Moving beyond mass-based parameters for conductivity analysis of sulfonated polymers

The proton conductivity of polymer electrolytes is critical for fuel cells and has therefore been studied in significant detail. The conductivity of sulfonated polymers has been linked to material characteristics to elucidate trends. Mass-based measurements based on water uptake and ion exchange capacity are two of the most common material characteristics used to make comparisons between polymer electrolytes, but they have significant limitations when correlated to proton conductivity. These limitations arise in part because different polymers can have significantly different densities and because conduction occurs over length scales more appropriately represented by volume measurements rather than mass. Herein we establish and review volume-related parameters that can be used to compare the proton conductivity of different polymer electrolytes. Morphological effects on proton conductivity are also considered. Finally, the impact of these phenomena on designing next-generation sulfonated polymers for polymer electrolyte membrane fuel cells is discussed.