Synthesis And Studies Of Two Dimensional Transition Metal-based Materials For Electrocatalytic Carbon Dioxide Reduction

In recent years,electrocatalytic reduction technology can transform the excess carbon dioxide in the atmosphere into chemicals and fuels with high added value,which can effectively improve the environmental problems and alleviate the energy crisis.However,there are still many fundamental challenges in electrochemical CO₂reduction:the commonly used electrocatalysts show high over-potential and slow reaction rate;it is still difficult to selectively control the reaction pathways that produce carbon-based products;competitive reactions with the formation of H₂are common in aqueous media.At present,how to prepare high efficiency catalyst with high Faraday efficiency,low overpotential,good selectivity to target products and high stability has become the key of research,and is also the research hotspot in the field of electrochemical CO₂reduction.Therefore,in this academic dissertation,for the purpose of synthesizing low-cost catalysts with high catalytic performance,two-dimensional metallic organic framework（MOF）nanosheets were successfully prepared by different methods and used as electrocatalysts.The electrocatalytic performance of these catalysts in the process of electrochemical CO₂reduction was systematically investigated.The research work is divided into the following three parts:1.Manganese,as a transition metal element,exists widely in nature and is largely used in electrocatalytic reactions,but its activity is low.Therefore,we attempted to prepare a two-dimensional MOF nanosheet by using surfactant assisted synthesis method,and synthesized nanosheets of different thickness by means of ultrasonic stripping,and studied the catalytic effect of this kind of catalyst in electrochemical reduction of CO₂.The results showed that the electrochemical CO₂reduction activity of the nanosheets increased with the increase of ultrasonic time,and the nanosheets had the highest electrochemical activity when the ultrasonic time was 4 h,and the maximum Faraday efficiency of formic acid reached 58.0%.2.In order to overcome the shortcomings（such as poor electrical conductivity,less surface active sites,low catalytic activity）of the bulk MOF,we take advantage of the characteristics of two-dimensional nanosheets,which are ultra-thin,strong charge transfer ability and many surface active sites,through a bottom-up approach to prepare a new type of double metal（cobalt/zinc）imidazole zeolite framework（ZIF）.After the high temperature heat treatment,its hexagonal shape can also maintain.The catalyst is studied in detail in the electric catalytic reduction of CO₂.The results show that,as a kind of bifunctional catalysts,wrapped in nitrogen doping carbon nanotubes hexagon piece of Co nanoparticles have excellent catalytic performance.In 0.5 M KHCO₃electrolyte,when the potential was-1.40 V（vs.Ag/AgCl）,the maximum Faraday efficiency of formic acid reached 68.0%.3.As a cheap metal,tin has a high performance of electrocatalytic reduction of CO₂to formate.In order to further improve the selectivity of formate,we successfully prepared two-dimensional tin-based MOF catalyst by ion exchange between Zn node and Sn ion adsorbed in MOF cavity.The catalyst has a good reduction effect on carbon dioxide,the Faraday efficiency of formic acid could reach more than 70.0%when the potential ranged from-1.60 V to-1.70 V（vs.Ag/AgCl）.