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 has an important influence on the losses of the conversion from chemical to electrical energy. Especially for the reaction kinetic 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 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 as electrochemical methods, the surface science analysis methods for the physical characterization are very helpful tools. Surface science analytical methods such as x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) provide important insight 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 electrochemical impedance spectroscopy and the ex-situ methods have to be combined. The results of combined investigations of different cathode catalysts will be presented. The catalysts were characterized by polarization curves, electrochemical impedance spectra and physically by surface analysis methods.