ARTICLE pubs.acs.org/JPCC
A Comparative Study of the Electrochemical Characteristics of [Emimþ][BF4] and [Bmimþ][BF4] Ionic Liquids at the Surfaces of Carbon Nanotube and Glassy Carbon Electrodes Jianping Zheng,† Surya S. Moganty,‡ Pubudu C. Goonetilleke,† Ruth E. Baltus,‡ and Dipankar Roy†,* †
Department of Physics and ‡Department of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, New York 13699, United States
bS Supporting Information ABSTRACT: The electrochemical interfaces of a glassy carbon (GC) electrode and a carbon nanotube (CNT) paper electrode have been studied in 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) and 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) ionic liquids (ILs) using direct current (DC) cyclic voltammetry (CV) and alternating current (AC) electrochemical impedance spectroscopy (EIS). These four electrode/electrolyte combinations serve as representative models of the relatively unconventional, spatially inhomogeneous ILcarbon interfaces that are potentially useful for electrochemical energy-storage/conversion devices. The present work explores in detail the double-layer capacitances as well as other essential electrochemical features of these interfaces. The voltage-dependent double-layer capacitances, and the interfacial charge transfer resistances have been determined within the nonfaradaic windows of these systems, independently, by using CV and EIS. The results of the DC and AC measurements have been compared to demonstrate how the material properties of such complex systems can affect their characterization procedures if they are simply treated as classical ideally polarized interfaces.
1. INTRODUCTION Room-temperature ionic liquids (ILs) have received considerable attention in recent years as potential electrolytes for electrochemical energy storage and conversion devices14 such as Li ion batteries,2 dye-sensitized solar cells,4 and electrochemical double-layer capacitors (EDLCs).1 However, owing to their rather complex physical and chemical properties, most ILs tend to exhibit nontrivial electrochemical behaviors that often are unpredictable within the conventional formalism of aqueous electrochemistry developed for dilute solutions.5,6 This makes it difficult to adequately correlate the IL properties with various performance indicator parameters of the IL-based devices. Furthermore, IL-supported electrochemical systems frequently use carbonaceous electrode materials such as composite/activated carbons, and carbon nanotubes (CNTs).7,8 These electrodes have large surface areas to efficiently support electrochemical processes, but at the same time, contain significant porosities and/or spatial inhomogeneities that additionally complicate the properties of the IL-electrode systems.9,10 Another related aspect of IL-electrode combinations for EDLC applications is that such a system should be ideally polarizable (strictly capacitive) within its electrochemical window.79 However, this often is difficult to achieve due to faradaically active impurities present in the electrolyte1012 and, in some cases, also due to impurities of the electrodes.1315 r 2011 American Chemical Society
Rigorous purification of ILs may not always be practical for costeffective utilization of such electrolytes. Removal of impurities from most carbonaceous electrodes requires additional experimental steps14,15 that may also be similarly preventive in practical terms for large scale commercial applications of these materials. The residual species at carbonIL electrochemical interfaces often make these systems deviate from ideal nonfaradaic behaviors, and in such cases, it becomes necessary to adequately account for the faradic leakage effects in the analysis of experimental capacitance data. The present work addresses these issues of IL-carbon electrochemical systems, focusing mostly on their EDLC applications. Four model cases of carbon-IL interfaces are studied here. These include a paper electrode of multiwalled (MW) CNT and a glassy carbon (GC) electrode, coupled individually with two solvent free IL electrolytes, 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emimþ][BF4] or EmimBF4) and 1-butyl-3methylimidazolium tetrafluoroborate ([Bmimþ][BF4] or BmimBF4). The CNT electrode is chosen because of its substantial charge storage capacity, which is a particularly attractive feature for EDLCs.8,16,17 The results for the porous CNT Received: December 28, 2010 Revised: February 10, 2011 Published: March 29, 2011 7527
dx.doi.org/10.1021/jp1123162 | J. Phys. Chem. C 2011, 115, 7527–7537
The Journal of Physical Chemistry C
Figure 1. (a) DC and (b) AC circuit models of faradaically active and spatially inhomogeneous interfaces of ILelectrode systems.
electrode are compared with those obtained under similar conditions using a flat-surface GC electrode. The two ILs with a common anion [BF4] are chosen to examine their different cation induced properties,18 because the cations of ILs strongly govern the performances of IL based supercapacitors.1,19,20 The techniques of direct current (DC) cyclic voltammetry (CV) and alternating current (AC) electrochemical impedance spectroscopy (EIS), are combined to probe the electrochemical windows, double-layer capacitances, polarization resistances, and certain essential electrochemical features of the spatially inhomogeneous interfaces.
2. THEORETICAL CONSIDERATIONS Circuit Models of the ElectrodeIL Interfaces. Electrode equivalent circuits (EECs) are commonly used to study IL electrochemistries.10,11,16 Figure 1a shows such a circuit representing the situation for DC CV, where E denotes the linearly swept voltage applied to the experimental interface and i is the net DC electrode current. Ru and Cdl are the uncompensated electrolyte resistance and the double-layer capacitance, respectively; ic is the current of double-layer charge/discharge; if is the faradaic (capacitor leakage) current, which, within the apparent electrochemical window, may come from impurities, and outside the window, can be generated by both the impurities and the oxidation/reduction of the anion/cation of the IL; R represents a net polarization resistance of these faradaic processes.17 The AC response of the EEC of Figure 1a is considered in Figure 1b, where the applied DC voltage E is combined with a frequency (ω) dependent sinusoidal perturbation voltage E~ for EIS. In going from parts a to b of Figure 1, the resistive elements Ru and R remain unchanged, but to account for frequency dispersion in the AC case, the double-layer capacitance is replaced by the constant phase element (CPE) Qdl. The complex
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admittance, Y(Qdl), of the CPE has the form:17,21,22 Y(Qdl) = Y0(jω)n, where j = (1)1/2; Y0 and n are frequency independent CPE parameters, with 0.5 e n e 1.0 for solidliquid interfaces.2225 The value of n provides a measure of the roughness and spatial inhomogeneity of the electrochemical interface, with lower values of n typically representing rougher/less homogeneous electrode surfaces.2123 For n = 1, one has Y0 = Cdl, and then the CPE takes the simple form of a capacitance.24 The AC perturbation currents through R and Qdl are denoted as ~i Q and~i f, respectively; ~i is the total AC current. The frequencydependent complex impedance of the interface is Z, which has a real (Z0 ) and an imaginary (Z00 ) component. The individual impedance parameters in Figure 1b can be determined through complex nonlinear least-squares (CNLS) analysis of experimentally measured Nyquist (Z0 vs Z00 ) plots.17,23 For a strictly nonfaradaic, ideally polarizable electrode, if≈0, ~i f≈0, and then R is infinitely large (open branch) in parts a and b of Figure 1. In addition, if the electrode surface is spatially homogeneous and flat at the atomic level (n = 1, with Qd1 Cdl), then both the EECs of parts a and b of Figure 1 reduce to a simple series combination of Ru and Cdl.2125 DC and AC Measurements of Cdl for IL-Based Systems. The double-layer capacitance of an ILelectrode interface can be taken as a series combination of two capacitors, CC and CD, representing the compact and diffuse layers of the interface, respectively5,6,26 ½Cdl 1 ¼ ½CC ðε, dÞ1 þ ½CD ðε, E, FÞ1
ð1Þ
where CC (ε*/d), with ε* and d representing the effective values of the compact layer’s dielectric function and width, respectively. The value of CD is determined by those of the bulk dielectric function (ε), the ion density of the IL, and the DC potential E. Both ε* and d are voltage dependent due to voltage-induced variations in the polarization and orientation of interfacial ions. Thus, CC is implicitly voltage dependent through the term (ε*/d).1,5,27 The value of ε*can often be substantially different from that of ε, and the compact Helmoltz layer can be thicker than one molecular layer.5 Moreover, depending on the ILelectrode system used, the observed voltage dependence of Cdl may be dictated predominantly by that of CC.1,27 Measuring Cdl as a function of E is an essential goal of most electroanalytical experiments designed to explore EDLC applications of IL electrode systems. As demonstrated previously, Cdl for carbon IL interfaces can be determined by combining EIS with scanspeed-(vdc)-dependent CV measurements and by analyzing the data in the framework of Figure 1a.17 The protocol for doing this is briefly outlined below. Neglecting the extreme upper and lower bounds of a given voltammogram (iE plot) recorded in CV,11
i ¼ ir ( vdc Cdl
ð2Þ
* = Cdl[R/(RuþR)]; the positive and negative signs where Cdl before vdc correspond to CV scans for increasing or decreasing voltages, respectively. If i(E) is plotted against vdc from a series of voltamograms obtained through multiscan CV experiments then, according to eq 2, the resulting graph should yield a straight line of slope C*dl and an ordinate intercept of ir(E). By measuring these ir data against E, the polarization resistance can be determined as17 R ¼ ½dir =dE1 7528
ð3Þ
dx.doi.org/10.1021/jp1123162 |J. Phys. Chem. C 2011, 115, 7527–7537
The Journal of Physical Chemistry C
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The values of R evaluated in this way can be used in the * , of experimental evaluation of Cdl(E) from the slopes, Cdl (iνdc) graphs by writing11,17
Cdl ¼ Cdl ½1 þ ðRu R 1 Þ
ð4Þ
where Ru can be found from EIS. Equation 4 shows how the accuracy of DC measurements of Cdl is affected by the relative values of Ru and R. For most IL electrolytes, the solution resistance cannot be neglected,12 and hence, unless the polarization resistance is very large, the capacitance C*dl measured using straightforward CV will substantially deviate from the actual value Cdl of the double-layer capacitance.11 CarbonIL EDLC interfaces often exhibit this non-IPE behavior. Only for an IPElike system, ir ≈ 0 and Ru , R f ¥, where Cdl ≈ C*dl in eq 4, and then Cdl can be evaluated directly from iE voltammograms by simplifying eq 2 as11 Cdl ≈ i(νdc)1 . EIS based,17 as well as single/fixed frequency impedance26,28 measurements of Cdl for ILelectrode systems have also been reported. While the polarization resistance plays a critical role in determining the accuracy of DC measurements of Cdl, AC measurements of this capacitance are affected by both the polarization resistance and the spatial inhomogeneity of the electrochemical interface. The fixed frequency approach uses Z00 measured with an AC perturbation voltage at a fixed angular frequency ω. The working formula used to calculate Cdl from such data comes from a special case of the following general expression obtained by analyzing the EEC of Figure 1b17
1 Y0 ω n 1 2 1 cotðnπ=2Þ þ 2 þ ¼ 00 ωZ R Y0 ω1 þ n sinðnπ=2Þ sinðnπ=2Þ ωR ð5Þ
Equation 5 shows how the value of R, determined by the strength of faradaic side reactions, as well as that of n, determined by the degree of surface inhomogeity, can prevent the left-hand side of this expression to be equated to Cdl. Only in the special case of a spatially homogeneous (n = 1) electrode exhibiting IPE behavior (Rf¥) does eq 5 simplify to Cdl ðωZ00 Þ1
ð6Þ
In this particular situation, Cdl can be evaluated from eq 6 by using Z00 measured at a single value of ω.3,28 EIS-based AC measurement of Cdl is straightforward through simple CNLS fitting of experimental impedance spectra, as long as the electrode does not exhibit CPE behavior. However, if CPE effects are detected (n < 1), extraction of Cdl from the measured values of Y0 and n is no longer straightforward, especially in the presence of finite polarization resistances. Determination of Cdl in such cases typically requires system dependent models to correlate Cdl with Y0 and n.21,22 This approach can provide satisfactory results if the approximations used for such model calculations are compatible with the experimental system used, and if n is close to 1.24 A relatively simple “ladder” model proposed by Brugg et al. often has yielded satisfactory results for Cdl form such calculations, in agreement with those of independent CV measurements.21 According to this model " #1=n Ru R 1 n Cdl ¼ Y0 ð7Þ Ru þ R
and the system criteria necessary to meet the underlying assumptions of this model have been reviewed recently.17 In the present work, CV measurements, in combination with eq 4, are used as the primary means of determining Cdl(E), and EIS is performed to independently obtain Cdl employing the framework of eq 7. The two sets of results are then compared to check how the presently used ILcarbon systems match the description of the CPE ladder model. The single frequency impedance method of measuring Cdl also is tested in this context.
3. EXPERIMENTAL SECTION Materials. MWCNT paper sheets as described elsewhere11,17,29
were obtained from Nanolab and assembled on a Cu current collector in the form of a working electrode. The GC electrode, obtained from BASi (model MF-2012), was cleaned/polished using standard procedures.15 The impurities (reported by the manufacturer) in the CNT samples included 1.65% Fe, 0.71% Si, 0.61% S, 0.6% Na, and 0.51% Al. A geometric surface area of 0.07 cm2 for both the CNT and the GC electrodes was exposed to the electrolyte, with silver and Pt wires (2 and 1 mm diameters, respectively) from Alfa Aesar used as reference and counter electrodes, respectively. EmimBF4 (98þ%) and BmimBF4 (99þ%), obtained from Alfa Aesar, were kept in a vacuum oven (80 C and ∼6 Torr) for 24 h and then purged with ultrahighpurity Ar for 1 h before each experiment. The electrode and electrolyte materials were used as-received to allow and probe their intrinsic impurity effects. Electrochemical Measurements. A Solartron 1287A potentiostat/galvanostat, operated with a model 1252A frequency response analyzer, was employed for CV and EIS experiments (performed at room temperature). The measured CV/EIS parameters were normalized with respect to the geometric electrode areas. CV data were recorded at various scan rates between 1 and 50 mVs1 after the OCP of the system was stabilized, and the voltage ranges necessary to determine the electrochemical windows were identified following previously discussed steps.22 AC signals of 10 mV amplitude with frequencies between 0.01 Hz and 10 kHz were used for EIS. Experimental Nyquist (Z0 vs Z00 ) plots, collected at different DC voltages and validated using the frequency-comparison method,30 were CNLS analyzed using ZSimpWinTM to determine EEC models of the interface. The modulus weighting factor23 was used to CNLS-fit the EIS data, and only those results yielding
