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Catalytic Graphitization of Carbon Aerogels by Transition Metals F. J. Maldonado-Ho´dar,† C. Moreno-Castilla,*,† J. Rivera-Utrilla,† Y. Hanzawa,‡ and Y. Yamada‡ Grupo de Investigacio´ n en Carbones, Departamento de Quı´mica Inorga´ nica, Facultad de Ciencias, Universidad de Granada, 18071, Granada, Spain, and National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki, 305-8569, Japan Received August 9, 1999. In Final Form: January 5, 2000 Carbon aerogels and Cr-, Fe-, Co-, and Ni-containing carbon aerogels were obtained by pyrolysis, at temperatures between 500 and 1800 °C, of the corresponding aerogels prepared by the sol-gel method from polymerization of resorcinol with formaldehyde. All samples were characterized by mercury porosimetry, nitrogen adsorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. Results obtained show that carbon aerogels are, essentially, macroporous materials that maintain large pore volumes even after pyrolysis at 1800 °C. For pyrolysis at temperatures higher than 1000 °C, the presence of the transition metals produced graphitized areas with three-dimensional stacking order, as shown by HRTEM, XRD, and Raman spectroscopy. HRTEM also showed that the metalcarbon containing aerogels were formed by polyhedral structures. Cr and Fe seem to be the best catalysts for graphitization of carbon aerogels.
Introduction Electrochemical double-layer capacitors or supercapacitors are considered to be energy storage devices with high power densities. They store charge at a polarized solid/electrolyte interface and, consequently, their capacity depends on the available surface area of the porous materials accessible to the electrolyte. The porous electrodes of supercapacitors may consist of metal oxides obtained mainly from sol-gel polymerization of transition metal alkoxides and functionalized with ferrocene derivatives.1-5 Metal oxides perform well but are expensive to prepare. Many different types of carbons, prepared from different starting materials and with different carbonization procedures, have also been tested for electrode production. Activated carbons,6-8 glassy carbons,9,10 carbon foams and other soft carbons,11-14 graphite or graphitized carbons,15,16 * Corresponding author. E-mail:
[email protected]. † Universidad de Granada. ‡ National Institute for Resources and Environment. (1) Sarangapani, S.; Lessner, P.; Forchione, J.; Griffith, A.; Laconti, A. B. J. Power Sources 1990, 29, 355. (2) Audebert, P.; Demaille, C.; Sanchez, C. J. Chem. Mat. 1993, 5, 911. (3) Audebert, P.; Calas, P.; Cerveau, G.; Corriu, R. J. P.; Costa, N. J. Electroanal. Chem. 1994, 372, 275. (4) Cattey, H.; Audebert, P.; Sanchez, C. New J. Chem. 1996, 20, 1023. (5) Cattey, H.; Audebert, P.; Sanchez, C.; Hapiot, P. J. Phys. Chem. B 1998, 102, 911. (6) Sanada, K.; Hosokawa, M. No. 55; NEC Research and Development, 1979. (7) Oren, Y.; Soffer, A. Electrochem. 1983, 13, 473. (8) Andelman, M. D. U.S. Patent 5,415,768, May 16, 1995. (9) Lausevic, Z.; Jenkins, G. M. Carbon 1986, 24, 651. (10) Markovic, V.; Vukelic, N.; Markovic, D. Fuel 1989, 68, 1039. (11) Wang, J.; Brennsteiner, A.; Sylwester, A. P. Anal. Chem. 1990, 62, 1102-1104. (12) Sato, K.; Noguchi, N.; Demachi, A.; Oki, N.; Endo, M. Science 1994, 264, 556. (13) Zheng, T.; Mckinnon, W. R.; Dahn, I. R. J. Electrochem. Soc. 1996, 143, 2137. (14) Mochida, I.; Ku, C. H.; Yoon, S. H.; Korai, Y. J. Power Sources 1998, 75, 214. (15) Weisshaar, D. E.; Tallman, D. E. Anal. Chem. 1983, 55, 1146. (16) Dahn, J. R.; Fong, R.; Spoon, M. J. Phys. Rev. B. 1990, 42, 6424.
and more recently, carbon aerogels,17-25 perform differently in relation to their structure and properties. Activated carbons are cheap, but the electrical conductivity is not satisfactory. Soft carbons, heat-treated between 600 and 800 °C, exhibit a capacity per weight two or three times that of graphite but present several disadvantages, such as high irreversible capacity or poor cycle stability.12,14 Advantages of carbon aerogels include controllable pore size distributions, high surface areas, and low electrical resistivity. This high electrical conductivity, in contrast with activated carbons, loosely bonded carbon powder, or even activated carbon fiber cloths (ACFCs), is attributable to its monolithic structure, which is composed of covalently bonded small carbonaceous particles.19,22 Moreover, increasing graphitization of carbon aerogels, preserving to the maximum their high pore volumes and surface areas, is expected to enhance their electrical conductivity. Certain organic monomers can be used to prepare carbon aerogels,17-28 mainly from melamine/formaldehyde or resorcinol/formaldehyde mixtures. Thus, polycondensa(17) Pekala, R. W. J. Mater. Sci. 1989, 24, 3221. (18) Wang, J.; Agnes, L.; Tobias, H.; Roesner, R. A.; Hong, K. C.; Glaus, R. S.; Kong, F. M.; Pekala, R. W. Anal. Chem. 1993, 65, 2300. (19) Pekala, R. W.; Mayer, S. T.; Poco, J. F.; Kaschmitter, J. L. Mater. Res. Soc. Symp. Proc. 1994, 349, 79-85. (20) Pekala, R. W.; Mayer, S. T.; Kaschmitter, J. L.; Kong, F. M. In Sol-Gel Processing and Applications; Attia, Y. A., Ed.; Plenum Press: New York, 1994; p 369. (21) Pekala, R. W.; Alviso, C. T.; Lu, X.; Groβ, J.; Fricke, J. J. NonCryst. Solids 1995, 188, 34. (22) Farmer, J. C.; Fix, D. V.; Mack, G. V.; Pekala, R. W.; Poco, J. F. J. Appl. Electrochem. 1996, 26, 1007. (23) Farmer, J. C.; Fix, D. V.; Mack, G. V.; Pekala, R. W.; Poco, J. F. J. Electrochem. Soc. 1996, 143(1), 159. (24) Pajonk, G. M.; Rao, A. V.; Pinto, N.; Dolle, F. E.; Bellido Gil, M. Preparation of Catalysts VII; Elsevier Science: New York, 1998; pp 167-174. (25) Saliger, R.; Fischer, U.; Herta, C.; Fricke, J. J. Non-Cryst. Solids 1998, 225, 81. (26) Hanzawa, Y.; Kaneko, K.; Pekala, R. W.; Dresselhaus, M. S. Langmuir 1996, 12, 6167. (27) Maldonado-Ho´dar, F. J.; Ferro-Garcı´a, M. A.; Rivera-Utrilla, J.; Moreno-Castilla, C. Carbon 1999, 37, 1199. (28) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J.; Rivera-Utrilla, J.; Rodrı´guez-Castello´n, E. Appl. Catal. 1999, 183, 345.
10.1021/la991080r CCC: $19.00 © 2000 American Chemical Society Published on Web 03/24/2000
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tion of resorcinol with formaldehyde in aqueous solutions leads to gels that can be supercritically dried with CO2 to form aerogels that, after pyrolysis in an inert atmosphere, yield carbon aerogels. In general, most phenolic-like resins do not completely graphitize even upon heating above 2500 °C. Thus, when resorcinol-formaldehyde carbon aerogels are pyrolyzed at 1050 °C, they show a largely amorphous structure that resembles that of turbostratic carbons.20 We have shown recently27,28 that it is possible to prepare transition-metal-containing carbon aerogels with different pore textures and surface characteristics by dissolving the corresponding metallic salt in the initial resorcinolformaldehyde mixture. During the heat treatment to produce the carbon aerogel, some of these transition metals can induce graphitization of the material, since they act as catalysts in this process.29-31 Thus, the objective of this work was to determine the changes in surface area, porosity, and graphitization of Cr-, Fe-, Co-, and Ni-containing carbon aerogels when they were heat-treated between 500 and 1800 °C. For this purpose, the samples obtained were analyzed by different techniques, which include gas adsorption, mercury porosimetry, X-ray diffraction, highresolution transmission electron microscopy, and Raman spectroscopy. Experimental Section Aerogels were synthesized by dissolving resorcinol (R) and formaldehyde (F) in water; the R/F molar ratio used was 1/2, and the concentration of the reactants in the aqueous solution was 64.5 wt %. Chromium nitrate and Fe, Co, and Ni acetates were added to the solution, and the metal concentration was 1 wt %. Another aerogel was prepared with a zero metal content and was used as a blank. The solutions obtained were cast into glass molds of 0.5 cm inner diameter and cured for 7 days up to a maximum temperature of 80 °C. After the cure cycle, the gels were cut into pellets of about 5-mm length and placed in acetone (2 days) to remove the water inside the pores. The gels were then supercritically dried with carbon dioxide. The resulting aerogels were pyrolyzed under nitrogen flow up to both 500 and 1000 °C, with a soak time of 5 h. Different portions of the samples prepared at 1000 °C were subsequently heated at 1400 and 1800 °C in Ar flow with a soak time of 4 h. The carbon aerogels will be referred to in the text by the following nomenclature: AM-T, where A indicates the origin of the organic aerogel, M the metal present, and T the pyrolysis temperature. The samples were characterized by adsorption of N2 at 77 K and mercury porosimetry by using a Quantachrome Autoscan 60, which provides the pore size distribution of pores (PSD) with a diameter greater than 3.7 nm; the external surface area contained in these pores, Sext; and the particle density, Fp. The BET equation was applied to the N2 adsorption isotherms to obtain the nitrogen surface area, SN2. X-ray diffraction (XRD) experiments were carried out with a Phillips PW1710 diffractometer (40 kV and 40 mA) using Cu KR radiation and high-resolution transmission microscopy (HRTEM) experiments with a STEM Phillips CM-20 (200 kV) microscope. Raman spectra were collected with a Jobin Yvon T6400 spectrometer using an Ar laser light source (λ ) 514.5 nm).
Results and Discussion Surface Characteristics. The weight loss (WL) percentage of the samples during pyrolysis is shown in Table 1. WL increased with pyrolysis temperature, but the release of volatile matter occurred largely when heating at 500 °C. Thermogravimetric analysis of these samples (29) Oberlin, A.; Roucky, J. P. Carbon 1971, 9, 39. (30) Audier, M.; Oberlin, A.; Oberlin, M.; Coulon, M.; Bonneatin, L. Carbon 1981, 19, 217. (31) Inagaki, M.; Okada, Y.; Vignal, V.; Komo, H.; Oshida, K. Carbon 1998, 36, 1706.
Maldonado-Ho´ dar et al. Table 1. Weight Loss Percentage during Pyrolysis at Different Temperatures weight loss (%) muestra
500 °C
1000 °C
1400 °C
1800 °C
A ACr AFe ACo ANi
43.6 42.7 53.4 50.1
53.8 54.1 54.3 55.2 53.7
57.4 58.9 59.0 57.9 56.9
58.8 59.8 60.7 62.8 60.4
Table 2. Surface Properties of Carbon Aerogel and Transition-Metal-Containing Carbon Aerogelsa sample
metal content (%)
Fp g cm-3
V2 V3 cm3 g-1
Sext SN2 m2 g-1
A-1000 A-1400 A-1800
nil nil nil
0.72 0.68 0.58
0.000 0.628 0.000 0.639 0.000 0.859
18 470 18 28 18 20
ACr-500 ACr-1000 ACr-1400 ACr-1800
3.6 4.2 Not determined Not determined
0.56 0.64 0.57 0.59
0.000 0.000 0.000 0.000
0.994 0.991 0.891 0.897
ACr > AFe ≈ A. In conclusion, the preparation of highly porous materials, with different pore size distributions and surface areas, depends on the metal present and the heat treatment used. This porosity is sufficiently open, for example, to be accessible to electrolytes such as lithium to produce supercapacitors. The release of volatile matter during carbonization generally produces an increase in macropore volume and a reduction in surface area, indicating that shrinkage of the porous texture of carbon aerogels takes place mainly in the micropore range. The PSD of samples obtained at 1000 °C is mainly maintained after subsequent heat treatments at higher temperatures. Therefore, the structure of the open porosity of the samples seems to be stabilized during heat treatment up to 1000 °C. Similar
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Figure 6. HRTEM micrographs of carbon aerogel: (a) blank sample A-1000; (b) ANi-1000; (c) AFe-1800; (d) AFe-1400; (e) ACo1400; (f and g) ANi-1400; and (h) ANi-1800.
results have been observed previously26 showing that, even after intense CO2 activation of carbon aerogels, the basic network was maintained. Structure and Morphology. The structure and morphology of the heat-treated carbon aerogels were studied by HRTEM, XRD, and Raman spectroscopy. HRTEM micrographs of selected samples are shown in Figure 6a to 6h. The blank sample, Figure 6a, did not show any 002 lattice fringes, and the structure of the
carbon aerogel was formed by interconnected spherical and noncrystalline particles. This structure was previously observed by scanning electron microscopy (SEM) of some carbon aerogels27 and is also in accordance with that previously proposed for a carbon aerogel obtained with Na2CO3 as polymerization catalyst.20 In the case of metal-containing carbon aerogels, the formation of polyhedral structures is evident in Figures 6b to 6g, which essentially revealed, in the case of samples
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Figure 7. Selected-area electron diffraction patterns: (a) ACr1800; (b) AFe-1800; and (c) ANi-1800.
heated at 1400 and 1800 °C, the presence of crystalline graphite, showing the 002 lattice fringes. This graphitic structure sometimes appeared around metal particles (Figure 6g). The formation of similar structures was observed previously in other spherical carbon materials such as carbon blacks or carbon shells.33 Although iron and chromium seem to be better catalysts for the graphitization of the carbon aerogels, as is shown later, Ni-containing samples show specific regions in the sample that have graphitic particles with very well-defined 002 lattice fringes, as shown in Figure 6h. Electron diffraction patterns of selected graphitized areas, Figures 7a to 7c, show the crystallinity of the samples. All these figures demonstrate the presence of graphite crystals with three-dimensional stacking order. However, disordered and poorly crystallized regions were also observed. XRD patterns of the different Fe-containing carbon aerogels are shown in Figure 8, as an example. The XRD pattern of AFe-500 shows two broad bands around 2θ ) 22 and 43°, indicating a largely amorphous structure of the organic matrix. Together with these bands, the XRD diffraction peaks corresponding to Fe2O3 (at 2θ ) 35.6, 57.1, and 62.7°) also appeared, and its mean particle size was about 16 nm. This particle size was calculated from Scherrer’s equation (with Warren’s correction for instrumental broadening) applied to the half-height of the maximum intensity diffraction peak. For sample AFe1000, the broad bands corresponding to the carbonaceous matrix become narrower, with higher intensity. The diffraction angles are now around 26 and 44°, corresponding to the 002 and 101 diffraction peaks of graphite, respectively. At this temperature, a mixture of Fe2O3 and metallic Fe (peaks at 44.6 and 65.2°) particles coexist inside the carbon matrix, indicating that the iron oxide particles are partially reduced by the organic matrix during the heat treatment. At higher temperatures, iron oxide is completely reduced, and only the peaks corresponding to Fe appear together with those of graphite. The other metal-containing carbon aerogels heat-treated at 500 °C did not show any diffraction peak corresponding to the metallic phase, implying that this phase is welldispersed in the carbon matrix. XRD patterns of samples obtained at 1000 °C are depicted in Figure 9. The catalytic effect of the metals on graphitization of the carbon aerogels is evident; whereas the XRD pattern of the
A-1000 still presents two broad bands, the rest of the samples present defined graphitic diffraction peaks. In addition, whereas iron showed a mixture of metal and oxide, cobalt and nickel were completely reduced at this temperature and presented mean crystallite sizes between 15 and 20 nm. In sample ACr-1000, no diffraction peaks corresponding to the metallic phase were observed, also indicating a good metal dispersion, with the metal being intimately linked or trapped inside the organic matrix and forming either amorphous or microcrystallites less than 4.0 nm in size. XPS measurements of this sample showed that Cr was in two oxidation states (III) and (VI), and the percentage of both species was 71% and 29%, respectively.28 An increase in the pyrolysis temperature up to 1800 °C (Figure 8) produces a reduction in the intensity of the peaks corresponding to the metallic phase with respect to the graphitic ones; this change can be due to both the progressive graphitization of the sample and, probably, a degree of sublimation of the metal at high temperatures. In all cases, the 002 diffraction peak showed a tail to the low-angle side and the 101 peak a tail to the highangle side, indicative of the coexistence of highly crystalline graphite with less-ordered carbon materials. This was also previously demonstrated by HRTEM. The mean stack height of the graphite crystallites, Lc, was obtained by applying Scherrer’s equation to the 002 diffraction peak. The values obtained are compiled in Table 3. In the case of the blank sample, this parameter slightly increased with the pyrolysis temperature. The data agree with those obtained by Hanzawa et al.34 The metal-carbon
(33) Oberlin, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol 22, Chapter 1.
(34) Hanzawa, Y.; Yoshizawa, N.; Hatori, H.; Yamada, Y. In Inter. Symp. Carbon, Tokyo, Japan, 1998, A11-09, pp 292.
Figure 8. XRD patterns of iron-containing carbon aerogels.
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Figure 10. Raman spectra of blank carbon aerogels.
Figure 9. XRD patterns of metal-carbon aerogels obtained at 1000 °C. Table 3. Stack Height and Interlayer Spacing of Graphite Microcrystallites from X-ray Diffraction Experiments sample
Lc (nm)
d002 (nm)
g
A-1000 A-1400 A-1800 ACr-500 ACr-1000 ACr-1400 ACr-1800 AFe-500 AFe-1000 AFe-1400 AFe-1800 ACo-500 ACo-1000 ACo-1400 ACo-1800
0.80 1.00 1.11 0.79 6.45 7.06 7.06 0.78 4.05 5.17 5.19 0.86 5.16 5.17 5.18
0.378 0.364 0.364 0.382 0.337 0.337 0.337 0.379 0.338 0.337 0.336 0.388 0.342 0.339 0.338
0.814 0.814 0.814 0.698 0.814 0.930 0.233 0.581 0.698
ANi-500 ANi-1000 ANi-1400 ANi-1800
0.67 4.83 7.06 7.06
0.390 0.340 0.339 0.339
0.465 0.583 0.581
aerogels treated at 500 °C showed Lc values similar to those found for the blank at 1000 °C. In these samples, Lc presented a strong increase when the pyrolysis temperature increased from 500 up to 1000 °C. This rise was more moderate between 1000 and 1400 °C and remained practically unchanged between 1400 and 1800 °C. Results reported in the literature10 indicate that, in carbon aerogels synthesized in the presence of Na2CO3 and pyrolyzed at 2100 °C, the graphite microcrystallites grew to around 4 nm. The evolution of the 002 interlayer spacing (d002) is similar to that described for Lc. The blank and metalcontaining samples obtained at 500 °C show d002 values
greater than 0.344 nm, corresponding to highly disordered carbon materials. However, this value decreased to 0.335 nm, corresponding to pure graphite, as the pyrolysis temperature increased. From these data, values of the graphitization degree parameter, g,35 were obtained, by applying the equation g ) 0.344 - d002/0.0086, and are compiled in Table 3. Iron and chromium were the best graphitization catalysts studied. The in-plane width of graphitic layers, La, could be analyzed from the 101 diffraction peak. However, this peak was very near to or even overlapped the maximum intensity peak of the metal phases, and, consequently, could not be used to determine this parameter. Instead, it was obtained from the Raman spectral data. The use of Raman as a method for the characterization of graphite was first reported by Tuinstra.36 A band around 1350 cm-1 (D-band) is associated with the disorder-induced scattering produced by imperfections or loss of hexagonal symmetry in the carbon structure. This band is attributed to the A1g mode37,38 and does not appear in perfect graphite crystals. Therefore, this band has been used to evaluate the degree of imperfection or crystallinity of graphite. On the contrary, another band around 1580 cm-1 (G-band) assigned to the Raman active 2E2g mode in twodimensional network structure is always observed in all carbon and graphite materials. Raman spectra of blank samples treated at 1000, 1400, and 1800 °C is shown in Figure 10. The line width (half-width) of both bands was lowered by the treatment at higher temperatures and, at the same time, the intensity ratio of the D- to the G-band, R, (R ) ID/IG), was always greater than the unity and gradually increased up to 1800 °C (Table 4). A similar behavior was also observed in glassy carbons from polyfurfuryl alcohol39 and in carbon aerogels,34 in which an increase of the R values was observed from 1000 to 2000 °C. In our case, the increase in the R values can be due to the shrinkage of the carbon aerogel particles with the heat treatment, which induces the loss of the microporosity previously described and imperfections in the carbon structure, enhancing in this way the D-band intensity. Raman spectra of Cr-containing samples are shown in Figure 11, as an example. The results obtained for all metal-containing samples are included in Table 4. Both G- and D-bands for metal-containing carbon aerogels were (35) Maire, J.; Mering, J. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, pp 125. (36) Tuinstra, F.; Koening, J. L. J. Chem. Phys. 1970, 53, 1126. (37) Wang, Z.; Lu, Z.; Huang, X.; Xue, R.; Chen, L. Carbon 1998, 36(1-2), 51. (38) Asari, E.; Kitajima, M.; Nakamura, K. G. Carbon 1998, 36, 1693. (39) Nakamizo, M. Carbon 1991, 29, 757.
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Table 4. Characteristics of the D and G Raman Bands and In-Plane Width of the Graphite Microcrystallites sample
band width ∆D (cm-1) ∆G (cm-1)
relative intensity R ) ID/IG
La (nm)
A-1000 A-1400 A-1800
33 21 12
18 17 13
1.08 1.29 1.48
4.07 3.41 2.97
ACr-1000 ACr-1400 ACr-1800
26 16 11
15 11 7
0.87 0.76 0.71
5.06 5.79 6.20
AFe-1000 AFe-1400 AFe-1800
19 16 12
16 14 11
1.07 0.92 0.78
4.11 4.78 5.64
ACo-1000 ACo-1400 ACo-1800
21 17 11
16 13 10
1.08 0.97 0.96
4.07 4.54 4.58
ANi-1000 ANi-1400 ANi-1800
24 18 13
16 15 9
0.99 0.97 1.01
4.44 4.54 4.36
narrower than those for the blanks when they were compared at the same temperature. The width of both bands also decreased when the temperature increased. The intensity ratio, R, in this case decreased with the increase in the treatment temperature (except in Nicontaining samples), becoming lower than unity; this fact is favored by the improvement of the stacking of the graphitic layers by the catalytic effect of the metals and by the smaller shrinkage of the microporosity of these samples. Tuinstra et al.36 reported a linear relationship between the inverse of the in-plane width of the graphene layers, La, and the relative integrated intensity ratio, R, such as La (nm) ) 4.4/R. The data obtained are shown in Table 4. These values have to be considered with certain caution because, as shown recently by Cuesta et al.,40 the above equation is valid only as a first approximation to La when considering materials with a certain degree of disorder. (40) Cuesta, A.; Dhamelincour, P.; Laureybs, J. Martinez-Alonso, A.; Tascon, J. M. D. J. Mater. Chem. 1998, 8, 2875.
Figure 11. Raman spectra of chromium-containing carbon aerogels.
Conclusions Results obtained in this work clearly show that Cr, Fe, Co, and Ni can catalyze the partial graphitization of carbon aerogels when these are pyrolyzed above 1000 °C. The resulting transition-metal-containing carbon aerogels are essentially highly porous solids in the macroporous range with nitrogen surface areas ranging between 300 and 400 m2 g-1 after heat treatment at 1000 °C. The surface area decreases significantly at a higher pyrolysis temperature, especially for Cr- and Fe-containing carbon aerogels. HRTEM, XRD, and Raman spectroscopy show the formation of polyhedral structures that, above 1000 °C, show the presence of graphitized areas with a three-dimensional stacking order. Cr and Fe seem to be the best catalysts for the graphitization of carbon aerogels. Acknowledgment. Grupo de Investigacio´n en Carbones acknowledges DGESIC for financial support (Project No. PB97-0831). LA991080R
