Synthesis and properties of novel sulfonated polyimide membranes for direct methanol fuel cell application

Two sulfonated diamine monomers, 4,4′-bis(4-aminophenoxy)benzophenone-3,3′-disulfonic acid (BAPBPDS) and 4,4′-bis(4-aminophenylthio)benzophenone-3,3′-disulfonic acid (BAPTBPDS), were synthesized via two-step reactions and a series of sulfonated polyimides (SPIs) were prepared from these sulfonated diamines, 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and common non-sulfonated diamines via conventional one-step polymerization approach. The resulting SPIs showed good solubility, high thermal stability, high proton conductivity (comparable to that of Nafion 112) and rather low methanol permeability (one-fifth to one-tenth of that of Nafion 112). BAPTBPDS-based SPI membranes displayed better water stability than BAPBPDS-based ones because of the higher basicity of BAPTBPDS moiety.