VOLUME 102, NUMBER 22, MAY 28, 1998
© Copyright 1998 by the American Chemical Society
LETTERS Hydrogen Storage in Graphite Nanofibers Alan Chambers, Colin Park, R. Terry K. Baker, and Nelly M. Rodriguez* Department of Chemistry, Hurtig Hall, Northeastern UniVersity, Boston, Massachusetts 02115 ReceiVed: NoVember 6, 1997; In Final Form: April 6, 1998
Graphite nanofibers are a novel material that is produced from the dissociation of carbon-containing gases over selected metal surfaces. The solid consists of very small graphite platelets, 30-500 Å in width, which are stacked in a perfectly arranged conformation. We have discovered that the material is capable of sorbing and retaining in excess of 20 L (STP) of hydrogen per gram of carbon when the nanofibers are exposed to the gas at pressures of 120 atm at 25 °C, a value that is over an order of magnitude higher than that found with conventional hydrogen storage systems. This behavior is rationalized in terms of the unique crystalline arrangement existing within the graphite nanofiber structure, where the platelets generate a system comprised entirely of slit-shaped nanopores, in which only edge sites are exposed. Since the interplanar distance within the material is 3.37 Å, sorption of molecular hydrogen, which possesses a kinetic diameter of 2.89 Å, is a facile process owing to the short diffusion path. In addition, owing to the weak (van der Waals) bonding of the platelets, these nonrigid wall nanopores can expand to accommodate hydrogen in a multilayer configuration. Subsequent lowering of the pressure to nearly atmospheric conditions results in the release of a major fraction of the stored hydrogen at room temperature.
Hydrogen storage has been the subject of extensive research for many years as evidenced by the vast literature on the subject.1-6 One method that has been rigorously studied involves the use of metals and alloys, where the solids are reacted with hydrogen to form metal hydrides. Another alternative to the storage problem involves the use of cryogenic conditions for adsorption on various solids including carbon.7-9 Schwarz and co-workers studied the adsorption of hydrogen on molecularly engineered carbon at -150 °C8,9 and reported that this material exhibited a capacity of 0.5 g of H2/kg of carbon at 20 atm pressure. In a recent publication, Dillon and coworkers10 used 1 mg of unpurified soot that consisted of a mixture of unidentified carbonaceous materials, purported to contain 0.1-0.2 wt % of single-walled carbon nanotubes as well as a large fraction of cobalt catalyst particles (∼20 wt %), and claimed that this sample adsorbed 5% of hydrogen at 0 °C. * To whom correspondence should be addressed.
An ideal solid for hydrogen storage would possess a structure consisting of slit-shaped nanopores having a width slightly higher than the kinetic diameter of hydrogen, 2.89 Å. To this end, metal-intercalated graphite has been considered as a possible candidate for the storage of hydrogen.11,12 It has been reported that up to 0.137 L (STP) of hydrogen per gram of carbon can be adsorbed between the layers of alkali-intercalated graphite,13,14 and no evidence of damage to the structure upon repeated adsorption and desorption cycles was observed at liquid nitrogen temperature. The interaction of molecular hydrogen and its various isotopes with graphite surfaces has been the subject of extensive research.15-17 Experimental and theoretical studies have indicated that hydrogen chemisorbed on graphite adopts a symmetrical x3 × x3 superstructure at submonolayer coverage. At near monolayer coverage, this configuration disappears and is replaced by an incommensurate triangular phase.
S1089-5647(98)00114-X CCC: $15.00 © 1998 American Chemical Society Published on Web 05/12/1998
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Figure 1. (a) Schematic representation of the arrangement of platelets in a catalytically grown graphite nanofiber; (b) an enlarged section showing the detail of area marked in (a).
In recent years, graphite nanofibers (GNF), a new type of carbon material, have been developed in our laboratory. These structures possess a cross-sectional area that varies from 30 to 500 Å2 and have lengths of between 10 and 100 µm. GNF were prepared by the catalyzed decomposition of carboncontaining gases and their mixtures over selected metal and alloy surfaces at temperatures over the range 450-750 °C as described in previous papers.18-20 Some of the key features of the growth process were first elucidated from dynamic studies using controlled atmosphere electron microscopy.21 The fibrous solid formed during these reactions, consists of graphite platelets perfectly aligned parallel to the precipitating faces of the metal particle.22 We have been able to produce nanofibers where the graphite platelets are arranged parallel, perpendicular, or at an angle with respect to the fiber axis. In this paper these structures are designated as “tubular”, “platelet” and “herringbone”, respectively. In these various conformations, the layers are separated at distances that are dependent upon the nature of the catalyst and the gas phase as well as the reaction conditions, where the minimum value possible is that of single-crystal graphite, 3.35 Å. The unique conformation of GNF, consisting of platelets having virtually only edges exposed, bestows this material with unique properties that are highly desirable for gas sorption applications. Indeed, these structures comprise an array of slitshaped pores separated by a distance g3.35 Å and, as such, possess the ideal configuration for use in the storage of hydrogen, whose kinetic diameter is only 2.89 Å. A clearer understanding of the structural features of the nanofibers are shown in the schematic renditions, Figure 1a,b. X-ray diffraction analysis coupled with electron diffraction indicated that prior to adsorption experiments the spacing between platelets for nanofibers that were suitable for hydrogen storage was 3.37 Å. High-pressure sorption studies of various types of graphite nanofibers were performed in a specially built apparatus that is shown schematically in Figure 2. The unit consists of a conventional high-pressure stainless steel sample cell, measuring 22.66 mL including void space, which was connected to a 75.31 mL high-pressure hydrogen reservoir container via a highpressure bellow valve. Several blank tests were carried out over a 24 h period by pressurizing the unit, which confirmed that the system was leak-free. Calibration of the system to account for the pressure drop brought about by the increase in volume upon opening of the valve between the reservoir and the evacuated sorption cell at various initial pressures was also carried out at this stage. The volume of gas contained in the
Letters system at various pressures in the absence of graphite nanofibers was precisely determined by allowing it to exit the system and then measuring by displacement of water. Although the metal content of the “as-received” graphite nanofibers was less than 1%, prior to the adsorption experiments, the catalyst particles were removed from the graphite nanofiber structure by dissolution in mineral acid. Following this protocol, approximately 0.2 g of different types of graphite nanofibers, produced from a variety of metal/hydrocarbon systems, was loaded into the sorption cell, and the sample was evacuated to 10-3 Torr while being heated to 150 °C. Hydrogen was then introduced into the reservoir container and subsequently allowed access to the sample chamber for interaction with the nanofibers. The drop in pressure of the system from an initial value of up to 112 atm was measured at regular intervals with a Cole Parmer meter. The amount of hydrogen stored in graphite nanofibers was calculated from the changes in pressure following interaction of the material with the gas. In a second set of experiments, an identical procedure was used to determine the hydrogen sorption characteristics of palladium powder, two metal alloys (LaNi5 and MnNi4.5Al0.5), and active carbon and graphite. In all cases the experiments were performed at room temperature. When purified graphite nanofibers samples were allowed to interact with hydrogen at 25 °C and an initial pressure of 112 atm, a drop in the pressure was observed over a period of 24 h, the degree of which was found to be dependent upon the origin of the sample, and this feature is presented in Figure 3, where the profiles of a selected number of samples are shown. The volumes (STP) and corresponding weight percentages of hydrogen uptake by these and other samples as determined from pressure decline are given in Table 1. The data obtained from these experiments for graphite nanofibers can be compared with those found for the other set of materials. In these latter systems it is apparent that only modest changes in the pressure were observed over the same time period. The dramatic decrease in pressure that occurs in the presence of certain types of graphite nanofibers is at first sight overwhelming. Indeed, in the best case a reduction of 71 atm was achieved in the presence of only 0.2914 g of nanofibers. The reasons for the observed variations in the hydrogen adsorption capacities of similar types of nanofibers are currently being investigated. When the pressure of the system reached steady state, stored hydrogen was allowed to exit by opening a regulating valve, which enabled one to control both the pressure and rate of release of the gas. The volume of the desorbed fluid was subsequently measured by displacement of water. Analysis of samples of the discharged gas by GC-MS indicated that hydrogen was the only component present, dismissing the possibility that any catalyzed or uncatalyzed reactions had occurred between the graphite nanostructures and hydrogen during any of the operations. The difference in the volume of hydrogen recovered following interaction with graphite nanofibers was compared to the volume present in the system in the absence of the solid at equivalent pressures. This procedure enabled us to make an assessment of the amount of reversibly sorbed hydrogen stored in the nanofibers. The rate of release at room temperature of this fraction of hydrogen was, in most cases, extremely fast (5-10 min). This process was, however, normally conducted in a controlled fashion over a period of 1 h, to ensure that the integrity of the graphite nanofiber structure was preserved for subsequent adsorption cycles. Examples of these desorption data, which were obtained from a separate set of experiments, are presented in Table 2. It was of interest to find that under the same conditions no hydrogen was released
Letters
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Figure 2. Schematic representation of the apparatus used for the measurement of hydrogen uptake in various materials.
TABLE 2: Desorption Data for Selected Carbons and Hydridesa sample
sample weight (g)
released H2 (L/g)
released H2 χH2 × 100
Pd LaNi5 MnNi4.5Al0.5 GNF herringbone GNF herringbone GNF herringbone GNF herringbone
0.2229 0.9956 0.7420 0.1090 0.1080 0.1032 0.1037
0.15 0.03 0.28 15.69 8.61 9.89 10.10
0.66 0.13 1.27 58.37 43.48 44.15 45.09b
b a Weight fraction: χ H2 ) wtH2/(wtH2 + wtadsorbent). The adsorption and desorption data were obtained using the same sample.
Figure 3. Change in hydrogen pressure as a function of time in the presence of b, aPd; ], bLaNi5; !, cMnNi4.5Al0.5; O, dNorit activated carbon; 0, eGNF tubular; [, fGNF herringbone; 2, gGNF herringbone; ", hGNF herringbone. Superscripts correspond to data presented in Table 1.
TABLE 1: Adsorption Data for Selected Carbons and Hydridesi sample a
Pd
bLaNi
5
c
MnNi4.5Al0.5 dactivated carbon graphite tubular fGNF herringbone gGNF herringbone hGNF herringbone GNF platelet GNF platelet eGNF
sample weight (g)
adsorbed H2 (L/g)
adsorbed H2 χH2 × 100
0.2229 1.0582 0.7420 0.8979 0.2434 0.1214 0.3188 0.2914 0.1037 0.1032 0.1014
0.24 0.05 0.38 0.18 0.53 1.42 13.35 23.33 13.57 12.98 9.37
2.07 0.44 3.33 1.63 4.52 11.26 57.85 67.55 60.58j 53.68 45.63
i Weight fraction: χ j H2 ) wtH2/(wtH2 + wtadsorbent). The adsorption and desorption data were obtained using the same sample.
from the palladium or alloy samples until the temperature was raised to 300 °C. In all cases, it was observed that there was a difference in the amount of hydrogen adsorbed and desorbed at room temperature from the graphite nanofibers, suggesting the possible existence of chemisorbed hydrogen. In an attempt to gain a deeper insight into this phenomenon, samples of graphite nanofibers that had ostensibly been discharged of hydrogen were examined using temperature-programmed desorption (TPD). In this step nitrogen was passed over the sample as the temperature was ramped at 10 °C/min, and the evolved gases were measured using a TCD detector. It is evident from these data that a
significant fraction of hydrogen is retained in the nanofibers and moreover, since high temperatures are required for complete desorption, one may conclude that chemical forces are involved in the adsorption of this strongly held hydrogen. There is no doubt that the unique structural conformation of graphite nanofibers constitutes one of the most effective media for the sorption and retention of vast amounts of hydrogen. In addition, owing to the small cross-sectional area of graphite platelets within the structure coupled with the enormous number of edges, diffusion limitations can be easily overcome. The extraordinary hydrogen storage capacity exhibited by graphite nanofibers is not totally unexpected in view of the calculations performed by Gubbins23,24 and Steele25 that are based on modern statistical mechanical fluids theory and computer simulation, which have indicated that when fluids are confined within narrow nanopores, they display behavior that does not conform to that predicted by classical thermodynamic methods26 such as Kelvin, BET, and Dubinin. Once the hydrogen molecules are confined within the pores, a strong interaction with both the graphite walls and other neighboring molecules will be established, leading to phase transitions and capillary condensation at abnormally high temperatures. Moreover, we believe that during the adsorption process, expansion of the graphite lattice occurs, a process that allows for multilayer coverage within the mobile pore walls. Under these circumstances, mobility is suppressed and the fluid adopts a liquidlike characteristic, which accounts for the extremely high adsorption capacity of the solid. It is therefore probable that the presence of delocalized π-electrons on the graphite layers is a major contributory factor to the enhancement of the hydrogen adsorption observed in the present system. Although the absolute density of graphite nanofibers can approach that of single-crystal graphite,19 the packing density of the material is considerably smaller than this value. The lack and difficulty of obtaining direct data regarding the final volume of the hydrogen-soaked nanofibers following the
4256 J. Phys. Chem. B, Vol. 102, No. 22, 1998 adsorption step makes it impossible to estimate with any degree of certainty the density of hydrogen. Despite this shortcoming, we believe that this value is quite high. Current research efforts are being focused on the use of spectroscopic techniques in an attempt to obtain information on the perturbations to the structure of graphite nanofibers in the presence of adsorbed hydrogen, as well as the possibility of detecting the existence of charge-transfer reactions. Acknowledgment. The authors thank H-Power Corporation of New Jersey for providing metal alloy samples and gratefully acknowledge permission to publish from Catalytic Materials Limited. References and Notes (1) Maeland, A. J. In Hydrides for Energy Storage; Andressen, A. F., Maeland, A. J., Eds.; Pergamon: Oxford, 1978; p 447. (2) Shilov, A. L.; Padurets, L. N.; Kost, M. E. Russ. J. Phys. Chem. 1985, 58, 1103. (3) Bambakidis, G.; Bowman, R. C., Jr. Hydrogen in Disordered and Amorphous Solids; Plenum: New York, 1986. (4) Maeland, A. J. In Metal Hydrides; Bambakidis, G., Ed.; Plenum: New York, 1981; p 177. (5) Suzuki, K. J. Less-Common Met. 1983, 89, 183. (6) Bowman, R. C., Jr. In Hydrogen Storage Materials; Materials Science Forum; Barnes, R. G., Ed.; Trans Tech Publications: Aedermannsdorf, Switzerland, 1988.
Letters (7) U.S. Patent 4,580, 404. (8) Amankwah, K. A. G.; Schwarz, J. A. Int. J. Hydrogen Energy 1989, 14, 437. (9) Jagiello, J.; Bandosz, T. J.; Putyera, K.; Schwarz, J. A. J. Chem. Soc., Faraday Trans. 1995, 91, 2929. (10) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (11) Akuzawa, N.; Amari, Y.; Nakajima, T. J. Mater. Res. 1990, 5, 2849. (12) Ichimura, K.; Sano, M. J. Vac. Sci. Technol. A 1992, 10, 543. (13) Watanabe, K.; Soma, M.; Ohishi, T.; Tamaru, K. Nature 1971, 233, 160. (14) Lagarnge, P.; Metror, A.; Herold, A. Comp. Rend. 1971, 275, 160. (15) Freimuth, H.; Wiechert, H.; Lauter, H. J. Surf. Sci. 1987, 189, 548. (16) Novaco, A. D. Phys. ReV. Lett. 1988, 60, 2058. (17) Cui, J.; Fain, S. C.; Jr., Freimuth, H.; Wiechert, H.; Schildberg, H. P.; Lauter, H. J. Phys. ReV. Lett. 1988, 60, 1848. (18) Kim, M. S.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1991, 131, 60. (19) Rodriguez, N. M. J. Mater. Res. 1993, 8, 3233. (20) Krishnankutty, N.; Rodriguez, N. M.; Baker, R. T. K. Catal. Today 1997, 37, 295. (21) Baker, R. T. K.; Barber, M. A.; Feates, F. S.; Harris, P. S.; Waite, R. J. J. Catal. 1972, 30, 86. (22) Rodriguez, N. M.; Chambers, A.; Baker, R. T. K. Langmuir 1995, 11, 3862. (23) Cracknell, R. G.; Gubbins, K. E.; Maddox, M.; Nicholson, D. Acc. Chem. Res. 1995, 28, 281. (24) Balbuena, P. B.; Gubbins, K. E. Langmuir 1993, 9, 1801. (25) Rao, M. B.; Jenkins, R. G.; Steele, W. A. Langmuir 1985, 1, 137. (26) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982.
