Compatible ionic crosslinking composite membranes based on SPEEK and PBI for high temperature proton exchange membranes

Highly sulfonated poly (ether ether ketone) (SPEEK)/polybenzimidazole (PBI) composite high temperature proton exchange membranes were prepared. To enhance the compatibility of SPEEK and PBI, SPEEK was transformed into SPEEK-Na and the prepared SPEEK-Na/PBI was immersed in dilute acid to obtain desired ionic crosslinking SPEEK/PBI membrane. Scanning electron microscope results showed that PBI formed a dense structure. Energy dispersive X-ray spectroscopy of the cross-section of composite membranes and X-ray diffraction declared that PBI were uniformly distributed in SPEEK without phase separation. Fourier-transform infrared spectroscopy indicated the intense ionic bonds between SO3H and N-basic group. Because of the intense interaction, SPEEK/PBI composite membranes could not only significantly improve the mechanical strength and oxidation resistance, but also reduce the swelling degree and methanol crossover. Thermogravimetric analysis results revealed that the membranes could be used below 200 °C. The proton conductivity of the SPEEK/PBI (20 wt%) composite membranes could reach 0.1985 S cm−1 at 170 °C and 100% relative humidity (RH). Proton transfer activation energy was 49.31 kJ mol−1. At 50% RH, the conductivity was 0.099 S cm−1 at 180 °C and it remained the value of 0.0084 S cm−1 at 180 °C in dry oven. The methanol permeability of SPEEK/PBI (20 wt%) membrane was 3.96 × 10−8 cm2 s−1 at 70 °C. The composite membrane appeared to be potential for usage in direct methanol fuel cells.