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Highly mesoporous carbonaceous material of activated carbon beads for electric double layer capacitor
Electrochimica Acta 55 (2010) 7334–7340
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Highly mesoporous carbonaceous material of activated carbon beads for electric double layer capacitor Zhenhe Feng a , Ruisheng Xue b , Xiaohong Shao a,∗ a b
College of Science, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 20 January 2010 Received in revised form 25 June 2010 Accepted 26 June 2010 Available online 3 August 2010 Keywords: EDLCs Activated carbon beads KOH activation Porosity structure
a b s t r a c t The activated carbon beads (ACB) are prepared by a new preparation method, which is proposed by mixing the coal tar pitch and fumed silica powder at a certain weight ratio and activation by KOH at different weight ratios and different temperatures. The BET surface area, pore volume and average pore size are obtained based on the nitrogen adsorption isotherms at 77 K by using ASAP 2010 apparatus. The results show that our samples have much high specific surface area (SSA) of 3537 m2 g−1 and high pore volume value of 3.05 cm3 g−1 . The percentage of mesopore volume increases with the weight ratio of KOH/ACB ranging from 4% to 72%. The electrochemical double layer capacitors (EDLCs) are assembled with resultant carbon electrode and electrolyte of 1 mol L−1 Et4 NBF4 /PC. The specific capacitance of the ACB sample could be as high as 191.7 F g−1 by constant current charge/discharge technique, indicating that the ACB presents good characteristics prepared by the method proposed in this work. The investigation of influence of carbon porosity structure on capacitance indicates that the SSA plays an important role on the capacitance and all the pore sizes of less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. Mesopore structure is beneficial for the performance at high current density. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitor has been recognized as an efficient electric power storage device for its higher cycle life, good operation in a wide temperature range and rapid charging–discharging. As a new device with a great prospect, it has become a research hot topic in recent years [1]. Although the supercapacitors with metal oxides [2,3] or conducting polymers [4,5] have been increasingly investigated, electrochemical double layer capacitors (EDLCs) have attracted considerable attention for the reason that there are no Faradic reactions occurring during the charge–discharge process [6]. Among the electrode materials, activated carbons and carbon nanotubes have been paid much attention [7–13]. With the higher electrical conductivity, carbon nanotubes are considered as ideal electrodes. Nevertheless, high cost, low density and low specific capacitance of carbon nanotubes prevent their use in commercial products. By contrast, activated carbons with high BET specific surface area (SSA) prepared from different precursors and by different activation processes are especially attractive as electrodes for capacitors from the economical point of view.
∗ Corresponding author at: College of Science, Beijing University of Chemical Technology, 15 Beisanhuandonglu, Beijing, China. E-mail address: [email protected] (X. Shao). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.071
Large BET SSA has been considered to be a basic guiding principle for larger capacity [14,15]. Thus, it is vital to explore new preparation techniques to enhance the BET SSA for EDLCs. In addition, one of the physical characteristics of activated carbon materials is the wide distribution of pore size, which may be essential for capacity. Many researchers consider mesopores are favorable for capacitance for the reason that the pore size smaller than the size of solvated electrolyte ions may be incapable of contributing to charge storage [16,17]. On the other hand, Chmiola et al. [18] prepared the carbidederived carbon with the average pore sizes from 0.6 to 2.25 nm, in which the maximum capacitance reached more than 140 F g−1 . They found an anomalous increase in carbon capacitance at the pore size less than 1 nm. Fernández et al. [19] found that an average width around 1.2 nm appeared to optimize the rate capability of carbons at high current density and the diffusion of aprotic cations was strongly hindered by the smaller micropores. The different results maybe are associated with precursors, preparation method and porosity structure, which can affect the capacitances. Therefore, it is still a challenging task to study the relationship between porosity structure and the electrochemical behavior. In this work, carbon beads are formed by mixing the coal tar pitch and fumed silica powder at a certain weight ratio. The resultants are then activated by KOH at different weight ratios and different temperatures. The effect of BET surface area and porosity structure on electrochemical behavior is investigated. To understand the relationship between pore size and capacitance, the
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Table 1 Serial number, preparation conditions and composition of the ACB. Sample
Temperature of activation (◦ C)
KOH/ACB weight ratio
ACB-800-3 ACB-800-5 ACB-800-7 ACB-800-9 ACB-900-3 ACB-900-5 ACB-900-7 ACB-900-9
800 800 800 800 900 900 900 900
3 5 7 9 3 5 7 9
density functional theory (DFT) SSA is employed and divided into three parts according to the pore size distribution.
2. Experimental 2.1. Materials Coal tar pitch was purchased from Anshan Steel. Co. Ltd., China. The softening point is 80 ◦ C Fumed silica powder (SiO2 ) with a specific surface area of 200 m2 /g and an average particles size less than 50 nm was obtained from Shandong Yihao Co. Ltd., China. Potassium hydroxide (KOH) was purchased from Beijing Chemicals Co. Ltd. All chemicals are AR grade.
2.2. Preparation of activated carbon beads We mixed the coal tar pitch and fumed silica powder (the weight ratio is about 4:1). Then, the mixture was heated at 1 ◦ C min−1 up to 360 ◦ C in air and maintained for 0.5 h. The mixture was then carbonized in a nitrogen at 500 ◦ C for 1 h. The carbonized resultant was sifted by the sieve of 200 meshes and the sifting (<200 mesh) was mixed with KOH at the KOH/carbon beads weight ratios from 3 to 9. The mixture was heated up from 800 to 900 ◦ C and was then held for 1 h in the nitrogen environment too. After cooling down, the sample was washed with deionized water and then dried in vacuum at 120 ◦ C for 12 h. The preparation conditions, composition and serial numbers of ACB are listed in Table 1. As is shown, samples of ACB800-3, ACB-800-5, ACB-800-7 and ACB-800-9 were activated at the same temperature of 800 ◦ C and the KOH/ACB ratios were 3, 5, 7 and 9, respectively. In addition, ACB-900-3, ACB-900-5, ACB-900-7 and ACB-900-9 were activated at the same temperature of 900 ◦ C and the KOH/ACB ratios were also set to 3, 5, 7 and 9, respectively.
2.3. Characterization of ACB The morphologies of the samples were observed by scanning electron microscope (SEM, Hitachi S-4700). X-ray diffraction (XRD) was recorded on a RigakuD/max-2500B2+/PCX system using Cu K␣ radiation ( = 1.54056 Å). The elemental composition of ACB (CHN) was carried out by CE440 Elemental Analyzer. The nitrogen, carbon, hydrogen content was determined directly, while the oxygen content was calculated by difference. Ash was determined according to the standard procedures (General standard of China, GB2295-80). The porous texture was characterized by adsorption/desorption isotherms of nitrogen at 77 K using an automatic adsorption apparatus (ASAP2010). Eight samples were applied in the measurements and all the samples were outgased under vacuum at 300 ◦ C for at least 24 h. The pore size distribution was obtained by a density functional theory (DFT) package built-in the apparatus. The SSA values were calculated by applying the BET (SBET ) model and the regularized DFT (SDFT ) model.
Composition (wt%) C
H
N
O
Ash
91.84 94.25 96.29 93.90 – – 92.81 –
0.10 0.00 0.00 0.08 – – 0.60 –
0.25 0.20 0.15 0.24 – – 1.00 –
7.27 4.93 2.80 5.00 – – 4.81 –
0.54 0.62 0.76 0.78 – – 0.78 –
2.4. Electrochemical performance First, the ACB powders were mixed with 10% (mass fraction) acetylene black and 10% (mass fraction) polytetrafluoroethylene (PTFE) in appropriate ethanol until slurries were obtained. The blended slurries were pressed and cut to the electrodes with diameter of 14 mm. Then, the disk of electrodes was pressed onto the stainless steel current collector and foam nickel. The thickness of the electrodes is about 0.1 mm (±0.02 mm). The mass of the electrodes ranges from 9 to 12 mg. Finally the electrodes were dried in vacuum at 120 ◦ C for 12 h. The test capacitor consists of two identical electrodes separated by a porous membrane with 1 mol L−1 Et4 NBF4 /PC as electrolyte. The assembly of the capacitors was prepared in a dry Ar-filled glove box (Mbraun Germany). Galvanostatic charge/discharge was carried out by a battery-test apparatus (CT2001A, LAND China) in this work at room temperature. The testing current density was from 0.35 to 70 mA cm−2 (about from 0.04 to 8 A g−1 ) and the voltage was set from 0 to 2.5 V. Capacitance was deduced from the slope of the V (t) curves after 20 times cycled, is given by Ccell =
I dV /dt
(1)
Specific capacitance Cg in Farad per gram of ACB (F g−1 ) can be obtained by Cg =
2Ccell m
(2)
where Ccell is the total capacitance of the cell (F), m is the mass of activated carbon in one electrode (g). Similarly, the volumetric capacitance is given by the equation [20] Cv =
2Ccell Ve
(3)
where Ve is the volume of electrode (cm3 ). Electrochemical impedance spectroscopy (EIS) was carried out at a electrochemical working station (Versastat3-200 Princeton Applied Research USA). EIS was performed on 2-electrode cells by applying 10 mV RMS sine wave at frequencies from 100 kHz to 1 mHz. 3. Results and discussion 3.1. Porosity characterization The morphologies of the ACB were observed by SEM and their images in high and low magnification are shown in Fig. 1. Of all the images, Fig. 1a–d represent the ACB activated with the ratios of KOH/ACB of 3, 5, 7 and 9 at 800 ◦ C and Fig. 1e–h images show the ACB activated with the ratios of KOH/ACB of 3, 5, 7 and 9 at 900 ◦ C. As is seen from images of the low magnification, all the samples mainly are of sphere shapes. According to our results [8], SiO2 is beneficial to the formation of beads. In the activation procedure,
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Fig. 1. SEM images of ACB: (a) ACB-800-3; (b) ACB-800-5; (c) ACB-800-7; (d) ACB-800-9; (e) ACB-900-3; (f) ACB-900-5; (g) ACB-900-7 and (h) ACB-900-9.
the KOH react with SiO2 , after washed with deionized water, the production of K2 SiO3 was removed away from the ACB [11]. Consequently, the main elemental component of the samples is carbon (see Table 1), which is in accordance with the XRD analysis of representative samples in Fig. 2. The (0 0 2) diffraction peak of ACB becomes obviously weak, indicating the ACB has changed into disordered structure by activation treatments. In addition, there is no diffraction peak of other compound in the patterns. It is implied that the KOH, SiO2 and the reactant (for example K2 SiO3 , K2 CO3 and so on) were removed away from the samples by washing with deionized water. The activation mechanism will be further discussed in our future work. With the increase of temperature and KOH/ACB
ratio, the pore size becomes larger, which will be discussed in the following part. As is seen there are many rather large pores on the surface of ACB-900-3 (see Fig. 1e), which possibly result from the fierce gasification of the low-molecular-weight component of carbon beads during activation at high temperatures. It is found that this structure resulting from the gasification disappears by further etching of KOH with the increase of KOH/ACB ratio. The isotherms of N2 adsorption/desorption at 77 K are presented in Fig. 3a and b. It is found that the nitrogen adsorption capacities are impressively high for all the samples and the uptake of ACB-800-9 approaches 2000 cm3 g−1 . From Fig. 3a, the nitrogen adsorption capacity increases significantly with the weight ratio
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Table 2 The parameters of the porous texture of ACB. Sample
SBET (m2 g−1 )
SDFT (m2 g−1 )
Vt (cm3 g−1 )
Vmeso (cm3 g−1 )
Vmicro (cm3 g−1 )
Vmeso /Vt (%)
Average pore size (nm)
ACB-800-3 ACB-800-5 ACB-800-7 ACB-800-9 ACB-900-3 ACB-900-5 ACB-900-7 ACB-900-9
1778 3455 3537 3017 1473 3071 2688 2543
1511 1929 1967 1797 1023 1852 1631 1535
0.85 1.97 2.56 3.05 0.75 2.47 2.62 2.82
0.03 0.97 1.53 2.14 0.19 1.57 1.82 2.02
0.82 1.00 1.03 0.91 0.56 0.90 0.80 0.80
4 49 59 70 25 63 70 72
1.91 2.28 2.89 4.04 2.04 3.21 3.92 4.43
SBET and SDFT were calculated by the BET method and DFT method. Vt represented the total pore volume. The mesopore volume (Vmeso ) was obtained by subtracting the microporous volume from total pore volume. The micropore volume (Vmicro ) was determined by applying Dubinin-Astakhov (DA) analysis.
Fig. 2. X-ray diffraction patterns of representative samples for ACB. (—) ACB-900-7; ) ACB-800-7; ( ) ACB-800-9. (
of KOH/ACB. The same phenomenon occurs in Fig. 3b. These indicate that high weight ratio of KOH/ACB is favorable for nitrogen adsorption. In addition, it is interesting that there are two kinds of isotherms shown in Fig. 3a and b. Gas adsorption achieves saturation at low pressures for the samples of ACB-800-3 and ACB-900-3. The two isotherms belong to type I isotherm for micropore materials on the IUPAC classification. The capillary condensation occurs in the isotherms of the samples of ACB-800-5, ACB-800-7, ACB800-9, ACB-900-5, ACB-900-7, and ACB-900-9. In addition, the six isotherms show clear hysteresis loops. The phenomena above indicate the existence of mesopores in these samples, which can be further proved by the pore size distribution as shown in Fig. 4 and Table 2. For comparison, the BET SSA, DFT SSA, total pore volume, mesopore fraction and average pore size of all the samples are listed in Table 2. The results indicate that the BET SSA of our samples covers a very wide range from 1472 m2 g−1 up to more than 3500 m2 g−1 . It is worth emphasizing that ACB-800-7 can achieve a high BET SSA of 3537 m2 g−1 , which is a very high value for mesoporous. In our former study, a rather higher result of 3180 m2 g−1 was obtained after KOH activation [10]. The results show that adding the fumed silica powder is beneficial for high BET SSA. It is interesting that the percentage of mesopore volume, total volume and average pore size increase with the increasing of the weight ratio of KOH/ACB. In addition, sample of ACB-800-9 has large pore volume values of 3.05 cm3 g−1 . ACB-900-9 can achieve a high percentage of mesopore volume of 72%. The average pore size is about 2–4 nm for all the samples. The sample of ACB-900-9 has the largest average pore diameter of 4.43 nm. Fig. 4 shows the pore size distributions obtained by applying the non-local density functional theory based on the nitrogen adsorp-
Fig. 3. Nitrogen adsorption/desorption isotherms at 77 K. (a: activation temperature of 800 ◦ C. 䊉: ACB-800-3, : ACB-800-5, : ACB-800-7, : ACB-800-9) (b: : ACB-900-7, activation temperature of 900 ◦ C. : ACB-900-3, : ACB-900-5, : ACB-900-9).
tion isotherms. There are three peaks of pore diameters of 0.7–0.9, 1–2 and 3–5 nm. The pore size distribution becomes wider with the ratio of KOH/ACB, which is in accord with the nitrogen adsorption capacity and can explain the large loop sizes of ACB-800-5, ACB-800-7, ACB-800-9, ACB-900-5, ACB-900-7, and ACB-900-9 in Fig. 3a and b. 3.2. Electrochemical properties The gravimetric and volumetric capacitances for samples at the current density of 0.35 mA cm−2 are given in Table 3. Seen from Table 3, the gravimetric and volumetric capacitances for samples are relatively high. The sample of ACB-800-7 has the highest gravimetric capacitance (191.7 F g−1 ) among all the samples. The gravimetric and volumetric capacitance values of ACB-800-5 are 182.0 and 137.7 F cm−3 .
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trode/electrolyte interface. The semicircle of ACB-800-9 is the smallest among the eight samples, indicating the lowest impedance on electrode/electrolyte interface. The magnitude of the equivalent series resistance (ESR) was estimated from the x-intercept of the Nyquist plot [21], which is shown in Table 3. The lower ESR values are about 1.60 and 1.63 for ACB-800-5 and ACB-800-7, respectively. The specific capacitances of the samples at different current densities were studied in a constant current charge–discharge test and the results are given in Fig. 7. It is obvious that the
Fig. 4. Pore size distribution of ACB by density function theory (a: activation tem: ACB-800-5, : ACB-800-7, : ACB-800-9) perature of 800 ◦ C. : ACB-800-3, : ACB-900-7, (b: activation temperature of 900 ◦ C. : ACB-900-3, : ACB-900-5, : ACB-900-9).
Fig. 5 shows an example of the galvanostatic charge/discharge characteristics for the capacitor built from ACB-800-7 at the current density of 0.35 and 7 mA cm−2 , respectively. Fig. 5a shows a typical triangular shape without ohmic drop, which confirms excellent capacitive properties at the current density of 0.35 mA cm−2 . The presence of the mesopores makes it possible that the electrolyte ions can easily access the pore of the electrode. When the current density increases to 7 mA cm−2 , the curve also shows a triangular shape. However, a little ohmic drop happens on 2.25–2.5 V for the large current density at the discharge process. To further investigate the electrochemical performance of the capacitors, electrochemical impedance spectroscopy was employed. Fig. 6 shows the Nyquist plots of the electrodes. A semicircle in the high frequency range and a sloped line in the low frequency range have been observed. To our knowledge, the smaller radius of semicircle, the lower impedance on elec-
Fig. 5. Galvanostatic charge/discharge characteristics of a capacitor built from ACB800-7: (a) 0.35 mA cm−2 and (b) 7 mA cm−2 .
Table 3 The specific capacitance and equivalent series resistance (ESR) of ACB. Sample
Cg (F g−1 )
Cv (F cm−3 )
ESR ()
ACB-800-3 ACB-800-5 ACB-800-7 ACB-800-9 ACB-900-3 ACB-900-5 ACB-900-7 ACB-900-9
133.8 182.0 191.7 173.8 105.8 164.6 161.0 155.4
103.4 137.7 60.8 70.3 93.8 76.2 64.7 44.9
2.00 1.60 1.63 2.18 7.52 1.71 7.53 2.54
Cg : gravimetric specific capacitance; Cv : volumetric specific capacitance; ESR: equivalent series resistance.
Fig. 6. Nyquist impedance plots for different electrodes (: ACB-800-3, 800-5, : ACB-800-7, : ACB-900-9).
: ACB-800-9;
: ACB-900-3,
: ACB-900-5,
: ACB-
: ACB-900-7,
Z. Feng et al. / Electrochimica Acta 55 (2010) 7334–7340
: ACB-800-9;
: ACB-900-3,
: ACB-
Fig. 9. Specific capacitance as a function of BET surface area for ACB ( and : ACB-900-5).
: ACB-800-5,
: ACB-800-7,
: ACB-
Fig. 10. Capacitance by experiment and calculation as a function of DFT surface area (: capacitance by experiment and : capacitance by calculation).
Fig. 7. Specific capacitance at various current densities for the capacitors (: ACB800-3, 900-5,
: ACB-800-5, : ACB-800-7, : ACB-900-7, : ACB-900-9).
Fig. 8. Cycle life of ACB (: ACB-800-3, 800-9;
: ACB-900-3,
: ACB-900-5,
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: ACB-900-7,
: ACB-800-9
: ACB-900-9).
capacitances of ACB-800-3 and ACB-900-3 decrease rapidly with the increase of current density and cannot be measured at the current density of 70 mA cm−2 . As is discussed above, the percentages of micropore for ACB-800-3 and ACB-900-3 are higher than other samples. The sharply reduced capacitance of microporous carbon as the discharge rate rises up not only reflects the slow diffusion of ions within micropores, but also concerns with the solvation/desolvation process of ions as they pass through the interface between porous carbon and electrolyte [22]. By contrast, the capacitances of other samples decrease slightly with the current density. Especially, the capacitance of ACB-800-7 is 131.2 F g−1 at the current density of 70 mA cm−2 and can still maintain 70.7% of original capacitance of 191.7 F g−1 at the low current density of 0.35 mA cm−2 . The excellent performance of ACB-800-7 may be due to its particular pore structure. The sufficient but not severe activating condition of ACB-800-7 likely produces high surface area and short length of pores, which will be studied in our future work. Fig. 8 presents the capacitance versus cycle number curves of ACB samples under current density of 3.5 mA cm−2 . Most samples have relatively stable capacitance with the increase of cycle number. But the capacitance of ACB-800-3 and ACB-900-3 show the obvious decline with the increase of cycle number. The results of Figs. 7 and 8 imply that, for ACB samples, mesopore plays an important role on high current performance and cycle life. In order to investigate the influence of SSA on capacitance, Fig. 9 shows the capacitance results against the BET SSA for all the sam-
ples. It is observed that the specific capacitance increases with the BET SSA except the sample of ACB-900-5. The BET SSA values of ACB-800-9 and ACB-900-5 are 3017 and 3071 m2 g−1 , respectively. However, the capacitance of sample ACB-800-9 (red color) is higher than that of sample ACB-900-5 (green color) despite having similar BET areas. As is discussed in Fig. 4 and Table 2, ACB-800-9 shows a wider pores size distribution than ACB-900-5, which implied that besides the BET SSA, the pore size distribution maybe affect the capacitance. Although the BET method plays important role on porous materials, it is a restriction to analyze the SSA with different pore size ranges. Based on solid modern statistical physics, the DFT method can resolve such problem. In order to discuss the influence of porosity on capacitance, the relationship between capacitance and surface area is further investigated by the DFT SSA, as is seen in Fig. 10. The SSA is divided into three parts: the surface area of the pore size below 1 nm (S1 ), the surface area of the pore size between 1 and 2 nm (S2 ) and the surface area of the pore size larger than 2 nm (S3 ). In Fig. 10 the specific capacitance results by experiment and calculation (using equation 4) are plotted against the DFT SSA for the ACB studied. We find that the calculated results agree well with the experiment data. Using the Least Square Method, S1 , S2 , S3 and capacitance (F) were calculated by the following equation F = 0.0874 × S1 + 0.0993 × S2 + 0.0969 × S3
(4)
where F is the capacitance, F g−1 . S1 , S2 , S3 are the surface areas with the pore size below 1 nm, between 1 and 2 nm, larger than
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2 nm m2 g−1 , respectively. The coefficients of S1 , S2 , S3 are 0.0874, 0.0993 and 0.0969 Fm−2 , respectively. As is seen that all the pore size less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. It can be seen from the coefficient of S1 , the average surface specific capacitance of the pore sizes below 1 nm is 8.74 F cm−2 (i.e. 0.0874 F m−2 ), which is close to the results (the surface specific capacitance from 5 to 14 F cm−2 ) of ref [18]. The pores below 1 nm can exhibit the high capacitance due to the fact that the micropores can adsorb de-solvated or partially de-solvated electrolyte ions [18]. The coefficient of the relatively narrow pores (1–2 nm) is slightly higher than that of the mesopores (>2 nm). It is implied that the contribution of the pores (1–2 nm) to the performance of supercapacitors is slightly higher than the pores above 2 nm. It confirms the general trends reported earlier [19,20,23]. 4. Conclusions A series of activated carbon beads have been prepared by mixing the coal tar pitch and fumed silica power and activation by KOH. The high BET specific surface area of 3537 m2 g−1 and the high pore volume values of 3.05 cm3 g−1 are obtained. The percentage of mesopore volume, total pore volume and average pore sizes increase with the weight ratio of KOH/ACB. The ACB-900-9 exhibits a high percentage of mesopore volume of 72%. The electrochemical double layer capacitors (EDLCs) are assembled with resultant ACB electrode and electrolyte. The investigation of gravimetric and volumetric capacitances for samples shows that ACBs exhibit good capacitances. ACB-800-7 gives the excellent capacitance of 191.7 F g−1 at the charge–discharge current of 0.35 mA cm−2 , and it still keeps a high value of 131.2 F g−1 at 70 mA cm−2 . The BET SSA and the pore size distribution present significant influences on the electrochemical performance of the samples. Detailed discussion indicates that all the pore size less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. In particular, the effect of micropores (<1 nm) on capacitance is slightly
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Highly mesoporous carbonaceous material of activated carbon beads for electric double layer capacitor Zhenhe Feng a , Ruisheng Xue b , Xiaohong Shao a,∗ a b
College of Science, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 20 January 2010 Received in revised form 25 June 2010 Accepted 26 June 2010 Available online 3 August 2010 Keywords: EDLCs Activated carbon beads KOH activation Porosity structure
a b s t r a c t The activated carbon beads (ACB) are prepared by a new preparation method, which is proposed by mixing the coal tar pitch and fumed silica powder at a certain weight ratio and activation by KOH at different weight ratios and different temperatures. The BET surface area, pore volume and average pore size are obtained based on the nitrogen adsorption isotherms at 77 K by using ASAP 2010 apparatus. The results show that our samples have much high specific surface area (SSA) of 3537 m2 g−1 and high pore volume value of 3.05 cm3 g−1 . The percentage of mesopore volume increases with the weight ratio of KOH/ACB ranging from 4% to 72%. The electrochemical double layer capacitors (EDLCs) are assembled with resultant carbon electrode and electrolyte of 1 mol L−1 Et4 NBF4 /PC. The specific capacitance of the ACB sample could be as high as 191.7 F g−1 by constant current charge/discharge technique, indicating that the ACB presents good characteristics prepared by the method proposed in this work. The investigation of influence of carbon porosity structure on capacitance indicates that the SSA plays an important role on the capacitance and all the pore sizes of less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. Mesopore structure is beneficial for the performance at high current density. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitor has been recognized as an efficient electric power storage device for its higher cycle life, good operation in a wide temperature range and rapid charging–discharging. As a new device with a great prospect, it has become a research hot topic in recent years [1]. Although the supercapacitors with metal oxides [2,3] or conducting polymers [4,5] have been increasingly investigated, electrochemical double layer capacitors (EDLCs) have attracted considerable attention for the reason that there are no Faradic reactions occurring during the charge–discharge process [6]. Among the electrode materials, activated carbons and carbon nanotubes have been paid much attention [7–13]. With the higher electrical conductivity, carbon nanotubes are considered as ideal electrodes. Nevertheless, high cost, low density and low specific capacitance of carbon nanotubes prevent their use in commercial products. By contrast, activated carbons with high BET specific surface area (SSA) prepared from different precursors and by different activation processes are especially attractive as electrodes for capacitors from the economical point of view.
∗ Corresponding author at: College of Science, Beijing University of Chemical Technology, 15 Beisanhuandonglu, Beijing, China. E-mail address: [email protected] (X. Shao). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.071
Large BET SSA has been considered to be a basic guiding principle for larger capacity [14,15]. Thus, it is vital to explore new preparation techniques to enhance the BET SSA for EDLCs. In addition, one of the physical characteristics of activated carbon materials is the wide distribution of pore size, which may be essential for capacity. Many researchers consider mesopores are favorable for capacitance for the reason that the pore size smaller than the size of solvated electrolyte ions may be incapable of contributing to charge storage [16,17]. On the other hand, Chmiola et al. [18] prepared the carbidederived carbon with the average pore sizes from 0.6 to 2.25 nm, in which the maximum capacitance reached more than 140 F g−1 . They found an anomalous increase in carbon capacitance at the pore size less than 1 nm. Fernández et al. [19] found that an average width around 1.2 nm appeared to optimize the rate capability of carbons at high current density and the diffusion of aprotic cations was strongly hindered by the smaller micropores. The different results maybe are associated with precursors, preparation method and porosity structure, which can affect the capacitances. Therefore, it is still a challenging task to study the relationship between porosity structure and the electrochemical behavior. In this work, carbon beads are formed by mixing the coal tar pitch and fumed silica powder at a certain weight ratio. The resultants are then activated by KOH at different weight ratios and different temperatures. The effect of BET surface area and porosity structure on electrochemical behavior is investigated. To understand the relationship between pore size and capacitance, the
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Table 1 Serial number, preparation conditions and composition of the ACB. Sample
Temperature of activation (◦ C)
KOH/ACB weight ratio
ACB-800-3 ACB-800-5 ACB-800-7 ACB-800-9 ACB-900-3 ACB-900-5 ACB-900-7 ACB-900-9
800 800 800 800 900 900 900 900
3 5 7 9 3 5 7 9
density functional theory (DFT) SSA is employed and divided into three parts according to the pore size distribution.
2. Experimental 2.1. Materials Coal tar pitch was purchased from Anshan Steel. Co. Ltd., China. The softening point is 80 ◦ C Fumed silica powder (SiO2 ) with a specific surface area of 200 m2 /g and an average particles size less than 50 nm was obtained from Shandong Yihao Co. Ltd., China. Potassium hydroxide (KOH) was purchased from Beijing Chemicals Co. Ltd. All chemicals are AR grade.
2.2. Preparation of activated carbon beads We mixed the coal tar pitch and fumed silica powder (the weight ratio is about 4:1). Then, the mixture was heated at 1 ◦ C min−1 up to 360 ◦ C in air and maintained for 0.5 h. The mixture was then carbonized in a nitrogen at 500 ◦ C for 1 h. The carbonized resultant was sifted by the sieve of 200 meshes and the sifting (<200 mesh) was mixed with KOH at the KOH/carbon beads weight ratios from 3 to 9. The mixture was heated up from 800 to 900 ◦ C and was then held for 1 h in the nitrogen environment too. After cooling down, the sample was washed with deionized water and then dried in vacuum at 120 ◦ C for 12 h. The preparation conditions, composition and serial numbers of ACB are listed in Table 1. As is shown, samples of ACB800-3, ACB-800-5, ACB-800-7 and ACB-800-9 were activated at the same temperature of 800 ◦ C and the KOH/ACB ratios were 3, 5, 7 and 9, respectively. In addition, ACB-900-3, ACB-900-5, ACB-900-7 and ACB-900-9 were activated at the same temperature of 900 ◦ C and the KOH/ACB ratios were also set to 3, 5, 7 and 9, respectively.
2.3. Characterization of ACB The morphologies of the samples were observed by scanning electron microscope (SEM, Hitachi S-4700). X-ray diffraction (XRD) was recorded on a RigakuD/max-2500B2+/PCX system using Cu K␣ radiation ( = 1.54056 Å). The elemental composition of ACB (CHN) was carried out by CE440 Elemental Analyzer. The nitrogen, carbon, hydrogen content was determined directly, while the oxygen content was calculated by difference. Ash was determined according to the standard procedures (General standard of China, GB2295-80). The porous texture was characterized by adsorption/desorption isotherms of nitrogen at 77 K using an automatic adsorption apparatus (ASAP2010). Eight samples were applied in the measurements and all the samples were outgased under vacuum at 300 ◦ C for at least 24 h. The pore size distribution was obtained by a density functional theory (DFT) package built-in the apparatus. The SSA values were calculated by applying the BET (SBET ) model and the regularized DFT (SDFT ) model.
Composition (wt%) C
H
N
O
Ash
91.84 94.25 96.29 93.90 – – 92.81 –
0.10 0.00 0.00 0.08 – – 0.60 –
0.25 0.20 0.15 0.24 – – 1.00 –
7.27 4.93 2.80 5.00 – – 4.81 –
0.54 0.62 0.76 0.78 – – 0.78 –
2.4. Electrochemical performance First, the ACB powders were mixed with 10% (mass fraction) acetylene black and 10% (mass fraction) polytetrafluoroethylene (PTFE) in appropriate ethanol until slurries were obtained. The blended slurries were pressed and cut to the electrodes with diameter of 14 mm. Then, the disk of electrodes was pressed onto the stainless steel current collector and foam nickel. The thickness of the electrodes is about 0.1 mm (±0.02 mm). The mass of the electrodes ranges from 9 to 12 mg. Finally the electrodes were dried in vacuum at 120 ◦ C for 12 h. The test capacitor consists of two identical electrodes separated by a porous membrane with 1 mol L−1 Et4 NBF4 /PC as electrolyte. The assembly of the capacitors was prepared in a dry Ar-filled glove box (Mbraun Germany). Galvanostatic charge/discharge was carried out by a battery-test apparatus (CT2001A, LAND China) in this work at room temperature. The testing current density was from 0.35 to 70 mA cm−2 (about from 0.04 to 8 A g−1 ) and the voltage was set from 0 to 2.5 V. Capacitance was deduced from the slope of the V (t) curves after 20 times cycled, is given by Ccell =
I dV /dt
(1)
Specific capacitance Cg in Farad per gram of ACB (F g−1 ) can be obtained by Cg =
2Ccell m
(2)
where Ccell is the total capacitance of the cell (F), m is the mass of activated carbon in one electrode (g). Similarly, the volumetric capacitance is given by the equation [20] Cv =
2Ccell Ve
(3)
where Ve is the volume of electrode (cm3 ). Electrochemical impedance spectroscopy (EIS) was carried out at a electrochemical working station (Versastat3-200 Princeton Applied Research USA). EIS was performed on 2-electrode cells by applying 10 mV RMS sine wave at frequencies from 100 kHz to 1 mHz. 3. Results and discussion 3.1. Porosity characterization The morphologies of the ACB were observed by SEM and their images in high and low magnification are shown in Fig. 1. Of all the images, Fig. 1a–d represent the ACB activated with the ratios of KOH/ACB of 3, 5, 7 and 9 at 800 ◦ C and Fig. 1e–h images show the ACB activated with the ratios of KOH/ACB of 3, 5, 7 and 9 at 900 ◦ C. As is seen from images of the low magnification, all the samples mainly are of sphere shapes. According to our results [8], SiO2 is beneficial to the formation of beads. In the activation procedure,
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Fig. 1. SEM images of ACB: (a) ACB-800-3; (b) ACB-800-5; (c) ACB-800-7; (d) ACB-800-9; (e) ACB-900-3; (f) ACB-900-5; (g) ACB-900-7 and (h) ACB-900-9.
the KOH react with SiO2 , after washed with deionized water, the production of K2 SiO3 was removed away from the ACB [11]. Consequently, the main elemental component of the samples is carbon (see Table 1), which is in accordance with the XRD analysis of representative samples in Fig. 2. The (0 0 2) diffraction peak of ACB becomes obviously weak, indicating the ACB has changed into disordered structure by activation treatments. In addition, there is no diffraction peak of other compound in the patterns. It is implied that the KOH, SiO2 and the reactant (for example K2 SiO3 , K2 CO3 and so on) were removed away from the samples by washing with deionized water. The activation mechanism will be further discussed in our future work. With the increase of temperature and KOH/ACB
ratio, the pore size becomes larger, which will be discussed in the following part. As is seen there are many rather large pores on the surface of ACB-900-3 (see Fig. 1e), which possibly result from the fierce gasification of the low-molecular-weight component of carbon beads during activation at high temperatures. It is found that this structure resulting from the gasification disappears by further etching of KOH with the increase of KOH/ACB ratio. The isotherms of N2 adsorption/desorption at 77 K are presented in Fig. 3a and b. It is found that the nitrogen adsorption capacities are impressively high for all the samples and the uptake of ACB-800-9 approaches 2000 cm3 g−1 . From Fig. 3a, the nitrogen adsorption capacity increases significantly with the weight ratio
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Table 2 The parameters of the porous texture of ACB. Sample
SBET (m2 g−1 )
SDFT (m2 g−1 )
Vt (cm3 g−1 )
Vmeso (cm3 g−1 )
Vmicro (cm3 g−1 )
Vmeso /Vt (%)
Average pore size (nm)
ACB-800-3 ACB-800-5 ACB-800-7 ACB-800-9 ACB-900-3 ACB-900-5 ACB-900-7 ACB-900-9
1778 3455 3537 3017 1473 3071 2688 2543
1511 1929 1967 1797 1023 1852 1631 1535
0.85 1.97 2.56 3.05 0.75 2.47 2.62 2.82
0.03 0.97 1.53 2.14 0.19 1.57 1.82 2.02
0.82 1.00 1.03 0.91 0.56 0.90 0.80 0.80
4 49 59 70 25 63 70 72
1.91 2.28 2.89 4.04 2.04 3.21 3.92 4.43
SBET and SDFT were calculated by the BET method and DFT method. Vt represented the total pore volume. The mesopore volume (Vmeso ) was obtained by subtracting the microporous volume from total pore volume. The micropore volume (Vmicro ) was determined by applying Dubinin-Astakhov (DA) analysis.
Fig. 2. X-ray diffraction patterns of representative samples for ACB. (—) ACB-900-7; ) ACB-800-7; ( ) ACB-800-9. (
of KOH/ACB. The same phenomenon occurs in Fig. 3b. These indicate that high weight ratio of KOH/ACB is favorable for nitrogen adsorption. In addition, it is interesting that there are two kinds of isotherms shown in Fig. 3a and b. Gas adsorption achieves saturation at low pressures for the samples of ACB-800-3 and ACB-900-3. The two isotherms belong to type I isotherm for micropore materials on the IUPAC classification. The capillary condensation occurs in the isotherms of the samples of ACB-800-5, ACB-800-7, ACB800-9, ACB-900-5, ACB-900-7, and ACB-900-9. In addition, the six isotherms show clear hysteresis loops. The phenomena above indicate the existence of mesopores in these samples, which can be further proved by the pore size distribution as shown in Fig. 4 and Table 2. For comparison, the BET SSA, DFT SSA, total pore volume, mesopore fraction and average pore size of all the samples are listed in Table 2. The results indicate that the BET SSA of our samples covers a very wide range from 1472 m2 g−1 up to more than 3500 m2 g−1 . It is worth emphasizing that ACB-800-7 can achieve a high BET SSA of 3537 m2 g−1 , which is a very high value for mesoporous. In our former study, a rather higher result of 3180 m2 g−1 was obtained after KOH activation [10]. The results show that adding the fumed silica powder is beneficial for high BET SSA. It is interesting that the percentage of mesopore volume, total volume and average pore size increase with the increasing of the weight ratio of KOH/ACB. In addition, sample of ACB-800-9 has large pore volume values of 3.05 cm3 g−1 . ACB-900-9 can achieve a high percentage of mesopore volume of 72%. The average pore size is about 2–4 nm for all the samples. The sample of ACB-900-9 has the largest average pore diameter of 4.43 nm. Fig. 4 shows the pore size distributions obtained by applying the non-local density functional theory based on the nitrogen adsorp-
Fig. 3. Nitrogen adsorption/desorption isotherms at 77 K. (a: activation temperature of 800 ◦ C. 䊉: ACB-800-3, : ACB-800-5, : ACB-800-7, : ACB-800-9) (b: : ACB-900-7, activation temperature of 900 ◦ C. : ACB-900-3, : ACB-900-5, : ACB-900-9).
tion isotherms. There are three peaks of pore diameters of 0.7–0.9, 1–2 and 3–5 nm. The pore size distribution becomes wider with the ratio of KOH/ACB, which is in accord with the nitrogen adsorption capacity and can explain the large loop sizes of ACB-800-5, ACB-800-7, ACB-800-9, ACB-900-5, ACB-900-7, and ACB-900-9 in Fig. 3a and b. 3.2. Electrochemical properties The gravimetric and volumetric capacitances for samples at the current density of 0.35 mA cm−2 are given in Table 3. Seen from Table 3, the gravimetric and volumetric capacitances for samples are relatively high. The sample of ACB-800-7 has the highest gravimetric capacitance (191.7 F g−1 ) among all the samples. The gravimetric and volumetric capacitance values of ACB-800-5 are 182.0 and 137.7 F cm−3 .
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trode/electrolyte interface. The semicircle of ACB-800-9 is the smallest among the eight samples, indicating the lowest impedance on electrode/electrolyte interface. The magnitude of the equivalent series resistance (ESR) was estimated from the x-intercept of the Nyquist plot [21], which is shown in Table 3. The lower ESR values are about 1.60 and 1.63 for ACB-800-5 and ACB-800-7, respectively. The specific capacitances of the samples at different current densities were studied in a constant current charge–discharge test and the results are given in Fig. 7. It is obvious that the
Fig. 4. Pore size distribution of ACB by density function theory (a: activation tem: ACB-800-5, : ACB-800-7, : ACB-800-9) perature of 800 ◦ C. : ACB-800-3, : ACB-900-7, (b: activation temperature of 900 ◦ C. : ACB-900-3, : ACB-900-5, : ACB-900-9).
Fig. 5 shows an example of the galvanostatic charge/discharge characteristics for the capacitor built from ACB-800-7 at the current density of 0.35 and 7 mA cm−2 , respectively. Fig. 5a shows a typical triangular shape without ohmic drop, which confirms excellent capacitive properties at the current density of 0.35 mA cm−2 . The presence of the mesopores makes it possible that the electrolyte ions can easily access the pore of the electrode. When the current density increases to 7 mA cm−2 , the curve also shows a triangular shape. However, a little ohmic drop happens on 2.25–2.5 V for the large current density at the discharge process. To further investigate the electrochemical performance of the capacitors, electrochemical impedance spectroscopy was employed. Fig. 6 shows the Nyquist plots of the electrodes. A semicircle in the high frequency range and a sloped line in the low frequency range have been observed. To our knowledge, the smaller radius of semicircle, the lower impedance on elec-
Fig. 5. Galvanostatic charge/discharge characteristics of a capacitor built from ACB800-7: (a) 0.35 mA cm−2 and (b) 7 mA cm−2 .
Table 3 The specific capacitance and equivalent series resistance (ESR) of ACB. Sample
Cg (F g−1 )
Cv (F cm−3 )
ESR ()
ACB-800-3 ACB-800-5 ACB-800-7 ACB-800-9 ACB-900-3 ACB-900-5 ACB-900-7 ACB-900-9
133.8 182.0 191.7 173.8 105.8 164.6 161.0 155.4
103.4 137.7 60.8 70.3 93.8 76.2 64.7 44.9
2.00 1.60 1.63 2.18 7.52 1.71 7.53 2.54
Cg : gravimetric specific capacitance; Cv : volumetric specific capacitance; ESR: equivalent series resistance.
Fig. 6. Nyquist impedance plots for different electrodes (: ACB-800-3, 800-5, : ACB-800-7, : ACB-900-9).
: ACB-800-9;
: ACB-900-3,
: ACB-900-5,
: ACB-
: ACB-900-7,
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: ACB-800-9;
: ACB-900-3,
: ACB-
Fig. 9. Specific capacitance as a function of BET surface area for ACB ( and : ACB-900-5).
: ACB-800-5,
: ACB-800-7,
: ACB-
Fig. 10. Capacitance by experiment and calculation as a function of DFT surface area (: capacitance by experiment and : capacitance by calculation).
Fig. 7. Specific capacitance at various current densities for the capacitors (: ACB800-3, 900-5,
: ACB-800-5, : ACB-800-7, : ACB-900-7, : ACB-900-9).
Fig. 8. Cycle life of ACB (: ACB-800-3, 800-9;
: ACB-900-3,
: ACB-900-5,
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: ACB-900-7,
: ACB-800-9
: ACB-900-9).
capacitances of ACB-800-3 and ACB-900-3 decrease rapidly with the increase of current density and cannot be measured at the current density of 70 mA cm−2 . As is discussed above, the percentages of micropore for ACB-800-3 and ACB-900-3 are higher than other samples. The sharply reduced capacitance of microporous carbon as the discharge rate rises up not only reflects the slow diffusion of ions within micropores, but also concerns with the solvation/desolvation process of ions as they pass through the interface between porous carbon and electrolyte [22]. By contrast, the capacitances of other samples decrease slightly with the current density. Especially, the capacitance of ACB-800-7 is 131.2 F g−1 at the current density of 70 mA cm−2 and can still maintain 70.7% of original capacitance of 191.7 F g−1 at the low current density of 0.35 mA cm−2 . The excellent performance of ACB-800-7 may be due to its particular pore structure. The sufficient but not severe activating condition of ACB-800-7 likely produces high surface area and short length of pores, which will be studied in our future work. Fig. 8 presents the capacitance versus cycle number curves of ACB samples under current density of 3.5 mA cm−2 . Most samples have relatively stable capacitance with the increase of cycle number. But the capacitance of ACB-800-3 and ACB-900-3 show the obvious decline with the increase of cycle number. The results of Figs. 7 and 8 imply that, for ACB samples, mesopore plays an important role on high current performance and cycle life. In order to investigate the influence of SSA on capacitance, Fig. 9 shows the capacitance results against the BET SSA for all the sam-
ples. It is observed that the specific capacitance increases with the BET SSA except the sample of ACB-900-5. The BET SSA values of ACB-800-9 and ACB-900-5 are 3017 and 3071 m2 g−1 , respectively. However, the capacitance of sample ACB-800-9 (red color) is higher than that of sample ACB-900-5 (green color) despite having similar BET areas. As is discussed in Fig. 4 and Table 2, ACB-800-9 shows a wider pores size distribution than ACB-900-5, which implied that besides the BET SSA, the pore size distribution maybe affect the capacitance. Although the BET method plays important role on porous materials, it is a restriction to analyze the SSA with different pore size ranges. Based on solid modern statistical physics, the DFT method can resolve such problem. In order to discuss the influence of porosity on capacitance, the relationship between capacitance and surface area is further investigated by the DFT SSA, as is seen in Fig. 10. The SSA is divided into three parts: the surface area of the pore size below 1 nm (S1 ), the surface area of the pore size between 1 and 2 nm (S2 ) and the surface area of the pore size larger than 2 nm (S3 ). In Fig. 10 the specific capacitance results by experiment and calculation (using equation 4) are plotted against the DFT SSA for the ACB studied. We find that the calculated results agree well with the experiment data. Using the Least Square Method, S1 , S2 , S3 and capacitance (F) were calculated by the following equation F = 0.0874 × S1 + 0.0993 × S2 + 0.0969 × S3
(4)
where F is the capacitance, F g−1 . S1 , S2 , S3 are the surface areas with the pore size below 1 nm, between 1 and 2 nm, larger than
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2 nm m2 g−1 , respectively. The coefficients of S1 , S2 , S3 are 0.0874, 0.0993 and 0.0969 Fm−2 , respectively. As is seen that all the pore size less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. It can be seen from the coefficient of S1 , the average surface specific capacitance of the pore sizes below 1 nm is 8.74 F cm−2 (i.e. 0.0874 F m−2 ), which is close to the results (the surface specific capacitance from 5 to 14 F cm−2 ) of ref [18]. The pores below 1 nm can exhibit the high capacitance due to the fact that the micropores can adsorb de-solvated or partially de-solvated electrolyte ions [18]. The coefficient of the relatively narrow pores (1–2 nm) is slightly higher than that of the mesopores (>2 nm). It is implied that the contribution of the pores (1–2 nm) to the performance of supercapacitors is slightly higher than the pores above 2 nm. It confirms the general trends reported earlier [19,20,23]. 4. Conclusions A series of activated carbon beads have been prepared by mixing the coal tar pitch and fumed silica power and activation by KOH. The high BET specific surface area of 3537 m2 g−1 and the high pore volume values of 3.05 cm3 g−1 are obtained. The percentage of mesopore volume, total pore volume and average pore sizes increase with the weight ratio of KOH/ACB. The ACB-900-9 exhibits a high percentage of mesopore volume of 72%. The electrochemical double layer capacitors (EDLCs) are assembled with resultant ACB electrode and electrolyte. The investigation of gravimetric and volumetric capacitances for samples shows that ACBs exhibit good capacitances. ACB-800-7 gives the excellent capacitance of 191.7 F g−1 at the charge–discharge current of 0.35 mA cm−2 , and it still keeps a high value of 131.2 F g−1 at 70 mA cm−2 . The BET SSA and the pore size distribution present significant influences on the electrochemical performance of the samples. Detailed discussion indicates that all the pore size less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. In particular, the effect of micropores (<1 nm) on capacitance is slightly
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