PNNL

Abstract

The ability to harness the metal-metal and metal-oxygen coordination structures of nanoalloy catalysts is critical for catalyzing the oxygen reduction reaction because such a detailed atomic-scale structure dictates the surface binding site and strength for molecular oxygen and oxygenated intermediate species in the electrocatalytic process. This report describes the results of an investigation of the metal-metal and metal-oxygen coordination structures of ternary nanoalloys and their manipulation to enhance the electrocatalytic activity for oxygen reduction reaction. The basic hypothesis is that such atomic-scale structure can be manipulated by oxidative-reductive thermal treatment to influence the binding site and strength of molecular oxygen and oxygenated species on the nanoalloy surface. The results have revealed remarkable increases in both mass activity and specific activity for the catalysts processed by the oxidative-reductive treatment over those treated under non-reactive or low-degree oxidative atmospheres before the reductive treatment. In comparison with non-reactive-reductive treatment, an increased degree of heteroatomic alloying among the three metal components in the ternary catalysts and a decreased percentage of oxides (NiO and CoO) have been revealed by X-ray absorption fine structure spectroscopy for the catalysts treated by the oxidative-reductive treatment. An enrichment of surface Pt has also been detected by x-ray photoelectron spectroscopy for such catalysts. A combination of the increase in the heteroatomic alloying, the decrease in metal oxides, and the enrichment of surface Pt by the oxidative-reductive thermal treatment has therefore been concluded to be responsible for the enhanced electrocatalytic activity. The demonstration of this new approach to manipulating the metal-metal and metal-oxygen coordination structures forms the basis for an effective strategy in engineering ternary nanoalloy catalysts, and has provided new insights into the role of such structures in the enhancement of the electrocatalytic activity.