Double-Layer Capacitance at Ionic LiquidâBoron-Doped Diamond

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Double Layer Capacitance at Ionic Liquid – Boron Doped Diamond Electrode Interfaces Studied by Fourier Transformed Alternating Current Voltammetry Anthony J Lucio, Scott K. Shaw, Jie Zhang, and Alan M. Bond J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00272 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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The Journal of Physical Chemistry

Title Double Layer Capacitance at Ionic Liquid – Boron Doped Diamond Electrode Interfaces Studied by Fourier Transformed Alternating Current Voltammetry 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 This paper reports the electrochemical double layer (EDL) behavior at the interfaces of ionic liquids (IL) and a boron doped diamond (BDD) electrode as measured by large amplitude Fourier transformed alternating current voltammetry (FT-ACV). Data are collected over a ≥2 V potential range and fitted to a simple RC circuit model. The absence of significant higher-order AC harmonic components implies nearly ideal capacitive behavior in the potential ranges examined. Capacitance values for two protic ILs and three aprotic ILs range from 3 to 8 µF cm-2, and generally increase (1 to 2 µF cm-2 V-1) as the potential is swept from negative to positive values. Capacitance-potential data display little dependence on the composition of the IL. The generally featureless, linear dependence of capacitance on potential over a wide potential is similar to that reported for BDD electrodes in aqueous electrolyte media, suggesting the BDD electrode is largely insensitive to the nature of the electrolyte media. The present study concludes that FT-ACV affords an efficient approach to probe the IL – electrode interface, with minimal capacitive hysteresis based on the potential scanning direction. Introduction Ionic liquids (IL) are similar to high-temperature molten salts (e.g. NaCl) in the sense they contain no molecular solvent. However due to the large, flexible and polarizable characteristics of the molecular ions which hinder traditional crystal lattice structures, many ILs are liquid at room temperature.1 Physiochemical properties of ILs can vary significantly, or even be tuned, based on their chemical composition. Many ILs exhibit large electrochemical potential windows (e.g. 3 to 6 V), low volatility (negligible vapor pressure), moderate to high viscosity (e.g. 10 to 1000 mPa s), and moderate conductivity (e.g. 0.01 to 1 S m-1).2 This makes ILs advantageous solvents and materials in battery applications and carbon dioxide capture / conversion technologies.3-5 Each of these uses, necessitates an understanding of the IL – electrode surface chemistry that governs these relevant processes. In electrochemistry, the formation and structure of the near-surface region, the electrochemical double layer (EDL), is of particular interest. Underlying questions regarding IL interfacial regions include understanding the distal extent and dynamics of the IL structures formed at polarized metal electrode surfaces. Currently, several descriptions of the IL EDL exist. It has been suggested that the IL EDL at a highly conducting metal electrode is only one ion-layer thick following a Helmholtz layer model.6-7 This has been contested8-9 and corroborated10-12 by others. Conversely, an arrangement of the IL EDL into a 1 ACS Paragon Plus Environment

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traditional double layer structure (i.e. Helmholtz layer plus diffuse layer) has also been postulated.13-16 There are also published reports17-20 that predict a multilayer arrangement extending several ionic radii (e.g. 2 to 7 ion pairs) from the IL – electrode interface. Long-range (ca. 5 to 10 nm) electrostatic forces have also been reported14, 21-22 in ILs and the effective Debye length is reported to be on the order of 1 to 10 nm from surface force apparatus measurements.1415, 23 Collectively, these studies suggest there is still much to understand about the complex surface chemistry of ILs, particularly within the electrochemical context. Electrochemistry provides a direct probe of the EDL, and has supported the development of models for aqueous electrolyte media for over a century. The Gouy-Chapman-Stern (GCS) model provides theoretical estimates of experimental capacitance-potential behaviors for a wide range of data.24 The GCS model includes three regions: 1) the inner-most layer (Helmholtz or Stern layer) where ions interact with the electrode surface, 2) the diffuse layer over which mobile ions move to accommodate larger overpotentials, and 3) the bulk region which is out of range from influence of electrode polarization. Capacitance-potential curvature on metallic electrodes in aqueous electrolyte show a wide range of responses but U-shape25 and camel-shape26-27 are typically reported. This scenario, however, is different when a semiconducting electrode is employed. Unlike a metal, where the excess charge resides at the electrode surface, a semiconductor (e.g. boron doped diamond, BDD) develops a space-charge region akin to the diffuse double layer model (GCS). This space-charge region contains the majority of the potential drop, which can dominate the capacitance response (i.e. Csc). However, when doped to a sufficiently high level to impart metallic like characteristics, the potential drop occurs at the semiconductor – solution interface in a similar manner to a metal.28 Interestingly, faradaic processes at heavily doped BDD electrodes do indeed display the same characteristics as found at metal electrodes, but not capacitance current. Capacitance-potential data with BDD electrodes in aqueous electrolyte generally show relatively featureless, linearly sloped curvature over a wide (1.4 to 2 V) potential range.29-35 The literature contains two reports29, 36 of capacitance-potential data for BDD electrodes (carrier concentration ~1021 B atoms cm-3) in ILs. Swain and coworkers studied the 1butyl-3-methylimidazolium tetrafluoroborate (Bmim BF4) – BDD interface and report a potential independent capacitance (~0.73 µF cm-2) from -1.0 to +0.1 V vs. Ag and a sloped, linearly dependent region from +0.1 to +1 V vs. Ag.29 Cannes et al. describes a linearly sloped capacitance-potential plot that increases from 25 µF cm-2 to 40 µF cm-2 over a 2.3 V range as the potential is swept from positive to negative potentials for the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Bmim TFSI) – BDD electrode interface. These types of capacitance responses on BDD, reminiscent of the response in aqueous electrolyte,29-35 are not predicted with current theoretical models for the IL – metal electrode interface. Mean field theoretical predictions suggest the capacitance-potential response in ILs with metal electrodes should display bell- or camel-shaped curvature, where the potential of zero charge (PZC) is hypothesized to represent the maximum or local minimum, respectively.37-40 The bellshaped curvature is hypothesized to arise from a very concentrated ionic system where only a limited increase in the concentration of ions in the EDL is possible due to steric effects with an 2 ACS Paragon Plus Environment

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increasing potential.40-41 As the electrode potential increases the EDL thickness increases with larger polarizations, which results in a lower capacitance. The camel-shape can be described in three parts: 1) at low overpotentials the capacitance response increases from the local minimum (i.e. PZC) due to an initial ‘compression’ of the alkyl tail substituents (acting as latent voids) and counter ions encroaching in on these sites, where 2) further electrode polarization induces lattice saturation effects (e.g. steric limitations) and the capacitance reaches a local maxima (the two peaks are believed to represent the cation and anion), when finally 3) at large overpotentials the EDL thickness swells to accommodate an increasing electrode potential, effectively lowering the capacitance.38 While there are experimental reports for both bell-8, 42 and camel-shaped36, 43-44 capacitance curves in ILs on metallic and carbon electrodes, researchers have also demonstrated parabolic U-shaped,7, 13, 45-47 relatively featureless,48 linearly sloped,36 and complex49 capacitance-potential curvature in IL systems. This range of capacitance-potential responses highlights a known unknown within IL electrochemistry: why do seemingly similar chemical systems display such different physical behaviors? Answering this question will allow significant progress towards uniting theoretical predictions with experimental results. One complicating factor in measuring and comparing capacitance-potential data is the presence of hysteresis, where successive curves do not retrace themselves.44, 50-53 Capacitive hysteresis in IL systems is thought to have several origins (e.g. potential scan direction, electrode surface variability, and measurement technique / method). However, these hysteresis effects are not exclusive to IL systems, as aqueous-based systems also demonstrate54 this phenomenon. An early report of capacitive hysteresis in ILs was from Lockett et al. for the 1-methyl-3hexylimidazolium chloride – glassy carbon electrode interface, which established a high level of hysteresis when scanning from negative-to-positive potentials compared to scanning from the open circuit potential (OCP) with data obtained from single frequency impedance measurements.44 It has been suggested that once the potential is well removed from the OCP it is difficult for the electrochemical system to return to its original, natural OCP state.50 Roling and coworkers reported a change from a local minimum to a local maximum in the 1-ethyl-3methylimidazolium tris(pentafluoroethyl)-trifluorophosphate (Emim FAP) – Au(111) interface when changing from positive to negative potential scan directions with EIS.51 However, work from our laboratory55 examining the Emim FAP – polycrystalline Au interface suggests that capacitive hysteresis need not be a general feature of ILs at an electrified interface. Even though our understanding of capacitive hysteresis at the IL – electrode interface is just emerging, the collective capacitance-potential behavior suggests the IL – BDD electrode interface is inherently different than that at the IL – metal electrode interface, and this difference is the focus of our present study. In our earlier work,13 the benefits of using large amplitude Fourier transformed alternating current voltammetry (FT-ACV) to measure capacitive currents in ILs were demonstrated. Specifically, we found minimal capacitive hysteresis at the Bmim BF4 – polycrystalline Au electrode interface based on the potential scan direction.13 When using large amplitude (e.g. 50 to 200 mV) single frequency perturbations, the FT-ACV power spectrum highlights well-resolved, individual harmonic components. The fundamental (first) harmonic (radial frequency, ω) provides details on both capacitive and faradaic currents present, while higher-order AC harmonics (e.g. 4ω, 5ω, etc.) are ideally devoid of charging current and 3 ACS Paragon Plus Environment


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highlight any faradaic processes present or non-ideal capacitive behavior. This allows the nonfaradaic, capacitive current to be probed. In this report, we describe capacitance-potential relationships derived from FT-ACV measurements for three aprotic and two protic ILs at a polycrystalline BDD electrode over an applied potential range (≥2 V) that does not reveal any faradic current. We contextualize the capacitance-potential behavior and capacitive hysteresis effects relative to existing literature, and highlight the benefits of large amplitude FT-ACV as a probe of electrochemical capacitance in ILs. The ferrocene (Fc/Fc+) redox couple is used to quantify the reference potential scale so that data can be easily compared and reproduced in other laboratories. Experimental Section Materials: The aprotic ILs in this study are 1-butyl-3-methylimidazolium tetrafluoroborate (Bmim BF4), 1-butyl-3-methylimidazolium hexafluorophosphate (Bmim PF6), and 1-butyl-3methylimidazolium methylsulfate (Bmim MeSO4) (IoLiTec Ionic Liquids Technologies GmbH). The protic ILs propylammonium nitrate (PAN) and triethylammonium bis (trifluoromethylsulfonyl)imide (NH222 TFSI) are synthesized in-house via previously reported methods.56 Excess solvent in the protic ILs is removed via rotary evaporation and the freshly synthesized protic ILs are placed under vacuum at 40 °C overnight for further drying. Ferrocene (Fc, Sigma-Aldrich, ≥98 %) is used to calibrate the potential of the platinum quasi-reference electrode. The residual water content in the ILs, prior to degassing with dry nitrogen, is determined in duplicate using a Metrohm 831 Karl Fischer titrator with 300 µL of IL sample. A BAS Epsilon EC-2000-XP electrochemical workstation is used to carry out the direct current (DC) voltammetric measurements. Large amplitude Fourier transformed alternating current voltammetry (FT-ACV) measurements are performed using purpose-built instrumentation described elsewhere.57 All experiments are conducted at 20 ± 2 °C. A three-electrode arrangement with a 2.5 mL conical glass vial with a custom sealed plastic cap serves as the electrochemical cell. The cell is cleaned by rinsing with copious amounts of high purity water (Milli-Q water purification system), followed by acetone rinsing, and dried thoroughly with high purity nitrogen gas. The BDD working electrode (Windsor Scientific Ltd, Slough Berkshire, UK) is a macrodisk electrode (nominal diameter = 3.0 mm) with an effective surface area of 0.07 cm2, sealed in Teflon. The BDD has been grown using chemical vapor deposition, is polycrystalline and is doped with sufficient boron to show metal-like behavior.58-59 The BDD electrode is polished with an aqueous slurry of 0.3 µm MicroPolish II aluminum oxide powder (Buehler) on microcloth pads (Buehler), sonicated for 5 minutes in high purity water, rinsed with copious amounts of water, and dried with nitrogen gas before use. Platinum wires serve as both the auxiliary and quasi-reference electrodes. These wires are cleaned by rinsing with copious amounts of water and acetone and dried with nitrogen gas prior to placing into the electrochemical cell.

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Methods: The platinum quasi-reference electrode is calibrated at the end of data collection by adding ferrocene to the IL and using the midpoint potential of the ferrocene/ferrocenium (Fc/Fc+) reversible couple derived from DC cyclic voltammetry scaled to 0.000 V. Before each electrochemical experiment, the IL is dried by vigorous purging with nitrogen gas for 1 hour after which a nitrogen blanket is maintained in the electrochemical cell headspace above the solution throughout experiments. The water content prior to nitrogen degassing can be as high as 7000 ppm (0.7 % water), however, extended periods of degassing with nitrogen lowers this to the 100 to 200 ppm range. All DC and AC voltammetric measurements are recorded in triplicate and data are reported as a mean with a standard deviation. Potential regions of primarily double layer charging current are determined first using DC cyclic voltammetry at a scan rate of 100 mV s-1. This helps identify a potential window devoid of significant faradaic contributions to the current (e.g. oxide formation, solvent or impurity redox chemistry). The purely capacitive nature of the potential window is supported by the absence of higher-order harmonic FT-ACV components. The large amplitude FT-ACV measurements use a sinusoidal perturbation with an amplitude (∆Eamp) of 80 mV and at a frequency ( f ) of 9.0 Hz, 55 Hz, 207 Hz, 607 Hz and 1017 Hz. The scan rates (ν) used are 78.20 mV s-1 (PAN), 93.10 mV s-1 (NH222 TFSI), 78.20 mV s-1 (Bmim MeSO4), 74.50 mV s-1 (Bmim BF4), and 74.50 mV s-1 (Bmim PF6). Details of our FT-ACV data treatment are published13 in our earlier work. Briefly, the capacitance is derived from a method based on analysis of the fundamental harmonic component from FT-ACV data,60 which is assumed to contain purely double layer charging current. This calculation utilizes a model of the capacitance as a simple equivalent circuit consisting of a solution resistance (Rs) in series with a double layer capacitance (Cdl) at each potential. The capacitance-potential data are generated and fitted60 with a polynomial equation (Equation 1). Equation 1: Polynomial Capacitance Equation Equation 1 describes the potential dependence of a non-linear capacitor61 where Cj represents the jth polynomial capacitance coefficient that applies for a given frequency and is generated from the optimized fit using software available with the FT-ACV instrument as described in reference 60 but converted to the Fc/Fc+ potential scale. Results and Discussion The chemical structures of each of the five ionic liquids (IL) used in this study are provided in Table 1, along with literature values62 of viscosity and conductivity at 25 °C, in addition to molar ion concentrations. We have included information for 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Bmim TFSI) in Table 1 for relevant discussion purposes. We note that the ion concentrations are calculated from molar mass and density of the respective IL, and hence only provide an approximation of the true ionicity.63 The first two ILs listed in the table are protic, ammonium-based ILs, while the other four are aprotic ILs that share a common 1-butyl-3-methylimidazolium cation. DC and AC Voltammetric Data: BDD electrodes generally show an extended potential window compared to metal and carbon based electrodes in aqueous electrolytes.58-59 Yet, the 5 ACS Paragon Plus Environment


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useable potential range still depends on several conditions such as pH, BDD surface termination and amount of non-diamond carbon (e.g. sp2 graphitic carbon). On BDD electrodes there are a lack of binding sites to mediate electron transfer for water electrolysis (HER) and oxygen reduction (ORR) pointing to the inertness of BDD electrode surfaces, however, these reactions become more favorable in the presence of sp2 carbon impurities.58 The DC potential windows for each of the five ILs examined in this study are displayed in Figure 1. They range from 4.5 V (Bmim BF4) > 4.4 V (Bmim PF6) > 4.2 V (NH222 TFSI) > 3.7 V (Bmim MeSO4) > 3.2 V (PAN). All cyclic voltammograms show minimal features outside of the faradaic current associated with the oxidation and reduction limits. There is no evidence of the oxygen reduction reaction in the cathodic regime, which is expected given the extensive nitrogen gas purging used prior to electrochemical measurements. There are a few small oxidative features around 0.75 V and 1.2 V vs. Fc/Fc+ in PAN and around 1.25 V and 1.6 V vs. Fc/Fc+ in Bmim PF6 possibly due to a small level of sp2 graphitic carbon (impurity) material on the BDD electrode surface (refer to Figure S1). In aqueous electrolyte, these features have been identified to be a consequence of oxidation of sp2 carbon in BDD electrodes.58-59 Only two of the ILs studied show these features but it also should be noted that these oxidative features can be seen on metal electrodes in ILs which typically point to residual water or other impurities in the IL.13, 64 However, together these characteristics suggest the BDD electrode surface is of relatively good quality and nearly inert. The data for the cathodic and anodic potential limits for the five ILs used in this study are summarized in Table S1. While there is no formal method for defining the IL electrochemical potential window, in practice a current density cut-off in the range of 1 to 5 mA cm-2 is often used to define the limits.65 In this spirit we used the method by Roling and coworkers, where they suggest use of a geometric scheme with parallel lines to the rising RED/OX currents to define the limits.49 One often overlooked component, however, with respect to the electrochemical solvent window in ILs is do the RED/OX limits correspond to simple electrolyte breakdown or are there other underlying processes? Howlett et al. demonstrated that the [TFSI]anion commences in a series of reductive reactions beginning around -2 V vs. Fc/Fc+, well before reported cathodic limits for [TFSI]-containing ILs.66 These findings were later corroborated by molecular dynamics simulations67 and these researchers also found the [Bmim]+ cation to be less stable against oxidation than the respective anions in Bmim BF4 and Bmim PF6. Kroon et al. analyzed the electrochemically generated decomposition products of Bmim BF4 and found that 1-butyl-3-methylimidazolium radicals are formed that react with each other in radicalradical coupling and disproportionation reactions.68 These studies suggest that the electrochemical decomposition can be more complex than simple cation/anion breakdown in the cathodic/anodic potential limits. In the present work the cathodic limits for imidazolium-based ILs are similar but slightly more negative than the ammonium-based ILs. The stability against oxidation is larger for the more hydrophobic anions. Zhao et al. reported the potential windows of several ammonium-based, protic ILs on BDD and they ranged from 2.13 to 3.03 V.69 While the potential window of PAN (3.2 V) in our study is in agreement with the literature, the results also highlight the significantly larger potential window afforded with NH222 TFSI (4.2 V) on a BDD electrode. Swain and coworkers found the potential window (using a current density cutoff of 40 µA cm-2) of a BDD electrode in Bmim BF4 to be 3.6 V.70 Comparing the same IL (protic or aprotic) we typically find (data not shown) the accessible potential range to be the 6 ACS Paragon Plus Environment

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largest in BDD electrochemical systems, where BDD > GC > metal (Au, Pt, Cu) electrodes. Furthermore, in our experience BDD and GC electrodes provide the ‘cleanest’ electrochemical potential windows (i.e. relatively featureless DC voltammograms) compared to metallic electrodes in the presence of small amounts of adventitious water. Nonetheless, in relation to the present work with BDD, it is likely that discrepancies in assigning potential window limits vary with factors that include variability of the electrode surfaces (i.e. the type of BDD, its carrier concentration, surface preparation, sp2 graphitic carbon content, etc.), and the IL purity. The cyclic voltammograms shown here (Figure 1) are indicative of relatively clean and dry IL solvents. A simple means to judge the capacitive nature of an electrochemical system can be accomplished via analysis of scan rate dependent CVs in a selected potential range devoid of faradaic current. Figure 2a shows scan rate dependent CVs for a BDD electrode in PAN within a 1.1 V potential range (within the larger potential range examined below in FT-ACV experiments). The response has a characteristic rectangular shape principally ascribed to non-faradaic charging current with larger magnitudes in the positive region. A stacked plot of all five ILs on a BDD electrode as used in this study is provided in Figure S2 and all demonstrate a rectangular response with very low currents ≤1 µA cm-2 at 100 mV s-1. Utilizing the scan rate dependent CVs, a log i (units of A cm-2) versus log ν (units of V s-1) plot (at a carefully selected potential) has a slope of unity for a purely capacitive system.24 A representative log i – log ν plot is shown for a BDD electrode in PAN in Figure 2b and demonstrates an excellent linear relationship over the entire range with a slope of 0.91 which corresponds to a double layer capacitance (Cdl) of 8.1 µF cm-2. The compiled analysis for all five ILs used in this study from these plots are provided in Table 2 and demonstrate slopes ranging from between 0.91 to 0.97 (R2 = 0.999) and Cdl values from ~8 to 9 µF cm-2. These results are in good agreement with the literature in aqueous electrolyte.32-33, 59, 71 In our FT-ACV EDL studies, a smaller potential range, compared to the full electrochemical window, was examined to minimize faradaic current contributions from any AC or DC reduction or oxidation processes. Even with this conservative approach, the BDD electrode allows a relatively large EDL potential range to be explored: 2.5 V (NH222 TFSI), 2.1 V (PAN), 2.1 V (Bmim MeSO4), 2.0 V (Bmim BF4), and 2.0 V (Bmim PF6) in FT-ACV measurements, which should be amenable to analysis in terms of a simple RC circuit model. An expanded results and discussion section for the FT-ACV data work-up and analyses, similar to our previous work,13 are provided in the Supporting Information section. Briefly, the total current-time data (see Figure S3) are relatively featureless and symmetric about the switching potential (t = 27 s). The power spectra (refer to Figure S4) illustrate the major contributor in all cases here is the fundamental harmonic component at 9 Hz. As a representative example, the magnitudes of the fundamental through fifth harmonic components for a BDD electrode in PAN show (see Figure S5) that the third harmonic is close to zero and no fourth or fifth harmonic signal is detected. Overall, these metrics demonstrate the electrochemical systems examined in the present work are behaving similar to an ideal capacitor and justify the use of an RC circuit to model the data. Other studies using electrochemical impedance spectroscopy (EIS),50 single-



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frequency impedance,8, 46, 52 AC voltammetry,53 and scan rate dependent DC cyclic voltammetry29, 69 have also used an RC circuit to model the interfacial capacitance in IL solvents. Capacitance-Potential Behavior: One feature of BDD electrodes is their relatively small charging currents compared to metal and carbon based electrode materials, which is reflected in their capacitance-potential relationships.32, 71 The capacitance-potential plots for all five ILs at 9 Hz generated from a fit to Equation 1 are provided in Figure 3 and generally display an increase in capacitance as the potential becomes more positive. Refer to Figure S6 for an overlay plot of these capacitance-potential curves. Capacitance values range from 3 to 8 µF cm-2 in good agreement with the values extracted from the DC voltammograms in the previous section. The low background currents and capacitance values observed on BDD electrodes have been hypothesized to result from two main factors. Namely, the relative absence of electroactive surface groups and a lower density of states.33 With respect to the shape of the capacitancepotential curvature, the rising response from negative to positive potentials could be a result of more charge carriers becoming available at progressively higher anodic potentials. The capacitance curvature also suggests that there are minimal interactions (e.g. adsorption) with IL ions again likely due to the inert nature of the BDD electrode material. It should be noted that we have measured five different frequencies with the FT-ACV technique and find (data not shown) the capacitance-potential curvature to be the same but the capacitance magnitude exhibits frequency dispersion by decreasing with increasing frequency (in the frequency range examined here) analogous to our previous13 study. Swain and coworkers used single-frequency (10 Hz) impedance measurements with a hydrogen-terminated nanocrystalline BDD film (0.5 to 1 µm thick) electrode (carrier concentration ~1021 B atoms cm-3) grown on quartz to study the capacitance-potential dependence in Bmim BF4.29 They report a constant capacitance (~0.73 µF cm-2) region from -1.0 to +0.1 V vs. Ag and a rising capacitance (~0.73 to 1.2 µF cm-2) region from +0.1 to +1.0 V vs. Ag.29 The authors attribute the order of magnitude lower capacitance found relative to aqueous electrolytes to the lower dielectric constant of the IL (i.e. 80 in water versus 14.5 in Bmim BF4). For reference, our Bmim BF4 capacitance-potential data increase from (~3.9 to 6.4 µF cm-2) for the potential range -1.18 to +0.82 V vs. Fc/Fc+. The principal difference between the BDD electrode used by Swain and coworkers compared to the one used in our study is the surface termination (Swain and coworkers used a hydrogen-terminated BDD electrode, whereas the one used in our study is oxygen-terminated). Hydrogen surface termination is considerably more hydrophobic compared to an oxygen terminated BDD electrode and this could significantly affect surface interactions. Researchers have demonstrated that hydrogen-terminated BDD electrodes typically give lower capacitive currents compared to oxygen-terminated BDD electrodes but increasing the boron content (doping density) acts to increase the measured capacitance in aqueous electrolye.59, 72 Nonetheless, while our capacitance data for the Bmim BF4 – BDD electrode interface is higher than in data from Swain and coworkers, the capacitance-potential curvature features are in qualitative agreement. A report by Cannes et al. describes a linearly sloped capacitance-potential plot that increases from 25 to 40 µF cm-2 over a 2.3 V range as the potential is swept from positive to negative for the Bmim TFSI – BDD (hydrogen-terminated) electrode (carrier concentration ~1021 B atoms cm-3) interface.36 It is interesting that Cannes et al. report a qualitatively different type of 8 ACS Paragon Plus Environment

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response in which the capacitance-potential trend is inverted relative to our work and other IL studies. While our work did not include studies with the Bmim TFSI IL, we have data from three aprotic ILs with a [Bmim]+ cation and one protic IL with a [TFSI]- anion, which all provide a similar qualitative capacitance-potential response (i.e. increasing from negative to positive potentials). The origin of this inconsistency is not known, however, we note the use a more complex equivalent circuit to model their capacitance data. Macpherson and coworkers58-59 examined in detail the factors that affect the electrochemical performance of oxygen-terminated polycrystalline BDD electrodes, in aqueous solutions. The impact of electrode features such as surface termination, boron dopant density, sp2 content and crystallography were emphasized in their work and implies the impact of these features on capacitance-potential curvature data is expected to be significant, making direct comparisons of data in different studies difficult. Furthermore, a major uncertainty in data published so far is the wide diversity of reference potential scales used, making direct comparisons of data challenging. We hope adoption of the Fc/Fc+ scale will assist in making it possible to properly compare results from different laboratories. Another important feature of the capacitance-potential data shown in Figure 3 is the very minimal hysteresis found with respect to the scan direction (positive versus negative) in all five ILs, even when scanning potential ranges in excess of 2 V. Swain and coworkers also report little capacitive hysteresis at the Bmim BF4 – BDD electrode interface.29 Thus, it appears that in addition to displaying little dependence on the nature of the IL, the BDD electrode specifically is not plagued by capacitive hysteresis within these potential ranges. However, it should be noted that most published IL capacitance data examine only a single potential scan direction. Some studies on metallic electrodes report significant capacitive hysteresis with respect to the potential scan direction 44, 50-53, 73 but the technique / protocol used to collect capacitance data could also play a role.53, 74 Our prior studies on the Bmim BF4 – polycrystalline Au electrode interface using FT-ACV display only a small level of capacitive hysteresis as well.13 Where capacitive hysteresis has been demonstrated in IL electrochemical studies, its origin is still unknown. Surface reconstruction,18, 51 ion-electrode interactions,50, 52 and slow re(organization) / movement of the interfacial IL structure51, 53, 73, 75-76 have all been proposed to account for capacitive hysteresis. The slow relaxation of the EDL is hypothesized to be a general feature of ILs at electrified interfaces.76 Additionally, it has been suggested that capacitancepotential data greatly depend on the initial potential, as well as the potential scan rate because reorganization processes in ILs are quite slow.50 Interestingly, our FT-ACV measurements of the IL – electrode interface do not show significant capacitive hysteresis traits. Traditional broadband EIS studies of the IL – electrode interface can take several hours,73 which encompasses timeframes noted for slow (minutes to hours) processes to occur.18, 49, 77-78 Our FT-ACV measurements take Bmim BF4 (1.25 ± 0.08 µF cm-2 V-1) > Bmim PF6 (1.09 ± 0.23 µF cm-2 V-1) > NH222 TFSI (0.98 ± 0.02 µF cm-2 V-1). For comparison, a rough estimate from the Bmim BF4 – BDD electrode work by Swain and coworkers29 over the sloped, linear portion of their capacitance data (i.e. 0 V to +1.0 V vs. Ag wire) suggests a slope of ~0.3 11 ACS Paragon Plus Environment


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µF cm-2 V-1. As noted above, these authors report an order of magnitude lower capacitance values for their Bmim BF4 – BDD electrode system. The Bmim TFSI – BDD electrode interface studied by Cannes et al. displays a ~5 µF cm-2 V-1 increase from positive to negative potentials (cathodic direction).36 Interestingly, the sloped linearity of capacitance-potential curves in aqueous and IL electrolytes is not exclusive to a BDD electrode. Silva and coworkers published relatively featureless capacitance curves for polycrystalline Au and Pt electrodes in imidazoliumbased ILs48 and our studies of the Emim FAP – polycrystalline Au electrode interface55 displays a sloped linear relationship with capacitance increasing by ~1.5 µF cm-2 V-1 from positive to negative (cathodic direction) potentials over a 2 V range. These sloped linear capacitancepotential relationships represent a challenge in uniting a myriad of experimental observations with theoretical predictions for IL capacitance behavior. Mott-Schottky Analysis: The traditional method of analysis used for semiconductor electrodes is based on a Mott-Schottky plot. While the BDD electrode used in the present study is a p-type material that has been degenerately doped above the metallic conduction threshold (i.e. > 3×1020 B atoms cm-3),58-59 we reveal the electrochemical system still can retain semiconductor-like characteristics. However, applying Mott-Schottky principles to a highly doped BDD electrode should be done with caution as the potential drop no longer occurs within the semiconductor material but rather on the solution side. Therefore, the below Mott-Schottky analysis is solely provided as an academic curiosity. Mott-Schottky plots (and ultimately capacitance-potential plots) are well known85-87 to exhibit frequency dispersion (i.e. frequency dependence). Typically frequency values ≥1 kHz are used when making a Mott-Schottky plot to extract carrier concentrations and flat-band potentials. Therefore, we have used our FT-ACV data acquired at 1017 Hz to construct our Mott-Schottky analyses. A representative Mott-Schottky plot of C-2 (in µF-2 cm4) versus potential (in V vs. Fc/Fc+) for a BDD electrode in PAN is illustrated in Figure 5b and does not exhibit the linear relationship over the entire EDL potential range, as found for the capacitance-potential data (Figure 5a). There are several factors that can affect the shape of Mott-Schottky plots such as surface termination, carrier concentration, measurement frequency, equivalent circuit used to model data.72 Additionally, the assignment of linearity in these plots is somewhat ‘user defined’ and the literature is far from consistent. Nonetheless, in general Mott-Schottky plots in aqueous electrolyte media exhibit linearity over a range of potentials from as little as ~0.4 V35, 86-88 to as high as ~2 V,36 where the flat-band potential (EFB) and free carrier concentration (NA) can be estimated from extrapolation to the potential axis (i.e. zero capacitance) and using the slope, respectively. Equation 2 is used to estimate NA from the slope [d(C-2)/dE] of the linear portion in the Mott-Schottky plots, where ε is the dielectric constant89 of the BDD (5.7), ε0 is the permittivity of free space (8.85 × 10-14 F cm-1), and e is the electron charge (1.602 × 10-19 C). Equation 2: Mott-Schottky equation The flat-band potential is the potential at which the electrostatic potential is constant throughout the semiconductor (i.e. no excess charge exists in the semiconductor and it starts to behave like a metal) and is equivalent to the point of zero charge, akin to that in metallic electrode systems.24 Figure 5b suggests a flat-band potential of 1.1 ± 0.1 V vs. Fc/Fc+ with a free carrier 12 ACS Paragon Plus Environment

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concentration of 1.0 ± 0.1×1021 B atoms cm-3 for our BDD electrode in PAN. Compiled MottSchottky analyses are provided in Table 4 for all five ILs. The linear portions (noted in parentheses) for all of the ILs span 0.8 to 1.3 V, representing substantial portions of the EDL potential ranges examined. The extrapolated flat-band potentials range from 0.9 to 1.7 V vs. Fc/Fc+, which are consistent with literature reports72, 88, 90 in aqueous electrolytes from similarly doped BDD electrodes. To our knowledge, the only other flat-band potential reported in the literature (3.0 V vs. Ag/Ag+) for an IL electrolyte was by Cannes et al. for the Bmim TFSI – BDD (3×1021 B atoms cm-3) electrode interface.36 The authors justify their very positive flatband potential to be due to the presence of a thin dielectric layer on the diamond surface. Finally, the carrier concentrations reported for our BDD electrode are on the order of ~8×1020 B atoms cm-3 and show a minor dependence on the IL in contact with the BDD electrode surface. Certainly the simple Mott-Schottky analysis applied here could overestimate these parameters yet it is interesting that our results are in qualitative agreement with expectations from true semiconducting electrodes materials. The fact that our IL capacitance-potential data matches well with previous data in aqueous electrolyte suggests that the BDD electrode shows only slight dependence on the nature of the electrolyte, which also has previously been reported32 in extensive studies in a range of aqueous electrolytes. It is intriguing that the BDD electrode behaves as a metal with respect to faradaic processes but has characteristics of a semi-conductor with respect to the charging current response. The boron content in the electrode is not uniform, so perhaps the origin of this dual metallic-semi-conductor behavior lies in the heterogeneous nature of the material. Thus, overlap of diffusion layers allows the faradaic response, which will be dominant at boron rich areas, to mimic that excepted for planer diffusion while the capacitance response retains features expected from the low boron semiconducting regions. Conclusions This report describes the capacitance-potential relationships for five ILs on BDD electrodes using large amplitude FT-ACV. Simple RC circuit models provide high quality fits to our data, allowing excellent analysis and interpretation. The capacitance-potential data, collected over ≥2 V potential ranges, exhibit relatively linear but increasing capacitance values (1 to 2 µF cm-2 V-1) from negative to positive potentials. We observe very little dependence on the IL composition in capacitance-potential data and values range from 3 to 8 µF cm-2. Our data on BDD electrodes do not create responses that match theoretical models for ILs derived for metallic (conducting) electrodes. Rather, our results advise that the capacitance-potential relationships of BDD in an IL mimic those found in traditional aqueous electrolyte media suggesting a media-independent response of the BDD electrode which we find intriguing. Simple Mott-Schottky analysis of the FT-ACV generated capacitance data is provided and further supports the finding that IL responses are similar to those in aqueous electrolytes. Importantly, capacitive hysteresis is not a major feature of data acquired here, using FT-ACV at a 100 mV s-1 sweep rate and a perturbation potential of 80 mV. Overall, our work with large amplitude FT-ACV provides an excellent method for probing the capacitive current and potential dependence in IL media. Associated Content Supporting Information 13 ACS Paragon Plus Environment


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Zoomed in region of anodic regions in DC potential windows for all five ILs (Figure S1), compiled data from CV potential windows (Table S1), DC CVs demonstrating ‘rectangular shape’ (Figure S2), expanded discussion and results for large amplitude FT-ACV data analysis, total current (AC plus DC) versus time plots (Figure S3), frequency-domain power spectra plots (Figure S4), representative plot of the first through fifth harmonic components derived from FTACV measurements for PAN at a BDD electrode (Figure S5), and an overlay of the capacitancepotential curves for the five ILs used in this study (Figure S6). The Supporting Information is available free of charge … Author Information Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgements AJL gratefully acknowledges financial support provided to him for this project by the National Science Foundation East Asia and Pacific Summer Institutes (NSF-EAPSI, award #1514836) and the T. Anne Cleary International Dissertation fellowships. AJL also expresses sincere gratitude to the Australian Academy of Science and the Monash Electrochemistry group for hosting his three-month research visit that enabled the FT-ACV experiments described in this paper to be undertaken. The authors thank Prof. Julie Macpherson (Department of Chemistry, University of Warwick) for helpful discussion and Cameron Bentley for assistance in synthesizing the protic ionic liquids at CSIRO. This work was also financially supported by the ACS-PRF and Iowa Energy Center under awards 55279-DNI5 and OG-15-002, respectively and the Australian Research Council. References 1. Krossing, I.; Slattery, J. M.; Daguenet, C.; Dyson, P. J.; Oleinikova, A.; Weingaertner, H., Why Are Ionic Liquids Liquid? A Simple Explanation Based on Lattice and Solvation Energies. J. Am. Chem. Soc. 2006, 128, 13427-13434. 2. Zhang, S.; Sun, N.; He, X.; Lu, X.; Zhang, X., Physical Properties of Ionic Liquids: Database and Evaluation. J. Phys. Chem. Ref. Data 2006, 35, 1475-1517. 3. MacFarlane, D. R., et al., Ionic Liquids and Their Solid-State Analogues as Materials for Energy Generation and Storage. Nature Reviews Materials 2016, 1, 15005. 4. MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D.; Davis, J. H.; Watanabe, M.; Simon, P.; Angell, C. A., Energy Applications of Ionic Liquids. Energy Environ. Sci. 2014, 7, 232-250. 5. Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B., Ionic-Liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621-629. 6. Baldelli, S., Interfacial Structure of Room-Temperature Ionic Liquids at the Solid-Liquid Interface as Probed by Sum Frequency Generation Spectroscopy. J. Phys. Chem. Lett. 2013, 4, 244-252. 14 ACS Paragon Plus Environment

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Evaluation of Resistance, Capacitance and Faradaic Current by Fourier Transform Ac Voltammetry. J. Solid State Electrochem. 2008, 12, 1301-1315. 61. Kennedy, G. F. B., Alan M.; Simonov, Alexandr N., Modelling Ac Voltammetry with Mecsim: Facilitating Simulation-Experiment Comparisons. Current Opinion in Electrochemistry 2017, 1, 140-147. 62. Zhang, S.; Lu, X.; Zhou, Q.; Li, X.; Zhang, X.; Li, S., Ionic Liquids: Physicochemical Properties; Elsevier: Oxford, UK, 2009. 63. Tokuda, H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M., How Ionic Are RoomTemperature Ionic Liquids? An Indicator of the Physicochemical Properties. J. Phys. Chem. B 2006, 110, 19593-19600. 64. Zhao, C.; Bond, A. M.; Lu, X., Determination of Water in Room Temperature Ionic Liquids by Cathodic Stripping Voltammetry at a Gold Electrode. Anal. Chem. (Washington, DC, U. S.) 2012, 84, 2784-2791. 65. O'Mahony, A. M.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G., Effect of Water on the Electrochemical Window and Potential Limits of Room-Temperature Ionic Liquids. J. Chem. Eng. Data 2008, 53, 2884-2891. 66. Howlett, P. C.; Izgorodina, E. I.; Forsyth, M.; MacFarlane, D. R., Electrochemistry at Negative Potentials in Bis(Trifluoromethanesulfonyl)Amide Ionic Liquids. Z. Phys. Chem. (Muenchen, Ger.) 2006, 220, 1483-1498. 67. Ong, S. P.; Andreussi, O.; Wu, Y.; Marzari, N.; Ceder, G., Electrochemical Windows of RoomTemperature Ionic Liquids from Molecular Dynamics and Density Functional Theory Calculations. Chem. Mater. 2011, 23, 2979-2986. 68. Kroon, M. C.; Buijs, W.; Peters, C. J.; Witkamp, G.-J., Decomposition of Ionic Liquids in Electrochemical Processing. Green Chem. 2006, 8, 241-245. 69. Zhao, C.; Burrell, G.; Torriero, A. A. J.; Separovic, F.; Dunlop, N. F.; MacFarlane, D. R.; Bond, A. M., Electrochemistry of Room Temperature Protic Ionic Liquids. J. Phys. Chem. B 2008, 112, 6923-6936. 70. Kim, D. Y.; Yang, J. C.; Kim, H. W.; Swain, G. M., Heterogeneous Electron-Transfer Rate Constants for Ferrocene and Ferrocene Carboxylic Acid at Boron-Doped Diamond Electrodes in a Room Temperature Ionic Liquid. Electrochim. Acta 2013, 94, 49-56. 71. Swain, G. M.; Ramesham, R., The Electrochemical Activity of Boron-Doped Polycrystalline Diamond Thin-Film Electrodes. Analytical Chemistry 1993, 65, 345-351. 72. Zivcova, Z. V.; Petrak, V.; Frank, O.; Kavan, L., Electrochemical Impedance Spectroscopy of Polycrystalline Boron Doped Diamond Layers with Hydrogen and Oxygen Terminated Surface. Diamond Relat. Mater. 2015, 55, 70-76. 73. Drueschler, M.; Huber, B.; Passerini, S.; Roling, B., Hysteresis Effects in the Potential-Dependent Double Layer Capacitance of Room Temperature Ionic Liquids at a Polycrystalline Platinum Interface. J. Phys. Chem. C 2010, 114, 3614-3617. 74. Small, L. J.; Wheeler, D. R., Influence of Analysis Method on the Experimentally Observed Capacitance at the Gold-Ionic Liquid Interface. J. Electrochem. Soc. 2014, 161, H260-H263. 75. Uysal, A., et al., Structural Origins of Potential Dependent Hysteresis at the Electrified Graphene/Ionic Liquid Interface. J. Phys. Chem. C 2014, 118, 569-574. 76. Yasui, Y.; Kitazumi, Y.; Mizunuma, H.; Nishi, N.; Kakiuchi, T., A Comparison of the Ultraslow Relaxation Processes at the Ionic Liquid|Water Interface for Three Hydrophobic Ionic Liquids. Electrochem. Commun. 2010, 12, 1479-1482. 77. Chu, M.; Miller, M.; Douglas, T.; Dutta, P., Ultraslow Dynamics at a Charged Silicon-Ionic Liquid Interface Revealed by X-Ray Reflectivity. J. Phys. Chem. C 2017, 121, 3841-3845. 78. Roling, B.; Drueschler, M.; Huber, B., Slow and Fast Capacitive Process Taking Place at the Ionic Liquid/Electrode Interface. Faraday Discuss. 2012, 154, 303-311.


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79. Parr, D.; Chrestenson, J.; Malik, K.; Molter, M.; Zibart, C.; Egan, B.; Haverhals, L. M., Structure and Dynamics at Ionic Liquid/Electrode Interfaces. ECS Transactions 2015, 66, 35-42. 80. Malik, M. K.; Chrestenson, J. R.; Parr, D.; Gray, D.; Mitiku, H.; Kahila, T.; Malinowski, A.; Sánchez, J.; Haverhals, L. M., Probing Ionic Liquid/Electrode Interfaces by Hyperspectral Imaging. ECS Transactions 2016, 75, 545-553. 81. Patten, H. V.; Meadows, K. E.; Hutton, L. A.; Iacobini, J. G.; Battistel, D.; McKelvey, K.; Colburn, A. W.; Newton, M. E.; MacPherson, J. V.; Unwin, P. R., Electrochemical Mapping Reveals Direct Correlation between Heterogeneous Electron-Transfer Kinetics and Local Density of States in Diamond Electrodes. Angew. Chem., Int. Ed. 2012, 51, 7002-7006, S7002/1-S7002/15. 82. Rao, T. N.; Yagi, I.; Miwa, T.; Tryk, D. A.; Fujishima, A., Electrochemical Oxidation of Nadh at Highly Boron-Doped Diamond Electrodes. Anal. Chem. 1999, 71, 2506-2511. 83. Kornyshev, A. A.; Luque, N. B.; Schmickler, W., Differential Capacitance of Ionic Liquid Interface with Graphite: The Story of Two Double Layers. J. Solid State Electrochem. 2014, 18, 1345-1349. 84. Fedorov, M. V.; Lynden-Bell, R. M., Probing the Neutral Graphene-Ionic Liquid Interface: Insights from Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2012, 14, 2552-2556. 85. Pleskova, Y. V.; Evstefeeva, Y. E.; Krotova, M. D.; Elkin, V. V.; Mazin, V. M.; Mishuk, V. Y.; Varnin, V. P.; Teremetskaya, I. G., Synthetic Semiconductor Diamond Electrodes: The Comparative Study of Electrochemical Behavior of Polycrystalline and Single Crystal Boron-Doped Films. J. Electroanal. Chem. 1998, 455, 139-146. 86. Pleskov, Y. V.; Elkin, V. V.; Abaturov, M. A.; Krotova, M. D.; Mishuk, V. Y.; Varnun, V. P.; Teremetskaya, I. G., Synthetic Semiconductor Diamond Electrodes: Elucidation of the Equivalent Circuit for the Case of Frequency-Dependent Impedance. J. Electroanal. Chem. 1996, 413, 105-110. 87. Pleskov, Y. V.; Mishuk, V. Y.; Abaturov, M. A.; Elkin, V. V.; Krotova, M. D.; Varnin, V. P.; Teremetskaya, I. G., Synthetic Semiconductor Diamond Electrodes: Determination of Acceptor Concentration by Linear and Non-Linear Impedance Measurements. J. Electroanal. Chem. 1995, 396, 227-32. 88. Latto, M. N.; Riley, D. J.; May, P. W., Impedance Studies of Boron-Doped Cvd Diamond Electrodes. Diamond Relat. Mater. 2000, 9, 1181-1183. 89. Spear, H. E.; Dismukes, J. P., Synthetic Diamond: Emerging Cvd Science and Technology; J. Wiley & Sons: New York, 1994. 90. Suffredini, H. B.; Pedrosa, V. A.; Codognoto, L.; Machado, S. A. S.; Rocha-Filho, R. C.; Avaca, L. A., Enhanced Electrochemical Response of Boron-Doped Diamond Electrodes Brought on by a Cathodic Surface Pre-Treatment. Electrochim. Acta 2004, 49, 4021-4026.



+ +

+ or

or

Boron Doped Diamond FT-ACV Cdl (μF cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

E (V vs. Fc/Fc+)

TOC Graphic

1

ACS Paragon Plus Environment

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


௠

‫ܥ‬ௗ௟ ሺ‫ܧ‬ሻ ൌ ෍ ‫ܥ‬௝ ‫ ܧ‬െ ‫ܧ‬஼ௗ௟

௠

௝ୀ଴

Equation 1.


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Ionic Liquid Structure

O

Symbols

F3C

Viscosity (mPa s)*

Ion Conductivity Concentration (S m-1)* (mol L-1)

PAN

67

---

9.5

NH222 TFSI

48

0.44

3.7

Bmim MeSO4

180

---

4.8

Bmim BF4

180

---

5.3

Bmim PF6

312

0.15

4.8

Bmim TFSI

55

0.41

3.4

O

N S

S

NH

Page 22 of 31

CF3

O O

O N

N

O

S

O

O

O N

N

F3C

O

N S

S O O

CF3

Table 1. Chemical structures, symbols, viscosity, conductivity, and ion concentration of the ionic liquids used in this study (* denotes literature obtained values at 25 °C from ref. 62). Bmim TFSI is included for comparison purposes with previous literature. Note: The ion concentrations are calculated from the molar mass and density of the relevant IL and are only to be used as a guide to compare their ionicity.


3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


Current Density (mA cm-2)

Page 23 of 31

1 0 -1 0.2 0.0 -0.2

(a)

(b)

1 0 -1 0.7 0.0 -0.7 0.08 0.00 -0.08 -3

(c)

(d)

(e)

-2

-1

0

1

2

3

Potential (V vs. Fc/Fc+)

Figure 1. Potential windows available in (a) PAN, (b) NH222 TFSI, (c) Bmim MeSO4, (d) Bmim BF4 and (e) Bmim PF6 with the BDD electrode. The DC scan rate is 100 mV s-1.


4

Page 24 of 31

10 5 0 -5 -10

(a) -0.8

-0.4 0.0 Potential (V vs. Fc/Fc+)

0.4

10-5 Log Current (A cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Current Density ( A cm-2)


10-6 Slope = 0.91 R2 = 0.999 10-7

(b) 10-2

10-1 100 -1 Log Scan Rate (V s )

Figure 2. (a) Representative scan rate dependent CVs at 800, 500, 300, 100, 50 and 20 mV s-1 for BDD electrode in PAN. The black arrow points towards increasing scan rates. (b) Representative plot of log of current versus log of scan rate at -0.3 V vs. Fc/Fc+ for BDD in PAN and its corresponding linear fit. Note: the error bars in (b) are contained within the symbol.

5


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


Cdl (μF cm2)

Slope log(μF cm-2)

R2

PAN

8.1

0.91

0.999

NH222 TFSI

8.6

0.93

0.999

Bmim MeSO4

8.9

0.95

0.999

Bmim BF4

9.3

0.97

0.996

Bmim PF6

8.5

0.93

0.999

IL

Table 2. Capacitance values and linear regression details from log current (A cm-2) vs. log scan rate (V s-1) plots. The data are taken at 0.3 V vs. Fc/Fc+ except for Bmim MeSO4 which is taken at -0.4 V vs. Fc/Fc+.


6


8 (a)

0 8 Capacitance (µF cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Page 26 of 31

(b) 0 8 (c)

0 8

(d)

0 8

(e) 0 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Potential (V vs. Fc/Fc+)

Figure 3. Capacitance versus potential data generated from the FT-ACV fundamental harmonic data using equation 1 for (a) PAN, (b) NH222 TFSI, (c) Bmim MeSO4, and (d) Bmim BF4, and (e) Bmim PF6 with a BDD electrode. Positive (____) and negative (----) scan directions. ΔEamp = 80 mV, f = 9 Hz.


7

Page 27 of 31

25 Au

20 Capacitance (µF cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


15

10

5

BDD

0 -1.5

-1.0 -0.5 0.0 0.5 + Potential (V vs. Fc/Fc )

1.0

Figure 4. Capacitance versus potential data generated from the FT-ACV fundamental harmonic data using equation 1 for Au and BDD electrodes in Bmim BF4. Positive (____) and negative (----) scan directions. ΔEamp = 80 mV, f = 9 Hz.


8


Capacitance (µF cm-2)

12

(a)

10 8 6 4 2

Slope = 1.74 ± 0.06 μF cm-2 V-1 R2 = 0.994 ± 0.003

0.07 0.06 1/Cdl2 (μF-2 cm4)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

0.05 0.04

(b) NA = 1.0 ± 0.1×1021 atoms cm-3 EFB = 1.1 ± 0.1 R2 = 0.996 ± 0.003

0.03 0.02 0.01 0.00 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Potential (V vs. Fc/Fc+)

1.5

2.0

Figure 5. (a) Linear regression analysis of capacitance versus potential data (9 Hz) derived from the fundamental harmonic FT-ACV voltammogram and (b) Mott-Schottky plot from 1017 Hz capacitance data with PAN at a BDD electrode. (____) represent fits to the linear portion and (----) represents an extrapolation to zero capacitance. 9


Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


Slope V-1)

R2

PAN

1.74 ± 0.06

0.994 ± 0.003

NH222 TFSI

0.98 ± 0.02

0.966 ± 0.004

Bmim MeSO4

1.36 ± 0.18

0.952 ± 0.007

Bmim BF4

1.25 ± 0.08

0.988 ± 0.013

Bmim PF6

1.09 ± 0.23

0.963 ± 0.063

Ionic Liquid

(μF

cm-2

Table 3. Summary of statistical parameters for ILs when a linear fit is applied to the entire capacitance versus potential (9 Hz) data for both positive and negative scan directions.


10


0º L F

t ÝÝ4 A @ % ?6 ¤@'

(TXDWLRQ


Page 30 of 31

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


Linear Range (V vs. Fc/Fc+)

EFB (V vs. Fc/Fc+)

PAN

-1.1 to +0.0 (1.1)

1.1 ± 0.1

1.0 ± 0.1 × 1021

0.996 ± 0.003

NH222 TFSI

-1.5 to -0.4 (1.1)

0.9 ± 0.1

8.1 ± 0.7 × 1020

0.997 ± 0.001

Bmim MeSO4

-0.9 to +0.4 (1.3)

1.7 ± 0.2

7.9 ± 0.6 × 1020

0.995 ± 0.004

Bmim BF4

-1.0 to -0.2 (0.8)

1.3 ± 0.4

7.2 ± 0.4 × 1020

0.997 ± 0.001

Bmim PF6

-1.2 to -0.2 (1.0)

1.5 ± 0.4

5.2 ± 0.7 × 1020

0.999 ± 0.001

IL

NA (atoms cm-3)

R2

Table 4. Summary of parameters derived by linear regression analysis of the Mott-Schottky plots from both the positive and negative potential scan direction data obtained at 1017 Hz. The magnitude of the width of the linear region is given in parentheses. The carrier concentrations are calculated using equation 2.


Double-Layer Capacitance at Ionic LiquidâBoron-Doped Diamond

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