Preparation strategies towards selective Ru-based oxygen reduction catalysts for direct methanol fuel cells

Carbon supported catalysts for the selective oxygen reduction in fuel cells have been prepared by three different methods using ruthenium oxalate, Ru-3(CO)(12) and ruthenium colloids as precursor. The morphology of the different Ru-nanoparticle preparations were analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD), which shows that the different catalysts consist of crystalline ruthenium particles with median sizes in the range from 7 mnto 2.5 nm with a very good dispersion over the applied carbon substrate. The corresponding catalytic activity for oxygen reduction was analyzed by cyclic voltammetry and rotating disc electrode (RDE) measurements for each particular catalyst. The results suggest that the concentration of surface sites on the substrate which bind catalytic particles is limiting the overall catalytic activity under our experimental conditions of 20% ruthenium/80% carbon. However, the presence of selenium was found to have a significant effect on current density and may act as electron transfer bridge between substrate and catalytic center. Furthermore, ruthenium nanoparticles are chemically surface modified by selenium and become stable against oxidation. However, under such conditions the overvoltage for oxygen reduction achieved is approximately 0.12 V higher than for a comparable platinum catalyst.