Effect of side chain on the electrochemical performance of poly (ether ether ketone) based anion-exchange membrane: A molecular dynamics study

Ionic conductivity and alkaline stability are the key properties that limited the widespread application of anion-exchange membranes (AEM) in electrochemical energy conversion/storage systems. In recent years, quaternary ammonium functionalized poly (ether ether ketone) (PEEK) membranes serve as a promising solution due to the good mechanical and chemical properties. Varied ionic conductivity and alkaline stability could be obtained when the membrane contains different functionalized side chains. However, it's still a challenge to understand the mechanism from the experimental study because various parameters could affect the electro-chemical performance of the membranes. In this work, we conduct coarse-grained molecular dynamics simulations to investigate two PEEK-based membranes, in which the side chains contain one (SQ) or two (GQ) quaternary ammonium groups. The simulation results indicate the self-diffusion coefficients in SQ and GQ are quite similar which should not be the main reason for the improved ionic conductivity of GQ, while the obviously increased ion-exchange capacity of GQ should result in the improved ionic conductivity. Furthermore, the simulation reveals that more water molecules wrap around the OH⁻ in GQ, which could lead to the improved alkaline stability in comparison to that of SQ. This work provides a deeper understanding for the design of grafted copolymer based AEM with QA functional side chains.