Temperature-Dependence of CO Desorption Kinetics from PtRu/C PEM Fuel Cell Anodes

摘要

Platinum-ruthenium catalysts have attracted considerable interest in recent years as highly active and more CO tolerant anode catalysts in Proton Exchange Membrane Fuel Cells (PEMFC). Three possible effects have been identified to explain increased CO tolerance of platinum-ruthenium catalyst compared to pure platinum: the bifunctional mechanism, the ligand effect mechanism and the "detoxification" mechanism. The issue of CO tolerance should be considered from the point of view of both the electrochemical CO oxidation and the equilibrium attained through the adsorption/desorption process, since the kinetically predominant of these two processes will govern the surface coverage of the CO species at steady-state. In recent studies, CO exchange rates at room temperature and under dry conditions were measured for a CO concentration of 100ppm in argon or hydrogen on both Pt/C and PtRu/C. These CO exchange rates are considered as high compared to the equivalent CO oxidation rate measured under the operating conditions of the fuel cell, suggesting that the adsorption/desorption process may have a significant influence with regards to the poisoning effect of CO on the PEM fuel cell anode. The aim of the presented work is to investigate for a real industrial catalyst the effect on the CO exchange rate of increasing temperature over a range that is of relevance to the operating PEM fuel cell. A temperature range of 25-150°C has been used for the measurements, due to the drive by the research community to develop new membranes for the PEM fuel cell capable of operating at higher temperatures than the current industry standard, the Nafion membrane, which is limited by its need for humidification by water to less than 100°C. In this study, the kinetics of CO desorption on PtRu/C catalyst have been investigated as a function of temperature and flow rate. Desorption rate constants have been determined for the temperature range 25-150°C. These rates appear to be significantly higher than previously published CO oxidation rates obtained at similar concentration and temperatures. This would imply that in the fuel cell environment it is the adsorption/desorption equilibrium that has the greater influence on the steady state CO coverage. It must however be stressed that the experiments performed in this study do not contain humidification or a potential field and therefore provide a simplified adsorption environment.