Bismuth-based Catalysts For Carbon Dioxide Electrochemical Reduction To Formate

The electrochemical reduction of CO₂ into high value-added chemical products and clean fuels driven by renewable energy sources is a key technology to reduce the high concentration of CO₂ in the atmosphere and promote the recycling of global carbon resources.Formate is an important C₁ product in the CO₂electrochemical reduction.Technoeconomic analysis shows that the CO₂ electroconverting formate seems to be the most viable as cost competitive and economic value.However,CO₂ electroreduction technology still faces many challenges:（1）The low solubility of CO₂ in the aqueous electrolyte leads to serious mass transfer limitations;（2）CO₂ with thermodynamic stability needs to overcome a high activation energy barrier;（3）Both multi proton-electron coupling and hydrogen evolution reaction reduce the selectivity of the reduction products.Therefore,designing and preparing cheap and efficient electrocatalysts are crucial to improve the catalytic selectivity and activity of CO₂ electroreduction to formate,and promote CO₂ reduction to large-scale industrial applications.This thesis focuses on the effects of Bi-based catalysts on the catalytic activity and reaction mechanism in the electrochemical reduction of CO₂ and explores the structure-activity relationship between catalyst structure and catalytic performance.Especially using the flow cell with gas diffusion layer electrode solves the problem of CO₂ mass transfer limitation.The research contents and results are as follows:（1）Bi-based nanospheres is constructed by using the ethylene glycol reduction method,and the role of the enhanced Bi-O crystalline structure is investigated for catalytic activity and reaction mechanism for the electrochemical reduction of CO₂.The performance results show that the enhanced Bi-O crystalline structure in Bi₂O₃catalyst shows a higher product selectivity and catalytic activity for CO₂electrochemical reduction to formate.At-0.9 V（vs.RHE）,Bi₂O₃-5 h with stronger Bi-O crystalline structure exhibites a Faradaic efficiency of 91%and a partial current density of 8 mA cm^-2 for the formate product,and could maintain stable catalytic performance for 24 h.Both the in-situ Raman characterization technique and DFT calculations indicate that the stable existence of Bi-O crystalline structure is the key factor for the enhancing electrochemical reduction of CO₂ toward formate.（2）In order to further improve the catalytic activity of Bi-based catalysts with Bi-O crystalline structure,the carbon nanorods encapsulated Bi₂O₃ nanoparticle catalyst（Bi₂O₃@C）have been prepared by using a simple yet novel space-limited pyrolysis method with BiBTC precursor.By adjusting the pyrolysis temperature and oxidation treatment,Bi₂O₃@C-800 shows an excellent catalytic performance for CO₂ electrochemical reduction to formate.In the H-type electrolytic cell,the Faradaic efficiency of Bi₂O₃@C-800 could reach 92%at-0.9 V（vs.RHE）,and the partial current density of formate is 7.5 mA cm^-2.In a flow cell with gas diffusion electrode,Bi₂O₃@C-800 could maintain a formate Faradaic efficiency of more than 90%over a wide range of potentials,and a current density of 200 mA cm^-2 or more at-1.1 V （vs.RHE）,which shows catalytic performance close to commercialization requirements.（3）On the basis of the relevant invesitigation of Bi-based MOFs,in order to further improve the atomic utilization of Bi-based catalysts,BiBTC precursors are used to prepare Bi single atoms catalysts（Bi SAs@C）supported on carbon nanorods at high temperature.With a flow cell equipped with gas diffusion electrode and alkaline electrolyte,Bi SAs@C exhibits a Faradaic efficiency of 92%for formate products at -0.7 V（vs.RHE）,and current density could be maintained above 125 mA cm^-2.More importantly,the formate partial current density of Bi SAs@C is 2 times than that of Bi@C,and the mass activity of Bi SAs@C is 122 times than that of Bi@C.