Sulfonated naphthalene dianhydride based polyimide copolymers for proton-exchange-membrane fuel cells: II. Membrane properties and fuel cell performance

Selected properties of two series of sulfonated naphthalene dianhydride based polyimide copolymers were studied to assess their potential for fuel cell applications. The copolyimides were synthesized in m-cresol from a novel disulfonated diamine 3,3′-disulfonic acid-bis[4-(3-aminophenoxy)phenyl] sulfone (SA-DADPS), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NDA), and one of two nonsulfonated diamines (4,4′-oxydianiline (ODA) or bis[4-(3-aminophenoxy)phenyl] sulfone (m-BAPS)). Both systems produced tough, ductile cast films. The membrane parameters studied include water sorption, proton conductivity, water stability, methanol permeability and direct methanol fuel cell (DMFC) performance. It was found that the molecular structure of the nonsulfonated diamine significantly influenced the hydrolytic stability of the membrane in water at 80°C. However, the water sorption and proton conductivity were primarily a function of the ion exchange capacity (IEC) and were independent of the structure of the nonsulfonated diamine. The copolyimide membranes utilizing m-BAPS as the nonsulfonated diamine displayed the best water stability at 80°C. Unfortunately, the best hydrolytic stability achieved was still much lower than Nafion or analogous poly(arylene ether)s. At relatively high ion exchange capacities, the proton conductivities of the polyimides in water at 30°C were equivalent to Nafion 1135. An IEC of ∼1.9 (BAPS-80, ODA-70) was necessary to provide conductivities close to 0.1 S/cm in water at 30°C. It was demonstrated that the proton conductivity of the membranes increased at elevated temperature and high relative humidity. The initial DMFC performance of several copolyimides was investigated, and it was found that these membranes had much lower methanol permeability and performed comparably to Nafion 117 over short-term testing at 80°C.