Study On Electroreduction Of Carbon Dioxide By Three-dimensional Copper-tin Bimetallic Materials

Renewable energy driven CO2 electroreduction technology provides a potential solution for stabilizing CO2 concentration in the atmosphere,realizing the conversion and storage of renewable energy and the synthesis of high added-value fuel molecules.In the electroreduction of CO2 system,a suitable electrocatalyst is of great significance for reducing the overpotential of the CO2 conversion reaction and improving the selectivity and generation rate of the product.It is one of key factors to save cost and improve the efficiency of the system.Among various metal electrocatalysts,Cu has attracted much attention because of its high value-added products such as CO,small-molecule organic acids,alcohols,hydrocarbons,and other products.However,there are also many problems,such as low overpotential of hydrogen evolution,diverse products difficult to separate,poor stability and easy deactivation.The non-toxic,cheap,and high-hydrogen-overpotential Sn modified on the surface of the Cu catalyst can greatly inhibit the hydrogen evolution side reaction and selective produce CO.At present,these Cu-Sn bimetallic materials exist either in the form of powder catalysts or based on electrodeposited Sn on plate copper foils.The limited electrocatalytic reaction interface limits the increase of current density and CO generation rate.It is one of the major obstacles to achieving industrialization.In this paper,three-dimensional copper-tin bimetal(aSaCN)was obtained as follows:in-situ electrodepositon of copper on nickel foam network skeleton,the copper nanowires obtained by a simple annealing and electroreduction treatment;Then tin was simply attached by electroless deposition;Finally,the redistribution of tin particles and the average diameter of tin were achieved by near-tin-melting point annealing followed with electroreduction reteatment.The morphology,composition and surface structure analysis results showed that aSaCN has a secondary structure of "three-dimensional network skeleton+in situ grown hybrid nanowires",in which the hybrid nanowire consists of a copper nanowire core,a Cu-Sn alloy intermediate layer of and a uniform tin nanoparticle thin shell layer.At a potential of-0.5 V vs.RHE,aSaCN achieved a 90%CO current efficiency,a total current density of 12.8 mA cm-2(2 to 25 times of the literature data),and a CO generation rate of 2.162 mol h-1 m-2(8?40 times of the literature data).The effects of different tin deposition time and annealing temperature on the performance of electroreduction CO2 by aSaCN were investigated.The optimum preparation conditions as follows:tin deposition time is 10 s,annealing temperature is 200 ?.Performance testing of the optimized material shows that aSaCN can selectively produce CO in a wide potential,which marked the potential advantage of resisting potential fluctuation.After repeated use for eight times at same potential,the performance of CO2 electroreduction maintained at a high level,besides,the microstructure of the material was not changed obviously after electrolysis test,which indicates that aSaCN has good activity and stability.By comparing the characterization results and performances in CO2 electroreduction by the three materials in the preparation process,the reasons for the differences in product distribution were explored.The results showed that the secondary structure has a significant effect on the current density and is almost free relation to nanowire surface composition.Distribution status of tin on the copper surface and particle size of tin have a great influence on products distribution.Comparing the performance in CO2 electroreduction process by several kinds of copper-tin materials,the explanations for the improvement of aSaCN performance was explored.The results showed that nickel as the support has no contribution to the activity of CO2 electroreduction,but nickel foam as a three-dimensional mesh skeleton template provides support for the construction of secondary structure with large specific surface area.The results of double-layer capacitance measurement showed that the higher current density caused by the secondary structure of "nanowires on three dimensional reticular skeleton".Improvement of aSaCN performance was evident in the two annealing treatments.In this thesis,three-dimensional composite nanowire arrays were prepared by inexpensive and resource-rich non-precious metals Cu and Sn through a simple and easy-to-scale method for the electroreduction of CO2,achieving high current density and high CO selectivity and generation rate at lower overpotentials.A new method for improving the dispersion state of low melting point metals by near-melting point annealing treatment is proposed.