Emerging membrane materials for high temperature polymer electrolyte fuel cells: Durable hydrocarbon ionomers

A high temperature-operable ionomer membranes using non-fluorinated copolymers combined with aliphatic building blocks was investigated. The improvement of hydrophobic stability increased the electron density on imide nitrogen atoms as hydrolysis was evoked by the nucleophilic attack of water molecules onto the imide linkages. It was suggested that the imide nitrogen atom attached to an aliphatic group should have a higher electron density than that attached to an aromatic group. It was found that the incorporation of bulky hydrophobic groups can effectively introduce water holding capability of polyimide membranes. The copolymer composition was set at 50 mol% so that the equivalent weight per sulfonic acid group was to be 549 g per eq. The sulfonated polyimide (SPI) membrane showed comparable proton conductivity to the perfluorinated ionomer membrane at high humidity. The behavior was observed for non-fluorinated hydrocarbons based ionomers due to less hydrophillic domains.