Synergistic Enhancement Of Electrocatalytic Oxygen Evolution Of Cobalt/Nickel Ferrite By Vanadium Doping And Heterojunction

The oxygen evolution reaction（OER）has limited the progress of water electrolysis for hydrogen production due to the complex kinetic process involving the transfer of 4 electrons and various oxygen intermediates,so the development of low-cost and efficient electrocatalysts is very important.In this thesis,aiming at the problems of few reactive sites and too high e_g occupancy of transition metal cation in spinel nanoferrite catalysts,we designed amorphization induced by vanadium（V）doping to provide a large number of reactive sites.The catalysts with excellent OER activity were prepared under the synergistic effect of phosphide heterojunction to enhance the catalyst conductivity and tune the electronic structure.The main research results are as follows:Vanadium was homogeneously doped into spinel nickel ferrite Ni Fe₂O₄ and cobalt ferrite Co Fe₂O₄ catalysts by a simple hydrothermal method.Vanadium weakens the crystallization ability of spinel nanoparticles,thereby inducing the formation of amorphous/crystalline mixed heterostructures,and the large amount of oxygen vacancies contained in the amorphous phase can provide abundant reactive sites.In addition,vanadium doping induces electronic structure rearrangement,which transfers electrons from cobalt/nickel to vanadium,which promotes the formation of higher oxidation state cobalt/nickel and optimizes the adsorption strength of oxygen intermediates.By adjusting the doping amount of vanadium,the crystal structure and electronic structure are regulated,and the oxygen evolution activity is significantly improved.The modified vanadium-doped nickel ferrite catalyst NFO-V_0.3 shows an overpotential of 309m V at the current density of 20 m A cm^-2 and a Tafel slope of 54 m V dec^-1,vanadium-doped cobalt ferrite CFO-V_0.3 showed an overpotential of 308 m V at 20 m A cm^-2 and a Tafel slope of 40 m V dec^-1,both showing better oxygen evolution activity and better stability than commercial ruthenium oxide catalyst.The vanadium doping-induced amorphization strategy enhanced the catalytic activity,but this single strategy alone cannot further enhance the catalytic activity due to the poor conductivity of the amorphous phase.Therefore,the electrical conductivity was improved by constructing metal phosphide heterojunctions based on CFO-V_0.3 and NFO-V_0.3.It was found that the nickel phosphide/cobalt phosphide heterojunction grown when the ratio of catalyst to sodium hypophosphite was 1:3 and the phosphating temperature was 300°C showed the best oxygen evolution activity.On the one hand,the phosphide heterojunction contributes to the improvement of the overall conductivity.On the other hand,the electronic structures of the amorphous/crystalline hybrid hetero-interface and the active site at the phosphide hetero-interface are synergistically optimized,resulting in a synergistic optimization of the two strategies.The transition metal cations have higher valence states,which ultimately lead to electrocatalysts with higher oxygen evolution activity.The as-prepared phosphide heterojunction cobalt ferrite catalyst CVFO-P3-300 exhibited an overpotential of 286 m V at the current density of 20 m A cm^-2 and a Tafel slope of 43 m V dec^-1,the phosphide heterojunction nickel ferrite catalyst NVFO-P3-300 achieved an overpotential of 277 m V at20 m A cm^-2 and a Tafel slope of 45 m V dec^-1.Both exhibited excellent stability and durability in alkaline media.