pubs.acs.org/Langmuir © 2009 American Chemical Society
Preparation and Electrochemical Performance of Novel Ordered Mesoporous Carbon with an Interconnected Channel Structure Yeru Liang,† Dingcai Wu,*,† and Ruowen Fu*,†,‡ †
Materials Science Institute, PCFM laboratory, School of Chemistry and Chemical Engnieering, Sun Yat-sen University, Guangzhou 510275, P. R. China, and ‡Institute of Optoelectronic and Functional Composite Materials, Sun Yat-sen University, Guangzhou 510275, P.R. China Received May 10, 2009. Revised Manuscript Received June 16, 2009
A novel ordered mesoporous carbon with an interconnected channel structure (OMC-IC) has been successfully fabricated by adding proper hydrophilic SiO2 nanoparticles to the solution of triblock copolymer F127 and phenolformaldehyde resol. Experimental results show that the presence of SiO2 nanoparticles does not hamper the self-assembly of F127 and resol to form an ordered two-dimensional hexagonal mesostructure. The neighboring channels of the OMC-IC are interconnected after removing SiO2 nanoparticles with a diameter larger than the thickness of the carbon wall. Such an interconnectivity of channels is beneficial in improving ion diffusion properties. The as-prepared OMC-IC exhibits much lower impedance to ion transport within both the channels and the micropores in the carbon wall, and thus has better electric double layer performance as compared to the conventional OMC with an unconnected channel structure.
Supercapacitors have attracted considerable attention in recent years due to their many benefits, such as high power density compared with secondary batteries, large energy density relative to conventional capacitors, long cycle life, short charging time, wide operating temperature, safety, and so on. Nowadays, supercapacitors have been widely used in many fields, including hybrid vehicles, memory back-up systems, and other devices that need high-pulse discharge profile.1,2 The core technology of supercapacitors is the fabrication of high performance electrode materials. Up until now, nanostructured carbons are recognized as the most promising electrode materials because of their excellent physicochemical stability and good conductivity.3 Therefore, design, preparation, and electrochemical characterization of various novel nanocarbon materials have become the hot topic of research in the field of supercapacitors.1-9 Ordered mesoporous carbon (OMC) materials have been proven to have good electrochemical properties, especially power performances because of their unique ordered mesostructure.10-13 *Corresponding authors. Telephone: þ86-020-84112759. Fax: þ86-02084115112. E-mail:
[email protected] (D.W.);
[email protected] (R.F.). (1) Jurewicz, K.; Vix-Guterl, C.; Frackowiak, E.; Saadallah, S.; Reda, M.; Parmentier, J.; Patarin, J.; Beguin, F. J. Phys. Chem. Solids 2004, 65, 287. (2) Raymundo-Pinero, E.; Leroux, F.; Beguin, F. Adv. Mater. 2006, 18, 1877. (3) Wang, D. W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H. M. Angew. Chem., Int. Ed. 2008, 47, 373. (4) Ania, C. O.; Khomenko, V.; Raymundo-Pi~nero, E.; Parra, J. B.; Beguin, F. Adv. Funct. Mater. 2007, 17, 1826. (5) Xia, K. S.; Gao, Q. M.; Jiang, J. H.; Hu, J. Carbon 2008, 46, 1718. (6) Kim, C.; Ngoc, B. T. N.; Yang, K. S.; Kojima, M.; Kim, Y. A.; Kim, Y. J.; Endo, M.; Yang, S. C. Adv. Mater. 2007, 19, 2341. (7) An, K. H.; Kim, W. S.; Park, Y. S.; Moon, J.-M.; Bae, D. J.; Lim, S. C.; Lee, Y. H.; Lee, Y. S. Adv. Funct. Mater. 2001, 11, 387. (8) Fang, B.; Binder, L. J. Phys. Chem. B 2006, 110, 7877. (9) Pr€obstle, H.; Schmitt, C.; Fricke, J. J. Power Sources 2002, 105, 189. (10) Xing, W.; Qiao, S. Z.; Ding, R. G.; Li, F.; Lu, G. Q.; Yan, Z. F.; Cheng, H. M. Carbon 2006, 44, 216. (11) Vix-Guterl, C.; Frackowiak, E.; Jurewicz, K.; Friebe, M.; Parmentier, J.; Beguin, F. Carbon 2005, 43, 1293. (12) Wang, D. W.; Li, F.; Fang, H. T.; Liu, M.; Lu, G. Q.; Cheng, H. M. J. Phys. Chem. B 2006, 110, 8570. (13) Li, H. Q.; Lu, L. R.; Zhao, D. Y.; Xia, Y. Y. Carbon 2007, 45, 2628.
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Recently, a novel type of two-dimensional hexagonal OMC with a channel structure has been successfully prepared.14-18 The channel structure of OMC may allow a facile mass transport. However, these channels are not connected to each other. This will block the transfer/diffusion of ions between the neighboring channels to a certain extent and thus reduce high-rate charging/ discharging properties of supercapacitors. Therefore, simple, effective, and low-cost methods to interconnect these neighboring channels of OMC materials are urgently needed. We report herein the fabrication of a novel OMC with an interconnected channel structure (OMC-IC) by adding hydrophilic SiO2 nanoparticles with a proper size to the mixture solution of the triblock copolymer F127 and the phenol-formaldehyde resol. The following removal of the SiO2 nanoparticles with diameters larger than the carbon wall thickness leaves mesopores that connect the neighboring channels. The overall synthetic procedure is illustrated in Scheme 1. The addition of appropriate amounts of hydrophilic SiO2 nanoparticles does not interrupt the self-assembly between the F127 and the resol. Figure 1 displays the X-ray diffraction (XRD) patterns of a conventional OMC with an unconnected channel structure (OMC-UC) and that of the corresponding novel OMC-IC samples prepared here. It can be seen that the OMC-UC possesses an intense diffraction peak (10) and two resolved diffraction peaks (11) and (20), indicating the formation of a highly ordered two-dimensional hexagonal mesostructure.16,17 Parallel arranged channels can be observed in the transmission electron microscopy (TEM) image of the OMC-UC (Figure 2A). For such a conventional OMC-UC, these channels are not connected to each other (Figure 2A). When adding hydrophilic SiO2 nanoparticles (about (14) Liang, C. D.; Hong, K. L.; Guiochon, G. A.; Mays, J. W.; Dai, S. Angew. Chem., Int. Ed. 2004, 43, 5785. (15) Tanaka, S.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Chem. Commun. 2005, 2125. (16) Meng, Y.; Gu, D.; Zhang, F. Q.; Shi, Y. F.; Yang, H. F.; Li, Z.; Yu, C. Z.; Tu, B.; Zhao, D. Y. Angew. Chem., Int. Ed. 2005, 44, 7053. (17) Meng, Y.; Gu, D.; Zhang, F. Q.; Shi, Y. F.; Cheng, L.; Feng, D.; Wu, Z. X.; Chen, Z. X.; Wan, Y.; Stein, A.; Zhao, D. Y. Chem. Mater. 2006, 18, 4447. (18) Liang, C. D.; Dai, S. J. Am. Chem. Soc. 2006, 128, 5316.
Published on Web 06/24/2009
DOI: 10.1021/la9016646
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Figure 1. XRD patterns of OMC-UC and OMC-IC samples.
Figure 2. TEM images of OMC-UC (A) and OMC-IC (B,C). Scheme 1. Overall Preparation Procedure of OMC-IC
8 nm in diameter, Figure S1 in the Supporting Information) with a SiO2/phenol mass ratio of 7%, the resulting OMC-IC has a lower order degree, judging from the weaker diffraction peak (11) and peak (20) in Figure 1. However, the peak (10) is still intense; and the TEM image in Figure 2B also shows that the ordered structure is maintained. This indicates clearly that the addition of SiO2 nanoparticles does not hamper the self-assembly of the triblock copolymer F127 and the phenol-formaldehyde resol to form an ordered two-dimensional hexagonal mesostructure, which gives us the possibility to make the pristine channels interconnected. Indeed, it is shown in Figure 2 that the neighboring channels of the original OMC-UC can be connected to each other (see arrows in Figure 2C) after removing the added SiO2 nanoparticles because the diameter of the SiO2-templating mesopores (centered at 7.6 nm, Figure 3) is larger than the thickness of carbon wall (5.6 nm, Table S1 in the Supporting Information). It can also be seen in Table S1 in the Supporting Information that the added SiO2 nanoparticles have the ability to increase slightly the thickness of the carbon wall and the unit cell parameter (a). The pore structures of the OMC-UC and OMC-IC samples are evaluated in detail by N2 adsorption-desorption isotherms (Figure 3). For the OMC-UC, a distinct hysteresis loop occurs at the relative pressure of about 0.4, resulting from the capillary 7784 DOI: 10.1021/la9016646
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Figure 3. N2 adsorption-desorption isotherms of OMC-UC and OMC-IC samples (inset shows their BJH pore size distributions).
condensation of N2 within the channels. The diameter of these channels is calculated to be 3.4 nm by the Barrett-JoynerHalenda (BJH) method (see the inset in Figure 3). When SiO2 nanoparticles are added, the as-prepared OMC-IC exhibits two hysteresis loops at relative pressures of ca. 0.4 and ca. 0.7 in its N2 adsorption-desorption isotherm, demonstrating that the OMC-IC has two types of mesopores: channels of the OMC matrix (about 3.4 nm) and SiO2-templating mesopores (centered at 7.6 nm). The latter ones interconnect the parallel channels. It should be noted that there exist a very small amount of mesopores of 15-25 nm in OMC-IC (see the inset in Figure 3), demonstrating that slight aggregation of SiO2 nanoparticles can happen. These above results are in agreement with the TEM observations (Figure 2). As a result, the OMC-IC with its bimodal mesopore structure has a higher Brunauer-Emmett-Teller (BET) surface area and larger mesopore surface area and volume than the OMC-UC (Table S1 in the Supporting Information). In addition, micropore surface area and volume basically remains unchanged because of the same carbon source and carbonization process (Table S1 in the Supporting Information). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to investigate whether and how the above channel interconnectivity affects the electrochemical performances. Figure S2 in the Supporting Information presents the cyclic voltammograms at different sweep rates for the OMC-UC and OMC-IC samples. As is well-known, for an ideal nanoporous carbon electrode, its nanoporous structure is able to provide very fast ion transport pathways, and thus, the electrical double layer can be reorganized quickly at the switching potentials (e.g., -0.9 and 0.1 V in Figure S2 in the Supporting Information). As a result, in the CV measurements, its response current keeps constant at a constant sweep rate, leading to a rectangular-shaped current versus potential curve.8 The rectangle degree can be used to reflect the ion diffusion rate within a nanoporous carbon structure. The higher the rectangle degree, the faster the ion diffusion rate. At slow sweep rate, the electrolyte ions have enough time to penetrate into the unconnected channels, and thus, the OMC-UC has a good rectangular-shaped CV curve at 10 mV/s of sweep rate (Figure S2A in the Supporting Information). However, with increasing the sweep rate to 300 mV/s, the rectangle degree of the OMC-UC gradually decreases (Figure S2B-D in the Supporting Information), demonstrating that these two-dimensional hexagonal channels are not perfect ion diffusion routes in the case of rapid charge-discharge operations. However, when making these channels interconnected by introducing the SiO2templating mesopores to the carbon wall, the resulting OMC-IC has better rectangular-shaped CV curves at various sweep rates as compared to those of the original OMC-UC. More importantly, the rectangle degree for the OMC-IC is still satisfactory even at Langmuir 2009, 25(14), 7783–7785
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Figure 5. Capacitance at various sweep rates for OMC-UC and OMC-IC samples. Figure 4. Nyquist plots (left ordinate) in the range of 10 kHz to 10 mHz and the corresponding RC plots (i.e., capacitance versus real impedance, right ordinate) for OMC-UC and OMC-IC samples.
a very high sweep rate of 300 mV/s. These results show that the ion diffusion rate within the interconnected channels of the OMC-IC is much faster than that within the unconnected channel of the OMC-UC, especially during large current charging/discharging process. Such a faster ion diffusion rate inside the interconnected channels can be further revealed by Nyquist plots and corresponding capacitance versus real impedance (RC) plots in Figure 4. At the high-frequency region of more than ca. 92.1 Hz (Figure S3 in the Supporting Information), the electrolyte ion transport into nanopores is inhibited, and thus, charge aggregation only occurs on the surface of OMC-UC or OMC-IC powder electrode, leading to negligible capacitance values. After that, the electrolyte ions begin to diffuse into the channels and subsequently into the micropores in the carbon wall, resulting in an increasing capacitance with decreasing frequency. It is known that the impedance to ion diffusion within different pore systems (e.g., mesopores and micropores) can be obtained according to the slope of the RC plot (i.e., the C/R ratio).9 Therefore, the two distinct slopes at the medium-frequency region (ca. 92.1 to ca. 0.34 Hz) and the low-frequency region (less than ca. 0.34 Hz) can be ascribed to the ion diffusion inside the channels and inside the micropores, respectively. For the OMC-UC, the ions diffuse into its channels only through the two entrances at the top and bottom of every channel. In contrast, for the OMC-IC, besides the above two types of entrances, the ions can also diffuse into its exterior and interior channels by means of the additional SiO2-templating mesopores which drill through the carbon walls. The ion diffusion in OMC-IC is accelerated through the much more ion diffusion routes, and thus, OMC-IC has much lower impedances to ion transport inside the interconnected channels (0.08 Ω) and within the micropores (2.04 Ω) compared to the OMC-UC (0.27 and 2.82 Ω, respectively, Figure 4). As a result of the improved ion diffusion performance, the capacitances of the OMC-IC from CV measurement basically stay constant (136-143 F/g) at the sweep
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rate range of 10-300 mV/s (Figure 5). In comparison, the OMCUC has a decreasing capacitance with an increase of the sweep rate and only retains 76% of its initial capacitance at 300 mV/s (Figure 5). The above results show that the as-prepared OMC-IC is a novel and very promising electrode material of supercapacitor for large current applications. In summary, we have successfully fabricated a novel OMC-IC material by adding proper hydrophilic SiO2 nanoparticles to the solution of triblock copolymer F127 and phenol-formaldehyde resol. Experimental results show that the presence of SiO2 nanoparticles does not interrupt the self-assembly of F127 and resol to form an ordered two-dimensional hexagonal mesostructure. The neighboring channels of the OMC-IC are interconnected after removing SiO2 nanoparticles with a diameter larger than the thickness of the carbon wall. Such an interconnectivity of channels is beneficial in improving ion diffusion properties. The as-prepared OMC-IC exhibits much lower impedance to ion transport within both the channels and the micropores in the carbon wall, and thus has better electric double layer performance as compared to the conventional OMC-UC. Our method for preparing the interconnected OMC provides a way to fabricate new materials for charge storage and to replace the traditional OMC in various electrochemical fields and other applications where rapid mass transport is required. Acknowledgment. This research was supported by the Project of NNSFC (50472029, 50632040, 50802116), the Specialized Research Fund for the Doctoral Program of Higher Education (200805581014), the Natural Scientific Foundation of Guangdong Province (8451027501001421), the Scientific Foundation of Guangzhou (2007Z2-D2041), and the SRF for Youth Scholars of SYSU (1131211). We thank Dr. Dangsheng Su at Fritz Haber Institute of the Max Planck Society, Germany for helpful discussions. Supporting Information Available: Experimental details, nanostructure parameters, and CV curves. This material is available free of charge via the Internet at http:// pubs.acs.org.
DOI: 10.1021/la9016646
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