Graphitic Carbon Nanofibers Synthesized by the Chemical Vapor

Energy & Fuels 2008, 22, 4139–4145

4139

Graphitic Carbon Nanofibers Synthesized by the Chemical Vapor Deposition (CVD) Method and Their Electrochemical Performances in Supercapacitors Denisa Hulicova-Jurcakova,† Xiang Li,† Zhonghua Zhu,*,† Roland de Marco,‡ and Gao Qing Lu† Australian Research Council (ARC) Centre of Excellence for Functional Nanomaterials and DiVision of Chemical Engineering, UniVersity of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia, and Department of Applied Chemistry, Curtin UniVersity of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia ReceiVed June 4, 2008. ReVised Manuscript ReceiVed August 1, 2008

Graphitic carbon nanofibers were synthesized by chemical vapor deposition of methane and acetylene on the γ-alumina-supported nickel catalyst. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption/desorption, X-ray diffraction (XRD), and Raman spectroscopy were used to examine the structure and the graphitic degree of carbons. The results show that carbons derived from methane consist of a more graphitic structure compared to acetylene-based carbons. The temperature and the catalyst loading affected the graphitic structure further; i.e., the higher the temperature and the catalyst loadings, the better the graphitic structure. The electrochemical performance of synthesized carbons in 1 M H2SO4 revealed that the methane-based carbons show very stable charge/discharge performance in the whole range of investigated current loadings (viz., 0.05 and 3 A g-1), owing to the graphitic structure and thus resulting from the good charge propagation, particularly at high loads. On the other hand, acetylene-based carbons provide greater gravimetric capacitance values as a result of structural defects, but consequently, the capacitance drops at high current loads.

1. Introduction Chemical vapor deposition (CVD) has been accepted as the most suitable method for large-scale, economically viable production of various nanostructured carbons, including carbon nanotubes and carbon nanofibers.1 The principle of the method is the decomposition of a gaseous precursor (hydrocarbons) on the substrate forming the solid (carbon) deposition. Transitionmetal catalysts are often employed in CVD to lower the decomposition temperature of the carbon precursor and to make the synthesis more economical. The properties of carbons derived from hydrocarbons highly depend upon the catalyst composition, decomposition temperature, and the carbon precursor. Transition metals, such as Ni, Co, and Fe, and their alloys have been traditionally used as the catalysts for the hydrocarbon decomposition.2-8 Especially, the Ni-based catalysts are active * To whom correspondence should be addressed: Division of Chemical Engineering, School of Engineering, University of Queensland, St. Lucia, Brisbane 4072, Queensland, Australia. Telephone: +61-7-3365-3528. Fax: +61-7-3365-4199. E-mail: [email protected]. † University of Queensland. ‡ Curtin University of Technology. (1) See, C. H.; Harris, A. T. Ind. Eng. Chem. Res. 2007, 46, 997–1012. (2) Takenaka, S.; Ogihara, H.; Otsuka, K. J. Catal. 2002, 208, 54–63. (3) Otsuka, K.; Ogihara, H.; Takenaka, S. Carbon 2003, 41, 223–233. (4) Chen, J. L.; Li, Y. D.; Li, Z. Q.; Zhang, X. X. Appl. Catal., A 2004, 269, 179–186. (5) Reshetenko, T. V.; Avdeeva, L. B.; Ushakov, V. A.; Moroz, E. M.; Shmakov, A. N.; Kriventsov, V. V.; Kochubey, D. I.; Pavlyukhin, Y. T.; Chuvilin, A. L.; Ismagilov, Z. R. Appl. Catal., A 2004, 270, 87–99. (6) Jablonski, G. A.; Geurts, F. W.; Sacco, A.; Biederman, R. R. Carbon 1992, 30, 87–98. (7) Chambers, A.; Rodriguez, N. M.; Baker, R. T. K. J. Mater. Res. 1996, 11, 430–438.

at lower temperatures and provide higher yields of deposited carbons from the decomposition of methane.9 Until now, various catalytic supports, such as Al2O3,8,10 SiO2,11 carbon fibers,12 and zeolites of Ni-based catalyst,13 have been examined in relation to methane decomposition. It has been reported that alumina is one of the most effective supports for the formation of carbon nanofibers.14,15 Moreover, the reaction temperature is also an important factor to optimize the properties and yields of carbon filaments. Generally, the reaction temperature is between 550 and 1000 °C according to the catalystsupport system.16-18 Variations in carbon-containing gases (CH4, C2H4, C2H2, and CO) can affect the final structure and the graphitic degree of deposited carbons.19 (8) Li, Y. D.; Chen, J. L.; Chang, L. Appl. Catal., A 1997, 163, 45–57. (9) Reshetenko, T. V.; Avdeeva, L. B.; Ismagilov, Z. R.; Chuvilin, A. L.; Fenelonov, V. B. Catal. Today 2005, 102, 115–120. (10) Murata, K.; Inaba, M.; Miki, M.; Yamaguchi, T. React. Kinet. Catal. Lett. 2005, 85, 21–27. (11) Ermakova, M. A.; Ermakov, D. Y. Appl. Catal., A 2003, 245, 277– 288. (12) Otsuka, K.; Abe, Y.; Kanai, N.; Kobayashi, Y.; Takenaka, S.; Tanabe, E. Carbon 2004, 42, 727–736. (13) Park, C.; Keane, M. A. Langmuir 2001, 17, 8386–8396. (14) Yang, Y. L.; Xu, H. Y.; Li, W. Z. J. Nanosci. Nanotechnol. 2004, 4, 891–895. (15) Chen, D.; Lodeng, R.; Holmen, A. Catal. Deact. 1999, 126, 473– 476. (16) Ermakova, M. A.; Ermakov, D. Y.; Chuvilin, A. L.; Kuvshinov, G. G. J. Catal. 2001, 201, 183–197. (17) Reshetenko, T. V.; Avdeeva, L. B.; Ismagilov, Z. R.; Chuvilin, A. L.; Ushakov, V. A. Appl. Catal., A 2003, 247, 51–63. (18) Rodriguez, N. M. J. Mater. Res. 1993, 8, 3233–3250. (19) Otsuka, K.; Kobayashi, S.; Takenaka, S. Appl. Catal., A 2001, 210, 371–379.

10.1021/ef8004306 CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

4140 Energy & Fuels, Vol. 22, No. 6, 2008

HulicoVa-JurcakoVa et al.

Graphitic carbon fibers possess high electrical conductivity, which is one prerequisite for a supercapacitor electrode material. Good charge propagation has to be achieved to operate the supercapacitor under strenuous conditions. The electrodes for electric double-layer capacitors, which are often called supercapacitors, have been studied intensively in recent years. Although new electrode materials have been reported, carbon is still considered as the most promising material, owing to its commercial availability, price, and abundance.20-22 Highly porous activated carbon has been the mostly studied material in supercapacitors, but its electrical conductivity is limited because of the amorphous structure. The power density of a supercapacitor is defined as P ) V 2/(4ESRm)

(1)

where V is the operational voltage, m is the total mass of the electrodes, and ESR is the equivalent series resistance, which is directly dependent upon the electrical resistance of the electrode material. Consequently, the development of supercapacitors with high power density necessitates a minimization of the ESR of the electrode material, or in other words, this change is required to improve the electrical conductivity of the electrode material. In this work, we employed CVD in the preparation of highly electrically conductive graphitic carbon nanofibers through the decomposition of methane and acetylene over an aluminasupported Ni catalyst at low temperatures. Synthesized carbons were characterized and tested as the electrode materials of supercapacitors. The relationship between the processing conditions and the microstructure of synthesized carbon samples was systematically investigated with particular focus on the graphitic degree of formed carbons. 2. Experimental Section 2.1. Synthesis of Catalysts. The Ni/Al2O3 catalysts with nominal composition of 15 and 25 wt % nickel were prepared by wet impregnation of nickel nitrate (Fluka, assay > 97%) on γ-Al2O3 (Merck, 187 m2/g). γ-Al2O3 was mixed with an aqueous solution of Ni(NO3)2 at room temperature. The slurry was heated up to 90 °C and stirred until most of the water had evaporated. The synthesized solids were dried at 110 °C overnight and calcined at 550 °C for 6 h in air. Before synthesis of carbon fibers using the CVD method, catalysts were reduced by pure H2 (70 mL/min) at the same temperature as the CVD temperature for 3 h. 2.2. Synthesis of Carbons. Carbon samples were produced by CVD in a horizontal tubular furnace. The carbon precursor gas was methane (CH4, 99.9% purity) or acetylene (C2H2, 99.9% purity). Each time, 1 g of catalyst was used, and the decomposition time was 3 h, at a heating rate of 10 °C/min. The final products were purified with 5 M HNO3 solution in a reflux system for 6 h to remove the catalysts. The carbon solutions were then filter-washed with distilled water and dried at room temperature. The synthetic carbons are denoted as “C1-x-y” and “C2-x-y”, where C1 and C2 are the carbon produced from pure CH4 and pure C2H2, respectively; “x” is the CVD temperature (in °C), and “y” is the loading of Ni (in %). 2.3. Characterization of the Catalysts and Carbon Samples. The crystalline parameters, textural properties, and morphology of prepared catalysts and carbon materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption/ (20) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; van Schalkwijk, W. Nat. Mater. 2005, 4, 366–377. (21) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11–27. (22) Frackowiak, E. Phys. Chem. Chem. Phys. 2007, 9, 1774–1785.

desorption at 77 K, SEM, and TEM. Electrical ohmmetry was used in determination of electrical conductivity of carbon materials. XRD analyses were performed on a Rigaku Miniflex X-ray diffractometer (40 kV and 30 mA) with Co KR radiation at a scanning rate of 2°/min in the 2θ range from 10° to 100°. The average size of carbon crystalline was calculated from the Debye-Scherer equation. The K values of 0.9 and 1.84 were used in Lc and La determinations, respectively, with Lc, the layer dimension perpendicular to the basal plane, obtained from the (002) reflection, and La, the layer dimension parallel to the basal plane, calculated from the (100) reflection. The d002 values were obtained from the Rigaku XRD software. To quantitatively analyze the size of quasi-graphitic crystallite units, we assumed that each graphitic crystallite is a nanosized cylinder. The cylindrical volume of each nanocrystallite can be designated as Vnano

π Vnano ) La2Lc 4

(2)

Raman spectroscopy was performed with the 633 nm line of a He-Ne laser (Renishaw-1000) at room temperature. The incident laser power at the sample was 5 mW, and the spectra were recorded with a resolution of 2 cm-1. The nitrogen adsorption/desorption was carried out in NOVA 1200 adsorption analyzer (Quantachrome, Boynton Beach, FL). All samples were degassed at 200 °C for at least 6 h prior to measurement. The specific surface area (SBET) was evaluated using the traditional Brunauer-Emmett-Teller (BET) technique, which was applied in the relative pressure range of 0.05-0.25. The pore size distribution of carbon samples was evaluated by the desorption isotherm using the Bopp-Jancso-Heinzinger (BJH) method. The catalyst residue was determined by thermogravimetric analysis (TGA) of the washed samples as the amount of ash after heating at 1000 °C in air. The morphology of samples was characterized by a JEOL 6400 field emission scanning electron microscope, with an accelerating voltage of 15.0 kV. The apparatus for determining the electrical conductivity of carbons consisted of a hollow cylinder constructed from a nonconducting material. Two copper pistons in both ends of the cell are used to press the samples in the cell and to connect to an ohmmeter. The electrical conductivity (σ) is given by

σ ) l/RA

(3)

where R is the electrical resistance in Ω, which was measured at room temperature using an ohmmeter (Hioki 3220 HiTester, range from 20 mΩ to 20 kΩ and accuracy of 0.2%), A is the surface area of the piston in cm2, and l is the distance between the two pistons in the hollow cylinder in centimeters. Because the electrical resistivity of powder beds is highly affected by the applied pressure, a constant pressure was applied by supporting a steel cylinder having a mass of 3 kg on the upper piston. 2.3. Electrochemical Measurements. The electrochemical measurements were performed in a two- and three-electrode cell in 1 M H2SO4. Working electrodes were prepared by mixing the carbons with 5 wt % polytetrafluoroethylene (PTFE) and pressing at 250 kg cm-2 for 5 min. The testing cell was constructed from two carbon pellets attached to platinum current collectors separated by a glass filter, and a Ag/AgCl reference electrode was used in a threeelectrode measurement. Cyclic voltammetry at 500 mV/s and galvanostatic charge/ discharge cycling with current loads between 0.05 and 3 A g-1 were conducted on a multichannel Solartron 1480 potentiostat/ galvanostat. The specific gravimetric capacitances were calculated from the discharge process of the galvanostatic charge/discharge profile. A commercial carbon black sample (Mitsubishi Chemicals, Inc.) was

Graphitic Carbon Nanofibers from the CVD Method

Energy & Fuels, Vol. 22, No. 6, 2008 4141

Figure 1. XRD profiles of (a) 15% Ni/Al2O3 catalyst and (b) 25% Ni/Al2O3 catalyst before reduction and reduced at 500 and 700 °C.

Figure 2. XRD profiles of commercial carbon black and all investigated samples after catalyst wash-out.

used as the comparative sample, and the electrode preparation and measurement conditions were identical to those used for synthesized carbons.

3. Results and Discussion 3.1. Characterization of Ni/Al2O3 Catalysts. The nitrogen adsorption/desorption isotherms of all Ni/Al2O3 catalysts (not shown here) are of type IV, typical for mesoporous material. The increase in both the reduction temperature and the Ni loadings decrease the surface area and the pore volume of catalysts, which can be explained on the basis of alumina support shrinkage at higher temperatures and blocking of the alumina pores at higher nickel loadings. X-ray powder diffraction analyses (Figure 1) revealed that the peaks of unreduced NiO are present in the Ni/Al2O3 catalyst reduced at 500 °C. At 700 °C, however, Ni2+ is completely reduced to Ni0, characterized with the peaks at 52°, 61°, and 92°. As expected, diffraction peaks of the γ-Al2O3 support were detected in all samples. 3.2. Characterization of Carbons. XRD profiles of all investigated carbons and the comparative carbon black are shown in Figure 2, and the structural parameters, catalyst residues, and electrical conductivities of synthesized carbons are summarized in Table 1. From the XRD profiles, it is clear that, even after the wash-out process, the catalyst residue is present in all samples in the form of reduced Ni0, characteristic of (111) and (200) diffractions at 52° and 61°, respectively. The total residue obtained from TGA-DTA is between 2.8 and 6.4% (Table 1). It is interesting to note that the samples prepared at 700 °C contain a greater amount of residue than the ones synthesized at 500 °C, which is believed to be due to the

Table 1. Characteristics of Synthesized Carbon Nanofibers sample C1-500-15 C1-700-15 C1-500-25 C1-700-25 C2-500-15 C2-700-15

residual conductivity SBET d002 Lc La Vnano (S/cm) (m2/g) (nm) (nm) (nm) (nm3) catalyst (%) 53 119 47 109 120 96

0.345 0.343 0.342 0.341 0.345 0.342

4.32 4.62 4.98 4.87 4.13 4.30

6.21 7.45 7.23 7.97 5.53 6.84

130.8 201.3 204.4 242.8 99.1 157.9

3.5 4.9 2.8 6.4 3.7 5.7

1.38 4.14 1.75 5.67 1.22 1.96

encapsulation of catalysts within the graphitic structure formed at higher temperatures, preventing a washing out of the catalyst. Regarding the carbon black sample, the 002 diffraction peak is broad corresponding to low crystallinity carbon. On the other hand, the 002 diffraction peak of synthesized samples becomes narrower and sharper with an increase in both the Ni loading and the temperature, reflecting the higher degree of structural order. Particularly, the favorable effect of the Ni catalyst on the formation of an ordered and compact graphene layer structure can be seen when comparing the d002 values of C1500-15 and C1-500-25 (Table 1). Carbon prepared by 25 wt % Ni on Al2O3 support (C1-500-25) possesses better graphitic structure with the d002 of 0.342 nm in contrary to 0.345 nm of C1-500-15 carbon prepared at the same temperature and from the same carbon precursor but on the catalyst with less (15%) nickel loading. Regarding the carbon precursor and its influence on d002, the results suggest no significant role of carbon precursor gas on this characteristic, because similar or identical d002 values were obtained in carbons prepared from CH4 and C2H2 under the same conditions and using the same catalyst.


Figure 3. D band to G band ratios ID/IG of all investigated carbon nanofibers as a function of the synthesis temperature.

However, the lattice parameters La, Lc, and Vnano change with the type of precursor gas. The sizes of crystallites (Lc and La) are larger in carbons prepared from methane (C1 series) than from acetylene (C2 series). The difference is even more significant when looking at the Vnano values, which represent the crystallite volume. For instance, Vnano of C1-700-15 is 201.3 nm3, whereas only 157.9 nm3 is calculated for C2-700-15. These results clearly show the importance of precursor gas selection in the preparation of highly ordered carbons because different decomposition mechanisms apply to different precursor gases, as indicated in a previous report.19 It is well-known that another important factor affecting the degree of graphitization is the synthesis temperature. At higher temperatures, larger crystallites are formed regardless of the carbon precursor gas. As expected, the same trend was observed in our samples when d002 decreased with temperature, while Lc, La, and Vnano increased. To further confirm the graphitic degree, Raman spectroscopic characterization was also undertaken. Two typical bands were observed in all samples: the D band at 1350 cm-1 and the G band at 1580 cm-1. The former is ascribed to the structural disorder within the graphite planes, while the later is characteristic of vibrational mode of the hexagonal lattice of the graphite.23 The intensity ratio of the D band and G band ID/IG denote the graphitic degree of carbon, and the values calculated for investigated samples are shown in Figure 3 as a function of the synthesis temperature. The ID/IG value decreased from 3.2 to 1.5 with the synthesis temperature, indicating a higher graphitic order in carbons synthesized at higher temperatures, which is in agreement with the above-discussed results. In addition, the values of ID/IG are strongly affected by the carbon precursor gas, with an observation of significantly higher values in samples derived from acetylene compared to those prepared using methane. The amount of nickel catalyst influenced the graphitic order; i.e.,


the higher the nickel content, the lower the ID/IG value. When the results on XRD and Raman spectroscopy are taken into account, it can be concluded that carbons derived from methane were more graphitic than those synthesized from acetylene, while elevated temperature resulted in a higher degree of graphitization in the synthetic carbons. A higher catalyst content further enhanced the formation of a crystalline structure. Graphite is an excellent electrical conductor because of its perfect two-dimensional crystalline structure, and the conductivity measurement is therefore a reliable method for assigning the degree of graphitization in synthesized carbons. The results are presented in Table 1. It can be concluded that the conductivities are congruent with the observed XRD and Raman spectroscopic structural data. For example, the most conductive sample C1-700-25 (5.67 S cm-1) is associated with the carbon derived from methane over a 25% Ni/Al2O3 catalyst at 700 °C. The C1-700-15 sample prepared using the same precursor at a similar temperature but over the catalyst with only 15% Ni/ Al2O3 is less conductive, i.e., 4.14 S cm-1; however, this value is about double the value measured for C2-700-15 (i.e., 1.96 S cm-1). The BET surface areas of samples increased at elevated synthesis temperatures. The explanation can be found in different structures formed at 500 and 700 °C, respectively. Figure 4 displays the SEM images of C1-500-25 and C1-700-25. Fibrous structures with smaller diameters and larger surface areas are observed in C1-700-25. The isotherms (not shown here) are of type IV, similar to those of Ni/Al2O3, indicating the presence of mesopores. TEM revealed that some of the nanofibers contain the internal cavities, resembling the bamboo-like morphology;24 however, the cavity is not located throughout the whole nanofiber (Figure 5a). Catalyst residue particles can be clearly observed. Parts b-d of Figure 5 represent high-resolution images of three different regions: (b) a high degree of ordering along the fiber axis with the catalyst particle attached, (c) a graphitic structure with a defect on the outer wall, and (d) the disordered carbon with localized ordering. These images are in agreement with XRD and Raman spectroscopy, confirming the presence of both the crystalline and disordered structures, with different contribution depending upon the synthesis conditions. 3.3. Performances of Synthesized Carbon Nanofibers in Supercapacitors. The cyclic voltammetry of all carbons including the carbon black were recorded using a two-electrode cell at the scan rate of 500 mV s-1, and the corresponding voltammograms are shown in Figure 6. The measurement at high scan rate is a reliable way of estimating the performance of a supercapacitor under strenuous operating conditions, particularly important in real applications. This characteristic can be deduced from the CV profile; a rectangular-shaped CV represents a quick charge propagation and fast charge/discharge kinetics, whereas any deviation in the CV is indicative of a worsening in performance.

Figure 4. SEM images of carbon samples: (left) C1-500-25 and (right) C1-700-25. The scale bar represents 200 nm.



Figure 5. TEM images of C1-700-15: (a) low-magnification image and (b-d) high-resolution images of three different regions.

Figure 6. Cyclic voltammograms (CVs) of synthesized carbon nanofibers and the commercial carbon black recorded in 1 M H2SO4 in twoelectrode cell at the scan rate of 500 mV/s.

The CVs are clearly indicative of samples prepared using methane (C1 series) possessing superior capacitive performance compared to acetylene-derived carbons (C2 series) in terms of quick charge/discharge at high scan rates. This can be seen in the shape of the CVs for the C1 series carbons appearing rectangular-shaped with no or a minimal slope at limited

potentials. Particularly, the C1-700-15 shows ideal capacitive performance. On the other hand, the CVs of the C2 series of carbons are ellipsoidal in shape. A similar distorted shape of the cyclic voltammogram is obtained from carbon black. The capacitance values at each current loading are summarized in Table 2. The gravimetric capacitances were calcu-



Table 2. Gravimetric Capacitance (Cg) at Current Loads between 0.05 and 3 A/ga sample

Cg (F/g) 0.05 A/g

Cg (F/g) 0.1 A/g

Cg (F/g) 0.2 A/g

Cg (F/g) 0.5 A/g

Cg (F/g) 1 A/g

Cg (F/g) 2 A/g

Cg (F/g) 3 A/g

∆Cg (%)

C1-500-15 C1-700-15 C1-500-25 C1-700-25 C2-500-15 C2-700-15

6.9 8.3 2.3 6.8 14.3 9.7

6.8 8.1 2.2 6.5 13.7 9.2

6.6 8.0 2.1 6.4 12.9 8.8

6.3 7.8 2.0 6.2 12.4 8.4

5.9 7.5 2.0 6.1 11.9 8.1

5.9 7.5 2.0 6.0 11.7 7.6

5.7 7.5 2.0 5.9 11.5 7.0

17 10 14 13 20 28

a

The capacitance drop ∆Cg (%) between 3 and 0.05 A/g is shown in the right column.

Figure 7. (a) Galvanostatic charge/discharge profiles of C1-700-15 and C2-500-15 recorded at the current load of 3 A/g in a three-electrode cell using Ag/AgCl electrode as the reference electrode. (b) Detail of the selected area near the limited potential of 0.9 V. An IR drop of about 0.02 V is seen in C2-500-15, while no significant potential drop is observed in C1-700-15.

lated from galvanostatic charge/discharge profiles. The capacitance drop ∆Cg (%) between the lowest (0.05 A g-1) and highest (3 A g-1) current loading, respectively, is calculated for each sample and is also presented in Table 2. Several conclusions can be drawn from these results. First, acetylene-derived samples (C2 carbons) possess greater gravimetric capacitances than the methane-based carbons (C1 carbons), and the best performance sample was C2-500-15 with 14.3 F/g at 0.05 A/g. Second, no linear relation is observed between the gravimetric capacitances and the BET surface areas as the C2-500-15 with the highest Cg, which has a moderate BET among all of the samples (i.e., 65 m2 g-1); however, as one expects, C1-500-25 with the lowest BET of 27 m2 g-1 gives the lowest Cg, being 2.3 F g-1 at 0.05 A g-1. The highest gravimetric capacitance of C2-500-15 despite lower BET surface area might be the consequence of structural defects helping in ion adsorption and their migration within the capacitor electrode as reported by Portet and colleagues.25 The authors also observed that capacitance drops more remarkably in samples with structural defects than in those with well-crystalline and ordered structure. The biggest capacitance drops of C2-500-15 and C2700-15, being 20 and 28%, respectively, are in good agreement with this conclusion. The smallest capacitance drop (10%) is measured in C1-70015, which is the sample with a perfectly rectangular CV and thus superior charge propagation properties. In fact, these results support the cyclic voltammetry data, confirming that C1-70015 shows advanced electrical conductivity properties crucial for quick charge propagation and fast charging and discharging at high current loads and scan rates. The gravimetric capacitance of the carbon black sample was less than 1 A/g at the lowest current load of 50 mA/g. (23) Wang, Y.; Serrano, S.; Santiago-Aviles, J. J. Synth. Met. 2003, 138, 423–427. (24) Lee, C. J.; Park, J. Carbon 2001, 39, 1891–1896.

Because of the fact that the electrical conductivity of C1700-15 (4.14 S cm-1) is just the second highest after its counterpart prepared from the same precursor and at the same temperature but with the highest nickel loading (C1-700-25; 5.67 S cm-1), one would expect the most stable performance from C1-700-25. In addition, this is the sample with the highest degree of graphitic structure, further supporting this expectation. In reality, however, a 13% capacitance drop is observed in contrast to 10% with C1-700-15. The possible reason for this phenomenon is probably due to a high amount of residual catalytic material, which could affect the electrical conductivity and the charge propagation at high current loads. Despite this consideration, it should be noted that the catalytic residue did not cause any significant Faradic (redox) reactions because no redox waves were observed in CVs. Figure 7 shows the galvanostatic charge/discharge profiles of C1-700-15 and C2-500-15. Triangular-shaped galvanostatic charge/discharge profiles are obtained with both samples (Figure 7a), even at high current load of 3 A g-1, indicating excellent charge propagation. However, a close look at the limited potential 0.9 V reveals the current-resistance drop of ca. 0.02 V, often called an IR drop, in C2-500-15 (Figure 7b). The IR drop is related to the internal resistance of the electrode material (or ESR).26 Consequently, this experiment supports the above-mentioned conclusions about the poorer conductivity properties of acetylene-based carbons compared to methane-based carbons. 4. Conclusions In conclusion, graphitic carbon nanofibers were prepared by CVD from methane and acetylene over the nickel-loaded Al2O3 catalyst at 500 and 700 °C. Various characterization methods were used in the evaluation of the structural parameters, and it was found that methane resulted in carbons with better graphitic


degree and higher crystallinity compared to acetylene-derived carbons. The temperature and the nickel loading affected the microstructure of carbons, leading to larger crystalline size and Vnano values in carbons synthesized from the same gas (methane) at 700 °C and 25% Ni/Al2O3 versus 500 °C and 15% Ni/Al2O3, respectively. The electrochemical measurements revealed a stability in performance for all of the synthesized carbons in 1 M H2SO4, even at high current loads of 3 A/g. This was primarily due to the graphitic nature and good electrical conductivity of the synthesized carbons. However, detailed analysis reveals that methane-based carbons possess superior charge/discharge performance to acetylene-based carbons, which is a result of their (25) Portet, C.; Yushin, G.; Gogotsi, Y. Carbon 2007, 45, 2511–2518. (26) Conway, B. E. Electrochemical SupercapacitorssScientific Fundamentals and Technological Applications; Kluwer: New York, 1999.


higher graphitic degree. However, the best gravimetric capacitance despite lower BET surface area was measured in C2-50015, which is the sample derived from acetylene. This behavior is due to less ordered structure and surface defects that help in ion adsorption and migration within the carbon electrode. The electrochemical performance of graphitic carbon nanofibers was superior to that of commercial carbon black, and therefore, they can find applications in different areas, such as catalyst supports, where quick electron transfer is required, or as conductive fillers in electronic applications. Acknowledgment. Financial support from the ARC Discovery Grant and the ARC Centre of Excellence for Functional Nanomaterials is appreciated. EF8004306

Graphitic Carbon Nanofibers Synthesized by the Chemical Vapor

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