Synthesis Of Bismuth/Carbon And Their Composite Materials For Electrocatalytic Reduction Of Carbon Dioxide

With the continuous development of human economy and society,the demand for resources is gradually increasing,and the excessive consumption of fossil fuels has led to the rising CO₂ concentration in the atmosphere,which causes serious environmental problems such as climate change and sea level rise.The reduction of CO₂ can reduce the concentration of CO₂in the atmosphere,and also produce valuable fuels and chemicals as raw materials for the chemical industry to alleviate the ever-growing energy crisis,which is crucial to realize energy and environmental sustainability.Electrochemical CO₂ reduction reaction（CO₂RR）is one of the most promising strategies in terms of its mild reaction conditions and high conversion efficiency as well as the easy operation conditions,it has been paid more and more attentions by researchers.Bismuth is considered as one of the most promising material because of its unique properties like eco-friendly,cost-effective and inferior to H₂ evolution.Bismuth based catalysts have been found with excellent performance towards CO₂ electroreduction to formate,but facing great challenge in exploring simple synthesis methods.In addition,the diaphragm is required in the traditional H-type electrolytic cell,which raises the overall resistance and mass transfer resistance,and increases the cost of electroreduction of CO₂.Therefore,it is of great significance for its industrial application to develop a reaction system that can efficient electrochemical CO₂ reduction reaction in a membrane free system.For the above-mentioned problems,this dissertation mainly completed the following tasks:（1）Bismuth nanospheres（Bi NSs）were prepared by a simple solvothermal method using bismuth nitrate as the main raw material,and the catalysts with different crystallinity were obtained by annealing at different temperatures using argon as the protective gas.A series characterization and the performance of electrocatalytic reduction of CO₂ were studied.We demonstrate that the catalytic performance of Bi NSs was closely related to the crystallinity.With the increasing of calcination temperature,the Faradaic efficiency of formate increases gradually.When the calcination temperature rises to 500℃,the Faradaic efficiency of formate reaches 95%at-1.2 V.In a broad potential range from-1.0 to-1.4 V（vs.RHE）,a high faradaic efficiency of 91～95%towards formate was achieved on the high crystallinity Bi NSs.High crystallinity endows Bi NSs with more distinct grain boundaries,providing more active sites for CO₂ electroreduction.（2）Bismuth carbon composites were prepared by coating bismuth nanospheres with dopamine and calcining at 700℃,800℃ and 900℃ respectively.A series characterization and the performance of electrocatalytic reduction of CO₂ were studied.The Faradaic efficiency of formate reaches 94%at-1.2 V（vs.RHE）in a single cell.Carbon coating enhances the conductivity and electrochemical surface area（ECSA）.The graphitic-N on the carbon layer has strong adsorption ability for CO₂ intermediates and increases the coverage of intermediates on the surface of the material,thus inhibiting the hydrogen evolution reaction and improving the selectivity of formate.（3）Nitrogen doped porous carbon materials were prepared by simple pyrolysis and ammonia etching using petroleum pitch as raw material and magnesium oxide as template.The series characterization and the performance of electrocatalytic reduction of CO₂ were studied.We found mesopores are favored formation by removing of asphaltene from petroleum pitch during the carbonation process.Simultaneously,ammonia etching can not only increase the pyridinic-N content,but also upgrade the ratio of meso-to micro-pores of carbon materials.The Faradaic efficiency of carbon monoxide reaches 83%at-0.9 V（vs.RHE）in 0.1 M KHCO₃.Compared with the carbon material without asphaltene removal under the same conditions,the performance of the carbon material is improved by 30%.This superior performance is attributed to the synergistic effects of highly pyridinic-N content in conjunction with the hieratically porous architecture,rendering abundant exposed and accessible active sites for electroreduction of CO₂.