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Large Amplitude Fourier Transformed AC Voltammetric Study of the Capacitive Electrochemical Behavior of the 1-Butyl-3-Methylimidazolium Tetrafluoroborate - Polycrystalline Gold Electrode Interface Anthony J Lucio, Scott K. Shaw, Jie Zhang, and Alan M Bond J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017
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TITLE Large Amplitude Fourier Transformed AC Voltammetric Study of the Capacitive Electrochemical Behavior of the 1-Butyl-3-Methylimidazolium Tetrafluoroborate Polycrystalline Gold Electrode Interface
AUTHORS Anthony J. Lucio,1 Scott K. Shaw,1* Jie Zhang,2 and Alan M. Bond2 1
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States
2
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
ABSTRACT In this paper, the capacitive electrochemical behavior of the 1-butyl-3-methylimidazolium tetrafluoroborate (Bmim BF4) - polycrystalline gold electrode interface is reported over the potential range from -0.37 to 0.53 V vs. Fc/Fc+ (Fc = ferrocene). Experimental results are generated by analysis of data (RC model) obtained from large amplitude Fourier transformed alternating current voltammetry (FT-ACV) over the frequency range of 10 Hz to 1 kHz. Results suggest a parabolic, U-shaped capacitance versus potential relationship in stark contrast to present ionic liquid (IL) electrochemical double layer (EDL) theory. The potential range analyzed was carefully selected to be free of Faradaic current and displays minimal hysteresis with respect to the potential scan direction. Over the selected potential window spanning 0.9 V, the capacitance versus potential curve at 9 Hz exhibits a U-shape, with a capacitance minimum of 19.9 ± 1.3 µF cm-2 at 0.13 ± 0.04 V, flanked by maximum values of 21.2 ± 1.3 µF cm-2 and 20.8 ± 1.4 µF cm-2 at -0.37 V and 0.53 V vs. Fc/Fc+, respectively. This capacitance versus potential profile is consistent with traditional Gouy-Chapman-Stern theory for dilute aqueous electrolyte solutions and high temperature molten salts but distinctly misaligned with bell- or camel-shaped relationships that have recently been proposed in IL model systems. The minimum capacitance exhibits a small level of frequency dispersion which increases linearly versus the logarithm of the applied frequency. The potential at which the minimum capacitance is located is also slightly dependent on frequency. This work demonstrates that large amplitude FT-ACV provides a sensitive probe of the EDL from a single experiment, and advances the convergence between theoretical predictions and experimental observations of IL – electrode EDL systems.
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INTRODUCTION Ionic liquids (ILs) are defined as salts that have a melting point below 100 °C. They represent an interesting class of electrolyte because in principle they contain no molecular solvent. As concentrated salts acting as both the electrolyte and solvent, ILs display higher levels of Coulombic forces than those found in their aqueous electrolyte counterparts (e.g. aqueous 0.01 M KCl), and their constituent cation and anion components do not have traditional solvation spheres. These properties add to their range of interesting behaviors not displayed by traditional molecular solvents. The generally large size and asymmetric shapes of organic cations and the wide functional space of available counter anions create large variability in physiochemical properties of ILs such as viscosity, polarity, and electrochemical window. It has been suggested that ILs might be ‘tuned’ to particular applications.1-4 For electrochemical applications, ILs can be selected to have very large electrochemical potential windows (e.g. 6 V) that provide access to previously inaccessible reduction/oxidation potentials, facilitating applications such as the separation of rare earth metals,5 and in the development of IL based capacitive energy storage devices.3 Since ILs represent a highly concentrated ionic medium, one might expect that theoretical models based on dilute aqueous electrolyte solutions would not properly describe the capacitive behavior of the IL electrochemical double layer (EDL).6 Creating a broadly applicable theoretical model for the IL EDL, accounting for the various physical properties and testing the models experimentally has proven challenging. Specifically, literature reports of IL electrochemical properties show significant variation for protic versus aprotic ILs and for specific electrode surfaces (metallic, non-metallic, and semiconductor).7-8 Variability in purity (water and halides are common contaminants), however, is also a major issue in many IL studies. A search of the literature describing capacitance curves at the IL – electrode interface reveals a wide range of dependencies of capacitance on potential. Experimental data from sum frequency generation spectroscopy and electrochemical impedance spectroscopy (EIS) suggest the Bmim BF4 - platinum interfacial region is simply a Helmholtz layer, one ion layer thick.9 Ohsaka and coworkers have undertaken a number of EDL studies in several classes of ILs using impedance measurements and obtained capacitance-potential curves which in some cases display parabolic10-12 features across several electrode materials (mercury, gold, glassy carbon), while other data exhibits bell-shape curvature,13 as predicted by theoretical models. Camel-shaped features on several electrode materials (gold, platinum, and glassy carbon) in several imidazolium-based ILs have been reported,14-17 a response which is also supported by theoretical models. Interestingly, a linear capacitance dependence on potential was reported with a semiconducting boron-doped diamond electrode in 1-butyl-3-methylimidazolium 18 bis(trifluoromethylsulfonyl)imide. In other work, Silva and coworkers describe relatively featureless (or potential independent) capacitance curves for several imidazolium-based ILs in contact with metallic gold and platinum electrodes.19 In contrast capacitance-potential curvature that is more complex than standard bell- or camel-shape has been suggested for imidazoliumand pyrrolidinium-based ILs on Au(111), where there is a relatively weak dependence of fast EDL formation on the electrode potential with typical capacitance values being in the range from 2 ACS Paragon Plus Environment
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6 – 12 µF cm-2.20 This breadth of predictions and results suggest that ILs exhibit very rich and complex electrochemistry, which may account for the lack of a single, unifying model to describe their capacitance behavior. However, the wide range of presently reported behavior may ultimately be attributed in part to experimental issues such as IL purity, electrode surface structure, potential windows examined, and the experimental technique employed to obtain capacitance data. Another intriguing property of ILs is the slow dynamics that IL – electrode interfaces exhibit when relaxing from an external applied potential. The slow dynamics give rise to interesting behaviors over timeframes from milliseconds to minutes.21 For example, recent work by Haverhals and coworkers described dynamic structural hysteresis behavior at the 1-ethyl-3methylimidazolium bis (trifluoromethylsulfonyl)imide - gold electrode interface with surface enhanced infrared absorption spectroscopy. Upon repetitive cycling of the potential over a 3.2 V range, the spectra indicate that the anion concentration increases (relative to the starting concentration) at an electrode surface even while poised to a strongly negative potential.22 In another study employing EIS and probe microscopy, two distinctively different processes were detected at the interface between 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ILs and Au (111). A faster, millisecond process is governed by bulk ion transport in the IL due to double layer charging, while a slower process hypothesized to be surface reconstruction of the single crystal electrode took place over several seconds.23 Also, using potential step chronoamperometry and linear sweep voltammetry, Kakiuchi and coworkers suggested that ultraslow (on the order of seconds) relaxation of the IL EDL is a common feature, to be expected across many classes of ILs at electrified interfaces.24 EIS is commonly applied with small perturbation amplitudes (≤10 mV) at a constant DC potential using a multi-frequency waveform, and analyzed via equivalent circuit analysis.25 Traditional AC voltammetry also employs small perturbation amplitudes, scanning from an initial to a final potential onto which a periodic, single-frequency waveform is superimposed.25 Small perturbation amplitudes and phase randomizations are used to minimize contributions from nonlinear, second and higher harmonic components to model the electrochemical system in a mathematically advantageous linear fashion. However, with large amplitude (50 mV to 200 mV) single-frequency perturbations, Fourier transformed alternating current voltammetry (FTACV) has significant advantages over existing DC methods and small amplitude AC counterparts by providing access to the information rich second harmonic and higher order harmonic elements.26-28 Specifically, under large amplitude conditions, the fundamental (first) harmonic (radial frequency ω) provides information on the capacitance and Faradaic currents, while higher order AC harmonics (4ω, 5ω, etc.) provide sensitive probes of Faradaic contributions separated from non-Faradaic double layer charging current. The 2ω and 3ω harmonic components can also contain contributions from double layer charging current (capacitance) arising from non-linearity or non-ideality. Using this technique ensures we are specifically probing non-Faradaic double layer charging currents to study the IL EDL and also confirms that departures from the ideal RC model applied in this study are small. FT-ACV has been successfully used to study processes occurring at electrode interfaces in both molecular and 3 ACS Paragon Plus Environment
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IL solvents.29-30 Data over a wide potential range are also collected from a single experiment making this a powerful technique to study a multitude of electrochemical processes. In this paper, we investigate the potential dependence of the capacitance response derived from the 1-butyl-3-methylimidazolium tetrafluoroborate (Bmim BF4) - polycrystalline gold electrode interface using large amplitude FT-ACV. We focus specifically on measurement and analysis of capacitance over a potential range which is shown to be devoid of any Faradaic contributions, and where hysteresis effects associated with the potential scan direction are minimal.
EXPERIMENTAL SECTION Reagents: 1-butyl-3-methylimidazolium tetrafluoroborate (Bmim BF4) of purity ≥99 % is obtained from IoLiTec and used as received. Ferrocene (Fc, Sigma-Aldrich, ≥98 %) is purchased from Merck-Schuchardt. At the end of a series of FT-ACV measurements with a Pt quasireference electrode ferrocene is added to the IL and the mid-point potential of the Fc/Fc+ reversible couple derived from cyclic voltammetry is used to calibrate the potential scale (Fc/Fc+ potential set equal to 0.000 V). It should be noted that the mid-point potential of the Fc/Fc+ redox couple can be different from the formal potential (by ≥10 mV) due to unequal diffusion coefficients between the charged and neutral redox species.31 Instrumentation: Direct current (DC) voltammetric measurements are undertaken with a BAS Epsilon EC-2000-XP electrochemical workstation. FT-ACV measurements are achieved with home-built instrumentation described elsewhere.28 The temperature used to obtain voltammetric data, unless stated otherwise, is 20 ± 2 °C. IL water content prior to degassing with nitrogen is determined using a Metrohm 831 Karl Fischer (KF) titrator. The average sample size in these measurements is 300 µL of neat IL and the water content is determined in duplicate. In situ monitoring of the much lower water content present in the IL at the time experiments were undertaken after 60 minutes of degassing with nitrogen is achieved at the gold working electrode using the cathodic stripping response of gold oxide generated at positive potentials.32 Water used for calibration and other purposes is obtained from a Milli-Q water purification system. Methods: The electrochemical cell consists of a three-electrode arrangement with a 2.5 mL (total volume) conical glass vial with a custom sealed plastic cap. Electrochemical experiments are undertaken in a 500 µL volume of IL. The cell is cleaned by rinsing with copious amounts of high purity water and acetone and dried thoroughly. The gold working electrodes (CH Instruments) are polycrystalline disks with surface areas of 9.59×10-3 or 3.20×10-2 cm2. The effective surface areas of these electrodes are calculated from the Randles-Sevcik equation (Equation 1) using the DC voltammetric peak current for reduction of 1.00 mM hexaammineruthenium chloride [Ru(NH3)6]3+/2+ redox couple ([Ru(NH3)6]Cl3, Strem Chemicals, 99%) in 0.1 M KCl (Merck) with a diffusion coefficient of 6.7×10-6 cm2 s-1.25 Randles-Sevcik Equation
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Polycrystalline gold working electrodes are polished with an aqueous slurry of 0.3 µm MicroPolish II aluminum oxide powder (Buehler) on microcloth pads (Buehler), sonicated in water for 5 minutes to remove excess aluminum oxide powder, rinsed with copious amounts of water, and dried. When not in use, the gold electrodes are stored in water after polishing to preserve the electrode surface. The counter and quasi-reference electrodes are made of Pt wire and cleaned by rinsing with water and acetone. Finally they are dried with high purity nitrogen gas before introduction into the electrochemical cell. Before each FT-ACV experiment the IL is dried by purging with high purity nitrogen gas for 60 minutes after which a nitrogen blanket is maintained above the solution during experiments. The water content prior to nitrogen degassing is around 4000 ppm (Karl Fisher titration), but extended periods of nitrogen degassing lowered this to the 100 to 200 ppm range (gold oxide stripping peak measurement) which is the water content relevant to reported experimental data.32 DC cyclic voltammograms are initially obtained to establish an electrochemical window containing negligible contributions from Faradaic processes, e.g. gold oxide formation, or solvent or impurity redox chemistry, which is then further confirmed by the absence of 4th and higher order harmonic components of FT-ACV. All electrochemical measurements have been recorded in triplicate. The FT-ACV measurements employ a sinusoidal perturbation having an amplitude (∆Eamp) of 80 mV and a frequency ( f ) of 9, 55, 207, 607, or 1020 Hz and a DC scan rate (υ) of 67.10 mV s-1. The capacitance, which is assumed to be derived primarily from the double layer response, is calculated at each frequency from the fundamental (first) harmonic component of the FT-ACV data using the equations described in detail elesewhere.33 This calculation assumes the capacitance behavior can be modelled purely in terms of a simple RC equivalent circuit. The level of frequency dispersion or non-conformance to the simple RC model is estimated by the apparent dependence of capacitance on frequency. Finally, the capacitance-potential data are fit33 with a polynomial (Equation 2). In this equation, Cj represents the jth polynomial capacitance coefficient that applies for a given frequency and is generated from the optimized fit to Equation 2. The polynomial capacitance coefficients are empirical values that apply for a given frequency, such that no physical meaning should be attached to their values.34 E’(t) is the potential which incorporates the Ohmic drop and is calculated from the DC component by E’(t) = iRu. Lastly, ECdl is also obtained from the DC potential component where the potential is set to the Fc/Fc+ reference scale. Capacitance Polynomial Equation Initially, a fourth-order polynomial form of Equation 2 is used to describe the capacitancepotential relationship, but we find that a second-order polynomial expression is sufficient to generate capacitance curves of interest in this work.
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RESULTS AND DISCISSION Due to the objectives of this work it is useful to briefly review EDL models for comparison with our experimental data. Table 1 complies the major EDL theories, the principle theoretical postulation(s), major characteristic(s), and the predicted capacitance versus potential curve based. Additionally, we have provided a more detailed background discussion of the EDL theories in the supporting information. The structure of Bmim BF4, a water miscible IL, and its physiochemical properties are provided in Table 2. The theoretical ionic concentration based on the molar mass and density of Bmim BF4 is 5.30 M, which highlights the significant difference in ionic strength compared to typical electrolyte solutions (e.g. 0.01 M) used in many aqueous EDL studies. Watanabe and coworkers report the effective ionic concentration of Bmim BF4 to be 3.4 M.35 These authors also found the ionicity of several classes of ILs to be between 50% to 80% (e.g. 1.5 to 3.4 M) “ionic” species, (defined as the ratio of the molar conductivity measured from EIS to that estimated from pulsefield-gradient spin-echo NMR diffusion coefficients).35 This implies that ILs also contain a significant concentration of neutral ion pairs, however, this viewpoint has been disputed36 in recent literature. The ionicity of ILs has led to debate in the literature, where some data suggests that ILs behave as dilute electrolyte solutions and others suggest ILs are strongly dissociated with ions interacting with several nearby counterions.37-39 Because ILs are a concentrated, highly dense media that lacks classical solvent molecules, the mathematical expressions from GCS theory should not apply. Nevertheless, there are numerous reports9-12, 40-41 that provide data for IL systems that is similar to what occurs in dilute electrolyte solutions. This is spite of significantly different ions, diverse chemical interactions, and the minimization of solvent molecules. Water is an adventitious impurity that must be monitored closely in IL studies. Working under an inert atmosphere is paramount to minimize the concentration of water present. In this study the IL in the electrochemical cell was purged with dry nitrogen gas for > 60 minutes prior to commencing experiments, and a nitrogen atmosphere was maintained over the IL during the course of experiments. This procedure produces ILs with water contents in the range of 100 – 200 ppm. Water is an interesting impurity within the context of the IL EDL, as well as altering electrochemical potential windows, viscosity, and in some cases degrading the electrode surface.42 Additionally, the tetrafluoroborate [BF4-] anion is known to hydrolyze in the presence of water, even under mild conditions.43 Our IL solutions have up to 4000 ppm water but when extensive degassing44 is used (prior to electrochemical measurements) this decreases substantially. However, if electrochemical measurements are made before the nitrogen purge time, the ~200 mM (4000 ppm) water concentration the hydrolysis products could be problematic. However, the lack of significant faradaic current in our data, as well as previous work45 suggesting that similar concentrations of water does not significantly disrupt the liquid structure in Bmim BF4, mitigate these concerns. Previous work from our laboratory has shown that within the range of 100 – 4000 ppm water, the IL electrochemical system only shows minor differences in DC voltammograms and capacitive currents.41 However, neither in our work nor any other experimental study is all water removed. Since it is very challenging and/or impossible 6 ACS Paragon Plus Environment
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to remove the last traces of water, our data represent a practical experimental environment for the majority of research up to the present time. Motobayashi and coworkers provided the first experimental evidence of water condensing at the electrode surface.46 These authors suggested that water condensing at the IL – electrode interface does not affect the EDL structure but rather accelerates the potential dependent cation/anion exchange in the first ionic layer by reducing the Columbic interaction among and between ions and the electrode surface.46 Endres and coworkers examined much higher concentrations of water and found a well-defined water-in-IL to IL-inwater transition in the range of 20 – 30 vol % (200,000 – 300,000 ppm) water using vibrational spectroscopy.47 Certainly, water is expected to play a role at the IL – electrified interface but its effects at concentrations
