Construction Of Cobalt-based Spinel Oxygen Electrocatalysts And Their Applications In Zinc-air Batteries

The use of catalysts to promote oxygen reduction and oxygen precipitation,which has great application value in metal-air batteries,especially zinc-air batteries,has been one of the hot research topics in recent years.Most of the state-of-the-art oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）catalysts are composed of noble metal catalysts such as platinum-based or iridium-based,but they are expensive and prone to agglomeration and toxicity in practical applications,which hinders their commercialization.The current challenge is to replace these noble metal-based catalysts with non-precious metal materials without compromising catalytic efficiency.Cobalt-based spinel oxide has the advantages of low cost,easy valence tuning and flexible structure,and is a highly promising non-precious metal oxygen electrocatalyst.The aim of this paper is to optimize the structure of cobalt-based spinel oxides through the tuning of elemental A/B sites and to achieve efficient oxygen electrocatalytic reactions with cobalt-based spinel oxides.In this paper,the effect of A/B-site substitution on the electron transfer of cobalt-based spinel oxide is systematically investigated,and the reaction mechanism is revealed with the help of DFT calculations,and it is confirmed that both cobalt-based spinel oxide A/B-site substitution strategies can effectively regulate the catalytic activity of Co _Oh³⁺octahedral active center.The details of the study are as follows.（1）In spinel oxides（AB₂O₄）,atomic substitution at the tetrahedral site(A_Td)can theoretically be achieved through A_Td-O-B_Oh interactions for effective optimization of the octahedral site(B_Oh)charge.Although substantial progress has been made,precise control and tuning of the spinel oxide crystal structure remains challenging due to its complexity.In this work,we used a simple solvent method to tune the spinel oxide structure and used spinel oxide composites（ACo₂O₄/NCNTs,A=Mn,Co,Ni,Cu,Zn）for oxygen electrocatalysis.The optimized Mn Co₂O₄/NCNTs exhibited high activity and good stability in the oxygen reduction/oxygen evolution reaction.Notably,the rechargeable liquid Zn–air battery equipped with a Mn Co₂O₄/NCNTs cathode exhibited a specific capacity of 827 m Ah g _Zn^-1and a power density up to 74.63 m W cm^-2,with no significant voltage drop after 300 cycles at a high charge/discharge rate(5 m A cm^-2).Density flooding theory（DFT）calculations show that the A-site substitution can modulate the Co³⁺/Co²⁺ratio,which leads to a modulation of the electronic structure accompanied by a shift of the d-band center.The interactions between tetrahedral and octahedral sites through Mn-O-Co,lead to an optimal charge structure of Co³⁺in Mn Co₂O₄,which optimizes the binding interaction between the active center and oxygen-containing species and improves the oxygen electrocatalytic performance of cobalt-based spinel oxides.（2）Based on the previous work,we further investigated the modulation of the octahedral site active center by octahedral site substitution.By using the less electronegative Mn³⁺to partially replace the Co³⁺at the octahedral site of Zn Co₂O₄,the electrocatalytic activity was found to be significantly increased.After characterization tests and DFT calculations,it was demonstrated that bond competition has a key role in regulating the cobalt valence state and the electrocatalytic activity of Zn Co₂O₄.The replacement of octahedral-site occupied Co³⁺by octahedral-site occupied Mn³⁺tends to effectively modulate the adjacent（Co-O）bond and induce the Jahn-Teller effect,thus changing the original stable structure of the crystal and thus optimizing the binding strength of the active center to the reaction intermediate.The manganese-substituted Zn Mn_1.4Co_0.6O₄/NCNTs show higher electrocatalytic activity in the oxygen reduction reaction（ORR）than Zn Co₂O₄/NCNTs and Zn Mn₂O₄/NCNTs,indicating that（Co-O）bond covalency plays a major role in optimizing the oxygen reduction reaction activity of Zn Co₂O₄.This study shows that bond competition occurs between adjacent（Co-O）and（Mn-O）by the B_Oh-O-B_Oh edge-sharing geometry.The substitution of octahedral sites by less electronegative cations could be a new and effective way to improve the electrocatalytic performance of cobalt-based spinel oxides.