Water Dynamics and Proton-Transport Mechanisms of Nafion 117/Phosphotungstic Acid Composite Membrane: A Molecular Dynamics Study.

Abstract

The use of a Nafion/phosphotungstic acid composite membrane and the impact of varying concentration of heteropoly acid (HPA) on the well-known effective mechanisms of proton transport were investigated by using classical and quantum hopping molecular dynamics simulation. Our simulations demonstrated that the HPA particles have a favorable influence on the Grotthuss mechanism in proton transportation at low hydration levels. From radial distribution function examinations, it was found that HPA particles were solvated with water and also exhibited stronger affinity toward hydronium ions. It can be concluded that addition of hydrophilic particles such as HPA improved proton conductivity. To assess this effectiveness, lifetime and half-life of the hydrogen bond (H-bond) network in the formed water clusters were investigated at different HPA concentrations. The analysis of H-bond network stability revealed that the lifetime of H-bonds between water molecules and protons decreased with increasing HPA concentration. Moreover, we found that the H-bond network between water molecules was more stable, and the mismatch between simulated bulk water and those formed water clusters in the considered systems decreased upon HPA addition. Indeed, for HPA doped membrane, the activation energy of proton transfer process from a hydronium ion to a water molecule was lower than for the undoped system. The water diffusion coefficient decreased and that of the hydronium ion enhanced with an increase in HPA doping level.