Physical and Electrochemical Characterization of Catalysts for Oxygen Reduction in Fuel Cells

Kurzfassung

In low temperature fuel cells the oxygen reduction reaction (ORR) at the cathode is responsible for a large part of the losses of the conversion from chemical to electrical energy. Especially for the reaction kinetics the catalyst is the crucial factor, while the structure of the electrodes is important for catalyst utilization, efficient mass transport and durability of the cell. In particular, the cathode catalysts currently used are associated with major cell efficiency losses, because of the kinetic limitation of the oxygen reduction reaction. To solve this problem a better understanding of the key steps in the oxygen reduction process on commonly used cathode catalysts as well as of alternative catalysts is required. For this purpose physical and electrochemical methods are used to characterize the catalysts. Besides the V-i-characteristics and the electrochemical impedance spectroscopy (EIS) as electrochemical methods, the physical methods nitrogen adsorption, porosimetry by mercury intrusion and temperature programmed reduction (TPR) are used to characterize the catalysts. Additionally, surface science analysis methods are very helpful tools for the physical characterization. Surface science analytical methods such as x-ray photoelectron spectroscopy (XPS) provide important insights into the structural and chemical properties of the electrodes. In order to perform a complete surface science analysis of the catalysts it is not sufficient to investigate freshly prepared catalysts, but also necessary to study the catalysts after operation in fuel cells or after electrochemical treatment. Comparing the catalysts before and after the electrochemical stressing the alterations of the catalysts can be determined. The electrochemical characterization by V-i-characteristics yields basic information about the performance of the electrodes and about the kinetics of the oxygen reduction reaction. Unfortunately, this information is not very detailed and insufficient for a distinct assessment of the catalysts. In order to achieve more detailed understanding it has to be combined with the electrochemical impedance spectroscopy and the ex-situ methods. The results of combined investigations of different cathode catalysts, namely carbon-supported Pt catalysts and Pt-alloy catalysts as well as RuSe catalysts, will be presented. The catalysts were characterized electrochemically by polarization curves and electrochemical impedance spectra (EIS). The physical characterization comprises the investigation by means of x-ray photoelectron spectroscopy (XPS), nitrogen adsorption, porosimetry by mercury intrusion and temperature programmed reduction (TPR).