Hierarchical dynamics of hydronium ions in polymer electrolyte membranes revealed by all-atom molecular dynamics simulations

Abstract

Understanding the transport dynamics of hydronium ions (H3O+) in polymer electrolyte membranes is critical for improving the performance of polymer electrolyte fuel cells. In this study, we performed all-atom molecular dynamics simulations of hydrated Nafion to investigate the relationship between the H3O+ diffusion mechanisms and the membrane morphology at various water uptakes (λ = 6, 10, and 14). To capture the intrinsic heterogeneity of the water channels and the long-time dynamics, large-scale simulations were required. Therefore, we used a classical H3O+ model that excludes the Grotthuss mechanism. Free energy maps revealed that H3O+ are strongly trapped near SO3− groups, while water molecules (H2O) exhibit broader free energy wells. Conduction path and structural factor analyses indicated that the water channels in Nafion form interconnected tubular networks, whose tube diameter increases with hydration. The simulated structure factors quantitatively reproduced the experimentally observed correlation length of the water channels. Mean square displacement and probability distributions revealed hierarchical dynamics modes for both H3O+ and H2O. The suggested transport processes were the following three modes with different time/spatial scales: (i) localized binding to the sulfonate groups on Nafion, (ii) confined diffusion within the water channels, and (iii) normal diffusion along the water channels. From these findings, the proton diffusion modes in Nafion are highly localized and governed by multiscale mechanisms owing to the membrane morphology and hydration. These results provide a detailed, molecular-level understanding of the proton transport in perfluorosulfonic acid membranes, and they provide valuable insight into optimizing materials for fuel cell applications.