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

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 (-¼5 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.