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.

Analysis of Aged Polymer Electrolyte Fuel Cell (PEFC) Components by Non Traditional Methods

“…For recording quantitative data the magnitude of the different forces and the dissipation energy is calibrated before measurement. This mode has been successfully applied to the investigation of microporous layers (11). For the analysis of polymer samples like Nafion the Peak Force Quantitative Nanomechanical Property Mapping (PeakForce QNM™, Bruker Corp.)) is better suited since it uses standard AFM tips with adjusted spring forces, including conductive tips.…”

“…In this respect, the degradation of the different fuel cell components can also be investigated. AFM has already been successfully applied to the investigation of microporous layers before and after fuel cell operation (11). A microporous layer is a two-component material which mainly consists of carbon and PTFE.…”

“…With AFM the surface topography as well as different properties such as i.e. electronic and ionic conductivity, adhesion force, elasticity, and dissipation energy can be measured and statistically analyzed (9)(10)(11). In this respect, the degradation of the different fuel cell components can also be investigated.…”

Atomic Force Microscopy Investigation of Polymer Fuel Cell Gas Diffusion Layers before and after Operation

“…With AFM the surface topography as well as different properties such as i.e. conductivity, adhesion force, elasticity, dissipation energy can be measured and statistically analyzed (8)(9)(10).…”

Analysis of Aged Polymer Electrolyte Fuel Cell (PEFC) Components by Nontraditional Methods

“…Atomic force microscopy (AFM) is a versatile tool for the analysis of surface properties and has been shown to be quite valuable for the investigation of components and materials used in PEM fuel cells [1][2][3][4][5][6]. Especially, the novel technique based on fourier synthesis of the force separation curve at every image point deliver material sensitive information (HarmoniX mode by Bruker Corp.).…”