Short-side-chain proton conducting perfluorosulfonic acid ionomers: Why they perform better in PEM fuel cells

Short-side-chain (SSC) perfluorosulfonic acid ionomers of different ion exchange capacity, IEC, (Dow 840 and Dow 1150) are characterized with respect to water sorption, transport (proton conductivity, electroosmotic water drag and water diffusion), microstructure and visco-elastic properties as a function of temperature and degree of hydration. The data are compared to those of Nafion 117 (Nafion 1100), and the implications for the use of such ionomers as separator materials in direct methanol and hydrogen fuel cells are discussed. For this purpose, a scheme is used which allows for the simulation of the water distribution and the resulting solvent (water, methanol) fluxes and potential losses across the membranes under transient and steady state conditions. The lower solvent (water, methanol) transport across Dow 1150 in direct methanol fuel cells is essentially the result of the reduced swelling. The smaller potential losses across Dow 840 compared to Nafion 117 in hydrogen fuel cells is mainly the result of the higher charge carrier concentration. The present comparison demonstrates that combining high IEC with high mechanical stability, e.g. by increasing crystallinity as in the present case, or by increasing molecular weight, cross-linking or introducing interacting particles, results in an increase in proton conductivity and lower electroosmotic drag of water. The improved stability is also anticipated to preserve the morphology of the membrane/electrode interface under conditions at which Nafion 117 starts to show plastic deformation. Hence, this may prove to be a suitable approach to improve membrane performance in both direct methanol and PEM fuel cells.