Current sensing atomic force microscopy study of aging mechanism of the nafion membranes due to thermal annealing

Abstract

Proton transport properties of Nafion 212 and Nafion 115 membranes have been studied using current sensing atomic force microscopy to reveal changes of the ionic domains and the local ionic conductivity distribution in the membranes due to the aging. The current sensing atomic force microscopy (CSAFM) is a novel technique which can map morphological and electronic structures of materials on nanometer scales. The technique has been employed in this work to study the ionic structures of Nafion membranes which consist of proton conductive hydrophilic side chains embedded in non-conductive hydrophobic backbones. The hydrophilic side chains aggregate forming ionic domains which could be interconnected when the membrane adsorbs water. We used CSAFM to image the ionic channel network and monitor their changes in a thermal aging process. The results reveal that the thermal aging process undergoes two steps: First, the ionic channels on the membrane surface changes from cluster-like to chain-like structure, accompanied by an increase of the conductance of the membrane. The observed changes can be explained in terms of reorientation of ionic channels near the membrane surface from perpendicular to parallel to the surface as the annealing temperature approaches the glass transition of the membranes. Second, as the annealing continues, the chain-like structure of the proton channels persists but the conductance of the membranes decreases. The observed conductance decreasing can be explained by sulfonic acid group decomposition due to produce sulfonic acid anhydride.