Surface structured platinum electrodes for the electrochemical reduction of carbon dioxide in imidazolium based ionic liquids.

The direct CO 2 electrochemical reduction on model platinum single crystal electrodes Pt( hkl ) is studied in [C 2 mim + ][NTf 2 (cid:2) ], a suitable room temperature ionic liquid (RTIL) medium due to its moderate viscosity, high CO 2 solubility and conductivity. Single crystal electrodes represent the most convenient type of surface structured electrodes for studying the impact of RTIL ion adsorption on relevant electrocatalytic reactions, such as surface sensitive electrochemical CO 2 reduction. We propose here based on cyclic voltammetry and in situ electrolysis measurements, for the first time, the formation of a stable adduct [C 2 mimH–CO 2 (cid:2) ] by a radical–radical coupling after the simultaneous reduction of CO 2 and [C 2 mim + ]. It means between the CO 2 radical anion and the radical formed from the reduction of the cation [C 2 mim + ] before forming the corresponding electrogenerated carbene. This is confirmed by the voltammetric study of a model imidazolium-2-carboxylate compound formed following the carbene pathway. The formation of that stable adduct [C 2 mimH–CO 2 (cid:2) ] blocks CO 2 reduction after a single electron transfer and inhibits CO 2 and imidazolium dimerization reactions. However, the electrochemical reduction of CO 2 under those conditions provokes the electrochemical cathodic degradation of the imidazolium based RTIL. This important limitation in CO 2 recycling by direct electrochemical reduction is overcome by adding a strong acid, [H + ][NTf 2 (cid:2) ], into solution. Then, protons become preferentially adsorbed on the electrode surface by displacing the imidazolium cations and inhibiting their electrochemical reduction. This fact allows the surface sensitive electro-synthesis of HCOOH from CO 2 reduction in [C 2 mim + ][NTf 2 (cid:2) ], with Pt(110) being the most active electrode studied.


Introduction
Carbon dioxide (CO 2 ) emission arising from human activities is considered as a major contributor to the global greenhouse effect. Thus, the CO 2 conversion into useful chemical products and/or energy is a critical concern nowadays. 1 CO 2 reduction can yield a large variety of useful products, such as C 1 species like carbon monoxide (CO), formic acid (HCOOH), methane (CH 4 ) and methanol (CH 3 OH), or C 2 products like oxalic acid (H 2 C 2 O 4 ), ethane (C 2 H 6 ), ethanol (C 2 H 5 OH), and ethylene (C 2 H 4 ). Indeed, different alternatives have been studied to transform CO 2 , 2,3 which include electro-synthesis of pharmacological or industrial valuable compounds, 4,5 chemical reduction by heterogeneous catalysis, 6,7 photocatalytic conversion, 8,9 and electrochemical reduction, which comprises molecular catalysed electrochemical reduction of CO 2 10-12 and direct electrocatalytic reduction of CO 2 . [12][13][14][15][16] In particular, direct electrochemical conversion and recycling of CO 2 represents a promising pathway, where product selectivity strongly depends on two main factors; (i) the electrode material employed 13,15 and (ii) the nature of the solvent (aqueous/non-aqueous) supporting electrolyte system. 14,17 The type of solvent is an important factor for the electrochemical CO 2 reduction, since most studies in aqueous media present some difficulties in controlling the concomitant side reaction of hydrogen evolution, which takes place at similar thermodynamic potentials. 18 On the other hand, conventional aprotic non-aqueous media, such as dimethyl sulfoxide (DMSO), N,Ndimethyl formamide (DMF) or acetonitrile (AN), present several advantages, since they avoid hydrogen evolution and present high CO 2 solubility (see Table 1). However, their main drawbacks are their high toxicity and volatility, the costly electrolytes employed and the difficulty in recycling the solvent and the electrolyte. A special category of non-aqueous solvents is comprised of room-temperature ionic liquids (RTILs), which have been pointed out as the most suitable solvents for carrying on the future developments in electrochemistry. 19 RTILs are commonly defined as materials composed entirely of organic cations and organic or inorganic anions, which melt at or below 100 1C. 20 This low melting point is their main difference versus molten salts, which require high working temperatures. Moreover, RTILs present a unique set of properties, which includes suitable ionic conductivity, high thermal stability, negligible vapor pressure and non-flammability. 21 All these properties allow RTILs to perform simultaneously the role of a solvent and an electrolyte in electrochemical applications and rule out any safety concern. RTILs comprise a large number of different solvents, but depending on whether the cation employed presents an available proton or not, two main subdivisions are possible; protic ionic liquids (PILs) 22 and aprotic ionic liquids (AILs). 23 Imidazolium based cations represent an example of that, since depending on the degree of substitution within the imidazolium ring they belong either to the PILs (1,2-dialkyl-imidazoliums) or AILs (1,3-dialkyl-imidazoliums and 1,2,3-trialkylimidazoliums). One of the major drawbacks of RTILs in electrochemical applications is their high viscosity, since this produces low diffusion of the electroactive species and limits the maximum current density in the process. The viscosity of RTILs varies with the type of cation and anion. In particular, 1,3-dialkyl-imidazolium [C n mim + ] cations are one of the most common RTIL cations for electrochemical purposes, since they exhibit both high ionic conductivity and low viscosity values in comparison with all other RTIL cations. Conductivity and viscosity are inversely related, the alkyl chain length (n) of the imidazolium cation being responsible for limiting both. For this reason, the short alkyl chain (n = 2) cation, 1-ethyl-3methylimidazolium [C 2 mim + ], presents both low viscosity and high conductivity values and is selected as a suitable solvent in the present work. This cation is combined with an asymmetric anion like bis(trifluoromethylsulfonyl)imide [NTf 2 À ], which also presents one of the lowest viscosity values among the most common anions in RTILs. 21 Thus, [C 2 mim + ][NTf 2 À ] exhibits a low viscosity (29 cP at 298 K) 21 combined with a high solubility of CO 2 , comparable to the one exhibited in non-aqueous solvents (see Table 1). In principle, the gas solubility in RTILs is mainly controlled by the nature of the RTIL anion. Thus, a strong interaction of CO 2 with the RTIL anion is the best indicator of CO 2 solubility, with [NTf 2 À ] being the anion presenting one of the greatest affinities for CO 2 . 24  ] anions absorbed alternatively next to each other on the electrode substrate. 29,30 For this reason, single crystal electrodes represent the most convenient type of surface structured electrodes for studying the impact of RTIL ion adsorption on electrocatalytic reactions, since they provide site distribution control at the electrode surface. In particular, we propose studying the electrocatalytic activity for CO 2 reduction as a function of surface sites using Pt single crystal (Pt(hkl)) electrodes, since it has been already proved that imidazolium cation adsorption is a surface-sensitive process 31 and this may drastically affect the CO 2 reduction activity at the electrode.
Electrochemical reduction of CO 2 on Pt(hkl) electrodes in acid aqueous solution takes place at potentials where hydrogen adsorption also occurs. Moreover, this reaction presents high sensitivity towards the surface site available at the electrode surface by displaying low activity at highly ordered Pt(111) and Pt(100), but high activity at Pt(110) kinked and stepped surfaces. [32][33][34] More recently, several Pt(hkl) electrodes selectively modified with adsorbed adatoms (Bi, Te and Sb) have also exhibited high catalytic activity for CO 2 reduction in buffered neutral aqueous media. 16 In contrast, electrocatalytic studies for CO 2 reduction in different RTILs have been only reported using Pt polycrystalline surfaces. 35,36 But, as far as the author's knowledge, the electrochemical reduction of CO 2 on Pt(hkl) electrodes in a RTIL medium has not been reported yet. Only a few articles devoted to studying conducting polymers 37 or electrochemical hydrogen 38,39 and carbon monoxide 40 oxidation reactions on Pt(hkl) electrodes in different RTILs can be found in the literature.
It is well established in the electrocatalytic CO 2 reduction reaction that the first reaction step consists of the CO 2 adsorption at the electrode surface and the subsequent electron transfer to form the corresponding radical-anion (CO 2 À ), as described in eqn (1), where M stands for the metal electrocatalyst. 15 Moreover, in liquid electrolytes it has been also described that CO 2 may form adducts by coupling with different anions present in solution, such as BF 3 Cl À or 1-isoquinolinecarboxylic acid. 35,41 The formation of those adducts weakens the CQO bond and makes the reduction of CO 2 more favorable. For this reason, it has been recently suggested that a convenient way to lower the potential necessary for achieving the first electron transfer for CO 2 reduction is decreasing the CO 2 À free energy of formation. This is achieved by forming a complex between the CO 2 À and an imidazolium based cation such as [C 2 mim + ], since this will help to decrease the overall energy barrier. 42 The formation of such a complex has been proposed to be mainly responsible for the highly active ionic-liquid mediated electrochemical conversion of CO 2 into CO at low overpotentials in a mixture of water and an imidazolium based ionic liquid. After the first electron transfer forms CO 2 À , several possible reaction pathways may be followed depending on the solvent conditions. They can be mainly divided into those that need the abundant presence of available protons and a second electron transfer, which may form either HCOOH (eqn (2)), if the proton is attached to the CO 2 À , or CO (eqn (3)), if the proton removes one of the oxygen atoms within the CO 2 À . In contrast, if protons are not available, as usually happens in nonaqueous systems, the dimerization reaction mainly produces C 2 O 4 2À (eqn (4)).

Chemicals and electrodes
The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C 2 mim + ][NTf 2 À ] 98% was supplied by Aldrich. This was used straight from the supplier, and its water content was minimized by maintaining it under an argon atmosphere within a silica gel container. Pt polycrystalline electrodes and Pt(100), Pt(110) and Pt(111) single crystal electrodes 43 were made from single crystal platinum beads (2.5 mm in diameter), obtained by fusion and subsequent slow crystallization of a 99.999% platinum wire (0.5 mm in diameter). For the Pt(hkl) electrodes, after careful cooling, the resulting single crystal beads were oriented, cut and polished following the procedure described by Clavilier et al.

Electrochemical characterization
A one compartment electrochemical glass cell with a threeelectrode configuration at room temperature and atmospheric pressure was employed for performing cyclic voltammetry experiments. All measurements were made using a computer controlled Autolab PGSTAT30 potentiostat driven by NOVA software. The electrocatalytic activity for the CO 2 reduction reaction was studied in the presence and the absence of a proton source using [C 2 mim + ][NTf 2 À ] as a solvent supporting electrolyte system. Prior to any experiment, the electrochemical cell was dried in hot air and Ar gas was flowed into the empty cell in order to remove any trace of water. A small volume of [C 2 mim + ][NTf 2 À ] (B3.5 mL) was used in each experiment and this was saturated with either Ar or CO 2 gas for at least 20 min before each experiment. A platinum wire, 0.5 mm diameter, was used as a counter electrode and the reference electrode was also another Pt wire (quasi-reference electrode). This reference was placed inside of a glass cylinder with a glass frit separator at the bottom and immersed in a solution identical to the one initially used within the electrochemical cell. This is necessary to isolate the quasi-reference electrode as much as possible from potential chemical interference derived from the faradic processes occurring in solution during cyclic voltammetry. 46 This is a crucial issue here, since the highly negative potentials reached in voltammetries may provoke H 2 evolution to some extent, which in contact with the Pt wire quasi-reference electrode may modify its open circuit potential and subsequently, the reference potential in the electrochemical cell.

Results and discussion
RTILs generally exhibit a wide electrochemical potential window. But, in the absence of any other electroactive species, they may undergo electrochemical degradation. The electrochemical stability of the imidazolium cation limits the cathodic polarization window, since the cation [C 2 mim + ] may be reduced to form the corresponding radical [C 2 mim ]. The carbon atom between the two nitrogen atoms in the imidazolium ring is the most electron deficient atom and is hence the most active site for reduction as shown in eqn (5). 47  38 However, if the cathodic limit of potential is extended up to À3.1 V, (Fig. 2), this quasi-reversible behaviour disappears. This has been already assigned in the literature to the formation of the corresponding imidazolium Furthermore, it is possible to observe in Fig. 2 the H 2 evolution reaction in all three low index planes of platinum. Oxidation of the newly formed H 2 can be more clearly observed on Pt(100) and Pt(110) electrodes in the reverse potential scan (inset of Fig. 2, oxidation peak between À1.7 and À1.0 V). In contrast, the oxidation peak shown at À0.5 V in Fig. 2 has been assigned in the literature 49 to the oxidation of the electrogenerated imidazolium carbene back to the initial cation. In particular, Feroci and co-workers have proposed the use of those electrogenerated imidazolium carbenes as a synthetic tool for capturing CO 2 and forming an imidazolium-2-carboxylate product by a coupling reaction between the electrogenerated carbene and CO 2 as described in eqn (8). 49,50 This reported reaction takes place outside the electrochemical cell, by CO 2 saturation of [C 4 mim + ][BF 4 À ] after performing a galvanostatic reductive electrolysis on glassy carbon, Pt, Pd or Au for electrogenerating the corresponding imidazolium carbene. This CO 2 coupling reaction is reported to produce a diminution in the oxidation current at À0.5 V assigned to the carbene oxidation back to the initial cation when the voltammogram in the CO 2 presence is compared with the one in the CO 2 absence, since most of the carbene is consumed by reaction with CO 2 . For this reason, the imidazolium-2-carboxylate product formed in eqn (8) is reported as an electroinactive adduct, 49 which may only give back CO 2 and imidazolium cations by heating induced decomposition.  (Fig. 3), the oxidation peak current at À0.5 V increases for Pt(110) and Pt(100) in the presence of CO 2 , and decreases only for Pt(111), which points out that different processes are taking place. Firstly, H 2 evolution is inhibited in all three Pt(hkl) electrodes when CO 2 is present in the electrochemical cell, since no subsequent oxidation current peaks are displayed in the reverse potential scan between À1.7 and À1.0 V (see the inset of Fig. 3). Secondly, the comparison between the insets in Fig. 2 and 3 shows an important increase in the oxidation peak current displayed at À0.5 V for Pt(110) and Pt(100) electrodes when CO 2 is present, but none for Pt(111). These facts indicate that the process taking place at potentials more negative than À2.0 V is not only the [C 2 mim + ] reduction following eqn (6) or (7). In fact, this corresponds to the simultaneous reduction of [C 2 mim + ] and CO 2 species on the Pt(100) and Pt(110) electrodes, which produces the formation of a new stable species [C 2 mimH-CO 2 À ] by a radical-radical coupling of both reaction   products formed following eqn (1) (CO 2 À ) and (5) [C 2 mim ], as globally described in eqn (9) for the first time. In contrast, at the Pt(111) electrode, which has been already reported to be less active in acid aqueous solutions for the CO 2 electrochemical reduction, 34 this radical-radical coupling reaction may not take place in [C 2 mim + ][NTf 2 À ], since À3.1 V is not a cathodic enough potential for significantly reducing CO 2 on Pt(111) following eqn (1). Thus, the reported coupling reaction by Feroci and co-workers between the electrogenerated carbene and CO 2 described in eqn (8) takes place at Pt(111). However, the imidazolium-2-carboxylate formed is electroinactive and for this reason the peak current at À0.5 V decreases in the presence of CO 2 (1.1 mA cm À2 ) with respect to the absence of CO 2 (1.6 mA cm À2 ), as described by Feroci et al. 49,50 In contrast, the new [C 2 mimH-CO 2 À ] adduct described in eqn (9) and formed on Pt(110) and Pt(100) is electroactive and may be easily oxidized back to the initial imidazolium cation at À0.5 V. For this reason, the oxidation peak current for Pt(110) and Pt(100) displayed in the inset of Fig. 3 is 2-3 times larger than the one displayed in the inset of Fig. 2.
(9) Fig. 4 shows the electrochemical behaviour of 1,3dimethylimidazolium-2-carboxylate on the Pt polycrystalline electrode as a model carboxylic product of the CO 2 coupling reaction reported in eqn (8). Fig. 4 shows the total electroinactivity of that compound within the potential range of interest, from À0.75 V to 1.0 V, in agreement with the literature, since the current density displayed in Fig. 4 is negligible in comparison with all other voltammograms and no oxidation or reduction peaks appear. This fact demonstrates that the product generated at À3.1 V by simultaneous reduction of [C 2 mim + ] and CO 2 at Pt(110) and Pt(100) electrodes does not follow eqn (8) and thus, it is not the imidazolium-2-carboxylate derivate already described in the literature.
In order to demonstrate that the formation of that new [C 2 mimH-CO 2 À ] adduct requires the initial electrochemical reduction of both [C 2 mim + ] and CO 2 , as described in eqn (9) and was never proposed before, Fig. 5 shows a short in situ electrolysis at À3.4 V in the presence and the absence of CO 2 followed by a linear voltammogram from À3.  (3)). 42 In the absence of CO 2 , Fig. 6 displays a clear reduction feature at À1.0 V, corresponding to the reduction of protons to form H 2 at the Pt(hkl) electrode surface and its corresponding reversible oxidation on the reverse potential scan. 38 This process seems to inhibit the [C 2 mim + ] reduction, because no reduction peak is shown at À0.5 V. Thus, the electrode-RTIL interface has been drastically modified by the presence of a high concentration of available protons, which are preferentially adsorbed and have  displaced the imidazolium cations from the surface of the electrode, catalyzing the proton reduction and inhibiting the [C 2 mim + ] reduction.
In contrast, Fig. 7 shows the voltammetric profiles in the presence of dissolved CO 2 and protons with different lower potential limits. When the lower potential limit is set at À2.5 V (Fig. 7A), no significant changes are observed in the presence of CO 2 in solution, which indicates that the CO 2 reduction reaction is not taking place. Only when the lower potential limit is set below À3.0 V, changes in the voltammogram are observed, which indicates that CO 2 reduction reaction is taking place. Fig. 7B and C show a new oxidative peak appearing at 0.3 V (see the inset of Fig. 7B and C), which is univocally linked to the reduction of the CO 2 species present in solution, since this peak is absent in an argon saturated solution (Fig. 6). This new oxidation peak does not fit our previous findings on the study of the CO electrooxidation reaction in [C 2 mim + ][NTf 2 À ], since the CO oxidation peak appears at more positive potentials. 40 Furthermore, a very similar peak has been assigned in a recent paper by Compton's group 36 to the oxidation of the HCO 2 H/ HCO 2 À species formed at a polycrystalline Pt electrode surface after CO 2 reduction in [C 2 mim + ][NTf 2 À ]. Thus, herein we prove that the electro-synthesis of HCOOH from CO 2 reduction in [C 2 mim + ][NTf 2 À ] is a surface sensitive process, since the three different basal planes of Pt produce different amounts of HCOOH, which is denoted by the different peak area displayed in Fig. 7B and C and quantified in Table 2. Thus, the results in Table 2 show that the Pt(110) electrode is the most active basal plane for this reaction and Pt(111) and Pt(100) electrodes exhibit a similar low activity (3 times lower than that of the Pt(110) electrode at À3.5 V).

Conclusions
We study here the direct electrochemical conversion and recycling of CO 2 in one of the most used AILs, [C 2 mim + ][NTf 2 À ], which is mainly hydrophobic and presents a suitable set of physical properties for electrochemical purposes such as moderate viscosity, high conductivity and high CO 2 solubility. Moreover, we use Pt single crystal electrodes, which represent  ]. Scan rate: 0.05 V s À1 . Plots A, B and C represent the same experiment, but reaching a different limit in negative potential: À2.5 V for plot A, À3.1 V for plot B and À3.5 V for plot C. Table 2 Charge associated with HCOOH formed by CO 2 reduction, which is quantified by integrating its oxidation peak at 0.3 V as shown in Fig. 7B