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 CoSe_1.8/C∼CoSe_2.7/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 (E_ORR) 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 E_ORR = 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