Regulating The Microstructure Of Two-dimensional Bismuth And Copper Materials For Studying Their Electrocatalytic CO₂ Reduction

Electrocatalytic CO₂ reduction reaction（ECO₂RR）using electric energy,which is a kind of clean energy,to transform CO₂ into higher-value fuels and chemical feedstocks offers a promising strategy to achieve CO₂ resource utilization.While,as CO₂ is a linear symmetric molecule with high stability,the high reduction potential is required to activate it,resulting in slow kinetics of most electrode materials.Two-dimensional nanomaterials have large surface areas and more surface active sites exposed,which is conducive to surface diffusion of reactants.In recent years,a variety of two-dimensional nanomaterials have been used as catalysts for ECO₂RR,and microstructure regulation strategies have also proved useful for enhancing the ECO₂RR activity.However,the intrinsic relationship between the microstructure regulation and the corresponding catalytic activity enhancement and the effects of microstructure regulation on the ECO₂RR microenvironment have not been fully studied.In this paper,the target products are C₁ product formic acid（2e^-）with high added value per kilowatt-hour of electrical energy input,and C₂ product ethylene（12e^-）which is with high widely used.Metal Bi is found to electrocatalyze CO₂ reduction to formic acid due to its easy adsorption of*OCHO.Cu can electrocatalyze CO₂ reduction to multi-carbon products including ethylene because of its suitable adsorption energy towards*CO.Therefore,Bi nanosheet（Bi NSs）and Cu nanosheet（Cu NSs）were used as model catalysts in this paper to investigate the internal relationship between microstructure regulations and the induced local microenvironment improvement for enhancing ECO₂RR activity,which can provide theoretical guidance for design of high efficiency catalyst and optimization of electrode.The main research achievements of this paper are as follows:（1）（003）basal plane oriented Bi NSs and（012）edge plane oriented Bi NSs were used to analyse the effective active sites for ECO₂RR on Bi NSs.Compared with（012）-oriented Bi NSs,（003）-oriented Bi NSs exhibit lower HER selectivity,with higher partial current densities and FEs for HCOOH under different applied potentials.（012）-oriented Bi NSs show higher hydrogen evolution activity than（003）-oriented Bi NSs.DFT calculations show that the adsorption free energies of ECO₂RR intermediate*OCHO on Bi（003）and Bi（012）are similar,whereas the kinetically favored HER is suppressed on Bi（003）.The experimental and theoretical calculation results indicate that increasing the proportion of basal planes on Bi NSs or Bi based two-dimensional catalysts helps inhibit the HER and promote the selectivity and efficiency of ECO₂RR to formic acid.（2）In order to increase the ratio of basal plane on Bi NSs,Bi NSs with a lateral size in micrometer-scale（Bi LNSs）exposed the（003）basal plane were synthesized by electrochemical exfoliation method.As-prepared Bi LNSs possess high selectivity of formic acid from 90.2%to 97.6%within a broad potential window from-0.44 to-1.10 V vs.RHE and a superior partial current density for formic acid about 590 m A mg^-1 in the flow cell electrolyzer,which are much higher than those on the commonly reported Bi nanoparticles and nano-lateral sized Bi nanosheets.Bi LNSs tend to lay flat on the electrode,exposing more basal active sites and inhibiting the competition reaction of hydrogen evolution.Moreover,the ions intercalation exfoliation process caused the lattice tensile in Bi LNSs structure.Structure analysis,ECO₂RR results and density functional theory calculations demonstrate that due to the existence of appropriate beneficial lattice tensile in Bi LNSs structure leads to less overlap of d orbitals,a narrower d-band width and an upward shift in the center of d band which lowers the binding energies of ECO₂RR intermediate*OCHO,therefore promoting ECO₂RR to HCOOH.（3）In order to promote the ECO₂RR activity to ethylene,porous structures with nanosizes were constructed on Cu NSs（with a lateral size in micrometer-scale）.The highest selectivity of C₂H₄ on rough porous Cu NSs（with the average pore size of about 13 nm）reaches（53.54±1.97）%with a high partial current density for C₂H₄ of（272.3±10.5）m A cm^-2 at-1.27 V vs.RHE,which is about 1.6 times larger than that on snooth Cu NSs without obvious porous structure.By measuring the wettability of the electrode surface and analyzing the results of in situ Raman spectroscopy and ECO₂RR,it is found that nanoporous structure exhibiting hydrophobic and CO₂-philic properties is conducive to improving the surface microenvironment,enhancing the accessibility of CO₂ on the catalyst layer and building abundant triple phase boundaries for increasing the reaction rate of ECO₂RR.This microstructure also can adjust the reaction microenvironment by attracting and aggregating OH^-anions to enhance the local p H,which helps lower the energy barrier for C-C coupling and hydrogenation of COCO*to form C₂H₄,improving the selectivity of ECO₂RR to C₂H₄.The stability of porous Cu NSs for ECO₂RR was preliminarily evaluated in the membrane electrode assembly.After operating for 70 hours,there was no significant change in the structure and catalytic activity of porous Cu NSs,indicating that the porous Cu NSs catalyst has good stability.（4）In order to further enhance the selectivity of C₂H₄,the porous Cu NSs electrode was modified with different types of organic polymers to regulate the local microenvironment.At the applied potential of-1.05 V vs.RHE,the selectivity of C₂H₄ on porous Cu NSs electrode without modification and modified with PSMIM（anionic organic polymer）is（46.27±1.72）%and（62.34±1.48）%,respectively.It is only（25.41±1.80）%on Nafion（cationic organic polymer）modified porous Cu NSs electrode.The effects of different organic polymer on the adsorption of CO and*CO on the surface of Cu NSs electrodes were analyzed by in-situ CO DRIFTs,ATR-SEIRAS measurements,and molecular dynamics simulation.When the porous Cu NSs electrode is modified with Nafion,CO gas and*CO intermediate are easy to desorption from the electrode.It is found that porous Cu NSs electrode modified with PSMIM is more likely to adsorb CO gas and*CO intermediate,and the higher*CO coverage is beneficial to form*CO dimerization,exhibiting higher ECO₂RR selectivity to C₂H₄.Furthermore,it has been found through comparison that porous Cu NSs modified with PSMIM can electrocatalyze CO₂ to C₂H₄with high selectivity and stability.Therefore,using PSMIM to modify porous Cu NSs electrodes is expected to further accelerate the development process of practical applications of porous Cu NSs for electrocatalytic CO₂ reduction to C₂H₄.