Electrocatalytic Reduction Of CO₂ By Bi₂O₃- Based Catalysts

With the massive consumption of fossil energy and the intensification of human activities,the content of CO₂ in the atmosphere is increasing rapidly,which is breaking the carbon cycle of the original ecological environment,resulting in a series of environmental problems.Therefore,it is urgent to stabilize the carbon cycle of the ecosystem and reduce the CO₂ content in the atmosphere.The electrocatalytic reduction of CO₂ to valuable energy has attracted much attention of researches at home and abroad.It is significant to select and design catalysts with high-efficiency?low-cost?high-stability for electrocatalytic reduction of CO₂.Based on the previous research of our group,this research selected low-cost,high-stability and non-toxic Bi₂O₃-based materials as the research object,combining with organic and inorganic materials by simple method to synthesis PANI-Bi₂O₃ and Co₃O₄-Bi₂O₃ catalysts,which can achieve high-efficiency of ethanol and formate in electrocatalytic reduction of CO₂.The main research contents and conclusions of this research are as follows:Firstly,the organic-inorganic PANI-Bi₂O₃ catalytic material was synthesized by one-pot method,and its ratio control and acid treatment were carried out.PANI-Bi₂O₃with different morphologies was obtained by adjusting the molar ratio of PANI and Bi₂O₃,and the performance of electrocatalytic reduction of CO₂ of different molar ratios PANI-Bi₂O₃ catalysts with HCl and HNO₃ treatment were investigated.Electrochemical experiments shown that with the introduction of PANI to Bi₂O₃,the main product was changed from formate of Bi₂O₃ to ethanol of PANI-Bi₂O₃.When the molar ratio of PANI to Bi₂O₃ was 3.48,it has good catalytic performance.The Faraday efficiency of ethanol was arrived at 69.1%at an overpotential of-1.6 V.vs.SCE.Further,when the molar ratio of PANI to Bi₂O₃ was 1.16 with HCl-treated,the Faraday efficiency of obtained ethanol was 79.6%at same overpotential and when its molar ratio of PANI to Bi₂O₃ was 2.32treated by HNO₃,the Faraday efficiency of ethanol was 65.2%in same condition.It was found that PANI-Bi₂O₃ has better performance of electrocatalytic reduction of CO₂ than that of Bi₂O₃ and PANI.With the content of PANI increased,the specific surface area of PANI-Bi₂O₃ increased correspondly.In addition,the presence of PANI in PANI-Bi₂O₃can be found-N=?-NH⁺and other N species by XPS characterization,and it also can be found that the characteristic peak of the pyridine-N species.The presence of this substance can effectively promote electrocatalytic reduction of CO₂ to ethanol.Furthermore,different ratios of PANI-Bi₂O₃ treated with HCl or HNO₃,which can provide proton acid doping and surface modification,resulting in its electrocatalytic performance improvement,while the effect of HNO₃ treatment PANI-Bi₂O₃ is worse than that of HCl,because of the internal catalytic structure changed of PANI-Bi₂O₃ in HNO₃treated,which reduces its electrocatalytic performance.Secondly,the Co₃O₄-Bi₂O₃ composite catalyst was synthesized by coordination precipitation method,and the calcination time,calcination temperature and catalyst loading of catalytic materials were studied.It was found that when the calcination temperature,calcination time and loading amount were 500°C?5 h and 0.75 mg.cm^-2,respectinvely.Which can obtain the best catalytic performance of electrocatalytic reduction of CO₂.Subsequently,the different molar ratios of Co/Bi?3/1,2/1,1/1,1/2,1/3?of Co₃O₄-Bi₂O₃ composite catalytic materials were synthesized and its structure and catalytic performance were subjected by XRD,Raman,SEM,BET and XPS.It was found that with the molar ratios of Co/Bi increasing,the specific surface area gradually increases,and the surface structure of the Co₃O₄-Bi₂O₃ catalyst gradually changes from a smooth network structure to a sheet-like intercalation structure.When the Co/Bi molar ratio was2/1,it has a distinct and unique intercalation structure and lattice distortion or residual stress is the largest.Electrochemical experiments shown that Co₃O₄-Bi₂O₃?2/1?electrocatalytic reduction of CO₂ performance is the best and the electric double layer capacitance value is the largest and the Tafel value is the smallest.the oneset potential of Co₃O₄-Bi₂O₃?2/1?catalyst electrocatalytic reduction of CO₂ is-0.88 V vs.SCE,compared with Bi₂O₃,which decreased 468 mV,and the current density at-1.4 V vs.SCE increased from-0.8 mA·cm^-2 to-34.8 mA·cm^-2,which is up to 40 times.It is indicated that the introduction of Co₃O₄ can significantly improve the performance of Bi₂O₃ electrocatalytic reduction of CO₂.In addition,Co₃O₄-Bi₂O₃?2/1?can achieve a Faraday efficiency of 93.1%at a lower overpotential of-1.05 V vs.SCE,and the Faraday efficiency and current density keep stablility after continuing electrolysising 12 h.,while,under this condition,Bi₂O₃dose not produce formation.It can be found that the best catalytic performance of Co₃O₄-Bi₂O₃?2/1?catalyst may be related to the unique intercalation structure,which can induce the interaction of a small amount of Bi₂₅CoO₄₀,Co₃O₄ and Bi₂O₃ and generate large lattice distortion in this condition.