Efficient and stable proton conduction achieved by accommodation of the membrane-wide cross-linking and branching strategies

Fabrication of high-temperature proton exchange membranes (HTPEMs) with high and durable proton conductivity, and sufficient mechanical/oxidative stability is of great significance but challenging. Herein, a flexible branched polymer (broPBI), a cross-linkable water-soluble proton conductor (PBSA), and a multi-functional cross-linker with high content of imidazole rings (BrABPBI) were designed to fabricate HTPEMs with good comprehensive performances. The branching structure of broPBI provided large free volume, high content of terminal groups, and interconnected main-chains, which are conducive to efficient proton conduction and improved mechanical property. BrABPBI covalently crosslinked broPBI and PBSA to form membrane-wide cross-linking structure, improving the mechanical strength and oxidative resistance while avoiding leaching of PBSA. The secondary amines in broPBI and PBSA became tertiary amines after cross-linking, which have higher basicity and thus higher doping capacity of PBSA. However, overusing of cross-linking or branching can deteriorate membrane performances. To achieve comprehensive good performance, the two strategies were appropriately accommodated in broPBI-BrABPBI-PBSA membranes. At 180 °C, the proton conductivity of broPBI-BrABPBI(20%)-PBSA(50%) composite membrane reached 0.140, 0.0701, and 0.0310 S cm−1 at 100% RH, 50% RH, and 0% RH, respectively. The membrane has excellent mechanical properties, proton conductivity, and oxidative stability, with low leaching of PBSA, demonstrating promising application prospects.