An Improved EVB Model for Proton Transport in Polymer Electrolyte Membrane

Abstract

The heart of Polymer electrolyte fuel cells (PEFCs) is polymer electrolyte membrane (PEM) that separated the reactant gases and conducts protons. In the nanoscopic structure of the membrane, ion clusters are formed by water molecules gathered vicinity of sulfonate groups which are hydrophilic parts of Nafion membrane, and proton transport (PT) are largely attributed to the nanoscopic structure of polymer membranes and water aggregations. Thus, many macroscopic simulations that are based on continuum theory have found it difficult to discuss the relation between the structural and dynamical properties of protons within the membrane in previous experimental studies. It is critical to understand the PT mechanisms through polymer membrane and clarify an important link between the membrane nanostructure and the PT properties. In the bulk aqueous solution, it is observed that proton mobility is roughly 5 times higher than other cations such as K+ which have an ionic radius similar to the hydronium ions. This property of PT is attributed to a combination of proton hopping, known as the Grotthuss mechanism, and vehicular mechanism where protons diffuse in solution as a hydrated form (e.g. hydronium ions). Many research efforts have been focused on understanding the PT mechanism using theoretical basis for the proton hopping models such as the empirical valence bond (EVB) approaches which allow PT phenomena to be simulated accurately in MD simulations. In Nafion membranes, it is considered that proton hopping transport still takes place although the water dynamic property in the bulk aqueous solution is inhibited by a lack of bulklike water structure at low water contents. Although the PT properties including hopping mechanism in hydrated polymer membrane have been studied using the original two-state EVB method by Walbran et al., the diffusion of water was found to be significantly higher than that of protons as well as that of real water in their model. In this study, a revised EVB model has been developed based on the previous study of the two-state EVB model, the impact of the proton hopping mechanism on the structure and dynamics properties of protons in hydrated Nafion membrane has been investigated at different hydration levels. The new EVB model in bulk water predicts a significantly enhanced transport in comparison with previous hopping models, which largely improves the agreement with the available experimental data. We have determined the diffusion coefficients of protons and water molecules in hydrated Nafion membrane as a function of hydration level, and the results are in good agreement with experimental data for the studied hydration levels. Proton hopping mechanism is found to become more significant at higher hydration levels. The influence of the sulfonate groups on the PT properties have also been investigated in this study. Our simulation results provide insight into quantitative information about PT properties in atomic level.