Design, Synthesis And Electrocatalytic Performance Of Iron-based Spinel-structured High-entropy Oxides

The electrochemical oxygen reduction reaction(ORR)is an important reaction involving the interconversion of electrical energy to chemical energy in the production of hydrogen peroxide(H₂O₂),proton exchange membrane fuel cells(PEMFC),rechargeable metal-air cells,etc.The ORR reaction can be carried out in two pathways.The 2e process(2e^-ORR)can be used to produce H₂O₂,and the 4e process(4e^-ORR)reduces O₂ to water(H₂O).Spinel oxides are abundant and readily available materials on Earth.These materials have an abundance of variable compositions,electronic configurations and valence states and have been extensively investigated as potential oxygen reduction electrocatalysts.In this paper,a series of Fe-based high-entropy spinel oxide(M_0.2Mg_0.2Co_0.2Ni_0.2Cu_0.2)Fe₂O₄(M=Fe,Mn,Zn)were synthesised by a glycine-nitrate combustion method by combining spinel oxides with the concept of high-entropy and applied for the first time to electrocatalytic oxygen reduction to generate H₂O₂.The results of the study showed that due to the lattice distortion effect of the high-entropy oxide,resulting in a large number of oxygen vacancies(27.1%)in the material.The electrochemical results show that the spinel structure of the high-entropy oxides has good 2e^-ORR activity and excellent stability.(Fe_0.2Mg_0.2Co_0.2Ni_0.2Cu_0.2)Fe₂O₄exhibited 81.1-85.0%H₂O₂ selectivity in the potential range of 0.50-0.60 V and the current density did not change significantly during a 12 h long electrolysis at 0.46 V potential.The results of this study offer great promise for the application of high-entropy spinel oxide for 2e^-ORR.As Mg²⁺is an electrocatalytically inert element,in order to improve the 2e^-ORR performance of the materials,a series of(X_0.2Y_0.2Co_0.2Ni_0.2Cu_0.2)Fe₂O₄(X,Y=Fe,Mn,Zn)materials without Mg²⁺was synthesised by the glycine-nitrate combustion method.XPS dates showed that such high-entropy materials formed a high concentration of oxygen vacancies(31.4%).Electrochemical tests with 2e^-ORR showed that(Fe_0.2Zn_0.2Co_0.2Ni_0.2Cu_0.2)Fe₂O₄(FZCNC)exhibited excellent performance.The H₂O₂selectivity is found to range from 69.5%to 85.8%under a wide potential window of 0.2?0.65 V with excellent durability up to 24 h.The improved performance is attributed to the high concentration of oxygen vacancies created by lattice distortion and the multicomponent co-interaction in the high-entropy oxide.In order to increase the specific surface area of the material and expose more active sites,spinel-phase(Fe_0.2Zn_0.2Co_0.2Ni_0.2Cu_0.2)Fe₂O₄(H-FZCNC)nanoparticles were synthesised by hydrothermal methods.The electrochemical results show that the H-FZCNC nanoparticles exhibit 4e^-ORR activity.In a 0.1 M KOH electrolyte,the initial potential of the H-FZCNC was 0.731 V,the half-wave potential 0.636 V and the limiting current density-5.50 m A cm^-2.The current density retention of 89%after 20,000 s by chronoamperometry at 0.46 V is better than that of Pt/C(20,000 s,78%),demonstrating excellent stability.At the same time,this high-entropy spinel oxide material exhibits superior electrochemical performance compared to spinel low-entropy oxides.This is due to the presence of a large number of oxygen vacancies(39.4%)in the high-entropy spinel oxide,which contributes to oxygen adsorption.In addition,the formation of nanoparticles helps to increase the electrochemically active surface area,resulting in a substantial improvement in electrochemical performance.