Durable Membrane Electrode Assemblies for Proton Exchange Membrane Electrolyzer Systems Operating at High Current Densities

Galvanostatic Electroshock Synthesis of Low Loading Au-Pt Nanoalloys Onto Gas Diffusion Electrodes as Multifunctional Electrocatalysts for a Glycerol-Fed Electrolyzer

Water electrolysis is increasingly considered a viable solution for meeting the world's growing energy demands and mitigating environmental issues. An inventive strategy to mitigate the energy requirements involves substituting the energy-intensive oxygen evolution reaction (OER) with biomass-derived glycerol electrooxidation. Nonetheless, the synthesis of electrocatalysts for controlling the selectivity towards added-value chemicals at the anode and efficient H2 generation at the cathode remains a critical bottleneck. Herein, we implemented a galvanostatic electroshock synthesis approach to control the reduction kinetics of Au(III) and Pt(IV) to grow ultra-low amount of gold-platinum alloys on a gas diffusion electrode (12-26 μgmetal cm-2) for glycerol-fed hydroxide anion exchange membrane based electrolyzer. The symmetric GDE-Au100-xPtx||GDE-Au100-xPtx systems showed a notable improvement in electrolyzer performance (GDE-Au64Pt36=201 mA cm-2) as compared to monometallic versions (GDE-Au100Pt0=18 mA cm-2, GDE-Au0Pt100=81 mA cm-2). Chromatography (HPLC) analysis underscores the critical importance of bulk electrolysis methodology (galvanostatic vs potentiostatic) for the efficient conversion of glycerol into high-value-added products. Regarding the electrical energy required to produce 1 kg of H2 for such an electrolyzer fed at the anode with glycerol, our results confirm a drastic decrease by a factor of at least two compared with conventional water electrolysis.

Insight into Oxygen Transport in Proton Exchange Membrane Water Electrolyzers by In Situ X-Ray Characterization

The proton exchange membrane water electrolyzer (PEMWE) is one of the most promising electrochemical energy conversion devices for hydrogen production, while still limited by performance bottlenecks at high current densities, due to the lack of mass transfer insights. To investigate the mechanisms of oxygen transport inside the PEMWE at high current density and its relation to electrolytic performance. Operational in situ x-ray imaging is utilized to simultaneously characterize the bubble behavior and voltage response in a novel designed visual mini-cell, and it is identified that oxygen evolution and transport in the PEMWE follow the process of bubble nucleation, growth, and detachment. Based on the results of mini-cells with three porous transport layers (PTLs) up to 9 A cm-2 operation, it revealed that critical current densities exist for both carbon-based and titanium-based PTLs. Once exceeding the critical current density, the cell voltage can no longer be stabilized and the cell exhibits a significant oxygen overpotential. To illustrate this, the concept of interfacial separation zone (ISZ) is first proposed, which is an effective pathway for bubble growth and separation and the pattern of the ISZ exhibits specific regimes with the critical current density. Ultimately, a new approach for better understanding the mechanisms of oxygen transport is revealed.

Advances in Noble Metal Electrocatalysts for Acidic Oxygen Evolution Reaction: Construction of Under-Coordinated Active Sites

Renewable energy-driven proton exchange membrane water electrolyzer (PEMWE) attracts widespread attention as a zero-emission and sustainable technology. Oxygen evolution reaction (OER) catalysts with sluggish OER kinetics and rapid deactivation are major obstacles to the widespread commercialization of PEMWE. To date, although various advanced electrocatalysts have been reported to enhance acidic OER performance, Ru/Ir-based nanomaterials remain the most promising catalysts for PEMWE applications. Therefore, there is an urgent need to develop efficient, stable, and cost-effective Ru/Ir catalysts. Since the structure-performance relationship is one of the most important tools for studying the reaction mechanism and constructing the optimal catalytic system. In this review, the recent research progress from the construction of unsaturated sites to gain a deeper understanding of the reaction and deactivation mechanism of catalysts is summarized. First, a general understanding of OER reaction mechanism, catalyst dissolution mechanism, and active site structure is provided. Then, advances in the design and synthesis of advanced acidic OER catalysts are reviewed in terms of the classification of unsaturated active site design, i.e., alloy, core-shell, single-atom, and framework structures. Finally, challenges and perspectives are presented for the future development of OER catalysts and renewable energy technologies for hydrogen production.

Grain-Boundary-Rich RuO2 Porous Nanosheet for Efficient and Stable Acidic Water Oxidation

RuO2 has been considered as the most likely acidic oxygen evolution reaction (OER) catalyst to replace IrO2, but its performance, especially long-term stability under harsh acidic conditions, is still unacceptable. Here, we propose a grain boundary (GB) engineering strategy by fabricating the ultrathin porous RuO2 nanosheet with abundant of grain boundaries (GB-RuO2) as an efficient acid OER catalyst. The involvement of GB induces significant tensile stress and creates an unsaturated coordination environment, effectively optimizing the adsorption of intermediates and stabilizing active site structure during OER process. Notably, the GB-RuO2 not only exhibits a low overpotential (η10=187 mV) with an ultra-low Tafel slope (34.5 mV dec-1), but also steadily operates for over 550 h in 0.1 M HClO4. Quasi in situ/operando methods confirm that the improved stability is attributed to GB preventing Ru dissolution and greatly inhibiting the lattice oxygen oxidation mechanism (LOM). A proton exchange membrane water electrolysis (PEMWE) using the GB-RuO2 catalyst operates a low voltage of 1.669 V at 2 A cm-2 and operates stably for 100 h at 100 mA cm-2.

Optimizing Ionomer Distribution in Anode Catalyst Layer for Stable Proton Exchange Membrane Water Electrolysis

The high cost of proton exchange membrane water electrolysis (PEMWE) originates from the usage of precious materials, insufficient efficiency, and lifetime. In this work, an important degradation mechanism of PEMWE caused by dynamics of ionomers over time in anode catalyst layer (ACL), which is a purely mechanical degradation of microstructure, is identified. Contrary to conventional understanding that the microstructure of ACL is static, the micropores are inclined to be occupied by ionomers due to the localized swelling/creep/migration, especially near the ACL/PTL (porous transport layer) interface, where they form transport channels of reactant/product couples. Consequently, the ACL with increased ionomers at PTL/ACL interface exhibit rapid and continuous degradation. In addition, a close correlation between the microstructure of ACL and the catalyst ink is discovered. Specifically, if more ionomers migrate to the top layer of the ink, more ionomers accumulate at the ACL/PEM interface, leaving fewer ionomers at the ACL/PTL interface. Therefore, the ionomer distribution in ACL is successfully optimized, which exhibits reduced ionomers at the ACL/PTL interface and enriches ionomers at the ACL/PEM interface, reducing the decay rate by a factor of three when operated at 2.0 A cm-2 and 80 °C. The findings provide a general way to achieve low-cost hydrogen production.

Stability of Ni-Fe-Layered Double Hydroxide Under Long-Term Operation in AEM Water Electrolysis

Anion exchange membrane water electrolysis (AEMWE) is an attractive method for green hydrogen production. It allows the use of non-platinum group metal catalysts and can achieve performance comparable to proton exchange membrane water electrolyzers due to recent technological advances. While current systems already show high performances with available materials, research gaps remain in understanding electrode durability and degradation behavior. In this study, the performance and degradation tracking of a Ni3Fe-LDH-based single-cell is implemented and investigated through the correlation of electrochemical data using chemical and physical characterization methods. A performance stability of 1000 h, with a degradation rate of 84 µV h-1 at 1 A cm-2 is achieved, presenting the Ni3Fe-LDH-based cell as a stable and cost-attractive AEMWE system. The results show that the conductivity of the formed Ni-Fe-phase is one key to obtaining high electrolyzer performance and that, despite Fe leaching, change in anion-conducting binder compound, and morphological changes inside the catalyst bulk, the Ni3Fe-LDH-based single-cells demonstrate high performance and durability. The work reveals the importance of longer stability tests and presents a holistic approach of electrochemical tracking and post-mortem analysis that offers a guideline for investigating electrode degradation behavior over extended measurement periods.

Constructing Robust 3D Ionomer Networks in the Catalyst Layer to Achieve Stable Water Electrolysis for Green Hydrogen Production

The widespread application of proton exchange membrane water electrolyzers (PEMWEs) is hampered by insufficient lifetime caused by degradation of the anode catalyst layer (ACL). Here, an important degradation mechanism has been identified, attributed to poor mechanical stability causing the mass transfer channels to be blocked by ionomers under operating conditions. By using liquid-phase atomic force microscopy, we directly observed that the ionomers were randomly distributed (RD) in the ACL, which occupied the mass transfer channels due to swelling, creeping, and migration properties. Interestingly, we found that alternating treatments of the ACL in different water/temperature environments resulted in forming three-dimensional ionomer networks (3D INs) in the ACL, which increased the mechanical strength of microstructures by 3 times. Benefitting from the efficient and stable mass transfer channels, the lifetime was improved by 19 times. A low degradation rate of approximately 3.0 μV/h at 80 °C and a high current density of 2.0 A/cm2 was achieved on a 50 cm2 electrolyzer. These data demonstrated a forecasted lifetime of 80 000 h, approaching the 2026 DOE lifetime target. This work emphasizes the importance of the mechanical stability of the ACL and offers a general strategy for designing and developing a durable PEMWE.

Advanced membrane-based electrode engineering toward efficient and durable water electrolysis and cost-effective seawater electrolysis in membrane electrolyzers

Researchers have been seeking for the most technically-economical water electrolysis technology for entering the next-stage of industrial amplification for large-scale green hydrogen production. Various membrane-based electrolyzers have been developed to improve electric-efficiency, reduce the use of precious metals, enhance stability, and possibly realize direct seawater electrolysis. While electrode engineering is the key to approaching these goals by bridging the gap between catalysts design and electrolyzers development, nevertheless, as an emerging field, has not yet been systematically analyzed. Herein, this review is organized to comprehensively discuss the recent progresses of electrode engineering that have been made toward advanced membrane-based electrolyzers. For the commercialized or near-commercialized membrane electrolyzer technologies, the electrode material design principles are interpreted and the interface engineering that have been put forward to improve catalytic sites utilization and reduce precious metal loading is summarized. Given the pressing issues of electrolyzer cost reduction and efficiency improvement, the electrode structure engineering toward applying precious metal free electrocatalysts is highlighted and sufficient accessible sites within the thick catalyst layers with rational electrode architectures and effective ions/mass transport interfaces are enabled. In addition, this review also discusses the innovative ways as proposed to break the barriers of current membrane electrolyzers, including the adjustments of electrode reaction environment, and the feasible cell-voltage-breakdown strategies for durable direct seawater electrolysis. Hopefully, this review may provide insightful information of membrane-based electrode engineering and inspire the future development of advanced membrane electrolyzer technologies for cost-effective green hydrogen production.

Review on Bubble Dynamics in Proton Exchange Membrane Water Electrolysis: Towards Optimal Green Hydrogen Yield

Water electrolysis using a proton exchange membrane (PEM) holds substantial promise to produce green hydrogen with zero carbon discharge. Although various techniques are available to produce hydrogen gas, the water electrolysis process tends to be more cost-effective with greater advantages for energy storage devices. However, one of the challenges associated with PEM water electrolysis is the accumulation of gas bubbles, which can impair cell performance and result in lower hydrogen output. Achieving an in-depth knowledge of bubble dynamics during electrolysis is essential for optimal cell performance. This review paper discusses bubble behaviors, measuring techniques, and other aspects of bubble dynamics in PEM water electrolysis. It also examines bubble behavior under different operating conditions, as well as the system geometry. The current review paper will further improve the understanding of bubble dynamics in PEM water electrolysis, facilitating more competent, inexpensive, and feasible green hydrogen production.

The thermodynamic and life-cycle assessments of a novel charging station for electric vehicles in dynamic and steady-state conditions

The current study performs the thermodynamic and life-cycle assessments (LCA) of a novel charging station in two system designs. The goal is to design an efficient charging station for electric vehicles with high efficiencies and low environmental impacts using Solid Oxide Fuel Cell (SOFC) technology. SOFC is considered a sustainable and environmentally friendly technology to generate electricity compared to combustion engines. To ameliorate the performance, the exhaust heat of the SOFC stacks will be recovered for hydrogen production in an electrolyzer. The system uses four SOFCs to charge the electric vehicles while the output heat is recovered by an Organic Rankine Cycle (ORC) to generate further electricity for hydrogen production in an electrolyzer. In the first design, it is assumed that the SOFC stacks will work full-load during the 24 h of the day, while the second design considers full-load operation for 16 h and part-load (30%) operation for 8 h. The second design of the system analyzes the possibility of integrating a [Formula: see text] lithium-ion battery stores the excessed electricity once the power load is low and acts as a backup in high power demands. Results of the thermodynamic analysis calculated the overall efficiencies of 60.84% and 60.67% for the energy and exergy, respectively, with the corresponding power and hydrogen production of 284.27 kWh and 0.17 g/s. It was observed that higher current density would increase the output of SOFC while reducing the overall energy and exergy efficiencies. In dynamic operation, the use of the batteries can well balance the change of the power loads and improve the dynamic response of the system to the simultaneous changes in the power demand. LCA results also showed that the 284.27kWh system leads to global warming (kg [Formula: see text] eq) of 5.17E+05, 4.47E+05, and 5.17E+05 using Solid Oxide Electrolyzer (SOE), Proton Exchange Membrane Electrolyzer (PEME), and Alkaline Electrolyzer (ALE), respectively. In this regard, the usage of PEME has the lowest impact on the environment in comparison to SOEC, and ALE. A comparison between the environmental impacts of different ORC's working fluids also suggested against the usage of R227ea while R152a showed promising results to be used in the system. The size and weight study also revealed that the battery benefits from the lowest volume and weight in comparison to the other components. Among the considered components in this study, the SOFC unit and the PEME have by far the highest volume.

IrOx·nH2O with lattice water-assisted oxygen exchange for high-performance proton exchange membrane water electrolyzers

The trade-off between activity and stability of oxygen evolution reaction (OER) catalysts in proton exchange membrane water electrolyzer (PEMWE) is challenging. Crystalline IrO₂ displays good stability but exhibits poor activity; amorphous IrO_x exhibits outstanding activity while sacrificing stability. Here, we combine the advantages of these two materials via a lattice water-incorporated iridium oxide (IrO_x·nH₂O) that has short-range ordered structure of hollandite-like framework. We confirm that IrO_x·nH₂O exhibits boosted activity and ultrahigh stability of >5700 hours (~8 months) with a record-high stability number of 1.9 × 10⁷ n_oxygen n_Ir^-1. We evidence that lattice water is active oxygen species in sustainable and rapid oxygen exchange. The lattice water-assisted modified OER mechanism contributes to improved activity and concurrent stability with no apparent structural degradation, which is different to the conventional adsorbate evolution mechanism and lattice oxygen mechanism. We demonstrate that a high-performance PEMWE with IrO_x·nH₂O as anode electrocatalyst delivers a cell voltage of 1.77 V at 1 A cm^-2 for 600 hours (60°C).

Analysis of PEM Water Electrolyzer Failure Due to Induced Hydrogen Crossover in Catalyst-Coated PFSA Membranes

Polymer electrolyte membrane water electrolysis (PEMWE) is a leading candidate for the development of a sustainable hydrogen infrastructure. The heart of a PEMWE cell is represented by the membrane electrode assembly (MEA), which consists of a polymer electrolyte membrane (PEM) with catalyst layers (CLs), flow fields, and bipolar plates (BPPs). The weakest component of the system is the PEM, as it is prone to chemical and mechanical degradation. Membrane chemical degradation is associated with the formation of hydrogen peroxide due to the crossover of product gases (H₂ and O₂). In this paper, membrane failure due to H₂ crossover was addressed in a membrane-focused accelerated stress test (AST). Asymmetric H₂O and gas supply were applied to a test cell in OCV mode at two temperatures (60 °C and 80 °C). Electrochemical characterization at the beginning and at the end of testing revealed a 1.6-fold higher increase in the high-frequency resistance (HFR) at 80 °C. The hydrogen crossover was measured with a micro-GC, and the fluoride emission rate (FER) was monitored during the ASTs. A direct correlation between the FER and H₂ crossover was identified, and accelerated membrane degradation at higher temperatures was detected.

PGM-Free Electrocatalytic Layer Characterization by Electrochemical Impedance Spectroscopy of an Anion Exchange Membrane Water Electrolyzer with Nafion Ionomer as the Bonding Agent

Low-cost anion exchange membrane (AEM) water electrolysis is a promising technology for producing “green” high-purity hydrogen using platinum group metal (PGM)-free catalysts. The performance of AEM electrolysis depends on the overall overvoltage, e.g., voltage losses coming from different processes in the water electrolyzer including hydrogen and oxygen evolution, non-faradaic charge transfer resistance, mass transfer limitations, and others. Due to the different relaxation times of these processes, it is possible to unravel them in the frequency domain by electrochemical impedance spectroscopy. This study relates to solving and quantifying contributions to the total polarization resistance of the AEM water electrolyzer, including ohmic and charge transfer resistances in the kinetically controlled mode. The high-frequency contribution is proposed to have non-faradaic nature, and its conceivable nature and mechanism are discussed. The characteristic frequencies of unraveled contributions are provided to be used as benchmark data for commercially available membranes and electrodes.

Highly active and stable OER electrocatalysts derived from Sr2MIrO6 for proton exchange membrane water electrolyzers

Proton exchange membrane water electrolysis is a promising technology to produce green hydrogen from renewables, as it can efficiently achieve high current densities. Lowering iridium amount in oxygen evolution reaction electrocatalysts is critical for achieving cost-effective production of green hydrogen. In this work, we develop catalysts from Ir double perovskites. Sr2CaIrO6 achieves 10 mA cm-2 at only 1.48 V. The surface of the perovskite reconstructs when immersed in an acidic electrolyte and during the first catalytic cycles, resulting in a stable surface conformed by short-range order edge-sharing IrO6 octahedra arranged in an open structure responsible for the high performance. A proton exchange membrane water electrolysis cell is developed with Sr2CaIrO6 as anode and low Ir loading (0.4 mgIr cm-2). The cell achieves 2.40 V at 6 A cm-2 (overload) and no loss in performance at a constant 2 A cm-2 (nominal load). Thus, reducing Ir use without compromising efficiency and lifetime.

Ultrafine IrRu Nanoparticles toward Efficient Oxygen Evolution Reaction in Acidic Media

Proton exchange membrane water electrolyzers (PEMWEs) are capable of mass-producing green hydrogen with renewable and wave-trough electricity, but confront the challenge of the lack of advanced electrocatalysts to accelerate sluggish oxygen evolution reaction (OER). Herein, we report the synthesis of ultrafine IrRu alloy nanoparticles (1.6 ± 0.3 nm) by coprecipitation of IrCl₃, RuCl₃, and HCOONa in water to allow Ir³⁺ and Ru³⁺ to be well dispersed and enclosed in the matrix of crystalline HCOONa, followed by heat treatment of HCOONa to reduce Ir³⁺ and Ru³⁺. Remarkably, the overpotential of IrRu toward acidic OER at 10 mA cm^-2 is merely 230 and 194 mV at 51 and 204 μg_IrRu cm^-2, respectively. The high electrochemically active surface area (ECSA) of 577.1 m² g^-1 and high specific activity (SA) of 22.7 μA cm^-2 at 1.45 V vs RHE would contribute to the exceptional OER activity. In addition, the electron transfer from Ir to Ru in IrRu should significantly boost the OER activity according to X-ray photoelectron spectroscopy (XPS). IrRu also shows an excellent stability during 10 h of a chronopotentiometry (CP) test at 10 mA cm^-2. Eventually, the high OER activity of IrRu was verified in a PEMWE.

Kinetic Insights of Proton Exchange Membrane Water Electrolyzer Obtained by Operando Characterization Methods

Benefiting from the merits of short response time, high current density, and differential pressure, the proton exchange membrane water electrolyzer (PEMWE) is attracting increasing attention from both academic and industry researchers. A limiting factor that impedes the widespread application of the PEMWE is its reliance on the rarest elements, such as iridium and platinum. In order to optimize the device performance as well as to reduce the usage of rare elements, it is important but difficult to directly observe the reaction within the electrolyzer under working conditions. Thus, operando characterization methods are urgently needed to probe in real time the water electrolysis process during operation. In this perspective, we highlight the important role and summarize the recent advances of operando characterization methods in obtaining kinetic insights about PEMWEs. Based on the demands of kinetic optimization, an outlook of future characterization methods is given at the end.

Zirconia Toughened Alumina-Based Separator Membrane for Advanced Alkaline Water Electrolyzer

Hydrogen is nowadays considered a favorable and attractive energy carrier fuel to replace other fuels that cause global warming problems. Water electrolysis has attracted the attention of researchers to produce green hydrogen mainly for the accumulation of renewable energy. Hydrogen can be safely used as a bridge to successfully connect the energy demand and supply divisions. An alkaline water electrolysis system owing to its low cost can efficiently use renewable energy sources on large scale. Normally organic/inorganic composite porous separator membranes have been employed as a membrane for alkaline water electrolyzers. However, the separator membranes exhibit high ionic resistance and low gas resistance values, resulting in lower efficiency and raised safety issues as well. Here, in this study, we report that zirconia toughened alumina (ZTA)-based separator membrane exhibits less ohmic resistance 0.15 Ω·cm2 and low hydrogen gas permeability 10.7 × 10-12 mol cm-1 s-1 bar-1 in 30 wt.% KOH solution, which outperforms the commercial, state-of-the-art Zirfon® PERL separator. The cell containing ZTA and advanced catalysts exhibit an excellent performance of 2.1 V at 2000 mA/cm2 at 30 wt.% KOH and 80 °C, which is comparable with PEM electrolysis. These improved results show that AWEs equipped with ZTA separators could be superior in performance to PEM electrolysis.

Clean production of porous-Al(OH)3 from fly ash

Fly ash is one of the largest solid waste and causes serious environment problems. Extraction of Al(OH)₃ from fly ash is beneficial to environment and economy. We developed a clean electrolysis method to generate hydroxyl groups in situ to extract Al(OH)₃ from fly ash leachate without adding chemicals or using expensive membranes, avoiding the introduction of new impurities, secondary pollutants generation, and membrane limitations. Batch experiments yielded porous electrolytic products with BET surface areas from 11.7610 to 25.5267 m²/g, pore volumes from 0.1935 to 0.1643 cm³/g and pore sizes from 65.7960 to 25.7434 nm. The composition of the electrolytic products was 86.43 wt% Al(OH)₃, 9.00 wt% SO₃, 1.67 wt% Fe(OH)₃, and 0.29 wt% Ca(OH)₂. The current efficiency was 90.51 % under optimized conditions of c (Al³⁺) = 0.1 M, t =2 h, and J = 750 A/m². Mean particle size was from 24.1-98.1 μm. Impurities mainly affected the composition of the electrolytic products. The OH- generated by H₂O reduction reacted with Al³⁺, Fe³⁺, and Ca²⁺ to generate a hydroxide. Fe³⁺ preceded Ca²⁺ into the hydroxide. H₂ released continuously from H₂O reduction, resulting in a porous hydroxide. The wastewater was reused as a leaching reagent to promote zero-pollution discharge.

Electrochemical Impedance Spectroscopy as a Diagnostic Tool in Polymer Electrolyte Membrane Electrolysis

Membrane–electrode assemblies (MEAs) designed for a polymer electrolyte membrane (PEM) water electrolyser based on a short-side chain (SSC) perfluorosulfonic acid (PFSA) membrane, Aquivion^®, and an advanced Ir-Ru oxide anode electro-catalyst, with various cathode and anode noble metal loadings, were investigated. Electrochemical impedance spectroscopy (EIS), in combination with performance and durability tests, provided useful information to identify rate-determining steps and to quantify the impact of the different phenomena on the electrolysis efficiency and stability characteristics as a function of the MEA properties. This technique appears to be a useful diagnostic tool to individuate different phenomena and to quantify their effect on the performance and degradation of PEM electrolysis cells.

Operando Evidence for a Universal Oxygen Evolution Mechanism on Thermal and Electrochemical Iridium Oxides

Progress in the development of proton exchange membrane (PEM) water electrolysis technology requires decreasing the anode overpotential, where the sluggish multistep oxygen evolution reaction (OER) occurs. This calls for an understanding of the nature of the active OER sites and reaction intermediates, which are still being debated. In this work, we apply synchrotron radiation-based near-ambient pressure X-ray photoelectron and absorption spectroscopies under operando conditions in order to unveil the nature of the reaction intermediates and shed light on the OER mechanism on electrocatalysts most widely used in PEM electrolyzers-electrochemical and thermal iridium oxides. Analysis of the O K-edge and Ir 4f spectra backed by density functional calculations reveals a universal oxygen anion red-ox mechanism regardless of the nature (electrochemical or thermal) of the iridium oxide. The formation of molecular oxygen is considered to occur through a chemical step from the electrophilic O^I- species, which itself is formed in an electrochemical step.

Highly active nano-sized iridium catalysts: synthesis and operando spectroscopy in a proton exchange membrane electrolyzer

A stable and cost effective oxygen evolution reaction (OER) catalyst is crucial for the large-scale market penetration of proton exchange membrane (PEM) water electrolyzers. We show that the synthesis of iridium nanoparticles in either low purity ethanol or water, or in the absence of a surfactant, is detrimental to the electrocatalytic properties of the materials. Adding NaBH4 in excess improves the purity of the catalyst enhancing the OER activity up to 100 A gIr-1 at 1.51 V vs. RHE, the highest value reported so far for high purity Ir nanoparticles. The measured OER activity correlates with the capacitive current rather than with the charge corresponding to the IrIII/IrIV oxidation peak. Operando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) on membrane electrode assemblies (MEAs) with the synthesized catalysts reveals a metallic core surrounded by a thin layer of IrIII/IV oxides/hydroxides. Oxidation of IrIII leaves behind a porous ultrathin layer of IrIV oxides/hydroxides, which dominate the surface during the OER, while IrV was not detected.

The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation

This Review addresses the technical challenges, scientific basis, recent progress, and outlook with respect to the stability and degradation of catalysts for the oxygen evolution reaction (OER) operating at electrolyzer anodes in acidic environments with an emphasis on ion exchange membrane applications. First, the term "catalyst stability" is clarified, as well as current performance targets, major catalyst degradation mechanisms, and their mitigation strategies. Suitable in situ experimental methods are then evaluated to give insight into catalyst degradation and possible pathways to tune OER catalyst stability. Finally, the importance of identifying universal figures of merit for stability is highlighted, leading to a comprehensive accelerated lifetime test that could yield comparable performance data across different laboratories and catalyst types. The aim of this Review is to help disseminate and stress the important relationships between structure, composition, and stability of OER catalysts under different operating conditions.

Low-Cost and Durable Bipolar Plates for Proton Exchange Membrane Electrolyzers

Cost reduction and high efficiency are the mayor challenges for sustainable H₂ production via proton exchange membrane (PEM) electrolysis. Titanium-based components such as bipolar plates (BPP) have the largest contribution to the capital cost. This work proposes the use of stainless steel BPPs coated with Nb and Ti by magnetron sputtering physical vapor deposition (PVD) and vacuum plasma spraying (VPS), respectively. The physical properties of the coatings are thoroughly characterized by scanning electron, atomic force microscopies (SEM, AFM); and X-ray diffraction, photoelectron spectroscopies (XRD, XPS). The Ti coating (50 μm) protects the stainless steel substrate against corrosion, while a 50-fold thinner layer of Nb decreases the contact resistance by almost one order of magnitude. The Nb/Ti-coated stainless steel bipolar BPPs endure the harsh environment of the anode for more than 1000 h of operation under nominal conditions, showing a potential use in PEM electrolyzers for large-scale H₂ production from renewables.