Preparation And Performance Study Of Carbon-based Catalysts For Electrochemical Reduction Of Carbon Dioxide

In recent years,the excessive consumption of traditional fossil energy leads to a sharp increase of CO₂concentration in the atmosphere,and the resulted“greenhouse effect”has caused serious environment problems.As the world’s largest energy consumer and carbon emitter,China plays an indispensable role in the process of“carbon neutrality”.At the United Nations General Assembly in 2020,China announced its own“double carbon”goal,that is,to reach the peak of carbon emissions by 2030 and achieve carbon neutrality by 2060,respectively.Therefore,it is necessary to find clean and renewable energies to drive the decrease of CO₂emissions in the atmosphere.Thus,it is imminent to reduce the CO₂concentration in the atmosphere.Converting CO₂into chemicals or fuels by electrochemical reduction can not only reduce the CO₂concentration in the atmosphere,but also convert clean renewable energy such as solar energy,wind energy and tidal energy into chemical energy for storage and utilization.However,due to the inertia of CO₂molecular and the low solubility in aqueous solution,the electrochemical reduction of CO₂is subject to high reaction overpotential,slow reaction kinetic process,inevitable side reaction of hydrogen evolution,and poor stability.Therefore,the design of catalysts with high activity,selectivity and stability is the basis for the industrialization of electrochemical reduction of CO₂.With the advantages of low-cost,rich sources,easily controlled structure and good stability,carbon materials served as substrates through different modification strategies exhibit high activity in various catalytic reactions.In this sense,this paper aims to synthesize carbon-based catalysts with low cost and good catalytic performance.A series of catalysts were prepared by metal loading,heteroatom doping,single atom sites,and investigated their behaviors towards electrochemcial CO₂reduction.In addition,combined with density functional theory（DFT）calculations,the relationship between catalyst structure and catalytic performance is deeply analyzed.The main research contents and results are as follows:1.Carbon substrates with Cu nanoparticles evenly loaded（Cu NPs@C）were fabricated by using a solvent-free ball milling method.The green method can not only shorten the time consuming,but also avoid the pollution of organic solvents to the environment.The exposure degree of Cu（111）facet can be adjusted by changing the loading amount of Cu precursors.As-obtained catalysts can selectively reduce CO₂to formate,typically,Cu NPs-3@C catalyst with the highest exposure degree of Cu（111）facet exhibits the highest Faraday efficiency of formate of 78%at-1.0 V versus RHE(V_RHE)and yield of 82.8μmol h^-1cm^-2at-1.2 V_RHE.DFT calculation results demonstrates that Cu（111）facet is conducive to the conversion of CO₂to formate,meanwhile,the existence of carbon substrate promotes the adsorption of*OCHO intermediate on the catalyst surface and the formation of formate.2.Ternary heteroatoms（N,S,P）-doped carbon electrocatalyst（NSP-HPC）was prepared in a facile and scalable manner.The multi-heteroatoms with different sizes and electronegativities can modulate the electronic properties of carbon frameworks and provide abundant active sites for CO₂reduction reaction.It exhibits high activity toward CO₂electroreduction to CO activity with a low onset overpotential of 270 m V and high Faradaic efficiency of 92%at-0.7 V_RHE.The promising potential of industrial application is manifested by the high current density of 245 m A cm^-2and stable Faraday efficiencies of CO above 98%in a flow-cell configuration.Moreover,in-situ Raman spectroscopy demonstrates that*COOH is the key intermediate in CO₂-to-CO conversion.DFT calculations reveal that the synergistic effect of N,S,and P heteroatoms boosts the catalytic activity by greatly decreasing the free energy barrier of*COOH formation.3.A novel single Ni atom catalyst with Ni-N₁-C₃configuration loaded on N-doped carbon nanotube is prepared.The obtained Ni-N₁-C₃catalyst exhibits a superior CO Faradaic efficiency of 97%and turnover frequency of 2890 h^-1at-0.9V_RHE,as well as long-term stability over 45 h.High current densities exceeding 200m A cm^-2and CO Faradaic efficiency of 99%are achieved in flow-cell.Moreover,in-situ potential-and time-dependent Raman spectra identify the key intermediates of*COOH and*CO during CO₂-to-CO conversion.DFT calculations reveal that the upward-shifted d-band center and charge-rich Ni sites of Ni-N₁-C₃facilitate the electron transfer to*COOH and thus reduce the*COOH formation energy barrier.4.A P-doped graphene aerogel as a self-supporting electrocatalyst for CO₂reduction to ethanol is reported.High ethanol Faradaic efficiency of 48.7%and long stability of 70 h are achieved at-0.8 V_RHE.Meanwhile,an outstanding ethanol yield of 14.62μmol h^-1cm^-2can be obtained,outperforming most reported electrocatalysts.In-situ Raman spectra indicate the important role of adsorbed*CO intermediates in CO₂-to-ethanol conversion.Furthermore,the possible active sites and optimal pathway for ethanol formation are revealed by DFT calculations.The graphene zigzag edges with P doping enhance the adsorption of*CO intermediate and increase the coverage of*CO on the catalyst surface,which facilitates the*CO dimerization and boosts the ethanol formation.