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Supercapacitive Properties of Composite Electrode Consisting of Activated Carbon and Di(1-aminopyrene)quinone
Supercapacitive Properties of Composite Electrode Consisting of Activated Carbon and Di(1-aminopyrene)quinone
ETRI Journal. 2016. Apr, 38(2): 252-259
Copyright © 2016, Electronics and Telecommunications Research Institute (ETRI)
  • Received : June 10, 2015
  • Accepted : September 30, 2015
  • Published : April 01, 2016
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About the Authors
Kwang Man Kim
Young-Gi Lee
Jeong Ho Park
Jang Myoun Ko

Abstract
Di(1-aminopyrene)quinone (DAQ) as a quinone-containing conducting additive is synthesized from a solution reaction of 1-aminopyrene and hydroquinone. To utilize the conductive property of DAQ and its compatibility with activated carbon, a composite electrode for a supercapacitor is also prepared by blending activated carbon and DAQ (3:1 w/w), and its supercapacitive properties are characterized based on the cyclic voltammetry and galvanostatic charge/discharge. As a result, the composite electrode adopting DAQ exhibits superior electrochemical properties, such as a higher specific capacitance of up to 160 F·g −1 at 100 mV·s −1 , an excellent high-rate capability of up to 1,000 mV·s −1 , and a higher cycling stability with a capacitance retention ratio of 82% for the 1,000th cycle.
Keywords
I. Introduction
Energy-storage devices, such as rechargeable batteries and supercapacitors, are now demanding higher energy and power densities for applications from small-scale ubiquitous electronic devices to large-scale electric vehicles and energy storage systems. Various electrode materials and electrolytes have been attempted to enhance the performance of energy-storage devices. In particular, electrolyte additives using redox-active chemical species [1] , [2] were introduced into a conventional electrolyte solution to improve the electrochemical performance of supercapacitors. It was also notable that quinone-containing organic species might play a role in enhancing the supercapacitive properties from their redox-active behaviors through a quinone-hydroquinone transition [3] [14] . Moreover, an activated carbon supercapacitor adopting 2,5-bis((2-(1H-indole-3-yl)ethyl) amino)cyclohexa-2,5-diene-1,4-dione (HBU) as a quinone-containing conducting additive was recently proved to achieve a higher specific capacitance of 130 F·g −1 within the range of 100 mV·s −1 to 1,000 mV·s −1 [15] . The addition of the HBU was beneficial in the enhancement of the electrochemical stability of the electrolytes and the cycle performance of the supercapacitor. These results were due to the single-step, four-electron (4 e ), four-proton (4H + ) redox process of a quinone-hydroquinone couple and the −NH groups of indole structures within the HBU.
On the other hand, aromatic amine-quinone complexes have shown higher electrical properties than pure component materials [16] . Of these, solid complexes of 1-aminopyrene with quinone have exhibited a high electrical conductivity as mostly radical-ion salts [17] . An aminopyrene-cation radical was recently proved to be important at controlling the formation of conducting polyaminopyrene materials [18] . Amino-group-substituted oligopyrene also showed a good redox activity for an enhanced fluorescence property [19] . It can thus be expected that the use of a conducting compound consisting of aminopyrene and quinone components would be a promising additive for enhancing the electrochemical performance of an activated carbon supercapacitor, similar to the case of using HBU as a conducting additive [15] . That is, the adoption of aminopyrene as a source compound of an additive with quinone, instead of tryptamine [15] , will be more beneficial because the aminopyrene has its own conducting property [17] , [18] and the synergistic effect in the supercapacitive properties can be expected by combining with quinone as another conducting component.
In this study, di(1-aminopyrene)quinone (DAQ) is synthesized using a solution reaction at room temperature and used as an additive with activated carbon to yield a composite electrode as an active material of a supercapacitor. The electrochemical properties of the symmetric supercapacitor fabricated using composite electrodes are characterized using a cyclic voltammetric measurement. Further enhancements of the supercapacitive performance can be expected by including DAQ, rather than adopting HBU [15] .
II. Experiments
DAQ as an additive for the composite electrode with activated carbon was prepared through a solution reaction of hydroquinone and 1-aminopyrene. First, 500 mg of hydroquinone (> 99%, Aldrich) was dissolved in 15 mL of ethanol. A solution containing 530 mg of 1-aminopyrene (97%, Aldrich) and 84 mg of cerium(III) chloride heptahydrate (CeCl 3 ·7H 2 O, Aldrich) in 10 mL of ethanol was added and stirred for 12 h at room temperature to provide a precipitate-containing solution. The precipitate was obtained by centrifuging the solution. It was then washed successively with deionized water, 2-propanol, and hexane. The precipitate was finally dried overnight at 60 °C to 80 °C to provide the DAQ powder. Figure 1(a) shows the chemical structures of the main substances used in this study.
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(a) Chemical structures of substances (hydroquinone, 1-aminopyrene and DAQ) used in this study and (b) redox reaction of DAQ.
To use the obtained DAQ as a conducting additive for a supercapacitor electrode, a viscous slurry for the composite electrode was prepared by mixing activated carbon (MSC-30, specific surface area of 3,000 m 2 ·g −1 , Kansai Cokes, 60 wt.%) as the active material, DAQ (20 wt.%) as a conducting additive, and poly(vinylidene fluoride) (Aldrich, 20 wt.%) as a polymeric binder with N -methyl-2-pyrrolidone as a dispersion solvent in a ball-mill at ambient temperature for 3 h. The composite electrode was fabricated by coating the slurry on a platinum current collector (1.0 cm × 1.0 cm) and drying at 100 °C in an oven to evaporate the solvent component. The surface morphologies of activated carbon, DAQ powder, and their composite were observed using a scanning electron microscope (Hitachi S-4800). As a result, 0.12 mg was confirmed as the loading level for the nonvolatile component applied to the surface of the platinum current collector. For comparison, a sample of the activated carbon electrode was also prepared in the same manner without DAQ. To compare the electrical conductivity of each sample in an aqueous electrolyte solution of 1 M H 2 SO 4 , a complex impedance spectroscopy was also performed using an Autolab instrument (PGstat 100, Eco Chemie) in the frequency range of 10 −2 Hz to 10 5 Hz with a stimulus potential of 0.45 V. The chemical species of hydroquinone, 1-aminopyrene, and the synthesized DAQ powders were characterized using Fourier-transform infrared spectroscopy (Bomem MB100).
The cyclic voltammetry measurement was conducted in a three-electrode cell, which was equipped with a reference electrode made up of Ag/AgCl saturated with KCl, platinum as a counter electrode, and samples (DAQ, the activated carbon, and their composite) as a working electrode in an aqueous electrolyte solution of 1 M H 2 SO 4 , using an Autolab instrument (PGstat 100, Eco Chemie) at different scan rates of 100 mV·s −1 to 1,000 mV·s −1 at a potential range of −0.2 V to 0.8 V versus Ag/AgCl. The specific capacitance ( C ) was calculated as a function of the scan rate using the equation
(1) C = | q a  +  q c |/( 2mΔV ),
where q a , q c , m , and Δ V denote the anodic and cathodic charges on each scan, the mass of the active material, and the potential window of the cyclic voltammetry, respectively. To compare the potential capacitances of the composite and activated carbon electrodes, a galvanostatic charge/discharge cycle test was also carried out at a constant current density of 5.0 mA·cm −2 using a cycler (Toscat 3000, Toyo Systems).
III. Results and Discussion
The synthesized DAQ can be identified by comparing the characteristic peaks of the Fourier-transform infrared spectra in Fig. 2 . The hydroquinone has characteristic sharp infrared bands at 759 cm −1 and 1,520 cm −1 , corresponding to C-O stretching and C-H in-plane bending vibrations, respectively [20] . Meanwhile, the infrared spectra of 1-aminopyrene show a sharp peak at 831 cm −1 , corresponding to the bending vibration of N-H. The broad bands at 3,028 cm −1 are attributed to the C-H stretching vibration of the aromatic ring, whereas the C=C vibrations of the aromatic ring are shown at 1,620 cm −1 and 1,483 cm −1 [19] . As a result, DAQ is known to be well-synthesized because the infrared spectra of DAQ adequately contain the characteristic peaks of both hydroquinone and 1-aminopyrene. That is, it is expected that DAQ as a conducting additive can help with the conduction of electrons and protons within the electrode material of the activated carbon supercapacitors. Moreover, the composite electrode composed of activated carbon and DAQ (3:1 w/w) will exhibit concomitantly the dominant supercapacitive behavior of activated carbon, and the minor but important influence of DAQ on the electrochemical performance.
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(a) Fourier-transform infrared spectra of hydroquinone, 1-aminopyrene and DAQ synthesized and (b) enlarged spectra in wavenumber range of 650 cm−1 to 1,700 cm−1.
The fact that solid complexes of 1-aminopyrene with quinone exhibit a high electrical conductivity [17] may help to increase the electrical conductivity of the composite with activated carbon and DAQ (3:1 w/w), which includes aminopyrene and quinone species. The electrical conductivity values obtained at room temperature are 2.8 S·cm −1 and 3.8 S·cm −1 for the samples of activated carbon and composite with DAQ, respectively. The increases in the electrical conductivity, and thereby the specific capacitance through the addition of DAQ, may be dominated by the quinone-hydroquinone redox reaction in an acidic medium. The reduction of quinone into hydroquinone is a single-step two-electron (2 e ), two-proton (2H + ) process [4] [14] . The redox reaction of the quinone-hydroquinone couple may also occur in the DAQ, as shown in Fig. 1(b) , to enhance the electrochemical performance of the supercapacitor. In addition, another redox reaction involving a two-electron (2 e ), two-proton (2H + ) process may occur at different sites in the DAQ species, as shown in Fig. 1(b) , which is described precisely in our discussion of the cyclic voltammetry results.
The synthesized DAQ consists of primary particles with a size range of 1 μm to 3 μm to show a rough morphology, as shown in Fig. 3(b) . Though the primary particles of the activated carbon have an average size of 15 μm, their size can be reduced in a composite electrode consisting of activated carbon and DAQ (3:1 w/w) owing to the mechanical ball-milling during the slurry preparation, as shown in Fig. 3(c) . Thus, the surface of the composite electrode shows a homogeneous morphology including smaller DAQ particles (average size of 1 μm) distributed on the surfaces and in the gaps of the large activated carbon particles (average size of 5 μm). Pores distributed among the small DAQ and large activated carbon particles will help the synergistic proton migration to promote the surface redox reactions and form electric double layers, thereby enhancing the supercapacitive properties of the composite electrode.
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Scanning electron microscopic surface images of (a) activated carbon, (b) synthesized DAQ, and (c) composite with activated carbon and DAQ (3:1 w/w).
Cyclic voltammograms recorded at a low scan rate of 100 mV·s −1 are shown in Fig. 4 for the DAQ, activated carbon, and their composite electrodes. It is notable that the potential windows used for the supercapacitor materials differ from each other: from −0.1 V to 0.8 V (versus Ag/AgCl) for the DAQ electrode and from −0.2 V to 0.8 V (versus Ag/AgCl) for the activated carbon and composite electrodes. The cyclic voltammograms of the activated carbon electrode in the first and 1,000th cycles exhibit a highly rectangular shape, which is a typical electric-double-layer-capacitor behavior, as shown in Fig. 4(a) . In contrast, although the amplitude of the current density response becomes about 35-times smaller than that of an activated carbon electrode, a DAQ electrode shows simple sharp peaks at both ends of the potential range from −0.1 V to 0.8 V (versus Ag/AgCl) in the first cycle, as shown in Fig. 4(b) , indicating a reversible redox process of nitrogen in the amino group (−NH 2 ) of the 1-aminopyrene molecules. Meanwhile, a DAQ electrode in the 1,000th cycle also exhibits another pair of redox peaks at 0.48 V (anodic)/0.30 V (cathodic), corresponding to a quinone-hydroquinone redox reaction [5] , [21] [23] . It can be expected from such results that the redox behaviors of DAQ are dominated by the amine group in the initial cycles, but as the cycle number increases, the influence of the quinone-hydroquinone couple is highly intensified. Overall, a redox reaction of DAQ also occurs through a single-step, four-electron (4 e ), four-proton (4H + ) process, as shown in the mechanism in Fig. 1(b) .
On the other hand, the cyclic voltammogram of the composite electrode in the first cycle contains influences of both the activated carbon and the DAQ; that is, a rectangular shape at both ends of the potential range of −0.2 V to 0.8 V (versus Ag/AgCl) and two pairs of redox peaks at 0.37 V/0.3 V (quinone-hydroquinone couple) and 50 mV/0.0 V (amino group of DAQ), respectively. As the cycle number increases, the influence of the quinone-hydroquinone redox reaction becomes more significant, whereas the other effects of the activated carbon and the amino group remain nearly unchanged. That is, the quinone-hydroquinone couple in the DAQ structure is definitely active for the repeated oxidation-reduction processes, and is capable of expecting a superior specific capacitance for a long cycle life. Moreover, it should be kept in mind that such a result of the quinone-hydroquinone couple can be recognized in the composite electrode, not in the sole DAQ, indicating the existence of a synergistic effect for activating the quinone-hydroquinone couple, and thus boosting the current density response at the redox peak potentials. In addition, the cyclic voltammograms of the composite electrode for a supercapacitor as a function of the scan rate are also shown in Fig. 4(c) . As the scan rate increases, the anodic peak potential of the quinone-hydroquinone couple moves positively, and the cathodic peak potential shifts negatively, indicating a slow, less-reversible ion-diffusion by undergoing a surface-controlled redox process [24] .
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(a) Cyclic voltammograms of DAQ, activated carbon, and composite with activated carbon and DAQ (3:1 w/w) at 100 mV·s−1 after first and 1,000th cycles, (b) enlarged cyclic voltammograms of DAQ, and (c) cyclic voltammograms of composite electrode as function of scan rate.
Figure 5 shows the high-rate capability and cycling stability of the specific capacitances calculated from the cyclic voltammogram data. In the first cycle measured at a low scan rate of 100 mV·s −1 , higher specific capacitances were obtained; that is, about 160 F·g −1 for the composite electrode with activated carbon and DAQ, and about 100 F·g −1 and 45 F·g −1 for the electrode adopting activated carbon and DAQ, respectively. It is notable that the specific capacitance of DAQ can be greatly increased (45 F·g −1 to 160 F·g −1 ) by compositing with activated carbon, compared with a small increase (110 F·g −1 to 130 F·g −1 ) in HBU [15] . This is probably due to the difference in the electrical conductivity between DAQ and HBU as a major factor, and in the surface morphology as a minor factor. Although the precise data are presently unavailable, the electrical conductivity of DAQ is higher than HBU because 1-aminopyrene as another species in DAQ, with the exception of quinone, has a high electrical conductivity [16] , [17] compared to the 1H-indole-3-ethanamine or tryptamine as another species in HBU, which is intrinsically insulating but conducting in its polymer phase when proton-doped [25] [27] . Thus, the increased specific capacitance when using DAQ is due to its highly conducting property and the surface affinity formed by DAQ and activated carbon together.
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Specific capacitances of supercapacitor electrodes as functions of scan rate and cycle number when using activated carbon, synthesized DAQ, and composite of activated carbon and DAQ (3:1 w/w). Effect of cycle number was measured at scan rate of 100 mV·s−1. Specific capacitance of composite electrode adopting HBU is also indicated for comparison.
Moreover, the specific capacitance of the composite electrode remains mostly constant at a higher scan rate, whereas the DAQ electrode shows a steeply decreasing specific capacitance with an increase in the scan rate. In addition, the composite electrode with activated carbon and DAQ maintains a nearly constant specific capacitance (about 130 F·g −1 in the 1,000th cycle) for the prolonged cyclic voltammetry. The capacitance retention ratio is about 82% at the 1,000th cycle for the composite electrode, whereas the DAQ electrode reaches at most 17% (45 F·g −1 in the first cycle to 8 F·g −1 in the 1,000th cycle). Such a superior high-rate capability and cycling stability may be due mostly to the activation of the quinone-hydroquinone redox couple and the activation persistence within the composite electrode against a higher scan rate of 1,000 mV·s −1 , and the 1,000 repeated potential scans between −0.2 V to 0.8 V (versus Ag/AgCl). In addition, Fig. 6 shows the galvanostatic charge/discharge profiles of the composite electrodes containing DAQ, recorded at a constant current density of 5.0 mA·cm −2 . As expected, the average cycle time of the charge/discharge on a composite electrode containing DAQ is proved to be 83 s·mg −1 , whereas those of the activated carbon and composite containing HBU are 50 s·mg −1 and 70 s·mg −1 , respectively. A longer cycle time means that more electric energy is stored in the supercapacitor, thereby showing a higher specific capacitance.
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Initial charge/discharge profiles of supercapacitor electrodes of activated carbon and composite with DAQ. Charge/discharge profile of composite with HBU is also included for comparison.
IV. Conclusion
In the present study, DAQ was synthesized and used as an additive with activated carbon to yield a composite electrode for a supercapacitor. The prepared composite electrode adopting DAQ exhibits superior electrochemical properties in a stable potential window of −0.2 V to 0.8 V (versus Ag/AgCl); for example, an enhancement of the specific capacitance of up to 160 F·g −1 at 100 mV·s −1 , compared to 130 F·g −1 for the composite electrode adopting HBU. The composite electrode is also very stable against a high scan rate and prolonged 1,000-fold cycling. These findings are due to the higher conducting behavior of DAQ and to the decent compatibility with activated carbon, demonstrating that DAQ can be useful at enhancing the specific capacitance of activated carbon for supercapacitor application. That is, DAQ is a conducting additive and simultaneously a redox species that suffers a single-step four-electron four-proton process by quinone-hydroquinone couple and amine functional groups within its chemical structure.
This work was also supported by Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korean Government (MSIP) (No. B0186-15-001, Form factor-free multi input and output power module technology for wearable devices).
BIO
Corresponding Author kwang@etri.re.kr
Kwang Man Kim received his BS degree in chemical engineering from Yonsei University, Seoul, Rep. of Korea, in 1985 and his MS and PhD degrees in chemical engineering from the Korea Advanced Institute of Science and Technology, Seoul and Daejeon, Rep. of Korea, in 1988 and 1995, respectively. He was with the Research Institute of Industrial Science & Technology (RIST), Pohang, Rep. of Korea, from 1988 to 1991 and the Hyosung R&D Center, Gumi, Rep. of Korea, from 1997 to 1998. He joined ETRI in 1999 and has been working with the Research Section of Power Control Devices, where he has studied various materials and fabrication processes for versatile types (for example, wearable, flexible, and solid-state) of power source devices including supercapacitors and lithium rechargeable batteries. He has published over 110 papers in SCI journals and has over 60 international and domestic patents.
lyg@etri.re.kr
Young-Gi Lee received his BS degree in chemical engineering from Pusan National University, Rep. of Korea, in 1995 and his MS and PhD degrees in chemical engineering and polymer materials from the Korea Advanced Institute of Science and Technology in Daejeon, Rep. of Korea, in 1997 and 2001, respectively. He joined ETRI in 2001, where he has been working with the Research Section of Power Control Devices. His current research topics are the establishments of various solid electrolyte systems for lithium rechargeable batteries and wearable, flexible primary and secondary batteries and their fabrication.
jhpark@hanbat.ac.kr
Jeong Ho Park received his BS and MS degrees in chemistry from Seoul National University, Rep. of Korea, in 1987 and 1989, respectively. He received his PhD degree in chemistry from Texas A&M University, College Station, USA, in 1996. He was a research scientist at the Korea Institute of Science & Technology, Seoul, Rep. of Korea, from 1990 to 1991 and a postdoctoral fellow at the University of Florida, Gainsville, USA, from 1996 to 1998. He joined the Organic Materials Laboratory, Hanbat National University, Daejeon, Rep. of Korea, in 1998, where he has been working on various synthesized organic materials and their applications. He has published over 50 papers in SCI journals and has over 10 domestic patents.
jmko@hanbat.ac.kr
Jang Myoun Ko received his BS degree in chemical engineering from Korea University, Seoul, Rep. of Korea, in 1984 and his MS and PhD degrees in chemical engineering from the Korea Advanced Institute of Science and Technology, Seoul and Daejeon, Rep. of Korea, in 1989 and 1995, respectively. He has been a full professor at Hanbat National University, Daejeon, Rep. of Korea, since 1997. He was a postdoctoral researcher at the University of California, Los Angeles, (UCLA), USA, in 2000, and a visiting professor at the University of Wollongong, Australia, in 2007 and at UCLA in 2013. He received many awards from the Korean Government and Hanbat National University for his work on advanced energy-storage materials including rechargeable electrodes for lithium batteries, supercapacitor electrodes, separators, polymer electrolytes, and gel electrolytes.
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