Recent advances in non-precious group metal-based catalysts for water electrolysis and beyond

Abstract

As the demand for green hydrogen (H₂) rapidly increases, the development of water electrolysis technology has been receiving great attention. Indeed, recent remarkable advances in catalyst materials increased the feasibility of water electrolysis for a future H₂ economy and technology. In this review, we summarize representative non-precious group metal-based materials for achieving active and stable water electrolysis performances. Our comprehensive range of the state-of-the-art catalysts includes doped carbon catalysts, metal borides, metal carbides, metal oxides, metal phosphides, metal sulfides, and single-atom catalysts. For each class of materials, we focus on the synthesis and catalytic performances of the state-of-the-art materials toward water electrolysis and present the current challenges and outlooks of such materials, along with prospective insights to develop and realize practical systems.

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Jin Young Kim

Jin Young Kim is a principal research scientist at the Center for Hydrogen and Fuel Cell Research at the Korea Institute of Science and Technology (KIST). He received a PhD degree in Materials Science and Engineering from the Massachusetts Institute of Technology under the supervision of Prof. Francesco Stellacci. Prior to joining KIST in 2013, he was a postdoctoral associate with Prof. Edward Sargent at the University of Toronto and was a research staff member at the Samsung Advanced Institute of Technology. His research area is the development of functional nanomaterials for electrochemical-based energy and environmental applications.

Kwangyeol Lee

Professor Kwangyeol Lee obtained his PhD degree (1997) in chemistry from the University of Illinois at Urbana-Champaign. After fulfilling his military obligation, he joined Korea University in 2003 as a chemistry faculty member, before being appointed as a professor. He is the recipient of the 2009 Wiley-KCS Young Scholar Award and 2019 Excellent Research Award (KCS Inorganic Chemistry Division). His current interests include the development of synthetic methodologies for nanoscale materials, the application of nanomaterials in biomedical fields, and the development of nanotechnologies to support the environment by creating sustainable energy sources.

Sang-Il Choi

Professor Sang-Il Choi received his PhD degree in inorganic chemistry from the KAIST (2011). After his postdoctoral research in the Xia group at the Georgia Institute of Technology, USA, he joined Kyungpook National University in 2015 as an assistant professor. He is the recipient of the 2018 POSCO TJ Park Science Fellowship. His research interests include the design and synthesis of nanomaterials and exploration of their applications in electro-catalysis.

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1. Introduction

Hydrogen (H₂) has been considered as one of the promising alternative energy sources to fossil fuels. However, since the steam reforming process is the major process for H₂ production, significant CO₂ emission, separation of the gases produced, and environmental impacts are the main drawbacks of the relevant technology.^1,2 Therefore, water electrolysis, a carbon-free H₂ production method, has been reconsidered significantly over the past decade, although technical limitations such as low power-to-gas generation efficiency and poor long-term durability have been revealed. Indeed, the remarkable advances in catalyst materials in recent years have enabled the re-evaluation of water electrolysis for future H₂ economy and technology.^3–8

Since water electrolysis occurs by supplying electric power to overcome the thermodynamic barrier, both the anode and cathode must adapt suitable electrocatalysts to minimize the overpotential (η, the potential difference between the theoretical and experimental values for a redox reaction).⁹ For the hydrogen evolution reaction (HER) at the cathode, Pt-based catalysts have been utilized as the most efficient electrode materials thanks to their optimal free energy of atomic H adsorption (ΔG_H), achieving H₂ production with a high exchange current density (j₀) and a small Tafel slope value.^7,8 Meanwhile, Ir- and Ru-based catalysts form active surface oxide layers during the anode reaction, providing optimal catalytic sites towards the oxygen evolution reaction (OER).^10,11 However, owing to the high price and scarcity of Pt, Ir, and Ru, the practical application of water electrolysis on a global scale has been limited. Given this situation, tremendous effort has been made to develop highly active, durable, and non-precious group metal (PGM) catalysts. Notable examples of the state-of-the-art catalysts include doped carbon catalysts, metal borides, metal carbides, metal oxides, metal phosphides, metal sulfides, and single-atom catalysts.^12–17

Although there have been extensive interest and research development of catalyst materials for water electrolysis, most of the review articles on the relevant topics have a limited coverage for half-cell reactions and material classes. Therefore, to provide a comprehensive overview of electrocatalysts for the anode and cathode reactions of water electrolysis, this review paper has compiled information from technical levels on the performance of the HER/OER in eight different classes of the state-of-the-art non-PGM-based catalysts. We first introduce the fundamental understanding of the HER and OER and the performance of representative catalysts. Then, we categorize eight different classes and introduce the current status and challenges of synthesis and catalysis with a critical view on the performance and stability. The scope of this review excludes the applications of bifunctional catalysts. Finally, we provide outlooks and future directions of model catalysts to realize highly active and durable catalysts. With the explosive increase in the demand for current technological developments, we believe that this review will provide promising and very interesting prospects for future research related to water electrolysis.

2. Water electrolysis

Water electrolysis, or electrochemical water splitting to produce H₂ (and O₂), has been considered to be a clean and sustainable system to replace fossil fuels. Water electrolysis includes two half-cell reactions, namely the HER and OER at the cathode and anode, respectively, and the electrolysis can be performed at various pHs. Fig. 1 shows a scheme of a water electrolyser system and the corresponding chemical equations for the electrode reactions in acidic or alkaline electrolytes are presented below.^18–20

Fig. 1 A scheme of a two-electrode system for overall water electrolysis.

In an acidic electrolyte:

Cathode: 2H⁺ + 2e⁻ → H₂, E₀ = 0 V

Anode: 2H₂O → O₂ + 4H⁺ + 4e⁻, E₀ = 1.23 V

In an alkaline electrolyte:

Cathode: 2H₂O+ 2e⁻ → H₂ + 2OH⁻, E₀ = −0.83 V

Anode: 4OH⁻ → O₂ + 2H₂O+ 4e⁻, E₀ = 0.40 V

A voltage of 1.23 V is a theoretical value required for water electrolysis. However, additional η must be applied practically to address the potential losses because of the kinetic constraints in the electrode reactions. Developing efficient electrocatalysts can therefore reduce the η of the HER and OER to kinetically accelerate water electrolysis.²¹

Water electrolysis can take place in different pathways at different pHs. In the case of the HER in acidic electrolytes, H₂ is generated by the reduction of H⁺ (2H⁺ + e⁻ → H₂) and for this, two mechanisms of Volmer–Heyrovsky and Volmer–Tafel mechanisms have been widely accepted. Both the mechanisms firstly involve the adsorption of H⁺ on the surface of the electrode (Volmer step: H⁺ + e⁻ → H_ads). The second step occurs in different ways; when H_ads is stable, the Tafel step (2H_ads → H₂) is predominant, and vice versa, the Heyrovsky step (H⁺ + H_ads + e⁻ → H₂) can occur. Therefore, a catalyst with ΔG_H close to zero shows the most optimal conditions for the HER process.^22,23 Meanwhile, the HER in alkaline electrolytes also proceeds through the Volmer–Heyrovsky and Volmer–Tafel mechanisms, but their reaction paths are totally different from those under acidic conditions. As water dissociation occurs in the alkaline HER, the reaction kinetics is about two to three orders of magnitude lower than that in acidic media. Based on the Tafel slope of the polarization curve in linear sweep voltammetry (LSV), it is possible to assume whether the Volmer or the Tafel/Heyrovsky step is rate determining.^8,24,25

The reaction pathway of the OER also depends on the pH of the electrolyte. In acidic media, O₂ is evolved by the oxidation of two water molecules. However, in alkaline and neutral solutions, the OER involves the oxidation of four OH⁻ to water and O₂. Many research groups have proposed possible mechanisms for the OER under acidic and alkaline conditions. Most of the proposed mechanisms include the same intermediates such as MOH, MO, and MOOH (M = electrode material surface). The major difference is the circumstances surrounding the O₂ evolution site as shown with blue and red arrows in Fig. 2. In addition, there is another O₂ evolution pathway from a MO intermediate under acidic conditions. As marked with a yellow route, the direct combination of 2MO produces O₂, which is different from the red route involving the formation of the MOOH intermediate, which subsequently decomposes to O₂. Despite these differences, the common consensus is that the M–O bond interactions within the intermediates (M–OH, M–O and M–OOH) are crucial for the overall OER performance.^19,21,26

Fig. 2 The OER mechanisms under acidic (red route) and alkaline (blue route) conditions. Under acidic conditions, the red route indicates that the oxygen evolution involves the formation of a peroxide (M–OOH) intermediate, while the yellow route shows the direct reaction of two adjacent M–O intermediates to produce O₂.

Although PGM-based electrocatalysts have demonstrated the most optimal activities toward the HER/OER, their scarcity and high cost impede practical applications on a large scale. Thus, research has been focused on the development of earth-abundant and cheap, yet efficient catalysts. Thereafter, the effects of different factors and different ways of improvement are thoroughly studied for alternative catalysts. An electrocatalyst should possess moderate adsorption energy to reaction intermediates on the surface. In addition, faster electron transfer during the HER/OER process in acidic/alkaline media is considered a desirable feature.^7,27,28 The surface phase stability is also an important factor since the elemental dissolution and surface structure degradation are easily observed under elevated potentials.^29,30 To date, the development of new designs and viable model electrocatalysts has represented a breakthrough in replacing PGMs. To improve the electrocatalytic responses, numerous experimental and theoretical results of doped carbon catalysts, transition metal borides (TMBs), transition metal carbides (TMCs), transition metal oxides (TMOs), transition metal phosphides (TMPs), transition metal sulfides (TMSs), and single-atom catalysts have made significant advances. Therefore, we here summarize and review the recent developments in non-PGM HER/OER electrocatalysts with particular attention paid to the discussion of the structural design, relationship between the structure/composition and specific/intrinsic activities, and various tactics to improve the performance.

3. Doped carbon catalysts

As described in the preceding section, PGM catalysts have been the mainstay for water electrolysis reactions (Pt for the HER and Ir oxide or Ru oxide for the OER). To replace these PGM-based catalysts, various non-metallic catalysts such as doped carbon, carbon nitride, boron nitride, phosphorus carbide, and black phosphorous are being studied for water electrolysis reactions. Among them, doped carbon electrocatalysts have received considerable attention due to their inexpensiveness, earth-abundance, and high conductivity.^12,31–34 Since pristine carbon materials have poor activity due to the unfavourable binding energies of reaction intermediates (H*, O*, OH*, and OOH*) on carbon surfaces, the introduction of dopants is imperative to endow carbon materials with appropriate binding energy and therefore improved performances for the HER and OER.

Doped carbon catalysts can be classified into two major classes: metal-free, heteroatom-doped carbon (heteroatom-doped carbon) catalysts and transition-metal and nitrogen co-doped carbon (M–N/C) catalysts. In the heteroatom-doped catalysts, the heteroatoms of p-block elements (O, N, P, and S) are doped in carbon host materials, including graphene, carbon nanotubes (CNTs), and porous carbons. By introducing a heteroatom having different electronegativity and electron affinity of carbon, the electronic structure, chemical state, and electrocatalytic activity of carbon adjacent heteroatoms can be modulated.^12,31,33 In M–N/C catalysts, a coordinated moiety of a transition metal (Fe, Co, and Ni) and nitrogen (M–N_x) is introduced into carbon. In this structure, nitrogen can be replaced with other heteroatoms or their multiple combinations. M–N/C catalysts are inspired from natural enzymes or homogeneous metal-complexes. The metal centre has been proposed as the active site and its electronic structure depends on the type of transition metal and its coordination environment.^32,34

In this section, we introduce the key strategies for rationally designing highly active doped carbon electrocatalysts for the HER and OER from both experimental and theoretical viewpoints. Also, we discuss the kinetics, mechanism, and critical factors for enhancing the catalytic activities of doped carbon catalysts. Finally, based on this understanding, we summarize the challenges of doped carbon catalysts.

3.1 The state-of-the-art catalysts and their HER/OER performances

In the early stage of doped carbon-based OER catalyst research, various dopants were investigated to enhance the OER performance of doped carbon catalysts. N-doped carbon showed the highest performance;^35,36 however its performance was far inferior to that of other types of non-PGM-based catalysts. As a means of further boosting the OER activity, a dual-doping strategy was exploited. Most dual-doped carbon catalysts were synthesized by introducing secondary heteroatoms (S and P) along with N.^37–39 As a notable example, Zhu et al. developed efficient dual-doped carbon nanofiber catalysts with the distribution of N and P on carbon paper (NPC-CP) (Fig. 3a).³⁸ To reveal the effect of dual-doping, N-doped carbon paper (NC-CP) was also prepared, and pristine CP and IrO₂ catalysts served as the standards. The LSV curve in Fig. 3b indicates that NPC-CP exhibited the highest activity among the compared catalysts. NPC-CP required the lowest η of 310 mV to drive 10 mA cm⁻² (η₁₀), followed by NC-CP (370 mV) and CP (560 mV). This result clearly suggests a role of the additional P dopant in enhancing the catalytic activity over mono-doped NC-CP. The Tafel slope of NPC-CP was 87.4 mV dec⁻¹, which is lower than those of IrO₂ (92.1 mV dec⁻¹), NC-CP (105.3 mV dec⁻¹), and CP (131.8 mV dec⁻¹), implying much more favourable electrochemical OER kinetics on NPC-CP. Density functional theory (DFT) calculations suggested that the pyridinic N and P dopants located at the edges in the NPC-CP catalyst could activate the adjacent edge C atoms, and the edge C atoms were proposed as the active sites of the OER. When the P atoms are doped, the adsorption energy of OH*, O* and OOH* intermediates on the edge C atoms adjacent to the P atoms could be appropriately modulated, thereby enhancing the OER activity. In another example, Hu et al. developed a conductive and porous hydrogel catalyst composed of phytic-acid-doped polypyrrole (P, N-doped) on carbon cloth (PA-PPy/CC), which showed efficient and stable catalytic performances. The PA-PPy/CC catalysts exhibited the highest activity (η₁₀ = 340 mV) among the CC (pristine), PPy/CC (N-doped), and PA/CC (P-doped) catalysts, demonstrating the efficacy of dual-doping.³⁹

Fig. 3 (a) Schematic illustration of N and P-doped carbon nanofibers on carbon paper. (b) OER LSV curves of NPC-CP, NC-CP, IrO₂ and pristine carbon catalysts in 1 M KOH. Reproduced with permission,³⁸ Copyright 2017, Wiley-VCH. (c) High-resolution TEM image of Ni-NHGF. (d) OER LSV curves and (e) Tafel plots of NHGF, Fe-NHGF, Co-NHGF, Ni-NHGF, and RuO₂/C catalysts in 1 M KOH. Reproduced with permission,⁴⁰ Copyright 2018, Springer Nature. (f) OER and ORR polarization curves of GNS/MC, Ni-MC, Fe-MC, OMC, Ir/C, and Pt/C catalysts in 0.1 M KOH. (g) SWV profiles of GNS/MC, Ni-MC, Fe-MC, and OMC catalysts collected in 0.1 M KOH. Reproduced with permission,⁴⁴ Copyright 2016, Wiley-VCH.

While heteroatom-doped carbons can serve as non-PGM catalysts for the OER, their intrinsic OER activity is unsatisfactory compared to their rival PGM-based catalysts. Instead, M–N/C catalysts emerged as more promising OER catalysts with enhanced activity.^40–42 A DFT calculation study predicted that Ni coordinated with pyridinic N (Ni-N/C) and Co coordinated with pyrrolic N (Co-N/C) can function as active sites for the OER.⁴³ Fei et al. validated this conjecture with a combined experimental and computational study.⁴⁰ A series of 3d transition metals embedded in N-doped graphene frameworks (M-NHGFs, M = Fe, Co, or Ni) were prepared and exploited as model catalysts. The combination of spectroscopic analyses of M-NHGFs clearly revealed that the structure of all catalysts comprised single metal centres with four adjacent N atoms and four C atoms in the second coordination sphere (MN₄C₄ structure). As shown in Fig. 3c, the annular dark-field scanning transmission electron microscopy (ADF-STEM) image of Ni-NHGF clearly visualised the atomic structure within single-layer graphene, which was further confirmed by the spectroscopic results. The LSV curves of the catalysts revealed that dopant-free NHGF exhibited poor OER activity (η₁₀ = 494 mV) and with the addition of a metal, the activity increased in the order of Fe (η₁₀ = 488 mV) < Co (η₁₀ = 402 mV) < Ni (η₁₀ = 330 mV) (Fig. 3d). The Tafel slope of Ni-NHGFs (63 mV dec⁻¹) was smaller than those of Co-NHGFs (80 mV dec⁻¹) and Fe-NHGFs (164 mV dec⁻¹). This work established the trend of OER activity in M–N/C catalysts and suggested an important role of the metal in enhancing reaction kinetics for the OER (Fig. 3e).

Toward achieving dual catalytic functionality, two different types of transition metals (M₁ and M₂) were exploited in the design of M–N/C catalysts. In this structure, each of the two metal-N species catalyses the OER and oxygen reduction reaction (ORR), respectively. The resulting OER–ORR bifunctional catalysts can be used as electrode materials in metal–air batteries and electrode catalysts in unitised regenerative fuel cells.^44–46 As representative work, Cheon et al. synthesized a graphitic nanoshell/mesoporous carbon comprising Fe and Ni-N species (GNS/MC) and used as a bifunctional OER–ORR catalyst. To experimentally identify the reason for high catalytic performance, mono-metallic Fe-N doped (Fe-MC) and Ni-N doped (Ni-MC) catalysts and N-doped carbon (OMC) were also compared.⁴⁴ The OER polarization curve indicated that GNS/MC showed the best activity with η₁₀ = 340 mV, which was followed by Ni-MC (η₁₀ = 480 mV), Fe-MC (η₁₀ = 500 mV), and OMC (Fig. 3f). In situ X-ray absorption spectroscopy (XAS) suggested that the Ni centre is the active site of GNS/MC and both Ni and Fe species are associated with the superior OER activity. Also, square wave voltammetry (SWV) measurement revealed that the Ni centre in GNS/MC is more amenable to the adsorption of the OH group than Ni-NC catalysts, which means that Fe species induce an appropriate binding energy between Ni and OH groups (Fig. 3g).

Turning to HER catalysts, most of the developed catalysts have been based on M–N/C catalysts, whereas only a limited number of studies were carried out for doped carbon catalysts due to their much lower activity than that of other benchmarked non-PGM-based catalysts. Hence, the main focus was directed towards identifying the origin of HER activity in doped carbons and establishing the catalyst design principle to enhance the HER performance.^37,47–49 Representatively, Jiao et al. carefully investigated a series of heteroatom-doped graphene materials (a-G; a = B, N, O, S, and P) as efficient HER electrocatalysts by combining spectroscopic characterization, electrochemical measurement, and DFT calculation.⁴⁸ Fifteen different doping configurations in the graphene matrix were identified by near edge X-ray absorption fine spectroscopy (NEXAFS) analyses (Fig. 4a). Next, the most favourable sites for the HER which include heteroatoms themselves and several carbon sites around them on the 15 doping models were assessed by DFT calculations. The results indicated that, for each dopant, the graphitic doping sites exhibited higher HER activity than edge type sites with lower ΔG_H* (Fig. 4b). Examining the reaction mechanism for each heteroatom doping model with the lowest ΔG_H*, it was found that all the doping models followed the Volmer–Heyrovsky mechanism, with the Volmer step being the rate determining step (Fig. 4c). The plot of j₀ and ΔG_H* showed the volcano-shaped plot of a-G catalysts and indicated that all a-G catalysts showed lower HER activity than the state-of-the-art HER catalysts (Pt and MoS₂) (Fig. 4d). Calculations were then performed to predict a new catalyst with higher HER performance, and it was found that dual doping and structural engineering can enhance the HER activity. On the basis of these findings, dual doped graphene materials (a,b-G/a = N, b = B, S, and P) were synthesised. As shown in the HER polarization curves, N,S-G exhibited a lower η than N-G and G, agreeing well with the calculation results (Fig. 4e). The general volcano plot including pure (G), a-G, a,b-G samples also supported the above results (Fig. 4f). Finally, it was predicted that the tuning dopant level and surface area of heteroatom-doped carbon catalysts can lead to improved HER activity.

Fig. 4 (a) Schematic summary of the heteroatom doping configurations: (top row, from left to right) pr-N, py-N, g-N, N–O, B-2C-O, B-3C, B-C-2O, P-3C(–O) and P-2C(–2O); (bottom row, from left to right) th-S, S-2O, py-O, C–O–C, C–OH, C=O, g-C, z-C and a-C. Green/grey, pink, blue, red, gold, purple, and white represent C, B, N, O, S, P and H atoms, respectively. (b) The computed lowest ΔG_H* for different models. The ΔG_H* values on graphitic type doping models are labelled with solid bars, and those on edge doping models are labelled with shaded bars. (c) Free energy diagram for the HER following the Volmer–Heyrovsky pathway on various graphene models. (d) Volcano plot between i_o and ΔG_H* with the charge-transfer coefficient, α = 0.125 (black solid line). The open symbols represent i_o obtained from Tafel plots and DFT-derived ΔG_H* for each graphene sample/model. (e) Electrochemical HER measurements on various graphene-based materials and MoS₂ in 0.5 M H₂SO₄. (f) A volcano plot that includes pure (G), single-doped (a-G) and dual-doped (a,b-G) graphene samples. The i_o values are those measured without normalization. Reproduced with permission,⁴⁸ Copyright 2016, Springer Nature.

M–N/C catalysts for use in the HER were derived from the studies of hydrogenase enzymes.⁵⁰ By mimicking these enzymes, several homogeneous metal complexes such as cobaloxime and cobalt diamine-dioxime were synthesized and studied for the HER.^51,52 Through these efforts, Co-N_x sites were proposed as the active sites of the HER. Inspired by these early studies, various heterogeneous Co-N/C catalysts are being investigated as potent non-PGM catalysts. Morozan et al. attempted to determine the HER activity trend of various M–N/C catalysts and suggested that Co-N/C showed the highest HER activity in both acidic and alkaline electrolytes.⁵³ However, their catalysts contained both Co-N_x and Co@C sites, which made the determination of exact active sites difficult. These problems could be resolved by subsequent efforts. Liang et al. successfully synthesized Co-N/C catalysts that contained molecular CoN_x centres by pyrolysis of several types of cobalt complex molecules (Fig. 5a).⁵⁴ Then, the HER performance of the prepared CoN_x/C, Co/C, and N/C catalysts was analysed. Interestingly, CoN_x/C exhibited the highest HER activity under all electrolyte pH conditions. The HER activity trend of the above catalysts under acidic conditions was in the order of CoN_x/C (η₁₀ = 133 mV) > Co/C (η₁₀ = 310 mV) > N/C (η₁₀ = 460 mV) (Fig. 5b). The Tafel slopes of the CoN_x/C, Co/C, and N/C catalysts were 57, 106, and 98 mV dec⁻¹, respectively, suggesting that CoN_x/C catalysts show faster kinetics than Co/C and N/C catalysts (Fig. 5c). An acid-leaching experiment clearly reconfirmed that CoN_x sites contribute to HER performance predominantly compared to Co nanoparticle sites (Fig. 5d). In order to further clarify the active sites in Co-N/C catalysts, Sa et al. carried out a systematic study by preparing Co-N/C model catalysts with controlled Co-N_x and Co@C site densities to clarify their contribution to HER activity (Fig. 5e).⁵⁵ Through extended X-ray absorption fine structure (EXAFS) analysis, the coordination numbers (CNs) of Co-N/C and Co@C were quantified to determine the correlation between the CN ratio of Co-N/C (CN_Co-N/C) and HER mass activity in both acidic and alkaline electrolytes. In both electrolytes, linear relationships between CN_Co-N/C and mass activities were established, suggesting that the Co-N/C sites play a crucial role in the acidic and alkaline HER, whereas Co@C sites exhibit negligible catalytic performance (Fig. 5f and g). According to DFT calculations, the CoN₄C₁₀ structure was considered as the best active site and the Co@C structure is inactive for the HER (Fig. 5h).

Fig. 5 (a) Schematic illustration of the porous CoN_x/C electrocatalysts. (b) HER polarization curves and (c) Tafel plots of the CoN_x/C, N/C, Co/C and Pt/C catalysts in 0.5 M H₂SO₄. (d) Comparison of the HER activity of the CoNPs/CoN_x/C and CoN_x/C catalysts showing the influence of acid leaching. Reproduced with permission,⁵⁴ Copyright 2015, Springer Nature. (e) Schematic illustration of CNT/Co-PcC catalysts with controlled Co-N_x and Co@C site densities. Correlation between the Co-N/O coordination number (CN_Co-N/O) and the HER mass activity measured in (f) 0.5 M H₂SO₄ and (g) 1 M KOH. (h) Comparison of calculated ΔG_H values for Co-N_x and Co@C sites. For Co-N_x sites, Co-N₄ clusters and Co-N₅ clusters composed of additional axial ligands were considered. For Co@C models, N dopants (denoted as H–N, black bars) and adjacent carbon sites (denoted as H–C, gray bars) in the graphene models (i.e., pyridinic (N₆), pyrrolic (N₅), graphitic (NG), and oxidized (NO) species) were considered as different hydrogen adsorption sites. Reproduced with permission,⁵⁵ Copyright 2019, American Chemical Society.

3.2 Challenges in synthesis and catalysis

During the last decade, there have been continuous efforts to develop doped carbon catalysts for use in the OER and HER. Although considerable advances have been made, further improvements in their synthesis and catalysis are still needed. First, it is important to develop synthetic methods that can create active sites in a high density. Currently developed doped carbon catalysts commonly require a pyrolysis step, which induces the decomposition of some heteroatom and transition metal precursors. Moreover, during the synthesis of M–N/C catalysts, the transition metal atoms aggregate together in the pyrolysis step. For these reasons, the active sites of doped carbon catalysts are generated in a low density. Next, it is also important to synthesize a catalyst which has only desirable sites. In the case of doped carbon catalysts, various sites are generated simultaneously. This can lead to ambiguity on the true active sites for the OER or HER. Finally, it is necessary to widen the pH range of the OER and HER in which doped carbon catalysts can effectively operate. Doped carbon catalysts are generally studied under alkaline conditions for the OER and acidic conditions for the HER. These research trends are due to the low stability of the acidic OER due to the corrosive environment, and low alkaline HER activity due to the additional water dissociation step. OER and HER studies in wider pH regions facilitate the universal use of doped carbon catalysts in water electrolysis devices.

4. Metal borides

As a class of intermetallic inorganics, TMBs have special chemical and physical properties, including high conductivity, superior hardness, and excellent corrosion resistance in both acidic and alkaline media. Recently, with the increase in research interest in electrochemical water splitting, TMBs began to gain more attention and demonstrated tuneable catalytic activity for the HER and OER. The catalytic activity of TMBs is affected by the electron transfer between the metal and boron, which modulates the electronic structure and changes the surface properties, thereby regulating the reaction energy barrier involved in the catalytic process. So far, a variety of strategies have been developed to optimize the catalytic activity of TMBs, such as component regulation (including the boron content and metal species/contents), crystallinity regulation, and nanostructure construction.

In this part, we review and compare the catalytic activities of different TMBs and analyse the important parameters that are related to the intrinsic catalytic activity. In addition, some effective methods that can regulate catalytic activity are summarized and discussed. Furthermore, we also analysed the structure and component changes of TMBs during the OER process.

4.1 The state-of-the-art catalysts and their HER/OER performances

Understanding the trend and mechanism of the catalytic activity of TMBs is helpful to get design guidance for the development of advanced catalysts. In order to study the difference in the catalytic activity of TMBs based on different metal elements, Mazánek et al. evaluated the electrochemical HER catalytic behaviour of different TMBs with an MB₂ structure, including AlB₂, CrB₂, HfB₂, MgB₂, NbB₂, TaB₂, TiB₂, VB₂, and ZrB₂.¹³ These borides were purchased from reagent companies without any treatment, which are all bulk materials on the microscale and are helpful to compare the intrinsic catalytic activity and eliminate the influence of factors like the porosity and defective structure. Notably, the test results suggested that TiB₂, ZrB₂ and HfB₂ exhibit better HER performance than bare glassy carbon (Fig. 6a). ZrB₂ exhibited the best catalytic activity and showed an η₁₀ of 970 mV. Their corresponding Tafel slopes were all larger than 120 mV dec⁻¹, suggesting that they all showed sluggish reaction kinetics (Fig. 6b). Recently, Li et al. conducted an elaborate study to unravel the relationship between the d-band centre of TMBs and catalytic activity.⁵⁶ They investigated 12 MB₂ that included 9 AlB₂-type borides (TiB₂, ZrB₂, HfB₂, VB₂, NbB₂, TaB₂, CrB₂, MoB₂, and WB₂), 2 RuB₂-type borides (RuB₂ and OsB₂), and ReB₂. Their theoretical calculations suggested that the ΔG_H* absolute value generally decreases from Group IV TMBs to Group VII TMBs, namely increasing their HER catalytic activity (Fig. 6c). Remarkably, they found that the d-band centres of these TMBs show similar trends, which are almost linearly related to the ΔG_H* absolute value. The reason for this is that if the d-band centre is more negative with respect to the Fermi level, the antibonding states will shift to lower energy and become more occupied, which helps to reduce the binding energy of H* on the surface. The predicted catalytic performance by theoretical calculations was further verified by their experimental results. As shown in Fig. 6d, the Group VIII TMBs (RuB₂ and OsB₂) exhibited much better HER performance than other synthesized borides under both acidic and alkaline conditions, and RuB₂ with the best catalytic activity only needed an ultra-low η₁₀ of 18 and 28 mV in 0.5 M H₂SO₄ and 1 M KOH, respectively. Their study demonstrated that the d-band centre of TMBs can be used as an important indicator to predict and guide the synthesis of advanced TMB catalysts. Ai et al. reported that the H* adsorption energy on metal-terminated MB type borides is weaker than their corresponding pure metal phase, which is caused by the strong hybridization between the d-orbitals of metal atoms and the sp orbitals of B atoms.⁵⁷

Fig. 6 (a) The η₁₀ in 0.5 M H₂SO₄ and (b) corresponding Tafel slopes of AlB₂, CrB₂, HfB₂, MgB₂, NbB₂, TaB₂, TiB₂, VB₂ and ZrB₂. Reproduced with permission,¹³ Copyright 2018, Royal Society of Chemistry. (c) The relationship between the d-band center of metal borides and ΔG_H* absolute value and (d) the η₁₀ of different metal borides in 0.5 M H₂SO₄ and 1 M KOH. Reproduced with permission,⁵⁶ Copyright 2018, Wiley-VCH. (e) SEM image and (f) TEM image of amorphous Ni-B; the inset shows the corresponding SAED pattern. Reproduced with permission,⁵⁹ Copyright 2018, Wiley-VCH. Energy band schematic diagram of (g) pure MoB and (h) MoB/g-C₃N₄. Reproduced with permission,⁶¹ Copyright 2018, Wiley-VCH. (i) DFT-optimized model of the Ni₃B/MoB heterostructure, and the charge density distribution and electronic local function in the corresponding structure. Reproduced with permission,⁶² Copyright 2021, Elsevier.

Among the reported borides, PGM-based borides showed excellent HER catalytic performance that exceeds that of benchmark Pt/C, such as Pd₂B, PdB, RuB, ReB, RuB₂ and OsB₂.^56–58 However, the catalytic activity of non-PGM-based TMBs has been limited. Researchers have developed some effective strategies to improve the intrinsic HER catalytic activity of non-PGM-based TMBs, like crystallinity engineering, component regulation, nanostructure control, interface constructing, etc. For example, Zeng et al. synthesized amorphous Ni–B nanoparticles with an average diameter of ca. 80 nm (Fig. 6e and f) by the electroless plating method.⁵⁹ The X-ray photoelectron spectroscopy (XPS) results suggested that the Ni/B ratio is 2.7 : 1 and electrons are transferred from B to Ni. The synthesized Ni-B showed a low η₂₀ of 123 mV in a 1 M HClO₄ electrolyte. Xu et al. constructed Ni-ZIF/Ni-B ultra-thin nanosheet arrays with numerous crystalline–amorphous phase boundaries.⁶⁰ DFT calculation results for the HER process demonstrated that compared with Ni-ZIF and Ni-B, the Ni-ZIF/Ni-B catalyst shows a more optimal ΔG_H* that is closer to zero. Benefiting from the structural advantage, Ni-ZIF/Ni-B required an η₁₀ of 67 mV for the HER. Zhuang et al. developed a type of Schottky catalyst by constructing an interface between metallic MoB and g-C₃N₄ so that a Schottky junction can be formed.⁶¹ Since the Fermi energy levels of metals and semiconductors are very different, it causes the charge flow propagated by electrons or holes at the interface to reach equilibrium, which eventually lead to band bending and the formation of a Schottky barrier (Fig. 6g and h). As a result, the electron density on the MoB surface was increased and the kinetic barriers for the HER process were dramatically lowered. Huang et al. developed a method for synthesizing a Ni₃B/MoB heterostructure with rich grain boundaries, which has an enhanced intrinsic catalytic activity.⁶² Ni₃B/MoB exhibited much better HER performance than pure Ni₃B and pure MoB and showed a low η₁₀ of 75 mV. Their DFT calculations revealed that the Ni atoms near the interface donate more electrons to the B atoms, and electrons are transferred from Ni₃B to MoB at the grain boundary, which results in a high local density of electrons at the grain boundary and a low density of electrons distributed on both sides (Fig. 6i). Such interface electric dipole formation strengthened the electronic interactions between MoB and Ni₃B so that the ΔG_H* of Ni₃B/MoB was closer to zero than that of pure Ni₃B and MoB.

The atomic ratio of B/M in TMBs also significantly affects their morphology, structure and surface properties, which ultimately affects their intrinsic catalytic activity. In 2017, Park et al. used the arc melting method to synthesize molybdenum boride with different B contents and crystal phases, including Mo₂B, α-MoB, β-MoB, and MoB₂.⁶³ As shown by the model structure in Fig. 7a, with the increase of the B content, the B–B connectivity changed from 0-D isolated B atoms (Mo₂B) to 1-D zigzag B chains (α-MoB and β-MoB) and further to 2-D graphene-like boron layers (MoB₂). For electrocatalysis, MoB₂ with the highest boron content showed the best HER catalytic activity, followed by β-MoB, α-MoB, and Mo₂B (Fig. 7b). Chen et al. revealed the reason for the high activity of MoB₂ with a subunit B layer (borophene) by experimental results and theoretical calculations.⁶⁴ They found that the density and activity of HER active sites on the surface of α-MoB₂ are even higher than that of the Pt(111) surface. Similarly, the phenomenon of changes in catalytic performance caused by the B content difference of TMBs has also been found in iron boride. Li et al. synthesized and compared the catalytic activity of Fe₂B and FeB₂.⁶⁵ Their nanostructure and crystal phases were analysed by TEM and XRD (Fig. 7c, d), and the results showed that Fe₂B and FeB₂ have similar structures and even specific surface areas. For the HER, FeB₂ had much better performance than Fe₂B (Fig. 7e). They analysed the adsorption energy of H* at different sites of FeB₂ through DFT calculations (Fig. 7f) and compared that with that of Fe₂B. The results showed that the absolute ΔG_H* values on different sites of FeB₂ are all much closer to zero than that of Fe₂B. From the above results, it can be inferred that increasing the boron contents in TMBs may be an effective strategy to enhance their HER activity.

Fig. 7 (a) XRD patterns and model structures of Mo₂B, α-MoB, β-MoB, and MoB₂ and (b) their HER polarization curves obtained in 0.5 M H₂SO₄. Reproduced with permission,⁶³ Copyright 2017, Wiley-VCH. (c) TEM image and (d) XRD pattern of FeB₂; (e) HER polarization curves of Fe₂B, FeB₂ and benchmark Pt/C; (f) several stable structure models of H* absorbed on different sites of FeB₂; (g) calculated free–energy diagram of the HER on FeB₂ and Fe₂B. Reproduced with permission,⁶⁵ Copyright 2017, Wiley-VCH.

Recently, TMBs have also been reported to be used as efficient catalysts for the OER. As is well known, the OER is a half-reaction that occurs at the anode in the process of electrochemical water splitting, which involves oxygen generation and electrochemical oxidation. Similar to TMSs and TMPs, TMBs are also a class of compounds that are easily oxidized, so their composition and electronic structure change during OER processes. For example, Guo et al. boronized a series of metal foils (Ni, Co, Fe, NiFe alloy and SUS 304 steel) by high temperature treatment with amorphous boron powder as the boron source (Fig. 8a).⁶⁶ The electrochemical OER performance of boronized metal foils was evaluated and compared with that of their corresponding metal phases in 1 M KOH. As shown in Fig. 8b, all the boronized mono-metal foils possessed better OER performance than their metal foils, and the boronized Ni foil exhibited the best catalytic activity. These results proved that boronization is an effective method to improve the OER catalytic activity of metals. They further analysed the composition and structural changes of the boronized Ni foil after catalyzing the OER for 10 h at 10 mA cm⁻². Raman analysis confirmed that a B doped γ-NiOOH film is generated on the surface of boronized Ni foil after the OER process. XPS results (Fig. 8c) revealed that the metaborate species exists in the surface oxidized layer, which reduces the oxidation state of Ni. The high resolution TEM image (Fig. 8d) suggested that the thickness of the oxidized layer is about 2–5 nm. Therefore, it can be determined that TMBs will generate an oxidized layer containing metaborate in the process of catalysing the OER. Han et al. reported a V-doped Ni-Co boride hollow nanoprism through a self-templated ion exchange strategy followed by atomic layer deposition (Fig. 8e and f).⁶⁷ The doped V in the hollow layered Ni-Co boride structure induced more unsaturated atoms that can serve as active sites, which exhibited a better catalytic performance than Ni-Co boride (Fig. 8g). DFT calculations suggested that the enhanced OER catalytic activity comes from the synergistic catalysis effect of different elements: Co and B served as the active sites, Ni acted as the regulator of surface electronic structures, and V facilitated the charge transfer. The analysis of the samples after the stability test also showed that the boride catalyst surface is partially oxidized.

Fig. 8 (a) Schematic for the preparation of metal borides from metal foil; (b) The η of different metal foils and boronized metal foils; (c) XPS analysis of metal foils and boronized metal foils before and after the OER; (d) TEM image of boronized metal foils after the OER. Reproduced with permission,⁶⁶ Copyright 2019, Royal Society of Chemistry. (e) Schematic for the preparation of hollow V doped cobalt nickel boride; (f) EDX mapping of the synthesized hollow V doped cobalt nickel boride; (g) OER polarization curves of different samples. Reproduced with permission,⁶⁷ Copyright 2019, Wiley-VCH.

4.2 Challenges in synthesis and catalysis

So far, various TMBs have been synthesized by CVD, electrodeposition, solid-state reactions, liquid-phase methods, and ball milling.⁶⁸ Although lots of achievements have been made regarding developing efficient TMBs for the HER and OER, there are still some challenges that need to be addressed. (i) The morphologies and structures of the currently reported borides are mostly uncontrollable together with a low specific surface area, which is not conducive to the exposure of active sites and the progress of mass transfer. Therefore, more research work is needed in the future to develop porous TMBs with controllable nanostructures to enhance the catalytic activity and application areas. (ii) At present, the research of TMBs is mainly focused on regulating the metal species in TMBs. It is necessary to study the influence of the doping of non-metallic elements (N, P, S, etc.) on the catalytic activity of TMBs. (iii) In the case of the OER, the active components of TMBs are mainly surface-generated hydroxides or oxyhydroxides, but their activity is far superior to that of their corresponding metals or metal hydroxides used in the OER. More research work is needed to reveal the mechanism and the role of B in the OER process. In situ characterization is also needed to explore the real active species of TMBs during the catalytic process.

5. Metal carbides

The HER performance of TMCs is still restricted by the strong H_ads on their surfaces. Optimizing the electronic properties of TMCs to tune the H_ads energy is crucial to further improve their HER activity. Therefore, recent studies on TMCs mainly focused on designing MC-based materials with heterostructures (e.g., hybridizing with transition-metal compounds).⁶⁹ TMCs have also been regarded as promising OER catalysts due to their high chemical stability at high potentials. However, compared to the acceptable HER activity, the OER activity of TMCs is poor which seriously limited their practical application. This is mainly attributed to the strong adsorption of the OER intermediates on the surfaces of catalysts leading to the sluggish process of removing reaction intermediates and products. In this section, the recent progress in TMC development for the HER and OER is summarized. In addition, the synthesis method and mechanism of the enhanced catalysis are also discussed.

5.1 The state-of-the-art catalysts and their HER/OER performances

MoC and Mo₂C materials have been explored as promising HER electrocatalysts owing to their low cost and high chemical stability. The carbon atoms in molybdenum carbides are filled in the Mo lattice, leading to the increase of lattice spacing and contraction of its d-band.⁷⁰ The synthesis of MoC and Mo₂C with a highly crystalline structure was usually carried out by pyrolysis of compounds containing Mo and organic precursors at high temperatures.¹⁴ Unfortunately, this process leads to a low surface area and poor mass transport. Therefore, great effort was devoted to improving the HER performance by synthesizing novel structures.^71,72 Hu et al. synthesized β-Mo₂C for the first time and achieved a good HER performance with an η₂₀ of 225 and 210 mV under acidic and alkaline conditions, respectively.⁷³ Recently, Asefa et al. reported nanoporous η-MoC nanosheets on N doped graphene (np-η-MoC NSs).¹⁴ In the synthesis, the precursor Mo-CN preserved a tetragonal structure during the pyrolysis at 750 °C (Fig. 9a). The np-η-MoC NSs showed a low onset potential of 28 mV, a small η₁₀ of 112 mV, and a small Tafel slope of 53 mV dec⁻¹ in a 1 M KOH electrolyte, as well as a more favorable ΔG_H* (Fig. 9b).

Fig. 9 (a) High-resolution TEM images of np-η-MoC NSs synthesized by pyrolysis of the Mo-CN precursor at 750 °C. (b) ΔG_H* diagram of H_ads on different electrocatalysts. Reproduced with permission,¹⁴ Copyright 2019, Royal Society of Chemistry. (c) Schematic illustration of the synthesis process of the Ni-GF/VC catalyst. Reproduced with permission,⁷⁵ Copyright 2020, Wiley-VCH. (d) Schematic representation of the fabrication of ES-WC/W₂C from organic–inorganic tungsten precursors. (e) The free energy diagram for the alkaline HER process, with the schematic illustration of water activation and H* formation (I) and hydrogen generation (II) processes in inset. (f) Local state densities of W and H on WC, W₂C, c-WC/W₂C and ES-WC/W₂C catalysts. Reproduced with permission,⁸¹ Copyright 2020, Elsevier.

Doping foreign atoms into TMCs can further improve the HER performance by tuning the ΔG_H*. Yu et al. prepared a series of Mo₂C particles doped with Ni, Co, Fe, and Cr. Among them, Ni-Mo₂C@C showed the lowest η₁₀ of 72 mV in 0.5 M H₂SO₄. DFT calculations suggested that the ΔG_H* of these transition metal doped Mo₂C materials follows the trend: Ni-Mo₂C > Co-Mo₂C > Fe-Mo₂C > Cr-Mo₂C.⁷⁴ Li et al. doped Ni atoms into VC and Fe₃C nanosheets via a hydrothermal method and the subsequent magnesium thermal reaction (Fig. 9c).⁷⁵ The ΔG_H* values for the (100) and (111) planes of Ni_ads site were 0.161 and 0.254 eV, respectively, which were much lower than that of C and V sites, indicating an active Ni site in Ni-VC. Recently, Mu et al. fabricated Ni-Ni₃C heterostructures to adjust the adsorption of H*.⁷⁶ The Ni-Ni₃C/CC catalyst exhibited a low η₁₀ of 98 mV in a 1 M KOH electrolyte. Chen et al. reported N, B co-doped Co₃C spheres for multifunctional electrocatalysis. The N, B co-doped Co₃C catalyst exhibited superior HER performance with an η₁₀ of 154 mV in 1 M KOH, which was 70 mV lower than that of the Co₃C catalyst.⁷⁷ Fan et al. integrated transition metal carbides M₃C (M = Ni, Co, Fe) with graphitic shells supported on vertically aligned graphene nanoribbons (M₃C-GNRs) and found that the catalysts possessed good HER activities with an η₁₀ of 49, 91 and 48 mV for Fe₃C-GNRs, Co₃C-GNRs and Ni₃C-GNRs in 0.5 M H₂SO₄, respectively, due to the small size of M₃C and the microporous structure of M₃C-GNRs.⁷⁸ Mullins et al. demonstrated a more favourable hydrogen adsorption free energy and better conductivity of Co₂C than those of Co₃C, indicating superior HER activity of Co₂C.⁷⁹ Co₂C nanoparticles were synthesized by Ma et al. and exhibited high HER activity with an η₁₀ of 181 mV in 0.1 M KOH.⁸⁰

Tungsten carbides exhibited similar HER performance to molybdenum carbides.⁸¹ Because of the higher electronic density of states (DOS) at the Fermi level and more favourable H* adsorption, W₂C exhibits better HER activity than WC.^82,83 However, the chemical stability of W₂C is worse than that of WC. Li et al. synthesized carbon covered ultrafine WC/W₂C nanowires (WC/W₂C@C NWs) to combine the advantages of these two components.⁸² The WC/W₂C@C NW catalyst showed a low η₁₀ of 69 and 56 mV in 0.5 M H₂SO₄ and 1 M KOH electrolytes, respectively, as well as good long-term stability. Chen et al. fabricated a eutectoid WC/W₂C heterostructure (ES-WC/W₂C) with defect-rich WC/W₂C interfaces (Fig. 9d).⁸¹ The ES-WC/W₂C catalyst exhibited excellent HER activity in an alkaline electrolyte with a low onset potential of 17 mV, η₁₀ of 75 mV. DFT calculations presented a lower water dissociation barrier at the defect-rich WC/W₂C interface, indicating the easier cleavage of HO–H bonds for ES-WC/W₂C (Fig. 9e). As shown in Fig. 9f, hybridization peaks were only observed at higher energies (about −3.4 and −3.6 eV) for ES-WC/W₂C, implying a weakened W–H interaction.

Metal carbides, such as Fe₃C, Ni₃C, Co₃C, Mo₂C, and W₂C, are also promising for the OER due to their superior chemical stability. Xiao et al. prepared an Fe₃C-based electrocatalyst by electrochemical metallurgy using Fe₂O₃ and CO₂ as the feedstock (Fig. 10a).⁸⁵ The carbon deposition (CO₃²⁻ + 4e⁻ = C + 3O²⁻) and iron formation (Fe₂O₃ + 6e⁻ = 2Fe + 3O²⁻) on cathode resulted in highly dispersed Fe/Fe₃C (Fe/Fe₃C-MC) generation. The η₁₀ of the Fe/Fe₃C-MC catalyst was 320 mV which is lower than that of benchmark RuO₂ (353 mV). Chemical structure analyses confirmed the transition of Fe/Fe₃C into Fe₃C–FeOOH during the OER process. The *OOH intermediates were linearly adsorbed on the Fe₃C–FeOOH surface, which would be conducive to the formation of O₂ (Fig. 10b). The energy barrier for the rate determining step on the Fe₃C–FeOOH model was 1.37 eV, which is lower than that of Fe₃C–Fe (5.43 eV) and Fe₃C (2.73 eV), indicating that the in situ produced FeOOH improves the catalytic OER kinetics (Fig. 10c). To further clarify the structural changes, Mullins et al. characterized the elemental composition and microstructure of Co₃C particles at various times during the OER process. They found that the Co₃C particles are firstly converted to a transitory Co₃C core-amorphous Co oxide shell and then to an amorphous Co oxide particle.⁸⁴ Gou et al. reported the preparation of Co-doped Fe₃C@carbon nano-onions (FCC@CNOs) via a high-pressure annealing method.⁸⁶ The high resolution TEM images showed the lattice fringes of 0.24 nm, which are indexed to the (210) crystal plane of Fe₃C. The OER η₁₀ of FCC@CNOs was 271 mV, which is much better than that of the benchmark RuO₂. DFT results showed that Co tends to form weaker *O adsorption than Fe do. Coupling Ni with Ni₃C can optimize the adsorption energies of water and OER intermediates, benefiting the OER catalytic kinetics. A Ni-Ni₃C heterostructure composed of Ni NPs and Ni₃C NSs on carbon cloth (Ni-Ni₃C/CC) was obtained by annealing Ni(OH)₂ and melamine.⁷⁶ The OER η₂₀ of the Ni-Ni₃C/CC catalyst was 299 mV, which is lower than that of Ni₃C/CC and RuO₂. DFT calculations showed that the ΔG value of the rate determining step on Ni-Ni₃C is 1.815 eV, which is significantly lower than that of Ni₃C (3.701 eV) and Ni (3.877 eV), confirming more efficient OER kinetics on the Ni-Ni₃C surface.

Fig. 10 (a) Schematic of electrochemical co-reduction to produce cathodic Fe/Fe₃C-MC. (b) Geometric structures for the rate-determining *OOH step on Fe₃C–FeOOH models. (c) The calculated energy profile for the OER on several models at pH = 14 and U = 1.23 eV vs. SHE. Reproduced with permission,⁸⁵ Copyright 2021, Wiley-VCH. (d) The synthesis illustration of Co SAs/Mo₂C. Reproduced with permission,⁸⁷ Copyright 2020, Royal Society of Chemistry. (e) LSV curves of Co-Mo₂C toward the alkaline OER. Reproduced with permission,⁶⁹ Copyright 2020, American Chemical Society.

Bimetallic TMCs were found to be more efficient for the OER. For instance, Kou et al. synthesized metal single atom (e.g., Co, Ni, and Cu) doped Mo₂C nanosheets (M SAs/Mo₂C) by metallic cation exchange and carbonization using MoZn bimetallic imidazolate frameworks as the precursors (Fig. 10d).⁸⁷ Metal single atoms were coordinated with three Mo atoms on the surface of Mo₂C to form M-Mo₃ sites. Co SAs/Mo₂C provided an ultralow η₁₀ of 270 mV and high turnover frequency (TOF). Partial Mo K-edge XANES plots suggested a decrease in the average electron density of Mo after the decoration of Co atoms on Mo₂C, leading to a favourable OH* adsorption strength for an efficient OER. Wang et al. synthesized a Co-Mo₂C heterostructure in a similar way.⁶⁹ Due to the long-range ordered Co-O–Mo connectivity in the Co-ZIF-L-MoO₄ precursor, the in situ generated Co and Mo₂C further formed heterostructure nanoparticles with uniform distribution of Co and Mo₂C after pyrolysis. The η₁₀ of Co-Mo₂C was only 190 mV that is much lower than that of RuO₂ (Fig. 10e). After OER cycling, a γ-phase cobalt oxyhydroxide (γ-CoOOH) was formed. The newly created γ-CoOOH would boost the electron flow from Mo to Co through the bridging oxygen, benefiting the electrostatic adsorption of OH⁻, thus increasing the catalytic activity of the OER. Bimetallic cobalt tungsten carbide nanosheets embedded in N-doped carbon (Co₆W₆C@NC) were prepared for efficient OER catalysis with an η₁₀ of 286 mV.⁸⁸ Pan et al. synthesized trimetallic CoCuW-based carbide hybrids by pyrolysis of a core–shell CuWO₄@ZIF-67 precursor.⁸⁹ The catalyst was comprised of metallic Cu, hexagonal WC, and Cu-doped cubic Co₃W₃C, and exhibited a low η₁₀ of 238 mV for the OER. The introduction of Cu into Co₃Wo₃C could prevent the reduction of Co to a metallic state, maximize the use of the active Co sites, and modulate the electronic structure of catalysts for an efficient OER.

5.2 Challenges in synthesis and catalysis

Although TMC-based catalysts achieved enhanced HER and OER activities, it is still a great challenge to replace PGM catalysts. Mo₂C is considered an ideal substitute for Pt in the HER. DFT calculations suggest that the synergistic effect between N-doped carbon and Mo₂C can significantly improve the HER activity of Mo₂C. Accordingly, N-rich carbon precursors are usually used for preparing such materials. However, the Mo₂C particles synthesized by this method are bulky and usually encapsulated in carbon. Therefore, building Mo₂C/NC composites with a porous structure and low-dimensional morphology is expected to further improve the HER activity. Although tungsten carbide-based catalysts exhibit a good HER activity in acidic solutions, their activities in alkaline solutions are unsatisfactory. W₂C has a strong affinity for OH* intermediates in alkaline solutions, which will lead to the poisoning of active W sites into inert W_xO_y species. The doping of other metals, such as Co and Ni, is a common strategy to improve the HER activity of W₂C.^90,91 However, due to the low chemical stability of the doped metals, this strategy may have adverse effects on the durability of W₂C-based catalysts.⁹² Therefore, it is interesting to develop W₂C with a unique structure to obtain excellent HER activity and stability in alkaline solutions. For instance, Chen et al. introduced a eutectoid structure into the field of electrocatalysis and the eutectoid WC/W₂C heterostructure showed good HER catalytic performances under alkaline conditions.⁸¹

In the case of the OER, the component and structural changes of TMC-based catalysts during the OER process become crucial factors limiting the improvement of electrocatalysis. Therefore, it is of great significance for the rational design of TMC-based catalysts by considering the surface reconstruction during the OER process. A computational thermodynamic model can be used to predict the catalyst structure reconstruction in different reaction environments, which will be constructive for building efficient TMC-based OER catalysts.

6. Metal oxides

TMOs based on first-row 3d metals (Mn, Fe, Co, and Ni) have been commercially used as anode catalysts in mature alkaline electrolysers because they feature high alkaline stability, flexibility in oxygen vacancies, and distinctive electron distribution.^15,21 The high intrinsic activity and stability of TMOs in alkaline media allow them to be applied even in technically advanced and well-defined anion exchange membrane water electrolyser (AEMWE) systems. The high OER activity of TMOs originates from the overlap of the d-band of 3d metals and the p-band of oxygen.²¹ The electronic interaction among these band structures tunes the binding energies between each intermediate species and the oxide surface and determines the OER rate. In particular, the binding energy difference between O* and OH* (ΔG_O − ΔG_OH) has been considered a useful OER activity descriptor of the oxide surfaces.²¹ Compositional and structural optimization enabled producing various crystal structures (e.g. rock salt, spinel, layer-structured oxides, and perovskites) having optimal ΔG_O − ΔG_OH and excellent OER performances.^21,93–95 These advanced TMO catalysts have often demonstrated superior OER performances to PGM-based catalysts.

Recently, TMOs have also emerged as promising HER catalysts.⁹⁶ Since TMOs can possess diverse crystal structures, their electronic configuration is flexible. TMOs, which are considered inactive materials for the HER, can be activated by structural engineering and modifying electronic structures.^97–100 These strategies, including oxygen-vacancy tuning, phase transformation, and developing multimetallic composition, generated HER-reactive TMO catalysts with various crystal structures via boosting the low electric conductivity and optimizing the ΔG_H of oxide surfaces to promote the HER.

In this section, we review remarkable examples the TMO catalysts promoting the OER and HER. The underpinning mechanisms of enhanced catalysis are also discussed. We also summarize the remaining challenges in the TMO field.

6.1 The state-of-the-art catalysts and their HER/OER performances

During the past few decades, the advances of TMO-based OER catalysts have been accelerated by inspiration from the cubane-like CaMn₄O_x structure in the nature's oxygen evolution complex.¹⁰¹ An enormous number of TMO catalysts have been developed with diverse strategies. Because the class of TMOs is too broad, advances in perovskites and pyrochlores are introduced in the next section (Section 7). Among the developed catalysts, (oxy)hydroxide (MO_xH_y) materials represent the most promising OER electrocatalysts.^15,21 In particular, NiFeO_xH_y catalysts have demonstrated remarkable OER activity.^102–104 Interestingly, pure NiO_xH_y, which shows low OER activity, shows a drastic activity increase when Fe is intercalated into the structure, even the Fe amount is impurity-level.^105,106 Systematic studies using XAS or electrochemical scanning microscopy have suggested that the Fe moiety in the NiFeO_xH_y matrices is the OER active site.^107,108 Recently, Chung et al. revealed that Fe–MO_xH_y systems generate dynamically stable Fe active sites on the MO_xH_y clusters (Fig. 11).¹⁰⁹ A model experiment combined with isotopic labeling and inductively coupled plasma mass spectroscopy (ICP-MS) analyses exhibited that the dissolution and redeposition of Fe species take place continuously over the MO_xH_y host materials during the OER process with Fe–MO_xH_y catalysts. Balancing the rates of Fe dissolution/redeposition is pivotal for an active and stable OER. Markovic et al. also suggested two routes for boosting catalytic activity: (i) increasing the surface area of the host MO_xH_y cluster to achieve the maximized number of Fe species, and (ii) tuning Fe adsorption energy on the host materials (ΔG_Fe–M) to boost the number of effective Fe active sites.

Fig. 11 Schematic diagram of the dynamically stable active-site/host pair at the electrode/electrolyte interface, highlighting the role of M oxyhydroxide as a suitable host for Fe species to stay at the interface long enough to catalyse the conversion of OH⁻ into O₂ molecules, with the presence of Fe in the electrolyte ensuring that Fe species can return to the interface and redeposit at oxyhydroxide sites. Reproduced with permission,¹⁰⁹ Copyright 2020, Springer Nature.

Adopting multimetallic M₁M₂O_xH_y host materials can further boost the OER performance by tuning the ΔG_Fe–M of Fe-MO_xH_y. For example, Zhang et al. developed highly efficient ternary oxyhydroxide catalysts with a homogeneous atomic distribution.⁹⁵ They adopted FeCoW composition based on DFT calculations. Computational prediction estimated that the OER activity of bimetallic TMOs based on first-row 3d metals can be greatly improved by W doping. On the basis of DFT calculations, gelled FeCo (G-FeCo) and gelled FeCoW (G-FeCoW) oxyhydroxides with a homogeneous atomic distribution were prepared via a room-temperature sol–gel process (Fig. 12a). To elucidate the impact of the structural homogeneity of the G-FeCoW catalyst on the OER performance, an annealed G-FeCoW (A-FeCoW) sample was synthesized by heating G-FeCoW oxyhydroxide at 500 °C. The prepared A-FeCoW contains Co₃O₄ and CoWO₄ phases, exhibiting the heterogeneous distribution of elements. An FeCo layer double hydroxide (LDH) was also prepared for the activity benchmark. The OER activity of catalysts was measured on gold-plated Ni foam. The OER polarization curves (Fig. 12b) and η₁₀ of the catalysts clearly suggest an OER activity in the following order: G-FeCoW (191 mV) > G-FeCo (215 mV) > A-FeCoW (232 mV) > LDH FeCo (279 mV). The intrinsic activity of catalysts was further compared using TOFs. The TOF of G-FeCoW (0.46 s⁻¹) also surpassed those of G-FeCo (0.043 s⁻¹), A-FeCoW (0.17 s⁻¹), and LDH FeCo (0.0085 s⁻¹). In addition, G-FeCoW catalysts exhibited high stability preserving the initial η₃₀ for 550 h (Fig. 12c). Notably, the OER faradaic efficiency for G-FeCoW was well preserved during the continuous OER process. The electrochemical quartz crystal microbalance technique and ICP-atomic emission spectroscopy (ICP-AES) analysis also confirmed the excellent stability of G-FeCoW catalysts. This synthetic method has recently been extended to the preparation of ternary NiFeM and FeCoM oxyhydroxides (M = W, Mo, Nb, Ta, Re) and quaternary NiFeMoW and FeCoMoW by the same group.¹¹⁰ Among those catalysts, the best-performing NiFeMo oxyhydroxide catalysts demonstrated excellent activity and durability in an industrial alkaline electrolyser system (Fig. 12d). Under various applied cell potentials, the obtained current densities of NiFeMo electrode-base cells are 17 times higher than those of the commercial RANEY® Ni electrode-based one. Furthermore, the NiFeMo catalyst demonstrated high stability, exhibiting no appreciable increase in the cell voltage during the continuous operation of an alkaline electrolyser at 300 mA cm⁻² for 12 h (Fig. 12e).¹¹⁰

Fig. 12 (a) Schematic illustration of the preparation process for the gelled structure and pictures of the corresponding sol, gel, and gelled film. (b) The OER polarization curve of catalysts loaded on two different substrates with a 1 mV s⁻¹ scan rate, without iR correction. (c) Chronopotentiometric curves obtained with the G-FeCoW oxyhydroxides with a constant current density of 30 mA cm⁻², and the corresponding faradaic efficiency from gas chromatography measurement of evolved O₂. Reproduced with permission,⁹⁵ Copyright 2016, AAAS. (d) Photograph of an industrial electrolyser device. (e) The cell voltage of the electrolyser held at 300 mA cm⁻² for 12 h at 80–85 °C and 2 MPa. Reproduced with permission,¹¹⁰ Copyright 2020, Springer Nature.

In the case of the HER, various structures of active catalysts have been explored and evaluated. The most studied compositions for the acidic HER are MoO₂, MoO₃, WO₃, and TiO₂, perhaps owing to their acidic stability. However, it is well known that these pristine oxides with well-defined stoichiometry are usually inactive for the HER. In this vein, the creation of oxygen vacancies has been regarded as an efficient way to activate these materials. Oxygen vacancy tuning can modulate the electronic structure, electrical conductivity, and ΔG_H leading to enhanced intrinsic HER activity. For example, Li et al. showed that the activity of WO₃ was improved by facile structural engineering to form oxygen-defect WO_2.9 (Fig. 13a).⁹⁷ As shown in the HER polarization curves (Fig. 13b), the η₁₀ of WO_2.9 is much lower than that of pristine WO₃ (637 mV). DFT calculations also suggested that the vacancy engineering tuned the ΔG_H toward an optimal value to promote the HER. The vacancy engineering of TMOs is also applicable to improve alkaline HER performance. Ling et al. reported the activity enhancement of CoO nanorods (NRs) via surface strain engineering.⁹⁸ They demonstrated that the electronic structures of CoO can be optimized by the tensile strain effect induced by generating a large number of oxygen vacancies. These oxygen vacancies can facilitate the dissociation of water molecules and lowered the ΔG_H. Consequently, the surface-strained CoO NRs showed excellent alkaline HER activity, which is comparable to that of the benchmark Pt/C catalyst. Hu et al. also reported that vacancy engineering is effective in improving the HER activity.¹¹¹ They developed metal oxide nanofibers, which are composed of interconnected Co-Ni oxide NPs possessing a plethora of various lattice defects and unsaturated metal sites (D-CoNiO_x-P-NFs). D-CoNiO_x-P-NFs demonstrated enhanced HER performance compared to defect-free CoNiO_x-P-NFs. It originates from the promoted water dissociation by the defective structure, consistent with the above CoO NR catalysts.

Fig. 13 (a) Schematic illustration of the plausible reaction mechanism of the electrocatalytic HER for WO₃ and WO_2.9. (b) HER polarization curves of catalysts in 0.5 M H₂SO₄. Reproduced with permission,⁹⁷ Copyright 2015, Springer Nature. (c) Schematic of the PLD chamber where Ti₂O₃ polymorphs were fabricated using the same (α-Ti₂O₃) target. (d) Unit cells for γ-Ti₂O₃, o-Ti₂O₃, and α-Ti₂O₃ polymorphs from top to bottom, respectively. (e) HER polarization curves of catalysts in 0.5 M H₂SO₄. Reproduced with permission,⁹⁹ Copyright 2019, Springer Nature. (f) HER polarization curves of P-CoO, pure CoO, Zn-doped, Ni-doped, Ni, Zn dual-doped CoO NRs, and the benchmark Pt/C catalyst recorded in 1 M KOH. (g) Alkaline HER performance of Ni, Zn dual-doped CoO NRs compared to recently reported highly active alkaline HER catalysts. Reproduced with permission,¹⁰⁰ Copyright 2019, Wiley-VCH.

The phase transformation of TMOs is also a vital strategy for boosting catalytic activity via tuning the electronic structure. Li et al. highlighted the importance of phase engineering to achieve efficient HER catalysts. They prepared three different bulk-absent Ti₂O₃ polymorphs (trigonal α-Ti₂O, orthorhombic o-Ti₂O₃, and cubic γ-Ti₂O₃) using the pulsed laser deposition (PLD) technique (Fig. 13c). By recrystallization on substrates during epitaxial growth, electronic reconstruction was classified into three types (Fig. 13d). The prepared Ti₂O₃ thin films were utilized as the model catalysts for investigating the correlation between the electronic structure and HER activity. As shown in Fig. 13e, the HER activity of cubic γ-Ti₂O₃ was much higher than those of trigonal α-Ti₂O and orthorhombic o-Ti₂O₃. The higher activity of γ-Ti₂O₃ is attributed to the strongest hybridization of Ti 3d and O 2p orbitals.

As illustrated in OER catalysts, adopting a multimetallic composition is also an effective and easy way to activate TMOs towards the HER. Ling et al. elaborately tailored the electronic structure and electrical conductivity of inactive CoO NRs by dual doping of Ni and Zn elements, resulting in activity enhancement.¹¹⁰ In this work, the Ni dopant modulated the electronic environment of host CoO NRs, tuning the binding energy of intermediates. The Zn dopant also improves the electrical conductivity of the CoO matrix by tuning the bulk electronic structure. As shown in the HER polarization curves (Fig. 13f), the Ni, Zn dual-doped CoO NRs outperformed the pure CoO NRs, polycrystalline CoO NRs (P-CoO NRs), and M-doped CoO NRs (M = Ni, Zn). Notably, the η of Ni, Zn dual-doped CoO NRs was even smaller than that of the benchmark Pt/C catalyst at high-current density. More importantly, the comparison of η₁₀ and η₂₀ demonstrated that the activity of Ni, Zn-doped CoO NRs surpasses those of present state-of-the-art alkaline HER catalysts (Fig. 13g).

Recently, high-performance multimetallic (oxy)hydroxide catalysts for the HER have also been developed. Monometallic (oxy)hydroxide materials are considered to have low HER activity due to their inappropriate ΔG_H. However, they can dissociate water efficiently and achieve high HER activity by adopting multimetallic composition. For example, Fe-substituted VOOH hollow spheres composed of 2D flakes exhibited an η₁₀ of 93 mV.¹¹² DFT calculations suggested that the Fe site of the Fe-doped VOOH tunes the ΔG_H of V sites to an optimal value. In another example, the dynamically self-optimized NiFe LDH nanosheets reported by Edvinsson et al. showed an η₁₀ of 59 mV in 1 M KOH.¹¹³ The in situ Raman spectra revealed that the dynamically generated FeOOH species promote water dissociation and induce H_ad–NiO bonds, resulting in HER activation.

6.2 Challenges in synthesis and catalysis

Despite the great promise of TMO catalysts for water electrolysis, several critical issues remain toward widespread application. For OER catalysis, the use of TMO-based OER catalysts is only available in AEMWEs due to their unstable nature in acidic media. Developing robust TMO catalysts with excellent acid stability and corrosion resistance is crucial for accomplishing low-cost catalysts, which are alternatives to Ir- and Ru-based catalysts for proton exchange membrane water electrolysers (PEMWEs). As reported by Simonov et al., CoFePb multimetallic oxide shows excellent acid stability, and further advances in TMO catalysts for PEMWEs are also highly expected.

For HER catalysis, the catalytic activity of oxide materials is still not comparable to that of Pt catalysts. There are huge opportunities for exploring new HER catalysts for optimal ΔG_H, given a myriad of TMO compositions that are not yet realized. Further improvements in structural engineering strategies for modulating the electronic structure are also demanding. Combining systematic compositional screening with structural optimization can produce highly conductive, efficient, and robust TMO-based HER catalysts. Intensive research along this direction should be conducted in the near future.

Overall, single-cell studies should be performed to accelerate the growth of this field. Currently, the fabrication of TMO-based PEMWE or AEMWE cells has been rarely reported. In addition, the standard measurement conditions for the performance evaluation of TMO catalysts in a single-cell configuration are not established firmly. Furthermore, developing in situ techniques for the structural analyses of TMO catalysts under single-cell driving conditions is an urgent task to glean insights into practically useable catalysts.

7. Perovskite and pyrochlore

Functional oxides with perovskite and pyrochlore structures have been intensively studied as electrocatalysts for the ORR and OER due to their low cost, compositional flexibility, and tuneable properties. The general formula of perovskites is ABO₃, where the A-site is an alkaline, alkaline-earth or rare-earth cation in 12-fold coordination with O and the B-site represents a smaller transition or p-block metal ion in corner-sharing BO₆ octahedral units (Fig. 14a).²⁷ Pyrochlore is a complex oxide with a general formula of A₂B₂O_7−δ where A is an alkaline-earth or rare-earth cation and the B-site represents a transition metal cation.¹¹⁴ The formula of pyrochlore can also be described as with two types of oxygen, consisting of a network of corner-sharing octahedra with A and O′′ atoms occupying interstitial sites (A₂O′′) (Fig. 14b).¹¹⁴ The compositional flexibility of the A and B sites of these perovskite and pyrochlore oxides provide numerous opportunities to manipulate their electronic structures and corresponding catalytic properties.^27,114,115 Furthermore, the ability to substitute other ions for the A- and B-site elements while maintaining the crystal structure provides more opportunities for adjusting the electronic structure and oxygen vacancy concentration, and thus their catalytic activity and stability.¹¹⁴ In this section, we first discuss the recent advances in understanding the OER mechanism based on perovskites, and then introduce representative examples of perovskite and pyrochlore catalysts for the OER and HER. We also discuss the remaining challenges and emerging opportunities in these oxide materials.

Fig. 14 Structure of (a) perovskite, A₂B₂O₇ and (b) pyrochlore, A₂B₂O₇.

7.1 The state-of-the-art catalysts and their HER/OER performances

The conventional OER mechanism for metal oxides stems from the basis that the oxygen evolves only around the metal centre, where the main parameter controlling the reaction η is the binding strength of oxygenated adsorbates (Fig. 15a).^116,117 However, recent studies on perovskite oxide catalysts have revealed a new OER mechanism based on the involvement of lattice oxygens in perovskite, the so-called lattice oxygen evolution reaction (LOER) mechanism (Fig. 15a).^117–119 Particularly under alkaline conditions, perovskite oxide catalysts have recently been central to the study of the LOER mechanism to improve the OER activity. Suntivich et al. systematically investigated the OER activity of perovskite oxides and suggested that the e_g occupancy of the 3d electron can be employed as an effective descriptor.⁹⁴ Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) showed the highest OER activity in 0.1 M KOH due to its optimal e_g occupancy close to unity (Fig. 15b).⁹⁴ However, the e_g occupancy descriptor was established based on the ionic model and could not capture the electron sharing between the metal and oxygen atoms.¹¹⁵ Therefore, the metal–oxygen covalency was proposed as a more effective descriptor for predicting both OER activity and stability.¹¹⁵ The metal–oxygen covalency can be quantified by charge-transfer energy (energy difference between unoccupied metal 3d- and occupied O 2p-band centres) or the O 2p-band center.¹¹⁵ For example, Grimaud et al. used the O p-band centre to screen the OER activity and stability of perovskites (Fig. 15c).¹²⁰ Bringing the O p-band centre closer to the Fermi level can increase the OER activities under alkaline conditions, and moving the O p-band centre too close to the Fermi level reduces stability.¹²⁰ It should also be noted that the metal–oxygen covalency can provide the basis for adjusting the electronic structure of metal oxides to control the LOER mechanism.¹²¹ In a highly covalent network in which the metal d band penetrates the O p-band, the electrons from the p band can move to the d band, generating ligand holes.¹²¹ The generated ligand holes can facilitate the formation of oxygenated (O₂)ⁿ⁻ species, which tend to oxidize and release gaseous oxygen.¹²¹ Therefore, increasing the metal–oxygen covalency is expected to promote the LOER mechanism.²⁷

Fig. 15 (a) Conventional and lattice oxygen participating mechanisms of perovskite oxides for the OER. Reproduced with permission,¹¹⁶ Copyright 2007, Elsevier. (b) The relationship between the OER catalytic activity and the occupancy of the e_g-symmetry electron of perovskites. Reproduced with permission,⁹⁴ Copyright 2011, AAAS. (c) Schematic representation of the O p-band for transition metal oxides and the OER activity versus the O p-band center relative to the Fermi level for perovskite oxides. Reproduced with permission,¹²⁰ Copyright 2013, Springer Nature. (d) Tafel plot comparing the specific activity of IrO_x/SrIrO₃ in 0.5 M H₂SO₄. Reproduced with permission,¹²¹ Copyright 2016, AAAS. (e) Crystal structure of 6H-SrIrO₃, including face-sharing IrO₆ octahedral dimers and a corner-sharing, isolated IrO₆ octahedron. Reproduced with permission,¹²³ Copyright 2018, Springer Nature. (f) High-resolution TEM images of cycled SrCo_0.9Ir_0.1O_3−δ, where the white curve indicates the interfaces between the crystallized region and the reconstructed region.¹²⁴ Copyright 2019, Springer Nature. (g) TEM image of nanoscale Bi₂Ir₂O₇. (h) Stability measurements for various pyrochlore samples in 0.1 M HClO₄. Reproduced with permission,¹²⁸ Copyright 2017, American Chemical Society. (i) OER performance of Y₂Ru₂O_7−δ and reference RuO₂ catalysts in 0.1 M HClO₄. Reproduced with permission,¹²⁹ Copyright 2017, American Chemical Society.

Under acidic OER conditions, research efforts have been focused on Ir- and Ru-based oxides due to their balanced activity and stability. Seitz et al. first reported that IrO_x/SrIrO₃ can be formed via strontium (Sr) leaching from the surface layer of SrIrO₃ thin films under acidic OER conditions.¹²² The in situ formed IrO_x/SrIrO₃ catalyst delivered high OER activity with an η₁₀ of 270 mV in 0.5 M H₂SO₄ (Fig. 15d).¹¹³ In addition, Yang et al. showed that a 6H-SrIrO₃ perovskite catalyst exhibited a high OER activity with a low η₁₀ of 248 mV in 0.5 M H₂SO₄. This perovskite catalyst also showed excellent stability with only ∼1% Sr leaching over a 30 h-long OER test. These improved activity and stability of 6H-SrIrO₃ were attributed to its unique face-sharing IrO₆ octahedral subunits (Fig. 15e).¹²³ Chen et al. also introduced a pseudo-cubic SrCo_0.9Ir_0.1O_3−δ perovskite, containing corner-sharing IrO₆ octahedrons.¹²⁴ The Ir in SrCo_0.9Ir_0.1O_3−δ exhibited a significantly higher TOF than that of benchmark IrO₂ in 0.1 M HClO₄.¹²⁴ The enhanced activity was explained by surface reconstruction caused by Sr and Co leaching, which can lead to the formation of active IrO_x layers under acidic OER conditions (Fig. 15f).¹²⁴ Moreover, Retuerto et al. showed that A-site Na doping into SrRuO₃ can improve both the activity and stability of the catalyst under acidic conditions.¹²⁵ Specifically, Sr_0.95Na_0.05RuO₃ and S_0.90Na_0.10RuO₃ delivered high OER activity with a low η_0.5 of 120 mV in 0.1 M HClO₄.¹²⁵ For Na-doped SrRuO₃, the increase in activity was attributed to a weakened adsorption energy of the OER intermediates, and the improved stability was due to lower surface energy and higher dissolution potentials.¹²⁵

Ir and Ru-based pyrochlore oxides have also been actively investigated as acidic OER catalysts due to their low Ir/Ru content, high activity, and stability in acidic environments.¹¹⁴ For example, Sardar et al. synthesized nanoscale Bi₂Ir₂O₇ with an average crystal size of 10 nm as an acidic OER catalyst by a one-step hydrothermal reaction (Fig. 15g).¹²⁶ Later, the same group first synthesized mixed Ru/Ir pyrochlores, (Na_0.33Ce_0.67)₂(Ir_1−xRu_x)₂O₇, and showed that Ru is more active toward the OER than Ir.¹²⁷ Lebedev et al. also synthesized a series of Ir pyrochlores, (A, A′)₂Ir₂O_6.5+x (A, A′ = Bi, Pb, Y), with a high surface area of 40 m² g⁻¹ using the Adams fusion method at moderate temperatures (500–575 °C).¹²⁸ Electrochemical tests in 0.1 M HClO₄ showed that yttrium pyrochlore catalysts exhibited high OER activity and reasonable stability (Fig. 15h), which is due to the formation of an active IrO_x surface layer driven by the leaching of Y³⁺.¹²⁸ Moreover, Kim et al. reported that a pyrochlore yttrium ruthenate (Y₂Ru₂O_7−δ) could provide significantly enhanced OER activity and stability over RuO₂ in 0.1 M HClO₄ (Fig. 15i).¹²⁹ The improved activity and stability of Y₂Ru₂O_7−δ were attributed to its favourable electronic structures with a low valence state and a lower band centre energy for the overlap between Ru 4d and O 2p orbitals.¹²⁹ Motivated by this study, Kuznetsov et al. synthesized a series of A-site substituted yttrium ruthenium pyrochlores, Y_1.8M_0.2Ru₂O_7−δ (M = Cu, Co, Ni, Fe, Y), and correlated their OER activity with the concentration of surface oxygen vacancies.¹³⁰ Electrochemical tests in 1 N H₂SO₄ showed that Y_1.8Cu_0.2Ru₂O_7−δ had the highest OER activity due to the upshift of the O 2p band centre closer to the Fermi level.¹³⁰

In addition to their application to OER catalysts, researchers have recently explored perovskite oxides as HER catalysts under alkaline conditions. Xu et al. demonstrated for the first time that A-site praseodymium (Pr)-doped Pr_0.5(Ba_0.5Sr_0.5)_0.5Co_0.8Fe_0.2O_3−δ (Pr_0.5BSCF) perovskite delivers a high HER activity with an η₁₀ of 237 mV and a Tafel slope of 45 mV dec⁻¹, which is significantly superior to Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) in 1 M KOH (Fig. 16a).¹³¹ The enhanced HER activity of Pr_0.5BSCF was attributed to the combination of increased concentration of lattice oxygen and partially oxidized cobalt induced by Pr-doping, an increased electrochemical surface area (ECSA), and promoted electron transfer.¹³¹ Hua et al. also reported that a perovskite oxyfluoride catalyst, La_0.5Ba_0.25Sr_0.25CoO_2.9−δF_0.1 (LBSCOF), exhibits an enhanced HER activity compared to La_0.5Ba_0.25Sr_0.25CoO_3−δ (LBSCO) in 1 M KOH.¹³² Specifically, LBSCOF showed a decreased Tafel slope of 44 mV dec⁻¹ compared to that of LBSCO (51 mV dec⁻¹) in a low current density region (0.3–8 mA cm⁻²) (Fig. 16b).¹³² The improved HER activity of LBSCOF was explained by fluorine (F)-anion doping that can uplift the O p band centre and activate the redox capability of lattice O.¹³² Therefore, LBSCOF has a preferred ΔG_H* of −0.279 eV, which is close to zero compared to that of LBSCO (−0.634 eV) (Fig. 16c).¹³²

Fig. 16 (a) HER Polarization curves of BSCF, Pr_0.5BSCF and benchmark Pt/C catalysts in 1 M KOH. Reproduced with permission,¹³¹ Copyright 2016, Wiley-VCH. (b) Tafel plots and (c) computed Gibbs free energies of LBSCO and LBSCOF for the HER. Reproduced with permission,¹³² Copyright 2018, Elsevier. (d) Computational illustration for the alkaline HER processes on the RBaCo₂O_5.75 surface, including H₂O adsorption, H₂O dissociation, H* intermediate formation, and subsequent recombination of H* to form molecular H₂. Reproduced with permission,¹³³ Copyright 2019, Wiley-VCH. (e) Volcano plot of the η₁₀ as a function of the Co valence state showing the highest HER activity for Gd_0.5. Reproduced with permission,¹²⁴ Copyright 2019, Wiley-VCH. (f) Volcano plot of the Tafel slope as a function of A-site ionic electronegativity. Reproduced with permission,¹³⁴ Copyright 2019, Springer Nature. (g) High-magnification TEM image of Ni nanoparticle-decorated La_0.4Sr_0.4Ti_0.9O_3−δ perovskite. Reproduced with permission,¹³³ Copyright 2019, Wiley-VCH.

Double perovskite oxides with a general formula of AA′B₂O_6−δ, where A sites are alkaline-earth metal ions and A′  are lanthanide elements, have also been studied as alkaline HER electrocatalysts. Based on computational research, Guan et al. predicted that synergistic interactions between the ordered oxygen vacancies (at pyramidal high-spin Co³⁺ sites) and the O 2p ligand holes (at metallic octahedral intermediate-spin Co⁴⁺ sites) in RBaCo₂O_5.5+δ (δ = 0.25; R = lanthanides) can provide a near-optimal HER reaction pathway to adsorb H₂O and release H₂, respectively (Fig. 16d).¹³³ They also experimentally showed that (Gd_0.5La_0.5)BaCo₂O_5.75 (δ = 0.25; Gd_0.5) had the highest HER activity among the RBaCo₂O_5.5+δ catalysts in 1 M KOH (Fig. 16e).¹³³ In addition, the same group reported that A-site ionic electronegativity (AIE) can be employed as an efficient descriptor to predict the HER activities of Co-based perovskites via a volcano-type activity trend (Fig. 16f).¹³⁴ Based on this prediction, (Gd_0.5La_0.5)BaCo₂O_5.5+δ(Gd_0.5) with an AIE value of ∼2.33 exhibited superior HER activity with a small Tafel slope of 27.6 mV dec⁻¹ in 1 M KOH.¹³⁴ These studies represent a descriptor-based design that can accelerate the screening of highly active electrocatalysts towards the HER.

Perovskite and pyrochlore oxides have also been used as supporting materials for metal nanoparticle deposition in the design of hybrid catalysts for the alkaline HER. For example, metal nanoparticles can be deposited on a perovskite or pyrochlore oxide host through an in situ exsolution process.^135–137 Zhu et al. synthesized a Ni nanoparticle-decorated La_0.4Sr_0.4Ti_0.9O_3−δ perovskite through the exsolution process as an alkaline HER catalyst (Fig. 16g).¹³⁵ Similarly, Kim et al. designed NiRu alloy nanoparticle-supported Pb₂Ru_2−xNi_xO_6.5 pyrochlore through a temperature-controlled exsolution process.¹³⁷ The NiRu alloy nanoparticle supported pyrochlore oxide delivered a high HER activity with a small Tafel slope of 30 mV dec⁻¹ in 0.1 M KOH.¹³⁷ The enhanced HER activity of these hybrid catalysts under alkaline media can be attributed to the bifunctional mechanism where the oxide substrate can promote the dissociation of water molecules and provide H_ad to the neighbouring active site of metal nanoparticles for the recombination of H_ad into molecular hydrogen.^135,137 Hu et al. modified the surface composition and structure of K_0.469La_0.531TiO₃ perovskite (KLTO) by a hydrothermal reaction with a RuCl₃ solution.¹³⁸ During the hydrothermal treatment, ion exchange caused the incorporation of Ru into the surface of KLTO, and nucleation growth generated Ti-doped RuO₂ (TRO) nanoparticles on the oxide surface.¹³⁸ These hybrid catalysts exhibited a very high HER activity with a low η₁₀ of 20 mV in 1 M KOH.¹³⁸ DFT computation results showed that TRO nanoparticles can promote water dissociation and the oxide surface facilitates hydrogen evolution.¹³⁸

7.2 Challenges in synthesis and catalysis

Considering the numerous combinations of A- and B-site compositions of perovskite and pyrochlore oxides, a computer-based screening process is necessary to identify the desired material structure for the best catalytic activity and stability.¹¹⁴ Recently, a computation-based approach has been used to identify a series of pyrochlore oxides as OER catalysts by calculating their phase diagrams, Pourbaix diagrams, projected density of states, and band energy diagram.¹³⁹ In particular, DFT-based Pourbaix diagrams are found to be effective to predict the stability of oxide catalysts under acidic OER conditions.^139,140 Additionally, a recent study has shown that a machine-learning model can be used to screen the covalency competition in spinel oxides to predict the composition of a highly active OER catalyst.¹⁴¹ This machine-learning based computational approach is also essential for identifying the ideal composition of perovskite and pyrochlore oxides for the HER and OER.

In addition to the composition, oxide size control is also important because electrocatalytic reactions occur on or near the surface. The conventional synthesis of perovskite and pyrochlore oxides typically involves a high-temperature calcination process (>800 °C), resulting in particle sintering and surface area reduction.¹¹⁴ Therefore, it is highly desirable to synthesize nanoscale perovskites and pyrochlore oxides under low-temperature conditions. Recent studies have used a porogen method to synthesize nanostructured porous pyrochlore oxides¹⁴² and a polymer entrapment flash pyrolysis (PEFP) method to demonstrate the synthesis of phase-pure pyrochlore oxides at low temperatures (500 °C).¹⁴³ Such efforts represent an effective strategy for improving the active sites of oxides for electrocatalytic reactions.

Surface reconstruction has been shown to occur for perovskite and pyrochlore oxides during the OER.^140,144,145 The most recent studies have shown that understanding the dissolution of the A-site and the associated exposure of the B-site metal in perovskite and pyrochlore oxides is crucial for improving both the activity and stability of the OER.^140,145 For example, Sr-doped LaCoO₃ (La_1−xSr_xCoO₃) perovskite showed enhanced activity during the alkaline OER with surface evolution due to A-site Sr dissolution.¹⁴⁵ The enhanced OER activity was attributed to the formation of a Co hydr(oxy)oxide layer that interacts with trace amounts of Fe (aq.) in the alkaline electrolyte to generate dynamically active sites.¹⁴⁵ It has also been reported that the dissolution of the A-site element in A₂Ru₂O₇ pyrochlore oxides during the acidic OER can expose highly oxidized Ru sites that exhibit enhanced activity.¹⁴⁰ Therefore, it is very important to understand the actual active sites and associated reaction mechanisms for mixed metal oxide catalysts during the OER process.

Finally, the in situ growth of metal nanoparticles on perovskite or pyrochlore oxide hosts through the exsolution process can be a powerful strategy to design high-performance hybrid catalysts for both the HER and OER.^136,137 In detail, the transition metals incorporated into the B-site of perovskite or pyrochlore oxides can be exsolved from the oxide backbone as highly dispersed nanoparticles under a reducing environment. In particular, a recent study has revealed that the generated vacancies in the pyrochlore oxide support during the exsolution process can facilitate charge transfer between the exsolved metal nanoparticles and oxide support for enhanced catalytic reactions.¹³⁷ These results open up new opportunities to design a variety of hybrid catalysts by carefully controlling the compositions and reduction conditions of perovskite and pyrochlore oxides.

8. Metal phosphides

TMPs are increasingly being developed through various technologies owing to the synergistic effect of metals and phosphides realizing a high efficiency of water electrolysis. Most of these materials involved Fe-, Co-, and Ni-based phosphides, and their alloy forms have been shown to efficiently improve the performance of electrolysis by altering the composition. In the case of the OER, TMOs are well known to promote the performance when the interaction of M²⁺ with OH* intermediates is weakened.²¹ This is mainly attributed to the increased repulsion between the metal d-band and the coordinated oxygen p-band (3d–2p). However, TMPs are not promising as OER catalysts because electronegative P atoms located near the metal atoms result in the obstruction of reaction intermediate OH* coordination on TMPs and thus the OER.¹⁶ Instead, TMPs have been applied as pre-catalysts to form OER active metal-oxo/hydroxo species under anodic conditions. In contrast, TMPs have been proven as excellent catalysts for the HER. The negatively charged P atoms and the positively charged metal atoms serve as the proton and anion acceptors, respectively. Therefore, both influence the ensemble effect of cooperatively promoting the HER under alkaline/acidic conditions. In addition, increasing the P atomic content within an appropriate range was found to be more effective in enhancing HER activity. Thereafter, much effort has been made to understand the role of P as the catalyst for the HER. Recently, Shin et al. designed a synthetic process to obtain Fe_xNi_2−xP nanocatalysts with various Fe-to-Ni compositions while maintaining the crystal structure.²² This work demonstrated the alloying effect of TMPs, excluding the structural effect, and that Fe_0.5Ni_1.5P nanocatalysts required the lowest η₅₀ of 163 mV and a Tafel slope value of 65 mV dec⁻¹ among the various Fe_xNi_2−xP nanocatalysts (Fig. 17a). According to XPS analysis, the P atoms in Fe_0.5Ni_1.5P showed the most electron-deficient character among the P atoms in Fe_xNi_2−xP (Fig. 17b). By EXAFS, it was found that the highly ordered M–M (Ni–M, Fe–M) bonds and disordered Ni–P bonds at Fe_0.5Ni_1.5P led to reduced electronegativity of P and thus to significantly improved HER kinetics (Fig. 17c). As a result, the optimal electron distribution of P is important for improving HER performance.¹⁴⁶ However, the HER performance of currently developed TMP electrocatalysts has not reached the industry level, and much effort has been made to design new catalyst models to further improve HER performances.

Fig. 17 (a) The histogram of the b and the η at j = 50 mA cm⁻² of Ni₂P (wine), Fe_0.5Ni_1.5P (red), Fe_1.0Ni_1.0P (orange), Fe_1.5Ni_0.5P (green), and Fe₂P (blue) nanocatalysts. (b) XANES spectra of the Fe_xNi_2−xP nanocatalysts: Ni 2p, Fe 2p, and P 2p binding energies of Fe_xNi_2−xP nanocatalysts. Fourier transforms of the (c) Ni K-edge and Fe K-edge EXAFS spectra for Fe_xNi_2−xP nanocatalysts. Reproduced with permission,²² Copyright 2020, American Chemical Society.

8.1 The state-of-the-art catalysts and their HER performances

To significantly improve the HER performance, TMP development strategies, such as N doping to induce lattice contraction, heterostructure formation, and vacancy/defect engineering, have been the primary topics. Firstly, the N doping effect is introduced to improve the HER performance. As discussed, TMPs with abundant P sites have been predicted to be more favorable for HER catalysis. However, the actual activities of P-rich TMPs did not work as expected, and the fundamental understanding of P-rich conditions was unclear at the atomic level. Therefore, from a structural perspective, modulating the nonpolar P–P interaction to tailor the overlap of atomic wave functions has been considered to further boost the intrinsic HER activity of TMPs. Cai et al. developed N-doped CoP₂ nanowires (N-CoP₂ NWs) to increase the overlap of atomic wave functions via N-induced lattice contraction.¹⁴⁷ XRD patterns showed similar diffraction patterns of CoP₂ and N-CoP₂ NWs. The diffraction peaks were slightly shifted to the higher angle region with the N incorporation (Fig. 18a), suggesting that N doping does not change the crystal structure of CoP₂ but leads to slightly compressed interplanar distances. The HAADF-STEM and the corresponding EDX elemental mapping images indicated that Co, P, and N are homogeneously distributed over the NW (Fig. 18b). The broad XPS N 1s spectrum of N-CoP₂ can be deconvoluted into the chemical states of N–Co and N–P, suggesting that N is successfully doped into the CoP₂ lattices (Fig. 18c). Together, material analyses further demonstrated that N is substitutionally doped into the structure of CoP₂, which can essentially manipulate the electronic and coordination structures of CoP₂. The N-CoP₂ NWs showed high catalytic performance with an η₁₀ of 38 mV and a Tafel slope of 46 mV dec⁻¹, which are substantially smaller than those of CoP₂ NWs (125 mV and 73 mV dec⁻¹) and even close to those of benchmark Pt/C catalysts (28 mV and 29 mV dec⁻¹) (Fig. 18d). This catalyst also showed superior stability up to 30 h (Fig. 18e). Characterization and DFT calculations consistently revealed that the created strong N–P interaction in CoP₂ could increase the atomic wave function overlap via lattice contraction and consequently broaden the local energy bandwidth with the downshift of the valence band centre. Furthermore, the free energy of ΔG_H on N-CoP₂ NWs was consistently much closer to neutral than those on CoP₂ (Fig. 18f). As a results, N-CoP₂ NWs facilitated HER catalysis by weakening the electron coupling between CoP₂ and H_ads.

Fig. 18 (a) XRD patterns of CoP₂ and N-CoP₂ NWs and the corresponding magnification of the diffraction peaks of the (200) facet. (b) HAADF-STEM image and EDX elemental mapping of Co, P, and N for a single N-CoP₂ NW. (c) XPS N 1s spectrum of N-CoP₂. (d) Polarization curves for CC, CoP₂, N-CoP₂, and Pt/C. (e) Chronopotentiometric curve for N-CoP₂ at a current density of 20 mA cm⁻². (f) Calculated ΔG_H on CoP₂ and N-CoP₂ with various facets. Reproduced with permission,¹⁴⁷ Copyright 2020, AAAS. (g) High-resolution TEM image of Ni₁₂P₅–Ni₄Nb₅P₄/PCC. (h) The SEM images of CC treated with DBD plasma. (i) The XRD pattern of Ni₄Nb₂O₉/PCC and Ni₁₂P₅–Ni₄Nb₅P₄/PCC. (j) Polarization curves of PCC, CC, Pt/C/PCC, Ni₄Nb₂O₉/PCC and Ni₁₂P₅–Ni₄Nb₅P₄/PCC. (k) Polarization curves of Ni₁₂P₅–Ni₄Nb₅P₄/PCC before and after 3000 cycles, and its chronoamperometric curves at a current density of 15 mA cm⁻² for 100 h in the inset of (k). (l) Gibbs energy of different catalysts for the HER (dotted lines represent bonds facing the plane). Reproduced with permission,¹⁴⁸ Copyright 2020, Wiley-VCH.

Heterointerface engineering can be used to develop excellent catalysts through electronic coupling effects between different components or phases. In addition, carbon defects can be used to trap and stabilize reaction products, further promoting electrocatalytic performances. Based on this, Chen et al. reported that bi-phase Ni₁₂P₅–Ni₄Nb₅P₄ nanocrystals with rich heterointerfaces and phase edges are successfully fabricated on carbon cloth (PCC), which is utilized by intentional defect creation by atmospheric pressure dielectric barrier discharge (DBD) plasma.¹⁴⁸Fig. 18g exhibits the high-resolution TEM image of Ni₁₂P₅–Ni₄Nb₅P₄ featuring the crystal planes of Ni₁₂P₅ and Ni₄Nb₅P₄ are characterized. After the DBD plasma treatment, a large number of circular pits were formed on the carbon fiber surface (Fig. 18h). Fig. 18i shows the XRD patterns of Ni₄Nb₂O₉/PCC and Ni₁₂P₅–Ni₄Nb₅P₄/PCC. According to these results, a hetero-structured Ni₁₂P₅–Ni₄Nb₅P₄ compound was formed on the PCC. The Ni₁₂P₅–Ni₄Nb₅P₄/PCC electrocatalyst exhibited excellent HER performance, showing a low η₁₀ and η₅₀ of 81 and 287 mV, respectively, which outperforms the industry-relevant benchmark Pt/C/PCC catalyst (Fig. 18j). As shown in Fig. 18k, the polarization curves after 3000 cycles were close to the initial one. A long-term chronoamperometric measurement at a current density of j₁₅ (the inset of Fig. 18k) for 100 h was performed that the obtained Ni₁₂P₅–Ni₄Nb₅P₄/PCC catalyst presents an excellent stability. Therefore, it can be demonstrated that Ni₁₂P₅–Ni₄Nb₅P₄ tightly coated on the surface of PCC results in a stable electrocatalyst. As shown in Fig. 18l, the ΔG_H at the interface of Ni₁₂P₅–Ni₄Nb₅P₄ was about 0.16 eV, which is much lower than that at the surfaces of Ni₁₂P₅ (0.23 eV) and Ni₄Nb₅P₄ (0.35 eV), indicating favorable H* adsorption kinetics on the Ni₁₂P₅–Ni₄Nb₅P₄ heterostructure during the HER process. They claimed that the superior HER performances of the Ni₁₂P₅–Ni₄Nb₅P₄/PCC electrocatalyst are attributed to the well-defined heterointerface between the Ni₁₂P₅ and Ni₄Nb₅P₄ phases and strong bonding between the composite and PCC. This feature reduced the loss of current, facilitated the adsorption or activation of active species, and caused a faster electron transfer and kinetic process for the HER.

In view of vacancy/defect engineering, Duan et al. succeeded in creating P vacancies (P_v) in Ni₁₂P₅ through a thermal-annealing process.¹⁴⁹ As shown in Fig. 19a, the Ni₁₂P₅ with P_v (v-Ni₁₂P₅) showed a porous nanosheet texture with homogeneous dispersion of Ni and P atoms. Synchrotron-based XAS indicated that the higher intensity of the Ni K-edge in Ni₁₂P₅ than that in metal Ni suggests more probable 1s → 3d transition, while it was much lower than that of NiO, indicating that Ni in Ni₁₂P₅ is close to the metallic state with a slightly smaller number of electrons in Ni 3d (Fig. 19b). A slightly lower intensity of v-Ni₁₂P₅ than that of pristine Ni₁₂P₅ (p-Ni₁₂P₅) suggested a higher electron density in the Ni 3d band because of P_v. The v-Ni₁₂P₅ catalyst required a small η₁₀ of 27.7 mV and a Tafel slope of 30.88 mV dec⁻¹ in comparison with p-Ni₁₂P₅ (120.1 mV and 83.60 mV dec⁻¹), and even outperformed the benchmark Pt/C (32.7 mV and 30.90 mV dec⁻¹) (Fig. 19c). DFT calculations performed that conducted that a significant electron redistribution in the Ni₁₂P₅ is induced by P_v with considerable electron accumulation and depletion parts inside v-Ni₁₂P₅ (Fig. 19d). As shown in Fig. 19e, the electrons depleted in the neighboring area (colored blue) of Ni 3d and P 2p, while accumulated on the Ni and P atoms (colored red). This electron redistribution at the P_v site may have a trivial effect on the adsorption/desorption of reaction intermediates during the HER process. The free energy is proven to be a key descriptor to characterize the HER activity of the electrocatalyst. The ΔG_H values were −0.36 eV for v-Ni₁₂P₅ and −0.43 eV for p-Ni₁₂P₅, demonstrating that the H* desorption step can be boosted by P_v, while the OH* desorption step was unaffected owing to the unchanged energies (Fig. 19f). According to XAS characterization together with DFT calculations, the P_v can weaken the hybridization of Ni 3d and P 2p orbitals, enrich the electron density of Ni and P atoms nearby P_v, and facilitate the H* desorption process, contributing to outstanding HER activity and facile kinetics.

Fig. 19 (a) TEM image and TEM EDS elemental mapping images of Ni and P of v-Ni₁₂P₅ (inset: EDS spectrum). (b) XAS spectra (fluorescence mode) of nanocataylsts in energy and R-spaces. (c) Polarization curves of NF, p-Ni₁₂P₅, v-Ni₁₂P₂, and Pt/C. (d) Electron distribution; yellow area indicates electron accumulation while cyan means electron depletion. (e) Two-dimensional charge difference isosurface; red is the electron-rich area while blue is the deficient area. (f) DFT calculated reaction pathways of v-Ni₁₂P₅. Reproduced with permission,¹⁴⁹ Copyright 2020, American Chemical Society.

8.2 Challenges in synthesis and catalysis

Although many TMP-based catalysts have been developed to enhance HER performance, the scalable and cost-effective synthesis of catalysts still remain challenging. Fundamental methods to enhance the electrocatalytic HER performance of TMPs have been developed to increase the number of active sites and the intrinsic activity of active sites. Accordingly, the development of facet-controlled and hollow TMP nanostructures has received great interest because these material structures can achieve highly active and large surface areas due to their morphological characteristics. The facet effect on the HER has been reported by several researchers. As an example, Wu et al. reported that CoPS single crystals with exposed (100) and (111) facets exhibited different HER behaviors as a function of η.¹⁵⁰ In the low potential region (0–0.35 V), the HER catalytic activity of the (100) facet was superior to that of the (111) facet, and the trend was reversed in the high-potential region (>0.35 V). At low potentials, rate limiting H adsorption was faster on the (100) facet, yielding an enhanced HER performance. However, at high potentials, H₂ recombination/desorption became the rate determining step and thus the (111) facet, with lower H_ads barriers, showed better HER activity. Therefore, hollow TMP nanostructures enclosed with a specific facet may possess a larger density of active sites than solid ones, contributing to the improvement in HER electrocatalysis. However, morphology control of catalysts, including hollow structures, is much less explored for these TMP systems because of difficulties in the synthesis, which requires a deep understanding of the nanocrystal growth process. Therefore, by further developing morphology and facet-controlled nanostructures, we can expect a high-density of low-coordinated metals and abundant P atoms as catalytic sites, which can prove to be better water splitting catalysts.

9. Metal sulfides

TMSs have been widely developed as potential electrocatalyst candidates for water splitting because of their unique physical and chemical properties.^16,151,152 Compared to TMOs, TMSs usually have high electrical conductivities stemming from the small bandgap between their valence and conduction bands. More impressively, 2D layered MS₂ (M = Mo, W) have abundant exposed active sites and moderate adsorption energy for atomic H, leading to remarkable electrocatalytic performance for the HER. Another class of TMSs, namely, nonlayered M_xS_y (M = Fe, Ni, Co, etc.), have also emerged as economical HER catalysts due to their high electrical conductivity, fast charge transfer kinetics, and low cost. The catalytic performance of these catalysts can be enhanced by various strategies, including morphology control, doping heteroatoms, phase control, defect engineering, and composite engineering.

Compared to the electrocatalytic mechanism of the HER, the OER shows a different mechanism and thus different strategies to improve catalytic performance. TMSs are characterized by lower thermodynamic stabilities than metal oxides, indicating that TMSs can be easily oxidized to the corresponding metal oxides under strongly oxidative conditions of the OER.¹⁵³ Therefore, TMSs act as a pre-catalyst in forming the oxide/hydroxide surface as actual catalytic active sites. In this section, the recent progress in different classes of TMS for the HER and OER is summarized. Moreover, the mechanism to enhance the catalytic activities of TMSs is discussed in detail.

9.1 The state-of-the-art catalysts and their HER/OER performances

The study of TMSs for the electrochemical HER can be traced back to 2005, which was inspired by the composition and structure of nitrogenases and hydrogenases.¹⁵⁴ The electrocatalytic performance results showed that (1010) Mo-edged MoS₂ is highly active toward the HER. Furthermore, the computational result of the ΔG_H on a Mo-edge was close to that of Pt, indicating the great potential of MoS₂ as a promising HER electrocatalyst. This innovative material opened up a new door for economical PGM free electrocatalysts. To further study the active sites of MoS₂, Jaramillo et al. synthesized different sizes of MoS₂ with a systematically varied number of active sites.¹⁵⁵ They reported that the catalytic performance of MoS₂ is directly proportional to the edge length, regardless of the particle size. After numerous studies about MoS₂, the nonlayered first-row transition TMSs were studied as PGM-free HER electrocatalyst candidates. Kong et al. reported that pyrite FeS₂, NiS₂, and CoS₂ exhibit attractive activity and stability toward the HER in an acidic environment.¹⁵⁶ Moreover, Fe_0.43Co_0.57S₂ had enhanced catalytic activity compared to FeS₂ and CoS₂.

The strategies to enhance the HER performance of TMSs were classified into two distinct groups: increasing active sites and electronic structure modification. The most general method to increase the number of active sites was designing new kinds of morphologies with a large surface area.^157–160 Kibsgaard et al. synthesized mesoporous MoS₂ with a double-gyroid structure, exposing more Mo-edges.¹⁵⁷ This unique structure of MoS₂ exhibited an increased surface area with a longer electrodeposition time. After optimal deposition time, double-gyroid MoS₂ showed a 2.2-fold larger surface area compared to MoO₃–MoS₂ nanowires. Furthermore, various shapes of TMSs such as nanorods, nanosheets, nanowires, and hollow structures were synthesized.^158–163 These morphology control studies have recently expanded to hetero-interface structures.^164–167 Solomon et al. reported Ag₂S/MoS₂/RGO with a ternary composite by using a simple one-pot polyol method.¹⁶⁴Fig. 20a shows the high-resolution TEM image of the Ag₂S/MoS₂/RGO composite composed of some large Ag₂S nanoparticles (10–50 nm) embedded in MoS₂. In addition, Fig. 20b exhibits the fast Fourier transform (FFT) pattern, and extracted inverse FFT images from the spots in the FFT pattern (red and yellow, [0,0,1] and [1,1,0] zone axes of Ag₂S; green, [0,0,2] zone axis of MoS₂). Moreover, the HAADF-STEM and the corresponding EDX elemental mapping results indicated that Ag and S are homogeneously distributed over the nanoparticles, while Mo is located within the outer part (Fig. 20c). The XRD pattern and Raman spectrum of the Ag₂S/MoS₂/RGO composite exhibited distinct phases of Ag₂S and RGO signals (Fig. 20d and e). According to these results, the Ag₂S/MoS₂/RGO composite has a large number of hetero-interfaces between two phases. The η₁₀ and η₅₀ of the Ag₂S/MoS₂/RGO composite required to drive the HER were 190 and 300 mV, while the η₅₀ of MoS₂ and MoS₂/RGO was 460 and 400 mV, respectively (Fig. 20f). The smaller required η of the Ag₂S/MoS₂/RGO composite compared to those of MoS₂ and binary composites indicated that the hetero-interfaces in the composite promotes HER performance. In addition, the double-layer capacitance (C_dl) results could evaluate the ECSA of each catalyst based on the linear relationship between the C_dl and ECSA. The C_dl of the Ag₂S/MoS₂/RGO composite (23.7 mF cm⁻²) was larger than those of MoS₂ (8.9 mF cm⁻²) and MoS₂/RGO (16 mF cm⁻²), indicating a larger ECSA (Fig. 20g). This could imply that the larger ECSA of the Ag₂S/MoS₂/RGO composite contributed to an enhanced electrocatalytic HER. As shown in Fig. 20h, to demonstrate the improved HER activity, they performed electrochemical impedance spectroscopy and data were fitted using a two-series of constant phase elements (the inset of Fig. 20h). The charge transfer resistance of the Ag₂S/MoS₂/RGO composite was 96 Ω, which is much smaller than those of MoS₂ (495 Ω) and MoS₂/RGO (259 Ω). This dramatically reduced charge transfer resistance clearly indicated that fast electron transfer occurs at the hetero-interfaces, leading to superior HER performances of the Ag₂S/MoS₂/RGO composite.

Fig. 20 (a and b) High-resolution TEM image of the Ag₂S/MoS₂/RGO composite, and the corresponding FFT patterns and extracted inverse FFT images from the spots in the FFT pattern (red and yellow, [0,0,1] and [1,1,0] zone axes of Ag₂S; green, [0,0,2] zone axis of MoS₂). (c) HAADF-STEM image of the Ag₂S/MoS₂/RGO composite and the corresponding EDX elemental maps. (d) XRD pattern of the Ag₂S/MoS₂/RGO composite. (e) Raman spectra of the Ag₂S/MoS₂/RGO composite. (f) Polarization curves of the Ag₂S/MoS₂/RGO composite and other counter groups. (g) Linear fit for double-layer capacitance from cyclic voltammetry results at different scan rates. (h) Nyquist plots and equivalent circuit (inset) of MoS₂, MoS₂/RGO, and Ag₂S/MoS₂/RGO composites. Reproduced with permission,¹⁶⁴ Copyright 2019 American Chemical Society.

In view of electronic structure modification, alloying or doping with other transition metals has been widely used.^{101,158,168–172} Yin et al. synthesized NiS₂ nanosheets with various transition metal dopants (Co, Cu, and Fe) to modify the electronic structure.¹⁷² This work demonstrated the incorporating transition metal effect by the projected partial DOS (PDOS) of surface metal sites (Fig. 21a). The surface Ni sites adjacent to the dopants serve as electron-depleting centres and promote electron transfer. Especially, Co doped NiS₂ (Co-NiS₂) shows a smaller e_g–t_2g splitting gap compared to Fe and Cu doped samples, indicating a higher efficiency of electron transfer across the Fermi level. In addition, DFT calculations were performed to understand the correlation between the doping and overall energetic pathway of the HER. As shown in Fig. 21b, the transition state barrier of water splitting at Co-NiS₂ was 0.80 eV, which is much smaller compared to that of NiS₂ (1.38 eV), indicating an energetically favourable HER. Based on these computational results, Co-NiS₂ required the lowest η₁₀ of 80 mV and the smallest Tafel slope of 43 mV dec⁻¹, which is much smaller than that of Fe doped, Cu doped, and undoped NiS₂ (Fig. 21c). In addition, vacancy engineering has been used as a strategy to modify the electronic structure of catalysts. Jia et al. reported that sulfur vacancies (V_s) could be generated by Ar plasma etching.¹⁶⁸ As shown in Fig. 21d and e, Ni₃S₂ with V_s (V_s-Ni₃S₂) synthesized by Ar plasma treatment for 300 seconds shows a Ni₃S₂ crystal structure with a porous morphology, indicating that the Ar plasma does not affect the crystal structure. To further analyse the V_s, they performed electron paramagnetic resonance (EPR) and XPS measurements (Fig. 21f and g). V_s-Ni₃S₂ exhibited an approximately two times larger EPR signal compared to Ni₃S₂, corresponding to V_s. Moreover, Ni 2p spectra shifted negatively due to V_s. V_s-Ni₃S₂ showed much lower η₁₀ for the HER (88 mV) than Ni₃S₂ (274 mV) (Fig. 21h). As shown in Fig. 21i, V_s in Ni₃S₂ change the electron density of the Ni site and the d-band centre shifts positively toward the Fermi level, demonstrating stronger binding between adsorbate and active sites. Therefore, the calculated ΔG_H at the Ni and S sites of V_s-Ni₃S₂ was lower than that of Ni₃S₂ and these results back up the excellent HER performance of V_s-Ni₃S₂ (Fig. 21j).

Fig. 21 (a) PDOS of the 3d-bands of Co, Fe, and Cu doped NiS₂. (b) Polarization curves of undoped NiS₂, Co, Fe, and Cu doped NiS₂. (c) Transition state barrier for H₂O splitting on the surface of NiS₂ and Co-NiS₂. Reproduced with permission,¹⁷² Copyright 2019 Wiley-VCH. (d) TEM image of V_s-Ni₃S₂. (e) XRD pattern of V_s-Ni₃S₂ treated for different time durations and Ni₃S₂. (f) EPR spectra of Ni₃S₂ and V_s-Ni₃S₂. (g) XPS spectra of Ni₃S₂ and V_s-Ni₃S₂. (h) Polarization curves of V_s-Ni₃S₂ treated for different time durations and other counter groups. (i) PDOS of the d-band for the (110) Ni surfaces of Ni₃S₂ and V_s-Ni₃S₂. (j) Gibbs free energies of H adsorption on the S and Ni sites of the (110) surface for Ni₃S₂ and V_s-Ni₃S₂. Reproduced with permission,¹⁶⁶ Copyright 2020 Royal Society of Chemistry.

The scientific approach to enhancing catalytic activity toward the OER is quite different from that of the HER. As mentioned earlier, the surface of sulfides is converted into oxides/hydroxides at harsh OER potentials, indicating that the sulfide surface cannot act as an active site. Thus, TMS electrocatalysts were considered a pre-catalyst with a hetero-structure of metal sulfides and metal oxides/hydroxides.¹⁷³ Recently, researchers have introduced the concept of a high entropy alloy to implement stable metal sulfides at OER potentials. Cui et al. reported high entropy metal sulfide nanoparticles through a pulse thermal decomposition method.¹⁷⁴ They utilized Cr, Mn, Fe, Co, and Ni metal elements to prepare various compositions of metal sulfides ranging from unary to quinary. As shown in Fig. 22a, the HAADF-STEM and the corresponding EDX elemental mapping images indicated that all metal elements and sulfur are homogeneously distributed. Quinary high entropy metal sulfide nanoparticles (i.e., (CrMnFeCoNi)S_x) showed enhanced OER activity and stability compared to binary, ternary, and quaternary nanoparticles (Fig. 22b and c). Moreover, even after an OER stability test, the (CrMnFeCoNi)S_x nanoparticles maintained their pristine crystal structure (Fig. 22d). This enhanced OER performance was consistent with DFT calculation, which reveals that (CrMnFeCoNi)S_x has optimal adsorption energy ΔE_o* (Fig. 22e). As a result, high entropy metal sulfides offer new insights into utilizing TMSs toward OER electrocatalysts.

Fig. 22 (a) HAADF-STEM image of (CrMnFeCoNi)S_x and the corresponding EDX elemental maps. (b) Polarization curves of (CrMnFeCoNi)S_x and other counter groups. (c) Chronopotentiometry test of (CrMnFeCoNi)S_x at a constant current density of 100 mA cm⁻². (d) XRD patterns of (CrMnFeCoNi)S_x before (red) and after (black) the OER stability test. (e) Volcano plot of metal sulfides for the OER. Reproduced with permission,¹⁷⁴ Copyright 2021 Wiley-VCH.

9.2 Challenges in synthesis and catalysis

TMS based electrocatalysts have been widely studied to enhance HER and OER activity through the traditional strategies of increasing active sites and electronic structure modification. However, advanced and recent strategies, such as forming hetero-interfaces, vacancies, and high entropy alloys, are very underdeveloped. Especially, hetero-structured TMSs show great potential toward HER and OER nanocatalysts. Hetero-interfaces are able to modulate the electronic structure of the both sides of materials and also provide a synergistic effect between two materials, such as the spill-over effect of adsorbates.^167,175,176 These unique advantages of hetero-interfaces simultaneously improve the electrocatalytic performance toward the HER and OER. However, understanding of hetero-interfaces, vacancies, and high entropy alloys is still at their early stages in boosting electrocatalytic performance. Therefore, continuous effort on understanding the relationship between the aforementioned strategies and the catalytic performance is crucial for the further development of TMS-based electrocatalysts.

In addition to a poor understanding of these state-of-the-art strategies, the lack of single-cell performance evaluations shows a threshold for the practical utilization of TMS electrocatalysts toward water electrolysers. According to Kim et al., electrocatalytic performance in a half-cell test and single-cell test is similar in the low current density region, where the electrocatalytic performance is determined by the reaction kinetics.¹⁷⁷ On the other hand, in a high current density region, the mass transfer at the electrode surface becomes a significant factor affecting the electrocatalytic performance, which gives a disjunction between the half-cell performance and the single-cell performance. Therefore, in the field of TMSs, where most of the research is concentrated on half-cell evaluation, it is believed that sufficient research on whether there is practical applicability will be needed.

10. Single-atom catalysts

Water splitting under an electrochemical force is a promising strategy to convert electricity and water into a high-purity hydrogen fuel.^15,178 The key point of electrochemical water splitting is high-performance catalysts to catalyze the half reaction of the HER and OER. Overcoming the limitation of high reaction kinetic barriers needs to be given a priority for the rational design of target catalysts. Single-atom catalysts (SACs) show great application prospects in the fields of the HER and OER, and many breakthroughs have been made recently.

10.1 The state-of-the-art catalysts and their HER/OER performances

Recently, SACs showed comparable HER performance with maximal utilization efficiency. Therefore, many researchers show interest in the fields of SACs to improve the HER performance. Using carbon materials as the support to anchor isolated metal atoms to fabricate Pt-based SACs has attracted enormous attention due to their high electrical conductivity, stability in a harsh liquid-phase, structural diversity, and tailored surface chemistry. Although Pt is a PGM, Pt-based SACs dramatically reduce the utilization of Pt, resulting in cost savings comparable to non-PGM-based catalysts. Implanting foreign metal atoms into the defects of the carbon support not only stabilizes isolated metal atoms, but also modifies their electronic structures via metal–support interactions. For example, Qu et al. developed a facile route to access active and well-defined single atom site catalysts.¹⁷ In general, a thermal emitting method using Pt bulk as the precursor under ammonia derived from dicyandiamide pyrolysis was developed. Because of the strong chelating action between ammonia and Pt atoms, the atomically dispersed Pt atoms could be caught by ammonia to form Pt(NH₃)_x and anchored on the defective graphene surface, as illustrated in Fig. 23a. In this case, the zero-valent Pt was oxidized to Pt^δ (0 < δ < 4), and at the same time, the most oxygen-containing functional group on GO was removed through thermal treatment, generating defective graphene (DG). Fig. 23b and c show the HAADF-STEM images of the as-obtained Pt SAs/DG catalyst. As can be seen, the light spots in Fig. 23c are the atomically dispersed Pt atoms. The coordination environment of isolated Pt atoms was further demonstrated by EXAFS characterization (Fig. 23d). The resultant Pt SAs/DG catalyst has exhibited superior performance for the HER as shown in Fig. 23e. As seen from Fig. 23f, the Pt SAs/DG catalyst shows the lowest ΔG_H* in comparison with other catalysts. Yin et al. demonstrated a new-type Pt-based SAC via supporting two Pt atoms on graphdiyne (donated as Pt-GDY2) to tune the electronic effect of Pt.¹⁷⁹ From the HER performance results, the activity of Pt-GDY2 was 26.9 times higher than that of benchmark Pt/C. A deeper study revealed that the five-coordinated Pt metal centre possessing zero-value ΔG_H* showed an enhanced HER performance. Zhang et al. documented a novel strategy to anchor isolated Pt atoms in N-doped mesoporous carbon via high-temperature pyrolysis. The obtained SACs exhibited 25 times higher mass activity than benchmark Pt/C.¹⁸⁰ In addition, the electroplating deposition method was also adapted to prepare atomically dispersed metal atoms on carbon materials. Zhang et al. demonstrated a facile and general electrochemical method to construct Pt-based SACs by using CoP nanoarrays as the support,¹⁸¹ which show an excellent HER performance in neutral media.

Fig. 23 (a) Proposed reaction mechanism for the preparation of Pt SAs/DG. (b and c) HAADF STEM image, (d) R-space spectra from EXAFS, and (e) HER LSV curves of the catalysts. (f) Calculated Gibbs free energy diagram of the HER on Pt/C, Pt-SAs-graphene and Pt-SAs-C₄ at the equilibrium potential. Reproduced with permission,¹⁷ Copyright 2019, American Chemical Society.

Besides Pt-based SACs, other non-Pt based SACs were also designed and prepared. Earth-abundant catalysts, especially M–N/C, have been widely studied as appealing non-PGM catalysts for HER application.¹⁸² Isolated Mo atoms anchored on N-doped carbon materials were designed and fabricated using a versatile template and high-temperature pyrolysis route.¹⁸³ AC-STEM and EXAFS characterization revealed that the single Mo atom is successfully anchored. Benefiting from the unique structural features, Mo-NC SACs showed extraordinary HER performance compared with Mo₂C, MoN, and benchmark Pt/C. DFT calculations disclosed that Mo-NC SACs possess a low ΔG_H* and a large density of states. In addition, metal oxide/carbide/nitride and carbon encapsulated metal atoms showed enhanced HER performance. However, the synergistic effects among these active species make it difficult to uncover the real active site and electrocatalytic mechanism.

As the other half reaction of water dissociation, the OER is kinetically sluggish in comparison with the HER, and SACs showed promising potential for OER catalysis. Zhang et al. provided an atomically dispersed Ir and Ir cluster supported on Co(OH)₂ nanosheets via a facile NaBH₄ reduction strategy.¹⁸⁴ The atomic Ir/Co(OH)₂ catalyst showed an overpotential of 373 mV at 10 mA cm⁻² in 1.0 M PBS. After activation, the valence state of Ir species increased derived from the strong peak of Ir–O coordination. On the other hand, the Co K-edge showed an evident shift to higher energy, indicating that the Co species are oxidized to a higher valence. XRD results revealed that the α-Co(OH)₂ phase is transformed into β-phase Co oxyhydroxide (β-CoOOH). Deep characterization revealed that high-valence unique β-CoOOH with a low-coordination structure and Ir species under oxidation potential are the main active sites for the OER.

Hu et al. developed a hybrid amorphous/crystalline FeCoNi LDH-supported and -stabilized single Ru atom catalyst (Ru SAs/AC-FeCoNi) through facile self-templating cation-exchange tactics,¹⁸⁵ as shown in Fig. 24a. The combined characterization of AC-STEM and XANES disclosed that the Ru atoms are evenly dispersed over the whole FeCoNi LDH support (Fig. 24b–f). Under the synergistic effect of abundant defect sites and unsaturated coordination sites, as well as a highly symmetric rigid structure, the Ru SAs/AC-FeCoNi catalysts showed enhanced OER performance (Fig. 24g). The intrinsic strong metal–support interaction between Ru and FeCoNi LDH was the main reason for promoting the OER performance via triggering the local electronic configuration and internal electron rearrangement. DFT calculations revealed that Ru SAs/AC-FeCoNi could optimize the adsorption energies for catalytic intermediates, subsequently enhancing the OER activity (Fig. 24h and i).

Fig. 24 (a) Illustration for the synthesis of Ru SAs/AC-FeCoNi. (b) AC HAADF-STEM image of Ru SAs/AC-FeCoNi. (c) 3D AOGF mapping and (d) 3D isolines of 5 shown in (b). (e) Intensity profile along the middle of 6 from (b). (f) HAADF-STEM image and corresponding elemental mappings of Ru SAs/AC-FeCoNi. (g) LSV curves of RuO₂, AC-FeCoNi, Ru SAs/AC-FeCoNi, and Ru SAs/C-FeCoNi. (h) Proposed OER pathway on Ru SAs/AC-FeCoNi for DFT + U calculation. (i) Free energy on AC-FeCoNi, Ru SAs/C-FeCoNi, and Ru SAs/AC-FeCoNi at the equilibrium potential. Reproduced with permission,¹⁸⁶ Copyright 2017, American Chemical Society.

Similarly, compared with their PGM counterparts, theoretical and experimental studies on non-PGM based SACs for the OER have sparked much research interest because of their distinctive OER activity. Besides the verification of the single metal atom and related active moieties, the metal species evolution is also important. For the OER, due to high potential, the microenvironment of monatomic catalysts is complex and changeable, and the electronic structure and coordination environment of the metal active centre are also dynamically evolving in the process of catalysis, which leads to the variability of the catalyst structure and the complexity of the catalytic reaction mechanism. In this case, given the complex profuse pyridine-N in the C₃N₄ matrix, Zheng et al. designed a molecule-level Co-C₃N₄/CNT catalyst accompanied by a CoN₃C₂ ring via a direct grafting strategy.¹⁸⁶ The well-defined Co-C₃N₄/CNT showed an excellent OER performance.

10.2 Challenges in synthesis and catalysis

A great impact of SACs on energy generation is foreseeing, such as, providing catalysts for water splitting, whose realization depends on highly efficient materials for both the HER and OER, greatly influencing economically and technologically our society. However, seeking high-efficiency, low cost, and sustainable SACs is promising for their large-scale practical applications. In fact, advanced characterization has been used to reveal the underlying relationships between the single atom and support to give a fundamental understanding of rational design and construction of novel SACs.

Among the numerous substrates for SACs, carbon materials (such as, graphitic materials, CNTs, graphene and derivatives) show huge application prospects for electrocatalytic water dissociation, involving the HER and OER due to excellent mechanical and chemical properties. Their intrinsic catalytic performance could be further improved by implanting nitrogen to form N-doped carbon materials. Moreover, most HER or OER catalysts were prepared by combining metal atoms with a suitable support with improved electronic interactions and a unique coordination environment. Therefore, in the future, constructing different atomic interfaces and modulating the surrounding coordination structure of active sites of SACs will be the main subject to improve the performance of water splitting.

Compared with SACs, dual-atom catalysts have also received increasing interest owing to their higher metal loading, more versatile active sites and unique reactivity.¹⁸⁷ For example, Yang et al. prepared a dual-atom catalyst consisting of an O-coordinated W-Mo heterodimer embedded in N-doped graphene (W₁Mo₁-NG). Impressively, the obtained W₁Mo₁-NG catalysts exhibited Pt-like activity and excellent stability for the HER in a pH-universal electrolyte. DFT calculations demonstrated that electron delocalization of W₁Mo₁-NG could show desirable ΔG_H and good HER kinetics, thereby enhancing the HER activity.¹⁸⁸

11. Conclusion and outlooks

This review article has summarized recent research advances in the development of HER/OER electrocatalysts for water electrolysis. The major goal of this research field is to replace expensive and rare PGM-based electrocatalysts with low-cost and highly active non-PGM bifunctional ones or greatly reduce PGMs by their single atom dispersion. By far, the state-of-the-art catalysts including carbon-based catalysts, TMBs, TMCs, TMOs, TMPs, TMSs, and single-atom catalysts have been investigated toward both the HER and OER, and they showed comparable activities to PGM catalysts as listed in Tables 1 and 2. In addition, catalyst modeling has been proposed to advance electrocatalysts for HER/OER performances and their stability.

Table 1 Summary of the state-of-the-art non-PGM-based catalysts and their OER performances^a

Class	Catalysts	Method	Electrolytes	η @ 10 mA cm^−2 (mV)	Tafel slope (mV dec^−1)	Ref.
a N/A: not available.
Carbon	N/C	Pyrolysis + etching	KOH	380	N/A	35
	NPMC foam	Polymerization + pyrolysis	6 M KOH	300	193	36
	GNS/MC	Templating + pyrolysis + etching	0.1 M KOH	340	80	43
	SHG	Hybridization + pyrolysis + etching	0.1 M KOH	260	71	37
	NPC-CP	Polymerization + carbonization	1 M KOH	310	87.4	38
	Ni-NHGF	Hydrothermal + pyrolysis	1 M KOH	331	63	40
	A-Ni@DG	Annealing + etching	1 M KOH	270	47	41
	CoNi-SAs/NC	MOF + pyrolysis	0.1 M KOH	340	58.7	45
	Ni-N4/GHSs/Fe-N4	Deposition + annealing + etching	0.1 M KOH	390	81	46
	PHI-Co-0.5	Hydrothermal	1 M KOH	324	44	42
TMBs	Co2B/N-doped graphene	Chemical reduction	1 M KOH	360	45	192
	NixB	Chemical reduction	1 M KOH	380	89	193
	FeB2	Chemical reduction	1 M KOH	296	52.4	65
	Fe–Co-2.3Ni–B	Chemical reduction	1 M KOH	274	38	194
	CoBx@h-BN	Chemical reduction + annealing	1 M KOH	290	81.9	195
	Ni3B/Ni foam	Annealing	1 M KOH	300	43	196
	V doped Ni–Co boride	Chemical reduction + CVD	1 M KOH	340	58	67
	boronized NiFe foil	Annealing	1 M KOH	270	N/A	66
	FeCoNiBOx/PPy/rGO	Chemical reduction	1 M KOH	290	47	197
	W, P-FeB	Chemical reduction	1 M KOH	209	40	198
TMC	Fe/Fe3C-MC	Electrochemical reduction	1 M KOH	320	51	91
	FCC@CNOs/NF	High pressure annealing	1 M KOH	271	48.9	92
	PB@Met-700-acid	MOF derived method	1 M KOH	390	N/A	199
	Co SAs/Mo2C	MOF derived method	1 M KOH	270	74.9	87
	Co-Mo2C	MOF derived method	1 M KOH	190	94.3	69
	Co/Mo2C@NC-800	MOF derived method	1 M KOH	311	131.50	200
	Co6W6C@NC	MOF derived method	1 M KOH	286	53.96	88
	NC@CuCoW-C (S-2)	MOF derived method	1 M KOH	238	59	89
	Ni/WCx-CNFs-3	Electrospinning + carbonization	1 M KOH	350	93	201
	a-MnOx/TiC	Hydrothermal	0.1 M KOH	330	110	202
	Ni-Ni3C/CC	Annealing	1 M KOH	299 (@ 20 mA cm^−2)	43.8	76
	Co3C	Wet-chemistry	1 M NaOH	455	N/A	84
	CoNi–C/rGO-450	Annealing	1 M KOH	364	101.6	203
TMO	Ni0.9Fe0.1Ox/ITO	Solution casting + annealing	1 M KOH	336	30	103
	NiFe-LDH nanosheets	Hydrothermal + anion exchange	1 M KOH	300	40	204
	Co3O4/Ni foam	Hydrothermal + plasma	1 M KOH	339	108	205
	G-FeCoW/Au foam	Sol–gel	1 M KOH	191	37	95
	CoNi(OH)x/Cu foil	Electrodeposition + annealing + etching	1 M KOH	280	77	206
	NiCoFe LDH NA/carbon cloth	Electrodeposition	1 M KOH	239	32	207
	Fe^2+–NiFe LDH NA/carbon fiber paper	Co-precipitation	1 M KOH	195	40.3	208
	NiFeV LDH NA/Ni foam	Hydrothermal	1 M KOH	192	42	209
Pv/Py	Ba0.5Sr0.5Co0.8Fe0.2O3−δ	Co-precipitation	0.1 M KOH	∼360 (@ 10 mA cmoxide^−2)	∼60	94
	(Pr0.5Ba0.5)CoO3	Solid-state	0.1 M KOH		∼60	120
	IrOx/SrIrO3	Pulsed laser deposition + electrochemical leaching	0.5 M H2SO4	270 (@ 10 mA cmoxide^−2)	N/A	122
	6H-SrIrO3	Wet-chemistry	0.5 M H2SO4	248	N/A	123
	SrCo0.9Ir0.1O3−δ	Solid-state method	0.1 M KOH	∼320 (@ 10 mA cmoxide^−2)	∼50	124
	Sr1−xNaxRuO3	Wet-chemistry	0.1 M KOH	170 (@ 10 mA cmoxide^−2)	N/A	125
	Bi2Ir2O7	Hydrothermal	1 M H2SO4	∼350	45	126
	(Na0.33Ce0.67)2(Ir1−xRux)2O7	Hydrothermal	0.5 M H2SO4	N/A	48.6–85.6	127
	(A, A′)2Ir2O6.5+x, (A, A′ = Bi, Pb, Y)	Adams fusion method	0.1 M HClO4	N/A	45–46	128
	Y2Ru2O7−δ	Sol–gel	0.1 M HClO4	270 (@ 1 mA cmoxide^−2)	46–55	129
	Y1.8M0.2Ru2O7−δ (M = Fe, Co, Ni, Cu, Y)	Sol–gel	1 N H2SO4	325–395 (@ 1 mA cmoxide^−2)	52–63	130
TMS	Ni(Fe)S2@Ni(Fe)OOH	Hydrothermal	1 M KOH	230	42.6	173
	Co1−xNixS2/N-doped rGO aerogel	Hydrothermal	1 M KOH	330	47	210
	FeNiS2 nanosheets/rGO	Solvothermal + pyrolysis under NH3	1 M KOH	200	40	211
	N–(Ni,Fe)3S2 nanosheets/Ni–Fe alloy foam	Pyrolysis with sulfur	1 M KOH	167	33	212
	Hollow nanostructured CoxNi1−xS2/rGO	Solvothermal	1 M KOH	290	46	213
	Ni3S2	Solvothermal	1 M KOH	330	52	214
	FeSx/CoSx heterophase	Solvothermal	1 M KOH	304	48.7	215
	Fe–CoMoS	Hydrothermal + pyrolysis	1 M KOH	282	58	216
	(CrMnFeCoNi)Sx high-entropy metal sulfide nanoparticles	Pulse thermal decomposition	1 M KOH	295 (@ 100 mA cm^−2)	66	174

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Table 2 Summary of the state-of-the-art non-PGM-based catalysts and their HER performances^a

Class	Catalyst	Method	Electrolytes	η @ 10 mA cm^−2 (mV)	Tafel slope (mV dec^−1)	Ref.
a N/A: not available.
Carbon	C3N4@NG	Polymerization + annealing	0.5 M H2SO4	240	51.5	47
	CoNx/C	Pyrolysis + etching	1 M KOH	170	75	54
			1 M PBS	247	N/A
			0.5 M H2SO4	130	57
	N,S-G	Templating + etching	0.5 M H2SO4	310	120	48
	SHG	Hybridization + pyrolysis + etching	0.1 M KOH	310	112	37
	Hierarchical NS500	CVD + etching	0.5 M H2SO4	230	72	49
	A-Ni@DG	Annealing + etching	0.5 M H2SO4	70	31	41
	CNT/Co-PcC-1	Pyrolysis + etching	1 M KOH	219	78	55
			0.5 M H2SO4	202	82
	Co1/PCN	Polymerization + annealing	1 M KOH	89	52	217
TMBs	FeB2	Chemical reduction	1 M KOH	61	87.5	65
	Co-Ni–B@NF	Electroless plating	1 M KOH	205	N/A	218
	Mo3B film	CVD	0.5 M H2SO4	249 (@ 20 mA cm^−2)	52	219
	Etched MoAlB	Etching of MXenes	0.5 M H2SO4	301	68	220
	Co-B/Ni electrode	Electroless plating	1 M KOH	70	68	221
	MoB/g-C3N4	Physical grinding	0.5 M H2SO4	133	46	61
	RuB2	Quasi solid-state metathesis	1 M KOH	28	28.7	56
			0.5 M H2SO4	18	38.9
	Ni doped WB	Molten salt-assisted reaction	0.5 M H2SO4	144	63	222
	Pd2B	Hydrothermal	0.5 M H2SO4	15.3	22.5	58
	RuB	Annealing	0.5 M H2SO4	22	30.7	57
	Ni3B/MoB	Annealing	0.5 M H2SO4	75	61	62
TMC	Np-η-MoC NSs	Hydrothermal	0.5 M H2SO4	112	53	14
			1 M KOH	119	39
	Np-α-MoC1−x NSs	Hydrothermal	0.5 M H2SO4	158	54
			1 M KOH	147	43.6
	Ni-Mo2C@C	Annealing	1 M KOH	72	65.6
	Co-Mo2C@C	Annealing	1 M KOH	122	80.9	74
	Fe-Mo2C@C	Annealing	1 M KOH	129	102.4
	Cr-Mo2C@C	Annealing	1 M KOH	147	114.2	74
	WC/W2C@C NWs	CVD	0.5 M H2SO4	69	52	82
			1 M KOH	56	59
	Ni/WC@NC	Hydrothermal + annealing	0.5 M H2SO4	53	43.5	223
	Cu@WC	Wet chemical oxidation + in situ electroreduction	0.5 M H2SO4	92	50.5	224
			1 M KOH	119	88.7
			1 M PBS	173	118.3
	W-W2C/CNT	Say-drying process + carbonization	0.5 M H2SO4	155	56	90
			1 M KOH	147	51
	CoW/CN	Annealing	1 M KOH	98	125	91
	ES-WC/W2C	CVD	1 M KOH	75	59	81
	Ni/VC	CVD + hydrothermal	0.5 M H2SO4	111	86	75
			1 M KOH	239	80
	Ni/Fe3C	CVD + hydrothermal	0.5 M H2SO4	112	N/A
			1 M KOH	93	N/A
	Ni–Ni3C/CC	CVD + hydrothermal	1 M KOH	98	88.5	76
	Co2C	Bromide-induced wet-chemistry	0.1 M KOH	181	89	80
	N, B co-doped Co3C	Ball-milling	1 M KOH	154	56	77
	Fe3C-GNRs	Hot filament chemical vapor deposition	0.5 M H2SO4	49	46
	Co3C-GNRs		0.5 M H2SO4	91	57	78
	Ni3C-GNRs		0.5 M H2SO4	48	54
TMO	WO2.9	Annealing	0.5 M H2SO4	70	50	97
	Porous MoO2 NSs/Ni foam	Hydrothermal + annealing	1 M KOH	25	N/A	225
	3D urchin-like Mo–W18O49	Hydrothermal	0.5 M H2SO4	45	54	226
	S-CoO NRs/carbon fiber paper	Hydrothermal + cation exchange	1 M KOH	73	82	98
	Porous WO2 HNs/Ni foam	Hydrothermal + annealing	1 M KOH	48	43	227
	Ni, Zn-dual doped CoO NRs/carbon fiber paper	Hydrothermal + cation exchange	1 M KOH	53	47	100
	Ni0.35Mo0.65O2/carbon paper	Hydrothermal + annealing	0.5 M H2SO4	43	37	228
	F-CoO NWs/carbon cloth	Hydrothermal + annealing	1 M KOH	53	65	229
Pv/Py	Pr0.5(Ba0.5Sr0.5)0.5Co0.8Fe0.2O3−δ	Sol–gel	1 M KOH	237	45	131
	La0.5Ba0.25Sr0.25CoO2.9−δF0.1	Solid-state reaction	1 M KOH	256 (@ 100 mA cm^−2)	44	12
	(Gd0.5La0.5)BaCo2O5.75	Sol–gel	1 M KOH	185	27.6	133
	Ni NP/La0.4Sr0.4Ti0.9O3−δ	Sol–gel + exsolution	0.1 M KOH	∼430	97	135
	NiRu NP/Pb2Ru2−xNixO6.5	Sol–gel + exsolution	0.1 M KOH	35	30	137
	Ti-doped RuO2/Ru-doped K0.469La0.531TiO3	Hydrothermal	1 M KOH	20	30	138
TMP	3D urchin-like CoP NCs	Hydrothermal + phosphidation	0.5 M H2SO4	180	46	230
	NiCoPx holey nanosheet/Ni foam	Calcination + plasma CVD	1 M KOH	58	57	231
	Ni-doped FeP/carbon hollow nanorods	Hydrothermal + etching + annealing	1 M KOH	95	72	180
			1 M PBS	117	70
			0.5 M H2SO4	72	54
	Ni2P/Ni foam	Hydrothermal	1 M KOH	37	76	232
	Co0.6Fe0.4P nanoframe	Template growth + etching + phosphidation	1 M KOH	133	61	233
	N-CoP2 NWs/carbon cloth	Hydrothermal + phosphidation + N doping	0.5 M H2SO4	38	46	147
	Pv-modified Ni12P5/Ni foam	Annealing	1 M KOH	27.7	30.88	149
	CoPx@carbon nanosheets	Carbonization + phosphidation	1 M KOH	91	129	234
			0.5 M H2SO4	98	N/A
	Fe0.5Ni1.5P	Colloidal synthesis	0.5 M H2SO4	163 (@ 50 mA cm^−2)	65	22
	CoP–InNC@CNT	Carbonization + phosphidation	1 M KOH	159	56	235
			0.5 M H2SO4	153	62
	Ni12P5–Ni4Nb5P4/PCC	Plasma + hydrothermal + phosphidation	1 M KOH	81	76.4	148
	P-NM-CF HNRs/Ni foam	Hydrothermal + phosphidation	1 M KOH	100	54.3	236
			0.5 M H2SO4	130	51.3
	2D Ni–CoP ultrathin nanosheets	Phosphidation	1 M KOH	88	41	237
	2D porous MoP/Mo2N heterojunction/Ni foam	Polyethylene glycol-mediated assembly	1 M KOH	89	78	238
TMS	Fe@FeOxSy/Fe foam	Solvothermal	1 M KOH	300	77	239
	a-CoSx/FTO	Electrochemical activation	1 M PBS	168	76	240
	NiWS/carbon fiber	Electrodeposition + hydrothermal	1 M KOH	38	98	241
			0.2 M PBS	120	244
			0.5 M H2SO4	56	128
	Co-NiS2/Ni foam	Electrodeposition + annealing	1 M KOH	80	43	172
	F-Ni3S2/Ni foam	Solvothermal + pyrolysis	1 M KOH	38	78	242
	Ag2S/MoS2/rGO	Hydrothermal	0.5 M H2SO4	190	56	164
	N-NiS/MoS2/Ni foam	Hydrothermal + pyrolysis with thiourea	1 M KOH	71	79	166
	V-MoS2	Solvothermal	1 M KOH	206	89	243
			0.5 M H2SO4	194	59
	Fe-CoMoS	Hydrothermal + pyrolysis	1 M KOH	137	98	216
	Vs-Ni3S2/Ni foam	Hydrothermal + plasma	1 M KOH	88	87	168

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Although a large number of electrocatalysts have been developed with promising performances for water electrolysis, their catalytic mechanism is not yet fully understood. In general, ex situ experiments for catalysts in pre- and post-reaction states have been commonly carried out to explain the behaviors of active species. However, these results do not represent the real-time action of catalysts due to the short-lived intermediates during the reaction. Motivated by these challenges, in situ characterization techniques, such as XAS, Raman spectroscopy, FT-IR spectroscopy, electron microscopy, ambient pressure XPS, ICP-MS, and synchrotron radiation Fourier transform infrared spectroscopy, have been extensively developed to monitor the structural evolution of catalysts, the changes of electronic configurations, and key intermediates in real-time during HER/OER processes.¹⁸⁹ For examples, Wei et al. unambiguously probed the catalytically active sites and the stability of Co SACs during the HER by in situ XAS.¹⁹⁰ Using an in situ TEM, Yu et al. captured the morphology evolution of an amorphous CoS_x catalyst during the OER process.¹⁹¹ The results from these in situ characterization will provide in-depth understandings of the catalytic mechanism and help for designing next-generation catalysts. However, there are still limitations to the development, standardization, and wide application of these techniques. In addition, current reports focused on single in situ characterization. Therefore, combining multiple in situ experiments is highly desirable to synergistically provide comprehensive insights into electrocatalysts.

The next step should be the practical application of the prepared non-PGM-based electrocatalysts and SACs for water electrolysis technology, providing potential directions. To date, limited work has been carried out on electrolysers including PEMWEs and AEMWEs. Especially, issues such as electrode stability, power efficiency, membrane handling, and gas treatment have to be solved. In addition, the use of chemically stable and cheap PEMs/AEMs with PGM-free catalysts will be suitable for reducing the overall cost of H₂ production and fuel cell operation. Finally, solving the durability issue of PEMWEs and the efficiency issue of AEMWEs will accelerate the widespread application of water electrolysis technologies.

Author contributions

H. J. K., H. Y. K., J. J., J. Y. K., K. L., and S.-I. C. conceived and designed the review article. H. J. K. and S.-I. C. contributed to chapters 1, 2, 8, and 11. H. Y. K., B. Y. M., and J. Y. K. contributed to chapters 1, 6, and 11. J. J., S. C., Y. P., and K. L. contributed to chapters 1, 9, and 11. J. S. L. and S. H. J. contributed to chapters 3 and 11. H. H. and J. L. contributed to chapters 4 and 11. J. H. and M. S. contributed to chapters 5 and 11. M. K. and S. W. L. contributed to chapters 7 and 11. Y. W., J. L., Z. Z., and J. M. contributed to chapters 10 and 11. All authors contributed to the finalization of the text and figures of the manuscript and commented on the final version.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors at Kyungpook National University acknowledge support from the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT (NRF-2021R1A2C4001411 and 2020R1A4A1018393). S. H. J. and J. S. L. were supported by the NRF of Korea (NRF-2021R1A2C200749511). M. Shao acknowledges the support from the Research Grant Council (16308420 and HKUST C6011-20G) of the Hong Kong Special Administrative Region, China. J. Hu acknowledges the support from the National Nature Science Foundation of China (No. 21862011). The authors at the Georgia Institute of Technology acknowledge support from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (No. 20188550000440). The authors at Tianjin University acknowledge support from the National Natural Science Foundation of China (22071172).

Notes and references

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Footnote

† These authors contributed equally to this work.

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