Copper Cobalt Sulfide Nanosheets Realizing a Promising\nElectrocatalytic Oxygen Evolution Reaction

Abstract

Nanostructured CuCo₂S₄, a mixed metal thiospinel,\nis found to be a benchmark electrocatalyst for oxygen evolution reaction\n(OER) in this study with a low overpotential, a low Tafel slope, a\nhigh durability, and a high turnover frequency (TOF) at lower mass\nloadings. Nanosheets of CuCo₂S₄ are realized\nfrom a hydrothermal synthesis method in which the average thickness\nof the sheets is found to be in the range of 8–15 nm. Aggregated\nnanosheets form a highly open hierarchical structure. When used as\nan electrocatalyst, CuCo₂S₄ nanosheets offer\nan overpotential value of 310 mV at a 10 mA cm^–2 current density, which remains consistent for 10000 measured cycles\nin a 1 M KOH electrolyte. A chronoamperometric study reveals constant\noxygen evolution for 12 h at a 10 mV s^–1 scan rate\nwithout any degradation of the activity. Furthermore, the calculated\nmass activity of the CuCo₂S₄ electrocatalyst\nis found to be 14.29 A/g and to afford a TOF value of 0.1431 s^–1 at 310 mV at a mass loading of 0.7 mg cm^–2. For comparison, nanostructures of Co₃S₄ and\nCu_0.5Co_2.5S₄ have been synthesized\nusing a similar method followed for CuCo₂S₄.\nWhen compared to the OER activities among these three thiospinels\nand standard IrO₂, CuCo₂S₄ nanosheets\noffered the highest OER activities at the same mass loading (0.7 mg/cm²). Extensive X-ray photoelectron spectroscopy and electron\nparamagnetic resonance analyses for a mechanistic study reveal that\nintroduction of Cu into the Co₃S₄ lattice enhances\nthe oxygen evolution and kinetics by offering Cu²⁺ sites\nfor utilitarian adsorption of OH, O, and OOH reactive species and\nalso by offering a highly active high-spin state of octahedral Co³⁺ for OER catalysis.