Alkali-doped hyperbranched cross-linked polybenzimidazoles containing benzyltrimethyl ammoniums with improved ionic conductivity as alkaline direct methanol fuel cell membranes

Summary Development of anion exchange membranes (AEMs) with good performance, such as high conductivity, good alkaline stability and mechanical strength, has been a hot topic for the fuel cell application. Here, a novel kind of hyperbranched cross-linker decorated with quaternary ammonium groups was introduced to polybenzimidazole (PBI) membranes and QOPBI-x membranes (where x is the weight ratio of the hyperbranched cross-linker). Compared with the linear OPBI membrane (0.091 S cm −1 ), QOPBI-x membranes displayed an improved ionic conductivity (up to 0.122 S cm −1 ) at 60°C after they were doped in 6 M KOH for 7 days. The KOH-doped QOPBI-x membranes also exhibited a high tensile strength (54.5-61.7 MPa) and superior alkaline stability. There is almost no decline in the ionic conductivity after being immersed in a 6 M KOH solution for 30 days. In addition, the alkaline direct methanol fuel cell (ADMFC) performance based on the KOH-doped OPBI and QOPBI-x membranes is described. The QOPBI-15 membrane displayed good performance (75.6 mW cm −2 ), which is 33.3% higher than the OPBI membrane (56.7 mW cm −2 ).