Fabrication And Electrochemical Performance Of Bi-based Catalysts With High Activity For Electrocatalytic CO₂ Reduction

As we all known,with the excessive use of fossil fuels(such as coal,oil and natural gas),a large number of carbon dioxide(CO2)have been discharged,resulting in increasingly serious environmental problems and energy crises.CO2 is an extremely disturbing greenhouse gas,and it is also an important,rich and cheap C1 raw material.The issue about CO2 utilization and conversion has been attracting tremendous attention.Normally,CO2 conversion can be achieved by chemical methods,photocatalytic reduction,electrocatalytic reduction,biological methods,and by a few other means.The electrocatalytic reduction of CO2 into value-added chemicals and low carbon fuels(such as formic acid,methanol,CO,methane,etc.)has attracted great attention,because it can perform the carbon-neutral cycle by storing the intermittent renewable energy(i.e.,solar,tidal and wind energy)in the CO2 reduction products.And the process is controllable by changing the electrolysis conditions(such as electrode potential,electrolyte,catalyst)to get the target products.Therefore,CO2 electrocatalytic reduction has been regarded as a potential pathway for CO2 conversion.Formic acid(HCOOH)or formate(HCOO-)is the favored value-added reduction product owing to the broad application fields.In aqueous solution,several metals have high selectivity for CO2 electrocatalytic reduction to formate,such as In,Hg,Pb,Sn,Bi and so on.However,Sn has been identified to be the most promising catalyst for the production of formic acid by CO2 electrocatalytic reduction owning to its innocuous and economy low cost.Nevertheless,a large number of studies have shown that their activity,selectivity and stability are still unable to meet the needs of practical commercial applications.Therefore,selecting and developing a new type of catalyst with high activity,stability and simple preparation process is the key way for the industrialization of CO2 electrocatalytic reduction to formate.Bismuth(Bi)is a promising metal as a cathode catalysis material for the electrochemical conversion of CO2 to formate,beacuse it is an environmentally friendly and low cost metal.Actually,the Bi catalyst exhibits high selectivity for CO2 reduction to formate,and the overpotential is lower than the Sn-based catalysts.Unfortunately,few studies have been conducted on Bi-based catalysts.In this work,a series of Bi-based catalysts have been constructed using a facile chemical reduction route in aqueous solution.A Bi catalyst(Bi100-45)with micro-structured sheets was easily designed by adjusting the reaction temperature and reaction time.A Bi catalyst(Bi3-30-80)with particle size of 50~100 nm was successfully prepared using polyethylene glycol 10000(PEG 10000)as a stabilizing agent and by optimizing the reaction conditions.In order to improve the current densities and the dispersion of the catalysts,a series of carbon-supported Bi nanoparticles were prepared using sodium citrate as a stabilizer,and different carbon materials,such as multi-walled carbon nanotubes functionalized with-COOH(MWCNT),carbon black(Vulcan XC-72)and Graphene as the support of the catalysts.XRD,SEM,TEM,ICP-OES and XPS were used to characterize the catalysts' structure and composition.The catalytic performance of CO2 electrocatalytic reduction to formate has been studied by cyclic voltammetry(CV),linear scanning(LSV)and chronoamperometry(i-t)techniques,and the reaction kinetics and mechanism were analyzed.The electrolyte effect on electrocatalytic properties of Bi/MWCNT was also studied in details.The researched effects of electrolyte include the influence of the cation(Na+,K+),anion(HCO3-,SO42-,OH-)and concentration.The main research contents and conclusions obtained are as follows:(1)Micro-structured Bi100-45 catalyst has been designed using a simple chemical reduction route in aqueous solution,bismuth nitrate pentahydrate(Bi(NO3)3 5H2O)was used as Bi source precursor and hydrazine hydrate(N2H4 H2O)as reducing agent.The composition and morphology of the catalyst were tuned by adjusting the reaction temperature and reaction time.The Bi100-45 catalyst is assembled by irregular microstructure Bi sheets with average diameters of 150~250 nm,which is fluffier compared with commercial Bi powder(2~4 ?m microsphere).The Bi catalysts were mainly inclined to grow along with Bi(012)lattice plane.The obtained Bi100-45 catalyst exhibits high selectivity for formate production with a faradaic efficiency(FEHCOO-)of 90% at-1.45 V vs.SCE in CO2 saturated 0.5 M KHCO3 aqueous solution.The onset potential is-1.26 V vs.SCE.Even more encouraging,this Bi catalyst shows long term stability.After 20 h continuous electrolysis,the faraday efficiency of formate is 87%,the attenuation is 3.3%,and the current density shows no obvious decay.After 45 h continuous electrolysis,the Faraday efficiency of formic acid still remains at 72%.Bi100-45 has larger electrochemical active surface area than solid sphere structure of commercial Bi powder,exposing more active sites,reducing the mass transfer resistance of reactants and products and improving the utilization rate of the catalyst space.(2)The Bi nanoparticles are prepared by a PEG 10000 assisted hydrazine hydrate reduction method,using Bi(NO3)3 5H2 O as Bi source precursor.The Bi3-30-80 catalysts with particle sizes in the range of 50~100 nm have been successfully designed by adjusting the reaction component and the reduction time.The Bi3-30-80 sample shows the Bi(012)lattice planes.PEG 10000 has an important influence on the morphology and structure of the catalyst.On the one hand,it can encapsulate particles to prevent further agglomeration of particles,and on the other hand,it has agglomeration effect on particles.The results show that when PEG 10000 is 30 mg,the particle size of the catalyst(Bi3-30-80)is the smallest,and more dispersive.The nanostructured Bi3-30-80 catalyst can result in an increased electrochemical surface area that provides a larger number of catalytically active sites,contributing to the notably enhanced activity for CO2 reduction to formate.In CO2-saturated 0.5 M KHCO3 solutions,the maximum FEHCOO-of 94.7% is achieved with a current density of ?4.9 m A cm-2 at-1.5 V vs.SCE.The onset potential is-1.2 V vs.SCE.The deactivation of Bi3-30-80 catalyst shows a potential-dependent feature.At-1.45 V vs.SCE,the catalyst shows no obvious degradation over 20 hours.At-1.5 V vs.SCE,the catalysts retains their activity for only 10 h.The deactivation mechanism of Bi3-30-80 catalyst is identified by both SEM and XRD before and after the electrolysis.The results indicate that the deactivation of Bi3-30-80 catalyst is attributed to the morphology change and the formation of(Bi O)2CO3.After electrolysis at-1.5 V for 30 hours,the morphology of Bi3-30-80 catalyst is changed compared to the initial one,the petal-like sheets can be observed,which spread all over the surface.The particles are conglutinated together and form a flower-like structure,which is the rudiment of petal-like sheet.After electrolysis at-1.45 V for 45 hours,the nanoparticle structure of Bi3-30-80 catalyst is actually remained,except for a small increase in the average size of the particle,where the surface of particles is covered by a gooey film.(3)It has been proved that the electrocatalytic performance can be improved by employing carbon material as catalysts support.A series of carbon-supported Bi nanoparticles were prepared by sodium citrate assisted sodium borohydride reduction method,different carbon materials,such as multi-walled carbon nanotubes functionalized with-COOH(MWCNT),carbon black(Vulcan XC-72)and graphene as the support of Bi nanoparticles.TEM images show that the Bi nanoparticles were uniformly loaded on the carbon supports,with the exception of Vulcan XC-72,where a not uniform dispersion is obtained.The average particle sizes of Bi nanoparticles in Bi/MWCNT,Bi/Vulcan XC-72 and Bi/Graphene are 4.4 nm,3.5 nm and 2.1 nm respectively,which all show the Bi(012)lattice planes.The carbon materials play an important role in reducing the aggregation of Bi nanoparticles.Carbon materials increase the conductivity of the catalysts,especially Bi/MWCNT,owing to its unique hollow,tube-like structure.The carbon material helps to accelerate the rate of electron transfer,which resulting in improving the rate of electrochemical reaction.In CO2-saturated 0.5 M KHCO3 solutions,Bi/MWCNT exhibits a better catalytic performance of CO2 reduction to formate compared with Bi/Vulcan XC-72 and Bi/Graphene.The maximum FEHCOO-of 94.7% is achieved on Bi/MWCNT catalyst with a current density of ?10.7 m A cm-2 at-1.5 V vs.SCE.And the onset potential is-1.28 V vs.SCE.The current density is 2.2 times of the current density of Bi3-30-80.This extremely high Faraday efficiency and current density is attributed to the excellent electrical conductivity of MWCNT and the controlled nanostructure.Unique hollow tube-like structure increases the electron transfer rate.And the structure is similar to gas diffusion layer,which increase the gas-liquid-solid interface.Furthermore,long and curved cavity increases the partial pressure of CO2,enhancing the mass transfer of reactants and intermediates.The particle size of 4.4 nm provides a larger surface area than Bi3-30-80,exposing more active sites.The Bi/Graphene show poor selectivity for the product of formate,the FEHCOOis only 31% at-1.5 V vs.SCE,the poor performance is derived from its too small nanoparticle size.(4)In the range of low overpotential,the Bi/MWCNT has a Tafel slope of 87 m V dec-1.The mechanism for CO2 electrocatalytic reduction to formate is a proton-coupled-electron transfer mechanism.In this mechanism,the first proton-coupled-electron(H++e-)is transferred to CO2 to produce an adsorbed formate species(*OCHO),followed by the consecutive protonation(H++e-)of *OCHO to produce HCOOH,and the formation of *OCHO is the rate-determining step.The reduction peak current(Ip)is proportional to the scan rate(?)over the range from 10 m V s-1 to 100 m V s-1,suggesting that the reduction is a surface-control process.Nevertheless,when the scan rate is greater than 100 m V s-1,Ip is proportional to neither ? nor ?1/2.This possibly indicates that the process is dominated by the mass transfer rate and surface-control simultaneously.(5)The cation,anion and concentration in the electrolyte have an important influence on the catalytic performance of Bi/MWCNT.The SO42-anion shows higher faradaic efficiency for formate than HCO3-and OH-,while HCO3-anion favors higher current density and production rate for formate.The highest yield rate of formate reached is 2.4 ?mol cm-2 min-1 in 0.5 M KHCO3 at potentials of-1.5 V vs.SCE,which is larger than the yield rate in 0.5 M KOH(1.9 ?mol cm-2 min-1)and 0.25 M K2SO4(1.8 ?mol cm-2 min-1).This is because the equilibrium between HCO3-and CO2,which can provide sufficient,dissolved CO2 to the interface between the Bi catalyst and the electrolyte for the reaction.For the other two electrolytes,KOH and K2SO4,the current density and production rate are limited by the diffusion of the dissolved CO2.The cation K+ exhibits higher current density,formate yield and Faraday efficiency than Na+ cation.This is because K+ more easily adsorbed on the electrode surface than Na+,resulting in the higher potential of the outer Helmholtz plane(OHP)of K+ solution.With the increase of the concentration of KHCO3 in the electrolyte,the current density increase,while the Faraday efficiency and yield of formate increase first and then decreases.High concentration of HCO3-can afford more H+,accelerating the reaction rate.But when the concentration increased to a certain extent,the p H value will also increase in CO2 saturated solution,the concentration of H+ decreased accordingly,the reaction rate will be inhibited.Overall consideration of Faraday efficiency,current density and yield of formate,the best choice of electrolytes is 0.5 M KHCO3.