Spatial distribution and dynamics of proton conductivity in fuel cell membranes: potentialand limitations of electrochemical atomic force microscopy measurements

Abstract

The proton conductivity of a Nafion 112 membrane is measured with a high spatialresolution using electrochemical atomic force microscopy. Image analysis reveals aninhomogeneous conductivity distribution which is attributed to the limited connectivity ofhydrophilic domains. This information relates to the micro-morphology which is due tophase separation of the hydrophobic polymer backbone and the hydrophilic pendantgroups. The direct images relate to a different length scale and are complementary to thex-ray diffraction investigations which provide only average information. Furthermore, themeasured current values reveal an interesting correlation with the size of the conductiveareas. A bimodal conductivity distribution suggests that there are different mechanismswhich contribute to the proton current in Nafion. Additionally, time dependence in localconductivity is found and interpreted in terms of redistribution of water in themembrane. A statistical analysis of the current distribution is performed andcompared with theoretical simulations. Evidence is found for the existence of acritical current density. On a timescale of seconds the response of the conductivenetwork is probed by applying voltage steps to the atomic force microscope tip.