Synthesis of graft copolyimide for proton exchange membrane

Abstract

In this study, the novel phosphonated graft copolyimide based on NTDA and ODA monomer was synthesized. The phosphonated polyimide side chain was obtained from the series of bromination and lithiation reaction while the graft structure was achieved by novel use of 3,3’-diaminobenzidine monomer with 4 reactive amine group. This synthesis route leads to relatively simple and easily adjustable of molecular weight, chain length of backbone (PI) or side chain (paPI) segment as well as phosphonation or sulfonation level, if preferred. This novel structure of graft copolyimide can improve proton conductivity while maintaining good thermal and hydrolytic stability. The effects of microstructure e.g. phosphonation level and side chain length on water uptake, swelling ratio and proton conductivity were studied. Based on spray techniques for MEAs fabrication, the fPBPI-3 membrane with IEC of 1.43 meq/g showed the highest proton conductivity (0.045 s/cm) which comparable to Nafion(R)117, moreover, this membrane also showed more stable proton conductivity at operating temperature of 80-100°C than Nafion(R)117. The optimal condition of fuel cell test for fPBPI membrane in this study was reported at 90°C and 74%RH. In addition, the methanol permeability of fPBPI membrane was 5 times lower than Nafion(R)117 which increased the feasibility of this membrane for DMFC application.