Spacer-engineering construction of continuous proton transport networks for cardo poly(biphenyl indole) high-temperature proton exchange membranes

Proton-exchange membranes are considered the core of high-temperature proton-exchange membrane fuel cells (HT-PEMFCs) and determine the output power of the cells. However, current HT-PEMs, which are mainly doped with phosphoric acid (PA), face challenges such as low conductivity and loss of PA during cell operation. In this study, cardo poly(biphenyl indole) membranes (POXIA-QA) with quaternary ammonium salt terminals and tunable side-chain lengths were designed at the molecular level. The rational structural design enabled PA to form a continuous hydrogen-bonding network in POXIA-QA, which could be effectively immobilized and retained. The proton conductivity and activation energy patterns were found to be inconsistent with the PA doping level, suggesting that proton transportation depends not exclusively on the amount of PA but also on the PA distribution induced by the acidic spacer. The polymer membrane bearing a hexyl side chain spacer exhibited excellent proton conductivity (0.113 S cm−1 at 180 °C) and single-cell power density (1001.3 mW cm−2 at 180 °C). This study provides insights into the microphase separation, PA distribution, proton transfer, and high-temperature fuel cell performance of phosphonic acid-doped HT-PEMs with side chains and establishes structure–performance relationships. This study is expected to provide a theoretical basis and suitable candidate materials for the design of HT-PEMs.