Anomalous water sorption kinetics in supported Nafion thin-films as membrane-electrode assemblies

To understand the role of interfacial resistance to water transport in a system geometry comparable to that of a PEM fuel cell, water sorption kinetics in carbon-supported Nafion thin-films has been studied at different relative humidifies at 20 degrees C while varying the membrane thickness. We have found anomalous water sorption kinetics at short time scales, i.e. deviation from the Fickian diffusion characteristic of these systems. We observe two different time scales with a delayed onset to Fickian behaviour that can be understood as the existence of two regimes with different diffusion coefficients. We attribute them to an interfacial diffusivity, D-t, and a bulk diffusivity, D-b. The onset delay is relatively more pronounced on thinner films due to increased interfacial resistance. Here, we show with a simple mathematical model based on Crank's solution to Fick's second law, that the presence of two regimes where water transport at the interface is orders of magnitude slower than at the bulk can simulate the anomalous behaviour in sorption curves. Experimental estimations of the onset time show it decays rapidly with increasing thickness of the films and scales linearly with the resistance to water transport.