Enhancement of comprehensive performance of high-temperature and low-humidity proton exchange membranes: Crosslinking of trifunctional proton conductor with branched polybenzimidazole

Abstract

In order to improve the proton conductivity and antioxidant stability of proton exchange membranes (PEMs) at wide relative humidity (RH), composite membranes cross-linked with tribranched polybenzimidazole (TaPBI) and poly(azido-1-propanesulfonic acid −2,2′- methylsulfonic acid −5,5′-bis-benzimidazole) polyphosphazenes (PSBI) (TaPBI-PSBI) are prepared. TaPBI, as a branched polybenzimidazole, exhibits a large free volume. The trifunctional proton conductor PSBI-Na incorporates abundant flexible acidic groups and basic sites to enhance proton conductivity, and cross-linking groups to increase its binding strength with PBI. By casting and cross-linking, the proton conductivity, mechanical properties, and oxidation resistance of the TaPBI-PSBI membranes are significantly improved. The generation of cross-linked structures in the membranes is demonstrated by FT-IR. SEM shows that the surface of composite membrane is smooth and free of through holes. The elements are uniformly distributed. The TEM shows microphase separation occurs in the TaPBI-PSBI membrane, facilitating the formation of the continuous proton channels. The proton conductivity of TaPBI-PSBI (40) composite membrane reaches 0.170, 0.081, 0.059, and 0.037 S cm−1 at 180 °C, 100%, 50%, 30%, and 0% RH, respectively. The composite membrane is demonstrated the outstanding stability with the small decrease of proton conductivity and mass after durability testing of 96 h. The composite membrane exhibits high comprehensive performances as HT-PEM.