Proton conduction mechanisms at low degrees of hydration in sulfonic acid-based polymer electrolyte membranes

The need to operate polymer electrolyte membrane (PEM) fuel cells at temperatures above 100degreesC, where the amount of water in the membrane is restricted, has provided much of the motivation for understanding the mechanisms of proton conduction at low degrees of hydration. Although experiments have not provided any direct information, numerous theoretical investigations have begun to provide the basis for understanding the mechanisms of proton conduction in these nano-phase-separated materials. Both the hydrated morphology and the nature of the confined water in the hydrophilic domains influence proton dissociation from the acidic sites (i.e., -SO3H), transfer to the water environment, and transport through the membrane. The following molecular processes are discussed in connection to their role in the conduction of protons in sulfonic acid-based polymer electrolyte membranes (PEMs): (a) local chemistry of the hydrophilic side chains; its effect on the dissociation of the proton and eventual stabilization (separation) of the proton in the water; (b) the presence of neighboring sulfonic acid groups on proton transfer; and (c) the effect of the distribution of the sulfonate groups on the transport of protons in the channels/pores of the membrane.