Water electrolysis powered by renewable electricity produces green hydrogen and oxygen gas, which can be used for energy, fertilizer, and industrial applications and thus displace fossil fuels. Pure-water anion-exchange-membrane (AEM) electrolyzers in principle offer the advantages of commercialized proton-exchange-membrane systems (high current density, low cross over, output gas compression, etc.) while enabling the use of less-expensive steel components and nonprecious metal catalysts. AEM electrolyzer research and development, however, has been limited by the lack of broadly accessible materials that provide consistent cell performance, making it difficult to compare results across studies. Further, even when the same materials are used, different pretreatments and electrochemical analysis techniques can produce different results. Here, we report an AEM electrolyzer comprising commercially available catalysts, membrane, ionomer, and gas-diffusion layers operating near 1.9 V at 1 A cm–2 in pure water. After the initial break in, the performance degraded by 0.67 mV h–1 at 0.5 A cm–2 at 55 °C. We detail the key preparation, assembly, and operation techniques employed and show further performance improvements using advanced materials as a proof-of-concept for future AEM-electrolyzer development. The data thus provide an easily reproducible and comparatively high-performance baseline that can be used by other laboratories to calibrate the performance of improved cell components, nonprecious metal oxygen evolution, and hydrogen evolution catalysts and learn how to mitigate degradation pathways.

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 Co3O4 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.

Metrics & MoreArticle Recommendations CONSPECTUS: Catalyzing the oxygen evolution reaction (OER) is important for key energy-storage technologies, particularly water electrolysis and photoelectrolysis for hydrogen fuel production. Under neutral-to-alkaline conditions, first-row transitionmetal oxides/(oxy)hydroxides are the fastest-known OER catalysts and have been the subject of intense study for the past decade. Critical to their high performance is the intentional or accidental addition of Fe to Ni/Co oxides that convert to layered (oxy)hydroxide structures during the OER. Unraveling the role that Fe plays in the catalysis and the molecular identity of the true "active site" has proved challenging, however, due to the dynamics of the host structure and absorbed Fe sites as well as the diversity of local structures in these disordered active phases.In this Account, we highlight our work to understand the role of Fe in Ni/Co (oxy)hydroxide OER catalysts. We first discuss how we characterize the intrinsic activity of the first-row transition-metal (oxy)hydroxide catalysts as thin films by accounting for the contributions of the catalyst-layer thickness (mass loading) and electrical conductivity as well as the underlying substrate's chemical interactions with the catalyst and the presence of Fe species in the electrolyte. We show how Fe-doped Ni/Co (oxy)hydroxides restructure during catalysis, absorb/desorb Fe, and in some cases degrade or regenerate their activity during electrochemical testing. We highlight the relevant techniques and procedures that allowed us to better understand the role of Fe in activating other first-row transition metals for OER. We find several modes of Fe incorporation in Ni/Co (oxy)hydroxides and show how those modes correlate with activity and durability. We also discuss how this understanding informs the incorporation of earthabundant transition-metal OER catalysts in anion-exchange-membrane water electrolyzers (AEMWE) that provide a locally basic anode environment but run on pure water and have advantages over the more-developed proton-exchange-membrane water electrolyzers (PEMWE) that use platinum-group-metal (PGM) catalysts. We outline the key issues of introducing Fe-doped Ni/Co (oxy)hydroxide catalysts at the anode of the AEMWE, such as the oxidative processes triggered by Fe species traveling through the polymer membrane, pH-gradient effects on the catalyst stability, and possibly limited catalyst utilization in the compressed stack configuration. We also suggest possible mitigation strategies for these issues. Finally, we summarize remaining challenges including the long-term stability of Fe-doped Ni/Co (oxy)hydroxides under OER conditions and the lack of accurate models of the dynamic active surface that hinder our understanding of, and thus ability to design, these catalysts.

Among existing water electrolysis (WE) technologies, anion-exchange-membrane water electrolyzers (AEMWEs) show promise for low-cost operation enabled by the basic solid-polymer electrolyte used to conduct hydroxide ions. The basic environment within the electrolyzer, in principle, allows the use of non-platinum-group metal catalysts and less-expensive cell components compared to acidic-membrane systems. Nevertheless, AEMWEs are still underdeveloped, and the degradation and failure modes are not well understood. To improve performance and durability, supporting electrolytes such as KOH and K2CO3 are often added to the water feed. The effect of the anion interactions with the ionomer membrane (particularly other than OH–), however, remains poorly understood. We studied three commercial anion-exchange ionomers (Aemion, Sustainion, and PiperION) during oxygen evolution (OER) at oxidizing potentials in several supporting electrolytes and characterized their chemical stability with surface-sensitive techniques. We analyzed factors including the ionomer conductivity, redox potential, and pH tolerance to determine what governs ionomer stability during OER. Specifically, we discovered that the oxidation of Aemion at the electrode surface is favored in the presence of CO3 2–/HCO3 – anions perhaps due to the poor conductivity of that ionomer in the carbonate/bicarbonate form. Sustainion tends to lose its charge-carrying groups as a result of electrochemical degradation favored in basic electrolytes. PiperION seems to be similarly negatively affected by a pH drop and low carbonate/bicarbonate conductivity under the applied oxidizing potential. The insight into the interactions of the supporting electrolyte anions with the ionomer/membrane helps shed light on some of the degradation pathways possible inside of the AEMWE and enables the informed design of materials for water electrolysis.

Proton-exchange-membrane water electrolyzers (PEMWEs) produce high-purity H2, withstand load fluctuations, and operate with a pure-water feed but require platinum-group-metal catalysts for durability, such as IrO2 and Pt, due to the acidic environment. At the anode, the slow oxygen evolution reaction (OER) requires a high overpotential to achieve relevant current densities (>2 A·cm–2) even with a high loading of IrO2. Using a simple commercial 1,5-cyclooctadiene iridium chloride dimer precursor, we synthesized submonolayer-thick IrO x on the surfaces of conductive metal oxides to make every Ir atom available for catalysis and reach the ultimate lower limit for Ir loading. We show that the reaction on Sb/SnO2 and F/SnO2 conductive oxides is surface-limited and that a continuous Ir–O–Ir network provides improved stability and activity. We cover IrO x with a thin layer of acid-stable TiO x by atomic-layer deposition. The effects of TiO x on the catalyst’s performance were assessed by inductively coupled plasma mass spectrometry (ICP-MS) coupled in situ with an electrochemical flow cell and ex situ by X-ray photoelectron spectroscopy. Tuning the binding environment of IrO x by TiO x addition enhances the intrinsic activity of the active sites, simultaneously accelerating the dissolution of the catalyst and the metal-oxide support. We illustrate the interplay between the support, catalyst, and protection-layer dissolution with OER activity, and highlight the effects of annealing to densify the TiO x protection layer on stability/activity. These ultrathin supported Ir-based catalysts do not eliminate the long-standing issue of the catalyst and support instability during OER in acids, but do provide new insight into the catalyst–support interactions and may also be of utility for advanced spectroscopic investigations of the OER mechanism.