Synthesis and characterization of a phosphonated graft copolyimide for direct methanol fuel cells application

The synthesis method and characterization of novel phosphonate graft copolyimides series via 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) were proposed. A branched polyimide structure was achieved by coupling the hydrophilic phosphonated polymide and hydrophobic polyimide with 3, 3′-diaminobenzidine.A phosphonated branched copolyimide exhibited high thermal stability, with a starting decomposition temperature in nitrogen of ~450 °C. The proton conductivity and the methanol permeability of phosphonate graft copolyimide membrane were studied. At room temperature and hydrated condition, the phosphonated membrane with highest IEC ~ 1.43 meq/g showed slightly lower proton conductivity than Nafion®117, however its methanol permeability is 5 times lower than Nafion®117. The decrease in methanol permeability with desirable proton conductivity could increase the feasibility of polymer electrolyte membranes for DMFC application.