Simple microwave preparation of high activity Se-rich CoSe2/C for oxygen reduction reaction

Carbon supported cobalt selenide (CoSe2/C) nanoparticles were prepared by a simple microwave method using cobalt acetate and selenium dioxide as precursors with different molar ratios of Se/Co. The effects of Se/Co ratios on surface morphology, crystal structure, chemical composition and electrocatalytic activity toward oxygen reduction reaction (ORR) of CoSe2/C catalyst nanoparticles were systematically investigated. It was found that the experimental compositions of CoSe1.8/C∼CoSe2.7/C with average crystallite sizes of 15.4∼12.9 nm were obtained with the Se/Co ratios of 2.0∼4.0. The major phases of CoSe2/C nanoparticles were identified to be orthorhombic CoSe2 with minor cubic CoSe2. The potentials corresponding to ORR (EORR) reached 0.6∼0.7 V, while the electron transfer numbers (n) were 3.1∼4.0 in oxygen saturated sulfuric acid solutions. The formation of Se-rich CoSe2/C nanoparticles was confirmed with Se/Co ≥ 2.5, and the best ORR activities with EORR = 0.705 V and n = 4.0 could be achieved with Se/Co = 3.0. Slightly excess amount of Se oxide presented in CoSe2/C would prevent CoSe2 nanoparticles from growing and smaller sizes with less agglomerated Se-rich CoSe2/C were obtained, resulting in good ORR activity. However, too much Se oxide would cause severe aggregation of CoSe2 nanoparticles, leading to poor ORR activity.