Synthesis and characterization of sulfonic acid containing polyimides for polymer electrolyte membranes (PEMs) in fuel cells

Different polyimides bearing sulfonic acid groups were prepared for polymer electrolyte membranes (PEMs) in fuel cells. These polymers contained displacing, angled, multi-functional and/or fluorinated comonomers.; Displacing and angled comonomers produce imperfections in the long rigid liquid crystalline polyimide backbone, generating nanoscale pores lined with sulfonic acid groups. Such pores absorb water strongly even at relatively low humidity, resulting in higher proton conductivity than Nafion® . However, those copolyimides either dissolve or fragment easily in water at elevated temperatures. To overcome this drawback, a fluorinated comonomer was introduced into the polymer.; Long-term stability of the polyimides was evaluated for the PEMs. We observed that the sulfonic acid containing polyimides degraded slowly under ambient conditions, and severely at high temperatures. This was analyzed using FT-IR and viscosity measurements.