Design Of Ultrathin Nanosheet/single-atom Catalysts For Electrocatalytic Reduction Of CO₂/N₂

In 2008,the National Academy of Engineering（NAE）listed 14 major engineering and technological challenges facing humanity in the 21st century.Among them,"developing carbon dioxide storage technology"and"controlling the nitrogen cycle"were regarded as two major issues.It has become the goal of many researchers that how to use effective technology to store CO₂ and N₂ small molecules in the atmosphere and convert them into valuable fuel/chemical products.The electrochemical reduction technology has attracted wide attention because of its simple operation,high energy conversion efficiency,and its combination with renewable energy sources.However,the extremely strong chemical stability of CO₂ and N₂ molecules has brought great challenges to the development of this technology.Therefore,the development of electrocatalytic materials with high activity,selectivity and stability has become the key of the current CO₂/N₂ reduction field.Based on the above topic,in this thesis,ultrathin nanosheet/single-atom catalysts are designed and used as excellent CO₂/N₂reduction electrocatalysts.The main research contents are as follows:（1）The two-dimensional（2D）ultrathin In₂Se₃ nanosheets were successfully prepared by vapor deposition and liquid-phase exfoliation method,which was used in electrocatalytic CO₂ reduction reaction.The prepared 2D nanosheets have a lateral size of 200±50 nm and a thickness of 2.9±1.0 nm.In the electrocatalytic CO₂ reduction reaction test,the nanosheets have high CO selective activity.At-0.7 V vs RHE,the highest CO Faraday efficiency can reach 89%.And the ultrathin nanosheet can maintain a high CO selectivity of more than 70%in a wide potential range（-0.6 to-0.8 V）,which is very beneficial in practical applications.At the same time,the CO Faraday efficiency and reduction current density of these nanosheets did not decrease significantly during the continuous electrolysis for 24 hours.Theoretical calculations further analyze the main reason for the excellent activity of 2D ultrathin In₂Se₃ nanosheets.And the results show that the strong binding ability between electronegative Se atoms exposed on the nanosheet surface and positively charged C atoms can stabilize*COOH intermediates,which in turn lead to an improved activity for CO₂-to-CO conversion.（2）The metal electrocatalyst（named ISAS-Fe/NC）was synthesized by anchoring isolated Fe atomic site on the 3D N-doped carbon frameworks using hydrothermal and high-temperature calcination methods,which is applied to nitrogen fixation reactions.The spherical aberration-corrected transmission electron microscopy and X-ray bsorption near-edge structure showed that single Fe atoms coordinate with four N atoms of the 3D N-doped carbon skeleton.In the neutral PBS electrolyte at room temperature,ISAS-Fe/NC can achieve an NH₃ yield of 62.9±2.7μg h^-1 mg^-1_cat.,and corresponding Faraday efficiency can reach 18.6±0.8%at-0.4 V vs RHE.At the same time,the catalytic activity of ISAS-Fe/NC has a slight decay during the 24 hours continuous reduction process.Combining X-ray absorption fine structure analysis and theoretical calculation results,it is concluded that the Fe-N₄ configuration,formed by coordination of the atomic dispersion Fe site and four N atoms,is favorable for the activation of N₂molecules,which promoted the formation of NH₃.