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Synthesis of ordered mesoporous carbon films with a 3D pore structure and the electrochemical performance of electrochemical double layer capacitors
Colloids and Surfaces A: Physicochem. Eng. Aspects 449 (2014) 51–56
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of ordered mesoporous carbon films with a 3D pore structure and the electrochemical performance of electrochemical double layer capacitors Takahito Mitome a,∗ , Yoshiaki Uchida a,b , Yasuyuki Egashira a , Norikazu Nishiyama a a b
Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Ordered
mesoporous carbon films were synthesized by a soft-templating method. • The mesoporous carbon film has a 3D pore structure. • The mesoporous carbon film shows a high capacitance at high scan rates. • The mesoporous carbon film shows a good cycle stability.
a r t i c l e
i n f o
Article history: Received 30 October 2013 Received in revised form 10 February 2014 Accepted 14 February 2014 Available online 22 February 2014 Keywords: Ordered mesoporous carbon film Soft-templating method 3D pore structure Electrochemical double layer capacitor
a b s t r a c t Ordered mesoporous carbon films with a 3D pore structure (COU-3) were synthesized by a softtemplating method using resorcinol (R)/formaldehyde (F) as a carbon source and Pluronic F127 as a template agent. The carbon films were prepared by a spin-coating technique. From the results of X-ray diffraction pattern, field emission scanning electron microscope and transmission electron microscope observation, the structure of the COU-3 film was determined to be an Im3m structure whose c-axis is oriented perpendicular to the film surface. The pores run in the directions both perpendicular and parallel to the film surface. The porous structure of the film is different from the ordered mesoporous carbon powders and thick films (COU-1 and COU-2) which were synthesized using the same precursor solution. In the preparation of the COU-3 films, the self-assembly and condensation of a RF-F127 composite immediately occurred during the spin-coating. In this process, the RF-F127 composites are arranged on a substrate surface and an ordered structure spreads from the substrate surface to an upper region. The electrochemical performance of electrochemical double layer capacitor was examined. The CV curves are flat and rectangular in shape, and did not change even at a high scan rate, indicating that the internal resistance in grain boundary and the contact resistance with the substrate can be ignored in the COU-3 film. Additionally, the COU-3 film shows good cycle stability after 250 cycles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Tel.: +81 668506257. E-mail address: [email protected] (T. Mitome). http://dx.doi.org/10.1016/j.colsurfa.2014.02.041 0927-7757/© 2014 Elsevier B.V. All rights reserved.
Ordered mesoporous carbon materials have attracted much attention due to their high surface area, large pore volume, chemical inertness and electrical conducting property. Development of well-ordered mesoporous carbon films will lead to new
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applications such as adsorbent, electronic device, membrane separation, hydrogen storage and catalyst support. Especially, electrochemical double layer capacitors (EDLCs) using carbon materials have been extensively studied because of their potential to achieve rapid electrical charge and discharge compared to modern secondary batteries. In addition, EDLCs have much longer life cycle than batteries because no or negligible small chemical charge transfer reactions are involved. If porous carbon thin films are prepared on electrodes, a binder and conductive agent such as carbon black are not necessary to prepare working electrodes, which can avoid the problems of interparticle resistance and contact resistance between carbon electrode and current collector. Ordered mesoporous carbons have been first reported by Ryoo’s [1–4] and Hyeon’s [5] groups who synthesized mesoporous carbons through an inorganic hard-templating method. With respect to electrochemical performance of porous carbon films, Kwon’s group synthesized nitrogen-doped mesoporous carbon thin films by using mesoporous silica thin films as a hard template [6]. However, the hard-templating method includes the extra steps for the preparation of silica scaffolds and removal of the silica, which cause a multi-step process and a high cost. Therefore, another simple process is required for practical applications. In contrast to the hard-templating method, a soft-templating method can directly produce ordered mesoporous carbons. Bulk mesoporous carbons with several unique structures have been synthesized, including a two-dimensional (2D)-hexagonal phase p6mm [7–9], a body-centered cubic phase Im3m [7–9] and gyroidbased cubic phase Ia3d [8–10]. As for film preparation, Dai’s group synthesized ordered mesoporous carbon films with large mesopores by selfof a polystylene-block-poly(4-vinylpyridine) assembly (PS-P4VP)/resorcinol–formaldehyde (RF) mixture [11]. We have first used a triblock copolymer, Pluronic F127, as a soft template for the preparation of ordered mesoporous carbon [12]. Tanaka’s group synthesized ordered mesoporous carbon film with a facecentered orthorhombic Fmmm symmetry [13,14]. Bein’s group synthesized a highly ordered mesoporous polymer resin thin films with 2D-hexagonal (plane group p6mm) structure [15]. Vogt’s group introduced metal oxides to an ordered mesoporous carbon film synthesized by the soft-templating method and observed electrochemical properties [16]. Electrochemical performance of mesoporous carbons have been extensively studied and reported so far. For example, Wu et al. reported that an ordered 2D reverse hexagonal pore morphology facilitates rapid ion diffusion more than a disordered wormhole-like pore morphology, thus leading to superior electrochemical properties [17]. However, the value of capacitance should strongly depend on the way of a preparation of electrodes because the internal resistance in grain boundary and the addition of binder actually affect the conductivity. For the film electrode, on the other hand, these problems can be ignored. Here, we report a synthesis of ordered mesoporous carbon films with a 3D pore structure and the electrochemical performance of electrochemical double layer capacitor consisting of the mesoporous carbon film.
2. Experimental 2.1. Synthesis of ordered mesoporous carbon film Resorcinol (R) was dissolved in ethanol solution, and then triblock copolymer ‘Pluronic F127’ was added and stirred for 60 min. After that, formaldehyde (37 wt%) (F) was added to the above solution and the solution was stirred for 10 min. Finally, HCl (5 N) was added as a catalyst to the solution. The molar ratios of the solution were 0.0038 F127:14.5 EtOH:6.5 water:1 R:1.5 F:0.25 HCl. After
stirring for 60 min, the resultant solution was deposited dropwise onto a silicon substrate, spinning at 50 rpm, and then the substrate was spun up to 1000 rpm for 60 s. The deposited sample was heated at 90 ◦ C for 5 h. Then, the resultant sample was carbonized under a nitrogen atmosphere at different temperature (200–800 ◦ C) for 3 h at a heating rate of 1.3 ◦ C min−1 . 2.2. Characterization X-ray diffraction (XRD) measurement of carbon films was performed on a Philips X’Pert-MPD diffractometer using Cu-K␣ ˚ The copper anode was operated at 40 kV radiation with 1.5418 A. and 30 mA. Field emission scanning electron microscope (FE-SEM) images of mesoporous carbon films were recorded on a Hitachi S-5000L microscope at an acceleration voltage of 22–29 kV. Transmission electron microscope (TEM) images of the samples were obtained on a Hitachi H800 electron microscope. An acceleration voltage was 200 kV. The TEM samples were prepared on a lacy carbon film supported by a copper grid. Fourier transform infrared (FT-IR) spectra were measured in transmission mode with a Shimadzu IRAffinity-1. Spectra were obtained between 400 cm−1 and 4000 cm−1 with a scan number of 400 and a resolution of 4 cm−1 . 2.3. Electrochemical measurements Electrochemical measurements consisted of cyclic voltammetry (CV) experiments using VersaSTAT 3. A mesoporous carbon film as a working electrode was synthesized on a Pt-coated Si wafer by the same procedure as described above such as spin-coating, thermal treatment and carbonization. The electrochemical behavior of the carbon film was analyzed with a three-electrode configuration in an aqueous 1 M H2 SO4 solution. A Pt wire and Ag/AgCl (3 M NaCl) were used as the counter electrode and reference electrode, respectively. The capacitances were calculated according to the equation C = I/V, where I and V are the current and the scan rate, respectively [18]. The average specific capacitances were obtained from CV curves. The mass of the COU-3 film was calculated from the increase of mass after the synthesis. Amount of carbon on electrode was 0.5 mg/cm2 . 3. Results and discussion To track the change of the molecular structure in the carbon films, we measured the temperature dependence of the FT-IR spectra as shown in Fig. 1. The broad peak at 3300 cm−1 arises from the O H stretching of phenolic or aliphatic hydroxyl groups, indicating the existence of a large amount of phenolic OH group [19,20]. The broadness of the absorption is derived from intermolecular hydrogen-bonding. A strong peak at 1610 cm−1 can be related to the carbon–carbon bond stretching of 1,2,4- and 1,2,6trisubstituted and phenyl alkyl ether-type substituted aromatic ring structure [20,21]. The peaks at 2854 cm−1 and 2926 cm−1 are assigned to the aliphatic carbon-hydrogen stretching [19,20]. These peaks remained even after carbonization at 600 ◦ C. Pluronic F127 molecules decomposed at temperature from 300 ◦ C to 400 ◦ C. Therefore, the peaks at 2854 cm−1 and 2926 cm−1 can be related to the resorcinol–formaldehyde polymer. The carbon film carbonized at 600 ◦ C has some methylene bridges by crosslinking with resorcinol and formaldehyde, indicating that the polymer network remains at 600 ◦ C. The FT-IR spectra for the samples carbonized at 600 ◦ C and 800 ◦ C show a broad peak at 1085 cm−1 , which is associated with a polycyclic aromatic ring deformation [22]. The peak indicates the occurrence of many polyaromatic reactions when RF
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Fig. 1. FT-IR spectra of the carbon film heated at 90 ◦ C (a), carbonized at 200 ◦ C (b), 400 ◦ C (c), 600 ◦ C (d) and 800 ◦ C (e). For clarity, FT-IR spectra were shifted up.
Fig. 2. XRD patterns of as deposited film (a), the film carbonized at 400 ◦ C (b), 600 ◦ C (c), 800 ◦ C (d) and ordered mesoporous carbon powder (e). For clarity, XRD patterns were shifted up.
resin is treated at temperature above 600 ◦ C. These results indicate that RF resin is gradually converted to carbon. To examine the internal mesostrucure of the as-deposited film and carbon film, we measured XRD patterns of the synthesized structures as shown in Fig. 2. The XRD pattern of the as-deposited film shows a weak peak at 2 = 1.1◦ , indicating that an ordered mesostructured material was already formed before the polymerization process. A strong diffraction peak appeared at 2 = 1.3◦ for the sample treated at 400 ◦ C, suggesting that an ordered mesostructured polymer was constructed before carbonization. Pluronic F127 molecules decompose at temperature from 300 to 400 ◦ C [23] and mesopore is generated by the decomposition of F127 molecules. Therefore, a strong diffraction peak is derived from a density difference between mesopore and carbon pore wall. On the other hand, a new peak appeared at 2 = 1.8◦ and 2.0◦ for the sample carbonized at 600 ◦ C and 800 ◦ C. This peak could not be observed for the ordered mesoporous carbon powder [24]. The d-spacing was
calculated to be 8.0 nm (as deposited sample), d002 = 6.7 nm (400 ◦ C), d110 = 6.8 nm (600 ◦ C), d002 = 4.9 nm (600 ◦ C), d110 = 6.4 nm (800 ◦ C) and d002 = 4.3 nm (800 ◦ C). The peak assignments are described below. To obtain details of the mesostructure, we observed the ordered mesoporous carbon films by FE-SEM as shown in Fig. 3. The thickness of carbon film was estimated to be about 2 m. The pores were observed in the direction both parallel and perpendicular to the surface of the carbon film, indicating that carbon film has a 3D pore structure. A cross-section of pores of carbon film carbonized at 400 ◦ C is circular in shape without deformation. On the other hand, the deformation of the carbon pore wall occurred during carbonization. In the film preparation, the presence of a substrate helps to depress shrinkage of pore wall in the direction parallel to the substrate. On the other hand, shrinkage of pore wall occurred in the direction perpendicular to the substrate. Therefore, the pores became ellipsoidal in shape. The shrinkage of
Fig. 3. FE-SEM images of carbon film carbonized at 400 ◦ C (a and b), 600 ◦ C (c and d) and 800 ◦ C (e and f) (a, c, e: cross-section; b, d, f: surface). The surface was etched to observe the surface.
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Fig. 4. The unit cell parameter a and the d001 value of ordered mesoporous carbon film carbonized at 400 ◦ C (a) and 800 ◦ C (b).
mesostructure is in good agreement with mesoporous silica thin films [25–27]. The unit cell parameters estimated from the crosssection images were shown in Fig. 4. The d001 value which is the pore-to-pore distance perpendicular to the substrate changed from 12.5 nm (400 ◦ C) to 8.9 nm (800 ◦ C) due to the shrinkage of carbon pore wall. However, the unit cell parameter a parallel to the substrate did not largely change during carbonization. The d001 value estimated from the FE-SEM observation (12.5 nm at 400 ◦ C and 8.9 nm at 800 ◦ C) is almost twice as the d-spacing calculated from the XRD patterns (6.7 nm at 400 ◦ C and 4.3 nm at 800 ◦ C), indicating that the peak at 2 = 1.8◦ and 2.0◦ can be indexed as the 002 plane. This structure is different from the ordered mesoporous carbon powders with straight channel structure (COU-1) and 3D wormhole-like structure (COU-2) [24] even though they were synthesized using the same precursor solution. This ordered mesoporous carbon film with a 3D porous structure was defined as COU-3. Furthermore, to verify the mesostructure of the COU-3 films, we took some TEM measurements for the COU-3 films which were peeled off from the substrate. The pore size was estimated to be 6.3 nm (400 ◦ C), 6.6 nm (600 ◦ C), and 8.2 nm (800 ◦ C) and the wall thickness was estimated to be 5.3 nm (400 ◦ C), 5.4 nm (600 ◦ C) and 3.6 nm (800 ◦ C), respectively. As shown in Fig. 5(a) and (b), ordered pore structures were observed in the sample carbonized at 400 ◦ C. The distance between the layers was estimated to be 11.5 nm. This value is almost equal to the unit cell parameter a calculated from the FE-SEM images. On the other hand, a highly ordered cubic structure was observed in the samples carbonized at 600 ◦ C and 800 ◦ C. Fig. 5(e) and (f) were taken under in-focus and overfocus conditions,
respectively. The distance between the layers was estimated to be 7.6–8.3 nm (600 ◦ C), 12.3 nm (600 ◦ C), 8.8 nm (800 ◦ C) and 12.7 nm (800 ◦ C), respectively. From the XRD, FE-SEM and TEM results, we determined the COU-3 film structure as a body-centered cubic Im3m structure. We considered possible structures such as Fm3m, Im3m, p6m, Ia3d and R3m. Then, only Im3m structure agrees with both XRD patterns and TEM/FE-SEM results. The X-ray diffraction peaks at 2 = 1.3◦ and 1.8◦ can be indexed as the 1 1 0 and 0 0 2 plane, respectively [28,29]. The peak intensity of the 0 0 2 plane is higher than that of the 1 1 0 plane for the samples carbonized at 600 ◦ C and 800 ◦ C. This result indicates that the 1 0 0 direction of the COU-3 structure is oriented in the direction perpendicular to the film surface, which is consistent with the FE-SEM results. In the powder preparation (COU-1 and COU-2), the Im3m structure was not obtained even though they were synthesized using the same precursor solution. In the powder preparation, the precursor solution containing RF and F127 was stirred for 72 h. RF and F127 molecules were slowly assembled in EtOH solution. After stirring, the precursor solution separated into two phases. The transparent upper phase was ethanol–water rich and the yellow lower phase was polymerrich precipitate. The COU-1 and COU-2 carbon powder are formed in a very long period. On the other hand, in the film preparation, the assembly and condensation process immediately occurs during spin-coating process. In this process, the RF-F127 composites are arranged on the substrate surface and this structure spreads from the substrate surface to an upper region. Thus, the rapid assembly and condensation led to a structure difference between the ordered mesoporous carbon powders and films. The COU-3 structure appears only when the film thickness is thin. In the case of the COU-3 film, the solvent evaporation is faster than that in the COU-1 film preparation. Ordered mesostructures are formed under a non-equilibrium condition. Thus, a different pore structure was formed. The 3D porous structure is expected to be effective for electrochemical performance because of a reduction of diffusion resistance of ions. The capacitance properties of ordered mesoporous carbon film on Pt-coated Si wafer were investigated. Fig. 6 shows the cyclic voltammograms (CV) at different scan rates for the COU3 film carbonized at 800 ◦ C. The current increases proportionally with the scan rate. Additionally, the profiles are relatively flat and rectangular in shape without obvious redox peaks in the chosen potential range, which is ideal for EDLCs behavior [5,18]. The shape of the voltammogram does not change even at high scan rates. The rectangular-shaped cyclic voltammogram reveals a very rapid current response on voltage reversal at each end potential. The excellent performance of the capacitor is possibly attributed to the continuous and uniform carbon framework in the COU-3 film. Namely, the internal resistance in grain boundary and the contact resistance with the substrate can be ignored in the COU-3 film. A binder and conductive agent are not necessary to prepare a working electrode, which can avoid electron hopping among different particles in powders. The capacitances (capacitance per unit area) were plotted as a function of the voltage scan rate in Fig. 7. The capacitance slightly decreased with increasing the scan rate. However, the COU-3 film showed about 60% of the initial capacitance even at a scan rate of 100 mV/s. It suggests that the 3D pore structure and the existence of mesopores seem to contribute to the reduction of the diffusion resistance of ions in the porous electrode at a higher scan rate. In addition, the distance of electrolyte ion diffusion in the direction perpendicular to the film surface is very short in the thin film. The capacitance on a total surface area basis was assumed to be 0.10 ± 0.03 F/m2 when the total surface area of COU-3 is similar to that of the ordered mesoporous carbon powder, COU-1 and COU2 (500–700 m2 /g). The capacitance value is lower than that of the
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Fig. 5. TEM images of the carbon film carbonized at 400 ◦ C (a and b), 600 ◦ C (c and d) and 800 ◦ C (e and f).
Fig. 8. Cycle performance of the COU-3 film at 10 mV/s scan rate. Fig. 6. Cyclic voltammograms of the COU-3 film at different scan rate.
reported free-standing ordered mesoporous carbon [30], possibly because the pore size of COU-3 film is almost twice as that of the reported mesoporous carbon film. On the other hand, the COU-3 film shows a higher capacitance value than other ordered mesoporous carbons without the incorporation of metal oxide [31]. The capacitance could be further enhanced by activation using CO2 gas or by incorporating other components, such as conducting polymers and transition metal oxide. Fig. 8 illustrates the capacitance for 250 cycles at a scan rate of 10 mV/s. The capacitances are almost constant after the 250 cycles, indicating that the COU-3 film has a good cycle stability. 4. Conclusion
Fig. 7. Capacitance of the COU-3 film as a function of scan rate.
Ordered mesoporous carbon films with a body-centered cubic Im3m structure (COU-3) were synthesized by the soft-templating method. From the FE-SEM images, the pores were observed in the direction both parallel and perpendicular to the surface of the carbon film. This structure is different from the ordered mesoporous
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carbon powders (COU-1 and COU-2) which were synthesized by using the same precursor solution, because the assembly and condensation process immediately occurred during spin-coating process and this structure spread from substrate surface to upper region. The COU-3 film shows about 65% of the initial capacitance even at a scan rate of 100 mV/s. The 3D pore structure and existence of mesopores seem to contribute to the reduction of the diffusion resistance of ions in the porous electrode. The excellent performance of the capacitor implies that the internal resistance in grain boundary and the contact resistance with the substrate can be ignored in the COU-3 film. These advantages will make it a potentially attractive route for fabricating electrodes for electric double layer capacitors. Acknowledgements The authors acknowledge the GHAS laboratory at Osaka University for the XRD measurements and FE-SEM observation. The TEM measurements were carried out by using a facility in the Research Center for Ultrahigh Voltage Electron Microscopy, Osaka University. References [1] R. Ryoo, S.H. Joo, S. Jun, Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation, J. Phys. Chem. B 103 (1999) 7743–7746. [2] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, Synthesis of new, nanoporous carbon with hexagonally ordered mesostructure, J. Am. Chem. Soc. 122 (2000) 10712–10713. [3] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles, Nature 412 (2001) 169–172. [4] J.S. Lee, S.H. Joo, R. Ryoo, Synthesis of mesoporous silicas of controlled pore wall thickness and their replication to ordered nanoporous carbons with various pore diameters, J. Am. Chem. Soc. 124 (2002) 1156–1157. [5] J. Lee, S. Yoon, T. Hyeon, S.M. Oh, K.B. Kim, Synthesis of a new mesoporous carbon and its application to electrochemical double-layer capacitors, Chem. Commun. (1999) 2177–2178. [6] K. Park, J. Jang, J. Hong, Y. Kwon, Mesoporous thin films of nitrogen-doped carbon with electrocatalytic properties, J. Phys. Chem. C 116 (2012) 16848–16853. [7] Y. Meng, D. Gu, F. Zhang, Y. Shi, H. Yang, Z. Li, C. Yu, B. Tu, D. Zhao, Ordered mesoporous polymers and homologous carbon frameworks: amphiphilic surfactant templating and direct transformation, Angew. Chem. Int. Ed. 44 (2005) 7053–7059. [8] Y. Meng, D. Gu, F. Zhang, Y. Shi, L. Cheng, D. Feng, Z. Wu, Z. Chen, Y. Wan, A. Stein, D. Zhao, A family of highly ordered mesoporous polymer resin and carbon structures from organic–organic self-assembly, Chem. Mater. 18 (2006) 4447–4464. [9] F. Zhang, Y. Meng, D. Gu, Y. Yan, Z. Chen, B. Tu, D. Zhao, An aqueous cooperative assembly route to synthesize ordered mesoporous carbons with controlled structures and morphology, Chem. Mater. 18 (2006) 5279–5288. [10] F. Zhang, Y. Meng, D. Gu, Y. Yan, C. Yu, B. Tu, D. Zhao, A facile aqueous route to synthesize highly ordered mesoporous polymers and carbon frameworks with Ia3d bicontinuous cubic structure, J. Am. Chem. Soc. 127 (2005) 13508–13509. [11] C. Liang, K. Hong, G.A. Guiochon, J.W. Mays, S. Dai, Synthesis of a largescale highly ordered porous carbon film by self-assembly of block copolymers, Angew. Chem. Int. Ed. 43 (2004) 5785–5789.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of ordered mesoporous carbon films with a 3D pore structure and the electrochemical performance of electrochemical double layer capacitors Takahito Mitome a,∗ , Yoshiaki Uchida a,b , Yasuyuki Egashira a , Norikazu Nishiyama a a b
Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Ordered
mesoporous carbon films were synthesized by a soft-templating method. • The mesoporous carbon film has a 3D pore structure. • The mesoporous carbon film shows a high capacitance at high scan rates. • The mesoporous carbon film shows a good cycle stability.
a r t i c l e
i n f o
Article history: Received 30 October 2013 Received in revised form 10 February 2014 Accepted 14 February 2014 Available online 22 February 2014 Keywords: Ordered mesoporous carbon film Soft-templating method 3D pore structure Electrochemical double layer capacitor
a b s t r a c t Ordered mesoporous carbon films with a 3D pore structure (COU-3) were synthesized by a softtemplating method using resorcinol (R)/formaldehyde (F) as a carbon source and Pluronic F127 as a template agent. The carbon films were prepared by a spin-coating technique. From the results of X-ray diffraction pattern, field emission scanning electron microscope and transmission electron microscope observation, the structure of the COU-3 film was determined to be an Im3m structure whose c-axis is oriented perpendicular to the film surface. The pores run in the directions both perpendicular and parallel to the film surface. The porous structure of the film is different from the ordered mesoporous carbon powders and thick films (COU-1 and COU-2) which were synthesized using the same precursor solution. In the preparation of the COU-3 films, the self-assembly and condensation of a RF-F127 composite immediately occurred during the spin-coating. In this process, the RF-F127 composites are arranged on a substrate surface and an ordered structure spreads from the substrate surface to an upper region. The electrochemical performance of electrochemical double layer capacitor was examined. The CV curves are flat and rectangular in shape, and did not change even at a high scan rate, indicating that the internal resistance in grain boundary and the contact resistance with the substrate can be ignored in the COU-3 film. Additionally, the COU-3 film shows good cycle stability after 250 cycles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Tel.: +81 668506257. E-mail address: [email protected] (T. Mitome). http://dx.doi.org/10.1016/j.colsurfa.2014.02.041 0927-7757/© 2014 Elsevier B.V. All rights reserved.
Ordered mesoporous carbon materials have attracted much attention due to their high surface area, large pore volume, chemical inertness and electrical conducting property. Development of well-ordered mesoporous carbon films will lead to new
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applications such as adsorbent, electronic device, membrane separation, hydrogen storage and catalyst support. Especially, electrochemical double layer capacitors (EDLCs) using carbon materials have been extensively studied because of their potential to achieve rapid electrical charge and discharge compared to modern secondary batteries. In addition, EDLCs have much longer life cycle than batteries because no or negligible small chemical charge transfer reactions are involved. If porous carbon thin films are prepared on electrodes, a binder and conductive agent such as carbon black are not necessary to prepare working electrodes, which can avoid the problems of interparticle resistance and contact resistance between carbon electrode and current collector. Ordered mesoporous carbons have been first reported by Ryoo’s [1–4] and Hyeon’s [5] groups who synthesized mesoporous carbons through an inorganic hard-templating method. With respect to electrochemical performance of porous carbon films, Kwon’s group synthesized nitrogen-doped mesoporous carbon thin films by using mesoporous silica thin films as a hard template [6]. However, the hard-templating method includes the extra steps for the preparation of silica scaffolds and removal of the silica, which cause a multi-step process and a high cost. Therefore, another simple process is required for practical applications. In contrast to the hard-templating method, a soft-templating method can directly produce ordered mesoporous carbons. Bulk mesoporous carbons with several unique structures have been synthesized, including a two-dimensional (2D)-hexagonal phase p6mm [7–9], a body-centered cubic phase Im3m [7–9] and gyroidbased cubic phase Ia3d [8–10]. As for film preparation, Dai’s group synthesized ordered mesoporous carbon films with large mesopores by selfof a polystylene-block-poly(4-vinylpyridine) assembly (PS-P4VP)/resorcinol–formaldehyde (RF) mixture [11]. We have first used a triblock copolymer, Pluronic F127, as a soft template for the preparation of ordered mesoporous carbon [12]. Tanaka’s group synthesized ordered mesoporous carbon film with a facecentered orthorhombic Fmmm symmetry [13,14]. Bein’s group synthesized a highly ordered mesoporous polymer resin thin films with 2D-hexagonal (plane group p6mm) structure [15]. Vogt’s group introduced metal oxides to an ordered mesoporous carbon film synthesized by the soft-templating method and observed electrochemical properties [16]. Electrochemical performance of mesoporous carbons have been extensively studied and reported so far. For example, Wu et al. reported that an ordered 2D reverse hexagonal pore morphology facilitates rapid ion diffusion more than a disordered wormhole-like pore morphology, thus leading to superior electrochemical properties [17]. However, the value of capacitance should strongly depend on the way of a preparation of electrodes because the internal resistance in grain boundary and the addition of binder actually affect the conductivity. For the film electrode, on the other hand, these problems can be ignored. Here, we report a synthesis of ordered mesoporous carbon films with a 3D pore structure and the electrochemical performance of electrochemical double layer capacitor consisting of the mesoporous carbon film.
2. Experimental 2.1. Synthesis of ordered mesoporous carbon film Resorcinol (R) was dissolved in ethanol solution, and then triblock copolymer ‘Pluronic F127’ was added and stirred for 60 min. After that, formaldehyde (37 wt%) (F) was added to the above solution and the solution was stirred for 10 min. Finally, HCl (5 N) was added as a catalyst to the solution. The molar ratios of the solution were 0.0038 F127:14.5 EtOH:6.5 water:1 R:1.5 F:0.25 HCl. After
stirring for 60 min, the resultant solution was deposited dropwise onto a silicon substrate, spinning at 50 rpm, and then the substrate was spun up to 1000 rpm for 60 s. The deposited sample was heated at 90 ◦ C for 5 h. Then, the resultant sample was carbonized under a nitrogen atmosphere at different temperature (200–800 ◦ C) for 3 h at a heating rate of 1.3 ◦ C min−1 . 2.2. Characterization X-ray diffraction (XRD) measurement of carbon films was performed on a Philips X’Pert-MPD diffractometer using Cu-K␣ ˚ The copper anode was operated at 40 kV radiation with 1.5418 A. and 30 mA. Field emission scanning electron microscope (FE-SEM) images of mesoporous carbon films were recorded on a Hitachi S-5000L microscope at an acceleration voltage of 22–29 kV. Transmission electron microscope (TEM) images of the samples were obtained on a Hitachi H800 electron microscope. An acceleration voltage was 200 kV. The TEM samples were prepared on a lacy carbon film supported by a copper grid. Fourier transform infrared (FT-IR) spectra were measured in transmission mode with a Shimadzu IRAffinity-1. Spectra were obtained between 400 cm−1 and 4000 cm−1 with a scan number of 400 and a resolution of 4 cm−1 . 2.3. Electrochemical measurements Electrochemical measurements consisted of cyclic voltammetry (CV) experiments using VersaSTAT 3. A mesoporous carbon film as a working electrode was synthesized on a Pt-coated Si wafer by the same procedure as described above such as spin-coating, thermal treatment and carbonization. The electrochemical behavior of the carbon film was analyzed with a three-electrode configuration in an aqueous 1 M H2 SO4 solution. A Pt wire and Ag/AgCl (3 M NaCl) were used as the counter electrode and reference electrode, respectively. The capacitances were calculated according to the equation C = I/V, where I and V are the current and the scan rate, respectively [18]. The average specific capacitances were obtained from CV curves. The mass of the COU-3 film was calculated from the increase of mass after the synthesis. Amount of carbon on electrode was 0.5 mg/cm2 . 3. Results and discussion To track the change of the molecular structure in the carbon films, we measured the temperature dependence of the FT-IR spectra as shown in Fig. 1. The broad peak at 3300 cm−1 arises from the O H stretching of phenolic or aliphatic hydroxyl groups, indicating the existence of a large amount of phenolic OH group [19,20]. The broadness of the absorption is derived from intermolecular hydrogen-bonding. A strong peak at 1610 cm−1 can be related to the carbon–carbon bond stretching of 1,2,4- and 1,2,6trisubstituted and phenyl alkyl ether-type substituted aromatic ring structure [20,21]. The peaks at 2854 cm−1 and 2926 cm−1 are assigned to the aliphatic carbon-hydrogen stretching [19,20]. These peaks remained even after carbonization at 600 ◦ C. Pluronic F127 molecules decomposed at temperature from 300 ◦ C to 400 ◦ C. Therefore, the peaks at 2854 cm−1 and 2926 cm−1 can be related to the resorcinol–formaldehyde polymer. The carbon film carbonized at 600 ◦ C has some methylene bridges by crosslinking with resorcinol and formaldehyde, indicating that the polymer network remains at 600 ◦ C. The FT-IR spectra for the samples carbonized at 600 ◦ C and 800 ◦ C show a broad peak at 1085 cm−1 , which is associated with a polycyclic aromatic ring deformation [22]. The peak indicates the occurrence of many polyaromatic reactions when RF
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Fig. 1. FT-IR spectra of the carbon film heated at 90 ◦ C (a), carbonized at 200 ◦ C (b), 400 ◦ C (c), 600 ◦ C (d) and 800 ◦ C (e). For clarity, FT-IR spectra were shifted up.
Fig. 2. XRD patterns of as deposited film (a), the film carbonized at 400 ◦ C (b), 600 ◦ C (c), 800 ◦ C (d) and ordered mesoporous carbon powder (e). For clarity, XRD patterns were shifted up.
resin is treated at temperature above 600 ◦ C. These results indicate that RF resin is gradually converted to carbon. To examine the internal mesostrucure of the as-deposited film and carbon film, we measured XRD patterns of the synthesized structures as shown in Fig. 2. The XRD pattern of the as-deposited film shows a weak peak at 2 = 1.1◦ , indicating that an ordered mesostructured material was already formed before the polymerization process. A strong diffraction peak appeared at 2 = 1.3◦ for the sample treated at 400 ◦ C, suggesting that an ordered mesostructured polymer was constructed before carbonization. Pluronic F127 molecules decompose at temperature from 300 to 400 ◦ C [23] and mesopore is generated by the decomposition of F127 molecules. Therefore, a strong diffraction peak is derived from a density difference between mesopore and carbon pore wall. On the other hand, a new peak appeared at 2 = 1.8◦ and 2.0◦ for the sample carbonized at 600 ◦ C and 800 ◦ C. This peak could not be observed for the ordered mesoporous carbon powder [24]. The d-spacing was
calculated to be 8.0 nm (as deposited sample), d002 = 6.7 nm (400 ◦ C), d110 = 6.8 nm (600 ◦ C), d002 = 4.9 nm (600 ◦ C), d110 = 6.4 nm (800 ◦ C) and d002 = 4.3 nm (800 ◦ C). The peak assignments are described below. To obtain details of the mesostructure, we observed the ordered mesoporous carbon films by FE-SEM as shown in Fig. 3. The thickness of carbon film was estimated to be about 2 m. The pores were observed in the direction both parallel and perpendicular to the surface of the carbon film, indicating that carbon film has a 3D pore structure. A cross-section of pores of carbon film carbonized at 400 ◦ C is circular in shape without deformation. On the other hand, the deformation of the carbon pore wall occurred during carbonization. In the film preparation, the presence of a substrate helps to depress shrinkage of pore wall in the direction parallel to the substrate. On the other hand, shrinkage of pore wall occurred in the direction perpendicular to the substrate. Therefore, the pores became ellipsoidal in shape. The shrinkage of
Fig. 3. FE-SEM images of carbon film carbonized at 400 ◦ C (a and b), 600 ◦ C (c and d) and 800 ◦ C (e and f) (a, c, e: cross-section; b, d, f: surface). The surface was etched to observe the surface.
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Fig. 4. The unit cell parameter a and the d001 value of ordered mesoporous carbon film carbonized at 400 ◦ C (a) and 800 ◦ C (b).
mesostructure is in good agreement with mesoporous silica thin films [25–27]. The unit cell parameters estimated from the crosssection images were shown in Fig. 4. The d001 value which is the pore-to-pore distance perpendicular to the substrate changed from 12.5 nm (400 ◦ C) to 8.9 nm (800 ◦ C) due to the shrinkage of carbon pore wall. However, the unit cell parameter a parallel to the substrate did not largely change during carbonization. The d001 value estimated from the FE-SEM observation (12.5 nm at 400 ◦ C and 8.9 nm at 800 ◦ C) is almost twice as the d-spacing calculated from the XRD patterns (6.7 nm at 400 ◦ C and 4.3 nm at 800 ◦ C), indicating that the peak at 2 = 1.8◦ and 2.0◦ can be indexed as the 002 plane. This structure is different from the ordered mesoporous carbon powders with straight channel structure (COU-1) and 3D wormhole-like structure (COU-2) [24] even though they were synthesized using the same precursor solution. This ordered mesoporous carbon film with a 3D porous structure was defined as COU-3. Furthermore, to verify the mesostructure of the COU-3 films, we took some TEM measurements for the COU-3 films which were peeled off from the substrate. The pore size was estimated to be 6.3 nm (400 ◦ C), 6.6 nm (600 ◦ C), and 8.2 nm (800 ◦ C) and the wall thickness was estimated to be 5.3 nm (400 ◦ C), 5.4 nm (600 ◦ C) and 3.6 nm (800 ◦ C), respectively. As shown in Fig. 5(a) and (b), ordered pore structures were observed in the sample carbonized at 400 ◦ C. The distance between the layers was estimated to be 11.5 nm. This value is almost equal to the unit cell parameter a calculated from the FE-SEM images. On the other hand, a highly ordered cubic structure was observed in the samples carbonized at 600 ◦ C and 800 ◦ C. Fig. 5(e) and (f) were taken under in-focus and overfocus conditions,
respectively. The distance between the layers was estimated to be 7.6–8.3 nm (600 ◦ C), 12.3 nm (600 ◦ C), 8.8 nm (800 ◦ C) and 12.7 nm (800 ◦ C), respectively. From the XRD, FE-SEM and TEM results, we determined the COU-3 film structure as a body-centered cubic Im3m structure. We considered possible structures such as Fm3m, Im3m, p6m, Ia3d and R3m. Then, only Im3m structure agrees with both XRD patterns and TEM/FE-SEM results. The X-ray diffraction peaks at 2 = 1.3◦ and 1.8◦ can be indexed as the 1 1 0 and 0 0 2 plane, respectively [28,29]. The peak intensity of the 0 0 2 plane is higher than that of the 1 1 0 plane for the samples carbonized at 600 ◦ C and 800 ◦ C. This result indicates that the 1 0 0 direction of the COU-3 structure is oriented in the direction perpendicular to the film surface, which is consistent with the FE-SEM results. In the powder preparation (COU-1 and COU-2), the Im3m structure was not obtained even though they were synthesized using the same precursor solution. In the powder preparation, the precursor solution containing RF and F127 was stirred for 72 h. RF and F127 molecules were slowly assembled in EtOH solution. After stirring, the precursor solution separated into two phases. The transparent upper phase was ethanol–water rich and the yellow lower phase was polymerrich precipitate. The COU-1 and COU-2 carbon powder are formed in a very long period. On the other hand, in the film preparation, the assembly and condensation process immediately occurs during spin-coating process. In this process, the RF-F127 composites are arranged on the substrate surface and this structure spreads from the substrate surface to an upper region. Thus, the rapid assembly and condensation led to a structure difference between the ordered mesoporous carbon powders and films. The COU-3 structure appears only when the film thickness is thin. In the case of the COU-3 film, the solvent evaporation is faster than that in the COU-1 film preparation. Ordered mesostructures are formed under a non-equilibrium condition. Thus, a different pore structure was formed. The 3D porous structure is expected to be effective for electrochemical performance because of a reduction of diffusion resistance of ions. The capacitance properties of ordered mesoporous carbon film on Pt-coated Si wafer were investigated. Fig. 6 shows the cyclic voltammograms (CV) at different scan rates for the COU3 film carbonized at 800 ◦ C. The current increases proportionally with the scan rate. Additionally, the profiles are relatively flat and rectangular in shape without obvious redox peaks in the chosen potential range, which is ideal for EDLCs behavior [5,18]. The shape of the voltammogram does not change even at high scan rates. The rectangular-shaped cyclic voltammogram reveals a very rapid current response on voltage reversal at each end potential. The excellent performance of the capacitor is possibly attributed to the continuous and uniform carbon framework in the COU-3 film. Namely, the internal resistance in grain boundary and the contact resistance with the substrate can be ignored in the COU-3 film. A binder and conductive agent are not necessary to prepare a working electrode, which can avoid electron hopping among different particles in powders. The capacitances (capacitance per unit area) were plotted as a function of the voltage scan rate in Fig. 7. The capacitance slightly decreased with increasing the scan rate. However, the COU-3 film showed about 60% of the initial capacitance even at a scan rate of 100 mV/s. It suggests that the 3D pore structure and the existence of mesopores seem to contribute to the reduction of the diffusion resistance of ions in the porous electrode at a higher scan rate. In addition, the distance of electrolyte ion diffusion in the direction perpendicular to the film surface is very short in the thin film. The capacitance on a total surface area basis was assumed to be 0.10 ± 0.03 F/m2 when the total surface area of COU-3 is similar to that of the ordered mesoporous carbon powder, COU-1 and COU2 (500–700 m2 /g). The capacitance value is lower than that of the
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Fig. 5. TEM images of the carbon film carbonized at 400 ◦ C (a and b), 600 ◦ C (c and d) and 800 ◦ C (e and f).
Fig. 8. Cycle performance of the COU-3 film at 10 mV/s scan rate. Fig. 6. Cyclic voltammograms of the COU-3 film at different scan rate.
reported free-standing ordered mesoporous carbon [30], possibly because the pore size of COU-3 film is almost twice as that of the reported mesoporous carbon film. On the other hand, the COU-3 film shows a higher capacitance value than other ordered mesoporous carbons without the incorporation of metal oxide [31]. The capacitance could be further enhanced by activation using CO2 gas or by incorporating other components, such as conducting polymers and transition metal oxide. Fig. 8 illustrates the capacitance for 250 cycles at a scan rate of 10 mV/s. The capacitances are almost constant after the 250 cycles, indicating that the COU-3 film has a good cycle stability. 4. Conclusion
Fig. 7. Capacitance of the COU-3 film as a function of scan rate.
Ordered mesoporous carbon films with a body-centered cubic Im3m structure (COU-3) were synthesized by the soft-templating method. From the FE-SEM images, the pores were observed in the direction both parallel and perpendicular to the surface of the carbon film. This structure is different from the ordered mesoporous
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carbon powders (COU-1 and COU-2) which were synthesized by using the same precursor solution, because the assembly and condensation process immediately occurred during spin-coating process and this structure spread from substrate surface to upper region. The COU-3 film shows about 65% of the initial capacitance even at a scan rate of 100 mV/s. The 3D pore structure and existence of mesopores seem to contribute to the reduction of the diffusion resistance of ions in the porous electrode. The excellent performance of the capacitor implies that the internal resistance in grain boundary and the contact resistance with the substrate can be ignored in the COU-3 film. These advantages will make it a potentially attractive route for fabricating electrodes for electric double layer capacitors. Acknowledgements The authors acknowledge the GHAS laboratory at Osaka University for the XRD measurements and FE-SEM observation. The TEM measurements were carried out by using a facility in the Research Center for Ultrahigh Voltage Electron Microscopy, Osaka University. References [1] R. Ryoo, S.H. Joo, S. Jun, Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation, J. Phys. Chem. B 103 (1999) 7743–7746. [2] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, Synthesis of new, nanoporous carbon with hexagonally ordered mesostructure, J. Am. Chem. Soc. 122 (2000) 10712–10713. [3] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles, Nature 412 (2001) 169–172. [4] J.S. Lee, S.H. Joo, R. Ryoo, Synthesis of mesoporous silicas of controlled pore wall thickness and their replication to ordered nanoporous carbons with various pore diameters, J. Am. Chem. Soc. 124 (2002) 1156–1157. [5] J. Lee, S. Yoon, T. Hyeon, S.M. Oh, K.B. Kim, Synthesis of a new mesoporous carbon and its application to electrochemical double-layer capacitors, Chem. Commun. (1999) 2177–2178. [6] K. Park, J. Jang, J. Hong, Y. Kwon, Mesoporous thin films of nitrogen-doped carbon with electrocatalytic properties, J. Phys. Chem. C 116 (2012) 16848–16853. [7] Y. Meng, D. Gu, F. Zhang, Y. Shi, H. Yang, Z. Li, C. Yu, B. Tu, D. Zhao, Ordered mesoporous polymers and homologous carbon frameworks: amphiphilic surfactant templating and direct transformation, Angew. Chem. Int. Ed. 44 (2005) 7053–7059. [8] Y. Meng, D. Gu, F. Zhang, Y. Shi, L. Cheng, D. Feng, Z. Wu, Z. Chen, Y. Wan, A. Stein, D. Zhao, A family of highly ordered mesoporous polymer resin and carbon structures from organic–organic self-assembly, Chem. Mater. 18 (2006) 4447–4464. [9] F. Zhang, Y. Meng, D. Gu, Y. Yan, Z. Chen, B. Tu, D. Zhao, An aqueous cooperative assembly route to synthesize ordered mesoporous carbons with controlled structures and morphology, Chem. Mater. 18 (2006) 5279–5288. [10] F. Zhang, Y. Meng, D. Gu, Y. Yan, C. Yu, B. Tu, D. Zhao, A facile aqueous route to synthesize highly ordered mesoporous polymers and carbon frameworks with Ia3d bicontinuous cubic structure, J. Am. Chem. Soc. 127 (2005) 13508–13509. [11] C. Liang, K. Hong, G.A. Guiochon, J.W. Mays, S. Dai, Synthesis of a largescale highly ordered porous carbon film by self-assembly of block copolymers, Angew. Chem. Int. Ed. 43 (2004) 5785–5789.
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