Molecular-Dynamics Simulations of the Glass Transition in Hydrated Nafion Membranes

Nafion is a commonly used polyelectrolyte membrane (PEM) in fuel cells and flow batteries. The annealing of polymer electrolyte membranes is known to affect the membrane structure and proton conductivity. The observed changes depend drastically upon the annealing temperatures and cooling rates. In this paper, we have performed a fully atomistic classical molecular-dynamics simulation of hydrated Nafion at various hydration levels and annealing rates. Our simulations [1] show the compression of hydrophobic Nafion domains by larger water clusters, with a strong antiplasticization effect upon hydration, demonstrated by increasing the glass-transition temperature. The close-range proximity of sulfonate−sulfonate groups of Nafion pendant side chains remains unchanged with the simulated cooling rates. The water clusters in hydrated Nafion become more disconnected and larger in size with slower cooling rates/increased annealing time. This results in the decrease of water and hydronium diffusivity and the corresponding conductivity, thereby explaining qualitatively the experimental observations.



[1]. A. V. Lyulin S. Sengupta, A. Varghese, P. Komarov and A. Venkatnathan, Effect of annealing on structure and diffusion in hydrated Nafion membranes, ACS Appl. Polym. Mater., 2, 5058-5066, 2020.