Energy Fuels 2010, 24, 1889–1893 Published on Web 01/27/2010
: DOI:10.1021/ef901299c
Electric Double-Layer Capacitors from Activated Carbon Derived from Black Liquor Xiao-Yan Zhao, Jing-Pei Cao,* Kayoko Morishita, Jun-ichi Ozaki, and Takayuki Takarada Department of Chemical and Environmental Engineering, Gunma University, Kiryu 376-8515, Japan Received November 5, 2009. Revised Manuscript Received January 13, 2010
Black liquor derived from a hardwood tree was activated by potassium hydroxide, and the performance as electric double-layer capacitors in a non-aqueous electrolyte of activated carbons with high surface area was presented in this work. The effects of preparation parameters on the char yield, Brunauer-Emmett-Teller (BET) surface area, pore structure, and capacitance of the resulting products were investigated. The results show that the char yield and capacitance decrease with the increase of the carbonization temperature (600900 °C). Nitrogen adsorption isotherms of activated carbons indicate that they were mainly micropores. The BET surface area increases slowly with the increase of the activation temperature, reaching the maximum value of 3089.2 m2 g-1 at 900 °C, 3 times larger than a commercial activated carbon from charcoal (943.5 m2 g-1). Capacitance is found to be approximately proportional to the BET surface area, and the maximum capacitance of 41.4 F g-1 was developed. In addition, the results obtained in this study confirm that capacitance of activated carbons from black liquor depends upon not only the surface area but also the pore size.
exist in black liquor, which are effectively used to improve the activation process. The addition of potassium and sodium compounds is common practice in the production of ACs. Besides the enhancement of reactivity, they are known to promote the development of microporosity and high surface area.11 Therefore, using black liquor as a porous carbon precursor may provide an effective approach to carry out the value-added use of black liquor. In comparison to the above dispose methods, such as alkali recovery, it is more economical and effective, but to some extent, no literature about the preparation of EDLCs from porous carbon derived from black liquor has been reported in detail. In this paper, we tried to prepare ACs with a large surface area and good electric conductivity from black liquor through potassium hydroxide (KOH) activation. The effects of preparation parameters on the yield, surface area, and pore structure of the resulting products were investigated, and the capacitance of the ACs was evaluated with galvanostatic charge/discharge in 0.5 M TEABF4/PC.
1. Introduction Electric double-layer capacitors (EDLCs) are very attractive as a potential energy storage system because of their high power density, quick charge-discharge rate, free maintenance, long-life operation, and environmentally friendly energy technology.1,2 For the application, activated carbons (ACs) are recognized as an essential component because of their large surface area, highly porous structure, and good adsorption property. Therefore, a lot of research on ACs with excellent capacitive performance in aqueous3-6 and nonaqueous7,8 electrolytes were reported. Black liquor is one of the main byproducts of the pulp paper industry, which is considered as a pollutant because it contains about 50% lignin. At present, more than 10 kinds of technologies have been used extensively, including alkali recovery, acidic separation, biochemistry, membrane treatment, flocculating settling oxygenation, etc.9 Although these methods of treatment seem effective for the liquid waste processing of the paper industry, they present the disadvantage of being expensive because of their operating cost and/or the relatively high cost for the chemical reagents used.10 Dependent upon the paper process, a high content of compounds of Na, K, etc.
2. Experimental Section 2.1. Sample. Black liquor used in the experiment was obtained from the Mitsubishi Corporation in Japan, and it comes from a hardwood tree. The proximate and ultimate analyses are given in Table 1. 2.2. Thermogravimetric Analysis. Thermal degradation characteristic of black liquor was studied using a thermogravimetric method. The experiment was performed on a TGD9600S analyzer (RIKO, Japan), and the result is shown in Figure 1. A sample of about 7 mg was heated from room temperature to 1000 °C at a heating rate of 10 °C/min under Ar flow. 2.3. Preparation of ACs. The sample was carbonized in muffle under Ar flow at the rate of 10 °C/min and was held for 1 h at the carbonization temperature. The carbonization temperature was varied over the temperature range of 500-900 °C. After carbonization, the char was pulverized to pass through a 150 mesh sieve. Then, the char was thermally treated under continuous Ar flow in
*To whom correspondence should be addressed. Telephone: þ81277-30-1452. Fax: þ81-277-30-1454. E-mail: beyondcao_2000@163. com or
[email protected]. (1) Qiao, W. M.; Yoon, S. H.; Mochida, I. Energy Fuels 2006, 20, 1680–1684. (2) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937–950. (3) Qu, Q. T.; Shi, Y.; Tian, S.; Chen, Y. H.; Wu, Y. P.; Holze, R. J. Power Sources 2009, 194, 1222–1225. (4) Shiraishi, S.; Kurihara, H.; Tsubota, H.; Oya, A.; Soneda, Y.; Yamada, Y. Electrochem. Solid-State Lett. 2001, 4, A5–A8. (5) Wen, Z. B.; Qu, Q. T.; Gao, Q.; Zheng, X. W.; Hu, Z. H.; Wu, Y. P.; Liu, Y. F.; Wang, X. J. Electrochem. Commun. 2009, 11, 715–718. (6) Qu, Q. T.; Wang, B.; Yang, L. C.; Shi, Y.; Tian, S.; Wu, Y. P. Electrochem. Commun. 2008, 10, 1652–1655. (7) Liu, P.; Verbrugge, M.; Soukiazian, S. J. Power Sources 2006, 156, 712–718. (8) Rufford, T. E.; Hulicova-Jurcakova, D.; Fiset, E.; Zhu, Z. H.; Lu, G. Q. Electrochem. Commun. 2009, 11, 974–977. (9) Demirbas-, A. Energy Convers. Manage. 2002, 43, 877–884. (10) Zaied, M.; Bellakhal, N. J. Hazard. Mater. 2009, 163, 995–1000. r 2010 American Chemical Society
(11) Wigmans, T.; Hoogland, A.; Tromp, P.; Moulijn, J. A. Carbon 1983, 21, 13–22.
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Energy Fuels 2010, 24, 1889–1893
: DOI:10.1021/ef901299c
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Table 1. Proximate and Ultimate Analyses (wt %) of Black Liquor (BL) proximate analysis (wt %)a
ultimate analysis (wt %, daf) C
H
N
S
b
O
sample
59.4
5.3
0.2
5.7
29.4
BL500 BL600 BL700 BL800 BL900 BL600(500) BL600(600) BL600(700) BL600(800) BL600(900) BL(0.5) BL(1) BL(1.5) BL(2) BL(2.5) BL(3) 600char CC
sample Mad BL
Table 2. Preparation Conditions and Characteristic Results of the ACs in This Studya
83.3
Vd
Ad
FCdb
41.0
40.7
18.3
a
M, moisture content; V, volatile matter content; A, ash content; FC, fixed carbon; ad, air-dried basis; d, dried basis; daf, dried and ash-free basis. b By difference.
CT (°C) 500 600 700 800 900 600 600 600 600 600 600 600 600 600 600 600 600
BET surface pore AT activation area (m2 size capacitance (°C) time (h) g-1) (nm) (F g-1) 800 800 800 800 800 500 600 700 800 900 600 600 600 600 600 600
2 2 2 2 2 2 2 2 2 2 0.5 1 1.5 2 2.5 3
2694.5 2722.7 1369.7 1867.6 1495.4 1935.2 1909.9 2613.2 2722.7 3089.2 1556.2 1796.2 1790.7 1909.9 1936.1 2139.7 358.0 943.5
1.9734 1.8733 2.7812 2.2381 2.8228 1.7643 1.9422 1.9715 1.8733 2.2782 1.8617 1.9604 1.8575 1.9422 1.9586 2.0006 2.0696 3.3428
35.0 40.8 34.6 30.0 21.0 26.1 41.4 35.8 40.8 38.1 37.4 38.2 39.0 41.4 39.5 41.0 1.1 16.0
a CT, carbonization temperature; AT, activation temperature. BL600 = BL600(800); BL600(600) = BL(2).
Figure 1. Thermogravimetry/derivative thermogravimetry (TG/ DTG) plots of black liquor.
an electrically heated horizontal tube furnace. In each experiment, the activation temperature was reached at a 10 °C/min heating rate and then maintained for 2 h and the weight ratio of KOH/char was 2. Different activation temperatures within the 500-900 °C range were investigated. The activated samples were cooled inside the furnace, maintaining the Ar flow, and then washed with hydrochloric acid and distilled water until neutral pH. The resulting ACs were dried at 200 °C in vacuum for 2 h, and the BET surface area and porous structure of ACs were determined by N2 adsorption/ desorption isotherms at 77 K (BELSORP-max, Japan). 2.4. Electrochemical Performance Evaluation. A mixture of 87 wt % AC, 10 wt % acetylene black, and 3 wt % PTFE binder was pressed into pellets (13 mm in diameter) as the electrodes. Then, the electrodes were dried under vacuum at 200 °C for 2 h. A button-type capacitor was assembled with two AC electrodes using 0.5 M TEABF4/PC as the electrolyte. The capacitors were galvanostatically cycled between 0 and 2.5 V on a Land cell tester. The capacitance (C) of a single electrode was determined with the formula C = IdT/2dV, where I is the discharge current, dT is the discharge time variation, and dV is the voltage variation in discharge. The preparation conditions and characteristic results of all of the ACs in this study were exhibited in Table 2.
Figure 2. Char yields of black liquor at different temperatures.
stable components in small particles may not be readily decomposed, while at higher temperatures (700-900 °C), most of these constituents can be thermally devolatilized, leaving a comparable amount of char. According to the TG result, lignin is decomposed from 650 °C,; therefore, the char yield decreases rapidly from 700 °C, reaching the maximum value (52.1%, db) at 600 °C. 3.3. Characterization of ACs. 3.3.1. Effect of Carbonization Temperatures (BL500-900). The ACs were obtained by activating chars with KOH at 800 °C for 2 h (named with BL500-900, respectively), and the effect of carbonization temperatures on the BET surface area is shown in Figure 3. A commercial AC from charcoal (CC) shows the value of 943.5 m2 g-1, much lower than that of ACs from black liquor. The BET surface areas of BL500 and BL600 show higher values than others and reach the maximum value (2722.7 m2 g-1) at 600 °C, but those of BL700, BL800, and BL900 are much lower, especially for BL700. Figure 4 compares the nitrogen adsorption isotherms of the ACs, and all of the samples present type I isotherms, indicating its microporous features.12 The
3. Results and Discussion 3.1. Thermogravimetric Analysis. Thermogravimetric analysis of black liquor revealed that two major thermal decompositions occurred around 250-500 and 650-950 °C, as shown in Figure 1. Generally, black liquor consists of lignin, hemicellulose, cellulose, and extracts. From the derivative thermogravimetry (DTG) curve, initial weight loss corresponds to moisture removal, followed by a second degradation event around 250-500 °C, where the evolution of light volatile compounds occurs from the degradation of cellulose and hemicelluloses. Degradation of lignin takes place at 650-950 °C. Thermal degradation of these individual components may be superimposed to simulate the overall degradation of black liquor. 3.2. Char Yields. As shown in Figure 2, at a lower carbonization temperature (500-600 °C), some of the more
(12) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619.
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Figure 3. BET surface area of ACs at different carbonization temperatures.
Figure 5. Effect of carbonization temperatures on the capacitance of ACs.
Figure 4. Nitrogen adsorption isotherms of the ACs.
Figure 6. BET surface area of ACs at different activation temperatures.
enhanced volume of the ACs means the increase of the pore volume. The results of some previous investigations13-16 showed that there is a direct relationship between the BET surface area and the capacitance of porous carbons. Theoretically, the higher the specific surface area of an AC, the higher the specific capacitance should be expected. Practically, the situation is more complicated. A significant deviation from this simple law has been frequently observed.14-18 The effect of the carbonization temperature on the capacitance of ACs at a current density of 10 mA g-1 is illustrated in Figure 5. The capacitance of CC is only 16.0 F g-1. From 600 to 900 °C, as the carbonization temperature increases, the capacitance of ACs decreases continuously. BL600 with the highest BET surface area has the highest specific capacitance of 40.8 F g-1, twice as large as that of BL900 and 2.5 times larger than that of CC. This means that the enhanced capacitance can be mainly attributed to the enhancement of the surface area. According to the characterization results, 600 °C is the most suitable carbonization temperature for obtaining
predominant porous carbon with the highest char yield, a higher BET surface area, and the highest capacitance than at other carbonization temperatures. 3.3.2. Effect of Activation Temperatures [BL600(500900)] and Times [BL(0.5-3)]. The BET surface area values corresponding to the ACs obtained at different activation temperatures (500-900 °C) but with a fixed carbonization temperature (600 °C) are plotted in Figure 6. It can be observed that this series of samples covers a wide range of surface areas (from 358 to more than 3000 m2 g-1), and the BET surface area of the char obtained in 600 °C (named 600char) is only 358.0 m2 g-1. The BET surface area was found to be increased slowly with the increase of the activation temperature, reaching 3089.2 m2 g-1 at 900 °C. Because the activation reaction with KOH is an endothermic reaction, the enhancement of the temperature is propitious to the process of the activation reaction and improves the activation effects. The effect of the activation temperature on the capacitance of ACs is illustrated in Figure 7. It can be observed that a capacitance value as high as 41.4 F g-1 was obtained, which indicates that micropores (pore size less than 2 nm) contribute to the EDLC, and it is also roughly proportional to BET surface area, reaching the maximum value at 600 °C. Besides, the capacitance of 600char is only found to be 1.1 F g-1. The effect of activation times from 0.5 to 3 h on BET surface area and capacitance was also investigated. According to Figure 8, the BET surface area slightly increases with the increase of the time, reaching the maximum value (2139.7 m2 g-1) with the time of 3 h. Figure 9 shows the
(13) Xu, B.; Wu, F.; Su, Y. F.; Cao, G. P.; Chen, S.; Zhou, Z. M.; Yang, Y. S. Electrochim. Acta 2008, 53, 7730–7735. (14) Gryglewicz, G.; Machnikowski, J.; Lorenc-Grabowska, E.; Lota, G.; Frackowiak, E. Electrochim. Acta 2005, 50, 1197–1206. (15) Lozano-Castell o, D.; Cazorla-Amor os, D.; Linares-Solano, A.; Shiraishi, S.; Kurihara, H.; Oya, A. Carbon 2003, 41, 1765–1775. (16) Barbieri, O.; Hahn, M.; Herzog, A.; K€ otz, R. Carbon 2005, 43, 1303–1310. (17) Endo, M.; Maeda, T.; Takeda, T.; Kim, Y. J.; Koshiba, K.; Hara, H.; Dresselhaus, M. S. J. Electrochem. Soc. 2001, 148, A910–A914. (18) Salitra, G.; Soffer, A.; Eliad, L.; Cohen, Y.; Aurbach, D. J. Electrochem. Soc. 2000, 147, 2486–2493.
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Figure 7. Effect of activation temperatures on the capacitance of ACs.
Figure 10. Correlations between the pore size (e2 nm) and capacitance.
carbon materials with a lower surface area.19-21 These results indicate that, although the BET surface area is a very important parameter, the EDLC also seems to depend upon other characteristics of the porous carbon materials, probably the pore size distribution and surface chemistry. Because the ACs are obtained from the same precursor and by the same method, they should have similar surface chemistry. In a previous study,22 where the EDLC performance of ACs with BET surface areas lower than 2000 m2 g-1 were measured, it was shown that mesoporosity is beneficial for EDLC. However, according to the results obtained with samples that show a BET surface area larger than 2000 m2 g-1, it seems that the presence of mesopores in materials with a high surface area (>2000 m2 g-1) is not very effective for double-layer capacitance. Figure 10 shows the correlations between the pore size (e2 nm) and capacitance. All ACs with the pore size smaller than 2 nm were displayed in this figure. It was found that there was a peak maximum of capacitance when the pore size was about 1.92 nm, which explained why BL600(600) [also named BL(2)] with the pore size of 1.9422 nm shows the highest capacitance, although its BET surface area is only 1909.9 m2 g-1. Capacitance is higher for a sample with a wider micropore size distribution than for a sample with a higher surface area but too narrow micropore size distribution. Both samples BL600(600) and BL600(500) have a similar BET surface area (see Table 2), whereas the capacitance of sample BL600(600) is much higher than BL600(500). This different performance of both samples can only be explained considering their pore sizes. As given in Table 2, BL600(600) shows a wider micropore size than BL600(500), even though both samples were prepared by similar conditions. The result shows that the existence of a wider micropore size causes sample BL600(600) to have a higher capacitance, which fits the general trend obtained for the other ACs. The existence of a very small pore size makes the entry of the electrolyte into the pores difficult.9 Then, the non-accessible pores do not contribute to the total capacitance, and a larger pore size will also produce non-accessible
Figure 8. BET surface area of ACs at different activation times.
Figure 9. Effect of activation times on the capacitance of ACs.
relationship between the BET surface area and capacitance. It is observed that from 0.5 to 2 h capacitance is also slightly increased with the increase of times but decreases at 2.5 h and then increases at 3 h are lower than that at 2 h. 3.3.3. Correlation between EDLCs and Pore Sizes. According to the characterization results, a general trend between the capacitance and BET surface area exists for all of the samples, although it is not a perfect linear relationship. This trend was also observed in previous studies using other
(20) Morimoto, T.; Hiratsuka, K.; Sanada, Y.; Kurihara, K. J. Power Sources 1996, 60, 239–247. (21) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1999, 146, 3639–3643. (22) Shiraishi, S.; Kurihara, H.; Shi, L.; Nakayama, T.; Oya, A. J. Electrochem. Soc. 2002, 149, A855–A861.
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density, it can still remain as high as 36.8 F g at a higher current density of 300 mA g-1. The good performance of AC demonstrates that black liquor will be a promising electrode precursor for EDLCs. 4. Conclusions ACs with a high surface area and capacitance derived from black liquor have been prepared by a chemical activation process. The results obtained with ACs of very different surface areas (from 1360 to 3090 m2 g-1) show that capacitance approximately increases with the surface area, reaching a maximum of 41.4 F g-1 for a surface area of 1909.9 m2 g-1. In addition, the results indicate that micropores contribute to the EDLC and confirm that capacitance depends upon not only the surface area but also the pore size distribution. The important microporosity development in the ACs prepared in the present work probably produces a mean micropore size (about 1.92 nm) wide enough to allow for electrolyte access inside the porosity. Besides, the amount of activating agent used in this study was extremely economized, compared to previous research.23,24 Accordingly, this study just provides an effective approach to carry out the value-added use of black liquor.
Figure 11. Capacitance against current density.
pores. These results show the importance of the accessibility of narrow micropores to electrolytic solution. To characterize the capacitive performance of AC derived from black liquor, Figure 11 exemplifies the capacitance of the sample BL600(600) and CC in 0.5 M TEABF4/PC as a function of the discharge current density. In comparison to CC, BL600(600) presents a higher capacitance of 41.4 F g-1 at a current density of 10 mA g-1. Although the capacitance decreases slightly with increasing the discharge current
Acknowledgment. Xiao-Yan Zhao and Jing-Pei Cao thank the China Scholarship Council (Project [2007]3020) for financial support.
(23) Mitani, S.; Lee, S. I.; Yoon, S. H.; Korai, Y.; Mochida, I. J. Power Sources 2004, 133, 298–301. (24) Lillo-R odenas, M. A.; Cazorla-Amor os, D.; Linares-Solano, A. Carbon 2003, 41, 267–275.
Supporting Information Available: Charge-discharge curves of the AC-based EDLCs (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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