Ionic liquids for CO2 electrochemical reduction

Fangfang Li,Francesca Mocci,Xiangping Zhang,Xiaoyan Ji,*,Aatto Laaksonen,4,5,6,*

1 Energy Engineering,Division of Energy Science,Lule? University of Technology,Lule? 97187,Sweden

2 Department of Chemical and Geological Sciences,University of Cagliari,Monserrato 09042,Italy

3 CAS Key Laboratory of Green Process and Engineering,Beijing Key Laboratory of Ionic Liquids Clean Process,State Key Laboratory of Multiphase Complex Systems,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

4 Division of Physical Chemistry,Department of Materials and Environmental Chemistry,Arrhenius Laboratory,Stockholm University,Stockholm 10691,Sweden

5 Center of Advanced Research in Bionanocojugates and Biopolymers,‘‘Petru Poni”Institute of Macromolecular Chemistry,Iasi 700469,Romania

6 State Key Laboratory of Materials-Oriented and Chemical Engineering,Nanjing Tech University,Nanjing 211816,China

Keywords:Carbon dioxide Ionic liquids Electro-reduction Electrolyte Electrocatalytic material Computer simulation

ABSTRACT Electrochemical reduction of CO2 is a novel research field towards a CO2 -neutral global economy and combating fast accelerating and disastrous climate changes while finding new solutions to store renewable energy in value-added chemicals and fuels.Ionic liquids(ILs),as medium and catalysts(or supporting part of catalysts) have been given wide attention in the electrochemical CO2 reduction reaction(CO2 RR)due to their unique advantages in lowering overpotential and improving the product selectivity,as well as their designable and tunable properties.In this review,we have summarized the recent progress of CO2 electro-reduction in IL-based electrolytes to produce higher-value chemicals.We then have highlighted the unique enhancing effect of ILs on CO2 RR as templates,precursors,and surface functional moieties of electrocatalytic materials.Finally,computational chemistry tools utilized to understand how the ILs facilitate the CO2 RR or to propose the reaction mechanisms,generated intermediates and products have been discussed.

1.Introduction

Global warming is now widely recognized as being the biggest global issue facing human beings,and the increased greenhouse gas (GHG) concentrations in the atmosphere are believed to be the main cause.The most significant anthropogenic GHG is CO2,and an effective framework for mitigating CO2emission needs to be established urgently.To mitigate CO2emissions,different options have been proposed,among which CO2capture,utilization,and storage is promising [1].On the other hand,currently,the carbon-based products,including fuels and chemicals,are mainly produced from fossil fuels.Due to the limited available fossilresources,developing an alternative and sustainable method to produce carbon-based products is essential.In fact,CO2is a non-toxic C1-feedstock,which can be converted into a variety of fuels and chemicals,such as syngas[2],hydrocarbons(lower olefins,gasoline,liquefied petroleum gas,aromatics) [3],alcohols,and carbamates[4].Therefore,CO2conversion (i.e.,CO2utilization via conversion)can be one of the most important solutions to control CO2emissions and produce carbon-based products for developing a sustainable carbon-based economy.Furthermore,according to recent estimations,such a solution would not only be environmentally friendly,but also cheaper than the presently used processes[5].

Recently,different strategies have been proposed,developed and commercialized for CO2conversion,for example,chemical conversion via hydrogenation [6],enzymatic conversion [7],electrochemical reduction[8],and photocatalytic reduction[9].Among them,the electrochemical reduction becomes more and more important in developing,owing to the mild reaction conditions,flexible and controllable process,and,in particular,it can be combined with renewable energy,avoiding the addition of H2and serving as energy storage [10].A typical electrochemical reduction mainly includes electrode,electrolyte,and electrolytic cell.A lot of researches have been conducted to develop catalysts to increase selectivity,activity,and stability,as summarized in the review articles[11].As another essential component,electrolyte also strongly determines the process performance (e.g.,overpotential,selectivity,reaction rate) [12].Therefore,developing novel electrolyte has become an important research topic.

Among the developed electrolytes,ionic liquids(ILs)have been considered as one of the most promising ones,due to their unique properties,such as high thermal and electrical stability,high CO2solubility,intrinsic conductivity,wide electrochemical window,and adjustable structure[13,14].Additionally,it has been observed that ILs can be served as co-catalysts in electrochemical reduction,for example,Rosen et al.[15]reported that electrochemical CO2reduction reaction (CO2RR)in aqueous 1-ethyl-3-methylimidazolium tetrafluoroborate [Emim][BF4]electrolyte exhibited high selectivity for CO with a quite low overpotential because of the formation of [Emim]-CO2complex on the surface of Ag electrode.Since the properties of ILs depend on the constituent ions,the proper choice of ions combined with functionalization will make it possible to develop multiple-function ILs,including CO2-absorbent to enhance CO2solubility,co-catalyst to activate CO2,electrolyte to offer reaction medium,and modifier to tune the microstructure of electrocatalytic materials.Research work has been conducted to develop ILs as electrolytes,and several relevant review articles have been published.For example,Alvarez-Guerra et al.[16]reviewed the electrochemical valorisation of CO2in IL-based electrolytes using CO2alone or with other carbon reactants,respectively.Lim et al.[17]and Feng et al.[18]summarized the electrochemical reaction mechanism of CO2in IL-based media.However,to the best of our knowledge,no review article with a specific focus on ILs for electrochemical reduction of CO2from the functions of ILs to tune the morphology and properties of electrocatalytic materials as well as theoretical insights is available.

This mini-review aims to make a comprehensive summary of ILs used in electrolytes and electrocatalytic materials and to illustrate their effect on the electrochemical performance.Then,the theoretical studies relating to the CO2RR in IL-based media will be summarized to provide guidelines for the development of novel electrocatalytic systems.

2.Ionic Liquids Used in Electrolytes

Electrolytes are considered as significant promotors for CO2dissolution,activation,and conversion in CO2RR.The most commonly used electrolytes are aqueous solutions with sodium and potassium salts,including those with halides,(bi-)carbonates,nitrates,(hydrogen-,dihydrogen-)phosphates,sulfates,and hydroxides[19].In general,electrolytes with high CO2solubility tend to show high CO2activation extent and low reduction barrier for CO2conversion.CO2solubility in water is,however,only 0.34 mmol·L-1under the ambient conditions,and in general,CO2solubility in aqueous solutions will be further decreased due to the saltingout effect induced by sodium and potassium salts in water [20].As a novel type of green and adjustable solvents,ILs exhibit excellent CO2absorption capacity.Rosen et al.[15]firstly reported an electrocatalytic system with aqueous IL electrolyte,and found that IL could reduce the free energy of CO2-·intermediate,thereby lowering the overpotential for CO formation.Since then,various conventional and functionalized ILs have been widely studied in CO2RR to convert CO2into value-added products.Table 1 gives a summary of recent studies where ILs were used in CO2RR.

2.1.CO2 electroreduction to CO

Although different chemicals have been produced from electrochemical conversion of CO2in IL-based electrolytes,CO is particularly attractive since CO2electro-reduction to CO requires just two electrons,thus being much easier to achieve compared to others[30–33].Meanwhile,CO is also an important starting material and carbon intermediate for the production of acetic acid,phosgene,aldehydes,and fine chemicals [77].In the form of syngas(CO/H2mixture),it can be further reacted to produce liquid hydrocarbons and alcohols via the Fischer–Tropsch routes [78].

The formation of CO2-·radial anion is the thermodynamic barrier and rate-determining step for CO2conversion to CO,and always needs high overpotential to reach a considerable reaction rate.Rosen and coworkers explored the reduction of CO2on Ag electrode with [Emim][BF4]as the electrolyte,and an extremely low overpotential of 0.2 V with high Faradaic efficiency of 96%was observed [15].Their research results implied that [Emim]+cation is beneficial for promoting CO2reduction into CO as well as inhibiting H2generation owing to the formation of [Emim]-CO2complex(Fig.1)on the surface of electrode[79].Subsequently,more and more works related to CO2RR in IL-based electrolytes have been reported.Sun et al.[80]studied CO2conversion with a Pb cathode in 1-ethyl-3-methylimidazolium bis(trifluoromethylsul fonyl)imide ([Emim][Tf2N]) electrolyte.It is suggested that the interaction between CO2-·and the [Emim]+covering the cathode surface could prevent the C-C coupling of CO2-·to oxalate,and thus facilitate CO formation.The presence of[Emim][Tf2N]switched the pathway of CO2reduction process,indicating that ILs can also be served as co-catalysts to improve the selectivity of CO2electroreduction reaction.

Considering the distinct advantages of ILs in reducing overpotential,inhibiting H2generation,and enhancing reaction selectivity,various studies that using ILs as electrolytes in combination with different electrocatalytic materials have been published.DiMeglio et al.[21]performed CO2electrochemical reaction on a bismuth-carbon monoxide evolving catalyst(Bi-CMEC) in imidazolium-based ILs,including [Emim][BF4],1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]),1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6])and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate([Bmmim]-[BF4]).The system with 20 mmol·L-1[Bmim][BF4]exhibited optimal reduction performance to CO with high Faradaic efficiency(FE)of(95±6)%and partial current density of(5.51±1.2)mA·cm-2at-1.95 V vs.saturated calomel electrode(SCE),which results in a nearly 50-fold higher current density than that in the electrolyte absence of IL.By increasing the content of ILs to 300 mmol·L-1,the current density further increased to 25–30 mA·cm-2[22].Zhou’s group found that the synergistic effect between Agmodified Cu cathode and imidazolium-based ILs is probably the main reason for the high CO selectivity in CO2RR [26].Chen et al.[28]found that the metals of In,Sn,Zn,Ag,and Au could catalyze CO2RR to CO and formate in IL dimethylammonium dimethylcarbamate (dimcarb);while most of transition and post-transition metals only reduced the proton of dimcarb into H2.Similar research was reported by Rudnev’s group via comparing cyclic voltammetry (CV) data in 13 metal electrodes with three imidazolium-based ILs,which demonstrated that the overall electrocatalytic activity depends on the chemical nature of both metal electrodes and IL cations [81].Moreover,hierarchically structured Au [24],Fe porphyrin [82],dichalcogenides [25,31,38],surfaceactivated Bi nanoparticles [34],bimetallic Zn-Cu cathode [83],Ag2S nanowires[42],CuSn[46],Bi50Sn22Pb28alloys[51],etc.,were also used as electrocatalysts in CO2RR with different imidazoliumbased IL electrolytes.All these articles confirmed that ILs as the absorbents and proton donors could promote CO2capture,as well as reduce the overpotential to CO formation via stabilizing CO2-·intermediate with imidazolium cations.It means that the interplay effect between electrocatalytic materials and ILs could significantly improve CO2conversion.

Numerous studies have also focused on the effect of anion and cation of ILs on the electrocatalytic CO2reduction[27,84].Medina-Ramos et al.[22]studied CO2electro-reduction in [Bmim]-based ILs with different anions,including [BF4]-,[PF6]-,chloride ([Cl]-),bromide ([Br]-) and trifluoromethanesulfonate ([OTf]-).Amongthem,[Bmim][OTf]exhibited the highest CO selectivity of(87±8)%and the ILs with [BF4]-and [PF6]-anions showed the best current densities with high FEs for CO.This is because that the strong interaction between the F atom of IL-anions and CO2results in much higher efficiency than the IL-anions without F atom in CO2RR[29].However,the ILs containing[Cl]-and[Br]-displayed the lowest efficiencies and kinetics,which may be attributed to the fact that these two ILs are more hygroscopic,leading to the formation of other products (such as formate) in the presence of water [22].Interestingly,an opposite conclusion was drawn by Chen et al.,[43]who performed CO2reduction in propylene carbonate(PC)/tetrabutylammonium perchlorate electrolytes with various imidazolium-based ILs.The results showed that [Bmim][Cl]has higher electrocatalytic efficiency than [Bmim][BF4].They believed that the strong H-bond between [Cl]-and water could inhibit H2generation;at the same time,the strong H-bond between[Bmim]+and water would facilitate O elimination from CO2.It means that the anion-water and cation-water interactions in aqueous[Bmim][Cl]are the dominating factors in promoting CO2electrocatalytic efficiency to CO and preventing H2formation [23].Zhao et al.[85]studied CO2reduction in Ag electrode and ILs with imidazolium,pyrrolidium,ammonium,phosphonium,and(trimethhylamine)-(dimethylethylamine)-dihydroborate cations(Fig.2).The CV results showed that all the above ILs can increase reaction kinetics,and imidazolium and pyrrolidium are most active.Among the aforementioned cations,imidazolium can directly act as co-catalyst with Ag in the form of the reduced imidazolium radical complex with CO2,while the others influence the kinetics by modifying the electrochemical double layer.Zhang et al.[37]reported a hydroxyl functionalized imidazolium-based IL,1-(3-hydroxypropyl)-3-methylimidazolium tetrafluoroboride ([POHmim][BF4]),and found that the overpotential in [POHmim][BF4]was decreased by 90 and 490 mV compared to [Bmim][BF4]and MeCN without IL additives,respectively.Moreover,hydroxyl group promoted reaction rate and CO selectivity since it could bridge an H-bonding chain for proton transport,thus lowering the activation barrier of CO2.Atifi et al.[39]reported that the CO2selectivity of 2e-reduction can be controlled by choosing suitable IL promoters via altering IL-cations.For example,in the system of bismuth cathode with[Bmim][PF6]electrolyte,the FE for CO is about 85%.While the addition of protic IL with the same anion,1,8-diazabicyclo[5.4.0]undec-7-ene hexafluorophosphate([DBUH][PF6]),suppressed CO formation,and the main product changed to formate with a FE and current density of～80% and 25–40 mA·cm-2,respectively.

Table 1 ILs used in CO2 RR

Table 1 (continued)

Fig.1.A schematic to reveal the free energy changes during CO2 RR to CO in H2 O or acetonitrile(solid line)or[Emim][BF4 ](dashed line).Reprinted with the permission from Ref.[15].Copyright 2011 American Association for the Advancement of Science.

Besides the electrocatalytic materials and structure of ILs,the water content,organic solutions,reaction conditions as well as cell types also have a significant effect on CO2RR.Rudnev et al.[86]found that a certain amount of water is beneficial for enhancing CO2reduction performance in[Bmim][BF4]-based electrolytes.This improvement can be attributed to two reasons,the addition of water (1) makes it easier to provide H+for CO2electro-reduction;and (2) could increase CO2diffusion by reducing the viscosity of electrolytes.Nevertheless,at a higher water content,the interaction between CO2and cation would be weakened,resulting in an increase of H2evolution reaction [40].Atifi et al.[49]performed CO2reduction in various electrolyte solutions,including acetonitrile (MeCN),N,N-dimethylformamide (DMF),dimethyl sulfoxide(DMSO) and PC containing [Bmim][OTf].Current density results showed that the CO2reaction rate highly depends on the choice of solvents with an order of [Bmim][OTf]with PC ＜DMSO ＜DMF ＜MeCN.Messias et al.[87]conducted CO2reduction at 3.0 MPa for producing green syngas on an inexpensive commercial foil with aqueous [Bmim][OTf]electrolyte,and high productivity of H2/CO with a ratio of 1/1 to 4/1,i.e.,syngas suitable for the production of several value-added chemicals,was achieved.When working on a porous Ag modified Al flow-through electrode in [Emim][BF4]-based electrolyte,an ultrahigh current density of 36.6 mA·cm-2for CO was obtained at the electrolyte flow rate of 100 ml·min-1,a 73-fold increase compared with the same system without electrolyte flow [50].The possible reason was that the flow-driven mass transport could promote dissolved CO2to the surface of porous electrode,inhibit the local increase of pH,effectively reducing H2generation,and remove the product CO timely for further CO2RR on the surface of electrode,making high CO selectivity [50].

Fig.2.Structures of the ILs used as potential promoters for the CO2 RR.Reprinted with the permission from Ref.[85].Copyright 2016 American Chemical Society.

2.2.CO2 electroreduction to HCOOH

HCOOH,as a liquid fuel with high energy density,can be produced from CO2with a two electron-proton transfer reaction in CO2RR,and it shows great commercial value in many industrial productions.The most common uses of HCOOH are in the leather tanning,animal feed,and agricultural chemical markets.Besides,it is also a probable H2carrier,which loads 580 times more H2than the gas with the same volume under ambient conditions.The formate salt is one of the effective and environmentally friendly antiicing agents [57].Furthermore,the direct formic acid fuel cell shows great potential for storing and releasing electric energy[88].

Watkin et al.[52]studied CO2electroreduction using various post-transition-metal electrodes with 1-ethyl-3-methylimidazolium trifluoroacetate ([Emim][TFA]) electrolyte,and excellent yield (ca.3 mg ·h-1·cm-2) was obtained in In,Sn,and Pb electrodes owing to the stabilized CO2intermedia with IL.Besides,the comparison of voltammograms with C-2-methylated IL ([EDmim][TFA]) confirmed that the high-yielding reduction was caused by the direct CO2reduction with In electrode,while being independent of the C-2-bound carboxylate intermediate as reported in other articles [15].Pb and Sn are inexpensive posttransition metals,and thus Pb-and Sn-based materials are often used as electrocatalysts in CO2RR.Han’s group adopted lead phytate(Pb-PhyA)[59]and PbO2[60]as electrocatalysts,respectively,in CO2reduction with 1-benzyl-3-methylimidazolium tetrafluoroborate ([Bzmim][BF4])/MeCN-H2O electrolytes.Both systems showed high current densities and FE values for HCOOH(30.5 mA·cm-2and 92.7% in Pb-PhyA;40.8 mA·cm-2and 95.5%in PbO2).The benzyl group in the imidazolium ring could stabilize the [Bzmim]-CO2complex,which results in high performance of[Bzmim][BF4][59,60].This group also synthesized SnO2nanosheets supported on N-doped porous carbon catalysts(SnO2@N-PC),a high FE for HCOOH of 94.1%with a current density of 28.4 mA·cm-2was obtained in IL-MeCN system at a moderate overpotential of 0.31 V[58].Besides,Sn and Pb metal alloys(Pb50-Sn50) [65],MoP supported on In-doped porous carbon (MoP@In-PC) [62],Single Mo atom loaded ultrathin N-doped graphene(Mo@NG)[63],and flowerlike In2S3[64]electrocatalysts were also used in CO2RR to HCOOH with various imidazolium-based IL electrolytes,and all of them exhibited effective reduction.Huan et al.[57]firstly reported a porous dendritic Cu for the electrocatalytic reduction of CO2into formate in IL-based electrolyte.For longterm electrolysis,a stable and high current density of 6.5 mA·cm-2and FE for formate of 87%were obtained owing to the combination of dendritic porous structure of Cu and the use of aqueous[Emim][BF4],indicating the importance of both the catalytic materials and electrolytes.Recently,Zhu et al.[66]reported a 3D hierarchical Cu dendrites (d-Cu-1) by using hollow copper metal-organic framework as mediator.The maximum current density and FE for HCOOH reached 102.1 mA·cm-2and 98.2%,respectively,at -1.85 vs.Ag/Ag+in IL/MeCN/H2O system,which far surpasses all the values reported so far.The authors suggested that the dendritic structure of d-Cu-1 could increase its active sites.Besides,the direct connection between catalyst and substrate is a benefit for electron transfer,and the vertically grown Cu on 3D conductive substrate leaves plentiful exposed edges,which provides more active sites for CO2reduction.

The effects of ILs and water on electrocatalytic performance to HCOOH with a Pb or Sn electrode using imidazolium-based IL catholyte mixtures were also investigated [54].By comparing the reduction in different ILs/MeCN-H2O electrolytes,the ILs containing[PF6]-and[BF4]-displayed higher current density and catalytic selectivity,while the anions without F atom,such as [NO3]-and[H2PO4]-,showed lower catalytic efficiencies.This result indicates that IL-anion plays a significant role in CO2RR via changing the interaction strength between the IL and CO2.Besides,the addition of water significantly enhanced the reduction performance by increasing CO2mass transfer and the electrolyte conductivity,decreasing the double-layer capacitance,and reducing the onset potential.A maximum partial current density (37.6 mA·cm-2)and FE (91.6%) for HCOOH were observed in [Bmim][PF6](30 wt%)/MeCN-H2O (5 wt%) on the Pb electrode.Zhang et al.[56]performed CO2electro-reduction in pure [Emim][N(CN)2]and its aqueous solutions at an Sn electrode.In the IL aqueous solution,HCOOH could be actively produced,while it cannot be detected in the pure IL system,indicating that the required proton (H+) for HCOOH formation comes from water.Moreover,the concentration of IL also showed significant influence on selectivity.It was found that FE for HCOOH firstly increased and then decreased with the increase of IL concentration,and the maximum FE of 81.9% was achieved with 0.5 mol·L-1[Emim][N(CN)2]in water.Actually,the high concentrations of both dissolved CO2and proton are desirable for the reduction to HCOOH.With increasing IL concentration,CO2solubility increases gradually,but at the same time proton would decrease as the hydrolysis [N(CN)2]-anion produces more OH-.

Hollingsworth et al.[53]firstly reported that chemisorbed CO2can be reduced to formate with an extremely low overpotential of 0.17 V using anion-functionalized tetraalkyl phosphonium IL,[P66614][124Triz]at an Ag electrode.In order to confirm that the choice of anions has a great influence on selectivity,[P66614]-based IL with a non-reactive anion of [NTf2]-was also used as an electrolyte for comparison,and the main product changed to CO.This observation indicates that the reactive anions can provide a new pathway with low activation energy for CO2electroreduction,which results in low applied potential (-0.7 V vs.Ag/Ag+) for formate formation.However,the current density is quite low (≤1 mA·cm-2).Considering the excellent performance of imidazolium-based ILs in enhancing current density,Feng et al.[61]synthesized a novel anion-functionalized IL with imidazolium cation,1-butyl-3-methylimidazolium 1,2,4-triazolide ([Bmim][124Triz]).The FE and current density for HCOOH reached 95.2%and 24.5 mA·cm-2,respectively.The main reason is that the[124Triz]-anion could efficiently activate and stabilize the intermediate of CO2,and then the activated CO2was easily transferred to the surface of electrode,leading to a low-energy pathway for CO2reduction to HCOOH (see Fig.3).These findings confirmed the importance of ionic microhabitat in enhancing CO2reduction.

2.3.CO2 electroreduction to CH3 OH and CH4

CH3OH and CH4are significant platform molecules with high energy densities,and can be directly used as clean fuels or used as reactants for producing different chemicals [89].However,only a few articles relevant to the electrocatalytic reduction of CO2into CH3OH and CH4have been reported due to the slow kinetics of multiple electron transfer processes(six electron-transfer for CH3-OH,and eight electron-transfer for CH4).The unsatisfactory current density and selectivity are the main obstacles towards the breakthrough of CO2electrochemical conversion to CH3OH and CH4so far.

Fig.3.The role of the[Bmim][124Triz]in CO2 capture,activation,and reduction.(a)Dissolution of CO2 in IL.(b)Formation of the chemical bond between CO2 and the anion of the IL.(c)Nyquist plots for a Pb electrode in various electrolytes.(d)Activation and reaction of CO2 at the electrode surface with the aid of[Bmim][124Triz].Reprinted with the permission from Ref.[61].Copyright 2018 John Wiley and Sons.

Sun et al.[67]performed CO2electro-reduction using an efficient and stable Mo-Bi bimetallic chalcogenide nanosheet (Mo-Bi BMC) electrode with [Bmim][BF4]/MeCN electrolyte.The current density and FE for CH3OH are high up to 12.1 mA·cm-2and 71.2%,respectively.In this system,the excellent electrocatalytic performance results from the interplay effect between Mo and Bi,in which Bi sites could stabilize the CO2-· intermediate adsorbed on electrode surface in the form of[Bmim]-CO2complex,and thus promote the reduction of CO2to CO.The Mo sites are favorable for producing H2and binding CO,which facilitates the further reaction of CO to CH3OH.Later,another bimetal,Pd83Cu17aerogel,was reported as an outstanding electrocatalyst for CO2reduction towards CH3OH with high current density (31.8 mA·cm-2) and FE(80.0%) at a low overpotential of 0.24 V in aqueous [Bmim][BF4]electrolyte [68].The cooperative effect between Pd and Cu with special valence states,as well as the aerogel network structure are the main reasons for achieving superior reduction performance.The same group also studied CO2reduction on Cu1.63Se(1/3) nanocatalyst in [Bmim][PF6]-based electrolyte,and found that the current density can be as high as 41.5 mA·cm-2with FE of 77.6% at the applied potential of -2.1 V vs.Ag/Ag+.The current density in this system is higher than the reported values for producing CH3OH [69].The superior performance of Cu1.63Se(1/3) is attributed to the synergistic effect between Cu and Se in the catalyst for catalyzing the reaction.

Sun et al.[70]reported the first work on CO2RR to CH4using metal-free electrodes,N-doped graphene-like materials prepared from 3-pyridinecarbonitrile (NGM-1).It was found that pyridinic N species in NGM-1 catalysts are the main active species for CO2reduction.The current density and FE for CH4can reach 1.42 mA·cm-2and 93.5%,respectively,in bulk [Bmim][BF4]electrolyte,which is about 6 times higher compared with the current density on the Cu electrode under similar reaction conditions.Besides,adding a trace amount of water could further increase current density because of the decreased viscosity and pH value.When 3 wt% H2O was added in [Bmim][BF4],the current density increased to 3.26 mA·cm-2with a high FE of 90.1%.Moreover,they also used Zn-1,3,5-benzenetricarboxylic acid metal-organic frameworks (Zn-MOFs) as electrodes for CO2conversion [71].It was found that CH4is the dominating product in [Bmim][BF4]with the maximum current density of 3.1 mA·cm-2at the overpotential of 0.25 V.The high electrochemical activity originates from the fact that imidazolium-based ILs are beneficial for absorbing and activating CO2,and,at the same time,the porous structure of Zn-MOF can provide more active surface areas for CO2reduction.Liu et al.[72]found that electro-reduction of CO2on ultrathin MoTe2layers could efficiently produce CH4with high current density(25.6 mA·cm-2)and FE(83%)at the potential of-1.0 V vs.reversible hydrogen electrode (RHE) in [Bmim][BF4]/H2O binary electrolyte,and these values are about 2.5 and 2.4 times higher than those of bulk MoTe2,respectively.This is because the ultrathin layers are favorable for enhancing mass transfer and providing more catalytically active sites,as well as IL can be served as co-catalyst to improve CH4selectivity.The above observations indicate that both catalytic materials and electrolytes play crucial roles in producing CH4.

2.4.CO2 electroreduction to C2 products

Considering the high energy density and added value of multicarbon chemicals,the generation of products containing two carbon atoms(C2products)is more desirable.However,CO2reduction towards C2products,such as hydrocarbons and alcohols,is challenging because of the complicated synthetic steps and energyintensive processes.Electrochemical reduction of CO2is an alternative way of simplifying reaction processes and reducing energy utilization,while C-C coupling during reduction remains as the main difficulty in CO2electrocatalytic conversion for producing C2products.

Cu-based materials are the most promising electrocatalysts for producing multi-carbons from CO2.It has been shown that N-based Cu(I)/C-doped boron nitride(Cu 1/BN-C30)composites can convert CO2into acetic acid in[Emim][BF4]-LiI-H2O electrolyte with FE and current density as high as 80.3% and 13.9 mA·cm-2,respectively[73].In this system,N in the catalyst could induce defect sites and thus facilitate the formation of CO2-·intermediates,and Cu(1) can help to bind CO2·-and COadsfor further converting to C2products.Meanwhile,IL plays a significant role in stabilizing CO2-· ·intermediates,and the promoter LiI favors the coupling of C-C bonds for acetic acid formation.The synergistic effect of Cu(1),BN-C,IL,and LiI promoter leads to the efficient reduction of CO2to acetic acid.Zarandi et al.[74]studied CO2electroreduction on a Cu nanoporous foam with IL-based electrolyte,and ethanol was obtained at the potential of -1.6 V vs Ag/AgCl with the FE of 49%.Experimental results showed that the large surface area of Cu foam along with the contribution of IL on the stabilization of CO2-· intermediates could strongly promote the reduction of CO2to ethanol.

3.Ionic Liquids for Developing Electrocatalytic Materials

Highly efficient electrocatalytic materials are the key factors for enhancing CO2reduction kinetics and selectivity.Nowadays,a large amount of metal-based and carbon-based materials have exhibited excellent electrocatalytic performance in CO2RR [90].The unique nanostructures that exist in ILs,due to their microstructural and dynamical heterogeneities [91],can tune the morphology and properties of the catalysts [92].Therefore,ILs have also been used as templates,precursors,and surface functional moieties to synthesize catalytic materials to realize effective CO2electro-reduction.

3.1.Ionic liquids used as templates and precursors

Porous metal-based materials can be more advantageous in electrochemical conversion of CO2because of their large surface areas.Kang et al.[55]synthesized a hierarchical mesoporous Prussian blue analogues (Cu-PBA) in the [Bmim][BF4]/H2O/MgCl2system for CO2electrochemical conversion.It was found that the pore properties and morphology of the catalysts can be easily controlled by adjusting the microstructure of the solution.The catalyst was also synthesized in the absence of IL for comparison,and no mesopore was observed,suggesting that the IL in the solution plays a template role in preparing mesoporous Cu-PBA.Electrolysis experiments showed higher activity and selectively for HCOOH using the mesoporous Cu-PBA electrode compared to the bulk materials,because more active sites (Cu2+) are exposed in the mesoporous Cu-PBA.Besides,Feng et al.[64]compared the CO2conversion performance on flowerlike and bulk In2S3catalysts synthesized via ionothermal and hydrothermal methods using 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim][BF4])and water as the solvents,respectively.They found that the flowerlike In2S3exhibited high electrochemical performance,reflecting by its high current density and FE for HCOOH of 25.6 mA·cm-2and 86%at-2.3 V,which is believed to be owing to the modification of IL on the microstructure of catalysts.

Zhang et al.[44]reported that using 1-butyl-3-methylimidazolium tetrachloroferrate [Bmim][FeCl4]as a precursor to synthesize single-atom catalysts(Fe-N-G/bC)by introducing atomic Fe into both the N-doped graphene nonosheets and bamboo-carbon nanotubes (CNTs) as seen in Fig.4.The synthesized catalysts could realize an efficient reduction of CO2to CO with a high FE of 95.8%for over 12 h.The graphene in this catalyst plays a crucial role in the formation of isolated Fe atoms and Fe carbide phase;the doped CNT growth could increase the surface area over graphene and improve the kinetics of ions and gases;the presence of Fe dopants is favorable for enhancing edge/defect ratio and graphitization process in the formation of the single atom catalysts with hierarchical pores.It means that the interplay effect of atomic Fe,N-doped graphene,and CNT could provide desirable conductivity and porosity for CO2conversion with high catalytic performance.Cheng et al.[48]prepared N-doped metal-dispersed carbon-based materials (M/C-N) using different metal ILs as precursors.The results showed that microstructure and properties of M/C-N materials can be easily designed and adjusted by varying the cations and anions of metal ILs.Specifically,the Ni/C-N material synthesized from 1-butyl-3-methylimisazolium tetrachloronickelate ([Bmim]2[NiCl4]) has the highest reaction rate and selectivity for CO generation because of the higher mesoporous C-N frameworks,the smallest interfacial charge-transfer,and the largest surface area compared with the ILs with [NiBr4]-and[NiI4]-anions.

3.2.Ionic liquids used as surface functional moieties

ILs can be used as surface modifiers of carbon-based materials to enhance their electrocatalytic performance.For example,Tamilarasan et al.[93]synthesized a polymerized ionic liquid (PIL)[Vmim][BF4]functionalized graphene as the cathode for CO2conversion.It was found that the HCOOH formation rate increased 2–3 fold compared to the pure graphene support,which is attributed to the improvement of interactions between CO2and the catalyst support.The same PIL was also used as the surface functional moiety of multiwalled carbon nanotube(MWNTs) [94].This novel material with high surface area exhibited good affinity towards CO2.Electrolysis experiments showed that PIL functionalized MWNTs could significantly improve CO2conversion rate to HCOOH.By inducing Cu2O nanocubes onto IL functionalized graphite sheets (ILGS) with the interface-induced method (Fig.5),Wang et al.[75]prepared novel Cu2O/ILGS composites.It was found that Cu2O nucleus tends to generate on the surface of ILGS,and the nanostructures of ILGS can stabilize Cu2O particles and prevent their aggregation.Based on the interface effect of ILGS,Cu2O/ILGS-100 (synthesized under 100 mmol·L-1CuCl2) showed good CO2reduction performance into C2H4with a FE of 31.1%.

Tamura et al.[76]performed CO2reduction on an imidazolium ion-terminated self-assembled monolayer-modified Au electrode(SAM-modified Au electrode).Electrolysis experiments showed that the modification of Au electrodes using imidazolium could facilitate CO2reduction to ethylene glycol (EG) with a maximum FE of 87%.While the main product on pure Au was CO,indicating that the reaction field of CO2reduction to EG is the imidazolium ion monolayer on the SAM-modified Au electrode.Iijima et al.[95]developed a strategy for the synthesis of phosphonium-type IL modified Au electrode incorporated with aromatic N-heterocycles(ANH@IL/Au)(Fig.6).The CV results of ANH@IL/Au,pure Au,and IL/Au electrodes showed that both ANH and the modified IL are beneficial for lowering the overpotential of CO2conversion.The electrocatalytic performance was also investigated on a modified iron porphyrin with local IL environment[45],evidencing that the modification using methylimidazolium groups not only results in advantageous anodic shift for electron addition but also enhances two-electron reduction of CO2to CO with a selectivity of 91%.

Fig.4.Illustration for introducing atomic Fe into N-doped graphene nanosheets and then forming atomic Fe,N co-doped bamboo-CNTs (Fe–N–G/bC).Reprinted with the permission from Ref.[44].Copyright 2018 Royal Society of Chemistry.

Fig.5.Illustration for the preparation of Cu2 O/ILGS.Reprinted with the permission from Ref.[75].Copyright 2019 Elsevier.

4.Theoretical Studies

Fig.6.Schematic illustration of the procedure for incorporation of ANH into the IL/Au electrode.Reprinted with the permission from Ref.[95].Copyright 2018 American Chemical Society.

In this part we review theoretical work connected to electroreduction of CO2facilitated or made possible by ILs.Such investigations are still scarce,especially pure computational publications are very limited.Therefore,we have also included in the discussion in silico studies performed in the experimental investigations,for example to find optimal geometries of molecular systems or clusters,to detect intermediates in the elementary steps,and this way proposes a possible chemical reaction,also,to compute spectroscopic and other experimental quantities as well as to assist in the characterization work.The section below discusses firstprinciples quantum chemistry calculations and the section after classical Molecular Dynamics investigations (both ab initio and force-field based) of ILs or solvent medium in the conversion of CO2.There is a review by Lim and Kim [17]which surveys pre-2017 work of IL-based electrochemical CO2reduction.It also discusses some of the earlier theoretical work.

4.1.First-principles quantum chemical studies of CO2 conversion

Most of pure electronic structure studies Quantum Chemical(QC) calculations are performed at 0 K and often in a vacuum(gas phase) without an explicit environment.Solvent effects are normally added by different solvation models,and polarizable continuum models (PCM) are commonly used.Standard thermodynamic and thermochemical quantities can be based on harmonic approximations and by assuming equipartition decomposed to contributions from translation,vibration,rotation,and electronic excitations ignoring couplings between them.To study chemical reactions by searching transition states for reactants and intermediates and proposing the steps in a chemical process is among the most important tasks for QC.Such investigations are beyond the routine use of QC to optimize geometries and to compute molecular properties of equilibrium structure or ensembles,and require experience and skills combined with good knowledge in Chemistry.QC becomes highly useful in studies of catalyzed reactions and in particular heterogeneous catalysis on surfaces with physior chemisorption involved.Two types of QC strategies or methods are normally applied in Materials Chemistry,depending if periodicity is applied on the studied systems.Using software not handling the periodicity,so called cluster approximation is used where a piece is carved around the reaction site with some surrounding and sometimes even including solvent molecules.Preferably big enough to have some‘‘bulk”atoms at the periphery.Using periodicity a cell of regular crystalline character is chosen and multiplied in different directions thereby becoming a‘‘supercell”.Cluster approach is traditionally the chemist’s choice,while supercell has been more of the physicist’s choice.Both have their pros and cons,and the most realistic method would be somewhere in between.In the articles reviewed below both approaches are used.

CO2is a very inert and thermodynamically stable molecule.To add the first electron in the electro reduction CO2to produce the CO2anion radical is a highly demanding step,mainly due to the energy needed to reorganize a linear molecule to a bent radical:

Not only the standard redox potential E0for this reaction is exceptionally high,-1.90 V,but in practice overpotentials are needed together with the catalysts.In the following we describe the theoretical studies aimed at understanding how the potential can be reduced by the interaction of CO2with IL (4.1.1) or with the electrode surface in the presence of IL (4.1.2).

4.1.1.Interactions of CO2and the ionic liquids

The role of imidazolium cation in the electro-reduction of CO2on a silver electrode in a DMF solution was studied by Niu et al.[96].The use of DFT calculations allowed to understand that the ion-pairing between the [Bmim]+cation and CO2radical anion has an important effect on the process.The ion pair formation is promoted by electrostatic interactions,and the DFT calculations showed that the pairing is due to attraction between four positively charged hydrogen atoms in the vicinity of the nitrogen atoms of the imidazolium cation and the negatively charged species originated by the electroreduction (Fig.7).The strength of the ion-pairing is indicated as a dominant factor in the improvement of the electrocatalytic process.Increasing the alkyl chain length of the substituent to the N atoms of the imidazolium ring hinders the ion-pairing,and thus leads to increasingly negative potentials.

In a subsequent study,Wang et al.[97]studied the catalytic cycle to produce CO from CO2using another imidazolium based IL,[Emim][BF4],as a catalyst,and,differently from Niu et al.[96],they included also the anion in the calculations,since the above discussed study of Medina-Romos et al.[22]that was had one year earlier revealed the anion influence on the FE of the CO2reduction to CO.They performed an extensive DFT study of different reaction pathway to the catalysis of the reduction to CO2,considering various mechanisms and their intermediates.As schematized in Fig.8,the authors find that [Emim][BF4]interacts efficiently with CO2forming an intermediate [Emim-COOH]-before a decomposition to CO.In the formation of this intermediate the H2hydrogen atom(i.e.that bound to the carbon atom bridging the two nitrogen atoms of the imidazolium ring) has a key role,constituting the proton source attaching to CO2.The proposed catalytic cycle is a proton coupled electron transfer (PCET) to CO2of sequential proton-electron transfer (SPET) type in which water may act as a co-catalyst for the proton transfer.Also,the occurrence of the intermediate Emim-CO2...BF4complex seen in sum frequency generation (SFG) experiments was confirmed in their calculations.They pointed out the importance of adding the electrode surface in the calculations,which was not done in this study.

The proton transfer proposed by Wang et al.[97]has a rather high activation barrier.To reduce the barrier,Zhang et al.[37]verified the possible role of functionalizing the alkyl chain at the N site of the imidazolium with an OH group,by studying both experimentally and with DFT calculations of the[POHmim][BF4](POHIL).Comparing the behaviour of this IL with that of [Bmim][BF4],they observed a significant enhancement of the catalytic activity of the ILs.As schematized in Fig.9,the DFT calculations suggest that the propanol hydroxyl could bridge a local hydrogen-bonding chain as a shortcut for proton transfer,drastically lowering the activation barrier for the catalytic reduction of CO2in the IL.

Fig.7.The ion pair structure between the [Bmim]+ cation and the CO2 ·-.Reprinted with the permission from Ref.[96].Copyright 2015 Elsevier.(1 ?=0.1 nm)

Fig.8.Catalytic cycle mechanism of type ET →PT →ET for the conversion of CO2 to CO using[Emim][BF4 ]as catalyst.26 is a neutral radical even if not indicated in the figure.Reprinted with the permission from Ref.[97].Copyright 2015 Royal Society of Chemistry.

Fig.9.Intrinsic reaction coordinate calculations for the proton-transfer to form imidazolium carboxylates using[Bmim][BF4 ](indicated as BIL)and POHIL;for the latter two mechanisms (2 and 3) are considered.Reprinted with the permission from Ref.[37].Copyright 2017 John Wiley and Sons.(1 kcal=4.186 kJ)

The interaction between CO2and the imidazolium cation discussed in previous computational studies can be strengthened by a proper choice of the anion.Feng et al.[61]use DFT calculations to explain how the 1,2,4-triazolide used as anion in their novel IL,[Bmim][124Triz],can enhance the performance in CO2.The computations highlight the important role of the interaction between the CO2carbon atom and a nitrogen atom of [124Triz]-(see Fig.3);such interaction induce large electronic and structural modifications of CO2,similar to the bending described by Wang et al.[97]for the interaction with the imidazolium cation.They also verified how the interactions alter the carbon hybridization of CO2molecule in the complex,in the latter being 75%sp2 hybridized,and only 25% with sp character.Interaction of CO2with other commonly used anion,such us [BF4]-,[PF6]-,[Tf2N]-,and[NO3]-,are not capable of inducing this rearrangement.The carbon hybridization of CO2in the complex is close to that in HCOOH,which will possibly facilitate the formation of HCOOH.Based on the results,they hypothesize that the 1,2,4-triazolide–CO2complex provides a low-energy pathway for CO2electro-reduction.

Although not involving electrochemical process,we report here also a few studies whose theoretical insight might be useful for understanding the electrocatalytic process by clarifying at the molecular level the interactions between CO2and the cation or the anion of ILs.

Using various functionals and basis sets combinations,Danten et al.[98]studied CO2or its iso-electronic triatomic counterparts OCS and CS2interactions with 1-butyl-3-methylimidazolium acetate ([Bmim][Ac]).The anion [Ac]-is shown to have an important role in the degradation of the sulphur containing molecules and in the capturing of those containing oxygen.In agreement with previous studies from the same group[99]and other experimental and computational investigations[100,101],the calculations show how the capturing of CO2can be induced by a proton exchange between the imidazolium cation and the acetate,which leads to acetic acid(ACH)and the neutral 1-butyl-3-methylimidazole-2-yli dene [IYC]able to form a complex with CO2or COS leading to imidazolium-2-carboxylate (see Fig.10),while the same reaction is not favoured with CS2.

The involvement of the IL in proton exchange is observed to have a key role also in the study of Qiu et al.[102]on the interaction between CO2and the protic IL 1,8-diazabicyclo-[5.4.0]-7-unde cenium 2-methylimidazolide [DBUH][MIm]and propargylic alcohols (PA).They use DFT calculations to propose a reaction mechanism where both cation and anion are important in catalysing the reaction by promoting the deprotonation of the hydroxyl of the alcohol,thus favouring the CO2electrophilic attack to the alcohol,and also the following intramolecular cyclization steps by both capturing and providing hydrogen protons.Calculations also reveal that the strongly basic anion[MIm]-can capture CO2and form the ion pair [DBUH][MImCOO]with the CO2interacting to one of the nitrogen atoms of the anion and adopting a bended configuration,closely reminding of the interaction observed by Feng et al.[61]with [Bmim][124Triz](see Fig.3).

Fig.10.Bmim Carboxylation pathway in presence of acetate anion.Reprinted with the permission from Ref.[98].Copyright 2016 American Chemical Society.

Moya et al.[103]studied other two aprotic heterocyclic anion ionic liquids (AHA-IL),the triethyl(octyl)-phosphonium indazole([P2228][Inda]),and triethyl(octyl)-phosphonium 2-cyanopyrrolide ([P2228][2-CNPyr]) and their reaction with propylene oxide to form propylene carbonate (PO) (see Fig.11).According to their calculation,the anion activates CO2with the formation of the carbamate complex through an exothermic reaction,which releases more energy with the [Inda]-anion compared to [2-CNPyr]-,and for this reason the former anion has better catalytic performance.

According to the DFT calculations of Ziaee et al.[104],who studied an urea-functionalized imidazolium-based ionic polymer(UIIP)based on 1,3,5-Triyl-tris(1-(3-aminopropyl)imidazol-3′-ium-bro mide)2,4,6-trimethyl-benzene (TAIB) for the conversion of carbon dioxide to cyclic carbonates without metals,solvent,or additives.Also,urea can play an important role in activating CO2.According to their calculation,the interaction of CO2with the NH group of urea induces a variation in the C-O bonds lengths and the bending of the O＝C＝O bond angle (see Fig.12).The structural reorganization of CO2is clearly much more limited with respect to those observed following the stronger interactions with strong basic anions (see Figs.3 and 11),or with imidazolium-2-ylidene (see Fig.10).

Fig.11.Reaction pathway for the CO2 reaction with PO catalyzed by [P2228 ][Inda].The IL’s anion activates CO2 by binding it to a nitrogen atom.Reprinted with the permission from Ref.[103].Copyright 2018 Elsevier.

Recently,Yu et al.[105],considering that[Emim][BF4]can stabilize the high-energy CO2-·radical anion intermediate,employed this IL as a medium for the green-light-driven synthesis of C1–C3 hydrocarbons from CO2and water on plasmonic Au NPs.They used DFT calculations to study the complex between CO2and the ylidene IYC,formed by the loss of a proton from [Emim]+.In such complex,as previously observed for this Emim-CO2complex[98,99],or in other CO2complexes with IL discussed above[37,61,97,102,103],CO2structural and electronic organization is deeply affected by the interaction with the IL,with the O＝C＝O angle reducing to 133.7° and C＝O bonds lengthening to 0.124 nm.Furthermore,the partial charge on CO2when in the complex is -0.73.The geometry observed in the complex is similar to that of the radical anion CO2-·,for which the bond angle is calculated being 137.8°and the bond length of 0.123 nm,thus indicating that the complexation prepares CO2to accept electrons from the photo excited Au NPs.

4.1.2.Interaction of CO2/ionic liquid with the electrode

To understand how ILs can improve the CO2reduction is important to know how they interact with the electrode,how much they cover it,and how they are structurally organized.Urushihara et al.[106]have studied imidazolium ILs by using Quantum Espresso with periodic boundaries to compute adsorption energies and free energies with solvation corrections and GAMESS-US both at DFT level.They calculated [Emim]+adsorption on silver surface and produced the surface Pourbaix diagrams as a function of the IL concentration and of the applied potential.The study shows that,under the experimental condition where [Emim]+is able to enhance the CO2electrochemical reduction,it densely covers the electrode surface,altering the electric field (EF) and improving CO2reduction capabilities.The authors hypothesize that the EF due to the IL coverage of the electrode could stabilize the CO2-·or alter the barriers for proton transfer.In a later investigation,Feaster et al.[107]study the influence of adding small amounts of 1-ethyl-3-methylimidazolium chloride ([Emim][Cl]) to suppress the hydrogen evolution reaction (HER) using polycrystalline Ag,Cu,and Fe electrodes in aqueous acidic and basic media in order to catalyse CO2electro-reduction reaction.The DFT calculations show that the IL changes the availability of active sites on the catalyst surface,and the authors ‘‘postulate that an adsorbed [Emim]+cation displaces H3O+at the interface,which would suppress HER activity”.

Fig.12.Structural modifications due to 1-methyl-3-phenylurea interaction with CO2 .Adapted and reprinted with the permission from Ref.[104].Copyright 2019 American Chemical Society.(1 ?=0.1 nm)

In a recent investigation,Wang et al.[108]report the role of a reduced imidazolium cation (rEmim+) layer in facilitating CO2reduction on a Cu electrode surface while suppressing the hydrogen evolution.Their DFT calculations showed that the electric field formed by solvated cations stabilizes the adsorbed CO2with a large dipole or polarizability shifting the free-energy landscape.Under these conditions,the imidazolium cations are assumed to remain positively charged during the catalytic cycle.The results strongly suggest that the reduction of [Emim]+cation to the corresponding neutral radical initiates the CO2reduction on metal electrodes.The role of the [Emim]+cation on the electrode is also studied by Liu et al.[42],who synthetized Ag2S nano wires as catalysts for the electroreduction of CO2in [Emim][BF4],and performed periodic DFT calculations using VASP.The complex formed by the imidazolium cation and CO2interacts with the surface of the catalyst,which is negatively charged,and leads to a substantial lowering of the energy barrier.As shown in the simplified scheme in Fig.13,the calculation indicates that the COOH*formation is energetically more favorable on(111)and(121)facets of Ag2S than that on Ag(111)and Ag55.They,therefore,conclude that the enhanced performance in IL is solvent-assisted and specific facet-promoted in a synergetic way.

Dai et al.[109]use DFT B3LYP calculations to study metal cluster functionalized ILs for CO2conversion.They investigate the interaction between CO2and Au-Pd atoms or clusters functionalized with [Emim]+and [Hmim]+,showing that the presence of the metals leads to a large increase in interaction with CO2.They found that the composition and size of the cluster can be tuned for the conversion performance.Au1Pd2did show the strongest interaction and performance.Batista et al.[110]investigated the adsorption properties and activation of CO2on transition-metal(TM)13-atom clusters(TM=Ru,Rh,Pd,Ag),based on DFT calculations using PBE with van der Waals D3 corrections.CO2is found to adopt two configurations:a bent CO2configuration corresponding chemisorption and a linear CO2corresponding physisorption.

Transition-metal chalcogenides and dichalcogenides (TMC and TMDCs) in combination with IL have shown very good performances as catalysts for CO2RR.Salehi-Khojin and co-worker [31]performed periodic DFT calculations using VASP to gain insight into the catalytic properties of TMDC nano-flakes.The reaction free energies of the CO2→CO pathway showed the COOH* formation being highly endergonic and rate limiting step for silver clusters,which was interpreted as the reason for the lower overpotential of Ag nanoparticles compared to bulk Ag.In a following investigation from the same group,Abbasi et al.[38]carried out similar DFT calculations to study the doping effects of Nb in Mo1-xMxS2(M=Nb and Ta) structure by comparing the reaction pathways of CO2reduction to CO on pure NbS2and MoS2,as well as Mo1-xNbxS2,finding that Nb doped MoS2could lead to a faster turnover for CO desorption than pure MoS2.Yang et al.[69]studied the CO2electroreduction to methanol using a copper selenide electrocatalyst and [Bmim]-based IL in CH3CN/H2O solution,with different compositions and different IL anions,as supporting electrolytes.To verify the reaction pathway,they did perform DFT calculations using the VASP package,investigating several of the elementary steps and proposing a reaction mechanism based on the free energy diagram,see Fig.14.They found that the formation of intermediate(*COOH)on the Cu1.63Se(1/3)surfaces with the help of two neighbouring Cu atoms through Cu-C and Cu-O bonds showed a stable configuration with lower free energy.The *COOH intermediate binds to the active sites on the surface of catalysts,accelerating the formation of adsorbed *CO species.In addition,the Cu1.63Se(1/3)catalyst also binds moderately*CO,which is beneficial for the CO2transformation.

Feng et al.[64]have created flowerlike In2S3nanoflakes with electrochemically active large surface area and enhanced mass transfer rate for CO2electro-reduction to formate using the ionothermal method.They show high Faradaic efficiency of 86%and high formate formation rate in [Hmim][BF4]electrolyte.DFT calculations,performed with Quantum Espresso and using plane wave basis set,indicate that the improved performance is due to the large adsorption energy of CO2* and OCHO* intermediates on the (440) facet and the main exposed crystal facet of flowerlike In2S3.

Yang et al.[111]have found indium selenide synthesized by an electrosynthesis method on carbon paper(γ-In2Se3/CP)as a highly efficient electrocatalyst for the reduction of CO2to syngas.Syngas(CO/H2) is a good raw material to produce valuable chemicals and fuels,and electrochemical reduction of CO2to syngas is very promising,although with high efficiency is still difficult.Several[Bmim]-based ILs having different anions were tested,and the anion was found to have an important role in the electrocatalytic process,with better results with [Bmim][PF6]as supporting electrolyte with respect to [Bmim][BF4],[Bmim][NO3],[Bmim][ClO4],[Bmim][OAc],and [Bmim][TF2N].The amount of IL in the electrolyte mixture also has an important impact,affecting the balance between CO2RR and HER.The authors used DFT calculations to obtain Gibbs free energy for the reaction with multiple elementary reaction steps with the γ-In2Se3catalysts with or without selenium vacancies.Calculations indicate that introduction of Se vacancies promotes CO2activation,as a result of the possibility for the carbon atom in*COOH species,the higher energy intermediate in the syngas formation,to occupy the selenium vacancy coordinating the nearby metals.

Fig.13.Mechanism for CO2 RR to CO on Ag2 S(111)and Ag2 S(121)in[Emim][BF4 ].Reprinted with the permission from Ref.[42].Copyright 2018 American Chemical Society.

Fig.14.(a) Mechanism for CO2 reduction to methanol.(b) Free energy diagrams on Cu1.63 Se(1/3) electrode.Reprinted with the permission from Ref.[69].Copyright 2019 Springer Nature.

4.2.MD simulations

Molecular dynamics (MD) simulations are used as the main method to study condensed matter,where particles(atoms,molecules,molecular systems,and coarse-grained beads representing them)are moved based on Newton’s second law.They can be carried out applying classical physics where the forces are calculated from molecular mechanical(MM)force fields giving a fully empirical description of the interactions between the atoms and molecules and also within the molecular particles.Classical all-atom MD simulations are the main work-horse to study materials from nano-materials and solids to biological systems and all mixtures thereof.However,classical MD with standard MM force fields cannot be used to study chemical reactions or anything involving electrons.In the MM force fields,the chemical bonds are harmonic wells and cannot be broken.This ‘‘defect”can be used to simulate at elevated temperatures where the molecule would dissociate.Simulating at high temperatures is a simple technique to speed up many processes and to improve sampling.There are empirical reactive force fields developed,which allow breaking and forming of chemical bonds,so called‘‘Reactive”force fields,have been fairly successful but to parameterize them is still a hurdle and only a few groups have developed the skill for doing it.Classical MD can be used in simulations of molecular systems with enough quantum mechanics involved to study chemical problems.By moving the nuclei using classical Newton dynamics but recalculating the electronic total energy after each(short)time step and,thereafter,calculating the gradient of the energy with respect to coordinates gives the force to move the nuclei to the next time point.The nuclei are this way moving in the complex and constantly changing energy landscape created by the electrons in the system.Schemes like the Car-Parrinello (CP) and many types of Born-Oppenheimer MD(BO-MD)are based on this approximation.They are also called ab initio or first-principles simulations as they don’t need any empirical information to have started,going other than physical and chemical conditions as well as systems size which is much smaller than in MM-based simulations due to very heavy computations performed repeatedly.There are also schemes to include hydrogens as quantum particles based on path integral methods in so called centroid MD methods.However,these latter methods are out of interest in studies of CO2conversion.However,CP and BO-MD both are very important in revealing the catalytic effects and proposing reaction mechanisms.So called QM/MM methods should also be mentioned as hybrids with QM methods applied on the reactive part while the surrounding is treated with classical MD.This requires a good description of cross interactions between the QM and MM domains,but something still does not seemed to be solved satisfactorily.QM/MM simulations are common as most standard MD and QC software are combined to perform these calculations.Below we start by ab initio simulations and continue with fully classical simulations.

4.2.1.Ab initio simulations

We start here with a nice example of the strength of ab initio simulations even if it is out of scope of this review.In fact,the reaction below produces more CO2than H2but it shows the effect of using ILs as a medium.By choosing another type of IL would increase the production of hydrogen.In this example,Bhargava et al.[112]did use BO-MD to simulate at 3000 K to find a reaction of formic acid decomposition in 1,3-dimethylimidazolium formate[mmim][HCOO].Where the formate anion dissociates to hydride anion and CO2and then the hydride abstracts the acidic proton from formic acid.All together this provides a reversible reaction HCOOH=CO2+H2in IL.The hydride is stabilized in the IL due to electrostatic interactions.

Holloczki et al.[113]present a theoretical study using DFT calculations and Car-Parrinello (CP2K) simulations of carbene formation from the 1-ethyl-3-methylimidazolium acetate with or without CO2in gas and liquid phases,introducing many interesting questions about carbene formation.

Klyukin and Alexandrov[114]have studied CO2adsorption and reactivity on rutile TiO2(110)surface in water using Car-Parrinello(CPMD) combined with metadynamics.CO2is found to adsorb at the bridging oxygen and becomes spontaneously protonated forming*COOH,which breaks to CO and OH–enhanced by Ti3+polaron.They also discuss the mechanisms of forming HCO3-in bulk water near the rutile surface.

Ocambo et al.[115]use ab initio simulations and DFT calculations for the adsorption of CO2on the Fe13,Co13,Ni13,and Cu13transition metal clusters,highlighting the relevance of the charge transfer for the activation of the adsorbed CO2and CO promoted by the adsorption on the clusters.This also provides interesting insights into the CO2reduction mechanism when the modified Fischer–Tropsch synthesis is used.

Using the Vienna ab initio simulation package (VASP),He et al.[116]report a systematic study of the active-site dependent activity and selectivity for CO2electrochemical reduction over Fenclusters with a very few atoms (n=1–4) doped graphdiyne,finding that Fe dimers and trimers show the highest catalytic activity and selectivity with the remarkably low rate-determining barrier.This work does not use ILs but we wanted to add it as a good example of using ab initio simulations in electroreduction studies.By showing that the catalytic activity and selectivity can be significantly tuned by controlling the number of Fe atoms.It is the first report of catalyst size effects for a precise number of atoms (Fe1-4),making it very interesting to nano-catalyst design for electrochemical reduction of CO2.

4.2.2.Force-field based simulations

Lim et al.[117]have performed multiscale simulations to investigate the atomic origin of the catalytic effects of IL in CO2conversion.They find that in contrary to the conventional picture assuming a particular intermolecular coordination of IL component,it is a collective tuning of the reaction microenvironment together with critically important bulk properties and electrochemical interfaces including resistance,gas solubility,diffusivity,viscosity and others,rather than the detailed chemical variations of the IL itself,being important in the design of an optimal electrolyte.They build the solvent environment and perform simulations with LAMMPS and TIP3P water,and OPLS-AA force field is used for ILs.DFT calculations are performed with the SIESTA code.The effect of solvent is always important to speed up chemical reactions,although it is always difficult to obtain clear picture from the experiments about how it should be.In this work,multiscale simulations are performed to follow how,by adding IL in the aqueous solution,is important to optimize the conditions leading to a lower reaction barrier(see Fig.15).The QM part for the key adsorbates is performed at the DFT-CES/2PT level,giving insight to the catalytic promotion effect of the IL on the CO2conversion.By screening the strong interionic electrostatic interaction,they find that a certain amount of water is important in facilitating the proton transfer by developing a percolated hydrogen bond network and improving the mass transport of RTIL components.

Instead of studying single ion or ion pair,Tan et al.[118]investigate the catalytic effect of clusters of ILs on the fixation of CO2.They use MD simulations to study the intermolecular interactions and microstructures of nine systems of (1-(2-hydroxylethyl)-3-m ethylimidazolium bromide) [Hemim][Br]clusters,finding that three-ion clusters to six-ion clusters were common.But the number of clusters did decrease as the cluster size increased.More insight into the catalytic effects of IL clusters on the fixation of CO2was obtained by DFT calculations.Indeed using clusters enhances the activity of ILs to some extent.

Medina-Ramos et al.[119]study CO2reduction on bismuth electrodes in acetonitrile solution containing 1-butyl-3-methylimidazolium IL electrolytes using DFT and MD simulations with ReaxFF reactive force field(see Fig.16).Both methods suggest the formation of Bi...[Im]+complexes through a partial electrode corrosion of the Bi film,favoring the catalytic reduction of CO2.

Yang et al.[120]have studied the mechanisms of CO2fixation with PO catalyzed by N,N-diethyl-2-hydroxyethanaminium bromide([HMEA][Br]),N-ethyl-2-hydroxy-N-(2-hydroxyethyl)-ethanaminium bromide ([HDEA][Br]),and tris(2-hydroxyethyl)ammonium bromide ([HTEA][Br]) by both Single-IL and Double-IL models(depending on the number of IL involved in the catalytic mechanism).The solvent effect of ILs is considered by using the ONIOM model with MD simulations.It is found that H-bonds stabilize the interaction between the anion and cation.The highest catalytic activity is found for[HTEA][Br]and could be attributed to having the strongest nucleophilic attack as well as showing Hbonding between IL and reactant.Key characteristics from the calculations are collected in Table 2.

Fig.15.Reaction free energy profiles for the reduction of CO2 to CO in pure water solution (red) and in IL solution (blue),showing a large reduction of the energy barrier.Reprinted with the permission from Ref.[117].Copyright 2018 American Chemical Society.

Fig.16.Formation of Bi...[Im]+complexes through partial cathodic corrosion of the Bi film promoting catalytic reduction of CO2 on applied potential in acetonitrile solutions containing 1-butyl-3-methylimidazolium ([Bmim]+) electrolytes.Reprinted with the permission from Ref.[119].Copyright 2018 American Chemical Society.

Table 2 Details of calculations discussed in the Theoretical part

5.Conclusions and Future Perspectives

This review gives a comprehensive survey of the experimental and theoretical studies concerning CO2electrochemical reduction to value-added chemicals,where ILs play either a key or supporting role in the catalysis.The cation and anion of ILs,the electrocatalytic materials,the operating conditions (pressure,electrolyte components,etc.),and the cell types,together with the most remarkable theoretical studies in relevant articles are discussed.

Despite many in-depth studies and applications of ILs on CO2RR systems,the available work is mainly focused on the imidazoliumbased ILs,and the understanding of the CO2reaction mechanisms/-pathways in the IL-based systems is not adequate enough.Moreover,the structure-property-function relationship of ILs is still largely unclear.Therefore,future studies should focus more on identifying the key factors of ILs on improving the electrocatalytic performance and the collective role of the medium in physical and chemical terms at the surface or in the region above the surface of electrodes by combining the experimental and theoretical methods.It is vital to screen and design novel task-specific ILs for producing more value-added products from CO2with a high selectivity and efficiency.

Besides,the products generated from CO2in the electroreduction of IL-based systems are mainly limited to C1-chemicals,(e.g.,CO,HCOOH,CH3OH,and CH4).In the future,more efforts should be put into the generation of high value-added C2(e.g.,C2H4,C2H6,C2H5OH,etc.) and multi-carbon (e.g.,C3H6,C3H8,C3H7OH,etc.) products.

Furthermore,separating products from liquids or gases is difficult and costly,and thus more attention should be put on developing one-pot production and separation technologies on CO2electrochemical conversion.

Finally,the maturity of technologies for CO2RR is still far from industrial applications.Current research studies are mainly carried out on lab-scale,the on-going and future research should operate more under practical reaction conditions to make it easier to scale it up,and the sustainability of CO2electrochemical conversion approaches will have to be carefully concerned from the economic to society integral point of views to create a balance between economy and environment.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

F.Li and X.Ji thank the financial support from the Swedish Energy Agency (P47500-1).A.Laaksonen acknowledges the Swedish Research Council for financial support (2019-03865),and partial support from a grant from Ministry of Research and Innovation of Romania (CNCS -UEFISCDI,project number PN-IIIP4-ID-PCCF-2016-0050,within PNCDI III).F.Mocci thanks the Fondazione di Sardegna,Project:‘‘Precious metal-free complexes for catalytic CO2reduction”(CUP:F71I17000170002)for the financial support.