Influence of PDMS chain length on proton conductivity in polysiloxane based membranes for HT-PEMFC application

Siloxane based membranes were produced with benzimidazole as proton conducting group by cross-linking siloxane monomers with two or three cross-linking groups. The network structure was further modified by adding a constant amount of silanol terminated linear polydimethylsiloxanes (PDMS) with varying chain lengths. Their influence on glass transition temperature, network formation and proton conductivity at varying relative humidities was investigated at temperatures from 30 to 160 °C. An increasing PDMS chain length decreases the glass transition temperature and changes the hydrophobicity of the membranes. The increase of the number of cross-linking groups leads to additional uncross-linked groups and consequently to a wider network. The new membrane materials possess high proton conductivities of up to 1.2 × 10−3 S/cm at 160 °C at relative humidities of less than 8% with thermal stabilities of up to 286 °C.