Structural Transition in an Ionic Liquid Controls CO2 Electrochemical

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The Journal of Physical Chemistry

A Structural Transition in an Ionic Liquid Controls CO2 Electrochemical Reduction Natalia García Rey and Dana D. Dlott* School of Chemical Sciences and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Il 61801, USA Abstract Broadband multiplex vibrational sum-frequency generation spectroscopy (SFG) was used to study CO2 reduction on an polycrystalline Ag electrode with a room-temperature ionic liquid (RTIL) electrolyte, 1-ethyl-3-methylimidazolium tetrafluorborate (EMIM-BF4) with 0.3 mol% water. The Ag/RTIL/H2O system has been shown to reduce CO2 with low overpotential and, depending on water concentration, with Faradaic efficiency of nearly 100% (Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I., Science 2011, 334, 643-644). The adsorbed CO created by CO2 reduction was probed with infrared (IR) pulses tuned to the CO stretch. Nonresonant (NR) SFG was used to probe the electrified interface. SFG showed CO binds weakly to Ag at the CO2 reduction threshold of 1.33V (vs. Ag/AgCl), so CO does not poison the surface. At potentials equal to or more negative than the threshold, the curvature of the parabolic potential-dependent NR intensity significantly increased, and the Stark shift of adsorbed CO, a measure of the surface field, more than doubled. The curvature increase indicates a potential-driven structural transition in the RTIL within the double layer. This transition was a property of the RTIL itself, since it occurred whether or not CO2 was present.

Significantly, the RTIL transition and the increased surface field occurred

precisely at the CO2 reduction threshold.

Thus we have demonstrated a close association

between an electrochemically-driven structural transition of the RTIL and low overpotential CO2 reduction. *

Author to whom correspondence should be addressed. Electronic mail [email protected]

1 ACS Paragon Plus Environment


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1. Introduction In this study, we used nonlinear laser spectroscopy to investigate the fundamental mechanisms of electrochemical reduction of CO2 to CO on a polycrystalline Ag electrode1,2 with a room-temperature ionic liquid (RTIL) electrolyte.3,4 The RTIL used here was 1-ethyl-3methylimidazolium tetrafluorborate (EMIM-BF4)3 with a small quantity (0.3 mol%) of water. The Ag/EMIM-BF4/H2O system has been shown to reduce CO2 with a low overpotential, and to operate for extended periods of time. Depending on the water content, it can produce CO with Faradaic efficiencies approaching 100%.3,4 The water is a necessary component for the efficient reduction of CO2.4 A broadband multiplex vibrational sum-frequency generation spectroscopy method5,6 (hereafter SFG) was used to investigate two issues: (1) the potential-dependent structural changes that occur within the double layer to catalyze CO2 reduction, and (2) the nature of adsorbed CO on the surface and the lack of apparent CO poisoning. The former was investigated using potential-dependent nonresonant (NR) SFG. NR SFG is closely related to NR secondharmonic generation (SHG) measurements frequently used to study electrified interfaces.7-11 The latter was investigated using resonant SFG measurements of the CO stretching transition.12-15 We discovered that SFG was ineffective to probe adsorbed CO2 or adsorbed RTIL since the CO2 concentration and the CO2 and RTIL absorbance in the electrolyte were so high. The electrochemical reduction of CO2 to CO is a key element of artificial photosynthesis,16-19 but its application so far has been hindered by the large overpotential, 600 mV or more,20,21 needed for the reaction. Large overpotentials waste energy and move the CO2 reduction potential above that of water and many organic solvents, which degrades the Faradaic efficiency.

In 2011, Rosen et al.3 demonstrated that EMIM-BF4 with added water4 could


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significantly reduce the overpotential at an Ag electrode. The mechanism of overpotential lowering was believed to be co-catalytic. It was suggested that RTIL creates a structure near the electrode surface that helps stabilize adsorbed high-energy intermediates such as CO2-.3,22 Overpotential lowering for CO2 reduction using RTIL is apparently not limited to Ag electrodes. In 2014, efficient CO2 reduction using MoS2 in the presence of the same RTIL23 has been demonstrated, and a closely-related RTIL has been shown to lower the overpotential and increase the Faradic efficiency with Pb electrodes.24 The common element in these lower overpotential reactors was RTIL. SFG was used in 2012 to investigate CO2 reduction at a Pt (rather than Ag) electrode with EMIM-BF4 electrolyte.22 It was shown that EMIM suppressed the formation of H2 and enhanced CO2 reduction. SFG was able to detect CO produced at, and adsorbed upon the Pt electrode. The long-term buildup of CO on Pt after cycling the potential 40 times suggested that CO might act as a catalytic poison on the Pt surface. A CO2 transition at 2348 cm-1 was observed whose intensity was potential-dependent. The CO2 SFG intensity increased near the onset of CO2 reduction. This transition was interpreted to arise from a CO2-EMIM(ad) complex at the electrode surface. The double-layer structures that result from RTILs near metallic electrodes have been discussed by several authors.12,25-29 Baldelli12 used SFG to study CO adsorbed at the Pt-EMIMBF4 interface. From the magnitude of the CO Stark shift (30-35 cm-1V-1), Baldelli estimated the field at the electrode and concluded that the electrified interface consisted of a Helmholtz layer one ion thick. Subsequent studies, based on STM measurements27 and molecular dynamics simulations,28 have suggested that the RTIL ordering near the electrode may, depending on potential, be either the monolayer suggested by Baldelli,12 or a few multilayers of alternating



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counter- and co-ions.28 These studies indicate that RTIL double layers at electrode surfaces may undergo potential-dependent monolayer-to-multilayer structural transitions. CO generally adsorbs weakly on Ag, and is not often seen by vibrational spectroscopy.30 There have been observations of CO on Ag during CO2 reduction, by Oda and co-workers31 and Schmitt and Gewirth,32 using either IR spectroscopy or surface-enhanced Raman spectroscopy (SERS). Schmitt and Gewirth observed potential-dependent adsorption of CO on roughened Ag by SERS. They observed CO adsorption on bridge and 3-fold hollow sites and measured Stark shifts for several CO sites. With the addition of 3,5-diamino-1,2,4-triazole, CO adsorption to less-coordinated sites was found, including a physisorbed or noncoordinating site that exhibited no Stark shift. In the present study, our NR SFG measurements indicated the existence of a potentialdependent structural transition of the RTIL. The resonant SFG studies of adsorbed CO showed a sudden increase in the Stark shift associated with the RTIL transition, that indicated the surface field more than doubled. The RTIL transition occurred even when CO2 was not dissolved in the RTIL, so the transition was a property of the RTIL itself, irrespective of the presence of CO2 or CO2 reduction to CO. Significantly, the RTIL transition and the increased Stark shift occurred at precisely the same potential that CO2 reduction began. Thus we have demonstrated a close association between low overpotential CO2 reduction and an electrochemically-driven structural transition of the RTIL. 2. Experimental A. Electrochemical cell The spectroelectrochemical apparatus, diagrammed in Fig. 1, was an improved version33 of what was used in previous works.5,14 The working electrode was a polycrystalline Ag cylinder


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5 mm in diameter and 2 mm high. The electrode was pressed against a 25 µm Teflon spacer14 and a downward-facing CaF2 window, with a gap filled with RTIL electrolyte. The Kel-F base had a feed-through for a Pt wire counter electrode, a Luggin capillary for the Ag/AgCl reference electrode, and a gas inlet for bubbling Ar or CO2 (UHP grade > 99.% from S. J. Smith Co.). Procedures for cleaning the cell and preparing the Ag electrode are described in supporting online material. Dissolved gases were purged from the ionic liquid EMIM-BF4 (98% Sigma-Aldrich) by bubbling Ar through the RTIL for at least 30 min before each experiment. A Karl-Fischer titration method described in supporting online material was used to determine that the Arpurged RTIL contained 0.3 mol% water. Water was allowed to remain in the RTIL, because water was needed to facilitate the CO2 reduction process.3,4 The electrochemical cell used an Ag/AgCl reference electrode with 3M saturated KCl electrolyte. Cyclic voltammetry (CV) measurements were made by sweeping the potentiostat output from -0.5V to -2.0V relative to this reference electrode. Since the cell electrolyte was RTIL, rather than 3M aqueous KCl, a correction to the reference potential had to be made. We followed the procedures outlined previously3 to correct the cell potential based on the known potential for ferrocene reduction. This led to a potential correction of +139 mV, as described in supporting online material. Thus the corrected cell potentials were swept from -0.36V to -1.86V. B. Laser apparatus The spectroelectrochemical apparatus was described previously.33

The broadband

infrared (IR) pulses were tunable in the 2.5-11 µm range, with a spectral width of 200 cm-1 fwhm. The narrowband (11 cm-1 fwhm) visible (800 nm) pulses were generated by filtering broadband visible pulses through a Fabry Perot étalon, to produce time-asymmetric pulses with



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an ~200 fs rise time and a 1 ps fall time.5,6 As described previously,5,6 a time delay ∆t between the IR and time-asymmetric visible pulses of typically 0.4 ps could be introduced, when desired, to suppress the nonresonant (NR) SFG signals. The CO-stretch resonances were measured using NR suppression. The IR and visible beams were somewhat elliptical, and the mean circularized beam diameters (1/e2) were 310 µm and 400 µm, respectively, and the pulse energies were each 5 uJ. All spectra were obtained using the ppp polarization condition with the incident beams approximately 60° from the surface normal. Other polarization combinations yielded too-weak signals, since s-polarized optical fields vanish at metal surfaces. SFG spectra were acquired during cyclic voltammetric (CV) scans by a computer-controlled CCD camera. The potentiostat (PAR 263A) was commanded by computer to scan the potential at the selected rate, and at the beginning of the scan the potentiostat triggered the CCD camera that was programmed to obtain spectra at the selected rate. A typical acquisition would use a 5 mV s-1 scan rate and a 10 s integration time for each spectrum. Spectroelectrochemical measurements were performed first with Ar-saturated RTIL. Then Ar was shut off and the RTIL was saturated with CO2. To insure CO2 flowed into the gap between electrode and window, the plunger holding the electrode was retracted for a short time as CO2 was bubbled into the cell. Then the plunger was pushed back so the electrode was pressed against the 25 µm spacer. CV scans performed during SFG measurements involved immersing the entire Ag working electrode in the RTIL electrolyte. In that configuration, current could flow through the electrode face and also through the edges and sides (see Fig. 1). Since SFG probes only a small ~300 µm diameter spot on the electrode face, it was useful to also obtain CV data where current flow was restricted to the center of the electrode face. That was accomplished by slightly


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withdrawing the plunger to create a meniscus contact between the electrolyte and the center of the electrode face. Meniscus CV measurements were obtained offline from the SFG apparatus. 3. Theoretical section In the SFG experiments described here, the femtosecond IR pulses pass through 25 µm of electrolyte on their way to the working electrode. Figure 2 is the IR spectrum of a 25 µm thick layer of CO2-saturated RTIL. The intense, sharp absorption at 2342 cm-1 (fwhm = 8 cm-1) is due to the asymmetric stretch of CO2 solute in the form of a CO2-RTIL weakly-associated complex.34-37 The doublet near 3600 cm-1 is due to associated water. The NR measurements were made with IR pulses tuned to 2630 cm-1, and the CO spectra with IR pulses tuned to 2100 cm-1. Figure 2 shows that at these frequencies, the RTIL electrolyte absorptions were broad and unstructured, and the absorbances were ~0.1. The RTIL attenuated the IR pulses by ~20% on their way to the electrode surface.

Due to the unstructured

absorbance, the RTIL only minimally distorted the temporal and spectral envelopes of these IR pulses. The absorption coefficient for the IR pulses was on the order of 100 cm-1, and the IR fluence at the beam center was 1.3 x 10-2 J cm-2, so the temperature jump14 in the electrolyte induced by a single IR pulse was small (~0.3K). Nonresonant SFG could be generated from the metallic electrode surface and from RTIL and other species such as H2O or CO2 within the double layer.

CO in solution exists in

centrosymmetric environments, and there was no significant electrolyte IR absorption that could be attributed to CO, so NR-suppressed SFG signals from CO could be generated only by CO adsorbed onto the electrode. The SFG intensity at frequency ω + ωvis, where ω represents the IR frequency, will vary as the potential φ is scanned. Scanning the potential changes the field within the double layer, and it also changes the structure and composition of the double layer. 7 ACS Paragon Plus Environment


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With a bit of oversimplification that ignores directional qualities due to the tensor nature of the nonlinear susceptibilities and the polarizations of the optical beams,38-40 the SFG intensity can be written as,41-45 2

(2) I SFG (ω + ωvis , φ ) ∝ I IR (ω ) I vis (ωvis ) χ R(2) ( ω , φ ) + χ NR (ω , φ ) ,

where R denotes resonant and NR denotes nonresonant.

(1)

In Eq. (1), we have ignored the

dependence of the second-order nonlinear susceptibilities χ(2) on the visible frequency, because the visible frequency was fixed and did not change during the experiments. The spectral profile of the broad-band IR pulses centered at ω will be denoted IIR(ω), where 2

I IR (ω ) = I 0 exp  −4 ln ( 2 )( ω − Ω ) / δ 2  .  

(2)

The center frequency is Ω and the fwhm (here 200 cm-1) is δ. The resonant susceptibility for CO adsorbates is frequently approximated as,14 2

χR

(ω ) = Σ

NAν , ν ω − ων + iΓν / 2

(3)

where N is the molecular number density, and the spectrum is taken to be a sum of Lorentzian lineshapes characterized by amplitudes Av, central frequencies ωv and linewidths Γ.

The

amplitude factors are proportional to the orientation-averaged hyperpolarizabilities. In the dipole approximation, the hyperpolarizabilities are written as the product of the transition dipole moments and the first-order (Raman) polarizabilities.46

4. Results A. Cyclic voltammetry of CO2 reduction Figure 3 shows CV data taken when the cell potential was scanned from -0.36V to 1.86V and the RTIL was saturated with either Ar or CO2. Such CV measurements have been


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reported previously,47 and our results are consistent with prior works. The dotted curves in Fig. 3 were obtained during the SFG measurements reported below, and the solid curves during offline meniscus CV measurements. The inset, comparing meniscus CV data with Ar or CO2, shows the CO2 current diverges from the Ar current at -1.33 (±0.05)V. We say this potential, where the reduction current suddenly increases, signifies the threshold for significant CO2 reduction to CO. In saying this, we recognize that smaller quantities of CO detectable by gas chromatography may be produced at lower, even significantly lower potentials.3,4,47

The

threshold potential -1.33V denotes the potential where CO2 reduction can be detected by increased current density, which is the point where CO2 is reduced rapidly at high current density on a planar electrode.

B. Nonresonant SFG Figure 4 shows three cycles of the wavelength-integrated NR SFG signal (2630 cm-1), as a function of potential, with Ar or CO2-saturated electrolyte, along with the corresponding meniscus CV. The IR pulses were tuned to 2630 cm-1. The NR intensities have maxima at the extrema of the potential scans, -0.36V and -1.86V, with minima near -1.33V. The NR signals with Ar or CO2 saturated RTIL electrolyte were identical, within experimental error. In Figs. 5a,b, we compared the NR signals to the meniscus CV data for the second potential cycle.!! We drew dotted circles in the CV data at the -1.33V threshold for CO2 reduction and at the NR intensity minimum. Note the threshold is a few mV different for forward and reverse scans. Figure 5 shows a close and clearly-evident association between the reduction threshold and the NR intensity minima. The NR intensity minima appeared at the same potential whether or not CO2 was present, so the location of the minima was unrelated to



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the presence of CO2 or the CO2 reduction process. The appearance and location of the minima was a property of the RTIL itself. We fit the NR signal intensities in Fig. 5 to parabolic functions of potential (the “parabolic model”)8,11 in Fig. 6, as suggested by earlier studies.7,8,11 The minimum was quite close to -1.33V, but we could not fit the data to a simple parabola. Instead we had to use two half-parabolas having a sudden change in curvature at the minimum. As shown in Fig. 6, the curvature in the CO2 reduction region at and below -1.33V was significantly greater than the curvature above -1.33V.

C. SFG spectra of adsorbed CO Figure 7 shows NR-suppressed spectra in the CO region during the first two CV scans, where the CO was generated by electrochemical reduction of CO2. The CO resonant frequencies were potential dependent and were in the 2080-2100 cm-1 range, about the same range reported by Baldelli for CO on Pt with RTIL.12 The arrows in the figure indicate the threshold potential for CO2 reduction. The CO observed in Fig. 7 must be attributed to CO adsorbed on the Ag electrode.2,30,48 Further support for this attribution is provided in Fig. 8, which plots the CO wavenumber versus potential for the first three CV scans. Figure 8 shows CO has a prominent Stark shift. At potentials where CO2 was not reduced (>-1.33V), the Stark shift was 24 (±1) cm-1 V-1.

In the CO2 reduction region (

Structural Transition in an Ionic Liquid Controls CO2 Electrochemical

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