(Invited) First-Principles Molecular Dynamics Investigations of Proton and Hydroxide Transport in Model Anion-Exchange- and Proton-Exchange Membranes in Nanoconfined, Low-Hydration Environments

Abstract

Fuel cell-based anion-exchange membranes (AEMs) and proton exchange membranes (PEMs) have considerable potential as cost-effective, clean energy conversion devices. However, a fundamental atomistic understanding of the hydroxide and hydronium diffusion mechanisms in AEM and PEM environments is an ongoing challenge. In recent years, nano-confined structures have been exploited in the study of cost-effective and reliable polymer architectures for electrochemical devices. In this study, first-principles molecular dynamics simulations are employed to investigate the influence of the water distribution, temperature, internal geometry, cation chemistry, and presence of carbon dioxide on the diffusion rate of protons and hydroxide ions in nanoconfined geometries that serve as mimics of PEM and AEM environments and to reveal key design principles1-8. The simulations indicate that the water distribution is a determinative factor in the diffusion process, which is also arrested when carbon dioxide is present. Moreover, it is found that hydroxide diffusion in AEMs is largely vehicular or a combination of vehicular and structural while proton diffusion in PEMs is largely structural. Pairs of cations in AEMs create bottlenecks for hydroxide diffusion while the anion groups in PEMs become active participants in proton diffusion. It is found that the temperature dependence of hydroxide diffusion in AEMs is non-monotonic, exhibiting a “kink” over a particular temperature range at which dD OH ─/dT < 0, a finding that is confirmed experimentally. The presence of carbon dioxide is found to suppress hydroxide diffusion, also in agreement with experiment. Looking forward, we investigate the influence of the solvent chemistry on proton transport and propose new nanoconfined environments with the potential to enhance proton transport rates considerably using these liquids.