In situ immobilization of copper oxide thin-layer on Zeolitic Imidazolate Framework-67-derived cobalt oxide@nitrogen-doped carbon with multi-level architecture and versatile active sites for enhancing oxygen evolution/reduction reactions

Design of multi-hierarchical hybrid catalyst with high activity (bifunctional active-sites) is urgently needed by oxygen evolution/reduction reactions (OER/ORR). This study reports a feasible method for anchoring the copper oxide nanoparticles on the surface of Zeolitic Imidazolate Framework-67 derived cobalt oxide@nitrogen-doped graphitized carbon (Co₃O₄@NGC@CuO). For OER, Co₃O₄@NGC@CuO-0.6 (mass ratio of CuCl₂·2H₂O to Co₃O₄@NGC = 0.6) requires only 210 mV overpotential to achieve a current density of 10 mA cm⁻² and a faraday efficiency of 93.5%. Owing to the synergies between CuO (Cu²⁺) shells and Co₃O₄ (Co²⁺/Co³⁺) cores, Cu²⁺ active sites can moderate the adsorption of oxygen-containing substances and balance the Co²⁺/Co³⁺ ratio. Tetrahedral Co²⁺ (Co²⁺_Td) can be served as the active sites to induce the μ-OOH moieties (Octahedral Co³⁺, CoOOH) to enhance OER activity and stability. For ORR, Co₃O₄@NGC@CuO-0.6 has a half-wave potential of 0.865 V, comparable to that of commercial Pt/C (0.868 V). CuO with Cu²⁺/Cu⁺ can increase the O₂ adsorption sites, while the active Co_Td²⁺ sites can boost the breakage of O–O bonds to produce OH⁻ via a 4e⁻ ORR pathway. The bifunctional activity [ΔE = E₁₀(OER)-E_1/2(ORR)] of Co₃O₄@NGC@CuO-0.6 is only 0.58 V, confirming excellent ORR/OER activities. This study provides a promising strategy for constructing versatile active sites on catalysts.