Preparation And Application Of Isolated Metal Atom Catalysts For Electrocatalytic Carbon Dioxide Reduction

The excessive use of fossil fuels has led to the continuous increase of the global atmospheric CO2 concentration.For this reason,the human society is facing severe energy crises and environmental challenges.As an effective CO2 utilization technology,electrocatalytic CO2 reduction can help to lower the CO2 concentration in the atmosphere and alleviate the energy crisis,and is thus of great significance for the realization of carbon peaking and carbon neutrality goals.However,the direct CO2 reduction on an electrode often suffers from high reaction overpotential and low product selectivity,due to the thermodynamic stability of CO2 molecule and the complex reaction pathway involving multi-proton coupled electron transfer.Therefore,to promote the development of CO2 utilization technology,it is critical to design and develop efficient electrocatalysts to lower the overpotential,activate CO2 molecule and convert it to specific products with high selectivity.Among various electrocatalysts,metal complex molecular catalysts and metal single-atom catalysts containing isolated metal sites have exhibited great potential for electrocatalytic CO2 reduction due to their high reactivity and maximum metal utilization efficiency.In this context,the thesis is intended to design and construct isolated metal atom catalysts for efficient electrocatalytic CO2 reduction.The main research contents and conclusions are as follows:1.Isolated Co-N4 sites have been successfully constructed on a carbon-based support for electrocatalytic CO2 reduction by loading cobalt phthalocyanine(CoPc)molecules on the surface of mesoporous carbon(MC).The catalytic activity can be regulated by adjusting the mass ratio of CoPc and MC during the synthesis process.When the mass ratio is 0.01,CoPc/MC-0.01 exhibits better intrinsic activity,with the turnover frequency for CO production(TOFco)reaching 16.1 s-1 at-1.17 V vs.reversible hydrogen electrode.In addition,the concept of solvent recycling is proposed for the catalyst synthesis,which can minimize the consumption of organic solvent and maximize the recycling of CoPc molecules,and is thus beneficial to the large-scale synthesis of such catalyst.The CoPc/MC-0.01 prepared in this way exhibits good stability(continuous and stable operation for 12 h)and applicability for large current density(250 tA cm-2),which demonstrates its great potential in large-scale production and industrial applications.Furthermore,the SCoPc/MC catalyst with comparable performance to CoPc/MC0.01 is prepared in aqueous solution using the sulfonated cobalt phthalocyanine/water system,providing an alternative for the synthesis of supported molecular catalysts.This synthesis method is expected to be coupled with the treatment of sulfonated phthalocyanine wastewater,which has great significance for green production.2.In order to further improve the surface utilization of porous carbon supports and explore the effect of the distribution of isolated metal atom sites on the performance of electrocatalytic CO2 reduction,a hollow carbon sphere(HIE/Ni-N-C)with isolated Ni-Nx active sites anchored on both internal and external surface has been prepared based on the combined top-down and bottom-up strategies.The catalyst exhibits good activity in the electrocatalytic CO2 reduction,and its current density(|jCO|)and selectivity(FECO)for CO production are much higher than those of solid sphere structures or isolated Ni-Nx sites supported on only internal or external surface.The results of electrolysis experiments and electrochemical impedance spectra demonstrate that the presence of hollow structure as well as active Ni sites on both external and internal surfaces can effectively promote the mass transport and charge transfer in the process of CO2 reduction,thereby greatly improving the HIE/Ni-N-C activity.Furthermore,we demonstrate that a secondary pyrolysis of the material can improve its activity and stability for CO2 reduction.Such treatment can increase the degree of graphitization and reaction kinetics of the material during electrocatalysis.The final material exhibits a large current density(150 mA cm-2)and a high Faradaic efficiency for CO production(94%)in a flow cell,showing good potential for practical applications.This thesis provides new ideas for the controllable preparation of emerging isolated metal atom catalysts.It deals with the structure-performance relationship of the catalysts,the effect of the parameters of the reaction system on the electrocatalytic CO2 reduction,and the application potential of these catalysts under industrially relevant conditions.The results presented in this thesis may have great significance for the development and application of electrocatalytic CO2 reduction technology.