Current Sensing Atomic Force Microscopy Study of Thermal Aging of Nafion Membranes

Abstract

We have investigated thermal aging in the ionic conductivity of Nafion membranes being annealed at elevated temperatures using current sensing atomic force microscopy (CSAFM), a unique technique that allows simultaneous imaging surface morphology and surface conductance of the materials. The local current flows on the membranes show an increase after initial annealing, followed by a gradual decrease as the annealing continues. The current distribution extracted from CSAFM images shows that the decrease in the conductance follows approximately an exponential decay which occurs uniformly across the membrane. Least-square fits to the measured conductance versus time t yield time constants at the annealing temperature. Considering aging is a thermally activated process, we used an Arrhenius equation to describe the relationship between the time constant and the annealing temperature, which gives activation energies for the membranes. The activation energy yields following this process are 54 and 83 kJ/mol for...