Dependence of electroosmosis on polymer structure in proton exchange membranes

Abstract

Effects of polymer structure on the electroosmosis in proton exchange membranes (PEMs) have been investigated using a reactive molecular dynamics simulation. An anharmonic two-state empirical valence bond (αTS-EVB) model has been used to describe efficiently excess proton transport via the Grotthuss hopping mechanism. The electroosmotic drag coefficients (i.e., the number of water molecules transferred through the membrane per proton) has been evaluated directly in PEMs consisting of various equivalent weights (EWs). The electroosmotic drag coefficients from the MD simulations are in good agreement with the available experimental values at studied levels of hydration. It is shown that for low water contents the connectivity of the water clusters decreases with increasing EW, leading to a decrease in the electroosmotic drag coefficients. The proton and water mobility is also found to decrease with increasing EW because of the structural changes in the water cluster domains. The present simulations provide quantitative information about the direct link between electroosmosis and nanoscopic water domain structure in different polymer structures