetic acid and peroxodicarbonate for disinfection and bleaching, dual redox transformations in wastewater treatment, and the electrosynthesis of hydrogen peroxide and organic acids from renewable feedstocks. The incorporation of advanced electrode materials—such as functional material-based anodes and gas diffusion cathodes—expands the operational range by offering extensive electrochemical windows, exceptional catalytic stability, and adjustable surface characteristics that facilitate selective electrosynthesis. Although it holds significant potential, various factors presently constrain the scalability of parallel paired electrolysis. The primary obstacle is the potential for cross-contamination or parasitic interactions between anodic and cathodic products, which may reduce selectivity and yield. Ensuring effective chemical isolation alongside sufficient ionic conductivity necessitates meticulous selection of membranes and strategic compartmentalization of cells. Moreover, accurate potential alignment between the two half-reactions is crucial to avoid energy inefficiencies, unintended side reactions, or degradation of the electrodes. Overcoming these challenges by enhancing membrane engineering, optimizing reactor design, and implementing in situ process monitoring will be crucial for attaining high selectivity, durability, and operational robustness.

In conclusion, parallel paired electrolysis stands out as a strategically important and sustainable method in contemporary electrochemical synthesis. Through the complete utilization of both anodic and cathodic reactions, this approach facilitates energy-efficient, atom-economical, and environmentally friendly production methods, leading to the development of integrated electrochemical manufacturing systems that reduce energy waste and enhance product value.

2.2 Sequential (linear) paired electrolysis

Sequential, or linear, paired electrolysis is an electrochemical approach in which anodic and cathodic half-reactions are interconnected via a common intermediate, enabling cooperative contributions from both electrodes to produce a single value-added product. In contrast to parallel systems that produce two independent products, sequential electrolysis facilitates stepwise electron transfer during single-substrate transformation. The process can occur in two modes: first, a substrate is reduced at the cathode to create an intermediate that is then oxidized at the anode, or conversely, it begins with anodic oxidation followed by cathodic reduction.³¹ The intrinsic coupling of redox processes streamlines reactor design and facilitates the efficient use of both electrodes for a cohesive synthetic result. A clear example of this concept is presented by a linear paired electrolysis system facilitated by multiple redox processes, demonstrating how coordinated anodic and cathodic reactions can collectively improve energy and electron efficiency. The intrinsic advantage of sequential paired electrolysis is its high electron utilization and energy efficiency, realized through synchronized anodic and cathodic processes that collectively facilitate a single transformation. These systems are especially important in biomass valorization and fine chemical synthesis, where the selective activation of multifunctional molecules necessitates controlled, stepwise electron transfer.

The successful implementation of sequential paired electrolysis depends on the stabilization of reaction intermediates and the maintenance of kinetic synchronization between the electrode processes. Mismatched reaction rates may result in intermediate decomposition or parasitic reactions, thereby reducing the overall efficiency. The durability of redox mediators and the optimization of mass transport parameters are essential for maintaining long-term operational stability. Sequential paired electrolysis provides a cohesive and energy-efficient approach for the electrosynthesis of a single product. Utilizing shared intermediates and cooperative redox events leads to increased conversion efficiency, streamlined reactor design, and enhanced sustainability. This positions it as a developing and scalable platform for the electrochemical upgrading of biomass-derived compounds and fine chemicals, connecting laboratory-scale innovation with industrially relevant electrochemical synthesis.

2.3 Convergent paired electrolysis

Convergent paired electrolysis enhances synthetic capabilities by integrating two different substrates in a single electrochemical system to produce a cohesive, structurally intricate product. In this configuration, each substrate participates in an independent half-reaction at the anode or cathode, generating complementary reactive intermediates that subsequently combine in situ to produce the final molecule.³² This method establishes a robust foundation for the formation of carbon–carbon and carbon–heteroatom bonds, demonstrating considerable promise in organic synthesis, pharmaceutical development, and materials chemistry, where complex molecular structures are frequently necessary. The primary benefit of convergent electrolysis is its synthetic versatility, facilitating bond formations that are difficult or unattainable through traditional single-electrode or stepwise approaches. This versatility is, however, counterbalanced by strict selectivity requirements. Multiple reactive species are generated simultaneously within a confined electrochemical environment. Therefore, careful regulation of electrode potential, solvent polarity, and mass transport is essential to ensure selective combination of desired intermediates while suppressing competitive side reactions or overoxidation events.³³ An illustrative case is the convergent paired electrolysis of methylarene derivatives with 1,4-dicyanoarenes, facilitating arylative functionalization of benzylic C(sp³)–H bonds to yield 1,1-biarylmethane derivatives, which are fundamental structures in various pharmaceuticals and biologically active compounds. This example illustrates the synthetic diversity and mechanistic complexity of convergent paired electrolysis, emphasizing that the overall reaction efficiency and selectivity are determined by the intricate interaction between electron-transfer kinetics and the local electrochemical microenvironment. The efficiency and selectivity of these systems are fundamentally influenced by the interaction between electron-transfer kinetics and the local electrochemical microenvironment. Convergent electrolysis offers a streamlined and atom-efficient method for synthesizing complex molecular architectures by integrating oxidative and reductive processes within a single reactor. However, its practical implementation relies on accurate electrochemical modulation and reactor design strategies that stabilize transient intermediates while directing them toward the desired coupling pathway.

2.4 Divergent paired electrolysis

Divergent paired electrolysis converts a single substrate into two separate products, with one produced at the anode and the other at the cathode, thus optimizing resource use and energy efficiency. This approach, similar to divergent synthetic strategies in organic chemistry, facilitates the concurrent production of multiple value-added products via complementary redox pathways within a unified electrochemical system. In this configuration, a single feedstock experiences oxidative transformation at one electrode and reductive transformation at the other, ensuring that both electrodes actively participate in productive chemical conversion instead of merely serving as counter-electrodes.

Divergent electrolysis is notably resource- and energy-efficient, providing a viable method for converting biomass-derived platform molecules into compounds of industrial significance. This method integrates oxidation and reduction processes in one operation, resulting in optimal atom economy, minimized energy waste, and improved process intensification. The dual activity of both electrodes presents selectivity challenges, particularly regarding the control of electrode potentials, suppression of product crossover, and maintenance of reaction isolation to prevent undesired side reactions or product degradation. These factors are particularly important in flow-cell architectures, as mass transfer and inter-electrode communication significantly affect the overall system performance. An illustrative case is the varied electrochemical conversion of furfural, a compound derived from renewable biomass, performed in a continuous-flow electrochemical reactor featuring a narrow inter-electrode gap to improve mass transport and reaction kinetics.³⁴ This system facilitates the anodic oxidation of furfural to generate 2(5H)-furanone, while the cathodic reduction concurrently produces furfuryl alcohol and/or hydrofuroins. The two reaction zones are separated by an ion-exchange membrane to inhibit cross-reaction. The simultaneous production of these three bio-based derivatives highlights the potential of varied strategies to utilize oxidation–reduction complementarity for sustainable, multi-product synthesis. 2(5H)-Furanone is an important intermediate and monomer in the synthesis of poly(γ-butyrolactone) and can be hydrogenated to produce γ-butyrolactone, a commonly utilized green solvent.³⁵ Divergent paired electrolysis is an efficient and sustainable method for converting single substrates into multiple high-value products within a unified system. The successful implementation relies on precise electrochemical control, selective electrode design, and meticulous reactor engineering to ensure product specificity. Divergent electrolysis facilitates simultaneous oxidative and reductive transformations of a single feedstock, representing an integrated and energy-efficient method for green chemical production and biomass enhancement.

3 Anodic oxidation as an accelerator for value-added products

Paired electrolysis is an efficient electrochemical method that concurrently produces target chemicals via anodic and cathodic processes.³⁶ Electrochemistry offers a viable option, utilizing electricity as a redox agent to facilitate intricate reactions under standard chemical or photochemical conditions.³⁷ In contrast to conventional electrooxidation or electroreduction methods, paired electrolysis, which employs both anodic and cathodic reactions, presents a more pragmatic and energy-efficient solution. In this section, we will discuss the anodic oxidation reaction. The AOR of organics involves modest faradaic efficiency under near-ambient conditions.³⁸ Anodic oxidation reaction (AOR) is a thermodynamically favorable alternative to the traditional OER because it operates at a lower potential, thus producing an alternate avenue for H₂ production in an accelerated mode.³⁹ The AOR involves directly oxidizing organic compounds and other chemical species at the anode surface. For example, Fig. 4a displays the conventional water splitting in an alkaline medium. Similarly, the coupling of formaldehyde oxidation (FOR) with H₂ production in an alkaline medium offers a transformative shift to the HCHO-oxidation-paired HER (Fig. 4b) under alkaline conditions, and hence, a paired electrolysis is desired for accelerated H₂ production at the cathode.

Fig. 4 (a) Conventional electrocatalytic water splitting under alkaline conditions. (b) Electrocatalytic water reduction coupled with HCHO oxidation under alkaline conditions.

A thermodynamically favorable AOR pathway for supplying electrons for the coupled cathodic HER process, including enhanced stability of the integrated electrochemical system, is the primary need that drives the design of a series of paired electrolysis, which requires tremendous attention for the advantages of high process capacity. Most HER or CO₂RR being paired with selective oxidation of organics with the AOR, modest faradaic efficiency (FE), has opened new directions. Table 1 describes the performance metrics analysis with AOR overpotentials against cathodic counterparts.

Table 1 A series of materials for anodic oxidation in the paired electrolysis process

S. no.	Material	Type of reaction	Type of electrolyser	Electrolyte	Current density	Overpotential	Stability	Tafel slope (mV dec^−1)	Faradaic efficiency	Effects	Ref.
1	Boron-doped diamond thin film electrode (BDD)	Anodic oxidation of 2-naphthol	Flow cell	2-Naphthol + 1 M H2SO4	60 mA cm^−2	0.65–2.25 V	—	—	—	• At 0.65–2.25 V (versus SHE), oxidation occurs	40
										• Oxidation heights: two oxidative peaks appear: initial peak: +1.12 V vs. SHE. 2nd peak: about +2.0 V against SHE
										Dependence: peak currents rise linearly with 2-naphthol concentration in the examined range
2	Iridium nanotubes (Ir NTs)	Oxygen evolution reaction (OER) and nitrate reduction reaction (NO3-RR)	H-type cell	0.1 M HClO4	10 mA cm^−2	245 mV	—	49.02	84.7%	• OER has increased activity and durability, requiring just 245 mV for 10 mA cm^−2	41
										• 84.7% Faraday efficiency and 921 µg h^−1 mg^−1 ammonia output rate for NO3-RR indicates high activity
										• OER and NO3-RR electrocatalysts, Ir NTs, are advanced
3	Pd@Ir3L nanocubes	Oxygen evolution reaction (OER)	Rotating disk electrode (RDE) method	0.1 M HClO4	10 mA cm^−2	245 mV	—	59.3	—	• Pd@Ir3L nanocubes surpass commercial Ir/C catalysts with optimal activity and durability	42
										• 245 mV overpotential at 10 mA cm^−2 and 3.33 A mgIr^−1 mass activity at η = 300 mV
4	Ru1–NiO	Oxidation of 5-hydroxymethylfurfural (HMF)	H-type cell	1 M KOH	10 mA cm^−2	1.283 V	—	—	70%	• In the electrooxidation of 5-hydroxymethylfurfural (HMF)	43
										• Ru1–NiO has a low potential of 1.283 V at 10 mA cm^−2
										• Ideal 90% 2,5-diformyl furan (DFF) selectivity
5	Pt/Ni (OH)2	5-Hydroxymethylfurfural (HMF) oxidation	H-type cell	1.0 M KOH	37.31 mA cm^−2	1.50 V	—	38.7	98.7%	• The Pt/Ni (OH)2 electrode has a lower Tafel slope (14.3 mV dec^−1) than Ni (OH)2 (38.7 mV dec^−1), indicating faster Ni^3+ formation	44
										Pt/Ni (OH)2-catalyzed hydrogen evolution process (HER) and HMF oxidation convert energy more efficiently than water splitting because the two-electrode design lowers cell voltage by 200 mV
6	MnOx	5-Hydroxymethylfurfural (HMF) oxidation	H-type cell	0.1 M H2SO4	4 mA cm^−2	1.6 V	—	—	33.8%	• Positive potentials (1.6 and 2.0 V vs. RHE) assist electrochemical oxidation	45
										• Annealing at 400 °C for 2 h was the most effective method, but it produced some Mn^3+ ions
										• The MnOx anode spontaneously produced 53.8% FDCA in a pH-1 H2SO4 solution without altering pH or other components
7	Ruthenium-titanium alloy oxide (RuTiO)	Iodide oxidation reaction	Rotating disk electrode (RDE) method	0.1 M HClO4	10 mA cm^−2	1.09 V	—	—	100%	• For IOR-based electrolysis, the RuTiO catalyst requires just 1.09 V to generate a current density of 10 mA cm^−2	46
										• Faradaic efficiency for cathode hydrogen generation is around 100%
										• IOR-based hydrogen synthesis is more energy-efficient than water electrolysis
8	PdAg/NF catalyst	Ethylene glycol oxidation	Rotating disk electrode (RDE) method	0.5 M KOH + 1 M ethylene glycol	10 mA cm^−2	980 mV	2 h	252	92%	• In situ-grown PdAg electrocatalyst on nickel foam	47
										• Electro-oxidation of ethylene glycol requires only 0.57 V compared to a reversible hydrogen electrode at 10 mA cm^−2
										• Effective glycolic acid synthesis at 300 mA cm^−2 current density without oxygen evolution
										• For 100% faradaic hydrogen synthesis using a Pt cathodic catalyst, a cell potential of 1.02 V is needed
9	NiF3/Ni2P@CC-2	Urea oxidation reaction (UOR)	H-type cell	1.0 M KOH + 0.33 M urea	50 mA cm^−2	1.83 V	5000 cyclic	75	95.07%	• A current density of 10 mA cm^−2 requires a potential of 1.36 V vs. RHE	48
										• Keeps catalytic activity after 5000 cyclic voltammetry tests
										• Provides 50 mA cm^−2 at 1.83 V
										• Runs continuously for over 10 hours
10	CoS2/MoS2	Urea electrooxidation	H-type cell	1.0 m KOH + 0.5 m urea	10 mA cm^−2	1.29 V	60 h	165	—	• The CoS2–MoS2 catalyst can generate 10 mA cm^−2 at 1.29 V	49
										• Over 60 hours, the catalyst is durable
										• Surface charge distribution modification in CoS2/MoS2 Schottky heterojunctions allows urea molecule chemical group adsorption and fracture
11	Nickel ferrocyanide (Ni2Fe (CN)6)	Urea electrooxidation	H-type cell	1.0 M KOH with 0.33 M urea	255 mA cm^−2	1.35 V	—	—	90%	• High activity and stability of Ni2[Fe(CN)6] surpass typical Ni-based catalysts, and are supported on Ni foam	50
										• Achieves 100 mA cm^−2 anodic current density at 1.35 V (overpotential 0.98 V)
12	Mo doped Co4N nanoarrays (Mo–Co4N	Methanol oxidation reaction (MOR)	H-type cell	1 M KOH	10 mA cm^−2	1.356	60+ hours	42	100%	• Overpotential: 45 mV at 10 mA cm^2	51
										• Tafel slope: 42 mV dec^−1
										• Oxidation potential: 1.356 V at 10 mA cm^2
										• Cell voltage: 1.427 V for 10 mA cm^2
										• Hydrogen evolution and formate production are nearly 100%
										• Excellent stability for 60+ hours
13	MnO2	Alcohol oxidation	H-type cell	0.005 M H2SO4	10 mA cm^−2	270 mV	10 h	352.6	100%	• At 10 mA cm^−2, oxidizing glycerol into value-added chemicals like formic acid requires a potential of 1.36 V (vs. RHE), 270 mV lower than the oxygen evolution reaction (OER)	52
										• Manganese oxide retains electrocatalytic hydrogen generation with glycerol oxidation for 865 hours, but the OER lasts only 10 hours

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3.1 Anodic organic oxidation in large-scale electrochemical processes

Industrial electrolyzers at large current densities, wherein the built-in outer electric field enables multiple charge redistribution.⁵³ Both H₂ production and decarbonization/CO₂RR at the cathode can be paired with the AOR to reduce energy consumption, with significantly enhanced clean fuel production, and for effective CO₂ mitigation at ambient temperature.⁵⁴ Significant steps to integrate input electrical energy-savings, simultaneously cathodic H₂ production, and upgrade of anodic oxidation for value-added production warrant highly electroactive material design and fabrication in electrode cells.^55,56 A strategy that can lower the cell voltage has been developed recently by chemical-assisted water splitting, in which easily oxidized species such as urea, ammonia, hydrazine, and methanol, as sacrificial agents for electrochemical oxidation, replace the OER.⁵⁷ In urea–oxidation-assisted HER, the cleavage of the C=N bond in urea should be avoided for the selective conversion of urea to N₂ during the electrochemical UOR, which involves electrons to the terminal O moiety.²² The scope of paired electrolysis has been broadened to significantly lower full-cell voltage compared with the state-of-the-art process at the same current density. Gas cross-over during water electrolysis turned H₂ production cost (US$4 Kg⁻¹) through water splitting, which can be tackled by mitigating the conventional OER half-cell process with alkaline electrooxidation of the organic substrate, which challenges favourable thermodynamics consideration.⁵⁸ The standard electrooxidation potential of HMF is much lower than the standard potential for the OER (1.23 V, which enables the replacement of the OER with the HMFOR for combined ∼200% FE). Furthermore, HMFOR-HER, competitive OER restricts the potential range for HMFOR, resulting in low current densities and charge utilization efficiencies (low FE). It is crucial to limit the degradation of HMF to achieve an industrial-level current density (>400 mA cm⁻²) from the electrooxidation half-cell process, via faster mass transfer and expedited activity of anodic electrode for rapid electrolysis.⁵⁹ While defect and interface engineering are key factors for a more in-depth effect of electronic structure for simultaneous organic electrooxidation half-cell, the choice of organic substrate for expedited current density is desirable. The faster surface deprotonation capability of the HMFOR through defect site engineering could be dominant, suppressing the OER at high potentials.⁶⁰ Therefore, the principles of the electrooxidation half-cell for expediting advancements and associated challenges will continue to re-investigate the effect on the cathodic production of H₂ or similar products. The inherent sluggish kinetics and complex development paths due to the electrooxidation half-cell reaction of organic oxidants with multiple electrons and intermediate transfer electrolysis make the process challenging. A comparison of the anode/cathode pairs is presented in Table 2 to highlight the pairing of anode–cathode for anodic oxidation with the HER on the cathode, involving intertwined electrochemical processes involving intermolecular electron transfer with reduced cell potential. The amperometric measurement at fixed voltage vs. RHE is carried out using the anodic electrode material coating, the signals of intermediates and products. Current density range and FE% enable the determination of the efficiency of pairing two electrodes with suitable materials for larger-scale production, as the significant scope of these simultaneous processes is listed in Table 2.

Table 2 Comparison of anode/cathode pairs

S. no.	Anode	Type of reaction	Type of electrolyser	Anolyte	Cathode	Type of reaction	Catholyte	Current density	Faradaic efficiency	Cell voltage	Ref.
1	Nickel–iron on nickel foam (NiFe/N)	Furfuryl oxidation	H-type cell	1 M KOH + 50 mM furfural	Cobalt catalyst (Co/Mo2TiC2-700)	HER	1 M KOH	400 mA cm^−2	120% to 150%	3.899 V	61
2	A-Co-Ni2P	HMFOR	Membrane electrode assembly (MEA electrolyzer)	1 M KOH + 0.1 M HMF	Pt-coated Ti	HER	1 M KOH + 0.1 M HMF	1290 mA cm^−2	100%	1.50 V	62
3	CeF3@Ni3N/CC	Methanol oxidation reaction (MOR)	H-type cell	1.0 M KOH + 1.0 M MeOH	CeF3@Ni3N/CC	HER	1.0 M KOH + 1.0 M MeOH	300 mA cm^−2	93%	1.86 V	63
4	CA–Ni/NiO@NCS	Urea oxidation reaction (UOR)	H-type cell	1 M KOH + 0.5 M urea	CA–Ni/NiO@NCS	HER	1 M KOH + 0.5 M urea	10 mA cm^−2	—	1.475 V	64
5	N–NiFe/WRIF	UOR	H-type cell	1 M KOH + 0.33 M urea	N–NiFe/WRIF	HER	1 M KOH + 0.33 M urea	100 mA cm^−2	97.4%	1.58 V	65
6	Phosphorized CoNi2 S4 yolk–shell spheres (PCoNi2 S4 YSSs)	UOR	H-type cell	1 M KOH + 0.5 M urea	P–CoNi2 S4 YSSs	HER	1 M KOH + 0.5 M urea	10 mA cm^−2	—	1.402 V	66
7	NiO/Ru	Hydrazine oxidation reaction	H-type cell	1.0 M KOH + 0.5 M N2H4	NiO/Ru	HER	1.0 M KOH	100 mA cm^−2	99%	0.22 V	67
8	NiMo	Hydrazine oxidation reaction	H-type cell	1 M KOH + 0.5 M N2H4	Ni2P	HER	1 M KOH + 0.5 M N2H4	500 mA cm^−2	—	343 mV	68
9	Ni–Co(OH)2 NSAs	Iodide oxidation reaction	H-type cell	1 M KOH + 0.33 M KI	Ni–Mo	HER	1 M KOH + 0.33 M KI	10 mA cm^−2	117.7%	1.34 V	69

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4 Critical factors of paired electrolysis

Despite significant progress in demonstrating paired electrolysis for sustainable energy conversion and chemical synthesis, several core challenges continue to impede its practical implementation. The major issues—electrode stability, product crossover, and system scalability—are often acknowledged but rarely quantified or discussed with sufficient rigor. Addressing these aspects through systematic and quantitative analysis is crucial for transforming paired electrolysis from a laboratory curiosity into a viable industrial process. Addressing these challenges requires coordinated methodological reform. Alternatively, minimum reporting protocol can be adapted are as follows (i) activity metrics normalized by geometric area or electrochemically active surface area (ECSA); (ii) energy consumption per mol product; (iii) long-term stability data (≥100 h); and (iv) quantified crossover and product purity. Integrating these standards will enable meaningful cross-comparison among studies and accelerate the rational design of durable, selective, and scalable paired-electrolysis systems. In the upcoming subsections we will discuss these parameters in brief.

4.1 Electrolytic cells

The commonly used electrolyzers include the H-cell configuration and the continuous flow cell. In paired electrolysis, the H-cell design comprises two electrolytic cells linked in series, usually organized horizontally like the letter “H.” The H-cell comprises two primary components: the electrolytic cell and the porous barrier or salt bridge. Electrolytic cells serve as the containers or compartments that contain the electrolyte solution and electrodes. To accomplish electrolysis, an electrical current is passed through a system in which an electrode is immersed in the electrolyte in each cell;⁷⁰ a porous barrier or salt bridge is a component that separates the two electrolytic cells in the H-cell arrangement. It enables the transfer of ions between the cells while maintaining the separation of the electrolytes. The barrier may consist of a permeable substance, such as a ceramic membrane or a salt bridge composed of an electrolyte solution that does not react chemically (Fig. 5a).⁷¹ The mass transport limits the testing to current densities of about <100 mA cm⁻², making these configurations less commercially relevant. Continuous flow reactor studies circumvent H-cell mass transport restrictions by cycling reactants and products to and from electrodes. These “flow cells” can be created and performed in a lab, and their designs can be more easily expanded to electrolyzer stacks. It encourages using flow cells instead of H-cells to investigate CO₂RR electrocatalysis due to the dynamic nature of flow parameters, ion and electron transfer/transport, and catalysis.⁷² The thermodynamics and kinetics of the CO₂RR in flow reactors differ from H-cells due to higher CO₂ concentrations at the electrode interface. Most CO₂ flow cell research involves membrane-containing reactors (Fig. 5b). These methods translate H-cell research using liquid electrolytes into low-temperature water electrolysis or fuel cell systems. In a typical flow cell, CO₂ is supplied to the cathode in a mildly basic solution (e.g., aqueous bicarbonate) form.⁷³ The polymer electrolyte membrane (PEM) separates cathodic and anodic chemistry to allow ion passage and reduce product crossover in the case of eCO₂RR. Although cathode potentials are reported, the high ohmic resistance from the distance between the anode and the cathode (a few centimeters), ion exchange membrane, and dilute electrolytes (0.1 mol L⁻¹) leads to high cell voltages and low energy efficiency. The H-cell electrolyzer has a low upper current density (a few tens of mA cm⁻²) and limited CO₂ solubility (33 mmol L⁻¹ at 25 °C and 101 325 Pa) in aqueous electrolytes. Such constraints restrict scaling and practical use (Fig. 5c). The flow cell has two chambers constructed from polytetrafluoroethylene, each with a thickness of ∼2 cm. The substantial electrolyte gaps were employed to guarantee that the anode reaction is influenced solely by electrode characteristics, such as porosity and oxidation state, rather than the cell layout (Fig. 5d). Ethylene propylene diene monomer (EPDM) gaskets can be employed to seal the contact points of electrodes, membranes, and flow fields.

Fig. 5 (a) Two-electrode configurations in an H-type electrochemical cell. (b) Flow cell electrolyzer. (c) Three-electrode setup for the CO₂ reduction process in the H-cell. (d) Flow cell electrolyzer for the CO₂ reduction or conversion. (d) Adapted with permission,⁷¹ copyright 2021, Curr. Opin. Electrochem.

The compartments in H-cell can be separated by the ion exchange membrane, which facilitates the ion flow. The chemical inertness and machinability of materials like polytetrafluoroethylene (PTFE) make it a preferred material for flow-cell chambers in laboratory-scale applications. The variety of materials utilized in flow-cell assemblies is significantly broadened in practical and industrial contexts, heavily influenced by operational parameters including electrolyte composition, pH, operating potential, and temperature. Materials including titanium, nickel, stainless steel, graphite, and their alloys or coatings are commonly selected under diverse electrochemical conditions to achieve a balance among mechanical stability, corrosion resistance, and conductivity. The variety in material selection guarantees compatibility across diverse electrochemical environments, encompassing acidic, neutral, and alkaline electrolytes. Fig. 5d presents a representative example of a flow-cell assembly. In a recent study, a titanium (Ti) flow field is utilized on the anode side, whereas a graphite flow field is implemented on the cathode side.⁷⁴ Assembly begins at the anode, where polyethylene terephthalate (PETE) gaskets, with thicknesses of approximately 0.047″ for BPMWE and approximately 0.037″ for PEMWE, each featuring a 1 cm × 1 cm opening, are sequentially affixed to the Ti flow field.⁷⁴ Subsequently, the PiperION membrane (1.5 × 1.5 cm) is attached to the PTL, and a TiO₂-coated Nafion membrane is positioned above that, with the TiO₂-coated side oriented downward toward the PiperION layer. This configuration results in the electrochemical performance being predominantly influenced by intrinsic electrode characteristics including porosity, morphology, and oxidation state, rather than by geometric or assembly-related factors. The generation of local pH gradients between the anode and the cathode during operation may contribute to changes in cell voltage, attributed to the depletion of reactant species (H⁺ and OH⁻) at the interface. The pH of the electrolyte has a significant impact on the kinetics, selectivity, and thermodynamics of coupled electrolysis processes for hydrogen evolution and CO₂ reduction. In alkaline environments, hydrogen evolution and CO₂ reduction frequently encounter elevated overpotentials due to limited proton availability; yet, the electrode stability is enhanced. Conversely, acidic environments facilitate proton transfer and hydrogen production but may result in catalyst deterioration and restricted CO₂ solubility. Therefore, controlling pH is essential for balancing activity, selectivity, and the long-term durability of electrodes.

4.2 Electrode stability and durability

Electrode deterioration constitutes a significant and under-investigated challenge in paired electrolysis. Most reports indicate encouraging performance during brief periods (usually several hours), while industrial electrolysers require uninterrupted operation for thousands of hours. Failure modes commonly seen include catalyst dissolution, phase reconstruction, passivation, and mechanical separation. The processes can be intensified in dual-reactant environments typical of paired systems, where the cathode and anode function under significantly differing pH, voltage, and reactant conditions. Quantitative stability evaluation is rarely standardized. Prolonged chronoamperometric or chronopotentiometric evaluations at industrially pertinent current densities (≥100 mA cm⁻²) must be supplemented by post-mortem analyses—such as XPS, XRD, or TEM—to associate morphological or compositional alterations with performance degradation. Creating stable catalysts that preserve activity in both oxidation and reduction conditions is essential for the practical implementation of paired electrolysis.

4.3 Product crossover and selectivity

The interchange of products between the anodic and cathodic chambers compromises both selectivity and energy efficiency. This problem is most acute in systems where gaseous and liquid products coexist or when soluble intermediates (e.g., formate, alcohols) permeate membranes. Crossover not only diminishes product production but may also result in adverse side reactions and electrode poisoning. Subsequent research should implement a consistent mass-balance methodology that encompasses crossover flow, comprehensive carbon balance (for CO₂-based systems), and product purity assessment (measured in ppm or wt%). Moreover, membrane selectivity—the ratio of the transport of desired to undesired species—should be assessed under operational settings rather than by ex situ permeability evaluations. The creation of ion-exchange membranes with customized hydrophobicity or charge density, along with bipolar or multilayer structures, presents effective strategies to reduce crossover while preserving low ohmic resistance. Ultimately, reducing crossover is essential for achieving the inherent efficiency benefits of combining oxidation and reduction events within a single cell.

4.4 Scalability and system integration

Although laboratory-scale demonstrations effectively validate concept feasibility, scalability is predominantly conjectural. Most studies are performed in small H-type cells or low-flow reactors with electrode surfaces of ∼1–2 cm². Scaling such systems to industrially relevant dimensions presents numerous additional constraints: heightened ohmic resistance, mass transport limits, bubble management, temperature regulation, and uniform current distribution across extensive electrodes. Furthermore, paired electrolysis complicates process design as both half-reactions yield value-added products, necessitating dual separation and purification processes. To facilitate scale-up, investigations must document geometric electrode area, reactor layout, flow rate, pressure drop, and electrolyte conductivity. Flow-cell or membrane-electrode-assembly (MEA) configurations, which replicate commercial electrolyzers, ought to supplant static cells as the experimental benchmark. A preliminary techno-economic evaluation (TEA) should accompany high-performance promises beyond device engineering, translating laboratory metrics into cost predictions (e.g., kWh per mol of product, anticipated cost per kg output). A simple TEA can determine if the energy savings from reaction pairing exceed the additional expenses of separation and maintenance.

5 Techno-economic analysis

The techno-economic analysis (TEA) is a thorough method that evaluates the technical feasibility and economic sustainability of emerging energy technologies by examining essential factors such as capital investment, maintenance, installation, input chemicals, and operational expenses.^75–77 It provides an essential foundation for comprehending the economic viability of a process, from initial proof-of-concept to comprehensive industrial implementation. The allocation of costs among these parameters is significantly affected by variables including reaction type, geographic location, energy supply, feedstock availability, and the efficiency of product recovery and separation. In electrochemical systems, the operational cost of the electrolyzer is the primary factor in total expenditure. The expenses are primarily influenced by performance measures such as energy efficiency (EE), current density, cell voltage, specific power consumption (SPE), and electrolyzer durability.^71,78 Enhancing these factors immediately correlates with diminished energy requirements and decreased hydrogen production expenses. Paired electrolysis systems, which concurrently generate hydrogen at the cathode and convert CO₂ or biomass-derived substrates at the anode, exhibit particularly potential techno-economic advantages. Substituting the energy-demanding AOR/OER with low-overpotential oxidation reactions (such as glycerol, ethanol, or formate oxidation) in paired electrolysis can markedly decrease the cell voltage (by 0.3–0.8 V) and overall power consumption.^7,79 Furthermore, the production of value-added co-products like formate generates supplementary revenue streams, enhancing economic competitiveness relative to traditional water electrolysis.^80,81

Recent studies demonstrate that paired electrolysis can attain a 15–30% decrease in energy expenses, potentially reducing the levelized cost of hydrogen (LCOH).^82,83 From a scaling perspective, pilot-scale thermodynamic electrochemical analysis was performed for the oxidation of glycerol in conjunction with nitrate reduction. They determined a profit of $1217 per tonne of NH₃ by substituting the OER with the production of value-added formic acid. Moreover, the transformation of formic acid into the highly valuable potassium diformate (KDF) enhanced the profitability of the NO3RR ‖ GOR system to around $4474 per tonne of NH3.⁸⁴ Similarly, a bifunctional electrode achieved an industrial-grade current density of 500 mA cm⁻² at 0.49 V, which saves at least 53.3% energy consumption compared to conventional alkaline water electrolysis.⁸⁵ Moreover, a faradaic efficiency (FE) of 75–85% for glycerol oxidation products (oxalate, glycerate, tartronate, lactate, glycolate, and formate) was attained at a total current density of 200 mA cm⁻², while cathodic CO production occurred with nearly 100% FE.⁸⁶ Nonetheless, further enhancement of catalyst stability, product separation, and stack engineering is crucial for commercialization. Consequently, a thorough techno-economic analysis of paired electrolysis must encompass both technical performance metrics (energy efficiency, current density, and durability) and economic factors (capital expenditure, power cost, and product valuation). Such analyses offer a framework for advancing paired electrolysis from experimental innovation to industrial-scale hydrogen and CO₂ utilization systems.

6 Thermodynamic and theoretical insights into anodic oxidation reactions (AORs)

Both the OER and AOR are thermodynamically favorable processes. Reducing the overpotential and enhancing the reaction kinetics rely heavily on rational catalyst design, emphasizing the anode materials and their associated catalytic active sites. Fig. 6a outlines the key reductive and oxidative reactions relevant to CO₂/CO electrolysis, categorized by their standard equilibrium potentials.⁸⁷ It highlights that anodic oxidation reactions (AORs), including alcohol, aldehyde, and carboxylate oxidation, have significantly lower equilibrium potentials (<0.25 V vs. RHE) than the competing oxygen evolution reaction (OER), making them thermodynamically favorable. This supports experimental trends (described in Tables 1 and 2). Fig. 6b presents a theoretical framework for understanding how equilibrium potentials in membrane-electrode assembly (MEA)-based systems can deviate from standard values. For example, ethanol oxidation may appear favorable thermodynamically, but the potential depends on dynamic concentration changes due to ethanol crossover.

Fig. 6 (a) Relevant reduction reactions (including the HER, COR, and CO₂R) and oxidation reactions (including the OER and AOR) with their standard equilibrium potentials, during the CO and CO₂ electrolysis process. (b) Flowchart for understanding anodic oxidation reactions from a theoretical perspective. (a and b) Reproduced with permission,⁸⁷ copyright 2023, Wiley-VCH.

To this end, identifying and characterizing these catalytic hotspots requires careful application of advanced analytical techniques under in situ operando or ex situ conditions and density functional theory (DFT) calculations. However, deriving reliable insights from DFT is inherently challenging. Therefore, it is essential to critically evaluate these computational predictions against experimental observations and existing literature to obtain meaningful and consistent interpretations.

7 Electrode material design

7.1 Role of single-metal atoms

The strong interactions between isolated metal atoms and the adsorbed substrate on the electrode create favorable electronic environments for activating substrates in the electrolyte, which results in high accelerated activity, for example, in the case of H₂ production.⁸⁸ When isolated, atomically dispersed metals offer a bifunctional activity for both the eHER and the eOER. However, oxidizing organic substrate can accelerate HER, wherein the configuration of the metal single-atom remains vital throughout the process.⁸⁹ Moreover, Ru single atoms and Ni/NiO nanoparticles which can regulate the electron distribution with strong orbital couplings to activate NC nanotubes, which boosts the overall water splitting. Such acceleration can also be used in simultaneously paired electrolysis to enhance the HER coupled with an electrochemical oxidation process.⁹⁰ Simultaneous electrolysis of CO₂ and glycerol in a two-electron flow system needs bifunctional NiNPs/Ni–N–C on both electrodes for anodic oxidation and CO₂RR, which faces an insufficient protonation process in an alkaline medium.⁹¹ Concomitant Ru nanoclusters and single atoms on cubic α-MoC/N-doped carbon achieve efficient coupling of hydrazine oxidation and HER for multifold acceleration of H₂ production by the overall HzOR process.⁹² The bifunctionality of two Ru-species, in which the co-existence of SA balances the H adsorption/desorption for the HER during HMFOR develops to a self-power HER via simultaneous electrolysis. Therefore, Ru-SA and Ru nanoparticles on a conductive NC offer a multifold HER while being coupled to HMFOR by replacing the OER in a cell-integrated electrolyzer to achieve bifunctionality.

Incorporating single atoms and heteroatoms (N, S) into a reduced graphene oxide framework could provide a conductive backbone. This achieved both the aggregation of Ru clusters and the provision of high-density anchoring sites for Ru³⁺ ion capture. As shown in Fig. 7a, the Co element concentration was low, at roughly 2.05 wt%, as determined by the ICP data; in TEM images, shining spots, darker than the Ru particles, surrounded the Ru nanoclusters (Fig. 7b). The cobalt-based oxide in the precursor likely became single atoms after loading with Ru. As shown in Fig. 7c, the electrolyzer using commercial Pt/C/RuO₂ electrodes required an overpotential of 1.59 V, whereas the CoSARuNC@NSG//RuO₂ electrolyzer required an overpotential of 1.56 V at 10 mA cm⁻². CoSARuNC@NSG//RuO₂ displays outstanding water-splitting capabilities that are on par with the industry standard Pt/C//RuO₂ system. As shown in Fig. 7d, the electrolytic stability of the CoSARuNC@NSG//RuO₂ electrolyzer was evaluated over more than 25 hours. The long-term stability in current density throughout testing shows that the CoSARuNC@NSG//RuO₂ electrodes are viable options for practical applications.³³

Fig. 7 (a) The scheme representing the synthesis of Co_SARu_NC@NSG. (b) AC-STEM analysis image of Ru nanoclusters and Co single atoms. (c) The paired electrolysis setup demonstrating the water-splitting performance test and the LSV curve at 5 mV s⁻¹. (d) Long-term stability performance for Co_SARu_NC@NSG//RuO₂. Reprinted with permission,³³ copyright 2024, J. Colloid Interface Sci. (e) The schematic shows the formation of a Co₃O₄/NC-250 three-dimensional self-supported electrode. (f) The in situ ATR-FTIR spectra of 3D Co₃O₄/NC-250 in a 0.5 M H₂SO₄ solution at different applied potentials. (g) The plots depict the Co–K-edge XANES spectra. (h) Three-dimensional Co₃O₄/NC-250 overpotential active site LOM mechanism free energy. (i) The plot depicting the polarization curves for 2D Co₃O₄-250, 3D Co₃O₄/NC-250, 3D Co₃O₄/NC-300, 3D Co₃O₄/NC-400, 3D Co₃O₄/NC-500, and commercial RuO₂ in a 0.5 M H₂SO₄ solution. Reproduced with permission,⁹³ copyright 2023, J. Alloys Compd.

X-ray absorption spectroscopy results activation over single-atom Rh sites oxidized by electrophilic OH_ads species on the Ni-atoms involving d–d orbital coupling interactions between atomic Rh and surrounding Ni atoms in forming intermediates of eHMFOR-HER coupled electrolysis.⁹⁴ Two-electrode cell systems with an HMFOR involving an OER intermediate determined by the in situ Raman spectral analysis pave the way for maximal production of an intermediate that accelerates the cathodic HER coupled with anodic oxidation.⁹⁵ Simultaneous generation of chemicals and reduction products from the eCO₂RR at both the anode and cathode makes efficient use of electrical energy in the combined anodic oxidation–cathodic reduction process. Therefore, such a level of integration of the electrochemical system can be further electrified using high-atom efficiency of single-atom consisting materials as a simultaneous electrode.³⁶ Prospects of single-atom-containing active sites of electrodes can be extensively tested for continuous production of ethylene and hydrogen peroxide via paired cathodic eCO₂RR-anodic water oxidation in integrated cells for overall electrical energy consumption savings.⁹⁶ Electrochemically generated active metal species due to vacancy-mediated electrooxidation in the anode can be further accelerated using defect engineering of electrode materials with an oxygen vacancy (V_o) to accelerate the HMFOR.^96,97 Therefore, even with limited investments in the actual acceleration of paired electrolysis using simultaneous single-atom metal-based electrodes, both anodic oxidation and cathodic reduction can be accelerated to maximize electrical energy savings.

Moreover, independent tunability of the electronic interface and kinetics enables a hetero-interface of simultaneous electrodes to accelerate the electro-production of H₂ at a faster rate than the individual HER, which finds an analogy with ex situ electro-organic synthesis with unrestricted reaction control.⁹⁸ Moreover, paired electrolysis can conduct the process without any external substrate as free-standing electrodes, which can provide stability and activity under high industrial current. With such 3D free-standing electrodes, the binding strength of each component, charge transfer promotion across interfaces, and modulation of electronics and nanostructural features of electrodes may be possible. The above change should deliver current densities as high as 500 to 1000 mA cm⁻² for the OER side of water-splitting. To better understand the mechanism of the acidic OER,⁹³ the nitrogen-doped layered Co₃O₄/NC-250 self-supported electrode on carbon cloth exhibits high electrical conductivity, which makes the effective transfer of electrons with reduced energy losses and increased electrolysis efficiency, which guarantees a reduced voltage drop across the electrode–electrolyte contact (Fig. 7e).⁹⁹ A novel metallic heterostructure, Co₉S₈@Ni₃S₂/NF, was designed for H₂ production coupled with the HMFOR, and it requires a low potential of 1.61 V with an initial current density of 50 mA cm⁻², which is accelerated by the electron transfer from Ni₃S₂ to Co₉S₈ at the heterogeneous interface.¹⁰⁰ In an acidic medium, in situ ATR-FTIR experiments revealed three distinct absorption bands at 1.3 V, indicating the development of crucial intermediates in the acidic OER process (Fig. 7f). DFT calculations indicate that the LOM mechanism reduces the adsorption energy of an oxygen-containing intermediate and enhances the OER kinetics. The multilayer structure displays several edge locations and enhances structural stability. In addition, XAFS spectral observations indicate that N doping modulates Co atom electronic states and enhances the OER kinetics (Fig. 7g). In the acidic OER, a characteristic absorption peak at 1.4 V reveals the formation of a critical intermediate O*O group, showing that the lattice oxygen mechanism (LOM) is activated by oxygen vacancies (Fig. 7h). Thus, the 3D Co₃O₄/NC-250 exhibits superior OER performance in acidic environments, with an overpotential of 225 mV at 10 mAcm⁻² current density (Fig. 7i). Table 3 presents a series of HER materials for their paired electrolysis with critical evaluation of their performance based on the lowering of overpotential and current density.

Table 3 HER materials in paired electrolysis

S. no.	Material	Electrolyte η (mV)	Overpotential (HER)	I (mA cm^−2)	Tafel slope (mV dec^−1)	Stability	Ref.
1	Ni-MOF/NF	1.0 M KOH	320 mV	100 mA cm^−2	—	500 cycles	101
2	MoS2	0.5 M H2SO4	300 mV	126.5 mA cm^−2	55	3000 cycles	102
3	Ir1@Co/NC	1.0 m KOH	1.603 V	10 mA cm^−2	119	5 h	103
4	20-NMWNT	0.1 M NaOH	340 mV	10 mm^−2	68	24 h	104
5	CeFeCoP/NF	1 M KOH	97 mV	10 mA cm^−2	147	10 h	105
6	NF@G-5@Ni3S2	1 M KOH	119 mV	10 mA cm^−2	64.8	1000 cycles	106
7	PtNi–O/C	1 M KOH	41.7 mV	10 mA cm^−2	78.8	—	107
8	CoFePo	1 M KOH	87.5 mV	10 mA cm^−2	38.1	—	108

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It is recognized that the interfacial charges induced by the work function promote HER kinetics, and that manipulation of electron redistribution via changes in coordination facilitates water oxidation at high current densities. The induced electron redistribution involves the hetero-surface contact through a hydrogen bond for enhanced photoinduced charge separation and transfer. Rapid transfer of electrons/ions favors electrolyte permeation on the cathode surfaces, which creates sufficient active sites for the OER and ORR. Lattice matching of two components helps in forming discharge products due to the reduced adsorption energy of the intermediates on the built-in electric fields, wherein the formation of a Li₂O₂ film by the surface path limits the electrolytic activity in lithium oxygen/air batteries.¹⁰⁹

Therefore, the above-mentioned results on electron redistribution depend on the extent of lattice-matching engineering of cathode materials, which is an effective approach to tune the formation of SEI. Moreover, tuning the electronic and structural engineering can improve the perfomence of electrodes.

7.2 Nanosheet arrangement

Considering electrode composition and surface features in paired electrolysis with the accelerated HER, a nanosheet array with tunable compactness on carbon cloth is designed via a facile two-step process. Nanosheet arrays with superhydrophobicity and superhydrophilicity surfaces have conductivity, accelerating mass transport. This facilitates the access of electrolytes and enables the bubble shielding effect of gas bubble diffusion to result in an outstanding HER performance in 1.0 M KOH at low overpotential at 35 mV to 10 mA cm⁻² at a high current density.¹¹⁰ The nanosheet arrays offer improved aerophobic features, resulting in high-efficiency tunable compactness on carbon cloth due to hierarchically nanostructured interfaces for accelerating HER. The intricate arrangement of atoms within these thin two-dimensional materials is crucial in nanosheet-enabled paired electrolysis. Thin two-dimensional materials with complex atomic arrangements are the building blocks of nanosheet-enabled paired electrolysis. Two-dimensional (2D) nanosheets are attractive due to their modified electronic qualities and distinctive structures. To facilitate electrochemical processes, the atomic structure of nanosheets results in a very high surface area-to-volume ratio. Because increasing reaction rates and efficiency in paired electrolysis depends on making the most of the available reaction surface, this property is beneficial.¹¹¹ It is also possible to tune the electronics of nanosheets by manipulating their atomic arrangements. When combined with the large surface area, this improved mass transport guarantees optimal reactant usage and minimizes mass transfer limits, ultimately leading to improved performance in paired electrolysis.¹¹²

In the utilization of the solvothermal synthesis technique, a succession of ultrathin nanosheets of binary Ni–M-MOFs (where M = Fe, Al, Co, Mn, Zn, and Cd) were synthesized in a solution of N,N-dimethylacetamide (DMA) and water. These nanosheets were subsequently used as anodic OER electrodes, as shown in Fig. 8a. The DMA/H₂O mixture was the key to success in creating these ultrathin bimetallic Ni–M-MOF NSs by a one-step solvothermal process. Fig. 8b shows the amorphous nanoflowers. In contrast, porous and fluffy powders were produced using DMA as the sole solvent. DMA dissolved the organic ligands during the solvothermal process to prevent nanoparticle formation. Meanwhile, water served as an effective exfoliant, occupying the surface metal coordination sites and limiting the formation of coordination polymers. The produced substance exhibited rapid electron transfer capability, and was highly exposed to the active site. A combination of the synergy between the metals and the one-of-a-kind coordination environment surrounding the active site intensified the catalytic activity. The ultra-thin Ni–Fe-MOF NSs (Fig. 8c) demonstrated the most improved activity and longevity for the OER. Fig. 8d shows the proposed universal synthesis method for TMDs, which relies on the solvothermal method.¹¹¹ This method allows for control over the composition (Mo and W), phase (semiconductor and metal), and morphology (edge and base plane orientation). Ni/TiO₂@Ni₃S₂ exhibits long-term stability for the HMFOR and HER, especially Ni₃S₂ nanosheet TiO₂ nanorod arrays grown on a conductive Ni foam.¹¹³ Au-modified Ni foam electrodes exhibit exceptional chemical stability for anodic H₂ production and HMFOR, which involves an H⁻ transfer at room temperature.¹¹⁴ NiCoB_x follows the “reactant-induced activation mechanism” with an accelerated dehydrogenation process to generate abundant active sites (Ni/B-doped CoOOH), resulting in efficient HMFOR performance.¹¹⁵

Fig. 8 (a) The ultrathin metal–organic framework (MOF) nanosheets and their use for the OER. (b) TEM images of NiFe-Metal organic framework (MOF) Nanosheets. (a and b) Reproduced with permission,¹¹⁶ copyright 2019 Wiley-VCH. (c) Schematic of the synthesis of transition metal dichalcogenides (TMDs) (MS₂, M = Mo/W) on carbon cloth. Reprinted with permission,¹¹⁷ copyright 2019, Elsevier. (d) Schematic of the synthesis of Zn-TCPP. Reproduced with permission,¹¹⁸ copyright 2015, Wiley-VCH.

A self-supported heterostructure, Ni₃P–Cu₃P/CF, is fabricated where a high range of activity was witnessed, such as the HMFOR with sizeable current density and higher selectivity. X-ray photoelectron spectroscopy and theoretical calculations reveal that the interface charge redistributes at the Ni₃P–Cu₃P heterointerface, resulting in the charge-deficiency Ni₃P unit and charge-accumulation Cu₃P. The charge-deficiency Ni₃P induced by charge-attracting Cu₃P favors the form of high-valence Ni species, which facilitates the optimization of the adsorption of HMF and OH* species for improving the current density and decreasing the potential. At the same time, the charge accumulation in Cu₃P broadens the potential window by suppressing the competitive oxygen evolution reaction.¹¹⁹ Hierarchical porous O-ZIS-120, composed of O-doped/ZnIn₂S₄ nanosheet, possesses abundant atomic-scale edge steps and lattice defects, which are beneficial for electron accumulation and molecule adsorption, wherein the weak interaction between O-ZIF-120 and the final product further modulates the electronic structure.¹²⁰ In a two-electrode H-cell using a bifunctional nickel phosphide nanoparticle/nickel cobalt sulfide nanosheet framework (NCSP150) as both the cathode and the anode, the cell voltage required to afford a current density of 10 mA cm⁻² is less than 153 mV compared to water electrolysis when adding 5 mM HMF in 0.1 M KOH.¹²¹ Ni₂P/NCS nanosheet frameworks were fabricated on Ni foams through a low-temperature route involving one-step chemical bath deposition of NCS nanosheet frameworks followed by vapor phosphorization for the surface formation of Ni₂P nanoparticles.

7.3 Role of nanowire array

Paired electrolysis is further enhanced using nanowire arrays, which possess exceptional structural and electrical properties. Nanowire arrays as electrodes improve stability, selectivity, mass transport, tuneability, surface area, and electron transmission and tuning.¹²² The substantial increase in surface area that nanowire arrays offer is a significant advantage of their use in electrolysis. With traditional flat electrodes, surface exposure is restricted, reducing the number of active sites available for electrochemical reactions. The accessible surface area is exponentially increased by nanowire arrays, which are composed of densely packed components with a high aspect ratio. Therefore, this increased surface area directly translates into more active sites for the electrochemical reactions, increasing the total reaction rate.¹²³ For instance, there is a correlation between a larger surface area and more efficient hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs).¹²⁴

Nanoscale dimensions and electrical properties boost the electrode kinetics of nanowire arrays. Nanowire's high aspect ratios and quantum confinement effects can reduce the electrochemical reaction activation energy. The metal nanowire network offers mechanical stability at the breaking edges, which results in a free-standing lamellae structure that is distinct from Cu nanoparticles, wherein Cu nanowires are embedded both in the Cu network (∼2 µm) and within the GDL (infiltration in pores ∼5 nm deep). Cu nanowires do not infiltrate the finer pores of the rough carbon fibers; however, they create a web-like network on their surface by spanning larger crevices.⁹⁶ Nanowire arrays maintain high reaction rates and ensure that reactants are always available at the active locations. Nanowire composition, diameter, length, and spacing may be precisely regulated during synthesis. Nanowire dimensions can also be changed to enhance the surface area and mass transmission. This level of control allows nanowire arrays to be tailored to specific pair electrolysis applications, improving efficiency and effectiveness. Electrolysis requires efficient electron transport to reduce resistive losses.¹²⁵

Nanowire materials such as metals and conductive oxides are electrically conductive. This high conductivity allows electrons to flow fast and with less resistance, enhancing the electrolysis efficiency. Nanowire arrays can be tailored to choose the desired reaction products, reducing side reactions and yield loss.¹²⁶ Titanium dioxide¹²⁷ forms stable nanowire arrays that can endure electrochemical cycling. Fig. 9a and b depict a core–shell nanowire array on a nickel foam. The core of the nanowires consists of Co₃O₄, while the outside layer comprises NiFe LDHs. The HRTEM image of the Co₃O₄@NiFe LDH nanowire, shown in Fig. 9c, allows for observing both the lattice fringe spacings of Co₃O₄ and NiFe LDH. The Co₃O₄@NiFe LDH exhibited the lowest OER overpotential of 226 mV at 35 mA cm⁻², as depicted in Fig. 9d and e.¹²⁸

Fig. 9 A nickel foam core–shell nanowire array with NiFe LDHs coated on the exterior and Co₃O₄ inside. (a) A schematic depicting the steps used to create the hybrid nanowire arrays of Co₃O₄@NiFe LDH. (b and c) TEM pictures of the Co₃O₄@NiFe LDH nanowires. (d and e) The LSV curves with a scan rate of 1 mV s⁻¹ for the OER and ORR. Reproduced with permission,¹²⁸ copyright 2020, Wiley-VCH.

A nanowire network of various electrodes made of Cu₂O at a broad range of ultralow applied potentials offer heterointerface-modulated *CO adsorption for enhanced C–N coupling dynamics for electrosynthesis. Such concepts of electrolysis can be implemented for simultaneous electrolysis.¹²⁹ Bifunctional heterointerface Co–N/Co₃O₄@N-carbon nanowires of hollow structures offer improved electrical conductivity with shortened electron/reactant transport pathways for oxygen reduction and oxygen evolution electrolysis.¹³⁰ Besides many combinations of paired electrolysis for anodic chemical production and cathodic reduction, H₂ production at the cathode is a key process for further advancements. The limitations of electron transfer rates and the kinetic overpotential of the electrode affect the H₂ production in neutral and near-neutral electrolytes, which is relatively slow.¹³¹ The improvement in limited ionic conductivity and the use of nanowire arrays for a range of electrode materials opens up more scope for the simultaneous electrolysis of a wide range, including the tuning of electrode electronics.

8 Regulation of electronic distribution and lowering of overpotential

The interfacial electronic structure lowers the energy barrier of paired electrolysis in both kinetics and thermodynamics, especially when using heterojunction materials with surface reconstruction.¹³² The anodic oxidation and cathodic reduction reactions within a paired electrolysis system can optimize the electrode potential, potentially achieving an overall theoretical faradaic efficiency (FE) of up to 200 for paired electrolysis of H₂O₂ electrosynthesis at high atomic efficiency.¹³³ Thermodynamic favor of electrooxidation of organics replaces the OER for coupled electrolysis with the HER, which significantly reduces the overpotential. This happens not only due to the coupling of electrooxidation with the HER but also due to the electronic tuning of electrode materials, such as the deposition of a single ruthenium atom on the molybdenum selenide.¹³⁴ Fabricating an electrode with a hydrophilic–aerophobic feature allows for the quicker release of adsorbate gas–phase molecules from the surface during the HER without any extra energy input; such simultaneous modulation of factors on the Ni-based electrode is known for promoting accelerated HER.¹³⁵ An ultrathin nanosheet structure with a higher conductivity in PdO bimetal line offers sufficient exposed sites through electronic effects, which achieved a lower voltage than water splitting (1.96 V) in the electrooxidation of acetate-coupled HER. The curved architecture of the materials enhanced the stronger electron interaction with higher atomic exposure.¹³⁶ Synergistic modulation of electronic features such as defect creation directly improves the kinetics of electrooxidation, which is achieved by topological doping of rare-earth single-atom Tb on defective Co₃O₄ for the process of urea oxidation (eUOR) at a lower overpotential 1.27 V for the UOR.¹³⁷ Balanced adsorption–desorption of intermediates with favorable charge transfer for the UOR empowers a hybrid electrolyzer with a triboelectric generator suitable for favorable electronic stabilization. Electrooxidation of glycerol on the Ni foam electrode needs Pt nano dendrites, which achieve a 235 mA cm⁻² at only 0.92 V vs. RHE, wherein Ni plays a significant role in altering the activity.¹³⁸ The presence of adjacent Ni/Ni(OH)₂ species and low Pt sites modulates the binding energy of intermediates with Pt wires, leading to maximum efficiency of the glycerol electro-oxidation reaction (GOR) and maximum current densities. Such GOR has significance when paired with the HER or CO₂RR.¹³⁹ The regulation of electronics includes surface reconstruction driven by stronger adsorption of OH to boost the HMFOR by V-doped Ni₃N, which exhibits a high HMFOR rate with a lower overpotential toward the HER at a current density of 10 mA cm⁻². Further, the required cell voltage reaches a current density of 200 mA cm⁻² in a continuous HMFOR-HER system.^121,140 Parasitic oxygen evolution reaction can be suppressed in the PEC case in simultaneous photoelectrochemical oxidation of glycerol coupled with dark HER or CO₂ reduction in a membrane-separated continuous flow cell, wherein a maximum photo-current density of 110 mA cm⁻² was achieved.¹⁴¹ In gas-diffusion-electrode (GDE) systems with an ion-exchange membrane, critical operational parameters—mass transport dynamics, cell impedance, pressure resilience, operating temperature, and flow rate characteristics—require meticulous assessment. Furthermore, integrating the oxygen reduction reaction (ORR) with the polymer's electrochemical oxidation reaction (EOR) markedly improves H₂O₂ electrosynthesis with bimetallic Ni–Mn-MOF-Se/NF anion-type electrodes.¹⁴² In continuous coupled electrolysis, the measurement of transferred charge at a constant voltage of 1.4 V with the refreshment of electrolyte in 10 h was carried out, and consistent current responses are achieved. The pairing of anodic ethylene glycol electro-oxidation (EOR) with an appropriate cathodic reaction results in a comparative analysis of current density and cell voltage. This analysis indicates that the EOR–ORR coupled system offers a notable energy-saving benefit compared to traditional H₂O₂ electrosynthesis. Electron delocalization is an integral part of the process in which heteroatoms B and N enrich defects as active sites, along with the increased N content. Therefore, the introduction of B and N contributes to the formation of a B–N bond for promoting the adsorption of intermediates followed by dissociation for electrosynthesis in the paired mode. Therefore, energy saving and upcycling of polyethylene terephthalate (PET) in the hybrid electrolysis mode such as ORR‖PET configuration in an H-cell offer multiple advantages including electrosynthesis on an industrial scale. The cell voltage vs. current density analysis reveals the ORR‖EOR paired system that worked at lower current densities with energy saving potentials without any increase in cell voltage. Electron conduction occurs in an inbuilt local microenvironment, in which the reservoir of proton can promote proton relays at the heterojunction interface through the cascade construction of hybrid electrolysis at two electrodes simultaneously. The thin-layer nanostructure of the phthalocyanine–porphyrin covalent organic framework (COF) on a ultrathin carbon nanotube acts as a bifunctional heterojunction for simultaneous electrolysis in a built-in microenvironment.¹⁴³ Therefore, the regulation of electron flow for lowering the overpotential of the pair electrolysis is a significant key control tool, in which the structural features of the electrode heterojunction are observed as the major factors for gaining an improved understanding.

9 Electrode kinetics of hybrid electrolysis

On the electrode surface, the electrode kinetics indicates how fast electron transfer processes occur. In the electrochemical H-cell, these half-reactions occur simultaneously.¹⁴⁴ Faraday's law of electrolysis asserts that the amount of material that undergoes oxidation or reduction at each electrode is directly proportionate to the amount of electric charge transferred through the electrolyte and forms the basis of electrode kinetics. The basic concept of the electrode kinetics of paired processes focuses on electrode reactions of species that lose electrons (oxidize) at the anode in an anodic reaction.¹⁴⁵ The amount of current per unit area of the electrode surface controls the electrochemical reaction rate, making it a crucial electrode kinetics parameter. The overpotential (η) differentiates between the electrode potential and the equilibrium potential. Ohmic losses, activation, and concentration contribute to overpotential, which drives the reaction at a quantifiable pace.¹⁴⁶ Exchange current density (i₀) is where forward and reverse reaction rates are equal at the equilibrium.¹⁴⁷ It is an essential electrode reactivity characteristic and the Butler–Volmer equation is a fundamental equation that relates the electrochemical reaction current density to overpotential. The Butler–Volmer reaction kinetics is characterized by exponential dependence on the activation overpotential. The net reduction current describes the equation given by α, a symmetry coefficient, and i₀, the exchange current density, which represents the rate of reaction in both directions when the reaction is in equilibrium. Depending on the system, the exchange current density can be considered a constant or a function of species concentrations or activities.¹⁴⁸ It is possible to characterize the anodic kinetics of the oxidation reaction at the anode using the Butler–Volmer equation's anodic branch:

i = i₀(exp (−αeη_eff/k_BT) − exp (1 − α)eη_eff/k_BT) (1)

Anodic transfer coefficient and exchange current density are two critical parameters. As with the anodic branch of the Butler–Volmer equation, cathodic kinetics describes the rate of reduction at the cathode. Important variables include the exchange current density and cathodic transfer coefficient. The electron transfer rates and corresponding overpotentials are balanced in paired electrolysis when the cathodic and anodic processes are symmetrical.¹⁴⁹ Disparities in electrode materials, reactant concentrations, and other variables frequently lead to asymmetry in practical applications. The approach involves enhancing the kinetics of oxygen evolution reaction (OER) through the utilization of nanospheres and first-generation electrodes, achieving a potential reduction to ∼1.8 V to provide a current density of 10 mA cm⁻².⁵⁵ The current phase has arrived, in which megawatt levels of current densities are desired from paired cathodic–anodic electrolyzers, which need a shift in strategies oriented toward accelerated charge transfer and optimized adsorption of reactants/intermediates via modulating d-band centers.¹⁶ Heterojunction with dual active sites to enhance the HMFOR involves the electronic interaction of two components in dual-site heterostructures that drive the reversible redox of metal ions such as Ni⁺² ↔ Ni⁺³. Two such similar electrode systems, namely, Ni(OH)₂–CuO/CF and Ni(OH)₂–Cu₂O(S)/CF were identified, which boost the HMFOR-HER activity.¹⁵⁰ Therefore, in situ, electronic reconstruction during the electrochemical process with advanced heterojunction-rich electrodes will be promising for devising the scale-up process of paired electrolysis for simultaneous products from both electrodes.

10 Prototype of the paired electrolysis device

A paired electrolysis apparatus generally has two half-cells interconnected to facilitate the movement of ions while maintaining the separation of the solutions. The anodic and cathodic electrode reactions should ideally be “paired” in electrosynthesis. Two simultaneous reactions at the anode and cathode are combined into one overall reaction in paired electrosynthesis; this allows for more complex reaction pathways, such as oxidation and reduction in a multi-step reaction pathway.¹⁵¹ It is possible to resolve waste and energy recovery issues by setting the anode and cathode processes as components of the synthetic transformation. Two products should be formed simultaneously at the anode and cathode, so the reactor can be designed as simply as possible. This configuration is essential in many applications, encompassing hydrogen generation, electroplating, and purification procedures.¹⁵² The selectivity and design of electrodes are crucial for paired electrode techniques to avoid undesirable reactions. When choosing a material for the cathode or anode, it is essential to consider the chemical interaction of an electroactive species produced from the substrate and the electrode.

A flow-cell arrangement typically consists of an electrolyzer utilizing a cation-exchange membrane to couple cathodic CO₂ reduction with the anodic oxidation of allyl alcohol to acrolein in an acidic electrolyte.⁵⁴ A CO₂RR-OER paired system, with a full-cell voltage of 3.55 V at 100 mA cm⁻² at pH 1, is switched to an organic oxidation system at the anode with allyl alcohol to access acrolein. This needs optimized analytes for the simultaneous cathodic CO₂RR and anodic AOR. Such full-cell flow electrolysis cells exhibit a decreased voltage at the same current density of 100 mA cm⁻² as found for the CO₂RR-OER pairing for 10 h, with the evaluation of energy consumption for producing 1 kg of CO at the cathode. This system reduces energy consumption by 44 mJ kg⁻¹ CO₂, lowering the energy requirement for flow cells with CEM.¹⁵³ Microfluidic flow electrolyzer cells without any ion-exchange membrane and with the Pt nanoparticle anode used for CO₂RR-glycerol oxidation pairing with the optimized electrolyte flow rate achieved 5–85% FE for glycerol oxidation products at lower full-cell voltage and at 200 mA cm⁻² current density for maximum five hours of continuous operation.¹⁵⁴ The electrochemical flow-reactor of a customized electrical, electrochemical flow reactor generates C₂H₄via a paired CO₂RR-OER for 370 h chronoamperometry at 200 mA cm⁻².⁹⁶Fig. 10a shows a device of H-cell divided into a flow reactor assembled with a 10 cm⁻² cathode for evaluating the Cu-based gas diffusion electrode (GDE) under microflow conditions. Fig. 10b shows the two-electrode flow reservoir electrolyzer under recirculation flow conditions.

Fig. 10 (a) A customized electrocell flow reactor design with the membrane and CuDE electrode. (b) Two-reservoir and two-electrode flow electrolyzer under recirculation flow conditions. Reproduced from a ref. 96 copyright 2024, Wiley-VCH.

10.1 Scalability challenges at the industrial current density

A UOR-HER device cell with a cathodic H₂ production rate of 22.0 mL h⁻¹ is coupled to the Pt counter electrode at a current density of 213 mA cm⁻². Further scale-up from this stage would require more efficient anode N₂ production acceleration and selectivity above 55%. In order to attain a higher current density of 540 mA cm⁻² at 1.5 V vs. RHE, the two-electrode membrane-free flow electrolyzer requires an absolute current of 4.8 A at an applied potential of 2.0 V. An intermittent potential strategy is employed in these membrane electrolyzers for long-term alcohol oxidation at a high current density (>250 mA cm⁻²) for 24 hours, resulting in a higher H₂ production rate. Therefore, a membrane-free flow electrolyzer can be an alternative to H-cell prototypes for achieving industrial-level current density while scaling up the H₂ production accelerated by an anodic oxidation reaction. Therefore, long-term electrooxidation at high current density with reduced energy consumption is a further challenge to the scalability of H₂ production in membrane-free flow-cell electrolyzers.¹⁵⁵ In large-scale electrolyzer cells with extensive electrode areas significant challenges when operated in a one-compartment configuration, as they must simultaneously support H₂ production and wastewater treatment.¹⁵⁶ Methanol oxidation has been shown to enhance the H₂ production at the industrial grade of current density (6.86 mA cm⁻²) for 20 000 cycles at a large current density of ∼250 mA cm⁻² for above 110 h.¹⁵⁷

10.2 Technical barriers

The electrooxidation of organic nucleophiles for coupled systems can operate continuously at stepwise ampere-level current densities for more than 500 h without degradation; however, the reduction of cell voltage with suitable anode materials at higher current densities of 1.0 to 1.5 mA cm⁻² than those of a conventional alkaline water splitting system is a challenge for facilitating the production of high-value products in the paired mode of electrolysis. A precise matching of the electrochemical potentials and reaction rates at the anode and cathode includes additional challenges. In addition, electron transfer at the heterojunction from the electrode to the substrate can be a challenge while attempting to merge it with the asymmetric electrochemical process.

10.3 Mitigation strategies for long-term operation

The challenges can be multifold for long-term operation of electrooxidation-paired production of H₂ and similar other simultaneous electrolysis processes in their respective prototypes. Cost-effective urea oxidation-paired H₂ production meets Cl-induced corrosion. However, by avoiding Cl-corrosion, it is possible to enable the system for running over 2000 hours of operation at reduced voltages.¹⁵⁸ The engineering of the electrodes for competitive adsorption of organic oxidants and anions on the active sites of electrodes accelerates the high-performance electrolysis under large current density operations, which can be further extended for a long range of organic electrooxidation-paired H₂ production in aqueous electrolytes.

11 Outlook and conclusion

This article highlights the recent advancements in the development of paired electrochemical systems utilizing H-cell configurations, which significantly reduce overall overpotentials and energy consumption relative to traditional single-electrode processes. Significant progress has been made by integrating the hydrogen evolution reaction (HER) and CO₂ reduction reaction (CO₂RR) with anodic organic oxidation, resulting in enhanced electrode stability, energy efficiency, and product selectivity. Future research must prioritize the development of rational catalyst design principles that incorporate active-site engineering, electronic structure modulation, and interface optimization to enhance the catalytic activity and durability. Emphasis must be placed on optimizing surface chemistry and adsorption properties, facilitating charge redistribution via heterostructure design, and developing hierarchical electrode architectures to improve mass transport and current density. These strategies, informed by an enhanced comprehension of reaction kinetics and electrode stability in operational environments, aim to direct the development of future catalysts and paired-cell systems towards scalable and energy-efficient electrochemical technologies (Fig. 11).

Fig. 11 Summary of the critical features associated with paired electrolytes and the energy conversion process.

11.1 Electronic structure modification

In this respect, extensive investigation of local electronic distribution at the heterointerface accelerates the charge transfer with optimized adsorption of intermediates. In this process, d-band center modulation and corresponding investigations are advisable to understand the thermodynamics and kinetics better. The potential needed to achieve a current density of 10 mA cm⁻² in a 1 M KOH medium with 50 mM HMF is 1.38 V for Ce–NiFe electrodes compared to electronically non-modified NiFe 1.51 V.¹⁵⁹ Therefore, lowering the overpotential to achieve higher current densities for the OER and HMFOR will continue to be at the forefront of the challenges in paired electrolysis. The current research on any of the advanced electrode systems such as Ni₃S₂-based materials in the HMFOR-assisted mode for H₂ production remains yet to be optimized by including factors like current density enhancement, further lowering the on-set potential of HMFOR, cycling stability, and flow-cell continuous H₂ production in the pairing mode. Therefore, it is desirable to modulate the electronic distribution through the effect of interfacial engineering and element doping, alter the distance between active sites, and enhance the active sites' entropy to advance the further electrode capability.

11.2 Electrode and device modification

A high current density is essential for enhancing electrochemical processes, and this study tackles the challenge of achieving such a rate. Adjustments to the electrical properties of the electrodes and the reaction kinetics are crucial methods for improving the performance characteristics. The enhanced system demonstrates improved rates of hydrogen generation and carbon dioxide mitigation. This investigation aims to improve the efficiency and affordability of sustainable energy generation and reduce greenhouse gas emissions by combining these approaches. This study focuses on the design and modification of electrode materials to manipulate the electronic structures. The electronic distribution at atomic sites and interfaces enhances charge transfer and optimizes the adsorption of reactants and intermediates through modulation of the d-band center. H-type and flow cell electrolyzers operate through an electron transfer reaction mechanism at both electrodes, employing optimal conditions to improve the adsorption of intermediates for a selective process at each electrode. The enhancement of these characteristics results in more excellent catalytic activity, better selectivity, and heightened electron transfer efficiency.

11.3 Ion exchange membrane and separator-free paired electrolysis

Ion-exchange membranes play a crucial role in electrochemical systems, particularly in applications such as electrodialysis, reverse electrodialysis, fuel cells, and redox flow batteries. A well-established class of such membranes is based on Nafion, a perfluorosulfonic acid (PFSA) polymer. Nafion is synthesized through the random copolymerization of tetrafluoroethylene (TFE) and sulfonyl fluoride vinyl ether.¹⁶⁰ This results in a network of nanometer-scale water-filled channels that facilitate ion transport. Paired electrolysis employs an applied potential bias to simultaneously drive the anodic and cathodic redox reactions on both electrodes while inhibiting reverse reactions. Separator-free paired electrolysis in a single cell is rare because the membrane minimizes the loss of products from reverse reactions. Accurate tuning of the electricity-driven hydrogen evolution reaction (HER) in conjunction with anodic oxidation is essential to enhance the efficiency of separator-free paired electrolysis utilizing an electrochemical mediator.¹⁶¹ The local electronic distribution at the atomic sites/interface accelerates the charge transfer and optimizes the adsorption of reactants/intermediates with the modulation of the d-band center. This method can yield favorable electrochemical performance for anodic oxidation reactions and cathodic reduction, achieving high faradaic efficiency and low overpotential.

11.4 Atom-utilization efficiency for paired electrolysis

Paired electrolysis optimizes atom and energy efficiencies by employing both electrodes. The selectivity and atom utilization are promising with single-metal-atom catalysts in paired electrolysis. Atomic-scale active sites help optimize cathodic reduction processes and biomass-derived molecule oxidation by improving surface electronic transport and maximizing atom usage. The unique atomic dispersion, changeable electronic structures, and vigorous catalytic activity of single-atom catalysts make them suitable for paired electrolysis. First, the determination of SAC dispersibility and stability is essential for long-term performance. Innovative synthesis approaches can improve the catalytic efficiency by tailoring the electronic structure. High-loading SACs with many active sites are intriguing, but precise synthetic techniques are needed to minimize agglomeration and maintain uniform dispersion. High-entropy SACs with numerous metal sites and various local coordination conditions revolutionize catalyst design. High-entropy SACs can increase the activity and stability using their complexity, spurring SAC innovation. Under industrially relevant conditions like high current densities, the catalyst stability remains a significant issue. Industrial applications require structural durability and degradation mitigation strategies. Scaling SAC production for industrial processes requires unique cost-effectiveness-performance strategies. Zinc–air batteries and other renewable energy technologies demonstrate SACs' ability to bridge lab and field advances. Single-atom catalysts in anodic oxidation reactions will require foundational investigations, creative synthesis, improved computational tools, and industrial-scale deployment. Together, these initiatives will enable efficient, resilient, and economically viable SACs, unleashing their full promise in sustainable energy technology.

11.5 Catalyst design principles for paired electrolysis

The effective design of catalysts for paired electrolysis necessitates the establishment of robust structure–function relationships to align anodic and cathodic reactions at lower cell voltages. Essential design principles include active-site engineering to customize coordination environments and oxidation states, modulation of electronic structures via doping or heterojunction formation to improve charge transfer, and interface or defect engineering to facilitate charge redistribution and stabilize intermediates.¹⁶² Enhancing surface chemistry to achieve a balance between adsorption and desorption, along with the development of hierarchical conductive structures with elevated surface area, significantly advances mass transport and electron mobility. For example, the intersite distance is the distance known as the distance between two isolated single atoms. Recent studies show that variation in metal loading with various intersite distance effects can offer different catalytic activities for the oxygen reduction reaction.¹⁶³ Similarly, the variation in the superstructure of single atoms, dual single atoms and alloy materials may offer variation in catalytic activity.¹⁶⁴ Single atoms and single-atom alloy materials offer high atom utilization efficiency for energy conversion, whereas dual single atoms offer different metal sites to promote the overall activity of catalysts.¹⁶⁵ For example, the single-atom material has more exposed surface area than the nanoparticles, resulting in low overpotential for the OER activity or energy conversion reactions.¹⁶⁶ Moreover, MXenes, when combined with metals, metal sulfides, and such compounds via metal–substrate interactions (MSIs), lead to orbital hybridization and enhanced charge transfer at the interface, consequently altering the adsorption behaviors of reactants and lowering the energy barriers of the reaction steps. MXenes enable the specific distribution of loaded metals to remain stable under electrochemical conditions, which helps achieve an industrially relevant current density.³ Ensuring long-term electrochemical stability through the use of corrosion-resistant materials and robust supports is crucial for the development of scalable and energy-efficient paired electrolysis systems.

11.6 Reaction kinetics integration

The integration of reaction kinetics is essential for optimizing paired electrolysis. It ensures that anodic oxidation and cathodic reduction occur at compatible rates, thereby minimizing the energy losses and maximizing the overall efficiency.¹⁶⁷ Precise control over charge-transfer dynamics, adsorption–desorption equilibria, and mass transport processes at both electrodes is required. The integration of kinetic modeling with experimental data facilitates the correlation of current density, overpotential, and intermediate formation, providing insights into rate-determining steps and mechanistic pathways. Additionally, integrating kinetic analysis with in situ and operando characterization can uncover transient species and interfacial dynamics, which can inform the rational design of catalysts and reactor configurations for optimized, high-rate paired electrochemical systems.

11.7 Paired cell engineering

Paired cell engineering aims to enhance the configurations of electrochemical reactors to facilitate synchronized anodic and cathodic reactions, ensuring minimal energy losses and maximizing product selectivity. The performance of the cell is influenced by critical parameters including electrode arrangement, membrane selection, electrolyte composition, and flow-field design, which regulate ion transport and current distribution. Recent developments in flow-cell architectures, bipolar membrane assemblies, and membrane-free designs have enhanced scalability, current density, and reaction compatibility, while effectively reducing crossover and stability challenges. The integration of engineering approaches with catalytic and kinetic insights facilitates the potential alignment of half-reactions, resulting in a reduction of overall cell voltage and an enhancement of energy efficiency. Ongoing advancements in modular and continuous-flow paired-cell designs are essential for achieving robust, high-rate, and industrially feasible electrochemical systems aimed at sustainable fuel and chemical production.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available from the authors.

Acknowledgements

SD wishes to acknowledge the research funding support by the Science & Engineering Research Board (SERB), Department of Science & Technology, Government of India, through a core research grant (CRG/2023/000044, 2024–2027). SD further acknowledges research funding support from the Department of Biotechnology, Ministry of Science & Technology, Government of India, through the DBT-Energy Bioscience-Biofuels research grant (2022–2025, BT/PR38594/PBD/26/795/2020). MM acknowledges the Department of Science & Technology, Ministry of Science & Technology, Government of India, for the Inspire Doctoral Fellowship (DST/INSPIRE/03/2022/006243). AB acknowledges the support from European Union, project MERGE (No. 101159582). The authors also acknowledge the support from the European Union under the REFRESH—Research Excellence for Region Sustainability and High-tech Industries project, number CZ.10.03.01/00/22_003/0000048, via the Operational Programme Just Transition of the Ministry of the Environment of the Czech Republic.

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