Degradation of Anion Exchange Membranes by Cation Elimination: Impact on Water Uptake, Nanostructure, and Ionic Mobility

Abstract

Anion exchange membranes (AEMs) are anattractive platform for fuel cell and electrolysis technologies sincethey enable the use of cheaper, nonplatinum group metalelectrodes and components. However, the widespread adoptionof AEM-based devices is limited by chemical degradation of theAEM in the highly alkaline medium of operation. Experimentalstudies report three major pathways of degradation, includingsubstitution (SN2), elimination (E2), and backbone cleavage. Thedecline in membrane performance is likely due to a cumulativeeffect of several pathways, which has proven to be difficult to disentangle through experiments. Here we use coarse-grainedmolecular simulations to isolate the impact of E2 degradation, where cationic sites are replaced by an alkene group, on AEM wateruptake, nanoscale morphology, structure of the ion-conducting channels, water mobility, and ionic conductivity. Our studies focuson the well-studied model AEM polyphenylene oxide with tetraalkylammonium cationic sites (PPO-TMA). We find that about halfof the hydrophobic groups resulting from the degradation retract into the hydrophobic domains of the membrane. The reduction inion exchange capacity (IEC) and the hydrophobicity of the alkene groups decrease both the equilibrium water uptake (%WU) andthe number of water molecules per cation (λ) of the membrane. Interestingly, the evolution of λ with the IEC seems to follow that ofundegraded PPO-TMA membranes. We ascribe this to the similarity between the structure of the degraded monomer and theneutral one in the polymer. We conclude that most of the performance losses originate in the lower hydration of the degradedmembrane.