Cathodic Corrosion at the Bismuth–Ionic Liquid Electrolyte Interface

Article Cite This: Chem. Mater. 2018, 30, 2362−2373

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Cathodic Corrosion at the Bismuth−Ionic Liquid Electrolyte Interface under Conditions for CO2 Reduction Jonnathan Medina-Ramos,*,† Weiwei Zhang,‡ Kichul Yoon,‡ Peng Bai,§ Ashwin Chemburkar,§ Wenjie Tang,§ Abderrahman Atifi,∥ Sang Soo Lee,† Timothy T. Fister,† Brian J. Ingram,† Joel Rosenthal,*,∥ Matthew Neurock,*,§ Adri C. T. van Duin,*,‡ and Paul Fenter*,† †

Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States § Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States ∥ Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States ‡

S Supporting Information *

ABSTRACT: Bismuth electrodes undergo distinctive electrochemically induced structural changes in nonaqueous imidazolium ([Im]+)-based ionic liquid solutions under cathodic polarization. In situ X-ray reflectivity (XR) studies have been undertaken to probe well-ordered Bi (001) films which originally contain a native Bi2O3 layer. This oxide layer gets reduced to Bi0 during the first cyclic voltammetry (CV) scan in acetonitrile solutions containing 1-butyl-3-methylimidazolium ([BMIM]+) electrolytes. Approximately 60% of the Bi (001) Bragg peak reflectivity is lost during a potential sweep between −1.5 and −1.9 V vs Ag/AgCl due to a ∼ 4−10% thinning and a ∼40% decrease in lateral size of Bi (001) domains, which are mostly reversed during the anodic scan. Repeated potential cycling enhances the thinning and roughening of the films, suggesting that partial dissolution of Bi ensues during negative polarization. The mechanism of this behavior is understood through molecular dynamics simulations using ReaxFF and density functional theory (DFT) calculations. Both approaches indicate that [Im]+ cations bind to the metal surface more strongly than tetrabutylammonium (TBA+) as the potential and the charge on the Bi surface become more negative. ReaxFF simulations predict a higher degree of disorder for a negatively charged Bi (001) slab in the presence of the [Im]+ cations and substantial migration of Bi atoms from the surface. DFT simulations show the formation of Bi···[Im]+ complexes that lead to the dissolution of Bi atoms from step edges on the Bi (001) surface at potentials between −1.65 and −1.95 V. Bi desorption from a flat terrace requires a potential of approximately −2.25 V. Together, these results suggest the formation of a Bi···[Im]+ complex through partial cathodic corrosion of the Bi film under conditions (potential and electrolyte composition) that favor the catalytic reduction of CO2.

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the Bi/[BMIM]+ interface during CO2 reduction remain unexplored. In a recent study, we followed the detailed changes in the Bi surface structure as a function of potential during CO2 reduction via in situ X-ray reflectivity (XR) measurements. In that work, we demonstrated that bismuth (001) oriented thin film electrodes (∼6 nm thick) undergo a dramatic restructuring in acetonitrile solutions containing 1-butyl-3-methylimidazolium triflate ([BMIM]OTf) upon cathodic polarization. Specifically, this restructuring was characterized by the initial electrochemical reduction of Bi2O3 into Bi0 at potentials between approximately −0.3 and −1.4 V, followed by a drastic decrease in reflectivity at the Bi (001) film Bragg peak of ∼60% when sweeping the potential from approximately −1.5 V to −1.9 V (at a scan rate of 10 mV/s).5 Notably, this loss in Bi

INTRODUCTION

Bismuth-based materials exhibit high electrocatalytic activity and selectivity for the conversion of CO2 to CO in nonaqueous acetonitrile solutions containing millimolar concentrations of alkyl-imidazolium ionic liquids.1−4 Rosenthal and collaborators provided a comprehensive evaluation of the catalytic performance of bismuth thin film cathodes (also known as Bi-CMEC, or bismuth carbon monoxide evolving catalysts) for CO evolution in acetonitrile solutions of 1-butyl-3-methylimidazolium ([BMIM]+)-based ionic liquids under variable conditions of applied potential and ionic liquid concentration.1,2,4 The onset potential for CO2 reduction over Bi in the presence of [BMIM]+ occurs at approximately −1.9 V vs SCE. The negative potentials are believed to be necessary, not only to formally reduce CO2 to CO2·− at the electrode surface but also to reduce Bi2O3 into Bi0 and enable the separation of the cations and the anions of an imidazolium-based ionic liquid electrolyte to facilitate CO2 activation. However, the processes taking place at © 2018 American Chemical Society

Received: January 4, 2018 Revised: March 7, 2018 Published: March 8, 2018 2362

DOI: 10.1021/acs.chemmater.8b00050 Chem. Mater. 2018, 30, 2362−2373

Article

Chemistry of Materials

thin film working electrode, a platinum wire counter electrode, and a leakless Ag/AgCl reference electrode. The potential applied between working and reference electrode was controlled by a Gamry Reference600 potentiostat. The Bi thin film working electrode (10 × 3.0 mm) was contacted from one end of the Bi covered surface by a stainless steel post insulated from solution by a Teflon sleeve. The specular Xray reflectivity (XR) signal was measured as a function of vertical momentum transfer Q, where Q = |Q| = | Kout − Kin | = (4π/λ) sin(2θ/ 2), where Kin and Kout are the incident and reflected X-ray wave vectors and 2θ is the scattering angle (Figure S1).7,8 X-ray reflectivity is defined as R(Q) = Ir(Q)/Io, where Io and Ir are the incident and reflected X-ray beam intensities as a function of momentum transfer, Q. In this work, we report the absolute values so that 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). The incident X-ray beam was horizontally aligned with respect to the 3 mm-long dimension of the working electrode. X-ray scattering experiments were conducted at the Advanced Photon Source (APS) in Argonne National Laboratory, in beamlines 12ID-D, 33BM-C, and 33ID-D. The monochromatic X-ray photon energy was 20 keV (λ = 0.62 Å). 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 (rms) roughness factors.9 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 roughness factors, in which the occupation factor of each layer in the films was defined by an error function profile characterized by an interface location and width (see Supporting Information for more details).8,10−12 The electrolyte fluid structure was modeled as an error function. ReaxFF Reactive Molecular Dynamics (MD) Simulations. A three-layer model was employed to simulate the Bi (001) restructuring process in solution. In this model, the bottom layer consisted of one sheet of graphene, followed by a Bi (001) layer (neutral or negatively charged with an average charge of −0.05 e−/Bi atom) and an electrolyte solution layer on top (containing either [EMIM]+, [BMIM]+, or TBA+). When a total charge equivalent to −0.05 e−/ Bi atom is applied to the entire Bi (001) slab, the EEM method is used to determine the individual charge on each Bi atom, thus enabling the Bi surface to develop an image-charge in response to the adsorption/ binding of the ionic liquid cations.13 The dimensions of the simulation box were 27.67 × 31.96 × 150.00 Å, including a vacuum slab above the three layers lined up along the z-direction. First, two separate simulations were conducted to equilibrate the Bi and graphene layers at 650 K, and the electrolyte solution layer at room temperature with a relatively high density (1.00 g/cm3). Afterward, the Bi/graphene and the solution layer were brought together with an initial 10 Å vacuum slab between the Bi surface and the solution front to prevent their interaction during the relaxation stage, and the evolution of the system was simulated at 650 K for 100 ps. All of the simulations were conducted by implementing the ADF code,14 using a NVT ensemble with a time step of 0.25 fs. The temperature was controlled by the Berendsen thermostat with a damping constant of 100 fs. Density Functional Theory (DFT) Simulations. All DFT calculations reported herein were carried out using the Vienna Ab Initio Software Package (VASP), version 5.3,15 with a plane-wave basis set and a cutoff energy of 400 eV. The core electrons were treated by pseudopotentials with the projector augmented-wave (PAW) method.16,17 Electron correlation and exchange was evaluated within the generalized gradient approximation using the Perdew−Burke− Ernzerhof functional.18 van der Waals (VDW) interactions were described using the D2 correction method of Grimme.19 Spinpolarization was employed when necessary. The Bi surface terrace was modeled using a 6-layer (3 × 3) unit cell of Bi (001) surface, which is the natural cleavage plane of Bi crystal and the preferred direction of epitaxial growth.20 To model Bi step edge sites, an 8-layer (6 × 3) unit cell of Bi (001) surface was employed with half of the top two layers

(001) reflectivity could be reversed almost completely during the subsequent anodic half-cycle. While this restructuring phenomenon was observed in solutions of [BMIM]OTf in both the presence and absence of CO2, it did not occur with Bi thin film cathodes in solutions containing tetrabutylammonium (TBA+)-based electrolytes such as TBAOTf or TBAPF6.5 Together, these findings demonstrated a dynamic restructuring of the Bi/electrolyte solution interface, particularly in the presence of the alkyl-imidazolium-based ionic liquid, within the potential window where CO2 reduction takes place. Nevertheless, the nature of the structural changes and the driving force for this dramatic restructuring of the Bi/[BMIM]+ interface remain unclear.5 Herein, we present a detailed study of the atomic-level structure of the Bi/[BMIM]+ interface through coordinated experimental and computational activities, including analysis of in situ XR data performed during and after cycling the potential of the bismuth electrodes, rotating ring-disk electrode (RRDE) studies, as well as computational approaches including ReaxFF and potential-dependent density functional theory simulations. This study provides new insights into the dramatic structural changes that take place at the Bi/[BMIM]+ interface at negative applied potentials in the presence of [Im]+ electrolytes and provides a proposed mechanistic pathway for these changes. Specifically, the results reveal that strong Bi···[BMIM]+ interactions at the negatively charged Bi electrode lead to significant restructuring of the electrode surface through partial solvation of the Bi by [BMIM]+ cations, which is supported by results from RRDE analyses. The onset potential for this restructuring is found to be sensitive to the nature of the electrode surface (e.g., terrace vs step) and confirms that it preferentially occurs at defect sites at potentials consistent with those seen experimentally.

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EXPERIMENTAL SECTION

Room Temperature Ionic Liquids (RTILs), Other Supporting Electrolytes and Solutions. The ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTf, ≥98%) was purchased from Sigma-Aldrich and used as received. Solutions of this RTIL were prepared in acetonitrile at a concentration of 100 mM. Experiments in the absence of this RTIL were performed in acetonitrile-based solutions containing 100 mM tetrabutylammonium hexafluorophosphate (TBAPF6, from Sigma-Aldrich, for electrochemical analysis ≥99.0% or from TCI America and purified by recrystallization from ethanol) or tetrabutylammonium triflate (TBAOTf, from Sigma-Aldrich, ≥99.0%) as the supporting electrolyte. Acetonitrile (MeCN, UHPLC plus grade for elution ≥99.9%) and bismuth(III) triflate (Bi(OTf)3) were purchased from Sigma-Aldrich. All electrochemical potentials in this work are reported with respect to the Ag/AgCl (1 M KCl) reference electrode unless otherwise stated. Growth of Bismuth Thin Films for XR Measurements. Bismuth-based thin films were grown on SiC previously coated with epitaxial graphene. Epitaxial graphene films were grown on N-doped, Si face epi-ready, 6H-SiC (0001) purchased from Cree Inc. or UniversityWafer (330 μm-thick; 0.02−0.2 Ω·cm resistivity). The SiC substrates (diced into 3 × 10 mm slides) were placed inside an MTI GSL1700X tube furnace and heated up to 1550 °C under argon flow (600−900 mbar) to grow smooth, epitaxial graphene (rms ≤0.5 nm) off of the Si-terminated face. Bismuth films were deposited onto the surface of graphene-coated SiC via DC magnetron sputtering at a low power (12 W; AJA International Inc. equipment) to ensure a deposition rate of ca. 5 nm/min. The thickness of the as-deposited Bi thin films was determined through low-angle reflectivity analysis. In Situ X-ray Scattering Experiments. A custom-made X-ray transmission cell was used for in situ X-ray reflectivity under voltammetric control (Figure S1).6 The cell was equipped with a Bi 2363

DOI: 10.1021/acs.chemmater.8b00050 Chem. Mater. 2018, 30, 2362−2373

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Chemistry of Materials removed. The two model systems have a z-dimension of 26.56 and 32 Å, respectively, and were evaluated with a (3 × 3 × 1) and a (2 × 4 × 1) k-point mesh, respectively.21 Due to the relatively small supercell sizes, and to minimize the displacement of solvent acetonitrile molecules, [EMIM]BF4 was used instead of [BMIM]OTf or [BMIM]PF6, and the resulting solution has a concentration of 1.2 M. For geometry optimizations, atoms in the bottom half of the Bi (001) surface models were fixed to their lattice positions, while those in top half and in the solution were allowed to fully relax until the forces on each atom were less than 0.05 eV/Å. Potential-dependent energies were calculated using the double-reference method on model Bi (001) surface.22,23 In this method, the electrode potentials are aligned to the Galvani potential of the electrolyte solution away from the metal-solution interface, which is assumed to be independent of the amount of excess charge in the system due to electrostatic screening. The Galvani potential of the solution is then aligned to the vacuum level in a noncharged calculation with an additional solutionvacuum interface.22,23 To isolate and better understand the nature and energetics of surface-cation interactions (Figure S8), gas-phase, nonperiodic DFT calculations were performed for a 37-Bi model using the Gaussian 09 software package, revision D.01,24 with the LanL2DZ basis set and effective core potential.25,26 Since only one layer was incorporated in our cluster calculations, excess charge will localize over this surface layer. The amount of polarization in these cluster models cannot be rigorously converted to electrode potentials and is thus expressed as a function of average excess charge per atom. Rotating Ring-Disk Electrochemistry (RRDE) Measurements. Rotating ring disk electrochemistry experiments were performed using a CHI-720D bipotentiostat. The working electrode was a rotating-ringdisk electrode (RRDE, Pine Instrument), comprised of a glassy carbon (GC, 5 mm) disk electrode surrounded by a platinum ring. A platinum wire (Goodfellow, 99.9%) was used as the counter electrode. All potentials were measured against a Ag/AgCl reference electrode (1 M KCl). The RRD electrode was polished sequentially with slurries of 0.3 and 0.05 μm alumina powders in Millipore water. The polished electrode was placed in an electrodeposition bath containing 20 mM Bi(OTf)3 and 100 mM TBAPF6 dissolved in N2 saturated MeCN. The disk electrode was preconditioned by cycling the applied potential (3 cycles) from −0.3 to −2.0 V vs Ag/AgCl at a sweep rate of 100 mV/s, and was then briskly agitated in the deposition solution to remove any exfoliated material from the GCE surface. Controlled potential electrolysis (CPE) was initiated using the conditioned disk in the quiescent Bi(OTf)3 solution at −1.25 V vs SCE until 1.0 C/cm2 of charge had been passed. The Bi-modified GC disk was rinsed with MeCN and then dried under a gentle stream of nitrogen.

Figure 1. X-ray reflectivity in the (a) low-angle and (b) crystal truncation rod (CTR) regimes of a Bi thin film immersed in a CO2saturated acetonitrile solution containing 100 mM [BMIM]OTf before (yellow circles) and after running 1 cyclic voltammogram (blue squares) between −0.32 and −1.90 V vs Ag/AgCl at 10 mV/s (the data after one CV scan has been scaled by a factor of 102 to improve data visualization). (c) CTR of the Bi (001) Bragg peak region recorded with the Bi thin film in solution at open circuit potential and polarized at −1.9 V. In all three figures, the solid black lines represent the best fit model of the respective reflectivity data, and the dotted line in panel b corresponds to the fitting result of the CTR signal prior to the CV scan.

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RESULTS AND DISCUSSION In situ X-ray Reflectivity at the Bi/[BMIM]+ Interface. In situ XR measurements were conducted with the purposes of (1) characterizing the atomic-level structure of the Bi/ [BMIM]+ interface before and after cycling the potential between open circuit potential (approximately −0.3 V) and −1.9 V, which is where the onset of CO2 reduction takes place and (2) probing the structure of the Bi thin film cathode while it is polarized at −1.9 V (Figure 1). The specular low-angle reflectivity and crystal truncation rod (CTR) signal of a bismuth film immersed in the CO2-saturated [BMIM]OTf solution collected before and after cyclic voltammetry (scanned between −0.32 and −1.90 V, at 10 mV/s) are plotted in Figures 1a and 1b, respectively (along with their best fit model calculations). The reflectivity signal near the Bi (001) thin-film Bragg peak was measured at open circuit potential (OCP) and under cathodic polarization at −1.9 V, as shown in Figure 1c. The CTR profiles measured after two and three consecutive CVs are included in the Supporting Information.

The low-angle reflectivity data presented in Figure 1a show signs of significant changes to the Bi/[BMIM]+ interface after one CV scan. The reflectivity signal for the pristine Bi film in Figure 1a has 3 well-defined oscillations corresponding to a ∼6.8 nm-thick film with an atomically flat surface. In comparison, the reflectivity profile for the same film after one CV scan shows two oscillations shifted to higher Q values, corresponding to a ∼6.0 nm-thick film, and an overall lower reflectivity above the critical angle which is indicative of an increase in surface roughness. The electron density profiles that describe the Bi/[BMIM]+ system (including the SiC substrate, the graphene layer underneath the bismuth film, and the fluid layer) were also extracted from the MotoFit analysis of the 2364

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Figure 2. Electron density profiles showing all the layers that comprise the Bi thin film used as a working electrode (SiC substrate, graphene, Bi (001) and the amorphous bismuth component) immersed in a CO2-saturated acetonitrile containing 100 mM [BMIM]OTf (fluid layer). (a) Electron density profile of the pristine Bi film/electrolyte interface and (b) after one CV between −0.3 and −1.9 V at 10 mV/s. Overlaid to each profile is the electron density profile of the working electrode/electrolyte system obtained through fitting of the low-angle reflectivity signal (solid black line).

differences that point to electrochemically driven structural changes to the film. For instance, a 73% increase in peak reflectivity (R at the Bi Bragg peak increases from 1.38 × 10−5 to 2.39 × 10−5) and a smaller fwhm is seen for the Bi (001) Bragg peak after one CV scan, as compared to that of the pristine film, which is consistent with an increase in Bi (001) thickness from 3.64 to 4.45 nm, as determined via fitting of the experimental CTRs (see Figures 2 and S2). Likewise, this thickening of the Bi (001) film after one CV scan causes the Kiessig fringes to shift closer to the Bi Bragg peaks. On the basis of the analysis of the low-angle reflectivity in Figure 1a and the CTR in Figure 1b, even though the Bi film becomes ∼1 nm thinner, approximately 90% of the restructured film is comprised of Bi (001) after one CV scan, as compared to ∼55% before running the voltammetry experiment. X-ray absorption (XANES and EXAFS) results obtained previously for this system demonstrated that this increase in thickness of the Bi (001) layer materializes upon electrochemical reduction of bismuth oxides originally found at the topmost layers of the film.5 Running the second and third CV scans causes the Bi (001) Bragg peak reflectivity to drop by ∼10 and ∼22%, respectively, when compared to the peak reflectivity achieved after the first CV scan (see Supporting Information). This decreased reflectivity is consistent with a decrease in Bi (001) layer thickness from 4.45 nm after the first voltammogram to 4.36 nm after the second and 3.94 nm after the third CV scan (see Figure S2). In addition, the bismuth film becomes increasingly rougher after each CV experiment, as demonstrated by the loss and attenuation of the remaining Kiessig fringes and by the changes in low-angle reflectivity discussed above. The electron density profiles obtained from fitting the lowangle reflectivity signal of the Bi film plotted in Figure 2a (solid black trace) describe a graphene-coated SiC substrate topped by a bismuth-based bilayer, while the profiles extracted from fitting the reflectivity in the CTR regime (red trace) describe a Bi (001) film (∼80% coverage) on top of graphene-coated SiC with a smaller amount of intermixed amorphous bismuth material. Given the discrepancies between the electron density profiles obtained through fitting of the low-angle reflectivity (solid black trace) and higher-angle CTR (red trace) signals of the pristine film, it is evident that the topmost bismuth layer detected in the low-angle regime consists of an amorphous bismuth material (most likely Bi2O3) with no contribution to the reflectivity signal in the higher-angle CTR regime. This hypothesis is supported by the fact that both the low-angle and

curves shown in Figure 1a and plotted in Figure S2 as a function of height, where the zero-height point corresponds to the top surface of the SiC substrate (Figure S2, solid black trace). The fitting of the reflectivity curves in Figure 1a using MotoFit reveals that the bismuth thin films are made up of metallic Bi0 coated by a lower-density layer of Bi2O3, consistent with the results from previous X-ray photoelectron spectroscopy (XPS) studies.5 The best fit (χ2 = 25) for the low-angle reflectivity curve corresponding to the pristine Bi film (Figure 1a, yellow circles) is achieved with a model in which the film is comprised of two Bi-based layers, a 3.36 nm-thick layer directly in contact with the graphene surface, which has an electron density of 2.14 e−/Å3, and a second layer on top that is 2.72 nm-thick with a density of 1.74 e−/Å3. In contrast, the thickness and electron density of these Bi-based layers after one CV scan (Figure 1a, blue squares) change to 3.47 nm and 1.72 e−/Å for the bottom layer and 1.63 nm and 1.54 e−/Å for the top layer, respectively (χ2 = 8.5). The expected electron densities of bulk Bi0 and Bi2O3 are 2.3 and 2.1 e−/Å, respectively; however, the lower electron density values determined by fitting the low-angle reflectivity data in Figure 1a could, in principle, be attributed to lower than ideal electron densities that are commonly found for sputtered materials. Furthermore, when comparing the two traces in Figure 1a, it is clear that the Bi film becomes thinner by ∼1 nm after one CV scan, and there is a significant roughening of the bismuth film, which provides an explanation for the decrease in electron density. The structural details of the Bi film, both before and after a full CV scan can be fully appreciated in the electron density profiles shown in Figure 2, which are derived from fitting the low-angle reflectivity and CTRs in Figures 1a and 1b, respectively. The CTR of the pristine bismuth film shown in Figure 1b (yellow circles) exhibits two broad Bragg peaks centered at 1.55 and 3.13 Å−1 corresponding to Bi (001) and Bi (002) reflections, respectively. The 6H-SiC substrate gives rise to a sharp Bragg peak centered at 2.49 Å−1. The wellresolved Kiessig fringes on both sides of the Bi (001) and Bi (002) Bragg peaks indicate that the surface of the film is very smooth. In addition, a broad oscillation between 1.7 and 2.2 Å−1 corresponding to a very thin graphene layer (∼10 Å-thick) is obscured by the presence of the Bi (001) Bragg peak and nearby Kiessig fringes. The CTR measured after the first CV in Figure 1b (blue squares) shows similar features to those observed with the pristine film (yellow circles), except for a few important 2365

DOI: 10.1021/acs.chemmater.8b00050 Chem. Mater. 2018, 30, 2362−2373

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Figure 3. Change in lateral size of Bi (001) domains and reflectivity as a function of potential applied for bismuth thin film cathodes in CO2saturated acetonitrile containing either 100 mM [BMIM]OTf (a−c) or 100 mM TBAOTf (d−f). Figures a and d describe the potential function applied during voltammetry; b and e show the Bi (001) lateral domain size change (open red diamonds), and c and f plot the total reflectivity (open blue circles) as a function of time.

(001) Bragg peak shape in a CV scan between OCP (approximately −0.3 V) and −1.9 V was performed to determine the relationship between the loss of Bi Bragg peak intensity and any morphological changes in the film in [BMIM]OTf and TBAOTf solutions. The average lateral Bi (001) domain size is complementary to the changes observed along the surface normal of the bismuth film (Figure S2). The variation in the average lateral Bi domain size is probed through its change in lateral width through the Debye−Scherrer equation as a function of applied potential for Bi thin films immersed in [BMIM]+ and TBA+ containing MeCN solutions, respectively (details on the calculation of Bi (001) domain size are included in the Supporting Information, Figure S4). The Bi (001) lateral domain size (Figure 3b) remains unchanged until the potential is reduced to at least −1.5 V, at which point the domains become smaller by up to ∼37% when the potential is reduced all the way to −1.90 V. This behavior is nearly completely reversed when the applied potential is raised to approximately −1.4 V. The changes in lateral Bi (001) domain size closely follow the variation in reflectivity as a function of time (Figure 3c). Because the peak reflectivity is proportional to the product of the square of the film thickness with the lateral Bi (001) film coverage, θBι(001), (eq 1) the observed decrease in these parameters by approximately 4−10 and 37%, respectively, at a potential of −1.9 V, almost fully accounts for the ∼60% reduction in the film reflectivity as compared to its value at −1.45 V. The trends in lateral domain size and reflectivity changes seen in Figures 3b and c are very similar to those measured for a bismuth film immersed in Ar-saturated [BMIM]OTf solution, as seen in Figure S5. This observation indicates that the primary factor driving the restructuring of the Bi thin film is the interaction between [BMIM]+ and the negatively charged Bi surface, further demonstrating that CO2 is not required for the observed restructuring phenomenon to take place. The observed changes in the bismuth film immersed in a MeCN solution containing TBAOTf (Figures 3e−f) exhibit a very different relationship as a function of externally applied

CTR derived electron density profiles begin to converge after one CV scan, which induces the reduction of the superficial Bi2O3 into metallic Bi (001) as seen in Figure 2b, and an even closer match is achieved after the second and third CV experiments (Figure S2, Supporting Information). Similar electron density profiles to those seen in Figures 2 and S2 were obtained from XR measurements with the bismuth thin films immersed in Ar-saturated [BMIM]OTf solution (see Figure S3), demonstrating that CO2 is not central to the restructuring of the Bi/[BMIM]+ interface, as has been previously reported.5 Additional in operando measurements of the CTR near the Bi (001) Bragg peak were performed at OCP and while the film was polarized at −1.9 V to probe the structure of the Bi film under polarization and determine the nature of the restructured film (Figure 1c). These measurements were performed rapidly (within 1 min) to minimize the impact of beam-induced film degradation that occurs within minutes at these applied potentials. The Bi (001) peak reflectivity is 60% lower when held at −1.9 V, which is consistent with the reported decrease in reflectivity observed during a cathodic scan to this negative potential.5 Moreover, analysis of these CTR data show that the Bi film’s thickness decreases by approximately 4−10% due to polarization at −1.9 V, which accounts for only a fraction of the ∼60% decrease in Bi (001) peak reflectivity. This substantial change in reflectivity due to cathodic polarization of the Bi film suggests that the loss of signal is not simply due to a reduction in the film thickness. The reflectivity at the Bi (001) Bragg peak is directly proportional to the coverage of Bi (001), θBi(001), and the square of the film thickness: RBi(001) ≈ θBi(001)[Bi(001)thickness]2

(1)

Assuming the coverage of Bi does not change, a ∼4−10% decrease in film thickness would correspond to a decrease in Bi (001) Bragg peak reflectivity of only ∼8−20%, which is much smaller than the experimentally measured (∼60%) loss. Variation in Bismuth (001) Lateral Domain Size as a Function of Applied Potential. Further analysis of the Bi 2366

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Figure 4. Snapshots of a charged Bi (001) structure in the presence of (a) [BMIM]+ and (b) TBA+ containing solutions after 30 ps of simulation (bismuth atoms are shown in mauve, hydrogen atoms in gray, nitrogen atoms in blue and carbon atoms in cyan; the black dashed line indicates the initial surface boundary). Atomic density profile (c) and total Bi migration number (d) for Bi (001) along the z-direction in [BMIM]+ (red) and in TBA+ (blue) containing solutions after 100 ps of simulation (the gray line represents the initial Bi (001) atomic distribution).

[BMIM]+ or TBA+ cations. ReaxFF simulations are an attractive way to explore the complex structural changes observed and understand the role of applied charge, choice of ionic liquid, and Bi electrode structure. Snapshots of the structures of Bi/[BMIM]+ and Bi/TBA+ interfaces after 30 ps of running simulation are shown in Figures 4a and 4b, respectively. We note that these snapshots show a higher degree of distortion to the Bi surface in the presence of [BMIM]+ compared to that seen for TBA+. This is further corroborated by comparing the atomic density profiles (ADPs) of these interfaces along their z-axis (parallel to the surface normal), presented in Figure 4c. Here, the ADP of the pristine Bi (001) structure is used as a reference, where the topmost layer of Bi atoms is located at a height of ca. 0.0 Å. The crystalline structure of the Bi surface after 100 ps of simulation becomes disordered in the presence of either [BMIM]+ or TBA+ cations, but the degree of disorder is significantly higher in the simulation with [BMIM]+, with some Bi atoms drifting away from the thin film’s surface (density peaks appear at positions as high as ≥5.0 Å). To quantify the distortion of the Bi (001) surface, we calculated the total Bi migration number (i.e., the number of Bi atoms for which coordination along the z-axis is larger than or equal to the position of the initial surface + 1.5 Å, given in arbitrary units) which was determined as a function of time in the presence of either [BMIM]+ or TBA+, as shown in Figure 4d. Overall, the migration of Bi atoms is more pronounced in the presence of [BMIM]+ than with TBA+, even though in both cases the migration numbers increase with time during the 100 ps-long simulations. Notably, the initial rate of Bi migration in the presence of [BMIM]+ is approximately 3 times higher than with TBA+, and the total loss of Bi atoms is also higher with the imidazolium-based cation case, even though it tends to level off after 60 ps. Likewise, simulations involving charged Bi (001) in low density electrolyte models yielded similar results with a higher degree of surface restructuring (distortion) and atomic migration in solutions of [EMIM]+ or [BMIM]+, as compared

potential as compared to those observed in the presence of [BMIM]OTf. The intensity of the Bi (001) Bragg peak and the lateral size of Bi (001) domains remains unchanged as the potential is scanned negatively from −0.3 V, until the applied potential was scanned from −1.4 to −1.7 V, at which point the peak reflectivity was observed to increase by ∼100% and the lateral domain size by ∼38%. Notably, there is no evidence for any loss of Bi (001) signal at the most negative potentials. The expansion of Bi (001) lateral domain size with increasingly cathodic potentials and the associated increase in reflectivity can be attributed to electrochemical annealing of the bismuth film, during which bismuth oxides are reduced and transform into crystalline metallic Bi0. The decrease in Bi (001) domains size and Bragg peak reflectivity during the anodic scan can be explained by the opposite phenomenon, that is reoxidation of the Bi film surface, which leads to loss of crystalline bismuth and increasing film roughness. The distinct structural changes at the Bi/[BMIM]+ interface revealed by the reflectivity measurements discussed above (vide supra) can be summarized as follows: (1) polarization of the Bi thin film in solutions containing [BMIM]+ to −1.9 V where the onset of CO2 reduction to CO occurs, leads to a decrease in film thickness of approximately 4−10%; (2) cathodic polarization of the Bi thin film also leads to a ∼37% decrease in Bi (001) lateral domain size; (3) when taken together, these structural changes amount to a ∼60% loss in total reflectivity, which is almost completely reversed upon a subsequent anodic potential scan. Because restructuring of Bi (001) is not seen in TBA+ containing solutions, we hypothesize that the interactions between [BMIM]+ cations and the negatively charged Bi surface are primarily responsible for these phenomena. ReaxFF and DFT Simulations of a Charged Bi (001) Surface in Contact with [BMIM]+ and TBA+ Cations. To test the above hypothesis, we conducted ReaxFF and DFT simulations to model the atomic and molecular-level dynamics at the Bi (001)/[BMIM]+ and Bi (001)/TBA+ interfaces, considering both neutral and negatively charged Bi (001) surfaces in contact with variable concentrations of either 2367

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Figure 5. Potential-dependent energies for models of a Bi (001) surface (a) terrace site and (b) step edge site, before and after the removal of a Bi atom as a Bi···[EMIM]+ complex, along with the corresponding molecular structure models (right). The legend indicates the different configurations of the solvent and ionic liquid molecules: for pristine and pristine-1, both [EMIM]+ cations are located near the surface, while for pristine-2, only one [EMIM]+ cation is close to the surface; for restructured-1, the free [EMIM]+ cation sits on top of an intact region of the surface, while for restructured-2, the free [EMIM]+ cation sits on top of the vacancy left behind by the removal of a Bi atom.

to simulations involving TBA+ (see Figures S6 and S7 in the Supporting Information). These trends in restructuring and Bi atom migration for the simulated Bi/[BMIM]+ and Bi/TBA+ interfaces are consistent with the results of in situ XR measurements under electrochemical control described in the preceding section (vide supra) and in our previous report.5 The loss of reflectivity from the Bi (001) film suspended in a nonaqueous MeCN solution containing [BMIM]OTf subjected to increasingly negative applied potentials (Figure 3c), is in agreement with the increase in disorder and Bi atomic migration predicted by ReaxFF simulations. On the other hand, the lower degree of Bi migration and distortion of the Bi (001) lattice in contact with TBA+, as predicted by ReaxFF, suggests that the interactions between TBA+ and the charged Bi surface are not strong enough to disrupt the Bi film structure and cause a drop in reflectivity akin to that measured in [BMIM]+ containing MeCN solutions under the same conditions of applied potential. By contrast, when the Bi thin film surface is immersed in TBA+ containing MeCN solutions under cathodic polarization, in situ XR indicates that the reduction of bismuth oxide into metallic Bi0 leads to a higher crystallinity of the film,

consistent with the observed increase in Bi (001) Bragg peak reflectivity (Figure 3f). Higher accuracy DFT calculations were also carried out to determine the gas phase binding energies of [BMIM]+ and TBA+ to the Bi (001) surface as a function of surface charge and thus provide an estimate of the strength of these interactions upon cathodic polarization of the Bi thin film. The binding energies are defined herein as the difference between the total energy of the cation bound to the surface (EC+/Bi) and the energies of the free cation (EC+) and Bi surface (EBi) ΔE bind = EC + /Bi − [EC + + E Bi]

(2)

Based on this equation, negative binding energies, ΔEbind, indicate that adsorption is thermodynamically favorable, and the formation of an attractive interaction between Bi and the [BMIM]+ cation. The DFT and the ReaxFF calculated binding energies for [BMIM]+ and TBA+ adsorbed onto the Bi (001) surface are plotted in Figure S8 as a function of Bi surface charge. While the binding energies calculated by DFT are higher than those for ReaxFF, the energetic trends are very similar as a function of 2368

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Chemistry of Materials Bi surface charge. At zero Bi surface charge, the TBA+ and [BMIM]+ binding energies were calculated to be within 0.13 eV of one another with TBA+ binding being slightly stronger than that for [BMIM]+ using both the DFT (−0.74 eV for TBA+ and −0.61 eV [BMIM]+) and the ReaxFF (−0.22 eV for TBA+ and −0.13 eV for [BMIM]+) methods. The stronger binding of TBA+ at zero charge is the result of the stronger VDW interactions between the alkyl chains on TBA+ and the bismuth surface compared to the weaker VDW interactions between the aromatic imidazolium ring and the metal. The addition of negative charge into the Bi slab reverses the relative affinity of the two cations for the Bi surface, with [BMIM]+ binding more strongly than TBA+. The binding between the positively charged cation and the negatively charged surface is predominantly Coulombic in nature and controlled, in part, by the 1.8 eV higher electron affinity of [BMIM]+ (−3.8 eV) over that of TBA+ (−2.0 eV). In addition, the positive charge on [BMIM]+ is delocalized over the imidazolium ring, thus allowing [BMIM]+ to adsorb closer to the negatively charged Bi surface. In contrast, the positive charge on the TBA+ cation is buried at the central N atom at the core of the tetrabuylammonium cation and, as such, remains much further away from the negatively charged Bi surface. Charging the surface to −0.05 e−/Bi atom resulted in a DFTcalculated [BMIM]+ binding energy of −4.51 eV, which is higher in magnitude than the value of −3.42 eV for TBA+ (see Figure S8). Overall, while the corresponding ReaxFF binding energy values are smaller, they show similar increases in the strength of adsorption for [BMIM]+ vs TBA+ upon polarization of the Bi cathode to −0.05 e−/Bi. To further model the effect of the presence of electrolytes in the condensed phase, periodic DFT calculations were performed as described previously. Our results indicate that the Bi (001) domains can undergo partial dissolution into the [BMIM]+ containing solution. The dissolution of Bi in ionic liquid solutions could potentially involve adsorbate-induced removal of either individual or clusters of Bi atoms in the form of Bix··· [Im]+y complexes, analogous to the process that controls the dissolution and corrosion of metals at anodic potentials in the presence of coordinating anions. To probe the extraction of Bi, we examined the binding of [EMIM]+ and removal of a Bi··· [EMIM]+ complex from a flat terrace or an edge site on the Bi (001) surface, as a function of potential, in an acetonitrile solution containing 1.2 M [EMIM]BF4. In these calculations, [EMIM]BF4 was used as a simpler (i.e., less computationally intensive) imidazolium-based ionic liquid model. The Bi···[EMIM]+ complex can form via the strong adsorption of a free [EMIM]+ cation from the electrolyte solution onto the negatively charged Bi surface, analogous to the gas phase binding of [BMIM]+ discussed above. The cathodic polarization of Bi in the ionic liquid solution was modeled by adding fractional electrons into the metal slab to establish an electrochemical potential which was subsequently converted to the Ag/AgCl electrode scale by using the ab initio potential-dependent double-reference method.22,23 The energies of pristine and restructured Bi (001) terrace and edge sites were examined as a function of applied potential and are compared in Figure 5. More specifically, in the traces labeled pristine and pristine-1, both of the [EMIM]+ cations are located near the Bi surface. For the pristine-2 trace, only one of the [EMIM]+ cations is close to the surface. For the restructured-1 trace, the free [EMIM]+ cation sits on top of an intact region of

the Bi surface, while for the restructured-2 trace, the free [EMIM]+ cation sits on top of the vacancy left behind by the removal of a Bi atom. At simulated potentials near E = −1 V, the removal of a Bi atom from both terrace and edge sites is highly unfavorable with an associated energetic penalty of ∼2 eV. As the simulated potential is made increasingly negative, the energy of the restructured Bi film states decreases (becoming more favorable) more rapidly than that of unrestructured states. The pristine-1, restructured-1, and restructured-2 curves in Figure 5a intercept near E = −2.25 V, indicating that the detachment of the Bi··· [EMIM]+ complex from a flat terrace site becomes favorable. The pristine-2 curve also appears to intercept the restructured states but at a slightly more negative potential. The dissolution of Bi···[EMIM]+ complex from a Bi step-edge, on the other hand, occurs approximately at potentials that are approximately 0.3−0.6 V less negative (i.e., E = −1.65 V for the restructured-2 model or E = −1.95 V in the case of the restructured-1 model), as seen in Figure 5b. The earlier crossover between the pristine and restructured curves in Figure 5b compared to what is seen in Figure 5a is a direct result of the stronger binding of the [EMIM]+ cations to the coordinatively unsaturated Bi atoms at edge sties, which facilitates the formation of Bi···[EMIM]+ complexes and their subsequent detachment from the Bi film. To understand the differences in the binding energies of Bi atoms at in-plane terrace and step-edge sites, we calculated the gas-phase vertical detachment energy (where the surface is prevented from reoptimizing) for Bi removal from these different sites. The detachment of Bi from the step edge site (4.6 eV) was calculated to be 1 eV more favorable than from the terrace site (5.6 eV). This difference is consistent with the larger loss of Bi (001) lateral domain size (∼40%) compared to vertical film thickness (∼4−10%) upon polarization of the Bi cathode at −1.9 V because a lower energy barrier for dissolution from step-edge sites would result in the size of Bi (001) domains decaying faster sideways than vertically, as was observed experimentally (see Figure 3). In principle, the vacancies created at under-coordinated sites can lead to [EMIM]+ cations binding more strongly to the Bi surface. The energy stabilization for a free [EMIM]+ cation on top of a Bi vacancy compared to that of [EMIM]+ on top of an intact region of the Bi surface, however, is small. This difference is slightly larger for step-edge sites compared to terrace sites (i.e., the crossover observed for restructured-1 with the pristine state in Figure 5b was shifted to more positive potentials by ∼0.3 V). While these simple gas phase and solution binding energies are important in showing the trends for dissolution, they do not take account for possible changes in the interactions between the solvent molecules near the site of dissolution, which are important and tend to moderate such interactions. The DFT results presented here indicate that partial dissolution of the Bi film electrodes becomes thermodynamically favorable at cathodic potentials that precede or coincide with the onset of CO2 reduction. This is a rather unusual observation because most metals corrode under anodic conditions via the formation of their corresponding metal oxide(s). In fact, applying a cathodic potential to a metal is a widely used technique for corrosion protection. For reducible metals such as Bi, however, the severe cathodic polarization may lead to the population of higher-energy bands (i.e., those formed by the anti-bonding p states on Bi; see Figure S9) and result in weakening of the material’s metal−metal bonds. The presence of stable cations at such negative potentials (i.e., 2369

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Table 1. Reaction Potential Relative to Ag/AgCl for a Series of Reduction Steps, Showing the Transition of Overall Charges of the Bi···[EMIM]+ Composite

[BMIM]+ or [EMIM]+) that can adsorb and bind to the negatively charged metal surfaces would ultimately aid in the abstraction of Bi atoms from the surface, thereby lowering the barrier for partial dissolution of the Bi film. This phenomenon is analogous to the strong interactions between anions and positively polarized metal electrodes that drive anodic corrosion and, as such, can be thought of as cathodic corrosion. There are a few reports in the literature on the subject of cathodic corrosion, and they involve systems invariably comprised of late transition27−32 or p-block33 elements. In many of these cases, dry aprotic solvents were used, and an insoluble “amalgam” of reduced cation−metal composite was formed on the electrode. More recent efforts describe the use of cathodic treatments and alternating currents to leach out nanoparticles of Pt, Au, Cu, Ag, Ni, and Rh from bulk metals.31,32 In line with what we discuss here for the Bi/ [BMIM]+ and Bi/[EMIM]+ systems, these other studies also emphasized the role of cations and the relative insensitivity of cathodic corrosion toward anions present in solution. Furthermore, the dissolution process under cathodic polarization of the metals mentioned above was also found to be reversible, although their recovery led to changes in surface morphology and eventually became incomplete as the electrode’s potential was cycled more than a few times. Another puzzling aspect of the restructuring of Bi thin film cathodes in [BMIM]+ containing solutions is the oxidation state of Bi in the Bi···[Im]+ species that is formed. To probe this question, a series of 1e− redox reactions involving the Bi··· [EMIM]+ complex were simulated using an implicit solvent model for acetonitrile, with the net charge on the complex varying from +2 to −2 (i.e., with the formal oxidation state of Bi varying from +1 to −3). The results which are shown in Table 1 suggest that at the most negative potentials in the applied CV experiments (−1.9 V vs Ag/AgCl), the Bi atom in the model complexes may prefer to be in an oxidation state of 0, −1, or perhaps a mixture of both. The Bi atom in such a complex can form a Bi−C covalent bond at the C-2 position on [EMIM]+, which would distort the planar geometry of the [Im]+ aromatic ring.34,35 Probing the Cathodic Dissolution of Bi via RRDE. Rotating-ring-disk electrochemistry experiments were employed to probe the manner in which imidazolium ([Im]+)based ionic liquids influence the potential controlled restructuring/dissolution of a Bi film electrode on a glassy carbon (GC) support. Using a published method,1,2,4 a thin Bi film was electrodeposited onto a GC RRDE surrounded by a platinum ring. The RRDE experiment was first carried out in the absence of an ionic liquid electrolyte using an N2-saturated MeCN solution containing 0.1 M TBAPF6. In this experiment, the disk potential was scanned from −1.0 to −2.1 V vs Ag/AgCl at a scan rate of 10 mV/s while rotating the electrode assembly at 1600 rpm and holding the ring potential at −1.65 V (Figure 6). During this experiment, the disk current showed a small wave

Figure 6. Current traces obtained using a rotating ring disk electrode in which the glassy carbon disk electrode (5 mm) was coated with a Bi film. RRDE experiments were conducted in N2 saturated MeCN solution containing 0.1 M TBAPF6 (blue and red traces). Dashed yellow and green traces were recorded under identical conditions except that 0.1 M [BMIM]OTf was added to the electrolyte solution. The linear sweep voltammograms recorded for the Bi modified GC disk electrode were obtained using a scan rate of 10 mV/s and as indicated were polarized from −1.0 to −2.1 V. The platinum ring electrode was poised at −1.65 V vs Ag/AgCl. All RRDE data were recorded using a rotation rate of ω = 1600 rpm. Given the direction of the potential scans, reductive current appears upward, while oxidative current appears downward, as indicated by the legend in the upper right-hand corner.

with a peak potential at about −1.55 V, while no detectable current was observed at the platinum ring electrode, indicating that no electrochemically active species were dissolving from the Bi film in the potential range between −1.0 and −2.1 V in the absence of [Im]+. The small wave observed at the disk electrode at approximately −1.55 V was found to persist even after repeatedly scanning the Bi film modified disk electrode within this potential window even though no appreciable current was observed at the platinum ring electrode, which was held at potentials between −1.0 and −2.0 V. This RRDE experiment was repeated following the addition of [BMIM]OTf (100 mM) to the N2-saturated MeCN-based electrolyte while maintaining the scan and rotation parameters specified above (vide supra). The presence of [BMIM]+ in the electrolyte solution resulted in a shift of the observed cathodic peak at the Bi modified GC disk electrode to less negative potentials by approximately 200 mV. This shift in cathodic peak potential was accompanied by significantly larger current densities compared to those in the absence of [BMIM]OTf. Also in contrast to the RRDE experiments conducted in the absence of an [Im]+-based ionic liquid, a significant current was 2370

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Chemistry of Materials observed at the platinum ring electrode (E = −1.65 V). The onset of this ring response is correlated to the redox event detected at the Bi modified GC disk electrode at approximately −1.25 to −1.30 V. We note that the measured ring current corresponds to a reduction process, suggesting that a species released from the Bi modified GC disk at potentials more negative than approximately −1.25 V is reduced at the platinum ring. Consistent with this interpretation is the observed loss of signal at the platinum ring when its potential was stepped more positive than approximately −1.15 V. When taken together, the results of the RRDE experiments described above indicate that Bi film electrodes can undergo a cathodic process that is dependent on the chemical nature of the cations present in the electrolyte solution, as the RRDE signals observed for the Bi modified GC disk electrode and platinum ring are either attenuated or absent when only TBA+ cations are present in the MeCN-based electrolyte. These observations are consistent with a strong interaction of the negatively polarized Bi film with the [BMIM]+ cation, which generates a species that can be transported away from the working disk electrode by electrolyte flow and be subsequently reduced/detected at the ring electrode (i.e., cathodic corrosion). Cathodic Corrosion of Bi Thin Film Electrodes in RTIL Solutions. The combined results from in situ XR and RRDE experiments along with the DFT and ReaxFF calculations reveal that Bi electrodes in [BMIM]+ containing electrolyte solutions are susceptible to cathodic corrosion, leading to the formation of solvated Bix···[BMIM]+y complex(es) at potentials that favor CO2 reduction. The corrosion of Bi cathodes in [BMIM]+ solutions is driven by the negative polarization of the electrode, and the reversibility of the restructuring process suggests that the Bix···[BMIM]+y complex(es) are stabilized near the electrode surface through electrostatic interactions, in similar fashion to the adsorption of multivalent ions onto minerals in aqueous systems.36 The evolution of catalyst surface structure and morphology under working conditions is a widely acknowledged phenomenon. For example, the catalytic hydrogenation of CO2 over metallic nanoparticles often involves changes to the catalyst morphology, underscoring the role that under-coordinated defect sites play in the reactivity and product distribution that such systems display.37 Likewise, more recent studies of electrochemical reduction of CO2 and oxygen evolution (OER) reactions on cobalt and cobalt oxide materials, respectively, claim differences in catalyst activity as a result of changes in oxidation state and morphology of the cobalt-based electrodes under catalytic potentials.38,39 The present study reveals an unusual change in catalyst structure in which a portion of the active material is solvated due to its strong interaction with the ionic liquid cations present in the electrolyte. Furthermore, this interaction is most intense at negative potentials under which charge accumulation on the Bi electrode surface favors the binding of [Im]+ cations. This binding then leads to solvation of Bi, likely in the form of a Bi0···[BMIM]+ complex that remains within the electrical double layer. At this point, we do not know the exact structure of the Bix··· [BMIM]+y complex(es) that form at the Bi/[BMIM]+ interface nor whether they play a role in the reduction of CO2. Mechanistic models of this reaction show that the 2H+/2e− reduction of CO2 to CO in the presence of [BMIM]+ involves co-organization of CO2 with two neighboring [BMIM]+

species. The restructuring of the Bi electrode to produce a Bi···[BMIM]+ solvated species under catalytically active conditions opens up the possibility that this interfacial solvated species operates as a reaction site for CO2 activation and conversion.

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CONCLUSION AND FUTURE DIRECTIONS The results presented in this study demonstrate that the structure and chemical composition of Bi film cathodes are highly dependent on applied potential and on the nature of the electrolyte present in solution. In situ XR measurements show that varying the applied potential between OCP (approximately −0.3 V) and −1.9 V in MeCN-based solutions containing [BMIM]+ leads to significant structural changes to Bi film electrodes composed primarily of Bi (001) (∼80%) and Bi2O3. During the first CV scan, the Bi2O3 component is reduced into metallic bismuth as the potential is scanned negatively from OCP, causing an increase in the coverage of Bi (001). Furthermore, a previous report and results from this work demonstrate that, as the potential becomes more negative than −1.5 V, a loss of Bi (001) material causes a decrease in reflectivity at the Bi (001) Bragg peak that amounts to a loss of ∼60% in reflectivity once the potential reaches −1.9 V. Although ∼95% of the reflectivity is recovered during the subsequent anodic half-cycle, overall, the Bi film is thinner by ∼1 nm, rougher, and almost completely crystalline after one CV scan. Repeated CV scans induce further roughening and thinning of the Bi film surface, which suggests that some of the Bi is lost to the electrolytic solution as the potential is cycled. ReaxFF and DFT simulations were carried out to probe the interactions between Bi (001), [Im]+, and TBA+ cations. Both methods show that the binding of imidazolium-based cations ([Im]+) such as [BMIM]+ and [EMIM]+ onto a Bi (001) surface intensifies as the negative charge on the Bi surface increases. A similar effect is observed for a Bi (001) slab in contact with TBA+ cations, but the binding of [Im]+ cations is significantly stronger than that of TBA+ at all nonzero charges. In addition, ReaxFF predicts a more dramatic distortion of the Bi (001) surface and eventual dissociation of Bi atoms due to highly favorable interactions with [Im]+ cations. DFT results indicate that formation of a Bi···[Im]+ complex that readily dissociates from the Bi (001) surface is favorable at negative potentials similar to those applied during CV and XR measurements, which may explain the loss of some Bi during voltammetric experiments. Indeed, the DFT work described above predicts that Bi dissolution due to formation of Bix··· [Im]+y complexes at step-edge sites can take place at potentials between −1.65 and −1.95 V, while more negative potentials in the vicinity of −2.25 V would be required for this dissolution to occur at flat terrace sites. These computational results are consistent with the analysis of in situ XR, given that vertical thinning of the Bi film due to changes in height of Bi (001) terraces is less pronounced than changes in lateral domain size of Bi (001) crystallites, which presumably occur due to Bi dissolution from step-edge sites. RRDE experiments confirm the formation of an electroactive species at a Bi modified carbon electrode polarized at potentials more negative than −1.25 V when in the presence of an electrolyte solution containing [BMIM]+. RRDE experiments also demonstrate that the electroactive species generated at the Bi cathode can be reduced at the ring electrode when it is held at potentials more negative than −1.65 V. This is not the case when the same experiment is carried out in electrolyte solutions 2371

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Chemistry of Materials that contain only TBA+, in which case no species are released from the Bi disk electrode that can be detected at the ring electrode. Taken together, the results from this diverse suite of measurements and calculations reveal that strong binding of [Im]+ cations onto the surface of Bi (001) film cathodes employed for CO2 reduction becomes favorable at the negative potentials required for the electrocatalytic conversion of CO2 to CO. Further, the strong Bi···[Im]+ interactions appear to disrupt the structure of the Bi films to a higher degree than interactions with TBA+ cations. The combined DFT, ReaxFF, and RRDE studies reported herein strongly suggest that formation of a Bix···[Im]+y species at the cathodic potentials required for CO2 activation coincide with the onset of Bi (001) restructuring, as observed via in situ XR measurements. The role that the cathodic formation of a purported Bix··· [Im]+y complex(es) at the Bi-CMEC/electrolyte interface might have on CO2 reduction is a focus of ongoing study. In particular, the relative importance of an ideal heterogeneous process that takes place either on flat terrace sites or at stepedge (defect) sites at the Bi/[BMIM]+ interface, versus one that might involve a solvated Bi···[BMIM]+ species needs to be clarified to understand how Bi-based electrocatalysts for CO2 activation function and, ultimately, to guide the design of new electrocatalysts for a variety of CO2 reduction processes that show high efficiency and selectivity. Such efforts may also help to improve our understanding of the catalytic plasticity that Bicathodes exhibit toward CO2 in the presence of varied ionic liquid promoters.40

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Department of Energy (DOE), Office of Science, Basic Energy Sciences. 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 DE-AC0206CH11357. The X-ray reflectivity work was carried out at the Advanced Photon Source, sectors 12-ID-D, 33-BM-C, and 33-ID-D. The computational work was carried out using resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract DE-AC02-05CH11231. Additional computational resources were provided by the Minnesota Supercomputing Institute. A.A. was supported through a Camille and Henry Dreyfus postdoctoral fellowship in Environmental Chemistry.

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(1) Medina-Ramos, J.; Pupillo, R. C.; Keane, T. P.; DiMeglio, J. L.; Rosenthal, J. Efficient Conversion of CO2 to CO Using Tin and Other Inexpensive and Easily Prepared Post-Transition Metal Catalysts. J. Am. Chem. Soc. 2015, 137, 5021−5027. (2) Medina-Ramos, J.; DiMeglio, J. L.; Rosenthal, J. Efficient Reduction of CO2 to CO with High Current Density Using in Situ or ex Situ Prepared Bi-Based Materials. J. Am. Chem. Soc. 2014, 136, 8361−8367. (3) Zhang, Z. Y.; Chi, M. F.; Veith, G. M.; Zhang, P. F.; Lutterman, D. A.; Rosenthal, J.; Overbury, S. H.; Dai, S.; Zhu, H. Y. Rational Design of Bi Nanoparticles for Efficient Electrochemical CO2 Reduction: The Elucidation of Size and Surface Condition Effects. ACS Catal. 2016, 6, 6255−6264. (4) DiMeglio, J. L.; Rosenthal, J. Selective Conversion of CO2 to CO with High Efficiency Using and Inexpensive Bismuth-Based Electrocatalyst. J. Am. Chem. Soc. 2013, 135, 8798−8801. (5) Medina-Ramos, J.; Lee, S. S.; Fister, T. T.; Hubaud, A. A.; Sacci, R. L.; Mullins, D. R.; DiMeglio, J. L.; Pupillo, R. C.; Velardo, S. M.; Lutterman, D. A.; Rosenthal, J.; Fenter, P. Structural Evolution of Bismuth Electrodes during Electrochemical Reduction of CO2 in Imidazolium-Based Ionic Liquid Solutions. ACS Catal. 2017, 7, 7285− 7295. (6) Fister, T. T.; Hu, X.; Esbenshade, J.; Chen, X.; Wu, J.; Dravid, V.; Bedzyk, M.; Long, B.; Gewirth, A. A.; Shi, B.; Schleputz, C. M.; Fenter, P. Dimensionally Controlled Lithiation of Chromium Oxide. Chem. Mater. 2016, 28, 47−54. (7) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley Publishing Company, Inc.: Menlo Park, CA, 1978. (8) Fenter, P. A. X-ray Reflectivity as a Probe of Mineral-Fluid Interfaces: A User Guide. In Reviews in Mineralogy and Geochemistry; GeoScienceWorld: 2002; Vol. 49, pp 149−221. (9) Nelson, A. Co-refinement of Multiple-Contrast Neutron/X-ray Reflectivity Data Using MOTOFIT. J. Appl. Crystallogr. 2006, 39, 273−276. (10) Uysal, A.; Zhou, H.; Feng, G.; Lee, S. S.; Li, S.; Cummings, P. T.; Fulvio, P. F.; Dai, S.; McDonough, J. K.; Gogotsi, Y.; Fenter, P. A. Interfacial Ionic ″Liquids″: Connecting Static and Dynamic Structures. J. Phys.: Condens. Matter 2015, 27, 1−9. (11) Uysal, A.; Zhou, H.; Feng, G.; Lee, S. S.; Li, S.; Fenter, P. A.; Cummings, P. T.; Fulvio, P. F.; Dai, S.; McDonough, J. K.; Gogotsi, Y. Structural Origins of Potential Dependent Hysteresis at the Electrified Graphene/ Ionic Liquid Interface. J. Phys. Chem. C 2014, 118, 569− 574. (12) Callagon, E. B. R.; Lee, S. S.; Eng, P. J.; Laanait, N.; Sturchio, N. C.; Nagy, K. L.; Fenter, P. Heteroepitaxial Growth of Cadmium Carbonate at Dolomite and Calcite Surfaces: Mechanisms and Rates. Geochim. Cosmochim. Acta 2017, 205, 360−380. (13) Mortier, W. J.; Ghosh, S. K.; Shankar, S. Electronegativity Equalization Method for the Calculation of Atomic Charges in Molecules. J. Am. Chem. Soc. 1986, 108, 4315−4320.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00050. Scheme of the electrochemical cell used for X-ray reflectivity measurements along with electron density profiles, X-ray reflectivity, ReaxFF, and DFT supporting data and fitting parameters (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail: *E-mail:

REFERENCES

[email protected]. [email protected]. [email protected]. [email protected]. [email protected].

ORCID

Jonnathan Medina-Ramos: 0000-0002-2998-5136 Peng Bai: 0000-0002-6881-4663 Abderrahman Atifi: 0000-0002-0163-5660 Sang Soo Lee: 0000-0001-8585-474X Timothy T. Fister: 0000-0001-6537-6170 Joel Rosenthal: 0000-0002-6814-6503 Paul Fenter: 0000-0002-6672-9748 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS 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. 2372

DOI: 10.1021/acs.chemmater.8b00050 Chem. Mater. 2018, 30, 2362−2373

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DOI: 10.1021/acs.chemmater.8b00050 Chem. Mater. 2018, 30, 2362−2373