A Thermodynamic Approach to Model Proton Conductivity of Nafion-117 Membranes: Temperature and Water Content Effects

Abstract

A thermodynamic model-combined ionic conductivity equation was used to predict the proton conductivity of Nafion 117 membranes at different water contents and temperatures. Both vehicle and Grotthuss (hopping) mechanisms were considered to be responsible for the proton transport, from which protons and hydronium ions diffusion coefficients and conductivities were then calculated. Moreover, a correction was proposed for calculating the total concentration of ions in the above-mentioned model. It was found that with increasing the temperature and water content, the diffusion coefficients and conductivities enhance and the temperature dependence of the conductivity of Nafion 117 membrane follows a simple Arrhenius behavior. A good agreement between the theoretical and previously reported experimental proton conductivity data was also observed only when the corrected concentration was used in the calculations. The results obtained in the present work showed that the Grotthuss mechanism, which is usually ignored in the theoretical predictions, plays a significant role in the total proton conductivity of Nafion 117 membrane, especially at higher operational temperatures and the membrane water volume fractions.