Preparation Of Zinc-silver Alloy And Copper-indium Oxide Composites For Electrocatalytic Reduction Of CO₂

With the rapid development of our society,much more CO2 than before has been produced,which is disturbing carbon cycle on the earth and leads to many environmental problems such as greenhouse effect and so on.In addition,due to the development of industrilization,the continuous exploitation of fossil fuels has also brought about energy crisis.As CO2 is one kind of carbon source,it can be converted into some hydrocarbons.So,if the excess CO2 in atmosphere can be converted to fuel and organic compounds,not only the environmental problems but also part of the energy issues can be sovled.Among the methods for CO2 reduction,electrochemical method has received widespread attention because of its mild reaction condition,controllable process,and environmental friendly.There are many products for CO2 electrocatalytic reduction,among which carbon monoxide(CO)is not only one of the main raw materials for syngas,but also an important raw material for synthesis of organic compounds.Therefore,the conversion of CO2 to CO has attracted much attention,which is conducive to the study of the reaction mechanism of electrocatalytic reduction of CO2.Among many electrocatalysts for CO2 reduction to CO,zinc(Zn)-based catalysts have become one of the important catalysts due to their ability to reduce CO2 to CO.In addition,copper(Cu)-based catalysts have attracted much attention due to their abundance on our planet,unique electronic structure,and highly CO2 reduction ability.However,for Zn-based metal,its catalytic activity is not high,which needs to be improved,and its starting potential is still too high,which is not suitable for industrial applications.Nevertheless,the products of Cu-based catalysts are mostly hydrocarbons,and it is hard to produce CO.In order to solve the above problems,herein,the alloying of metal Ag with Zn is introduced to improve the catalytic activity,and copper oxide is composited with indium oxide to facilitate the formation of CO product.The specific study is divided into two parts as shown below.(1)One-step electrodeposition of ZnAg alloy for efficient electrocatalytic reduction of CO2To improve the catalytic activity of Zn-based metals for the electrocatalytic reduction of CO2 to CO,in this thesis,ZnAg alloy nanoparticle electrodes were deposited on Zn substrates by a simple one-step multipotential deposition method.For the control of pure Zn nanoparticle sample,the CO Faraday efficiency at-1.2 V relative to the reversible hydrogen electrode(vs.RHE)was 40%.For the control of pure Ag nanoparticle sample,the CO Faraday efficiency at-1.3 V(vs.RHE)was 52%.However,for the ZnAg alloy catalyst,its CO Faraday efficiency reached 71.2%at the optimum bias voltage of-1.1 V(vs.RHE),and its current density at the optimum bias voltage was 1.5 and 1.22 times higher than that of the control Zn and Ag sample respectively.In addition,the possible mechanism based on detailed characterization test for the increased catalytic activity of the ZnAg alloy material compared to the control samples is shown as follow.First,the alloy sample has a larger electrochemically active surface area(ECSA),which provides more active sites for the catalytic reaction.Secondly,it is because the electron effect on the surface of the ZnAg alloy catalyst has changed,and more surface electrons move from Zn to Ag,which enriches electrons on Ag and thus promotes the activation of CO2.(2)Preparation of CuIn oxide composites for electrocatalytic reduction of CO2In order to change the selectivity of Cu-based catalysts for the electrocatalytic reduction of CO2 to CO,we prepared Ox-CuIn for the electrocatalytic reduction of CO2 to CO by a three-step synthesis method.It was found based on characterizations that the molar ratio of Cu and In oxides affects the ability for electrocatalytically reduce CO2,and the best sample was Ox-CuIn50 with a CO Faraday efficiency of 78.3%at-0.8 V(vs.RHE).Compared to the control CuO and In2O3 samples,the Ox-CuIn showed a significant improvement in both CO2 catalytic performance and current density.Combined with the electrochemical characterizations,the electrocatalytic mechanism of CuIn oxide composites is twofold:first,the change in the surface morphology of CuIn oxide composites thus leading to an increase in the electrochemically active specific surface area.The second is that it is the metallic state that plays a main role in CO2RR,and the change of the active site due to the coating of indium metal leads to high selectivity of CO.