Insights Into the Composition and Function of a Bismuth Based

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Insights Into the Composition and Function of a Bismuth Based Catalyst for Reduction of CO to CO 2

Abderrahman Atifi, Thomas P. Keane, John L DiMeglio, Rachel C. Pupillo, David R Mullins, Daniel A Lutterman, and Joel Rosenthal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00504 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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The Journal of Physical Chemistry

Insights Into the Composition and Function of a Bismuth Based Catalyst for Reduction of CO 2 to CO Abderrahman Atifi,§ Thomas P. Keane,§ John L. DiMeglio,§ Rachel C. Pupillo,§ David R. Mullins,† Daniel A. Lutterman† and Joel Rosenthal*,§ §

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States †

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Corresponding author: [email protected]

Abstract The electrochemical conversion of CO 2 to CO is a process important for generation of carbon-based fuels using energy produced from intermittent, renewable energy sources. Electrodeposited thin films of bismuth-based materials are promising platforms for this transformation when used for CO 2 electrolysis in the presence of imidazolium-based ionic liquids including [BMIM]OTf and [EMIM]OTF. In this study, the composition and function of a bismuthbased carbon monoxide evolving catalyst (Bi-CMEC) has been probed. In particular, the composition of both the electrodeposited bismuth material and the catholyte has been scrutinized in an effort to understand the factors that drive efficient catalyst operation. A combination of XPS depth profiling, XANES, and EXAFS experiments were employed to identify the bulk composition of the bismuth catalyst, which is shown to be comprised of both metallic and oxidized phases. The identity of the catholyte solvent has been shown to influence the nature and efficacy of CO 2 reduction by the Bi-based catalyst system, as mass transport of both CO 2 and imidazoliumbased ionic liquid to the electrode surface dramatically impacts the kinetics and selectivity for CO production at the cathode/electrolyte interface.

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Introduction The electrochemical reduction of CO 2 to generate CO is an important process to the development of successful storage schemes for energy produced from intermittent, renewable sources. 1,2 CO 2 taken either from the atmosphere or a point source of emission can be converted into CO via the following energetically uphill electrochemical reaction:

CO2 + 2H + + 2e−  CO + H2 O

(1)

The resulting CO can then be upconverted using Fischer-Tropsch chemistry or electrochemical processes to synthesize artificial petroleum and liquid fuels. 3- 5 However, in order for such a process to be economically viable on an industrial scale, a catalytic system is needed to selectively reduce CO 2 with high current density at low applied overpotential. 6 While several noble metal catalysts, including Au and Ag, have long been established as being able to drive CO production under favorable conditions, 7– 11 the high cost of these metals is prohibitive to their use in economically practical systems. An inexpensive alternative to noble-metal based catalysts is a bismuth-based carbon monoxide evolving catalyst (Bi-CMEC), 12 which consists of an electrodeposited bismuth film submersed in an acetonitrile (MeCN) electrolyte solution containing low concentrations of ionic liquids (ILs) comprised of appropriately substituted imidazolium cations ([Im]+) such as 1-butyl-3-methylimidazolium ([BMIM]+) or 1-ethyl-3-methylimidazolium Chart 1. Structures of Imidazolium Ionic Liquids Employed in This Study.


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([EMIM]+) (Chart 1). This system has been shown to convert CO 2 to CO at a low overpotential (η < 500 mV) with Faradaic efficiencies (FEs) and current densities (j) comparable to those observed with Au or Ag catalysts. Work to date carried out on the Bi-CMEC platform and other systems involving IL promoted CO 2 reduction has demonstrated that the identity of the catalyst surface 13– 16 and the composition of the catholyte 17– 19 are both critical to the nature and efficacy of catalysis. Previous X-ray Photoelectron Spectroscopy (XPS) studies showed that the surface of the Bi-CMEC catalyst material is comprised of a mixture of Bi3+ containing compounds interspersed with metallic Bi0, but did not give information about the material’s bulk composition.12 Moreover, studies to date have only focused on the electrochemistry and catalysis of the Bi-CMEC system in the presence of [BMIM]+ or [EMIM]+ in MeCN based electrolytes. As such, in addition to gaining finer insight into the bulk composition of the electrodeposited Bi-CMEC material we have also sought to determine how varying the solvent composition of the catholyte solution might impact the ability of the Bi cathode to activate CO 2 and efficiently promote CO evolution. To this end, in addition to reporting new XPS, Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) experiments aimed at characterizing the electrodeposited Bi-based catalyst, this study also examines the function of this system in several different solvents in order to better understand how changing solvent dielectric and viscosity influences how the Bi/[Im]+ interface mediates efficient CO production, and provide further insight into the nature of ILmediated electrocatalysis at the Bi-CMEC surface.

Results and Discussion As has been reported previously, thin films of Bi-based materials may be electrodeposited from MeCN solutions containing 20 mM of Bi(OTf) 3 and 0.1 M TBAPF 6 under ambient



conditions by polarizing a glassy carbon electrode (GCE) or graphite electrode at potentials more negative than –1.3 V versus the saturated calomel electrode (SCE; all potentials herein are referenced to SCE). Detailed XPS analysis of these resulting materials shows that the surface is largely oxidized as indicated by the strong Bi 4f 7/2 signal centered around 159.5 eV (Figure 1a red) with a significantly smaller amount of metallic Bi0 present (~2%). 20 In order to determine whether the large proportion of Bi3+ containing compounds (as indicated by surface XPS of the Bi film) permeated the bulk of the electrodeposited film, depth profiling XPS analysis was undertaken for the bismuth containing material. The Bi film was etched within the XPS chamber using an Ar+ monatomic ion beam operating in low current mode at an energy of 2 keV and an angle of 30°. Etching cycles were 25 seconds in duration and XPS spectra were acquired before and after each etching cycle. The results of this XPS depth profiling experiment are shown in Figure 1a, which illustrates that the composition of the Bi film’s bulk is significantly different than that of the surface. The XPS spectra recorded after each sequential etching cycle shows progressively smaller relative ratios of Bi3+ to Bi0, which plateau at a ratio of ~1:2 within the bulk of the Bi-based film. As such, these XPS etching experiments indicate that while the surface of the electrodeposited Bi film is largely oxidized, the bulk of the material is

Figure 1. (a) Depth profiling Bi 4f XPS spectra of an electrodeposited Bi-based film recorded prior to sputtering (red), after successive 2 minute intervals of sputtering (grey), and after 14 total minutes of sputtering (blue). Also shown are Bismuth LIII – edge (b) XANES and (c) EXAFS spectra recorded for samples of bismuth metal powder (red traces), bismuth oxide powder (green traces) and Bi-CMEC (blue traces).


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predominantly metallic, though a significant amount of oxidized Bi-materials permeates the entire Bi-film. In an effort to corroborate the XPS depth profiling experiments, XANES and EXAFS were carried out to probe the bulk composition of the Bi-based films. Figure 1b shows the XANES spectrum recorded for the Bi-CMEC powder along with those recorded for metallic Bi0 and Bi 2 O 3 powders, which served as reference materials. The position of the leading absorption edge, and the intensity of the peak between 13435 eV and 13440 eV are indicative of the degree of Bi oxidation.21,22 As Bi becomes more oxidized the edge position shifts toward greater photon energies and the peak intensity increases. Relative to the reference samples with Bi0 (Bi metal) and BiIII (Bi 2 O 3 ) the nominal oxidation state of the electrodeposited bismuth sample appears to be intermediate between Bi0 and BiIII, suggesting that the Bi film consists of both metallic and oxide containing bismuth materials. The Fourier transforms of the k3-weighted EXAFS spectra of the electrodeposited Bi film and reference samples are displayed in Figure 1c. The Fourier transform k-range is k = 2–14 Å-1. The metallic Bi0 reference produces a peak near 3 Å. 23 This feature can be modeled by Bi–Bi single scattering at 3.07 Å with a nominal coordination number of 3. This is consistent with the Bi nearest neighbor separation of 3.07 Å in rhombohedral Bi metal. 24 The width and doublet appearance result from the k-range used in the Fourier transform. The Fourier transform of Bi 2 O 3 has peak intensity in two regions; the peak at approximately 1.6 Å results from the Bi–O single scattering and a broad feature between 3.0 Å and 3.8 Å that results from the nearest Bi–Bi neighbors. The Bi–O peak can be modeled by a single Bi–O scattering pair at 2.15 Å with a nominal coordination number of 3. Despite the wide range of Bi–O distances in α-Bi 2 O 3 and the actual larger number of O neighbors, this fit is consistent with what has been found by other



investigators22,25 We note that the scattering phase shift moves the apparent Bi–O distance in Figure 1c to smaller R relative to the actual Bi–O distance. The Bi–Bi peak between 3.0 Å and 3.8 Å can be modeled by a Bi–Bi pair at 3.57 Å, also with a nominal coordination of 3. As was true for the XANES experiments (vide supra) EXAFS analysis of the electrodeposited Bi film is also consistent with this powder being a mixed-phase material containing both metallic Bi0 and oxidized BiIII domains. As demonstrated by the EXAFS spectra shown in Figure 1c, the electrodeposited bismuth material contains features that resemble the Bi–O scattering in Bi 2 O 3 near 1.6 Å and Bi–Bi scattering in metallic Bi0 near 3 Å. Reasonable fits were obtained for the electrodeposited Bi sample using three single scattering pairs; Bi–O and Bi–Bi as in Bi 2 O 3 , as well as the Bi–Bi pair in metallic Bi0. Based on the intensities derived from the fits of the Bi film data as compared to those obtained for the two reference

Figure 2. Cyclic voltammograms recorded using Bi-modified GCEs in either MeCN (red), DMSO (yellow), DMF (blue) or PC (green) containing 100 mM TBAPF6 and 100 mM [BMIM]OTf under an atmosphere of CO2.


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materials, we can estimate that the relative ratio of oxidized to metallic bismuth in the electrodeposited film is ~1:2, which is consistent with the results obtained for the XPS etching experiments described above (vide supra). Having obtained a more complete understanding of the composition of the Bi-CMEC material, we turned our attention to probing how the composition of the electrolyte impacts the ability of the bismuth-based catalyst to promote CO 2 reduction. Previous work has shown that electrodeposited Bi-films12,13,17 and Bi nanoparticles 26 engender efficient CO 2 reduction using MeCN catholytes in the presence of millimolar concentrations of [Im]+ promoters. While there has been some work devoted to understanding how varying the identity of electrolyte cations and water content influences heterogeneous CO 2 electrocatalysis 27,28 and related processes, 29 little work has been devoted to improving our systematic understanding of how variations in electrolyte viscosity and polarity influence such processes. This is especially true for the Bi/[Im]+ system, which has only been probed in acetonitrile based electrolytes. To address this limit in our understanding, we have investigated whether the use of solvents other than acetonitrile might result in different reactivity for the Bi/[Im]+ system. The catholyte systems evaluated in this study contained 100 mM tetrabutlyammonium hexafluorophospate (TBAPF 6 ) as a supporting electrolyte and 100 mM 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTf) dissolved in several organic solvents under an atmosphere of CO 2 . Each of the solvents tested were polar aprotic solvents, including acetonitrile (MeCN), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and propylene carbonate (PC). Initial experiments employed cyclic voltammetry (CV) to probe the basic electrocatalytic response of the Bi-modified GCE under 1 atm of CO 2 in each of the electrolyte solutions described above. Inspection of the CO 2 reduction wave in the CVs recorded in each of the different solvents



(MeCN, DMF, DMSO, PC), as shown in Figure 2, demonstrates that the identity of the solvent does not have a significant effect on the onset potential of the catalytic wave. The peak current of the catalytic wave, however, did vary dramatically depending on the electrolyte solvent. In all cases, the catalytic peak current can be attributed to electrochemical CO 2 activation as repetition of the CV experiments in each solvent, but under an atmosphere of N 2 , does not lead to a large catalytic wave in the potential region –1.75 to –2.0 V vs SCE (Figure S1). Having noted that the electrolyte solvent impacts the current associated with CO 2 reduction (i.e., reaction rate), we sought to further examine how the catholyte solvent impacted the reaction kinetics of this system and whether solvent choice affected the selectivity and efficiency of the CO 2 electrolysis. Controlled potential electrolysis (CPE) experiments were carried out for CO 2 saturated solutions containing 100 mM [BMIM]OTf and 100 mM TBAPF 6 dissolved in each of the solvents employed for the voltammetry experiments of Figure 2. CPE experiments were performed at –1.95 V and –2.05 V to probe the solvent and potential dependency of Bi/[BMIM]+ catalysis. We note that the two electrolysis potentials surveyed are near the mid-point and peak of the catalytic wave, respectively. The total current responses for representative CPE experiments

Figure 3. Total current density (jtot) profiles for Bi-CMEC modified GCEs in either MeCN, DMSO, DMF or PC containing 100 mM TBAPF6 and 100 mM [BMIM]OTf under 1 atm of CO2 at applied potentials of (a) E = –1.95 V versus SCE (b) E = –2.05 V versus SCE.


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are shown in Figure 3. Similar to the voltammetry in Figure 2, the CPE current is highly dependent upon the solvent choice, increasing in the order PC < DMSO < DMF < MeCN. This trend is observed at both –1.95 and –2.05 V with the latter providing a larger overpotential, resulting in the higher current densities observed in Figure 3b. During electrolysis, gas chromatography was used to periodically sample the headspace of the cell and quantify any gaseous products formed. Quantification of the CO generated from 2e−/2H+ reduction of CO 2 afforded the Faradaic efficiencies (FE CO ) and partial current densities (j CO ) for CO production for each of the catholyte solutions shown in Table 1. Consistent with previous work, electrolysis of CO 2 saturated solutions of [BMIM]OTf in MeCN at –2.05 V has a FE CO of ~90%.17 CO remains as the dominant CO 2 reduction product formed for each of the electrolyte solvents surveyed, however, the observed FE CO values are lower than those observed in the MeCN electrolyte. Measured FE CO values upon CPE at –2.05 V were ~81%, 66%, and 63% when the catholyte is based upon DMF, DMSO, and PC, respectively. This trend in decreasing FE CO values mirrors the kinetic trend observed for these solvents (vide supra). For each of the catholyte solvents surveyed, CO was the only gaseous product detected. Formic acid is generated as a minor 2e–/2H+ CO 2 reduction side product for the Bi/[Im]+ system. 30 Table 1. Metrics for CPE experiments performed with Bi-modified GCE in CO2 saturated solutions of the indicated solvents containing 100 mM TBAPF6 and 100 mM [BMIM]OTf at applied potentials of E = –1.95 V and –2.05 V (versus SCE). Solvent MeCN DMF DMSO PC

Applied Potential (E) –1.95 V –2.05 V –1.95 V –2.05 V –1.95 V –2.05 V –1.95 V –2.05 V

FE

CO

(%)

j (mA•cm–2) tot

82 ± 4 90 ± 3 72 ± 3 81 ± 4 61 ± 5 66 ± 6 58 ± 7 63 ± 6


12 ± 1 22 ± 3 7±2 15 ± 1 5±1 8±1 2±1 6±1

j

CO

(mA•cm–2)

10 ± 1 20 ± 2 5±1 12 ± 2 3±1 5±1 1 ± 0.5 4±1


The solvent employed for the CO 2 electrolysis experiments has a profound effect on both the kinetics and efficiency of CO evolution at by the Bi/[BMIM]+ catalyst system. In an effort to understand which factors governed the differences in observed reaction rate and product selectivity for the various solvents studied, we initially worked to confirm that the CO evolution reaction occurred via the same rate determining process for each. CPE experiments were performed in order to determine the dependence of current density on applied overpotential for each of the solvents surveyed. Tafel analysis of the resulting data produced linear plots with slopes ranging from 112– 119 mV/dec (Figure S2). These Tafel slopes are all close to 118 mV/dec, and as such are consistent with a rate-limiting single electron transfer to CO 2 at the Bi-CMEC/catholyte interface.8–11,31,32 With evidence suggesting that conversion of CO 2 to CO proceeds via the same fundamental rate determining step at Bi-modified GCE in the presence of 100 mM [BMIM]OTf in each of the four solvents used in this study, we considered which solvent properties may be directing the disparate reactivity metrics shown in Table 1. Since charge transfer to CO 2 at the Bi/catholyte interface is critical to catalysis, we hypothesized that differences in solvent polarity might impact the extent to which reduced CO 2 intermediates are stabilized at the electrode surface and give rise to the disparate rates and selectivities for CO production. 33


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Table 2. Physical properties and jss values obtained for each of the solvents employed in this study. Dielectric Constant (ε)

Dipole Moment (D)

Viscosity (η)

[CO2]a

MeCN

36.6

3.92

0.37 cP

DMF

38.3

3.82

0.79 cP

DMSO

47.2

3.96

66.1

4.90

neat [BMIM]OTf

13.1

N/A

neat [EMIM]OTf

15.1

N/A

Solvent

PC

j (mA•cm–2)b ss

270 mM

E = –1.95 V 13.5 ± 6.3

E = –2.05 V 22.1 ± 6.1

186 mM

7.1 ± 1.2

10.5 ± 0.5

1.99 cP

129 mM

3.1 ± 0.1

4.7 ± 0.1

2.5 cP

144 mM

2.3 ± 0.2

4.0 ± 0.1

76 cP

86 mM

0.8 ± 0.1

1.1 ± 0.1

45 cP

74 mM

1.0 ± 0.2

1.1 ± 0.2

a

Concentration of dissolved CO2 in each solvent under 1 atm of CO2. bjss values determined via chronoamperometry in CO2 saturated solutions. Chronoamperometry experiments carried out in MeCN, DMF, DMSO or PC, contained 100 mM TBAPF6 and 100 mM [BMIM]OTf.

In order to correlate the current associated with CO 2 conversion with catholyte properties and to minimize complications stemming from analysis of electrode dynamics under manual convection, chronoamperometry was performed for Bi-CMEC films on glassy carbon submerged in quiescent solutions of each of the solvents of Figure 3 containing 100 mM [BMIM]OTf and 100 mM TBAPF 6 that were saturated under 1 atmosphere of CO 2 (i.e., the electrocatalytic conditions of Figures 3 and Table 1). Representative chronoamperometric traces are reproduced in Figure S3 and averaged steady state current densities (j ss ) recorded at applied potentials of E = –1.95 V and –2.05 V are listed in Table 2. Plotting the average steady state j ss values obtained in this way against either the dipole moment (D) 34 or dielectric constant (ε) 35,36 of the four solvents studied showed a relatively weak correlation (see Figures 4a and 4b, respectively). 37,38 Both of these physical parameters are also reproduced in Table 2 for each solvent considered. The absence of strong correlations in Figure 4 suggests that some other properties of the solvents may be manifest in the varied kinetics and selectivities observed for CO evolution.



Given that the Bi film employed in these studies is a heterogeneous catalyst, one would expect mass transfer effects to play a significant role in the system’s performance. The general equation for a diffusion-limited steady-state current response is given by Equation 2, which is shown below, 31 𝑛𝑛𝑛𝑛𝐷𝐷𝑂𝑂 𝐶𝐶𝑂𝑂 ∗ 𝑗𝑗𝑠𝑠𝑠𝑠 = 𝛿𝛿𝑂𝑂

(2)

where j ss is the steady-state current density, n is the number of electrons transferred in the reaction, F is Faraday’s constant, D O is the diffusion coefficient of the reactant(s), C O * is the bulk concentration of the reactant(s), and δ Ο is the length of the diffusion layer, beyond which the concentration profile of the reacting species is constant and equivalent to the bulk concentration. For conditions under which the diffusion layer thickness reaches a constant value due to either manual convection (i.e., in the CPE experiments), or to non-manual convection in the bulk solution as a result of density/concentration gradients (i.e., under quiescent conditions), one would expect a linear dependence of steady-state current on both D O and C O *. 39

Figure 4. Steady state current density (jss) values for Bi-modified GCEs in either MeCN, DMSO, DMF or PC containing 100 mM TBAPF6 and 100 mM [BMIM]OTf under 1 atm of CO2 at applied potentials of either E = –1.95 V (red) or E = –2.05 V (blue) are correlated versus (a) the dipole moments and (b) dielectric constants of the solvents studied.


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The Einstein-Stokes equation (Equation 3), which is shown below, relates solution viscosity to the diffusion coefficient of a reacting species (D O ). 40 𝐷𝐷𝑂𝑂 =

𝑘𝑘𝑏𝑏 𝑇𝑇 6𝜋𝜋𝜋𝜋𝑟𝑟𝑂𝑂

(3)

Here, k b is Boltzmann’s constant, T is the solution temperature, η is the viscosity of the solvent, and r 0 is the approximate radius of the diffusing species. Combining Equations 2 and 3 gives Equation 4, shown below, which describes the relationship between steady-state current density (j ss ) and solvent viscosity (η) 36,41 and establishes a linear relationship between j ss and C O */η for systems with a constant diffusion layer thickness. 𝑛𝑛𝑛𝑛𝑘𝑘𝑏𝑏 𝑇𝑇𝐶𝐶𝑂𝑂 ∗ 𝑛𝑛𝑛𝑛𝑘𝑘𝑏𝑏 𝑇𝑇 𝐶𝐶𝑂𝑂 ∗ =� �� 𝑗𝑗𝑠𝑠𝑠𝑠 = 6𝜋𝜋𝑟𝑟𝑂𝑂 𝛿𝛿𝑂𝑂 𝜂𝜂 6𝜋𝜋𝑟𝑟𝑂𝑂 𝛿𝛿𝑂𝑂 𝜂𝜂

(4)

Based on this expression, we expect that the steady-state current obtained from the catalytic reduction of CO 2 to CO at the Bi /catholyte interface should show a linear dependence on the bulk concentration of substrate(s)/reactant(s) and an inverse linear dependence on catholyte viscosity, assuming that the Einstein-Stokes equation holds over the range of solvents examined in this study. 42 Given that the rate limiting step for CO evolution at the Bi/catholyte interface is single electron transfer to CO 2 (vide supra), we first considered how variations in diffusion of CO 2 to the cathode surface may impact the disparate kinetics we observe for CO evolution in MeCN, DMF, DMSO and PC at both –1.95 and –2.05 V. Since the solubility of CO 2 (under 1 atm of CO 2 ) is different for each of the four solvents, 43 C CO2 */η must take into account how the molarity of CO 2 ([CO 2 ]) and the ability of the dissolved gas molecule to diffuse to the electrode (η) varies for each catholyte solution. Table 2 reproduces these values and Figure 5a plots the relation between j ss and C CO2 */η for each of the solvents studied. Satisfyingly, a linear trend is observed, indicating



that the current density observed for the Bi/[Im]+ catalyst system is limited by diffusion of CO 2 , and that the reaction at the cathode surface is fast compared to mass transport in each solvent considered.39 Since diffusion and availability of the [Im]+ promoter at the electrode surface is critical to the efficient evolution of CO at the Bi cathode, we also considered how j ss values measured for each of the solvents (at E = –1.95 and –2.05 V) varied as a function of the ability of [BMIM]+ to diffuse to the cathode surface under electrocatalytic conditions (100 mM [BMIM]OTf and 100

Figure 5. Plots of (a) averaged steady state current density (jss) and (b) Faradaic Efficiency of CO production (FECO) for Bi-modified GCEs in either MeCN, DMSO, DMF or PC containing 100 mM TBAPF6 and 100 mM [BMIM]OTf under 1 atm of CO2 at applied potentials of either E = –1.95 V (red) or E = –2.05 V (blue) versus the product of the concentration of dissolved CO2 and the inverse of the catholyte viscosity for each solvent studied. Panels (c) and (d) show how the same jss and FECO values correlate with the product of the concentration of dissolved [BMIM]OTf (100 mM) and the inverse of the catholyte viscosity for each solvent studied.


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mM TBAPF 6 ). These data are plotted in Figure 5c and once again show a linear correlation between the current associated with CO 2 reduction and C [BMIM] */η, demonstrating that the chemistry at the Bi film’s surface, which results in CO evolution, is fast compared to transport of [BMIM]+ to the cathode/electrolyte interface in each solvent studied. The dependence of j ss on [BMIM]+ mass transport was further verified by measuring j ss for solutions containing different concentrations of [BMIM]+ in the MeCN solvent for which the Bi/[Im]+ mediated conversion of CO 2 to CO is most rapid and efficient. Figure 6 plots j ss versus C [BMIM] */η for solutions containing 50 mM, 75 mM, 100 mM, and 150 mM [BMIM]+ in MeCN. As the value of C [BMIM] * is changed independently of η (i.e., in a single solvent), we continue to observe a linear dependence of j ss on C [BMIM] */η, consistent with the ability of [BMIM]+ to diffuse to the Bi cathode underpinning the activity of the Bi/[BMIM]+ system.

Figure 6. Plot showing how averaged steady state current density (jss) recorded for a Bi- modified GCE varies as a function of the product of the concentration of dissolved [BMIM]OTf in MeCN and the inverse of the catholyte viscosity (η–1 = 2.7 cP–1) at applied potentials of either E = –1.95 V (red) or E = –2.05 V (blue).



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Table 3. Metrics for CPE experiments performed with Bi-modified GCEs in CO2 saturated solutions of the indicated imidazolium ILs (neat) at an applied potential of E = –1.95 V. Solvent

Viscosity44,45

[BMIM]OTf

76 cP

[EMIM]OTf

45 cP

FE

CO

(%)

88 ± 4 90 ± 3

j (mA•cm–2) tot

j

CO

(mA•cm–2)

2.2 ±0.3

1.9 ±0.3

3.8 ±0.2

3.4 ±0.2

Having established that transport of imidazolium is a significant factor controlling the kinetics of CO 2 reduction at the Bi/[BMIM]+ interface, we wondered whether similar phenomena might be contributing to the disparate selectivities for CO evolution observed for the four solvents studied, which range from FE CO ~ 60% in PC to FE CO ~ 90% in MeCN. While a plot of observed FE CO versus the ability of CO 2 to diffuse to the electrode surface (Figure 5b) is moderately correlated, the corresponding plot constructed for [BMIM]+ clearly shows that as solvent viscosity decreases and the ability of [BMIM]+ to diffuse to the electrode increases, more selective CO generation is realized (Figure 5d). This trend is consistent with the [Im]+ playing a critical role in shaping the outcome of CO 2 activation/reduction at the surface of the Bi cathode. Since the data shown in Figures 5 and 6 suggest that both transport and accumulation of the [Im]+ promoter at the Bi cathode surface is critical to achieving high selectivities and current densities for CO generation, we considered how this system would operate if neat imidazoliumbased ionic liquid was used as the catholyte. We expected that electrolysis of CO 2 using a Bimodified GCE in neat imidazolium would ensure that a high concentration of [Im]+ is maintained near the cathode surface during catalysis, leading to relatively high selectivities for CO production. CPE experiments were carried out at –1.95 V in either neat [BMIM]OTf or neat 1-ethyl-3methylimidazolium trifluoromethanesulfonate ([EMIM]OTf). [EMIM]OTf was selected for its comparably low viscosity and structural similarity to [BMIM]OTf. 44,45 Representative current density versus time traces observed during these experiments are reproduced in Figure S4 and


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averaged current densities and selectivities for CO production are listed in Table 3. Consistent with a model in which accumulation of imidazolium at the Bi cathode is critical to achieving high selectivities for CO evolution, the CPE experiments conducted in neat [Im]OTf delivered metrics of FE CO = 88 ± 3.7 % and 90 ± 3.5 % for [BMIM]OTf and [EMIM]OTf, respectively. This level of selectivity for CO is comparable to that observed for the least viscous solvents probed in this study (i.e., MeCN and DMF) in which the imidazolium can most readily diffuse and be supplied to the Bi/catholyte interface. Although mass transport should be slow for neat [BMIM]OTf and [EMIM]OTf, which are relatively viscous, the high concentration of imidazolium (4.53 and 5.32 M for [BMIM]OTf and [EMIM]OTf, respectively) in the neat ionic liquids ensures that the [BMIM]+ or [EMIM]+ promoter is always available at the surface of the Bi cathode to facilitate CO 2 reduction and ensure selective CO production. Chronoamperometry experiments carried out for CO 2 saturated solutions of neat [BMIM]OTf or [EMIM]OTf using a Bi cathode polarized at E = –1.95 V or E = –2.05 V provide results that are consistent with the CPE experiments described above. Steady state current densities of j ss = 0.79 ± 0.05 mA/cm2 and 1.14 ± 0.09 mA/cm2 were recorded in [BMIM]OTf at applied potentials of E = –1.95 V and –2.05 V, respectively. Measured j ss values were slightly higher for experiments conducted in neat [EMIM]OTf (j ss = 1.01 ± 0.20 mA/cm2 and 1.17 ± 0.22 mA/cm2 at E = –1.95 V and –2.05 V, respectively) consistent with the reduced viscosity of this ionic liquid compared to [BMIM]OTf. When considered together, these experiments demonstrate that Bi film electrodes can operate in neat [BMIM]+/[EMIM]+ based ionic liquids. More generally, the results of these experiments are also consistent with those carried out in the various solvents of Figures 2 and 3, and support the observed trends between the rate and selectivity of CO evolution by the Bi/[Im]+ catalyst system as a function of solvent viscosity.



Conclusions and Future Directions The development of electrocatalyst platforms that can efficiently and selectively promote the conversion of CO 2 to CO is a key goal toward carbon recycling and the renewable production of carbon-based fuels. Recent work has shown that Bi film electrodes are inexpensive catalysts for this transformation when operated in the presence of imidazolium-based promoters, such as [BMIM]OTf dissolved in MeCN. In an effort to better understand the function of this Bi/[BMIM]+ system, we have sought to gain a more precise understanding of: 1) the composition of the Bi-film electrode formed upon electrodeposition of Bi3+ salts on carbon-based electrodes and 2) how variations to the catholyte solvent might impact the extent to which the Bi-film catalyst promotes the reduction of CO 2 to CO. Combined EXAFS, XANES and depth-profiling XPS analyses, have revealed the bulkcomposition of the electrodeposited Bi films. Although the surface of these Bi materials is only comprised of ~2% metallic Bi0, as judged by surface XPS, the composition within the bulk of the material is vastly different. Depth profiling XPS experiments demonstrate that the interior bulk of the electrodeposited film is made up of both metallic Bi0 and Bi3+ based oxides. The ratio of metallic to oxidized bismuth within the Bi-film is ~2:1. Analysis of the same Bi-films by EXAFS and XANES, which provide information about the composition of the bulk material, is consistent with the Bi0/Bi3+ ratio obtained from the XPS experiments. As such, these combined compositional studies provide a mutually consistent and unambiguous analysis demonstrating that the deposited Bi film is a mixed phase system with both metallic and oxidized components. Prior work on the ability of Bi cathodes to activate CO 2 in the presence of [BMIM]+ and [EMIM]+ have all utilized MeCN as the catholyte solvent. In the present study we show that the identity of the catholyte solvent is critical to ensuring efficient CO 2 reduction and CO evolution at


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the Bi/[BMIM]+ interface. Parallel voltammetric and CPE studies of the reduction of CO 2 to CO by Bi/[BMIM]+ show that the rate and selectivity of this process is strongly dependent on solvent choice. In surveying a series of polar aprotic catholyte solvents, we found that although MeCN and DMF based catholytes give rise to rapid and selective CO evolution, the use of DMSO or PC results in much lower j CO (i.e., slower kinetics) and FE CO (i.e., reduced selectivity) for the Bi/[BMIM]+ system. Analysis of the steady state current density (j ss ) as measured by chronoamperometry and the efficiency for CO production (FE CO ) shows that both these metrics can be correlated with the ability of CO 2 to diffuse to the electrode surface. Upon accounting for the differing solubilities of CO 2 in the different solvents studied, it is clear that both j ss and FE CO are inversely related to the viscosity of the catholyte solvent. This dependency is to be expected since CO 2 is the substrate for electrocatalysis and the rate-limiting step of CO generation involves electron transfer to CO 2 at the Bi cathode. A more surprising finding is related to the role of the imidazolium in the Bi/[BMIM]+ system. In addition to the catalytic production of CO being dependent on the mass transport of CO 2 to the cathode surface, the ability of the [Im]+ to diffuse to the Bi cathode is strongly correlated with the efficacy of the electrocatalytic reduction. Moreover, the ability of [Im]+ to access the Bi/catholyte interface not only strongly influences the rate at which CO is produced by the Bi electrocatalyst, but also is well correlated with the selectivity displayed for CO generation in each of the electrolyte solvents surveyed. Recognition that the accumulation of [Im]+ at the Bi cathode is critical to rapid conversion of CO 2 to CO prompted us to study CO 2 electrolysis using the Bifilm electrode in neat imidazolium-based ionic liquids, which ensured that large concentrations of [Im]+ are maintained at the cathode during catalysis. These experiments showed highly selective



conversion of CO 2 to CO, with FE CO ~ 90% being obtained, despite the high viscosities of the neat IL electrolyte. In addition to demonstrating that Bi cathodes for CO 2 reduction can operate in neat imidazolium ionic liquids (a finding which may have implications for the design of new CO 2 electrolyzer assemblies), these results suggest that the role of imidazolium in the Bi/[Im]+ system is complex. Rather than simply serving as a supporting electrolyte or just providing the protons required to balance the conversion of CO 2 to CO (Eq 1), the dynamics of the [Im]+ and its interactions with the polarized Bi surface, likely underpin the manner in which CO 2 is activated at the cathode. Such phenomena may also control the dynamic processes that Bi films have been observed to undergo upon cathodic polarization, 46,47 and may be of critical import to facilitating the generation of CO 2 reduction intermediates at the Bi/electrolyte interface. Obtaining a more precise understanding of how the interfacial interactions/dynamics between electrolyte additives and polarized materials dictate the efficacy/outcome of electrocatalytic processes will be important to developing catalytically plastic systems for CO 2 conversion.30 This general pursuit, along with the design of new electrocatalyst platforms that utilize ionic liquid additives to engender tunable reactivity profiles for a variety of small-molecule activation schemes, remains the subject of intense focus in our laboratory.

Supporting Information Experimental procedures, voltammetric data, Tafel analyses, and controlled potential electrolysis current traces. The Supporting Information is available free of charge on the ACS Publications website.


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Acknowledgements Portions of this work were supported by Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. A.A. was supported through a Camille and Henry Dreyfus postdoctoral fellowship in Environmental Chemistry. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DEAC02-98CH10886 with additional support through the Synchrotron Catalysis Consortium under Grant DE-FG02-05ER15688. XPS data were acquired at UD using instrumentation obtained with assistance from the NSF (CHE 1428149).

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TOC



Author’s Biography

Joel Rosenthal - A native of New York City, Rosenthal received his BS degree from New York University, where he conducted undergraduate research with Prof. David I. Schuster. He completed a Ph.D. in Inorganic Chemistry from the Massachusetts Institute of Technology as a Fannie and John Hertz doctoral fellow, working with Prof. Daniel G. Nocera on the mechanistic study of proton-coupled electron transfer reactions as applied to energy conversion processes. Rosenthal studied bioinorganic chemistry and metalloneurochemistry with Prof. Stephen J. Lippard at MIT as a Ruth L. Kirschstein NIH postdoctoral fellow, where he developed detection methods for reactive nitrogen species and neuronal signaling agents. Rosenthal currently holds the rank of Associate Professor and is the Associate Chair for Graduate Studies and Research in the Department of Chemistry and Biochemistry at the University of Delaware. His research group holds expertise in areas related to energy, catalysis, chemical synthesis, electrochemistry and photochemistry. A point of emphasis within Rosenthal’s labs is identification of the molecular design principles that drive the energetically demanding conversion of stable molecules to fuels and other value-added chemicals.


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Insights Into the Composition and Function of a Bismuth Based

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