Imaging and Reactivity Measurements by Surface Probe Microscopy Techniques with relevance to Fuel Cells

Kurzfassung

The structure of gas diffusion electrodes in fuel cells is complex: The porous electrode often consists of platinum nanoparticles, carbon support, the ionomer (e.g. Nafion) and hydrophobic additive (e.g. Teflon). The reaction is assumed to take place at the three-phase boundary between gas, catalyst surface and ionomer. However, the details of the structure/activity relationship of the electrode remain unknown. Especially, it is very interesting to investigate the structure and distribution of the catalyst particles as well as the extent of ionomer coverage of the particles. The local electrode structure is probably very important for the ORR activity. In order to provide insight into these relationships, a novel method to analyse the local reactivity of a catalyst/electrode interface is being developed with a high lateral resolution. The method is based on a scanning tunneling microscope working in an electrochemical cell. Using STM the structure of the electrode surface can be imaged on the nanoscale. The novel research strategy is to combine local structural and reactivity measurements by using the STM tip as an oxygen generator and subsequently as a sensor electrode for the ORR. First results of the surface structure of carbon supported commercial catalysts will be presented as well as first results on local reactivity at various catalysts concerning the ORR. In order to investigate the fundamental interaction between platinum and the ionomer a method is herein described that enables the sharp, conically-shaped tip of a short Pt wire attached to a conventional scanning tunneling microscope (STM) head to be gradually inserted into a Nafion solid polymer electrolyte membrane. A comparatively much larger Pt electrode placed on the underside of the membrane allows for the area of the Pt tip in contact with the Nafion to be determined by coulometric analysis of the cyclic voltammetric features. Preliminary results have shown that this novel tactic makes it possible to examine clean Pt surfaces involving as few as ca. 94,000 active metal sites to contact Nafion, opening new prospects for probing the Pt|Ionomer|Gas three-phase interface under conditions which replicate those found in fuel cells.