Polyphenylquinoxaline-based proton exchange membranes synthesized via the PPA Process for high temperature fuel cell systems

Abstract

A novel method for the successful, high temperature (125 °C) polymerization of high inherent viscosity polyphenylquinoxaline (PPQ) homopolymer and copolymers with polybenzimidazole (PBI) was investigated. PPQ homopolymer and PPQ/PBI copolymer membranes were prepared via the Polyphosphoric Acid (PPA) Process for use in high temperature (>120 °C) fuel cell systems. PPQ homopolymer membrane with a polymer content of 8.00 wt% was shown to have a phosphoric acid loading of 36.2 mol phosphoric acid per mol of polymer repeat unit (PA/r.u.) and a Young's modulus of 21 MPa, indicating a rigid gel membrane. PPQ homopolymer membranes with high PA loadings were, however, found to be dimensionally unstable at temperatures greater than 120 °C and reverted to a sol state. To increase the dimensional stability of a PPQ-based proton exchange membrane (PEM), a series of copolymer films containing PPQ and the highly dimensionally stable p-PBI polymer were studied. The series of PPQ/PBI copolymer membranes ranged in PPQ content from 10 to 95 mol% and were found to possess phosphoric acid doping levels between 19 and 39 mol PA/r.u. and proton conductivities up to 0.26 S cm−1. Several of the copolymer membranes were also shown to have enhanced rigidity over numerous other membranes developed by the PPA Process as exhibited by Young's moduli between 1.9 and 31.1 MPa. An advantageous balance of properties was found for a PPQ/p-PBI copolymer membrane composition of 58 mol% PPQ and 42 mol% p-PBI, denoted as PPQ-58. This membrane was found to have a phosphoric acid doping level of 39.2 mol PA/r.u. and a proton conductivity of 0.24 S cm−1 at 180 °C. PPQ-58 membranes also showed excellent long-term stability in a fuel cell operating under non-humidified conditions at 160 °C utilizing hydrogen and air (1.2:2.0 stoich.) at a current density of 0.2 A cm−2. Under these conditions, PPQ-58 exhibited a voltage degradation rate of 30 μV h−1 during a 2900 h lifetime performance test.