Battery and Fuel Cell Technology

The impact of greenhouse gases on the environment and the scarcity of natural resources demand for sustainable solutions to maintain our steadily increasing demand for energy supply; one possibility are fuel cells. In particular for the transportation and stationary sector, polymer-electrolyte-membrane fuel cells are used. Their centerpiece is the membrane-electrode assembly which is composed of several functional layers: the polybenzimidazole-based membrane and two electrodes. Generally the lifetime of fuel cells is limited due to harsh operation conditions leading to several degradation mechanisms that affect all functional layers. In our studies, we focused on alternative materials for the fuel cells´ electrodes. We combined new catalyst or support materials with standardly used material systems and investigated the catalyst´s growth behavior during preparation and its aging properties in various operation conditions. In one study, octahedral shaped, high-surface-area platinum networks were grown on a tungsten suboxide support layer via a template-free synthesis route. Using various transmission electron microscopy (TEM) based techniques, we were able to explain their growth mechanism and evaluate their higher stability and lower degradation rate during fuel cell operation. In another study we investigated the structural characteristics and the stability

of Pt/Ru catalyst nanoparticles on a high-surface-area carbon support material. Dynamic fuel cell operation was simulated using cyclic voltammetry experiments and site specific degradation mechanisms of single nanoparticles were evaluated by performing intermitted TEM studies on identical locations. Our results reveal dissolution and agglomeration to be the main degradation

mechanisms. Continuous fuel cell operation gives rise to a further degradation mechanism: dissolution of the Pt and Ru catalyst particles promote the diffusion of the concomitantly formed ions into the membrane where they precipitate with different crystal structure and composition. Finally a band of nanoparticles was observed in the membrane adjacent to the cathode catalyst layer.