Fabrication Of Silver-based Bimetal And Tin Oxide Nanocomposite As Electrocatalysts For CO₂ Reduction

In the past hundred years,with the increase of the use of fossil fuels,carbon dioxide emission into atmosphere has increased dramatically,leading to the greenhouse effect and a series of environmental problems.Converting carbon dioxide into chemical fuel can not only solve the current problems of carbon dioxide over emissions but also ease the energy crisis around the world.Thus,finding efficient methods to convert carbon dioxide has become a hot international research area currently.Many efforts have been made to develop technologies to convert carbon dioxide into useful products.Among them,the electrochemical conversion of carbon dioxide is widely investigated because clean and renewable electrical energy can be used as an energy source and the reaction process can be controlled at normal temperature and pressure.There are many products evolved from carbon dioxide electroreduction,among which C1 products CO and HCOOH have become two of the most popular target products mainly because CO is an important raw material for organic chemicals,and HCOOH is easy to be stored and has high energy density.Owing to its excellent catalytic activity,Ag is widely used as an electrocatalyst for CO2 conversion to CO.At the same time,tin oxide has become a focus of electrocatalyst for CO2 reduction to HCOOH due to its advantages of good physical and chemical stability and low cost.However,Ag as a precious metal is difficult to be applied in large-scale due to its high cost,and the catalytic activity of tin oxide needs to be improved for future application.In order to address the above issues,this thesis focuses on reducing the cost of silver catalysts while improving catalyst activity through the formation of bimetal(Ag-non precious metal),and enhancing the catalytic activity of tin oxide by nano-engineering method.The main research content in the thesis includes the following two parts:(1)Efficient electrochemical reduction of CO2 on AgX(X=Cu,Zn,Ni)bimetallic catalyst by one-step replacementThe silver-based bimetallic catalyst is formed by a one-step ion exchange method.Compared with pure silver,the bimetallic catalysts have low cost and enhanced catalytic activity.The Faraday efficiency of electrocatalytic reduction of CO2 to CO under the optimal bias voltage of copper-silver bimetal is 82%,which is 1.5 times of that of pure silver;the current density of the copper-silver bimetal under the optimal bias is 5.1 6 times of that of pure copper.XPS analysis showed that there was electron transfer between the copper-silver bimetal and the electronic structure of silver changed.DFT calculations showed that the copper-silver bimetal changed the binding energy of the intermediate product*COOH and reduced the highest free energy barrier of the intermediate reaction compared with pure silver,increasing the catalytic activity.The Faraday efficiency of CO2 reduction to CO over the zinc-silver bimetallic catalyst is 74%,which is 1.37 times of that of pure silver.The CO Faraday efficiency over nickel-silver bimetallic catalyst is 33%.It is found that there are two reasons for the catalytic activity difference of silver-based bimetals with different components.One is the morphology difference for the different bimetals,resulting in different electrochemical active surface areas.The other is the synergy and electronic effect difference for different silver-based bimetallic catalysts.For zinc-silver bimetallic catalyst,the addition of zinc may inhibit hydrogen evolution reaction and thus enhance the activity of CO2 reduction reaction.For nickel-silver bimetallic catalyst,the base metal nickel facilitates the competitive hydrogen evolution reaction and reduces the catalytic activity of CO2 reduction.The performance enhancement of the copper-silver bimetal is related to the electronic effect between the bimetals.The electronic structure of silver in the copper-silver bimetal has changed,which changes the binding energy of the intermediate product and thus improves the CO2 reduction activity.(2)Controllable synthesis of tin oxide nanorods for electrocatalytic reduction of carbon dioxideUsing hydrothermal method,flower-shaped tin oxide nanorods were grown on copper substrate for electrocatalytic reduction of carbon dioxide.It is found that the concentration of reaction precursor,reaction time and temperature all have an effect on the morphology and size of the prepared tin oxide nanorods.Through a series of characterization and CO2 catalytic reduction performance tests,the best sample SnO2-2 was obtained,with a formate Faraday efficiency of 62%at-1.1 V.By CV test for the different samples,it is shown that the change of the size of the tin oxide nanorods led to the change of the electrochemical active surface area and SnO2-2 had the largest electrochemical active surface area.