New Materials for Oxygen Reduction Electrodes

This thesis is concerned with the discovery, characterisation and testing of new catalysts for the oxygen reduction reaction (ORR). A theroretical screening study was performed, in close collaboration with the theory group at the Center for Atomicscale Materials Design (CAMD), searching for catalysts which were both active and stable. The screening study identified Pt3Y and Pt3Sc as interesting candidates for the ORR.
The activity of the ORR on sputter cleaned bulk polycrystalline electrodes of Pt3Y and Pt3Sc was probed in an electrochemical cell using a rotating disk electrode setup. Pt3Y and Pt3Sc showed an enhancement in ORR activity by a factor of 1.5{1.8 and 6-10, respectively, in the range 0.9-0.87 V.
Ultra high vacuum (UHV) annealing of bulk polycrystalline samples of Pt3Y, Pt5Y and Pt3La did not result in a Pt overlayer formation. For the bulk polycrystalline Pt3Sc sample, on the other hand, a Pt overlayer was formed upon annealing in UHV. The reactivity of the Pt overlayer was tested by temperature programmed desorption (TPD) of CO, which yielded a lower desorption temperature compared to Pt(111). The ORR activity of the annealed Pt3Sc sample was found to be on the same order as the sputter cleaned Pt3Sc.
The deposition of Y on a Pt(111) crystal was then investigated. It was found that when annealing the crystal above 800 K a Pt overlayer was formed on top of a PtxY structure. Low energy electron diffraction (LEED) was used to probe the ordering of the surface and the LEED patterns indicated a 1:88 x 1:88 R30 degree structure compared to Pt(111). CO TPD measurements showed a distinct peak at 295 K and a general weaker bonding of CO (on the order of 120 K) compared to Pt(111). The ORR activity of the annealed Y/Pt(111) was measured to be comparable to that of sputter cleaned bulk polycrystalline Pt3Y. The angle resolved x-ray photoelectron spectroscopy (ARXPS) depth profile after the electrochemical measurements showed the formation of a thick Pt overlayer.
Finally, the possible changes in the surface morphology of Pt nanoparticles by annealing in high pressures of CO was correlated with measurements done on a sputtered Pt(111) crystal. For the Pt(111), crystal high pressures of CO was seen to enhance the annealing of the defect sites, whereas for nanoparticles a slight roughening was observed.