Research On Regulating Electronic Structure Of Copper-Ulfur-Based Electrocatalysts And Their Performance In Carbon Dioxide Reduction

Environmental issues are becoming more serious as a result of excessive energy use and greenhouse gas emission.Countries worldwide are devoted to the development of technology and scientific research on“carbon reduction”.Meanwhile,the target of"achieving emission peak by 2030 and carbon neutrality peak by 2060"has been proposed by China.Electrocatalytic CO₂ reduction as a green“carbon reduction”technology is anticipated to accomplish low-carbon and green-development energy and chemicals based on the requirement for the“emission peak and carbon neutrality peak”aim.Cu-S-based materials as catalysts for electrocatalytic CO₂ reduction face problems such as the lack of structural changes and theoretical research,the unclear methods for regulating product selectivity and so on.The structural changes of Cu S during the electrocatalytic CO₂ reduction process and its mechanism for selective generation of formate are investigated in this thesis.The problems of the severe side reaction of hydrogen evolution reaction（HER）and the poor stability for Cu S due to total Faraday efficiency（FE）of below 100%for its reduction and the reoxidation of the reduced Cu S were improved by introducing Sn O₂ to construct a composite structure and designing a Cu-S material covered by metal Bi to regulate the catalyst structure and the adsorption strength of intermediates for electrocatalytic CO₂reduction.The improved catalysts increase the formate selectivity and obtain a high partial current density of format,while improving the problems of poor stability and the serious HER.In addition,the structural changes of Cu₂S during the electrocatalytic CO₂ reduction and the mechanism of selective ethanol generation were investigated.The specific studies are as follows:1.Cu S nanosheet arrays were prepared on brass mesh substrate（Cu S/BM）by a chemical bath deposition method and applied to electrocatalytic CO₂ reduction with a total current density of-72.5 m A·cm^-2and a formate FE of 70.2%at-0.7 V.During the reduction process,Cu S/BM is reduced.After the structural reconstruction,the nanowire structure of Cu（111）/Cu S（102）is formed.The surface of the nanowire is Cu and the interior is Cu S.DFT calculations show that the formation of Cu（111）/Cu S（102）during the reduction process is responsible for the high selectivity of formate.Compared with the Cu（111）plane,Cu（111）/Cu S（102）weakens the adsorption of*COOH,increases the energy barrier of*COOH to form*CO,and greatly inhibits the formation of CO.Meanwhile,the S element under the Cu effectively promotes the desorption of*HCOOH.However,Cu S faces the challenge of severe HER and poor stability.2.The composite of Cu S and S-doped Sn O₂（Cu S/Sn O₂-S）was synthesized by a redox reaction at room temperature.Cu S/Sn O₂-S nanoparticles with a Cu/Sn atomic ratio of 5.42 are structurally reconstructed to form nanowires of Cu_6.26Sn₅/Cu/Sn bulk phase during the initial process of electrocatalytic CO₂ reduction,and the exposed dominant crystalline plane is Cu_6.26Sn₅（110）.The structural reconstruction of the electrode material decreases the energy barrier of CO₂ adsorption,promotes the desorption of*HCOOH and enhances the adsorption of*H,thus suppressing the competing reaction HER and optimizing the performance of selective formate generation.The composite weakens the reduction degree of Cu S,inhibits the reoxidation of Cu S after reduction,and improvs the stability of catalyst.The results show that the FE of electrocatalytic CO₂ reduction to formate is 84.9%at-0.8 V,and the local current density reaches-15.8m A·cm^-2 with a yield of 8860μmol·h^-1·cm^-2.3.Bi/Cu-S/BM electrode material for electrocatalytic CO₂ reduction was prepared by electrodepositing Bi on the surface of Cu-S/BM nanowires obtained from Cu S/BM after the electroreduction.The FE of the formate reaches the maximum of 94.3%at-0.9 V with a high partial current density of-50.7 m A·cm^-2 and a yield of 30.7 mmol·h^-1·cm^-2.Meanwhile,its FE of formate is higher than 90%in the voltage range of-0.8～-1.0 V.It exhibits good stability because the FE of formate remains above 93%after 10 h reaction at-0.9 V.DFT calculations demonstrate that the Bi/Cu-S structure promotes the adsorption of CO₂ and effectively inhibits HER through enhancing the adsorption of*H to a great extent,thus promoting the selective conversion of CO₂ to formate.Bi/Cu-S/BM electrode material exhibits better performance than Cu S/BM and Cu S/Sn O₂-S in electrocatalytic CO₂reduction.4.Cu₂S nanorod arrays/copper mesh（Cu₂S/CM）were prepared by the gas-phase sulfide of Cu（OH）₂/CM nanorod arrays.During the electrocatalytic CO₂ reduction at a specific voltage of-0.8 V,Cu₂S/CM is structurally reconstructed and transferred to Cu/Cu₂S/CM with abundant Cu⁰-Cu⁺sites.Although its selectivity for C₂H₅OH only reaches 13.5%at-0.8 V,it achieves a high C₂H₅OH yield of 986.4μmol·h^-1·cm^-2 and a high partial current density of-13.3 m A·cm^-2.The reaction intermediates are detected by in-situ FTIR and are consistent with the C₂H₅OH pathway.Furthermore,DFT calculations demonstrate that the presence of Cu⁰-Cu⁺sites in Cu/Cu₂S enhances the adsorption of the key intermediate*CH₂CHO and favours the pathway for C₂H₅OH generation.