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Mesoporous Carbon Nanofibers for Supercapacitor Application Kaixue Wang, Yonggang Wang, Yarong Wang, Eiji Hosono, and Haoshen Zhou* Nano Energy Materials Group, Energy Technology Research Institute, AIST Tsukuba, Central 2, Ibaraki 305-8568, Japan ReceiVed: August 20, 2008; ReVised Manuscript ReceiVed: October 17, 2008
Mesoporous carbon nanofibers have been prepared by a confined self-assembly of triblock copolymers with soluble low molecular weight phenolic resol within the channels of anodic alumina membranes. SEM and TEM observations showed that hexagonally arranged mesoporous channels were coiled concentrically around the longitudinal axis of the carbon nanofibers. These carbon nanofibers with BET surface area over 1424 m2 g-1 have been used as electrode materials for electrochemical capacitors tested in KOH and EC/DEC electrolyte solutions. Compared to mesoporous carbon prepared from the same precursor sol, these one-dimensional nanofibers could provide a shortened diffusion distance for electrolyte ions. The better performance of these mesoporous nanofibers greatly benefited from their high specific surface area, shortened diffusion distance, mesoporous openings on the outer surface, and well-controlled pore size. These mesoporous carbon nanofibers have been proved to be promising electrode materials for electrochemical supercapacitors in high-rate charge/ discharge operations. 1. Introduction Electric double layer capacitors (EDLCs) have evoked wide interest in recent years due to their ability to supply high power in short-term pulse, which make them very good energy storage devices for applications such as hybrid power sources for electrical vehicles, portable electronic devices, uninterruptible power supply (UPS), and pulse laser techniques.1-3 The working function of EDLCs is based on the quick formation of a double layer of charges or opposite ions at the electrode/electrolyte interface.1,4,5 For a better performance, the electrode materials for EDLCs should have large surface area to accumulate large amount of charges and size-controllable porous channel system for the easy access of the electrolyte. So far, activated carbons with high surface area have been widely used as the electrode materials for EDLCs.6-8 However, the small disordered micropores of activated carbons cannot be readily accessed by the electrolyte and also constrict the space for charge accommodation inside the pore wall, which limits their gravimetric capacitance and performance.9 Recently, mesoporous carbons have been prepared through a hard template and a soft template pathway by using mesoporous silica as hard templates10,11 and triblock copolymers as soft templates,12,13 respectively. Mesoporous carbons have a variety of properties, including high specific surface area, large size-controllable mesopores, and easily accessed ordered pore channels, which greatly facilitate their applications in EDLCs2,14 and hydrogen storage.2,15 The electrochemical applications of mesoporous carbons will even benefit from their one-dimensional (1D) nanostructures, such as nanofibers and nanotubes, with well-controlled dimensions. Besides the extra surface area derived from the 1D nanostructures, the 1D nanostructures provide a shortened path for the electron transportation. Meanwhile, the ordered mesoporous structures can facilitate the penetration of the electrolytes from the direction perpendicular to the longitudinal axis of the * To whom correspondence should be addressed. Tel: 0081-29-8615795. Fax: 0081-29-8615799. E-mail:
[email protected].
nanostructures. Thus, 1D mesoporous carbon nanostructures with shortened diffusion length for both the electrons and the ions are highly desirable for the EDLCs with both high capacitance and high rate capability. Recently, we have reported the fabrication of mesoporous carbon nanofibers arrays within the pore channels of anodic alumina (AAO) membranes through a confined self-assembly of triblock copolymers with soluble low molecular weight polymer precursors.16 Herein, we report the electrochemical performance of mesoporous carbon nanofibers as a promising electrode material for high-rate EDLC applications. 2. Experimental Section 2.1. Mesoporous Carbon Nanofibers by a Confined SelfAssembly. First, the phenol/formaldehyde resol was prepared from the reaction of phenol with formaldehyde in a basecatalyzed process following a previously reported procedure.12 Then, 1.0 g of Pluronic copolymer surfactant F127 (EO106PO70EO106, Mav ) 12 600) was dissolved in 20.0 g of ethanol. Then 5.0 g of the resol precursor solution in ethanol was added. After stirring for 10 min, a homogeneous sol was obtained. The sol infiltrated the pores of the AAO membrane after the AAO membrane immersed into the sol and changed into gel during the aging periods at room temperature and at 60 °C. After that, the AAO membrane was calcined at 600 °C and subsequently 900 °C for approximately 3 h, under Ar atmosphere to decompose the surfactant molecules and carbonize the mesoporous walls. The AAO membrane was dissolved by 30 wt % HF acid, and the residual product was washed with a large amount of distilled water and then ethanol. For comparison, mesoporous carbon was also prepared from the same surfactant/resol ethanolic precursor sol following the procedure described in ref 12. 2.2. Characterization. X-ray diffraction (XRD) patterns were recorded in a θ-2θ mode on M03XHF22 diffractometer from MAC Science Corp. Nitrogen adsorption-desorption isotherms were collected at 77 K for mesoporous carbon nanofibers released from AAO membranes using BELSORP 18
10.1021/jp807463u CCC: $40.75 2009 American Chemical Society Published on Web 12/30/2008
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Figure 1. (a) Low-magnification and (b) high-magnification scanning electron microscope images of mesoporous carbon nanofibers within the pores of AAO membranes.
adsorption apparatus. The morphology of the carbon nanofiber arrays was characterized by scanning electron microscopy (SEM) performed on a field-emission scanning microscope (FESEM; Carl Zeiss Gemini Supra) operating at 10 kV. Transmission electron microscopy (TEM) images were recorded using a JEOL 2010F microscope operated at 200 kV. 2.3. Electrochemical Measurement. The electrodes were prepared by mixing 80 wt % mesoporous carbon nanofibers, 15 wt % acetylene black (AB) and 5 wt % poly(tetrafluoroethylene) (PTFE) powder. The mixture was spread and pressed onto a nickel mesh. The electrochemical performance of mesoporous carbon nanofibers in 1 M KOH electrolyte was measured in a beaker-type electrochemical cell equipped with the working electrode, a nickel counter electrode, and a standard calomel reference electrode (SCE). Their electrochemical performance was also investigated in a three-electrode cell using lithium metal as counter and reference electrodes, and 1 M LiClO4 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) as an electrolyte. Cyclic voltammetry were conducted on the electrodes using an Autolab PGSTAT 12 potentiostat/galvanostat analyzers. Galvanostatic charge/discharge measurement was performed on a Solartron 1480 potentiostat/galvanostat analyzer. 3. Results and Discussion 3.1. Mesoporous Carbon Nanofibers. A precursor sol containing a soluble low molecular weight phenol/formaldehyde resol and triblock copolymer F127 was first prepared following the procedure described previously.12 Then 1D mesoporous structures formed within the pore channels of commercial AAO membranes through a confined self-assembly process. Figure 1 shows the FE SEM images of mesoporous carbon nanofibers embedded inside the AAO membrane. The nanofibers are observed occupying almost all of the pores of AAO membrane. The diameter of the nanofibers is approximately 200 nm, consistent with the pore size of alumina membranes employed. The highly magnified SEM images clearly show the circular mesoporous channels concentrically coiled around the axis of the carbon fibers. It is also noted that the surface of the nanofibers is not smooth and a lot of openings exist on the outside wall of the carbon fibers. Low-angle X-ray diffraction pattern of the carbon nanofibers released by dissolving AAO membranes in HF solution clearly shows one ill-defined diffraction peak, indicating the mesoporous nature of the carbon nanofibers prepared (Figure 2). It is impossible for mesoporous nanofibers released from the matrix of AAO membranes to form long-range ordering in the mesoporous scale. Thus, the appearance of the ill-defined broad diffraction peak in the low-angle region is reasonable. The well-ordered mesoporous structure of the carbon nanofibers was confirmed by transmission electron microscopy (TEM)
Figure 2. Low-angle XRD pattern of mesoporous carbon nanofibers released from the matrix of AAO membranes.
(Figure 3). Uniform mesopores are observed hexagonally arranged over the whole nanofibers even after calcination at a temperature of 900 °C. The presence of the elliptical shaped mesopores indicates that the nanofibers are subjected to an anisotropic shrinkage during the high-temperature calcination process. Consistent with the SEM observation, the hexagonalarranged mesochannels are observed concentrically coiled around the longitudinal axis of the carbon nanofibers. No obvious graphitic fringe was observed in the HRTEM images, indicating the amorphous nature of the mesopore wall of the carbon nanofibers even after calcination at a temperature as high as 900 °C. However, two broad diffraction peaks located at 24.3 and 44.8 (2θ) can be observed in the high-angle XRD pattern (Figure 4) which can be assigned to the (002) and (100) of the graphite structure. The broadened peaks indicate that very small domains of stacked crystalline graphite, which cannot be detected by TEM technique, exist in the mesopore wall of the nanofibers. The Raman spectrum of mesoporous carbon fibers exhibits two peaks centered at approximately 1335 and 1590 cm-1 corresponding to the D- and G-bands of polycrystalline carbon materials (see the Supporting Information). The low intensity of the G-band indicates the small domains of stacked graphene sheets in the mesopore walls, consistent with the XRD studies. The texture properties of the carbon nanofibers were further confirmed by the nitrogen adsorption/desorption measurements conducted at 77 K. The nitrogen adsorption/desorption isotherms as shown in Figure 5 for the carbon nanofibers exhibit the combined characteristics of multimodal porosity, micropores, and mesopores together with macropores. The sharp increase in the adsorption curve at the low relative pressure is characteristic of microporous materials, indicating the presence of micropores in carbon nanofibers. The curves at a relative pressure of 0.6-0.9 can be classified as a type IV, typical for mesoporous materials. The following abrupt increase in the
Mesoporous Carbon Nanofibers as Supercapacitors
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Figure 3. High-resolution transmission electron microscopy images of mesoporous carbon nanofibers released from the matrix of AAO membranes, showing circular hexagonal-ordered mesoporous structures.
Figure 4. High-angle XRD pattern of mesoporous carbon nanofibers.
Figure 6. Cyclic voltammograms of (a) mesoporous carbon nanofibers and (b) mesoporous carbon in 1 M KOH at sweep rate from 3 to 50 mV/s.
Figure 5. (a) Nitrogen adsorption-desorption isotherm and (b) pore size distribution of mesoporous carbon nanofibers.
curves at high relative pressure, above 0.9, can be attributed to the multilayer adsorption of nitrogen in macropores formed among the carbon nanofibers. Brunauer-Emmett-Teller (BET) surface area and micropore volume are 1424 m2 g-1 and 327 cm3 g-1, respectively. The pore size distribution of the carbon nanofibers as provided in Figure 5b is relatively broad. The
mesopores with pore size approximately 3.4 nm is in good agreement with the mesoporous carbon prepared from the same surfactant/resol precursor sol.12 Moreover, a lot of mesopores with pore size ranging from 5 to 8 nm exist in the nanofibers. The ellipse-shaped mesopores as shown in the TEM image indicate that the broad pore size distribution might result from the anisotropic contraction of the nanofibers during the calcination within the channels of AAO membranes. 3.2. EDLC Performance of Mesoporous Carbon Nanofibers. The electrochemical performance of mesoporous carbon nanofibers was characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge techniques in comparison with that of mesoporous carbon prepared from the same precursor sol following a previously reported procedure (see the Supporting Information for the details of mesoporous carbon).12 Cyclic voltammetry is a useful tool to determine the faradaic and nonfaradaic behaviors and evaluate the capacitance of the electrode materials. Figure 6 shows the CV curves of mesoporous carbon and carbon nanofibers in 1 M KOH solution at a variety of sweep rates varying from 3 to 50 mV/s in the potential
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Figure 8. Representative galvanostatic charge-discharge curves of the capacitor made of mesoporous carbon nanofibers at various rate ranging from 10 to 60 A/g.
Figure 7. Variation of capacitance of mesoporous carbon nanofibers (MCF) and mesoporous carbon (MC) plotted as a function of sweep rates in (a) 1 M KOH and (b) 1 M LiClO4 in EC/DEC electrolytes.
range from -1.1 to 0.0 V vs SCE. The CV curves of the mesoporous carbon nanofibers as shown in Figure 6a maintain a relatively good rectangular shape even at a sweep rate as high as 50 mV/s, indicating their excellent capacitor behaviors. The deviation from regular rectangular shape might result from the inherent resistivity of the nanofiber electrode, the heteroatoms such as oxygen remained in the porous carbon wall, the surface functionality enriched by the HF acid, and the electrochemical hydrogen storage reaction of carbon materials in KOH.3,4 In contrast, those of the mesoporous carbon as shown in Figure 6b gradually lose its regular rectangular shape with the increase of the sweep rates. The improved capacitive performance can be ascribed to the formation of 1D nanofibers. Given the small diameter of the nanofibers, the mesoporous nanofibers provide a shortened electron transport path and electrolyte penetration path from the direction perpendicular to the longitudinal axis of the nanostructures. Meanwhile, the ordered large mesopores along with the mesoporous opening on the external surface of the nanofibers as shown in the SEM images will greatly facilitate the distribution of the electrolytes from the external surface of the nanofibers to the interior mesoporous channels and the ion transportation in the mesoporous channels. The better capacitor behavior of mesoporous carbon nanofibers over mesoporous carbon suggests that mesoporous carbon nanofibers are more suitable for high-rate charge-discharge operations. The CV measurement was also used to calculate the specific capacitance of both mesoporous carbon and carbon nanofibers. The capacitance was plotted as a function of voltage sweep rate. Figure 7a presents the capacitance of mesoporous carbon and carbon nanofibers in 1 M KOH at different voltage sweep rates. Mesoporous carbon nanofibers show the capacitance of charge and discharge of 90 and 152 F/g, respectively, at a sweep rate of 5 mV/s. At low sweep rate, the specific capacitance is closely related to the active surface area of the electrodes. The capacitances of mesoporous carbon with specific surface area
of 698 m2/g are obviously lower than those of carbon fibers at low sweep rate. For mesoporous carbon nanofibers, the capacitances of charge and discharge are kept at a constant value in the range of 90-152 F/g in a wide range of sweep rates from 3 to 50 mV/s, while for the mesoporous carbon, it experiences a dramatic decay in capacitance with the increase of sweep rates. As pointed out by Xing et al. that at low sweep rate, the ions have enough time to diffuse into the inner surface of mesopore channels and even the micropores.17 Thus, the penetration distance and easily accessed pore channels plays a less important role at such stage, while at high sweep rate, the ions can only penetrate into the inner surface of relatively large pores, which indicates that less active surface area of the pores is taking part in the electrochemical processes. Compared to the carbon nanofibers, the long electron transport distance of mesoporous carbon also gives a significant impact on the capacitance at high sweep rate. As a result, the capacitance of mesoporous carbon decreases with the increase of sweep rate. The importance of the penetration distance and easily accessed porous structures on the capacitor performance of mesoporous carbon and carbon nanofibers was also demonstrated by using 1 M LiClO4 in EC/DEC (1:1 by volume) as an electrolyte. The variation of the capacitance with the voltage sweep rate was presented in Figure 7b. It is noted that there is significant difference in capacitance for mesoporous carbon and carbon nanofibers even at low sweep rate. Compared to the aqueous electrolytes, the penetration ability of EC/DEC electrolyte into the inner active surface of small pores is quite limited due to the large steric size of EC and DEC molecules even at slow sweep rate. Under such circumstance, a short penetration distance and easily accessed porous structure are essential, which might account for the better performance of mesoporous carbon nanofibers in nonaqueous electrolyte. Galvanostatic charge/discharge method was also applied to evaluate the capacitance of mesoporous carbon nanofibers. Plots of potential versus time for the mesoporous carbon nanofibers at various rates are shown in Figure 8 and the Supporting Information. A symmetric charge/discharge characteristic of triangular shape typical for ideal capacitor behavior was observed even at high current load of 60 A/g, demonstrating a fast charge propagation with a very small ohmic drop. High electrochemical stability of the capacitor made of mesoporous carbon nanofibers was evaluated by repeated galvanostatic charge/discharge cycling at a rate of 10 A/g (see the Supporting Information). No obvious discharge and charge capacitance decay was observed even after 1000 cycles, indicating the capacitor has a very good cycle life.
Mesoporous Carbon Nanofibers as Supercapacitors 4. Conclusion Mesoporous carbon nanofibers have been prepared by a confined self-assembly of triblock copolymers F127 with soluble low formula weight phenolic resol within the channels of anodic alumina membranes. Hexagonally arranged mesoporous channels were observed concentrically coiled around the longitudinal axis of the carbon nanofibers. These mesoporous nanofibers with shortened electron transport path showed good capacitor performance even at high sweep rate. The shortened diffusion distance, the mesoporous openings on the outer surface, and the well-controlled pore size could greatly facilitate the diffusion and transportation of electrolyte ions inside the mesoporous channels to the active surface area. The better capacitor performance of mesoporous carbon nanofibers over mesoporous carbon suggests that mesoporous carbon nanofibers are promising electrode materials for supercapacitors in high-rate charge/ discharge operations. Acknowledgment. K.W. thanks the Japan Society for the Promotion of Science (JSPS) for the fellowship. The authors also give special thanks to Dr. Masaki Ichihara for HRTEM assistance. Supporting Information Available: Raman spectra of mesoporous carbon and carbon nanofibers, N2 adsorption/ desorption analysis and TEM image of mesoporous carbon, galvanostatic charge-discharge curves of mesoporous carbon nanofibers at rates from 1 to 10 A/g, and cycling performance of mesoporous carbon nanofibers at a rate of 10 A/g. This
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