Water diffusion and ion conductivity in polymer electrolyte membranes (PEMs), such as Nafion, is not well understood but is important for advancing technologies such as fuel cells and water purification. Nafion is a fluorinated polymer with sulfonate anions attached to side chains. In hydrogen fuel cells, the counter ions are protons. High proton conductivity is essential for optimal fuel cell performance and strongly linked to water content and mobility in Nafion. Unfortunately, commercialization of fuel cells requires high temperature and low humidity. Therefore, understanding water diffusion and ion conductivity in Nafion is critical.
Fourier transform infrared - attenuated total reflectance (FTIR-ATR) spectroscopy can resolve different states of water in Nafion at the molecular level in real time. Thus, diffusion of each state can be measured simultaneously. This work identified vapor-phase mass transfer resistance and non-Fickian diffusion as causes of variation in reported diffusion coefficients. By accounting for these effects, accurate diffusion coefficients were obtained. In addition, proton conductivity of Nafion was measured at different water contents. The correlation between proton conductivity and different states of water were examined and used to identify promising alternative PEMs.