Pyrazolium Ionic Liquid Co-catalysts for the Electroreduction of CO2

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Pyrazolium Ionic Liquid Co-catalysts for the Electroreduction of CO2 Dmitry Vasilyev, Erfan Shirzadi, Alexander Rudnev, Peter Broekmann, and Paul J. Dyson ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01086 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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ACS Applied Energy Materials

Pyrazolium Ionic Liquid Co-catalysts for the Electroreduction of CO2 Dmitry Vasilyev,1‡ Erfan Shirzadi,1‡ Alexander V. Rudnev,2,3 Peter Broekmann,2 Paul J. Dyson1* 1

Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland 2 Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland 3 A. N. Frumkin Institute of Physical Chemistry and Electrochemistry Russian Academy of Sciences Leninskii prospekt 31, 119991 Moscow, Russia KEYWORDS carbon dioxide reduction, electrochemistry, ionic liquids, pyrazolium cations, cyclic voltammetry.

ABSTRACT: Pyrazolium ionic liquids (Pz ILs) were employed as co-catalysts for electrochemical conversion of CO2 to CO on a silver disk electrode, leading to a significant decrease in the onset potential for the reduction (ca. 500 mV). The electrochemical conversion of CO2 to CO proceeds in acetonitrile-based electrolytes containing Pz IL co-catalysts with Faradaic efficiencies (FEs) of nearly 100% over a range of at least 0.5 V and the Pz cations remain intact over prolonged CO2 electrolysis. The impact of alkylsubstituents on the Pz-ring and the influence of water on the process are also discussed.

Due to the growing abundance of CO2 levels in the atmosphere, which is leading to global warming,1 CO2 has become an attractive C1 feedstock chemical. Various catalytic approaches to valorize CO2 have been commercialized and many others are under development.2–4 In addition to traditional catalytic approaches, electrocatalytic activation represents a promising option to directly transform CO2 into a range of products including CO, formate/formic acid, oxalate, methanol, methane and more complex alcohols and alkanes.5–11 The electrochemical reduction of CO2 into CO is a particularly important reaction. The high reduction overpotential for this reaction, however, is problematic.12 In this respect, imidazolium-based ionic liquids (ILs) were shown to act as cocatalysts, promoting the reduction of CO2 to CO on silver electrodes, significantly reducing the reaction overpotential and suppressing the hydrogen evolution reaction (HER).12 In addition, IL additives are able to switch the selectivity of the reduction, e.g. resulting in the formation of carbon monoxide instead of oxalate when a lead electrode is employed.13 In a series of publications the influence of the electrolyte composition,14,15 electrode material,15,16 task-specific 17 and superbasic 18 IL additives was investigated. The nature of the IL cocatalysis has also been probed by spectroscopic 19–21 and computational 22,23 methods and although structure-activity relationships have been proposed it remains difficult to predict the impact of different IL structures on the electroreduction of CO2 to CO. In our previous study, we demonstrated the influence of the substitution pattern on the performance of imidazolium ILs, which contributes to the understanding of the role of ILs in the electrochemical reduction of CO2.24 Herein, we report on an even more dramatic change related to the structure of the cation. By switching from imidazolium salts to the pyrazolium (Pz) ILs the reduction of CO2 to CO proceeds with higher Faradaic efficiencies (FEs) without decomposition while

maintaining the same high current densities, therefore making a further step towards a commercial viable electrochemical CO2 conversion process.11,25 To the best of our knowledge, this is the first example describing the use of pyrazolium ILs as cocatalysts in the electroreduction of CO2, although protic Pz ILs were employed as catalysts for the conversion of CO2 into cyclic carbonates.26,27 Despite certain similarity of imidazolium and Pz rings, the latter possess different resonance structures, charge distributions and are usually less likely to form carbenes and hydrogen bonds. The methyl-substituted Pz ILs used in the electrochemical investigation are shown in Figure 1. Methyl groups were systematically introduced onto the ring to investigate the influence of the substitution pattern on the activity. In all cases bis(trifluorosulfonyl)imide (Tf2N-) was used as the anion unless stated otherwise.

Figure 1. Pz cations used in this study.

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Electrochemical measurements were performed in dry acetonitrile (MeCN) containing 0.1 M NBu4PF6. This system dissolves significant amounts of CO2 (eight times higher than water)28 and possesses a potential window exceeding 4 V.29 Moreover, conducting the studies in MeCN allows the influence of water on the reaction to be probed separately. A silver commercially available disk electrode was employed as the cathode, since it exhibits high FEs for CO production,12 is easy to handle and, moreover, there is a wealth of experimental data accumulated on such electrodes. The Pz ILs were used in the concentration of 0.02 M in order to promote the reduction. Under our experimental conditions we neglect the changes of the physical properties of the system (e.g. conductivity, CO2 adsorption, etc.), which appear due to additives of the cocatalysts, as the properties of the bulk solution are mainly defined by the nature of supporting electrolyte.

Figure 2. CVs for the Pz ILs under Ar, illustrating the differences in stability of diversely substituted Pz cations (Ag polished disk electrode, 0.1 M NBu4PF6 in dry acetonitrile, 0.02 M IL additive).

The stabilities of the Pz ILs were investigated by cyclic voltammetry (CV) under an argon atmosphere. The decomposition onset potential tends to shift towards more negative values with the introduction of methyl groups on the Pz ring (Figure 2), i.e. electron-donating substituents and/or increased steric hindrance of the ring result in higher stabilities. Methyl groups at positions C3 and C5 influence on the stability to a greater extent than methylation of the C4-position, therefore it seems likely that decomposition occurs at C-N or N-N sites. CV experiments were then performed under a CO2 atmosphere. A significant shift of the onset potential towards more positive values compared to the CVs recorded under Ar (ca. 150-250 mV depending on the Pz IL), followed by a steep increase in the current density is observed under these conditions (Figure 3; Pz IL 12 was selected as it has the simplest structure. Comparison with the system without IL is shown in the inset).

Figure 3. Example of the change in the CV after addition of CO2 using Pz IL 12 as the co-catalyst (red – argon atmosphere, black – carbon dioxide atmosphere). The performance of the silver electrode towards CO2 reduction with and without Pz IL additives are compared in the inset. CVs for the other Pz ILs under CO2 are provided in the SI (Figures S1 – S5).

Different variations of the Pz ILs structures affect the cocatalytic performance in the reduction of CO2 to CO in the following ways: (1) the introduction of methyl groups on the Pz ring leads to a slight increase in the reduction onset potential (see SI, Figure S1 – S5); (2) substitution of the N-methyl group by a butyl group has minor positive impact on catalytic performance (see SI, Figure S6). Therefore, neglecting the minor impact in onset potential with substitution, and taking into consideration the increase in their stability, the more substituted Pz ILs are the most attractive co-catalysts. We did not consider the influence of the anions on the catalytic performance of the Pz IL additives. At negative potentials (where reduction of CO2 takes place) cations form the innermost adsorption layer at the surface of the electrode and, for imidazolium co-catalysts, the negatively charged CO2.- is believed to be stabilized by the positively charge cations.20 Therefore, we expect the effect of the cations to dominate over the anions in the CO2 reduction reaction. Although we cannot exclude that the anions may influence the IL double layer structure at the electrode surface, which in turn may have an impact on CO2 reaction rate, we did not observe such effect when comparing Tf2N- and TfO- salts (Figure S7). The reduction waves in the presence of CO2 for the Pz ILs do not reach a current limitation peak at the decomposition potential of the co-catalyst (Figure 3). This is particularly striking for co-catalyst 1235, which affords current densities ≈ 30 mA/cm2 (Figure S4). CVs using a glassy carbon electrode were recorded employing the same conditions both under Ar and under a CO2 atmosphere. Under Ar, the onset potentials for the CVs do not differ from those recorded on the silver electrode (see SI, Figure S8). Moreover, the voltammetric response of the glassy carbon remains unchanged when the solution is saturated with CO2. Combined, this demonstrates that the Ag electrode is the actual catalyst for the reduction, and that the IL behaves as a co-catalyst in the reduction of CO2 to CO. Notably, the introduction of CO2 does not result in destabilization of the Pz ILs. In order to confirm that the changes of the current registered are related to the CO2 reduction process, rather than catalyst decomposition, chronoamperometric experiments followed by online gas chromatography analysis were performed using a two-compartment H-type cell and IL 1235B, chosen as a com-


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ACS Applied Energy Materials promise between activity and stability of the Pz IL containing a butyl group, which also makes it more similar to the conventional BMIM ILs. An aqueous 0.5 M sulfuric acid solution was used in the compartment with the counter-electrode in order to decrease the impedance of the system and avoid the formation of MeCN decomposition products, which interfere with the separator (a Nafion 117 membrane). As Nafion is able to conduct protons, hydrogen-containing products or hydrogen itself could be expected if the HER takes place. The results of the measurements are presented in Figure 4 and the values are summarized in Table S1.

Figure 5. Decrease of the electrochemical reduction onset potential for the conversion of CO2 to CO using co-catalyst 1235 in the presence of water.

Figure 4. Dependency of the Faradaic efficiency in the formation of the detected products on voltage employing Pz IL 1235B. Conditions: cathode – Ag foil; anode – Pt foil; catholyte – 0.1 M NBu4PF6 in MeCN, 0.02 M IL, saturated with CO2; anolyte – 0.5 M H2SO4 (aq); separator – Nafion 117.

FEs for CO formation are around 100% with current densities up to 30 mA/cm2, which suggests that CO is the major product formed. Small amounts of hydrogen (from 1 – 4 %) and formic acid (< 4%) were detected by GC and 1H NMR spectroscopy, respectively. 1H NMR spectroscopy revealed no decomposition of the Pz IL over the time of the electrolysis (Figure S61). Notably, small amounts of methane (around 0.5 – 1 % FE) were observed as well by GC at low overpotentials (see Table S1). The formation of methane has been detected as well by GC during CO2 reduction at a Ag foil electrode in aqueous solution,30 but to the best of our knowledge not in acetonitrile solution. The maximum amount of methane was obtained at the potential when active decomposition of the catalyst (according to the CVs recorded under Ar, Figure 2) does not take place. Thus, the methane must be derived from carbon dioxide. The protons, necessary for formation of methane and formic acid, might be supplied to the system from the anolyte by diffusion through the Nafion membrane. Water may impact on the electroreduction process depending upon the nature of the catalyst.31–33 For Pz ILs, water shifts the CO2 reduction onset potential towards more positive values. The influence of water on catalysis employing 1235 is shown in Figure 5. 1235 was selected as its properties are representative of the general trends of the Pz ILs studied. As the water content increases, the onset potential for the reduction of CO2 gradually decreases, until a concentration of 0.30.6 mM is reached (ca. 200 mV of the potential decrease, the value depends on the Pz IL employed).

The activity of the Pz ILs was compared to [BMIm][BF4] (where BMIm = the 1-butyl-3-methyl-imidazolium cation, Figure S10) using Pz IL 1235B as a representative example. [BMIm][BF4] is structurally similar to [EMIm][BF4] (where EMIm = the 1-ethyl-3-methyl-imidazolium cation), which is considered as the benchmark IL co-catalyst for this reaction.12 Under equivalent conditions 1235B exhibits a comparable onset potential and current density to [BMIm][BF4], displaying a steeper slope for the reduction wave. In order to demonstrate consistency of the working procedures, chronoamperometric experiments followed by GC analysis were performed for the [BMIm][BF4] and Pz IL systems under the same conditions and employing the same equipment (see SI, Figure S10). The [BMIm][BF4] co-catalyst results in good FE for CO (ca. 90% for low overpotentials, ca. 80-90% at higher voltages, see the SI), which is consistent with the literature.12 However, in case of [BMIm][BF4] the only reduction products observed are CO and negligible amounts of H2. No traces of methane or other products were detected by 1H and 13C NMR spectroscopy or GC. The protons in Pz-based ILs should not be readily reduced to form carbenes, which excludes the likelihood of the formation of covalent adducts with CO2, e.g. carboxylates, as is the case of imidazolium ILs.13 Moreover, the fully substituted Pz IL 12345 displays high catalytic performance, excluding the possibility of covalent interactions between the Pz IL and CO2. As mentioned above, catalysis occurs when a silver electrode is used as the cathode and not with a glassy carbon electrode. Therefore, the initial step of the reduction most probably corresponds to the formation of a CO2-based radicalanion on the surface of the silver electrode (see Figure 6). The effect of the co-catalyst should then result from stabilization of this intermediate by ion pair, dipole-ion interactions or change of the local microenvironment. According to previous reports, IL cations are able to arrange on the silver surface in a coplanar fashion to form a double layer.20,21 The nitrogen-containing domain of the Pz cation is the most positively charged region and substitution at C3 and C5 positions changes the activity of the system more than at the C4 position. Therefore, it is reasonable to assume that the CO2- intermediate interacts with the positively charged region of the Pz ring (Figure 6), which helps to stabilize the intermediate and lower the overpotential of the reaction.



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Figure 6. Possible intermediate structures between the Pz cations and the CO2●- radical anion leading to a lowering of the overpotential in the reduction of CO2 to CO.

In conclusion, a series of substituted Pz ILs was synthesized and shown to be efficient co-catalysts for electrochemical reduction of CO2 to CO, superior to the benchmark imidazolium-based ILs in terms of FE for CO formation (100% vs. 8090%). High current densities (30 mA/cm2 during electrolysis) were achieved. The influence of the substitution pattern on the Pz ring on stability and activity was investigated, with substituents generally stabilizing the co-catalyst, but decreasing performance. Water decreases the reduction onset potential for Pz co-catalysts. Additionally, during chronoamperometric experiments the unexpected formation of small quantities of methane and formic acid was observed, which is highly encouraging and warrants further investigation of Pz ILs with other electrode materials.

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ASSOCIATED CONTENT Supporting Information. Full experimental details, synthetic and electrochemical procedures, CV, chronoamperometric and NMR spectroscopic data are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author

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*E-mail: [email protected]

ORCID (14)

Dmitry Vasilyev: 0000-0001-7956-0307 Alexander V. Rudnev: 0000-0002-3502-0322 Peter Broekmann: 0000-0002-6287-1042 Paul J. Dyson: 0000-0003-3117-3249

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Author Contributions ‡D.V. and E.S. contributed equally. (16)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

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The work was supported by the Swiss National Science Foundation, EPFL, University of Bern, Frumkin Institute of Physical Chemistry and Electrochemistry RAS and the Swiss Competence Center for Energy Research. Dr. Daniel Ortiz is thanked for help with mass spectrometry.

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ABBREVIATIONS CV cyclic voltammetry; FE Faradaic efficiency; IL ionic liquid; HER hydrogen evolution reaction; Pz pyrazolium.

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Pyrazolium Ionic Liquid Co-catalysts for the Electroreduction of CO2

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