Synthesis Of Copper-based MOFs Materials And Its Derived Cu-N-C For Electroreduction Of CO₂

Electrochemical reduction of CO2(ECR-CO2)is one of the most useful methods owing to its higher energy efficiency and milder reaction conditions.However,it requires a large amount of energy to activate CO2 to a CO2 radical anion with ESHE=-1.90 V,which is the key step in the whole reaction process because that CO2 is a stable molecule in thermodynamics.At the same time,the energy for the hydrogen evolution reaction(ESHE=-0.42 V)is much less than the energy for activating CO2,so that the hydrogen evolution reaction(HER)is accompanied by the entire process,resulting in the low selectivity to the desired product.Currently,copper(Cu),an earth-abundant metal,is the only electrocatalyst known to reduce CO2 into value-added products in aqueous electrolytes during the ECR-CO2.However,copper-based catalysts also have obvious disadvantages,such as high overpotential,poor reactivity and stability,and low selectivity to specific products.Therefore,it is necessary to develop effective copper-based electrocatalysts.Metal organic frameworks(MOFs)with controllable porous structure and large specific surface area are considered to be excellent catalyst precursors for electrochemical reduction of carbon dioxide.Copper-based MOF,with its special ability to produce hydrocarbons,carboxylic acids and alcohols,is currently the most promising MOF material.Thermal decomposition of MOF in an inert atmosphere can provide many active sites for ECR-CO2.In addition,some heteroatoms such as B,N,P,and S added into the carbon material has been shown to effectively change the electronic properties,material structure,and chemical state of the catalyst.These heteroatoms combine with carbon atoms as new active sites to enhance the catalytic activity of carbon dioxide electroreduction.However,at present,nitrogen-doped copper-base MOF materials are mainly HKUST-1 modified with additional nitrogen-containing aromatic heterocyclic ring,few nitrogen-bearing organics are directly used as ligands to introduce nitrogen source in situ.Herein,we designed and synthesized a new copper based MOF material(Cu-NBDC)with 2-amino-terephthalic acid containing amino group as the organic ligand and Cu(OH)2 nanowires as the copper source.The main work is as follows:(1)one-dimensional Cu(OH)2 NA/CF was pepared by electrochemical anodic oxidation of copper foam with good conductivity.Copper hydroxide nanowires were used as copper source and growth template,and 2-amino-terephthalic acid as ligand to explore the synthesis of new MOF(Cu-NBDC).We characterized it with SEM,XRD,FTIR and BET.We studied how the concentration,reaction time and solvent influence the growth of Cu-NBDC.By regulating the solvent,we finally obtained Cu-NBDC with a book-shaped morphology of 170 m2/g specific surface area synthesized with DMF(EtOH)as the reaction solvent and a spherical flower morphology of 1210 m2/g specific surface area synthesized with DMF and H2O as the mixed double solvent.H2O not only plays the role of dissolving reactants,but also participates in the self-assembly process of Cu-NBDC,which can change the morphology of MOF and effectively increase its specific surface area.(2)We selected the spherical flower morphology of Cu-NBDC with high specific surface area as the precursor to prepare the catalyst.The composite catalyst of Cu2O/Cu@NC that Cu2O/Cu nanoparticles anchored in the nitrogen-doped porous carbon framework was prepared by annealing Cu-NBDC at different temperatures.And it was characterized with SEM?XRD?TEM?TGA?Raman and XPS.XPS analysis proved that Cu2O/Cu@NC material has a structure of Cu-N.The ECR-CO2 of catalysts prepared at different temperatures was tested in a CO2-saturated 0.1 M KHCO3 electrolyte.Compared to Cu2O/Cu@C,Cu2O/Cu@NC could change the kind of reduction product,inhibiting HER effectively and improve the activity of the ECR-CO2 and the selectivity of the catalyst to the product.Under the annealing treatment at 400?,the CO2 catalytic efficiency of Cu2O/Cu@NC was higher than 86%(-1.4?-1.6 V vs.RHE),among which 20.4%of FEC2H4(-1.4 V vs.RHE)and 23.9%of FECH4(-1.6 V vs.RHE),while Cu2O/Cu@C obtained only 2.33%of FECH4(-1.6 V vs.RHE),and no C2H4 was found.With the increase of annealing temperature,the selectivity of formate gradually increased,and finally the Faradaic efficiency reached a maximum of 67.9%at 800? at-1.2 V vs.RHE.We believe that these significant catalytic differences were attributed to the fact that Cu-N is conducive to the stable adsorption of*CH2 intermediate in the ECR-CO2,thus inhibiting the evolution of H2.These results indicate that the path of Cu-based MOF derived catalysts can be effectively changed and their catalytic performance can be improved by regulating the doping of nitrogen.