High-performing catalysts for energy-efficient commercial alkaline water electrolysis

Abstract

‘Green’ hydrogen produced from water electrolysis powered by renewable energy will play a critical role in the future global energy transition to ‘net zero’ carbon emissions. To this end, intensive efforts are needed to improve the energy efficiency with which green hydrogen can be made and thereby reduce its cost. A key required effort in this respect involves developing the most efficient and durable possible catalysts to facilitate the ‘hydrogen evolution reaction’ (HER) at the cathode and the ‘oxygen evolution reaction’ (OER) at the anode in alkaline water electrolysers. Most work in this regard has focused on improving the activity of catalysts at or around a standard current density of 10 mA cm⁻² for both the HER and OER. However, to be practically useful, electrocatalysts must operate efficiently at commercial current densities, which are typically much higher; for example, commercial alkaline water electrolysers routinely operate at current densities of 200–700 mA cm⁻². Reviews of such electrocatalysts and their suitability for commercial-scale water electrolysis are rarely reported. This work presents an overview of recent progress in this respect. The most energy efficient and durable electrocatalysts for the HER, the OER, and for overall water splitting, are identified, discussed, and prioritized with a view towards enhancing commercial alkaline water electrolysis. The major challenges involved in their preparation and operation, as well as potential avenues for further performance improvements as reliable, robust electrocatalysts for commercial alkaline electrolysis are also highlighted. The latest work on new technology development in alkaline water splitting is also presented. The high-performing catalysts that are most likely to accelerate the prospects of green hydrogen are listed in a comparative table. A discussion of future directions in this emerging field is also provided.

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1. Introduction

The recent COP26 meeting¹ has sparked much interest in the global community about the need to attain ‘net zero’ carbon emissions by 2050. ‘Green’ hydrogen, which is hydrogen produced from water using electrolysis powered by renewable electricity, is poised to play a key role in this energy transition as it offers the only viable means of decarbonizing a host of ‘hard-to-abate’ industries, including long-haul transport, shipping, aviation, and steel, amongst others.

The production of green hydrogen is currently an active field of research that has been explored in detail in the scientific literature. While numerous water electrolysers have been developed over the last 200 years, hydrogen produced by electrolysis still comprises only a tiny fraction (<5%) of the worldwide production of hydrogen due to its inefficient energy consumption and high cost.² Of the different methods of producing hydrogen by electrolysis, alkaline electrolysis is one of the most promising; it offers a mature technology that combines a capacity for high production volumes with the lowest-available production costs.²

Given the need to reduce the cost of producing green hydrogen, intensive efforts are being made to develop innovative processes that increase the efficiency of alkaline electrolysis. New advances include the development of membraneless alkaline electrolysis systems that do not require an inter-electrode separator.³ Electrolysis cells comprising two porous gas diffusion electrodes that use capillary effects to directly extract the generated gases through the electrodes without visible bubble formation have also been developed.^3–7 Most recently, an innovative ‘capillary-fed’ alkaline electrolysis cell, which offers the promise of a breakthrough energy efficiency, has been described.⁸ These efforts necessarily involve developing high-performing electrocatalysts for the oxygen-evolution reaction (OER) (at the anode in a water electrolysis cell) and the hydrogen-evolution reaction (HER) (at the cathode in a water electrolysis cell). Improving catalytic performance offers a viable means of making the entire process more energy efficient.

An efficient catalyst should provide high catalytic activity (i.e. large current density at low overpotential) and long-term durability in operation. Recently, platinum group metal (PGM) catalysts had the highest activity for HER and OER, respectively.⁹ However, their scarcity and high cost have hindered application in large-scale deployment.^10,11 Therefore, the exploration of cheap, robust, and earth-abundant electrocatalysts has received much attention. Various earth-abundant materials including transition-metal based compounds, oxides and non-oxide catalysts have been reported as promising electrocatalysts for energy-efficient alkaline water electrolysis.^12–14

From an industrial point of view, it is essential to develop high-performing, robust electrocatalysts that can sustain high current densities (for example, 200–700 mA cm⁻²) to end-of-life in water electrolyser systems without significant electrochemical degradation.⁹ Despite the efforts directed to electrocatalyst development, the strict requirement for durability in commercial applications (10 years or more) has largely been overlooked in previous studies.

This review is intended to inspire further scientific advancements in alkaline water electrolysis. The development of OER electrocatalysts, which has been a bottleneck in the water splitting process, is emphasised, with particular attention to those catalysts capable of operating steadily at high current densities. The development of architectures and designs of water electrolysis system as well as related economic aspects have been well reviewed¹⁵ in the past and are not discussed in detail in this review.

The work commences with an exposition of the fundamentals of electrochemical water electrolysis. It then reviews and concentrates on high-performing electrocatalysts that: (i) overcome the high energy barriers involved in the HER and OER, to thereby: (ii) improve the kinetics and long-term stability of the cathode and anode electrodes, as well as (iii) potentially providing high catalytic performance and efficiency suitable for commercial alkaline water electrolysis. The review further extends to an exploration of the production of ‘green hydrogen’, which will be a turning point in human history if the goal of net zero emission is achieved.

The work concludes with a ranked tabulation of what seem, to the authors, to be the most promising and high performing HER, OER, and overall water-splitting systems available today for alkaline electrolysis. We recommend that researchers seeking to accelerate the prospects of green hydrogen, focus on further studies of these electrocatalysts.

2. Mechanism of the electrochemical hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline media

Water electrolysis is the process whereby water is decomposed into hydrogen (H₂) and oxygen (O₂) by applying an electrical current within an electrochemical cell, known as an electrolysis cell or an electrolyser cell. Typically, an electrolysis cell consists of an anode, a cathode, a liquid electrolyte and an ion-permeable, gas-impermeable “separator” between the electrodes (Fig. 1). When a direct current is passed through the cell, redox reactions occur at the electrodes. Hydrogen is produced at the cathode and oxygen is produced at the anode, usually in the form of gas bubbles within the liquid electrolyte. The electrolyte provides a pathway for ion movement between the two electrodes; the ions pass through the separator. Beyond ion conduction, the main task of the separator is to keep the gas bubbles produced at each electrode from mixing with each other since a hydrogen stream containing >4.6% oxygen, or an oxygen stream containing >3.8% hydrogen, comprises an explosive mixture (at 80 °C, which is a common operating temperature for an electrolyser).

Fig. 1 Schematic diagram of an electrolysis cell, or an electrolyser cell.

The chemical reactions that occur at the cathode and anode in alkaline condition are:

Anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻E⁰ = 0.40 V (1)

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

Overall: 2H₂O → 2H₂ + O₂E⁰ = 1.23 V (3)

where E⁰ is the standard potential vs. the standard hydrogen electrode (SHE) under standard condition (298 K, 1 atm = 1.013 × 10⁵ Pa).

In order to split water into hydrogen and oxygen, the voltage applied to the cell must generally be significantly higher than the standard equilibrium potential for water electrolysis (E⁰ 1.23 V). The additional energy input is known as the overpotential.

The OER that occurs at the anode involves a 4-electron transfer to produce a molecule of O₂, as shown in eqn (1). The HER at the cathode involves a 2-electron transfer to form a molecule of H₂ as depicted in eqn (2). A much higher overpotential is required for the OER than for the HER because it has a higher kinetic barrier resulting in a higher energy consumption. The anodic reaction therefore poses the main challenge to improving the energy efficiency of electrolysers.¹⁶ The development of high performing electrocatalyst for the OER is therefore more critical than for the HER. Understanding of the electrochemical reaction process is, however, crucial for designing efficient catalysts, whether they facilitate the OER or the HER. Several recent works have examined and reviewed the mechanisms of the HER and OER process.^17–21

2.1 Mechanism of the hydrogen evolution reaction

The HER at the cathode is one of the most studied processes in electrochemistry. The HER is described by the Volmer–Heyrovsky–Tafel reaction mechanism.¹⁰ In alkaline solution, hydrogen is generated by the electrochemical reduction of water, in the half-reaction in eqn (2).

The reaction pathway in alkaline solution is: (M = active surface site)

M + H₂O + e⁻ → MH_ads + OH⁻ Volmer step (4)

MH_ads + H₂O + e⁻ → M + OH⁻ + H₂ Heyrovsky step (5)

2MH_ads → 2M + H₂ Tafel step (6)

The symbol MH_ads denotes hydrogen atoms adsorbed on the electrode surface. The first step involves the adsorption of hydrogen atoms on the electrode surface (Volmer step) through the reduction of water in alkaline solution (eqn (4)). H₂ is then formed through desorption by either the Heyrovsky step or the Tafel step. The Heyrovsky reaction involves the direct bonding of a hydrogen atom with a second water molecule in alkaline media (eqn (5)). The Tafel reaction involves, effectively, the recombination of two hydrogen atoms adsorbed on the surface (eqn (6)).²²

The steps of adsorption (Volmer step) or desorption (Heyrovsky step or Tafel step) can be rate determining depending on the electrode surface. If the bond between the hydrogen atom and the catalytic surface is too strong, the second step is rate limiting. If that bond is too weak, the first step is rate limiting.^23,24

2.2 Mechanism of the oxygen evolution reaction

The OER is the most important process in water electrolysis. Its mechanism, which involves a 4-electron process, is more complex than the HER, which is a 2-electron process, and includes many different intermediate states in the reaction. Several kinds of activation steps may determine the rate of the OER. Therefore, the mechanism may not always be fully understood and can depend strongly on the catalyst used.²⁵ Generally, the OER in alkaline conditions can be described in the following steps as shown below.

M + OH⁻ → MOH_ads + e⁻ (7)

MOH_ads + OH⁻ → MO_ads + H₂O + e⁻ (8)

2MO_ads → 2M + O₂ (9)

MO_ads + OH⁻ → MOOH_ads + e⁻ (10)

MOOH_ads + OH⁻ → M + O₂ + H₂O + e⁻ (11)

The M-bound OH⁻, O²⁻, and OOH⁻ in the above steps are the reaction intermediates adsorbed on the active site of catalyst surface (M = active surface site).⁹ A number of reaction pathways have been proposed in the literature with the most widely accepted involving the same intermediates of M–OH and M–O.^26,27 At the first step, an OH⁻ species is adsorbed at the active surface site (eqn (7)), whereafter it is transformed to a MO_ads species by oxidation of the metal site (eqn (8)). The major differences in mechanism likely revolve around the mechanism to produce oxygen. Two different approaches have been considered for the formation of oxygen from a MO_ads intermediate.¹⁶ One approach, described early on by Bockris, involves an electrochemical oxide path (eqn (7)–(9)) as oxygen is formed directly by combination of two MO_ads species.²⁸ An alternative mechanism has been reported by Yeager and colleagues,^29,30 wherein the MO_ads species reacts with another OH⁻ ion (eqn (10)) resulting in an oxy-hydroxide MOOH_ads intermediate that is then oxidized to release O₂ (eqn (11)) in final step.³¹

3. General measures for evaluating catalytic performance

3.1 Overpotential

The cell overpotential is defined as the difference between the experimentally-required cell potential to drive an electrochemical reaction at a certain rate and its thermodynamically-expected cell potential at that rate.

Overpotential is one of the most important quantities to consider when evaluating catalytic performance in water electrolysis.³² It is directly related to the energy efficiency of the water electrolysis cell since a small overpotential results in a higher electrochemical conversion efficiency. It is experimentally determined by measuring the potential at which a particular current density is produced. This is usually done using linear sweep voltammetry (LSV), which is typically performed under steady-state conditions and at low scan rates (e.g., 2 mV s⁻¹, 5 mV s⁻¹ and 10 mV s⁻¹) to reduce capacitive current contributions.³³ The overpotential required to produce each current density can be considered to ‘activate’ the reaction, with the activation step then comprising the sum of the mass transfer and the charge transfer resistance. In order to precisely compare catalytic activities, the activation overpotential should be corrected by iR-compensation (eqn (12)), where i is the current flow in the circuit and R_s is the sum of the series resistances: namely, mass transfer, charger transfer resistance, etc.

E_corrected = E_measured − iR_s (12)

Generally, the overpotential required to achieve a current density of 10 mA cm⁻² corresponds to 10% efficiency in the solar-conversion within a photoelectrochemical water splitting device. For this reason, 10 mA cm⁻² has been widely used as a benchmark for evaluating catalyst performance.^34–36 However, to meet industrial standards for practical water splitting, electrocatalysts must afford commercial current densities, which typically operate at much higher currents; for example, commercial alkaline water electrolysers routinely operate at current densities of 200–700 mA cm⁻² which may require cell potentials of 1.8–2.4 V.⁹ Such electrocatalysts must also demonstrate high durability, with lifetimes of 90 000 hours, or around 10 years, typically required.

It is a significant challenge to develop high performing water electrolysis electrocatalysts that exhibit the necessary long-term stability at high current densities (200–700 mA cm⁻²). For this review, catalyst performances has been judged by comparisons of the overpotential required to produce a benchmark current density of 10 mA cm⁻² and a commercial operating current density in the range 200–700 mA cm⁻².

3.2 Electrode

There have been extensive efforts to develop catalysts that are highly active, cost-effective, and that display stable long-term performance under harsh working conditions. High performing catalysts are almost always synthesized without supporting structures, such as in the form of nanoparticle, nanosheet, core–shell nanostructure, and other ingenious nanostructures.^37–42 However, in industrial practice, high performing catalyst development should be integrated with electrode design to exhibit long-term stability.

The design of electrodes/supports play a critical role in increasing the efficiency of both the HER and OER in alkaline water electrolysis. Based on the development of electrode structure, electrode design can be divided into two categories: flat surface electrodes (2D) and 3D electrodes. Both electrode designs have their own advantages and limitations.

Flat surface electrodes, such as Cu/Ti foil, glassy carbon (GC) and 2D ultrathin sheets provide good mechanical stability but they limit the catalyst loading on the substrate, inhibit mass and charge transport, and thereby diminish the overal catalytic activity.^43,44 As a result, these electrodes generally produce low current densities and require high overpotentials for both HER and OER.

To address these challenges, 3D electrodes supported by metal foams (e.g. Ni, Fe, and Cu), stainless steel meshes/mats, carbon cloth, carbon paper, carbon fibers and carbon nanotubes have been widely employed to improve the catalytically active surface area and mass transport properties, as well as enhance gas diffusivity. Nickel foam (NF) has been commonly used as an ideal support for HER/OER catalyst due to its high electrical conductivity. It is true that the porous structure of NF can facilitate the electrolyte transfer and promote generated gas bubbles release during water electrolysis.^35,45 However, an inappropriate fabrication method may affect the interfacial adhesion between the catalyst and substrate, resulting in poor stability. Hydrothermal, solvothermal, and electrodeposition approaches have been widely used to synthesize nanostructure electrodes grown on NF substrate.^46–48

3.3 Electrolyte

The electrolyte provides the ionic conduction pathway between the electrodes. Different electrolytes influence the performance of the electrodes and the electrolysis reaction in general. Liquid alkaline solution of sodium and potassium hydroxide with a typical concentration of 20–40% have been widely used as the electrolyte in alkaline electrolysis due to its high conductivity.⁴⁹ However, aqueous solution of KOH are preferred over NaOH because of their higher specific conductivity.⁵⁰ The maximum conductivity of KOH aqueous solutions is obtained at 30 wt%. This concentration is commonly used in modern alkaline electrolysers.

3.4 Tafel slope and exchange current density

A Tafel plot is produced by graphing the overpotential as a function of the logarithm of the current density (from a corresponding LSV curve). The slope of the linear region of a Tafel plot is known as the Tafel slope. The Tafel slope is normally determined at low overpotentials using eqn (13):

η = b log j + a (Tafel equation) (13)

where η is the overpotential; j is the current density and b is the Tafel slope.

A Tafel plot depicts the additional potential required to increase the catalytic current by an order of magnitude, with typical units of mV per decade. A small Tafel slope corresponds to a large change in the electrocatalytic current density. Therefore, low Tafel slopes are desirable when developing catalysts with high performance in water electrolysers.

The exchange current density is defined as the equilibrium potential at which the anodic and cathodic currents are equal. It is another key parameter with which to evaluate catalytic efficiency. The exchange current density reflects the intrinsic activity of charge transfer between the electrode and electrolyte. It can be determined by extrapolating the linear part of a Tafel plot to the x-axis. The point of intersection is the exchange current density.

It should be noted that the exchange current density varies significantly with the catalyst used and the experimental temperature. The exchange current density is also considered to be proportional to the (electro)catalytically active surface area (ECSA). The larger the exchange current density, the larger the surface area and the faster the rate of electron transfer.

Given that a high performing catalyst will ideally exhibit a small overpotential, a low Tafel slope and a large exchange current density, this combination of metrics may appear to offer an ideal means of appraising the electrocatalytic activity of catalysts.

However, these parameters are not independent of each other and this may complicate the analysis. For instance, two HER electrocatalysts with different Tafel slopes could require the same overpotential to produce a current density of 10 mA cm⁻². In such a case, the Tafel slope does not identify the best catalyst, at least at 10 mA cm⁻².

The Tafel slope may, under certain conditions, provide some valuable insights into the possible reaction mechanism.³⁶ However, it is difficult to elucidate the operating mechanism for multi-step electron transfer processes including many possible intermediates that may change with applied potential, such as OER.^32,51 In most cases, additional experiments are required in combination with Tafel slope to identify the catalytic mechanism and materials.^52,53 Nevertheless, a better electrocatalyst will always shows a smaller overpotential at the current density of interest.

3.5 Turnover frequency

The overpotential required to produce a current density of interest is normally used for activity comparisons of catalysts. However, different catalyst mass loadings may lead to changes in active site density and active surface area. It is generally challenging to evaluate the intrinsic activity of different catalysts when different mass loadings have been used. To normalise for such variations, the ‘turnover frequency’ (TOF) is used as a measure of the intrinsic site activity of a catalyst.^54,55 TOF is defined as the number of product molecules generated per active site per unit time (eqn (14)):^56–58where j is the current density at a certain overpotential, A is the electrode area, F is Faraday's constant (a value of 96 485 C mol⁻¹) indicating the number of moles of electron consumed to produce one mole of product, and m is the number of moles of active sites.

The main drawback of the TOF metric is finding a precise way to calculate the total number of active sites accurately because this may not be proportional to the mass loading due to morphology variations in the catalyst. To estimate the number of active sites, the total surface area or electrochemically active surface area (ECSA) may also be employed.²³

3.6 Durability

The long-term operational stability, or durability, of a catalyst is one of the most important properties from the point of view of high performance. A catalyst cannot be considered high performing if it only has a short operational lifetime.

The durability of a catalyst is best evaluated by extended testing under an applied overpotential (chronoamperometry test) or at a given current density (chronopotentiometry test), and then measuring the change in overpotential or current density over time.

Indications of likely durability can also be assessed by considering the change in repeated LSV curves at current densities of interest. Such tests may be carried out before and after the longer-term stability tests noted above. If a catalyst loses its activity quickly, an increase in overpotential or decrease in current density will become apparent.

Generally, a fixed current density of 10 mA cm⁻² has been employed by researchers to evaluate catalyst stability. However, this is practically inadequate, especially given the fact that many catalysts degrade slowly over time due to electrochemical corrosion.^59–63 For industrial applications, much higher current densities, such as 200 mA cm⁻², 500 mA cm⁻², or even 1000 mA cm⁻² have recently been used to test catalyst durability. Long term testing duration of 10–2000 h have been considered as benchmarks for comparison.^64–67

4. Overview of hydrogen-evolving catalysts in alkaline media

4.1 Noble metal catalysts

Several studies over the years have focused on developing efficient alkaline catalysts for the HER. Pt and Pt-group metals (i.e., noble metals) have been most widely studied; these have generally demonstrated very low overpotentials for the HER process. However, the expensive nature and the scarcity of Pt group metals (PGMs) have hindered their applications in large-scale industrial applications.^10,11 Additionally, noble-metal catalysts also suffer from scarcity (high cost) and may display poor stability in alkaline electrolysis systems, especially if they become contaminated with organic materials that bind irreversibly.⁶⁸

Many studies have demonstrated that Pt exhibits lower HER activity in alkaline than in acid media. It also displays more rapid decreases in activity over long term measurements.^51,69–71 Such losses in activity are largely due to surface contamination from trace organic materials in the cells used in the measurements.^72,73

As the most popular commercial HER catalyst, Pt typically displays an excellent HER activity with a low Tafel slope (30 mV dec⁻¹) in alkaline electrolytes.^74,75 A current density of 10 mA cm⁻² can be achieved at a low overpotential of 36 mV in 1 M NaOH at room temperature. Higher current densities of 50 mA cm⁻² and 100 mA cm⁻² may be achieved at 81 mV and 92 mV, respectively.⁷⁶ In order to increase performance, higher loadings of Pt are required. In the case of Pt nanoparticles, agglomeration may then occur, decreasing the active surface area and impeding the catalytic performance.^77,78

This may be counteracted by using carbon support materials, such as carbon black, carbon nanotubes, carbon nanofibers or graphene, which not only decrease the amount of Pt needed, but increase its electrocatalytic activity.⁷⁹ For example, Chang et al.⁸⁰ reported that a commercial Pt/C, combined with 5% Nafion solution in a mixture of water and ethanol (% v : % v = 1 : 1) loaded on carbon fiber paper, delivered 20 mA cm⁻² at an overpotential of 45 mV in 1 M KOH electrolyte. Wang et al.⁸¹ noted that the activity of Pt/C strongly depends on the OH⁻ concentration of alkaline electrolyte, such that the Tafel slope declines from 180 mV dec⁻¹ in 0.01 M KOH electrolyte to 94 mV dec⁻¹ in 0.1 M KOH and only around 30 mV dec⁻¹ in 1 M KOH.

Combining Pt with other metals may also be used to realize highly active and low-cost catalysts. For instance, the Pt₃Ni alloy introduced by Chen et al.⁸² showed impressive HER activity that was 22-times higher than that of commercially available Pt/C.

Efforts have also focused on developing new catalysts for alkaline media that exhibit improved energy efficiency and narrow the gap in overpotential for Pt in alkaline vs. acidic electrolytes. A plausible approach has been to combine Pt with transition metal hydroxides that enhance the HER activity.

Subbaraman et al.⁷⁴ successfully synthesized ultrathin Ni(OH)₂ nanoclusters (height 0.7 nm, width: 8 to 10 nm) on modified, pristine Pt(111) single crystal surfaces by electrochemical deposition and studied the effect of Ni(OH)₂ on HER activity. Scanning tunneling microscopy (STM) (Fig. 2(a–d)) shows how the morphology changes from atomic structures of Pt(111) to Ni(OH)₂ nanocluster-modified Pt(111)/Pt-islands. The presence of Pt islands and Ni(OH)₂ nanoclusters enhanced the actives sites thereby accelerating HER catalysis (Fig. 2(e)). The authors expressed the opinion that Ni(OH)₂ clusters played a promoting role in water adsorption by accelerating interaction of O atoms with Ni(OH)₂ and H atoms with Pt at the boundary between Ni(OH)₂ and Pt domains. They then further assisted water dissociation when two H_ads atoms, adsorbed on the Pt surface, recombined into H₂ molecules during the H₂ desorption stage. In addition, the presence of Li⁺ cation may have further increased the interaction between hydrated metal cations and adsorbed OH_ads species in the alkaline environments, resulting in an enhancement in the concentration of OH_ad clusters at the interface.⁸³ The resulting hybrid material displayed a performance that was comparable to the activity of Pt in acidic solution.

Fig. 2 (a–d) STM images (60 nm by 60 nm) and CV traces for (a) Pt(111), (b) Pt(111) with 2D Pt islands, (c) Pt(111) modified with 3D Ni(OH)₂ clusters, (d) Ni(OH)₂/Pt-islands/Pt(111) surface. Clusters of Ni(OH)₂ in the STM images appear ellipsoidal with particle sizes between 4 and 12 nm. (e) Comparison of HER activities with Pt(111) as the substrate, with incremental HER activity in sequence: the bare Pt(111) (black line), Pt(111)/Ni(OH)₂ (blue line), Pt(111)/Ni(OH)₂/Pt-islands (green line), Pt(111)/Ni(OH)₂/Pt-islands/10⁻³ M Li⁺ (red line) and Pt(111) in 0.1 M HClO₄ (dashed line), electrolyte 0.1 M KOH, sweep rate of 50 mV s⁻¹. (f) Schematic representation of the HER process on β-Ni(OH)₂/Pt electrode for Volmer and Heyrovsky steps. Images (a–e) reproduced from ref. 74 with permission from the American Association for the Advancement of Science, Copyright ©2011. Reproduced from ref. 84 with permission from The American Chemical Society, Copyright© 2017.

In separate work, Yu et al.⁸⁴ also proved that the HER activity of pristine Pt in alkaline solution was higher when α-Ni(OH)₂ or β-Ni(OH)₂ nanostructures were incorporated. A lower overpotential and smaller Tafel slope was observed with a β-Ni(OH)₂/Pt electrode due to the stronger interactions between the β-Ni(OH)₂ clusters and the Pt substrate with greater water dissociation ability. The proposed HER mechanism of the β-Ni(OH)₂/Pt electrode in alkaline media is shown in Fig. 2(f). It involved a Volmer step in which water molecules dissociated into H_ads species on the Ni(OH)₂ lattice, followed by a Heyrovsky step in which H_ads species combined with H atoms to form H₂ molecules on the Pt surface.

Other research⁸⁵ reported on the enhancement of catalytic stability by growing ultrathin Pt nanowires on 2-D Ni(OH)₂ nanosheets. Although this method contributed to an increase in the activity of the Pt group catalysts in alkaline media, it still had a limitation in practical applications due to its complex synthesis method and poor stability in long-term operation.

Ruthenium (Ru) is a potential alternative to Pt as the strength of the Ru–H bond is similar to that of Pt–H (∼65 kcal mol⁻¹). This may enable Ru-based catalysts to exhibit comparable HER activities to Pt.^86–88

Baek et al.⁸⁸ prepared Ru@C₂N catalyst by dispersing Ru nanoparticles within a nitrogenated, holey, 2-D carbon structure. The C₂N layer frameworks had nano sized holes, which provided large surface areas, anchoring sites, and conductive platforms for the Ru nanoparticles. The Ru nanoparticles nucleated and grew, producing an abundance of uniformly distributed coordination sites. In 1 M KOH solution, the as-prepared Ru@C₂N required an overpotential of only 17 mV to produce a current density of 10 mA cm⁻². To generate 20 mA cm⁻², an overpotential of 35.5 mV was required; this was lower than the overpotential of commercial Pt/C under the same conditions.^83,84 It also exhibited a higher TOF (0.76 H₂ s⁻¹ at 25 mV; 1.66 H₂ s⁻¹ at 50 mV) than commercial Pt/C (0.47 H₂ s⁻¹ at 25 mV; 0.95 H₂ s⁻¹ at 50 mV) and excellent stability in 1 M KOH solution with negligible degradation.

Noble metal-based catalysts may, potentially, not be ideal for large scale industrial use due to high material cost and the scarcity of their sources. Therefore, development of non-noble metal catalyst for the HER have seen increasing commercial and research activities.

4.2 Earth-abundant (non-noble) metal catalysts

4.2.1 Metal composites: transition metal-based catalysts. Transition metals have been widely used to create noble metal-free catalysts for the HER; these mainly include iron (Fe), nickel (Ni), cobalt (Co), copper (Cu), molybdenum (Mo) and tungsten (W). Ni and Fe are the most abundant and lowest cost metals in this group. Thus, HER catalysts based on Ni and Fe offer a lower cost of hydrogen production. Ni atoms have excellent properties for enhancing water dissociation in the Volmer step;⁸⁹ for this reason and because of its higher stability compared to Fe or Co in alkaline media, Ni is considered the most promising non-noble metal catalyst. However, Ni alone is only weakly catalytic as a cathode. Therefore, Ni alloys or its composites have been investigated in the past decades as an alternative, to improve catalytic activity.^90,91 Ni–Mo binary or ternary species/alloys have been studied by many research groups.

Raj et al.^92–94 investigated the HER activity of several Ni-based binary composites such as Ni–Mo, Ni–Zn, Ni–Co, Ni–W, Ni–Cr and Ni–Fe, coated on a steel substrate by electrodeposition. They found that Ni–Mo was the best electrocatalyst with long-term stability and resistance to corrosion.⁹¹ They also showed that an optimized Ni-ternary composite of Ni–Mo–Fe, coated on a mild steel substrate, exhibited a steady current density of 300 mA cm⁻² at an overpotential of 187 mV at 80 °C for over 1500 h of electrolysis. This is 300 mV lower in the cathodic overpotential when compared with commercial mild steel plate under the same operating condition.

From an industrial point of view, catalyst that can be easily prepared by coating onto a conductive substrate may be preferred. For example, McKone et al.⁹⁵ developed a method to coat Ni–Mo nano powders onto various substrates with different loadings. Firstly, powders were prepared by slow decomposition of mixed Ni and Mo salts into mixed Ni–Mo oxides/hydroxides, followed by annealing at high temperature under an atmosphere of 5% H₂ and 95% N₂. The resulting powders were then suspended in common solvents and cast onto the substrate, followed by annealing at 400–450 °C under the above gas mixture, to improve the adhesion and conductivity of the catalytic layer.

In 2 M KOH solution, with a very low loading of 1.0 mg cm⁻², the Ni–Mo nano powder electrode required an overpotential of 70 mV to generate 20 mA cm⁻². With a higher loading of 13.4 mg cm⁻², 130 mA cm⁻² was produced at an overpotential of 100 mV. No degradation was observed at 20 mA cm⁻² after testing for 100 h in alkaline solution.

The optimum Mo loading in Ni–Mo alloys has also been examined in electrocatalytic HER. In their patent, Brown et al.⁹⁶ described a method to prepare Ni–Mo on a metal substrate. For 30 atom% of Mo in the Ni–Mo, by optimizing the Ni–Mo electrode loading to 20 mg cm⁻², a current density of 1000 mA cm⁻² could be achieved at an overpotential of only 80 mV at 70 °C in 30 wt% aqueous KOH solution.

In separate work, Xiao and colleagues found an optimal loading for a Ni–Mo electrode to be 40 mg cm⁻². The electrode generated a current density of 400 mA cm⁻² at 110 mV overpotential in 1 M KOH electrolyte at 40 °C.⁹⁷ However, the authors also noted that a thick electrode may contribute to voltage drops during stability tests.

Zhang et al.⁶⁶ reported a high performing MoNi₄ electrocatalyst supported by MoO₂ cuboids on nickel foam (MoNi₄/MoO₂@Ni). The catalyst was prepared by growing NiMoO₄ on nickel foam via a hydrothermal reaction, followed by a calcining process in a H₂/Ar (v/v, 5/95) atmosphere at 500 °C for 2 h (Fig. 3(a–g)). A substantially enhanced HER activity with a low Tafel slope of 30 mV dec⁻¹ in 1 M KOH electrolyte (Fig. 3(i)) was observed. In addition, MoNi₄/MoO₂@Ni delivered current densities of 10 mA cm⁻², 200 mA cm⁻², and 500 mA cm⁻² at overpotentials of 15 mV, 44 mV, and 65 mV, respectively (Fig. 3(h)). Fig. 3(j) and (k) display the energy barrier of the Volmer step (H₂O dissociation) and Tafel step (formation of adsorbed hydrogen) from DFT calculations. The free energy of MoNi₄ for the Volmer step was lower than on Pt (0.39 eV vs. 0.44 eV).⁷⁵ The results demonstrated a largely decreased energy barrier for both the Volmer and Tafel step for MoNi₄ electrocatalysis. In addition, to understand the origin of the high HER activity, the authors measured the HER onset potential of pure MoO₂ nanosheets and MoNi₄ supported by MoO₂ cuboids on carbon cloth in 1 M KOH. They concluded that the excellent HER performance of MoNi₄/MoO₂@Ni derived from the MoNi₄ nanoparticles rather than from supporting MoO₂ cuboids. A series of HER stability measurements at different current densities (10 mA cm⁻², 100 mA cm⁻², and 200 mA cm⁻²) were performed over a total of 30 h but no change in the catalyst structure was observed, indicating that the structure of the MoNi₄ electrocatalyst was robust.

Fig. 3 (a–c) Typical SEM and (d–f) HRTEM images of MoNi₄/MoO₂@Ni; (g) corresponding elemental mapping images of MoNi₄ electrocatalyst and MoO₂ cuboids. The inset image in (d) is the related selected-area electron diffraction pattern of the MoNi₄ electrocatalyst and the MoO₂ cuboids. Scale bars, (a) 20 μm; (b) 1 μm; (c) 100 nm; (d–f) 2 nm; inset in (d) 11 nm; (g) 20 nm; (h) LSV curves of various catalysts as indicated, electrolyte: 1 M KOH, scan rate: 1 mV s⁻¹; (i) Tafel plots of the MoO₄ electrocatalyst supported by the MoO₂ cuboids, pure Ni nanosheets, and MoO₂ cuboids on the nickel foam; (j) calculated adsorption free energy diagram for the Volmer step and (k) Calculated adsorption free energy diagram for the Tafel step of various catalysts as indicated. Reproduced from ref. 66 with permission. Copyright ©2017, The Authors, some rights reserved; exclusive licensee Springer Nature. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://protect-au.mimecast.com/s/4B4DCRON66hym235F9sxMx?domain=creativecommons.org/.

In the latest work of Alber and colleagues,⁹⁸ MoNi₄–MoO₂ was successfully coated on Ni foam substrate via two heat treatment steps. Ni with a small amount of Mo and MoO₂ were firstly introduced on the Ni foam by reduction of NiMoO₄ powder, followed by sintering at 800 °C to improve the adhesion of the catalyst to the substrate. The resulting catalyst required an overpotential of only 95 mV to deliver 300 mA cm⁻² in 30 wt% KOH electrolyte at 60 °C. It also exhibited a small Tafel slope of < 30 mV dec⁻¹ and good stability at 300 mA cm⁻² after 10 h operation under the same condition.

Chen et al.⁹⁹ successfully synthesized a NiMo–NH₃/H₂ catalyst that exhibited a Tafel slope of 35 mV dec⁻¹. In 1 M KOH, this catalyst required overpotentials of only 11 mV and 107 mV to produce 10 mA cm⁻² and 500 mA cm⁻², respectively. Durability was assessed at a fixed voltage of −0.1 V vs. RHE with a negligible change in current density observed after 20 h. Analogous to Zhang's report, the NiMo–NH₃/H₂ catalyst was prepared by annealing NiMoO₄ on nickel foam under a gas mixture of H₂/NH₃ (5% H₂) at 550 °C for 2 h. The H₂ in the gas not only efficiently reduced NiMoO₄ but enhanced the active surface area. In addition, the appearance of the NiMoN_x phase, by reacting with NH₃, increased the HER activity.¹⁰⁰

Wang et al.¹⁰¹ reported that Ni–Mo electrodeposited on Cu foam exhibited 20 mA cm⁻² at an overpotential of 34 mV in 1 M NaOH at room temperature; the HER performance was improved by optimizing the Ni/Mo ratio.

The nature of the catalyst substrate may also contribute to decreasing the overpotential for overall water splitting. Highly electrically conductive substrates, including carbon-based and metal-based materials, have been widely used to reduce the electrical resistance of electrodes. Zhang et al.¹⁰² reported that a MoS₂-based catalyst loaded on Cu foam exhibited a higher HER performance than on carbon cloth and Ti foam. MoS₂ on Cu foam needed 541 mV to deliver 1000 mA cm⁻² in 1 M KOH electrolyte while with carbon cloth and Ti foam 665 and 838 mV, respectively, was required to produce the same current density. Thermal treatment improved the mechanical adhesion between the catalyst and substrate resulting in decreased electrical resistance at their interface and enhanced mechanical stability at high current densities.

In another report, Yu and colleagues developed bimetallic nitrides of NiCoN on different substrates in 1 M KOH electrolyte.¹⁰³ They found that the use of Ni foam as catalyst support produced higher HER performance than catalyst grown on Cu foam, carbon paper, or stainless-steel mat. NiCoN grow on Ni foam delivered a current density of 100 mA cm⁻² at 149 mV overpotential. In the same way, low electrical resistance at the catalyst–substrate interface improved catalytic performance.

RANEY® Ni is known as sponge nickel and was first prepared by Murray Raney in 1927¹⁰⁴ from an original composition of 50 : 50 wt% Ni : Al. RANEY® Ni is still widely used today in alkaline electrolysis. It is derived from Ni–Al or Ni–Zn alloy. The Al or Zn is removed by leaching in alkaline solution, resulting in a high surface area with a far more porous structure than regular Ni.

Tanaka et al.¹⁰⁵ prepared RANEY®-Ni by leaching Al out of different precursor Ni–Al alloys, such as Ni₃Al, Ni₂Al₃, NiAl and NiAl₃ in concentrated NaOH. These authors found that the hydrogen overpotential of Al-rich alloys (i.e., NiAl₃ and Ni₂Al₃) were lower than those of Ni-rich alloys (i.e., NiAl and Ni₃Al) in 1 M NaOH at 303 K. They reported that NiAl₃ was the most active substrate for catalytic RANEY®-Ni cathode preparation due to the large surface area of the electrode, caused by the presence of micropores in the Ni phase after leaching the Al out. Wu et al.,¹⁰⁶ on the other hand, developed porous Ni₃Al electrodes for the HER and their results showed good stability in long-term experiments at 100 mA cm⁻² under industrial conditions (6 M KOH at 80 °C).

HER catalytic activity can also be improved by adding Mo to RANEY® Ni.^107–109 Birry et al.¹⁰⁷ studied the change in electrode activities with different fractions of Mo and Al in the electrode components. They showed that the highest activity at 30 mV overpotential and a current density of 250 mA cm⁻², was obtained with NiAl₃Mo_0.306 in 1 M KOH at 70 °C.

4.2.2 Doping with non-metallic atoms. Along with the development of transition metal-based catalysts, doping with non-metallic elements, such as N, P, S, and B, has also been examined. However, it is challenging to incorporate the light non-metallic dopants into the host catalysts homogeneously. The presence of non-metallic atoms can change the surface structure, resulting in catalytic sites of enhanced activity.^110,111 Among these elements, N and P have attracted more studies in doping non-metallic atoms onto non-noble metal-based catalysts due to their properties. For example, the N atom has a small atomic radius that can be readily embedded into the lattice of the host catalyst without affecting the overall electrical conductivity.¹¹² Compared to N, P has a larger atomic radius but stronger electron-donating ability, which can affect the electronic states of the metal elements of the host catalyst and provide new active site after doping.¹¹³

Wu et al.¹¹⁴ introduced N atoms into a NiCo₂S₄ host catalyst by treating with NH₃ at high temperature. The results showed that incorporation of N can decrease the energy barrier of the water dissociation reaction on NiCo₂S₄ and thereby improve activity for the HER. N–NiCo₂S₄ produced 10 mA cm⁻² at an overpotential of 41 mV, with a small Tafel slope of 37 mV dec⁻¹. Stability measurement in 1 M KOH, indicated that 68.2% of the initial current density was retained after 1000 h at 50 mV vs. RHE. As its structure was conserved, this indicates the gas bubbles accumulation on the electrode surface led to a drastic decrease in the HER current.

Non-noble metal-based phosphide catalysts have recently shown a comparable performance to commercial Pt/C catalysts.¹¹⁵ For example, Cao et al.¹¹⁶ studied Ni_xP deposited on Ni foam substrate by a facile, one-step co-electrodeposition method at constant potential of −0.8 V vs. Ag/AgCl. The results demonstrated that the Ni_xP/NF with 20 min electrodeposition time can yield 10 mA cm⁻² at overpotential of 63 mV, and a low Tafel slope of 55 mV dec⁻¹ in 1 M KOH, room temperature. It also required only 150 mV to deliver 400 mA cm⁻² and no degradation was observed after 20 h operation at 10 mA cm⁻² at the same condition. The use of electrodeposition method to grow Ni_xP on 3D Ni foam structure without presence of polymer binder not only increases adhesion between catalyst layer and substrate but creates more accessible active area, resulting in high catalytic activity and increased stability.

Yu et al.¹¹⁷ reported Ni₂P nanoarray catalyst grown on a Ni foam substrate which exhibited outstanding performance in 1 M KOH electrolyte. The Ni₂P/NF nanoarrays required only 306 and 368 mV overpotential to produce 1000 and 1500 mA cm⁻², respectively. It also maintained its stability under a high current density of 2500 mA cm⁻² for 10 h of continuous electrolysis. The super-aerophobic property of the Ni₂P/NF catalyst enhanced the rate of release of generated H₂ bubbles from the catalyst surface. As a result, the original catalyst active sites were maintained and stable at high current densities.

To enhance the active site and conductivity, Ni₂P can be effectively modulated by doping with Fe, resulting in increased HER activity.¹¹⁸ Sun and co-workers developed Fe-doped Ni₂P embedded in carbon nanotubes using metal–organic framework arrays on nickel foam; these demonstrated excellent HER performance, requiring overpotentials of only 183 mV to generate 1000 mA cm⁻² current density.¹¹⁸ The authors also demonstrated that Fe doping provided more benefits to HER activity than phosphorization. But both Fe doping and phosphorization enhanced the electrochemically surface area of the electrode. As proof of this statement, Yu et al.¹¹⁹ reported that Fe₂P/N₂P hybrid grown on NF exhibited an extremely low overpotential of 14 mV at 10 mA cm⁻² and required only 270 mV to deliver 1000 mA cm⁻² in 1 M KOH electrolyte, which was much higher performance than bare N₂P grown on NF (150 mV overpotential at 10 mA cm⁻² in the same condition).

In another study, Chen and co-workers¹²⁰ reported on N- and P-co-doped molybdenum carbide nanofibers prepared by the pyrolysis of phosphomolybdic acid-doped polyaniline nanofibers at 900 °C under an Ar atmosphere. The results suggested that the ultrasmall N,P-doped molybdenum carbide nanoparticles can provide abundant catalytic active sites, thereby amplifying the performance of this electrocatalyst. Catalytic performance with an overpotential of 135 mV for a current density of 10 mA cm⁻² in 1 M KOH, was reported.¹²⁰

A brief summary of the non-noble metal-based catalysts for the HER reported in the scientific literature is presented in Table 1.

Table 1 Selected non-noble metal-based catalysts for the HER in alkaline solution

Catalyst	Method	Substrate	Operating conditions	Performance	Tafel slope (mV dec^−1)	Ref.
Ni–Mo	Steady-state deposition	Mild-steel foil	6 M KOH, 80 °C	300 mA cm^−2 @ 187 mV	110	92
Ni–Mo nanopowder	Precipitation and subsequent thermal treatment	Ti foil	2 M KOH, 23 °C	130 mA cm^−2 @ 100 mV	NA	95
Ni–Mo	Thermal decomposition and curing	80 mesh Ni substrate	30% KOH, 70 °C	1000 mA cm^−2 @ 80 mV	NA	96
Ni–Mo	Codeposition	Stainless steel fibre felt	1 M KOH, 40 °C	400 mA cm^−2 @ 110 mV	NA	97
MoNi4/MoO2	Hydrothermal	Ni foam	1 M KOH, 23 °C	10 mA cm^−2 @ 15 mV	30	66
				200 mA cm^−2 @ 44 mV
				500 mA cm^−2 @ 65 mV
MoNi4–MoO2	Thermal treatment	Ni foam	30% KOH, 60 °C	300 mA cm^−2 @ 95 mV	< 30	98
NiMo–NH3/H2	Thermal treatment	Ni foam	1 M KOH, 23 °C	10 mA cm^−2 @ 11 mV	35	99
				500 mA cm^−2 @ 107 mV
NiAl3-type RANEY® Ni	Thermal treatment		1 M NaOH, 30 °C	250 mA cm^−2 @ 186 mV	90	105
NiAl3Mo0.306	Thermal treatment		1 M KOH, 70 °C	250 mA cm^−2 @ 30 mV	150	87
NixP	Electrodeposition	Ni foam	1 M KOH, 23 °C	10 mA cm^−2 @ 63 mV	55	116
				400 mA cm^−2 @ 150 mV
Ni2P	Hydrothermal	Ni foam	1 M KOH, 23 °C	1000 mA cm^−2 @ 306 mV	76	117
				1500 mA cm^−2 @ 368 mV
Fe-doped N2P in CNT	Metal–organic framework	Ni foam	1 M KOH, 23 °C	1000 mA cm^−2 @ 183 mV	30	118
Fe2P/N2P	Thermal treatment	Ni foam	1 M KOH, 23 °C	10 mA cm^−2 @ 14 mV	24.2	119
				1000 mA cm^−2 @ 270 mV

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5. Catalysts of the oxygen evolution reaction (OER) in alkaline water electrolysis

The OER is kinetically more sluggish than the HER due to its multi-electron charge transfer reaction, which results in a high overpotential.^121,122 Therefore, the OER is the key electrochemical reaction that must be accelerated to realize high performance water electrolysis. Numerous studies have examined how to enhance the electrocatalytic kinetics of the OER.

5.1 Oxide catalysts for the OER

5.1.1 Precious metal oxides. Currently, IrO₂ and RuO₂ are often considered to be the benchmark for OER catalysis in both acidic and alkaline environments.^123,124 However, while numerous studies have shown that Ir- and Ru-based materials exhibit the best performance in acidic medium under industrial conditions, IrO₂ reportedly suffers dissolution in strongly alkaline media.^125,126 Therefore, IrO₂ cannot be the benchmark for OER catalysis in alkaline solution.

RuO₂ is more active than IrO₂, but it is less stable at high anodic potentials during OER catalysis.¹²⁷ Under anodic conditions, RuO₂ is further oxidized to form the hydrous compounds RuO₂(OH)₂ and RuO₄ which are not stable and can dissolve in the solution, coloring the electrolyte.^{16,27,125,126,128}

The instability of IrO_x in alkaline solution has been previously observed in the growth of an oxide film at the electrode–electrolyte interface (over a number of applied cycles) or the loss of material under oxidizing condition.^125,129,130 Interestingly, most noble-metal-based catalysts perform well for either one of the half-cell reactions (either HER or OER) under specific electrolytic conditions.⁶⁸ To improve the activity and stability of noble metal oxide catalysts, a doped bimetal oxide Ru_xIr_1−xO₂ was investigated.^131,132 The highest performance was obtained when x fell in the range 0.5 to 0.8.¹³³

Although the performance and stability improved, these catalysts are scarce and expensive, which affects their practicality in large-scale applications. It can be shown that if the entire annual production of Ir was diverted to water electrolysis, only 10 GW of electrolysers could be produced each year; this is a tiny fraction of the electrolyser capacity needed to realize net-zero carbon emissions by 2050.

5.1.2 Perovskites. Perovskites have ABO₃ structures, where A is an alkaline metal or rare-earth metal (La, Ca or Sr) and B is a transition metal (Co, Fe or Mn). Table 2 lists selected perovskite-type OER catalysts for alkaline conditions. LaNiO₃ and SrCoO₃ display excellent oxygen evolution activity.¹³⁴

Table 2 Selected perovskite-type catalysts for the OER in alkaline media

Catalyst	Method	Substrate	Operating conditions	Performance	Tafel Slope (mV dec^−1)	Ref.
LaNiO3	Solid-state reaction	Sintered rod	1 M NaOH; 25 °C	100 mA cm^−2 @ 300 mV	43	155
SrCoO2.85−δF0.15	Ink (drop casting)	GCE	1 M KOH	10 mA cm^−2 @ 380 mV	60	156
Ba0.5Sr0.5Co0.8Fe0.2O3−δ	Nitrate combustion method	GCE	0.1 M KOH	10 mA cm^−2 @ 370 mV	NA	135
La0.95FeO3−δ	Ink (drop-casting)	GCE	1.1 0.1 M KOH, 23 °C	10 mA cm^−2 @ 400 mV	48	157
La0.4Sr0.6FeO3	Solid-state reaction	Sintered rod	1 M KOH; 25 °C	40 mA cm^−2 @ 622 mV	58	158
La0.2Sr0.8FeO3				40 mA cm^−2 @ 502 mV	53
La0.2Sr0.8Fe0.2Co0.8O3				40 mA cm^−2 @ 402 mV	80
SrFeO3	Ink (drop-casting)	GCE	1.1 M KOH, 23 °C	10 mA cm^−2 @ 410 mV	63	159
La0.6Sr0.4CoO3	Ink (drop-casting)	GCE	1.1 M KOH, 23 °C	10 mA cm^−2 @ 468 mV	NA	160
La0.6Sr0.4Co0.6Fe0.4O3				10 mA cm^−2 @ 424 mV	NAp
SrSc0.025Nb0.025Co0.95O3−δ	Solid state reaction + ink (drop-casting)	GCE	1.1 M KOH, 23 °C	10 mA cm^−2 @ 350 mV	55	161

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Park et al.⁶⁸ reported on a noble metal-free catalyst which comprised of perovskite oxides (La_0.5Sr_0.5CoO_3−δ, LSC) and potassium ion-bonded MoSe₂ (K–MoSe₂). The authors reported that the enhanced HER kinetics was due to the modulated electronic structure of MoSe₂ with high metallic-phase purity and with improved electrical conductivity. The electrocatalyst outperformed the state-of-the-art noble-metal pair of Pt/C‖IrO₂, with lower cell overpotential at 10 and 100 mA cm⁻², improved energy efficiency, and excellent operational stability over 2500 h.⁶⁸

Suntivich et al.¹³⁵ found that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) exhibited the best OER performance of any perovskite in alkaline media. However, this material exhibited instability under harsh alkaline conditions due to the amorphization and dissolution of Ba and Sr.^136–138

A few studies have reported amplified anodic currents at high pHs when carbon is included in the electrode composition.^139–141 However, carbon will typically be oxidized (slowly or fast) at high pH under catalytic OER conditions, so that the high anodic currents are, in all likelihood, due to carbon corrosion currents and not to the OER.¹⁴² Industrial electrolysers typically scrupulously avoid the presence of carbon in their anodes.

Recently, a few research groups have examined double perovskites as a means of improving stability in alkaline medium. Grimaud et al.¹³⁷ demonstrated that the double perovskite catalysts Ln_0.5Ba_0.5CoO_3−δ (Ln = Pr, Sm, Gd and Ho) are, as a group, amongst the most efficient OER catalysts reported to date, exhibiting activities greater than BSCF.

5.1.3 Spinels. Spinel oxides have the general formula AB₂O₄ with A^II and B^III cations. They are usually prepared in combinations of transition metals (i.e. Cr, Fe, Mn, Ni and Co). Among the spinel oxides, Co₃O₄⁻ based electrodes^143,144 and NiCo₂O₄ (ref. 145 and 146) have received great interest due to their high activity and stability in alkaline solution. NiCo₂O₄, which shows comparatively high electronic conductivity, has been of particular interest. Table 3 lists selected spinel catalysts for the OER in alkaline solution.

Table 3 Selected spinel-type catalysts for the OER in alkaline solution

Catalyst	Method	Substrate	Operating conditions	Performance	Tafel slope (mV dec^−1)	Ref.
NiCo2O4	Deposition and phosphorisation	Ni foam	1 M KOH, 23 °C	10 mA cm^−2 @ 250 mV	58	162
NiFeCrO4	Hydroxide precipitation at 70 °C + sintering at 400 °C for 24 h + slurry painting	Ni plate	1 M KOH, 25 °C	100 mA cm^−2 @ 285 mV	40	163
CoFe2O4	Electrospinning + thermal treatment + ink (drop-casting)	GCE	1.1 M KOH, 23 °C	10 mA cm^−2 @ 370 mV	82	150
Co3O4	Electrodeposition at 50 °C	Au	1 M KOH, 23 °C	10 mA cm^−2 @ 400 mV	49	164
CoCr2O4/CNT	Molten salt calcination at 700 °C + ink (drop-casting)	GCE	1 M KOH, 23 °C	10 mA cm^−2 @ 326 mV	51	165

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Chien et al.¹⁴⁷ observed high performance by an aerogel NiCo₂O₄, which required an overpotential of 184 mV to produce 100 mA cm⁻² in 1 M KOH at 25 °C. Besides that, La-doped Co₃O₄ in the form of a thin film on Ni, prepared by Singh et al.¹⁴⁴ exhibited an overpotential of 224 mV under the same conditions. Srivastava et al.¹⁴⁸ reported that an increase in performance was observed when the NiCo₂O₄ was mixed with carbon. However, as noted above, the increase in activity was likely due to the additional current arising from carbon oxidation, so that the evaluation of catalytic activity in this case appears to be unreliable.²²

Shi et al.¹⁴⁹ utilised a defect engineering strategy to create Ce-substituted spinel CuCe_δ Co_2−δO_x (δ = 0.45, 0.5 and 0.55) nanoparticles which showed an increased OER activity in comparison to commercial RuO₂. The particles in nano dimensions were prepared using a phase-transfer co-precipitation strategy. The authors reported that the electrocatalyst required a low overpotential of 294 mV to produce 10 mA cm⁻², and had a Tafel slope of 57.5 mV dec⁻¹. The catalytic performance could be attributed to the substituted Ce, which produced high oxygen vacancies and more efficient Co²⁺ sites, and thereby improved electron transport and decreased the energy barrier of the rate-determining step of the OER.

Beyond cobalt-based compounds, various studies have investigated iron-based (ferrite) compounds with other transition metals such as Mn, Ni and Cu.^150,151

Li et al.¹⁵⁰ reported that the trend of OER catalytic activities for MFe₂O₄ (M = Co, Ni, Cu, Mn) was: CoFe₂O₄ > NiFe₂O₄ > CuFe₂O₄ > MnFe₂O₄. Regarding the incorporation of Mn in Co-based spinel Mn_3−xCo_xO₄ (0 ≤ x ≤ 1): the OER electrocatalytic activity significantly improved with increasing Co content.¹⁵²

Although good performance may be achieved with spinel metal oxides, an important issue that needs to be considered is the phase transformation of spinels into metal oxyhydroxides on the catalyst surface during OER.^16,153,154 This has inspired many researchers to develop high performing OER catalysts based on the layer structure-type oxides.

5.1.4 Layered metal oxides. Layered metal oxides consist of metal hydroxides M(OH)₂, oxyhydroxides MOOH and layered double hydroxides (LDH), where M represents transition metals.

These compounds have attracted considerable attention recently because many transition metals form layered rather than a pure oxide phase.^137,166,167 However, it is difficult to determine the phase accurately. Some metal hydroxides or oxyhydroxides are commonly referred to as having layered double hydroxide (LDH) structures.²⁵ Among transition metals, Ni is one of the most promising catalysts for the OER in alkaline solution¹⁶⁸ and its phase transforms at high potential in alkaline medium. For example, Ni(OH)₂ is instantaneously formed on the top of NiO when Ni is placed in air-oxygenated aqueous solution; Ni(OH)₂/NiOOH then grows upon that layer with continuous potential cycling.¹⁶⁹

Corrigan has pointed out that the OER activity of NiOOH can be significantly increased by introducing an impurity of Fe in the electrolyte.¹⁷⁰ The beneficial role of Fe ions in the NiOOH structure was also investigated by Trotochaud et al.¹⁷¹ The authors found that Fe incorporation not only increased NiOOH conductivity by >30-fold but affected the NiOOH electronic structure by inducing partial-charge transfer, which activated the Ni centers throughout the catalyst film.

In recent years, many researchers have developed nickel-iron (oxy)hydroxide-based catalysts that are, today, considered to be the state-of-the-art catalyst for the OER in alkaline solution.^66,172–174 In order to maximize the electrocatalytic activity of Ni–Fe (oxy)hydroxide, the Fe content should be taken into account, which depends on the catalyst substrate and the method of preparing the Ni–Fe (oxy)hydroxide. The highest activity can be achieved with Fe content in the range of 15–50% for electrodeposited films evaluated in 0.1 M KOH.⁴² Li et al.¹⁵⁰ have, however, reported that 10% is the optimum Fe composition in electrodeposited NiFe-based film. In other work, 12–17% was said to be an optimum value of Fe incorporated into Ni oxyhydroxide film on Au substrate, as described in a work of Klaus et al.¹⁷⁵

Lu and Zhao developed a very efficient NiFe composite by electrodeposition on Ni foam substrate.⁶⁴ The Tafel slope of NiFe/NF were 33 mV dec⁻¹ and 28 mV dec⁻¹ in 0.1 M and 1 M KOH electrolyte, respectively; these are lower than the benchmark IrO₂ and RuO₂ catalysts.¹²⁷ According to the Tafel plot, a notable OER performance was achieved at current densities of 500 mA cm⁻² and 1000 mA cm⁻² with overpotentials of only 240 and 270 mV, respectively, in 10 M KOH. In addition, the NiFe/NF electrode exhibited good durability in different electrolyte concentration (100 mA cm⁻² in 1 M KOH for 10 h and 500 mA cm⁻² in 10 M KOH for 2 h), indicating that the electrode was catalytically stable and mechanically robust. Microscopy measurement showed that the porous structure of NiFe/NF electrode (Fig. 4(a)) involved ultrathin, amorphous, mesoporous Ni–Fe nanosheets interconnected across the whole microporous NF substrate (Fig. 4(c) and (d)). The amorphous structure had a larger surface area and higher electrical conductivity, resulting in high activity for OER catalysis.¹⁷⁶ Furthermore, the macroporous structure of the NF substrate allowed large oxygen bubbles to disperse rapidly into the electrolyte at high current densities, which avoided the phenomenon of attachment of gas bubbles on the electrode surface.

Fig. 4 Microscopy measurements of the NiFe/NF electrode. (a) SEM image of NiFe/NF electrode. (b) High resolution SEM image of the area squared in (a). (c and d) TEM images of NiFe nanosheets scratched off from the NiFe/NF (the inset shows the corresponding selected area diffraction pattern). Scale bars, 200 μm, 100 nm, 50 nm and 10 nm in (a), (b), (c), and (d), respectively. Reproduced from ref. 64 with permission. Copyright ©2015 The Authors, some rights reserved; exclusive licensee Springer Nature. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://protect-au.mimecast.com/s/4B4DCRON66hym235F9sxMx?domain=creativecommons.org/.

In their patent, Hongjie Dai¹⁷⁷ reported OER electrodes with NiFe-LDH electrocatalyst coating on different passivation layers/substrates, which all exhibited excellent stability in alkaline electrolyte containing chloride. For example, NiFe hydroxide carbonate (NiFe-HC) was obtained during anodization of NiFe foam by running at constant current density of 250 mA cm⁻² at 85 °C in 0.1 M KHCO₃ electrolyte for 16 h. The NiFe-HC electrode produced 400 mA cm⁻² with overpotential of 370 mV in 1 M KOH electrolyte, room temperature. In addition, when pairing with Ni mesh as cathode in a 2-electrode electrolysis, the NiFe-HC electrode displayed excellent durability at 400 mA cm⁻² for 1500 h under industrial condition at 80 °C and electrolyte containing 6 M KOH + 2 M K₂CO₃ + 0.5 M NaCl.

Inspired by this work, Zhou et al.¹⁷⁸ reported, in 2018, an amorphous mesoporous NiFe (oxy)hydroxide film grown on Ni foam, which produced 500 mA cm⁻² and 1000 mA cm⁻² at overpotentials of 259 and 289 mV, respectively, in 1 M KOH electrolyte at room temperature. This (Ni,Fe)OOH catalyst exhibited much higher OER activity than NiFe LDH nanosheets/NF, the benchmark IrO₂ electrode, and the Ni foam substrate. For example, it required an overpotential of only 174 mV to produce 50 mA cm⁻², which is 75 mV, 165 mV, and 179 mV less than the overpotential of the NiFe LDH nanosheets/NF (249 mV), IrO₂ electrode (339 mV) and the Ni foam (353 mV), respectively, under the same condition. This catalyst also exhibited a small degradation during stability measurement at 500 mA cm⁻² and 1000 mA cm⁻² over 44 h in 1 M KOH. The overpotential drifts were only 14 mV and 59 mV for current densities of 500 mA cm⁻² and 1000 mA cm⁻², respectively.

The authors claimed that the excellent catalytic behavior was mostly due to the amorphous (Ni,Fe)OOH film, rather than the Ni foam. The mixed composite of Ni(OH)₂ and FeOOH was grown uniformly forming an amorphous mesoporous film on the NF surface, which created more active sites for OER. In other measurements, the amorphous (Ni,Fe)OOH showed smaller double layer capacitance but larger TOFs at 300 mV than that of NiFe LDH nanosheets/NF and IrO₂ catalyst, which corroborated a higher intrinsic catalytic activity for the OER.

Recently, a paper by Liang et al.¹⁷⁹ reported a comparable result to Lu and Zhao.⁶⁴ They found that NiFe oxyhydroxide on NiFe alloy nanowire exhibited current densities of 500 mA cm⁻² and 1000 mA cm⁻² at overpotentials of only 248 and 258 mV, respectively, in 1 M KOH. However, the electrodes were prepared using a magnetic-field-assisted chemical deposition method, which is more complicated than the methods used by Lu and Zhao.⁶⁴ By using a uniform magnetic field, reduced ferromagnetic Ni_xFe_1−x nanoparticles were aligned and coalesce with each other on the nickel foam substrate (Fig. 5(a)). This resulted in the creation of an ultrathin amorphous layer (1–5 nm) of NiFe oxyhydroxide in a nanowire structure with wire axes parallel to the magnetic field direction.¹⁸⁰ The SEM images (Fig. 5(b) and (c)) showed that a large number of Ni_xFe_1−x nanowires were rooted vertically on the Ni foam substrate with the gaps between the nano wires offering a strong capillary action to thereby improve wettability and the active surface of electrodes. In addition, the nanowire array structure reduced the contact between bubbles and electrodes thereby facilitating bubble release from and ion transfer to the electrodes.

Fig. 5 Morphological and structural characterisation of Ni_xF_1−x alloy-ultrathin amorphous oxyhydroxide nanowire arrays (denoted as Ni_xF_1−x-AHNAs). (a) Schematic of the synthesis of the Ni_xF_1−x-AHNAs nanowires arrays and its catalytic function for the OER. (b and c) SEM images of Ni_xF_1−x-AHNAs at different magnifications. Inset of (c) is a low magnification TEM of a single nanowire. Reproduced from ref. 179 with permission from the Royal Society of Chemistry, Copyright ©2020.

Wu and colleagues¹⁸¹ demonstrated outstanding activity of NiFe₂O₄/NiFe LDH on Ni foam in 1 M KOH, which required only low overpotentials of 242 mV and 265 mV to produce 500 mA cm⁻² and 1000 mA cm⁻², respectively. The current densities were 97% retained during 20 h stability test at overpotentials of 202 mV, 230 mV, and 242 mV with barely any change in morphology and microstructure. The authors proposed a NiFe₂O₄ nanoparticles/NiFe LDH nanosheet array on Ni foam. The NiFe LDH nanosheets were firstly formed on the Ni foam substrate by a solvothermal method. Then NiFe₂O₄ particles were deposited on the NiFe LDH nanosheet via a hydrolysis approach. This process not only provided good electrical contact between the catalyst and Ni foam substrate but enhanced the catalytically active surface area, which indicated high intrinsic electrochemically activity.

A slightly higher overpotential was reported by Teng et al.¹⁸² for NiFeO_x nanotube arrays, which achieved 100 mA cm⁻² and 1000 mA cm⁻² in 1 M KOH at overpotentials of 260 mV and 350 mV, respectively, with a Tafel slope of 47 mV dec⁻¹. Little degradation in the OER performance with negligible change in morphology was observed after operating at a fixed overpotential of 260 mV for 12 h.

Xiao and colleagues¹⁸³ constructed the porous composite electrode NiFe/NiCo₂O₄/NF via a 2-step deposition procedure. The catalyst produced a current density of 1200 mA cm⁻² with an overpotential of 340 mV in 1 M KOH solution, at room temperature. This electrode comprised three levels of porous structure, including macroporous Ni foam substrate (∼500 μm thickness), an intermediate vertically aligned macroporous layer of NiCo₂O₄ nanoflakes (∼500 nm thickness) formed in a hydrothermal reaction, and the top layer of NiFe(oxy) hydroxide mesoporous nanosheets (∼5 nm thick) formed by an electrodeposition method. The authors claimed that the catalytic activity was predominantly contributed by the topmost NiFe layer, while the presence of NiCo₂O₄ nanoflakes layer increased the active surface areas significantly.

Jiang et al.¹⁸⁴ showed that Ni₃Fe_0.5V_0.5 ultrathin nanosheets grown on hydrophilic carbon fiber paper by a hydrothermal method exhibited a very low overpotential of 300 mV to produce 1000 mA cm⁻² in 1 M KOH electrolyte at room temperature, with a Tafel slope of 39 mV dec⁻¹. As mentioned above however, the presence of carbon as a substrate in OER catalysts leads to false and misleading anodic currents that do not reflect the OER activity. A question therefore exists over this result.¹⁴²

Developing NiFe LDH-based catalyst with more active sites on different conductive skeletons has been considered as an alternative approach to improve OER activity and stability.

Liu et al.¹⁸⁵ fabricated a thin film NiFe LDH nanosheet array on a Fe foam substrate via a corrosion reaction of the Fe foam with a corrosive solution of nickel salts (e.g. NiSO₄·6H₂O) at room temperature for 12 h. In this solution, Fe-layered double hydroxides were spontaneously formed rather than the formation of common Fe rust (Fig. 6(a)) due to the presence of divalent Ni²⁺ in the corrosive solution. The layered double hydroxides were well-oriented with abundant grain boundaries to create a nanosheet array architecture, which was believed to enhance the electrochemical reaction (Fig. 6(b)). The NiFe LDH nanosheet array, prepared by a corrosion engineering method, displayed higher OER performance than NiFe LDH prepared by the electrodeposition method. At room temperature, this electrode required only 300 mV and 340 mV to generate 500 mA cm⁻² and 1000 mA cm⁻² in 1 M KOH, respectively, and 257 mV and 280 mV for the same current densities in 10 M KOH. It also showed robust stability at room temperature with negligible decay at 1000 mA cm⁻² for 5000 h in 1 M KOH (Fig. 6(c)), followed by 1050 h in 10 M KOH (Fig. 6(d)). Iron substrates may, however, be expected to be subject to rapid corrosion by dissolved oxygen (forming iron oxides),¹⁸⁶ so that the practical utility of this result is also open to question.

Fig. 6 (a) Schematic diagram depicts specific reaction of the formation NiFe LDH catalyst on iron substrate by a corrosion engineering method. (b) The formation of grain boundary-enriched layered double hydroxide (LDH) nanosheet arrays on the Fe substrate surface. Right side of (b) shows the representative crystal structure of LDH. (c and d) Chronopotentiometric curves at room temperature of NiFe LDH (3D-O₂-Cat-1) for 5000 h (c) in 1 M KOH, and then for 1050 h (d) in 10 M KOH at current density of 1000 mA cm⁻². Reproduced from ref. 185 with permission. Copyright ©2018. The Authors, some rights reserved; exclusive licensee Springer Nature. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://protect-au.mimecast.com/s/4B4DCRON66hym235F9sxMx?domain=creativecommons.org/.

Yu et al.¹⁸⁷ reported NiFe-LDH nanostructures grown vertically on Cu nanowire arrays, which were able to generate 1000 mA cm⁻² at 315 mV overpotential in 1 M KOH at room temperature, with a very small Tafel slope of 27.8 mV dec⁻¹. The shell of the NiFe LDH nanosheets grew vertically around the core of the Cu nanowires, forming a highly porous structure that provided many channels for electrolyte diffusion and gas diffusion. There was no increase of overpotential during 48 h stability test at current densities of 10 mA cm⁻² and 100 mA cm⁻² in 1 M KOH at room temperature. However, a small change in the morphology and phase of the original 3D porous structure was observed after the stability test. It is also probably due to the fact that Cu likely dissolved in 1 M KOH electrolyte during the test.¹⁸⁸

Shen et al.¹⁸⁹ developed Ni(Fe)O_xH_y-coated nano-cone array stainless steel meshes, which achieved high current densities of 500 mA cm⁻² and 1000 mA cm⁻² at overpotentials of 280 mV and 303 mV, respectively, in 1 M KOH electrolyte at 25 °C, with a small Tafel slope of 34.9 mV dec⁻¹. However, the nano-cone on the electrode surface dropped off after 82 h of electrolysis at 500 mA cm⁻², resulting in an increase in the overpotential of 15 mV.

In order to improve the activity of NiFe (oxy)hydroxide, the incorporation of a third metal has also been studied. Wang et al.¹⁹⁰ fabricated NiFe (sulfur) oxyhydroxide on Ni foam substrate by a facile one-step, wet synthesis method. This electrode produced a current density of 1000 mA cm⁻² at 260 mV overpotential in 1 M KOH at room temperature.

Liu et al.¹⁹¹ also reported carbon-free nano-NiCoFe layered double hydroxides (LDHs) with MoO₄²⁻ anions to alleviate the issues of carbon corrosion and catalytic stability of most electrocatalysts of OER.

Zou et al.¹⁹² reported a NiFe hydroxide integrated with a Ni₃S₂ nanostructure supported on Ni foam via a novel 2-step method, which possessed high catalytic activity in both 1 M KOH and 30 wt% KOH solution. Firstly, Ni₃S₂/NF was synthesized via a hydrothermal method in a solution containing a source of sulfur.¹⁹³ Then, amorphous Ni–Fe bimetallic hydroxide was grown on the Ni₃S₂/NF skeleton by immersion of as-prepared Ni₃S₂/NF into a pre-heated (100 °C) solution containing Fe³⁺ ion for 5 s. The resulting electrode generated 500 mA cm⁻², 1000 mA cm⁻², and 1500 mA cm⁻² at overpotentials of 370 mV, 469 mV, and 565 mV, respectively, in 30 wt% KOH electrolyte at room temperature. The catalytic stability was also assessed by recording chronopotentiometric curves at 100 mA cm⁻² and 500 mA cm⁻² in 1 M KOH and 1000 mA cm⁻² in 30 wt% KOH solution, with negligible loss after 50 h of operation under ambient condition. The 3D Ni₃S₂ nanosheet arrays were said to provide not only faster electron transport but also higher electrochemically active surface areas to the electrode.

During the drafting of this publication, Jiang et al.¹⁹⁴ reported Ni₃Fe LDH synthesized by a co-precipitation method from Ni(NO₃)₂ and Fe(NO₃)₃ in alkaline solution, which exhibited a low overpotential of 249 mV at 10 mA cm⁻² in 1 M KOH electrolyte, room temperature, with an ultra-small Tafel slope of 24 mV dec⁻¹. To evaluate stability, Ni₃Fe LDH with a 0.2 mg cm⁻² loading on Ni foam was tested at a fixed voltage of 1.6 V for 400 h in 1 M KOH. This catalyst displayed good stability with a degradation in current of 0.0261 mA cm⁻² h⁻¹, equating to current density loss of 10.44 mA cm⁻² after 400 h operation. However, the Ni₃Fe LDH catalyst required a relatively long preparative process including drying in a vacuum oven overnight at 25 °C, which may limit its practicality in large scale applications.

Several approaches to synthesize metal-hydroxide catalyst for OER in the literature are described in Table 4.

Table 4 Selected metal (oxy)hydroxide-type catalysts for the OER in alkaline solution

Catalyst	Method	Substrate	Operating conditions	Performance	Tafel slope (mV dec^−1)	Ref.
Ni(OOH)2	Hydrothermal at 120 °C	Ni foam	0.1 M KOH, 23 °C	30 mA cm^−2 @ 450 mV	65	65
				30 mA cm^−2 @ 280 mV	50
NiFe-LDH				Compared with NF (520 mV for 30 mA cm^−2)
NiFe(OH)2	Electrodeposition	Nickel microdisc	1 M NaOH, 80 °C	500 mA cm^−2 @ 265 mV	33	173
NiFe-hydroxide	Electrodeposition	Ni foam	10 M KOH, 23 °C	500 mA cm^−2 @ 240 mV	32	64
NiFe-LDH (Ni : Fe = 3 : 1 wt%)	Electrodeposition	Stainless steel	30% KOH, 23 °C	300 mA cm^−2 @ 396 mV	40	195
NiFe-LDH	Electrodeposition	Ni foam	1 M KOH, 23 °C	10 mA cm^−2 @ 224 mV	52	196
NiFe film	Electrodeposition	Au	0.1 M KOH, 23 °C	10 mA cm^−2 @ 280 mV	40	174
NiFe-LDH/X where X refers to different anion intercalation into LDH structure	Hydrothermal at 25 °C in different Ni salts solution	Fe foam	1 M KOH, 23 °C			197
NiFe-LDH/Cl^−	NiCl2			10 and 100 mA cm^−2 @ 230 and 260 mV (for NiCl2)	53.1
NiFe-LDH/NO3^−	Ni(NO3)2			10 and 100 mA cm^−2 @ 210 and 240 mV (for Ni(NO3)2)	40.4
NiFe-LDH/SO4^2−	NiSO4			10 and 100 mA cm^−2 @ 235 and 270 mV (for NiSO4)	56.5
NiCo-LDH	Electrodeposition	Stainless steel	1 M KOH, 23 °C	10 mA cm^−2 @ 270 mV	61	198
NiMn-LDH nanosheet	Coprecipitation + hydrothermal	GCE	0.1 M NaOH, 23 °C	10 mA cm^−2 @ 330 mV	47	199
CoFe-LDH	Coprecipitation	Ni foam	0.1 M KOH, 23 °C	10 mA cm^−2 @ 320 mV	45	200
Ultrathin CoFe-LDH	Coprecipitation	Ni foam	1 M KOH, 23 °C	100 mA cm^−2 @ 310 mV	47	201
Ultrathin CoMn-LDH	Coprecipitation	CFP (pretreated with O2 plasma)	1 M KOH, 23 °C	10 mA cm^−2 @ 324 mV	43	202

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5.2 Non-oxide catalysts for the OER

Non-oxide catalysts for the OER have received less attention than metal-oxides (or hydroxides/oxyhydroxides). Transition metal phosphides, sulfides and selenides have been reported as electrocatalysts for the OER. Of most interest are Ni- and Co-based compounds because they possess high conductivity.¹⁶ Among these, phosphides and sulfides have exhibited good catalytic activity. For instance, Zheng et al.²⁰³ reported Ni₂P grown on Ni foam via hydrothermal treatment method with reaction time of 6 h, which needed an overpotential of 142 mV to deliver 10 mA cm⁻² in 1 M KOH at room temperature. In addition, their long-term stability may be improved by uniformly doping Fe into the Ni₂P phase; the resulting electrode exhibited an overpotential of 150 mV at 10 mA cm⁻².²⁰⁴ In addition, according to Xu et al.²⁰⁵ NiFe-selenides can drive higher OER activity because they transform to an oxide and oxyhydroxide phase on their surface.

One of the highest performing catalysts, Fe(PO₃)₂/Ni₂P supported on Ni foam, has been reported by Zhou and colleagues.²⁰⁶ This electrode yielded current densities of 10 mA cm⁻² at an overpotential of 177 mV, and 500 mA cm⁻² at 265 mV. At room temperature, it also produced 1705 mA cm⁻² at only 300 mV in 1 M KOH solution and demonstrated good stability at 100 mA cm⁻² and 500 mA cm⁻² for 20 h. The predominant contribution to the large current density was by the Fe(PO₃)₂. The catalytic activity improved after the first 1000 cycles of cyclic voltammetry testing and maintained mostly the same performance after 10 000 cycles due to the formation of amorphous FeOOH on the Ni foam during OER operation.

Ren et al.²⁰⁷ reported on the NiMoN@NiFeN transition metal nitride (TMN) as an electrocatalyst for the OER, where NiFeN nanoparticles were decorated on NiMoN nanorods supported on porous Ni foam. Very low overpotentials of 337 mV and 368 mV were required to deliver large current densities of 500 and 1000 mA cm⁻², respectively, in 1 M KOH at 25 °C. The authors attributed the high performance of this electrocatalyst to the in situ evolved amorphous layers of NiFe oxide and NiFe oxyhydroxide on the electrode surface. In addition, the integrated 3D core–shell TMN nanostructure allowed ions or molecules to access the interior volumes, increasing active sites and promoting OER performance.

In a recent report, Chen and Hu⁹⁹ developed a highly activity OER catalyst from HER catalyst NiMo–NH₃/H₂ by doping Fe ions into it to create Fe–NiMo–NH₃/H₂. To prepare this catalyst, NiMoO₄ was first annealed on a Ni foam in a mixture of H₂/NH₃ (5% H₂) at 550 °C for 2 h to form NiMo–NH₃/H₂. The as-synthesized NiMo–NH₃/H₂ was then dipped into a fresh FeCl₃ solution (1 mM) for 15 min for the growth of FeOOH nanoclusters onto the NiMo-based electrode. Finally, the Fe–NiMo–NH₃/H₂ electrode was obtained after drying in an oven at 70 °C, followed by in air for 1 h. This electrode catalyzed the OER at 500 mA cm⁻² at an overpotential of only 244 mV in 1 M KOH solution at room temperature. It also exhibited stable performance in 18 h for electrolysis at 1.46 V with negligible change in the current density. After catalytic OER operation, Mo and N was observed to be lost, suggesting that an amorphous NiFe oxyhydroxide layer formed on the electrode surface; this may explain the high OER performance. Although the original catalyst was not an oxide, during OER electrolysis, a structural transformation to an oxide or oxyhydroxide phase occurred on the electrode surface, thereby producing high OER activity. It is, accordingly, essential to accurately understand the catalytic mechanism in order to design high performing catalytic materials.

6. Bifunctional catalysts for the HER and OER

Noble Pt/C, RuO₂, and IrO₂ or even the transition metal-based catalysts that are functional for only single half-cell reactions make it cost efficient to develop efficient bifunctional catalysts capable of catalyzing both the HER and OER.

Using zeolitic imidazole framework (ZIF) structures, Song et al.,²⁰⁹ reported a hybrid nanostructure with CoP nanoparticles embedded in an hollow N-doped carbon nanocage which they termed as h-CoP@NC. With these novel structures, the authors could able to address some of the problems usually encounter in catalysis such as mass transport and agglomeration of nanoparticles. The optimal electrocatalyst exhibited superior bifunctional activities for both the HER and OER with overpotentials of 196 mV and 339 mV respectively to drive a 10 mA cm⁻² current; an cell voltage of 1.764 V was required for overall water splitting.²⁰⁹

Ji et al.²⁰⁸ on the other hand, incorporated heteroatoms such as Ni and Co in the N-doped carbon matrix to create (Ni,Co)₂P nanoframes (NFs) that were studied as HER and OER catalysts. The work showed that when the (Ni,Co)₂P NF catalyst was employed as both the cathode and anode for overall water splitting, a remarkably low cell voltage of 1.54 V was required to achieve a current density of 10 mA cm⁻², with overpotentials of 92 mV for the HER and 283 mV for the OER in 1 M KOH at room temperature, respectively.²¹⁰ Interestingly the same synthetic strategy (see Fig. 7) for preparing nanoframes could be extended to the preparation of other sulphides and selenides, such as (Ni,Co)S₂ NFs and (Ni,Co)Se₂ NFs.

Fig. 7 Schematic synthesis route for (Ni,Co)₂P NF_s. Reproduced from ref. 208 with permission from American Chemical Society, Copyright ©2021.

Yu et al.¹¹⁹ constructed metallic iron and dinickel phosphides (FeP/N₂P) on commercial Ni foam for both HER and OER catalysts. The cell required only 1.42 V to generate 10 mA cm⁻² with a low Tafel slope of 69.5 mV dec⁻¹ in 1 M KOH electrolyte, room temperature. It also delivered current densities of 500 and 1000 mA cm⁻² at low voltage of 1.72 and 1.78 V, respectively, and exhibited excellent stability at 500 mA cm⁻² during 40 h of operation in 1 M KOH at room temperature with negligible degradation. The work showed that FeP particles grown on top of a highly conductive, high surface area of Ni₂P/Ni foam support enhanced OER performance due to improvement of catalytic activity with Fe incorporation.¹⁷⁸ Density functional theory (DFT) measurement showed a significant decrease in hydrogen adsorption energy of FeP/Ni₂P catalysts compared to pure Ni₂P, indicating that this catalyst was efficient for HER. This conclusion was also supported by TOF calculations, with TOF values for FeP/Ni₂P calculated to be 0.163 s⁻¹ at 100 mV overpotential compared to 0.006 s⁻¹ for pure Ni₂P catalyst at the same overpotential.

7. High-performance alkaline water electrolysis

The next step toward industrial applications would be to investigate full-cell, 2-electrode water electrolysis, with simultaneous generation of hydrogen at the cathode and oxygen at the anode. The development of high performing electrocatalysts for efficient and durable overall water splitting in commercial alkaline electrolysers are of critical importance in further decreasing the cost of green hydrogen.

The hydrogen production rate is directly proportional to the current density employed. However, increasing the current density decreases the energy efficiency. Both parameters have a direct impact on the cost of hydrogen production. In practice, the energy efficiency and the hydrogen production rate should be considered when designing an electrolyser system.

Chen and Hu⁹⁹ reported a high-efficiency anion exchange membrane (AEM) water electrolysis cell, which employed NiMo–NH₃/H₂ as a cathode and Fe–NiMo–NH₃/H₂ as an anode. The cell produced 1000 mA cm⁻² at 1.57 V at 80 °C in 1 M KOH electrolyte, which corresponds to 93.6% energy efficiency relative to the higher heating value (HHV) of hydrogen. In addition, this cell displayed excellent stability when operated at 50 mA cm⁻² at 80 °C but at a constant current density of 500 mA cm⁻² degradation was observed after 25 h operation.

Masel's group developed a ‘Sustanion’ anion exchange membrane-based alkaline electrolyser, which generated 1000 mA cm⁻² at 1.9 V at 60 °C with a NiFe₂O₄ anode and FeNiCo cathode catalyst on stainless steel fibers. The cell maintained excellent stability at 1000 mA cm⁻² for 2000 h giving a degradation rate of only 5 μV h⁻¹.²¹¹ With a precious metal Pt/C catalyst at the cathode and IrO₂ at the anode on the Sigracet carbon fiber paper, the cell delivered 1000 mA cm⁻² at 1.63 V, however its stability was not reported.²¹¹ The carbon used at the anode likely produced an artefact result reflecting carbon corrosion rather than OER catalysis.

Alchemr's water electrolyser cells (5 cm²) employed a Sustanion membrane, NiFe₂O₄ catalyst on a stainless-steel gas diffusion layer at the anode, and a modified RANEY® Ni on Ni fiber paper at the cathode. The cell produced a stable performance of 1000 mA cm⁻² at 1.85 V at 60 °C in 1 M KOH, with voltage decay of less than 1 μV h⁻¹ over 10 000 h.²¹⁰ Regarding the industrial requirement of limiting the performance loss of the catalysts to less than 10% of the initial performance at end-of-life (from 1.85 V to 2.035 V): the authors assumed that a commercial system must maintain >90% of initial performance (at the end the system will be at 2.035 V), then 1 μV h⁻¹ degradation gives (2.035–1.85)/10⁻⁶ = 185 000 h = ∼21 years lifetime. This met the industrial performance requirement.

In another report, a cell consisting of a RANEY®-type NiMo cathode, RANEY®-Ni anode and a m-PBI inter-electrode separator membrane (40 μm thick) developed by Kraglund et al.²¹² generated 1700 mA cm⁻² at 1.8 V in 24% KOH solution at 80 °C. However, the cell failed after 120 h of stability test due to mechanical failure of the membrane. When using an 80 μm thick membrane, the failure was observed at roughly double the lifetime of 230 h.

Zhou and colleagues¹⁷⁸ constructed a highly efficient electrolyser with a (Ni,Fe)OOH catalyst at the anode and a robust MoNi₄ catalyst at the cathode. Both electrodes used Ni foam as a substrate. The cell produced current densities of 500 mA cm⁻² and 1000 mA cm⁻² at only 1.586 V and 1.657 V, respectively, in 1 M KOH at room temperature. It sustained that performance with only a small voltage drift for 40 h when operated at constant current densities of 500 mA cm⁻², 1000 mA cm⁻², and 1500 mA cm⁻².

In a separate study, Liang et al.¹⁷⁹ combined a Ni_0.8Fe_0.2 amorphous oxyhydroxide nanowire array at the anode with a Ni nanowire array at the cathode in an alkaline electrolyser, that produced current densities of 500 mA cm⁻² and 1000 mA cm⁻² at low cell voltages of 1.70 V and 1.76 V, respectively, in 1 M KOH at room temperature.

By using a commercially available polyethersulfone (PES) ultrafiltration membrane with 0.2 μm pore size and 140 μm thickness as a separator between two Ni-based electrodes in a zero-gap configuration, Schalenbach et al.²¹³ reported a high current density of 2000 mA cm⁻² at a cell voltage of 1.85 V at 80 °C in 30 wt% aqueous KOH solution. However, the voltage increased by 20 mV during an 8 h stability test at 500 mA cm⁻² at 80 °C. The performance recovered after the cell was shut down for the night. This could be explained by the formation of bubbles, which covered the electrodes, resulting in masking of the active sites of the electrodes. The performance only fully recovered when the gas bubbles were released from electrode surface after the cell was shut down.

Interestingly, using a similar PES membrane but different pore size (8 μm), a unique concept of water electrolysis, namely, the capillary-fed electrolyser developed by Swiegers and colleagues,⁸ produced the highest performance yet reported for an electrolysis cell at 80–85 °C. In this configuration, water was supplied to both electrodes via capillary-induced transport up and along the PES membrane. Commercial catalysts were employed, with Pt/C on Sigracet carbon fiber paper at the cathode and NiFeOOH with PTFE on Ni mesh at the anode. This cell exhibited outstanding performance at 500 mA cm⁻² and 1000 mA cm⁻² requiring cell voltages of only 1.506 and 1.575 V, respectively, at 85 °C in 27 wt% aqueous KOH; this equated to a cell energy efficiency of 98% and 93% (HHV) respectively. In addition, when the cell voltage was fixed at the thermoneutral voltage for water electrolysis, 1.47 V at 85 °C, which equates to 100% energy efficiency (HHV), the cell produced a constant current density of 300 mA cm⁻². No water electrolysis, either alkaline or PEM, has ever produced 300 mA cm⁻² at 100% energy efficiency, HHV. The capillary-fed cell also demonstrated sustained stable performance from 1 working day to 30 days continuously at 80 °C and room temperature, respectively.

Very early on, Schiller et al.¹⁴³ tested a 10 kW electrolyser system with activated Mo-containing RANEY® nickel cathodes and RANEY® nickel/Co₃O₄ matrix composite anodes in a bipolar electrolyser with zero-gap configuration. The cell achieved 300 mA cm⁻² and 500 mA cm⁻² at 80 °C in 30 wt% KOH solution, at 1.60 and 1.67 V, respectively. The electrolyser stack was also tested in constant and intermittent current mode at 300 mA cm⁻², 80 °C over 15 000 h duration with stable performance.

Asahi Kasei demonstrated a medium size zero-gap configuration (0.3 m² per cell), which, arguably, constitutes the state-of-the-art in commercial alkaline electrolysis. The cell operates at up to 1000 mA cm⁻² and required less than 1.8 V to produce 600 mA cm⁻².²¹⁴

Table 5 summaries the state-of-the-art HER, OER and overall water splitting performance.

Table 5 Tabulation of the highest performing HER, OER catalysts and overall water splitting performance to date^a,b

	Cathode catalyst	Anode catalyst	Electrolyte	Temperature	Current density	Durability	Ref.
a The HER and OER catalysts were studied in 3-electrode configurations. The catalysts listed in the ‘Overall water splitting’ section were studied in 2-electrode cells. b AHNA = Amorphous oxyhydroxide nanowire arrays; CF = Carbon fibre; CNT = Carbon nanotube; HC = Hydroxide carbonate; LDH = Layered double hydroxide; NF = Nickel foam. c The value is calculated from the curves shown in literature.
HER (cathode)	20 mg cm^−2 NiMo		30% KOH	70 °C	1000 mA cm^−2 @ 80 mV	78.7 mg cm^−2 NiMo, 70 °C, 600 h @ 1000 mA cm^−2, 16.6^c μV h^−1	96
	3.4–3.6 mg cm^−2 NiMoFe		8.32 M NaOH	80 °C	600 mA cm^−2 @ 230 mV	1500 h, 80 °C @ 300 mA cm^−2, 9^c μV h^−1	93
	NiMo (75 : 25 wt%)		6 M KOH	80 °C	300 mA cm^−2 @ 185 mV	1500 h, 80 °C @ 300 mA cm^−2, negligible degradation	94
	40 mg cm^−2 NiMo		1 M KOH	40 °C	400 mA cm^−2 @ 110 mV	8 h, water, 70 °C @ 400 mA cm^−2, 6250^c μV h^−1	99
	43.4 mg cm^−2 MoNi4/MoO2@Ni		1 M KOH	23 °C	500 mA cm^−2 @ 65 mV	10 h, 23 °C @ 200 mA cm^−2, 500^c μV h^−1	66
	2 mg cm^−2 NiMo–NH3/H2		1 M KOH	23 °C	500 mA cm^−2 @ 107 mV	20 h, 23 °C @ 0.1 V (∼410 mA cm^−2), negligible degradation	100
	NiMoN		1 M KOH	23 °C	500 mA cm^−2 @ 127 mV	24 h, 23 °C @ 127 mV (∼500 mA cm^−2), ∼ −0.7^c mA cm^−2 h^−1	207
	35 mg cm^−2 RANEY® Ni		28% KOH	80 °C	500 mA cm^−2 @ 100 mV	NA	215
	35 mg cm^−2 Ni–Cr RANEY®		28% KOH	80 °C	500 mA cm^−2 @ 80 mV	NA
	8 mg cm^−2 NiO/Ni-CNT		1 M KOH	23 °C	100 mA cm^−2 @ 100 mV	2 h, 23 °C @ 20 mA cm^−2, no degradation	^216
	8 mg cm^−2 FeP/Ni2P		1 M KOH	23 °C	350^c and 1000^c mA cm^−2 @ 200 and 270 mV	24 h, 23 °C @ 100 mA cm^−2, no degradation	119
	1.45 mg cm^−2 Pt/C/CF		1 M KOH	23 °C	20 mA cm^−2 @ 45 mV	NA	81
OER (anode)		2.78 mg cm^−2 NiFe LDH on Fe foam	1 M KOH	23 °C	500 and 1000 mA cm^−2 @ 300 and 340 mV	5000 h, 23 °C @ 1000 mA cm^−2, negligible degradation	185
			10 M KOH		500 and 1000 mA cm^−2 @ 257 and 280 mV	1050 h, 23 °C @ 1000 mA cm^−2, negligible degradation
		4.0 mg cm^−2 NiFe(OOH)/NF	1 M KOH	23 °C	500 and 1000 mA cm^−2 @ 259 and 289 mV	44 h, 23 °C @ 500 mA cm^−2, 318^c μV h^−1	178
						44 h, 23 °C @ 1000 mA cm^−2, 1341^c μV h^−1
		2.5 mg cm ^−2 Ni0.8Fe0.2-AHNA	1 M KOH	23 °C	500 and 1000 mA cm^−2 @ 248 and 258 mV	120 h, 23 °C @ 5 constant current densities (10, 100, 500, 1000 and 10 mA cm^−2), negligible degradation	179
		2.8 mg cm^−2 NiFe2O4/NiFe LDH/NF	1 M KOH	23 °C	500 and 1000 mA cm^−2 @ 242 and 265 mV	20 h, 23 °C @ 242 mV (∼500 mA cm^−2), 97% retaining (∼−0.75^c mA cm^−2 h^−1)	181
		NiFeOx on	1 M KOH	23 °C	1000 mA cm^−2 @ 300 mV	10 h, 10 M KOH, 23 °C @ 480 mV (∼1400 mA cm^−2), negligible degradation	217
		Fe foam
		NiFe/NiCo2O4/NF	1 M KOH	23 °C	1200 mA cm^−2 @ 340 mV	10 h, 23 °C @ 50 mA cm^−2, negligible degradation	183
		Ni(Fe)OxHy on stainless steel	1 M KOH	23 °C	500 and 1000 mA cm^−2 @ 280 and 303 mV	82 h, 23 °C @ 500 mA cm^−2, 182.9^c μV h^−1	189
		NiFeOH@Ni3S2/NF	30% KOH	23 °C	500 and 1000 mA cm^−2 @ 370 and 469 mV	50 h, 1 M KOH, 23 °C @ 500 mA cm^−2, negligible degradation	192
						50 h, 30% KOH, 23 °C @ 1000 mA cm^−2, negligible degradation
		8 mg cm^−2 Fe(PO3)2/Ni2P/NF	1 M KOH	23 °C	500 and 1705 mA cm^−2 @ 265 and 300 mV	20 h, 23 °C @ 500 mA cm^−2, negligible degradation	206
		8 mg cm^−2 FeP/Ni2P	1 M KOH	23 °C	1000 mA cm^−2 @ 293 mV	24 h, 23 °C @ 100 mA cm^−2, no degradation	119
		NiMoN@NiFeN/NF	1 M KOH	23 °C	500 and 1000^c mA cm^−2 @ 337 and 370 mV	24 h, 23 °C @ 337 mV (∼500 mA cm^−2), −0.775 mA cm^−2 h^−1	207
		NiFe-HC	1 M KOH	23 °C	400 mA cm^−2 @ 370 mV	NA	176
Overall water splitting	NiMo–NH3/H2	Fe–NiMo–NH3/H2	1 M KOH	80 °C	1000 mA cm^−2 @ 1.57 V	25 h, 80 °C @ 50 mA cm^−2, negligible degradation	100
						25 h, 80 °C @ 500 mA cm^−2, degradation observed
	FeNiCo on stainless-steel fiber	NiFe2O4 on stainless-steel fiber	1 M KOH	60 °C	1000 mA cm^−2 @ 1.9 V	2000 h, 60 °C @ 1000 mA cm^−2, 5 μV h^−1	212
	RANEY® Ni on Ni fiber paper	NiFe2O4 on stainless-steel	1 M KOH	60 °C	1, 000 mA cm^−2 @ 1.85V	10 000 h, 60 °C @ 1000 mA cm^−2, 1 μV h^−1	200
	MoNi4	(Ni,Fe)OOH	1 M KOH	23 °C	500 and 1000 mA cm^−2 @ 1.586 and 1.657 V	40 h, 23 °C @ current densities of 500, 1000 and 1500 mA cm^−2, small degradation	178
	Ni nanowire array	Ni0.8Fe0.2-AHNA	1 M KOH	23 °C	500 and 1000 mA cm^−2 @ 1.70 and 1.76 V	24 h, 23 °C @ 500 mA cm^−2, negligible degradation	179
	Pt/C	NiFeOOH/PTFE	27% KOH	80 °C	300 mA cm^−2 @ 1.47 V	30 days, 23 °C @ 400 mA cm^−2, negligible degradation	8
					1000 mA cm^−2 @ 1.59 V
	RANEY®-type-NiMo	RANEY® Ni	24% KOH	80 °C	1700 mA cm^−2 @ 1.8 V	The cell failed after 120 h of operation at 80 °C	212
	Ni mesh	NiFe-HC	6 M KOH + 2 M K2CO3 + 0.5 M NaCl	80 °C	NA	1500 h, 80 °C @ 400 mA cm^−2, negligible degradation	179
	Mo-containing RANEY® Ni	RANEY® Ni/Co3O4	30% KOH	80 °C	300 mA cm^−2 @ 1.6 V	15 000 h, 80 °C @ 300 mA cm^−2, negligible degradation	143
	FeP/Ni2P	FeP/Ni2P	1 M KOH	23 °C	500 and 1000 mA cm^−2 @ 1.72 and 1.78 V	40 h, 23 °C @ 500 mA cm^−2, negligible degradation	119

---

8. Conclusions and perspectives

‘Green’ hydrogen has been identified as an alternative energy carrier to fossil fuels, that could help achieve global net-zero carbon emission in the coming decades. However, green hydrogen is, today, still expensive due to the low energy efficiency of its production process. Reducing the cost of green hydrogen is critical for effective translation of hydrogen solutions in the transport and energy sector. Improving the efficiency of alkaline water splitting via high-performing catalysts offers a viable means of reducing the cost of green hydrogen.

It is vital however, to develop electrocatalysts that are suitable for industrial electrolysers. That is, they need to be high performing under industrially relevant conditions, which include high current density (for example, 200–700 mA cm⁻²), long-term durability without significant degradation to end-of-life, and at desired pressures and temperatures. Despite the efforts directed to electrocatalyst development, the absolute necessity of durability in commercial applications (10 years or more) has largely been overlooked in previous studies. Studies of catalysts at low current densities over short periods of testing, during which degradation may often be observed, as has been widely practiced in the field, are, frankly, not useful in this regard.

This review has summarised recent progress in the development of such electrocatalysts for the HER and OER, with the aim of identifying systems for high-efficiency water electrolysis in alkaline environment. The work has focused on materials, synthesis methods, electrochemical performance and the related mechanism of activity enhancement.

With regard to hydrogen evolution, Pt-group catalysts stand as the state-of-the-art electrocatalyst due to their low overpotential for HER. However, the high cost and scarcity of precious metals limit their application in industrial water splitting. A move toward developing earth-abundant materials for HER is still required. These materials should exhibit not only high catalytic activity but be easily scaled-up for practical application in alkaline medium.

NiMo-based alloys or its alloy doped with Fe constitute the most promising alternatives to noble metal-based catalysts for the HER. These catalysts exhibit high catalytic activity and stable performance at the operating conditions of commercial electrolysers.

There has been great interest in developing a high performing OER electrocatalyst due to the sluggish kinetics of the anode, which results in a significant loss of energy. Many studies have focused on Ni, Fe, Co, and Mn oxide materials and their combinations since these metal oxide electrocatalysts generally exhibit excellent catalytic activities for oxygen evolution in alkaline media. Among them, NiFe-based catalysts have been shown to exhibit great potential in large-scale commercial water electrolysis due to their easily scalable fabrication and stable performance at large current densities for long times. It appears to the authors that NiFe oxides, double hydroxides, and oxyhydroxides offer the most promising OER catalysts for efficient, high-performing oxygen evolution.

Non-oxide catalysts in the form of transition metal phosphides and selenides also demonstrate enhanced catalytic activity for both the HER and OER, showing promising potential for bifunctional electrocatalysts that facilitate overall water splitting.

Although intensive efforts have been made to develop high-performing catalysts that increase the efficiency of alkaline electrolysis, some challenges still need to be overcome, including:

(i) 3D electrode fabrication: generally, catalysts can be easily prepared by coating on a conductive substrate with or without a polymer binder, followed by an annealing process at high temperature to improve adhesion between catalyst and substrate. Growing electrocatalyst nanoarrays as 3D structures on a catalyst support is considered as an alternative, more preferred solution, which not only increases the number of active sites but accelerates mass and charge transfer processes; it also enhances electrical conductivity between the catalyst and substrate. However, to be practically useful in commercial electrolysers, such fabrication processes need to be amenable to high-volume electrode manufacturing techniques. While significant and promising progress has been made, further work is needed on fabrication methods that impart the electrode structure with optimum catalytic performance whilst also being capable of mass manufacturing for commercial electrolysers.

(ii) Developing standards for catalyst evaluation relevant to commercial-scale electrolysers: significant differences in catalytic performance may be observed by different groups when examining catalysts having the same composition and structure. Such differences may be due to different fabrication methods, catalyst loadings, reactant electrolytes, testing conditions, and so on. Therefore, benchmarking and assessment of the performance of catalysts under industrial-relevant conditions is very much needed. Newly developed catalytic electrodes should sustain current densities of 200–700 mA cm⁻² or higher (1000 mA cm⁻²) without electrochemical degradation (at least for several days) in 25–30% KOH electrolyte at elevated temperatures (60–90 °C).¹⁴ Additionally, stability and durability tests need to be taken into consideration in a more efficient and time-saving manner. The trade-off between catalytic performance and electrode stability needs to be explicitly considered.

(iii) The economic cost of catalysts/electrodes in industrial-scale applications: translation of fabrication methods from the laboratory to industry needs to consider the cost (and practicality) of large-scale electrode production for commercial electrolysers. Catalytic electrodes must be manufacturable by low-cost procedures that require little time to complete.

In fact, developing highly performing catalysts should be combined with improvements in the inter-electrode separator membrane as well as changes in cell design to obtain further reductions in cell voltage for overall water splitting.

Abbreviations

AEM	Anion Exchange Membrane
CFP	Carbon Fiber Paper
CNT	Carbon Nanotube
COP26	Conference of the Parties 26
ECSA	Electrochemically Active Surface Area
GCE	Glassy Carbon Electrode
HER	Hydrogen Evolution Reaction
HHV	Higher Heating Value
LSV	Linear Sweep Voltammetry
LDH	Layered Double Hydroxide
NF	Nickel Foam
OER	Oxygen Evolution Reaction
PES	Polyether Sulfone
RHE	Reversible Hydrogen Electrode
SHE	Standard Hydrogen Electrode
STM	Scanning Tunneling Microscopy
TOF	Turnover Frequency

Conflicts of interest

The authors have no interests to declare in respect of this manuscript.

Acknowledgements

The authors gratefully acknowledge support from the Australian Renewable Energy Agency (ARENA), Grant number 2018/DM015 (entitled: Ammonia Production from Renewables) (G. F. S. & G. G. W.). This activity received funding from ARENA as part of ARENA's Research and Development Program – Renewable Hydrogen for Export. Support from the Australian Research Council Centre of Excellence Scheme (grant number CE140100012) (G. G. W. and others) and the Australian National Fabrication Facility (ANFF) Materials Node is also acknowledged. The authors acknowledge the assistance of the University of Wollongong Electron Microscopy Centre. This research used equipment funded by the Australian Research Council—Linkage, Infrastructure, Equipment, and Facilities grant LE160100063.

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