Structural Dynamics and Evolution of Bismuth Electrodes during

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Structural Dynamics and Evolution of Bismuth Electrodes during Electrochemical Reduction of CO in Imidazolium-Based Ionic Liquid Solutions 2

Jonnathan Medina-Ramos, Sang Soo Lee, Timothy T. Fister, Aude A. Hubaud, Robert L. Sacci, David R Mullins, John L DiMeglio, Rachel C. Pupillo, Stephanie M. Velardo, Daniel A. Lutterman, Joel Rosenthal, and Paul Fenter ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01370 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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Structural Dynamics and Evolution of Bismuth Electrodes during Electrochemical Reduction of CO2 in Imidazolium-Based Ionic Liquid Solutions

Jonnathan Medina-Ramosa,*; Sang Soo Leea; Timothy T. Fistera; Aude A. Hubauda; Robert L. Saccib; David R. Mullinsc; John L. DiMegliod; Rachel C. Pupillod; Stephanie M. Velardod; Daniel A. Luttermanc,*; Joel Rosenthald,*; Paul Fentera,* a

b

Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439 Materials Science and Technology Division and

c

Chemical Sciences Division, Oak Ridge National

Laboratory, Oak Ridge, TN 37831 d

Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716

ABSTRACT Real-time changes in the composition and structure of bismuth electrodes used for catalytic conversion of CO2 into CO were examined via X-ray absorption spectroscopy (including XANES and EXAFS), electrochemical quartz crystal microbalance (EQCM) and in situ X-ray reflectivity (XR). Measurements were performed with bismuth electrodes immersed in acetonitrile (MeCN) solutions containing a 1-butyl-3-methylimidazolium ([BMIM]+) ionic liquid promoter or electrochemically inactive tetrabutylammonium supporting electrolytes (TBAPF6 or TBAOTf). Altogether, these measurements show that bismuth electrodes are originally a mixture of bismuth oxides (including Bi2O3) and metallic bismuth (Bi0), and that the reduction of oxidized bismuth species to Bi0 is fully achieved under potentials at which CO2 activation takes place. Furthermore, EQCM measurements conducted during cyclic voltammetry revealed that a bismuth-coated quartz crystal exhibits significant shifts in resistance (∆R) prior to the onset of CO2 reduction near −1.75 V vs. Ag/AgCl and pronounced hysteresis in frequency (∆f) and ∆R, which suggests significant changes in roughness or viscosity at the Bi/[BMIM]+ solution interface. In situ XR performed on rhombohedral Bi (001) oriented films indicates extensive restructuring of the bismuth film cathodes takes place upon polarization to potentials more negative than −1.6 V vs. Ag/AgCl, which is characterized by a decrease of the Bi (001) Bragg peak intensity of ≥50% in [BMIM]OTf solutions in the presence and absence of CO2. Over 90% of the reflectivity is recovered

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during the anodic half-scan, suggesting that the structural changes are mostly reversible. By contrast, such a phenomenon is not observed for thin Bi (001) oriented films in solutions of tetrabutylammonium salts that do not promote CO2 reduction. Overall, these results highlight that Bi electrodes undergo significant potential-dependent chemical and structural transformations in the presence of [BMIM]+ based electrolytes, including the reduction of bismuth oxide to bismuth metal, changes in roughness and nearsurface viscosity.

Keywords: bismuth electrocatalyst; electrochemical CO2 reduction; ionic liquid; electrode restructuring; X-ray reflectivity; XANES; EXAFS; EQCM.

INTRODUCTION The increase in CO2 emissions is a leading cause for the rise in temperatures around the globe, due to the negative impact it has on the amount of heat retained by Earth’s atmosphere.1 In the U.S. alone, the latest reports from the US Environmental Protection Agency (EPA) show that emitted CO2 corresponds to 81% of the total greenhouse gas emissions, which in turn are equivalent to ~7000 million metric tons of CO2 entering the atmosphere.1-3 Carbon dioxide is the primary product of burning fossil fuels (coal, natural gas, and oil), solid waste, trees and wood, as well as an outcome of other activities including farming, deforestation and industrial processes.1-3 As such, there is a growing demand for new technologies to improve the sequestration and conversion of this greenhouse gas into more useful and valuable products.1-2, 4-5 The electrocatalytic conversion of CO2 is an attractive strategy to transform this greenhouse gas into more valuable products such as carbon monoxide (CO)6-13, which can be used for generation of syngas (CO + H2)14, formate/formic acid15-18 as well as short-chain alcohols (e.g., methanol, ethanol)19-20, and hydrocarbons (e.g., methane, ethane, ethylene).20-27 Gold and silver have historically been the benchmark electrode materials for reduction of CO2 to CO with high Faradaic efficiencies at low overpotentials, but the high cost of these coinage metals has precluded their use at large-scale.21, 28-32 In contrast, recent work has demonstrated that more abundant and inexpensive post-transition metals such as bismuth and tin show exceptional performance as selective and energy-efficient electrocatalytic platforms for CO2 conversion when millimolar concentrations of an imidazolium-based ionic liquid such as 1-butyl-3methylimidazolium is dissolved in CO2-saturated electrolyte.33-36 Electrolysis of CO2 using bismuth electrodes

in

acetonitrile

solutions

containing

20-300

mM

1-butyl-3-methylimidazolium

trifluoromethanesulfonate ([BMIM]OTf) gives rise to Faradaic efficiencies (FECO) and partial current densities for CO production (jCO) of ~75-90% and 5-25 mA/cm2, respectively. Similar FECO and jCO 2 ACS Paragon Plus Environment

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metrics are observed for these bismuth electrodes (also known as bismuth-based carbon monoxide evolving catalysts or ‘Bi-CMECs’) when the solution contains imidazolium cations paired with other counter-anions (i.e. PF6−, BF4−, Cl−, Br−, etc.).33-35 The reduction of CO2 to CO at the Bi/[BMIM]+ interface is a complex proton coupled electron transfer reaction.21, 33-35 Previous reports have proposed that alkyl-imidazolium ionic liquids dissolved in non-aqueous solvents help stabilize the CO2•− intermediate formed after the first electron transfer into the CO2 molecule via complexation with the imidazolium cation.29, 37 Subsequently, the protonation of this complex intermediate by a free imidazolium cation would take place, followed by a second electron and proton transfer which lead to CO and H2O as the final products of the reaction.29, 37 Little is known, however, about changes of the Bi catalyst structure and morphology under potentials favorable for electrocatalysis. Numerous studies have demonstrated that imidazolium-based room temperature ionic liquids (RTILs) exhibit potential-dependent structural rearrangements at electrode surfaces due to the formation of charge separated layers under polarization.38-39 Furthermore, it is known that certain metal surfaces actively adsorb RTILs. For example, the adsorption of [EMIM]+ on Ag electrodes40 has been proposed based on density functional theory (DFT) studies, and spectroscopic and electrochemical measurements show that imidazolium cations adsorb at the surface of Au (100), Au (111) and Pt electrodes upon polarization to the negative potentials where CO2 reduction is favored.41-43 In addition, an in situ scanning tunneling microscopy (STM) study revealed the potential-dependent restructuring of Au (111) surfaces in neat [BMIM]BF4.44 Lust and coworkers investigated the electrochemical and structural properties of Bi (001) electrodes in neat 1-ethyl-3-methylimidazolium tetrafluroborate ([EMIM]BF4) and in a 1 wt.% mixture of [EMIM]I + [EMIM]BF4 using cyclic voltammetry (CV), impedance and in situ STM.45-48 These measurements were confined to a potential window between −0.2 V and −1.2 V vs. Ag/AgCl, where no surface Bi reconstruction was observed.45-48 The adsorption of iodide and imidazolium cations appeared to generate a 2D-superstructure that was visible via STM in the potential range between −0.7 V and −0.3 V vs. Ag/AgCl.46 Adsorption of these ionic components also contributed to the roughening and dissolution of Bi (001) at potentials more positive than −0.3 V vs. Ag/AgCl.46 None of these studies, however, probed the Bi electrodes at potentials that would promote the reduction of CO2. Herein, we seek to understand how potential-dependent processes, under conditions suitable for CO2 electroreduction, influence the composition and morphology of Bi electrodes for thin film Bi-CMEC cathodes. In particular the evolution of Bi electrodes in acetonitrile solutions containing millimolar concentrations of an imidazolium-based ionic liquid, in the presence and absence of CO2, were 3 ACS Paragon Plus Environment

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interrogated by in situ X-ray absorption spectroscopy (XAS), electrochemical quartz crystal microbalance (EQCM), X-ray reflectivity (XR) and CV. This study provides the first direct observations of compositional and structural changes of Bi electrodes, in a nonaqueous solvent containing an imidazolium-based electrolyte. The Bi electrode undergoes significant restructuring at negative potentials, including the reduction of a native Bi2O3 layer to crystalline Bi and a subsequent reversible electrochemically-driven loss of crystalline Bi that begins prior to the onset of CO2 reduction. This work demonstrates that the Bi-CMEC undergoes significant potential-dependent changes under conditions at which CO2 is catalytically reduced into CO.

EXPERIMENTAL AND COMPUTATIONAL METHODS Room Temperature Ionic Liquids (RTILs), Other Supporting Electrolytes and Solutions. Two imidazolium-based ionic liquids, 1-butyl-3-methylimidazollium trifluoromethanesulfonate ([BMIM]OTf, ≥ 98%) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6, ≥ 98.5%) were used. These RTILs were purchased from Sigma-Aldrich, Alfa Aesar or Acros Organics and used as received. Solutions of these imidazolium-based ionic liquids were prepared in acetonitrile, at a concentration of 100 mM. Experiments in the absence of RTILs were performed in acetonitrile based solutions containing 100 mM tetrabutylammonium triflate (TBAOTf) or tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. Tetrabutylammonium hexafluorophosphate was purchased from either SigmaAldrich (≥ 99.0%) or TCI America (> 98.0%, recrystallized from ethanol prior to use). Tetrabutylammonium triflate (≥ 99.0%), acetonitrile (MeCN, UHPLC plus for gradient elution, ≥ 99.9%) bismuth (III) nitrate pentahydrate (99.999% trace metal basis) hydrochloric acid (HCl, 36.5-38.0% w/w) and potassium chloride (KCl, ≥ 99.0%) were purchased from Sigma-Aldrich. X-ray Absorption Spectroscopy (XAS). X-ray absorption spectroscopy (XAS) was applied to determine the oxidation state and coordination chemistry of Bi film cathodes in situ, under electrochemical control.49 X-ray absorption experiments were carried out at Beamline X19A of the National Synchrotron Light Source (NSLS) in Brookhaven National Laboratory (BNL). The X-ray absorption data were collected in transmission mode at the Bi LIII-edge (13419 eV). A Si (111) double crystal monochromator was used and detuned by 30% to reject higher harmonics. Ion chambers for measuring I0 and IT were filled with nitrogen and a 50:50 mixture of N2:Ar, respectively. A third absorption cell, IR, was placed downstream of IT, and metallic Bi powder was placed between these last two absorption cells in order to calibrate the photon energy.


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Bismuth-based thin films (Bi-CMECs) were prepared via electrodeposition by immersing a glassy carbon substrate in a quiescent solution of 20 mM Bi(NO3)3 + 1.0 M HCl, at a fixed potential of −0.30 V vs. Ag/AgCl until ~3.6 C/cm2 had been passed. The XAS and electrochemical measurements were performed using a two-compartment cell with the Bi-CMEC working electrode and a Ag/AgCl (1.0 M KCl) reference electrode in the cathode compartment, and a platinum mesh counter electrode in the anode compartment. Both sides of the cell were filled with equal volumes of electrolyte solution, which consisted of either 100 mM [BMIM]OTf or 100 mM TBAPF6 dissolved in acetonitrile. A Nafion membrane was used to separate the anode and cathode compartments. The cell was equipped with a gas inlet and an outlet to allow for saturation of the solution with N2 or CO2. The X-ray beam was directed onto the bismuth working electrode, and an absorption (µ(x)) of ca. 0.3 was measured for all the bismuth films utilized. The XAS data was analyzed using ATHENA (available through DEMETER software package, version 0.9.17)50 for fitting pre-edge and post-edge backgrounds, normalization, and spectral averaging. Electrochemical Quartz Crystal Microbalance (EQCM). In situ EQCM experiments were conducted to assess changes in the Bi-film mass and elasticity caused by adsorption, oxidation/reduction or restructuring at the Bi/[BMIM]+ interface as a function of applied potential. Measurements were carried out using an oscillating 5 MHz AT-cut quartz crystal microbalance (QCM200, SRS) connected to a Biologic SP-200 potentiostat. A ~1 µm-thick bismuth layer was electrodeposited onto the Au-coated quartz crystal (Maxtek) following the same procedure used to prepare the bismuth-based films for XAS. Scanning electron microscopy (SEM) images of the film show that bismuth deposits in the form of small platelets spread uniformly onto the Au surface (Figure S1). Changes in resonance frequency (∆f) and resistance (∆R) of the bismuth-coated quartz crystal were measured while performing CV with the crystal immersed in CO2-saturated 100 mM [BMIM]OTf/MeCN solution (scan rate = 20 mV/s). Growth of Bismuth Thin Films for XR Measurements. Model Bi thin films were grown on epitaxial graphene surfaces. Epitaxial graphene films were grown on N-doped, Si face epi-ready, 6H-SiC (0001) wafers purchased from Cree Inc. and UniversityWafer (330 µm-thick; 0.02-0.2 Ω•cm resistivity). Smooth, epitaxial graphene (rms ≤ 0.5 nm) was grown via thermal decomposition at 1550 °C in an MTI GSL1700X tube furnace, under argon flow (600−900 mbar).38-39,

51

DC magnetron sputtering (AJA

International Inc.) was used to deposit thin bismuth films (~6 nm-thick) onto the surface of graphenecoated SiC under low power (12 W) and room temperature to minimize the growth rate to ca. 5 nm/min. The thickness of the as-deposited Bi thin films was determined through low-angle X-ray reflectivity analysis. 5 ACS Paragon Plus Environment

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Atomic Force Microscopy (AFM) Imaging of Substrates and Bismuth Thin Films. Ex situ AFM imaging of SiC substrates and graphene-coated SiC samples before and after deposition of bismuth thin films was carried out using a MFP3D Asylum Research Microscope. The images were collected under tapping mode, using Olympus microcantilevers (OMLC-AC160TS-W2; resonant frequency = 300 Hz; spring constant = 42 N/m). In Situ X-ray Scattering Experiments. A previously described, custom-made electrochemical ‘transmission X-ray’ cell was used for in situ X-ray reflectivity (XR) measurements under voltammetric control (Fig. S2).52 The cell was equipped with a Bi thin film working electrode, a platinum wire counter electrode and a Leakless Ag/AgCl reference electrode (eDAQ Inc.). The potential applied between working and reference electrodes was controlled by a Gamry Reference-600 potentiostat. The Bi thin film working electrode (10 mm × 3.0 mm) was contacted from one end of the Bi-covered surface by a stainless steel post insulated from the solution by a Teflon sleeve. The incident X-ray beam was aligned along the shorter 3 mm-long dimension of the working electrode to minimize absorption and background scattering. X-ray reflectivity experiments were conducted at the Advanced Photon Source (APS) in Argonne National Laboratory, in sectors 12-ID-D, 33-BM-C and 33-ID-D. The monochromatic X-ray photon energy was 20 keV (λ = 0.62 Å). The specular XR signal was measured as a function of vertical momentum transfer Q, where Q = Kout − Kin = (4π/λ) sin(2θ/2) where Kin and Kout are incident and reflected X-ray wave vectors, and 2θ is the scattering angle (Figure S2).53-54 The low-angle X-ray reflectivity data were analyzed using the MotoFit software package using a density profile consisting of slabs that were characterized by electron densities, thicknesses and interfacial roughnesses.55 The XR data in the crystal truncation rod (CTR) regime were analyzed separately using fully atomistic models comprising the SiC substrate, epitaxial graphene and two Bi layers. Given the limited range of data, the films were modeled as ideal Bi and graphene films having a roughness defined by an error function profile that modulates the occupation factor of atomic layers near each interface (see supporting information for more details).56 The electrolyte fluid structure was modeled as an error function. The reflectivity is reported in units of absolute reflectivity, R(Q) = Ir(Q)/Io, where Io is the incident beam flux and Ir(Q) is the reflected intensity at the detector as a function of momentum transfer, Q. Therefore, variations in the reflectivity magnitude (e.g., near the bismuth film Bragg peak) reflect differences in the structure of each sample (e.g., thickness and crystallinity of the Bi film). We note that all electrochemical potentials in this work are reported with respect to the Ag/AgCl (1M KCl) reference electrode, unless otherwise stated. Slight variations in potential (~3-20 mV) may however arise due to small differences in setup and experimental conditions.


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RESULTS AND DISCUSSION X-ray Absorption: In Situ XANES and EXAFS of the Bi-CMEC.

Figure 1. Bi LIII-edge XANES spectra of Bi-CMEC films in acetonitrile-based solutions containing (a) TBAPF6 and (b) [BMIM]OTf. The electrode potential and the gas that was sparged through the electrolyte solutions are indicated in the legend; measurements were run sequentially in the order indicated in the figure’s legend from top to bottom. For referencing purposes, the XANES for metallic Bi and Bi2O3 standards are shown as dotted black and dashed blue dotted lines, respectively.

The X-ray absorption spectroscopy of bismuth in electrodeposited Bi-CMECs was measured in situ, at the near-edge (XANES) and extended fine structure (EXAFS) regimes to probe the oxidation state and coordination structure of Bi in the films. The Bi-CMECs were immersed in [BMIM]+ or TBA+ containing solutions, that were saturated with either CO2 or N2. Measurements were conducted at open circuit potential (OCP) or −2.0 V, the potential at which catalytic CO2 reduction occurs (the OCP ranged between −0.1 V and −0.3 V in the N2- and CO2-saturated solutions). The XANES and EXAFS spectra of metallic bismuth (Bi0) and bismuth (III) oxide (α-Bi2O3) were also collected as references to estimate the oxidation state of bismuth within the Bi-CMEC films. The XANES Bi LIII-edge spectra of the electrodeposited Bi-CMECs immersed in TBA+ and [BMIM]+ containing solutions are shown in Figure 1 (individual XANES spectra are provided in Figure S3). The XANES spectra of metallic Bi0 and Bi2O3 are also shown for reference (dotted black and dashed blue lines, respectively). The position of the Bi absorption edge and the intensity of the peak near 13.44 keV 7 ACS Paragon Plus Environment

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are indicative of the degree of Bi oxidation: as bismuth becomes more oxidized, the edge position shifts toward higher photon energies and the intensity of the peak increases.57 Relative to the spectral references for Bi0 and Bi3+, the nominal oxidation state of the initial Bi-CMECs (purged with N2) at open circuit potential appears to be intermediate between Bi0 and Bi3+ (solid black line, Figures 1a and 1b). When the potential of the Bi-CMECs in either [BMIM]+ or TBA+ containing solution is stepped from OCP to −2.0 V under an atmosphere of N2 (gray triangles, Figures 1a and 1b) the position of the absorption edge shifts to a lower photon energy, and the XANES spectrum in both cases becomes virtually identical to that of metallic bismuth. Repeating this experiment in CO2-saturated solutions shows no significant change in the XANES spectrum of the Bi-CMECs, either at OCP (solid red line, Figures 1a and 1b) or after switching the potential to −2.0 V (solid green line, Figures 1a and 1b) indicating that once reduced, the bismuth within the film remains as Bi0. Similar results were obtained in the EXAFS measurements, presented in Figure S4 (see the Supporting Information). Together, XANES and EXAFS revealed that the Bi-CMECs were initially of mixed composition, containing both metallic (Bi0) and oxidized bismuth (Bi3+) phases (Figures 1 and S4, solid black trace). After the Bi-CMECs were polarized to −2.0 V, while immersed in N2-saturated solutions containing [BMIM]+ or TBA+, the Bi-O feature was replaced by the Bi-Bi features associated with metallic Bi, both in shape and intensity (Figure S4, gray triangles). It can therefore be concluded that the Bi-CMEC film is fully reduced to metallic bismuth under the conditions upon which CO2 electroreduction is known to take place, which is consistent with the XANES spectra (Figures 1 and S3).

Electrochemical Quartz Crystal Microbalance (EQCM) Measurements. EQCM was used to probe the evolution of ~1 µm-thick electrodeposited Bi-CMEC films due to phenomena such as ion adsorption, changes in roughness and/or viscosity at the Bi/[BMIM]+ interface as a function of applied potential. Typically, changes in the crystal’s resonance frequency (∆f) are inversely proportional to shifts in electrical resistance (∆R), although each of these parameters has a different sensitivity to various physical properties of the film. For example, ∆f is typically related to changes in mass due to adsorption/desorption of species (e.g. electrodeposition/dissolution at the electrode surface) whereas ∆R is more sensitive to changes in near-surface electrolyte viscosity (∆η), surface roughness (∆hr) and adsorbed particle size.58 The evolution in frequency (∆f) and resistance (∆R) of the bismuth-coated quartz crystal immersed in CO2-saturated 100 mM [BMIM]OTf/MeCN solution, as measured during a potential cycle between 0.05 V and –1.95 are shown in Figure 2, and can be categorized into three ‘regimes’. 8 ACS Paragon Plus Environment

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Figure 2. EQCM and CV (scan rate = 20 mV/s) measurements conducted for a 1 µm-thick Bi-CMEC film coated onto a quartz crystal electrode in a CO2 saturated acetonitrile solution containing 100 mM [BMIM]OTf. The parameters ∆f (in Hz) and ∆R (in Ω) represent the change in frequency and resistance of the bismuth-coated crystal, respectively, as measured during the cathodic (solid lines) and anodic sweeps (dotted lines) voltammetric scans (red circles). Assuming that ∆R is solely due to changes in surface morphology, the black scale bar shows the change in surface rms roughness (∆hr). The EQCM response is described by three regimes, I, II and III; regime I involving the Bi3+/Bi0 redox activity and ionic liquid adsorption/desorption; regime II corresponds to Bi restructuring, and regime III includes Bi restructuring along with CO2 reduction. Regime I corresponds to the potential region where bismuth oxidation/reduction and ionic liquid adsorption/desorption take place (approx. 0.05 V to –0.8 V); regime II covers the onset of bismuth restructuring (from –0.8 V to –1.4 V) and regime III comprises both bismuth restructuring and the onset of CO2 reduction (from –1.4 V to –1.95 V). In regime I of the voltammogram shown in Figure 2, cathodic 9 ACS Paragon Plus Environment

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activity is first detected at 0.05 V, followed by a larger cathodic peak centered at –0.75 V, which can be attributed to the reduction of native bismuth oxide(s), which is consistent with the results obtained via XAS (vide supra) and XR measurements (vide infra). Simultaneously, a slow increase in ∆f takes place between 0.05 V and –0.4 V, followed by a steeper drop until the potential reaches –0.8 V. The initial rise in frequency at the beginning of the potential scan indicates a loss of mass in the film, consistent with an expected loss of oxygen during the conversion of bismuth oxide to metallic Bi0. As the Bi-CMEC film is polarized to more negative potentials, the measured frequency between –0.4 and –0.8 V decreases, suggesting a mass increase which is likely driven by adsorption of [BMIM]+ onto the negatively charged bismuth surface. At first, ∆R increases during regime I before plateauing between −0.4 V and −0.7 V, which suggests an early roughening of the surface due to non-uniform reduction of bismuth oxide(s) into metallic bismuth. Assuming that the change in ∆R is solely due to variation in roughness, ∆hr would increase by ~1.5 nm. The resistance response is also influenced by changes in local fluid viscosity, which can be controlled by adsorption of IL at the interface. For instance, a sudden change in ∆R at –0.3 V could be interpreted as an increase in viscosity offsetting the effect of surface roughening on the resistance. Furthermore, hysteresis found in ∆R may also be seen in ∆f. In regime II (from –0.8 to –1.4 V), both current density and ∆f remain mostly constant through the anodic and cathodic scans, while variations in ∆R appear during the cathodic sweep. A change in potential from –0.8 to –1.2 V leads to a decrease in ∆R, suggesting a possible decrease in surface roughness or viscosity. Given that there are no noticeable changes in ∆f, the latter explanation seems unlikely. The estimated decrease in roughness of ~1.5 nm would indicate that the surface topography recovered almost fully in this regime. This recovery can be related to electrochemical annealing of the film surface, as expected from changes in surface atom dynamics with applied potentials.59-60 On the other hand, at –1.2 V ∆R begins to increase, indicating re-roughening of the surface. In particular, there are no significant variations in both voltammetry and ∆f, which suggests that this increase in resistance is primarily due to structural changes in the Bi film rather than reduction of Bi or adsorption/desorption of IL ions. In situ XR measurements discussed in the following section show that, within this potential region, a major restructuring of the Bi film takes place, leading first to an electrochemical ‘annealing’ of the film followed by an increase in surface roughness as the potential becomes more negative. Within regime III (from −1.4 V to −1.9 V), an enhancement in cathodic current due to CO2 activation is seen between −1.65 V and −1.9 V, while ∆f decreases slightly by ~2.5 Hz. It is possible that changes in


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mass due to CO2 adsorption and CO desorption are too small within regime III to cause shifts in frequency. In contrast, ∆R increases at potentials more negative than –1.4 V, indicating that the bismuth surface becomes rougher or the near-surface solution becomes more viscous. Given that ∆f is decreasing in this regime, it is not possible to distinguish between these two possible scenarios. In the anodic half-scan, the frequency remains fairly constant until it begins to decrease as the potential is swept from –1.1 V to –0.2 V (moving from regime II into I). This drop in frequency indicates that the interfacial mass (adsorbed layer) is changing, presumably due to the onset of bismuth oxidation. However, as the potential rises above –0.2 V the frequency increases again, which suggests that desorption of [BMIM]+ species from the electrode interface is favored as the potential becomes more positive. We note that neither the measured frequency nor the resistance of the Bi film returns to its original values, revealing that the Bi electrode undergoes an irreversible change during the electrochemical cycle.

X-ray Reflectivity of the Bi Thin Film Electrodes. X-ray reflectivity probes the structure and morphology of thin films, providing insight into fundamental processes, in real time and under electrochemical control. As such, in situ reflectivity in the Fresnel regime and higher-angle crystal truncation rod (CTR) measurements were undertaken at the Bi/electrolyte interface under potential control, in an effort to understand the differences in sensitivity of the Bi-CMEC films to applied potential in imidazolium-based ionic liquid solutions and solutions of electrochemically inactive tetrabutylammonium salts.

The structure of Bi films on graphene-coated SiC (001). Ex situ atomic force microscopy (AFM) images of the graphene-coated SiC before and after bismuth deposition were obtained to assess the topography of the bismuth-based films and the underlying substrate. The AFM images of bare graphene and bismuth-coated graphene are shown in Figure S5. The graphene-coated SiC surface (Figure S5a) exhibits terraces that are several hundreds of nm to µm-wide, separated by steps in height of up to ~2 nm. The bismuth-covered graphene (Figure S5b) surface resembles the pristine graphene surface in that the micrometer-sized terraces are still visible, but with a textured surface consistent with partial coverage of the top film layer. The atomic scale structure of the Bi-based films deposited on the graphene-coated SiC was determined via in situ X-ray reflectivity measurements, conducted in an acetonitrile solution containing 100 mM [BMIM]OTf prior to an external potential being applied to the substrate. The reflectivity in the 11 ACS Paragon Plus Environment

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low-angle Fresnel (inset) and the crystal truncation rod regimes for the Bi film in the ionic liquid solution are shown in Figure 3. The analysis of the low-angle reflectivity signal (Figure 3a, inset) reveals a 6.1 nm thick Bi film deposited on top of a ~1.1 nm-thick epitaxial graphene layer grown on a SiC substrate. This Bi film is composed of two distinct layers with thickness of 3.4 nm and 2.7 nm and electron densities of 2.16 and 1.73 e−/Å3, respectively. The electron densities for the Bi-based layers are lower than the reported 2.34 e−/Å3 for Bi0. However, we note that sputtered materials often have lower than ideal electron densities,

Figure 3. X-ray reflectivity and electron density profile of a pristine Bi thin film immersed in a CO2saturated acetonitrile solution containing 100 mM [BMIM]OTf, at OCP. (a) Low-angle reflectivity (inset) and CTR showing Kiessig fringes and Bragg peaks corresponding to the Bi (001) layer, and the SiC substrate Bragg peak. (b) Electron density profile showing all the layers that comprise the working electrode (SiC substrate, graphene, Bi (001) and the amorphous bismuth component) and the ionic liquid solution; overlaid is the electron density profile of the working electrode obtained from fitting the lowangle reflectivity signal (solid black line).

especially when the films are nanostructured.61-62 This lower film density can also be explained by the presence of Bi2O3 which has a lower electron density (2.14 e−/Å3) than Bi0. The specular CTR data in Figure 3 shows three Bragg peaks centered at 1.55 Å-1, 2.49 Å-1 and 3.13 Å-1 which correspond to Bi (001), SiC (006) and Bi (002), respectively. The presence of these Bi Bragg peaks confirms that the films consisted primarily of crystalline bismuth. The prevalence of the (00L) reflections along the specular condition, i.e., the surface normal direction, indicates that the basal plane of crystalline Bi was parallel to the basal plane of SiC. The appearance of well-resolved Kiessig fringes on each side of the Bi (001) and Bi (002) peaks indicates that the bismuth-based film had an atomically smooth surface.


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The analysis of the CTR signal describes structural details of the interface that consists of the SiC (001) substrate, a ~1 nm-thick graphene layer and a bismuth-based film composed of ~80% Bi (001) with a laterally interspersed component that was assumed to be an amorphous bismuth containing phase, with a total Bi layer thickness of ~3.6 nm (Figure 3b). Both the location and thickness of this Bi layer match those of the first Bi layer determined via fitting of the low-angle XR data (Figure 3a, inset). By contrast, the second Bi layer invisible to the CTR analysis would correspond to a non-crystalline bismuth material (e.g. bismuth oxide) with a larger rms surface roughness than the crystalline phase (0.6 nm vs. 0.3 nm; see Table S1). High-resolution X-ray photoelectron spectroscopy of the as-deposited Bi thin film shown in Figure S6


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Figure 4. In situ X-ray reflectivity at the Bi (001) film Bragg peak and CV (scan rate = 10 mV/s) response of bismuth thin film cathodes immersed in three different acetonitrile-based electrolyte solutions containing (a) 100 mM [BMIM]OTf + CO2; (b) 100 mM [BMIM]OTf + Ar and (c) 100 mM TBAOTf + CO2. In all figures, the dotted blue lines correspond to reflectivity curves while their respective CVs are represented by red open circles (a), green open circles (b) or a solid black line (c). The arrows indicate the direction in which the potential scans were performed. These data correspond to the second CV and XR collected in the respective experiment. 14 ACS Paragon Plus Environment

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further confirms the presence of Bi2O3 on top of metallic bismuth with peaks at ca. 159 eV and 157 eV, respectively, and their corresponding doublets at higher binding energies. In summary, both the X-ray and AFM measurements showed that the Bi thin films were suitable for in situ reflectivity measurements under electrochemical control. The Bi film grown on top of the graphene layer was predominantly crystalline with a strongly preferred Bi (001) surface orientation and small roughness (i.e., rms = 0.3 nm), which effectively enhances the sensitivity of surface specific XR to changes in the Bi film structure. The presence of a graphene layer underneath the Bi film ensures good electrical conductivity across the electrode (attempts were made at using Bi films sputtered onto nonconducting sapphire, and it was observed that Bi films thicker than ~6 nm were required to observe signs of CO2 activation via CV).

Electrochemical and structural changes at the Bi electrode during cyclic voltammetry measurements.

The electrochemical behavior of the Bi thin films was probed by conducting CV in CO2-saturated acetonitrile containing 100 mM [BMIM]OTf. Control CV experiments were performed using Arsaturated electrolyte solution of the same composition. The voltammogram recorded for the CO2-saturated solution is shown in Figure 4a and depicts a sharp increase of cathodic current at potentials below ca. −1.7 V, which is consistent with the known activation of CO2 at the Bi/[BMIM]+ interface.33-36 As expected, such enhancement in cathodic current is not seen in the CV obtained in the Ar-saturated catholyte solution. A few broad cathodic waves of low current densities appear at potentials less negative than −1.6 V in the voltammograms recorded for the bismuth thin films in both CO2 and Ar-saturated [BMIM]OTf solutions (Fig. 4b). Based on the XAS and EQCM results (Figure 2) described above (vide supra), and the previous findings of Lust and coworkers, we attribute these broad cathodic waves to the reduction of bismuth oxides into Bi0.45-48, 63 Figure S7 directly compares the voltammetric responses of the Bi film in [BMIM]OTf solutions saturated with either CO2 or Ar. It is worth mentioning here the parallel between the behavior of these Bi thin film cathodes and that of oxide-derived Cu, Au and Sn-based electrocatalysts for CO2 reduction that have been investigated by Kanan and coworkers.32, 64-65 As it was observed for the Bi films probed in this work, these other metal-based cathodes also undergo reduction of their native oxides followed by surface roughening prior to the onset of CO2 reduction, which results in higher catalytic activity. However, to the best of our knowledge, the role of imidazolium-based ionic liquids on CO2 reduction using Cu, Au or Sn based cathodes in aqueous media, and the possible manner in which these ionic liquids modulate the structure of the metal electrodes at potentials at which CO2 reduction takes place have not been reported. 15 ACS Paragon Plus Environment

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The real time variations in the Bi (001) Bragg peak intensity were monitored to investigate the relationship between these electrochemical signals and the Bi film structure. Measurements were made during CV between the open circuit potential (corresponding to the initial potential on Figures 4a-c) and −1.9 V (at a scan rate of 10 mV/s). Potential-dependent variations in the Bi (001) peak intensity were used to characterize the evolution of the bismuth (001) film. The reflectivity and voltammetric experiments were performed using three different acetonitrile-based solutions under catalytic and non-catalytic conditions. These include (1) CO2-saturated MeCN containing 100 mM [BMIM]OTf; (2) Ar-saturated MeCN containing 100 mM [BMIM]OTf; and (3) CO2-saturated MeCN containing 100 mM TBAOTf. The lower potential limit of −1.9 V was chosen based on preliminary data, which showed that at potentials more negative than −2.0 V the Bi thin film electrodes were unstable. The second voltammograms and reflectivity scans measured during each experiment are presented in Figure 4. We note that similar trends to those shown in Figure 4 were reproduced in subsequent CV cycles. Although comparable electrochemical and reflectivity features are seen in the first scan collected in CO2-saturated [BMIM]+ based electrolyte (Figure S8), a significant increase in reflectivity signal is measured during the cathodic sweep of the first CV, which precedes the reversible loss of reflectivity with an onset at approximately −1.75 V. We attribute the net increase in reflectivity observed during this first CV to electrochemical ‘annealing’ of the Bi film, which increases the overall thickness and coverage of Bi (001) via reduction of bismuth oxides and subsequent reorganization of bismuth within the film. The same phenomenon was observed in the first CV measured with Bi films in Ar-saturated MeCN containing [BMIM]OTf and in the TBAOTf-containing solution. The cyclic voltammogram of the bismuth film electrode immersed in CO2-saturated [BMIM]OTf solution (Figure 4a) shows small currents between OCP and −1.4 V, which can be attributed to the reduction of the surface oxide, followed by a sharp increase in the CO2 activation current at potentials more negative than −1.7 V. Simultaneously, the Bi (001) reflectivity signal shows small increases between OCP and −1.4 V each corresponding to the reductive features in the CV, further confirming that these signals involve the conversion of a native bismuth oxide film into crystalline Bi0. This behavior is followed by a dramatic decrease in Bi (001) signal at potentials below −1.5 V, with a ~ 58% loss of signal at −1.9 V. However, this reflectivity loss is reversed during the anodic half-cycle, with an almost full recovery (~95%) being achieved after a cycle. As evidenced by the EQCM measurements discussed above, there are no drastic changes in mass at the Bi/[BMIM]+ interface occurring at potentials more negative than −1.5 V that help to explain these reflectivity responses (∆f does not change significantly within regime III). These results and the associated variation in resistance (∆R) observed within this


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potential region by EQCM suggest an increase in roughness and local viscosity, both of which would be expected to accompany the restructuring of the Bi film. Similar results were obtained for Bi films studied in Ar-saturated electrolyte containing 100 mM [BMIM]OTf (Figure 4b). The features in the voltammetric trace of Figure 4b are analogous to those seen in Figure 4a except for the lack of cathodic current associated with CO2 reduction. The Bi (001) reflectivity signal exhibits features that resemble those observed in the CO2-saturated [BMIM]OTf solution: the cathodic sweep is accompanied by an increase in reflectivity of about 10% between OCP and −1.3 V, followed by a steep decrease, beginning at around −1.5 V, that amounts to a ~65% loss of reflectivity signal at −1.9 V. Moreover, the anodic half-cycle on Figure 4b reveals an almost complete recovery of the initial reflectivity signal toward the end of the voltammogram (~5% net loss of signal). Overall, the parallel in electrochemically-driven reflectivity changes in CO2 as well as Ar-saturated [BMIM]OTf solutions indicate that the presence of CO2 does not drive the structural changes observed for the Bi film. Additional measurements were conducted in a CO2-saturated electrolyte solution containing 100 mM TBAOTf to determine whether restructuring was dependent on [BMIM]+. No evidence for CO2 reduction was observed in the CV, reflecting the absence of [BMIM]+ promoter, which is required for efficient activation of CO2 by bismuth cathodes. Furthermore, the XR signal recorded for this experiment (Figure 4c) is quite different compared to that in the presence of [BMIM]OTf (Figure 4a). During the cathodic sweep, there is no loss of reflectivity for the Bi thin film signal at potentials below −1.5 V as was seen when [BMIM]OTf was present in the electrolyte. Instead, a rise in Bi (001) Bragg peak reflectivity takes place at potentials more negative than −1.5 V, corresponding to the electrochemical annealing of the film until reaching −0.6 V on the anodic sweep, upon which time the film is re-oxidized. The same trends were observed when this experiment was conducted in a solution of TBAPF6, as shown in Figure S9. We note that the XANES measurements (Figure 1) indicate that, upon reduction of bismuth oxide to metallic Bi0, the oxidation state of the Bi film electrode remained unchanged even after opening the circuit, which seems to contradict the evidence for re-oxidation of the Bi film during the anodic scan of the EQCM experiment (Figure 2, regime I) and the reflectivity behavior especially in the presence of TBA+ at increasingly positive potentials (Figures 4c and S9). However, it must be taken into account that EQCM is mostly sensitive to surface phenomena while the XANES signal carries information from both surface and bulk structures. Likewise, the X-ray reflectivity measurements were performed with ~6 nm-thick Bi films while XAS was performed on films that are ~1-3 µm-thick; thus, the changes in reflectivity attributed to Bi re-oxidation which involve ~1 nm of the Bi film are not expected to have a significant effect in the XANES measurements. 17 ACS Paragon Plus Environment

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These results present a clear contrast in the structural behavior of the bismuth film cathodes in solutions containing the imidazolium-based ionic liquid versus the inert TBAOTf supporting electrolyte at potentials between −1.5 V and −1.9 V. The reduction in the Bi (001) Bragg peak reflectivity can be interpreted simply using kinematical diffraction theory. The magnitude of reflectivity at the Bi film Bragg peak is proportional to NBi2, where NBi is the number of Bi atoms in the crystalline film. The reduction of the XR signal therefore is an indication of the partial release of Bi atoms from the film at these cathodic potentials, which only ensues in the presence of the [BMIM]+ based electrolyte. The magnitude of these changes in reflectivity proves that they are not due simply due to a surface reconstruction of the Bi film, as this would not significantly change the Bi (001) Bragg peak intensity. Ongoing work is devoted to understanding the nature of this reversible restructuring process.

CONCLUSION In situ X-ray absorption spectroscopy, electrochemical quartz crystal microbalance and X-ray reflectivity experiments were performed to probe the structure of Bi-CMEC films in real time, under both catalytic and non-catalytic conditions for CO2 reduction. The results show that the Bi-CMEC coated electrodes (both electrodeposited and sputtered) are composed of metallic bismuth (Bi0) and oxidized bismuth phases (including Bi2O3) with the latter found primarily at the topmost layers of the films. The reduction of native bismuth oxides into metallic bismuth is observed as the potential is scanned negative from the open circuit potential, and is fully achieved at potentials more negative than –1.8 V, which coincides with the onset of CO2 reduction in acetonitrile solutions containing [BMIM]+. The results of EQCM measurements show that the most pronounced shifts in frequency and resistance at the bismuthcoated quartz crystal occur at potentials between 0.05 V and –1.0 V, where the interconversion between Bi3+ (primarily in the form of bismuth oxides) and Bi0 takes place. However, scanning toward more negative potentials only seems to have an effect on the resistance of the bismuth-coated crystal, which increases as the potential drops from –1.0 V to –1.95 V, indicating a possible increase in surface roughness and/or local viscosity of the electrolyte. A change in the electrolyte viscosity near the electrode surface may be explained by adsorption of imidazolium cations onto the surface of the bismuth film at more negative potentials. Low-angle X-ray reflectivity and crystal truncation rod (CTR) measurements revealed that the atomic-level structure of the Bi films is very sensitive to potential and electrolyte composition. The analysis of low-angle reflectivity signals showed that bismuth films sputtered onto graphene are primarily composed of crystalline Bi and a thinner bismuth oxide layer. Cathodic potentials more negative than −0.3 V drive the reduction of the bismuth oxide species into metallic Bi, in acetonitrile solutions containing either [BMIM]OTf or TBPAF6/TBAOTf. In addition, CTR 18 ACS Paragon Plus Environment

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measurements exposed that these bismuth films undergo a drastic structural change in the imidazoliumbased ionic liquid solutions when the potential is scanned from −1.5 V down to −1.9 V. This change is signaled by a loss of Bi (001) Bragg peak reflectivity, which decreases by at least 50% within the indicated potential window. The loss of reflectivity is reversed during the subsequent anodic sweep. Remarkably, the structural dynamics observed for the Bi/[BMIM]+ system is not observed when the voltammetry is conducted with the Bi film immersed in acetonitrile containing TBAPF6 or TBAOTf, which are supporting electrolytes that do not promote the reduction of CO2.66 Altogether, these observations point to a strong interaction between the bismuth cathode and the ionic liquid cation [BMIM]+ as the driving force behind the Bi film’s restructuring. The findings discussed in this paper underscore the fact that the atomic structure and oxidation state of Bi-CMECs are highly sensitive to applied potential and the specific interactions between cationic species dissolved in the electrolyte and the bismuth electrodes. The unexpected structural dynamics elucidated for the Bi/[BMIM]+ system are the subject of further investigation in our laboratories as we seek to develop a rigorous mechanistic understanding of the underlying forces that drive the activation of CO2 at the Bi/electrolyte interface. ASSOCIATED CONTENT Supporting Information. Additional SEM, EDS, XANES/EXAFS, XPS, AFM and X-ray reflectivity data and fitting parameters are included.

AUTHOR INFORMATION Corresponding Authors *[email protected]; [email protected]; [email protected]; [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This material is based upon work supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science (SC), and Office of Basic Energy Sciences (BES). Use of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) for XAS measurements was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886, with additional support through the Synchrotron Catalysis 19 ACS Paragon Plus Environment

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Consortium under Grant DE-FG02-05ER15688. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The X-ray reflectivity work was carried out at the Advanced Photon Source, sectors 12-ID-D (Hua Zhou), 33-BM-C (Evguenia Karapetrova) and 33-ID-D (Zhan Zhang). Special thanks to Ahmet Uysal (Argonne) for training J.M.R. on graphene growth and to Brian J. Ingram (Argonne) for allowing A.A.H. access to the sputtering chamber for Bi thin film deposition.

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ACS Catalysis

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Structural Dynamics and Evolution of Bismuth Electrodes during

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