Molecular Simulations of Proton and Hydroxide Transport in Fuel Cell Membranes

Acid/base reactions are ubiquitous in nature and they have received extensive experimental and theoretical attention over the past several decades. Even though we now understand reasonably well how the Grotthuss hopping mechanism is responsible for the rapid charge diffusion in aqueous acidic and basic solutions, the mechanisms for long-range proton and hydroxide transport in fuel cell membranes are not nearly as well understood. While there is similarity in the solvation structures for the hydroxide and the hydronium ions, the underlying hopping mechanisms in both cases have significant differences. Through careful parameterization based on ab initio calculations, accurate and efficient multistate reactive molecular dynamics (MS-RMD) models are constructed to study proton and hydroxide transport in both proton-exchange and anion-exchange membrane (PEM and AEM) systems. These MS-RMD models allow for the calculations of statistically converged properties and reveal important transport mechanisms in these energy materials. In this talk, the mechanistic pictures of proton and hydroxide transport in PEM and AEM will be discussed and compared.

Figure Caption: The left figure shows the typical solvation structures around hydronium ions (green) in the 3M PEM and illustrates how a hydronium bridges two sulfonates. The right figure shows the solvation around hydroxide ions (green) in poly(vinyl benzyl trimethylammonium) AEM.

Acknowledgment: This research used resources of Research Computing Center (RCC) at the University of Chicago, the Innovative and Novel Computational Impact on Theory and Experiment (INCITE), ASCR Leadership Computing Challenge (ALCC) programs, and the Argonne Leadership Computing Facility (ALCF) at Argonne National Laboratory (Supported by DOE contract DE-AC02-06CH11357).