Optimizing the Oxygen-Catalytic Performance of Zn-Mn-Co Spinel by Regulating the Bond Competition at Octahedral Sites

By using the more electro-negative Mn3+ ion to partially replace Co3+ at the octahedral site of spinel ZnCo2O4, i.e., forming ternary Zn-Mn-Co spinel oxide, the electrocatalytic oxygen reduction/evolution activity is found to be significantly increased. Considering the physical characterization and theoretical calculations, it demonstrated that the bond competition played a key role in regulating the cobalt valence state and the electrocatalytic activity. The partial replacement of octahedral-site-occupied Co3+ by Mn3+ can effectively modulate the adjacent Co-O bond and induce the Jahn-Teller effect, thus changing the originally stable crystal structure and optimizing the binding strength between the active center and reaction intermediates. Certainly, the Mn-substituted ZnMn1.4Co0.6O4/NCNTs exhibit higher electrocatalytic oxygen reduction reaction (ORR) activity than that of ZnCo2O4/NCNTs and ZnMn2O4/NCNTs, supporting that the Co-O bond covalency determines the ORR activity of spinel ZnCo2O4. This study offers the competition between adjacent Co-O and Mn-O bonds via the B-Oh-O-B-Oh edge-sharing geometry. The ion substitution at octahedral sites by less electronegative cations can be a new and effective way to improve the electrocatalytic performance of cobalt-based spinel oxides.

Automated summary

By partially replacing Co3+ with the more electro-negative Mn3+ ion at the octahedral site, a ternary Zn-Mn-Co spinel oxide is formed, leading to a significant increase in electrocatalytic oxygen reduction/evolution activity. Physical characterization and theoretical calculations show that bond competition plays a crucial role in regulating the cobalt valence state and the electrocatalytic activity. The partial replacement of Co3+ by Mn3+ effectively modulates the adjacent Co-O bond and induces the Jahn-Teller effect, resulting in a change in crystal structure and optimization of the binding strength with reaction intermediates. The Mn-substituted ZnMn1.4Co0.6O4/NCNTs exhibit higher electrocatalytic activity than ZnCo2O4/NCNTs and ZnMn2O4/NCNTs, indicating that the covalency of the Co-O bond determines the oxygen reduction reaction (ORR) activity of spinel ZnCo2O4. This study introduces the competition between adjacent Co-O and Mn-O bonds through the B-Oh-O-B-Oh edge-sharing geometry, demonstrating that ion substitution at octahedral sites with less electronegative cations can effectively enhance the electrocatalytic performance of cobalt-based spinel oxides.