Tailoring the electronic structure of ZnCo2O4 by incorporating anions with low electronegativity to improve the water oxidation activity

Electron configurations and conductivity are significant descriptors/characteristics for the oxygen evolution reaction (OER), which can be modulated with heteroatom doping. Given that metal substitution usually reduces the number of active sites of spinel electrocatalysts, the effect of anion doping on the electronic structure has been investigated by using ZnCo2O4 (ZCO) as a demonstration. Compared with Co3+-dominated ZCO, the substitution of oxygen with less electronegative sulfur raises the portion of Co2+ in the low-spin states (t2g6eg1), which is more OER-active than Co3+ (t2g6eg0). Co2+ in the sulfur-doped ZCO (ZCO-S) is associated with the redistribution of electron density from S toward Co due to the high covalent interaction of Co—S. The Co—S interaction also induces a fast charge transfer. ZCO-S outperforms pristine ZCO by 11 times in terms of specific OER activity at 1.65 V versus reversible hydrogen electrode. In contrast, doping fluorine with higher electronegativity and valence deteriorates OER activity. Our work establishes the correlation between the electronegativity of anion dopants and OER intrinsic activity in spinel oxides and provides a simple and effective method to modulate the electronic structure of spinel oxides by doping anions with different electronegativities, which can be a new avenue for the rational design of high-performance spinel electrocatalysts.