Study On Catalytic Performance And Electronic Property Of LaFeO₃ And Bi-doped LaFeO₃ Nanoparticles

Metal-air batteries have considerable potential development prospects which can be attributed to high energy conversion efficiency,high theoretical energy density and pollution-free.The oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）is cathode core reactions of renewable energy technologies,such as metal-air batteries and fuel cells.The investigation has demonstrated that the kinetics of the OER and ORR is slow,so the efficiency of the battery in nature is limited.The urgent task of the current research is to find a cathode electrode catalyst which has catalytic efficiency for OER and ORR reactions.Up to now,the best catalytic of ORR reaction catalysts currently are Pt and related alloys.But Pt is precious metal,and that directly leads to a great increase in the production cost of the battery.Moreover,it has low catalytic stability and low reserves.By contrast,metal oxides,such as Ru O₂ and Ir O₂ electrocatalysts have extraordinary OER activity,but poor ORR activity.Therefore,performance limitations,high cost and scarcity of these materials severely limit their application in practical development.So,it is important to find a catalyst which can replace precious metal-based catalysts.Perovskite materials have a definite research value in the field of electrocatalysis because of its large characteristics.It can be found that there are three main factors which can affect the catalytic activity in perovskite-type electrocatalytic materials:oxygen vacancy content,the strength of B-O bond and particle size.These factors can be controlled according to processes such as ion doping,sintering temperature,and sintering atmosphere.In this paper,the electrocatalytic properties of La_1-xBi_xFeO₃ particles which prepared by sol-gel method were studied.The details are as follows:（1）Firstly,La_1-xBi_xFeO₃ particles were obtained by annealing at 800°C in oxygen.Its structure and catalytic properties are studied.The lattice distortion of La FeO₃ is increased with the amount of Bi doping,and the density also changed significantly.Moreover,the vacancy of oxygen vacancies and the Fe ions in the La_1-xBi_xFeO₃ material also changed significantly with the amount Bi-doping.The Bi-doped resulted in a structural change of La FeO₃ which can significantly improve OER and ORR performance.The initial potential,limiting the current density,Tafel slope,and the number of transferred electrons of Bi-doped sample was significantly improved compared with the original sample.The results show that the optimal ORR performance is obtained when the doping amount of Bi is 0.15:the optimal starting potential is 0.698 V（vs.RHE）,the highest current density is 6.36 m A/cm²,and the lowest Tafel slope is 74 m V/dec^-1 and the number of transferred electrons is 3.8,which closest to 4 electron transfer model.（2）The oxygen vacancies in the sample are increased by changing the sintering atmosphere.The powders were calcined in an environment which has a low oxygen concentration.As a result,it could be found that ORR performance of pure La FeO₃ which sintering in air is better than that of sintering in oxygen.By contrast,the particle size of the calcined powder which sintering in air is much larger than that of oxygen.It can further reveal that the oxygen vacancies have a greater influence on the electrocatalytic performance than the specific surface area.When the amount of Bi-doping is 0.2 which was sintered in air at800°C,the limiting current density can reach 6.87 m A/cm².That is very close to the commercial Pt/Vulcan.Although the Tafel slope and the number of transferred electrons is inferior to those of Pt/C.（3）The electrocatalytic performance of La_1-xBi_xFeO₃ is studied through the difference of the specific surface area.La_1-xBi_xFeO₃ powders were obtained in vacuum at 600°C.The results show that the sample size of the sintered sample at 600°C is significantly lower than that in the other environments,but the crystallinity is not so that good.However,When the amount of Bi-doping is 0.2,the samples have better ORR and OER performances in the oxygen reduction property,Such as the optimum initial potential of 0.688 V（vs.RHE）,the highest limiting current density of 6.46 m A/cm² and a more favorable catalytic transfer electron number of 3.82.This further confirms that the number of oxygen vacancies have a great influence on the electrocatalytic performance of the perovskite material.（4）In order to investigate the influence of Bi-doping on the electrocatalytic performance of La FeO₃,a first-principles study on La_1-xBi_xFeO₃is conducted.The lattice model,density of states and energy bands of the La FeO₃and La_1-xBi_xFeO₃materials was obtained by construction model and theoretical calculation.It can be found that the addition of Bi and Vo caused the transforming of the original lattice and made a significant change in theβ-orbital electron motion,especially near the Fermi level.The Bi-doping and the addition of Vo can increase the conductivity of the material and the electronic transition orbit,which theoretically confirm the feasibility of Bi-doping in enhancing the catalytic performance of the material,and verify the correctness of the experimental investigation.