Yingying
Lv
a,
Fan
Zhang
*a,
Yuqian
Dou
b,
Yunpu
Zhai
a,
Jinxiu
Wang
a,
Haijing
Liu
a,
Yongyao
Xia
a,
Bo
Tu
a and
Dongyuan
Zhao
*a
aDepartment of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, Shanghai, 200433, P. R. China. E-mail: zhang_fan@fudan.edu.cn; dyzhao@fudan.edu.cn; Fax: +86-21-51630307; Tel: +86-21-51630205
bDepartment of Chemistry, Northeast University, Shenyang, 110819, P. R. China
First published on 11th October 2011
Activation of ordered mesoporous carbon orientates the development and application of new carbonaceous supercapacitor materials with high energy density and power density. Ordered mesoporous carbons FDU-15 are synthesized in large scale via a soft template method through evaporation induced self-assembly of mesostructure on the sacrificed polyurethane foam. Common activating agent potassium hydroxide (KOH) is utilized to improve the surface area and tailor the pore texture of the ordered mesoporous carbon by adjusting KOH/carbon mass ratio as well as activation time. At low KOH/carbon ratio, the generated micropores increase in volume and either connect to other micropores or eventually become mesopores. At high KOH/carbon ratio, an excess amount of micropores would be generated. Meanwhile, the continuous shrinkage of carbon framework is carried through as prolonged time at high activation temperature. Competition between KOH etching and shrinkage of mesopores is existed during the activation. The latter obviously preponderates over the former at low KOH/carbon ratio, which is reversed at high KOH/carbon ratio. Thus, an optimized micro-mesostructure is achieved under certain activation conditions: maintained ordered mesostructure, suitable microporosity, high surface area (1410 m2 g−1) and large pore volume (0.73 cm3 g−1). The activated sample exhibits improved electrochemical behavior with a gravimetric capacitance of 200 F/g, excellent rate performance and good cycling stability with capacitance retention of ∼98% over 300 cycles.
Our group has successfully developed a soft template method to synthesize highly ordered 2-D hexagonal mesoporous carbon FDU-15 by using amphiphilic triblock copolymers (PEO-PPO-PEO) as template phenolic resin as carbon precursor via a solvent evaporation induced self-assembly method (EISA) followed by a thermopolymerization process.10 Recently, a simple strategy has been reported for the synthesis of ordered mesoporous carbon by using polyurethane foam scaffolds as decomposable substrates for the EISA step.11 After carbonization, the substrates (polyurethane foam) are burned out, and ordered mesoporous carbon is obtained in large scale, which makes it possible for commercial use. However, the specific surface area of the obtained ordered mesoporous carbon FDU-15 is relatively low (660 m2 g−1), which is disadvantageous for charge storage. In order to obtain ordered mesoporous carbon with desirable electrochemical capacitive performance, further research into subsequent activation is required. Industrially, KOH activation is a well-established method that can be used for generating micropores in activated carbon.12,18–20 Up to now, some studies have been reported for the KOH activation of mesoporous carbon, and show the improvement of specific surface area.13–15KOH activated mesoporous carbon (KOH/carbon ratio of 6.0, activation temperature of 800 °C and activation time of 1 h) with high surface area of 1940 m2 g−1 at the expense of the mesostructural order, shows a improved capacitance of 188 F/g at a low scan rate of 1 mV s−1 in 0.1 M NaCl electrolyte.16 Under the conditions of KOH/carbon ratio of 4.0, activation temperature of 800 °C and activation time of 1 h, ordered mesostructure of the activated mesoporous carbon is retained. A capacitance of 180 F/g is obtained with a frequency of 1 Hz in 30% KOH electrolyte.17 Increasing the activation temperature up to 1000 °C, the activated mesoporous carbon demonstrates higher surface area of 2060 m2 g−1 but a relatively low capacitance of 60 F/g.17 Although increasing the KOH/carbon ratio or activation temperature can be utilized to improve the surface area of mesoporous carbon, it is unfavorable from both economical and environmental perspectives. Hence, it is quite important to systematically study the KOH activation on mesostucture or pore evolution and eventual effect on the electrochemical performance of soft template synthesized ordered mesoporous carbon. Moreover, given their great potential and ease of industry-scale production, it is desirable to generate new pores in such novel carbon materials for electro-chemical applications.
In this paper, the KOH activation of the ordered mesoporous carbon is systematically investigated by adjusting a series of activation conditions at various KOH/carbon mass ratios and activation time. It is found that FDU-15 shows excellent mesostructural stability even under severe activation conditions. The effects of KOH activation on the mesostructure, pore size and surface area of FDU-15 are conveniently studied in detail. Meanwhile, a large number of micropores or mesopores are generated, and their contents and properties are determined. Moreover, the activated mesoporous carbon can be used as high performance supercapacitor electrode material with high energy density and power density.
Fig. 1 SAXS patterns of FDU-15 and activated samples with KOH/carbon ratio of 1.0 (A), 4.0 (B), 6.0 (C) for 45, 60, 90 min, respectively. |
Sample | Unit cell size | BET surface area | Pore volume | Micropore surface area | Microporosity (surface area) | Micropore volume |
---|---|---|---|---|---|---|
(nm) | (cm2 g−1) | (cm3 g−1) | (cm2 g−1) | (%) | (cm3 g−1) | |
FDU-15 | 10.2 | 660 | 0.44 | 180 | 27 | 0.07 |
KF1-45 | 10.7 | 930 | 0.49 | 590 | 63 | 0.24 |
KF1-60 | 10.2 | 1030 | 0.52 | 660 | 64 | 0.27 |
KF1-90 | 9.8 | 1410 | 0.73 | 890 | 63 | 0.38 |
KF4-45 | 10.5 | 1150 | 0.56 | 830 | 72 | 0.34 |
KF4-60 | 10.3 | 1310 | 0.62 | 1030 | 79 | 0.43 |
KF4-90 | 10.3 | 1240 | 0.59 | 970 | 78 | 0.40 |
KF6-45 | 10.7 | 1280 | 0.59 | 990 | 77 | 0.41 |
KF6-60 | 10.6 | 1400 | 0.69 | 1020 | 73 | 0.42 |
KF6-90 | 10.6 | 1200 | 0.56 | 960 | 80 | 0.40 |
A more detailed structural characterization is revealed by TEM images, the pristine FDU-15 shows highly ordered stripe-like (from [11] direction) and hexagonally arranged (from [01] direction) images (Fig. 2A, B). After KOH treatment with KOH/carbon ratio of 1.0 for 90 min, the activated sample exhibits 2-D hexagonal regularity, suggesting good framework stability under a long activation time (Fig. 2C, D). Estimated from TEM images, the cell parameter decreases from ∼10.4 to 10.1 nm for the pristine FDU-15 and activated sample KF1–90, respectively, which are accordance with the shrinkage degree calculated from SAXS data. Compared with that of pristine FDU-15 (∼5 nm), the mesopore size of activated sample KF1–90 shrinks to ∼3–4 nm (Fig. 2D), which is in agreement with the degree of shrinkage of the framework. It should be noted that, the formation of KOH generated micro/mesopores in the wall is obvious in the activated sample under the conditions of KOH/carbon ratio of 1.0 and long activation time of 90 min (Fig. 2C).
Fig. 2 TEM images of ordered mesoporous carbon FDU-15 (A, B) and activated sample KF1–90 (C, D), KF6–90 (E, F), viewed from [11] (A, C, E) and [01] (B, D, F) directions. The insets are the corresponding FFT diffractograms. |
Nitrogen sorption isotherm (Fig. 3A) of the pristine FDU-15 is a representative type-IV curve for the typical H1 hysteresis loop (parallel and nearly vertical branches in the mesopore range) at the evident capillary condensation steps at P/P0 = 0.4–0.6, which are typical features of uniform mesopores. The BET surface area and total pore volume are calculated to be 660 m2 g−1 and 0.44 cm3 g−1, respectively. The pore size distribution (Fig. 3B) calculated from the adsorption branch by NLDFT (Nonlocal Density Functional Theory) method reveals bimodal pores with an average mesopore of ∼5 nm and a micropore of ∼1.5 nm, respectively. After the KOH activation treatment, all the activated samples with KOH/carbon ratio of 1.0 exhibit similar type-IV curves with H1 hysteresis loops at the same P/P0 range as that of the pristine sample FDU-15 (Fig. 3A), revealing that ordered mesostructure can be maintained under the applied activation conditions. However, the surface area and pore volume of all activated samples are larger than that of pristine FDU-15 (Fig. 3A). The surface area of KF1 samples increases from 930 to 1410 m2 g−1 as activation time is prolonged from 45 to 90 min (Table 1). Compared with the intrinsic microporosity of the pristine FDU-15 (27%), the microporosities of KF1 samples (with KOH/carbon ratio of 1) are dramatically increased to ∼63% (Table 1). The condensation steps become wider and the adsorption amounts obviously increase (Fig. 3A), suggesting that mesopore distribution becomes wider and pore volumes increase greatly (Fig. 3B). Remarkably, the activated sample with KOH/carbon ratio of 1.0 for 90 min shows a wider mesopore size distributed between 1.5–4 nm, and a high pore volume of 0.73 cm3 g−1 contributed mostly by micropores (Fig. 3B), which is coincident with TEM images (Fig. 2B).
Fig. 3 N2 sorption isotherms and pore size distribution of ordered mesoporous carbon FDU-15 and activated samples with KOH/carbon ratio of 1.0 (A, B), 4.0 (C, D), 6.0 (E, F) for 45, 60, 90 min, respectively. |
At low KOH/carbon ratio, the KOH generated micropores increase in volume and lead to the degradation of mesostrucutre. The etching action as well as continuous shrinkage of pore wall occurred in KOH activation, leads to expansion and shrinkage of framework, respectively. The small mesopores in the wall caused by activation with low KOH/carbon ratio under long activation time, imply that KOH is homogeneously distributed inside the pore walls on nanoscale. The results show that the activated sample obtained with a KOH/carbon ratio of 1.0 under long activation time of 90 min possesses the highest BET surface area of 1410 m2 g−1 and total pore volume of 0.73 cm3 g−1, respectively, indicating the effectiveness of KOH in developing new pores in mesoporous carbon skeleton.
The TEM images of the activated sample with high KOH/carbon ratio of 6.0 viewed from [11] (Fig. 2E) and [01] (Fig. 2F) directions, respectively, further confirm the well preserved 2-D hexagonal mesostructure. The cell parameter of the activated sample KF6–90 under long activation time of 90 min is estimated to be 10.3 ± 0.2 nm, in good agreement with that determined from the SAXS data. The mesopore size of KF6–90 is about 4.5 nm, which is indicative of smaller framework shrinkage of activated samples with high KOH/carbon ratio than low KOH/carbon ratio. At high KOH/carbon ratio of 6.0, the formation of overabundant micropores distributed in the carbon framework can be observed (Fig. 2F).
Nitrogen sorption isotherms of activated samples with different KOH/carbon ratio show the similar trends of structure evolution. The representative type-IV isotherms and H1 hysteresis loops are observed under the condition of KOH/carbon ratio between 1.0 and 6.0 with activation time of 45 min (Fig. 3A, C, E), reflecting the maintenance of ordered mesostructure even at severe activation conditions. As the KOH/carbon ratio continuously increases from 1.0 to 6.0, the surface area of activated samples is increased from 930 to 1280 m2 g−1 with activation time of 45 min. The sequence of initial parts of adsorption isotherms (Fig. 3A, C, E) as well as the NLDFT pore size distribution (Fig. 3B, D, F) show gradual increase in the micropore volume as KOH/carbon ratio increases (Table 1). The microporosity is increased to 77% as the KOH/carbon ratio is increased (4.0 or 6.0) (Table 1). It should be noted that, the BET surface area of KF6 samples, increases from 1280 to 1400 m2 g−1 with the increasing activation time from 45 to 60 min, but decreases to 1200 m2 g−1 with activation time of 90 min (Table 1). The KF6–90 samples show a highest microporosity of 80%, suggesting that a large amount of micropores are generated instead of being connected to form small mesopores at severe activation condition.
As KOH/carbon ratio increases, the dramatic increasing of micropores is attributed to a fairly larger extent of enlarging surface area and total pore volume. At high KOH/carbon ratio, the competition between the etching of mesopores and shrinkage of frameworks still exists, the former is more obvious. The etching action at high KOH/carbon ratio is more severe than that at low KOH/carbon ratio, so few small mesopores are generated. If activation time is too long, inner ultra-small micropores without the connecting by mesopores are difficult for N2 adsorption, inducing a decrease of surface area and pore volume.
Scheme 1 Illustration of KOH activation of ordered mesoporous carbon FDU-15. |
Fig. 4 The CV curves at the scan rates of 5 mV s−1 (A) and the dependence of retained capacitance (B) of ordered mesoporous carbon FDU-15 and activated samples with KOH/carbon ratio of 1.0 for 45, 60, 90 min, respectively; specific capacitance of FDU-15 and activated samples versusKOH/carbon ratio and activation time (C); cycle performance of activated sample KF1–90 at the current density of 1.0 A g−1 (D). |
Fig. 4B shows the specific capacitance of the pristine FDU-15 and KF1 samples at a wide range of current densities from 0.5 to 20 A/g. The curve of the retained specific capacitance of KF1 samples as the increasing current density is similar to that of pristine FDU-15, showing that the micro-mesostructure still performs with good rate capability. Because the enhanced specific surface area benefits EDLC formation, the specific capacitance of activated samples is larger than that of pristine FDU-15 (130 F/g). The specific capacitance is proportional to their specific surface area at different KOH/carbon ratio (Fig. 4C). The specific capacitances of KF1–45, KF1–60, KF1–90 are increased to 144, 172 and 200 F/g (Fig. 4B), respectively, which is consistent with the variation tendency of specific surface area. KF4 or KF6 samples with over-abundant micropores are less effective in EDLC forming than KF1, which could be ascribed to the resistance of electrolyte ion transportation into the inner micropores.
In order to evaluate the cycle stability of the activated carbon materials, galvanostatic charge-discharge studies were performed at a fairly high current density of 1.0 A g−1 between −0.8 and 0 V in the 6 M KOH electrolyte. After 300 cycles, the capacitance retention of KF1–90 is as high as ∼98%, indicative of excellent electrochemical stability (Fig. 4D). Therefore the activated mesoporous carbon presents optimized micro-mesoporous structure for enhanced power density especially at high current density. In addition, abundant micropores and small mesopores in the original mesoporous walls generated by the activation treatment lead to large specific surface area (1410 cm3 g−1), which also provide low-resistant pathways, short distance for ions through the porous particles and made great contributions for EDLC forming.
Footnote |
† Electronic supplementary information (ESI) available: See DOI: 10.1039/c1jm12742j |
This journal is © The Royal Society of Chemistry 2012 |