Characterization of morphological and transport behavior of solvated sulfonated poly(2,6 dimethyl 1,4 phenylene oxide) fuel cell membranes from molecular simulation

During recent years, polymer electrolyte membranes (PEMs) have attracted intensive research works thanks to their potential usages in both polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). Perfluorosulfonic Nafion membranes, which consists of poly(tetrafluoroethylene) (PTFE) backbone and a sulfonic acid terminated perfluorinated side chain are the most widely adopted membranes, because of their high proton conductivity at optimum hydration levels and acceptable mechanical stability. However, major drawbacks including high methanol crossover (for DMFCs), high production cost and lower proton conductivity at increased temperatures have limited their applications and motivated extensive studies toward the development of alternative membranes especially hydrocarbon-based PEMs such as sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO). Therefore, in the current research, molecular dynamics (MD) simulations were used to investigate the structure and dynamics of water and hydronium ion inside the solvated SPPO membranes as a function of hydration level. Simulation results have shown that SPPO membranes become phase separated when they uptake water molecules. Evaluation of radial distribution function revealed that water molecules and hydronium ions form larger aqueous clusters within the hydrated SPPO membranes at enhanced hydrations. Finally, the calculated diffusion coefficients for hydronium ions and water molecules inside SPPO based PEMs were increased as the hydration level is increased.