Excessive consumption of fossil fuels must be replaced with renewable ones, especially because the worldwide power demand will reach 24 or 26 TW by 2040, with emitted CO₂ emissions of 37–44 GT per year by 2040 ^[1]. Among the various fuel sources, hydrogen is often referred to as the “fuel of the future”, with a high energy density of 140 MJ kg⁻¹, which is more than twice that of conventional solid fuels (50 MJ kg⁻¹) ^[2]. Other benefits of promoting the hydrogen economy include energy security by reducing oil imports, less pollution and better urban airy quality, sustainability by taking advantage of renewable energy sources, and economic viability by potentially shaping future global energy markets ^[3].

Currently, total global hydrogen production is about 500 billion cubic meters (bcm), with the produced hydrogen being extensively used in petroleum refining processes, in the petrochemical and chemical industries, in fuel cells, and in the fertilizer industry. Most of the hydrogen (about 96%) is produced from non-renewable fossil fuels, mostly by steam reforming of methane. Therefore, the hydrogen produced in this way is characterized by a lower purity and a high concentration of harmful greenhouse gases ^[2]. Only 4% of the hydrogen produced worldwide is produced in a renewable way by water electrolysis, which is a promising way of producing high-purity hydrogen without carbon emissions ^[1].

Synergies between hydrogen, electricity, and renewable energy sources are urgently needed ^[4]. Over the past decades, the increasing prices of electricity have postponed and hindered the production of electrolytic hydrogen, but this low percentage of its application is expected to increase with recent growth in energy capacity based on renewable sources like wind turbines and photovoltaics ^[5]. This is also supported by the revised Renewable Energy Directive 2018/2001/EU ^[6], which has established a new binding target for renewable energy in the EU for 2030 of at least 32%.

Proton exchange membrane electrolysis (PEMWE) is considered the most promising form of hydrogen production based on high efficiencies and suitable current densities even at moderate temperatures ^[4]. Combined with renewable energy sources, PEM electrolyzers can produce electrolytic hydrogen that can work as an energy vector/carrier and energy storage medium and thus overcome the intermittency of typical renewable energy sources ^[5]. After being stored, the hydrogen produced by electrolysis can be converted back into electricity when needed or used to refill fuel-cell-based cars ^[4].

2. Electrocatalysts for Hydrogen Production

The question of using expensive materials for the fabrication of the electrodes in PEMWE technology dates back to the 1960s when the first PEMWE devices were developed. Regarding the empirical aspects, these early systems were considerably efficient, presenting performances of 1.88 V @ 1 A cm⁻² or 2.24 V @ 2 A cm⁻², with a cell life of over 15,000 h without substantial performance degradation. Back then, the question concerning the high cost of used catalysts was also raised, and for these systems, catalyst layers were based on Ir and Pt black with high metal loading ^[5]. This work will put aside the analysis of anode electrocatalysts, and the focus will be on cathode ones. Harsh electrochemical environments (high anodic overpotential, low pH, the presence of strong oxidants, possibility of operating at higher temperatures) require the use of precious metal compounds as electrocatalysts in PEMWE [92]^[7]. For HER, highly dispersed carbon-supported Pt-based materials, with low overpotential close to zero and a Tafel slope around 30 mV/decade, are currently benchmark catalysts, but the practical developments to satisfy the growing demands require the use of cheaper electrocatalysts [7]^[8]. The cathode catalyst represents a considerable portion of the total system cost, especially if degradation or corrosion of the carbon support occurs ^[5]. Nowadays, cathode side metal loading is maintained at approximately 0.5–1 mg cm⁻², and further decreases will be needed for values reaching below 0.2 mg cm^{−2 [2]}. The PEMWE with the non-noble cathodes exhibited the current density of 0.35–0.73 A cm⁻² at 2.0 V in the operating temperature range of 80–90 °C, which were still lower than that with noble Pt/C cathodes (1.46–2.71 A cm⁻²) [93]^[9]. Typically, electrocatalysts are chosen based on their specific characteristics such as particle size, pore structure, good electrical conductivity with high surface area, and corrosion stability under oxidizing conditions [94]^[10]. For better electrochemical performance and maximum consumption of catalyst surface, the electrocatalysts are usually supported on the carbon because of its superior electrical conductivity, mechanical and thermal stability, large surface area, environmental friendliness, and relatively low cost [94]^[10]. Different types of carbons are studied for the application in PEMWE, such as carbon black (CB), carbon nanomaterials (CNMs), graphene, fullerenes, carbon nanotubes (CNTs), and heteroatom-doped CNMs(N-CNTs) [94]^[10]. There are different proposed ways of decreasing Pt loading in PEMWE, but most of the solutions require a finding of cheaper non-noble catalyst, the use of specific supports to achieve better dispersion and a higher catalytic surface, and the formation of noble metals alloys by addition of new elements to the main compound [95]^[11].

2.1. HER Electrocatalysts with Substituted Noble Metals Content

According to the volcano-plot theory, the electrocatalytic activity is controlled by the H adsorption free energy for HER, and only noble metals can efficiently electrolyze the HER [7]^[8]. But, during the operation of PEMWE, conventional HER catalysts suffer fewer kinetic and stability problems due to relatively facile reduction in protons in acidic media and the negative potential window for operation [96]^[12]. This opens the possibility of using non-noble metals as HER catalysts for PEMWE. But, pure transition metals, such as Ni (−0.280 V_SHE), Co (−0.277 V_SHE), Fe (−0.440 V_SHE), and Mo (−0.200 V_SHE) are also less stable in PEMWE electrolysis because they can undergo dissolution during HER because of the values of their standard reduction potentials [4,96]^[4][12]. The use of these transition metals could be improved by composite formation with other promising materials since the main requirements within this field are the improvement of the surface area of the electrodes and the optimization of their ability to reduce protons to molecular hydrogen [95]^[11]. Therefore, transition metal compounds including carbides, nitrides, phosphides, and sulfides have been actively investigated for acidic HER. Their higher HER activities than those of pure transition metals can be attributed to the suitable energies for hydrogen adsorption on the catalyst surface [93]^[9]. On the other side, the insufficient intrinsic activity of non-noble metals could be improved by tuning properties such as the composition and morphology of the used materials [97]^[13]. Non-metallic elements (C, N, P, S, and Se) that are being used in the composites together with metallic elements have high electronegativity and therefore draw electrons from the metal components, as well as attract protons due to the partial negative charge [98]^[14]. Although the performances of such materials are still lower than those with noble cathodes, due to the relatively low price of non-noble metals, there is still great scientific interest for research within these alternatives. Incorporation of high-performance low-cost HER electrocatalysts into PEM electrolyzers is an emerging area of research with still limited reports of PEM electrolyzers that utilize non-precious catalysts at the H₂ electrode side being published [99]^[15]. Most of the published works still only deal with the investigation of electrocatalytic activity of novel materials in acidic media by the application of three-electrode systems without further application in PEM water electrolyzers. Both of the mentioned applications will be presented in further sections. Since the works of Hinnemann et al. [100]^[16] and Thomas et al. [101]^[17], who reported promising results of the HER electrolysis utilizing MoS₂, transition metal chalcogenides are among the most promising non-noble metal cathode electrocatalysts. The main characteristic of this family of materials is that they have a 2D lattice structure and, like graphene, the reactive sites are located along the edges while the basal plane is catalytically inactive [7]^[8]. Therefore, the aim of most of the works within this area is to increase the number of active sites by increasing the ratio of edges to the basal plane and to increase the electrical conductivity simultaneously while minimizing the overpotential required for HER [7]^[8]. For this purpose, many distinct molybdenum sulfide structures such as nanoparticles, nanowires, films, and mesopores were synthesized with the aim of maximizing the number of exposed edge sites [99]^[15]. Besides the aspect of active sites, electric conductivity is another crucial factor related to electrocatalytic activity because a high conductivity ensures fast electron transport during the catalytic process [102]^[18]. Because of that, MoS₂, which is more economical and 10⁴ times more abundant than Pt, can be chemically bonded to RGO via a facile solvothermal approach [103]^[19]. Such a composite that contains highly exposed edges can exhibit HER activity with a small overpotential of ~0.1 V, large cathodic currents, and a Tafel slope of 41 mV/decade. Another research work conducted by Corrales- Sánchez et al. [104]^[20] explored the electrocatalytic activity of MoS₂/RGO hybrids, as well as of pristine MoS₂ and MoS₂ physically mixed with an electrically conducting carbon material (Vulcan^® XC72) towards PEM electrolysis. As a reference, the performance of Pt black was also shown. Among tested MoS₂-based materials, 47 wt% MoS₂/Vulcan^® gave the best performance in terms of current density at an encouraging level for practical application, while the MoS₂/RGO hybrids showed higher HER activity than pristine MoS₂. The poor performance of pristine MoS₂ can be contributed to poor electrical conductivity. Although improvement in electrocatalytic activity for the mentioned composites can be noticed, results still did not exceed those achieved with the use of Pt black. Research conducted by Kumar et al. [102]^[18] confirmed that by controlling the reaction temperature and sulfur precursor employed, different MoS₂ nanostructures like nanosheets, nanocapsules, and nanoflakes could be obtained. Among all indicated materials, MoS₂ in the form of nanocapsules exhibits superior activity towards HER in 0.5 M H₂SO₄ with an overpotential of 120 mV vs. RHE. The following Mo-based catalyst was further incorporated into the PEM electrolyzer where the fabricated MEA consisted of a Nafion PEM sandwiched between iridium (IV) oxide and MoS₂-nanocapsules used as the anode and cathode catalyst. The designed cell was operated for 200 h at 2 V without any degradation of electrocatalytic activity. Recently, another efficient and stable electrocatalyst composed of earth-abundant TiO₂ nanorods decorated with MoS₂ thin nanosheets was recorded [105]^[21]. This composite possesses hydrogen evolution activity in acidic media at an overpotential of 0.35 V and a Tafel slope of 48 mV/decade. It is very important to measure the Tafel slope because it is a primary and inherent property of the catalyst which indicates the rate-determining step involved in the HER [105]^[21]. In this case, the measured Tafel slope is very close to the one of benchmarking Pt/C (32 mV/decade) and indicates that electrochemical desorption is the rate-limiting step for the HER in acidic media. Higher HER activity than MoS₂ can be obtained by the use of alternative Mo-based catalysts such as MoS_x, [MoS₃S₁₃]²⁻ nanoclusters, and sulfur-doped molybdenum phosphide (MoP|S), loaded onto CB support. These carbon-supported catalysts, synthesized by Ng et al. [99]^[15], are electrochemically tested in a standard three-electrode electrochemical and subsequently integrated into PEM electrolyzer systems and operated continuously for 24 h. Related to PEM electrolyzer testing, the performance of each electrolyzer was examined by stepping the potential from 1.2 to 1.0 V at 50 mV intervals at a cell temperature of 80 °C. The MoS_x-CB-based electrolyzer required 1.86 ± 0.03 V to reach 0.5 A cm⁻², while the MoS₃S₁₃-CB and MoP|S-CB-based electrolyzers both required 1.81 ± 0.03 V to reach 0.5 A cm⁻². The best overall performance, also including three-electrode electrochemical data, was achieved with the (MoP|S) electrolyzer. Such results suggest that Mo-based catalysts hold promise for commercial applications with the possibility of replacing the Pt-based cathodes currently being used in PEM electrolyzers. Other interesting transition metal chalcogenides applied as a cathode side within the field of PEMWE are iron sulfide materials, which have the great advantage of being widespread in nature. Pyrite (FeS₂) is the most abundant sulfide mineral, while pyrrhotite is an unusual iron sulfide mineral with variable iron content [Fe_(1−x)S_(x=0–0.2)] that often accompanies base metal sulfides in ore deposits [106,107]^[22][23]. The synthesis, characterization, and activity towards the HER of different stoichiometries of iron sulfide materials including the above-mentioned pyrite and pyrrhotite, as well as greigite (Fe₃S₄), were investigated by Di Giovanni et al. [108]^[24]. Finally, their performances were also investigated in situ in a PEM electrolyzer single cell under 80 °C. The MEAs were prepared by using pyrite, pyrrhotite, or greigite as the cathode catalyst and tested in an electrolysis single cell. The catalysts were not supported but were mixed with 20% of CB. Nafion 115 (125 µm) was used as the membrane and IrO₂ as the anode. According to the SEM results presented within the research, the thickness of the IrO₂ catalyst layer is ~6 µm, while the thickness of the FeS₂/CB catalyst layer is ~30–40 µm. Also, the experimental results have shown that all three catalysts allow ~2100 mV at 1 A cm⁻² to be reached, but both ex situ and in situ electrochemical experiments have revealed that pyrite (FeS₂) is more active than greigite (Fe₃S₄), which is more active than pyrrhotite (Fe₉S₁₀). Generally, all three catalysts allow ~2100 mV at 1 A cm⁻² to be reached. The electronic structure of metal sulfide materials can be modified by doping metal atoms, which can optimize hydrogen adsorption energy and enhance HER catalytic activity. Such an example can be seen in the work of Wang et al. [109]^[25], where Co-doped iron pyrite FeS₂ nanosheets were hybridized with carbon nanotubes (Fe_1−xCo_xS₂/CNT). HER was tested in 0.5 M H₂SO₄ acidic solution in a three-electrode system without further application in PEMWE. Electrochemical measurements showed a low overpotential of ~0.12 V at 20 mA cm⁻², a Tafel slope of ~46 mV/decade, and long-term durability over 40 h of operation using bulk quantities of Fe_0.9Co_0.1S₂/CNT hybrid catalysts at high loadings (~7 mg cm⁻²). Density functional theory (DFT) revealed that an increase in the catalytic activity comes from a large reduction in the kinetic energy barrier of H atom adsorption on FeS₂ surface upon Co doping in the FeS₂ structure. Transition metal phosphides, such as CoP, NiP, FeP, and MoP, are viewed as a promising replacement of Pt because of their good stability and high activity in acidic media, and further improvement of intrinsic HER activity can be realized by employing more than one transition metal [110]^[26]. Therefore, FeCoP shows a near optimal hydrogen adsorption free energy (ΔG_H) that is similar to that of Pt and is significantly affected by the Fe/Co ratio [110]^[26]. The development of NiP catalysts is also on the ascending path, with the composites being developed by different research groups. NiP catalysts electrodeposited on carbon support were developed by Kim et al. [98]^[14] and applied as a cathode for a PEMW electrolyzer. The performance of the water electrolyzer was evaluated in a galvanostatic mode in the range of 0.02–4 A cm⁻² after the activation process, and cell voltages of 1.96, 2.07, and 2.16 were required to obtain a current density of 1.2 and 3 A cm⁻². NiP nanoparticles, but with a different stoichiometric ration (Ni₂P) and support (multiwall carbon nanotubes), were designed by in situ thermal decomposition of nickel acetylacetonate as the nickel source and trioctylphosphine as the phosphorus source in an oleylamine solution of carbon nanotubes [111]^[27]. Electrocatalytic activity of this nanohybrid was evaluated in 0.5 M H₂SO₄ with an onset overpotential of 88 mV, a Tafel slope of 53 mV/decade, and an exchange current density of 0.0537 mA cm⁻². Besides nickel–phosphide materials, nickel–carbon-based catalysts were developed by Fan et al. [112]^[28]. This work reveals the new area of tuning structure and functionality of metal–carbon-based catalysts at an atomic scale that may help accelerate the large-scale application of PEM electrolyzers. By the use of electrochemical methods, the indicated composite can be activated to obtain isolated nickel atoms anchored on graphitized carbon, consequently displaying high activity and durability for HER. Owing to their low-coordination and unsaturated atoms, isolated metal atoms have demonstrated more catalytic active than nanometer-sized metal particles. Other attempts at the improvement of the “noble-metal-free” electrocatalysts can be noticed at the use of group VI transition metal carbides that exhibit catalytic properties analogous to platinum group materials (PGMs) because of their unique d-band electronic structures [113]^[29]. Catalytic properties of carbide materials strongly depend on their surface structure and composition, which are closely associated with their method of synthesis [113]^[29]. Therefore, by the use of a simple and environmentally friendly carburization process, Chen et al. [113]^[29] synthesized Mo₂C covalently anchored to carbon supports (carbon nanotubes and XC-72R carbon black). The electrochemical impedance spectroscopy (EIS) results demonstrated that the incorporation of Mo₂C onto carbon supports enhanced the exchange current density (measured overpotential of 63 mV applied for driving 1 mA cm⁻² of exchange current density), reduced charge-transfer resistance, and a change in the HER mechanism. Excellent chemical stability has opened the possibility of the application of transition metal oxides, such as WO₂ in the field of clean hydrogen energy production. Metallic WO₂–C mesoporous nanowires with a high concentration of oxygen vacancies (OVs) were synthesized by Wu et al. [114]^[30]. All tests were carried out in 0.5 M H₂-saturated H₂SO₄, and the products exhibited promising performance for hydrogen generation with a Tafel slope of 46 mV per decade. For comparison, as already noted, corresponding to the literature [7]^[8], the value of the Tafel slope for commercial Pt/C was about 30 mV per decade. Other interesting Pt-free alternatives with the corresponding features are listed in Table 71. ^]. Considering commercial PEMWE systems and their industrial applications, the typical catalyst loading is 1–2 mg cm⁻²; therefore, it is responsible for 25% of the PEMWE stack cost [140,141]^[56][57]. To reduce Pt content, different approaches are listed. Some of the solutions imply the development of novel thin and tunable gas diffusion electrodes with a Pt catalyst thickness of 15 nm and a total thickness of about 25 µm, which can enhance catalyst mass activity up to 58 times higher than conventional catalyst-coated membrane (CCM) at 1.6 V under the operating conditions of 80 °C and 1 atm. [142]^[58] On the other hand, core-shell structures with Pt on the surface and Ru forming the core of the particles were developed by Ayers et al. [140]^[56]. This composition enables appropriate electrocatalytic activity to be achieved; by utilizing Pt spontaneous deposition on metallic Ru nanoparticles, an ultralow Pt-content catalyst was made with a 20:1 Ru:Pt atomic ratio. Furthermore, reactive spray deposition technology (RSDT) enabled one-step fabrication of two MEAs (86 cm²) containing platinum group metal (PGM) loadings in amounts of only 0.2 and 0.3 mg_PGM cm⁻² loading in the cathode and anode electrodes, respectively. This assembly, involved in electrolysis operation conducted at 50 °C and 400 psi differential pressure with 1.8 A cm⁻² current, demonstrated durability for over 3000 h of operation at industrially relevant operating conditions [143]^[59]. Consumption of energy to produce hydrogen strongly depends on the current and voltage applied. The cell voltage is composed of the anode and cathode potentials and the IR drop in the electrolyte. The reduction in the cathode potential can be achieved by modifying the composition of the catalyst or by increasing the surface area of the catalyst by reducing the particle size [144]^[60]. Such an example can be seen in the work of Ravichandran et al. [144]^[60] where, during the impregnation reduction synthesis of Pt catalyst, the addition of nonionic surfactant reduced the particle size. The MEAs of 4 cm² coating area, with a loading of 0.4 mg cm⁻² Pt and 1.2 mg cm⁻² IrO₂, was tested for HER and operated as a single cell at 2 V and 80 °C, achieving the highest current density of 1.5 A cm⁻². Similarly, the stack of the hydrogen generation capacity of about 1 N m³ h⁻¹ capacity was assembled and tested by the integration of five single MEAs of 500 cm² area. The stack displayed a current density of about 1.18 A cm⁻² at 10.0 V and 80 °C; performance lasted up to 3000 h of operation with not much change in the current density noticed. The use of the impregnation reduction method of synthesis and surfactant are beneficial in reducing the particle size, which also confirmed earlier conducted studies [145,146]^[61][62]. Wang et al. [145]^[61] designed surfactant-stabilized Pt and Pt alloy electrocatalysts on carbon supports for the application in PEMFC and revealed the improved electrocatalytic activity due to the well dispersed and smaller catalytic particles; while Rajalakshmi et al. [146]^[62], through the impregnation reduction method, synthesized Pt-deposited Nafion^® membrane as cathode without using any surfactant. The obtained material showed improved fuel cell performance in comparison to other methods of synthesis. To create cathode formulations with cheaper characteristics than Pt, efforts are being addressed to develop Pd-, Rh-, and Ru-based catalysts. Pd is three times less expensive than Pt and was used in the work of Kumar et al. [94]^[10] to prepare a phosphorus-doped carbon-nanoparticles-supported palladium (Pd/P-CNPs) electrocatalyst. The structural modification of the carbon by phosphorus doping will be more effective than nitrogen doping since phosphorus has a much larger covalent radius (107 ± 3 pm) than carbon (73 ± 1 pm) compared with nitrogen (71 ± 1 pm). The synthesized electrocatalyst was used as the HER electrode for the fabrication of MEAs, and its performance was evaluated in house-fabricated PEMWE 25 cm² single-cell assemblies. The obtained results showed that the synthesized Pd/P-CNPs have shown similar electrochemical activity and stability compared to commercial Pt/C. Rh–P catalysts exhibit very good HER performances due to the introduction of P into the Rh catalyst material, which induces a ΔG_H* shift to more neutral values; this indicates greater active catalytic activity for a lower amount of Rh loading [97]^[13]. Facile fabrication of Rh and Rh–P electrodes on a carbon paper as substrate via electrodeposition at room temperature and ambient pressure was performed by Kim et al. [97]^[13] and evaluated for the acidic HER in terms of intrinsic and mass activity. Under the optimized deposition parameters, such as potential and time, a certain facet widespread at the surface of Rh electrodes (Rh (111) facet) demonstrated high intrinsic activity for HER in acidic medial, while the further enhancement of the catalyst performance was achieved by a modified electronic structure of Rh–P electrodes with intrinsic and mass activity greater than ones of Pt electrodes. Except for Rh–P, composites with different stoichiometric ratios of Rh and P are also highly represented within this field. Many different morphologies of Rh₂P are synthesized and tested for hydrogen production in acidic media with the potential of use in PEMWE. Yang et al. [147]^[63] performed a colloidal synthesis of monodisperse Rh₂P nanoparticles with an average size of 2.8 nm and with an overpotential of 140 mV achieved a current density of 10 mA cm⁻² in 0.5 M H₂SO₄, while Duan et al. [148]^[64] synthesized rhodium phosphide nanocubes supported on high surface area carbon (Rh₂P/NCs). In the case of Rh₂P/C, the overpotential at the current density of 5 mA cm⁻² is 5.4 mV, which is lower than Pt/C (8.0 mV) and Rh/C (68.4 mV). Carbon support was also used in the synthesis of wrinkled, ultrathin Rh₂P nanosheets (w-Rh₂P NS/C) for enhancing HER in 0.1 M HClO₄ [149]^[65]. To reach a current density of 10 mA cm⁻², the overpotential of 15.8 mV is required, which is 6.3 and 25.8 mV lower than those of commercial Pt/C (22.1 mV) and carbon-supported Rh nanosheets (Rh NS/C) (41.6 mV). The Tafel slope is 29.9 mV s⁻¹, which is comparable for commercial Pt/C and lower than that of Rh NS/C (37.4 mV/decade). The core-shell structure of obtaining composites used in hydrogen production is well known, and such morphology is also presented in the work of Pu et al. [150]^[66], who synthesized Rh₂P nanoparticles encapsulated in an N-doped carbon (NC) core-shell structure (Rh₂P@NC) achieving overpotential of 9 mV at 10 mA cm⁻² in 0.5 M H₂SO₄. Comparison in the electrocatalytic activity between Rh₂P@NC, Rh/NC, and Pt/C and belonging HER polarization curves shows that both Rh₂P@NC and Pt/C exhibit high HER catalytic activities with 0 mV onset overpotential (η_onset), which is much smaller than that of Rh/NC. Rh₂P@NC needs an overpotential (η₁₀) of 9 mV at the current density of 10 mA cm⁻² with the corresponding Tafel slope of 26 mV/decade. The last part of the electrochemical measurements examined stability test where, after 1000 cyclic voltametric (CV) cycles at a scan rate of 100 mV/s in 0.5 M H₂SO₄ solution, the polarization curve retains an almost similar performance to the initial test. Besides N-doping, carbon-supported materials can also be double-codoped with nitrogen and phosphorus. Two electrocatalysts composed of N and P codoped carbon (NPC) modified with noble metal phosphides (Rh_xP/NPC and RuP/NPC) with a low loading of Rh (≈0.4 wt%) and Ru (≈0.5 wt%) achieved promising electrocatalytic activities [139]^[55]. Rh_xP/NPC delivers Pt-like HER activity with an ultralow overpotential at 10 mA cm⁻² (19 mV) and a small Tafel slope (36 mV/decade), while the RuP/NPC requires overpotential of 125 mV to achieve 10 mA cm⁻² and a Tafel slope of 107 mV/decade. Besides metal phosphides, conductive oxides can also be considered as potential catalysts for the HER. The advantage of such materials is that, unlike their metallic counterparts and most prominently Pt, they are not prone to poisoning by underpotential deposition of less active metals that are always presented in the form of impurities in technological electrolytes [151]^[67]. Ru and Ir thin films, as well as their corresponding thermally oxidized RuO₂ and IrO₂ thin films, were developed by Cherevko et al. [151]^[67] and evaluated for HER in 0.1 M H₂SO₄. Metals, exhibit more extensive dissolution are found to be more active in catalyzing the hydrogen production, while metal oxides are easily blocked by hydrogen bubbles and show no dissolution during HER. Based on the results, it can be concluded that oxides may be considered to catalyze HER in case Pt contamination is an issue; even though metals are more active, their application as cathode materials is not feasible due to low stability. The dissolution of metals in acidic solutions is 2–3 magnitudes higher compared to their respective oxides. Table 82 contains selected cathode materials recently synthesized and tested under acidic conditions with a high potential for later application in PEM water electrolysis, according to listed electrochemical parameters (overpotential at current density, Tafel slope, and stability) that are very close to the benchmark 20% Pt/C cathode catalyst material.

2.2. HER Electrocatalysts with Reduced Noble Metals Content

To achieve an environmentally sustainable society, the reduction in the consumption of noble metals is a topic of great importance. In the section above, substituted noble metals alternatives are shown; even though much scientific effort has been put to develop new and prosperous materials, their activities are rarely comparable to that of the benchmark catalyst Pt/C, and their application is still limited in real energy devices. Also, a lot of developed materials are only being tested in acidic media, without further tests in PEMWE systems, which should be another step forward to fully understand their potential use. For these reasons, strategies to synthesize catalysts have mostly been focused on the reduction in the content of noble metals, which is considered a more practical strategy for accelerating their industrial application [139]^[55