ZIF-67-derived Co3O4@carbon protected by oxygen-buffering CeO2 as an efficient catalyst for boosting oxygen reduction/evolution reactions

Abstract

The synergies between transition metal oxides (bimetallic oxides) play important roles in determining the bifunctional catalytic activity for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline media. This study uses a facile hydrothermal method for uniformly coating CeO₂ shells on the surfaces of ZIF-67-derived porous Co₃O₄@Z67-NT (T-temperature, 500–900 °C) cores. Co₃O₄@Z67-N700@CeO₂ exhibits excellent bifunctional (ORR/OER) catalytic activity with a ΔE [E_j=10(OER) − E_1/2(ORR)] of 0.70 V. For ORR, Co₃O₄@Z67-N700@CeO₂ exhibits a higher half-wave potential of 0.88 V (vs. RHE) than that of commercial Pt/C (0.87 V), which is attributed to the synergies between CeO₂ (Ce³⁺) and Co₃O₄ (Co²⁺). The oxygen vacancies on CeO₂ can enhance O₂ adsorption on the interfaces to promote the activation of adsorbed O₂ to O₂⁻, and can alleviate O₂ deficiency during ORR, thereby improving ORR activity. For OER, Co₃O₄@Z67-N700@CeO₂ has a lower overpotential of 350 mV at 10 mA cm⁻² and a higher Faraday efficiency of 92.2% than those of RuO₂. An effective valence transition between Ce³⁺ and Ce⁴⁺ endows CeO₂ with high electrical conductivity and oxygen storage capacity, which greatly promote charge transfer and the generation/smooth transport of highly active species (O₂²⁻/O⁻). Moreover, the interactions between CeO₂ (Ce³⁺/Ce⁴⁺ and oxygen vacancies) and Co₃O₄ (Co³⁺/CoOOH) provide more electrochemically active sites for OER. This study provides a new strategy for constructing the stable core@shell structure based on MOF derivatives to improve the ORR/OER performance.