Design of Low Pt Concentration Electrocatalyst Surfaces with High Oxygen Reduction Reaction Activity Promoted by Formation of a Heterogeneous Interface between Pt and CeO_x Nanowire

Pt-CeO_x nanowire (NW)/C electrocatalysts for the improvement of oxygen reduction reaction (ORR) activity on Pt were prepared by a combined process involving precipitation and coimpregnation. A low, 5 wt % Pt-loaded CeO_x NW/C electrocatalyst, pretreated by an optimized electrochemical conditioning process, exhibited high ORR activity over a commercially available 20 wt % Pt/C electrocatalyst although the ORR activity observed for a 5 wt % Pt-loaded CeO_x nanoparticle (NP)/C was similar to that of 20 wt % Pt/C. To investigate the role of a CeO_x NW promotor on the enhancement of ORR activity on Pt, the Pt-CeO_x NW interface was characterized by using hard X-ray photoelectron spectroscopy (HXPS), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). Microanalytical data obtained by these methods were discussed in relation to atomistic simulation performed on the interface structures. The combined techniques of HXPS, TEM-EELS, and atomistic simulation indicate that the Pt-CeO_x NW interface in the electrocatalyst contains two different defect clusters: Frenkel defect clusters (i.e., 2Pt_i^•• - 4O_i″ - 4V_o^•• - V_Ce″″) formed in the surface around the Pt-CeO_x NW interface and Schottky defect clusters (i.e., (Pt_Ce″ - 2V_O^•• - 2Ce_Ce′) and (Pt_Ce″ - V_O^••)) which appear in the bulk of the Pt-CeO_x NW interface similarly to Pt-CeO_x NP/C. It is concluded that the formation of both Frenkel defect clusters and Schottky defect clusters at the Pt-CeO_x NW heterointerface contributes to the promotion of ORR activity and permits the use of lower Pt-loadings in these electrocatalysts.