Selenium Becomes Metallic in Ru−Se Fuel Cell Catalysts: An EC-NMR and XPS Investigation

We report the results of an investigation of the interaction between Se and Ru in Ru−Se electrocatalysts showing high activity and methanol tolerance in the oxygen reduction reaction (ORR). The results of both 77Se EC-NMR as well as the XPS measurements indicate that Se, a semiconductor in elemental form, becomes metallic when interacting with Ru in catalysts made both by direct Se deposition onto Ru and from Ru3(CO)12. The 77Se spin−lattice relaxation rates are proportional to T, the Korringa behavior characteristic of metals, an observation supported by the XPS binding energy shift results. The binding energy shifts are proposed to be due to Ru → Se charge transfer (resulting in the semiconductor → metal transition) which makes Ru less susceptible to oxidation, in the ORR. The results are of general interest since the combined use of EC-NMR and XPS should be applicable to the study of a broad range of other catalysts.