Highly enhanced bifunctional electrocatalytic activity of mixed copper–copper oxides on nickel foam via composition control

Abstract

Developing an efficient transition metal-based non-noble electrocatalyst is important for the industrial water splitting process and producing green sustainable hydrogen (H₂) fuel. Particularly, the direct fabrication of electrocatalysts on an electrode surface can exhibit strong electronic coupling that results in increased catalytic activity. Herein, copper (Cu) and cuprous oxide (Cu₂O) nanoparticles were directly deposited on a nickel foam (NF) substrate and the controlled calcination of the Cu produced mixed copper–copper oxides (CuO, Cu₂O–CuO and Cu–Cu₂O–CuO) that exhibited excellent composition-controlled bifunctional electrocatalytic (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) activity in an alkaline medium with good stability. Powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopic (XPS) studies confirmed the formation of pure and mixed copper–copper oxide nanomaterials on NF. Mixed copper oxide (Cu₂O–CuO@NF) and pure CuO@NF exhibited excellent electrocatalytic OER and HER activity, respectively, in alkaline media. To the best of our knowledge, Cu₂O–CuO@NF required an overpotential of 262 mV to achieve the lowest geometric current density (10 mA cm⁻²) among the pure copper-based electrocatalysts reported. Cu₂O–CuO@NF also produced a high current density (130.9 mA cm⁻²) at a relatively low applied potential (1.56 V) compared to other catalysts. The low Tafel slope (45.8 mV dec⁻¹) and charge-transfer resistance indicated that good electronic coupling and fast charge transport led to the higher activity. In the HER, pure CuO@NF required a much lower overpotential of 165 mV to produce geometric current density. It is worth noting that CuO@NF showed strong HER activity in the alkaline medium. The Tafel slope of 121.81 mV dec⁻¹ for CuO@NF suggests that the desorption process could be the rate-determining step in the HER process. Cu₂O–CuO@NF and CuO@NF exhibited good stability in the alkaline medium. The highly enhanced current density and good stability of Cu₂O–CuO@NF in 6.0 M KOH at 60 °C highlights its potential for practical applications. Overall, an earth-abundant copper-based electrocatalyst with excellent OER and HER activity in alkaline media has been developed by directly fabricating on a conducting substrate and controlling the copper oxides composition.