A Promising Proton-Exchange Membrane: High Efficiency in Low Humidity

Abstract

In this research, two new proton conductive membranes consisting of −SO3H groups are synthesized and their proton transfer properties are studied in different conditions. By indirect insertion of the sulfonate group onto the imidazolic nitrogen of poly(benzimidazole-imide) (PBII) by sultone, water uptake increases to 160% and maximum proton conductivity (0.067 S cm–1) is observed at 80 °C and RH = 60% (PBII2). However, at temperatures higher than 80 °C, the proton conductivity of PBII2 becomes similar to that of Nafion (lower proton conductivity at high temperatures). Nevertheless, when the sulfonate group is directly attached to the imidazolic nitrogen by ClSO3H (PBII3), water uptake drops to approximately 0% and shows very poor conductivity at ambient temperature. By increasing the temperature, proton conductivity is amplified and at 160 °C and RH = 0%, the proton conductivity of the membrane reaches 0.0251 S cm–1. At low temperatures, because of highly strong electrostatic interactions, the proton cannot transfer easily. Nevertheless, at high temperatures, sufficient energy is provided for proton transfer through the hopping mechanism. Finally, some theoretical calculations were conducted to support both the experimental findings and the nature of interactions.