Rigid Tröger’s Base-Containing Poly( p -terphenyl pyridine) Membranes with Enlarged Free Volume and Robust Mechanical Strength for High-Temperature Proton-Exchange Membrane Fuel Cells

Abstract

The plasticizing effect of phosphoric acid seriously deteriorates the mechanical properties of high-temperature proton-exchange membranes and hinders the practical application of fuel cells. Herein, we synthesize two Tröger’s base-containing aromatic monomers with tunable rigidity (named DMD and TMD) to prepare copolymers based on poly(p-terphenyl pyridine) (PTP). The proposed poly(p-terphenyl pyridine-co-TMD, PTPT) membrane presents a narrower micropore size and a higher micropore density than the original PTP membrane, as evidenced by the CO2 adsorption isotherm. The highly rigid and twisted structure of TMD endows the PTPT membrane with increased free volume and improved mechanical strength to counteract the plasticizing effect of the doped phosphoric acid (PA). The PTPT membrane doped with 276 wt % PA displays a volume swelling ratio of 121%, which is lower than that of 162% for the PTP membrane doping with 223 wt % PA. Moreover, this PA-doped PTPT membrane delivers an anhydrous proton conductivity of 139 mS cm–1 at 160 °C, which is higher than that of the PA-doped PTP membrane (99 mS cm–1). Besides the high PA doping level, the increased proton conductivity benefits from the formation of larger and well-connected ion transport channels within the PTPT membrane, as proved by SAXS profiles and AFM adhesion force images. The PA-doped PTPT membrane-based H2/O2 fuel cell demonstrates a peak power density of 1116 mW cm–2 at 180 °C under a Pt loading of 1.0 mg cm–2 without humidity or backpressure. When 1 bar backpressure is applied to both the anode and the cathode, the fuel cell delivers a peak power density of 1600 mW cm–2 at 180 °C under a Pt loading of 0.6 mg cm–2 corresponding to a specific power of 2667 mW mg–1. Additionally, the PTPT-membrane-based fuel cell works stably during start/stop cycling for hundreds of times.