Electrochemical performance of microporous and mesoporous activated carbons in neat and diluted 1-ethyl-3-methylimidazolium tetrafluoroborate

Journal of Power Sources 343 (2017) 303e315

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Electrochemical performance of microporous and mesoporous activated carbons in neat and diluted 1-ethyl-3-methylimidazolium tetrafluoroborate Seiji Kumagai a, *, Masaki Hatomi a, Daisuke Tashima b a b

Department of Mathematical Science and Electrical-Electronic-Computer Engineering, Akita University, Tegatagakuen-machi 1-1, Akita, 010-8502, Japan Department of Electrical Engineering, Fukuoka Institute of Technology, Wajiro-higashi 3-30-1, Higashi-ku, Fukuoka, 811-0295, Japan

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Effect of porosity on electrochemical performances of activated carbons.
EMIm$BF4, both neat and diluted with propylene carbonate, for EDLC electrolytes.
Electrochemical impedance spectroscopy with Kang's equivalent circuit model.
Mesoporosity may lead to unstable capacitive and resistive performances.
Lower mobility of ions in mesopores has been related to performance deterioration.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 September 2016 Received in revised form 9 January 2017 Accepted 14 January 2017

1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIm$BF4), neat and diluted with propylene carbonate to 1 mol L1, have been employed as electrolytes of electrical double-layer capacitors (EDLCs). The effects of microporosity and mesoporosity in activated carbon (AC) electrodes on the capacitive and resistive performances upon the use of neat and diluted EMIm$BF4 have been explored. In addition to cyclic voltammetry and galvanostatic chargeedischarge tests, electrochemical impedance spectroscopy has been performed employing Kang's equivalent circuit model consisting of three resistances, three constant phase elements, and one bounded Warburg impedance. The overall impedance of the EDLC cell was separated into components of intrinsic resistance, bulk electrolyte, diffusion layer, and Helmholtz layer. The specific capacitance and the equivalent series resistance (ESR) of mesoporous AC were found to be highly dependent on the rate of ionic transfer. Lower cell voltage was identified as being responsible for lower specific capacitance and larger ESR of mesoporous AC, which was similarly seen in the neat and diluted EMIm$BF4, and could be alleviated by increasing the cell voltage. The inferior rate performance and the cell-voltage-dependent performance of mesoporous AC, which were more distinctly observed in the neat EMIm$BF4, could be attributed to the lower mobility of EMImþ and BF 4 in mesopores. © 2017 Elsevier B.V. All rights reserved.

Keywords: Electrical double-layer capacitor Activated carbon Porosity Ionic liquid Electrochemical impedance spectroscopy Equivalent circuit

1. Introduction * Corresponding author. E-mail address: [email protected] (S. Kumagai). http://dx.doi.org/10.1016/j.jpowsour.2017.01.064 0378-7753/© 2017 Elsevier B.V. All rights reserved.

High-performance energy storage devices are strongly required in growing industries, such as renewable power generation

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systems, next-generation vehicles, and consumer electronics. Electrical double-layer capacitors (EDLCs), alternatively named as supercapacitors, exhibit attractive performances of higher power density (1e10 kW kg1), higher chargeedischarge efficiency (>99%), and longer operation life (>105 cycles) [1], when compared with modern secondary batteries. Application of a voltage between the positive and negative electrodes of an EDLC cell produces two layers of polarized ions, called an electrical double-layer, on the respective electrode [2,3]. The electrical double-layer consists of the Helmholtz layer, related to the adsorption of ions on the electrode surface, and the diffusion layer, allowing the concentration gradient of ions offshore. The number of adsorptionedesorption ions on the electrode determines the chargeedischarge capacity of an EDLC. Nanoporous materials with higher surface area are essential for wider double-layer formation, resulting in higher capacitance. Owing to their high specific surface areas (approximately 1000e3000 m2 g1), low cost, and high chemical stability, activated carbons (ACs) are widely accepted as electrode materials for EDLCs. The ionic motion contributing to double-layer formation is influenced by the AC electrode properties (i.e., pore structure and surface chemistry) and the electrolyte properties (i.e., conductivity, viscosity, and potential window width). Factors affecting the electrolyte properties are the type of cation, anion, and solvent, and their composition [4e6]. Non-aqueous or organic electrolytes, enabling higher withstanding voltage (>2.5 V) and thus higher energy density than conventional aqueous electrolytes, are now the mainstream of modern EDLCs. Quaternary ammonium salts dispersed in a solvent such as propylene carbonate or acetonitrile, e.g., tetraethylammonium tetrafluoroborate/propylene carbonate or acetonitrile (TEA$BF4/PC or AN) and triethylmethylammonium tetrafluoroborate/propylene carbonate or acetonitrile (TEMA$BF4/ PC or AN), are typical non-aqueous electrolyte systems [7]. Alternatively, spiro-(1,10 )-bipyrrolidinium tetrafluoroborate/propylene carbonate or acetonitrile (SBP$BF4/PC or AN) systems have been used [8e14]. However, in transportation and high-temperature applications, volatility and flammability due to organic solvents (PC or AN) should be lowered as much as possible to prevent explosive accidents. In recent decades, because of their outstanding merits of nonflammability, non-volatility, and wider potential window, roomtemperature ionic liquids (RTILs), which are molten salts at room temperature and consist only of cations and anions, have been a focus of attention as EDLC electrolytes [15]. However, RTILs have several drawbacks, such as higher viscosity, narrower operational temperature range, and higher cost [16e18]. Among the numerous types of RTILs, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm$BF4), exhibiting relatively high conductivity and low viscosity, has been extensively studied as an EDLC electrolyte [19e22]. Thus, EMIm$BF4 may be used as a benchmark of RTILs intended for EDLC application. The conductivity of EMIm$BF4 is comparable to those of the above non-aqueous electrolytes, whereas its viscosity is considerably higher [23]. Higher viscosity of the electrolyte, resulting from a strong interactive force between cations and anions, impedes the ionic transportation required for the formation of Helmholtz and diffusion layers. The attenuated ionic motion is manifested as internal resistance when the chargeedischarge current density or the cell voltage rate is increased [24]. The voltage allocated to the internal resistance decreases the voltage allocated to the double-layer formation, which is responsible for lowering the energy and power density of EDLC cells. There are some approaches to minimize the impact of the high viscosity of RTILs. One is to broaden the pore size distribution of the AC electrodes by properly incorporating mesopores [25]. The presence of mesopores could shorten the path length for ion

transport in micropores, and could alleviate pore blockage in micropores due to the aggregation of ions. However, the introduction of mesopores inevitably decreases the interfacial area between the electrode and electrolyte. Xu et al. reported that activated carbon fiber, 66.7% of the pore volume of which was contributed by small mesopores of 2e5 nm, showed good compatibility with an RTIL electrolyte (a blend of LiN(SO2CF3)2 and 2-oxazolidinone) [26]. A high specific capacitance of >180 F g1 was achieved from activated carbon fiber with a high specific surface area of 3291 m2 g1 at a low current density of 50 mA g1. However, a significant deterioration of the specific capacitance was observed on increasing the current density. The other approach for alleviating the impact of high viscosity is dilution with a polar organic solvent (typically PC or AN). This can reduce the cost and viscosity of RTILs, but the benefits of non-flammability and non-volatility diminish. The conductivity of the EMIm$BF4/PC system increases with increasing PC solvent ratio up to ca. 50 mass%, but a further increase in the PC content decreases the conductivity [27]. Kirchner et al. suggested that the factors determining the interfacial ordering in RTILs can be different from those in solvent-based electrolytes, in which ions are surrounded by a significant number of solvent molecules [28]. Thus, it is of academic and industrial significance to explore the capacitive and resistive performances of ACs with distinct pore structures in neat or diluted RTILs. Cyclic voltammetry (CV) and galvanostatic chargeedischarge (GCD) tests have been frequently employed for performance evaluations of EDLCs, providing their capacitances over the entire operational voltage range or in a specific voltage range. However, resistance information obtainable from CV and GCD test data is limited. Only the IR drop incurred at the beginning of current reversal in the GCD test indicates a total resistance, which is the sum of various ohmic contributions, namely electrolyte resistance, contact resistance (between AC particles and/or between current collector and AC particles), and component resistance (AC particles and current collector). Kumagai et al. mentioned that the specific capacitance of mesoporous ACs in neat EMIm$BF4 showed a significant dependence on the applied cell voltage (electric field across the double-layers) in CV curves [24,29], but a detailed explanation of the observed results has not been given. Electrochemical impedance spectroscopy (EIS) has been effectively utilized to obtain detailed information about capacitive, diffusive (mass transfer), and resistive (ohmic) elements in EDLCs on the basis of their different time constants [27,30,31]. A simple electrical equivalent circuit to describe an EDLC is a series connection of resistance and capacitor [32]. However, this equivalent circuit model is inadequate to interpret actual experimental EIS data. An ideal EDLC should operate independently of the frequency and magnitude of the applied voltage. However, such dependences are commonly observed in practical EDLCs owing to specific ionic adsorption, diffusion processes, and incomplete electrostatic polarization on the porous electrode (Faradaic charge transfer caused by potential difference across the electrode/electrolyte interface) [33,34]. The introduction of constant phase elements (CPEs) and Warburg impedance, considering the kinetics of ion diffusion, into equivalent circuits of EDLCs has improved the matching accuracy between the experimental data and the circuit simulation data, which has assisted understanding of the observed EDLC performances [30,31]. CPEs are associated with distributed surface reactivity, surface inhomogeneity, roughness, electrode porosity, and current and potential distributions [35]. Thus, CPEs are frequently employed to compensate for the non-ideal capacitive behavior of EDLCs. Recently, Kang et al. proposed a new equivalent circuit model for two-electrode EDLC cells by incorporating three resistances, three CPEs, and one Warburg (finite thin-layer diffusion model) impedance element, which were successively applied to

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characterize actual EDLC cells using aqueous and non-aqueous electrolytes [33]. Impedance spectra of EDLCs have been simulated and discussed using the transmission line equivalent circuit model [36,37], which is essentially equivalent to the diffusion process mathematically expressed by a hyperbolic cosine function [38]. In the present study, the effects of microporosity and mesoporosity on the electrochemical properties of ACs in neat EMIm$BF4 and in EMIm$BF4 diluted with PC are discussed. With specific material properties (comparable total pore volumes and surface chemistry), two types of ACs have been evaluated by CV, GCD tests, and EIS. Kang's equivalent circuit model, providing all of the elemental parameters related to the bulk electrolyte, the diffusion layer, and the double-layer, has been employed for EIS to understand in detail the behaviors of EMImþ and BF 4 in the pores of AC. Finally, the relationship between porosity and the ionic motion of EMIm$BF4 has been explored, with and without PC as solvent, providing a perspective towards utilizing the full potential of RTILs in EDLC applications.

2. Experimental 2.1. Sample ACs Two types of powder-state ACs (RP25, phenolic resin-based, Kuraray Chemicals Co., Ltd., Japan; and KLM, coconut shell-based, Japan Envirochemicals, Ltd., Japan) were used as the active materials of EDLC electrodes. RP25 has been used by Kumagai et al. for EDLCs working in TEMA$BF4/PC electrolyte [39]. KLM has also been employed by Kumagai et al. as a reference sample of rice-husk derived micro- and mesoporous ACs working in 1 mol L1 H2SO4 solution and 1 mol L1 TEMA$BF4/PC [40]. CV and GCD test results of a microporous AC and a mesoporous AC (KLM) in 1 mol L1 TEMA$BF4/PC or neat EMIm$BF4 electrolyte have been provided by Kumagai et al. [29]. However, interaction between the pore structure of AC and the ionic motion of EMIm$BF4 could not be discussed in depth because of incomparable material properties of the ACs used and insufficient measurement of electrolyte

305

properties [29]. Median diameters of RP25 and KLM powders evaluated using a laser diffraction particle size analyzer (SALD-300V, Shimadzu Corp., Japan) were 5 and 7 mm, respectively, indicating that the particle sizes of the AC samples were sufficiently comparable. The pore structures of RP25 and KLM were evaluated by acquiring nitrogen adsorptionedesorption isotherms at 196  C using a gas adsorption analyzer (Autosorb-iQ, Quantachrome Instruments Inc., USA) (Fig. S1, see Supporting Information). KLM showed a gradual increase in the isotherm slope at relative pressures of >0.1, and distinct hysteresis loops at a high relative pressure, which means that mesopores were significantly formed therein. A small increase in the isotherms of RP25 at relative pressures of >0.2 was observed, which suggests that micropores were primarily formed therein. BrunauereEmmetteTeller (BET) specific surface area was calculated on the basis of BET theory [41]. Assuming the pores are filled with liquid nitrogen at a relative pressure close to unity, the amount of vapor adsorbed at a relative pressure of 0.995 can be converted to the volume of liquid nitrogen contained in the pores, representing total pore volumes of the AC samples. Quenched solid density functional theory (QSDFT) [42,43] was applied to obtain the pore size distribution of the AC samples, in which a carbon slit pore equilibrium model installed in the QSDFT software (ASiQwin, version 1.11, Quantachrome Instruments Inc.) was used. The textural properties of RP25 and KLM are summarized in Table 1, which are based on QSDFT calculations. The QSDFT size distributions of narrower pores (<10 nm) in RP25 and KLM are shown in Fig. 1. The results of the micropore (or mesopore) volume fractions and the pore size distributions revealed microporous and mesoporous structures to be present in RP25 and KLM, respectively. Although they have different pore structures and BET specific surface areas (2587 m2 g1 for RP25, 1298 m2 g1 for KLM), their total pore volumes are comparable (ca. 1.4 cm3 g1). The surface chemistry of RP25 and KLM was analyzed by means of X-ray photoelectron spectrometry (XPS) using an Axis Ultra DLD instrument (Kratos Analytical Ltd., UK) with a monochromated AlKa excitation source. Prior to the analysis, the powder-state samples were degassed at 150  C for >6 h. Narrow scan spectra for the C1s

Table 1 Textural properties of RP25 and KLM. Sample Micropore volume (cm3 g1)

Mesopore volume (cm3 g1)

Total pore volume (cm3 g1) Micropore volume fraction (%) Mesopore volume fraction (%) BET specific surface area (m2 g1) Micropore specific surface area (m2 g1)

Mesopore specific surface area (m2 g1)

QSDFT total specific surface area (m2 g1) Micropore surface area fraction (%) Mesopore surface area fraction (%) w: pore width. Data for KLM have been previously reported elsewhere [40].

w  0.7 nm 0.7 < w  1.4 nm 1.4 < w  2 nm Total 2 < w  5 nm 5 < w  10 nm 10 < w  50 nm Total

w  0.7 nm 0.7 < w  1.4 nm 1.4 < w  2 nm Total 2 < w  5 nm 5 < w  10 nm 10 < w  50 nm Total

RP25 (microporous)

KLM (mesoporous)

0.14 0.46 0.41 1.01 0.19 0.02 0.02 0.23 1.35 74.8 17.0 2587 497 897 489 1883 152 6 2 160 2044 92.1 7.9

0.01 0.21 0.15 0.37 0.52 0.17 0.20 0.89 1.41 26.2 63.1 1298 55 374 169 598 335 50 20 405 1003 59.6 40.4

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Fig. 2. XRD patterns of RP25 and KLM.

Fig. 1. QSDFT pore size distributions of RP25 and KLM, calculated from nitrogen adsorptionedesorption isotherms.

region were deconvoluted into six peaks resulting from carbonrelated species using Compro10 software (Surface Analysis Society of Japan), whereby the spectra were fitted using Gaussian algorithms after Shirley background extraction (Fig. S2, see Supporting Information). These peaks represent components of carbide at 284.1 eV; graphitic carbon at 284.7 eV; carbon species in alcohol and/or ether groups (ReOH and/or CeOeC) at 258.8 eV; carbon in carbonyl groups (C]O) and/or quinone at 287.7 eV; carbon in carboxyl and/or ester groups (COOH and/or eC(O) eOeCe) at 289.4 eV; and a shake-up satellite due to a pep* transition in aromatic rings at 291.6 eV [44,45]. Table 2 shows the atomic percentages (at. %) of these components, which were obtained by calculating the area contribution of each deconvoluted curve to the total area of the C1s fitted curves. It is obvious that some carbon atoms constituted oxygen-containing functional groups. However, no critical differences in the concentrations of oxygen-containing functional groups could be discerned. The surface acidities and basicities of RP25 and KLM were also evaluated based on the Boehm titration method [46] (Table S1, see Supplementary Information). The surface acidities and basicities of RP25 and KLM were found to be sufficiently low [47,48] and the differences between the numbers of acidic and basic sites were slight. X-ray diffraction (XRD) analysis was performed to study the crystallinity of RP25 and KLM using a diffractometer (RINT-2020V, Rigaku Corp., Japan) with Cu-Ka radiation (wavelength: 0.15418 nm). XRD profiles of RP25 and KLM are shown in Fig. 2. Two broad peaks with 2q values of ca. 23 and 43 , attributable to the graphitic (002) and (100) planes, respectively, were similarly observed in both XRD patterns, indicating that both the ACs had low graphitic degrees. Variations in the XRD profiles over the 2q range from 10 to 30 suggested that RP25 was composed of a mixture of randomly oriented graphene single-layer and bilayer domains, and that KLM was composed of a mixture of bilayer and trilayer domains [49,50]. Referring to [51,52], the graphene

Table 3 Graphitic crystalline parameters of RP25 and KLM. Sample

d002 (nm)

La (nm)

RP25 KLM

0.379 0.371

4.4 3.9

d002: graphene interlayer spacing, La: apparent crystallite size along basal graphene planes.

interlayer spacing (d002) was calculated using the (002) peak and Eq. (1), and apparent crystallite size along basal graphene planes (La) was calculated using the (100) peak and Eq. (2):

d002 ¼

La ¼

l

(1)

2 sin qd

1:84l Ba cos qa

(2)

where l is the wavelength of X-ray used, qd and qa are the Bragg angles of the (002) and (100) peaks, respectively, Ba is the fullwidth at half-maximum of the (100) peak. d002 and La of RP25 and KLM are shown in Table 3. KLM showed smaller d002, formation of graphene bilayers and trilayers, and smaller La, implying that to some extent it maintained graphitic structure in the lateral direction, whereas RP25 did so in the basal direction. Although differences in the XRD patterns were observed, the small differences in d002 and La were likely to be insufficient to produce significant differences in the graphitic degree and thereby electrical conductivity of bulk carbonaceous parts of RP25 and KLM. Thus, it was confirmed that the powders of RP25 and KLM had comparable properties, apart from their pore structures (microporous or mesoporous).

Table 2 Assignment of convoluted peaks in XPS C1s spectra of RP25 and KLM. Sample

RP25 KLM

Assignment based on peak-area (at. %) Carbide (284.1 eV)

Graphite (284.7 eV)

ReOH, CeOeC (285.8 eV)

CaO, quinone (287.7 eV)

COOH, eC(O)eOeCe (289.4 eV)

p-p* in aromatic rings (291.6 eV)

4.5 4.4

45.9 46.6

34.1 34.6

3.0 4.5

10.2 8.3

2.3 1.6

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2.2. Electrolytes 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIm$BF4, Kanto Chemicals Co., Inc., Japan) was used as received as the neat RTIL electrolyte. EMIm$BF4 diluted with propylene carbonate (PC, Wako Pure Chemical Industries, Ltd.) at a concentration of 1 mol L1 (14 mass%, one EMIm$BF4 molecule to 11.8 PC molecules) was also tested as an electrolyte. The viscosities of the electrolytes were evaluated using a viscometer (VM-10A, Sekonic Corp., Japan). The conductivities of the electrolytes were also evaluated by means of a conductivity meter (MM-60R, DKK-TOA Corp., Japan). Both evaluations were carried out at 25  C. The conductivities and viscosities of the neat and diluted EMIm$BF4, as well as molecular size information, are given in Table 4. The viscosity of the diluted medium was ten times lower than that of the neat IL, but its conductivity showed little change.

2.3. Electrode preparation and cell assembly The sample AC was mixed with 10 mass% acetylene black (Denka Black, Denki Kagaku Kogyo Kabushikikaisya, Japan) and 10 mass% polyfluoroethylene (Polyflon D210-C, Daikin Industries, Ltd., Japan). The mixture was ground with a mortar and pestle with the addition of ethanol, and was pressed into a sheet. From this sheet, disks of 12 mm in diameter were punched out. Each disk was pressed and attached to a circular piece of Al mesh (dia.: 15 mm), which served as the current collector, degassed at 140  C for >6 h, and then employed as an EDLC electrode. A pair of electrodes with approximately similar active mass and thickness were used as the anode and cathode. The electrode active masses of RP25 and KLM were 17.1 mg and 16.1 mg, respectively, and their electrode thicknesses were both 0.36 mm. Detailed mass and thickness data are collected in Table S2 (see Supplementary Information). Two pieces of paper-based separator (diameter: 23 mm, thickness: 55 mm; TF4050, Nippon Kodoshi Corp., Japan) sandwiched by the positive and negative electrodes were sealed with the electrolyte (1 mL) in a two-electrode cell system, the outer body of which was made of aluminum alloy (HS cell, Hohsen Corp., Japan). The cell was assembled in an argon-filled glove box (GBJF080R, Glovebox Japan Inc., Japan).

2.4. Electrochemical tests and equivalent circuit model CV was performed at 25  C by using an electrochemical measurement system (HZ-5000, Hokuto Denko Corp., Japan). The cell voltage (potential difference between the positive and negative electrodes of the assembled EDLC cell) was repeatedly varied from 0 V to 2.5 V and then from 2.5 V to 0 V at a constant scan rate, during which the current was measured. The cell was thrice subjected to this voltage sweep. CV curves were acquired at scan rates of 100, 10, and 1 mV s1. The specific capacitance, CCV, of the sample AC at varying applied voltages was calculated according to Eq. (3):

Table 4 Conductivities and viscosities of electrolytes at 25  C.

Neat EMIm$BF4 Diluted EMIm$BF4 (in PC at 1 mol L-1)

Conductivity (mS cm1)

Viscosity (mPa s)

13.6 12.1

41.9 4.15

Ionic diameters of EMImþ and BF 4 are 0.60 and 0.46 nm, respectively [24]. Diameter of the PC solvent molecule is 0.55 nm [24].

CCV ¼

2ICV mAC v

307

(3)

where ICV is the current measured in CV at varying applied voltage, mAC is the active mass of AC used in the single electrode, and v is the voltage scan rate. The direction of ICV for cell-charging was defined as positive. Thus, negative specific capacitance could be observed during cell discharging. The voltammogram at the second cycle was used to evaluate the specific capacitance of the sample AC. In the present study, specific capacitance refers to gravimetric specific capacitance in units of F g1. After the CV, the cell was subjected to a GCD test, in which the cell voltage was increased from 0 V to 2.5 V (charge) and then decreased from 2.5 V to 0 V (discharge) at constant current, using a battery chargeedischarge system (HJ1005SD8, Hokuto Denko Corp., Japan). Cyclic chargeedischarge was executed at 25  C at various current densities (0.1e10 mA cm2) and for different numbers of cycles (2 and 5). The specific capacitance of the sample AC in the GCD test, CG, was obtained by calculating the charge quantities during charge or discharge according to Eq. (4):

CG ¼

2QG mAC ðE  VIR Þ

(4)

where QG is the time-integral of current in the charge or discharge process, E is the cell voltage range for chargeedischarge (2.5 V), and VIR is the IR drop incurred at the chargeedischarge or dischargeecharge switching. Simultaneously, the equivalent series resistance (ESR) of the sample AC, ESR, was calculated according to Eq (5):

ESR ¼

VIR IG

(5)

where IG is the actual current in the GCD test. The specific capacitance and the ESR at different current densities were evaluated under the specified conditions: 0.1, 1, and 10 mA cm2 (Table S3, see Supplementary Information). The specific surface capacitance, in units of mF cm2, was also evaluated by dividing the gravimetric specific capacitance by the BET specific surface area in order to ascertain the effect of pore structure on the capacitive performance irrespective of specific surface area [53]. The resistance of a disk-shaped single EDLC electrode into which the electrolyte had not been impregnated (dry conditions) was evaluated under constant compression. A single electrode having approximately similar mass and dimensions as the electrodes used for the electrochemical tests was tested. The electrode without an Al mesh current collector was degassed at 140  C for >6 h. Immediately thereafter, it was sandwiched between SUS304 steel plates, compressed at 0.3 MPa, and subjected to resistance evaluation using a digital multimeter (175, Fluke Corp., USA) at 25  C. Following the GCD test, EIS was performed at 25  C for the cells at different charging states using the abovementioned electrochemical measurement system. Each cell was subjected to AC voltage application biased at DC 0 V (fully discharged) and 2.5 V (fully charged). The frequency of the applied AC voltage was varied from 100 kHz to 10 mHz. The amplitude of the AC voltage was set at 10 mV (7.1 mVrms). Prior to the AC voltage application, the cell was subjected to a DC bias for 10 min to stabilize the charging stage. EIS provided the whole cell impedance, Z(u), through scanning of the AC voltage frequency, according to Eq. (6): 00

ZðuÞ ¼ Z 0 ðuÞ þ jZ ðuÞ

(6)

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can be written in complex form by Eq. (7) [54]: 00

CðuÞ ¼ C 0 ðuÞ  jC ðuÞ

(7)

where C0 (u) and C00 (u) are the real and imaginary parts, respectively, of the EDLC cell capacitance. When the EDLC cell is considered as the whole capacitance, C(u) can be expressed by Eq. (8) [54]:

CðuÞ ¼

Fig. 3. EDLC equivalent circuit proposed by Kang et al. [33]: (a) complete circuit, (b) bulk electrolyte, (c) diffusion layer, (d) Helmholtz layer.

1 juZðuÞ

(8)

Thus, the real and imaginary parts of the specific capacitance of the sample AC, defined as C0 EIS(u) and C00 EIS(u), respectively, can be described by Eqs. (9) and (10): 00

where j is an imaginary unit, Z0 (u) and Z00 (u) are the real and imaginary parts, respectively, of the whole cell impedance, and u is the angular frequency. The relationship between Z00 (u) and eZ00 (u), known as the impedance spectrum or Nyquist plot, was acquired for each EDLC cell. Fig. 3 shows the EDCL equivalent circuit model proposed by Kang et al. [33]. This circuit model is comprised of three resistances, three CPEs, and one bounded Warburg (finite thin-layer diffusion model) impedance element [30] (Fig. 3(a)), in which diffusion, adsorption, and bulk medium impedance are taken into account. The circuit can be separated into four components: intrinsic ohmic component, RS, bulk electrolyte (Fig. 3(b)), diffusion layer (Fig. 3(c)), and Helmholtz layer (Fig. 3(d)). The explanations and the impedance expressions for each element are summarized in Table 5. Using EIS simulation software (EIS, Version 1.0.14, Hokuto Denko Corp., Japan), the element parameters depicted in the equivalent circuit were determined so as to fit the actual impedance spectra with the simulated data over the frequency range from 100 kHz to 10 mHz. The impedance spectra of the bulk electrolyte, the diffusion layer, and the Helmholtz layer were also simulated, and provided assistance in understanding the EIS results. In addition to the impedance spectral analysis, the specific capacitance of the sample AC was also calculated based on the EIS results. The frequency-dependent capacitance, C(u), of an EDLC cell

0 CEIS ðuÞ ¼

00

CEIS ðuÞ ¼

2Z ðuÞ mAC ujZ ðuÞj2 2Z 0 ðuÞ mAC ujZ ðuÞj2

(9)

(10)

The real and imaginary parts of the specific capacitance in EIS were calculated and then simulated over the extended frequency range using the determined element parameters, enabling estimation of the quasi-DC specific capacitance and the dielectric relaxation time of the sample AC. When the imaginary parts of the specific capacitance in EIS yield a peak at frequency f0, the dielectric relaxation time, t0, is obtainable from Eq. (11):

t0 ¼

1 f0

(11)

Imaginary specific capacitance is indicative of the magnitude of dielectric loss (frequency-dependent energy loss). At a frequency f0, half of the low-frequency capacitance (CHLF) is reached. At f < f0, the capacitance is greater than CHLF, whereas at higher frequencies, the capacitance is less than CHLF [32]. Thus, the relaxation time is related to the capacitance stability with respect to higher frequency, indicating that shorter relaxation times can be responsible for better rate capabilities of AC samples [37].

Table 5 Elements and impedances used in the EDLC equivalent circuit, associated with Fig. 3. Element

Impedance expression ZR ¼ RS

Notes

Electrolyte resistance irrespective of charge/discharge process, contact resistance, component resistance, etc. Bulk electrolyte dominates at high frequencies, whereas interfacial electrochemical reactions related to the double-layer formation take place at low frequencies. The bulk process in the electrolyte is valid when the current overcomes the bulk impedance, leading to a capacitance in parallel with the bulk resistance [33]. Tbulk: CPE coefficient of bulk electrolyte, pbulk: exponent of bulk electrolyte (0 < pbulk  1). Resistance for the ZR ¼ Rbulk Connected with CPE for the bulk electrolyte in parallel and connected with the diffusion layer in series [33]. The bulk bulk electrolyte resistance can be considered as the resistance to supply the diffusion layer with excess ions. qffiffiffiffiffiffiffi  Warburg (diffusion) Diffusion model of thin layer with reflecting boundary condition [38]. u ffiffiffiffiffi ffi ZW ¼ pRW coth j uD j uu impedance Rw: diffusion resistance, uD: characteristic angular frequency of diffusion in a finite layer, corresponding to the transit D time of ions. 1 CPE for the Helmholtz ZCPE ¼ Capacitance due to Helmholtz layer formation of electrostatically attracted ions or solvated ions. TH ðjuÞpH layer TH: CPE coefficient of Helmholtz layer, pH: exponent of Helmholtz layer (0 < pH  1). Interfacial resistance ZR ¼ Rint The sum of the charge-transfer resistance and the adsorption resistance. The charge-transfer resistance is mainly attributed to incomplete polarization (Faradaic reactions) at the electrode/electrolyte interface, leading to current leakage. The adsorption resistance is related to specific adsorption of ions or solvated ions. Specifically adsorbed ions change the surface charge density, resulting in an interfacial current path. The interfacial resistance is connected with the adsorption capacitance in series and connected with the Helmholtz layer capacitance in parallel. CPE for the ion Capacitance due to the specifically adsorbed ions or solvated ions nearest the electrode surface, connected with the ZCPE ¼ T ðj1uÞpads ads adsorption Helmholtz layer capacitance in parallel. Tads: CPE coefficient of ion adsorption, pH: exponent of ion adsorption (0
Intrinsic resistance CPE for the bulk electrolyte

1 pbulk bulk ðjuÞ

ZCPE ¼ T

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3. Results and discussion 3.1. CV results Cyclic voltammograms of microporous RP25 and mesoporous KLM in neat and diluted EMIm$BF4 were acquired at scan rates of 100, 10, and 1 mV s1 (Fig. 4). A symmetrical rectangular voltammogram should be observed if the cell functions as an ideal capacitor that has no series resistor and no dependence of capacitance on the applied cell voltage. At the highest scan rate of 100 mV s1, RP25 showed a distorted parallelogram-shaped voltammogram and KLM showed a more distorted or collapsed voltammogram. The use of neat or diluted EMIm$BF4 led to insignificant differences in the CV profiles for the respective samples. On decreasing the scan rate, the distortion in the voltammograms of RP25 and KLM tended to be alleviated. The lowest scan rate of 1 mV s1 applied to RP25 provided an isosceles trapezoidshaped voltammogram, whereby higher cell voltage induced higher specific capacitance during either the charge or discharge processes. A higher specific capacitance of RP25 was achieved in neat EMIm$BF4 compared to that in the diluted EMIm$BF4. On the other hand, irrespective of the electrolyte type, KLM showed a right triangle-shaped voltammogram, even at the lowest scan rate (1 mV s1). A higher cell voltage led to higher specific capacitance, which was similarly seen for RP25. KLM displayed a gradual increase in specific capacitance during charging at a cell voltage of <1.5 V, which highlights a distinct difference between RP25 and KLM. Higher specific capacitance was observed for KLM in the diluted EMIm$BF4 than that in the neat EMIm$BF4 at a scan rate of 10 mV s1. However, an effect of electrolyte dilution on the CV profiles of KLM was barely detectable at the lowest scan rate of 1 mV s1. The collapsed CV profiles provided evidence of the increased ESR, which is generally related to electrolyte conductivity and ionic

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motion at the double-layers [55]. Mesoporous KLM displayed higher ESR than did microporous RP25, regardless of their similar total pore volumes and comparable surface chemistry. In particular, the gradual elevation of that of KLM during charging at a lower cell voltage (<1.5 V) indicated that this parameter was dependent on the cell voltage. At the lowest scan rate, small bulges were observed for all of the samples and the electrolytes at a higher cell voltage of >2.3 V during charging, whereas no bulges were observed during discharging. This implies that irreversible faradaic reactions occurred when a higher voltage was allocated to the interface between the AC surface and the electrolyte. Except for these reactions, the samples showed no local humps or dents in the voltammograms, indicating negligible faradaic reactions between the surface functional groups on the samples and the electrolyte. 3.2. GCD test results RP25 and KLM were galvanostatically charged and discharged at different current densities of 0.1, 1, and 10 mA cm2. Fig. 5 shows cell voltage vs. specific capacity for RP25 and KLM in neat or diluted EMIm$BF4. The changes in cell voltage under all conditions were approximately linear, except during chargeedischarge or dischargeecharge switching. Rapid voltage changes during the switching indicate an IR drop (VIR). At the lowest current density (0.1 mA cm2), slight VIR were observed for KLM at the beginning of charge. It was also found that the increase in the cell voltage faded at a higher cell voltage of >2.0 V, producing a significant irreversible capacity during chargeedischarge. With increasing current density, RP25 and KLM displayed a larger VIR and improved cell voltage linearity, regardless of the electrolyte type. It was observed that VIR of KLM at the beginning of charge was notably larger than that at the beginning of discharge, while the corresponding difference for RP25 was slight. A larger VIR was observed in neat EMIm$BF4 as opposed to the diluted medium, which was independent of the AC

Fig. 4. Cyclic voltammograms of RP25 and KLM in neat EMIm$BF4 and diluted EMIm$BF4 (in PC at 1 mol L1). (a) voltage scan rate: 100 mV s1, (b) 10 mV s1, (c) 1 mV s1. CV curve of KLM in neat EMIm$BF4 at 1 mV s1 have been previously reported in Ref. [29].

Fig. 5. Galvanostatic chargeedischarge properties of RP25 and KLM in neat EMIm$BF4 and diluted EMIm$BF4 (in PC at 1 mol L1). (a) chargeedischarge current density: 0.1 mA cm2, (b) 1 mA cm2, (c) 10 mA cm2.

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Table 6 Specific capacitance, specific surface capacitance, and equivalent series resistance of RP25 and KLM in neat EMIm$BF4 or diluted EMIm$BF4 (in PC at 1 mol L1) electrolyte. Sample

RP25

Electrolyte

Neat Diluted

KLM

Neat Diluted

Process

Charge Discharge Charge Discharge Charge Discharge Charge Discharge

Specific capacitance (F g1)

Specific surface capacitance (mF cm2)

Equivalent series resistance (U)

0.1 mA cm2

1 mA cm2

10 mA cm2

0.1 mA cm2

1 mA cm2

10 mA cm2

0.1 mA cm2

1 mA cm2

10 mA cm2

120 103 101 92.0 96.9 87.7 93.2 79.9

104 103 92.3 91.0 80.2 70.5 74.8 67.0

91.5 89.1 84.1 82.9 75.0 24.8 56.2 31.2

4.66 3.99 3.92 3.56 7.47 6.76 7.18 6.16

4.04 3.96 3.57 3.52 6.18 5.43 5.76 5.16

3.54 3.44 3.25 3.20 5.78 1.91 4.33 2.40

e e e e 305 e 177 e

16.0 14.2 10.3 9.16 289 43.8 180 40.2

16.4 12.1 10.8 8.97 167 61.8 127 52.0

: Not calculated because of negligible IR drop voltage.

type. RP25 and KLM in the neat EMIm$BF4 showed more moderate changes in cell voltage, indicating that higher specific capacity was obtainable from the neat electrolyte compared to the diluted electrolyte. Table 6 lists the specific capacitances, specific surface capacitances, and ESRs of RP25 and KLM based on the GCD test results. RP25 showed higher specific capacitance than did KLM, which was maintained even at increased current density. Electrolyte dilution was found to reduce the specific capacitance of RP25. The specific capacitance of KLM was seen to be highly dependent on the current density and the electrolyte type. KLM in the neat electrolyte showed higher specific capacitance at lower current density, whereas in the diluted electrolyte it showed higher specific capacitance at higher current density. The difference in specific capacitance for the charge and discharge processes at the lowest current density can be rationalized in terms of the irreversible capacity resulting from the faradaic reactions that were observed in the CV profiles during charging at a cell voltage >2.3 V. KLM displayed a large decrease in specific capacitance even when the current density was increased. No faradaic reactions were observed in its CV profiles at the higher scan rates (10 and 100 mV s1). Thus, lowering of the specific capacitance of KLM during discharge was attributable to the considerably large ESR leading to energy loss. In order to determine the effectiveness of the pore structures of RP25 and KLM, their specific surface capacitances were also evaluated. At low and medium current densities (0.1 and 1 mA cm2), higher specific surface capacitance was obtainable from KLM, indicating that the mesoporosity therein was responsible for effective surface utilization under moderate ionic transfer. At least 30% higher specific surface capacitance was observed on KLM. Lower specific surface capacitance during discharge at a higher current density (10 mA cm2) indicated that the advantage of mesoporosity in KLM was weakened under high-rate ionic transfer. ESR evaluation indicated that a considerably larger resistive component was produced in KLM. The difference between the ESR at the onset of charging and the onset of discharging was slight for RP25, whereas a more significant difference was observed for KLM. The two samples showed larger ESR values in neat EMIm$BF4 than in the diluted IL. The resistances of single disc-shaped electrodes of RP25 and KLM, without impregnation by the electrolyte, were evaluated under constant compression. The resistances of the RP25 and KLM electrodes were 28 and 3670 U, respectively. Considering the comparable material properties except for the textural properties, the development of mesoporosity was clearly related to the very high resistance of KLM, which was in particular observed upon dischargeecharge switching during the GCD tests. The mesoporefilling or penetration of the electrolyte was found to be important to reduce the cell resistance during the chargeedischarge process.

3.3. EIS results Impedance spectra of the EDLC cells were acquired at the fully discharged and fully charged states (DC bias voltages: 0 and 2.5 V). On the basis of the EDCL equivalent circuit model proposed by Kang et al., 11 circuit parameters associated with three resistances, three CPEs, and one bounded Warburg impedance element were estimated to fit as closely as possible the simulated lines to the actual impedance spectra. Table 7 lists the estimated equivalent circuit parameters of RP25 and KLM in the neat and diluted EMIm$BF4. Fig. 6 shows the actual impedance spectra, the simulated impedance spectra, and the simulated spectra separated into the components of the bulk electrolyte, the diffusion layer, and the Helmholtz layer. It was confirmed that the simulated impedance lines fitted the actual impedance data under all conditions. The contributions of intrinsic ohmic resistance (RS) and the impedances of the bulk electrolyte, the diffusion layer, and the Helmholtz layer to the impedance spectral composition were also identified. The intrinsic resistance (RS) corresponds to the left-hand-side Z0 -intercept of the flattened semicircle. The values of RS for RP25 and KLM were found to be comparable (1e2 U), irrespective of the electrolyte type or the bias voltage. A bulk electrolyte can be defined as an electrolyte displaying no concentration gradient of cations and anions at low AC frequencies, leading to certain doublelayer formation. The bulk process is valid particularly at high AC frequencies, producing a CPE in parallel with bulk resistance (Rbulk). The flattening degree of the semicircle was represented by the CPE exponent of the bulk electrolyte (pbulk). Values of pbulk under all conditions were found to be at a similar level (ca. 0.7). The CPE coefficients of the bulk electrolyte (Tbulk) under all conditions were very small (<~104 F s(p1)) in comparison with those of the Helmholtz layer and of ion adsorption. The value of Rbulk, which can be considered as the resistance to supply the diffusion layer with excess ions to create the double-layer capacitance, was equivalent to the length between the two Z0 -intercepts of the flattened semicircle. Because of high impedance contributions from the diffusion and Helmholtz layers, no right-hand-side Z0 -intercept was observed for KLM, the cell bias voltage of which was 0 V. In this situation, KLM displayed considerably large Rbulk (145 U for the neat EMIm$BF4, 55 U for the diluted EMIm$BF4). Increasing the cell bias voltage from 0 to 2.5 V reduced Rbulk (to about 10 U), which was seen regardless of the electrolyte dilution. Rbulk of RP25 in neat EMIm$BF4 was slightly larger than that in the diluted EMIm$BF4. A higher bias voltage resulted in a lowering of Rbulk. All of the estimated parameters regarding the bulk electrolyte suggested that cell charging was responsible for the decrease in the bulk resistance. Thus, the bulk resistance can reflect the ionic motion associated with formation of the diffusion layers and mesopore-filling (penetration) of ions, which was verified by the resistance

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Table 7 Estimated parameters of the equivalent circuit elements for RP25 and KLM in neat EMIm$BF4 or diluted EMIm$BF4 (in PC at 1 mol L1) electrolyte under bias voltages of 0 V and 2.5 V. Sample

Electrolyte

Bias (V)

R S ( U)

pbulk ()

Tbulk (F s(p1))

RP25

Neat

0 2.5 0 2.5 0 2.5 0 2.5

1.36 1.27 1.33 1.29 1.90 1.53 1.94 1.53

0.78 0.73 0.75 0.72 0.71 0.71 0.69 0.65

8.74 1.74 1.89 2.87 9.79 1.13 1.36 2.27

Diluted KLM

Neat Diluted

       

105 104 104 104 105 104 104 104

Rbulk (U)

RW (U)

uD (rad s1)

pH ()

TH (Fs(p1))

Rint (U)

pads ()

Tads (F s(p1))

4.13 2.52 2.13 1.62 145 13.8 55.1 10.6

24.4 17.5 21.1 20.2 4.92 9.23 1.46 4.54

0.0434 0.0251 0.0625 0.0348 0.0763 0.101 0.826 0.146

0.52 0.55 0.61 0.60 1.00 1.00 0.94 1.00

0.211 0.340 0.328 0.600 3.75  8.49  1.41  4.48 

6.42 19.7 1.36 3.13 3.52 0.906 24.0 0.595

0.99 1.00 1.00 1.00 0.26 0.47 0.32 0.64

0.680 0.262 1.35 0.607 7.50  103 0.340 0.0148 0.495

evaluation of a single electrode without electrolyte impregnation. The capacitance of the bulk electrolyte was found to be negligible in comparison to those related to the double-layer. The diffusion process can be characterized by the diffusion resistance (Rw) and the characteristic angular frequency of diffusion (uD), both of which are incorporated in the Warburg impedance (Zw). The inverse of uD gives the transit time of ions for the diffusion. It is obvious that Rw of RP25 was higher and uD of RP25 was smaller than those of KLM, implying that mesoporous KLM permitted higher-rate ionic transfer in the finite diffusion layer than did microporous RP25. Higher Rw was observed on KLM biased at 2.5 V than that at 0 V, irrespective of whether EMIm$BF4 was diluted or not. Charging imparted the mesoporous structure with higher congestive resistance at the diffusion layer. The lowest Rw and the highest uD were observed for the discharged KLM (0 V bias) in the diluted EMIm$BF4, validating that electrolyte dilution and charge neutrality accomplished the most fluent diffusion under all conditions. The results concerning the Warburg impedance revealed that the diffusion process in KLM was more fluent than that in RP25, and was significantly dependent on the charge state and the electrolyte type. The separated impedance spectra of the diffusion layers indicated that the contribution of the diffusion layer to the Z0 component was more limited in KLM than in RP25, and that the Warburg impedance strongly contributed to leaps in the eZ00 component at lower frequency. The Helmholtz layer in Kang's equivalent circuit model is composed of the two CPEs and the interfacial resistance (Rint). The two CPEs are derived from Helmholtz layer formation by electrostatically attracted ions or solvated ions (CPE coefficient: TH, CPE exponent: pH) and from specifically adsorbed ions or solvated ions nearest the electrode surface (CPE coefficient: Tads, CPE exponent: pads). Rint is defined as the sum of the charge-transfer resistance and the adsorption resistance, details of which are described in Table 5. Higher Rint was observed at a higher bias voltage for RP25, but at a lower bias voltage for KLM. The use of the neat electrolyte imparted RP25 with higher Rint. The Helmholtz layer capacitance of RP25 could be attributed to both the electrostatic attraction (TH) and the specific adsorption (Tads). pads and pH of RP25 were found to be 1 and 0.5e0.6, respectively. A lower CPE exponent (p) affects the real part of the CPE impedance, as shown in Eq. (12):

pp pp 1 1 1  j p sin cos p ¼ p T T u 2 u 2 TðjuÞ

(12)

A frequency-dependent resistive component could be generated through electrostatic attraction, associated with the inhomogeneity of the AC surface. On the other hand, TH of KLM under all conditions was found to be small (<~103 F s(p1)) in comparison with TH of RP25 and Tads of KLM and RP25, indicating that the contribution of electrostatic attraction to the Helmholtz layer capacitance of KLM was negligible, even if its pH was invariably approximately 1. It was found that Tads of KLM biased at 0 V was slight (<~102 F s(p1)) and

105 104 104 103

that its pads was also small (ca. 0.3). These results were consistent with the lower CCV of KLM observed at lower cell voltages and the higher ESR observed at dischargeecharge switching. Moreover, the Helmholtz layer capacitance of KLM biased at 2.5 V was mainly attributable to specifically adsorbed ions or solvated ions. The real and imaginary parts of EIS specific capacitance (C0 EIS and C00 EIS, respectively) for RP25 and KLM are plotted in Fig. 7 as a function of frequency. Using the estimated parameters of the equivalent circuit elements, simulated C0 EIS and C00 EIS lines were also generated over a more extended frequency region. The actual C0 EIS of RP25 under all conditions showed a slight difference in the frequency region >102 Hz. However, the simulated C0 EIS lines for RP25 suggested that higher bias voltage and no electrolyte dilution led to higher C0 EIS in the lower frequency region <103 Hz. A higher actual C00 EIS of RP25, indicative of dielectric loss (energy loss), was observed at higher bias voltages. The actual C00 EIS of RP25 biased at 0 V showed a shorter dielectric relaxation time (t0) than was the case at 2.5 V. The simulated lines of C00 EIS for RP25 implied that higher bias voltage and the neat electrolyte were responsible for the longer t0. The results for RP25 suggested that higher bias voltage and no electrolyte dilution imparted it with higher quasiDC specific capacitance, but longer t0. Longer t0 is associated with lowering of the rate capability of AC electrodes. On the other hand, the actual C0 EIS of KLM was found to increase when a higher bias voltage (2.5 V) was applied. The C0 EIS simulation of KLM indicated that higher C0 EIS above ca. 104 Hz was achievable with higher bias voltage and electrolyte dilution. Below ca. 104 Hz, the simulated C0 EIS of KLM biased at 0 V in the neat electrolyte increased anomalously. A higher actual C00 EIS was observed for KLM biased at 2.5 V than that at 0 V, which was similarly seen for RP25. Very long t0 (103104 s) for KLM under no bias voltage suggested that its very high quasi-DC specific capacitance in the neat electrolyte was realistically meaningless because of very long dielectric relaxation times. On the other hand, t0 of KLM biased at 2.5 V was estimated to be comparable to that of microporous RP25 (ca. 102 s), revealing that the charge state had a strong impact on the capacitive and resistive performance of mesoporous KLM.

3.4. Microporosity and mesoporosity of ACs in neat and dilute EMIm·BF4 The viscosity of the neat EMIm$BF4 (41.9 mPa s) was ten-fold higher than that of the diluted EMIm$BF4 (4.15 mPa s), while their conductivities were relatively comparable (12e13 mS cm1). Electrostatic interaction (Coulomb force) and hydrogen bonding between cations and anions are known to be sources of high viscosity of RTILs [56]. Thus, even though the conductivity difference between the neat and diluted EMIm$BF4 was slight, transportation mechanisms of EMImþ and BF 4 in the electrode pores should be discussed separately. Polarization mechanisms at the interface between carbon and RTILs have been extensively discussed on the

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Fig. 6. Impedance spectra of RP25 and KLM in neat EMIm$BF4 or diluted EMIm$BF4 (in PC at 1 mol L1), and their simulation lines obtained with equivalent circuits. (a) RP25, 0 V bias, (b) RP25, 2.5 V bias, (c) KLM, 0 V bias, (d) KLM, 2.5 V bias.

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Fig. 7. Real and imaginary specific capacitances of RP25 and KLM in neat EMIm$BF4 and diluted EMIm$BF4 (in PC at 1 mol L1), and their simulation lines. (a) RP25, real specific capacitances, (b) RP25, imaginary specific capacitances, (c) KLM, real specific capacitances, (d) KLM, imaginary specific capacitances.

basis of computational simulations [28,57,58]. The structural regime of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm$BF4) between two rigid graphene slabs separated by a distance of 10.4 nm, under different degrees of surface charge of graphene, has been comprehensively simulated by Ivanistsev et al. [59]. They suggested that a layer of BMImþ formed next to the surface as the surface charge was increased from neutral to more negative values. An oppositely charged layer of BF 4 and a subsequent layer of BMImþ then alternately formed (multilayer formation). More negative surface charging produced only a single layer of concentrated BMImþ closest to the graphene surface (monolayer formation) and an upper disordered region of cations and anions. When the BMImþ concentration in the monolayer was maximized, the dense monolayer could not provide a net counter charge. This layer transition is similarly observed on a positively charged surface. There is high structural similarity between EMIm$BF4 and BMIm$BF4. Only the size of BMImþ is slightly larger than that of EMImþ. The QSDFT pore size distribution (Fig. 1) revealed that most of the pores in RP25 and KLM had width <10.4 nm. Thus, structural transitions could occur in RP25 and KLM in the neat EMIm$BF4. On the other hand, the polarization mechanisms of the diluted EMIm$BF4 could be described by the conventional GouyeChapmaneStern model modified by Grahame in 1947 [60], which is applicable to double-layer formation from solvent molecules and solvated ions. The spatial density of cations and anions in neat EMIm$BF4 must be higher than that in diluted EMIm$BF4 because one molecule of EMIm$BF4 is allocated to 11.8 PC molecules. Because a larger number of cations and anions could be ordered close to the

electrode surfaces in the neat electrolyte, higher specific capacitance was obtainable therein, irrespective of the AC type. Increasing the rate of ionic transfer (current density) led to a decrease in the specific capacitance and an increase in the ESR of the mesoporous structure. Moreover, the mesoporous structure in its discharged state displayed very low specific capacitance and very large ESR. These features were observed for both the neat and diluted electrolytes, but were hardly observed for the microporous structure. It has been indicated that ionic transfer in a mesoporous structure is susceptible to an electric field [24]. A large driving force induced by an electric field across a double-layer is necessary to propel ions into mesopores, into micropores through mesopores, as well as to arrange layers of ions. The distance between the pore walls determines the adsorptive force (van der Waals force) that holds ions within the pores, which is closely related to the diffusive force to release ions from the pores. A larger poreewall distance of mesopores will certainly lower the pore occupancy, which can be derived from alleviation of the solideliquid interaction potential with increasing distance from a homogeneous flat pore wall, formulated by the 10-4-3 Steele potential equation [61]. Lowering the ion-retention ability of the mesopores restricted rapid layer formation by ions and caused ionic congestion. When the cell voltage was increased, ions could drift into mesopores or into micropores through mesopores due to the increased electric field. Once the ions had entered the pores, the adsorptive force assisted their retention [3,62]. As a result, mesoporous KLM displayed cellvoltage-dependent specific capacitance and ESR. On the other hand, higher specific surface capacitance was observed on the mesoporous KLM compared to the microporous RP25. Considering the diameters of EMImþ (0.60 nm), BF 4 (0.46 nm), and PC

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(0.55 nm), the smaller micropores in RP25, which contributed to its higher specific surface area, were not fully utilized in double-layer formation. The polarization mechanism in the neat EMIm$BF4, namely the ionic layer formation mechanism on the electrode surface, differed from that in the diluted EMIm$BF4. The chargeedischarge mechanism in the diluted EMIm$BF4 could be explained in terms of a modified GouyeChapmaneStern model. For the neat EMIm$BF4, it seems valid to consider the first monolayer of ions, which is oppositely charged with respect to the charged surface, as the Helmholtz layer. The upper layers, showing a positional concentration gradient of charges with alternating multilayer formation of cations and anions or with their disordered distribution, could be treated as the diffusion layer in the modified GouyeChapmaneStern model. The bulk electrolyte for the neat EMIm$BF4 could be similarly defined as the charge-neutral region. Lower mobility of EMImþ and BF 4 in mesopores under lower electric field (lower cell voltage) is likely to be connected to lowering of ion-trapping in mesopores and thereby greater congestion resistance in forming the diffusion layer, explaining the considerable bulk electrolyte resistance of the non-charged mesoporous structure as well as the greater bulk electrolyte resistance of the neat EMIm$BF4 compared to the diluted IL. The very long dielectric relaxation time of KLM in its discharged state could also be attributed to the lower mobility of EMImþ and BF 4 in mesopores. The lower resistive contribution of the diffusion layer in the mesoporous structure is associated with its lowered ion-trapping ability. The higher contribution of specific adsorption of ions or solvated ions to the Helmholtz layer capacitance of the mesoporous structure in the charged state is likely to be consistent with the increased ion-trapping ability of mesopores represented by increased diffusion resistance. Moreover, the electric field to form the Helmholtz layer was estimated to be mitigated by the additional allocation of potential for Rbulk, which was larger than that for microporous RP25. Thus, a higher contribution of specific adsorption was observed on KLM than on RP25. The low mobility of EMImþ and BF 4 in mesopores in approaching the pore surface is presumed to be more pronounced in the neat electrolyte than in the diluted electrolyte owing to the viscosity difference, suppressing the rate performance of the mesoporous AC in the neat EMIm$BF4. Previously, the introduction of mesopores into a microporous structure to retain the capacitive performance under high ionic transfer has been demonstrated, based on their broader path provision, to promote the transportation of ions or to alleviate micropore blockages [26,63,64]. However, from the present study, it is concluded that a mesopore-dominated structure is not effective to retain the capacitive performance, particularly in the discharged state. 4. Conclusions Many experimental results have been acquired to understand the behaviors of EMImþ and BF 4 in the electrode pores in relation to the capacitive and resistive performances of EDLC cells. The specific capacitance and the ESR of the mesoporous structure have been shown to be highly dependent on the rate of ionic transfer. A mesoporous structure under lower cell voltage showed very low specific capacitance and very large ESR, which were independent of the electrolyte dilution and could only be alleviated by increasing the cell voltage. Considerable bulk electrolyte resistance was observed for the discharged mesoporous structure, and the resistive contribution of the diffusion layer in the mesoporous structure was lower than that in the microporous structure. Results suggested that a larger poreewall distance of mesopores reduces the ability to retain ions within the pores, leading to

lowering of mesopore-filling of ions and to disordering and congestion of ions. The considerable bulk electrolyte resistance of the non-charged mesoporous structure as well as the higher bulk electrolyte resistance of neat EMIm$BF4 could be rationalized in terms of the lower mobility of EMImþ and BF 4 in mesopores under lower electric field. The lower resistive contribution of the diffusion layer in the mesoporous structure could also be attributed to the lower ion-trapping ability of the mesopores. Owing to the higher viscosity of neat EMIm$BF4, the lower mobility of EMImþ and BF 4 in the mesopores in approaching the pore surface suppressed the rate performance. Co-existence of micropores and mesopores, and their size distribution optimization, have been found to be important to exploit the maximum potential of EMIm$BF4 as an EDLC electrolyte. Acknowledgment This research was supported in part by JSPS KAKENHI Grant Number JP15K05925. We would like to thank Mr. Koji Mukaiyachi of Akita University for his help with the experiments. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.064. References [1] J.R. Miller, A.F. Burke, Electrochemical capacitors: challenges and opportunities for real-world applications, Interface 17 (2008) 53e57.
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