Anion exchange membranes (AEMs) find widespread applications as an electrolyte and/or electrode binder in fuel cells, electrodialysis stacks, flow and metal-air batteries, and electrolyzers. AEMs exhibit poor stability in alkaline media; their degradation is induced by the hydroxide ion, a potent nucleophile. We have used 2D NMR techniques to investigate polymer backbone stability (as opposed to cation stability) of the AEM in alkaline media. We report the mechanism behind a peculiar, often-observed phenomenon, wherein a demonstrably stable polysulfone backbone degrades rapidly in alkaline solutions upon derivatization with alkaline stable fixed cation groups. Using COSY and heteronuclear multiple quantum correlation spectroscopy (2D NMR), we unequivocally demonstrate that the added cation group triggers degradation of the polymer backbone in alkaline via quaternary carbon hydrolysis and ether hydrolysis, leading to rapid failure. This finding challenges the existing perception that having a stable cation moiety is sufficient to yield a stable AEM and emphasizes the importance of the often ignored issue of backbone stability.alkaline fuel cells | anion exchange membrane degradation | water electrolysis | quaternary benzyl ammonium cations T here have been intensive research efforts directed toward the development of anion exchange membranes (AEMs) for solidstate alkaline fuel cells (AFCs) and water electrolyzers in recent years (1-6). These devices permit the use of non-platinum-groupmetal electrocatalysts for the oxygen reduction/evolution and hydrogen oxidation/evolution reactions (7-11). Besides their use in AFCs, AEMs are highly relevant for other electrochemical energy conversion/storage devices such as redox flow batteries, electrodialysis stacks, and metal-air batteries (6,(12)(13)(14). The renewed interest in AFCs has led to many studies directed toward improving the hydroxide ion conductivity of AEMs, a property that is inherently limited by the lower intrinsic mobility of the hydroxide ion. Approaches have included investigating new fixed cation chemistries and/or modification of the polymer electrolyte membrane network (e.g., cross-linking, spacer chain pendants, and block copolymers) (15-17). These efforts have facilitated an order of magnitude gain in hydroxide ion conductivity (roughly 10) (15,18,19). AEMs have traditionally exhibited poor chemical stability in alkaline environments. The poor alkaline stability of AEMs is an important issue that has received much less attention than AEM ionic conductivity. This issue has limited the use of AEMs in applications that involve exposure to hydroxide ions, a potent nucleophile (12). The primary AEM degradation modes involve hydroxide ion attack on the fixed cation groups of AEMs, leading to Hoffman elimination (1, 10, 11), direct nucleophilic substitution (1-3), and chemical rearrangements induced through ylide intermediate formation (2,3,(20)(21)(22). Each of these degradation mechanisms results in rapid loss of ion exchange capacity, and hence ionic co...

The development of alkaline fuel cells over the past decade has led to exciting developments in low resistant and alkaline stable anion exchange membranes (AEMs). This Review highlights new material chemistries and macromolecular designs that have fueled AEMs with ionic conductivities greater than 100 mS cm–1, while demonstrating stability for extended periods in base bath solutions of 1 M potassium (or sodium) hydroxide solutions at temperature of 80 °C or greater. The new AEMs have led to AEM fuel cells (AEMFCs) with power density values that exceed 1 W cm–2 with hydrogen and oxygen. AEM research activities are motivated in large part by their prospect to realize fuel cells free of platinum group metals, which is paramount for cost reduction of fuel cell technology. In addition to highlighting the remarkable achievements of AEMs in the past 4 years, this Review discusses future priorities for the scientific community to address in AEM development. These priorities include stability and conductivity under low humidity or dry conditions, resisting carbonation and oxidation, and AEMFC device stability studies.

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Alkaline fuel cells containing anion exchange membrane electrolytes (i.e., solid-state alkaline fuel cells -SAFCs) promise to provide high power densities without platinum group metal catalysts. In the past decade, SAFC performance has improved substantially due to improvements in electrode binders that facilitate good membrane-electrode contact and ionic conductivity in the electrode layer. However, the alkaline (in)stability of AEMs is a long-standing challenge that currently precludes commercialization of this technology. To date, there have not been any satisfactory strategies or approaches to adequately assess an AEM's suitability for SAFC applications. Here, we report an all-encompassing "best practices" approach to evaluate a leading AEM candidate (poly(2,6-dimethyl 1,4-phenylene) oxide (PPO) with quaternary ammonium groups) for SAFCs. Additionally, this work reports an excellent peak power density of 294 mW cm −2 when the fuel cell was operated with hydrogen-oxygen. This high fuel cell performance was attained by painting the electrodes directly onto the membrane to minimize membrane-electrode contact resistance losses.

Solid-state alkaline water electrolysis using a pure water feed offers several distinct advantages over liquid alkaline electrolyte water electrolysis and proton exchange membrane water electrolysis. These advantages include a larger array of electrocatalyst available for oxygen evolution, no electrolyte management, and the ability to apply differential pressure. To date, there have been only a handful of reports on solid-state alkaline water electrolyzers using anion exchange membranes (AEMs), and there have been no reports that investigate loss in system performance over time. In this work, a solidstate alkaline water electrolyzer was successfully demonstrated with several types of polysulfone-based AEMs using a relatively expensive but highly active lead ruthenate pyrochlore electrocatalyst for the oxygen evolution reaction. The electrolysis of ultrapure water at 50 C resulted in a current density of 400 mA cm À2 at 1.80 V. We demonstrated that the short-term degradation of water electrolyzer performance over time was largely a consequence of carbon dioxide intrusion into the system and could be easily remedied, while longterm deterioration was a consequence of irreversible AEM polymer degradation.