Fabrication Of Cu Foam Based Nanocomposites As Electrocatalysts For Selective CO₂ Reduction

Since the Industrial Revolution,the amount of carbon dioxide(CO2)emissions from the consumption of fossil fuels has already exceeded their natural circulation capacity,leading to greenhouse effect and serious environmental problems.In order to reduce the concentration of carbon dioxide in atmosphere and alleviate the energy crisi.The reduction of carbon dioxide into high energy density carbon-based compounds by using renewable energy is one of the most promising ways.The electrochemical reduction of carbon dioxide(e-CO2RR)has the advantages of controllable reaction process,compact reaction system,and easy scale,etc.,which have attracted wide attention.Previous studies have shown that copper and copper-based catalysts have the ability to reduce CO2 to hydrocarbon fuels and chemicals,but there are many reduction products,and the selectivity is poor.Among them,ethylene(C2H4),as a two-carbon reduction product,has become the focus because of its energy density and commercial value,which is much higher than that of methane(CH4)and other one-carbon reduction products.In order to improve the activity of selectivity of CO2 reduction to C2H4,in this thesis,it has been attempted to increase the number of exposed active sites for copper-based catalysts and improve their intrinsic activity by the modification methods of nanostructurization,surface oxidation and selenylation.The detailed work is divided into the following two aspects.(1)In order to increase the number of exposed active sites,improve the intrinsic activity of the copper-based catalysts,and enhance the selectivity of the CO2 electrocatalytic reduction to the C2H4,through the combination of hydrogen bubble dynamic template method and thermal reduction treatment,the catalyst copper oxide nanowires(CuO NWs)have been deposited onto a self-made three-dimensional porous copper foam substrate for electrochemical CO2 reduction with high efficiency and selectivity.Compared with copper foam substrate,this new electrode shows better electrocatalytic activity,higher ethylene selectivity and lower initial potential(-1.4 V vs.RHE)than the foamed copper electrode.The Faraday efficiency of ethylene increased to 25.01%at-1.4 V(vs.RHE),and the current density was-24 mA·cm-2.The performance improvement of CuO NWs compared to Cu foam may be attributed to two reasons.First,the high surface roughness of the CuO NWs sample resulted in the increase of specific surface area,leading to more exposed active sites.Second,the surface of the CuO NWs sample has a relatively rich oxygen content after redox,which activates the catalytic activity and improves the selectivity.Second,the rich oxygen content on the surface of the CuO NWs sample activates the catalyst activity and improves the selectivity.Oxidized metals have rich grain boundaries on the surface of the catalyst.Under CO2RR conditions,copper oxide will be reduced to copper by electrolysis.This process activates the copper catalyst and forms specific low coordination active sites on the surface of the catalyst.The reaction site of the competitive hydrogen evolution reaction(HER)is blocked,which is positive for the adsorption and retention of the*CO surface group required for C-C coupling,thus promoting the two-carbon reduction product(ethylene)conversion,which enhances selectivity.(2)In order to improve the inherent catalytic activity of the catalyst,a chemical vapor deposition method was used to prepare Cu2O NWs with selenium modification on the surface of the copper foam substrate for CO2 electrocatalytic reduction.That is,on the copper foam substrate under fixed deposition conditions(deposition time),a Cu(OH)2 nanowire precursor was formed by electrochemical oxidation,and then chemical vapor deposition method was used for thermal reduction.Then,Se/Cu2O NWs composite on a three-dimensional porous substrate was prepared by selenium modification over Cu2O nanowires.The results show that Cu2O NWs modified with Se element enhances the conductivity of pure phase Cu2O NWs,thereby increasing the activity of CO2 electrocatalytic reduction.In addition,the composite catalyst also showed excellent stability.