NANO LETTERS
Studies into the Storage of Hydrogen in Carbon Nanofibers: Proposal of a Possible Reaction Mechanism
2002 Vol. 2, No. 3 201-205
Darren J. Browning,‡ Mark L. Gerrard,† J. Barry Lakeman,‡ Ian M. Mellor,*,† Roger J. Mortimer,† and Mark C. Turpin§ Department of Chemistry, Loughborough UniVersity, Loughborough, Leicestershire, LE11 3TU, UK, DERA Centre for Marine Technology, Haslar, Gosport, Hampshire, PO12 2AG, UK, and Morgan Materials Technology Ltd., Bewdley Road, Stourport-on-SeVern, Worcestershire, DY13 8QR, UK Received June 28, 2001; Revised Manuscript Received December 19, 2001
ABSTRACT Substantial levels of hydrogen, up to 6.5 wt %, have been stored in carbon nanofibers (CNFs) under conditions of 12 MPa pressure and ambient temperature. The magnitude of this result cannot be interpreted in terms of physisorption on the external surface alone. Kinetic studies indicate that a slow chemisorption process is involved. The rate of uptake corresponds to that of hydrogen dissociation on graphite edge sites. Such a finding proposes a novel mechanism, offering a plausible explanation for these unusually high experimental observations. This involves the initial dissociation of hydrogen, believed to be catalyzed by carbon edge sites, which constitute the majority of the nanofiber surface, a property which is probably an important contributory factor toward their high hydrogen storage capacities.
In recent years there has been considerable experimental and theoretical interest in the use of nanostructured carbon materials, especially in the form of tubes,1-16 fibers,15-24 and mechanically milled graphite,25 as potential hydrogen sorbents. The mechanistic details of hydrogen adsorption on the external surface of both single (SWNTs) and multiwalled (MWNTs) nanotubes is well understood. It is generally accepted that the majority of uptake can be explained in terms of physisorption at subambient temperatures on the exterior and interior surfaces, assuming prior removal of the end caps,2-5 and also in the interstitial channels between nanotube bundles.1-5 In certain cases it has been found that hydrogen can be retained in SWNT structures at room temperature.5-8 However, desorption measurements, also at 298 K, indicated that only roughly 80% of the stored hydrogen could be recovered,6 with temperatures in excess of 473 K required to reclaim the remaining gas. This would suggest the presence of some chemisorption, although neither a specific location nor possible mechanism to account for this adsorbed hydrogen was discussed. Later, theoretical work by Lee et al.7 highlighted top sites at the outer walls of SWNTs that could chemisorb atomic hydrogen. This was also true for * To whom correspondence should be addressed. E-mail: i.m.mellor@ bigfoot.com. Present address: Morgan Materials Technology Ltd, Bewdley Road, Stourport-on-Severn, Worcestershire, DY13 8QR, UK. † Loughborough University. ‡ DERA Centre for Marine Technology. § Morgan Materials Technology Ltd. 10.1021/nl015576g CCC: $22.00 Published on Web 01/18/2002
© 2002 American Chemical Society
the inner surface; however, upon relaxation the hydrogen present at these positions became unstable, resulting in molecular hydrogen formation, where it was shown to exist in the vacant space inside the tubes. Verification of these findings was attempted experimentally,8 both electrochemically and with the aid of Raman spectroscopy. Chen et al.9 thought that lithium, present in alkali-doped nanotubes, was acting as a catalytically active center for dissociative hydrogen adsorption, after which spillover occurred onto the graphite sheets of MWNTs. Efforts to reproduce these results have proved unsuccessful,10-11 attributing changes in weight observed during cycling to water gain/loss present in the hydrogen feedstream. Despite this, Yang10 acknowledged that the actual storage levels were still significant, warranting further investigation. Interestingly, to date there has been no satisfactory mechanism forwarded to adequately explain the proposed levels of storage within the highly ordered graphitic layers of MWNTs and CNFs at room temperature. Several authors have postulated that the interplanar spacing between adjacent platelets in both MWNTs14 and CNFs19-22 can accommodate considerable amounts of molecular hydrogen, although confusion surrounding the exact nature of this interaction is apparent. Instances citing chemisorption,20-22 physisorption,19 and a combination of both processes14 exist. The argument for the former arose from observations noted during desorption studies,22 as not all the adsorbed hydrogen was retrievable after prolonged evacuation at 298 K. Similar findings
Table 1: Summary of the Results from the Characterization and Hydrogen Storage of the CNF Samples metal ratio in catalyst Fe/Ni/Cu
carbon yield (g/gcat)
surface area (m2g-1)
TEM diameter rangea (nm)
TEM average diametera (nm)
uptake-as prepared fibers (wt %)
uptake-Ar treated fibers (wt %)
90:5:5 85:10:5
22 23
34 51
50-280 30-260
180 150
1.66 4.18
3.54 6.54
a Values correspond to measurements taken from solid nanofibers only exhibiting structures thought to be responsible for the observed hydrogen uptake,20 neglecting the diameters of any tubes or amorphous carbon present.
regarding this lack of total reversibility were reported by Chambers et al.,20 but no ensuing explanation for this was presented. Unfortunately, subsequent Grand Canonical Monte Carlo simulations by Wang and Johnson24 could not corroborate such theories. Calculations performed assuming the interaction between molecular hydrogen and nanofiber surfaces only yielded a maximum uptake of 0.46 wt %, being over 100 times less than the experimental value observed by Chambers et al.20 under identical conditions. Through the preparation of CNFs exhibiting high hydrogen storage capacities and interpretation of kinetic data, this paper attempts to address this problem concerning the lack of mechanistic detail, providing greater insight into the uptake process. CNFs26-28 used in uptake measurements, were synthesized at 873 K by passing pure ethylene over a series of Fe/Ni/Cu catalysts, prepared according to the procedure of Best and Russell.29 A total flow rate of 300 mL/min was used, and growth was monitored over a 3 h reaction period, after which the nanofibers were allowed to cool under an inert atmosphere, prior to their weight being recorded. In certain circumstances, small batches of the product were treated in a continuous flow of argon at 1273 K for 36 h, as it is reported the graphitic quality of the fibers can be considerably enhanced under such conditions.30 Characterization of the CNFs was performed with the aid of several techniques, particularly transmission electron microscopy (TEM), selected area diffraction, and nitrogen BET surface area determination at 77 K. Hydrogen storage was followed at 12 MPa and ambient temperature, utilizing a novel high-pressure apparatus (Figure 1) having four volumetrically balanced chambers, measuring uptake as a differential pressure between the sample relative to that of a blank.31 Adopting this technique, over that of using a single gauge,16-21 is considered important, as it eliminates potential errors due to low precision in high-pressure gauges. The method also eradicates problems associated with the expansion of nonideal hydrogen gas,32 which can introduce large errors when performing this type of measurement. The apparatus is capable of reproducing hydrogen uptakes as low as 0.1 wt % when using 100 mg of material. During a typical experiment, approximately 50-100 mg of nanofibers were initially loaded into the sample chamber while heating under vacuum (ca. 103 Pa) at 423 K, removing any physisorbed water. Following this the system was allowed to cool to room temperature, after which hydrogen was admitted and uptake determined. Initial testing included a blank experiment, without sample, which showed that the 202
Figure 1. Schematic diagram of the measurement system used.
stainless steel vessel did not give rise to hydrogen adsorption, while also ensuring that the instrument was leak free. In addition, a commercial MmNi4.5Al0.5 alloy (where Mm denotes a mismatch of lanthanides) with known hydriding properties was investigated, giving an uptake of 1.27 wt %, coinciding well with the manufacturer’s data. Unfortunately due to the present configuration of the apparatus, it is very difficult to perform desorption measurements with any great degree of accuracy. However, it is estimated that the amount of reversible hydrogen at 298 K is approximately 80%.20-22 Interestingly, it has recently been suggested that in practical applications, this remaining gas can be reclaimed by utilizing the waste heat produced during desorption to drive off the hydrogen.33 A summary of the characterization and hydrogen storage results for CNFs, grown on different Fe/Ni/Cu catalysts, is given in Table 1. TEM of the as prepared carbonaceous deposits (Figure 2) indicate that the majority are fibrous in nature, possessing diameters in the range 30-280 nm. Subsequent electron diffraction gives images indicative of a herringbone structure,28 Figure 2 inset, consisting of stacked plates, aligned at an angle (∼ 45°) to the fiber axis with an interlayer spacing of 0.34 nm, being close to that of graphite. Close inspection of the micrographs also shows the presence of nanotubes and amorphous carbon. Hydrogen uptake increased for nanofiber aliquots after heating at 1273 K under argon. However, electron diffraction proved inconclusive in confirming whether graphitic character had been improved. The role played by argon toward increasing the hydrogen storage capacity of carbon nanofibers Nano Lett., Vol. 2, No. 3, 2002
Table 2: Initial Rates of Hydrogen Uptake on the Various CNF Samples metal ratio in catalyst Fe/Ni/Cu
sample history
initial rate of uptake × 1017 molecules s-1 m-2
90:5:5
as prepared argon treated as prepared argon treated
1.1 ( 0.1 3.6 ( 0.2 2.3 ( 0.1 5.7 ( 0.4
85:10:5
Figure 2. TEM micrograph and electron diffraction (inset) of fibers derived from a 90:5:5 Fe/Ni/Cu catalyst.
Figure 3. Hydrogen uptake curves of CNF samples prepared from a 85:10:5 Fe/Ni/Cu catalyst, (]) argon treated, ([) as prepared, and 90:5:5 (4) argon treated, (2) as prepared.
is still unclear. It is conceivable that at the temperatures adopted, it aids desorption of surface groups that would otherwise inhibit hydrogen adsorption.21 Clearly the influence of argon treatment is not surface area related, as values only changed by roughly 15% during the procedure. As Table 1 shows, at present the best storage results have been achieved using nanofibers from a 85:10:5 catalyst following treatment in argon, giving an uptake of 6.54 wt %, equalling the gravimetric target as set by the American Department of Energy (DOE). Unfortunately, due to the relatively low density of these nanostructures, typically 700 kg/m3, they fall short of the desired volumetric storage requirement of 62 kg H2/m3 established by the DOE, delivering approximately 45 kg H2/m3.34 Previously, similar problems have been overcome in activated carbons by metal insertion into the materials.35 In certain cases where the bulk density was doubled, volumetric hydrogen adsorption capacities improved by 54%, which applied to the current situation would now satisfy the DOE criteria.36 Kinetics of hydrogen adsorption for the 6.54 wt % sample are illustrated in Figure 3, together with fibers obtained from a 90:5:5 alloy under the same conditions. Monitoring the uptake curve, it is apparent charging is virtually complete within 3 h. The slow rate of sorption would tend to indicate a chemisorption mechanism, as a physisorption process would tend to equilibrate quickly. Comparison of these data, with that available for activated carbons, confirms this point. In these materials it is accepted Nano Lett., Vol. 2, No. 3, 2002
that storage can be explained by physisorption on the external surface,18,35,37 and the limited amount of uptake observed is rapid.20 This can be further reiterated by calculating the wt % uptake corresponding to physisorption of hydrogen on the outer surface of the nanofibers. Using the characterization data from Table 1 for the 6.54 wt % material and assuming a kinetic diameter of 0.289 nm for hydrogen, an uptake of only 0.2 wt % is realized from a physisorbed monolayer. Similar findings were reported by Ahn et al.,17 using CNFs grown under comparable reaction conditions, as adopted in this work. Although uptakes of Ahn et al.17 were lower than those obtained here, ca. 0.2 wt %, and the authors claim CNFs are not attractive candidates for hydrogen sorption, their results are still intriguing. Total storage exceeded the amount of molecular hydrogen able to adsorb on the external surface by roughly a factor of 3. Intercalation of molecular hydrogen between the stacked graphite layers has been proposed as a possible mechanism20-21 but cannot account for the levels of uptake observed.24 The basal plane separation of 0.34 nm is too narrow to accommodate significant hydrogen at ambient temperature. Clearly a novel uptake mechanism has to be considered in order to explain the experimental results. It has been demonstrated by Ishikawa et al. that graphitized carbon black surfaces are capable of rapidly equilibrating hydrogen/deuterium mixtures.38 A dissociation rate of 2.5 × 1017 molecules s-1 m-2 ASA (ASA ) active surface area) was measured at ambient temperatures and pressures, irrespective of the nature of the carbon material under investigation. The ASA was described in terms of atoms located at edge positions on the graphite basal plane and was determined from the amount of oxygen able to chemisorb at these sites. The ASAs of the current samples have been calculated, assuming that the entire nanofiber surface is edge site, using measurements taken from electron micrographs (Table 1). From the uptake curves (Figure 3), this gives initial hydrogen adsorption rates of 3.6 × 1017 and 5.7 × 1017 molecules s-1 m-2 ASA, for nanofibers produced from 90:5:5 and 85:10:5 catalysts, respectively, following argon treatment (Table 2), their magnitude being comparable to those of equilibration.38 Furthermore, regarding equilibration, the total number of reaction events is double that of the observed rate, as only sequences leading to HD desorption are measurable, since steps leading to the recombination of H2 or D2 are not taken into consideration. The exchange rate at the carbon surface must be at least 5 × 1017 molecules s-1 m-2 in Ishikawa’s experiment, which is very close to the present observations. 203
The close coincidence of both the adsorption and equilibration rates possibly indicates that dissociation, leading to atomic hydrogen formation, is the rate determining step for hydrogen storage in CNFs. Unfortunately due to the complex nature of the experiment, it is difficult to perform temperature-dependent studies of uptake allowing determination of an activation energy (Ea). Calculations show that a temperature differential of only 1 K between the sample and blank chambers is equivalent to an uptake of 2 wt %. Therefore, maintaining constant temperatures on both sides, other than room temperature, would prove very challenging with the apparatus in its current guise, leading to the acquisition of unreliable results. These and other pitfalls associated with the accurate recording and interpretation of hydrogen storage data have been well documented by Tibbetts et al.39 Therefore, in an attempt to provide a more comprehensive mechanistic detail, parallels are drawn from the results of Ishikawa.38 It was found that all H2/D2 equilibration data obeyed a first-order rate expression with respect to hydrogen concentration over a wide temperature range; however, two distinct regions were observed in the Arrhenius plot. The first, in the 70-200 K range, had an Ea of less than 3 kJ mol-1 and preexponential factor of approximately 1018 molecules s-1 m-2. From this it was deduced the rate determining step did not require a high activation energy to split a hydrogen molecule, whereas the magnitude of the preexponential factor implies that either the number of active sites was much lower than the quantity of adsorbed hydrogen or there is a large entropy of activation involved in the transition. Second, at temperatures greater than 450 K, the process had an Ea of 57 kJ mol-1 and preexponential factor of 4 × 1020 molecules s-1 m-2. Here the transition state is of high activation energy but lower activation entropy. Clearly the present hydrogen uptake measurements, obtained at 298 K, fall into neither of these two regions; however, interpretation of the kinetic data suggests they belong in the first category. Recent results by Orimo et al.25 on mechanically ground graphite possessing an abundance of edge sites appear to validate the claim that hydrogen dissociation is involved in the uptake process. Milling under a deuterium atmosphere led to the formation of C-D bonds at terminal positions, as verified by neutron diffraction measurements. The actual mechanism of storage still remains a mystery. Taking the van der Waals radius of the hydrogen atom to be 0.12 nm, an uptake of only 0.4 wt % can be achieved by covering CNFs from a 85:10:5 catalyst (Table 1) with an atomic monolayer. Therefore, an alternative explanation of values exceeding 6.5 wt % is required. Speculatively, the possibility of intercalation of hydrogen atoms into the graphite structure might be considered. An extensive literature review dealing with many aspects of graphite intercalation compounds (GICs) has been published by Dresselhaus et al.40 It is generally acknowledged that the intercalant must initially find exposed graphene edges, and these sites then provide pathways for more intercalant to move inward, filling the space between the basal planes. Such a process corresponds well with the 204
argument regarding the involvement of atomic hydrogen in the storage mechanism, as outlined above. Recent findings, made by Cracknell33 during molecular simulations, further substantiate this theory. It was stated that if atomic hydrogen could interact with graphite-like surfaces through chemisorption, the possibility then exists that hydrogen, including its molecular form, could be intercalated in nanofibers. Experimental evidence of this has been provided by Orimo et al.25 In addition to observing C-D bonds at the periphery of carbon nanostructures, they also found deuterium atoms existing between adjacent graphite sheets, with an interlayer spacing of 0.36 nm. This observation coincides well with simulations of Lee et al.,8 showing that a hydrogen molecule in the vicinity of edge sites dissociates, leaving the two atoms in terminal positions. A consequence of this is that the graphite interlayer spacing increases, possibly allowing the insertion of hydrogen, in accordance with the results of Orimo et al.25 Watanabe et al.41 attempted electrochemical atomic hydrogen adsorption in various graphite materials, including carbon nanotubes. A distinct lack of storage, coupled with molecular orbital calculations, using benzene as a model, led Watanabe41 to predict that hydrogen GICs may not be stable under normal conditions. However, subsequent investigations have determined that carbon nanotubes can facilitate hydrogen uptake electrochemically,12-13 contradicting Watanabe’s conclusions.41 Studies in this laboratory42 are in agreement with Watanabe41 regarding graphite; however, we have reproducibly stored hydrogen electrochemically in CNFs under identical experimental conditions. Cyclic voltammetry of nanofiber electrodes, immersed in a 1 M H2SO4 electrolyte, provided direct evidence for the evolution and subsequent storage of hydrogen. Also very promising are the more in depth theoretical studies performed by Cracknell,33 modeling the participation of atomic hydrogen in the uptake process, borne out from the fact that no reasonable physisorption process can explain the levels of adsorption reported. Using a potential for chemisorption based on the calculation of interaction between atomic hydrogen and a graphitic surface, storage levels of 8 wt % are achievable in CNFs, with 70% being reversible under ambient conditions. Clearly this further corroborates our claim regarding the proposed involvement of atomic hydrogen in the adsorption process in CNFs, while providing further credence to our own experimental results, being of a similar magnitude. Clearly this approach, providing mechanistic details for hydrogen storage in CNFs involving atomic hydrogen, requires additional experimental and theoretical work to substantiate these claims. However, it is hoped that these predictions stimulate further research interest and discussion regarding this subject. Acknowledgment. The authors are grateful to DERA and EPSRC for financial support, which has allowed the conduct of this program of research. We would also like to thank Mr. J. S. Bates for assistance with the electron microscopy Nano Lett., Vol. 2, No. 3, 2002
and Professor S. Fletcher, Dr. C. E. Madden, and Dr. Y. F. Yin for their useful discussions. References (1) Wang, Q.; Johnson, J. K. J. Phys. Chem. B 1999, 103, 4809. (2) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D.; Smith, K. A.; Smalley, R. E. Appl. Phys. Lett. 1999, 74, 2307. (3) Darkrim, F.; Levesque, S. J. Phys. Chem. B 2000, 104, 6773. (4) Yin, Y. F.; Mays, T.; McEnaney, B. Langmuir 2000, 16, 10521. (5) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (6) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127. (7) Lee, S. M.; Lee, Y. H. Appl. Phys. Lett. 2000, 76, 2877. (8) Lee, S. M.; Park, K. S.; Choi, Y. C.; Park, Y. S.; Bok, J. M.; Bae, D. J.; Nahm, K. S.; Choi, Y. G.; Yu, S. C.; Kim, N.; Frauenheim, T.; Lee, Y. H. Synth. Met. 2000, 113, 209. (9) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. Science 1999, 285, 91. (10) Yang, R. T. Carbon 2000, 38, 623. (11) Pinkerton, F. E.; Wicke, B. G.; Olk, C. H.; Tibbetts, G. G.; Meisner, G. P.; Meyer, M. S.; Herbst, J. F. J. Phys. Chem. B 2000, 104, 9460. (12) Nu¨tzenadel, C.; Zu¨ttel, A.; Chartouni, D.; Schlapbach, L. Electrochem. Solid State Lett. 1999, 2, 30. (13) Rajalakshmi, N.; Dhathathreyan, K. S.; Govindaraj, A.; Satishkumar, B. C. Electrochim. Acta 2000, 45, 4511. (14) Zhu, H. W.; Chen, A.; Mao, Z. Q.; Xu, C. L.; Xiao, X.; Wei, B. Q.; Liang, J.; Wu, D. H. J. Mater. Sci. Lett. 2000, 19, 1237. (15) Dresselhaus, M. S.; Williams, K. A.; Eklund, P. C. MRS Bull. 1999, 24, 45. (16) Cheng, H. M.; Liu, C.; Fan, Y. Y.; Li, F.; Su, G.; Cong, H. T.; He, L. L.; Liu, M. Z. Metallkd. 2000, 91, 306. (17) Ahn, C. C.; Ye, Y.; Ratnakumar, B. V.; Witham, C.; Bowman, R. C., Jr.; Fultz, B. Appl. Phys. Lett. 1998, 73, 3378. (18) Stro¨bel, R.; Jo¨rissen, L.; Schliermann, T.; Trapp, V.; Schu¨tz, W.; Bohmhammel, K.; Wolf, G.; Garche, J. J. Power Sources 1999, 84, 221. (19) Fan, Y. Y.; Liao, B.; Liu, M.; Wei, Y. L.; Lu, M. Q.; Cheng, H. M. Carbon 1999, 37, 1649. (20) Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez, N. M. J. Phys. Chem. B 1998, 102, 4253. (21) Park, C.; Anderson, P. E.; Chambers, A.; Tan, C. D.; Hidalgo, R.; Rodriguez, N. M. J. Phys. Chem. B 1999, 103, 10572. (22) Klyamkin, S. N.; Sklovsky, D. E.; Metenier, K.; Bonnamy, S.; Be´guin, F. Mol. Mater. 2000, 13, 367. (23) Gupta, B. K.; Srivastava, O. N. Int. J. Hydrogen Energy 2000, 25, 825.
Nano Lett., Vol. 2, No. 3, 2002
(24) Wang, Q.; Johnson, J. K. J. Phys. Chem B 1999, 103, 277. (25) Orimo, S.; Meyer, G.; Fukunaga, T.; Zu¨ttel, A.; Schlapbach, L.; Fujii, H. Appl. Phys. Lett. 1999, 75, 3093. (26) Rodriguez, N. M. J. Mater. Res. 1993, 8, 3233. (27) Rodriguez, N. M.; Chambers, A.; Baker, R. T. K. Langmuir 1995, 11, 3862. (28) Krishnankutty, N.; Park, C.; Rodriguez, N. M.; Baker, R. T. K. Catal. Today 1997, 37, 295. (29) Best, R. J.; Russell, W. W. J. Am. Chem. Soc. 1954, 76, 838. (30) Che, G.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R.; Ruoff, R. S. Chem. Mater. 1998, 10, 260. (31) Mellor, I. M.; Mortimer, R. J.; Turpin, M. C. Proceedings of the 24th Biennial Conference on Carbon, Charleston, SC, 11-16th July. American Carbon Society 1999, 2, 622. (32) McCarty, R. D. Hydrogen: Its Technology and Implications. In Hydrogen Properties Volume III; CRC Press: Boca Raton, Florida, 1975; pp 47. (33) Cracknell, R. F. Phys. Chem. Chem. Phys. 2001, 3, 2091. (34) This value represents an estimated upper limit for volumetric storage. To accommodate significant hydrogen within the CNF structure, expansion of the graphite interlayer spacing is required. As a consequence of this, the nanofiber volume increases, resulting in a decrease in density. Ultimately, this is detrimental toward total volumetric storage, lowering it from the calculated 45 kg H2/m3 reported. (35) Noh, J. S.; Agarwal, R. K.; Schwarz, J. A. Int. J. Hydrogen Energy 1987, 12, 693. (36) Clearly any improvement in volumetric storage through the enhancement of bulk density is offset by the adverse effect upon the gravimetric uptake. This is undesirable considering present sorption levels, as 6.54 wt % only marginally exceeds the DOE target. Therefore, a sacrifice has to be made when attempting to optimize volumetric over gravimetric storage, or vice versa. However, assuming a similar nanofiber density as found here, future development obtaining CNFs exhibiting a capacity of approximately 9 wt % overcomes this problem, as both criteria can be satisfied simultaneously. (37) Chahine, R.; Bose, T. K. Int. J. Hydrogen Energy 1994, 19, 161. (38) Ishikawa, Y.; Austin, L. G.; Brown, D. E.; Walker, P. L., Jr. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Dekker: New York, 1975; Vol. 12, p 39. (39) Tibbetts, G. G.; Gregory, P.; Olk, H. Carbon 2001, 39, 2291. (40) Dresselhaus, M. S.; Dresselhaus, G. AdV. Phys. 1981, 30, 139. (41) Watanabe, M.; Tachikawa, M.; Osaka, T. Electrochim. Acta 1997, 42, 2707. (42) Mellor, I. M., in preparation.
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