Metal-Oxygen Hybridization Determined Activity in Spinel-Based Oxygen Evolution Catalysts: A Case Study of ZnFe2-xCrxO4

A good understanding of the correlation between electronic properties and catalytic performance is vital to the rational design of active oxygen evolution reaction (OER) catalysts. Here, a volcano-shaped correlation between the OER activity and the Cr substitution amount was found for spinel oxides ZnFe2-xCrxO4 (x = 0-2), in which Zn2+ resides in tetrahedral (Td) sites while Fe3+ and Cr3+ occupy octahedral (Oh) sites. Such a relationship probably is because Cr substitution tunes the e(g) occupancy of Oh-site transition metals (TMoh) via the Oh-Oh superexchange effect. Density functional calculations further revealed the hybridization degree between the TMoh 3d and the O 2p states, and a volcano-shaped trend was also found in the variation of TMoh 3d-O 2p hybridization with the amount of Cr substitution. The good correlation between the OER activity and the hybridization highlights the important role of metal-oxygen hybridization in determining the OER activity of these spinel oxides.