Exploring Side-Chain Designs for Enhanced Ion Conductivity of Anion-Exchange Membranes by Mesoscale Simulations

Anion-exchange membranes (AEM) are polyelectrolytes functionalized with cationic groups. Studies of AEM in the past few decades suggest that AEM is a competitive alternative to conventional proton-exchange membranes in fuel cell (FC) application, mainly because of its alkaline environment that allows the use of non-noble metal for electrocatalysts. Understanding AEM morphology and anion transport is a key for improving the performance of AEMFC. The present work uses dissipative particle dynamics to simulate the mesoscale structure of hydrated poly(phenylene oxide) (PPO) functionalized with tetramethylamine (TMA) groups on different hydration levels (HL) and ion-exchange capacities (IEC). Additional spacers are tethered onto PPO-TMA to enhance the nanosegregation of hydrophilic and hydrophobic subdomains, and therefore expand the pathways for ion transportation. A variety of spacers studied include alkyl spacers in PPO-C4-TMA, PPO-C8-TMA, and alkoxy spacers in PPO-E2-TMA. Simulation results show that the diffusivities of anions and water increase with the elevation of HL and IEC, which is consistent with experimental observations. Adding hydrophobic alkyl spacers intensifies the phase segregation and the formation of larger water clusters. The size of the clusters further increases due to the agglomeration with the increase of HL or the length of the alkyl spacers. Nevertheless, hydrophobicity from the side chains results in overaggregated water phase, and therefore forms bottleneck within the transport pathways that retards the anion diffusivity. The same issue is observed if the alkyl fragment is tethered on TMA as an extender. A suggested design using less hydrophobic alkoxy spacers, PPO-E2-TMA, outperforms all of the other types of AEM in this work in anion transport by forming narrower channels but more connected network. The provided fundamental information may be useful for designing more versatile AEMFC.