Degradation Mechanisms of Anion Exchange Membranes due to Alkali Hydrolysis and Radical Oxidative Species

Chemical degradation of anion exchange membrane (AEM) and anion exchange ionomer (AEI) due to alkali hydrolysis and radical oxidative species represents a daunting challenge in scale-ups of alkaline membrane fuel cell devices (AEMFCs) and alkaline membrane water electrolyzers (AEMWEs). Decades of research endeavor were devoted to find conditions where the nucleophile (i.e. OH − and O 2 ˙ − ) and nucleofuge (i.e. R 4 N + ) can co-exist indefinitely. It is envisaged that to overcome the stability setbacks, an in-depth understanding on the degradation mechanisms of functional cationic groups and polymers is important. In this chapter, an insight toward the degradation mechanisms of anion exchange functionalities of AEMs and AEIs at high pH is summarized. This chapter describes the major degradation pathways of various organic and inorganic cationic functionalities via β-hydride elimination, nucleophilic substitution reactions, and ring opening reactions in heterocyclic systems. Moreover, probable routes for oxidative degradation with reactive radical oxygen species are also summarized. The polymer backbone also plays a decisive role in designing the stable AEMs and AEIs. Thus, the mechanism and degradation phenomena in poly(arylene ethers), poly(fluoroalkanes), and poly(ionic liquids) is included. Due to recent advancements in AEM/AEI designs, the progress in cationic group's stability has achieved t 1/2 ≥ 10 000 h in strong alkaline pH. Thus, the designing rules and chemical structures of the recently reported high-performance anion exchange head group are outlined and tabulated. It is anticipated that the understanding of degradation mechanism and hydroxide breaching phenomena in AEMs can offer newer avenues to precisely design and functionalize polymers to fabricate state-of-the-art AEMs/AEIs for alkaline membrane fuel cell and water electrolyzers.