Rigid hydrophobic group-modified polybenzimidazole as proton exchange membranes for high-temperature fuel cells

Superior overall performance is crucial for advancing high-temperature proton exchange membrane fuel cells (HT-PEMFC). In this study, we report a series of polybenzimidazole copolymer membranes by incorporating various aromatic rigid hydrophobic units (phenylene, biphenyl, and terphenyl) into poly(2,2′-m-(phenylene)-5,5′-bibenzimidazole) (mPBI) main chains. These membranes show increased free volume and hydrophobic properties. As a result, after doping with phosphoric acid, the HT-PEMs demonstrate enhanced oxidative stability and good PA retention across different relative humidity and temperatures. The fuel cell based on PA-doped Tp-mPBI-1/9 with terphenyl exhibits the highest peak power density retention after cycling from 1 V to 0.1 V at different low temperatures (80 °C/40 °C). Furthermore, Tp-mPBI-1/9 delivers steadily increasing single-cell performance (675 mW cm⁻²) at 220 °C and effectively broadens the operating window. Stability testing of Tp-mPBI-1/9-based single cells at 0.3 A cm⁻²/160 °C resulted in a voltage decay rate of 73.2 μV h⁻¹. Based on their excellent comprehensive performance, fine-tuning of the PBI backbone structure optimized the PBI membranes for HT-PEM applications.