Structure, Properties, and Degradation of Ultrathin Ionomer Films in Fuel Cell Catalytic Layers

Kurzfassung

The performance of polymer electrolyte fuel cells and electrolysers depends on the nanostructure of its components. Mainly the nanostructured electrodes determine cell performance and degradation. In working fuel cell electrodes, the ionomer films that encapsulate the Pt/C agglomerates fall within the range of ultrathin films. In recent years, an oxygen transport resistance has been identified as a major hurdle in the development of low-loaded MEAs and the ionomer layer has been postulated to be the reason for additional resistance. The quantitative investigation of the nanostructure of fuel cell electrodes, especially the analysis of the correct dimensions of the ionomer component has proven to be exceptionally difficult. Using electron beam-based techniques that need vacuum the ionomer is drying and significantly shrinking, in addition to a low contrast between ionomer and carbon components. Also cryogenic TEM analysis may alter the sample by use of alcoholic solvents that have an input on structure, despite of beam damage. Atomic force microscopy (AFM) has the advantage to work in humid environment and at elevated temperatures, close to operational conditions. Using material-sensitive tapping mode, the high contrast between the ionomer- and the Pt/C phase in adhesion force mapping allows studying the distribution and thickness of the ionomer films that cover the Pt/C agglomerates. With electronic current mapping, the free Pt-surface can be evaluated (Figure 1a,b). In this contribution, the analysis of cross-sections of Nafion- and Aquivion–based fuel cell electrodes by material-sensitive AFM will be presented. Analysis of pristine electrodes delivers the distribution of the ionomer film thickness, small ionomer clusters, and larger agglomerates. A thickness distribution of the ionomer films ranging from roughly 4 to 20 nm was retrieved (Figure 1c). After operation, significant thinning of the ionomer films depending on location within the membrane-electrode-assembly and preparation was found. Differences of the swelling behavior of the ionomer films prior and after operation were used as a measure for ionomer degradation caused ma radical attack. A dependence of macroscopic cell degradation rate on initial electrode ionomer film thickness was observed. For further determination of the properties of such ultrathin films model layers of Nafion®, and Aquivion® PFSA were examined. Conductive AFM allowed investigations of through- and in-plane conductivity in dependence of the film thickness. Significant differences in conductivity for films below ~10 nm thickness were detected.