Three-Electrode Study of Electrochemical Ionomer Degradation Relevant to Anion-Exchange-Membrane Water Electrolyzers

Perovskite Catalysts for Pure-Water-Fed Anion-Exchange-Membrane Electrolyzer Anodes: Co-design of Electrically Conductive Nanoparticle Cores and Active Surfaces

Anion-exchange-membrane water electrolyzers (AEMWEs) are a possible low-capital-expense, efficient, and scalable hydrogen-production technology with inexpensive hardware, earth-abundant catalysts, and pure water. However, pure-water-fed AEMWEs remain at an early stage of development and suffer from inferior performance compared with proton-exchange-membrane water electrolyzers (PEMWEs). One challenge is to develop effective non-platinum-group-metal (non-PGM) anode catalysts and electrodes in pure-water-fed AEMWEs. We show how LaNiO3-based perovskite oxides can be tuned by cosubstitution on both A- and B-sites to simultaneously maintain high metallic electrical conductivity along with a degree of surface reconstruction to expose a stable Co-based active catalyst. The optimized perovskite, Sr0.1La0.9Co0.5Ni0.5O3, yielded pure-water AEMWEs operating at 1.97 V at 2.0 A cm-2 at 70 °C with a pure-water feed, thus illustrating the utility of the catalyst design principles.

Impact of ionomers on porous Fe-N-C catalysts for alkaline oxygen reduction in gas diffusion electrodes

Alkaline exchange membrane fuel cells (AEMFCs) offer a promising alternative to the traditional fossil fuel due to their ability to use inexpensive platinum group metal (PGM)-free catalysts, which could potentially replace Platinum-based catalysts. Iron coordinated in nitrogen-doped carbon (Fe-N-C) single atom electrocatalysts offer the best Pt-free ORR activities. However, most research focuses on material development in alkaline conditions, with limited attention on catalyst layer fabrication. Here, we demonstrate how the oxygen reduction reaction (ORR) performance of a porous Fe-N-C catalyst is affected by the choice of three different commercial ionomers and the ionomer-to-catalyst ratio (I/C). A Mg-templated Fe-N-C is employed as a catalyst owing to the electrochemical accessibility of the Fe sites, and the impact of ionomer properties and coverage were studied and correlated with the electrochemical performance in a gas-diffusion electrode (GDE). The catalyst layer with Nafion at I/C = 2.8 displayed the best activity at high current densities (0.737 ± 0.01 VRHE iR-free at 1 A cm⁻²) owing to a more homogeneous catalyst layer, while Sustainion displayed a higher performance in the kinetic region at the same I/C. These findings provide insights into the impact of catalyst layer optimization to achieve optimal performance in Fe-N-C based AEMFCs.

Ether-Free Alkaline Polyelectrolytes for Water Electrolyzers: Recent Advances and Perspectives

Anion exchange membrane (AEM) water electrolyzers (AEMWEs) have attracted great interest for their potential as sustainable, environmentally friendly, low-cost sources of renewable energy. Alkaline polyelectrolytes play a crucial role in AEMWEs, determining their performance and longevity. Because heteroatom-containing polymers have been shown to have poor durability in alkaline conditions, this review focuses on ether-free alkaline polyelectrolytes, which are more chemically stable. The merits, weaknesses, and challenges in preparing ether-free AEMs are summarized and highlighted. The evaluation of synthesis methods for polymers, modification strategies, and cationic stability will provide insights valuable for the structural design of future alkaline polyelectrolytes. Moreover, the in situ degradation mechanisms of AEMs and ionomers during AEMWE operation are revealed. This review provides insights into the design of alkaline polyelectrolytes for AEMWEs to accelerate their widespread commercialization.

Quantitative Analysis and Manipulation of Alkali Metal Cations at the Cathode Surface in Membrane Electrode Assembly Electrolyzers for CO2 Reduction Reactions

The stable operation of the CO2 reduction reaction (CO2RR) in membrane electrode assembly (MEA) electrolyzers is known to be hindered by the accumulation of bicarbonate salt, which are derived from alkali metal cations in anolytes, on the cathode side. In this study, we conducted a quantitative evaluation of the correlation between the CO2RR activity and the transported alkali metal cations in MEA electrolyzers. As a result, although the presence of transported alkali metal cations on the cathode surface significantly contributes to the generation of C2+ compounds, the rate of K+ ion transport did not match the selectivity of C2+, suggesting that a continuous supply of high amount of K+ to the cathode surface is not required for C2+ formation. Based on these findings, we achieved a faradaic efficiency (FE) and a partial current density for C2+ of 77 % and 230 mA cm-2, respectively, even after switching the anode solution from 0.1 M KHCO3 to a dilute K+ solution (<7 mM). These values were almost identical to those when 0.1 M KHCO3 was continuously supplied. Based on this insight, we successfully improved the durability of the system against salt precipitation by intermittently supplying concentrated KHCO3, compared with the continuous supply.

Protocol for Evaluating Anion Exchange Membranes for Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries often suffer from reduced battery lifetime and decreased coulombic efficiency due to crossover of the redox-active species through the membrane. One method to mitigate this undesired crossover is to judiciously choose a membrane based on several criteria: swelling and structural integrity, size and charge of redox active species, and ionic conductivity. Most research to date has focused on reducing crossover by synthesizing modified redox-active molecules and/or new membranes. However, no standard protocol exists to compare membranes and a comprehensive study comparing membranes has yet to be done. To address both these limitations, we evaluate herein 26 commercial anion exchange membranes (AEMs) to assess their compatibility with common nonaqueous solvents and their resistance to crossover by using neutral and cationic redox-active molecules. Ultimately, we found that all the evaluated AEMs perform poorly in organic solvents due to uncontrolled swelling, low ionic conductivity, and/or high crossover rates. We believe that this method, and the generated data, will be useful to evaluate and compare the performance of all AEMs─commercial and newly synthesized─and should be implemented as a standard protocol for future research.

Operando Raman Spectroscopy Reveals Degradation Byproducts from Ionomer Oxidation in Anion Exchange Membrane Water Electrolyzers

This work showcases the discovery of degradation mechanisms for nonplatinum group metal catalyst (PGM free) based anion exchange membrane water electrolyzers (AEMWE) that utilize hydroxide ion conductive polymer ionomers and membranes in a zero gap configuration. An entirely unique and customized test cell was designed from the ground up for the purposes of obtaining Raman spectra during potentiostatic operation. These results represent some of the first operando Raman spectroscopy explorations into the breakdown products that are generated from high oxidative potential conditions with carbonate electrolytes. We provide a unique design and fabrication method for three-dimensional (3D) printable flow cells that enable spatially resolved Raman spectra collection from the electrode surface into the bulk electrolyte. It is proposed that the generation of breakdown products from the hydroxide-conductive ionomers and membranes originates from a multistep, free radical reaction pathway resulting in chain scission of the poly aryl backbone. This hypothesis is backed by the detection of carboxylic and aromatic functional group Raman signals from small molecules that had dissolved and diffused into the bulk electrolyte.

Understanding the Effects of Anode Catalyst Conductivity and Loading on Catalyst Layer Utilization and Performance for Anion Exchange Membrane Water Electrolysis

Anion exchange membrane water electrolysis (AEMWE) is a promising technology to produce hydrogen from low-cost, renewable power sources. Recently, the efficiency and durability of AEMWE have improved significantly due to advances in the anion exchange polymers and catalysts. To achieve performances and lifetimes competitive with proton exchange membrane or liquid alkaline electrolyzers, however, improvements in the integration of materials into the membrane electrode assembly (MEA) are needed. In particular, the integration of the oxygen evolution reaction (OER) catalyst, ionomer, and transport layer in the anode catalyst layer has significant impacts on catalyst utilization and voltage losses due to the transport of gases, hydroxide ions, and electrons within the anode. This study investigates the effects of the properties of the OER catalyst and the catalyst layer morphology on performance. Using cross-sectional electron microscopy and in-plane conductivity measurements for four PGM-free catalysts, we determine the catalyst layer thickness, uniformity, and electronic conductivity and further use a transmission line model to relate these properties to the catalyst layer resistance and utilization. We find that increased loading is beneficial for catalysts with high electronic conductivity and uniform catalyst layers, resulting in up to 55% increase in current density at 2 V due to decreased kinetic and catalyst layer resistance losses, while for catalysts with lower conductivity and/or less uniform catalyst layers, there is minimal impact. This work provides important insights into the role of catalyst layer properties beyond intrinsic catalyst activity in AEMWE performance.

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

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.

Anion Exchange Ionomers: Design Considerations and Recent Advances - An Electrochemical Perspective

Alkaline-based electrochemical devices, such as anion exchange membrane (AEM) fuel cells and electrolyzers, are receiving increasing attention. However, while the catalysts and membrane are methodically studied, the ionomer is largely overlooked. In fact, most of the studies in alkaline electrolytes are conducted using the commercial proton exchange ionomer Nafion. The ionomer provides ionic conductivity; it is also essential for gas transport and water management, as well as for controlling the mechanical stability and the morphology of the catalyst layer. Moreover, the ionomer has distinct requirements that differ from those of anion-exchange membranes, such as a high gas permeability, and that depend on the specific electrode, such as water management. As a result, it is necessary to tailor the ionomer structure to the specific application in isolation and as part of the catalyst layer. In this review, an overview of the current state of the art for anion exchange ionomers is provided, summarizing their specific requirements and limitations in the context of AEM electrolyzers and fuel cells.

Hydrocarbon Ionomeric Binders for Fuel Cells and Electrolyzers

Ionomeric binders in catalyst layers, abbreviated as ionomers, play an essential role in the performance of polymer-electrolyte membrane fuel cells and electrolyzers. Due to environmental issues associated with perfluoroalkyl substances, alternative hydrocarbon ionomers have drawn substantial attention over the past few years. This review surveys literature to discuss ionomer requirements for the electrodes of fuel cells and electrolyzers, highlighting design principles of hydrocarbon ionomers to guide the development of advanced hydrocarbon ionomers.

Nanoscale Measurements of Charge Transfer at Cocatalyst/Semiconductor Interfaces in BiVO4 Particle Photocatalysts

Semiconductor photocatalyst particles convert solar energy to fuels like H₂. The particles are often assumed to provide crystalline-facet-dependent electron-hole separation. A common strategy is to deposit a hydrogen evolution reaction (HER) electrocatalyst on electron-selective facets and an oxygen evolution reaction (OER) electrocatalyst on hole-selective facets. A precise understanding of how charge-carrier-selective contacts emerge and how they are rationally designed, however, is missing. Using a combination of ex situ and in situ conducting atomic force microscopy (AFM) experiments and new ionomer/catalyst-semiconductor test structures, we show how heterogeneity in charge-carrier selectivity can be measured at the nanoscale. We discover that the presence of the water/electrolyte interface is critical to induce hole selectivity between the CoO_x water-oxidation catalyst and the BiVO₄ light absorber. pH-dependent measurements suggest that negative surface charge on the semiconductor is central to inducing hole selectivity. The work also demonstrates a new approach to control local pH and introduce water using thin-film ionomers compatible with conductive AFM measurements.

Anode Catalysts in Anion-Exchange-Membrane Electrolysis without Supporting Electrolyte: Conductivity, Dynamics, and Ionomer Degradation

Anion-exchange-membrane water electrolyzers (AEMWEs) in principle operate without soluble electrolyte using earth-abundant catalysts and cell materials and thus lower the cost of green H₂ . Current systems lack competitive performance and the durability needed for commercialization. One critical issue is a poor understanding of catalyst-specific degradation processes in the electrolyzer. While non-platinum-group-metal (non-PGM) oxygen-evolution catalysts show excellent performance and durability in strongly alkaline electrolyte, this has not transferred directly to pure-water AEMWEs. Here, AEMWEs with five non-PGM anode catalysts are built and the catalysts' structural stability and interactions with the alkaline ionomer are characterized during electrolyzer operation and post-mortem. The results show catalyst electrical conductivity is one key to obtaining high-performing systems and that many non-PGM catalysts restructure during operation. Dynamic Fe sites correlate with enhanced degradation rates, as does the addition of soluble Fe impurities. In contrast, electronically conductive Co₃ O₄ nanoparticles (without Fe in the crystal structure) yield AEMWEs from simple, standard preparation methods, with performance and stability comparable to IrO₂ . These results reveal the fundamental dynamic catalytic processes resulting in AEMWE device failure under relevant conditions, demonstrate a viable non-PGM catalyst for AEMWE operation, and illustrate underlying design rules for engineering anode catalyst/ionomer layers with higher performance and durability.