Versatile Synthesis of Ultrafine Ternary Spinel Oxides/Carbon Nanohybrids toward the Oxygen Reduction Reaction

Abstract

Designing cheap and highly efficient electrocatalysts for the oxygen reduction reaction (ORR) is vital to advance fuel cells or metal–air battery technologies. Although great progress have been obtained, facile and versatile synthesis of ternary spinel oxides (AB2O4) and carbon nanohybrids (NHs) remains a challenging work and their applications in ORR have not been systematically investigated. In this work, a series of ultrafine AB2O4 nanocrystals/Vulcan C NHs, including MnCo2O4/C NHs, CoFe2O4/C NHs, MnFe2O4/C NHs, NiCo2O4/C NHs, and NiFe2O4/C NHs, are synthesized by directly refluxing bimetallic precursors and carboxylic-functionalized Vulcan C in an environmentally friendly solvent, i.e., 1,3-dimethyl-3,4,5,6-tetrahydro-2­(1H)-pyrimidinone. This strategy does not need a high temperature, long reaction time, and post-annealing treatment, which is a simple, green, and easy method for scalable synthesis. In those NHs, all of the AB2O4 nanocrystals are ultrafine (∼5 nm) and dispersed uniformly on the C support. Among them, MnCo2O4/C NHs exhibit the highest catalytic activity, with an onset reduction potential of 0.96 V [versus reversible hydrogen electrode (RHE)] and a half-wave potential of 0.754 V (versus RHE). Related electrocatalytic dynamic tests reveal that the ORR mechanism follows the direct “4e–” process, and only 11.1% HO2– yield is generated at 0.5 V (versus RHE). As revealed from the microstructural and electrochemical measurement, the superior catalytic performance of MnCo2O4/C NHs can be attributed to their high specific surface area and low interfacial electron transfer resistance in relation to other AB2O4/C NHs.