Mechanisms of proton transport in alcohol-penetrated perfluorosulfonated ionomer membranes for fuel cells

In order to clarify the mechanisms of proton transport in alcohol-penetrated perfluorosulfonated ionomer (PFSI) membranes for fuel cells, six membranes having different equivalent weight (EW) values were examined. Water, water/methanol mixture (molar ratio: 1/1), methanol, ethanol and 2-propanol were penetrated to each membrane, and membrane swelling, methanol permeability, proton conductivity and mass (alcohol and proton) diffusion coefficients were measured systematically. The methanol permeability PM and the membrane expansion fraction θ showed that the PFSI membranes with smaller EW values were swelled larger by methanol and the permeation rates were also larger. The proton conductivity was reduced by methanol penetration into the membranes especially for the smaller EW value ones. To investigate the roles of CH3 group of methanol, self-diffusion coefficients of the alkyl group DCH3 and of OH (including protons) DOH of alcohols (methanol, ethanol and 2-propanol) were measured separately by the pulsed-gradient spin-echo (PGSE) 1H NMR method. Both D values increased with decreasing the EW value, and the DOH was always larger than the DCH3. In addition, the differences between the DOH and DCH3 increased with the decrease of the size of alkyl groups. These results indicate that protons transport faster than the alcohols by the Grotthuss (hopping) mechanism, and the faster proton transport was promoted more when the membrane was penetrated by smaller alcohol. The lipophilic nature of alcohols was found to be one of the factors that influence the mechanisms of proton transport in the membranes.