Pore–Filling and Thermally Cross–Linked Polyethersulfone Membranes with High Ion Exchange Capacity and Thermal Stability for PEFCs

Polymer electrolyte membrane fuel cells (PEFCs) have attracted considerable attention due to their high efficiency and clean electric generating system. Especially, in terms of the catalytic activity and water management, there is a great demand for operation under high temperature and low relative humidity. Under such situation, we recently succeeded in the development of heat–resistant electrolyte membrane (PI-CLSPES) with high ion exchange capacity (IEC) from porous polyimide (PI) substrates and highly sulfonated polyethersulfones (SPES150: IEC = 4.9meq/g) by using both polymer-filling and thermal cross–linking methods. The PI substrate can be prepared by using the viscolastic phase separation phenomenon: multi-stage solvent replacement of polyammic acid (PAA) cast film into N-methyl-2-pyrrolidone (NMP) and methanol, followed by thermal treatment. We found that the changes in viscosity of PAA solution, thickness of PAA cast film and immersion time afford the PI substrates with various pore sizes. On the other hand, the SPES150 membrane with high IEC is obtained by further sulphonation of sulfonated poly(arylene ether sulfone) (SPES50) in concentrated sulfuric acid, and allowed for impregnation into the pores of the porous PI substrate. The SPES150 inside the PI pores can be cross-linked between the sulfonic acid and phenyl groups by appropriate thermal treatment, and consequently, lead to the high thermal stability and suppression of membrane swelling. Moreover, the pore-filling and cross-linked membranes still retain the high IEC, and displayed the superior proton conductivity under high temperature comparable to that of Nafion. The detailed proton conducting behavior of the polymer electrolyte membranes as well as their structural analysis will be presented.