Cobalt\nTelluride: A Highly Efficient Trifunctional\nElectrocatalyst for Water Splitting and Oxygen Reduction

Abstract

Cobalt\ntelluride has been identified as an efficient multifunctional\nelectrocatalyst for oxygen and hydrogen evolution reactions and oxygen\nreduction reaction in alkaline medium. Both hydrothermally synthesized\nand electrodeposited, CoTe and CoTe₂ show efficient electrocatalytic\nactivities. CoTe shows better efficiency for OER with a low Tafel\nslope (43.8 mV dec^–1) and lower overpotential (200\nmV) at 10 mA cm^–2 compared to CoTe₂.\nDFT studies have also been performed which revealed that CoTe showed\nhigher adsorption energy for intermediate −OH adsorption on\nthe catalyst surface, which corresponds to the catalyst activation\nstep. Comparison of the −OH adsorption energies (<i>E</i>_ads) on different catalyst surfaces with the observed\noverpotential also revealed that this <i>E</i>_ads can be used as an appropriate descriptor for benchmarking catalytic\nefficiencies. Both CoTe and CoTe₂ exhibited improved OER\ncatalytic efficiency compared to Co₃O₄, confirming\nthe primary hypothesis that decreasing anion electronegativity enhances\ncatalytic efficiency by virtue of increasing lattice covalency around\nthe catalytically active site. The difference in OER catalytic activity\nbetween CoTe and CoTe₂ could be explained from fundamental\nmaterials chemistry concepts by comparing their lattice structures\nwhich showed different packing density of catalytically active Co\nsites as well as varying unsaturation of Co-terminated surfaces. Band\nstructure calculations also corroborated such differences and could\npotentially explain the difference in activity due to observed differences\nin electron density distribution around the catalytically active Co\nsite. The cobalt telluride compositions also showed moderate HER and\nORR activity in alkaline medium, making them trifunctional catalysts\nwhich can be used in practical devices. Both CoTe and CoTe₂ showed extensive functional and compositional stability for OER,\nHER, and ORR, under continuous operation in alkaline medium for over\n24 h with less than 5% degradation of current density. The excellent\ncompositional stability of each catalyst was revealed by detailed\nelectrochemical measurements and surface and bulk analytical characterizations,\nwhich confirmed that there was no catalyst leaching even with long-term\noperation and no other impurity enrichment in the electrolyte.