Anchoring Highly Sulfonated Hyperbranched PBI onto oPBI: Fast Proton Conduction with Low Leaching

Abstract

To achieve fast proton conduction with low leaching of the proton conductor, hyperbranched polybenzimidazole with abundant terminal diamino groups was synthesized and highly sulfonated to yield the cross-linkable proton conductor SHBPBI. A high-temperature proton exchange membrane (HTPEM) was prepared with ether containing polybenzimidazole (oPBI) and triallyl isocyanurate (TAIC). TAIC formed covalent bonds with N–H bonds in oPBI and SHBPBI, and effectively anchored soluble SHBPBI onto the oPBI-TAIC network without sacrificing proton-conducting sulfonic acid groups. oPBI-TAIC-SHBPBI exhibited good thermal stability, mechanical property, dimensional stability, oxidative resistance, membrane selectivity, and low methanol/H₂/O₂ crossover. The proton conductivity of oPBI-TAIC(5%)-SHBPBI(50%) achieved was 0.147, 0.074, and 0.034 S cm^–1 at 100%, 50%, and 0 RH at 180 °C, respectively. The conductivity of oPBI-TAIC(5%)-SHBPBI(50%) at 0 RH remained 97.1% after washing with water for 96 h, indicating low leaching of the soluble proton conductor SHBPBI. The composite membranes exhibited potential application in HTPEM fuel cells and direct methanol fuel cells (DMFCs).