Uncovering the prominent role of metal ions in octahedral versus tetrahedral sites of cobalt–zinc oxide catalysts for efficient oxidation of water

Abstract

The fabrication and design of earth-abundant and high-performance catalysts for the oxygen evolution reaction (OER) are very crucial for the development and commercialization of sustainable energy conversion technologies. Although spinel catalysts have been widely explored for the electrochemical oxygen evolution reaction (OER), the role of two geometrical sites that influence their activities has not been well established so far. Here, we present more effective cobalt–zinc oxide catalysts for the OER than ‘classical’ Co₃O₄. Interestingly, the significantly higher catalytic activity of ZnCo₂O₄ than that of Co₃O₄ is somewhat surprising since both crystallize in the spinel-type structure. The reasons for the latter remarkable difference of ZnCo₂O₄ and Co₃O₄ could be deduced from structure–activity relationships of the bulk and near-surface of the catalysts using comprehensive electrochemical, microscopic and spectroscopic techniques with a special emphasis on the different roles of the coordination environment of metal ions (octahedral vs. tetrahedral sites) in the spinel lattice. The vital factors influencing the catalytic activity of ZnCo₂O₄ over Co₃O₄ could be directly attributed to the higher amount of accessible octahedral Co³⁺ sites induced by the preferential loss of zinc ions from the surface of the ZnCo₂O₄ catalyst. The enhanced catalytic activity is accompanied by a larger density of metal vacancies, defective sites and hydroxylation. The results obtained here clearly demonstrate how a surface structural modification and generation of defects of catalysts can enhance their OER performance.