Recent progress of advanced electrocatalysts for hydrogen production via hydrazine-assisted water electrolysis

Abstract

Coupling the thermodynamically favorable hydrazine oxidation reaction (HzOR) with the hydrogen evolution reaction (HER) in a hybrid water electrolyzer is an effective strategy to improve the energy efficiency of large-scale high-purity H₂ production, while achieving pollutant degradation. Recently, various advanced materials have been exploited as electrocatalysts for hydrazine-assisted water electrolysis, but a fundamental understanding of them and a comprehensive summary are lacking to date. In this review, we provide a comprehensive review of advanced electrocatalysts available for HzOR-assisted water electrolysis, as well as various regulatory strategies based on precious metals and non-noble metal-based materials, such as doping, heterostructures, single-atoms, and alloying. Moreover, the structure–activity relationship including electronic structure, surface properties, and catalytic performance of the electrocatalysts is systematically discussed. Given the importance and unique advantages of direct hydrazine hydrate-assisted seawater electrolysis and self-powered electrolyzers, we also present systematic summaries of material design, performance evaluation, and mechanism studies. Finally, several key challenges and future perspectives about hydrazine-assisted water electrolysis are discussed to offer insight into large-scale H₂ production for energy-saving pathways.

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Yun Tong

Yun Tong is an associate professor at the School of Chemistry and Chemical Engineering at Zhejiang Sci-Tech University (ZSTU), China. She received her BS degree from Zhejiang Sci-Tech University in 2012 and obtained her Ph.D. in Inorganic Chemistry from the University of Science and Technology of China (USTC) in 2018. She joined ZSTU as a lecturer in 2018 and was promoted to associate professor in 2023. During this period, she received the China-Swiss Government Exchange Scholarship to conduct postdoctoral research at EPFL (2019–2021). Her research focuses on the development and application of inorganic materials for electrocatalysis.

Pengzuo Chen

Pengzuo Chen is a professor at the School of Chemistry and Chemical Engineering in Zhejiang Sci-Tech University (ZSTU), China. He received his Ph.D. in Inorganic Chemistry from the University of Science and Technology of China (USTC) in 2018 and obtained a BS degree from Zhejiang Sci-Tech University in 2012. He worked as a postdoctoral researcher at École Polytechnique Fédérale de Lausanne (EPFL) from 2018 to 2021. He joined ZSTU as a full professor in 2021. His research interests include the controllable synthesis of low-dimensional inorganic solid materials for electrocatalysis.

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1. Introduction

The detrimental environmental pollution issues caused by the overdependence on fossil fuels have led to an urgent requirement for renewable, green, and alternative energy sources to alleviate the huge pressure on humanity. Hydrogen (H₂) fuel is an important energy carrier and low-carbon energy system that has gained widespread attention as a renewable alternative energy resource due to its extremely high gravimetric energy density and pollution-free byproduct, water.^1–5 Recently, the steam reforming of fossil resources has become the main method of producing most of the H₂. However, the low conversion rate and high emission of greenhouse gases from this method result in high economic costs and environmental problems. In this regard, water electrolysis for the generation of hydrogen and oxygen has been considered a valuable alternative strategy to acquire high-purity H₂ at a large scale with efficiency and zero-CO₂ emission.^6–10 The anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) are the basic half-reactions for water electrolysis.^11–13 Particularly, the kinetically sluggish OER with a four-proton-coupled electron transfer process has been demonstrated as the key restraint for the conversion efficiency of water-to-hydrogen because of its high thermodynamic potential of 1.23 V. This high potential significantly hinders the commercialization of electrocatalytic water splitting technology.^14–17

Over the past few years, plenty of advanced electrocatalytic oxidation reactions of small molecules, such as alcohol, urea, furfural, glucose, and hydrazine hydrate, have emerged as promising and effective substrates for the OER.^18–25 The combination of the HER with favorable kinetics of the half-reactions, such as low theoretical potentials, is important to lower the overpotential at the anode, and thus, realize an energy-saving H₂ production.^26,27 Among these alternative reactions, the hydrazine oxidation reaction (HzOR) is regarded as an attractive option because of its small theoretical potential (−0.33 V vs. RHE) and high power density.^28–30 On the one hand, hydrazine is regarded as an important feedstock for various applications, such as rocket fuel, pharmaceutical products, fuel cells, and agrochemicals.^31–33 On the other hand, the excessive emissions of highly toxic hydrazine may cause irreversible effects on human health and ecosystem. Therefore, the construction of a HzOR-assisted HER system is beneficial to gain high-purity H₂ in an energy-saving manner, while maintaining environmental sustainability.^34,35 As early as 2001, T. I. Valdez and co-authors confirmed that the electrolysis of aqueous organic solutions is an effective way to generate H₂.³⁶ However, this phenomenon was only published as a United States patent. The related literature was lacking until Minoru Umeda's group reported the electrolysis of methanol to water solution and saved ∼60% electrical energy compared to direct water electrolysis.³⁷ Subsequently, with the rapid development of nanostructure control strategies, more and more high-performance electrocatalysts have been developed for hydrazine hydrate-assisted electrolysis for hydrogen production.

Among the important material systems are precious metal-based electrocatalysts with high intrinsic activity, such as Pt, Rh, Ru, Pd, Ir, and Ag, and their alloys.^38–46 However, the high prices and limited crustal reserves of these materials limit their wide-ranging applications. In addition, the formation of oxides/hydroxides of most precious metals on such materials’ surfaces limits the HzOR activity.⁴⁷ Therefore, it is of great research value to achieve a high-quality utilization of noble metal catalysts to ensure high catalytic activity. Furthermore, improving the activity and stability of noble metal-based catalysts through nanostructure engineering is also a widely reported and important issue in the literature.^48,49 On the other hand, non-precious metals, such as Fe, Co, Ni, Cu, and so on, have been widely used as alternatives to develop highly active electrocatalysts, including metal oxides, phosphides, nitrides, selenides, or their alloys for HzOR-assisted water electrolysis.^50–60 Contrary to the precious metal catalysts, surface oxidation of these materials to active species with high valence states (oxyhydroxides/peroxides) plays a critical influence on their catalytic performance.^61–63 However, the main challenge faced with non-precious metal materials is achieving noble metal-like activity, especially for the design of bifunctional electrocatalysts with high performance in both HER and HzOR.

Recently, inspired by pioneering works, various nanostructure engineering methods have been applied to rationally design efficient electrocatalysts. Great progress has been made in energy-saving H₂ production by hydrazine-assisted water electrolysis.^64–67 However, regardless of more and more significant achievements being reported, the comprehensive discussion and summary of research progress has been ignored. In this review, we focus on the nano-regulation strategies of advanced electrocatalysts based on precious metals, non-precious metals, or metal-free materials, to give a basic, but comprehensive, overview of the development of HzOR-assisted water electrolysis. Major nanostructure engineering approaches, such as doping, heterostructure, single-atom, and alloying, have been summarized to study the close relationship between the catalytic performance and the physicochemical properties of electrocatalysts. In addition, the promising applications of advanced electrocatalysts in various fields, including direct HzOR-assisted seawater electrolysis and self-powered electrolysis systems, have also been discussed. Finally, future challenges and opportunities have been proposed to encourage more in-depth study on this highly promising technology.

2. A fundamental understanding of hydrazine-assisted water electrolysis

2.1 The cathodic electrocatalytic hydrogen evolution reaction (HER)

The hydrogen evolution reaction (HER) is one of the most important cathodic half-reactions for traditional electrocatalytic water splitting. The HER process is related to the pH of the electrolyte, which is described by the following equations: 2H⁺ + 2e⁻ → H₂ in an acidic electrolyte and 4H₂O + 4e⁻ → 2H₂ + 4OH⁻ in an alkaline electrolyte.⁶⁸ Normally, several possible principal steps have been reported to occur on the electrode surface during the HER process, whether it is for the reduction of H⁺ or the conversion of H₂O in acidic and alkaline media, respectively. However, the adsorbed hydrogen atom (H_ads) intermediate is always the key to the study of HER mechanisms. Firstly, the Volmer reaction is regarded as the first discharge step that yields H_ads in an acidic electrolyte by receiving one electron on the electrode surface and reducing one proton at the same time.⁶⁹ However, the concentration of proton is low under neutral/alkaline conditions, leading H₂O to act as a H_ads source by receiving the transferred electron. Furthermore, H₂ gas can be produced by Heyrovsky or Tafel pathways, which is directly relative to the coverage ratio of H_ads on the electrode surface. The HER takes place via the Heyrovsky reaction when the coverage ratio of H_ads is low, leading to the preferable combination of proton/H₂O and single H_ads to form H₂ with the help of an extra electron.⁷⁰ On the contrary, the Tafel reaction of HER happens under a high coverage ratio of H_ads, resulting in the formation of H₂ by the combination of two neighboring H_ads atoms. Regardless of the electrolyte used, the fast kinetics of the two-electron transfer process is a unique advantage of HER, but the efficiency of H₂ generation via it is often limited by the anodic reaction.

2.2 The anodic electrocatalytic hydrazine oxidation reaction (HzOR)

Hydrazine (N₂H₄) fuel is not only a promising alternative to H₂ due to its high energy density (5.40 kW h L⁻¹, 1 atm), but it is also an attractive chemical as rocket fuel or reductant in organic synthesis. In the 1960s, direct hydrazine fuel cells (DHFCs) derived from hydrazine oxidation reaction (HzOR) emerged and gained attention because they benefited from the utilization of the energy-dense N₂H₄ fuel.⁷¹ HzOR process is a rapid four-electron transferred kinetics with an ultralow theoretical oxidation potential, which endows its high applicability and promising prospects. The fundamental reaction process of HzOR is directly related to the acidity and alkalinity of the electrolyte. Under acidic conditions, N₂H₄ is first converted predominantly to the hydrazinium (N₂H₄ + H⁺ + e⁻ → N₂H₅⁺) ion and further oxidized to the nitrogen gas and protons by the following equation (N₂H₅⁺ → N₂ + 5H⁺ + 4e⁻, E° = −0.23 V vs. SHE).⁷² Moreover, the hydrazine is relatively stable under alkaline conditions and can be further oxidized to harmless nitrogen gas and water according to a different reaction than the acidic pathway (N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻, E° = −0.33 V vs. RHE, pH = 14).⁷³ It is worth noting that the HzOR has small theoretical oxidation potentials in a pH universal range. This indicates that the HzOR is a promising half-reaction to couple with HER and can significantly reduce the required potential for water splitting.

2.3 The coupled overall reaction for energy-saving H₂ generation

The traditional water electrolysis consists of HER/OER with a high thermodynamic voltage of 1.23 V, leading to low conversion efficiency and high energy consumption for the commercialization of H₂ generation. To save energy and reduce the required voltage for water electrolysis, thermodynamically fast HzOR is a good alternative for the OER. It establishes a new electrochemical configuration for hydrazine-assisted H₂ production (HER/HzOR), namely overall hydrazine splitting (OHzS), which is of significance for sustainable energy storage and conversion systems (Fig. 1).⁷⁴ The HER and HzOR act as new cathodic and anodic reactions in this coupling system, and the overall reaction of OHzS can be presented as the following equations: anodic reaction is N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻, while the cathodic reaction is 4H₂O + 4e⁻ → 2H₂ + 4OH⁻, leading to the following overall reaction: N₂H₄ → N₂ + 2H₂, in alkaline system.⁷⁵ However, although the coupled overall reactions of HER/HzOR have potential advantages, their catalytic performance must be improved further by designing highly active and robust electrocatalysts for large-scale H₂ production. Recently, many electrocatalysts have been developed by using various regulation strategies that achieved significant progress for energy-saving H₂ generation.

Fig. 1 An illustration of the electrocatalytic hydrazine-assisted water electrolysis.

3. The design strategies of efficient electrocatalysts

3.1 Noble metal-based materials

Materials based on noble metals, including Pt, Au, Rh, Pd, and Ru, are promising electrocatalysts for various electrocatalysis reactions. However, the main obstacles to their practical application are still their high cost and scarcity. Therefore, devising feasible ways to improve the intrinsic mass activity of noble metal-based materials is urgent. Recently, the precious metal rhodium (Rh) emerged as a star material in the Pt-group metal family because of its unique and fascinating catalytic activity for energy-related electrocatalytic reactions.^76–78 Rh-based materials have also been widely developed as excellent electrocatalysts for HzOR. Most of the work reported to date demonstrates that the regulation of a desirable morphology of the catalysts is directly related to the optimization of the catalytic activity targeting HzOR. For example, the porous single-metal Rh nanosheets (Rh-NSs) are developed by the in situ transformation of metastable trigonal RhO₂ in an H₂ atmosphere (Fig. 2a and b).⁷⁹ The tunable lattice strain in Rh-NSs has been confirmed to be directly related to the reactivity of HzOR (Fig. 2c). The optimal Rh-NSs-300 shows better catalytic performance than pure Rh, which may be due to the reduced Gibbs free energy of the potential determining step on Rh-NSs as catalysts with tunable compressive strain (Fig. 2d and e). In addition, controlling the exposed crystal plane of the precious metal-based materials is another important means to improve the catalytic performance. For example, Rh nanocubes with specific exposed (100) planes are highly active bifunctional electrocatalysts for HzOR-assisted water splitting (Fig. 2f–h).⁸⁰

Fig. 2 (a) The scanning transmission electron microscopy (STEM) and (b) energy dispersive spectroscopy (EDS) mapping images of Rh nanosheets. (c) Relationships among the lattice strain, average pore size, and annealing temperatures for Rh nanosheets. (d) HzOR polarization curves of various samples in 1.0 M KOH/0.5 M N₂H₄. (e) Density functional theory (DFT) calculations of Rh nanosheets for HzOR.⁷⁹ Copyright 2023. Reproduced with permission from Elsevier. (f) Transmission electron microscopy (TEM) and (g) High-Resolution TEM (HRTEM) images of Rh nanocubes. (h) Polarization curves of HzOR-assisted water splitting for Rh nanocubes.⁸⁰ Copyright 2021. Reproduced with permission from Elsevier.

However, according to the Sabatier principle, monometallic materials usually suffer from limited catalytic performance, as well as instability, due to the single linear adsorption behavior of reaction intermediates under practical operating conditions.⁸¹ This phenomenon has been confirmed experimentally for the d-band center of metals, where the reaction kinetics can be adjusted by doping foreign non-metallic atoms.^82–84 For example, N-doped mesoporous Ru films and B-doped 2D Ir nanosheets array have been grown on nickel foam as excellent bifunctional electrocatalysts for HER and HzOR (Fig. 3a and b).⁸⁵ The OHzS devices thus built exhibit low voltages and high stability for H₂ production (Fig. 3c). On the other hand, metal cation doping is also an important strategy for regulating the electronic structure and catalytic activity of precious metals.^86–89 For example, trace Co is doped into the Ru matrix to form a highly dispersed RuCo material, which exhibits significantly boosted bifunctional activity due to the optimization of the rate-determining energy barrier of HER and HzOR reactions.⁹⁰ However, decreasing the amount of precious metal catalysts required is still a major concern in the field of electrocatalysis.

Fig. 3 (a) EDS mapping images and (b) the water contact angles of Ru–Cu₂O. (c) Polarization curves of OHzS and OWS.⁸⁵ Copyright 2023. Reproduced with permission from Elsevier. (d) The scanning electron microscopy (SEM) image of Ru–FeP₄/IF. (e) The device of drainage method and (f) the generated volume of N₂ and H₂ gases at different times. Free energy diagrams of (g) HER and (h) HzOR on FeP₄ and Ru–FeP₄ samples.⁹¹ Copyright 2022. Reproduced with permission from Elsevier.

The introduction of precious metals as dopants into non-precious metal supports can not only reduce the usage of precious metals, but also maintain excellent electrocatalytic performance. Therefore, this has proved to be an effective strategy for the construction of high-performance electrocatalysts.^92–94 For example, a 3D self-supported electrode decorated with Ru-doped Cu₂O nanochains was fabricated and exhibited superior bifunctional performance for HER and HzOR.⁸⁵ The improved reaction kinetics benefit from the unique porous structure, specific morphology with a hydrophilic surface, and Ru doping. Impressively, a low voltage of 17.4 mV was obtained at 10 mA cm⁻² for an HzOR-assisted water electrolyzer. Similarly, Ru-doped FeP₄ nanosheets are grown on iron foam as an efficient self-supporting bifunctional electrocatalyst for H₂ production coupled with HzOR (Fig. 3d–f).⁹¹ The small voltage of 0.90 V at 1 A cm⁻² and 100% faradaic efficiency of H₂ release are obtained for the OHzS reaction. Density functional theory (DFT) calculations suggest that the modulated electronic structure and the improved adsorption-free energy of intermediates are synchronously achieved by trace Ru doping (Fig. 3g and h). Inspired by this, more and more materials doped with precious metals, such as defective Ru-doped α-MnO₂ nanorods, Ru-doped VO_x/Ni₃S₂ amorphous/crystalline heterostructure, Ru-doped oxygen-defective Co₃O₄ nanosheet array, and microsphere-like Ir-doped Ni/Fe-based MOF arrays, have been widely developed as high-performance electrocatalysts for hydrazine hydrate-assisted electrolysis.^95–98

Alloying strategies have recently emerged as an effective way to rationalize the design of specific nanomaterials, which benefits from the increasing development of precision synthesis technology.⁹⁹ Alloying various metals not only adjusts the morphology and structure of the final material, but also increases the intrinsic electrocatalytic activity and stability of electrocatalysts to avoid deactivation.^100–102 For the HzOR-assisted water electrolysis, a series of advanced materials have been exploited via the alloying method. The design of ultrathin metallene may offer abundant favorable active sites and promote the utilization of metal atoms. For example, Wang and co-authors developed a trace Pt-doped Rhene by a simple solvothermal strategy (Fig. 4a).¹⁰³ This kind of Pt–Rhene shows excellent bifunctional performance with an ultralow cell voltage of 0.28 V at 100 mA cm⁻² for hydrazine-assisted water electrolysis, which mainly originates from the synergistic effect of dual active sites, the optimized d-band center, and improved reaction kinetics (Fig. 4b and c). In addition, similar bimetallic RhIr mesoporous nanospheres, AuPt alloys, and ultrafine RuPd alloy nanoparticles have been reported with superior electrocatalytic activity for OHzS.^104–106

Fig. 4 (a) EDS mapping images of Pt–Rhene. (b) The partial density of states (PDOS) of bulk and surface of Pt–Rhene. (c) Polarization curves of OHzS and OWS by using Pt–Rhene as catalysts.¹⁰³ Copyright 2023. Reproduced with permission from Elsevier. (d) Two-electrode polarization curves of Pt/C, PtCo, and Pt/C + IrO₂ for OHzS and OWS. (e) The volumes of N₂ and H₂ gases generated at different times. (f) Structural models of Pt and the PtCo alloy. (g) Free energy diagrams of (g) HER and (h) HzOR on various samples.¹⁰⁷ Copyright 2022. Reproduced with permission from Elsevier.

In addition to binary precious metal alloys, the use of non-precious metals to build alloys is an ideal way to improve catalytic performance and reduce electrolysis costs. For example, hydrogen bubbles are generated in situ and used as dynamic templates to construct surface-cleaned trimetallic nanosponges of Pt₅₃Ru₃₉Ni₈ and nanochain-like networks of PdNiRu alloy.^108,109 Both electrocatalysts display dramatically enhanced catalytic performance. In addition, nanosheets of the PtCo alloy are developed as bifunctional electrocatalysts for a proton exchange membrane hydrazine electrolyzer (PEMHE) with acidic electrolytes (Fig. 4d and e).¹⁰⁷ The optimized adsorption-free energy of H* and reduced dehydrogenation-free energy of N-based intermediates are realized by alloying Pt with Co (Fig. 4f–h). As a result, the PEMHE device achieves a high H₂ yield rate of 1.87 mmol h⁻¹ cm⁻² at 0.6 V, 100% removal efficiency of hydrazine, as well as excellent stability over 60 h at 100 mA cm⁻². In addition, there has been a rapid development of PdCo alloy nanoparticles, amorphous RhPb nanoflowers, Cu₂O-derived PtCu nanoalloy, and Cu₁Pd₃/C and RuSb nanobranches as excellent electrocatalysts for HER and HzOR. These innovative materials indicate the feasibility of improving the electronic structure and catalytic properties of noble metal hosts by alloying them with non-noble metals.^110–114

In recent years, high-entropy alloys have received extensive attention in the field of electrocatalysis due to their tunable multi-component and electronic structures.^116–120 For example, Xia's group designed and synthesized extremely small NiCoMoPtRu alloy nanoclusters with only seven atomic layers (1.48 nm) and high entropy for HzOR-assisted electrolytic H₂ generation (Fig. 5a and b).¹¹⁵ This high-entropy alloy exhibits a small overpotential of 9.5 mV at 10 mA cm⁻² and superior mass activity of 12.85 A mg⁻¹ of noble metals at −0.07 V. The OHzS electrolyzer thus constructed only needed a working voltage of 0.181 V to achieve 100 mA cm⁻², which outperformed the other well-known chemical-assisted H₂ generation systems (Fig. 5c–e). Theoretical calculations suggest that the fully active sites on high-entropy nanoclusters and some super-active sites are the key to achieving best-level performance (Fig. 5f and g).

Fig. 5 (a) High-angle annular-dark-field STEM (HAADF-STEM) image and (b) EDS elemental mapping images of high-entropy alloy nanoclusters (HEANCs). (c) Schematic illustration of the device and (d) polarization curves of OHzS and OWS by using HEANCs as catalysts. (e) The stability test of two-electrode electrolyzers. Free energy diagrams of (f) HER and (g) HzOR on various samples.¹¹⁵ Copyright 2024. Reproduced with permission from Wiley.

Interface engineering has been demonstrated as an effective strategy for material design to improve the performance of catalysts.^123–125 The coupling of different components can promote electron transfer, expose catalytic active sites at the interface, as well as optimize the physical and chemical properties of the catalysts, which are closely related to the electrocatalytic performance.¹²⁶ In recent years, carbon-based materials have been widely adopted as promising matrices to anchor metal-based catalysts.^127,128 For example, N, S co-doped hollow carbon spheres are used as supporters to anchor Ru clusters or Rh nanoparticles by regulating the coordination environment surrounding the metal–S bonds, leading to superior bifunctional activity and stability for HER and HzOR reactions.¹²⁹ In addition, other kinds of P, N-doped porous carbon or hollow dodecahedron carbons have also been developed for the decoration of RuP and Pd nanoparticles, respectively.^130,131 Both materials achieve remarkable performance due to the optimized adsorption/desorption sites on the electrocatalyst. Similarly, N-doped porous carbon supports ultrafine Ir nanoclusters and Ru nanoparticles, Rh₂S₃/N-doped carbon material, Ag nanoparticles-decorated Fullerene, and the porous carbon microsheets anchored with RuP₂ nanoparticles are synthesized by various chemical strategies; they show remarkable bifunctional performance for HER and HzOR.^132–136 However, it is difficult to form a strong coupling heterogeneous interface between metals and the carbon carrier, resulting in limited performance in practical applications.^137–139

Recently, oxide-supported metal catalysts have been used extensively to study the influence of metal–support interactions on catalytic performance.¹⁴⁰ The oxygen atoms at the metal/oxide interface are proven to have uniquely high reactivity, while the synergistic effect of metal/oxide contributes to the optimization of the reaction energy barrier. Therefore, the modulation of the metal/oxide interface has an indispensable role in electrocatalysis.¹⁴¹ For example, Rh/RhO_x nanosheets are synthesized by alkali-assisted synthesis, and the construction of highly active Rh–O–Rh interfaces endows ultra-low voltages in pH-universal conditions for H₂ generation (Fig. 6a–c).¹²¹ In addition, fabric-like Rh₂O₃–NiWO₄ nanosheets and RuO₂/Co₃O₄ are also developed as superior electrocatalysts due to an abundance of exposed active sites on the heterostructure, as well as the enhanced metal–support interactions.^52,142 Furthermore, the metal/metal interface is also important in regulating the binding energy of metal–hydrogen and N–N bond cleavage. For example, Duan's group reported Ru@Ag nanoparticles with abundant Ag–Ru interfaces, which acted as excellent bifunctional electrocatalysts for hydrazine-assisted water electrolysers (Fig. 6d).¹²² The optimized desorption energy of H₂ and N₂ and the higher barrier of N–N bond cleavage are confirmed in the Ru@Ag catalyst, leading to high electrocatalytic activity and selectivity (Fig. 6e–h). Meanwhile, the heterointerface Rh/Pd metallene and Au–Rh₂P mesoporous nanotubes are confirmed to have a strong electronic effect, highly exposed active sites, as well as optimal adsorption energy of intermediates, contributing to the excellent performance for HzOR-assisted H₂ production.^143,144

Fig. 6 (a) SEM image, (b) X-ray photoelectron spectroscopy (XPS) spectra and, (c) polarization curves of OHzS and OWS for the Rh/RhO_x-500 sample.¹²¹ Copyright 2022. Reproduced with permission from RSC. (d) EDS mapping images and (e) polarization curves of OHzS and OWS of Ru@Ag NPs. (f) DFT calculations of potential-determining steps for various catalysts. (g) Representative model of Ru@Ag. (h) Stability test of the Ru@Ag NPs‖Ru@Ag NPs electrolyser.¹²² Copyright 2024. Reproduced with permission from RSC.

An in-depth study and understanding of the reaction mechanism is significant for HzOR-assisted water electrolysis. A better knowledge in this field will drive the urgent requirement for precise design and construction of advanced electrocatalysts. In this regard, single-atom catalysts are undoubtedly an important model material due to the strong exposure of active metal atoms and the nearly 100% efficiency of atomic utilization, which boosts the development of electrochemical performance.^145–147 Typically, carbon-based materials are an important support for single metal atoms. For example, a single atom of iridium anchoring the N-doped carbon layer and single atoms/clusters of Ru decorating N-doped carbon have been developed as robust electrocatalysts for HER and HzOR.^148,149 However, the stability of such material remains an important challenge, especially in long-term electrolysis processes with high current density. Thus, metal-based oxides or sulfides have been exploited for the dispersion of single noble metal atoms. For example, the spinel Co₃O₄ with octahedral structure is adopted as a substrate for the lattice-confined Ru sites, causing a reduced energy barrier for the desorption of H* and the formation of N₂H₂* species on Ru–Co₃O₄ catalyst (Fig. 7a–d).¹⁵⁰ Remarkably, a small potential of −0.024 V at 100 mA cm⁻² and a high output of 0.48 kW h electricity per m³ H₂ is realized (Fig. 7e–h). In addition, urchin-like oxygen vacancy-rich WO₃ is also confirmed as an effective matrix for the immobilization of Ru single atoms, resulting in superior performance for pH-universal HzOR-assisted water splitting.¹⁵¹ The –O terminate Ti₃C₂O_x is regarded as a good electron promoter for enhancing the activity of heterojunctions. Luo and co-authors confirmed that the oxygen-functionalized Ti₃C₂O_x MXene is promising support for the decoration of highly dispersed single atoms of Rh. The Rh–SA/Ti₃C₂O_x catalyst achieves remarkable catalytic activities for pH-universal HER and HzOR reactions, as well as for the ultrahigh H₂ generation rate of 45.77 mmol h⁻¹.¹⁵²

Fig. 7 (a) Schematic illustration of the synthesis process, (b and c) HAADF-STEM images, and (d) corresponding intensity profiles of Ru–Co₃O₄. (e) The comparison of H₂ volume between theoretical and experimental tests for HER. (f) Techno-economic analysis of various systems. (g) Calculated models of Ru–Co₃O₄ and (h) free energy diagrams of HzOR on Ru–Co₃O₄ and Co₃O₄ samples.¹⁵⁰ Copyright 2024. Reproduced with permission from Wiley.

3.2 Non-noble metal-based materials

Considering some inherent shortcomings of precious metal-based catalysts, such as high prices, scarcity of reserves, and weak stabilities, researchers have devoted efforts toward exploiting non-noble metal-based materials for energy-saving H₂ generation by HzOR-assisted water electrolysis. Therefore, the development of advanced electrocatalysts with lower cost and superior activity has achieved significant progress, including non-precious metal single-atom catalysts.^154,155 For example, Yuan et al. developed a Ti₃C₂T_x–MXene rich in Ti vacancies as a trap that anchored substrates for the dispersion of individual Ni single atoms by a self-reduction strategy (Fig. 8a–c).¹⁵³ The strong Ni–C interaction around three neighboring C atoms is realized on the Ni atoms that occupy the Ti vacancies. As a result, the Ni SACs/Ti₃C₂T_x exhibits a small onset potential of −0.03 V and excellent catalytic stability (Fig. 8d). DFT calculations suggest that hydrazine adsorption is promoted and the energy barrier of HzOR is optimized from the redistributed electronic density of states around the Ni–C bonds (Fig. 8e). In addition, single atomic Mn sites are embedded in a B, N co-doped carbon matrix, which is confirmed to have superior bifunctional performance for HER/HzOR reactions under alkaline conditions. This Mn–SA/BNC catalyst delivers low potentials of −51 mV at −10 mA cm⁻² and 132 mV at 10 mA cm⁻² for the HER and HzOR, respectively.¹⁵⁶ In addition to the design of a single atom, non-noble metal-based alloys, such as Co–Mo alloy, nanotubular Ni–Cu alloy, Ni(Cu) cubic nanopore, and porous Ni(Cu) coatings, have also been developed via different chemical strategies; they are reported to demonstrate superior electrocatalytic activity for the HER and HzOR.^157–160

Fig. 8 (a) Schematic diagram of the synthesis and (b) HAADF-STEM image of Ni SACs/Ti₃C₂T_x. (c) The k²-weighted Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) spectra of Ni K-edge and (d) polarization curves of OHzS for various samples. (e) Free energy diagrams of HzOR on Ni SACs/Ti₃C₂T_x and Ni NPs/Ti₃C₂T_x samples.¹⁵³ Copyright 2022. Reproduced with permission from Wiley.

In recent years, both anionic modification and cationic doping strategies have been widely used to design various electrocatalysts with unique advantages in regulating catalytic performance.^161–164 Firstly, the anion modification strategy can achieve a synergistic optimization of the electronic structure and catalytic properties of the materials without the introduction of additional metal centers; it has been widely used in the field of HzOR-assisted H₂ production. For example, N-doped M/CoO (Ni, Co, and Mn) heterostructure microspheres, P-doped CoCO₃ nanosheets, S-doped tri-metallic NiCoFe phosphide, and S-rich MoS₂ materials have been synthesized to study their catalytic performance in the HER or HzOR.^58,165–167 On the other hand, metal cation doping can not only regulate the electronic structure of the supports but also introduce additional catalytic active centers, which has been considered to be an effective way to design high-performance bifunctional electrocatalysts. Recently, transition metal-based chalcogenides, such as sulfides and selenides, as well as other metal-based nitrides and phosphides, have gained extensive attention for various catalytic purposes due to their unique structural properties and intrinsic high stability.^168–172 For example, a series of metal-doped transition metal-based catalysts, including Mn-doped CoS₂, Fe-doped Ni₂P–Co₂P–Zn₃P₂ heterogeneous material, V-doped Ni₃N nanosheet, porous Ce-doped Ni₃N nanosheet arrays, Fe-doped Ni₂P/CoP, Fe-doped Co₃N, Ni_0.6Co_0.4Se, and Fe-doped CoS₂ nanosheets have been developed and their remarkable advantages as bifunctional electrocatalysts in HzOR-assisted seawater splitting have been confirmed.^173–180

Recently, a dual-modification strategy has emerged as an alternative way to further improve the performance of catalysts. The He group developed a dual-cation co-doping strategy to improve the HzOR and HER performance of Ni₂P via control lattice strain engineering.¹⁸² The Cu₁Co₂ co-doped Ni₂P induces an optimal compressive strain of −3.62% on the optimized d-band center and reduces the energy barrier of the catalytic process, thereby achieving superior bifunctional activity for both the HzOR and HER. In addition, Zhang and co-authors designed and fabricated an integrated electrode by growing P in situ and co-doping Co₃N nanowire arrays with W on a nickel foam substrate (Fig. 9a–c).¹⁸¹ DFT calculations revealed that the co-doping of P/W not only endows the improved dehydrogenation of adsorbed *NH₂NH₂, but also optimizes the free energy of adsorbed H*, resulting in the highly efficient bifunctional activity with an ultra-low operation voltage of 277 mV at 200 mA cm⁻² for the OHzS reaction (Fig. 9d–h).

Fig. 9 (a) The EDS mapping images and (b) HAADF-STEM images of PW-Co₃N nanowires. (c) W L₃-edge extended XAFS (EXAFS) fitting curves and wavelet-transformed (WT) contour plots of PW-Co₃N sample. (d) HzOR polarization curves of various samples. (e) Polarization curves of OHzS and OWS with PW-Co₃N as catalysts. (f) The comparison of H₂ volume between calculations and experimental tests. Free energy diagrams of (g) HER and (h) HzOR on various samples.¹⁸¹ Copyright 2020. Reproduced with permission from Nature.

In the past few decades, the design of heterostructures of non-precious metal-based materials has been an important research field. For HzOR-assisted water electrolysis, a variety of heterogeneous materials have been widely designed and studied for their catalytic performance. The application of transition metal oxides is limited by their relatively weak intrinsic conductivity. Therefore, heterogeneous interface manipulation based on non-noble metal oxides is an ideal way to design high-performance electrocatalysts.¹⁹⁵ Recently, various heterogeneous materials, such as Co(OH)₂/MoS₂ nanosheets, MoO₂/Co, MoO_x/MoS₂ core–shell nanowires, NiOOH@cobalt copper carbonate hydroxide, SNiC₂O₄–Nb₂O₅, FeWO₄–WO₃, Co₃O₄/Co, Co/amorphous LaCoO_x, and Ni(OH)₂/Ni₂P, have been developed as highly effective electrocatalysts for energy-saving H₂ generation via the HzOR-assisted process.^{189,196–203} In addition, transition metal nitrides/phosphides have emerged as promising models for heterostructure engineering because of their versatile advantages, such as high stability and high conductivity. For example, CoP/Co Mott–Schottky material, Ni₂P/Zn–Ni–P nanosheet, 1D/3D CoN–Co₂N, Ni(Cu)@NiFeP, NiCoP@CoFeP, and porous Ni₃N–Co₃N nanosheets have been assessed and confirmed to have superior bifunctionality toward HER and HzOR.^{64,184,188,191,204–206} However, the lattice mismatch for heterojunction engineering is a universal challenge for many heterostructures, which hinders electron transfer at the interface, leading to limited bifunctional performance. Thus, the development of lattice-matched dual-phase materials with abundant interfaces is required to boost the electron transfer efficiency.

Recently, Yang and co-authors constructed a heterojunction with biphasic metal nitrides (Ni₃N–Co₃N) containing highly lattice-matched interfaces (Fig. 10a).¹⁹¹ The electron redistribution at the enhanced local electric field between electron-rich Ni₃N and electron-deficient Co₃N is triggered by Mn doping (Fig. 10b and c). As a result, this Mn@Ni₃N–Co₃N/NF bifunctional electrocatalyst achieves a small voltage of 0.49 V at 500 mA cm⁻² and at least 53.3% energy consumption of H₂ generation compared to traditional water splitting (Fig. 10d–f). On the one hand, DFT simulations indicate that highly active Ni species and electron-deficient Co centers contribute to the optimization of the adsorption energy of H₂O, H*, and N₂H₄. On the other hand, researchers have confirmed that heterostructures designed using materials with intrinsically high HER activity can simultaneously improve the bifunctional activity of the HER and HzOR. In this regard, the NiMo alloy, being a typical high-efficiency HER material, has been widely studied.^207–210 For example, hierarchical NiMo/Ni₂P ohmic contact heterojunction was developed by the Shen group to achieve a low cell voltage of 343 mV at high current densities of 500 mA cm⁻² for the OHzS electrolyzer (Fig. 10g–i).¹⁸⁷ Moreover, an abundance of NiCo/MoNi₄ heterostructure interfaces and metallic hierarchical Co/MoNi heterostructures have been demonstrated to be highly active centers for both the HER and HzOR.^185,211 Multiple modulation strategies to optimize the electrocatalytic activity of materials have been proven to be feasible. For example, both heterostructure interfaces and heteroatom substitution have been adopted for the design and synthesis of Mo–Ni₃N/Ni, N–Ni₅P₄@CoP, and Fe/F–Ni₂P electrocatalysts for the OHzS systems.^190,212,213

Fig. 10 (a) HRTEM image and (b) average charge density difference of Mn@Ni₃N–Co₃N. (c) Schematic illustration of the enhanced charge transfer by Mn. (d) The picture of the gas-collection device and (e) the volumes of H₂ and N₂ at different times. (f) The stability tests of OHzS by Mn@Ni₃N–Co₃N/NF.¹⁹¹ Copyright 2024. Reproduced with permission from Wiley. (g) Polarization curves of OHzS with NiMo/Ni₂P and Pt/C as catalysts. (h) Free energy diagrams of HER. (i) The difference of energy bands between Schottky and ohmic contacts.¹⁸⁷ Copyright 2024. Reproduced with permission from Wiley.

As mentioned above, carbon supports are common materials for supported metal catalysts. As a typical representative, reduced graphene oxide is widely used to improve the catalytic activity and stability of catalysts due to its ultra-thin two-dimensional characteristics and high conductivity.²¹⁴ For example, N-doped NiZnCu-layered double hydroxides have been grown in situ on the reduced graphene oxide (N–NiZnCu LDH/rGO), representing a promising bifunctional catalyst for HER and HzOR.²¹⁵ Moreover, the active Ni–P species are cathodically deposited onto reduced graphene oxide nanosheets, achieving improved conductivity for excellent stability and catalytic activity for HzOR-assisted HER system.²¹⁶ On the other hand, direct pyrolysis of precursors to form carbon-supported metal catalysts is a widely used approach. The Li group utilized dual-heteroatom-containing metal–organic framework sheets to synthesize Co nanoparticles embedded with B and N co-doped carbon matrix to achieve a small voltage of 0.617 V at 100 mA cm⁻² for the OHzS electrolyzer.²¹⁷ However, for carbon-supported catalysts, to the accurate construction of the catalytic active sites and interface structure is key to improving their bifunctional activity and stability. Xie and co-authors confirmed that the controllable construction of dual nanoislands on Ni/C nanosheet array is an effective way to integrate dual active sites for both HER and HzOR (Fig. 11a–c).¹⁹³ DFT results suggest that a more thermoneutral H* adsorption on the C sites and favorable N₂H₄ dehydrogenation kinetics on the surface can expose the Ni centers, and thus, are the main factors for the boosted bifunctionality. Remarkably, this catalyst delivers low potentials of −37 mV and −20 mV at 10 mA cm⁻² for HER and HzOR, respectively (Fig. 11d–i).

Fig. 11 (a–c) The TEM, HRTEM, and EDS mapping images of Ni/C hybrid nanosheet array. (d) Polarization curves of OHzS by using Ni/C and Pt/C as catalysts. (e) The stability tests of OHzS and (f) the generated H₂ volume with different times. (g) A structural model of Ni/C, and free energy diagrams of (h) HER and (i) HzOR on various samples.¹⁹³ Copyright 2022. Reproduced with permission from Wiley.

The highly reactive metal species in metal-based catalysts are generally considered to be the main catalytically active centers of HER and HzOR. Therefore, most of the reported catalysts are based on noble or non-precious metal materials to develop high-performance electrocatalysts. Recently, some studies have shown that non-metallic materials can also be used for HzOR-assisted H₂ production, although the catalytic performance of such materials is limited. For example, the You group reported the fabrication of a metal-free pyridinic nitrogen-rich carbon paper (N-CP-800) by electropolymerization of aniline via an annealing treatment (Fig. 12a–e).¹⁹² The abundant pyridinic-N exists as zig and arm configurations in N-CP-800, allowing the favorable rate-limiting proton desorption of reaction intermediates for HzOR. As a result, this N-CP-800 catalyst shows a low potential of only 0.78 V at 10 mA cm⁻² for HzOR under neutral conditions (Fig. 12f–h). However, although the coupling of N-CP-800 and CoP can help achieve a small cell voltage of 1.56 V at 10 mA cm⁻² for HzOR-assisted seawater electrolysis, the potential application of this kind of metal-free material is still challenging.

Fig. 12 (a) The X-ray diffraction (XRD) patterns, (b) SEM mapping images, (c) TEM and HRTEM images of N-CP-800. (d) Fourier transform infrared spectroscopy (FTIR) spectra of CP, PANi-CP, and N-CP-800 samples. (e) XPS spectrum of N 1s for N-CP-800. (f) Polarization curves of various samples and (g) in situ FTIR spectra for OHzS. (h) Free energy diagrams of HzOR on different N-sites.¹⁹² Copyright 2024. Reproduced with permission from Springer.

To sum up, the improvement in performance of advanced electrocatalysts by designing heterostructures, doping, or alloying, may additionally benefit from their optimized electronic interaction and synergistic effects (Table 1). However, building precise structural models of these catalysts to study the structure–activity relationship is challenging. Moreover, the single-atom strategy is a promising model for studying the catalytic mechanism of HzOR-assisted water electrolysis. However, their catalytic activity and stability during long-term testing have been unsatisfactory. Therefore, the development of advanced synthesis methods and characterization techniques remains important for the development of high-performance electrocatalysts for HzOR and HER reactions.

Table 1 Performance comparison of representative non-noble metal-based electrocatalysts for HzOR-assisted water electrolysis

Catalysts	HzOR		HER		OHzS
	Potential at 10 mA cm^−2 (mV)	Tafel slope (mV dec^−1)	η 10 (mV)	Tafel slope (mV dec^−1)	Voltage at 10 mA cm^−2 (mV)
P, W co-doped Co3N^181	−55	14	41	40	28
Ce–Ni3N/NF^178	92	47	—	—	256
Fe–CoNiP@NC^175	280@1 A cm^−2	54.7	490@1 A cm^−2	86.09	560@1 A cm^−2
Cu1Co2–Ni2P/NF^182	−52	14.4	51	52.3	160
NiCoMoPtRu/C^115	50@3.26 A mg^−1	32	9.5	29.8	25
CoH–CoPV@CFP^183	−61	59.9	77	75.2	230@0.5 A cm^−2
CoN–Co2N@NF^184	−110	42	71	84	53
NiCo–MoNi4 HMNAs/NF^185	−30	28.5	68	67.5	630@0.25 A cm^−2
(P–Co/Ni3P)A3/NF^55	−79	1.8	10	45	50@0.3 A cm^−2
B–CoNS@CuPD^186	−146	21.7	70	76.6	60
NiMo/Ni2P^187	−17	35.24	15	29.45	181@0.1 A cm^−2
Ni3N–Co3N^188	−88	21.6	43	35.1	71
Ni(OH)2/Ni2P/NF^189	−14	22.0	72	82.7	80
Mo–Ni3N/Ni/NF^190	−0.3	45	45	48	55
Mn@Ni3N–Co3N/NF^191	1.65 V@0.5 A cm^−2	19.3	310@0.5 A cm^−2	117.2	490@0.5 A cm^−2
CoP/N-CP-800 (neutral condition)^192	780	99	140	108	780
Ni–C HNSA^193	−20	16.2	37	31.9	140@50 mA cm^−2
ZIF67@CoNiSe-3^194	130@0.4 A cm^−2	44.3	49	41.4	450@0.1 A cm^−2
Ni SACs/Ti3C2Tx ^153	−30	62	—	—	—
Mn–SA/BNC^156	132	75.9	51	51.2	0.41

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4. Hydrazine-assisted seawater splitting

Although hydrazine-assisted H₂ production by water electrolysis outperforms traditional petrochemical techniques due to its capacity as renewable, high processing efficiency, and environmental friendliness, the critical concerns focus on the massive requirement for high-purity water. In this regard, seawater has been regarded as a sustainable water resource to generate high-purity H₂ due to its great volume, i.e., ∼97% of all water on our planet. However, direct seawater electrolysis faces huge challenges in its application for large-scale hydrogen generation because of the complex ionic chemistry of seawater. An important problem is the competitive chlorine evolution reaction (CER) with OER on the anode.²²⁰ The electrolysis efficiency and sustainability are largely hindered due to anodic dissolution and contamination from the generation of toxic and corrosive chlorine species.²²¹ Another challenge in maintaining high electrolysis efficiency is preventing blocked active sites by the formed solid precipitates. Therefore, suppressing the CER process by reducing the overpotential of OER below 0.48 V in an alkaline medium is important. However, the demands of industry-level criteria for seawater electrolysis are far beyond this overpotential, forcing researchers to develop high-performance electrocatalysts or effective modification strategies to hinder the CER.

The alternative HzOR process has a far lower oxidation potential than CER, which contributes to eliminating the notorious problems of chlorine chemistry based on high current density and H₂-yielding efficiency (Fig. 13a).²¹⁸ Therefore, the coupling of HzOR with HER in a costless seawater electrolyte can become a promising and valuable avenue to achieve environmental sustainability and cost-effectiveness of H₂ generation. In recent years, researchers have spent a lot of effort to develop high-performance electrocatalysts for HzOR-assisted seawater electrolysis. For example, the Qiu group reported a super aerophobic–hydrophilic NiCo/MXene-based electrode for chlorine-free H₂ production. The hybrid hydrazine-assisted seawater electrolyzer only needs an ultrasmall voltage of 0.7 V, high H₂ yields of 9.2 mol h⁻¹ g_cat⁻¹, as well as high stability over 140 h at a high current density of 500 mA cm⁻² (Fig. 13b).²¹⁸ In addition, not only the chlorine-induced hazards on cell performance of NiCo/MXene-based electrolyzer can be effectively reduced, but also the consumption of electricity is significantly decreased by 30–52% at 500 mA cm⁻² compared to the traditional alkaline water electrolysis (Fig. 13c). The excellent bifunctional performance of NiCo/MXene materials is attributed to the improved water/hydrazine adsorption capability, interfacial conductivity, strong structure stability, and gas-releasing pattern at large current densities. Moreover, a cobalt/nitrogen co-doped carbon nanosheet array (CC@CoNC) is developed by an in situ pyrolysis strategy of metal–organic framework (MOF).²¹⁹ The hydrazine-assisted seawater electrolysis system delivers a small cell voltage of 0.557 V, as well as a low electricity expense of 1.22 kW h per cubic meter of H₂ at 200 mA cm⁻² (Fig. 13d–g). To improve the catalytic activity and stability of HzOR-assisted seawater electrolyzers, more and more regulatory strategies have been exploited to optimize the surface properties and electronic structure of electrocatalysts. For example, the noble metal-doped materials (Ru–Ni(OH)₂ nanowire network, Ru–(Ni/Fe)C₂O₄, NiRh–MOF), non-noble metal-doped materials (Cu–CoFe/Co/NC, Fe/P co-doped NiMoO₄, FeCo–Ni₂P@MIL–FeCoNi), and heterostructured materials (CoP/Ni₂P, MoNi/MoO₂, bimetallic Ni₄Mo/Ni₄W nanoalloys) have emerged as bifunctional electrocatalysts with remarkable catalytic performance for HzOR-assisted seawater splitting.^222–230

Fig. 13 (a) The Pourbaix diagram of different reactions on artificial seawater. (b) Polarization curves of HER//HzOR and HER//OER in neutral or alkaline seawater media. (c) Energy equivalent input and CO₂ equivalent emission of different H₂ generation techniques.²¹⁸ Copyright 2021. Reproduced with permission from Nature. (d) The schematic diagram of CC@CoNC-600 catalyst during seawater electrolysis. (e) Polarization curves of OHzS and OWS, and (f) the stability tests of OHzS using CC@CoNC-600 as a catalyst. (g) The comparison of the cell voltages between different catalysts.²¹⁹ Copyright 2023. Reproduced with permission from Wiley.

5. Self-powered systems for hydrazine-assisted H₂ production

Recently, considering the requirement of large-scale renewable energy systems for traditional seawater electrolysis, self-powered systems for hydrazine-assisted H₂ production have attracted more and more attention due to the low electricity consumption of HzOR-assisted seawater electrolysis. Small-scale renewable energy systems (fuel cells or solar cells) are feasible for integration with hydrazine-based hybrid electrolyzers. For example, the Wang group developed hydrazine-assisted self-powered seawater electrolysis by coupling direct hydrazine fuel cell (DHzFC) with OHzS system for the production of high-purity H₂.²³¹ This self-powered device delivers a maximum power density of 37.06 mW cm⁻² at 98.31 mA cm⁻² and high H₂ yield of 0.90 mmol h⁻¹ with 1.0 M KOH + seawater + 0.5 M N₂H₄ electrolyte without an external power source (Fig. 14a–e). In addition, the DHzFC-powered OHzS device is built by using 6W–O–CoP/NF as the anode and features a maximal power density of 142 mW cm⁻² and a superior H₂ production rate of 3.53 mmol cm⁻² h (Fig. 14f–h).²³² The performance of self-powered systems can further be improved by using catalysts decorated with noble metals. For example, Ru₁–NiCoP and RuFe–Ni₂P have been further developed to achieve a maximum power density of 60.1 mW cm⁻² at 157.8 mA cm⁻² and 176 mW cm⁻² at 0.40 V, confirming the feasibility of the application of self-powered H₂ production.^233,234

Fig. 14 (a) Schematic illustration and (b) digital photograph of self-powered H₂ production device integrated by driving the OHzS electrolyzer with DHzFC as power supply. (c) Discharge curve and power density of DHzFC. (d) The generation rates of H₂ in different electrolytes. (e) Ultraviolet-visible (UV-vis) spectra of the industrial N₂H₄ wastewater at different test times.²³¹ Copyright 2023. Reproduced with permission from Wiley. (f) Discharge curve and power density of W-CoP/NF and Pt/C for DHzFC. (g) Digital photograph of the self-powered device. (h) The yield and generation rate of H₂ by using a DHzFC-powered system.²³² Copyright 2023. Reproduced with permission from Springer.

In addition to the DHzFC-powered OHzS system, the solar cell is also applied to couple with OHzS by photovoltaic electrolysis (PV–EC) system. For example, Yong and co-authors demonstrate that the perovskite solar cells (PSC) can be used as a power source to drive the OHzS system.²³⁵ This kind of self-powered PV–EC system realizes excellent performance with a high photocurrent density of 23 mA cm⁻² and a considerable H₂ amount for hydrazine-assisted H₂ production. Furthermore, the Chen group recently developed a pre-protonated vanadium hexacyanoferrate (p-VHCF)–N₂H₄ liquid battery, which can power the decoupled electrolysis of H₂ generation and hydrazine oxidation at nighttime, providing a new idea for the renewables-to-hydrogen conversion.²³⁶

6. Conclusions and outlooks

In summary, this comprehensive review discusses state-of-the-art research progress on the cost-effective and efficient electrocatalysts for HzOR-assisted water electrolysis based on noble, non-noble metal-based, and metal-free materials to date. In terms of precious metals, many efforts have been devoted to optimizing the utilization efficiency through designing specific morphology, exposing high-active planes, or incorporating other low-cost transition metals or carbon materials to achieve a highly homogeneous dispersion of precious metal-based active sites. On the other hand, non-noble metal-based materials, such as oxides, phosphides, alloys, nitrides, and chalcogenides, as well as single-atom structures feature breakthroughs. Some of the advanced materials exhibit comparable performance to the benchmark noble metals. Specifically, the nanostructure engineering of electronic structure and surface properties of the advanced catalysts have been highlighted: (i) constructing specific structure or morphology to improve the exposed number of surface active sites, (ii) designing coupled heterogeneous interfaces or combining with highly conductive matrix to speed up electron transfer, thereby reducing the barriers to kinetic reactions, (iii) introducing heteroatoms to optimize the electronic configuration and improve the thermodynamic adsorption/desorption of reaction intermediates on the surface of catalysts. Although there are relatively few examples of metal-free catalysts, doping and design of defect structures are feasible ways to improve catalytic activity. With the development of various bifunctional electrocatalysts and the better catalytic performance of HzOR-assisted water electrolysis for H₂ production, significant progress has been made toward practical application-oriented systems, such as with seawater electrolyte or self-powered systems, that represent an important research direction in this field for future studies.

However, despite the significant progress made for the coupling of sustainable and energy-saving H₂ generation via small-molecule oxidation reactions with HER, there is still a long journey to continue before HzOR-assisted water electrolysis can be achieved for commercial and industrial applications. The co-existence of opportunities and challenges creates many possibilities for future studies as proposed below: (i) most of the reported synthetic strategies focus on studying the relationship between structure and activity while ignoring the enhanced promotion of industrial applications. Therefore, based on maintaining high activity and stability, preparing high-quality electrocatalysts or electrodes on a large scale and realizing the leap from the laboratory scale to the industrial level is of significance. (ii) Despite the coupling of thermodynamically favorable HzOR reactions that lower the cell voltage of a hybrid water electrolyzer, the anode feedstock of hydrazine is expensive, unstable, and toxic compared to the traditional water splitting. Moreover, the worthless oxidation products of HzOR also increase the separation costs. Therefore, the use of industrial wastewater with highly toxic hydrazine hazards instead of pure hydrazine hydrate solution is a more feasible option. Although some materials have been used in this research, the application of direct energy-saving electrolysis of industrial hydrazine hydrate wastewater for hydrogen production is rarely reported. (iii) Achieving high operational stability of electrocatalysts at large current density is a formidable task. Although the oxidation products (N₂) from HzOR can avoid the safety issues generally associated with the H₂/O₂ mixture, the membrane electrodes assembly (MEA) devices are still necessary to achieve the direct separation of high-purity H₂. Therefore, more high-performance electrocatalysts should be directly used for the application of actual MEA devices. (iv) More research efforts have been made to develop direct HzOR-assisted seawater electrolysis. However, most of the research is still based on evaluating performance in simulated alkaline seawater electrolytes. It is necessary to reveal whether some existing nanostructure-engineered electrocatalysts can be used for H₂ production from HzOR-assisted real seawater electrolysis.

Data availability

No primary research results, software or code have been included, and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22205205), the Zhejiang Provincial Natural Science Foundation of China (LQ22B030008), and the Science Foundation of Zhejiang Sci-Tech University (ZSTU) under Grant No. 21062337-Y.

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