Nanoscale Investigation of Nafion Membranes after Artificial Degradation

Abstract

In this contribution we report on the nanostructure and conductivity of freshly prepared as well as artificially degraded Nafion membranes investigated by contact atomic force microscopy (AFM), conductive AFM, and pulsed force-mode (PFM)-AFM. The different techniques can provide complementary information on structure and conductivity. Conductive AFM is used to investigate the conductivity of Nafion membrane surfaces. The images show a quite inhomogeneous distribution of current at the surface. The percentage of conductive surface increases with humi¬dity, but regions without any current are still present up to 80% relative humidity [1,2]. Comparison with PFM-AFM images, where differences in adhesion forces are measu¬red, indicates that hydrophobic regions, which are attribu¬ted to PTFE-like polymer back bone without conductivity, are present at the surface with comparable dimensions. Conductive AFM gives information about the internal structure of ionic clusters during current flow. High resolution current images of the membrane were used to directly compare the measured nanostructure of the single conductive channels with model predictions from the literature (Fig. 3). The influence of H2O2 treatment as a method for artificial degradation is investigated. The analysis of adhesion forces demonstrates a significant change of the surface properties with different membrane treatment. Topography and adhesion measurements clearly show materials changes with high resolution and correlate with changes in conductivity distributions. In particular, small protrusions in the topography correlating with strongly diminished conductivity in the current images are observed. These areas are interpreted as crystalline polymer backbone which higher stability against radical decomposition.