Study On Preparation Of Copper And Tin-based Nanomaterials And Their Performance For Electrocatalytic Reduction Of CO₂

Electrochemical reduction of CO₂ into high value-added chemicals is one of the effective technologies to solve many problems caused by the emission of CO₂.Electrocatalytic reduction of CO₂ could be driven by electricity generated from intermittent renewable energy sources,such as solar and wind.This can not only realize the efficient storage of clean energy,but also accelerate the economic transition from fossil fuel to renewable energy sources.However,CO₂ possesses the high thermodynamic stability and slow reaction kinetics,which make CO₂ reduction reaction（CO₂RR）need to overcome a large energy barrier.Meanwhile,hydrogen evolution reaction（HER）with fast dynamics usually occurs at the same applied potential during CO₂RR,further inhibiting the process of CO₂RR.Therefore,it is crucial to explore a kind of cheap,efficient and stable electrocatalyst.Many catalysts with nanostructure have been reported previously,but there are many problems still exist,such as high overpotential,slow reaction and poor stability,which make it unable to meet the needs of industrialization.In this context,it is significant for development strategy to unceasingly develop electrocatalysts with low overpotential,high current density and high selectivity for single product,and further expand the basic research in this field.The research of this paper focus on using the strategy of electronic structure regulation,grain boundry and lattice defects construction,etc.,with the regulation of Cu,Sn-based nanocomposite,to prepare a series of high-performance catalysts for CO₂RR.Furthermore,this research investigates the factors influencing the activity of each catalyst,analyzes the catalytic active sites and puts forward concrete solutions to some problems existing in catalytic materials.The specific research content of this paper is as follows:1.Sn-MOF was prepared by standing at room temperature,and a series of carbon-coated Sn-based nanomaterials were prepared by further heat treatment at different temperatures.Further,the carbon content in the composites was regulated by thermal oxidation.Among them,the ultrathin carbon coated Sn O₂ quantum dots（Sn O₂@u-C）have the best catalytic performance for CO₂RR,which the Faradaic efficiency and energy efficiency of formate around a wide range of applied potential are more than 80%and 50%,and the highest of them are 95.7%and 57.2%,respectively.It can effectively catalyze CO₂ into formate with high faradaic efficiency of 89.0%for 10 h under the potential of-1.17 V（vs.RHE）.This prominent electrochemical performance is mainly attributed to electronic structure of the ultra-thin carbon layer regulated by Sn O₂ quantum dots（Sn O₂QDs）,which facilitates the adsorption and conversion of CO₂.Moreover,the small size of Sn O₂@u-C is conducive to exposing more catalytic active area,while the charge transfer from catalyst to reactant molecules can be promoted by carbon layer,which further improve the reaction rate and protect the Sn O₂ QDs from reduction.2.Zn-doped Sn O₂,namely Zn-Sn O₂,with abundant lattice defects and oxygen vacancies,was prepared by the heat treatment of as-prepared Zn-Sn precursor.Under moderate reduction potential,Zn²⁺in Zn-Sn O₂ was further reduced into a kind of Sn O₂（Sn O₂-d）with abundant cation vacancy and oxygen vacancy,which showed excellent electrochemical performance for CO₂RR.Especially,Sn O₂-d electrode shows a general selectivity over 90%（the highest is 95.7%）for C1 product（CO and formate）under a range of applied potential,and can maintain the selectivity for single product of formate at 72.3%after 10 h continuous reaction.Besides,the products from electrolysis can realize the comprehensive utilization with formate and syngas to achieve nearly 100%product utilization efficiency.Such excellent electrocatalytic performance of Sn O₂-d could be mainly contributed to the?doping and de-doping‘of Zn atoms,which fabricates the lattice distortion and abundant vacancy defects on the surface of Sn O₂,providing a large number of actives for CO₂RR.Contrast experiments further confirm that the introduction of the cationic vacancies is beneficial for improving the Faradaic efficiency of CO at low overpotential.In this work,the strategy of introducing unstable heteroatoms to create vacancy defects is proposed,which provides a new idea for the preparation of high-efficiency electrocatalysts.3.Sn O₂/NC was prepared by the Sn O₂ QDs loading on the hollow porous carbon framework（NC）,which was prepared from the heat treatment of ZIF-8 synthesized by solvothermal method,was used as a carrier to load.Herein,the Sn O₂ QDs with the particle size of only about 5 nm,possessing abundant grain boundaries and edge sites,was fabricated by adjusting the distribution of Sn O₂ nanoparticles through changing the load of Sn O₂,which makes Sn O₂/NC exhibit excellent electrocatalytic performance.Under the potential of-1.13 V（vs.RHE）,Sn O₂/NC shows the Faradaic efficiency of 87.6%for generating formate from CO₂RR,with the partial current density of formate reaching at 16.1 m A cm^-2.Besides,it can efficiently keep stable for20 h under the potential of-1.03 V（vs.RHE）.Such excellent electrochemical performance could be mainly attributed to the strong interaction between the N-doped hollow porous carbon skeleton and Sn O₂ QDs,which makes the electron migrate from NC to Sn O₂ surface,further promoting the adsorption and activation of CO₂.On the other hand,the abundant grain boundaries of the Sn O₂ QDs could provide active centers for CO₂ conversion,while rich pores structure is conducive to electrolyte infiltration,further promoting mass transfer process.In this work,the strategy of electronic structure regulation and active site construction are simultaneously adopted to improve the catalytic performance of Sn O₂,which provided a new approach for the effective design of electrocatalysts for CO₂RR.4.Cu S/N,S-r GO,which is compromised of the small size Cu S nanoparticles loaded on N and S co-doped graphene oxide nanosheets,was prepared by one-step solvothermal methods.Cu S/N,S-r GO exhibits high current density,low reaction overpotential and the selectivity for formate reaching at 82.0%under the potential of-0.67 V（vs.RHE）.Moreover,it can be operated efficiently and stably at-0.83 V（vs.RHE）for 20 h.Such excellent electrochemical performance of Cu S could be mainly ascribed to the formation of S-doped Cu₂O through in situ transformation of crystal structure during the process of cathodic activation.In addition,the strong interaction between N,S co-doped graphene oxide and small size S-Cu₂O is conducive to the adsorption and transformation of CO₂.