AWE is a well-established technology for hydrogen production and this phenomenon was first found by Troostwijk and Diemann in 1789 [12]. AWE usually operates at lower temperatures between 30-90°C and uses alkaline KOH/NaOH aqueous solution with a typical concentration of 20-30% as the electrolyte. In alkaline electrolysers, the diaphragm and porous nickel-based materials are usually used as asbestos and electrode. Alkaline electrolysis is a mature process, which only needs low investment and operation costs. However, the AWE usually exhibits a low operating current density of 200-400 mA cm⁻² and an energy efficiency of approximately 62-82% [15]. Moreover, the pressure between the anode and cathode sides needs to keep balanced, avoiding the hydrogen or oxygen generated by the electrolysis penetrating the diaphragm to the other side and resulting in the explosion risk.

SOE has received an enormous amount of attention because of its high efficiency and environmental friendliness, the needed electrical power could be provided by sustainable energy sources [16]. However, SOE works at high temperatures (500-850°C) and pressure, water is in the form of steam during the SOE process. The most commonly used electrolyte material is yttria-stabilized zirconia, the conventional high-temperature oxygen-ion conductor. However, the high temperatures result in the fast deterioration of catalytic performance, and achieving the long-term operation of SOE is a challenge. Meanwhile, the produced hydrogen can form a mixture with steam and needs an additional procedure to acquire high purity hydrogen.

PEM electrolysis has been known for over sixty years and was developed by General Electric, where a solid sulfonated polystyrene membrane is used as the electrolyte. This technology is also named polymer electrolyte membrane electrolysis. The proton exchange membrane acts as both the gas separator and the electrolyte. In the electrolysis process, only deionized water is injected into the cell without any electrolytic additives. There are a variety of advantages of PEM electrolysis, such as high current density, greater energy efficiency, low gas permeability, wider operating temperatures (20-80°C) [12], easy handling and maintenance. However, the popular electrocatalysts for PEM electrolysis are precious metals (Pt, Ir, Ru), the exorbitant cost of these precious metal makes PEM electrolysis a relatively higher investment than alkaline electrolysis. In this regard, researchers have paid tremendous efforts for reducing the cost and improving the intrinsic activity of PEM electrocatalysts. As shown in Table 1, listing the typical characteristics of three water electrolysis technologies.

The predicament for HER in PEM electrolysis is the development of high-activity and stability catalysts for the cathode. Presently, platinum (Pt) and Pt-based materials are recognized as the state-of-the-art electrocatalysts for the cathode side in PEM electrolysers [17,18], which exhibit decent electrochemical activity for the HER in acidic electrolytes. However, the large-scale deployment of these catalysts is constrained by the high cost and limited reserves. For this, considerable efforts have been devoted to exploring various methods to reduce Pt mass loading and exploit other Pt-free electrocatalysts [19,20,21,22,23,24,25,26,27]. Lim and co-workers [28] prepared a Pt@N-containing carbon core-shell catalyst on carbon nanofibers (Pt@CS/CNF), which exhibited excellent catalytic activity and durability. They found that the graphitic N and pyridinic N on the carbon shell could not only offer additional catalytic active sites for HER, but also impede the active Pt core from dissolution and agglomeration, improving the stability (Fig. 2a-d). Zhong and co-workers [21] adopted an ultralow-temperature solution method to prepare a single atom Pt anchored on carbon nanotubes (SA-Pt-CNTs), which reduced Pt usage and improved the intrinsic HER activity in acidic media, a low overpotential of only 41 mV to achieve the current density of 10 mA cm⁻². The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) demonstrates Pt atoms were atomically dispersed. Density functional theory (DFT) demonstrated the high electrocatalytic activity is attributed to the interaction between CNTs and Pt atoms, which leads to the downshift of the d-band center and facilitates the H desorption (Fig. 2e-g). Xu et al. [22] rationally designed atomically dispersed Pt anchored on amorphous MoO_x (Pt-SA/α-MoO_x), which unfolded a low overpotential of 19 mV at 10 mA cm⁻² and excellent mass activity of 52.0 A mg_Pt⁻¹ at the overpotential of 50 mV in 0.5 M H₂SO₄. Additionally, it also exhibited a low Tafel slope of 123 mV dec⁻¹ and long-time durability. DFT results revealed that the remarkable performance is attributed to the synergistic effect between unsaturated Pt atoms and amorphous MoO_x, which are beneficial for atomic hydrogen adsorption. Xia and co-workers [29] reported a series of carbon-supported ultrasmall high-entropy alloy nanoparticles (us-HEA NPs) through a general and ultrafast chemical co-reduction method. In 0.5 M H₂SO₄ electrolyte, this us-HEA NPs exhibited an eminent mass activity of 28.3 A mg⁻¹ at -0.05 V, which shows a promising prospect for HER in PEM electrolysis.

In recent years, various transition metal dichalcogenides have been developed as acidic HER electrocatalysts [30], such as MoS₂ [31,32,33,34,35], WS₂ [36], MoSe₂ [37,38,39], and WSe₂ [40], which have shown to be splendid cathodic materials for HER. Nørskov and co-workers [25] investigated the HER performance of MoS₂ and fabricated an MEA based on a Nafion proton exchange membrane, which showed a lower current density of 10 mA cm⁻² at the overpotential of 175 mV. Kim et al. [41] reported a simple one-step electrodeposition method to prepare amorphous molybdenum sulfide (MoS_x) on carbon paper and used as the gas-diffusion layer for PEM water electrolysis, which displayed acceptable cell performance. Because sulfur atoms on the catalyst surface with different binding energy values, they could extremely affect the intrinsic activity of MoS_x and facilitate the HER performance. The 1T phase of TMDs possesses high electrical conductivity and a larger number of active sites compared to the 2H phase [42,43,44], which could result in a higher charger transfer rate in the electrochemical process. Recently, Kang and co-workers [45] successfully synthesized Mo_1-xV_xSe₂ alloy nanosheets through a colloidal reaction, which could lead to the phase transition from a semiconducting 2H phase MoSe₂ to the metallic 1T phase VSe₂ (Fig. 3). DFT calculations suggested the decent electrocatalytic activity attributed to the V and Se vacancies, which are beneficial for the phase transition from the 2H phase to the 1T phase and forming the intermediates in HER. Additionally, various cation-doping is also an effective strategy to tune the surface electronic structure of catalysts and optimize the hydrogen adsorption free energy, improving the electrocatalytic HER activity [46,47]. Morozan et al. [24] adopted microwave irradiation or heat treatment in a furnace under inert and reductive atmospheres to prepare the bimetallic FeMoS catalyst, which presented a low overpotential of 140 mV at the current density of 10 mA cm⁻². Combining with carbon nanotubes (CNTs), the composite catalyst could operate at 500 mA cm⁻² using a voltage of less than 300 mV compared to using Pt as the cathode catalyst in PEM electrolyser. Zhao et al. [48] prepared an ultrastable Pd-doped MoS₂ material and supported on a vertical graphene network (VGN@Pd-MoS₂), which exhibited a stable operation over 1000 h at 10 mA cm⁻² and provide a prospect to replace Pt for PEM electrolysis.

Apart from Pt and TMDs, researchers also devoted to developing various alternative electrocatalysts for the hydrogen evolution reaction in acidic electrolytes [26,27,49,50,51]. Li and co-workers [26] proposed a universal strategy to prepare a series of Ni-activated transition metal carbides (TMCs) on Ni foam coated with graphene (Ni-GF) and further optimize the HER activity of these TMCs. After the introduction of Ni atoms, these Ni-GF/TMCs exhibited much reduced overpotentials and intriguing durability toward HER, as displayed in Fig. 4a-c. They found that the superior HER performance hinges on the unique structural characteristics and optimized electronic synergy of Ni-activated TMCs. The doping of Ni atoms on the surface of TMCs could effectively regulate the d-electron structure, which can not only significantly enhance the exposed active sites, but also improve the intrinsic catalytic activity (Fig. 4d, e). Additionally, the high specific surface areas and unique 3D skeleton structure could facilitate a fast electron transfer rate and efficient release of hydrogen bubbles. Sapountzi et al. [50] prepared highly crystalline FeP NPs with predominantly exposed [010] facets on a conductive carbon support and presented high activity in HER. They also assembled a full-cell PEM water electrolysis by using FeP as cathode and a commercial IrRuO_x as anode, which could reach a current density of 0.2 A cm⁻² at a voltage of 2.06 V and operate steadily for up to 36 h continuously. Ito and co-workers [27] further improved the acid stablility through graphene-covering approach and investigated the effect of graphene-covering layers on HER activities of Ni and Cu. They found that the graphene could optimize the HER activity and impede the corrosion, and the ability of protons to penetrate graphene manipulates the HER performance of these graphene-covered non-noble metals.

Because of the pivotal role of OER in water electrolysis, the energy community has put substantial efforts into exploiting admirable OER catalysts. Due to the acidic and strongly oxidative environments, developing high-active and robust electrocatalysts for OER in PEM electrolysers is an arduous challenge. To date, Ir-based and Ru-based catalysts are the most popular OER electrocatalysts in acid. Ru-based catalysts exhibit super OER activity but unsatisfactory stability compared with Ir-based catalysts. In this section, we systematically summarize the reports about Ir-based and Ru-based catalysts in the past decades (Table 2).

Ir metal catalysts Ir metal is a vital electrocatalyst for OER under an acidic environment and has gained explosive interest because of its low overpotentials and unique stability. The catalytic performance is largely credited to the morphology, particle size, crystal structure, and substrate materials. As a result, various Ir-metal materials have been explored to optimize the OER activity and reduce the amount of precious metal.

Decreasing particle size or preparing ultrathin structure is an effective method to maximize surface area and expose more active sites [80], beneficial for improving the catalytic activity [81,82]. Ultrafine monodisperse Ir NPs with narrow size distribution and high dispersion were successfully synthesized through a colloidal method [52]. Due to the ultrafine particle size and uniform dispersion, this catalyst exhibited startling catalytic performance for acidic OER and exceeded commercial IrO₂ catalyst. Carbon nanobowls supported ultrafine Ir nanocrystal (Ir@HEDP/CNBs) catalyst was prepared by Chen and co-workers [53], Fig. 5a-c show the ultrafine size and homogeneous distribution of Ir nanocrystal, as well as the high surface area and conductivity of CNBs, rendering it low OER overpotential of 290 mV at 10 mA cm⁻² and excellent stability. Single atom catalysts (SACs) can realize the maximum atom utilization efficiency and reduce the amount of precious metal [83,84]. In view of the severe activity decay of SACs in harsh acidic solutions, high-stable Ir SAC onto Fe NPs coated with nitrogen-doped carbon nanotubes (Ir-SA@Fe@NCNT) was prepared, which realized the ultralow Ir loading and superior OER performance with an overpotential of only 250 mV to reach 10 mA cm⁻² in acidic solution. As shown in Fig. 5d-f, Qiao and co-workers [55] also reported a short-range ordered Ir SA integrated into Co oxide spinel structure and exhibited much higher acidic OER activity and excellent stability. A 3D Ir superstructure catalyst consisting of ultrathin Ir nanosheets to increase the accessible active sites and active surface area was reported by Huang and co-workers [56]. This 3D Ir catalyst exhibited an unexpectedly low overpotential of 240 mV and intriguing durability with ignorable voltage degradation (Fig. 5g-i).

A robust and appropriate catalyst support is also important to improve activity and stability. Graphene, with decent electrical conductivity and chemical stability, high mechanical strength, and flexibility, has been used as a promising substrate material in diverse fields. Velasco-Vélez and co-workers [85] prepared electrochemically active Ir NPs on graphene and improved the acidic OER activity and stability. Shao and co-workers further adopted graphitic carbon nitride (g-C₃N₄) and nitrogen-doped graphene (NG) as a substrate to anchor monodispersed Ir NPs (Ir/g-C₃N₄/NG). This g-C₃N₄/NG substrate could effectively prevent the aggregation of Ir NPs and improve the reaction kinetics. The Ir/g-C₃N₄/NG catalyst can reach the current density of 10 mA cm⁻² at a low overpotential of 287 mV, much superior to the pure Ir. Montmorillonite (MMT, (Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O) is a natural mineral, which is easily available and cost-efficient. Boshnakova et al. [86] proved that MMT is a distinguished substrate to replace conventional carbon supports, the unique morphology and layered structure are conducive to enhancing the accessible active sites and improving the stability.

The preparation of catalysts with abundant porous structures or defects is an effective approach to expose more active surface area to the electrolyte environment and enhance the amount of the active sites. As shown in Fig. 6, Yamauchi and co-workers [58] reported an unprecedented type of 2D mesoporous Ir nanosheets through the formic acid assisted method, which served as both a reducing agent and a shape-directing agent. They found that the mesoporous structure was robust and this mesoporous Ir nanosheets also presented unrivaled electrocatalytic OER activity. This work opened a new dimension in the preparation of other porous 2D materials and further exploration of the paramount of porous structure on 2D materials for OER performance. Amorphous catalysts are often prepared due to their abundant active sites, inherent disordering, and an unsaturated coordination structure, which usually displayed more impressive catalytic performance than corresponding crystalline materials [78,87,88]. Wu et al. [59] prepared a series of amorphous metal catalysts through a general and facile method. They found that the amorphous Ir nanosheets depicted intriguing acidic OER activity than crystalline Ir nanosheets and benchmark IrO₂ catalyst, which only required an overpotential of 255 mV to reach the current density of 10 mA cm⁻² and exhibited robust durability with ignorable activity alleviation even after 5000 cycles in 0.1 M HClO₄ solution.

Ru metal catalysts Ruthenium (Ru) has been evaluated to be one of the promising active materials for acidic OER by virtue of its appropriate adsorption strength toward oxygenated intermediate species. Additionally, Ru is abundant in natural resources and cheaper than Ir metal. Unfortunately, the reported Ru-based catalysts are unstable, which is likely due to the oxidation of lattice oxygen and generation of oxygen vacancy, leading to the over-oxidation of Ru-based materials [61,77]. To overcome this limit, great efforts have been dedicated to exploiting robust and efficient Ru-based electrocatalysts for OER in acid media [62,89,90].

Recently, Ru SACs have ignited much interest and exhibited extraordinary performance for acidic OER. It has demonstrated that the substrate and coordination environment have a significant effect on the electrocatalytic activity. As displayed in Fig. 7, a Ru SAC with the Ru₁-N₄ configuration anchored on nitrogen-carbon support displayed an exceptionally intrinsic activity, the mass activity could reach 3571 A g_metal⁻¹ and a high turnover frequency of 3348 O₂ h⁻¹, as well as a low overpotential [60]. Moreover, it also revealed decent stability and could operate steadily for 30 h in 0.5 M H₂SO₄ solution. Operando synchrotron radiation X-ray absorption fine structure (XAFS) spectroscopy and Fourier transform infrared (SR-FTIR) spectroscopy validated that adsorbed oxygen atom on Ru site to form O-Ru₁-N₄ site under the operating condition. The oxygen adsorption could downshift the Ru 4d band and increase the average valence state of Ru, which is responsible for reducing the limiting reaction barrier. Wu and co-workers [61] constructed a series of PtCu_x alloys that supported Ru SACs through sequential acid etching and electrochemical leaching. Ru₁-Pt₃Cu as the best catalyst in this work, only need a low overpotential of 220 mV to reach a current density of 10 mA cm⁻² and displayed ultra-long lifetime than benchmark RuO₂. From XPS and operando XAFS results, they demonstrated the valence state of Ru has no evident variation even after the stability test, preventing the over-oxidation of Ru. Further, they captured the OOH intermediate during the electrochemical process by using in-situ attenuated total reflection infrared (ATR-IR) spectroelectrochemical measurements, indicating the adsorbate evolution mechanism (AEM) instead of lattice oxygen evolution reaction (LOER) during the acidic OER reaction over Ru₁-Pt₃Cu. DFT calculations suggested that with the increase of Pt to Cu ratio, the compressive surface strain would lead to the upshift of the d-band center of Ru and enhancement of oxygen adsorption, resulting in the deteriorated performance.

Interface engineering is also an admirable approach to modulating the electronic environment, exposing abundant active sites, promoting electron transfer, and optimizing the electrocatalytic activity [91,92]. Based on this, Mu and co-workers [62] prepared a laminar Ru/RuS₂ heterostructure catalyst through the synchronous reduction and sulfurization under the eutectic salt system for the first time. Theoretical calculation results suggested engineering of this heterostructure would endow it with higher electrical conductivity, charge redistribution over interfaces, and optimized adsorption strength of key intermediates, sequentially reducing the thermodynamic energy barriers.

The OER catalytic performance of Ru nanocrystals with different structures was investigated to explore the relationship between catalytic activity and crystal structures [63]. It is reported that the face-centered cubic (fcc)-Ru octahedral nanocrystals exhibit the best performance compared with fcc-Ru truncated cubes, fcc-Ru particles, and conventional hexagonal close-packed (hcp)-Ru particles.

Metal alloy catalysts Although the OER performance of mono-metallic Ir or Ru has achieved some progress, the activity and stability are still far from practical application. Alloying is an outstanding approach to significantly alter the electronic structures and optimize the adsorption energy of reaction intermediates on the catalyst surface, which in turn extremely improves the OER activity. Simultaneously, the alloy strategy could noticeably reduce the usage of precious metals Ir and Ru.

A wide range of Ir-based alloy structures has been synthesized and applied in acidic OER, such as nanodendrite [93,94], thin film [95], nanowires [64], nanotube [96], and porous structure [65,97]. For example, a series of IrM (M = Ni, Co, Fe) nanoporous nanowires (IrM NPNWs) were prepared through a novel eutectic-directed self-templating method [64], the subsequent dealloying process introduced the abundant nanoporous structure. The IrM NPNWs after the introduction of Fe, Co, and Ni all have an eminent performance in 0.1 M HClO₄, of which the Ni is the most promising element and IrNi NPNWs displays the finest catalytic performance and durability. DFT calculations confirmed that alloy formation can shift down the d-band center and alter the adsorption strength of O-related intermediates, elucidating the relationship between the d-band center and bond strength. Apart from promoting the intrinsic activity of catalysts, increasing the number of active sites is another typical strategy to optimize catalytic performance via morphology and structure control. As shown in Fig. 8a-c, a nanovoid incorporated Ir₃Cu metallic aerogel was formed by a galvanic displacement reaction and gelation process [65]. The electrochemical active surface area (ECSA) derived from the double-layer capacitance of Ir₃Cu is much larger than that of commercial IrO₂ and other counterparts, revealing the much more active sites of Ir₃Cu toward OER. Additionally, the introduction of Cu weakens the binding energy of O-related species and promotes mass transfer. Downsizing the catalyst NPs diameter is also a general method to increase the specific surface area and active sites [66,67]. A scalable microwave-assisted method was used to prepare the ultrasmall (sub-10 nm) composition-tunable Rh-Ir alloy NPs [67], which displayed a significant enhancement of acidic OER performance to pure Ir NPs. Theoretical studies demonstrated that the introduction of foreign Rh atoms could regulate the electronic structure and strain, which could weaken the adsorption strength of oxygen-containing intermediates. The mass activity of Rh₂₂Ir₇₈ NPs could reach 1.17 A mg⁻¹_Ir at the overpotential of 300 mV and achieve a threefold increase compared with pure Ir NPs. Ru-based alloys also provided opportunities for optimizing the intrinsic OER activity and improving the stability. For example, combinatorial synthesis and high throughput screening method were used to prepare a series of bimetallic Ru-M (M = Pd, Ir, Cu, Co, Re, Cr, Ni) alloys [98]. The Ru-Co, Ru-Cu, and Ru-Ir exhibited distinctly improved OER activities compared to pure Ru catalyst. Liu and co-workers [68] reported a simple hydrothermal method to synthesize ultrasmall IrRu nanoclusters supported on amorphous tellurium (Te) NPs (IrRu@Te), which was high conductive and acid-stable. This IrRu@Te catalyst displayed distinguished catalytic performance and decent stability, only needing an overpotential of 220 mV to deliver 10 mA cm⁻², benefitting from the strong electronic coupling between IrRu nanoclusters and Te, which could effectively reduce the kinetic barrier for OER and prevent the dissolution of IrRu nanoclusters.

In addition to preparing bimetallic alloys, designing ternary or multi-metallic alloys is a plausible approach to altering the electronic environment and increasing the number of surface active sites [69,73,99]. A co-reduction polyol method was used to prepare transition-metal (TM) doped RuIr nanocrystals catalysts [69], the introduction of different TMs has a significant effect on the catalytic performance (Fig. 8d-h). To deliver a current density of 10 mA cm⁻², the Co-RuIr only required a low overpotential of 235 mV and achieved a long-term chronopotentiometry measurement at 10 mA cm⁻². Various multi-metallic-based hollow NPs such as nanoframes and nanocages have been designed and acted as ideal catalysts [70,71,72], owing to their highly exposed accessible active sites and readily tunable components. A cubic Pt₃₉Ir₁₀Pd₁₁ nanocage catalyst was prepared and applied as a bi-functional catalyst toward oxygen reduction reaction (ORR) and OER in acidic media [70], with an average edge length of 12.3 nm and porous walls of 1.0 nm. This catalyst exhibited an unexpected activity and durability ascribed to the highly open structure and electron coupling. Park et al. prepared a robust IrNiCu catalyst with a double-layered nanoframe (DNF) structure. The unique porous morphology endowed it with remarkable activity and stability for acidic OER. This work provided a simple synthetic method for engineering high-active catalysts with complex nanoframe structures.

Metal oxide catalysts Compared to metal catalysts, metal oxide catalysts are more active and stable under acidic and oxidizing OER conditions. Hartig-Weiss et al. [100] reported an Ir oxide catalyst supported on high surface area and high-conductivity antimony-doped tin oxide, which displayed a very compelling OER activity and reduced the usage of Ir metal, benefitting from the uniform catalyst dispersion and strong metal-support interaction (SMSI). The oxidized Ir quantum dots supported on the surface of graphdiyne (IrO_xQD/GDY) are synthesized and showed incredibly acidic OER performance and robust stability [74], which only need a small overpotential of 236 mV to reach a current density of 10 mA cm⁻². In this work, GDY could effectively improve the electrical conductivity, facilitate the charge transfer, easy gas releasing and protect the Ir catalysts from corrosion. The early transition metals have a relatively high resistance to corrosion in strong acids [101], thus, the introduction of early transition metals may have a significant effect on the acidic OER performance. An early transition metal hafnium-modified IrO_x (IrHf_xO_y) was successfully prepared and displayed intriguing OER activity in both alkaline and acid electrolytes [75]. In order to improve the OER performance of RuO_x, a low-cost rutile-structured chromium-ruthenium oxide catalyst (Cr_0.6Ru_0.4O₂) was prepared through pyrolysis of Cr-based MOF [76], which unfolded an ultra-low overpotential of 178 mV at 10 mA cm⁻², as shown in Fig. 9. Combining XANES results and DFT calculations, they found that the presence of electron transfer from Ru to Cr, and the high valence Ru has the plausible ability for the OER. The introduction of Cr could tailor the electron structure of the RuO₂ phase and has a profound effect on optimizing the durability and activity. It is well known that RuO₂ has relatively higher acidic OER activity derived from its appropriate binding ability of oxygen-contained intermediates, but poor stability. Many reports have demonstrated that the unsatisfactory stability is ascribed to the over-oxidation of Ru-based materials and the formation of the soluble Ru^x>4 derivatives [77,102]. In order to improve the stability, Zhang and co-workers [77] prepared a bimetallic-doped RuO_x (W_0.2Er_0.1Ru_0.7O_2-δ) catalyst as shown in Fig. 10. They found that the introduction of W and Er elements could modulate the electron structure of RuO₂ and distinctly increase the formation energy of oxygen vacancy, resulting in an excellent durability of up to 500 h in 0.5 M H₂SO₄, as well as unrivalled OER performance. Because the amorphous/crystalline (a/c) heterostructure usually unravels unique electronic synergy and exceptionally catalytic performance, Zhang et al. reported a sodium-decorated a/c-RuO₂ electrocatalyst [78]. This a/c-RuO₂ catalyst displayed a high resistance to acid corrosion and excellent OER activities in all pH environments. Since the relatively higher activity of RuO₂ and much higher stability of IrO₂, preparing IrRu bimetallic oxide or core/shell structure is a promising strategy to form a catalyst with high activity and stability. A highly active and stable RuIrO_x nano-netcage catalyst was reported by Li and co-workers [79] through a dispersing-etching-holing method using MOF as the template. This open nano-netcage structure with abundant mesoporous could simultaneously improve the atom utilization of Ru/Ir and increase the ECSA. With the unique morphology and component, this RuIrO_x catalyst displayed incredibly startling activity with a low overpotential of 233 mV for reaching 10 mA cm⁻².

Perovskite oxides have also received much attention in acidic OER due to the low usage of precious metals, compositional flexibility, and structural stability [103,104]. A pseudocubic SrCo_0.9Ir_0.1O_3-δ perovskite catalyst with IrO₆ octahedra was designed and prepared [105]. This SrCo_0.9Ir_0.1O_3-δ catalyst displayed a high turnover frequency of 2.56 ± 0.15 s⁻¹ at 1.5 V, which was two orders of magnitude higher than that in the IrO₂ catalyst. They found that the structural reconstruction of SrCo_0.9Ir_0.1O_3-δ was generated during the anodic oxidation process and accompanied by the Sr and Co elements leaching. Therefore, the catalytic species are corner-shared and under-coordinated IrO_x octahedrons derived from SrCo_0.9Ir_0.1O_3-δ. The alkaline OER is catalyzed by SrRuO₃ thin film with relatively higher activity [106], but the performance is poor in acidic OER. Based on this, a Na-doped SrRuO₃ (Sr_1-xNa_xRuO₃) was synthesized by a wet-chemistry procedure [102], which displayed both improved activity and stability in an acidic solution. The enhanced OER mechanism is demonstrated based on XANES and theoretical calculations, Na incorporation increased the valence state of Ru, weakened Ru-adsorbate bonds, and improved the dissolution potentials.

The membrane electrode assemblies are made up of proton exchange membrane, ionomer solution, and catalysts. Proton exchange membrane is also called polymer exchange membrane, which plays a critical role in separating product gases, transporting protons, and supporting catalyst layers [12]. The commercial membrane must have advantages of high mechanical strength and proton conductivity, outstanding oxidative and thermal stability. At present, these membranes are mainly focused on perfluorosulfonic acid polymer membranes, such as Nafion, Fumapem, and Flemion. However, the most commonly used are Nafion membranes (Nafion 115, 117, and 212) in PEM water electrolysers because of the advantages of high proton conductivity, high durability at higher current densities (2 A cm⁻²), but the high price and complex disposal when disused restrict its development. Research efforts have been devoted to reducing the cost of these polymer membranes and simultaneously optimizing the ion exchange characteristics and stability. In order to improve the performance of PEM and reduce the cost, the enhanced composite scheme can be used to improve the mechanical properties of PEM, which is beneficial to reduce the thickness of the membrane. Moreover, the membrane resistance and electrolytic energy can be reduced by increasing the ionic conductivity of the film, which is beneficial to improving the overall performance of the electrolyser. According to the different catalyst layer supports, membrane electrode preparation methods consist of the catalyst-coating substrate (CCS) method and catalyst-coating membrane (CCM) method. The CCS method directly coats the catalyst active components on the gas diffusion layer, while the CCM method directly coats the catalyst active components on both sides of the proton exchange membrane, which is the biggest difference between the two manufacturing processes. Compared with the CCS method, the CCM method has a higher catalyst utilization rate and greatly reduces the proton transfer resistance between the membrane and the catalytic layer. CCM is the most common method to prepare MEAs. In this process, the homogeneous catalyst slurry, which was performed by sonication of a mixture of catalysts, ionomer solution, and solvent, is directly coated on the membrane and hot-pressed at high pressure [107]. The sputtering method, spraying method, electrochemical deposition and decal method are usually used in the CCS and CCM methods.

Current collectors are porous media placed between the MEA and bipolar plate at both electrode sides. In PEM water electrolysers, water is usually input on the anode side of the cell, where the OER takes place. The water diffuses through the channels in the separation plate and through the current collectors. The major functions are electric conduction between the electrode and the bipolar plate, as well as transport of liquid/gas from the reaction site with as low as activation, thermal, ohmic, and fluidic losses [108]. The current collector should have energetic corrosion resistance and super electrical conductivity because of the strong acidic media and strongly oxidative environments. Additionally, the pore size and structure of the current collector are important, because the produced gases in the PEM water electrolysis process should be effectively expelled and water must be reaching the catalytic sites of the electrode surface [109]. The porous titanium plates are the most used current collectors for both sides of MEA due to their decent conductivity, excellent mechanical stability, and corrosion resistance.

Bipolar plate is an essential component in the PEM electrolyser stack, reducing the cost of bipolar plates is the key to controlling the cost of electrolysers. In the harsh working environment of the anode of the PEM electrolytic cell, if the bipolar plate is corroded, it will lead to the leaching of metal ions, thereby contaminating the PEM system. Therefore, the commonly used bipolar plate protection measure is to prepare a layer of anti-corrosion coating on the surface. The bipolar plates play multifunctional roles, such as separators, conduct heat and current between single cells in a stack, heat and water management [110]. Thus, the bipolar plate must have excellent strength, high thermal conductivity, low permeability, and high shock durability. Graphite has been previously used as a bipolar plate due to its excellent electrical conductivity. Nevertheless, there are many problems including the high corrosion rates, depressed mechanical strength, and exorbitant cost. Currently, the most commonly used bipolar plates are titanium and coated stainless steel [111,112]. But there are still some challenges, such as hydrogen embrittlement, passivation, and corrosion [113]. Therefore, exploiting cost-effective and stable bipolar plates is still under challenge.