München 2004 – scientific programme

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AKE: Energie

AKE 5: Alternative Treibstoffe

AKE 5.3: Invited Talk

Thursday, March 25, 2004, 12:30–13:15, HS 109

Energy Conversion in Fuel Cells and Processes in Nanometric Dimensions — •Ulrich Stimming — Department of Physics, E19: Interfaces and Energy Conversion, Technische Universität München — Bavarian Center for Applied Energy Research (ZAE), D-85748 Garching

Fuel cells are currently considered an important technology being able to save primary energy and reduce emissions associated with energy conversion. In addition, they may be ideally suited for energy conversion in a future hydrogen based energy economy. Currently, fuel cells are developed for stationary applications in large power plants but also for residential power, for vehicles in road traffic and also for portable applications to potentially replace rechargeable batteries.

There are various types of fuel cells ranging from low temperature cells such as the Proton Exchange Membrane Cell (PEMFC) operating between 50 and 100 ^∘C and high temperature cells such as the Solid Oxide Fuel Cell (SOFC) with temperatures up to 1000 ^∘C. The functional layers of electrodes and electrolyte have a typical thickness in the range between µm and some hundred µm but are often composite materials in themselves. A typical example is the PEMFC where the electrode is made up of a carbon diffusion layer with carbon supported noble metal catalysts, the latter being of nm sized particles.

It is thus important to understand electrochemical reactivity on small particles of nanometric dimensions. This was investigated using colloidal particles of Pt on Au(111) with respect to CO-oxidation which is important if a carbon containing fuel is used in fuel cell operation which has to be reformed which in turn produces CO in the feed-gas. Electrochemical measurements and spectroscopic results suggest a pronounced particle size dependence emphasizing the solid state physics in the energy conversion process in nanometric dimensions. Experimental results are, however, obscured by agglomeration of single particles leading to an actually broad size distribution of reacting particles.

In order to better understand such an influence of particle size a novel all-in-situ technique using a scanning tunneling microscope was developed that allows the preparation, structural characterization and investigation of the reactivity of a single particle. Pd-particles having a size in the range of a few to some hundred nm were investigated regarding hydrogen evolution. A pronounced effect of size was found with smaller particles being more reactive by orders of magnitude as compared to bulk flat surfaces.

These results are discussed in terms of their relevance for fuel cell operation and possible conclusions regarding the design of fuel cells.