Asymmetric proton-conducting membrane made by photopolymerization

Abstract

Proton-conducting membranes with interpenetrating polymer network morphology have been frequently considered in recent years for potential replacement of standard Nafion membranes in direct methanol fuel cells. Asymmetric membranes comprising protonconducting channels of cross-linked sulfonic acid functionalized ionomers embedded within a matrix of thermally-resistant, glassy polymer were prepared by photopolymerization starting from a polymer solution and evaluated in our laboratories. These membranes have an integral top skin layer with fine proton-conducting channels, which serves as a barrier against methanol crossover, on top of a coarser proton-conducting support. Conductivity of asymmetric membranes over a range of initial polymer concentrations and ion-exchange capacities (IEC) was just slightly lower than for the corresponding symmetric membranes. Methanol barrier properties of asymmetric proton-conducting membranes were better than that of the state-of-the art Nafion 115 membrane. The crosslinking agent functionality had a major effect on membrane conductivity. Use of trifunctional crosslinking agents resulted in significantly higher conductivities than those obtained with bifunctional agents, even surpassing the conductivity of Nafion membranes.