Facile Synthesis of Hierarchical CuS and CuCo₂S₄ Structures from an Ionic Liquid Precursor for Electrocatalysis Applications

Abstract

Covellite-phase CuS and carrollite-phase CuCo₂S₄ nano- and microstructures were synthesized from tetrachloridometallate-based ionic liquid precursors using a novel, facile, and highly controllable hot-injection synthesis strategy. The synthesis parameters including reaction time and temperature were first optimized to produce CuS with a well-controlled and unique morphology, providing the best electrocatalytic activity toward the oxygen evolution reaction (OER). In an extension to this approach, the electrocatalytic activity was further improved by incorporating Co into the CuS synthesis method to yield CuCo₂S₄ microflowers. Both routes provide high microflower yields of >80 wt %. The CuCo₂S₄ microflowers exhibit a superior performance for the OER in alkaline medium compared to CuS. This is demonstrated by a lower onset potential (∼1.45 V vs RHE @10 mA/cm²), better durability, and higher turnover frequencies compared to bare CuS flowers or commercial Pt/C and IrO₂ electrodes. Likely, this effect is associated with the presence of Co³⁺ sites on which a better adsorption of reactive species formed during the OER (e.g., OH, O, OOH, etc.) can be achieved, thus reducing the OER charge-transfer resistance, as indicated by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy measurements.