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

Abstract 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 CoTd2+ sites can boost the breakage of O–O bonds to produce OH− via a 4e− ORR pathway. The bifunctional activity [ΔE = E10(OER)-E1/2(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.