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Insight into the Microenvironments of the Metal-Ionic Liquid Interface during Electrochemical CO Reduction 2

Hyung-Kyu Lim, Youngkook Kwon, Han Seul Kim, Jiwon Jeon, YongHoon Kim, Jung-Ae Lim, Beom-Sik Kim, Jina Choi, and Hyungjun Kim ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03777 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Catalysis

Insight into the Microenvironments of the Metal-Ionic Liquid Interface during Electrochemical CO2 Reduction

Hyung-Kyu Lim1, Youngkook Kwon2,3, Han Seul Kim1, Jiwon Jeon1, Yong-Hoon Kim1, Jung-Ae Lim2, Beom-Sik Kim2,3, Jina Choi2,*, Hyungjun Kim1,4,*

1

Graduate School of EEWS, Korea Advanced Institute of Science and Technology, 291 Daehak-

ro, Yuseong-gu, Daejeon 34141, Republic of Korea 2

Carbon Resource Institute, Korea Research Institute of Chemical Technology, 141 Gajeong-ro,

Yuseong-gu, Dajeon 34114, Republic of Korea 3

Advanced Materials and Chemical Engineering, University of Science & Technology, 217

Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea 4

Department of Chemistry, Korea Advanced Institute of Science and Technology, 291 Daehak-ro,

Yuseong-gu, Daejeon 34141, Republic of Korea

*

Correspondence to: Jina Choi (+82-42-860-7396, [email protected]) and Hyungjun Kim (+82-42-350-

1725, [email protected]).

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Abstract Recently, many experimental and theoretical efforts are being intensified to develop highperformance catalysts for electrochemical CO2 conversion. Beyond the catalyst materials screening, it is also critical to optimize surrounding reaction medium. From vast experiments, inclusion of room-temperature ionic liquid (RTIL) in the electrolyte is found to be beneficial for CO2 conversion; however, there is yet no unified picture of the role of RTIL, prohibiting further optimization of the reaction medium. Using a state-of-the-art multiscale simulation, we here unveil the atomic origin of catalytic promotion effect of RTIL during CO2 conversion. Unlike the conventional belief, which assumes a specific intermolecular coordination by the RTIL component, we find that the promotion effect is collectively manifested by tuning the reaction microenvironment. This mechanism suggests the critical importance of the bulk properties (e.g., resistance, gas solubility and diffusivity, viscosity, etc.) over the detailed chemical variations of the RTIL components in designing the optimal electrolyte components, which is further supported by our experiments. This fundamental understanding of complex electrochemical interfaces will help to develop more advanced electrochemical CO2 conversion catalytic systems in the future.

Keywords: reaction mechanism, electrocatalysis, ionic liquids, multiscale simulation, solid-liquid interface

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INTRODUCTION There are various technical explorations of the sustainable future of the global carbon-based economy. The development of CO2 conversion technologies is imperative to successfully recycle the anthropogenically emitted greenhouse gas to produce future carbon resources of fuels and chemicals. Electrochemical CO2 conversion is considered one of the most feasible and promising technologies because of its high reactivity at ambient condition, wide product window according to the type of catalyst, and good applicability in commercial processes. Thus, many experimental and theoretical studies are extensively developing high-performance electrochemical catalyst systems for CO2 conversion to achieve a minimized overpotential, improved product selectivity, improved stability, etc.1-4 Beyond many efforts to optimize the catalyst material5-7, there has been an interesting report in 2011 by Rosen et al.8 They show that the activity of the electrochemical CO2-to-CO conversion reaction can be remarkably enhanced by the 18:82 mol% mixture electrolyte of 1-ethyl-3methylimidazolium tetrafluoroborate (EMIm-BF4), one of the room temperature ionic liquids (RTILs), and water. After this seminal work, many experimental reports have shown that the addition of RTIL into electrolyte generally promotes the CO2 reduction activity regardless of the choice of catalyst materials (Ag, Au and Pt)9-10. Because the electrochemical catalytic reaction is manifested by the interfacial chemistry, which occurs at the heterogeneous interface between the catalyst surface and the liquid medium, this study successfully demonstrated the importance of a particular design of the reaction medium. However, there is little understanding on how RTIL participates in and promotes the CO2 reduction chemistry, which limits further systematic design of the catalyst system. Rosen et al. 3


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attributed the improved reactivity to a greater proton availability by the hydrolysis of BF4− 11 and a layer of EMIm+ on the catalyst surface12 from follow-up experimental studies. In additions, the imidazole cation may have a direct chemical reaction with CO2 and serve as a proton donor, which promotes the proton-coupled electron transfer (PCET) process13-14. However, a recent experimental study has shown that methylation at the C2 position of imidazole promotes the CO2 reduction activity, which excludes the possibility of the direct catalytic role of proton at the C2 position15. The experimental screening approach to identify the catalytic role of RTIL mostly relies on investigating the correlation between the catalytic activity and the chemical modifications of RTIL15-16. Although this approach certainly provides valuable information about how the RTIL participates in the reaction, the concurrent change of bulk properties of the RTIL after the chemical modifications adds many complications in deciphering the experimental observations. For example, methylation at the C2 position of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) increases the melting temperature from -14.7 to 22.0 °C, and changes the viscosity from 21 to 74 mPa·s17. As summarized in our recent review article18, there have been various efforts to identify mechanisms through various experimental and theoretical studies, but no decisive conclusion has been drawn. Computer simulations based on the fundamental theory can supplement the experiments to provide direct mechanistic information about the promotion effect of RTILs. However, the application of the first-principle simulation to such a system is nearly impractical in terms of computational cost because of the high degrees of freedom in structures and slow dynamics of RTILs, which affect the complicated surface reactions. To overcome the practical limitations and elucidate the role of RTILs at the molecular level, our group recently developed a firstprinciples-based multiscale simulation method: density functional theory in classical explicit 4


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solvents (DFT-CES)19. The DFT-CES method is based on the theory of the mean-field type coupling between static DFT calculation under periodic boundary condition (PBC) and classical molecular dynamics (MD) simulations20. In this study, we unveil the atomic origin of the catalytic promotion effect of RTIL during electrochemical CO2 conversion by means of the first-principles based multiscale simulation method of DFT-CES. In contrast to the previous conception that a certain type of direct chemical interaction between the RTIL component and the reaction intermediate promotes the catalytic activity (which have often been assumed while interpreting experimental results), we find that RTIL components participate in the reaction in a collective manner by adjusting the reaction microenvironment during CO2 reduction. Based on our mechanism, we designed and performed a series of systematic experiments. This demonstrates the importance of bulk properties such as a bulk conductivity over the detailed chemical interactions of RTIL, suggesting a new design guideline of the reaction medium to enhance the CO2 reduction.

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Methods Multiscale simulation method: DFT-CES To briefly quote the key aspects of the DFT-CES method for the interested readers, the background real-space grid is used as a communication channel for the electrostatic interactions between two different simulations in the length scale. In addition, by completely decoupling the time scales of two simulations, we can include the environmental effect of the reaction medium into DFT calculations as fully equilibrated states (i.e., no biased information originating from the choice of initial locations of the solvent molecules). The advantage of the DFT-CES method is that it can reasonably handle both the electronic polarization of the DFT part and the structural reorganization of the MD part by bridging two different scales of simulations. Thus, we could simultaneously obtain appropriately polarized electronic structures with properly reorganized solvent structures. Because of such advantages, DFT-CES is a suitable method to readily and efficiently simulate the complex heterogeneous interface system without losing detailed information of the electronic structure from the DFT and realistic solvent effects from the MD. To efficiently calculate the free-energy quantities, we further incorporate the two-phase thermodynamic (2PT) model21, which properly approximated the total degree of freedom of the liquid system into the gas-like part (assumed as the hard-sphere model) and the solid-like part (assumed as the harmonic-oscillator model). We have validated our DFT-CES/2PT method for the calculations of solvation free energies of 17 small molecules. We note that the computational details are fully described in another publication19, which are also summarized in Supporting Information. 6


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Computational details First, we built 4 types of solvent models with different molar ratios of EMIm-BF4 to water: from pure water (0:100) to pure ionic-liquid (100:0) (Table S1). We used the LAMMPS (Largescale Atomic/Molecular Massively Parallel Simulator)22 code for the MD simulations. The TIP3P water model23 was used for water molecules and the OPLS-AA-based model optimized for ionicliquid systems24 was used for EMIm-BF4 molecules. We set the timestep as 1 fs, and performed isothermal-isobaric ensemble (NPT) simulations at 300 K and 1 atm with the Nosé-Hoover thermostat25-26 and Parrinello-Rahman barostat27, which were followed by canonical ensemble (NVT) simulations at 300 K to obtain the reference states for bulk solvent systems. We used 100 fs and 1,000 fs of damping parameters for the thermostat and barostat, respectively. DFT calculations were performed with the SIESTA code28 using the generalized-gradient approximation (GGA) exchange-correlation functional of Perdew, Burke, and Ernzerhof (PBE)29. Numerical atomic orbitals (NAOs)30 were used for the basis functions of the valence electrons, and the interactions between valence and core electrons were treated with norm-conserving scalar-relativistic pseudopotentials, including nonlinear partial-core corrections31. Double-zeta polarization basis functions were used for Ag, C, O, and H atoms, which were optimized to fit the experimental surface energy of the clean Ag slab, and theoretical COOH radical and CO binding energies. The selected mesh cutoff energy was 200 Ry. To evaluate the reaction thermodynamics of the electrochemical CO2 to CO reduction on the Ag(111) catalyst in various RTIL/water mixture solvent systems, the DFT-CES simulations were performed for each elementary reaction step (*, *COOH, and *CO states), which couples the DFT-based catalyst part and MD-based solvent part. The catalyst part is composed of an Ag(111) 7


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surface structure (108 atoms, 3 layers) and one adsorbed reaction intermediate molecule (COOH or CO), as shown in Figure S1. To address the proper interface interactions with large-sized ionic-liquid molecules, we had to make a catalyst structure as large as possible to be calculated with the DFT code. In this sense, the catalyst structure for the DFT calculations was extended to 2×2 supercells in the DFT-CES simulations. To describe the van der Waals interactions at the catalyst-solvent interface, we applied the OPLS-AA force-field parameters32 for adsorbate molecules and UFF force-field parameters33 for the Ag catalyst. In each DFT-CES iterative process, we averaged the solvent charge density at each timestep for the last 1-ns trajectory after the 1-ns equilibration step, and the electronic structure was updated under the external potential from the averaged solvent charge density after every 2-ns MD simulations. We note that 2-ns dynamics was turned out to be long enough to equilibrate the system and sample the equilibration state under a fixed external potential, as monitored from the converging behavior of the MD total energy (Figure S2). Finally, after obtaining the converged self-consistent DFT-CES state, we applied the 2PT method to evaluate the free energy of the reorganized MD solvent systems. The total free-energy of a particular DFT-CES state was determined by summing the DFT electronic energy, MD free energy, and translational, rotational, and vibrational free energy terms, if necessary. Additionally, the chemical potential of a proton and electron pairs in the elementary reaction steps was considered to be half of the H2 gas free energy34. Hence, the reaction free-energy change (∆G) could be converted into the reduction potential values (E0) vs. the standard hydrogen electrode (SHE) via Nernst equation (∆G°=−nFE0; n=1 and F=1 when ∆G° is given in eV). Electrochemical measurements 8


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Electrochemical experiments were performed in a three-electrode system. A silver foil (Alfa Acer, 99.998%) was polished before each measurement with aluminum oxide suspension (Ted Pella) up to 0.3 µm. The Pt coil (Alfa Acer, 99.999%) as the counter electrode was flameannealed before use. The reference electrode, which consisted of a silver wire immersed into a solution containing 0.01 M NaNO3 and 0.1 M TBAP in acetonitrile, was separated from the electrolyte by a glass frit. Then, 20 mL of 0.2 M electrolyte (20:80 mol% of ILs:H2O) in acetonitrile was introduced to the anodic and cathodic chambers, which were separated by an anion exchange membrane (Selemion AMV). The electrolyte was purged with argon (99.9999%) or carbon dioxide (99.999%) for at least 15 min prior to each measurement. The Ohmic resistance of each IL was obtained using a frequency resistance analysis method in the range of 100,000-100 Hz with 10 mV amplitude using the Auto Lab Electrochemical System (PGSTAT 302N), and the IR drop was corrected. Linear sweep voltammograms were collected at a scan rate of 20 mV s-1. Chronoamperometry was performed at -2.4 V vs. Ag/Ag+ (before IR correction), and gaseous reaction products were monitored by an online gas chromatographer (µTCD, Molecular Sieve 5A). The reliability of the Ag/Ag+ reference electrode was obtained from the Fc/Fc+ redox couple as described in Figure S3 and Table S2.

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RESULTS AND DISCUSSION Reaction thermodynamics Using the DFT-CES/2PT method, we investigate the reaction thermodynamics for the CO2-toCO reduction on the Ag(111) surface when the electrolyte phase is water or EMIm-BF4/water mixture with 20:80 mol%. The simulation cell in Figure S1 includes 432 Ag atoms and 4 adsorbate molecules for the surface reaction calculations (DFT part) and 1,300 water molecules or 110/440 EMIm-BF4/water molecules for the solvent simulations (MD part). In Figure 1, we show the free-energy profiles at the applied potential of U = −0.11 V vs. SHE using the computational hydrogen electrode (CHE) method34. Depending on the choice of electrolyte, we find that a substantial modification occurs in the energetics of the 1st PCET step: * + CO2(g) + [H+ + e−] → *COOH(solv.), whereas the remaining reaction steps show marginal changes. Thus, the EMIm-BF4/water mixture electrolyte system has a significantly decreased overpotential energy by ~310 meV compared with the aqueous system. Our simulation results elucidate that the EMIm-BF4/water mixture electrolyte provides an optimal solvation environment and specifically stabilizes *COOH, which is the energetically most unfavorable intermediate. Hence, the mixed electrolyte system has a high catalytic activity, which is consistent with previous experimental report8. Then, the next apparent question is: what is the nature of interaction at the molecular level, which effectively stabilizes the *COOH intermediate in the EMIm-BF4/water mixture electrolyte? Water molecule coordinating *COOH Surprisingly, we find that the local coordinating environment of *COOH is nearly identical for aqueous and EMIm-BF4/water systems; a single water molecule binds to *COOH by forming a 10


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hydrogen bond (Figures 2a and 2c). This phenomenon is contrary to the previous conception that the formation of a distinct local coordination with RTIL components, particularly cations, would cause a specific intermediate stabilization15-16, 35. However, considering that the water molecule serves as a primary proton source, this local atomic geometry is responsible for the facile proton transfer. If the intermediate species does not directly interact with the water molecule, the protontunneling transfer barrier will exponentially increase and deteriorate the electrochemical reaction rate. More interestingly, because of the strong electrostatic interaction with *COOH, the water molecule is nearly immobilized and shows no exchange event in the entire MD trajectory, which is 0.5~1 ns (Figure S4). Thus, a more correct expression of the 1st PCET step should include a process that immobilizes one of the bulk-phase water molecules: * + CO2(g) + H2O(solv.) + [H+ + e−] → [*COO--OH3+(solv.) ↔ *COOH-OH2(solv.)]. Using the 2PT analysis (Table S3), the entropy of this bound water (S°wat(bound)) is calculated as 42.60 J mol-1 K-1 for the aqueous case and 38.02 J mol-1 K-1 for the EMIm-BF4/water mixture electrolyte case, both of which are comparable with the entropy of ice (S°wat(ice) = 41 J mol-1 K-1). We further estimate the entropic cost required for the water-immobilizing process from the bulk phase to *COOH. In the bulk phase of the electrolyte, the entropy of a water molecule solvated by RTIL is calculated to be S°wat = 55.71 J mol-1 K-1, which is lower than the entropy of a water molecule immersed in the bulk water, i.e., S°wat = 66.07 J mol-1 K-1. This phenomenon occurs because of the confinement effect; the motion of water molecules is restricted within the free volume space between bulky RTIL components36. Therefore, in the water-binding process, the mixture electrolyte accompanies a less entropic reduction (55.71 → 38.02 J mol-1 K-1; ∆S° = 17.69 J mol-1 K-1) than the aqueous electrolyte (66.07 → 42.60 J mol-1 K-1; ∆S° = 23.47 J mol-1 K-1). Thus, the EMIm11


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BF4/water mixture electrolyte has a certain thermodynamic advantage compared to pure water in terms of reducing entropy cost. However, in free-energy quantities, the magnitude of the thermodynamic gain from this entropic cost is at most < 20 meV, under 300K; 17.69 × 300 = 5.307 kJ mol-1 = 55.00 meV for mixed electrolyte and 23.47 × 300 = 7.041 kJ mol-1 = 72.97 meV for aqueous electrolyte. Thus, a significant overpotential drop of ~310 meV in the EMImBF4/water mixed electrolyte is not entirely due to the *COOH-water complex formation process. Collective change of reaction microenvironment Beyond the comparison of the direct coordinating environments, we investigate more structural details consisting of the microenvironment of *COOH solvation shell. For the mixture electrolyte system, we find that approximately 1~2 water and BF4− molecules closely locate next to the adsorbed water molecule (Figures 2b and 2d). This result is ascribed to the small size of the BF4− anion, which is harder than the imidazole cation (EMIm+) and develops a stronger iondipole interaction with the water molecule. Approximately 4 EMIm+ surrounds a water-BF4− complex because of the strong cation-anion electrostatic attraction (Figure 2e). The combination of these complex non-covalent interactions among water-BF4−-EMIm+ forms an exotic microenvironment for *COOH, which consists of charge-alternating layers of the solvation shell structure. As a result, the 3-dimensional atomic arrangement of this alternating solvation shell structure makes EMIm+ directly contact the metal surface surrounding the key reaction intermediate (Figure 3a). By analyzing the electronic structure change using the results of the DFT part, we find a significant polarization of the electron density on the catalyst surface, which is manifested by the image charge effect of EMIm+ on the metal surface. We evaluate the differential charge density 12


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maps by subtracting the DFT electron density optimized in vacuo from the DFT electron density optimized with the solvation effect (Figure 3b). In contrast to the aqueous system, a significant amount of metal electron is polarized toward *COOH in the mixed electrolyte system. This induces a strong local field effect at the electrical double layer (EDL) interface, which stabilizes the *COOH intermediate via the field-dipole interaction37-40. To quantify the contribution of the interfacial image charge effect on the electrochemical activity, we perform another set of calculation, where the electronic polarization in response to the solvent effect is not permitted, i.e., a non-self-consistent DFT-CES calculation. With no image charge effect, we find that (1) the charge-alternating layer formation tendency becomes significantly weakened (Figure S5), and (2) the thermodynamic barrier for the CO2 reduction increases by 140 meV (Figure S6), which shows the importance of the unique electrical-doublelayer (EDL) structure of the EMIm-BF4/water mixed electrolyte in catalyzing the CO2 reduction. Interestingly, this strong electron polarization is only induced when the COOH intermediate is adsorbed on the catalyst surface. For the bare catalyst surface or when the CO intermediate is adsorbed on the catalyst surface, neither the formation of the alternating solvation shell structure nor the significant electron polarization is observed (Figure S7). We further find an appropriate structural adaptation of the solvation environment targeted to the *COOH intermediate. From the density profile analyses of the MD trajectories, we find that ionic species become strongly bound on the catalyst surface only for the *COOH intermediate (Figure S8). Therefore, we conclude that there is a dynamic structural transformation of the RTIL/water mixed electrolyte over the course of reaction steps, because the electrolyte structure is needed to be significantly reorganized in specific response to the formation of *COOH (Table S4). This forms the

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cooperative microenvironment and aids a specific stabilization of the *COOH intermediate. In terms of developing a strong interaction between the RTIL components and the solid surface, where catalytic reaction is manifested, we further conceive that our mechanism shares conceptually some similarities with the SILP (supported ionic liquid phase) technology41-42. Water channel formation: facile proton supply chain Furthermore, we investigate the effect of the RTIL contents in the mixed electrolyte systems. By varying the EMIm-BF4 concentration from 20 mol% to 100 mol%, we calculate the reaction free-energy profiles (Figure S9), which shows no noticeable difference in terms of the thermodynamic barrier for the 1st PCET step. Thus, the favorable reaction thermodynamics for the CO2 reduction can be achieved if the RTIL contents becomes only sufficient to locally develop the suitable microenvironment. However, as shown in Figure 4, the hydrogen bond network among water molecules, which is the proton supply chain to the reaction plane at the interface, becomes significantly disturbed when the RTIL content exceeds 60 mol%. The MD trajectories show that the probability of the percolation path from bulk to catalyst surface (details about the percolation analysis are given in Figure S10) shows a dramatic decrease from 96 % to 0 % when the RTIL content increases from 20 mol% to 60 mol%. Therefore, increasing the RTIL contents can substantially decrease the proton conductivity, cause a critical kinetic problem of the continuous proton transfer, and degrade the catalytic activity and/or stability. This conclusion is consistent with the previous experimental report that the CO2 reduction current density dramatically increases when the water contents exceed 50 mol%, although at 96 mol%, the HER becomes dominant43. Effect of chemical variations of RTILs 14


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Our mechanism suggests that the detailed chemical composition and structure of the RTIL components may not significantly affect the intermediate stabilization, since the RTIL components collectively participate in the reaction by modifying the EDL character and reaction microenvironment. Thus, we also design a series of systematic experiments to unveil the dependence of the catalytic promotion effect of RTILs on the chemical details of cation and anion components. For the experiments, in brief, a mechanically polished 1-cm2 Ag foil is used as the working electrode, and the Pt coil and Ag/Ag+ electrodes are used as the counter and reference electrodes, respectively. To compare the catalytic promotion effect of the RTIL with minimizing the bias originating from the change of the bulk properties of the medium (e.g., gas solubility and diffusivity, viscosity, and ion mobility), the mixture of RTILs and water (0.2 M) is diluted in anhydrous acetonitrile as the supporting electrolyte. Then, this mixture is purged with argon to remove any dissolved oxygen, and the CO2 saturated solution is prepared by the CO2 purging process for at least 15 min. More details of the experimental conditions are provided in the Methods section. As the linear sweep voltammograms in Figure 5a show, the cationic effect is evaluated by altering the length of the alkyl chain of the imidazolium cation with a representative anion of BF4-, where the onset potentials and trend of current densities are marginally changed. Interestingly, the anions of RTILs significantly contribute to the current density of the CO2 reduction and a minor shift of onset potentials as shown in Figure 5b, where the catalytic activity increases with the increase in size of the anion. However, the activity variation for different anions is mainly attributed to the cell resistance derived from the bulk intrinsic property of anions, as summarized in Table S5. Thus, the IR correction makes the CO2 reduction activity of the Ag electrode nearly insensitive to the choice of anions (Figure 5d). Chronoamperometric 15


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experiments at an applied potential of -2.4 V vs. Ag/Ag+ generate purely carbon monoxide, regardless of the anionic and cationic compositions.

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CONCLUSION Based on the atomistic details elucidated from our state-of-the-art simulations and systematic experiments, the catalytic promotion effect of RTILs is manifested by the collective participation of both cations and anions of RTILs. The RTIL at the EDL interface provides a suitable microenvironment to solvate the intermediate, which can be well preserved over the variations in chemical details because the solvation energy is dominated by the non-specific interaction. We further anticipate that our comprehensive study on the nature of the complex electrochemical interface will be useful for designing enhanced electrochemical CO2 reduction systems in the future. For example, the bulk properties of the reaction medium, such as the electrical resistance, gas solubility, gas diffusivity, and viscosity, are more critically optimized than a specific interaction pair. In addition, a certain amount of water is indispensable to facilitate the proton transfer by developing a percolated hydrogen bond network and improve the mass transport of RTIL components (Table S6) by screening the strong interionic electrostatic interaction36.

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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Authors Hyungjun Kim ([email protected]), Jina Choi ([email protected])

Notes The authors declare no competing financial interest.

SUPPORTING INFORMATION Supporting figures about the simulation cell, additional analyses of the DFT-CES results, and experimental cyclovoltammograms; tables summarizing the water entropy, diffusion constants, details of the mixed solvent models, and experimental bulk electrolyte resistances; and summary of the DFT-CES method. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS This research has been performed as a project SI1701-05 supported by the Korea Research Institute of Chemical Technology (KRICT). This work was also supported by the Creative Materials Discovery Program (Grant 2017M3D1A1039378), and the National Research Foundation of Korea (NRF) grant (No. NRF-2017R1A5A1015365) funded by the Korea 18


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government (MSIT).

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(13) Cabaco, M. I.; Besnard, M.; Danten, Y.; Coutinho, J. A. P., Carbon Dioxide in 1-Butyl-3methylimidazolium Acetate, I. Unusual Solubility Investigated by Raman Spectroscopy and DFT Calculations. J. Phys. Chem. A 2012, 116, 1605-1620. (14) Sun, L. Y.; Ramesha, G. K.; Kamat, P. V.; Brennecke, J. F., Switching the Reaction Course of Electrochemical CO2 Reduction with Ionic Liquids. Langmuir 2014, 30, 6302-6308. (15) Lau, G. P. S.; Schreier, M.; Vasilyev, D.; Scopelliti, R.; Gratzel, M.; Dyson, P. J., New Insights Into the Role of Imidazolium-Based Promoters for the Electroreduction of CO2 on a Silver Electrode. J. Am. Chem. Soc. 2016, 138, 7820-7823. (16) Zhao, S.-F.; Horne, M.; Bond, A. M.; Zhang, J., Is the Imidazolium Cation a Unique Promoter for Electrocatalytic Reduction of Carbon Dioxide? J. Phys. Chem. C 2016, 120, 23989-24001. (17) Noack, K.; Schulz, P. S.; Paape, N.; Kiefer, J.; Wasserscheid, P.; Leipertz, A., The Role of the C2 Position in Interionic Interactions of Imidazolium Based Ionic Liquids: a Vibrational and NMR Spectroscopic Study. Phys. Chem. Chem. Phys. 2010, 12, 14153-14161. (18) Lim, H.-K.; Kim, H., The Mechanism of Room-Temperature Ionic-Liquid-Based Electrochemical CO2 Reduction: A Review. Molecules 2017, 22, 536. (19) Lim, H. K.; Lee, H.; Kim, H., A Seamless Grid-Based Interface for Mean-Field QM/MM Coupled with Efficient Solvation Free Energy Calculations. J. Chem. Theory Comput. 2016, 12, 5088-5099. (20) Nakano, H.; Yamamoto, T., Accurate and Efficient Treatment of Continuous Solute Charge Density in the Mean-Field QM/MM Free Energy Calculation. J. Chem. Theory Comput. 2013, 9, 188-203. (21) Lin, S. T.; Blanco, M.; Goddard, W. A., The Two-Phase Model for Calculating Thermodynamic Properties of Liquids from Molecular Dynamics: Validation for the Phase Diagram of Lennard-Jones Fluids. J Chem Phys 2003, 119, 11792-11805. (22) Plimpton, S., Fast Parallel Algorithms for Short-Range Molecular-Dynamics. J. Comput. Phys. 1995, 117, 1-19. (23) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L., Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926-935. (24) Lopes, J. N. C.; Padua, A. A. H., Molecular Force Field for Ionic Liquids III: Imidazolium, Pyridinium, and Phosphonium Cations; Chloride, Bromide, and Dicyanamide Anions. J. Phys. Chem. B 2006, 110, 19586-19592. (25) Hoover, W. G., Canonical Dynamics - Equilibrium Phase-Space Distributions. Phys. Rev. A 1985, 31, 1695-1697. (26) Nose, S., A Unified Formulation of the Constant Temperature Molecular-Dynamics Methods. J. Chem. Phys. 1984, 81, 511-519. 21


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Figure 1. Calculated reaction free-energy profiles during CO2-to-CO reduction in aqueous and 20:80 EMIm-BF4/water mixture electrolytes at the applied potential U = −0.11 V vs. SHE. The primary energy barrier, the 1st proton-coupled electron transfer (PCET) step, has been significantly reduced by ~310 meV in the mixture electrolyte compare with the aqueous system.

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Figure 2. Structural characteristics of the 20:80 mol% EMIm-BF4/water mixture electrolyte surrounding the *COOH state. (a) The calculated radial distribution functions between the hydrogen atom in *COOH and the oxygen atoms in water under aqueous and mixture electrolyte conditions. (b) The calculated radial distribution functions between the oxygen atoms in the adsorbed water molecules on *COOH and the oxygen, boron, and carbon atoms in water, BF4-, and EMIm+ in the mixture electrolyte. (c-e) Different constituents of the mixed electrolyte form alternating layers to solvate the *COOH intermediate .

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Figure 3. Solvent charge density and the corresponding electronic polarization in the *COOH state. (a) Side view of the overall molecular structure around *COOH in the 20:80 mol% EMIm-BF4/water mixture electrolyte. The EMIm+ cations are closely located at the Ag surface by *COOH. (b) Iso-surfaces of the averaged solvent charge density (±0.006e/bohr3, red and blue) and differential electron density (±0.0006e/bohr3, yellow and green).

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Figure 4. Hydrogen-bond networks (blue dashed line) among water molecules in the 20:80 and 60:40 mol% EMIm-BF4/water mixture electrolytes. The probability of the percolation path from bulk to surface has completely dropped down to 0% in 60:40 EMIm-BF4/water mixture.

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Figure 5. Experimental linear sweep voltammograms for the electrochemical CO2 reduction on the Ag electrode at a scan rate of 20 mV s-1. Effect of different (a) cations with BF4- anions and (b) anions with BMIm+ cations. IR corrected voltammograms of different (c) cations with BF4- anions and (d) anions with BMIm+ cations.

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Insight into the Microenvironments of the Metal ... - ACS Publications

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