Electrochemical Tuning of CO 2 Reactivity in Ionic Liquids Using Different Cathodes: From Oxalate to Carboxylation Products

: There is currently quite a lot of scientific interest in carbon dioxide (CO 2 ) capture and valorization with ionic liquids (ILs). In this manuscript, we analyze the influence of the potential applied, the nature of the cathode and the electrolyte using different organic mediators, such as nitro or cyano aromatic derivatives, to promote the electrochemical activation of CO 2. An electrocatalytic process using a homogeneous catalysis is seen when nitroderivatives are used, yielding to oxalate in organic electrolytes and ILs. Turnover frequency (TOF) values and Farafay efficiencies were slightly higher in N,N’ -dimethylformamide (DMF) than in ILs probably due to the viscosity of the electrolyte. The use of cyano derivatives allows to tune the electrochemical reactivity in function of the reduction potential value applied from electrocarboxylated products (via a nucleophile-electrophile reaction) to oxalate. These electrochemical reactions were also performed using three different cathodes, organic electrolytes and ionic liquids. The use of copper, as a cathode, and ionic liquids, as electrolytes, would be a cheaper and greener alternative for activating carbon dioxide.


Introduction
Nowadays methane, nitrous oxide and carbon dioxide (CO2) emissions represents approximately 98% of the total greenhouse gas (GHG) inventory worldwide [1][2][3], and their share are expected to increase this twenty-first century. CO2 represents the most important GHG, which represents approximately the 77% of the global GHG emissions (considering its global warming potential) worldwide. Moreover, the change in atmospheric CO2 concentration has been considered the most important driver of global warming. In 2019 carbon dioxide emissions from energy stagnated at 33 Gt, which are mainly due to anthropogenic activities. Besides, it is expected to increase to 40.3 Gt by 2030 and to 50 Gt by 2050, if proper measures are not taken [4][5][6][7].
In the last decade, new concepts have been developed as a suitable approaches for facing the challenges of the current global scenario; so biomimetic [8] and circular economy models have been formulated. All the approaches involve a first capture step for an efficient removal of CO2 from common points sources prior to the release of gases into atmosphere [9][10][11][12].
Carbon capture and storage (CCS) approaches include planning the confinement of CO2 into depleted oil and gas wells, deep oceans, and aquifers [13]. Capture approaches based on chemical absorption and desorption using an aqueous amine solution are also one of the most promising option for separating CO2 from fossil-fuel-derived flue gas due to its simple operation, high absorption efficiency, cost-effectiveness and maturity [14,15]. However, all these strategies only partially solve the problem, so approaches to recover valuable products from its conversion through a circular economy vision are highly desirable [16,17]. In this sense, some conversion routes are designed for the reuse of fuels, especially when inexpensive renewable energy processes are available.
New approaches that transform CO2 in fine chemicals or value-added products are also being deeply investigating. The remunerability of the product is much higher and can economically support the development of the technologies involved. However, the volume of the potential market of these fine chemicals can hardly match with the volume of CO2 emissions, needing the development of a network of parallel CCU technologies. Hence, different strategies, including biological methods, have been developed either for directly producing reduced products (i.e., carbonic anhydrase, hydrogenation of CO2 to formate, reduction of CO2 to methane, CO2 conversion into methanol by enzyme cascade) or to store CO2 in biomass (e.g., algae) [18][19][20]. The photocatalytic reduction approaches also allow to synthetize a wide spectrum of CO2 reduction products, such as HCOOH, HCHO, CH3OH, or CH4, and can be effectively used by using visible responsive materials [21][22][23][24][25][26][27]. Adsorption, CO2 activation and further reduction to produce value-added products are crucial steps for photocatalytic processes. Sharma et al. reported a combined theoretical and experimental study describing the selective reduction of CO2 into methane through a robust visible-light photocatalyst based on single-phase ternary sulfide (CTS) [28]. Zhou et al. proposed the use of aqueous suspensions of cubic ZnS nanocrystals for the photocatalytic reduction of CO2 into formate [29]; and the development of heterostructures, such as Cu2O/TiO2, for artificial photosynthesis [30].
Chemical catalytic strategies for CO2 reduction have been proposed mainly based on the Sabatier reaction [31]. Finally, it is worthy to highlight that in the last years, it has been also reported that the development of electrochemical technologies to capture and transform CO2 into high-added value products is a suitable and green way for activating CO2 compared to others Carbon Capture, Utilization and Storage (CCUS) strategies [32].
Direct carboxylation of carbon nucleophile using CO2 as an electrophile is a straightforward route to prepare carboxylic acids. The main drawback associated to this process is related to the use of toxics reagents and the production of a large amounts of waste [33]. For instance, the conventional route for synthesizing 6-aminonicotinic acid from the corresponding nitriles involves low yields, hazardous chemicals (such as cyanide and ammonia gas) and high temperatures [34][35][36]. Nevertheless, using organic electrochemistry techniques improve environmental conditions [37][38][39][40][41][42][43][44][45][46][47][48][49][50]. One of the most widely used approaches for valorizing CO2, is to use organic halides with electrochemical techniques, which makes it possible to activate the CO2 (Scheme 1, Equations (1)-(4)); In a first step, a one electron transfer process generates the organic radical anion, which later converts to an anion though a second reduction electron transfer and a halide anion. The key to this approach relies on the reduction potential value of the organic halides and the stability of the organic anion formed after the reduction process [51−65]. Scheme 1. Electrochemical strategies to valorize CO2.
The development of selective electrocatalysis processes for the reduction of carbon dioxide (CO2) to yield C1 products (such as CO (2e − ), or higher Cn products) would allow the use of CO2 as a carbon feedstock (Scheme 1). Nevertheless, it is necessary to overcome high-energy barriers for the direct reduction of CO2, which commonly implies reduction potential values significantly more negative than the corresponding thermodynamic reduction potential value of CO2. In this sense, different types of electrodes have been studied for direct reduction of CO2. These cathodes are classified based on the nature of the main product obtained in the electrochemical process in aqueous and nonaqueous supporting electrolytes [66]. Moreover, it has also been developed other kinds of modified or doped electrodes (with immobilized enzymes, nanoparticles or metallic oxides) to catalyze CO2 direct reduction [67][68][69]. Hence, the improvement of electrocatalytic processes could lower overpotential requirements while maintaining appropriate catalytic rates and selectivity .
One of the most common techniques to analyze electrocatalysis is cyclic voltammetry, which provides direct and rapid information regarding the relationship between driving force (i.e., overpotential) and catalytic turnover (i.e., current) compared to rather complicated alternative methods monitored by spectroscopy or other means. However, understanding detailed mechanistic schemes behind voltammograms is quite challenging due to complicated and intertwining processes, such as mass transport, electron transfer, chemical reaction and interfacial chemistry between electrode and reactants. In this sense, developing the foot of the wave analysis (FOWA) of cyclic voltammetry by Savéant and Costentin has provided a feasible method to benchmark molecular electrocatalysts by turnover frequency (TOF) and turnover number (TON) [92][93][94][95][96].
Molecular catalysts are an attractive option owing to the high degree of tunability of electronic and geometric parameters, which allows for systematic reactivity studies that can lead to new catalyst design guidelines. The well-defined structure of the catalytic sites also allows the establishment of a precise structural model for better analysis of the multiple proton-coupled electron transfer processes involved in CO2 reduction and better understanding of the CO2 reduction mechanism. For example, Leung et al. proposed a mechanism for CO production using Co-based porphyrins [97,98], where the [Co(P)-(CO2)] 2− intermediate will be protonated to form [Co(P)-(COOH)] − . Later, it will decompose to provide CO. Koper and co-workers proposed a different mechanism [99], where the formation of the CO2 − , anion intermediate, will be protonated yielding to [Co(P)-(COOH)] 0 intermediate. Then, [Co(P)-(COOH)] 0 will decompose to CO. Finally, Yao et al. proposed a pre-activation process to form a local proton source could facilitate CO2 reduction [100].
The electrocatalysts participates in an electron transfer reaction (on the electrode surface) and facilitates acceleration of a chemical reaction. Both electron transfer and chemical kinetics must be fast for an efficient electrocatalyst. In addition, an optimal electrocatalyst must display a good thermodynamic match between the redox potential (E 0 ) for the electron transfer reaction and the chemical reaction that is being catalyzed (in this work; CO2 reduction). Chemical tuning of the electrocatalyst can optimize these factors. A general approach for an electrocatalytic system is given in Scheme 2.
The electrocatalytic activity can be analyzed in terms of in cyclic voltammetry (CV). In a CV under a dry inert atmosphere, an electrocatalyst should show a reversible redox couple. Upon addition of the reagent which is catalyzed (i.e., CO2), the diffusion limited current should increase significantly, while the potential shifts anodically, and the reversibility in the return oxidation wave is lost due to the chemical reaction between reagent and electrocatalyst. Electrocatalyst offer critical solutions to lowering the overpotentials, improving selectivity, and increasing the reaction kinetics of carbon dioxide conversion [101]. There are a lot of different catalysts reported in literature, although most of them involve the use of metal complexes, such as salen ones [102,103], offering highs values of TOF with orders of h −1 . On the other side, ionic liquids have been developed as a new catalyst, with a high dependence between TOF and the nature of the cation and anion which form the ionic liquid [104]. The order of TOF in these cases are in order of s −1 , allowing avoiding but the use of metals.
Therefore, this work shows different strategies to valorize CO2 using electrochemical technologies in ionic liquids. In one hand, with a series of nitro-compounds ( Figure 1) using three different cathodes (carbon, copper and silver). One the other hand, the replacement of the nitrogroup for a cyano-group, involves changes in the reaction mechanism and tune the CO2 reduction products.

Cyclic Voltammetry
All electrochemical experiments were performed in an electrochemical conical cell with a set-up of the three-electrode system. For CV experiments, the working electrode is a vitreous carbon disk (1 mm diameter), silver disk (3 mm diameter) and copper disk (3 mm electrode). The counter electrode is a Pt disk (<1 mm diameter). All electrodes are polished using a 1 mm diamond paste. All the electrochemical potentials were measured using a saturated calomel electrode, SCE (+0.2411 V vs. SHE) isolated from the working electrode compartment by a salt bridge (salt-solution of the reference calomel electrode is separated from the electrochemical solution by a salt-bridge ended with a ceramic material frit, allowing ionic conduction between the two solutions and avoiding appreciable contamination). Ideally, the electrolyte solution present in the bridge is the same as the one used for the electrochemical solution in order to minimize junction potentials. When such a bridge is used, the ions in the bridge are present in large excess at the junction and they carry almost the whole of the current across the boundary. In our case were used DMF/0.1M TBA BF4 (trying to weigh the same amount of support electrolyte that in the electrolyte solution) or EMIM TFSI, in both cases without electroactive substance. The error associated with the potential values is less than 5 mV. The ohmic drop can be one of the main sources of error when ILs are used as solvents, since they are more resistive media than aprotic polar solvents with 0.1 M concentration of supporting electrolyte. Before and after performing any electrochemical experiments, the solution is purged with inert gas (nitrogen or argon) for 20 min to avoid secondary reactions associated to OER and ORR due to dissolved oxygen in the solution [143,144].
The number of electrons involved in the first reduction process of nitro-compounds were determined by comparison with very well-known one-electron reduction of 9-fluorenone (redox probe), in the same medium using the same electrochemical set-up, by terms of cyclic voltammetry.
It is used 9-fluorenone and nitrobenzene as probe because both compounds have the same magnitude value of diffusion coefficient without further limitations (DMF (10 −9 m 2 ·s −1 ) nor ionic liquids (10 −11 m 2 ·s −1 ) [145,146]. The number of electrons involved in this first electron transfer was also confirmed by controlled-potential electrolysis.

Electrocarboxylation Processes
Cyano-compounds were electrolyzed at a negative potential of ~0.1 V more negative than the Epc potential value under nitrogen/argon or carbon dioxide saturated solutions. When the reaction is completed, the resulting solution in the electrolysis is extracted with ether. The organic layer is dried with Na2SO4 and evaporated to yield a residue that is analyzed by gas chromatography-mass spectrometry, and Proton Nuclear Magnetic Resonance ( 1 H-NMR). Thus, all products obtained, and the commercial analogues were characterized by 1 H-NMR. Measurements were made using a DPX360 (250 MHz) spectrometer (Bruker, Billerica, MA, USA) spectrometer. Proton chemical shifts were reported in ppm (d) (CDCl3, δ 7.26, or CD3CN, δ 1.94). The J values are reported in Hz.  When pure IL is used as electrolyte, the products of the electrolyzed solution are extracted with ether, allowing one to recover almost an 80% of the IL at the end of the experiment [147].

Determination of CO2 Concentration
A thermal mass flowmeter of modular construction with a 'laboratory style' pc-board housing (EL-FLOW ® Mass Flow Meter/Controller) from Bronkhorst Hi-Tec (Ruurlo, Netherlands), was used to monitor the CO2 concentrations in the solution. Control valves were integrated to measure and control a gas flow from the lowest range of 0.2 up to 10 mL·min −1 .

Electrochemical Behaviour of Nitro-Compounds under Inert Atmposphere
The electrochemical behavior of a sequence of four p-nitro-compounds was studied in different solvents, and electrodes under inert atmosphere.
Cyclic voltammograms (CVs) of a 3-10 mL solution of 1-4 in DMF using 0.10 M of tetrabutylammonium tetrafluoroborate (TBA BF4), and 1-methyl-1-ethylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM TFSI) were recorded at different scan rates (from 0.10 a 1.0 V s −1 ) using glassy carbon (C, blue lines), Ag (black lines) and copper, Cu (red lines) as working electrodes under a N2 atmosphere ( Figure 2). Focusing in the first wave, the same general trend was observed in all electrodes and both solvents. Compounds 1-3 shows a monoelectronic fast reversible electron transfer, whereas compound 4 has an irreversible electron transfer (Table 1).  A closer look in CVs shows that of 4-iodonitrobenzene has irreversible wave at low scan rate, however it is possible to calculate the E 0 and the value of the kinetic constant associated to the chemical reaction (k2·10 4 s −1 ) by increasing the scan rate (Scheme 3).

Scheme 3. Electron transfer followed by compounds 1-4.
The cyclic voltammogram of 4 also shows a second monoelectronic reversible wave following the first irreversible one. To check the nature of the product obtained after this first electron transfer, a control potential electrolysis of 4 was performed, being nitrobenzene (5) the only product obtained. Considering cyclic voltammetry and control potential electrolysis experiments under inert atmosphere, it is possible to conclude that 4-iodonitrobenzene follows an electron transfer mechanism (E) followed by a chemical reaction (C). Hence, in a first electrochemical step (E) the radical anion, 4 · -, was generated, which evolves to cleavage C-I bond, obtaining nitrobenzene radical which produces nitrobenzene and iodine with an EC mechanism (Scheme 4).

Electrochemical Behaviour of Nitro-Compounds under CO2 Atmposphere
Compounds 1-4 shows a clearly different behaviour when the cyclic volammograms are recorded under CO2 atmosphere, although they show the same general trend in aprotic solvent and ionic liquid. Also the follow same tendency in C, Ag and Cu working electrodes; there was a rising in current value of the reduction peak and its reversibility was lost (Figure 3), which indicates that pnitrocompounds 1-4 follow a demeanor of an organic mediator for CO2 indirect reduction through a homogeneous catalytic process (Scheme 5).

Scheme 5.
Proposal mechanism for general catalytic process between CO2 and p-nitrobenzene halides.
In the case of carbon cathode, C, it is possible to see an oxidation peak in the corresponding anodic counter scan (copper and silver electrodes would be oxidized under this experimental conditions) between +0.19 and +0.44 V (vs. SHE). In all cases this peak corresponds to the oxidation of oxalate anion [149], which is formed after reducing the nitroaromatic compound. It is important to remark that the oxidation potential value obtained for oxalate is strongly dependent of the amount of water present in the solvent (Figure 4). The same results are obtained for a sample of pure tetrabutylammonium oxalate, since different amount of water are the oxalate oxidation peak shifts to higher oxidation potentials. Moreover, by using a carbon cathode it is possible to determine the concentration of oxalate formed upon reduction using CV, since the anodic peak current value is related with the oxalate concentration. Hence, we built a calibration curve using pure tetrabutylamonium oxalate for determining the amount of oxalate obtained upon reduction for each p-nitro compound. Finally, the TOF values were obtained using through FOWA's method. All the values were summarized in Table 2.  Knowing that with carbon cathode it is possible to determine oxalate's peak with CV, with a calibration curve is determined the amount of oxalate obtained with each p-nitro compound. Also, it is possible to obtain TOF values through Fowa's method [92][93][94][95][96]. All the values were summarized in Table 2. Note that changing a methyl group by halides, the standard potential is lowered between 80-200 mV depending on electrode and halide. TOF values and oxalate amount obtained in DMF were higher than in EMIM TFSI. Faraday efficiencies were slightly higher in DMF solutions (from 30% to 88%) than in EMIM TFSI (from 10 to 51%) depending on the organic mediator used. These results can be explained due to low viscosity of the ionic liquid and coefficient diffusion value of CO2 (Table 3).  [131].
Note, that it is possible to see an improvement in overpotential values with the use of ionic liquid instead DMF when p-nitrocompound has a halide group in its structure. Finally, note that it is possible to use small organic compound and ionic liquids to obtaining similar TOF values (in the order of s −1 ) to previously published one for organometallic and more sophisticated organic compounds [89,138,[148][149][150]. On the other hand, we observed that for compound 4 the anion radical generated after electron transfer evolves to nitrobenzene radical at the same time that the CO2 reduction is electrocatalyzed. To check if it is possible to obtain electrocarboxylation product of 5 upon reaction of its radical with CO2, a control potential electrolysis at −0.96 V (vs. SHE) was performed. After the passage of 1F, no electrocarboxylated aromatic compounds were formed, being oxalate the only electroreduced product of CO2 formed. These results seem to indicate that the nitrobenzene radical formed upon reduction of 4 (Scheme 4) show a low nucleophility, being not able to attack and capture CO2.

Electrocarboxylation of Cyano-Compounds 6 and 7 under CO2 Atmposphere
According to previous results obtained in our research group [151], 4-iodobenzonitrile follows an ECE reduction mechanism in ionic liquids (Scheme 6). In the presence of CO2 the electrogenerated anion intermediate leads to the corresponding carboxylation process. Hence, with the aim to obtain a stronger electrogenerated nucleophile, R − (Scheme 6), we decided to move from nitro to cyano aromatic compounds. Thus, two new different p-cyanobenzene halides (4-chlorobenzonitrile (6) and 4-bromobenzonitrile (7)) were explored in this section. Scheme 6. General overview for an ECE electrocarboxylation mechanism. Figure 5 shows CVs at DMF/0.1M TBA BF4 and one example in EMIM TFSI obtained under inert atmosphere and CO2 atmosphere. All electrochemical parameters are summarized in Table 4. The same general trend was observed in aprotic solvents and ionic liquids. Moreover, not significate differences were detected when the nature of the working electrode was changed.  As it was expected compounds 6 and 7 exhibit an irreversible bielectronic wave under nitrogen atmosphere for any of the working electrodes investigated (Figures 5a,b). The same electrochemical behavior is observed when DMF is replaced by EMIM TFSI. A closer look to cyclic voltammograms reveals the electrocatalytic properties of silver as a working electrode, since the peak potential value of the reduction peak is shifted to less negative potentials. In order to determine the nature of the product formed upon reduction, a controlled potential electrolysis was performed. In all the cases benzonitrile, 8, was the only product obtained after the passage of 2F. Taking into account CVs and the electrochemical data obtained from the electrolysis, it is possible to conclude that compound 6 and 7 follows an ECE mechanism. In a first electrochemical step, the radical anion of benzonitrile halide was generated. After that, a chemical reaction coupled to this first electron transfer led to a benzonitrile radical and the corresponding halide anion through a C-X cleavage reaction. Later, benzonitrile radical was reduced to its corresponding anion at the electrode surface (Scheme 7). It is important to remark that when cyano derivatives are used, it is possible to electrogenerate a strong nucleophile (benzonitrile anion), which is stable enough under our experimental conditions for reacting with CO2 leading to the corresponding electrocarboxylated product. Scheme 7. ECE proposal mechanism for the electrochemical reduction of 6 and 7.
Figures 5c,d show that halobenzonitriles showed the same electrochemical behavior, a first twoelectron reduction wave is observed for halobenzonitriles (Figures 5a,b) in nitrogen and carbon dioxide atmosphere. In none of the cases the presence of oxalate anion was detected.
Once the electrochemical reduction mechanism of halobenzonitriles under CO2 atmosphere was disclosed, electrocarboxylation process were performed using carbon graphite rod. For all the compounds the applied potential was 0.1 V more negative than potential of the first reduction wave. The results of carboxylation reactions were summarized in Table 5. It could observe that pcyanobenzoic acid (9) and benzoic acid (10) were obtained in moderated yields after the passage of 2F. Benzonitrile (8) is also obtained, showing that there was a competition between carboxylation and protonation process. Benzoic acid obtention would be explained because at the potential of electrolysis, p-cyanobenzoic acid is also reduced, since its reduction potential is c. a. −1.36 V (vs. SHE).  Figure 6 shows several cyclic voltammograms of 6 and 7 under nitrogen and CO2 atmosphere using silver and glassy carbon (GC) as a cathode. The use of DMF as a solvent and GC as a cathode allows to see a second monoelectronic fast reversible wave after first one ( Figure 5). This second wave corresponds to the electrochemical reduction of benzonitrile, which is the only product obtained after the reduction of compounds 6 and 7. It is noticeable that benzonitrile acts as a redox mediator for CO2 reduction when reaction is performed under CO2 atmosphere, since there is an increase of the peak current value and a loss of reversibility of this second reduction wave (homogeneous indirect CO2 reduction catalytic process (Figures 6a,c)). The use of silver electrode allows to tune the CO2 reduction mechanism. In this case the current value of the first reduction is increasing as well as the peak potential value is shifting when the concentration of CO2 is rising. These results can be explained taking into account that under these experimental conditions two electrochemical processes are happening at the electrode surface at same potential; the reduction of the cyanoaromatic halide and the direct reduction of CO2 (Figures 6b,d). The same trend is observed with the use of copper cathode and a ionic liquid with wide electrochemical window, such as 1-methyl-1-propylpiperidinium bis(trifluoromethylsulphonyl)imide ([PP13] TFSI). Note that in this cases EMIM TFSI cannot be used as a IL, since the reduction of the EMIM cation takes place c.a. −2.06 V vs. SHE. [151].

Conclusions
In conclusion, we describe an efficient approach for producing high value products using CO2 as building blocks. The methodology employed is based on electrochemical techniques and ILs, which provide eco-friendly chemistry solutions. These can be employed to offer a potential long-term strategy for using CO2 feedstocks. There are two different strategies to obtain a CO2 valorization product depending on the functional group, nitro or cyano, of the aromatic halide. An electrocatalytic process using a homogeneous catalysis, which provides an easy way of obtaining oxalate is seen when nitro derivatives are used, removing metal complexes as a catalyst. The use of cyano derivatives allows to tune the reactivity in function of the reduction potential value applied from electrocarboxylated products (via a nucleophile-electrophile reaction) to oxalate. These electrochemical reactions were performed with three different electrodes and in aprotic solvents and ionic liquids, which all showed the same trend. This opens the possibility of using a cooper electrode and ionic liquids to valorize CO2, which would be a cheaper and greener alternative.