Switching the Reaction Course of Electrochemical CO2 Reduction

Article pubs.acs.org/Langmuir

Switching the Reaction Course of Electrochemical CO2 Reduction with Ionic Liquids Liyuan Sun,† Ganganahalli K. Ramesha,‡ Prashant V. Kamat,*,†,‡,§ and Joan F. Brennecke*,† †

Department of Chemical and Biomolecular Engineering, ‡Radiation Laboratory, and §Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) offers new ways to modulate the electrochemical reduction of carbon dioxide. [emim][Tf2N], when present as the supporting electrolyte in acetonitrile, decreases the reduction overpotential at a Pb electrode by 0.18 V as compared to tetraethylammonium perchlorate as the supporting electrolyte. More interestingly, the ionic liquid shifts the reaction course during the electrochemical reduction of carbon dioxide by promoting the formation of carbon monoxide instead of oxalate anion. With increasing concentration of [emim][Tf2N], a carboxylate species with reduced CO2 covalently bonded to the imidazolium ring is formed along with carbon monoxide. The results highlight the catalytic effects of the medium in modulating the CO2 reduction products.

1. INTRODUCTION The steadily increasing atmospheric concentration of carbon dioxide (CO2), a major greenhouse gas, mainly generated from burning of fossil fuels, and its potential impact on climate has been the topic of much discussion.1−4 On the other hand, CO2 is recognized as a naturally abundant and inexpensive carbon source for the production of fuels and feed stocks for a variety of chemical syntheses in industry.2,4 Therefore, an attractive way to mitigate increasing atmospheric CO2 concentrations is through conversion of CO2 into fuels and useful chemicals using carbon-neutral energy resources.4 Approaches such as chemical, thermochemical, photochemical, biochemical, and electrochemical reduction methods have been investigated to achieve CO2 conversion.1,5−11 Of these methods, electrochemical reduction of CO2 is considered to be attractive because of its higher achievable conversion efficiency, its product selectivity, and its potential to store electrical energy that is extracted from renewable energy sources like the sun.4,12,13 CO2 is thermodynamically stable and kinetically inert; the main hurdle in the electrochemical CO2 reduction lies in the first step, the one-electron reduction of CO2. This activation of CO2 to form an anion radical (CO2−•) requires an unusually high reduction potential of −1.9 V vs NHE,14 associated with an overpotential that is dependent on the electrode and medium.11,13,15−17 Efforts have been made to achieve CO2 reduction at lower electrochemical potentials, e.g., using pyridinium-based homogeneous catalysts.18 Due to their low volatility, nonflammability, good solvating ability, and good thermal and chemical stability, ionic liquids (ILs) have been regarded as “green” alternatives for conven© XXXX American Chemical Society

tional toxic and volatile organic solvents in many chemical processes. Additionally, the high solubility of CO2 in ILs,19−21 intrinsic ionic conductivity, and wide potential windows make ILs attractive media for electrochemical reduction of CO2. Recently, Rosen et al. reported the effective lowering of the reduction overpotential for CO2 conversion into CO at Pt and Ag electrodes in an aqueous solution of the IL 1-ethyl-3methylimidazolium tetrafluoroborate ([emim][BF4]). They proposed that the effectiveness of this newly observed catalytic reduction relies on the ability of the IL to lower the energy barrier for the formation of CO2−• through the formation of a complex.11 Wipple et al. also proposed that the overpotential of CO2 reduction could be lowered by stabilizing the intermediate CO2−• using a suitable catalyst.4 Electrochemical reduction is a convenient approach to carry out reduction of CO2 in a controlled way. The product distribution varies depending on the type of metal electrodes employed in a certain electrolyte. For example, oxalate has been widely reported as the major product of CO2 reduction at a Pb electrode in nonaqueous media.12,22−24 Also, Pb is considered to be an inert electrode material that does not interfere with the reduction of CO2 and the competing pathways for product formation.17 Here we investigate the role that the IL 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N], shown in Figure 1) plays as a homogeneous catalyst for CO2 reduction using electrochemical reduction at a Pb cathode. Received: March 7, 2014 Revised: May 9, 2014

A

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2.2. Electrolysis at Controlled Potentials and Product Analysis. We carried out electrolysis at controlled potentials to monitor the CO2 reduction products. When the electrolysis was carried out for 60 min in 0.1 M TEAP/AcN we observed the formation of a white precipitate. Analysis of the precipitate using FTIR spectra (see Supporting Information, Figure S3) confirmed the formation of oxalate as the primary product of CO2 reduction. Oxalate generated from CO2 reduction in 0.1 M TEAP/AcN was quantified using HPLC (Supporting Information, Figure S4). The formation of oxalate anion is the result of dimerization of CO2−•. This selective reduction product formed at Pb electrodes agrees with earlier findings.22,23 The oxalate formation was significantly decreased when 0.1 M [emim][Tf2N]/AcN was used as an electrolyte. Efforts were made to isolate and identify reduction products using 1H and 13 C NMR. In addition, the gaseous reduction product, carbon monoxide (CO), was analyzed using GC analysis of the head space samples. The resonance lines of [emim][Tf2N] in the 1H NMR spectrum were assigned from their multiplet splitting patterns, as well as comparison with peak assignments for other [emim]+ ILs whose NMR spectra are available in the literature.26 The chemical shifts of the protons in [emim][Tf2N] are summarized in Table 1, according to the labeling

Figure 1. Structure of the ionic liquid [emim][Tf2N].

Specifically, we seek answers to the following questions: (1) how does [emim][Tf2N] mediate the product selectivity during CO2 reduction and (2) how does the mechanism of CO2 reduction switch from oxalate anion to CO in an IL-based nonaqueous medium?

2. RESULTS AND DISCUSSION 2.1. Voltammetry Study of CO2 Reduction in TEAP/ AcN and [emim][Tf2N]/AcN at a Pb Working Electrode. The cyclic voltammograms (CVs) measured using a Pb cathode in 0.1 M tetraethylammonium perchlorate/acetonitrile (TEAP/ AcN) and 0.1 M [emim][Tf2N]/AcN are shown in Figure 2. These voltammograms were recorded at a potential sweep rate of 50 mV/s at room temperature after the electrolyte solution was purged with N2 for 30 min to remove dissolved oxygen, followed by purging with CO2 for another 30 min. Parts a and b show the reductive scan of N2-saturated and CO2-saturated electrolyte solutions, respectively. The increased current seen in the CO2-saturated solutions confirms the ability of Pb electrode to reduce CO2 to CO2−•. The onset potential corresponding to the reduction of CO2 in the presence of 0.1 M [emim][Tf2N]/AcN is decreased by 0.18 V compared to that of 0.1 M TEAP/AcN. The onset potential is −2.30 V in 0.1 M TEAP/AcN and −2.12 V in 0.1 M [emim][Tf2N]/AcN with respect to the reference electrode (Ag/AgNO3), as seen in Figure 2. Since it is difficult to establish the exact reduction peak, we selected potentials that gave a current of 0.6 mA/cm2 for performing reduction of CO2 in acetonitrile. Potentials of −2.40 and −2.25 V (vs Ag/ AgNO3) were employed for CO2 reduction in 0.1 M TEAP/ AcN and 0.1 M [emim][Tf2N]/AcN, respectively, in our experiments, unless otherwise indicated. This approach of monitoring the apparent current density at a fixed potential is consistent with earlier reports.25 The difference in the two reduction potentials represents the decrease in overpotential in the [emim][Tf2N]/AcN system compared to TEAP/AcN.

Table 1. 1H NMR Chemical Shifts (DMSO, δ, ppm relative to TMS) of the Pure 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) and of the Solution Obtained by CO2 Electrochemical Reduction at −2.25 V vs Ag/AgNO3 in 0.1 M [emim][Tf2N]/AcN at Room Temperature, under Atmospheric Pressure for 1 h no. of H atom 7 8 6 5 4 2

pure [emim][Tf2N] COO−[emim][Tf2N] [emim][Gly]26 1.42 3.85 4.20 7.69 7.77 9.12

1.35 3.96 4.48 7.54 7.61

1.41 3.87 4.22 7.75 7.84 9.68

shown in Figure 3. As is shown in the spectra in Figure 3a−c, the original resonance lines of pure [emim][Tf2N], with the

Figure 2. Cyclic voltammograms (CVs) recorded using a Pb working electrode in an electrolyte containing (A) 0.1 M TEAP/AcN and (B) 0.1 M [emim][Tf2N]/AcN. The CVs were recorded in (a) N2-saturated and (b) CO2-saturated solutions. (Scan rate 50 mv/s. The vertical line in each plot is the potential employed for electrolysis experiments.) B

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Figure 3. 1H NMR spectra (in DMSO, reference to TMS) of (a) pure [emim][Tf2N], (b) 0.1 M [emim][Tf2N]/AcN after purging CO2 for 30 min, (c) 0.1 M [emim][Tf2N]/AcN electrolysis solution after 1 h of CO2 reduction, and (d) separated precipitate generated in CO2 reduction. The schematic representation of the [emim][Tf2N] ion pair with the labeling of the carbon atom used in the assignment is displayed.

solution after electrolysis (Supporting Information, Figure S6), which can be attributed to the carboxylate group covalently bound to C2 of the imidazolium ring,27 providing further confirmation that a carboxylate species was formed. It needs to be pointed out that electrospray ionization (ESI) MS provides further evidence of the formation of the carboxylate species by showing a peak with an m/z value of 155.0822 (Supporting Information, Figure S7). The 1-ethyl-3-methylimidazolium-2carboxylate species was quantified on the basis of integration of peak intensities in the 1H NMR spectra, and CO was quantified by GC (Supporting Information, Figure S5). Rosen et al. 10 suggested that an adsorbed neutral [emimCO2] complex was responsible for lowering the overpotential for the electrochemical conversion of CO2 to CO, lending support to the discovery presented here of a 1ethyl-3-methylimidazolium-2-carboxylate species in solution and as a bulk precipitate. From the 1H NMR spectra we cannot determine if the complex is neutral or negatively charged. We envision that the [emim]+ cation is deprotonated to form the neutral carbene (Figure 4). The carbene then reacts with the reduced CO2−•, which would form a negatively charged [emimCO 2]−. However, this complex may be reprotonated to form the overall neutral molecule shown in eq 1. The proton removed from the C2 position is not seen in the 1H NMR of the solution after electrolysis (c) or in the precipitate (d); i.e., there is no 2′ in either spectrum. However, if the H+ is attached to the complex, we would anticipate fast exchange of that proton between the carboxylate site and the polar solvent, so that it would not show up in the 1H NMR.27,30

exception of the proton on C2, become accompanied by a corresponding secondary line of lower intensity after CO2 electrochemical reduction is carried out in [emim][Tf2N]/ AcN. The composite spectrum of the electrolysis solution mixture indicates a substitution reaction at C2 of the imidazolium ring to form a reacted imidazolium species. Figure 3d shows the 1H NMR spectrum of a precipitate generated during the electrolysis of CO2-saturated 0.1 M [emim][Tf2N]/AcN after it was isolated from the electrolysis solution mixture, purified, and dissolved in DMSO. The main resonance lines in this spectrum correspond to the protons of the new reacted imidazolium species, which we believe to be a carboxylate complex. The less intense lines at the same chemical shifts as the pure IL can be attributed to residual unreacted IL in the precipitate. There have been abundant literature reports describing carboxylation at the C2 position of imidazolium-based ILs, triggered by reaction with neutral CO2 molecules.3,27−29 The reaction scheme is shown in Figure 4, although there is some debate regarding the reversibility of the reaction. We reason from the spectra in Figure 3 that electrochemically reduced CO2 is also capable of deprotonating the imidazolium ring to form a carboxylate species. In addition, we observe a resonance line at 155.1 ppm in the 13C NMR spectrum of the CO2-saturated 0.1 M [emim][Tf2N]/AcN

[emim] + CO2 + H+ + e− → ([emimCO2 ]− ···H+)

(1)

Figure 5 shows the products formed and the Faradaic efficiency as a function of time for electrolysis of CO2-saturated 0.1 M TEAP/AcN solution (A and B) and CO2-saturated 0.1

Figure 4. A schematic diagram of the carboxylation of C2 of the imidazolium ring by CO2. Redrawn according to refs 16−18. C

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Figure 5. Formation of the products (a) oxalate anion, (b) CO, and (c) carboxylate anion during the course of electrolysis and the corresponding Faradaic efficiency using two different electrolytes: (A and B) 0.1 M TEAP/AcN (applied potential of −2.40 V vs Ag/AgNO3) and (C and D) 0.1 M [emim][Tf2N]/AcN (applied potential of −2.25 V vs Ag/Ag NO3). The electrolyte was kept saturated with CO2. The Faradaic efficiency was monitored from the charges flowed during the reduction period.

Figure 6. Effect of [emim][Tf2N] concentration on the distribution of CO2 reduction products: (A) (a) oxalate and (b) CO, and (B) (b) CO and (c) carboxylate. The electrolysis was carried out at −2.34 V (vs Ag/AgNO3) in CO2-saturated AcN containing 0.1 M TEAP and varying concentrations of [emim][Tf2N]. The products were analyzed after 1 h of electrolysis.

Obviously, the presence of [emim][Tf2N] changes the course of the CO2 reduction reaction so that it produces CO and an imidazolium−carboxylate complex and suppresses the dimerization of CO2−• to form oxalate. We further explored the role of [emim][Tf2N] in switching the course of the reaction by varying its concentration in a 0.1 M TEAP/AcN solution and monitoring the product distribution of CO2 electrochemical reduction. 2.3. Shifting the Course of the Electrochemical Reduction of CO2 with [emim][Tf2N]. Cyclic voltammograms were recorded using a Pb working electrode (Supporting

M [emim][Tf2N]/AcN solution (C and D). In agreement with previous studies, the product of electrochemical reduction of CO2 in nonaqueous media using Pb as the working electrode and TEAP as the supporting electrolyte is predominantly oxalate anion.12,22−24,31 When we used 0.1 M [emim][Tf2N]/ AcN as the supporting electrolyte, CO and an imidazolium− carboxylate species become the prominent products. In the 0.1 M [emim][Tf2N]/AcN system, the Faradaic efficiency of the imidazolium carboxylate decreased with time and that of CO increased with time, until equilibrium values were reached after an hour or two. D

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Scheme 1. Reaction Pathways for the Electrochemical Reduction of CO2 in the (A) Absence and (B, C) Presence of [emim][Tf2N] at a Pb Electrode in AcN

hydroxide. This produced no change in the amount of CO produced, thus eliminating reaction 3 as the mechanism. Gu et al. reported a mechanism where N-heterocyclic carbenes (NHC) catalyzed CO2 splitting to form CO when aromatic aldehydes were used as oxygen acceptors. They proposed a reaction scheme where CO2 reacted with the NHC, resulting in the formation of an imidazolium carboxylate, which subsequently reacted with added aromatic aldehyde to generate an intermediate species that was likely stabilized by intermolecular hydrogen bonding. The H+ on the imidazolium ring of the intermediate species was trapped by the added base, leading to the splitting of CO2 and formation of the corresponding aromatic salt. Finally, CO was released from the NHC−CO complex, which regenerated the NHC, which could proceed to the next catalytic cycle.8 The detection of a 1ethyl-3-methylimidazolium-2-carboxylate complex in our experiments and the leveling off of the Faradaic efficiency of both the carboxylate and CO as a function of electrolysis time suggest that CO could be generated from the imidazolium carboxylate in our system, as well. To test this hypothesis, we used trimethylimidazolium bis(trifluoromethylsulfonyl)imide ([mmmim][Tf2N]) as the supporting electrolyte. In this IL the proton at the C2 position is replaced with a methyl group, making the formation of a carbene impossible. To our surprise, CO was produced with an Faradaic efficiency of 57.2% for 1 h electrolysis at −2.25 V (vs Ag/Ag NO3), even higher than that of 39.2% with [emim][Tf2N] as the electrolyte, which suggests that CO does not originate from the 1-ethyl-3-methylimidazolium-2-carboxylate; rather, the carboxylate may be a competing species formed concomitantly with CO. Finally, our observation that CO is formed preferentially over oxalate when [emim][Tf2N] is added to TEAP/AcN is similar to what occurs with the addition of a surfactant.35 This indicates that ILs ([emim][Tf2N] and [mmmim][Tf2N]) may play a role similar to that of surfactants, thus helping to stabilize

Information, Figure S8) in N2- and CO2-saturated 0.1 M TEAP/AcN solutions with different concentrations of [emim][Tf2N]. We applied a constant potential of −2.34 V (vs Ag/ AgNO3) to carry out CO2 electrolysis for a period of 1 h. The concentration of products and current that flowed through the circuit were monitored to estimate the Faradaic efficiency. As shown in Figure 6A, CO increases and oxalate anion decreases with increasing concentration of the IL [emim][Tf2N]. Another reduction product is imidazolium carboxylate, which is formed in significant amounts during the electrolysis (Figure 6B). It is evident from the product analysis that [emim][Tf2N] can switch the course of the electrochemical reduction reaction, leading to the formation of CO and suppressing the oxalate anion formation. It has been reported that CO is produced in the electrochemical reduction of CO2 in nonaqueous media at metal working electrodes, such as In, Zn, Sn, Au, Hg, Pb, and Pt, followed by the disproportionation of CO2−• (reaction 2).22,23,31−33 On the other hand, in aqueous media protons can interact with CO2−• to produce CO (reaction 3).1,7,10,11,23,34 2CO2−• → CO + CO32 −

(2)

CO2−• + 2H+ → CO + H 2O

(3)

We further probed the involvement of reactions 2 and 3 in our experiments. The 13C NMR spectrum of the electrolysis sample (Supporting Information, Figure S6) did not show any carbonate species, which would have to be produced in an amount equal to the amount of CO formed if reaction 2 were taking place. This rules out the disproportionation of CO2−• (reaction 2) in the electrolysis of CO2-saturated 0.1 M TEAP/ AcN plus [emim][Tf2N] mixtures. We also explored the possibility of CO being formed by reaction of CO2−• with H+ ions (reaction 3) by adjusting the proton ion concentration through addition of sulfuric acid or tetraethylammonium E

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CO2−•, which is responsible for the reduced overpotential for the CO2 reduction reaction. Moreover, the presence of ILs modifies the course of the electrochemical reduction of CO2 in nonaqueous solution. Analogous to Rosen et al.’s study,10 the formation of CO in our system can be attributed to the interaction between CO2 and the cation of the IL at the electrode surface. When only TEAP is used as the supporting electrolyte, CO2−• dimerizes on the surface of the Pb electrode to yield oxalate (shown in Scheme 1A). When an imidazolium IL is added, the cathode surface becomes covered with both CO2−• and the cations of the IL. The imidazolium cation stabilizes the CO2−•, prevents close approach of two CO2−•, and inhibits dimerization to form oxalate (Scheme 1B). In addition, CO2−• reacts with the [emim]+ cation to form carboxylate (Scheme 1C). Therefore, when [emim][Tf2N] is added, two competing pathways replace dimerization of CO2−• to form oxalate (Scheme 1A), with one leading to the formation of CO (Scheme 1B) and the other to the formation of the carboxylate (Scheme 1C).

electrode was manufactured from a 25 × 25 mm platinum gauze (Alfa, Aesar, 99.9%, 52 mesh). The working electrode and the reference electrode were immersed in 10 mL of electrolyte solution in the cathodic compartment of the cell, and the counter electrode was immersed in 5 mL of electrolyte solution in the anodic compartment. The applied potential between the reference and working electrodes was maintained by a Gamry potentiostat. Before each set of bulk electrolysis experiments, the cathodic compartment was purged with N2 for 30 min to remove any moisture and dissolved oxygen, followed by purging with CO2 for 30 min to saturate the electrolyte. Cyclic voltammograms of the Pb electrode in N2- and CO2-saturated electrolyte were taken between −1.50 and −2.50 V (vs Ag/Ag NO3) at a sweep rate of 50 mV/s right after purging the gas. Generally, the surface of the working electrode was electrochemically conditioned, and equilibrium was reached after 10 CV cycles with the electrolyte kept under the N2 or CO2 atmosphere. The data reported corresponds to the last cycle. Chronocoulometry was carried out in the closed cell with an applied potential of −2.40 and −2.25 V for 0.1 M TEAP/AcN and 0.1 M [emim][Tf2N]/AcN, respectively. 4.3. Product Characterization. Fourier transform infrared spectroscopy (FTIR) (Bruker Tensor 27) was used for the IR measurements of the precipitate generated from CO2 reduction with TEAP as the electrolyte. Sodium oxalate powder was used as a standard. The precipitate was separated and purified from the electrolysis solution in a centrifuge at 6000 rpm for 15 min, followed by rinsing with pure AcN. The separation and purification were repeated three times. Figure S3 (Supporting Information) shows the IR spectra of the blank, standard, and unknown samples. The NMR measurements were performed on a Varian spectrometer operating at 600 MHz Larmor frequency. The samples were dissolved in DMSO, wherein the electrolysis solution was dried on a rotary evaporator at 40 °C under vacuum. The precipitate generated in [emim][Tf2N]/AcN was separated and purified similarly. High-performance liquid chromatography (HPLC) was performed on an Alliance-Waters HPLC instrument with a C18 reversed phase (RP) column (250 × 4.6 mm i.d., OOG-4299-E0, Phenomenex). The mobile phase was AcN/10 mM KH2PO4 aqueous solution of pH 2.5 (5:95) at a flow rate of 0.5 mL/min. A standard calibration curve was developed to determine the concentration of oxalate generated in the unknown samples (Supporting Information, Figure S4). The gaseous product of CO2 reduction was analyzed with a gas chromatography (GC) instrument (Thermo Scientific) equipped with a thermal conductivity detector (TCD). A molecular sieve 5A column was used with helium as the carrier gas at a constant pressure of 3 psi. The temperature of the oven was set at 65 °C for 4.5 min. The standard calibration curve developed to quantify CO in unknown samples is shown in Figure S5 (Supporting Information).

3. CONCLUSIONS The electrochemical reduction of CO2 at a Pb electrode in the presence of ionic liquid [emim][Tf2N] shows the catalytic role of ionic liquid in shifting the course of the reduction pathway. The cyclic voltammograms confirm that imidazolium IL lowers the overpotential for CO2 reduction by about 0.18 V. Most importantly, our study shows that the course of the electrochemical reduction of CO2 can be altered dramatically by the presence of [emim][Tf2N]. Specifically, increasing the concentration of [emim][Tf2N] in 0.1 M TEAP/AcN switches the product from oxalate, the widely reported principle product of CO2 reduction on Pb in AcN, to CO and an imidazolium carboxylate complex. The results discussed in this investigation further ascertain the role of imidazolium-based ionic liquid as homogeneous catalyst for lowering the CO2 reduction potential as well as modulating the course of the reaction. 4. EXPERIMENTAL SECTION 4.1. Materials. Acetonitrile (AcN, Fisher Scientific, 99.9%), sodium oxalate powder (Na2C2O4, Alfa Aesar, A.C.S, 99.5+%), potassium phosphate monobasic (KH2PO4, Fisher Scientific, A.C.S, 99.9%), and dimethyl sulfoxide (DMSO, D, Cambridge Isotope Laboratories, 99.9%, +1% v/v TMS,) were used as purchased. Tetraethylammonium perchlorate (TEAP) (Alfa Aeser, 98%) was desiccated at 40 °C overnight in a vacuum oven before use. [emim][Tf2N] and [mmmim][Tf2N] were synthesized by standard techniques. N2 and CO2 were supplied from Airgas (purity 99.995%). The Nafion 117 membrane (manufactured by PuPont Fuel Cells) was treated as described in the literature before use:36,37 boiling in 3% H2O2 aqueous solution, deionized water, 0.5 M H2SO4 aqueous solution, and finally deionized water, successively, for 1 h for each step. 4.2. Electrochemical Measurements. Cyclic voltammetry and chronocoulometry were performed in a two-compartment, threeelectrode glass cell. Figure S1a,b (Supporting Information) shows the construction of the cell. The working electrode was a lead sheet (Alfa, Aesar, 99.998%), which was polished with sand paper with a grit designation of 320, followed by rinsing with AcN before use to remove any oxide impurities on the surface. A geometrical surface area of 2.0 cm2 of the lead working electrode was maintained in the solution in all electrochemical experiments. The Ag/AgNO3 electrode, made of a silver wire immersed in 0.01 M silver nitrate dissolved in 0.1 M TEAP/ AcN, was used as the reference electrode. The reference electrode was calibrated against a ferrocene/ferrocenium (Fc+/Fc) redox couple to make sure it gave the right potential in reference to SHE.38 Figure S2 (Supporting Information) shows the calibration curves. The counter

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ASSOCIATED CONTENT

S Supporting Information *

Construction of the electrochemical cell, IR spectra, and calibration curves are presented. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.F.B.), [email protected] (P.V.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Notre Dame Sustainable Energy Initiative for financial support during this project. We also thank the Notre Dame Center for Environmental Science and Technology (CEST) for free access to the facilities. F

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