Molecular dynamics simulations of Nafion and sulfonated polyether sulfone membranes. I. Effect of hydration on aqueous phase structure
We measured the water uptakes and proton conductivities of a Nafion membrane and three sulfonated polyether sulfone membranes (SPESs) with different values of ion-exchange capacity (IEC = 0.75, 1.0 and 1.4 meq/g) in relation to relative humidity in order to apply the findings to polymer electrolyte membrane fuel cells. The number of water molecules per sulfonic acid group lambda at each humidity level was independent of the relative humidity for all membranes, but the proton conductivities of the SPESs were inferior to that of Nafion for the same lambda value. Classical molecular dynamics simulations for the same membranes were carried out using a consistent force field at lambda = 3, 6, 9, 12 and 15. The structural properties of water molecules and hydronium ions at a molecular level were estimated from radial distribution functions and cluster size distributions of water. We found that the radial distribution function of S(sulfonic acid)-S(sulfonic acid) of Nafion at lambda = 3 indicated a significant correlation between the S-S pair, due to water channels, while the S-S pair of the SPESs showed a poor correlation. The cluster size distribution of water was also calculated in order to estimate the connectivity of the water channel. It is clear that some water is present in the SPESs as small, isolated clusters, especially when the water content is low.
molecular dynamics sulfonated polyether sulfone membrane aqueous phase structure nafion membrane proton exchange membrane fuel cell