Characteristics of powdered activated carbon treated with dielectric barrier discharge for electric double-layer capacitors

Electrochimica Acta 77 (2012) 198–203

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Characteristics of powdered activated carbon treated with dielectric barrier discharge for electric double-layer capacitors Daisuke Tashima a,∗ , Hiromu Yoshitama b , Tatsuya Sakoda c , Akihito Okazaki d , Takayuki Kawaji e a

Interdisciplinary Research Organization, University of Miyazaki, 1-1 Gakuenkibanadai-Nishi 1-1, Miyazaki 889-2192, Japan Interdisciplinary Graduate Schools of Agriculture and Engineering, University of Miyazaki, 1-1 Gakuen Kibanadai Nishi, Miyazaki 889-2192, Japan Department of Electrical and Electronic Engineering, Faculty of Engineering, University of Miyazaki, 1-1 Gakuenkibanadai-Nishi, Miyazaki 889-2192, Japan d Department of Mechanical Engineering, Nippon Institute of Technology, 4-1, Gakuendai, Miyasiro-machi, Minamisaitama-gun, Saitama 345-8501, Japan e UD Trucks Corporation, 1-1, Ageo-shi, Saitama 362-8523, Japan b c

a r t i c l e

i n f o

Article history: Received 10 February 2012 Received in revised form 18 May 2012 Accepted 30 May 2012 Available online 8 June 2012 Keywords: Surfaces Surface properties Electrochemical properties Photoelectron spectroscopy

a b s t r a c t The electrochemical properties of electric double-layer capacitors (EDLCs) made with plasma-treated powdered activated carbon (treated using a dielectric barrier discharge) were examined using cyclic voltammetry (CV), Cole–Cole plots, and X-ray photoelectron spectroscopy (XPS). The dielectric barrier discharge method, which operates at atmospheric pressure, dramatically reduces the processing time and does not require vacuum equipment, making it a more practical alternative than low-pressure plasma treatment. The experimental data indicate that the specific capacitance of the EDLCs could be improved by oxygen plasma treatment. Capacitance of EDLCs made with activated carbon treated for 15 s showed 193.5 F/g that 20% increase in the specific capacitance relative to untreated EDLCs. This result indicates that the plasma treatment yields EDLCs that are suitable for high-energy applications. The enhancement of capacitance was mainly attributed to an increase in the BET surface area of the activated carbon and the creation of carboxyl groups on the surface of the carbon. The carboxyl groups induced oxidation–reduction reactions in the presence of O2 which was included in the operation gas. In addition, the carboxyl groups improved the penetration of the electrolyte solution into the carbon electrodes. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Recently, research on EDLCs [1] has led to many improvements in their function [2–4]; however, there is still need for enhancement of certain characteristics such as the specific energy density and capacitance. EDLCs utilize a double layer formed at the interface between a nanoporous carbonaceous electrode and the electrolyte solution. Activated carbons have been used as the electrode material in EDLCs because of their relatively low cost and very high specific surface area. Because the movement of charge through EDLCs proceeds only by the physical adsorption of ions, EDLCs have good response characteristics for load changes. However, one limitation of EDLCs is their low energy density. In order to solve this problem, the development of new electrolytes and new carbon electrode materials is underway [5–7]. In our recent research on EDLCs, attempts were made to increase the capacitance of EDLCs by using carbon nanospheres [8] and by mixing either Ketjenblack or carbon nanotubes into the activated-carbon electrode material

∗ Corresponding author. Tel.: +81 985 58 7868; fax: +81 985 58 7868. E-mail address: [email protected] (D. Tashima). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.05.105

[9,10]. In addition, we attempted to modify the surface of carbon sheet electrodes by means of plasma chemical reactions [11–13]. Other recent attempts at improving the functionality of EDLCs have explored methods for the electrochemical treatment of activated carbons [14]. For example, Momma et al. reported that the capacitance of EDLCs increased when carbon activated by electrochemical oxidation processing at 1.15 V for 1 h. In addition, Okajima et al. reported that etching the surface layer of activated carbon fibers (ACFs) using oxygen plasma resulted in an increase in the capacitance [15]. The plasma-treated ACFs had a specific surface area of 1500 m2 /g. The capacitance increased by 28% in comparison with that of the untreated sample, and the highest capacitance, 142 F/g, was achieved with an oxygen feed concentration of 10 vol%. The specific surface area of 2103 m2 /g was 34% higher than that of an untreated sample. Furthermore, treatment of the ACFs with oxygen plasma generated functional groups on the surface of the ACFs, which increased the specific capacitance of EDLCs employing aqueous electrolytes. Carboxyl group, carbonyl group, quinone group and phenolic hydroxyl groups was reported as surface functional group of carbon materials [16]. Oda et al. also reported that oxygen containing groups such as carboxyl groups increase the specific capacitance of EDLCs with aqueous electrolytes [17].

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Teller (BET) method [18] and these pore size distributions were calculated by the Barrett, Joyner, and Halenda (BJH) [19] and micropore (MP) [20] method by using nitrogen adsorption isotherms at −196 ◦ C, which were measured using a Tristar 3000 analyzer (Shimadzu Co.). 2.3. CV for electrochemical analysis

Fig. 1. Experimental setup for plasma treatment.

However, the long processing time required for the electrochemical treatment method makes it unsuitable for practical application. Furthermore, the plasma treatment is applied at reduced pressure, which requires vacuum equipment. In order to make this technique suitable for practical use, we have used a dielectric barrier discharge operated at atmospheric pressure for the plasma treatment. In comparison to the low pressure plasma treatment, the dielectric barrier discharge can dramatically reduce the processing time and does not require vacuum equipment. The focus was placed on increasing the surface area and the surface functional groups of the activated carbon by using a dielectric barrier discharge with an operation gas mixture containing Ar and O2 . 2. Experimental methods

The electrochemical properties of the polarized electrode were evaluated by cyclic voltammetry (CV). The polarized electrode was made of plasma-treated activated carbon, polytetrafluoroethylene (PTFE), and acetylene carbon black (Streams chemicals, surface area: 66 m2 /g), which were mixed at weight ratio of 8.5:0.5:1. The activated carbon was dried at 20 Pa for 30 min in a vacuum chamber (As One Co., Ltd., dome-type acryl vacuum box) in order to allow for impregnation of the micro or mesopores with both anionic and cationic species from the electrolyte. After the vacuum treatment, the EDLCs were evaluated using a CV measurement system (HZ-5000, Hokuto Denko Co.). The measurements were carried out at 20 ◦ C in the voltage range of −0.5 to 0.5 V, with a scan rate of 5 mV/s, using an Ag/AgCl as the reference electrode and 200 mm2 platinum electrodes as the counter electrode. Polarized electrode as the working electrode had an area of 78.5 mm2 and thickness of all samples are approximately 550 ␮m measured by micrometer (MDC-25MJ, Mitsutoyo, Japan). A 0.5 mol/dm3 solution of H2 SO4 was used as an electrolyte. The complex impedance of the EDLC cells was measured with an impedance measurement system (FRA5020, Hokuto Denko Co.), using an AC two electrode method. A 0.2-mA AC current was applied across the EDLC cells with a frequency range of 10 mHz to 20 kHz at a natural potential of approximately 0.25 mV. We assume that when the potential, V, applied to the working electrode is swept anodically or cathodically from V1 to V2 at a constant scan rate, rs , the current, i, as a function of time, t, flows to yield a total charge, −Q or +Q in each scan. Then the total charge is given by half the area, Sc , of the rectangular i–V curve and hence the capacitance, C, of the EDLC can be evaluated by the following Eq. (1) using the total one-way scan time, ts .

 ts C=

 0V2 V1

[i dt] [dV ]

=

(i/rs )

 V2 V1

 V2 V1

[i dV ]

[dV ]

=

(1/2rs )Sc V2 − V1

(1)

2.1. Plasma treatment using dielectric barrier discharge 2.4. X-ray photoelectron spectroscopy (XPS) analysis The experimental setup for the plasma treatment shown in Fig. 1 is comprised of a chamber composed of a stainless steel rod covered with a glass tube having an inner diameter of 13.3 mm and an outer diameter of 16.0 mm, which in turn is covered with a glass tube having an inner diameter of 16.8 mm and an outer diameter of 20.0 mm. Copper tape was attached to the surface of the outer glass tube. The discharge gap length was 0.8 mm. The operating gas was a 9.5:0.5 mixture of Ar and O2 gases, respectively, which was kept at a total flow rate of 900 ml/min. The plasma treatments of activated carbon were carried out for 10 s, 15 s, 20 s, 25 s, and 30 s. An AC voltage of 2.0 kV with a frequency of approximately 8 kHz was applied to the copper electrode. Taikou P (surface area: 888 m2 /g) from Futamura Chemical Co., Ltd. was used as the activated carbon, which was mixed with the operating gas and supplied from the top of the glass tube. A combination of gravity and operating gas flow pressure allowed the activated carbon to move downward, and the material was plasma-treated as it moved down through the tube. The treated carbon was recovered from an acrylic vessel. 2.2. Pore size analysis of powdered activated carbon The surface area of the activated carbon with various plasma treatment times were calculated by the Brunauer, Emmett, and

X-ray photoelectron spectroscopy (XPS; AXIS-HS, Shimadzu/Kratos Co.) was performed in order to determine the types of oxygen-containing functional groups. The XPS spectra were recorded using Mg K␣ radiation through background processing. The analysis of the spectrum was performed using a non-linear least-squares fitting program with a symmetric Gaussian function. The surface composition of the samples was calculated with C 1s, O 1s and C O combination peaks and the appropriate sensitivity factors [21]. 3. Results and discussion 3.1. SEM image of powdered activated carbon Fig. 2(a)–(f) shows the scanning electron microscopy (SEM, Hitachi S-4800) images of the plasma-treated samples for various treatment times. A wood structure of the raw material are confirmed in Fig. 2(a) and (b), the wood structure is not understood easily in Fig. 2(c)–(f). There was a gradual decrease in the size of the plasma-treated carbon particles with increasing treatment time. The reason that carbon particles were connected with oxygen and release carbon monoxide at high temperature [22], which these

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Fig. 2. SEM images of plasma-treated samples for various treatment times. (a) No treatment, (b) 10 s, (c) 15 s, (d) 20 s, (e) 25 s and (f) 30 s.

phenomena may occurred by the heat of the plasma with increasing treatment times. 3.2. Pore size analysis of plasma-treated activated carbon Fig. 3(a) shows the nitrogen adsorption–desorption isotherms of the plasma-treated samples. Typically, type I and type IV isotherm were indicated as reported in IUPAC [23]. Three events were observed for the adsorption–desorption isotherms. The initial rapid adsorption at P/P0 ∼ 0 observed in each isotherm is mainly due to N2 uptake by micropores of ∼2 nm or less in size. The subsequent gradual adsorption over a wide range of relative pressures (P/P0 = 0–0.6) is due to capillary condensation of N2 in mesopores with a broad range of pore sizes. The adsorption–desorption isotherms of each sample at P/P0 = 0.9 or below, indicating that there is a little interaction between N2 and mesopores in carbon. The adsorption by the second mode was not very marked for these samples. In addition to

the adsorption–desorption isotherms described above, data were also obtained for MP and BJH pore-size distribution curves, BET surface area (SBET ), micropore volume (Vmicro ), and mesopore volume (Vmeso ). Fig. 3(b) shows pore-size distribution curves obtained from MP and BJH methods, indicate micro pore (<2 nm) volumes of the all plasma-treated samples increase compared to those of the untreated sample and meso pore (2–50 nm) volume of the all plasma-treated samples decrease for the untreated sample. Table 1 shows the results of SBET , Vmicro , and Vmeso analyses of the plasmatreated samples. The SBET of the samples plasma-treated for 15 s increased by approximately 16.2% compared to the untreated sample. Treatment for periods longer than 15 s resulted in a decrease in the SBET . These results are closely related to the pore sizes on the surface of particles of the activated carbon powder. Therefore, the large pore sizes of the activated carbon powder in the case of 25-s and 30-s plasma treatment accounts for the decrease in the SBET of these samples. It was also determined that Vmicro was larger than

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Fig. 3. (a) N2 adsorption isotherms and (b) MP and BJH pore size distribution curves of plasma-treated samples. Table 1 BET surface area (SBET ), micropore volume (Vmicro ), and mesopore volume (Vmeso ) of plasma-treated samples. BET surface area (SBET ) m2 /g No treatment 888 953 10 s 1032 15 s 960 20 s 901 25 s 903 30 s

Pore volume (Vmicro : <2 nm) cm3 g−1

Pore volume(Vmeso : 2–50 nm) cm3 g−1

0.370 0.398 0.432 0.402 0.379 0.382

0.297 0.250 0.248 0.217 0.261 0.274

Vmeso for all samples, and that plasma surface treatment generally increases the Vmicro of the activated carbon powders. Thus, the plasma surface treatment using dielectric barrier discharge is effective for increasing the amount of micropores in activated carbon powder. 3.3. Characterization of electrochemical capacitor Cyclic voltammograms and Cole–Cole plots of the plasmatreated activated carbon samples are shown in Fig. 4(a) and (b). The cyclic voltammograms show that the ion adsorption and desorption between the working electrode and the electrolyte occur with changes in the voltage. The current increases at both lower and higher potential values as the number of adsorbed ions increases; which means not only that the surface area of the activated carbon

Fig. 4. Electrochemical characterization of plasma-treated samples in 0.5 mol/dm3 H2 SO4 : (a) cyclic voltammogram and (b) Cole–Cole plot.

is large, but also that reduction–oxidation reactions are occurring with the functional groups on the surface of the plasma-treated activated carbon samples. Oda et al. reported that a large number of functional groups promote both the wettability of the electrodes as well as the negative charge of the electrodes, leading to an increase in capacity. Furthermore, a significant increase in the number of phenolic hydroxyl groups on the positive electrode was confirmed in long cycle test [17]. Therefore, it is considered that the wettability of the electrodes are influenced by the carboxyl groups generated by the dielectric discharge plasma, whereby the presence of carboxyl groups results in oxidation–reduction reactions and therefore in increased capacitance. The AC impedance spectra are shown in Fig. 4(b). Each impedance spectrum describes a semicircular path in the middle-frequency range of 2–20 kHz, and a line path inclines to the real axis in the low-frequency range of 10 mHz to 2 kHz. The distance between the high-frequency part of the semicircle and the starting point on the real-axis indicates the bulk resistance of the carbon electrode. The semicircular path indicates the reaction resistance. Based on the Cole–Cole plot, the internal resistance of the activated carbon treated for 15 s was the same as that of the parent sample. Fig. 5 shows the specific capacitance, calculated from the cyclic voltammograms, for various plasma treatment times which include 60, 90 s plasma treatments. The change in the capacitance

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Fig. 5. Specific capacitance calculated from cyclic voltammograms for various plasma treatment times.

is dependent on the treatment time. The capacitance of the EDLCs employing activated carbon treated for 15 s was 193.5 F/g, which increased by 20% in comparison with that of those in which the parent sample was used. In addition, the change of capacitance for various treatment times shows a trend similar to the change in surface area shown in Table 1. It was also observed that the specific capacitance of the samples that were plasma-treated for over 30 s gradually decreased with increasing treatment times. Although it was previously found that the specific capacitance increased linearly as the BET surface area increased [24,25], there was a 20% increase in the capacitance of the samples treated for 15 s, in spite of the fact that the percentage increase in the BET surface area was 16.2%. This discrepancy suggests that it is probable that the observed increase in the electrochemical capacitance results from the Faraday reaction, which is caused by the functional groups on the surface of the activated carbon.

Fig. 6. C 1s and C O combination spectra of plasma-treated samples for various treatment times. (a) No treatment, (b) 10 s, (c) 15 s, (d) 20 s, (e) 25 s and (f) 30 s.

D. Tashima et al. / Electrochimica Acta 77 (2012) 198–203 Table 2 Atomic ratios (O 1s)/(C 1s) and ratio of functional groups (C O)/(C 1s) on surface of plasma-treated samples.

No treatment 10 s 15 s 20 s 25 s 30 s

(O 1s)/(C 1s)

Functional group ratio (C O)/(C 1s)

0.089 0.299 0.283 0.217 0.208 0.393

0.285 0.328 0.310 0.284 0.270 0.310

3.4. XPS spectra of plasma-treated activated carbon Fig. 6(a)–(f) shows the C ls and C O combination spectra of the plasma-treated activated carbon following deconvolution of the obtained spectrum. The acquired spectrum was deconvoluted into the two component spectra with peak energies of 285 eV and 286 eV using a non-linear least-squares fitting program with a symmetric Gaussian function. The spectrum at around 285 eV corresponds to C 1s whereas the spectrum at around 286 eV is due to a C O combination, which indicates the presence of a carboxyl group. O 1s spectra at around 532 eV were also observed, which indicates the presence of an oxygen containing functional group. Table 2 shows the O 1s/C 1s atomic ratios and the C O/C 1s ratio on the surface of the plasma-treated activated carbon. The O 1s:C 1s atomic ratio and C O:C 1s ratios of the samples treated for 15 s are 3.2 times higher and 8.8% greater than the parent sample, respectively. On the other hands, the capacitance of the EDLCs using activated carbon treated for 10 s and 15 s were low in spite of high O 1s:C 1s atomic ratio and C O:C 1s ratios of these samples. This indicates that the capacitances were mainly affected by the BET surface areas and oxygen-containing functional groups, including the carboxyl group, were incorporated into the high BET surface area (1032 m2 /g) of the activated carbon sample which was plasma-treated for a period of 15 s. These carboxyl groups and the high surface area of the 15 s-treated samples led to the increased specific capacitance of these samples. 4. Summary In this study, the dielectric barrier discharge method, which is suitable for practical use, was used to modify powdered activated carbons to improve their capacitance in EDLCs. Optimal improvements in capacitance were achieved using a treatment time of 15 s at an AC voltage of 2.0 kV with a frequency of approximately 8 kHz in a mixed gas of Ar:O2 (9.5:0.5). The capacitance for the optimum treatment time of 15 s was found to be 20% higher than that of an untreated sample.

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Acknowledgements The authors thank Masatoshi Esaki and Ryotaro Hirakawa, graduate students from the Department of Electrical and Electronic Engineering at the Graduate School of Engineering of the University of Miyazaki, for his help with this study. This study was partially supported by a Grant-in-Aid for Trust Research from the UD Trucks Corporation, and a Scientific Research Grant from the Japan Society for the Promotion of Science and Program to Disseminate Tenure Tracking System from the Japanese Ministry of Education, Culture, Sports, Science and Technology and a grant for Scientific Research on Priority Areas from the University of Miyazaki. References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Plenum Press, New York, 1999. [2] H.-Y. Liu, K.-P. Wang, H. Teng, Carbon 43 (2005) 559. [3] G.-J. Lee, S.-I. Pyun, Electrochimica Acta 51 (2006) 3029. [4] T. Morishita, Y. Soneda, H. Hatori, M. Inagaki, Electrochimica Acta 52 (2007) 2478. [5] T. Sato, G. Masuda, K. Takagi, Electrochimica Acta 49 (2004) 3603. [6] C.-H. Hou, C. Liang, S. Yiacoumi, S. Dai, C. Tsouris, Journal of Colloid and Interface Science 302 (2006) 54. [7] I. Stepniak, A. Ciszewski, Electrochimica Acta 56 (2011) 2477. [8] D. Tashima, E. Yamamoto, N. Kai, D. Fujikawa, G. Sakai, M. Otsubo, T. Kijima, Carbon 49 (2011) 4848. [9] D. Tashima, K. Kurosawatsu, M. Uota, T. Karashima, M. Otsubo, C. Honda, Y.M. Sung, Thin Solid Films 515 (2007) 4234. [10] D. Tashima, H. Yoshitama, M. Otsubo, S. Maeno, Y. Nagasawa, Electrochimica Acta 56 (2011) 8941. [11] D. Tashima, K. Kurosawatsu, M. Otsubo, C. Honda, Plasma Processes and Polymers 4 (2007) S502. [12] D. Tashima, A. Sakamoto, M. Taniguchi, T. Sakoda, M. Otsubo, Vacuum 83 (2008) 695. [13] D. Tashima, A. Sakamoto, M. Taniguchi, T. Sakoda, M. Otsubo, Surface, Coatings Technology 202 (2008) 5560. [14] T. Momma, X. Liu, T. Osaka, Y. Ushio, Y. Sawada, Journal of Power Sources 60 (1996) 249. [15] K. Okajima, K. Ohta, M. Sudoh, Electrochimica Acta 50 (2005) 2227. [16] H.P. Boehm, Angewandte Chemie International Edition in English 5 (1966) 533. [17] H. Oda, A. Yamashita, S. Minoura, M. Okamoto, T. Morimoto, Journal of Power Sources 158 (2006) 1510. [18] S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, Journal of the American Chemical Society 62 (1940) 1723. [19] E.P. Barrett, L.G. Joyner, P.P. Halenda, Journal of the American Chemical Society 73 (1951) 373. [20] K.K. Aligizaki, Pore Structure of Cement-Based Materials: Testing, Interpretation and Requirements, Taylor & Francis, Abingdon, 2006. [21] C.-T. Hsieh, H. Teng, Carbon 40 (2002) 667. [22] A.M. Golovin, Y.G. Degtev, V.V. Kuryatnikov, V.R. Pesochin, Combustion Explosion and Shock Waves 30 (1994) 19. [23] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure and Applied Chemistry 57 (1985) 603–619. [24] L. Wang, M. Fujita, M. Inagaki, Electrochimica Acta 51 (2006) 4096. [25] L.-h. Wang, M. Toyoda, M. Inagaki, New Carbon Materials 23 (2008) 111.