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Design and synthesis of three-dimensional hierarchical ordered porous carbons for supercapacitors
Electrochimica Acta 154 (2015) 110–118
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Design and synthesis of three-dimensional hierarchical ordered porous carbons for supercapacitors Qinglan Zhao, Xianyou Wang * , Jing Liu, Hao Wang, Youwei Zhang, Jiao Gao, Qun Lu, Heye Zhou Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Hunan Province Key Laboratory of Electrochemical Energy Storage and Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 October 2014 Received in revised form 7 December 2014 Accepted 8 December 2014 Available online 10 December 2014
Three-dimensional hierarchical ordered porous carbons (3D HOPCs) have been successfully prepared through templating method using silica sphere nano-array as a hard template, triblock copolymer P123 as a soft template and sucrose as a carbon source, and used as the electrode materials for supercapacitors. The structure, morphology and physicochemical properties of the as-prepared 3D HOPCs are characterized by nitrogen adsorption–desorption isotherm, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) tests, and cycle life measurements. The results show that the 3D HOPCs possess well-interconnected 3D hierarchical ordered porous structure. Especially, the 3D HOPC-80, of which the pore size is 4.01 nm, shows the specific surface area of 1182 m2 g1 and a specific capacitance of 247 F g1 at the current density of 1 A g1. Besides, the supercapacitors based on 3D HOPCs exhibit excellent rate performance, high energy densities of 7.5 W h kg1 and 5.8 W h kg1 at the power densities of 500 W kg1 and 10000 W kg1, respectively. Moreover, the supercapacitors using 3D HOPCs as electrode materials hold high capacitance retentions over 91% even after 10000 cycles. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Supercapacitor Three-dimensional hierarchical ordered porous carbon Nano-array Template
1. Introduction There has been ever-increasing interest in exploring and developing more efficient energy storage devices for the past decades [1,2]. Supercapacitor, as a promising candidate for energy storage system, is attracting more and more attention owing to its advantages of high power density, long cycle life and fast charge– discharge rate [3,4]. Due to good conductivity, high specific surface area and low cost, the porous carbons have been widely used as electrode materials for supercapacitors, especially the electrical double layer capacitors (EDLCs) [5,6]. Recently, the porous carbons with three-dimensional (3D) interconnected ordered structure have been proposed as promising electrode materials for supercapacitors because of their distinctive structural features and good physicochemical properties [7,8]. The 3D interconnected ordered porous structure can improve the access of electrolytes to the inner of the electrode and promote the electrolyte transportation by providing shorter
* Corresponding author. Tel.: +86 731 58292060; fax: +86 731 58292061. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.electacta.2014.12.052 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
diffusion pathways [9–11]. For example, Su et al. prepared 3D interconnected ordered macroporous carbons by chemical vapor deposition of bezene with inverse silica opal as the template, and found that the 3D interconnected open pore structures can contribute to the enhancement of the electrochemical characteristics of the porous carbons [12]. However, although the 3D ordered porous carbons can facilitate the rapid ion transportation, the specific surface area is still relatively low due to the few micropores and mesopores, leading to a limited specific capacitance [13]. On the other hand, the porous carbon materials can usually be divided into three categories according to their pore sizes: micropores (<2 nm), mesopores (2–50 nm) and macropores (> 50 nm) [14]. The pore size will play an important role in the formation of the electrical double layer based on the energy storage mechanism of the EDLC [15]. In general, the micropores mainly contribute to the electrical double layer capacitance, while the mesopores and macropores facilitate the kinetics of electrical double layer formation and transportation of the electrolyte ions [16]. Thus, the carbons with hierarchical porous structure are of great potential in the EDLC, to which many efforts have been devoted [17–21]. Liu et al. found that the unique hierarchical
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structure of carbon materials can provide a favorable path for electrolyte penetration and transportation, which resulted in attractive capabilities in supercapacitors and fuel cells [22]. Xing et al. synthesized the hierarchical porous carbons through selfassembly of triblock copolymer, phloroglucinol and formaldehyde followed by KOH post-activation, which maintained the specific capacitance of 180 F g1 at the frequency of 1 Hz [17]. Zhang et al. obtained the hierarchical porous carbons by KOH activation of coal liquefaction residue and coal, and the specific capacitance can be highly up to 186 F g1 at the scan rate of 5 mV s1 [20]. Apparently, it will be very necessary and significant to combine the advantages of 3D ordered structure and those of hierarchical porous structure in the carbon materials for the application of high performance supercapacitors. However, great success has been achieved in the porous carbons with hierarchical structure and/or 3D structure with/without ordered porous structure [10,12,13,17,18,20–22], the porous carbons combined the advantages of the hierarchical structure and 3D ordered porous structure have rarely been reported in the studies of supercapacitors. In addition, the currently reported methods are too complex to effectively control the pore structural parameters, or the pore size of the carbon material is not very suitable for the application of supercapacitors [23]. Herein, we put forward a strategy using the silica sphere nanoarrays self-assembled by monodisperse silica particles as a hard template and a short triblock copolymer PEO20PPO70PEO20 (P123) as a soft template to synthesize 3D hierarchical ordered porous carbons and explored their application in supercapacitors for the first time. Moreover, the effects of size of silica particle templates on physicochemical properties and supercapacitive behaviors of the 3D HOPCs have also been investigated. 2. Experimental 2.1. Synthesis of silica sphere nano-arrays Firstly, the three solutions containing monodisperse silica spheres with sizes of 80, 150, and 230 nm were prepared according
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to the reported method [24]. The prepared silica spheres were uniform in size with deviation of less than 5%. Next, the solutions were centrifuged, rinsed with water, and dried to be slurry colloids. Then, the silica colloids were deposited onto the surface of clean glasses according to Colvin [25]. Subsequently, the ordered silica sphere nano-arrays were left on glasses after the solutions evaporated. Finally, the nano-arrays were dried at 60 C and sintered at 750 C for 2 h. 2.2. Preparation of 3D HOPCs The preparation strategy of 3D HOPCs is schematically shown in Fig. 1. The P123 and sucrose were introduced into the voids of the silica sphere nano-array. After carbonization, the soft template P123 was decomposed to leave some mesopores in the carbon/ silica composite. The composite was further immersed in HF solution to resolve the silica, resulting in the porous carbons. A typical procedure was as follows. The as-prepared silica sphere nano-arrays were put in a Büchner funnel attached to a vacuum of ca. 100 mTorr. Then, the aqueous solution with mass ratio of 10 sucrose:2.5 P123:1 sufuric acid was added into the voids of the silica sphere nano-arrays. After drying in oven at 60 C, the above composite nano-arrays were heated at 150 C for 6 h and carbonized at 700 C for 3 h with a heating rate of 5 C min1 under Ar flow. The resulting silica/carbon composites were immersed in 20 wt% HF, washed until pH = 7, and dried at 100 C. The prepared carbons were named as 3D HOPC-x, where “3D HOPC” denotes three-dimensional hierarchical ordered porous carbon, and x depends on the sizes of silica spheres. 2.3. Characterization and electrochemical measurements Both scanning electron microscopy (JSM-6610LV, JEOL) and transmission electron microscopy (JEM-2100, JEOL) were used to study the microstructure of the prepared 3D HOPCs. N2 adsorption–desorption isotherms were measured at 77 K (TriStar II 3020, Micromeritics) after being degassed in a vacuum at 200 C for 3 h. The specific surface areas (SBET) were evaluated by the
Fig. 1. Schematic illustration of preparation strategy of the 3D HOPCs.
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Fig. 2. (a) Nitrogen adsorption–desorption isotherms at 77 K of 3D HOPCs and (b) the corresponding BJH mesopore size distribution curves.
Brunauer–Emmett–Teller (BET) method. The pore size distributions were determined from the adsorption branch of the isotherms by the Barrett–Joyner–Halenda (BJH) method. The total pore volumes (Vtotal) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.995. X-ray photoelectron spectroscopy (K-Alpha 1063, Thermo Fisher Scientific) was used to study the surface chemistry. For the preparation of the electrode, 80 wt% the 3D HOPC sample, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder were dispersed in N-methyl-2-pyrrolidone to get a slurry. The slurry was then coated on the nickel foam substrate with a spatula. The electrode was dried at 80 C overnight, and then pressed at 16 MPa for 1 min to assure a good electronic contact between the nickel foam substrate and the active material. The electrode was finally dried at 80 C in vacuum for 24 h. The total mass of the materials on the nickel foam substrate is about 5 mg per electrode. The electrochemical performances of the prepared electrode were evaluated from 1.0 V to 0 V by CV and GCD using a threeelectrode system, in which the prepared electrode (radius = 0.5 cm), nickel sheet (10 cm 8 cm) and Hg/HgO electrode were used as the working, counter and reference electrodes, respectively. For supercapacitor tests in a two-electrode configuration, the button cell supercapacitor was assembled according to the order of electrode–separator–electrode and tested in a voltage window between 0 V and 1 V. The electrochemical impedance spectroscopy (EIS) of supercapacitors was measured in the frequency ranging from 105 Hz to 102 Hz with amplitude of 5 mV. The CV, GCD and EIS measurements were conducted on an electrochemical workstation (VersaSTAT3, Princeton Applied Research, USA). The cycle life was measured by a supercapacitor test station (SCTS, Arbin Instruments, USA). Besides, all the tests were measured in 6 mol L1 KOH electrolyte at room temperature.
3. Results and discussion The pore texture properties of 3D HOPCs are investigated by nitrogen adsorption–desorption isotherms. Fig. 2 presents the typical N2 adsorption–desorption isotherms and the BJH pore size distributions of 3D HOPCs. As shown in Fig. 2a, the N2 adsorption– desorption isotherms of the 3D HOPCs display a type IV isotherm with a H2 hysteresis loop. The small amounts of N2 adsorbed at the region of low relative pressure suggest the existence of micropores, which may be derived from the polymerization and carbonization of the sucrose. The hysteresis loops start at the relative pressure P/ P0 0.55, whereas the sharp capillary condensation steps shift gradually to high relative pressure from 3D HOPC-80 to 3D HOPC230, which may be attributed to the increase of the pore size. The similar result has also been observed by Zhang et al. [10]. The Hysteresis loop tails at high relative pressure region (P/P0 > 0.95) can be seen obviously in 3D HOPC-150 and 3D HOPC-230, especially the 3D HOPC-230, which might be mainly ascribed to the internal macropores [26]. The pore size distributions of 3D HOPCs are displayed in Fig. 2b. It can be found that all the samples show a peak at ca. 4 nm, which can be interpreted as the mesopores generated by the removal of P123. Besides, the broad peak observed at ca. 8 nm, 15 nm and 30 nm for 3D HOPC-80, 3D HOPC-150 and 3D HOPC-230 may correspond to the small windows connecting the spherical macropores. Compared with the other two samples, the 3D HOPC-230 shows a wider distribution in the mesopore region. This may be due to the increasing interstices and windows connecting the spherical macropores [10]. The corresponding BET specific surface areas, pore volumes and pore sizes are summarized in Table 1. It can be seen that all the samples possess high specific surface areas and large pore volumes. Particularly, the specific surface area of 3D HOPC-80 is
Table 1 The pore characteristics of 3D HOPCs. Samples
SBET (m2 g1)
Vtotal (m3 g1)
Mesopore size (nm)
Macropore window size (nm)
3D HOPC-80 3D HOPC-150 3D HOPC-230
1182 879 782
1.79 2.10 2.65
4.01 4.01 4.02
8 15 30
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Fig. 3. SEM images of (a) silica sphere nano-array template and (b) the 3D hierarchical ordered porous carbon after carbonization and soak in HF solution; (c) TEM image and (d) HRTEM image of the 3D hierarchical ordered porous carbon (All the samples are prepared based on 230 nm silica spheres.).
1182 m2 g1, which is the highest among all three samples. The specific surface areas will increase with the decrease of the sizes of silica sphere templates, therefore the specific surface areas can be controlled by adjusting the sizes of silica spheres. The typical SEM images of a silica sphere nano-array template and the porous carbon sample after removal of the templates are displayed in Fig. 3. As shown in Fig. 3a, the silica sphere of the nano-array is ca. 230 nm in diameter. There are a few defects in an occasional sphere vacancy, but generally, the silica sphere nanoarray is in a highly ordered arrangement. After removing the P123 and silica sphere nano-array templates through carbonization and soak in HF solution, the 3D ordered porous carbon is obtained. As shown in Fig. 3b, it can be found that the spherical macropore size is ca. 215 nm, revealing a small shrinkage of ca. 6.5% from a 230 nm silica sphere. This is probably due to the shrinkage of silica spheres in sintering or carbonization process. Besides, it is notable that the next lower layer of pores can be observed through small windows connecting the neighboring spherical pores, suggesting the 3D interconnection in the ordered porous structure. This connectivity may be attributed to the contacting points of the tightly arranged silica spheres in the nano-array after the sintering process. Fig. 3c is the TEM image of the 3D HOPC sample, in which a distinct ordered porous structure can be seen. In addition, it can be clearly observed from the high-resolution TEM (HRTEM) image in Fig. 3d that there are mesopores and micropores in the walls of the macropores, indicating the hierarchical structure of the 3D ordered porous carbon. Consequently, it can be concluded from the results of SEM and TEM that the as-prepared porous carbon owns a 3D hierarchical ordered structure.
XPS analysis was employed to further investigate the surface chemical compositions. Fig. 4 shows the wide scan XPS spectrum and the fitted C1s XPS spectrum of 3D HOPC-80. The wide scan XPS spectrum in Fig. 4a shows that the 3D HOPC-80 consists of C and O, peaking at the binding energies of around 284.8 and 532.9 eV with the O/C atomic ratio of 0.062. As shown in Fig. 4b, the C1s XPS spectrum of 3D HOPC-80 can be fitted by four component peaks at binding energies of about 284.4, 285.4, 286.4, and 289.2 eV, which may be respectively attributable to C—C, C—O, C¼O, and O—C¼O species [27–29]. The O1s XPS spectrum of 3D HOPC-80 is further shown in Fig. 4c, which can be also fitted by three component peaks. The first peak at 530.8 eV is assigned to the oxygen atoms in carboxyl or carbonyl groups. The second peak at 532.2 eV can be attributable to the oxygen atoms in epoxy, phenol or carboxylic groups. And the third one at 533.2 eV is related to the chemisorbed oxygen or moisture [30]. Therefore, there do exist some oxygenenriched functionalities coming from the carbonization of the sucrose. These oxygen-enriched functionalities can both improve wettability and produce pseudocapacitance, resulting in better supercapacitive performance [31]. 4. Electrochemical properties Cyclic voltammetry measurements are carried out to evaluate the electrochemical performances of 3D HOPCs. Fig. 5a shows the cyclic voltammograms of the prepared 3D HOPC electrodes in three-electrode system at the scan rate of 5 mV s1 in 6 mol L1 KOH. As shown in Fig. 5a, all the CV curves present a nearly rectangular shape, suggesting a typical EDLC behavior. The capacitance of the 3D HOPC electrodes can be calculated by Eq. (1):
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Fig. 4. (a) The wide scan XPS spectrum, (b) the C1s XPS spectrum and (c) the O1s XPS spectrum of 3D HOPC-80.
Cg ¼
Ia þ jIc j 2mðdV=dtÞ
(1)
where Cg,Ia, Ic, m and dV/dt are the specific capacitance (F g1), the current (A) of anodic and cathodic CV curves on positive and negative sweeps at the medium of the potential range, the mass of
active material (g), and the sweep rate (mV s1), respectively [32]. It can be found in Table 2 that the specific capacitances of the electrodes are 243.5 F g1, 171.1 F g1 and 194.2 F g1 for 3D HOPC80, 3D HOPC-150 and 3D HOPC-230, respectively. The highest specific capacitance of 3D HOPC-80 may be due to its suitable pore size and high specific surface area. Besides, the reason why 3D
Fig. 5. (a) CV and (b) GCD curves of the 3D HOPC electrodes in three-electrode system.
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Table 2 The comparison of capacitances calculated via different configurations of 3D HOPCs. Samples
Cga (F g1)
Cmb (F g1)
Cc (F g1)
Cd (F g1)
Ce (F g1)
3D HOPC-80 3D HOPC-150 3D HOPC-230
243.5 171.1 194.2
247 169 198
60.6 40.1 47.2
56.8 40.3 44.9
54.2 40.0 44.1
a b c d e
The capacitances calculated from CV at 5 mV s1 in three-electrode system. The capacitances calculated from GCD at 1 A g1 in three-electrode system. The capacitances calculated from CV at 5 mV s1 in two-electrode configuration. The capacitances calculated from cycle life in two-electrode configuration. The capacitances calculated from GCD at 1 A g1 in two-electrode configuration.
HOPC-230 has higher specific capacitance than that of 3D HOPC150 may be ascribed to its wider mesopore size distribution and higher utilization of specific surface area, although the specific surface area of 3D HOPC-230 is lower. The GCD tests are conducted to further confirm the specific capacitance. Fig. 5b gives the charge–discharge curves of the 3D HOPC electrodes in three-electrode system at the current density of 1 A g1. The GCD curves are slightly bent, which suggests a certain pseudocapacitance resulting from the oxygen-enriched functionalities [31]. The specific capacitances of the prepared electrodes can be estimated from Eq. (2): Cm
It ¼ V m
(2)
Fig. 6. The Nyquist plots of the 3D HOPC supercapacitors with the expanded highfrequency region of the plot inset.
where Cm is the specific capacitance (F g1), I is the discharge current (A), Dt is the discharge time (s), DV is the potential window (V), and m is the weight of active material (g) [33]. As listed in Table 2, the specific capacitances based on the mass load of an
Fig. 7. Cyclic voltammograms of (a) 3D HOPC-80, (b) 3D HOPC-150 and (c) 3D HOPC-230 supercapacitors at scan rates ranging from 5 mV s1 to 100 mV s1, and (d) their specific capacitances at different scan rates.
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Fig. 8. GCD curves of (a) 3D HOPC-80, (b) 3D HOPC-150 and (c) 3D HOPC-230 supercapacitors at current densities ranging from 1 A g1 to 20 A g1 and (d) Ragone plots of 3D HOPC supercapacitors measured at current densities varying from 1 A g1 to 20 A g1.
electrode are 247 F g1, 169 F g1 and 198 F g1 for 3D HOPC-80, 3D HOPC-150 and 3D HOPC-230, which are in good agreement with the CV observations. Electrochemical impedance spectroscopy is employed to analyze the impedance behaviors of porous carbon materials. Fig. 6 displays the Nyquist plots of the supercapacitors using 3D HOPCs as electrode materials. The Nyquist plots are similar with a semicircle in the high-frequency region, a line with a slope close to 45 in middle frequency region and a nearly vertical line in low frequency region. The impedance response is a typical capacitive behavior of porous carbon electrodes. The semicircle shape is determined by the adsorption kinetics of the ions at a micro/ mesoporous carbon electrode, the series resistance of a material, charge transfer resistance inside the macro/mesoporous carbon structure and the mass transfer resistance in the microporous carbon electrode (RCE) at higher frequency [34].The equivalent series resistance (ESR) values calculated from x-intercepts with the real axis of the Nyquist plots in high frequency part are 0.7 V, 0.9 V and 0.8 V for 3D HOPC-80, 3D HOPC-150 and 3D HOPC-230 supercapacitors, respectively. The sum of (ESR + RCE) and the internal distribution of the electrolyte resistance values in the pore matrix of carbon electrodes (Rpore), i.e., the total polarization resistance, depends on the extrapolation of the low-frequency linear part of Z00 vs. Z0 plots to Z00 = 0. The total polarization resistances of 3D HOPC80, 3D HOPC-150 and 3D HOPC-230 supercapacitors are 2.0 V, 2.4 V, and 2.2 V. As shown, all the 3D HOPC supercapacitors
exhibit low resistance. It can be attributed to the hierarchical ordered porous structure of the electrode materials promoting the ion transportation.
Fig. 9. Cyclic performance for supercapacitors using 3D HOPCs as electrode materials at a current density of 0.5 A g1.
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To estimate the rate performance, CV measurements of supercapacitors based on the 3D HOPCs as electrode materials at different scan rates from 5 mV s1 to 100 mV s1 are studied further, as presented in Fig. 7a–c. All the CV curves of the 3D HOPC supercapacitors show almost rectangular shapes without obvious distortion at high scan rates. Additionally, there exist a quick current response to the switching potential in the CV curves, indicating low equivalent series resistance, which can contribute to a high rate capability and power density [35]. Obviously, it can be found in Fig. 7d that the specific capacitances of all the supercapacitors decay very slowly even at high scan rate of 100 mV s1 with the capacitance retentions over 90%, which are obviously much higher than the result reported previously [19]. The good rate capability may be due to the 3D hierarchical ordered porous structure and excellent pore connectivity, which can facilitate the fast ion transportation at high scan rates. High energy density or high specific capacitance at high current densities is always expected for supercapacitors [36]. Fig. 8a–c show the GCD curves of different 3D HOPC supercapacitors at current densities ranging from 1 A g1 to 20 A g1. The energy density and power density are estimated from the GCD tests of supercapacitors, according to the following Eqs. (3) and (4) [37]: 1 E ¼ CV 2 2
P¼
E
Dt
(3)
(4)
where E is the energy density (W h kg1), C is the specific capacitance of the capacitor (F g1), V is the cell voltage (V), P is the power density (W kg1), and Dt is the discharge time (s). Fig. 8d shows the Ragone plots exhibiting the relationship of power density and energy density. It can be seen that the energy density of 3D HOPC-80 supercapacitor at the same power density is the highest among all three samples. The 3D HOPC-80 supercapacitor obtains the energy density of 7.5 W h kg1 and 5.8 W h kg1 at the power density of 500 W kg1 and 10000 W kg1, respectively. Therefore, the 3D HOPC-80 supercapacitor owns a great potential for high energy and power densities. Long cycle life is one of the most important parameters for the practical supercapacitors. As presented in Fig. 9, the cycle life performance of supercapacitors using 3D HOPC as electrode materials is tested for 10000 cycles at a current density of 0.5 A g1. The initial capacitances of the supercapacitors are listed in Table 2. Besides, the specific capacitance retentions even after 10000 cycles remain 91%, 92.6% and 93% for 3D HOPC-80, 3D HOPC-150, 3D HOPC-230 supercapacitors, respectively. It further demonstrates the excellent long-term cycling stability of the supercapacitors using 3D HOPCs as electrode materials. 5. Conclusion In summary, the novel porous carbons with a 3D hierarchical ordered structure have been prepared using silica sphere nanoarray and P123 as templates, and sucrose as a carbon source. The pore sizes are in a good agreement with the sizes of silica templates used. In addition, the silica templates can not only effectively control macropore size, but also affect the micro- and mesopore size distribution. The 3D HOPCs hold good pore connectivity and high specific surface area. The highest specific capacitance of 3D HOPCs is as high as 243.5 F g1 and 247 F g1 at the scan rate of 5 mV s1 and the current density of 1 A g1, respectively. Besides, the specific capacitance retentions of the supercapacitors based on 3D HOPCs remain over 90% when the scan rate increases from 5 mV s1 to 100 mV s1. Particularly, the energy densities of the 3D
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HOPC-80 supercapacitor are as high as 7.5 W h kg1 and 5.8 W h kg1 at the power densities of 500 W kg1 and 10000 W kg1, respectively. Concurrently, the 3D HOPC-80 supercapacitor holds the high specific capacitance retention of 91% even after 10000 cycles. The good performances of the 3D HOPC are ascribed to the 3D hierarchical ordered porous structure with high specific surface area, which can facilitate the electrical double layer formation and ion transportation. Accordingly, the porous carbons with 3D hierarchical ordered structure will have a great potential for the high-performance supercapacitors. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51072173, 51272221, 51302239 and 21203161), Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20134301130001), the Natural Science Foundation of Hunan Province, China (Grant No. 13JJ4051). Reference [1] X. Xia, Q. Xiong, Y. Zhang, J. Tu, C.F. Ng, H.J. Fan, Oxide nanostructures hyperbranched with thin and hollow metal shells for high-performance nanostructured battery electrodes, Small 10 (2014) 2419. [2] F. Cao, G.X. Pan, X.H. Xia, P.S. Tang, H.F. Chen, Synthesis of hierarchical porous NiO nanotube arrays for supercapacitor application, J. Power Sources 264 (2014) 161. [3] M.F. El-Kady, V. Strong, S. Dubin, R.B. Kaner, Laser scribing of highperformance and flexible graphene-based electrochemical capacitors, Science 335 (2012) 1326. [4] Q. Li, Z.L. Wang, G.R. Li, R. Guo, L.X. Ding, Y.X. Tong, Design and synthesis of MnO2/Mn/MnO2 sandwich-structured nanotube arrays with high supercapacitive performance for electrochemical energy storage, Nano Lett. 12 (2012) 3803. [5] D. Carriazo, F. Picó, M.C. Gutiérrez, F. Rubio, J.M. Rojo, F. del Monte, Blockcopolymer assisted synthesis of hierarchical carbon monoliths suitable as supercapacitor electrodes, J. Mater. Chem. 20 (2010) 773. [6] Z. Tai, X. Yan, Q. Xue, Three-dimensional graphene/polyaniline composite hydrogel as supercapacitor electrode, J. Electrochem. Soc. 159 (2012) A1702. [7] Y. Wang, S. Gai, N. Niu, F. He, P. Yang, Fabrication and electrochemical performance of 3D hierarchical b-Ni(OH)2 hollow microspheres wrapped in reduced graphene oxide, J. Mater. Chem. A 1 (2013) 9083. [8] J. Ji, L.L. Zhang, H. Ji, Y. Li, X. Zhao, X. Bai, X. Fan, F. Zhang, R.S. Ruoff, Nanoporous Ni(OH)2 thin film on 3D ultrathin-graphite foam for asymmetric supercapacitor, ACS Nano 7 (2013) 6237. [9] H. Pröbstle, C. Schmitt, J. Fricke, Button cell supercapacitors with monolithic carbon aerogels, J. Power Sources 105 (2002) 189. [10] S. Zhang, L. Chen, S. Zhou, D. Zhao, L. Wu, Facile synthesis of hierarchically ordered porous carbon via in situ self-assembly of colloidal polymer and silica spheres and its use as a catalyst support, Chem. Mater. 22 (2010) 3433. [11] E. Lee, T. Lee, B.S. Kim, Electrospun nanofiber of hybrid manganese oxides for supercapacitor: Relevance to mixed inorganic interfaces, J. Power Sources 255 (2014) 335. [12] F. Su, X.S. Zhao, Y. Wang, J. Zeng, Z. Zhou, J.Y. Lee, Synthesis of graphitic ordered macroporous carbon with a three-dimensional interconnected pore structure for electrochemical applications, J. Phys. Chem. B 109 (2005) 20200. [13] Y. Li, Z. Li, P.K. Shen, Simultaneous formation of ultrahigh surface area and three-dimensional hierarchical porous graphene-like networks for fast and highly stable supercapacitors, Adv. Mater. 25 (2013) 2474. [14] S. Harpalani, G. Chen, Influence of gas production induced volumetric strain on permeability of coal, Geotechn. Geol. Engng. 15 (1997) 303. [15] Y. Lv, L. Gan, M. Liu, W. Xiong, Z. Xu, D. Zhu, D.S. Wright, A self-template synthesis of hierarchical porous carbon foams based on banana peel for supercapacitor electrodes, J. Power Sources 209 (2012) 152. [16] Y. Li, Z. Fu, B. Su, Hierarchically structured porous materials for energy conversion and storage, Adv. Funct. Mater. 22 (2012) 4634. [17] W. Xing, C.C. Huang, S.P. Zhuo, X. Yuan, G.Q. Wang, D. Hulicova-Jurcakova, Z.F. Yan, G.Q. Lu, Hierarchical porous carbons with high performance for supercapacitor electrodes, Carbon 47 (2009) 1715. [18] P. Hiralal, H. Wang, H.E. Unalan, Y. Liu, M. Rouvala, D. Wei, P. Andrew, G.A.J. Amaratunga, Enhanced supercapacitors from hierarchical carbon nanotube and nanohorn architectures, J. Mater. Chem. 21 (2011) 17810. [19] Y. Liang, F. Liang, D. Wu, Z. Li, F. Xu, R. Fu, Construction of a hierarchical architecture in a wormhole-like mesostructure for enhanced mass transport, Phys. Chem. Chem. Phys. 13 (2011) 8852. [20] J. Zhang, L. Jin, J. Cheng, H. Hu, Hierarchical porous carbons prepared from direct coal liquefaction residue and coal for supercapacitor electrodes, Carbon 55 (2013) 221.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Design and synthesis of three-dimensional hierarchical ordered porous carbons for supercapacitors Qinglan Zhao, Xianyou Wang * , Jing Liu, Hao Wang, Youwei Zhang, Jiao Gao, Qun Lu, Heye Zhou Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Hunan Province Key Laboratory of Electrochemical Energy Storage and Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 October 2014 Received in revised form 7 December 2014 Accepted 8 December 2014 Available online 10 December 2014
Three-dimensional hierarchical ordered porous carbons (3D HOPCs) have been successfully prepared through templating method using silica sphere nano-array as a hard template, triblock copolymer P123 as a soft template and sucrose as a carbon source, and used as the electrode materials for supercapacitors. The structure, morphology and physicochemical properties of the as-prepared 3D HOPCs are characterized by nitrogen adsorption–desorption isotherm, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) tests, and cycle life measurements. The results show that the 3D HOPCs possess well-interconnected 3D hierarchical ordered porous structure. Especially, the 3D HOPC-80, of which the pore size is 4.01 nm, shows the specific surface area of 1182 m2 g1 and a specific capacitance of 247 F g1 at the current density of 1 A g1. Besides, the supercapacitors based on 3D HOPCs exhibit excellent rate performance, high energy densities of 7.5 W h kg1 and 5.8 W h kg1 at the power densities of 500 W kg1 and 10000 W kg1, respectively. Moreover, the supercapacitors using 3D HOPCs as electrode materials hold high capacitance retentions over 91% even after 10000 cycles. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Supercapacitor Three-dimensional hierarchical ordered porous carbon Nano-array Template
1. Introduction There has been ever-increasing interest in exploring and developing more efficient energy storage devices for the past decades [1,2]. Supercapacitor, as a promising candidate for energy storage system, is attracting more and more attention owing to its advantages of high power density, long cycle life and fast charge– discharge rate [3,4]. Due to good conductivity, high specific surface area and low cost, the porous carbons have been widely used as electrode materials for supercapacitors, especially the electrical double layer capacitors (EDLCs) [5,6]. Recently, the porous carbons with three-dimensional (3D) interconnected ordered structure have been proposed as promising electrode materials for supercapacitors because of their distinctive structural features and good physicochemical properties [7,8]. The 3D interconnected ordered porous structure can improve the access of electrolytes to the inner of the electrode and promote the electrolyte transportation by providing shorter
* Corresponding author. Tel.: +86 731 58292060; fax: +86 731 58292061. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.electacta.2014.12.052 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
diffusion pathways [9–11]. For example, Su et al. prepared 3D interconnected ordered macroporous carbons by chemical vapor deposition of bezene with inverse silica opal as the template, and found that the 3D interconnected open pore structures can contribute to the enhancement of the electrochemical characteristics of the porous carbons [12]. However, although the 3D ordered porous carbons can facilitate the rapid ion transportation, the specific surface area is still relatively low due to the few micropores and mesopores, leading to a limited specific capacitance [13]. On the other hand, the porous carbon materials can usually be divided into three categories according to their pore sizes: micropores (<2 nm), mesopores (2–50 nm) and macropores (> 50 nm) [14]. The pore size will play an important role in the formation of the electrical double layer based on the energy storage mechanism of the EDLC [15]. In general, the micropores mainly contribute to the electrical double layer capacitance, while the mesopores and macropores facilitate the kinetics of electrical double layer formation and transportation of the electrolyte ions [16]. Thus, the carbons with hierarchical porous structure are of great potential in the EDLC, to which many efforts have been devoted [17–21]. Liu et al. found that the unique hierarchical
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structure of carbon materials can provide a favorable path for electrolyte penetration and transportation, which resulted in attractive capabilities in supercapacitors and fuel cells [22]. Xing et al. synthesized the hierarchical porous carbons through selfassembly of triblock copolymer, phloroglucinol and formaldehyde followed by KOH post-activation, which maintained the specific capacitance of 180 F g1 at the frequency of 1 Hz [17]. Zhang et al. obtained the hierarchical porous carbons by KOH activation of coal liquefaction residue and coal, and the specific capacitance can be highly up to 186 F g1 at the scan rate of 5 mV s1 [20]. Apparently, it will be very necessary and significant to combine the advantages of 3D ordered structure and those of hierarchical porous structure in the carbon materials for the application of high performance supercapacitors. However, great success has been achieved in the porous carbons with hierarchical structure and/or 3D structure with/without ordered porous structure [10,12,13,17,18,20–22], the porous carbons combined the advantages of the hierarchical structure and 3D ordered porous structure have rarely been reported in the studies of supercapacitors. In addition, the currently reported methods are too complex to effectively control the pore structural parameters, or the pore size of the carbon material is not very suitable for the application of supercapacitors [23]. Herein, we put forward a strategy using the silica sphere nanoarrays self-assembled by monodisperse silica particles as a hard template and a short triblock copolymer PEO20PPO70PEO20 (P123) as a soft template to synthesize 3D hierarchical ordered porous carbons and explored their application in supercapacitors for the first time. Moreover, the effects of size of silica particle templates on physicochemical properties and supercapacitive behaviors of the 3D HOPCs have also been investigated. 2. Experimental 2.1. Synthesis of silica sphere nano-arrays Firstly, the three solutions containing monodisperse silica spheres with sizes of 80, 150, and 230 nm were prepared according
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to the reported method [24]. The prepared silica spheres were uniform in size with deviation of less than 5%. Next, the solutions were centrifuged, rinsed with water, and dried to be slurry colloids. Then, the silica colloids were deposited onto the surface of clean glasses according to Colvin [25]. Subsequently, the ordered silica sphere nano-arrays were left on glasses after the solutions evaporated. Finally, the nano-arrays were dried at 60 C and sintered at 750 C for 2 h. 2.2. Preparation of 3D HOPCs The preparation strategy of 3D HOPCs is schematically shown in Fig. 1. The P123 and sucrose were introduced into the voids of the silica sphere nano-array. After carbonization, the soft template P123 was decomposed to leave some mesopores in the carbon/ silica composite. The composite was further immersed in HF solution to resolve the silica, resulting in the porous carbons. A typical procedure was as follows. The as-prepared silica sphere nano-arrays were put in a Büchner funnel attached to a vacuum of ca. 100 mTorr. Then, the aqueous solution with mass ratio of 10 sucrose:2.5 P123:1 sufuric acid was added into the voids of the silica sphere nano-arrays. After drying in oven at 60 C, the above composite nano-arrays were heated at 150 C for 6 h and carbonized at 700 C for 3 h with a heating rate of 5 C min1 under Ar flow. The resulting silica/carbon composites were immersed in 20 wt% HF, washed until pH = 7, and dried at 100 C. The prepared carbons were named as 3D HOPC-x, where “3D HOPC” denotes three-dimensional hierarchical ordered porous carbon, and x depends on the sizes of silica spheres. 2.3. Characterization and electrochemical measurements Both scanning electron microscopy (JSM-6610LV, JEOL) and transmission electron microscopy (JEM-2100, JEOL) were used to study the microstructure of the prepared 3D HOPCs. N2 adsorption–desorption isotherms were measured at 77 K (TriStar II 3020, Micromeritics) after being degassed in a vacuum at 200 C for 3 h. The specific surface areas (SBET) were evaluated by the
Fig. 1. Schematic illustration of preparation strategy of the 3D HOPCs.
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Fig. 2. (a) Nitrogen adsorption–desorption isotherms at 77 K of 3D HOPCs and (b) the corresponding BJH mesopore size distribution curves.
Brunauer–Emmett–Teller (BET) method. The pore size distributions were determined from the adsorption branch of the isotherms by the Barrett–Joyner–Halenda (BJH) method. The total pore volumes (Vtotal) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.995. X-ray photoelectron spectroscopy (K-Alpha 1063, Thermo Fisher Scientific) was used to study the surface chemistry. For the preparation of the electrode, 80 wt% the 3D HOPC sample, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder were dispersed in N-methyl-2-pyrrolidone to get a slurry. The slurry was then coated on the nickel foam substrate with a spatula. The electrode was dried at 80 C overnight, and then pressed at 16 MPa for 1 min to assure a good electronic contact between the nickel foam substrate and the active material. The electrode was finally dried at 80 C in vacuum for 24 h. The total mass of the materials on the nickel foam substrate is about 5 mg per electrode. The electrochemical performances of the prepared electrode were evaluated from 1.0 V to 0 V by CV and GCD using a threeelectrode system, in which the prepared electrode (radius = 0.5 cm), nickel sheet (10 cm 8 cm) and Hg/HgO electrode were used as the working, counter and reference electrodes, respectively. For supercapacitor tests in a two-electrode configuration, the button cell supercapacitor was assembled according to the order of electrode–separator–electrode and tested in a voltage window between 0 V and 1 V. The electrochemical impedance spectroscopy (EIS) of supercapacitors was measured in the frequency ranging from 105 Hz to 102 Hz with amplitude of 5 mV. The CV, GCD and EIS measurements were conducted on an electrochemical workstation (VersaSTAT3, Princeton Applied Research, USA). The cycle life was measured by a supercapacitor test station (SCTS, Arbin Instruments, USA). Besides, all the tests were measured in 6 mol L1 KOH electrolyte at room temperature.
3. Results and discussion The pore texture properties of 3D HOPCs are investigated by nitrogen adsorption–desorption isotherms. Fig. 2 presents the typical N2 adsorption–desorption isotherms and the BJH pore size distributions of 3D HOPCs. As shown in Fig. 2a, the N2 adsorption– desorption isotherms of the 3D HOPCs display a type IV isotherm with a H2 hysteresis loop. The small amounts of N2 adsorbed at the region of low relative pressure suggest the existence of micropores, which may be derived from the polymerization and carbonization of the sucrose. The hysteresis loops start at the relative pressure P/ P0 0.55, whereas the sharp capillary condensation steps shift gradually to high relative pressure from 3D HOPC-80 to 3D HOPC230, which may be attributed to the increase of the pore size. The similar result has also been observed by Zhang et al. [10]. The Hysteresis loop tails at high relative pressure region (P/P0 > 0.95) can be seen obviously in 3D HOPC-150 and 3D HOPC-230, especially the 3D HOPC-230, which might be mainly ascribed to the internal macropores [26]. The pore size distributions of 3D HOPCs are displayed in Fig. 2b. It can be found that all the samples show a peak at ca. 4 nm, which can be interpreted as the mesopores generated by the removal of P123. Besides, the broad peak observed at ca. 8 nm, 15 nm and 30 nm for 3D HOPC-80, 3D HOPC-150 and 3D HOPC-230 may correspond to the small windows connecting the spherical macropores. Compared with the other two samples, the 3D HOPC-230 shows a wider distribution in the mesopore region. This may be due to the increasing interstices and windows connecting the spherical macropores [10]. The corresponding BET specific surface areas, pore volumes and pore sizes are summarized in Table 1. It can be seen that all the samples possess high specific surface areas and large pore volumes. Particularly, the specific surface area of 3D HOPC-80 is
Table 1 The pore characteristics of 3D HOPCs. Samples
SBET (m2 g1)
Vtotal (m3 g1)
Mesopore size (nm)
Macropore window size (nm)
3D HOPC-80 3D HOPC-150 3D HOPC-230
1182 879 782
1.79 2.10 2.65
4.01 4.01 4.02
8 15 30
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Fig. 3. SEM images of (a) silica sphere nano-array template and (b) the 3D hierarchical ordered porous carbon after carbonization and soak in HF solution; (c) TEM image and (d) HRTEM image of the 3D hierarchical ordered porous carbon (All the samples are prepared based on 230 nm silica spheres.).
1182 m2 g1, which is the highest among all three samples. The specific surface areas will increase with the decrease of the sizes of silica sphere templates, therefore the specific surface areas can be controlled by adjusting the sizes of silica spheres. The typical SEM images of a silica sphere nano-array template and the porous carbon sample after removal of the templates are displayed in Fig. 3. As shown in Fig. 3a, the silica sphere of the nano-array is ca. 230 nm in diameter. There are a few defects in an occasional sphere vacancy, but generally, the silica sphere nanoarray is in a highly ordered arrangement. After removing the P123 and silica sphere nano-array templates through carbonization and soak in HF solution, the 3D ordered porous carbon is obtained. As shown in Fig. 3b, it can be found that the spherical macropore size is ca. 215 nm, revealing a small shrinkage of ca. 6.5% from a 230 nm silica sphere. This is probably due to the shrinkage of silica spheres in sintering or carbonization process. Besides, it is notable that the next lower layer of pores can be observed through small windows connecting the neighboring spherical pores, suggesting the 3D interconnection in the ordered porous structure. This connectivity may be attributed to the contacting points of the tightly arranged silica spheres in the nano-array after the sintering process. Fig. 3c is the TEM image of the 3D HOPC sample, in which a distinct ordered porous structure can be seen. In addition, it can be clearly observed from the high-resolution TEM (HRTEM) image in Fig. 3d that there are mesopores and micropores in the walls of the macropores, indicating the hierarchical structure of the 3D ordered porous carbon. Consequently, it can be concluded from the results of SEM and TEM that the as-prepared porous carbon owns a 3D hierarchical ordered structure.
XPS analysis was employed to further investigate the surface chemical compositions. Fig. 4 shows the wide scan XPS spectrum and the fitted C1s XPS spectrum of 3D HOPC-80. The wide scan XPS spectrum in Fig. 4a shows that the 3D HOPC-80 consists of C and O, peaking at the binding energies of around 284.8 and 532.9 eV with the O/C atomic ratio of 0.062. As shown in Fig. 4b, the C1s XPS spectrum of 3D HOPC-80 can be fitted by four component peaks at binding energies of about 284.4, 285.4, 286.4, and 289.2 eV, which may be respectively attributable to C—C, C—O, C¼O, and O—C¼O species [27–29]. The O1s XPS spectrum of 3D HOPC-80 is further shown in Fig. 4c, which can be also fitted by three component peaks. The first peak at 530.8 eV is assigned to the oxygen atoms in carboxyl or carbonyl groups. The second peak at 532.2 eV can be attributable to the oxygen atoms in epoxy, phenol or carboxylic groups. And the third one at 533.2 eV is related to the chemisorbed oxygen or moisture [30]. Therefore, there do exist some oxygenenriched functionalities coming from the carbonization of the sucrose. These oxygen-enriched functionalities can both improve wettability and produce pseudocapacitance, resulting in better supercapacitive performance [31]. 4. Electrochemical properties Cyclic voltammetry measurements are carried out to evaluate the electrochemical performances of 3D HOPCs. Fig. 5a shows the cyclic voltammograms of the prepared 3D HOPC electrodes in three-electrode system at the scan rate of 5 mV s1 in 6 mol L1 KOH. As shown in Fig. 5a, all the CV curves present a nearly rectangular shape, suggesting a typical EDLC behavior. The capacitance of the 3D HOPC electrodes can be calculated by Eq. (1):
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Fig. 4. (a) The wide scan XPS spectrum, (b) the C1s XPS spectrum and (c) the O1s XPS spectrum of 3D HOPC-80.
Cg ¼
Ia þ jIc j 2mðdV=dtÞ
(1)
where Cg,Ia, Ic, m and dV/dt are the specific capacitance (F g1), the current (A) of anodic and cathodic CV curves on positive and negative sweeps at the medium of the potential range, the mass of
active material (g), and the sweep rate (mV s1), respectively [32]. It can be found in Table 2 that the specific capacitances of the electrodes are 243.5 F g1, 171.1 F g1 and 194.2 F g1 for 3D HOPC80, 3D HOPC-150 and 3D HOPC-230, respectively. The highest specific capacitance of 3D HOPC-80 may be due to its suitable pore size and high specific surface area. Besides, the reason why 3D
Fig. 5. (a) CV and (b) GCD curves of the 3D HOPC electrodes in three-electrode system.
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Table 2 The comparison of capacitances calculated via different configurations of 3D HOPCs. Samples
Cga (F g1)
Cmb (F g1)
Cc (F g1)
Cd (F g1)
Ce (F g1)
3D HOPC-80 3D HOPC-150 3D HOPC-230
243.5 171.1 194.2
247 169 198
60.6 40.1 47.2
56.8 40.3 44.9
54.2 40.0 44.1
a b c d e
The capacitances calculated from CV at 5 mV s1 in three-electrode system. The capacitances calculated from GCD at 1 A g1 in three-electrode system. The capacitances calculated from CV at 5 mV s1 in two-electrode configuration. The capacitances calculated from cycle life in two-electrode configuration. The capacitances calculated from GCD at 1 A g1 in two-electrode configuration.
HOPC-230 has higher specific capacitance than that of 3D HOPC150 may be ascribed to its wider mesopore size distribution and higher utilization of specific surface area, although the specific surface area of 3D HOPC-230 is lower. The GCD tests are conducted to further confirm the specific capacitance. Fig. 5b gives the charge–discharge curves of the 3D HOPC electrodes in three-electrode system at the current density of 1 A g1. The GCD curves are slightly bent, which suggests a certain pseudocapacitance resulting from the oxygen-enriched functionalities [31]. The specific capacitances of the prepared electrodes can be estimated from Eq. (2): Cm
It ¼ V m
(2)
Fig. 6. The Nyquist plots of the 3D HOPC supercapacitors with the expanded highfrequency region of the plot inset.
where Cm is the specific capacitance (F g1), I is the discharge current (A), Dt is the discharge time (s), DV is the potential window (V), and m is the weight of active material (g) [33]. As listed in Table 2, the specific capacitances based on the mass load of an
Fig. 7. Cyclic voltammograms of (a) 3D HOPC-80, (b) 3D HOPC-150 and (c) 3D HOPC-230 supercapacitors at scan rates ranging from 5 mV s1 to 100 mV s1, and (d) their specific capacitances at different scan rates.
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Fig. 8. GCD curves of (a) 3D HOPC-80, (b) 3D HOPC-150 and (c) 3D HOPC-230 supercapacitors at current densities ranging from 1 A g1 to 20 A g1 and (d) Ragone plots of 3D HOPC supercapacitors measured at current densities varying from 1 A g1 to 20 A g1.
electrode are 247 F g1, 169 F g1 and 198 F g1 for 3D HOPC-80, 3D HOPC-150 and 3D HOPC-230, which are in good agreement with the CV observations. Electrochemical impedance spectroscopy is employed to analyze the impedance behaviors of porous carbon materials. Fig. 6 displays the Nyquist plots of the supercapacitors using 3D HOPCs as electrode materials. The Nyquist plots are similar with a semicircle in the high-frequency region, a line with a slope close to 45 in middle frequency region and a nearly vertical line in low frequency region. The impedance response is a typical capacitive behavior of porous carbon electrodes. The semicircle shape is determined by the adsorption kinetics of the ions at a micro/ mesoporous carbon electrode, the series resistance of a material, charge transfer resistance inside the macro/mesoporous carbon structure and the mass transfer resistance in the microporous carbon electrode (RCE) at higher frequency [34].The equivalent series resistance (ESR) values calculated from x-intercepts with the real axis of the Nyquist plots in high frequency part are 0.7 V, 0.9 V and 0.8 V for 3D HOPC-80, 3D HOPC-150 and 3D HOPC-230 supercapacitors, respectively. The sum of (ESR + RCE) and the internal distribution of the electrolyte resistance values in the pore matrix of carbon electrodes (Rpore), i.e., the total polarization resistance, depends on the extrapolation of the low-frequency linear part of Z00 vs. Z0 plots to Z00 = 0. The total polarization resistances of 3D HOPC80, 3D HOPC-150 and 3D HOPC-230 supercapacitors are 2.0 V, 2.4 V, and 2.2 V. As shown, all the 3D HOPC supercapacitors
exhibit low resistance. It can be attributed to the hierarchical ordered porous structure of the electrode materials promoting the ion transportation.
Fig. 9. Cyclic performance for supercapacitors using 3D HOPCs as electrode materials at a current density of 0.5 A g1.
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To estimate the rate performance, CV measurements of supercapacitors based on the 3D HOPCs as electrode materials at different scan rates from 5 mV s1 to 100 mV s1 are studied further, as presented in Fig. 7a–c. All the CV curves of the 3D HOPC supercapacitors show almost rectangular shapes without obvious distortion at high scan rates. Additionally, there exist a quick current response to the switching potential in the CV curves, indicating low equivalent series resistance, which can contribute to a high rate capability and power density [35]. Obviously, it can be found in Fig. 7d that the specific capacitances of all the supercapacitors decay very slowly even at high scan rate of 100 mV s1 with the capacitance retentions over 90%, which are obviously much higher than the result reported previously [19]. The good rate capability may be due to the 3D hierarchical ordered porous structure and excellent pore connectivity, which can facilitate the fast ion transportation at high scan rates. High energy density or high specific capacitance at high current densities is always expected for supercapacitors [36]. Fig. 8a–c show the GCD curves of different 3D HOPC supercapacitors at current densities ranging from 1 A g1 to 20 A g1. The energy density and power density are estimated from the GCD tests of supercapacitors, according to the following Eqs. (3) and (4) [37]: 1 E ¼ CV 2 2
P¼
E
Dt
(3)
(4)
where E is the energy density (W h kg1), C is the specific capacitance of the capacitor (F g1), V is the cell voltage (V), P is the power density (W kg1), and Dt is the discharge time (s). Fig. 8d shows the Ragone plots exhibiting the relationship of power density and energy density. It can be seen that the energy density of 3D HOPC-80 supercapacitor at the same power density is the highest among all three samples. The 3D HOPC-80 supercapacitor obtains the energy density of 7.5 W h kg1 and 5.8 W h kg1 at the power density of 500 W kg1 and 10000 W kg1, respectively. Therefore, the 3D HOPC-80 supercapacitor owns a great potential for high energy and power densities. Long cycle life is one of the most important parameters for the practical supercapacitors. As presented in Fig. 9, the cycle life performance of supercapacitors using 3D HOPC as electrode materials is tested for 10000 cycles at a current density of 0.5 A g1. The initial capacitances of the supercapacitors are listed in Table 2. Besides, the specific capacitance retentions even after 10000 cycles remain 91%, 92.6% and 93% for 3D HOPC-80, 3D HOPC-150, 3D HOPC-230 supercapacitors, respectively. It further demonstrates the excellent long-term cycling stability of the supercapacitors using 3D HOPCs as electrode materials. 5. Conclusion In summary, the novel porous carbons with a 3D hierarchical ordered structure have been prepared using silica sphere nanoarray and P123 as templates, and sucrose as a carbon source. The pore sizes are in a good agreement with the sizes of silica templates used. In addition, the silica templates can not only effectively control macropore size, but also affect the micro- and mesopore size distribution. The 3D HOPCs hold good pore connectivity and high specific surface area. The highest specific capacitance of 3D HOPCs is as high as 243.5 F g1 and 247 F g1 at the scan rate of 5 mV s1 and the current density of 1 A g1, respectively. Besides, the specific capacitance retentions of the supercapacitors based on 3D HOPCs remain over 90% when the scan rate increases from 5 mV s1 to 100 mV s1. Particularly, the energy densities of the 3D
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HOPC-80 supercapacitor are as high as 7.5 W h kg1 and 5.8 W h kg1 at the power densities of 500 W kg1 and 10000 W kg1, respectively. Concurrently, the 3D HOPC-80 supercapacitor holds the high specific capacitance retention of 91% even after 10000 cycles. The good performances of the 3D HOPC are ascribed to the 3D hierarchical ordered porous structure with high specific surface area, which can facilitate the electrical double layer formation and ion transportation. Accordingly, the porous carbons with 3D hierarchical ordered structure will have a great potential for the high-performance supercapacitors. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51072173, 51272221, 51302239 and 21203161), Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20134301130001), the Natural Science Foundation of Hunan Province, China (Grant No. 13JJ4051). Reference [1] X. Xia, Q. Xiong, Y. Zhang, J. Tu, C.F. Ng, H.J. Fan, Oxide nanostructures hyperbranched with thin and hollow metal shells for high-performance nanostructured battery electrodes, Small 10 (2014) 2419. [2] F. Cao, G.X. Pan, X.H. Xia, P.S. Tang, H.F. Chen, Synthesis of hierarchical porous NiO nanotube arrays for supercapacitor application, J. Power Sources 264 (2014) 161. [3] M.F. El-Kady, V. Strong, S. Dubin, R.B. Kaner, Laser scribing of highperformance and flexible graphene-based electrochemical capacitors, Science 335 (2012) 1326. [4] Q. Li, Z.L. Wang, G.R. Li, R. Guo, L.X. Ding, Y.X. Tong, Design and synthesis of MnO2/Mn/MnO2 sandwich-structured nanotube arrays with high supercapacitive performance for electrochemical energy storage, Nano Lett. 12 (2012) 3803. [5] D. Carriazo, F. Picó, M.C. Gutiérrez, F. Rubio, J.M. Rojo, F. del Monte, Blockcopolymer assisted synthesis of hierarchical carbon monoliths suitable as supercapacitor electrodes, J. Mater. Chem. 20 (2010) 773. [6] Z. Tai, X. Yan, Q. Xue, Three-dimensional graphene/polyaniline composite hydrogel as supercapacitor electrode, J. Electrochem. Soc. 159 (2012) A1702. [7] Y. Wang, S. Gai, N. Niu, F. He, P. Yang, Fabrication and electrochemical performance of 3D hierarchical b-Ni(OH)2 hollow microspheres wrapped in reduced graphene oxide, J. Mater. Chem. A 1 (2013) 9083. [8] J. Ji, L.L. Zhang, H. Ji, Y. Li, X. Zhao, X. Bai, X. Fan, F. Zhang, R.S. Ruoff, Nanoporous Ni(OH)2 thin film on 3D ultrathin-graphite foam for asymmetric supercapacitor, ACS Nano 7 (2013) 6237. [9] H. Pröbstle, C. Schmitt, J. Fricke, Button cell supercapacitors with monolithic carbon aerogels, J. Power Sources 105 (2002) 189. [10] S. Zhang, L. Chen, S. Zhou, D. Zhao, L. Wu, Facile synthesis of hierarchically ordered porous carbon via in situ self-assembly of colloidal polymer and silica spheres and its use as a catalyst support, Chem. Mater. 22 (2010) 3433. [11] E. Lee, T. Lee, B.S. Kim, Electrospun nanofiber of hybrid manganese oxides for supercapacitor: Relevance to mixed inorganic interfaces, J. Power Sources 255 (2014) 335. [12] F. Su, X.S. Zhao, Y. Wang, J. Zeng, Z. Zhou, J.Y. Lee, Synthesis of graphitic ordered macroporous carbon with a three-dimensional interconnected pore structure for electrochemical applications, J. Phys. Chem. B 109 (2005) 20200. [13] Y. Li, Z. Li, P.K. Shen, Simultaneous formation of ultrahigh surface area and three-dimensional hierarchical porous graphene-like networks for fast and highly stable supercapacitors, Adv. Mater. 25 (2013) 2474. [14] S. Harpalani, G. Chen, Influence of gas production induced volumetric strain on permeability of coal, Geotechn. Geol. Engng. 15 (1997) 303. [15] Y. Lv, L. Gan, M. Liu, W. Xiong, Z. Xu, D. Zhu, D.S. Wright, A self-template synthesis of hierarchical porous carbon foams based on banana peel for supercapacitor electrodes, J. Power Sources 209 (2012) 152. [16] Y. Li, Z. Fu, B. Su, Hierarchically structured porous materials for energy conversion and storage, Adv. Funct. Mater. 22 (2012) 4634. [17] W. Xing, C.C. Huang, S.P. Zhuo, X. Yuan, G.Q. Wang, D. Hulicova-Jurcakova, Z.F. Yan, G.Q. Lu, Hierarchical porous carbons with high performance for supercapacitor electrodes, Carbon 47 (2009) 1715. [18] P. Hiralal, H. Wang, H.E. Unalan, Y. Liu, M. Rouvala, D. Wei, P. Andrew, G.A.J. Amaratunga, Enhanced supercapacitors from hierarchical carbon nanotube and nanohorn architectures, J. Mater. Chem. 21 (2011) 17810. [19] Y. Liang, F. Liang, D. Wu, Z. Li, F. Xu, R. Fu, Construction of a hierarchical architecture in a wormhole-like mesostructure for enhanced mass transport, Phys. Chem. Chem. Phys. 13 (2011) 8852. [20] J. Zhang, L. Jin, J. Cheng, H. Hu, Hierarchical porous carbons prepared from direct coal liquefaction residue and coal for supercapacitor electrodes, Carbon 55 (2013) 221.
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