Proton conductivity and micro-morphology of Nafion fuel cell membranes determined by Electrochemical Atomic Force Microscopy

The proton conductivity of a Nafion 112 membrane is measured with a resolution of ca. 10 nm using Electrochemical Atomic Force Microscopy. The observations are compared with the Gierke model of the micro-morphology due to phase separation of the hydrophobic polymer backbone and the hydrophilic pendant groups. The results demonstrate a highly inhomogeneous conductivity distribution which is attributed to the limited connectivity of hydrophilic domains. A pronounced peak was found in histograms representing the number of conductive areas with a certain current. We suggest that the measured current significantly depends on the formation of a continuous network in the membrane, so that the same conductivity can be measured at all pore exits. These pore exits to some extent seem to be quantized in size, even though they are of irregular shape.