Solvent-free Preparation of Electrochemical Capacitor Electrodes

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Solvent-free Preparation of Electrochemical Capacitor Electrodes Using Metal-free Redox Organic Compounds Hiroyuki Itoi, Yuka Yasue, Keita Suda, Seiya Katoh, Hideyuki Hasegawa, Shinya Hayashi, Masanao Mitsuoka, Hiroyuki Iwata, and Yoshimi Ohzawa ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01947 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 14, 2016

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ACS Sustainable Chemistry & Engineering

Solvent-free Preparation of Electrochemical Capacitor Electrodes Using Metal-free Redox Organic Compounds Hiroyuki Itoi,*,† Yuka Yasue,† Keita Suda,† Seiya Katoh,† Hideyuki Hasegawa,† Shinya Hayashi,† Masanao Mitsuoka,† Hiroyuki Iwata,‡ and Yoshimi Ohzawa† †

Department of Applied Chemistry, Aichi Institute of Technology, Yachigusa 1247, Yakusa-cho,

Toyota, 470-0392, Japan. ‡

Department of Electrical and Electronics Engineering, Aichi Institute of Technology, Yachigusa

1247, Yakusa-cho, Toyota, 470-0392, Japan. *E-mail: [email protected] KEYWORDS: Electrochemical capacitor, Solvent-free, Metal-free, Redox compound, Porous carbon

ABSTRACT: A Metal-free redox organic compound, 2,5-Dichloro-1,4-benzoquinone (DCBQ), was finely dispersed over the porous carbon substrate, Ketjen Black (KB), by a solvent-free method in only one step. By using this method, DCBQ was loaded inside the pores of KB with as much as ca. 60 wt % in the sample without any agglomeration. This high loading along with high dispersion can be attained due to the absence of solvent. And thus, solvent removal, at which

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process organic compounds tend to agglomerate as the concentration of organic compounds increases, and concomitant purification process are not necessary. The electrochemical behaviors of resulting composite materials were evaluated and found that the volumetric capacitance, which correlated with the capacitance per unit mass of KB, increased with the loading amount of DCBQ, with high power density and long cycle lifetime. The resulting volumetric capacitances reached 4.7 times higher than those of KB and exhibited high rate capability up to 5 A g−1, along with excellent cycle lifetime up to 10000 cycles. These results indicate the superiority of the pseudocapacitance induced inside the pores of porous carbon substrates by quinone derivatives over the electric double layer capacitance to gain both high power and high energy densities.

Introduction Electrochemical capacitors (ECs) utilize both an electric double layer and redox reactions for the electrical energy storage with high power density, and have a potential of the applications to a wide range of electrical appliances and electric vehicles.1 Thus far, considerable efforts have been devoted to enhance the energy density of ECs by complexing carbon substrates with redox materials such as metal oxides2-4 and conductive polymers.5,6 Also, as a substrate for the redox materials, nanostructured carbons, including carbon nanotubes,7-9 templated mesoporous carbons,10 graphenes,11-13 and etc., have been extensively examined. Indeed, ECs with enhanced energy density have been developed by the complexation of these materials, however, such redox materials suffer from poor cycle lifetime, low conductivity, and low power density. Low conductivity and low power density can be improved by the appropriate morphology and


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complexation of redox materials with carbon substrates, where a large contact area between them along with a short diffusion path for electrolyte ions are desirable.14 As a redox material, redox organic compounds have been also used for the enhancement in energy densities of ECs. Uchiyama and Kalinathan et al. have reported the enhancement of the electrochemical capacitance by using quinone/hydroquinone redox couples whereby redox organic compounds were chemically attached to porous carbon substrates.15,16 In these method, severe reaction conditions must be used to react organic compounds with chemically inert carbon substrates, however, not all quinone molecules cannot be attached to the carbon substrates. Also, not only the use of solvent but also purification process are indispensable for their electrode preparations. Redox organic compounds can be also immobilized onto porous carbon substrates by their physical adsorption ability. Leitner et al. have reported the enhanced capacitance via the combination of pseudocapacitance derived from 2-nitro-1-naphthol and electric double layer capacitance derived from a carbon substrate, with good redox reversibility of the compound.17 Isikli et al. have reported the ECs by using a quinone derivative, 1,4,9,10-anthraceneterraone, and carbon black or activated carbon as an carbon substrate.18 The quinone derivative was impregnated onto the carbon substrate and the resulting material showed the capacitance enhancement by more than 50% in comparison with a pristine carbon substrate, along with good cycle lifetime. Recently, Honma et al. have also reported the ECs decorated with quinone derivatives, anthraquinone (AQ) and tetrachlorohydroquinone (TCHQ), demonstrating high energy density and long cycle lifetime using asymmetrical cell configuration.19,20 In all cases, ECs still suffer from inferior capacitance compared to metal oxides and conductive polymers. Moreover, cycle lifetime was measured up to 1000 cycles, however, thousands of cycle lifetime is desirable for ECs.


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Though the electrochemical performances of ECs reported in this decade have been enhanced in terms of power density, energy density, cycle lifetime, etc., however, these advantages are not compatible with each other in most cases. Moreover, the structures of ECs have been getting more complex and their preparations include many steps, rendering their reproducibilities and practical applications difficult. Therefore, a simple procedure for the preparation of the electrode materials for ECs must be developed. In this work, we demonstrate a simple and feasible fabrication of the electrode materials for ECs without any chemical reactions and cumbersome procedures, whereby an organic redox compound, 2,5-Dichloro-1,4-benzoquinone (DCBQ), is simply adsorbed into a porous carbon substrate, Ketjen black (KB), in only one step. This procedure is free from not only organic solvents but also solvent removal and concomitant purification process. Furthermore, the absence of the solvent enhance the loading amount of redox organic compounds (up to 60.1 wt %), because solvent molecules together with redox organic molecules are incorporated into the pores of the porous substrate in a conventional liquid phase adsorption. Also, this solventfree preparation enables the fine dispersion of redox organic molecules inside the pores of the porous substrates without any agglomeration. Because the use of solvent needs the solvent removal at the end of preparation, where the concentration of redox organic compounds increases through the course of solvent removal, resulting in the agglomeration of organic compounds not only on the outer surface of the porous substrates but also inside the pores of the substrates, especially for the preparation of high loading samples. By using the solvent-free method, the ratio of the redox organic compound and the porous substrate can be accurately adjusted unless the amount of the organic compound does not exceed the saturation of the substrate. Fine dispersion of redox organic molecules, of which electrical conductivities are


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generally very low, enables fast redox reaction necessary for high power density due to the good contact with conductive carbon substrates. In addition, this method enhances the volumetric capacitance efficiently because the electrode volume remains constant, irrespective of the loading amount of DCBQ in the porous carbon substrate. Furthermore, appropriate selection of an insoluble redox organic molecule enables 10000 cycles of redox reactions with excellent capacitance retention. We believe this simple and ideal method will expand the utilization of abundant redox organic compounds beyond the electrode fabrication for ECs, regardless of low electricical conductivity and physical state (e.g., solid, liquid, and gas) of organic compounds. Experimental Materials. Ketjen black (KB, ECP600JD) was kindly provided by Lion Corporation. 2,5Dichloro-1,4-benzoquinone (DCBQ, ≥98%, Tokyo chemical industry CO., LTD.) was used as received Sample preparation. For the sample preparation, ca. 100 mg of Ketjen black was added into a 10 mL glass ampoule and dried at 150 °C under vacuum for 6 h to remove adsorbed water. Dried KB was transferred into the ampoule, which had been weighed in advance, and the accurate mass of dried KB was calculated from the weight difference.. Then, DCBQ was added to a glass tube, followed by cupping the tube with quartz wool to avoid the direct contact of DCBQ with KB. The weight of DCBQ was accurately adjusted using a microbalance so as to achieve DCBQ/KB weight ratios of 5:95, 10:90, 20:80, and 40:60. Subsequently, the glass tube was inserted into the ampoule and sealed off under vacuum (for details, see Figure S1 in the supporting information). Finally, the ampoule was kept in an incubator at 100 °C for 24 h to complete the adsorption of DCBQ into KB. This temperature is a little higher than the


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sublimation temperature of DCBQ (92 °C) and suitable to complete the adsorption within 24 h. The samples are labeled as KB/DCBQ (X %), where X stands for the weight percent of DCBQ in the obtained samples. The sample saturated with DCBQ was prepared by using the excess amount of DCBQ, and its loading amount of DCBQ was calculated from the weight difference between dried KB and the obtained sample. Characterizations. Nitrogen adsorption/desorption isotherms (at –196 °C) were collected with a Micrometrics ASAP 2020 apparatus. Before the measurement, KB was dried at 150 °C under vacuum for 6 h. On the other hand, DCBQ in the KB/DCBQ samples is desorbed upon heating under vacuum, which disturbs the accurate estimation of surface areas and pore volumes. Thus, the KB/DCBQ samples were collected in an argon atmosphere glove box after the preparation to prevent the adsorption of water. Each KB/DCBQ sample was added to an analysis cell and the cell was sealed with a special cap which prevented the air exposure until the analysis. The cell was degassed at room temperature for 0.5 h before the measurement. The specific surface area (SBET) was estimated by the Brunauer–Emmett–Teller (BET) method using an adsorption isotherm at P/P0 = 0.05~0.20. Total pore volume (Vtotal) was estimated using the adsorption amount at P/P0 = 0.96. The micropore volume (Vmicro) was determined by the Dubinin–Radushkevich method. X-ray diffraction (XRD) analysis was conducted with XRD6100 (Shimadzu) with a Cu Kα radiation (λ = 1.5418 Å). A transmission electron microscope (TEM) observation was performed with a JEM-2010 (JEOL) under an accelerating voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDS) was recorded with a JEM-2100Plus (JEOL) equipped with Noran System 7 EDS system and integrated with a scanning device comprising an annular dark-field (ADF) and a bright-field (BF) detectors.


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Electrochemical measurements. Electrochemical analyses were conducted with a threeelectrode cell using 1 M H2SO4 as an aqueous electrolyte at 25 ºC. An electrode sheet was prepared by mixing the sample, carbon black (Denka black, Denka Company Ltd.), and poly(tetrafluoroethylene) (PTFE; PTFE 6-J, Du Pont-Mitsui Fluorochemicals Company, Ltd.) with a weight ratio of KB in the sample, acetylene black, and PTFE = 90 : 5 : 5. To prepare a working electrode, the electrode sheet was sandwiched with an amount of 8.5 mg of KB in the sheet by stainless steel mesh (100 mesh, Nilaco) and pressed at 30 MPa. A counter electrode was prepared by using activated carbon (SX-2, Norit) in the same manner as in the case of a working electrode. As a reference electrode, a Ag/AgCl electrode (sat. KCl) was used. Electrochemical analyses were performed on a potentiostat/galvanostat instrument (VMP3, Bio-logic). The potential range of all analyses was set to –0.1 to 0.8 V vs. Ag/AgCl. Cyclic voltammetry (CV) was measured at a sweep rate of 1 mV s–1 for 4 cycles. Galvanostatic charge/discharge analysis (GC) was conducted at current densities ranging from 50 to 5000 mA g–1. The gravimetric capacitances were calculated from obtained GC curves from –0.1 to 0.8 V. The densities of electrodes were measured to calculate the volumetric capacitances. For a fair comparison, electrode sheets of KB and KB/DCBQs were pressed at 30 MPa for 300 s, which was accurately controlled by using a precision universal / tensile tester (AUTOGRAPH, AG-X plus, Shimadzu), to form the pellets with a diameter of 13 mm. This pressure is the same as that for preparing the working electrodes. The electrode thickness was accurately measured by a micrometer (DIGIMICRO, MF-501, Nikon). A cycle test was conducted by using a two electrode cell at a current density of 1000 mA g–1 with a potential range of −0.1 to 0.8 V using 1 M H2SO4 as an aqueous electrolyte.


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Results and discussion Figure 1 shows the results of nitrogen adsorption/desorption measurements for KB and KB/DCBQs. The BET surface areas and pore volumes together with the amount of DCBQ in the samples are summarized in Table 1. As shown in Figure 1a, KB displays steep uptake and hysteresis at low and high relative pressure regions, respectively, indicating a combination of type I and IV isotherms, and has a high surface area (1340 m2 g–1) along with a large mesopore volume (1.24 cm3 g–1). The uptake near P/P0 = 1 arises from capillary condensation of nitrogen at inter-particle spaces of KB particles (particle size is ca. 30 nm). The amount of adsorbed nitrogen at low relative pressure region (P/P0 < 0.20), which corresponds to micropore filling, decreases with the amount of DCBQ in the sample. This result is in accord with the result that the BET surface areas of KB/DCBQs decrease with the amount of DCBQ in the samples (Table 1). Only the isotherm of KB/DCBQ (60.1%), which is saturated with DCBQ, no longer shows hysteresis, indicating that both micropores and mesopores are alsmost filled with DCBQ. The pore size distributions (PSDs) for KB and KB/DCBQs calculated from the BJH method give the good information about the mesopore structures. The PSDs indicate that substantial mesopore filling is accompanied with micropore filling also for KB/DCBQ (40%). It should be noted that the maximum amount of DCBQ in the samples reached as much as 60.1 wt %, of which value is not attainable by a conventional liquid phase adsorption.


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Figure 1. Nitrogen adsorption/desorption isotherms (a), and PSDs calculated by the BJH method (b).

Table 1. The BET Surface Areas and Pore Volumes of KB and KB/DCBQs. DCBQ

SBET

Vtotal

Vmicro

Vmeso

(wt %)

(m2 g–1)

(cm3 g–1)

(cm3 g–1)

(cm3 g–1)

0

1340

1.72

0.48

1.24

KB/DCBQ (5%)

5.0

1050

1.51

0.36

1.15

KB/DCBQ (10%)

10.0

1000

1.43

0.35

1.08

KB/DCBQ (20%)

20.0

770

1.28

0.27

1.01

KB/DCBQ (40%)

40.0

460

0.88

0.17

0.71

KB/DCBQ (60.1%)

60.1a

70

0.18

0.03

0.15

samples KB

a

Maximum adsorption of DCBQ in the samples.

Figure 2 shows XRD patterns of KB and KB/DCBQs along with DCBQ. KB shows two broad peaks at around 25º and 44º derived from carbon (002) and (10) diffractions, respectively. On the other hand, each KB/DCBQ sample shows a broad peak at around 25º and its peak intensity


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increases with the amount of DCBQ in the sample without any distinct peak as shown in DCBQ. Note that not only KB but also any other porous carbons show increasing broad peaks with the amount of organic compounds incorporated by the solvent-free method and their peak positions depend on the kind of organic compound. In this study, the peaks derived from the carbon (002) and adsorbed DCBQ incidentally appear at almost the same position. The absence of any distinct peak indicates that there are very few DCBQ molecules on the particle surface of KB because DCBQ molecules, if any, easily agglomerate on such a non-interactive carbon surface, and adsorbed DCBQ is not in a crystalline state, i.e., DCBQ molecules were finely dispersed inside pores of KB particles, even in KB/DCBQ (60.1%) with very high loading amount of DCBQ. This result strongly emphasizes the superiority of the solvent-free method over conventional liquid phase adsorption methods in terms of dispersion and loading amount. If there exists any agglomeration or nanosized particle, inner molecules in the agglomerations or particles cannot take part in the redox reaction due to their low electrical conductivities, which results in the waste of redox compounds. Moreover, agglomerations and nanosized particles in the pores disturb the ion diffusion, resulting in low power density. Therefore, the fine dispersion of redox compounds is indispensable not only for the effective utilization of redox compounds but also providing the facile ion diffusion to both redox compounds and carbon surface, which deeply correlate with high power and high energy densities.


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Figure 2. XRD patterns of KB, KB/DCBQs, and DCBQ.

The TEM images of KB and KB/DCBQ (60.1%) are shown in Figure 3. As is seen in the TEM images of KB (Figure 3a, b), KB has chain-agglomerate morphology with a hollow sphere structure. KB/DCBQ (60.1%) with a maximum amount of DCBQ also shows the same morphology as KB without any particle and agglomeration (Figure 3c, d). Note that this result does not mean that DCBQ in KB desorbed under ultra-high vacuum condition. DCBQ contains two chlorine atoms and existence of chlorine in KB/DCBQ (60.1%) was confirmed by X-ray photoelectron spectroscopy (XPS) analysis, which also needs ultra-high vacuum condition (Figure S4 in the Supporting Information). As a matter of course, no agglomerations or nanosized particles were not observed in all the other KB/DCBQs by TEM observation (data not shown). However, DCBQ consists of light elements and such light elements are not well detected by TEM observation. So by using a scanning transmission electron microscope (STEM) equipped with an energy dispersive X-ray spectrometry (EDS), we further conducted the STEM/EDS elemental mapping for KB/DCBQ (60.1%). As presented in Figure 3e, it can be confirmed that DCBQ are uniformly dispersed over the KB particles.


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Figure 3. TEM images of KB (a, b), KB/DCBQ (60.1%) (c, d), and STEM image and EDS spectra of KB/DCBQ (60.1%) (e). All scale bars in Figure 3e are 200 nm. For the electrochemical analysis, working electrode sheets were prepared by mixing the same weight ratio of KB in KB/DCBQs, carbon black, and PTFE as in the case of KB. To ensure the same contact between the electrode sheets and current collectors for all electrodes, the amount of KB (not including DCBQ) in all electrode sheets was fixed to the same (i.e., 8.5 mg), which makes all electrode sheets the same thickness (vide infra). The cyclic voltammograms for KB and KB/DCBQs are illustrated in Figure 4a. The voltammogram of KB is characterized by a rectangular shape, derived from typical electric double layer behavior. In contrast, the


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voltammograms of KB/DCBQs were deviated from a rectangular shape. Their peaks between ca. 0.2 and 0.6 V (vs Ag/AgCl) are indicative of the redox reaction of DCBQ, and KB/DCBQs with high DCBQ loading show high and broadened peaks. On the other hand, the current derived from electric double layer behavior decreases with the amount of DCBQ in the samples (Figure 4a, inset). It is expected from their voltammograms that pseudocapacitance dominantly contributes to their total capacitance, mainly for high DCBQ loading samples. Gravimetric capacitances of KB and KB/DCBQs were determined by galvanostatic charge/discharge cycling (GC) after the CV measurement. Figure 4b displays the GC curves for KB and KB/DCBQs measured at a current density of 50 mA g–1. The GC curve for KB showed a linear characteristic typical for electric double layer behavior. In contrast, KB/DCBQs show plateau derived from the redox reaction of DCBQ. As shown in Figure 4c, the capacitance enhancement by DCBQ is clearly confirmed for KB/DCBQs. Due to the large mesopore fraction of KB, KB retains the capacitance to large extent up to the current density of 5000 mA g–1 because mesopores reduce the diffusion resistance of ions.21,22 Similarly, the capacitances of all KB/DCBQs show excellent rate capability even at 5000 mA g–1. It has been reported in many published studies that the pseudocapacitance lacks high power density.23-26 Our group has reported the excellent rate capability of quinone functionalized zeolite-templated carbon (ZTC), which is chemically bonded by quinone groups through an electrochemical oxidation.27 ZTC has a high surface area and many edge sites available for chemical functionalization,28 and therefore, ZTC can be functionalized with many quinone groups,29 thereby enhancing pseudocapacitance with high power density.27 In this study, physically adsorbed quinone molecules exhibit fast redox-reaction, probably due to good contact with carbon surface by their fine dispersion. The highest gravimetric capacitance was observed for KB/DCBQ (40%) over all the current densities.


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The lower gravimetric capacitance of KB/DCBQ (60.1%) than that of KB/DCBQ (40%) is attributed to the decrease in both electric double layer capacitance and the utilization ratio of DCBQ. The electric double layer formation is disturbed by the adsorbed DCBQ molecules, resulting in the decrease of the electric double layer capacitance, as observed in the voltammograms (Figure 4c, inset). The utilization ratio of adsorbed DCBQ can be calculated by integrating the current derived from the pseudocapacitance in the voltammogram (Figure S3). The decrease in the utilization ratio of DCBQ is probably owing to the blockage of proton diffusion, which is necessary for the redox reaction of DCBQ, because there remains almost no pore for KB/DCBQ (60.1%) (Figure 1a,b). Note that Figure 4c shows the gravimetric capacitances (i.e., the capacitances per 1 g-sample containing both KB and DCBQ). In this study, DCBQ molecules were dispersed not on the outer surface of KB particles but inside the pores of KB (i.e., inner part of KB particles). Hence, the volume per unit mass of KB does not increase upon adsorbing DCBQ, which was experimentally confirmed (for details, see Table S1 in the Supporting Information). For the practical use, the volumetric capacitance is more important for energy storage devices than the gravimetric capacitance. The volumetric capacitances were calculated from the electrode densities and gravimetric capacitances (Figure 4d). The tendencies of the capacitance enhancement observed in Figure 4c and Figure 4d are quite different, especially for high DCBQ loading samples. The volumetric capacitance increases with the amount of DCBQ in the samples (Figure 4d). By comparing KB/DCBQ (60.1%) with KB, the volumetric capacitance of KB/DCBQ (60.1%) at 50 mA g–1 reached 146 F cm–3 and retained 4.7 times higher than those of KB over all the current densities, suggesting that the high loading of redox compounds into porous carbons prepared by the solvent-free method offers a significant enhancement in the volumetric capacitance. These


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results also proved the superiority of the pseudocapacitance induced inside the pores of porous carbon substrates by quinone derivatives over the electric double layer capacitance to gain both high power and high energy densities. In this study, electrode densities of KB and KB/DCBQs were accurately measured for a fair comparison of volumetric capacitances. However, electrode densities can vary depending on the preparation conditions (e.g., the pressure, the amount of binder, and etc.) even using the same material. Therefore an universal index for the comparison of the volumetric capacitances is of importance. Here, we propose appropriate indexes, the capacitance per 1 g of KB (F g-KB–1) and a theoretical density, which can be estimated by the fact that the electrode volumes per unit mass of KB do not increase regardless of the adsorbed amount of DCBQ (Table S1). The theoretical electrode densities for KB/DCBQs were calculated by using the experimental electrode density of KB, and in agreement with the experimental values. This result suggests that only the electrode density of a porous carbon substrate can offer all the densities of the composite materials unless the electrode volume per unit mass of the substrate does not increase upon the adsorption of the compounds. Moreover, there is an excellent correlation between the volumetric capacitances and the capacitances per unit mass of KB (Figure 4d and Figure S2). From these results, this estimation is proved to be reliable and can eliminate the measurement of all the electrode densities for the composite materials, which is easily influenced by changing the conditions.


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Figure 4. Electrochemical behaviors for KB and KB/DCBQs measured in 1 M H2SO4 at 25 °C. (a) Cyclic voltammograms of KB and KB/DCBQs measured at a scan rate of 1 mV s–1. All voltammograms are for the fourth cycle. Inset: A magnified voltammograms between ‒0.15 and 0.15 V. (b) GC curves for KB and KB/DCBQs obtained at a current density of 50 mA g–1. (c) Gravimetric capacitances at current densities ranging from 50 to 5000 mA g–1. (d) Volumetric capacitances at current densities ranging from 50 to 5000 mA g–1. DCBQ is not chemically bonded to KB but simply adsorbed inside the pores of KB, and therefore, its endurance for thousands of charge/discharge cycling is a large curiosity. Figure 5 illustrates the capacitance dependences of KB and KB/DCBQs on the number of charge/discharge cycling measured by using a two electrode cell at a current density of 1000 mA


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g–1. Although KB/DCBQ (60.1%) shows inferior cycle lifetime (ca. 70% retention) than KB, however the excellent capacitance retention up to 10000 cycles were obtained for KB/DCBQs, probably due to the strong adsorption ability of the porous carbon substrate and insolubility of DCBQ in an aqueous electrolyte. From these results, redox active functional groups are not necessarily chemically bonded to active materials for retaining high power density along with long cycle lifetime. By the solvent-free preparation using porous carbon substrates and redox compounds, the total charge of the redox compounds can be adsorbed into the substrates with both fine dispersion and an accurate ratio in only one step unless the amount of redox compounds exceeds the saturated amount, which can be easily determined. Moreover, this method is free from any chemical reactions, solvent, and purification process, and can be applied to any porous carbon substrates. Therefore, the solvent-free method can be expected to further extend beyond the electrode preparations for ECs.

Figure 5. The dependence of the capacitance retentions for KB and KB/DCBQs on the number of charge/discharge cycling measured using a two electrode cell and at a current density of 1000 mA g–1.


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Conclusions A metal-free redox organic compound, DCBQ, was finely dispersed over KB, being incorporated into the pores of KB by the solvent-free method in only one step. This method is free from solvent along with purification process, and maximum loading of 60.1 wt % could be attained without any agglomeration. Moreover, the total charge amount of DCBQ could be incorporated into KB with an accurate ratio unless the amount DCBQ exceeded the saturated amount. Therefore, this method can eliminate the waste of not only solvent but also excess redox organic compounds. The electrochemical behaviors of obtained composite materials were evaluated and found that the volumetric capacitance, which correlates with the capacitance per unit mass of KB, increased with the loading amount of DCBQ. The resulting volumetric capacitance of KB/DCBQ (60.1%) reached 146 F cm–3,at 50 mA g–1 and retained 4.7 times higher than those of KB over the current densities up to 5000 mA g–1. These results proved the superiority of the pseudocapacitance induced inside the pores of porous carbon substrates by quinone derivatives over the electric double layer capacitance, in terms of energy and power densities. Furthermore, excellent cycle lifetimes up to 10000 cycles could be attained. By using the solvent-free method and appropriate redox compounds, it becomes possible to achieve high energy density, high power density, and long cycle lifetime, which have been incompatible for the electrode materials using metal oxides or conductive polymers. This method can be applied to any porous carbon substrates and have the potential to widen the diversity beyond the electrode preparations for ECs.


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ASSOCIATED CONTENT Supporting Information. Sample preparation, experimental and theoretical electrode densities, method to calculate gravimetric and volumetric capacitances, Cl 2p XPS spectrum. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Hiroyuki Itoi E-mail address: [email protected] Tel: +81 565 48 8121 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Number 15K21478. REFERENCES (1) Conway, B. E. Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Press: New York, 1999. (2) Bao, L.; Zang, J.; Li, X. Flexible Zn2SnO4/MnO2 core/shell nanocable-carbon microfiber hybrid composites for high-performance supercapacitor electrodes. Nano Lett. 2011, 11, 12151220.


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For Table of Contents Use Only

Solvent-free Preparation of Electrochemical Capacitor Electrodes Using Metal-free Redox Organic Compounds Hiroyuki Itoi,* Yuka Yasue, Keita Suda, Seiya Katoh, Hideyuki Hasegawa, Shinya Hayashi, Masanao Mitsuoka, Hiroyuki Iwata, and Yoshimi Ohzawa

Synopsis: A metal-free redox organic compound was finely dispersed over porous carbon substrate by the solvent-free method and electrochemical performance of resulting materials was evaluated.


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Solvent-free Preparation of Electrochemical Capacitor Electrodes

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