The Study Of Exsolution Regulation And Electrocatalytic Performance For Fe-based Perovskite Oxides

The depletion of fossil energy and the aggravation of environmental pollution have caused an increasing serious global energy crisis,which has aroused people’s interest in finding new clean and renewable energy sources.Clean hydrogen energy produced by electrochemical decomposition of water is one of the most promising alternative fuels to reduce CO₂ emissions and solve the global energy crisis.The oxygen evolution reaction in the electrochemical decomposition process of water is complex and difficult,and the kinetics is relatively slow.Therefore,the efficiency of water decomposition mainly depends on oxygen evolution reaction.Traditionally,Ru O₂ and Ir O₂ oxides are catalysts with high activity for oxygen evolution reaction,but because they are rare and expensive precious metals,it is very important to develop a low-cost and efficient electrocatalyst for oxygen evolution reaction to meet the needs of industrial hydrogen production by electrolysis.Perovskite oxide（ABO₃）is considered to be the most promising electrocatalyst for oxygen evolution reaction due to its excellent catalytic activity,flexible composition and environmental friendliness.In addition,the physical and chemical properties of perovskite oxides can be changed by doping other elements at A/B sites,which will affect its catalytic activity.Exsolution is a simple and environmentally friendly method.The nanoparticles prepared by exsolution are socked in the parent oxide and have a strong interaction with the parent,which greatly overcomes the shortcomings of the nanoparticles prepared by traditional methods which are easy to assemble due to their instability at high temperature.In this paper,the cobalt doped iron-based perovskite oxides with different contents were synthesized by sol-gel method,and the optimum proportion of cobalt doped iron-based perovskite oxide was determined by testing the electrocatalytic performance.The perovskite oxides were treated in the reducing atmosphere by the method of exsolution.The metal nanoparticles were produced on the surface of the perovskite oxides to form the metal nanoparticle/perovskite oxide composite.The composition of the nanoparticles can be adjusted in situ by adjusting the reduction temperature,which is of positive significance to the design of a wide range of efficient electrocatalysts.The main research contents are as follows:1.We synthesized different content of cobalt doped iron-based perovskite oxides with A site deficiency(La_0.95Fe_1-yCo_yO₃,y=0,0.1,0.2,0.3,0.4)by sol-gel method.Citric acid is a complexing agent.According to the corresponding stoichiometric ratio,the iron-based perovskite oxides were synthesized by adjusting the amount of Co（NO₃）₂·6H₂O and Fe（NO₃）₃·9H₂O.We characterized the morphology and structures of all samples,and the electrochemical performance of oxygen evolution reaction was also characterized.When the content of cobalt is 20%,it is the best doping ratio and oxygen evolution reaction performance has improved a lot.Under the current density of 10 m A cm^-2,La_0.95Fe_0.8Co_0.2O₃ has a low overpotential of 402 m V,which is 121 m V lower than that of undoped lanthanum ferrite,and a low tafel slope value of 83.9 m V dec^-1.The electrochemical impedance spectroscopy analysis showed that the charge transfer resistance was low and had good charge transfer ability.The sample is an excellent electrocatalyst for oxygen evolution reaction.2.We can adjust the annealing temperature of La_0.95Fe_1-yCo_yO₃（y=0.1,0.2,0.3）in reducing atmosphere by thermal reduction,so that nanoparticles can be exsolved from the perovskite oxides surface,and the composition of nanoparticles exsolved from La_0.95Fe_0.8Co_0.2O₃（LFCO）can be controlled in situ by adjusting the annealing temperature.In the whole reduction process of La_0.95Fe_0.9Co_0.1O₃,with the increase of temperature（600-800 ^oC）,the exsolved nanoparticles are all Co Fe alloys.In the whole reduction process of La_0.95Fe_0.7Co_0.3O₃,with the increase of temperature（500-800 ^oC）,the exsolved nanoparticles are all single Co metals.At lower temperature（500 ^oC）,the species exsolved from LFCO is Co Fe alloy,while the temperature rises above 600 ^oC,the composition of metal nanoparticles changes to single Co metal by cation exchange between Fe in metal nanoparticles and Co in perovskite oxide,forming Co/LFCO composite.This phenomenon could be owing to the higher co-segregation energy of Co than Fe cations,and La Fe O₃ is thermodynamically more stable at high temperatures.The oxygen evolution reaction performance of all the exsolution samples was tested.The oxygen evolution reaction performance of Co/LFCO composite formed by exsolving cobalt from LFCO with 20%Co dopant is the best.The high activity of the sample is due to the great improvement of conductivity of the formed Co/LFCO composite than that of Co Fe/LFCO.