Bismuth Ferrite Heterostructure Construction And Its Electrochemical Reduction Performance

Electrocatalytic reduction technology provides an effective way for the storage and conversion of clean renewable energy,which reduce CO₂ and NO₃^-to High value-added chemicals with water,wind energy and solar energy as the hydrogen and driving force.Besides this,meeting the recycling of carbon resources and future energy and environmental needs,have attracted much attention in recent years.However,there are still many challenges in the electrocatalytic reduction reaction（HER）,such as low Faradaic efficiency,high overpotential,low current density,and insufficient stability.Moreover,bimetallic bismuth-iron-oxygen（Bi-Fe-O）material,as a novel electrochemical reduction catalyst,exhibits excellent catalytic activity in electrocatalytic reduction.In this thesis,the high-efficiency electrocatalytic reduction conversion was achieved by adjusting the feeding ratio of the second active center Fe based on bimetallic bismuth-iron-oxygen catalysts.Improved heterojunction nano catalyst loading capacity and optimizing the reaction conditions of BFO,which significantly improved the electrocatalytic Faradaic efficiency and current density of the reduced products.The main results are as follows:（1）Five kinds of bismuth ferrite catalysts with different ligands were prepared as Bi Fe O₃,Bi₂Fe₄O₉,Bi₂₅Fe O₄₀,Bi Fe O₃/Bi₂Fe₄O₉ and Bi Fe O₃/Bi₂₅Fe O₄₀（BFO）,respectively.Heterojunction composite nano-structure materials were constructed by adjusting the feeding ratio of Bi and Fe for Bi₂₅Fe O₄₀ and Bi₂Fe₄O₉.In addition,BFO catalyst prepared by sol-gel method under high temperature environment,the structure-activity between the ratio and the atomic number ratio of the catalyst.The study found that stoichiometric ratio of Bi³⁺and Fe³⁺at 2:1,1:1 and 1:0.04,respectively,which corresponds to Bi₂Fe₄O₉,Bi Fe O₃ and Bi₂₅Fe O₄₀catalysts.Furthermore,heterojunction nano catalyst will be formed by Bi³⁺:Fe³⁺between 2:1,1:0.04 and 1:1.Additionally,the Bi Fe O₃/Bi₂₅Fe O₄₀ heterojunction catalyst has excellent catalytic performance with the smallest band gap about 1.77 e V when Bi³⁺/Fe³⁺is 1/0.75 at700°C.Moreover,the crystallite size of Bi₂₅Fe O₄₀ slower growth compare with Bi₂Fe₄O₉ and Bi Fe O₃ catalysts with gradually increase the calcination temperature.The existence of Bi Fe O₃/Bi₂Fe₄O₉,Bi Fe O₃/Bi₂₅Fe O₄₀ heterojunctions was proved by XPS,TEM,and UV-vis reflectance.（2）During the electrocatalytic reduction of NO₃^-ammonia synthesis（NO₃^-RR）,it was found that the FE of NH₄⁺was 98.4%under the premise of ensuring high current density around 800 m A cm^-2@-0.5 V vs.RHE.The Bi₂Fe₄O₉@-0.47 V vs.RHE has almost no by-product NO₂^-with the current density at 700±20 m Acm^-2,in addition the Faradaic efficiency of NH₄⁺is above 86%for 45 h.It is proved by TEM and other characterizations that Bi-Fe bimetallic interaction.（3）Performance changes of BFO catalysts in the electrocatalytic reduction of CO₂（CO₂RR）,in which Bi element is dispersed in the Bi-Fe alloy in the form of single atoms,showing low overpotential,wide potential window,and high HCOOH Faraday in the electrochemical reduction of CO₂ efficiency and high stability and other characteristics.The electrocatalytic experimental data show that the CO₂ selectivity of the heterojunction Bi Fe O₃/Bi₂₅Fe O₄₀ catalyst is higher than Bi Fe O₃ and Bi₂₅Fe O₄₀,which illustrating the higher Bi Fe O₃/Bi₂₅Fe O₄₀ loading,the better CO₂ selectivity.Among them,58%Bi Fe O₃/Bi₂₅Fe O₄₀catalyst FE of HCOOH exceeds 95%with 170±20 m A cm^-2 current density@-0.64 V vs.RHE with 80 h in the flow cell.It is proved by HAADF-STEM and other characterization methods that the heterojunction nano-structure of BFO is the joint action of Bi single atom and Bi-Fe alloy in situ reaction during CO₂RR.