Enhancement on both phosphoric acid retention and proton conduction of polybenzimidazole membranes by plasma treatment

In this study, plasma treatment is designed to solve phosphoric acid (PA) leaching problem as well as enhance the proton conduction and tensile strength of PBI membranes at high temperature. The hydrophily of polybenzimidazole (PBI) membrane surface is remarkably improved during plasma process according to the water contact angle decreasing from 84.0 degrees to 9.9 degrees. The effect of plasma generating power, gas pressure, Ar/O-2 gas proportion and exposure time on water contact angle was investigated. The optimized plasma treating condition was 350 W, 5 Pa, 50 % O-2 and 7 min. Fourier transform infrared spectroscopy (FT-IR) and x-ray photoelectron spectroscopy (XPS) were performed to investigate the effects of plasma treatment on the chemical composition of PBI membranes. The results of FT-IR and XPS reveal the formation of oxygen groups and a progressive increase of oxygen content on the PBI membrane surface due to the plasma modification. Furthermore, the pristine-PBI and plasma-PBI membranes were doped into phosphoric acid (PA) for high temperature proton exchange membrane use. Compared with PA doped pristine-PBI, the PA doped plasma-PBI membranes were found to present higher PA doping levels, higher proton conductivity and better PA retention ability. This indicates plasma treatment could promote acid doping ability, maintain good mechanical strength and reduce PA leaching of PBI membranes.