Rapid Proton Conduction through Unfreezable and Bound Water in a Wholly Aromatic Pore-Filling Electrolyte Membrane

We found that protons rapidly conduct through unfreezable and bound water in a pore-filling electrolyte membrane (PF-membrane), although many ions usually conduct through free water contained in polymer electrolytes. PF-membrane is a unique membrane that can suppress the swelling of filled sulfonated poly(arylene ether sulfone) (SPES) because of its rigid polyimide substrate. Based on low-temperature DSC measurements, this strong suppression of swelling resulting from the special structure of the polymer electrolyte results in unfreezable and bound water only; it does not contain any free water. Protons rapidly conduct through this structure. In addition, the activation energy of the proton conduction decreased from 16.3 to 9.1 kJ/mol in proportion to the increase in the ion exchange capacity (IEC) of the filled SPES, unlike the almost constant values of the SPES-cast membranes. This tendency of PF-membrane occurred because of the structure of the membrane, where the concentration of the sulfonic acid groups increased with increase in IEC, which became possible by squeezing free water using the swelling suppression of filled SPES. Without being constrained by the PF-membrane, this unique proton conduction through the structured water and highly concentrated sulfonic acid groups will help to develop future polymer electrolytes, particularly in the fuel cell field where protons need to conduct at various conditions such as temperatures below 0 °C, combined high temperature and low humidity, and the presence of fuels.