Programming ORR Activity of Ni/NiOx@PdElectrocatalysts via Controlling Depth of Surface-DecoratedAtomic Pt Clusters

摘要

Carbon nanotube supported ternary metallic nanocatalysts (NCs) comprising Nicore–Pdshell structure and Pt atomic scale clusters in shell (namely, Ni@Pd/Pt) are synthesized by using wet chemical reduction method with reaction time control. Effects of Pt⁴⁺ adsorption time and Pt/Pd composition ratios on atomic structure with respect to electrochemical performances of experimental NCs are systematically investigated. By cross-referencing results of high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption, density functional theoretical calculations, and electrochemical analysis, we demonstrate that oxygen reduction reaction (ORR) activity is dominated by depth and distribution of Pt clusters in a Ni@Pd/Pt NC. For the optimum case (Pt⁴⁺ adsorption time = 2 h), specific activity of Ni@Pd/Pt is 0.732 mA cm^–2 in ORR. Such a value is 2.8-fold higher as compared to that of commercial J.M.-Pt/C at 0.85 V (vs reversible hydrogen electrode). Such improvement is attributed to the protection of defect sites from oxide reaction in the presence of Pt clustersin NC surface. When adsorption time is 10 s, Pt clusters tends toadsorb in the Ni@Pd surface. A substantially increased galvanic replacementbetween Pt⁴⁺ ion and Pd/Ni metal is found to result inthe formation of Ni@Pd shell with Pt cluster in the interface whenadsorption time is 24 h. Both structures increase the surface defectdensity and delocalize charge density around Pt clusters, therebysuppressing the ORR activity of Ni@Pd/Pt NCs.