Molecular dynamics simulation analysis of hydration effects on microstructure and transport dynamics in sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) fuel cell membranes

Molecular dynamics simulations were used to study microstructure and transport behavior of hydrated sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) membranes at different hydration levels. Simulation results have shown that SPPO membranes swell upon hydration and become phase segregated into hydrophobic and hydrophilic domains with sulfonic acid groups located at their interface. Evaluation of radial distribution function revealed that with increasing the hydration level, sulfonic acid groups were found to be solvated with more water molecules which cause the sulfonic acid–sulfonic acid and sulfonic acid–hydronium ion distances to increase. By examining the water cluster size distribution, larger well connected clusters containing almost all the water molecules and hydronium ions were formed inside the hydrated SPPO membranes at increased water contents. Furthermore, the calculated hydronium ion and water diffusion coefficients in SPPO based membranes were increased by increasing the hydration level and were smaller than those in Nafion membrane.