Preparation Of Copper-based Organic Composite And Their Applications In Electrocatalytic Reduction Of CO₂

Carbon dioxide electrochemical reduction（CO₂RR）powered by renewable energy is promising to realize the carbon-neutral energy cycle.Due to the fact that copper（Cu）has a negative adsorption energy for^*CO but a positive adsorption energy for^*H,it is commonly considered as the most efficient electrocatalyst that reduces CO₂ to various>2e^-products.Polycrystalline copper has been reported to produce more than 16 different products.Despite its unique catalytic property,it is still challenging to control the product selectivity of copper catalyzed CO₂RR.Designing Cu catalyst with clear and controllable active sites is essential to understand the structure-activity relationship and overcome this challenge.This thesis is devoted to the preparation of copper-based organic composite materials,including single atom and metal-organic complex catalysts,and their application in the direction of CO₂electrochemical reduction.A series of copper-based organic composite materials are designed by changing the coordination environment and initiating tandem effect to control the selectivity of CO₂RR products,and the relationship between the catalyst structure and the product selectivity is constructed.The DFT calculation reveal the mechanism for the structure-activity relationship of the catalyst.The designed catalyst can effectively solve the problems of low selectivity and poor stability of copper based organic catalyst.The main research contents of this paper include:（1）The selectivity of CO₂RR products can be regulated by adjusting the coordination environment of metal Cu on the copper catalyst.Three different coordination environments of Cu-N,Cu-C and Cu-Cu were obtained by different synthesis methods with molecular support PDI as ligand and metal Cu as active site.In the process of electrocatalytic CO₂RR,the three coordination environments mainly produce carbon monoxide,methane,ethylene and other carbon-containing products.Three copper based organic catalysts were characterized in detail to obtain a clear structural model.The novel metal organic complex we designed in this work can exhibit excellent stability during the electrolytic process with more than 70 h operation in a membrane electrode assembly.According to the DFT calculations results,the influence of the Cu coordination environment on the selectivity of CO₂RR products can be summarized as the following aspects:the Cu-N sites weaken the adsorption energy of^*CO intermediate,which are beneficial for CO desorption.The Cu-C and Cu-Cu sites facilitate the formation of^*OCOH and^*OCCO intermediates respectively,favoring the CH₄ and C₂H₄ pathways.This work provides a stable and simple model system for studying the influence of the type of coordination elements on the product selectivity of CO₂RR.（2）Cu-N sites and Cu clusters tandem catalyst PDI-Cu/Cu for CO₂RR to C₂H₄ was synthesized by uniformly dispersing Cu clusters on coordination polymer PDI-Cu which has atomically isolated Cu-N sites.This catalyst achieves a tandem effect between the Cu-N site and the Cu cluster.The FE of C₂H₄ in PDI-Cu/Cu is the highest at-1.9 V vs.RHE,reaching31.4%.Compared to the non-tandem catalyst PDI/Cu,the C₂H₄ yield of PDI-Cu/Cu is increased by more than 2 times.We also proved that the Cu cluster content in PDI-Cu/Cu is the optimal content for forming the tandem effect.And DFT calculations reveal that the high concentration of^*CO which is formed by Cu-N sites can decrease the free energy of^*CO dimerization step on Cu cluster surface.DFT calculations are consistent with the tandem reaction mechanism,Cu-N sites firstly reduce CO₂ into high concentration CO and then CO migrate to Cu clusters surface for further conversion into C₂H₄,decoupling complex C₂H₄ generation pathway on single active site into a two-step tandem reaction.This work offers a rational approach to design electrocatalysts for further boosting the selectivity of CO₂RR to C₂₊products via a tandem route.（3）Metal organic coordination polymer Bix-Cu was prepared by hydrothermal reaction.It is formed by self-assembly of Cu²⁺and basic N donors.Bix-Cu was first used in the study of electrocatalytic reduction of carbon dioxide.The catalyst Bix-Cu has accurate crystallization model information,which can better analyze the relationship between structure and performance during electrochemical CO₂RR process.Electrochemical tests show that the main product of CO₂ reduction catalyzed by Bix-Cu is CH₄,the FE of CH₄ gradually increases with increasing potential.At-1.9 V vs.RHE,it exhibits the highest FE of CH₄（40.0%）.Bix-Cu has high CH₄ selectivity.At-2.1 V vs.RHE,the partial current density of CH₄ is as high as 93.5m A cm^-2.XRD tests on Bix-Cu samples after 5-,10-,20-,and 30-min electrolysis showed that the Bix-Cu catalyst could maintain the structure of the Cu active center unchanged and could be stably used for the study of electrocatalytic reduction of carbon dioxide.The electrochemical in situ Fourier transform infrared spectroscopy technology confirmed that Bix-Cu had a high^*CO intermediate coverage on its surface during the electrolysis process.We infer that the reason why Bix-Cu has high CH₄ selectivity is that the Cu-N₃ center can enhance the co-adsorption of^*CO and^*H intermediates,increasing the possibility of^*CHO formation,which is the key intermediate in the CH₄ generation path,ultimately promoting the formation of CH₄.In this work,a new metal organic catalytic material was first applied to the field of electrocatalytic CO₂RR,and its performance and active sites were verified through experimental characterization.