Key Components and Design Strategy for a Proton Exchange Membrane Water Electrolyzer

As the most attractive energy carrier, hydrogen production through electrochemical water splitting (EWS) is promising for resolving the serious environmental problems derived from the rapid consumption of fossil fuels globally. The proton exchange membrane water electrolyzer (PEMWE) is one of the most promising EWS technologies and has achieved great advancements. To offer a timely reference for the progress of the PEMWE system, the latest advancements and developments of PEMWE technology are systematically reviewed. The key components, including the electrocatalysts, PEM, and porous transport layer (PTL) as well as bipolar plate (BPP), are first introduced and discussed, followed by the membrane electrode assembly and cell design. The highlights are put on the design of the electrocatalyst and the relationship of each component on the performance of the PEMWE. Moreover, the current challenges and future perspectives for the development of PEMWE are also discussed. There is a hope that this review can provide a timely reference for future directions in PEMWE challenges and perspectives.


Introduction
In the past few decades, the extensive consumption of fossil fuels has caused deteriorating environmental pollution, and developing clean and renewable energy has become a matter of utmost urgency on a short-term scale. [1,2] Hydrogen, as an attractive alternative energy carrier, shows great potential to relieve the global excessive dependence on nonrenewable resources owing to its merits of high energy density (140 MJ kg À1 ) and ecofriendly characteristics. [3,4] However, most hydrogen is by far produced by steam reforming natural gas or other fossil fuels under rigorous conditions, [5,6] and the conversion efficiency is energy intensive and even accompanied by the release of adverse carbon dioxide byproducts. [7,8] Therefore, exploring efficient green hydrogen production technology is a crucial route to reduce carbon emissions and thus achieve carbon neutrality.
Electrochemical water splitting (EWS) using intermittent electricity generated by sustainable energy sources (e.g., solar and wind power) is deemed to be a promising strategy to realize green hydrogen production on a large scale. [9][10][11] Depending on different kinds of electrolytes, operating temperatures and ionic transport (OH À , and H þ ), EWS technologies can be divided into three main categories ( Table 1): alkaline water electrolyzer (AWE), anion exchange membrane water electrolyzer (AEMWE), and proton exchange membrane water electrolyzer (PEMWE). [12,13] Currently, the state-of-the-art large-scale hydrogen production via electrolysis is greatly dominated by AWE technology at an industrial level for its low cost with the employment of nonnoble electrodes and robust stack durability. [14,15] In AWE system, the anodic and cathodic electrodes are immersed in liquid alkaline electrolyte (usually the potassium hydroxide, KOH), and a diaphragm between the two electrodes serves to separate the hydrogen and oxygen gases. Nevertheless, the use of a liquid electrolyte and diaphragm separator in AWE will inevitably bring about low efficiency, low hydrogen purity, and possible factors of safety if critical contents of hydrogen in oxygen reached. Moreover, the diaphragm greatly impedes the ionic conduction (OH À transport) leading to low current densities (the maximum current density is usually limited to 0.6 A cm À2 ). Subsequently, the development of zero-gap cell configurations of AEMWE has been proposed to improve the current density and hydrogen DOI: 10.1002/sstr.202200130 As the most attractive energy carrier, hydrogen production through electrochemical water splitting (EWS) is promising for resolving the serious environmental problems derived from the rapid consumption of fossil fuels globally. The proton exchange membrane water electrolyzer (PEMWE) is one of the most promising EWS technologies and has achieved great advancements. To offer a timely reference for the progress of the PEMWE system, the latest advancements and developments of PEMWE technology are systematically reviewed. The key components, including the electrocatalysts, PEM, and porous transport layer (PTL) as well as bipolar plate (BPP), are first introduced and discussed, followed by the membrane electrode assembly and cell design. The highlights are put on the design of the electrocatalyst and the relationship of each component on the performance of the PEMWE. Moreover, the current challenges and future perspectives for the development of PEMWE are also discussed. There is a hope that this review can provide a timely reference for future directions in PEMWE challenges and perspectives. production efficiency. As the polymer anion exchange membrane (AEM) sandwiched by two electrodes transporting anions (OH À ) from cathode to anode, it can achieve higher current density of 2 A cm À2 at 2.0 V. [16] Moreover, the AEM is employed as a diaphragm favorably generating hydrogen gas at a high rate and reducing the ohmic losses compared to two-electrode AWE system. However, on account of the low anion conducting of OH À through AEM and highly expensive of the thin polymer membrane, the current density and hydrogen production efficiency are still far away from the another zero-gap cell configuration of the PEMWE system. Similar to AEMWE, the high proton conductivity (0.1 AE 0.02 S cm À1 ), low gas crossover, and high operating pressure of PEMWE render it a more competitive and promising hydrogen production technology. In particular, the fast response when coupled with renewable energy resources and the high efficiency of generating high-purity hydrogen make it much more attractive for future green hydrogen production.
To date, few reviews have reported this promising hydrogen production technology from fundamental to application. In this review, the newest developments in the field of PEMWE will be reviewed and summarized to provide a timely reference for researchers. The key components, including the electrocatalysts, proton exchange membrane (PEM), porous transport layers (PTLs), and bipolar plates (BPPs) for PEMWE, will be introduced. In particular, the advanced electrocatalysts used in the anode and cathode will be highlighted and compared in detail. Based on these components, the membrane electrode assembly (MEA) and the cell design strategy for PEMWE will be overviewed thereafter, and we will focus on the relationship of each component on the performance of the PEMWE system. Last but most importantly, some of the remaining challenges and promising research directions of PEMWE are proposed.

Key Components in the PEMWE
With the development of PEMWE technology, ever higher current densities (10 A cm À2 at 80°C) have been achieved. [17] Nevertheless, owing to the high capital cost for PEM stacks, [18] it became urgent to explore advanced materials that are suitable for PEMWE. The main components of the PEMWE are shown in Figure 1a. The MEA configuration usually consists of catalyst layers (CLs), proton exchange membranes (PEMs) and PTLs, or porous transport electrodes (PTEs) is a core component in PEMWEs, where electrochemical reactions take place. [19] Meanwhile, BPPs are employed as separators for single cells in a stack, providing electrical contact and conducting heat between cells. Hence, the PEM with the associated CLs and PTLs combined with BPPs are the key components determining the large-scale hydrogen yields at higher current densities in the PEMWE.
A typical PEMWE operates according to the following principles ( Figure 1b): water is supplied in the anode side of the cell and then successively flows across the channels of separator plates and PTLs and finally reaches the anodic CL. Then, the water molecules dissociate into oxygen, protons, and electrons. Furthermore, the generated oxygen evolves from the PEMWE system, and the formed electrons combine with the protons Alternating current power supply; b) Direct current power supply.
passing through the membrane to the cathodic CL to generate molecular hydrogen, which correspondingly releases from the PEMWE.

Electrocatalysts
Generally, EWS comprises two half-cell redox reactions: the hydrogen evolution reaction (HER) at the cathode (Equation (1)) and the oxygen evolution reaction (OER) at the anode (Equation (2)) according to the equation for overall water splitting (Equation (3)). [9] Theoretically, a standard thermodynamic potential of 1.23 V is required for EWS under standard condition (V vs reversible hydrogen electrode, RHE, 1 atm and 25°C), whereas in practice, higher operation voltage is actually demanded to overcome the overpotentials derived from reactants adsorption/desorption, contact and solution resistances of the catalysts, thus greatly impeding the thermodynamic and kinetics process for EWS. Hence, developing highly active and efficient electrocatalyst with low overpotentials and robust stability is of great significance to expedite the overall EWS.
Cathode∶2H þ þ 2e À ! H 2 (1) The precious group metal (PGM)-based catalysts are crucial in PEMWE, which drive the hydrogen and oxygen evolution process more thermodynamically and kinetically. Generally, platinumbased catalysts (e.g., Pt/C) are utilized for the hydrogen evolution reaction (HER) on the cathode, while iridium-based oxides (e.g., IrO 2 or RuO 2 ) are used for the oxygen evolution reaction (OER) on the anode. The typical loadings of Pt/C and IrO 2 are 0.4 mg Pt cm À2 and 1.54 mg IrþRu cm À2 , respectively. It is worth mentioning that the prices of Pt metal and Ir metal have drastically increased over the last 20 years. The trend of excessive consumption of noble metal catalysts contributes to a corresponding price increase in the overall cost of the PEMWE system. To date, extensive strategies have been developed to fabricate nonnoble electrocatalysts or reduce the loading of noble metals to decrease the capital cost of electrocatalysts. [20]

Cathodic HER Electrocatalysts
The HER is a classic two-electron transfer process that takes place at the interface between the cathodic catalyst layer and the membrane during the operation of a PEMWE. Generally, the reaction proceeds either via the Volmer-Heyrovsky mechanism or the Volmer-Tafel mechanism and is described as follows (in acidic solution) [21] In both mechanisms, the proton is first absorbed on the surface of the metal site to form M-H* (Equation (4)). Subsequently, the M-H* is then combined with a proton and an electron (the Heyrovsky step, Equation (5)) or another newly formed M-H* (the Tafel step, Equation (6)) to evolve gaseous H 2 . Notably, the protons are easily available in acidic media, especially in pH ¼ 0. [22] According to the density functional theory (DFT) calculations, [23] the free energy of hydrogen adsorption (ΔG H* ) is the most widely accepted descriptor for illustrating the binding strength of M-H* on the catalyst surface and an indicator of assessing the catalytic activity of HER. Moreover, if the value of ΔG H* is close to zero, the relationship between H* adsorption and desorption on the surface of catalysts are the most feasible. When ΔG H* > 0, the M-H* bond is too strong, rendering the Volmer step easy to occur, whereas making the Tafel and Heyrovsky steps difficult. On the other hand, if ΔG H* < 0, the M-H* bond is too weak to impede the Volmer step hindering the overall electrochemical reaction. While the HER rate strongly depends on the activity of the catalyst influenced by its electron structure. Based on the volcano curve plotted by Trasatti, [24] precious metals (e.g., Pt) show the highest catalytic activities compared to nonnoble metals (e.g., Fe, Co, Ni, Mo, and W); thus, the hydrogen evolution proceeding on the surface of noble metals is inclined to the Volmer-Tafel step, a much faster hydrogen evolution process. However, precious metals with high catalyst loading certainly increase the capital cost.  To effectively decrease the cost of noble metals with low catalyst loading while improving the performance, the following strategies have been explored and studied: 1) construction of a single atomic catalyst, 2) catalysts supported on conductive substrates, and 3) alloying with other metals to form bimetallic alloys. For example, Mahmood et al. [25] prepared Pt single atoms that were atomically anchored on onion-like carbon (OLC) nanospheres by reducing the dimensions and introducing curvature by using surface-oxidized detonation nanodiamonds (DNDs) as the precursor (Figure 2a). Further extended X-Ray absorption fine structure (EXAFS) measurements proved the formation of Pt single atoms (Figure 2b). Significantly, the obtained catalysts demonstrated a low η 10 of 38 mV with a Tafel slope of 36 mV dec À1 under 0.5 M H 2 SO 4 media, which is promising for use in PEMWE applications due to its considerably low catalyst loading of Pt (0.27 wt%). Recently, Yuan et al. [26] reported a laser strategy for anchoring Pt nanoparticles (<4 nm) on Mo 2 C micropillars (Pt/Mo 2 C-L/Mo). Owing to the high active interface between Pt and Mo 2 C and well-designed pores, Pt/Mo 2 C-L/Mo exhibited outstanding HER catalytic activity with low overpotentials of 21 mV at 10 mA cm À2 and excellent stability in acidic media. Moreover, the PEMWE assembled with Pt/Mo 2 C-L/Mo as the cathode demonstrated higher current densities (122 mA cm À2 ) under 1.8 V compared to the PEMWE constructed with commercial Pt/C catalysts (22 mA cm À2 @1.8 V) (Figure 2c). The alloying of Ru with Pt is an alternative to modulate the electron structure of Pt and reduce the Pt loading. On account of this, Li et al. [27] successfully synthesized Ru alloying with a trace loading of Pt (0.01% of commercial Pt/C), which was evenly embedded in resorcinol-formaldehyde (RF)-based carbon Figure 2. a) Schematic illustration of the preparation of Pt/OLC through dimension reduction and curvature strategies and b) Pt L 3 edge FT-EXAFS spectra of Pt/OLC. a,b) Reproduced with permission. [25] Copyright 2019, Springer Nature. c) Polarization curves of the PEMWE of Pt/Mo 2 C-L/Mo (À) || IrO 2 /Ti (þ) and the commercial PEMWE. Reproduced with permission. [26] Copyright 2022, Elsevier. d) Polarization curves of Ru@RFCS and other contrast samples. Reproduced with permission. [27] Copyright 2018, The Royal Society of Chemistry. e) Polarization curves of MEAs before and after 100 h. Reproduced with permission. [33] Copyright 2020, Wiley-VCH GmbH. f ) Photograph of the 86 cm 2 electrolyzer and g) the long-term durability test for the PEMWE with CoP as the cathode at 50°C compared with Ir black as the cathode at a constant current density of 1.78 A cm À2 . f,g) Reproduced with permission. [35] Copyright 2019, Springer Nature. h) Polarization curves of the PEMWE assembled with Cu 44.4 Ni 46 Mo 9.6 as the cathode (inset: counter plots for the HER geometric activity of the CuNiMo ternary system). Reproduced with permission. [37] Copyright 2021, Elsevier. spheres (PtRu@RFCS). The PtRu alloy nanoparticles with a large surface and porous structure were favorable for gas-liquid transfer and delivered a much lower η 10 and η 100 of 19.7 and 43.1 mV, respectively, in 0.5 M H 2 SO 4 ( Figure 2d). Therefore, it is expected to be promising for large-scale PEMWE applications in comparison to commercial Pt/C.
In parallel to the great achievements made in PGM-based HER electrocatalysts, much effort has also been devoted to the exploration of PGM-free HER catalysts. Among various nonnoble metal electrocatalysts, transition metal dichalcogenides, phosphides, and bimetallic or poly-metallic alloys show great promise as alternatives to traditional Pt/C. [28][29][30] Through a physical mixture of MoS 2 and carbon black, Corrales-Sánchez et al. [31] first demonstrated the cell performance of a PEMWE assembled with an MoS 2 -carbon composite as a cathode coated on a Nafion membrane. A cell voltage of 0.3 A cm À2 was achieved at 85°C and 2 V, significantly surpassing the bare MoS 2 . However, the performance of MoS 2 -based catalysts unsatisfied the high requirements of higher current densities at the same temperature and cell voltage in comparison to Pt-based metals. Subsequently, Mo 3 S 13 2À clusters were synthesized to enlarge the number of active sites of MoS x -based materials to enhance the activity of hydrogen production. [32] For instance, Holzapfel et al. [33] prepared Mo 3 S 13 2À nanoclusters supported on nitrogen-doped carbon nanotubes (NCNTs) as cathode catalysts for PEMWE. Due to the high chemical affinity between Mo 3 S 13 2À and NCNT and the excellent electrical conductivity provided by NCNT together with the enlarged active reaction area of the composed catalysts, the PEMWE constructed with Mo 3 S 13 2À nanoclusters (3 mg cm À2 ) as the cathode catalyst layer showed an extremely low cell voltage of 2.4 V to achieve a high current density of 4 A cm À2 at 80°C (Figure 2e), surpassing most of the nonnoble HER catalysts employed in PEMWE applications. [33] In another work, Giovanni et al. [28] investigated three kinds of iron sulfide, including pyrite FeS 2 , greigite Fe 3 S 4 and pyrrhotite Fe 9 S 10 . When used as cathodes in a PEMWE, cells assembled with pyrite FeS 2 exhibited the lowest cell voltage of 2.3 V at 2 A cm À2 and 80°C compared to other iron sulfides. Such outstanding performance was attributed to the higher S/Fe ratio and more catalytic active S 2 2À ions in pyrite FeS 2 . Apart from nonnoble metal sulfides, transition metal phosphides are also promising due to their intrinsic activity and excellent electron-transfer ability and durable stability during acidic conditions, which synergistically boost the thermodynamics and kinetics toward EWS. Through impregnation and sulfurization methods, sulfur-doped molybdenum phosphide supported on carbon black (denoted as MoP|S/C) was synthesized by Ng et al. [34] The MEA assembled with MoP|S/C showed the best performance, requiring a low cell voltage of 1.8 V to achieve 0.5 A cm À2 . King et al. [35] recently reported a promising cobalt phosphide (CoP) as an HER catalyst for commercial-scale PEM electrolyzers, which are expected to replace commercial PEM electrolyzers using expensive platinum group metals. The CoP-assembled MEA was able to generate 60 (Figure 2f,g).
Despite the capital costs of transition metal dichalcogenides and phosphides employed as cathodes in PEMWEs being significantly reduced, the current density of 0.1-1.48 A cm À2 at 2 V and 80°C is still far from the PGM-based cathodes (1.7-2.7 A cm À2 under the same conditions). Meanwhile, Ni-M (M ¼ Mo, Co, W, Cu, and Fe) alloy HER catalysts show excellent performance under acidic conditions owing to the modulable electronic state and excellent conductivity as well as good stability. For instance, by a simple electrodeposition method, Kim et al. reported the synthesis of NiW deposited on Cu/CP (where Cu nanowires were first electrodeposited on carbon paper). [29] Due to the highly porous structure of Cu nanowires and the intrinsic activity of the optimized Ni/W ratio (96:4), the obtained electrode assembled in the PEMWE delivered superior cell performance, requiring a low cell voltage of 2 V to achieve 1.79 A cm À2 at 90°C. In another study, Kim et al. [36] directly electrodeposited NiMo alloy on Cu foam, which displayed a current density of 2 A cm À2 at 2 V and 90°C, surpassing most of the PGM-free HER catalysts for the PEMWE and even exceeding some of the Pt-based catalysts. Recently, Choi et al. [37] investigated different compositions of CuMoNi ternary alloys by electrodeposition ( Figure 2h). When the atom ratio of Cu:Ni:Mo reached 45:45:10, the obtained Cu 44.4 Ni 46 Mo 9.6 showed the best HER catalytic activity for the PEMWE with an operation performance of 1.62 A cm À2 @1.9 V and 95°C.
To date, although the noble metal or carbon-supported noble metal composite shows superior HER performance and excellent durability, the trade-off between catalyst loading (capital cost) and HER properties is still not satisfactory. Despite the low-cost and earth-abundant transition metal is frequently reported, the efficiency of the catalysts assembled in PEMWE is inferior. It is highly worthwhile to design and nanostructure superior active and durable PGM-free HER electrocatalysts or reduce the catalyst loading and improve the utilization of precious metals for PEMWE. Moreover, recently reported full PEMWEs with PGM-free HER catalysts are summarized and compared in Table 2.

Anodic OER Electrocatalysts
The OER process, as another important half-reaction, significantly hinders the practical application of the EWS due to the more complicated four-electron multistep transfer, resulting in high kinetic barriers on the anode side. Thus, the OER process is of great significance to determine the overall water splitting efficiency and hydrogen yields; thus, much attention should be given to the OER process. The widely two accepted OER mechanism under acidic media is adsorbate evolution mechanism (AEM) and lattice oxygen evolution mechanism (LOM) shown below [38,39] AEM The difference between the AEM mechanism and LOM mechanism is that the produced oxygen derived from electrolyte in the former, whereas it is possibly generated from lattice oxygen (denoted as V o ) of the catalysts in the latter. [40] The main AEM mechanism includes the generation of M-OH* and M-O* intermediates, whereas there are two different routes to form oxygen from the M-O* intermediate: one way is the combination of two M-O* intermediates to directly convert to O 2 (Equation (9)), another route is the formation of M-OOH* intermediates through the reaction of M-O* coupled with H 2 O and then decomposing to O 2 . (Equation (11)). [39] However, the reaction route for AEM mechanism (M-OH* ! M-O* ! M-OOH* ! O 2 ) [38] shows that the theoretical overpotential limitation is 370 mV, which is not satisfied for the practical applications. This indicates that the moderate bonding interactions with active intermediates M-O*, M-OH*, and M-OOH* are of great significance to the fast evolution of oxygen. [41] Alternatively, electrocatalysts with high metal-oxygen bond covalency and orbital hybridization tend to adopt LOM mechanism (Equation (12)(13)(14)(15)). Significantly, LOM mechanism shows lower thermodynamic limitations compared to AEM mechanism for the highly intrinsic activity derived from oxygen lattice and undercoordinated metal sites, and thus leads to a more competitive strategies to design efficient OER electrocatalysts.
Currently, only iridium-and ruthenium-based oxides are deemed the most attractive and efficient OER catalysts for PEMWE due to their excellent catalytic activity and robust stability under acidic conditions. [42] However, the excessive consumption and limited Ir and Ru reserves in the Earth's crust cannot meet the GW-scale demands of hydrogen production requirements, expediting the development of low-cost and long-lifetime electrocatalysts for the PEMWE system. Hence, to further advance the commercialization application of PEMWE technology, it is urgent to design highly active, cost-effective and robust alternatives to current Ir/Ru-based catalysts.
Considering the excellent OER performance of Ir-and Ru-based catalysts, many researchers have devoted effort to designing novel Ir/Ru-based nanostructures by reducing their loading amount. Through technical assumptions, Minke et al. [43] predicted that the Ir loading will decrease to 0.40 mg cm À2 (0.05 Â g kW À1 ) for commercial PEMWE in 2035. At present, the reported strategies to construct low loading Ir/Ru catalysts include 1) designing the morphology of Ir/Ru materials to expose more active areas, 2) alloying Ir/Ru with the incorporation of other metals to modulate and tune the charge density and electronic structure, and 3) constructing advanced nanostructures with conductive substrates.
The nanostructure of Ir/Ru catalysts to some extent determines the performance of the overall PEMWE because suitable modification of the nanostructure will increase the electrochemical surface area and provide a high mass transfer capacity. Recently, Chatterjee et al. [44] synthesized porous Ir nanosheets (npIr x -NS) with thicknesses of 100 and 5 nm pore sizes by dealloying a NiIr alloy precursor through a top-bottom approach (Figure 3a). Although the rotating disk electrode (RDE)-OER polarization of npIr x -NS and commercial IrO 2 showed no significant difference under the same catalyst loading (Figure 3b), the obtained npIr x -NS with ultralow loading (0.06 mg Ir cm À2 ) employed as an anodic catalyst layer in the PEMWE displayed a cell voltage of only 1.98 V to achieve high current densities of 2.4 A cm À2 at 80°C (Figure 3c). Through the molten salt method, Lee's group [45] fabricated ultrathin IrO 2 nanoneedles (NNs) with improved conductivity and high mass activity. To further enhance the stability of Ir NN, the authors [46] successfully deposited Ir atomic clusters (AC) on IrO 2 NN (denoted as AC/NN-m). The PEMWE assembled with Ir AC/NN-m as the anode showed excellent operation performance with a high current density of 3 A cm À2 at 1.82 V, and no negligible degradation was observed even after 90 h. In another groundbreaking study, by repeating solvent-assisted nanotransfer printing technology, Kim et al. [47] developed woodpile (WP)-structured Ir with stacking nanowire arrays. The orderly macropores with 200 nm and high surface area could accelerate the rapid removal of generated bubbles and increase the exposed electrochemical active sites. Moreover, the 3D WP Ir catalysts with 30 layers by cross stacking showed the highest mass activity (140 A mg À1 ) at 1.8 V in a PEMWE and even achieved 5.2 A cm À2 at 2 V and 80°C, exhibiting the most remarkable PEMWE performance reported to date. Recently, Huang et al. reported novel self-standing porous RuO 2 nanosheet arrays anchored on a carbon fiber substrate (denoted as RuO 2 -NS/CF) (Figure 3d). [48] The hierarchical RuO 2 -NS/CF with abundant edges and defects possessed excellent electron/charge transfer ability and more active sites; thus, remarkable cell performance was achieved with a current density of 7.905 A cm À2 at 90°C and 2 V (Figure 3e). Nevertheless, it has been reported that RuO 2 is more active for OER than IrO 2 , whereas the stability of IrO 2 is relatively higher than that of  [44] Copyright 2021, Wiley-VCH GmbH. d) Schematic of RuO 2 -NS/CF by in situ conversion and further heat treatment and e) polarization curves of RuO 2 -NS/CF compared with commercial RuO 2 /CF. d,e) Reproduced with permission. [48] Copyright 2021, Elsevier. f ) Schematic diagram of IrO 2 by spray-drying and further calcination at 450°C and g) polarization of a PEMWE constructed with Ir 0.7 Ru 0.3 O 2 as the anode in comparison to IrO 2 with different catalyst loadings. f,g) Reproduced with permission. [50] Copyright 2018, Wiley-VCH GmbH. h) Mass activity comparison with an overpotential of 250 mV between Ir 0.7 Ru 0.3 O 2 (EC/TT). Reproduced with permission. [51] Copyright 2017, Elsevier. RuO 2 in acidic environments for the rapid conversion of RuO 2 to RuO 4 under high potential conditions during the OER process. An effective strategy is to develop mixed oxides of IrO 2 and RuO 2 to protect RuO x from dissolution and deactivation by tuning the ratios of Ir:Ru in iridium ruthenium oxide (Ir x Ru 1Àx O 2 ) to enhance the stability and catalytic activity at the same time. For example, Lv and coworkers synthesized a novel RuO 2 @IrO x core-shell structure. [49] The amorphous IrO x layer could modulate the inner Ru 3d shift to a higher binding energy. When employed as anode catalysts in the PEMWE, the cell exhibited excellent stability at 1 A cm À2 for 300 h and an ultralow voltage of 1.683 V at 1 A cm À2 and 80°C. In addition, by an industrial spray-drying combined calcination strategy, Faustini et al. [50] prepared hierarchically ultraporous Ir 0.7 Ru 0.3 O x nanoparticles with RuCl 3 and IrCl 3 as precursors (Figure 3f ). The Ir 0.7 Ru 0.3 O x with 1.8 mg cm À2 as anode CLs assembled in the PEMWE showed a lower cell voltage compared to commercial IrO 2 with high catalyst loading (Figure 3g). Similarly, Wang et al. [51] developed an Ir 0.7 Ru 0.3 O x nanostructure by electrochemical leaching of Ru from bimetallic IrRu. The mass activity of as-prepared Ir 0.7 Ru 0.3 O x (EC) was 13-fold higher than that of Ir 0.7 Ru 0.3 O x prepared by traditional thermal treatment ( Figure 3h). Meanwhile, a two-cell stack assembled with Ir 0.7 Ru 0.3 O x (EC) as the anode only required 1.6 V to achieve 1 A cm À2 at 80°C and showed negligible cell voltage change after 400 h at 1 A cm À2 . To further optimize the durability of the Ir 0.7 Ru 0.3 O x catalyst, Aricò's group employed a hot acid pretreatment combined thermal treatment process. As a result, an extremely stable Ir 0.7 Ru 0.3 O x catalyst with low loading (0.34 mg IrþRu cm À2 ) was fabricated. [52] The degradation rates were only 15 and 23 μV h À1 for 1 and 3 A cm À2 , respectively, even after 1000 h. Therefore, the synergistic effect between the interaction of Ir-Ru significantly improved the stability of the overall PEMWE.
In addition to alloying Ir with Ru, incorporating other elements, such as Ni, Co, Cu, Nb, Fe, Mn, W, and Sn, into Ir or Ir-Ru oxides is another effective strategy to change the electronic structure and tune intrinsic catalytic activity as well as durability in the PEM system. Li and coworkers [53] fabricated porous Ir 0.6 Sn 0.4 O 2 nanocatalysts by using surfactant hydrophilic triblock polymer (TBP, Pluronic F108, and Pluronic F123) to obtain an enlarged Ir surface and more catalytic active areas ( Figure 4a). The hydrophilic and large block length F108 surfactant could precisely modulate the structure of Ir 0.6 Sn 0.4 O 2 with smaller crystallite sizes. They also found that F108-Ir 0.6 Sn 0.4 O 2 with 0.88 mg Ir cm À2 showed better cell performance (1 A cm À2 @1.621 V) and excellent durability with a negligible degradation rate at 0.5 A cm À2 and 80°C for 200 h (Figure 4a). However, the doping of heterogeneous atoms is limited by the Hume-Rothery rule, significantly hindering the utilization of iridium alloying. [54] Alternatively, introducing chemically stable catalyst supports, such as TiO 2 , Ta 2 O 5 , SiO 2 , and Ti metal, into Ir/Ru catalysts is also a good strategy to enhance the performance of catalysts. [55] Recently, Silva et al. [56] studied the relationship between the activity and stability for 18 kinds of supported and unsupported IrO x . They proved that the supported IrO 2 showed higher OER catalytic activity in comparison to unsupported catalysts. Considering this, Pham et al. [57] developed novel IrO 2-coated core-shell microparticles with a loading of 0.4 mg Ir cm À2 by employing TiO 2 . With the unique core-shell structure and the boosted charge/electron transfer capacity as well as reduced interfacial resistance provided by TiO 2 supports, the hybrid catalysts achieved high current densities of 1 A cm À2 at 1.67 V and 80°C. In another study, Kim and coworkers [58] reported an IrRu alloy anchored on a TiO 2 -rGO (TG) substrate (where TiO 2 was first decorated on reduced oxide graphene by ultrasonic spray pyrolysis) by the polyol method ( Figure 4b). An obvious 0.4 eV negative shift of the Ir species could be observed, indicating electron transfer from TG to IrRu (Figure 4c). By investigating the degradation mechanism of IrRuO x , IrRu/rGO, and IrRu/T 90 G 10 , IrRu/T 90 G 10 successfully alleviated the problems occurring on IrRuO x and IrRu/rGO, such as metal dissolution/agglomeration and carbon corrosion from rGO ( Figure 4d). Moreover, the hybrid catalysts exhibited an ultralow cell voltage of 1.56 V at 1 A cm À2 and 80°C. The good performance of IrRu/TG catalysts was attributed to the introduction of the TG support, which hindered agglomeration and prevented IrRu from dissolving under acidic and oxidative conditions, thus improving the durability. In addition, Liu et al. [59] prepared a Sbdoped SnO 2 nanowire structure as a substrate to support IrO 2 (denoted as IrO 2 /Sb-SnO 2 ) through electrospinning combined with the Adams method. The porous IrO 2 /Sb-SnO 2 nanowires exhibited a high surface area and good electrical conductivity, which showed marvelous cell performance that achieved 2 A cm À2 at 1.62 V and remained stable for 650 h at 0.45 A cm À2 and 80°C. In parallel, Jiang et al. [60] developed a defective Ir film coated on WO x nanorods (Ir@WO x ) by hydrothermal and electrodeposition methods (Figure 4e). The optimized Ir film with 68 nm thickness was achieved by 100 cycles of CV electrodeposition, which maximized the exposed active area and avoided falling off (Figure 4f ). Due to the stable properties of WO x supports with a vertically aligned channel array and well-dispersed defective Ir coating, the electrolyzer assembled with Ir@WO x NRs-100 exhibited outstanding durability at 0.5 A cm À2 after 1030 h and excellent single-cell performance of 2.2 A cm À2 at 2 V ( Figure 4g). Therefore, to improve the performance and reduce the loading of Ir/Ru-based OER electrocatalysts, a suitable support should satisfy the requirements of high stability, low cost, high surface area, and excellent electrical conductivity. Furthermore, we also compared the recently reported PEMWE assembled with low-loading Ir/Ru-based catalysts as anodes in Table 3.
In addition to designing low-loading Ir/Ru-based OER catalysts, developing active and durable noble-metal-free catalysts for the anode of a PEMWE is necessary to achieve cost reduction and advance the large-scale application of the PEMWE. [61] To this end, numerous researchers have been committed to the exploration of noble-metal-free catalysts that are promising for practical PEM systems. [62] Carbon-based materials are one of the most investigated electrode catalysts due to their low cost, long-term durability, and good electrical conductivity. Gao et al. [63] reported a novel buckminsterfullerene (C 60 ) anchored on single-walled carbon nanotubes (SWCNTs). The as-prepared C 60 -SWCNTs exhibited good OER catalytic activity compared to commercial RuO 2 , achieving a current density of 10 mA cm À2 at a low overpotential of 400 mV with a Tafel slope of 38.4 mV dec À1 in 0.5 M H 2 SO 4 . This is expected to be promising for PEMWE as a highly active anode catalyst layer.
It is worth noting that these carbon-based catalysts will undergo corrosion and transform into carbon monoxide and carbon dioxide under acidic conditions. [64] Yi et al. [65] investigated the degradation mechanism of MWCNTs and observed the formation of oxygen-containing groups (carboxyl and hydroxyl) on the surface of MWCNTs during the OER process, which subsequently impeded the OER kinetics and decreased the current density. Hence, the challenge is how to enhance the long-term durability of carbon-based materials in acidic environments. Naturally, earth-abundant transition metal oxides, such as Mn-, Co, and Fe-based oxides, show excellent durability in acidic media and display good electrochemical properties. [66] With the aid of DFT calculations, Su et al. [67] confirmed that O* covered MnO 2 as the active surface for the acidic OER. Moreover, Mn 3þ was confirmed to make a great contribution to the performance and durability of MnO x because Mn 3þ is unstable if the pH < 9, and a disproportionation reaction will occur (Mn 2þ ← Mn 3þ ! Mn 4þ ). [68] Recently, Li et al. [69] prepared a novel gamma MnO 2 deposit on carbon paper (γ-MnO 2 /CP). The γ-MnO 2 phase was composed of pyrolusite (β-MnO 2 ) and ramsdellite (R-MnO 2 ). The as-prepared γ-MnO 2 /CP showed excellent OER catalytic activity with low overpotentials of 428 AE 5 mV to obtain 10 mA cm À2 and a remarkable stability at 10 mA cm À2 (where the potential was approximately 1.73 V vs RHE) for 8000 h under 1.0 M H 2 SO 4 (Figure 5a,b) Moreover, the author found that if the voltage was higher than 1.8 V vs RHE or lower than 1.4 V vs RHE, the MnO 2 would dissolve into MnO 4À and Mn 3þ agglomeration, significantly boosting the degradation of the catalysts under acidic media, while a stable potential window was between 1.6 and 1.75 V. Moreover, the PEMWE assembled with γ-MnO 2 /CP as the anode demonstrated excellent performance and good stability at 10 mA cm À2 for more than 350 h at 25°C (Figure 5c,d).
Recently, Huang et al. [70] incorporated nanocrystalline CeO 2 into  [53] Copyright 2016, Elsevier. b) Schematic illustration of the synthesis process IrRu/TG, c) Ir 4f XPS spectra of IrRuO x and IrRu/TG, and d) comparison of the degradation mechanisms of IrRuO x , IrRu/rGO, and IrRu/T 90 G 10 . b-d) Reproduced with permission. [58] Copyright 2021, Elsevier. e) TEM image of Ir@WOxNR-100, f ) diagram of the structure of single and cluster Ir aggregates, and g) long-term stability test for the PEMWE assembled with Ir@WO x NR-100 as an anodic CL at 80°C at 0.5 A cm À2 . e-g) Reproduced with permission. [60] Copyright 2021, American Chemical Society.  (Figure 5e,f ). Moreover, the well-designed Co 3 O 4 /CeO 2 showed excellent performance with a low η 10 (347 mV) and remained stable for more than 100 h at a constant current density of 10 mA cm À2 .
Owing to the good balance between the activity and stability of Co 3 O 4 /CeO 2, the catalyst is expected to be applied for PEMWE systems.
To date, numerous advancements have been achieved in noble-metal-free electrocatalysts that are promising for PEMWE; however, the performance of these noble-metal-free electrocatalysts is still inferior to meet the high demand of practical PEM systems, and continuous research efforts are needed to advance the exploitation of high-efficiency, low-cost and robust OER electrocatalysts for PEMWE. Furthermore, more in situ/operando characterizations and theoretical calculations should be explored deeply to illustrate the actually degradation mechanism for OER under acidic conditions to discover better and more suitable OER electrocatalysts for PEMWE.

Proton Exchange Membrane
The solid polymer electrolyte, also called the proton exchange membrane, that is utilized in PEMWE to replace the liquid electrolyte (KOH solution) can selectively allow the protons to pass through the membrane to participate in the reaction while preventing gas crossover. Currently, membranes of perfluorosulfonic acid (PFSA), such as Nafion, Aciplex, Flemion, 3M, and SCC, have been deemed state-of-the-art membranes for PEMs due to their compact system design, high-pressure operation, remarkable chemical-mechanical stability, sufficient water uptake and moderate swelling, as well as excellent proton conductivity at 90°C. [71] In addition, hydrocarbon-based non-PFSA membranes have also been developed to improve the structure and properties of PEMs.
In short, high-performance PEMs should satisfy the following requirements: 1) high proton conductivity, 2) low gas crossover capability, 3) good thermal stability, and 4) robust chemical-mechanical durability.

Nafion-Based Composed Membrane
Among various perfluorosulfonic acid (PFSA) membranes, Nafion is most attractive for its high proton conductivity, which is primarily attributed to the natural phase separation between the hydrophobic polymer backbones (polytetrafluoroethylene (PTFE)) and the hydrophilic side chains terminated by sulfonic acid functional groups (-SO 3 H) (Figure 6a). [72] The protons generated by liquid water through Nafion are considered effective hydrophilic transfer nanochannels induced by sulfonic acid functional groups. The protons conduction in membrane following two mechanisms: 1) Grotthus mechanism and 2) vehicular mechanism. [73] In Grotthus mechanism, protons hop from one proton donor to another proton acceptor, then protons combine with water molecule to form hydronium ion inside the membrane, thus protons transferring from one hydronium ion to others. Moreover, PFSA-based membranes tend to follow the first Grotthus mechanism. For vehicular mechanism, water molecules provide vehicle for protons, and the hydronium ions pass through the medium by electro-osmotic drag (when an electric field is applied, the protons passing through the PEM). Generally, the protons passing through the vehicular pathway is slower than Grotthus pathway. Moreover, the higher the proton conductivity provided by the thinner proton exchange membrane is, the higher the current densities that can be readily obtained. Furthermore, the hydrophobic surface can avoid excessive water uptake, contributing to lower swelling ratios and providing the membrane with superior mechanical stability. Meanwhile, Nafion is highly resistant to oxide, as the fluorine atoms can protect the sulfonated groups and C─C bonds, consequently improving the stability of the PEM. The maximum operating temperature of Nafion is approximately 90°C, which is favorable to the PEMWE system because the higher temperature  will accelerate the kinetic process and require less energy supported thermodynamically. For instance, the reversible voltage of the electrolysis cell is 1.23 V at 25°C, considerably larger than the cell voltage of 1.14 V at 200°C in the form of steam water. [74] Hence, the superior performance and durability of the PEMWE significantly depend on the properties of the PEM. Currently, some researchers have been dedicated to tackling the drawbacks of plain Nafion membranes at higher temperatures (>90°C). For example, Lee et al. [75] investigated the effect of changing the microstructure of a PFSA membrane through biaxially drawing. The stretched Nafion 117 with decreased thickness (28.2 AE 1.7 μm vs 180.4 AE 6.1 μm for plain Nafion 117) exhibited reduced ohmic resistance, improved dimensional stability owing to the generation of oriented hydrophilic channels through biaxially stretching Nafion 117 providing more hydrophilic pathways to accelerate the protons transfer and decrease the hydrogen permeability. Moreover, the cell performance was significantly enhanced after stretching Nafion 117 from 1.5 to 1.9 A cm À2 at 1.9 V and 80°C. In addition, to enlarge the gas permeation characteristics of the Nafion membrane, incorporating inorganic fillers (e.g., SiO 2 and TiO 2 ) or sulfated/sulfonated inorganic oxides into the Nafion matrix to fabricate a Nafion-based hybrid membrane has also been extensively studied. The main roles of the inorganic fillers in composite membranes are to enhance the water retention inside the membrane, improve conductivity and ion-exchange capacity (IEC), and maintain good mechanical and thermal stability. Compared with plain Nafion, a novel sulfated titania-modified hybrid Nafion membrane [76] with a uniformly distributed anatase lattice in the matrix exhibited a higher current density (4 A cm À2 vs 3 A cm À2 ) at 2 V and 100°C. Moreover, the introduction of highly acidic inorganic TiO 2 also enhanced the stability of the Figure 6. a) Chemical formula for the PFSA ionomers of different forms. Reproduced with permission. [72] Copyright 2017, American Chemical Society. b) Durability test for the PEM electrolyzer with hBN/Nafion at 50°C at 0.4 A cm À2 . Reproduced with permission. [77] Copyright 2021, American Chemical Society. c) Scheme preparation of the ATP@GO membrane via electrostatic layer-by-layer deposition. Reproduced with permission. [79] Copyright 2021, Elsevier. d) Cross-section SEM image of sPPs-MEA, e) Crossover current density over HFR of sPPs-MEA and Nafion-MEA and f ) Comparison of cell polarization of sPPs and Nafion membranes. d-f ) Reproduced with permission. [82] Copyright 2020, Wiley-VCH GmbH. g) Scheme of the synthesis of the Ce-IIL membrane and h) long-term stability test for the PEM electrolyzer with Ce-IIL-MEA at 1 A cm À2 . g,h) Reproduced with permission. [88] Copyright 2022, The Royal Society of Chemistry. functionalized hybrid membrane where the sulfate groups were anchored on TiO 2 surface and anatase lattice derived from sulfated TiO 2 endowed the Nafion membrane with more hydrophilic domains to enhance the protons conducting. In another important work, Taeeun and coworkers [77] prepared an hBN/Nafion hybrid membrane with low gas crossover and dimensional stability through a facile hot pressing and etching process. The mechanical properties of hBN/Nafion were significantly improved, and the obtained electrolyzer cell displayed good stability over 100 h (Figure 6b). In addition, Liu and coworkers designed a molecular-level composite membrane obtained by blending bismuth oxide clusters {H 6 Bi 12 O 16 } with a Nafion matrix through the solution-casting method. [78] Due to the introduced {H 6 Bi 12 O 16 } cationic clusters being well combined with the sulfonic acid groups in the Nafion matrix through electrostatic and hydrogen-bonding interactions, the as-prepared hybrid membrane exhibited an excellent proton conductivity of 0.368 S cm À1 at 80°C and enhanced physicochemical stability. Via electrostatic layer-by-layer deposition, adenosine triphosphate (ATP) immobilized on graphene oxide (GO) nanosheets hybridized with a Nafion membrane (Nafion/ATP@GO) (Figure 6c) was fabricated by Wang et al. [79] The orderly ATP-functionalized GO nanosheets dispersed in Nafion could provide efficient ATP channels for effectively transferring protons and stabilizing the physicochemical properties of the membrane. The obtained membrane also showed a superior proton conductivity of 0.345 S cm À1 at 80°C and low gas crossover. However, due to the poor interfacial bonding strength between the PEM and CLs, the electrochemical reaction will be impeded and thus degrade the performance of the cells.
In addition to designing hybrid Nafion membranes, it is also crucial to prevent the incorporated inorganic nanoparticles among the Nafion membranes from agglomerating. One efficient strategy is improving the usage of the catalytic dead zone brought about by the smooth surface of the three phase boundaries (TPB). To effectively boost the utilization efficiency of CLs and improve the properties of TPBs, Hrbek et al. [80] attempted to modify the surface morphology and physical properties of the membrane by plasma etching of the PEM followed by depositing a thin CeO x film. The modified PEM enhanced the utilization efficiency of the CL by enlarging the interfacial surface area between the CL and PEM while improving the proton conductivity. Hence, the advanced surface-modified Nafion membranes have many strengths compared to flat Nafion membranes, such as enhanced TPBs, enlarged surface area to expose more active sites for electrochemical reactions, and facilitated mass transfer ability.
To conclude, although current Nafion-based membranes with thinner thickness (25 μm) and superior conductivity have been significantly researched and the capital cost has been decreased through the incorporation of inorganic nanoparticles, the degradation of Nafion-based membrane (decomposition, deformation, aging, and contamination) is still required to be reconsidered and how to correspondingly improve the chemical and mechanical properties through in situ and operando characterization investigating their proton conducting mechanism and cause of degradation is a key issue needed to be solved in the future.

Hydrocarbon-Based Composed Membrane
Although Nafion-based PFSA membranes have achieved great advancements, the thickness of the majority of the stateof-the-art PFSA membrane is approximately 100 μm, resulting in low Faradaic efficiency and poor proton conductivity of PEMWE. [72] In contrast, hydrocarbon-based membranes are emerging as attractive candidates owing to their low fabrication cost, similar proton conductivity, low gas crossover, good thermal stability, and eco-friendly characteristics with free fluorine atoms. [71] Hence, the development of hydrocarbon-based membranes is of great significance to reducing cost and obtaining high-performance membranes for PEMWE. Currently, various hydrocarbon-based membranes fabricated from sulfonated derivatives have been explored, which are mainly composed of rich hydrophilic sulfonic groups, such as sulfonated poly(phenylene sulfone) (sPPS), sulfonated polysulfone (sPSf ), sulfonated poly(arylene ether sulfone) (sPAES), sulfonated poly(ether ether ketone) (sPEEK), and sulfonated polybenzimidazole (sPBI). [81][82][83][84] Owing to the negatively charged sulfonated group, the positively charged protons will be attracted; thus, the sulfonated groups significantly optimize the properties of hydrocarbon-based membranes.
By far, the performance of the hydrocarbon-based membrane still cannot surpass that of commercial Nafion until the introduction of hydrocarbon-based materials as membrane and ionomer solutions simultaneously. The sPPS-based hydrocarbon membrane containing sulfone units (-SO 2 -) and sulfonated phenyl ring owns high ion exchange capacity (IEC), superior oxidative and thermal stability, as well as low gas crossover capability. Klose et al. [82] developed a PEMWE with sPPS as the membrane and ionomer (Figure 6d) (4 wt% on the anode and 10 wt% on the cathode side). The obtained membrane assembled cell exhibited low high frequency resistance (HFR) and low gas crossover (Figure 6e), which can achieve 100 h of continuous operation at 1 A cm À2 under 80°C and achieved a current density of 3.5 A cm À2 at 1.8 V at 80°C compared to Nafion N115 (Figure 6f ). By grafting sPPS onto a polysulfone (PSf ) backbone, a novel nonfluorinated PSf-g-sPPS was synthesized by Li and coworkers. [85] They found that the proton conductivity and thermal and oxidation stability of the PEM were significantly improved after grafting modification. Compared with Nafionbased membrane, the hydrogen crossover of sPPS is comparatively low, and the protons transporting ability of sPPS-based membrane is enhanced by increasing temperature, but the durability is limited by temperature, in which the membrane becomes brittle to break. Moreover, the high water content of sPPS will lead to the soften phenomenon of membrane, which would result in high gas crossover and rapid degradation. Therefore, how to effectively improve the durability of sPPS-based membrane under higher temperature (e.g., 95°C) and the optimal water content is required to be explored.
Among hydrocarbon-based membrane, sPAES is widely used as PEM owing to its low-cost fabrication process and superior mechanical properties as well as thin thickness accelerating the protons conduction. Typically, the properties of sPAES membranes also significantly depend on the degree of sulfonation (DS) or IEC. High DS values can form large hydrophilic domains contributing to high proton conductivities, whereas low DS values will bring about undesired swelling and thus lead to poor chemical-mechanical stability due to the abundance of sulfonic acid groups. In addition, a low DS will decrease the high proton conductivities of the membrane. Via the typical solution casting method, Kim and coworkers [86] reported that a sPAES membrane and ionomer with a DS of 50 mol% (sPAES50) showed excellent performance. SPAES50 showed good mechanical stability and high proton conductivity (330.1 AE 6.0 mS cm À1 at 90°C) in the PEMWE. The achieved cell voltage of 1.7 A cm À2 at 1.6 V is attributed to the reduced ohmic resistance resulting from the high proton conductivity. In addition, the SPAES50 membrane (20 μm) displayed a much lower hydrogen crossover than Nafion N211 (25 μm) and Nafion N115 (125 μm). Apart from adjusting the value of DS, optimizing the IEC value is also an effective route to hydrocarbon-based composed membranes. For instance, Han and coworkers [83] synthesized random and block copolymers of biphenol-based sPEAS (called BPSH) by changing the ICE from 1.2 to 2.0 meq g À1 and compared their performance in PEMWE. They found that at similar IECs, block BPSHs exhibited inferior gas permeability to random BPSHs owing to their more hydrophilic domains. The obtained random BPSH with ICE of 1.9 meq g À1 showed the best performance of 5.3 A cm À2 at 1.9 V, significantly surpassing the commercial Nafion 212 (4.8 A cm À2 at 1.9 V), which also exhibited much lower hydrogen permeability than the Nafion membrane. Although the sPAES-based membranes are thinner than traditional Nafion and own high IEC values endowing it with high water uptake and excellent proton conduction ability, the degree of sulfonation significantly hinder the mechanical stability of sPAES membrane. Therefore, how to rationally control and optimize the degree of sulfonation is the key to enhance the performance and durability of sPAES membrane under long-term operation for PEMWE system. Sulfonated polyaromatic-based sPEEK membrane composed of phenyl rings interlinked with ether bonds and carbonyl groups and rigid benzene ring is also an alternative option to replace Nafion-based benchmark membrane due to their facile and low-cost fabrication process and good thermal-chemicalmechanical stability. The structure of sPEEK membrane is similar to Nafion when it is sulfonated, the nano-phase separation between hydrophobic backbone and hydropholic -SO 3 H side chain. The -SO 3 H groups aggregate to form ion channels allowing protons to transport in the SPEEK matrix and providing more proton transport sites to improve proton conductivity. Wei et al. [87] employed sPEEK blended with poly(ether sulfone) (PES) as a hybrid membrane for PEMWE, where sPEEK serves as a membrane and ionomer solution at the same time. When used at the PEMWE, the sPEEK displayed an excellent performance of 1.66 A cm À2 at 2 V and 80°C. However, the swelling and dissolution of the sPEEK membrane at high temperature significantly decreased the performance and accelerated the degradation of the cell. Hence, some sPEEK membranes blended with inorganic oxides (e.g., SiO 2 , CeO 2 , TiO 2 ) and organic fillers have been investigated to improve the properties of sPEEK membranes. Waribam and his coworkers [84] prepared MXene-Cu 2 O/sPPEK through a facile solution casting approach. The obtained hybrid membrane showed improved antioxidant ability for 72 h and boosted the mass transfer to increase the flow rate of hydrogen evolution compared to plain SPEEK. More importantly, the peroxide bonds will be cleaved to form radical species in polymer chains at elevated temperature, and the dissolved oxygen will generate hydrogen peroxide groups to cause chain scission, thus leading to the degradation of hydrocarbon-based membranes (hydrothermal degradation/oxygen-induced degradation/ degradation at the weak-link). [88] In addition, Choi et al. [89] embedded CeO 2 into the interlocking interfacial layer on sulfonated poly(p-phenylene ether sulfone) (sPPES) to form an interconnected ball-and-socket structure (Figure 6g). CeO 2 nanoparticles served as radical scavengers, and the interlocking interfacial layer improved the mechanical toughness to avoid interfacial delamination. Via UV spectra, they proved that the chemical degradation of the obtained hybrid membrane was enhanced. The cell exhibited a lower voltage increase rate than Nafion-MEA after long-term durability for 500 h (48 vs 53 μV h À1 ) (Figure 6h). Actually, the proton conductivity of sPEEK is also affected by the DS. Although the proton conductivity increases with the value of DS, high value of DS will negatively affect the dimensional and chemical stability of the membrane. How to construct cross-linked structures (e.g., functional sPEEK) and modulate the value of DS is very crucial to advance the performance and durability of the membrane.
These works demonstrated the tremendous potential of hydrocarbon-based membranes in PEMWE. However, practical PEMWE systems are required to operate for more than 50 000 h, and the actual H 2 permeability still must be minimized. The performance of hydrocarbon-based membrane greatly depends on the degree of sulfonation, and different degree of sulfonation will result in varied mechanical-chemical stability, thus directly influencing the durability during long-term operation in PEMWE system. Some recent reported full PEMWE with hydrocarbon-based membranes is compared in Table 4.

Porous Transport Layer
The PTLs, also called gas diffusion layers (GDLs) or liquid/gas diffusion layers (LGDLs) located between the CLs and BPPs, are another key component of the PEMWE. The PTLs in the PEMWE systems are mainly responsible for conducting electricity and heat from CLs to BPPs and providing mass transport pathways for the liquid-gas phase. Carbon materials (e.g., carbon felt, carbon cloth, or carbon paper) are usually not suitable for PTLs in the case of PEMWE on the anode side due to the harsh acidic conditions and high potential of the anodic side. The 1.8 V anode potential is higher than 0.207 V corrosion potential for carbon materials, which would result in rapid carbon oxidation. [64] In addition, the nanostructured catalyst particles will fall into the carbon-based substrate, thus decreasing the utilization of the catalysts and the performance of the PEMWE. Therefore, a good PTL should satisfy the following requirements: 1) suitable pore and porosity, 2) thin thickness for effective removal of product gases, 3) low contact resistance with other components, 4) excellent electrical and thermal conductivity, 5) high corrosion resistance under harsh anode side, and 6) good mechanical stability to avoid hydrogen embrittlement on cathode side.
Currently, titanium-based materials are often employed as anode PTLs, such as Ti meshes, Ti felts, Ti foams, and sintered Ti powders, due to their good mechanical-chemical stability and  [90] Compared with commercial PEMWE, the modified PSL/mesh-PTLs dispensed with a flow field in BPPs exhibited a high H 2 output pressure of 90 bar (Figure 7a), showing great promise for commercial applications. The constructed electrolyzer exhibited a cell voltage of 2.54 V at 6 mA cm À2 at 90°C, which was attributed to the hills and valleys in mesh PTLs and the serried surface of PSL synergistically decreasing mass transporting losses between CLs and BPPs (Figure 7b). Similarly, Hackemüller et al. [91] prepared 470 Â 470 mm 2 PTLs with 300 μm thickness by tape casting hydrogenation-dehydrogenation Ti powders at 1000°C for 2 h. The low cost and effective scaled level of PTLs assembled with an electrolyzer were expected to be employed for next-generation PEMWE devices. Modulating the pore diameters (porosity) and thickness are also a frequently used strategy to optimize the performance of PTLs. It is proven that large pore spaces are unfavorable for two-phase transport, especially when the generated oxygen accumulates at high current densities. Additionally, the large porosity would increase the resistance of PTLs. Grigoriev et al. [92] indicated that the optimized pore diameters might be between 10 and 13 μm, which would facilitate gas/water transport. In addition, the interface and contact resistance between PTLs and CLs are another key issue that should be considered because the interface contact will affect the high-frequency resistance (HFR) and thus influence the performance of the PEMWE. By modulating the open pore sizes from 1.16 to 0.2 mm in multilayer Ti mesh PTLs, Kim et al. [93] successfully improved the contact between CLs and PTLs and decreased the mass transport losses of PTLs. Based on this, Lopata et al. [94] investigated the influence of PTL/CL interface contact on mass and charge transport between the PTL and CL, as shown in Figure 7c. By adding patterned through pores (PTPs) under channels between PTLs and BPPs, the cell voltage was fourfold lower compared to commercial PTLs at the same high current density of 9 A cm À2 and 75°C. Furthermore, Lettenmeier et al. [95] carried out PEMWE experiments focusing on different pore-graded Ti-PTLs. The optimal pore diameter was between 6 and 11 μm, and the porosities were larger than 22%. In another interesting study, thin and tunable LGDLs with controlled morphologies were developed through conventional contact lithography and chemical wet etching technology. [96] The thickness of the LGDLs was decreased to 25 μm compared to that of conventional layers (350 μm), significantly reducing the ohmic resistance and improving the TPBs.
To decrease the loading of catalysts while improving their utilization in uncontacted and uncompressed domains, microporous layers (MPLs) are often employed. Titanium MPLs incorporated on thin/tunable LGDLs were fabricated by Kang and his coworkers. [97] The MPLs with nano/micro-Ti particles could activate the catalysts falling into the pores in LGDLs, thus increasing the active sites and improving the performance of the cell. In another study, Schuler and coworkers [98] prepared hierarchically multilayer PTLs (ML-PTLs) with high open porosity (%35%) for good liquid/gas transport. The tailored ML-PTLs with small surface roughness caused less damage to the membrane and provided higher interfacial contact (Figure 7d). In contrast, the single-layer PTLs had a disastrous effect on the membrane, and almost 60% of CLs were inactive sites, thus demonstrating a decreased performance. Furthermore, via X-Ray radiography technology and modeling based on the lattice Boltzmann method, Kulkarni et al. [99] investigated different catalyst loadings on two types of PTLs (fiber and sintered Ti-PTLs). They found that sintered PTLs with a uniform pore radius of 8.1 μm and 44% porosity as well as catalyst loadings above 0.5 mg cm À2 showed excellent two-phase interfacial transport and improved the reaction kinetics during electrolyzer operation.
Nevertheless, similar to carbon materials, Ti-PTLs still cannot resist corrosion in severe environments, and the gradual oxidation of Ti 0 will significantly increase the interface contact resistance and consequently result in drastic degradation of the electrolyzer. [100] For instance, Rakousky et al. [101] investigated the influence of PEMWE assembled with Ti-PTLs without any surface modification and found that the degradation rate was %194 μV h À1 , which was attributed to the increased interfacial ohmic resistance. For comparison, by employing Pt-coated PTLs in the electrolyzer, it is found that the degradation rate was decreased to 12 μV h À1 , proving the positive effect of Pt-coating on Ti-PTLs. Hence, extensive works have been conducted to enhance the lifetime of PEMWE by coating protective precious metals on the surface of anode PTLs. Through Au sputter coating and electroplating approaches, Kang et al. [102] recently fabricated a thin and tunable porous LGDL with a 180 μm Au film. The straight-well porous and planar surface structure of the novel LGDLs greatly reduced ohmic and activation losses as well as improved interfacial contacts. Moreover, the Au-sputtered-coated titanium LDGLs showed good performance with a value decreasing from 1.69 to 1.63 V at 2.0 A cm À2 and 80°C compared to the unmodified LGDLs. In another study, Liu et al. [103] developed a novel titanium PTL with a thin layer of iridium coating via a plasma-sputtering technique. The optimized PTL with an ultrasmall Ir loading of 0.025 mg cm À2 , which was 40-fold smaller than the conventional Au or Pt protective layer, could not only impede the phenomenon of Ti passivation but also serve as a severe catalyst to boost the performance of the electrolyzer (Figure 7e). Furthermore, Liu's group [104] proved that IrO x would appear on the Ir-coated PTLs after a 4000 h test, and the electrolyzer remained stable owing to the degradation prevention for PTLs coming from the generated IrO x film (<10 nm), which indicated that the Ir coating was oxidized under oxygen conditions during the first few hours and was not continuously oxidized (Figure 7f ). Although these rare metal coatings can significantly reduce the contact resistance from Ti oxidation and impede hydrogen embrittlement, attention must be paid to the thickness of noble-metal coatings to satisfy the balance between the capital cost and long-term durability for %50 000 h in practical PEMWE.
Recently, Stiber et al. [105] developed a Nb/Ti coating layer on permission. [90] Copyright 2021, Wiley-VCH GmbH. c) Depiction of transport phenomena at the interface between the PTLs and the CLs on the anode side in the PEMWE. Reproduced with permission. [94] Copyright 2020, Electrochemical Society. d) Mechanism of swelling ionomers in CLs for water vapor and liquid water. Reproduced with permission. [98] Copyright 2019, Wiley-VCH GmbH. e) Schematic diagram of Ir CLs with PTL and Ir-PTE. Reproduced with permission. [103] Copyright 2021, American Chemical Society. f ) The key effect of Ir-coated PTL for enhancing the stability of the PEMWE. Reproduced with permission. [104] Copyright 2021, Wiley-VCH GmbH. g) Accelerated stress test (AST) of the PEMWE with Ti-PTL, Nb/Ti/Ti-PTL and Nb/Ti/ss-PTL at 2 A cm À2 and 80°C. Reproduced with permission. [105] Copyright 2022, The Royal Society of Chemistry. stainless steel PTL by plasma spraying. The low-cost coating significantly improved the performance of the cell and remained stable for 1500 h under an accelerated stress test compared to the uncoated coating (Figure 7g). Moreover, the current density reached 6 A cm À2 at 80°C. In other words, the pore diameter and porosity have a great effect on the performance of PTL, the optimized pore diameters should be between 10 and 13 μm, and a porosity of %30% is conducive to boosting the mass transport between PTLs and CLs. Moreover, the phenomenon of anode Ti-PTL passivation is another key point that should be considered because the oxidation of Ti will directly influence the durability of the overall PEMWE. Although the noble metal coating on Ti PTL will enhance passivation, the high cost and scarcity of noble metals will drastically increase the capital cost. Hence, developing a lowcost and highly effective PTL or PTE with a specific structure for rapid two-phase flow and enhancing contact with CL and BPP are necessary for PEMWE commercialization. The reported full PEMWE with advanced anode PTLs/GDLs is compared and given in Table 5.

Bipolar Plate
The BPP, as a multifunctional component of the PEMWE stack, features flow field channels to ensure an average flow of the water to the MEA interface and rapid removal of the generated gaseous product as well as provide mechanical support. Moreover, another important function of BPPs is to separate the single electrolyzer in a stack, providing electrical contact and conducting heat between single electrolyzers in a stack. The BPP accounts for half the cost of the stack and occupies 80% of the stack weight as well as 50% of the volume, so the structure and composition of BPPs play a vital role in reducing the manufacturing capital and improving the performance of the cells. [106] The selection of suitable BPP materials is restrained by high corrosion conditions. Although graphite is often utilized as a BPP material in proton exchange membrane fuel cells (PEMFCs), graphite BPPs are not able to withstand harsh environments during PMEWE operation (high potentials up to 2 V) and tend to be oxidized to CO 2 , thus decreasing the performance of the cells. With excellent electrical and thermal conductivity, metallic materials have become the most suitable BPPs candidates in PEMWEs. Therefore, high corrosion resistance and good mechanical stability are key requirements for BPPs materials in PEMWE.
Corrosion-resistant metals, such as stainless steel (SS), titanium, and their alloys, are potential substitutes, as they can withstand the high anodic potential and acidic conditions during operation. Among the most widely investigated stainless steels, 316 L SS has attracted much attention because of its low cost and outstanding corrosion resistance as well as easy formability. Li and coworkers [107] melted 316 L SS with molybdenum addition and found that the appropriate Mo content could improve the performance of BPPs. However, these stainless steels are subject to the formation of an oxide layer on the surface, which will inevitably result in high ohmic resistance and poor performance. [108] Currently, extensive work has been focused on studying metal-coated BPPs, and Au/Pt-based composites are widely utilized as protective coating layers for enhanced performance. Gago et al. [109] investigated two types of modified stainless steel BPPs with different coatings. They observed that the contact area between coatings on BPP and www.advancedsciencenews.com www.small-structures.com PTL as well as TPBs was the most likely eroded area under harsh conditions during PEMWE (Figure 8a). The Ti-coated SS BPP with a thin Pt film (8 wt% Pt/Ti) demonstrated a decreased interfacial contact resistance (ICR) compared with Ti-coated/Pt-coated SS BPP (Figure 8b). Moreover, the cell with Pt/Ti-coated SS BPP showed good durability for 200 h at a constant 1.2 A cm À2 at high temperature with negligible degradation. In addition, Wakayama et al. [110] developed a low-cost and conductive titanium suboxide (Ti 4 O 7 )-sputtered titanium BPP to replace expensive Au/Pt coatings to avoid corrosion under high current density. The surface-modified BPPs exhibited extremely low resistance of %5 mΩ cm 2 , and the contact resistance was only %5.2 mΩ cm 2 even after long-term water electrolysis. Rojas and coworkers [111] investigated different surface-coated SS 316, SS 904 and SS 321 BPPs through PVD deposition. The Ti/TiN multilayered coating on SS 321 L BPP exhibited the best performance with negligible weight loss and low ICR, indicating the superior protection properties of Ti/TiN on SS BPPs. Moreover, by coating Nb and Ti on stainless steel BPPs via magnetron sputtering and vacuum plasma spraying, respectively, bifunctional coating layers were designed by Lettenmeier et al. [112] In this biccoated BPP, the Nb coating layer significantly decreased the contact resistance, while the Ti coating (50 μm) protected the stainless steel BPPs from corrosion under harsh environments. Apart from optimizing the composition, BPPs with well-designed flow channels are another strategy to improve  [109] Copyright 2016, Elsevier. c) Six types of flow channels. Reproduced with permission. [113] Copyright 2012, Elsevier. d) Schematic illustration of the integration of the current distributor, BPP, gasket, and LGDL into one AM plate via additive manufacturing and e) polarization curves of the integrated AM BPP with pin flow channels and LGDL and a conventional cell. d,e) Reproduced with permission. [115] Copyright 2018, Elsevier. f ) Configuration and reaction site distribution of conventional PEMWE (top) and PEMWE with AIOBE (bottom) and g) mass activities at 1.6 V comparison of the conventional electrode and AIOBE with different catalyst loadings. f,g) Reproduced with permission. [116] Copyright 2021, Elsevier.
www.advancedsciencenews.com www.small-structures.com the corrosion resistance and guarantee the uniform transport of reactants passing through the active area. Flow channels with different performances were reviewed by Manso et al. (Figure 8c). [113] Various fabrication methods for BPPs, especially additive manufacturing (AM), have been studied. [108] AM is able to print complicated flow channels, and the fabrication cost is rather low and time saving compared to other methods. Yang et al. [114] developed a multifunctional SS 316 L BPP with parallel flow channels through selective laser melting (SLM). When used as both the cathode BPP and current distributor, the flow channels of the obtained AM SS BPP were smoother, and the overall volume and weight were also smaller than those of the conventional BPP and current distributor. In another study, [115] a four-function integrated AM BPP with pin flow channels was fabricated with stainless steel powders by a 3D printing approach. The superior BPP was employed as the current distributor, BPP, gasket, and LGDL synchronously in one component (Figure 8d), thus directly decreasing the ICR between those components and saving time for assembling the cells into stacks. The PEMWE with this novel AM BPP showed excellent performance with a low voltage of 1.715 V at 80°C and 2A cm À2 , surpassing the conventional cell (1.715 V@1.24 A cm À2 ) ( Figure 8e).
Recently, an all-in-one bipolar electrode (AIOBE) was proposed by Yang et al. [116] via AM and sputtering technology. Five components, including CL, GDL, BPP, current distributor and gasket, were integrated into one PEMWE device (Figure 8f ). They found that the utilization of catalysts was lower than that of PEMWE with AIOBE, where the mass activity increased from 0.327 to 4.48 A mg Pt À1 with low catalyst loading (Figure 8g). At present, performance, lifetime, and cost are still the main challenges for BPPs. Although current researchers have improved the durability by coating noble metals (e.g., Pt, Cu) or abundant metals (e.g., Nb, Ti) to protect BPP from corrosion and passivation under acidic and high potential conditions and rendered them more conductive, the cost of coating materials increases the capital input for BPPs, and high ICR is still a bottleneck. In addition, the surface of BPP prepared from AM needs additional polishing, requiring extra time. Therefore, the BPP should meet the following requirements: 1) cost-effective fabrication craft as well as material 2) high corrosion resistance, 3) low ICR property, 4) high in-plane electrical conductivity, and 5) chemical-mechanical stability. The recently reported PEMWE with superior BPPs is well compared in Table 6.

Design Strategy and Membrane Electrode Assembly
The MEA fabricated by sandwiching a proton exchange membrane (PEM) with two CLs between two PTLs or with two PTEs directly coated with catalysts is a core component of the PEMWE. As MEA is responsible for electrochemical reactions (water molecule dissociation and proton and electron transportation) at triple-phase boundaries (TPBs), optimizing the structure of MEA is of great significance to improve the performance of PEMWE and reduce the capital cost. Currently, although individual advanced materials (CL, PEM, and PTLs) have been discovered, the fabrication of MEAs with superior performance is of great importance to enhance the utilization of catalysts and improve the durability of PEMWEs. The two most common processes to fabricate MEA are [117] 1) the porous transport electrode (PTE)-type or gas diffusion electrode (GDE)-type MEA configuration and 2) the catalyst-coated membrane (CCM)-type MEA configuration (Figure 9a). For the PTE(GDE)-type MEA, the catalysts are directly sprayed or printed onto both sides of the PTLs and then combined with the PEM via hot pressing. The advantage of this type of MEA configuration is that the deformation of the membrane can be avoided under the MEA fabrication process, and the fabrication methods are also simple and effective. However, the PTE(GDE)-type MEA usually exhibits poor cohesion between the catalysts and PEM, which would result in high resistance of proton transfer, low utilization of catalysts, and poor mechanical stability. To solve this problem, Choe et al. [118] applied a thin IrO 2 layer (loading: 0.4 mg IrO2 cm À2 ) on a Ti mesh substrate to form an anode PTE via electrodeposition for high-temperature PEMWE. The IrO 2 layer not only acted as a catalyst to participate in the anodic electrochemical reactions but also protected the Ti mesh from oxidation, thus exhibiting good stability of 1.5 mA cm À2 h À1 over 300 h at a constant cell voltage of 1.72 V and 120°C. Another novel PTE-MEA fabrication process involved spraying catalyst inks on two PTLs followed by a hot press against a Nafion 117 membrane. [117] The authors focused on the influence of the catalyst loading and Nafion ionomer in the CL and found that the ionomer content could influence the kinetics and ohmic resistance of the MEA. An optimum loading of 1.4 mg IrO2 cm À2 and 9% Nafion content for MEA exhibited robust durability after a 200 h long-term test at 2 A cm À2 (Figure 9b,c). Kang et al. [119] recently reported a Pt catalyst-coated GDE with tunable thickness by sputtering deposition, which exhibited excellent electrochemical performance. With the aid of in situ high-speed and microscale visualization, they proved that the rim of pores was the real active site where hydrogen was generated, and thus, the Pt coating was enough to provide the active sites needed in TPBs (Figure 9d). In parallel, Kim et al. [120] reported an Ir-based PTE via a self-terminated electrodeposition (SED) technique to deposit ultralow loading of Ir on dendritic Au/CP (where Au was first electrodeposited on carbon paper). The as-formed Ir-Au active interface and increased Ir coverage could provide more active sites for OER (Figure 9e). More importantly, they proved that higher Ir 3þ /(Ir 3þ þ Ir 4þ ) ratios can improve the intrinsic OER activity and enhance the OER stability (Figure 9f,g). On the cathodic side, the Pt metals directly coated on the GDLs prevented hydrogen embrittlement. Conventional CCM-type MEA is manufactured by spraying catalysts with proton conductive ionomer solutions to directly deposit on both sides of the membrane or through a decal transfer process. Such a configuration provides a zero gap between the catalysts and PEM and thus gives rise to a well-attached contact interface, good mechanical stability, and low ohmic losses. For example, Su et al. [121] developed a MEA with superior performance and a low IrO 2 loading of 0.38 mg cm À2 through catalyst inks directly sprayed onto both sides of the membrane under illumination. The good interface contacts between the catalysts and PEM, low ohmic resistance, and low mass transport losses rendered the CCM-type MEA an excellent cell performance of 1.633 V at 2 A cm À2 and 80°C and decent stability of almost 122 h without distinct degradation at 80°C (Figure 10a). In another work, the authors first combined TiC nanoparticles and Nafion ionomers onto Nafion N115 through hot pressing and then deposited a thin Ir layer on a TiC-based support sublayer via magnetron sputtering. [122] Not only was the loading of Ir  [117] Copyright 2019, The Royal Society of Chemistry. d) Scheme illumination of conventional MEA and novel thin GDEs. Reproduced with permission. [119] Copyright 2018, Elsevier. e) Ir 4f 7/2 peak binding energy and [Ir 3þ þ Ir 4þ þ Ir > 4þ ] area ratio of Ir/Au/CP as functions of Ir coverage, f ) specific and mass activities at 1.575 V vs RHE, and g) Specific activity as a function of the Ir 3þ /(Ir 3þ þ Ir 4þ ) ratio. e-g) Reproduced with permission. [120] Copyright 2021, Elsevier. extremely decreased through magnetron sputtering, but the MEA also showed excellent TPBs. The decal transfer method is also a frequently employed process to produce CCM-type MEA, in which catalyst inks are first brushed, sprayed, or doctor blade-coated onto polymer films (PTFE substrate) to generate CLs and then transferred onto both sides of the PEM through hot pressing. Previously, Xie and his coworkers [123] tried to prepare two kinds of MEAs via direct spraying deposition and a decal transfer approach. However, the results showed that the CCM fabricated by spraying deposition showed better cell performance than the CCM synthesized by decal transfer owing to the reduced interfacial contact between CLs and the PEM (Figure 10b,c). It has been reported that thin and uniform coatings via ultrasonic spray coating techniques can effectively reduce the loading of precious metals and increase the utilization efficiency of CCM-type MEAs. [124] In light of this, Sassin and coworkers [125] successfully fabricated an effective MEA by depositing catalysts directly on both sides of the PEM through automatic ultrasonic spray.
In addition to the abovementioned traditional assembly method, numerous advanced fabrication methods, including roll-to-roll (R2R), vapor-based processes, and slot dies, have been developed to decrease the loading of catalysts and enhance the utilization efficiency of catalysts. [126][127][128] R2R manufacturing is a full automatic strategy that can continuously offer high throughput and effectively enhance the interfacial interaction Figure 10. a) Stability test for MEA with a low loading of noble metal catalyst (0.38 mg cm À2 ). Reproduced with permission. [121] Copyright 2013, Elsevier. b) Polarization curves and c) EIS plots of cecal CCM and spray CCM. b,c) Reproduced with permission. [123] Copyright 2021, Elsevier. d) Schematic illumination of the process for R2R direct CL coating on PEM, e) photograph of R2R production of CCMs, and f ) comparison of cell polarization between R2R CCM and conventional ultrasonic sprayed CCM. d-f ) Reproduced with permission. [127] Copyright 2020, Elsevier. g) Long-term electrolysis operation of RSDT-based CCM-MEA and baseline MEA from Nel Hydrogen and h) mechanism of Pt-Ir dissolution and precipitation in PEM. g,h) Reproduced with permission. [131] Copyright 2020, Elsevier. i) Schematic of the novel 2D-patterned electrode. Reproduced with permission. [133] Copyright 2022, American Chemical Society. between the inks and membrane. Recently, Park and coworkers [127] developed a high-efficiency and low-cost CCM-based MEA for PEMWE via direct coating of an IrO 2 catalyst onto the membrane through a roll-to-roll method, potentially eliminating the decal-transfer step commonly used in the CCM process (Figure 10d,e). They discovered that the ratio of water and alcohol had an impact on the swelling of the membrane, and two optimized water/1-propanol ratios (90:10/75:25) exhibited low contact angles and better dispersion of the catalysts. Compared to the ultrasonic-sprayed CCM with pumping DI water at 80°C and a water flow rate of 80 mL min À1 , the performance of R2R direct-coated CCMs demonstrated a cell voltage of 1.91 V at 2 A cm À2 (Figure 10f ). Vapor-based processes with a specific uniform growth rate can decrease the loading of noble metals while dispensing with the drying process. Atomic layer deposition (ALD) is a vapor-based deposition technique with the ability to deposit nanofilms with uniform thickness. [129] For example, Ir was coated on highly ordered TiO 2 nanotubes through ALD to optimize the electrode geometry and control the loading of noble metals. [130] Another reactive spray deposition technology (RSDT), as a flame-based cost-effective methodology, allows for one-step fabrication of MEAs with direct deposition of nanomaterials onto a variety of substrates and precise control of the distribution of catalysts, further reducing the need for PGM catalysts. [128] Yu et al. [131] developed a high-performance MEA with ultralow loadings of 0.03 and 0.08 mg IrOx cm À2 and demonstrated robust durability after a long-term test of 4500 h at 1.8 A cm À2 and 80°C fabricated by a flame-based RSDT (Figure 10g). They also investigated the mechanism of cathode degradation (Figure 10h). During the electrochemical reactions, Ir would dissolve and migrate from the anode to deposit on the surface of Pt. Meanwhile, the dissolved high state of Ir nþ would pass through the PEM and combine with the dissolved Pt and the generated H 2 to form Pt-Ir particles in the middle of the membrane, thus resulting in performance degradation. Currently, numerous endeavors have been dedicated to developing nanostructure MEAs, which will further improve the utilization efficiency of the catalysts, provide more pathways for protons to pass through the membrane and enhance the stability of the MEAs. [132] A novel 2D-patterned MEA ( Figure 10i) with dimensional anode catalysts on a PEM was synthesized by Kang et al. [133] The modified MEA not only decreased the loading of anode catalysts by 61%, while the formed edge active sites increased the cell performance. For MEA, as the key component of PEMWE, the degradation of CLs and PEM is of great importance to influencing the performance of the cell. For long-term operation, the noble metals catalysts will agglomerate and aggregate on the two side of CLs, which would bring about the reduced active sites. Actually, due to the unqualified purification of water used in practical PEMWE application, trace contamination metals (Na þ , Ca 2þ , Cu 2þ , and Fe 3þ ) [134,135] will be introduced, indirectly leading to the generation of unfavorable particles/precipitates covering the surface of CLs. Moreover, theses contamination cation metals significantly decrease the proton transfer efficiency and lower the conductivity of PEM. [136,137] The unevenly heat distribution from GDL and the combination of generated gases through permeation will expedite the degradation of PEM. But fortunately, the catalyst poisoning and PEM contaminating could be recovered by acid treatment. Accordingly, the extra capital cost will be required. Currently, the improved strategies for MEA are to choose more appropriate coating methods (electrodeposition and ultrasonic spray) to evenly disperse catalyst ink on the surface of membrane and take the advantage of low-temperature decal method and other novel roll method to enhance the mechanical stability of MEA. Therefore, high-performance MEA should satisfy the following high requirements: 1) good combination between CLs and PEM; 2) low proton transfer resistance and reduced contact resistance between CLs and PEM; 3) the interface contact between PEM and CL should be as large as possible; 4) welldefined structure of MEA ensuring low mass transport losses; 5) abundant TPBs; and 6) superior chemical and mechanical stability. Significantly, an effective assembly strategy of MEA is vital for the enhanced performance of the PEMWE.
To conclude, all components should be considered for their jointly influence on PEMWE system. As shown in Figure 11, the performance of PEMWE is attributed to the system restriction derived from these key components of PEMWE, which can be separately ascribed to the thickness of PEM (also the ionexchange capacity), three-phase boundaries between electrocatalysts and PEM, and the ICR. Specially, the MEA design is of great significance because the properties of membrane directly influence the protons conducting, while the performance of PEMWE greatly depends on HER/OER electrocatalysts. Moreover, the Figure 11. Each component respectively effects on PEMWE for developing high-efficient hydrogen production. interface between PME and CLs should be in good contact by avoiding electrocatalysts agglomeration and detachment.

Summary, Challenges, and Outlooks
Hydrogen, as a clean and sustainable energy carrier, is taking the lead of fuels for its versatile applications. The PEMWE system is a more promising technology than other water electrolysis technologies. In this review, we first introduce the research status of the key components in a typical PEMWE system, including electrocatalysts, PEMs, PTLs, and BPPs. In particular, the development of electrocatalysts used in the anode and cathode is summarized and discussed in detail. Thereafter, the MEA and the cell design strategy for PEMWE are overviewed, especially the relationship of each component on the performance of PEMWE. Although PEMWEs have currently achieved great success, each component of PEMWEs is expensive, and their intolerance to harsh electrolysis environments directly hampers PEMWE applications. Many challenges, such as the degradation of catalysts and membranes, mechanical instability, and electrochemical corrosion of PTLs and BPPs leading to decreased cell performance, are a matter of great urgency to be resolved immediately.
The requirements for the activity and lifetime of anodic/cathodic electrocatalysts are equally important to the performance of the PEMWE system. Current reported electrocatalysts still cannot meet the demands of high current density (>200 mA cm À2 ) for industrial application to reach the desired value of 3 A cm À2 @1.9 V in 2025 is also a great challenge. Exploration of high-performance electrocatalysts remains a matter of utmost urgency on a short-term scale. High-throughput (HT) technologies combined with computational screening and experimental approaches for building material databases have been conducted, which would be helpful for discovering optimized electrocatalysts. Compared with traditional trial and error experimental methods, HT technologies take advantage of machine learning (ML) and study parallel synthesis and characterization with different parameters at one time to rapidly screen catalysts with high intrinsic activity and make recommendations for rationally designing promising catalysts. Meanwhile, the long-term stability of electrocatalysts is most important for the target durability for PEMWE is %80 000 h. Protecting active materials from corrosion and dissolution in acidic environments, such as alloying, introducing an oxidation layer or incorporating corrosion resistant metal into initial catalysts to hinder solubility, are all potential strategies to enhance the stability of catalysts.
For PEMs, composite membranes are more favorable and attractive for tunable properties. Inspired by the strategy utilized in PEM fuel cells, introducing more channels by designing nanoporous structures, such as the incorporation of covalent organic frameworks and inorganic polyoxometalates, is promising for developing high-performance PEMs for PEM water electrolyzers. For the porous transport layer, mass transport losses often occurred during the operation of the PEMWE system. Currently, the developments of operando X-Ray tomographic microscopy could provide visualization processes for the interface between PTL and CLs in three dimensions, which would be favorable for discovering the realistic instability and stability region and thus provide design direction for the PTLs. For BPPs, the emergence of 3D printing technology shows great promise for designing BPPs with complex flow channels and smaller volumes and weights. Moreover, the construction of nonmetal BPPs is expected to be realized through 3D printing technology to greatly reduce capital costs in the future. The design of the MEA configuration is of great significance for enhancing the three-phase boundaries where electrochemical reactions occur. Nanostructuring is one potential route for nanostructured CLs, and PTLs coated on PEMs can simultaneously increase the utilization of catalysts and mass transport. In particular, AST measurement is more meaningful and time consuming for researchers to investigate the degradation mechanism for the PEMWE system. Therefore, with the rapid development of intermittent electricity generated by renewable resources, the commercialization progress of PEMWE is expected to outperform the current EWS technologies and provide more pure hydrogen for numerous industrial applications, thereby tackling the escalated environment issues with zero carbon footprint.