Preparation Of Zinc Oxide Based Nanocomposites For Electrocatalytic Carbon Dioxide Reduction Reaction

With the rapid development of industrialization,the emission of carbon dioxide is consistently expanding,which has brought many negative impacts on environment.Electrocatalytic carbon dioxide reduction to value-added chemicals and fuels powered by renewable energy sources,can significantly promote carbon dioxide resource utilization and emission reduction,which is a crucial measure to realize the aim of"carbon neutrality".There are a variety of products of electrocatalytic carbon dioxide reduction.With the increase of carbon number of products,the number of transferred electrons required to obtain monomolecular products increases,which leads to the increased energy consumption.Thus,from the perspective of technology and economy,converting carbon dioxide into formic acid is considered to be one of the most promising ways to realize industrialization.However,carbon dioxide reduction reaction（CO₂RR）is a multi-path competitive process with multi-electron coupled proton transfer.Meanwhile,the potential of CO₂RR is much close to that of hydrogen evolution reaction（HER）,which leads to the strong competition between CO₂RR and HER.The above two aspects result in poor selectivity and low product yield of CO₂RR.Therefore,it is significant but challenging to design and develop high selectivity and low cost electrocatalysts to improve the selectivity and yield of carbon dioxide reduction products.As well known,the zinc element is non-toxic,environmentally friendly and inexpensive.Additionally,the ZnO shows some catalytic activity in the electrocatalytic reduction of carbon dioxide to carbon monoxide.In this dissertation,we developed SnSA/ZnO,ZnSnO₃/ZnO heterojunction and Zn₂SnO₄/ZnO composite catalysts for efficiently catalyzing CO₂ electroreduction.The potential catalytic mechanism and performance enhancement mechanism of carbon dioxide reduction were preliminarily investigated using the physical characterization and theoretical calculations.The structure-activity relationship between catalyst microstructure and carbon dioxide reduction performance was established.The main research achievements are as follows:（1）Preparation of single atom Sn anchored on ZnO nanosheets（SnSA/ZnO）catalyst and its application in CO₂ electroreduction to formic acid.The SnSA/ZnO catalyst was successfully synthesized by one-step hydrothermal method,using hexadecyl trimethyl ammonium bromide（CTAB）as template and Zn（CH₃COO）₂ and Sn Cl₂ as raw materials.Due to the Sn single atom introduced to the lattice of ZnO,the tensile strain is generated on ZnO surface.Moreover,the selectivity of ZnO in formic acid is greatly improved.At a wide voltage window（～1.0V）,the catalytic activity and selectivity of the SnSA/ZnO for carbon dioxide reduction to formic acid is better than that of commercial ZnO.The mass activity of the SnSA/ZnO normalized by Sn mass is about 205 times higher than that of commercial Sn at-1.7 V vs RHE.The SnSA/ZnO exhibits excellent stability with Faraday efficiency（FE）of formic acid maintaining above 80%after continuous stability test at-1.2 V vs RHE.And activity and stability can be maintained for 11 h.Theoretical calculations and experimental studies showed that the exposed Sn and O dual active sites enhance the capture and activation of carbon dioxide,while surface strain promote the adsorption of*HCOO intermediates,which synergistically improved the activity and selectivity of electrocatalytic carbon dioxide reduction to formic acid.This work constructs double active sites by regulating the electronic structure of inactive metal oxides using single atom doping,which provides a new idea for the design of high efficiency electrocatalyst for catalyzing carbon dioxide conversion to formic acid.（2）Construction of ZnSnO₃/ZnO heterojunction and its application in electrocatalytic carbon dioxide reduction to formic acid.The ZnSnO₃/ZnO heterojunction nanosheets was synthesized by a one-step hydrothermal method using CTAB as template agent.The physical characterization results show that there are oxygen vacancies on the catalyst surface and electron-deficient ZnO surfaces are formed due to electrons transfer to the interface.The ZnSnO₃/ZnO catalyst showed high FE of electrocatalytic reduction of carbon dioxide to formic acid and good stability.The FE of C₁（CO and HCOOH）product remained above 98%in the whole applied potential window.The total current density and FE_HCOOH of ZnSnO₃/ZnO showed no obvious change after continuous testing for 13 h at potential of-0.9 V vs RHE.The theoretical calculation combined with spectroscopy study show that the electron-deficient ZnO surface induced by the charge transfer in heterostructure is the active component of carbon dioxide reduction to formic acid.The electronic structure of ZnO is further regulated by the oxygen vacancy and synergistically promotes the formation of*HCOO intermediates.This work provides a reference for enhancing the selectivity of catalysts through the construction of heterointerface structure.（3）Synthesis of Zn₂SnO₄/ZnO composite catalyst for boosting electrocatalytic carbon dioxide reduction reaction.The Zn₂SnO₄/ZnO composite catalyst was synthesized by a simple hydrothermal reaction with precisely controlling the ratio of Zn and Sn sources.The physical characterization results show that the cubic Zn₂SnO₄ was supported on the ZnO nanosheet.The Zn₂SnO₄/ZnO composite catalyst exhibited a maximum FE of 98%for C₁（CO and HCOOH）products.At the potential of-1.0 V vs RHE,the Zn₂SnO₄/ZnO achieved a maximum FE_HCOOH of 90%.The advanced characterizations of XPS and XANES combined with electrochemical test results show that the interfacial charge transfer and oxygen vacancy synergically optimized the electronic structure of Zn₂SnO₄/ZnO composite catalyst,which facilitated the formation of*HCOO intermediates.Meanwhile,the formation of composite materials increased the electrochemical active area.The above two aspects synergically improved the activity and selectivity of electrocatalytic carbon dioxide reduction to formic acid.This work provides the practical support for improving the selectivity of carbon dioxide reduction reaction of electrocatalyst by the synergistic effect of the nanocomposites.