The state-of-the-art of polymer electrolyte membrane fuel cell (PEMFC) technology is based on perfluorosulfonic acid (PFSA) polymer membranes operating at a temperature of around 80°C. However, operating at a higher temperature has many benefits such as fast electrode kinetics and high CO tolerance. Phosphoric-acid-doped PBI membrane has been found to possess desirable characteristics including low humidity requirement, high proton conductivity at high temperature (0.13 at 160°C) and nearly zero electroosmotic drag. To further enhance the proton conductivity at high temperature, composite polymer membranes of phosphoric-acid-doped polybenzimidazole (PBI) and cesium hydrogen sulfate (CsHSO4) will be investigated.

Hydrogen sulfates, MHXO4, where M is large alkali species Rb, Cs, or NH4+, and X is S, Se, P, or As, are solid acids whose chemistry and properties lie between those of a normal acid (e.g. H2SO4) and a normal salt (e.g. Cs2SO4). One of the hydrogen sulfates, CsHSO4, transforms at 140°C into a superionic phase and exhibits a high proton conductivity about 10-2 . At the transition, the proton conductivity increases by several orders of magnitude. CsHSO4 also has high thermal (decomposes at 212°C) and electrochemical stability. Furthermore, its conductivity does not depend on humidity because CsHSO4 does not contain water in its structure. However, due to solubility in water, poor mechanical properties, extreme ductility, and volume expansion at higher temperature, it is not suitable for fuel cell operation. In order to avoid the water solubility problem of CsHSO4, operation at above 100°C is desirable. At these temperatures, any water in the fuel cell is in the form of steam and does not damage CsHSO4.

In this paper, the development of incorporating CsHSO4 into PBI and doped with phosphoric acid for high temperature operation will be attempted. Using PBI as the backbone, CsHSO4 acts as an assist in improving the proton conductivity at higher temperature. Physiochemical properties of the membranes will be investigated by measurements of CsHSO4 concentration, acid doping level, proton conductivity, mechanical strength and water drag coefficient. Moreover, fuel cell tests on the membranes will be demonstrated.