Reduction of Carbon Dioxide to Formate at Low Overpotential Using a Superbase Ionic Liquid

A new low-energy pathway is reported for the electrochemical reduction of CO2 to formate and syngas at low overpotentials, utilizing a reactive ionic liquid as the solvent. The superbasic tetraalkyl phosphonium ionic liquid [P66614][124Triz] is able to chemisorb CO2 through equimolar binding of CO2 with the 1,2,4-triazole anion. This chemisorbed CO2 can be reduced at silver electrodes at overpotentials as low as 0.17 V, forming formate. In contrast, physically absorbed CO2 within the same ionic liquid or in ionic liquids where chemisorption is impossible (such as [P66614][NTf2]) undergoes reduction at significantly increased overpotentials, producing only CO as the product.

Although CO 2 is agreenhouse gas thought to be involved in climate change, [1] it can also be considered as an abundant carbon building block for carbon neutral fuels and chemicals. [2] Electrochemical reduction is one route to achieve this goal. Indeed, the reduction of CO 2 at low applied overpotentials with high efficiencies is as ignificant current challenge owing to its thermodynamic stability and kinetic inertness. [3] Theh igh overpotential for CO 2 reduction is related to the large reorganization energy associated with reduction of linear CO 2 to the bent [CCO 2 ] À radical anion. Thus,avery negative reduction potential is required for the first electron reduction, that is, À1.9 Vv s. NHE, [4] rendering reduction highly energy inefficient. Materials that form complexes with CO 2 in an on-linear geometry can decrease this reorganization energy and catalyze the electrochemical CO 2 reduction. [5] Recently,p romising results have been reported utilizing room-temperature ionic liquids (RTILs) for the electrochem-ical reduction of CO 2 .H igh CO 2 solubility,i ntrinsic ionic conductivity,a nd wide potential windows of RTILs make them attractive solvents for CO 2 electroreduction. [6] Initial reports on CO 2 reduction in RTILs formed dialkyl carbonates through generation of CCO 2 radicals,which were reacted with alcohols using 1-alkyl-3-methylimidazolium ([C n mim] + ) based RTILs with arange of non-coordinating anions. [7] Further RTIL studies focused on CO 2 reduction to products other than dialkyl carbonates. Rosen 4 ], which was found to decrease the energy of formation of the [CCO 2 ] À radical anion through the complexation of CO 2 with the [C 2 mim] + cation. [8] This significantly reduced the overpotential for CO 2 reduction to CO to < 0.2 V. Furthermore,t he cation was shown to suppress the competing H 2 production reaction by forming am onolayer on the electrode. [9] Further decreases of the applied potential have been achieved by substitution of the Ag working electrode with MoS 2 ,giving overpotentials as low as 0.054 V. [10] Brennecke and co-workers also showed that the anion influenced the product selectivity,w ith oxalate formation favored over CO in [C 2 mim][NTf 2 ]. [11] This change in product selectivity was also shown by Watkins and Bocarsly, where formate was produced in [C 2 mim][TFA] using Pb and Sn working electrodes. [12] Therein, no evidence was found for a[C 2 mim] + -CO 2 complex; [13] however,the RTIL was thought to stabilize intermediates in formate production.
Although interesting, CO 2 reduction in non-coordinating [C n mim] + based RTILs is of limited applicability,owing to low CO 2 solubility.F or example,CO 2 solubility in [C 4 mim][NTf 2 ] is < 0.04 CO 2 mole fraction at 10 8 8Cand 0.1 MPa. [14] Theonly coordinating IL used to date for CO 2 reduction studies is [C 4 mim][OAc], [15] which has aC O 2 solubility of 0.274 CO 2 mole fraction at 25 8 8Ca nd 0.1 MPa. [16] Thel ow solubility affects the rate of product formation and limits the industrial significance of these systems.
These low solubilities have been overcome by the use of superbasic ionic liquids.Inthese systems,[P 66614 ][124Triz],for example,h as been shown to absorb equimolar quantities of CO 2 through the chemical interaction of CO 2 with the anion and physical absorption of CO 2 in the solution free space (Scheme 1). [6b,17] This set of ILs have been studied extensively for CO 2 capture but, to date,n or eports of their use in CO 2 conversion have been published. Akey feature of the anion-CO 2 interaction is that the CO 2 chemically binds without prior reduction to [CCO 2 ] À .Notably,since CO 2 is transformed from al inear to bent geometry on binding to the anion, this can significantly lower the CO 2 reduction potential.
In this study,wereport the first electrochemical reduction of CO 2 captured within the superbasic RTIL [P 66614 ][124Triz], providing an ew low-energy pathway for CO 2 reduction in RTILs.F ull product characterization of solution and gas phase products is reported using 1 HNMR spectroscopy and online GC analysis,r espectively.
Thei nfluence of the anion identity on CO 2 reduction was examined by recording cyclic voltammograms (CVs) at aA gw orking electrode in 0.1 mol L À1 [P 66614 ][NTf 2 ]a nd [P 66614 ][124Triz] in acetonitrile (MeCN). Ther eference electrode was 0.01 mol L À1 Ag + /Ag formed by dissolving AgNO 3 in [C 4 mim][NO 3 ]a nd separated from the bulk solution by ag lass frit, as reported previously. [18] As well as the reactive IL, [P 66614 ][NTf 2 ]was examined as acomparison because CO 2 is unable to chemically bind to this anion. However,b y keeping the [P 66614 ] + cation consistent, the physically absorbed CO 2 should be stabilized to the same extent once reduced to the radical anion ([CCO 2 ] À ), allowing reduction processes for the chemically and physically absorbed CO 2 to be distinguished. TheC Vs recorded at as can rate of 100 mV s À1 at 22 8 8Ci n[ P 66614 ][NTf 2 ]a nd [P 66614 ][124Triz] are shown in Figure 1a and b, respectively.F irst, the CVs were taken in RTIL solutions saturated with argon ( Figure 1). Consistent with previous reports of CVs using metal electrodes in dialkylimidazolium based RTILs,o nly small capacitive currents are observed in aw ide potential window,w ith no additional features other than the solvent or RTIL reduction in the cathodic range.
TheC Vs taken for solutions of both ionic liquids purged with CO 2 for 30 min ( Figure 1) exhibited cathodic features that can be associated with CO 2 reduction. As the onset potential is often difficult to define,i na greement with previous reports,the potentials that result in acurrent density of 6Am À2 were selected as the onset potential for CO 2 reduction. [11,19] Fort he [NTf 2 ] À -based RTIL, upon the addition of CO 2 ,the current starts to increase at À0.9 Vfollowed by arapid increase at À1.9 V. Therapid increase in current at À1.9 Vc an be attributed to reduction of physically bound CO 2 ,f orming [CCO 2 ] À stabilized by the [P 66614 ] + cation. The small current increases from À0.9 Va re attributed to trace water contamination (see below). Fort he [124Triz] À -based RTIL, upon the addition of CO 2 ,asmall current increase is observed at circa À0.9 Va nd af urther small increase is observed at circa À1.5 V; these features are likely to be due to the reduction of CO 2 bound to the [124Triz] À anion (see below). This is followed by ar apid increase in current at À1.9 Vo wing to the reduction of physically bound CO 2 to [CCO 2 ] À stabilized by the [P 66614 ] + cation. This potential is identical to the [NTf 2 ] À -based RTIL system and is consistent with the [P 66614 ] + cation providing the same stabilization to [CCO 2 ] À in both systems.O wing to the highly hygroscopic nature of RTILs [20] and the high solubility of O 2 within MeCN, the small features at less negative potential (À0.9 V) are attributed to trace amounts of moisture or oxygen within the system. Prior to this analysis,the RTILs were rigorously dried under high vacuum and purged with Ar for 1h to limit the influence of adventitious water;h owever, complete removal is very difficult.
These studies were compared with the response following the addition of H 2 O( 0.7 mol L À1 ), the presence of which is required to form protonated reduction products and CO.The presence of H 2 O + CO 2 within the basic IL should enable the formation of carbonate,which may itself be reduced. CVs of Ar and CO 2 saturated RTILs in the presence of water are shown in Figure 1. Fort he [NTf 2 ] À -based RTIL, the addition of H 2 Otothe Ar saturated sample (Figure 1a)shows asmall current increase at circa À0.9 V. This is in the same position that current increases were observed in the anhydrous CO 2 saturated sample and may suggest the presence of small amounts of moisture in the anhydrous CO 2 -saturated sample. Ar apid increase in current is then observed at circa À2.3 V, suggesting the reduction of H 2 OtoH 2 upon the Ag electrode. Thea ddition of H 2 O( 0.7 mol L À1 )t ot he CO 2 saturated solution shows as mall reduction feature at À0.7 Va nd as trong cathodic current from À1.9 V. Although as mall anodic shift is observed, the responses are similar. Thes mall reduction feature at À0.7 Vc an be associated with those observed at À0.9 Vi nt he anhydrous CO 2 saturated sample and in the Ar saturated sample containing H 2 O. Thel arge reduction feature at À1.9 Vi si dentical to that seen in the
Thea ddition of H 2 Ot ot he argon-saturated [124Triz] Àbased RTIL shows two small reduction peaks at circa À1.1 V and À1.5 Vf ollowed by ar apid current increase at À1.9 V. Thes uperbasic RTIL, [P 66614 ][124Triz] is able to react with H 2 Ot of orm [P 66614 ][OH] + [124Triz]ÀH, the latter of which provides an additional route to H + .The first reduction peak at À1.1 Visa tamore negative potential than that observed in the anhydrous CO 2 saturated sample (À0.9 V);h owever, the second current reduction is in an identical position (À1.5 V). Therapid reduction of current is due to reduction of H 2 Ot o H 2 .T his is at aless negative potential than that observed for the [NTf 2 ] À based RTIL (À2.3 V) suggesting the choice of anion has an effect on the H 2 Oreduction potential. Themost significant changes are observed in the addition of H 2 O (0.7 mol L À1 )t ot he CO 2 saturated [124Triz] À based RTIL, which shows an anodic shift and large enhancement of the two low potential reduction features at À0.7 Va nd À1.3 V (Figure 1b)f ollowed by al arge increase in current at À1.9 V. Thef irst reduction feature is attributed to the reduction of CO 2 bound to the [124Triz] À anion to form formate (Scheme 2, Equation (2)). Thes econd reduction feature is also associated with formate product formation via ad ifferent mechanism (see below). Thet hird reduction feature,a tÀ1.9 Vi nF igure 1b,i sa ssigned to both the reduction of the physically bound CO 2 stabilized by the [P 66614 ] + cation, consistent with the CV observed in [P 66614 ][NTf 2 ] ( Figure 1a)a nd the reduction of H 2 Ot oH 2 . To examine the origin of these features in more detail, an electrolysis study has been performed.
In Scheme 2, Equation (2) indicates that the anion is regenerated and, therefore,should be catalytic. However,any formic acid produced may protonate this anion and the system would not be recyclable.T oe xamine whether the feature at low potential is still present in the presence of the products,o ne mole equivalent of formic acid to [P 66614 ][124Triz] was added to the system (AcN + IL + CO 2 + H 2 O) and the voltammetry compared with the system without formic acid addition (Supporting Information, Figure S1). No reduction in the low potential feature was observed, suggesting no inhibition of the anion binding site.F urthermore, 1 HNMR spectrum of this sample revealed no evidence for the neutral 124-triazole.I ts hould also be noted that the addition of formic acid to the same system in the absence of CO 2 does not result in the appearance of peaks below À1V (not shown).
Electrolysis was performed using aAgworking electrode on the hydrated CO 2 -saturated RTILs within ag as-tight electrochemical cell. Fore ach potential, 10 Co fc harge was passed before analysis of the products b < 1 HNMR spectroscopy and online gas chromatography.T he variation in product selectivity with applied potential using the RTIL [P 66614 ][124Triz] is shown in Figure 2a nd the Supporting Information, Table S1. It should be noted that no degradation of the IL was observed in the 1 HNMR throughout the electrolysis studies.
In [P 66614 ][124Triz],f ormate was the only detectable product in the solution phase.A tÀ0.7 V, 0.05 mmol of formate is produced at aF aradaic efficiency of 95 %. The open circuit potential of the system is À0.53 V; therefore, holding at À0.7 Vcorresponds to an applied overpotential of 0.17 V. Production of formate at À0.7 Vr elates to the first reduction peak seen in the CV of the [P 66614 ][124Triz] + H 2 O + CO 2 system. No formate production was observed at less negative applied potentials.Asecond, albeit smaller,formate maxima is seen at À1.3 Vw ith aF aradaic efficiency of 39 %. This potential corresponds to the second reduction feature seen in the CVs.T he formation of formate at À1.3 Vi s plausibly formed via an alternative mechanism to that seen at À0.7 V, for example by the decomposition of formaldehyde to formate or ar adical reaction with superoxide.F urther experiments are ongoing to ascertain the origin of the second reduction feature.F inally,a tÀ1.9 Vf ormate production is dramatically reduced and syngas is detected. This corresponds to the rapid increase in reduction current noted on the CV.Asummary of the reductive process can be found in Scheme 2.
Formate production is greatly decreased on the onset of CO production, which suggests that reduction of physically absorbed CO 2 competes with the reduction of chemically bound CO 2 in this potential range.T his may be due to  blocking of the electrode surface by adsorbed CO,C O 2 or other reaction intermediates.
In contrast with the reactive RTIL, electrolysis at À0.7 V in ah ydrated CO 2 saturated [P 66614 ][NTf 2 ]/MeCN electrolyte showed no formate production. This is consistent with the postulation that no lower energy pathway for CO 2 reduction is available owing to alack of CO 2 binding site on the [NTf 2 ] À anion. Furthermore,t here is as ignificant decrease in the production of CO with only 0.2 mmol formed at À1.9 V; that is,over an order of magnitude lower than the amount formed in [ .P erforming electrolysis using this ionic liquid at À0.7 Vfor 10 Cofcharge resulted in the production of formate (0.021 mmol) as well as formaldehyde (0.006 mmol) with ac ombined Faradaic efficiency of 63 %. Thec ontrast of high versus low Faradaic efficiencies and product formation at low potentials when reactive and non-reactive anions are employed, respectively, provides support for the proposal that the reactive anions can provide anew low energy pathway for CO 2 electroreduction.
In conclusion, the superbasic RTIL [P 66614 ][124Triz] has been shown to provide an alternative low-energy pathway for CO 2 conversion to formate.This is the first time that chemical binding of the neutral CO 2 molecule to the anion of aR TIL has been shown to decrease the activation energy required for electrochemical reduction. This subsequently leads to the lowest reported applied potentials for CO 2 reduction to formate on Ag electrodes to date with high Faradaic efficiencies.