Eine Untersuchung von Größenselektierten Subnanometerkatalysatoren für Brennstoffzellen
Zusammenfassung der Projektergebnisse
Fuel cells convert chemical energy directly into electrical energy, without the formation of heat and mechanical energy as in conventional energy conversion based on the Carnot cycle (Carnot limitation ≈ 40 % efficiency). In a fuel cell the direct electrochemical conversions of H2 at the anode and O2 at the cathode occur spatially separated on two electrodes connected by an electrolyte, which allows for higher energy efficiency conversions. The proton exchange membrane fuel cell (PEMFC) owing to their relatively low operating temperature, rapid startup capability at ambient temperatures and highly dynamic response is the main focus for automotive applications. The main reason hampering commercial application is the high platinum content in the cells. The oxygen reduction reaction taking place at the cathode is very sluggish and at low temperatures needs to be catalyzed by platinum or platinum alloy particles. It is now clear that a size-effect on the catalytic activity of these particles exist, however, so far it is possible to measure the activity for a monodisperse platinum particle in order to determine the best catalyst for the oxygen reduction reaction. The project “A survey of size-selected subnanometer catalysts for fuel cells” focuses on the deposition of the smallest possible platinum structures on glassy carbon and the determination of their oxygen reduction onset potential. Via the Utah Ion Deposition Instrument (University of Utah, Salt Lake City, Utah) small platinum clusters are generated in the gas phase with a laser ablation source and mass-selected by a quadrupole mass filter and soft-landed on a glassy carbon electrode. It was shown that it is indeed possible to deposit platinum clusters on glassy carbon and investigate them via X-ray photoemission spectroscopy. An intended investigation using ISS was not possible due to the considerable roughness of the electrodes. The approach to use a Macor electrochemical cell with a platinum wire quasi-reference electrode, however, proved to be detrimental to the project. Both because of the material which is very hard to machine, so that a tight seal between the glassy carbon electrodes and the electrochemical cell could not be achieved and the quasi-reference electrode requiring potential sweeps into region where platinum particles are oxidized. Both failures led to a second approach where a PEEK electrochemical cell with a real silver/sliver chloride reference electrode is investigated now as part of the “Rückkehrstipendium”.
