Electrocatalytic properties of PtCo/C and PtNi/C alloys for the oxygen reduction reaction in alkaline solution

The oxygen reduction reaction (ORR) was studied on carbon-dispersed Pt3Co, PtCo, and Pt3Ni alloys in KOH electrolyte. A detailed discussion of the enhancement of the O-2 reduction kinetics based on the physical and electronic properties of the catalysts is presented. The characterization techniques considered were high-resolution transmission electron microscopy, XRD (X-ray diffraction), in situ XANES (X-ray absorption near edge structure), and EXAFS (extended X-ray absorption fine structure) measurements. Cyclic voltammetry and steady-state polarization measurements for the ORR were conducted using a rotating ring-disk electrode. The XANES results at the Pt L-3 edge at anodic potentials showed lower oxide coverage on the PtCo/C catalysts, while at the Co and Ni K edges the results evidenced compositions that include Co or Ni metals and oxides. The XRD and EXAFS analyses showed a slight reduction of the Pt-Pt interatomic distance in the Pt alloys compared to Pt/C. The polarization curves indicated that the 4-electron mechanism is mainly followed for the ORR on all materials. The highest electrocatalytic activity was obtained for the PtCo/C alloy (1:1 atomic ratio), and this was attributed to less Pt-OH formation or faster Pt-O- electroreduction due to a lower Pt d-band center.