Novel components in Proton Exchange Membrane (PEM) Water Electrolyzers (PEMWE): Status, challenges and future needs. A mini review

Recent Advances in Polymer Electrolyte Membrane Water Electrolyzer Stack Development Studies: A Review

Polymer electrolyte membrane water electrolyzers have significant advantages over other electrolyzers, such as compact design, high efficiency, low gas permeability, fast response, high-pressure operation (up to 200 bar), low operating temperature (20-80 °C), lower power consumption, and high current density. Moreover, polymer electrolyte membrane water electrolyzers are a promising technology for sustainable hydrogen production due to their easy adaptability to renewable energy sources. However, the cost of expensive electrocatalysts and other construction equipment must be reduced for the widespread usage of polymer electrolyte membrane water electrolyzer technology. In this review, recent improvements made in developing the polymer electrolyte membrane water electrolyzer stack are summarized. First, we present a brief overview of the working principle of polymer electrolyte membrane water electrolyzers. Then, we discuss the components of polymer electrolyte membrane water electrolyzers (base materials such as membranes, gas diffusion layers, electrocatalysts, and bipolar plates) and their particular functions. We also provide an overview of polymer electrolyte membrane water electrolyzer's material technology, production technology, and commercialization issues. We finally present recent advancements of polymer electrolyte membrane water electrolyzer stack developments and their recent developments under different operating conditions.

Interface-Strengthened Ru-Based Electrocatalyst for High-Efficiency Proton Exchange Membrane Water Electrolysis at Industrial-Level Current Density

Developing an OER electrocatalyst that balances high performance with low cost is crucial for widely adopting PEM water electrolyzers. Ru-based catalysts are gaining attention for their cost-effectiveness and high activity, positioning them as promising alternatives to Ir-based catalysts. However, Ru-based catalysts can be prone to oxidation at high potentials, compromising their durability. In this study, we utilize a simple synthesis method to synthesize a SnO2, Nb2O5, and RuO2 composite catalyst (SnO2/Nb2O5@RuO2) with multiple interfaces and abundant oxygen vacancies. The large surface area and numerous active sites of the SnO2/Nb2O5@RuO2 catalyst lead to outstanding acidic oxygen evolution reaction (OER) performance, achieving current densities of 10, 50, and 200 mA cm-2 at ultralow overpotentials of 287, 359, and 534 mV, respectively, significantly surpassing commercial IrO2. Moreover, incorporating Nb2O5 into the SnO2/Nb2O5@RuO2 alters the electronic structure at the interfaces and generates a high density of oxygen vacancies, markedly enhancing durability. Consequently, the membrane electrode composed of SnO2/Nb2O5@RuO2 and commercial Pt/C demonstrated stable operation in the PEM cell for 25 days at an industrial current density of 1 A cm-2. This research presents a convenient approach for developing a highly efficient and durable Ru-based electrocatalyst, underscoring its potential for proton exchange membrane water electrolysis.

Designing a Schottky Barrier-Free Interface for a Highly Conductive Anode in Proton Exchange Membrane Water Electrolysis

Iridium, the most widely used anode catalyst in proton exchange membrane water electrolysis (PEMWE), must be used minimally due to its high price and limited supply. However, reducing iridium loading poses challenges due to abnormally large anode polarization. Herein, we present an anode catalyst layer (CL) based on a one-dimensional iridium nanofiber that enables a high current density operation of 3 A cm-2 at 1.86 V, even at an ultralow loading (0.07 mgIr cm-2). The performance is maintained even with a Pt coating-free porous transport layer (PTL) because our nanofiber CL circumvents the interfacial electron transport problem caused by the native oxide on the Ti PTL. We attribute this to the low work function and the low-ionomer-exposed surface of the nanofiber CL, which prevent the formation of Schottky contact at the native oxide interface. These results highlight the significance of optimizing the electronic properties of the CL/PTL interface for low-iridium-loading PEMWE.

Acidic Oxygen Evolution Reaction: Fundamental Understanding and Electrocatalysts Design

Water electrolysis driven by "green electricity" is an ideal technology to realize energy conversion and store renewable energy into hydrogen. With the development of proton exchange membrane (PEM), water electrolysis in acidic media suitable for many situations with an outstanding advantage of high gas purity has attracted significant attention. Compared with hydrogen evolution reaction (HER) in water electrolysis, oxygen evolution reaction (OER) is a kinetic sluggish process that needs a higher overpotential. Especially in acidic media, OER process poses higher requirements for the electrocatalysts, such as high efficiency, high stability and low costs. This review focuses on the acidic OER electrocatalysis, reaction mechanisms, and critical parameters used to evaluate performance. Especially the modification strategies applied in the design and construction of new-type electrocatalysts are also summarized. The characteristics of traditional noble metal-based electrocatalysts and the noble metal-free electrocatalysts developed in recent decades are compared and discussed. Finally, the current challenges for the most promising acidic OER electrocatalysts are presented, together with a perspective for future water electrolysis.

New triazinephosphonate dopants for Nafion proton exchange membranes (PEM)

A new paradigm for energy is underway demanding decarbonized energy systems. Some of them rely on emerging electrochemical devices, crucial in hydrogen technologies, including fuel cells, CO2 and water electrolysers, whose applications and performances depend on key components such as their separators/ion-exchange membranes. The most studied and already commercialized Nafion membrane shows great chemical stability, but its water content limits its high proton conduction to a limited range of operating temperatures. Here, we report the synthesis of a new series of triazinephosphonate derivatives and their use as dopants in the preparation of new modified Nafion membranes. The triazinephosphonate derivatives were prepared by substitution of chlorine atoms in cyanuric chloride. Diverse conditions were used to obtain the trisubstituted (4-hydroxyphenyl)triazinephosphonate derivatives and the (4-aminophenyl)triazinephosphonate derivatives, but with these amino counterparts, only the disubstituted compounds were obtained. The new modified Nafion membranes were prepared by casting incorporation of the synthesized 1,3,5-triazinephosphonate (TPs) derivatives. The evaluation of the proton conduction properties of the new membranes and relative humidity (RH) conditions and at 60 °C, showed that they present higher proton conductivities than the prepared Nafion membrane and similar or better proton conductivities than commercial Nafion N115, in the same experimental conditions. The Nafion-doped membrane with compound TP2 with a 1.0 wt % loading showed the highest proton conductivity with 84 mS·cm-1.

Ce-radical Scavenger-Based Perfluorosulfonic Acid Aquivion® Membrane for Pressurised PEM Electrolysers

A Ce-radical scavenger-based perfluorosulfonic acid (PFSA) Aquivion® membrane (C98 05S-RSP) was developed and assessed for polymer electrolyte membrane (PEM) electrolyser applications. The membrane, produced by Solvay Specialty Polymers, had an equivalent weight (EW) of 980 g/eq and a thickness of 50 μm to reduce ohmic losses at a high current density. The electrochemical properties and gas crossover through the membrane were evaluated upon the formation of a membrane-electrode assembly (MEA) in a range of temperatures between 30 and 90 °C and at various differential pressures (ambient, 10 and 20 bars). Bare extruded (E98 05S) and reinforced (R98 05S) PFSA Aquivion® membranes with similar EWs and thicknesses were assessed for comparison in terms of their performance, stability and hydrogen crossover under the same operating conditions. The method used for the membrane manufacturing significantly influenced the interfacial properties, with the electrodes affecting the polarisation resistance and H2 permeation in the oxygen stream, as well as the degradation rate, as observed in the durability studies.

Alternative to Conventional Solutions in the Development of Membranes and Hydrogen Evolution Electrocatalysts for Application in Proton Exchange Membrane Water Electrolysis: A Review

Proton exchange membrane water electrolysis (PEMWE) represents promising technology for the generation of high-purity hydrogen using electricity generated from renewable energy sources (solar and wind). Currently, benchmark catalysts for hydrogen evolution reactions in PEMWE are highly dispersed carbon-supported Pt-based materials. In order for this technology to be used on a large scale and be market competitive, it is highly desirable to better understand its performance and reduce the production costs associated with the use of expensive noble metal cathodes. The development of non-noble metal cathodes poses a major challenge for scientists, as their electrocatalytic activity still does not exceed the performance of the benchmark carbon-supported Pt. Therefore, many published works deal with the use of platinum group materials, but in reduced quantities (below 0.5 mg cm-2). These Pd-, Ru-, and Rh-based electrodes are highly efficient in hydrogen production and have the potential for large-scale application. Nevertheless, great progress is needed in the field of water electrolysis to improve the activity and stability of the developed catalysts, especially in the context of industrial applications. Therefore, the aim of this review is to present all the process features related to the hydrogen evolution mechanism in water electrolysis, with a focus on PEMWE, and to provide an outlook on recently developed novel electrocatalysts that could be used as cathode materials in PEMWE in the future. Non-noble metal options consisting of transition metal sulfides, phosphides, and carbides, as well as alternatives with reduced noble metals content, will be presented in detail. In addition, the paper provides a brief overview of the application of PEMWE systems at the European level and related initiatives that promote green hydrogen production.

Dense-Packed RuO2 Nanorods with In Situ Generated Metal Vacancies Loaded on SnO2 Nanocubes for Proton Exchange Membrane Water Electrolyzer with Ultra-Low Noble Metal Loading

Proton exchange membrane water electrolyzer (PEMWE) is a green hydrogen production technology that can be coupled with intermittent power sources such as wind and photoelectric power. To achieve cost-effective operations, low noble metal loading on the anode catalyst layer is desired. In this study, a catalyst with RuO2 nanorods coated outside SnO2 nanocubes is designed, which forms continuous networks and provides high conductivity. This allows for the reduction of Ru contents in catalysts. Furthermore, the structure evolutions on the RuO2 surface are carefully investigated. The etched RuO2 surfaces are seen as the consequence of Co leaching, and theoretical calculations demonstrate that it is more effective in driving oxygen evolution. For electrochemical tests, the catalysts with 23 wt% Ru exhibit an overpotential of 178 mV at 10 mA cm-2 , which is much higher than most state-of-art oxygen evolution catalysts. In a practical PEMWE, the noble metal Ru loading on the anode side is only 0.3 mg cm-2 . The cell achieves 1.61 V at 1 A cm-2 and proper stability at 500 mA cm-2 , demonstrating the effectiveness of the designed catalyst.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.

Hydrocarbon-Based Composite Membrane Using LCP-Nonwoven Fabrics for Durable Proton Exchange Membrane Water Electrolysis

A new hydrocarbon-based (HC) composite membrane was developed using liquid crystal polymer (LCP)-nonwoven fabrics for application in proton exchange membrane water electrolysis (PEMWE). A copolymer of sulfonated poly(arylene ether sulfone) with a sulfonation degree of 50 mol% (SPAES50) was utilized as an ionomer for the HC membranes and impregnated into the LCP-nonwoven fabrics without any surface treatment of the LCP. The physical interlocking structure between the SPAES50 and LCP-nonwoven fabrics was investigated, validating the outstanding mechanical properties and dimensional stability of the composite membrane in comparison to the pristine membrane. In addition, the through-plane proton conductivity of the composite membrane at 80 °C was only 15% lower than that of the pristine membrane because of the defect-free impregnation state, minimizing the decrease in the proton conductivity caused by the non-proton conductive LCP. During the electrochemical evaluation, the superior cell performance of the composite membrane was evident, with a current density of 5.41 A/cm² at 1.9 V, compared to 4.65 A/cm² for the pristine membrane, which can be attributed to the smaller membrane resistance of the composite membrane. From the results of the degradation rates, the prepared composite membrane also showed enhanced cell efficiency and durability during the PEMWE operations.

An electrochemically fabricated cobalt iron oxyhydroxide bifunctional electrode for an anion exchange membrane water electrolyzer

For an anion exchange membrane water electrolyzer (AEMWE), exploring bifunctional electrodes with low cost and high efficiency is a crucial task for future renewable energy systems. Herein, we report a simple method to fabricate cobalt iron oxyhydroxide (Co_zFe_1-zO_xH_y) bifunctional electrodes for AEMWEs. The bifunctional electrodes were prepared via one-pot electrodeposition on Ti paper (TP). By adjusting the electrodeposition conditions, the morphology and composition of Co_zFe_1-zO_xH_y/TP could be controlled. The Co₆₅Fe₃₅O_xH_y/TP electrode demonstrated the highest activity for overall water electrolysis owing to the maximized synergy effect between Co and Fe. The bifunctional activities of Co₆₅Fe₃₅O_xH_y/TP were well retained at -50 and 50 mA cm^-2 for 12 h. Co₆₅Fe₃₅O_xH_y/TP, which shows the highest bifunctional activity, was employed in an AEMWE single cell as the anode and cathode. The AEMWE single cell employing Co₆₅Fe₃₅O_xH_y/TP showed a current density of 0.605 A cm^-2 at a cell voltage of 2.0 V_cell. The calculated energy efficiency of the single cell is 55.7% at 2.0 A cm^-2, which is comparable with those of the state-of-the-art AEMWE single cells with bifunctional electrodes. Furthermore, the cell voltage of the single cell with Co₆₅Fe₃₅O_xH_y/TP showed negligible degradation for 50 h at 0.6 A cm^-2.

Unveiling the Metal Incorporation Effect of Steady-Active FeP Hydrogen Evolution Nanocatalysts for Water Electrolyzer

Metal phosphides are promising noble metal-free electrocatalysts for hydrogen evolution reaction (HER), but they usually suffer from inferior stability and thus are far from the device applications. We reported a facile and controllable synthetic method to prepare metal-incorporated M-FeP nanoparticles (M=Cr, Mn, Co, Fe, Ni, Cu, and Mo) with the guide of the density functional theory (DFT). The evaluated HER activity sequence was consistent with the DFT predictions, and cobalt was revealed to be the appropriate dopant. With the optimization of the Co/Fe ratio, the Fe_0.67 Co_0.33 P/C only required overpotentials of 67 mV and 129 mV to obtain the cathodic current density of 10 and 100 mA cm^-2, respectively. It maintained the initial activity in the 10 h stability test, surpassing the other Co-FeP/C catalysts. Ex situ experiments demonstrated that the decreased element leaching and the increased surface phosphide content contributed to the high stability of the Fe_0.67 Co_0.33 P/C. A proton exchange membrane water electrolyzer was assembled using the Fe_0.67 Co_0.33 P/C as the cathodic catalyst. It showed a current density of 0.8 A cm^-2 at the applied voltage of 2.0 V and retained the initial activity in the 1000 cycles' stability test, suggesting the potential application of the catalysts.

Transition metal oxides with perovskite and spinel structures for electrochemical energy production applications

Transition metal oxide-based materials are an interesting alternative to substitute noble-metal based catalyst in energy conversion devices designed for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution reactions (HER). Perovskite (ABO₃) and spinel (AB₂O₄) oxides stand out against other structures due to the possibility of tailoring their chemical composition and, consequently, their properties. Particularly, the electrocatalytic performance of these materials depends on features such as chemical composition, crystal structure, nanostructure, cation substitution level, e_g orbital filling or oxygen vacancies. However, they suffer from low electrical conductivity and surface area, which affects the catalytic response. To mitigate these drawbacks, they have been combined with carbon materials (e.g. carbon black, carbon nanotubes, activated carbon, and graphene) that positively influence the overall catalytic activity. This review provides an overview on tunable perovskites (mainly lanthanum-based) and spinels featuring 3d metal cations such as Mn, Fe, Co, Ni and Cu on octahedral sites, which are known to be active for the electrochemical energy conversion.

Supported Iridium-based Oxygen Evolution Reaction Electrocatalysts - Recent Developments

The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

Nafion Composite Membranes Impregnated with Polydopamine and Poly(Sulfonated Dopamine) for High-Performance Proton Exchange Membranes

We prepared Nafion composite membranes by impregnating Nafion-212 with polydopamine, poly(sulfonated dopamine), and poly(dopamine-co-sulfonated dopamine) using the swelling-filling method to generate nanopores in the Nafion framework that were filled with these polymers. Compared to the pristine Nafion-212 membrane, these composite membranes showed improved thermal and mechanical stabilities due to the strong interactions between the catecholamine of the polydopamine derivatives and the Nafion matrix. For the composite membrane filled with poly(sulfonated dopamine) (N-PSDA), further interactions were induced between the Nafion and the sulfonic acid side chain, resulting in enhanced water uptake and ion conductivity. In addition, filling the nanopores in the Nafion matrix with polymer fillers containing aromatic hydrocarbon-based dopamine units led to an increase in the degree of crystallinity and resulted in a significant decrease in the hydrogen permeability of the composite membranes compared to Nafion-212. Hydrogen crossovers 26.8% lower than Nafion-212 at 95% relative humidity (RH) (fuel cell operating conditions) and 27.3% lower at 100% RH (water electrolysis operating conditions) were obtained. When applied to proton exchange membrane-based fuel cells, N-PSDA exhibited a peak power density of 966 mW cm^-2, whereas N-PSDA showed a current density of 4785 mA cm^-2, which is 12.4% higher than Nafion-212 at 2.0 V and 80 °C.