| Proton exchange membrane (PEM) NafionRTM has been studied using current sensing atomic force microscopy (CSAFM) to reveal ionic domains and the local ionic conductivity distribution in the membranes. Topography and current images were gathered under different relative humidity and external forces and their profiles and distributions are analyzed. Under different forces and humidity, the Current-Voltage (I-V) curves of the membrane were also investigated. Force-Distance (F-D) curves under different humidities and temperatures have been acquired and studied in order to investigate the mechanical stiffness of NafionRTM membranes under such conditions.;The study reveals that local conductivities have no correlation with the surface topography of the membranes nor are they much affected by the tip intrusion and the resulting contact area change. As a result, the most important factor in the conductivity of NafionRTM membranes is the extent of their hydration. The local ionic conductivity in the membranes follows a Gaussian distribution. The peak and half width of the distribution increase with relative humidity. It is believed these increases are a result of the increased sizes and interconnections of ionic clusters in the membranes. Quantitative methods are employed to describe the conductive channels of Nafion RTM membranes upon hydration. The characteristics of Current-Voltage (I-V) curves under different humidity and forces further demonstrate that the conductance images are related to the properties of ionic clusters and ionic conducting channels in the membranes and illustrate the intrinsic heterogeneity of NafionRTM membranes upon hydration, while the effects of tip intrusion into the membranes is insignificant compared with the effects of hydration.;CSAFM, with its ability to acquire high resolution images for the topography and current distribution across membrane surface and its ability to apply Current-Voltage and Force-Distance analysis, is proven in the research a powerful tool in the study of materials with heterogeneous structures.;Background information about fuel cells, NafionRTM membranes and atomic force microscopy are also provided. |
