Hydrogen Storage in Graphite Nanofibers: Effect of Synthesis Catalyst

Dec 23, 2003 - The nanofibers possessing a herringbone structure and a high degree of defects were found to exhibit the best performance for hydrogen ...
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Langmuir 2004, 20, 714-721

Hydrogen Storage in Graphite Nanofibers: Effect of Synthesis Catalyst and Pretreatment Conditions Angela D. Lueking,†,‡ Ralph T. Yang,*,† Nelly M. Rodriguez,§ and R. Terry K. Baker§ Department of Chemical Engineering, 2300 Hayward, University of Michigan, Ann Arbor, Michigan 48109, and Catalytic Materials LLC, 1750 Washington Street, Holliston, Massachusetts 01746 Received June 4, 2003. In Final Form: October 17, 2003 A series of graphite nanofibers (GNFs) that were subjected to various pretreatments were used to determine how modifications in the carbon structure formed during either synthesis or pretreatment steps results in active or inactive materials for hydrogen storage. The nanofibers possessing a herringbone structure and a high degree of defects were found to exhibit the best performance for hydrogen storage. These materials were exposed to several pretreatment procedures, including oxidative, reductive, and inert environments. Significant hydrogen storage levels were found for several in situ pretreatments. Examination of the nanofibers by high-resolution transmission electron microscopy (TEM) after pretreatment and subsequent hydrogen storage revealed the existence of edge attack and an enhancement in the generation of structural defects. These findings suggest that pretreatment in certain environments results in the creation of catalytic sites that are favorable toward hydrogen storage. The best pretreatment resulted in a 3.8% hydrogen release after exposure at 69 bar and room temperature.

Introduction The development of a hydrogen storage material capable of meeting the U.S. Department of Energy storage target of 6.5% would speed the commercialization of fuel cell vehicles, which in turn could alleviate the dependence on fossil fuels while reducing air emissions. One candidate for a hydrogen storage medium is based on a certain type of graphite nanofiber that appears to have the potential to store hydrogen under moderate temperature and pressure conditions. Research in this area has been reviewed elsewhere.1-4 Early claims of hydrogen storage implied 5-10% storage at moderate temperatures and pressures,5 but subsequent claims have illustrated the necessity of high pressure and/or cryogenic conditions.6,7 Other hydrogen storage claims in carbon materials have been highly disputed due to artifacts such as water contamination,8,9 metal contamination10,11 and/or tem* To whom correspondence should be addressed: Phone (734) 936-0771; fax (734) 764-7453; e-mail [email protected]. † University of Michigan. ‡ Present address: Department of Energy and Geo-Environmental Engineering, Pennsylvania State University, University Park, PA 16802. § Catalytic Materials LLC. (1) Darkrim, F. L.; Malbrunot, P.; Tartaglia, G. P. Int. J. Hydrogen Energy 2002, 27, 193. (2) Yang, R. T. Adsorbents: Fundamentals and Applications; Wiley: New York, 2003; Chapter 10. (3) Ding, R. G.; Lu, G. Q.; Yan, Z. F.; Wilson, M. A. J. Nanosci. Nanotech. 2001, 1, 7. (4) Dillon, A. C.; Heben, M. J. Appl. Phys. A 2001, 72, 133. (5) Dillon, A. C.; Jones, K. M.; Bewkkedahl, T. A.; Kiang, C. H.; Bethune, D. S. 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) Ye, Y.; Ahn, C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D.; Smith, K. A.; Smalley, R. E. Appl. Phys. Lett. 1999, 74, 2307. (8) Yang, R. T. Carbon 2000, 38, 623. (9) 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. (10) Hirscher, M.; Becher, M.; Haluska, M.; Dettlaff-Weglikowska, U.; Quintel, A.; Duesberg, G. S.; Choi, Y.-M.; Downes, P.; Hulman, M.; Roth, S.; Stepanek, I.; Bernier, P. Appl. Phys. A 2001, 72, 129.

perature effects.12 In 1998, Chambers et al.13 reported hydrogen storage in graphite nanofibers (GNF) of 20 L (STP) of hydrogen per gram (∼67%); in 1999, Park et al.14 reported up to 40% in these same materials while emphasizing the importance of adequate pretreatment to allow access of hydrogen to the pores in the GNF. These two reports remain contested, as several other laboratories have been unable to reproduce these results.12,15,16 Despite these doubts, there has been a recent resurgence in reports of hydrogen storage in GNF, ranging from 6.5% at 12 MPa (10 bar)17 to 10-15% at 120 atm (122 bar).18,19 These latest reports have highlighted the changes occurring in the GNF structure following exposure to high-pressure hydrogen.19 There have also been suggestions that the presence of defects in the GNF structure may play a role in the dissociation of hydrogen leading to subsequent intercalation.17 There have also been numerous papers dealing with hydrogen storage in multiwall carbon nanotubes (MWNT). The term MWNT is often used interchangeably with GNF, since the former materials are frequently constituted of tilted or conical tubes that closely resemble the structure of herringbone GNF. Recent claims of hydrogen storage in MWNT include 1.97% at 40 bar,20 3.7% at 69 bar,21 4% (11) Lueking, A.; Yang, R. T. J. Catal. 2002, 206, 165. (12) Tibbetts, G. G.; Meisner, G. P.; Olk, C. H. Carbon 2001, 39, 2291. (13) Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez, N. M. J. Phys. Chem. B 1998, 102, 4253. (14) Park, C.; Anderson, P. E.; Chambers, A.; Tan, C. D.; Hidalgo, R.; Rodriguez, N. M. J. Phys. Chem. B 1999, 103, 10572. (15) Ahn, C. C.; Ye, Y.; Ratnakumar, B. V.; Witham, C.; Bowman, R. C.; Fultz, B. Appl. Phys. Lett. 1998, 73, 3378. (16) Strobel, R.; Jorissen, L.; Schliermann, T.; Trapp, V.; Schu¨tz, W.; Bohmhammel, K.; Wolf, G.; Garche, J. J. Power Sources 1999, 84, 221. (17) Browning, D. J.; Gerrard, M. L.; Lakeman, J. B.; Mellor, I. M.; Mortimer, R. J.; Turpin, C. Nanoletters 2002, 2, 201. (18) Gupta, B. K.; Srivastava, O. N. Int. J. Hydrogen Energy 2001, 26, 857. (19) Gupta, B. K.; Srivastava, O. N. Int. J. Hydrogen Energy 2000, 25, 825. (20) Lee, H.; Kang, Y.-S.; Kim, S.-H.; Lee, J.-Y. Appl. Phys. Lett. 2002, 80, 577. (21) Lueking, A.; Yang, R. T. AIChE J. 2003, 49, 1556.

10.1021/la0349875 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/23/2003

Hydrogen Storage in Graphite Nanofibers

at 100 bar,22 and 6.3% at 148 bar.23 In all these cases it has been shown that the structure and purification of the MWNT are key factors for attaining optimum hydrogen storage levels. It is clear that there is a wide variation in the degree to which carbon structures can store hydrogen, and many of the reports are still disputed. The main objective of this paper was to determine the difference between GNF that are active for hydrogen storage and those that are inactive. To this end, the behavior of GNF produced from two different sources with diverse synthesis catalysts was investigated. The distinction between the catalysts used in generating these two materials and the resulting properties imposed upon the GNF can provide insights into the manner by which different catalysts produce either active or inactive structures. A secondary objective of the investigation was to ascertain the effect of various pretreatments on the hydrogen storage properties and to examine the concomitant modifications on the carbon structure. Thus, the overriding goal of this paper is to begin to explore how different attributes of GNF that arise either during the synthesis or pretreatment steps exert a positive or negative impact on the hydrogen storage properties of the material. A further aspect that must be taken into consideration is the fundamental differences in the methods used to perform the adsorption measurements, and the merits of the procedure used here are discussed. It should be stressed that the “herringbone” graphite nanofibers used in the present investigation were produced from a different catalyst formulation than that used in refs 13 and 14. While the respective materials possessed the same basic structural characteristics, the earlier nanofibers contained a large number of defect regions that appear to be desirable features for hydrogen adsorption. Furthermore, the hydrogen adsorption/desorption protocol followed here was not the same as that used in refs 13 and 14. Experimental Methods Materials. All gases were obtained from Cryogenic gases and had the following purities: CP-grade ethylene (99.5%), dry-grade air (99.99%), prepurified-grade helium (99.8%), and ultrahighpurity hydrogen (99.999%). Helium and hydrogen were both passed through a 3A zeolite (Grace Davison, Grade 564, 8-12 mesh) column for moisture removal prior to use in the highpressure apparatus. Preparation of Graphite Nanofibers. GNF-FC was synthesized by passing an ethylene-hydrogen (1:4) mixture over a 3:7 copper-iron catalyst at 600 °C.24 Two separate acid treatments were used to remove the associated metal catalyst from the GNF-FC. In one case, the nanofibers were treated in 6 N HNO3, and in the other, the materials were dispersed in 1 N HCl for 24 h at a ratio of 50 mL of acid/g of catalyst. Herringbone GNF structures were produced from the interaction of a 2:8 copper-nickel alloy (F166) and a 1:9 copper-nickel alloy (F168), respectively, with an ethylene-hydrogen (4:1) mixture at 600 °C. The herringbone GNF were not acid-treated, as the metal content was inherently low. Characterization Studies. The bimetallic composition of the various catalysts used for the synthesis of GNF was determined from measurements by neutron activation analysis (NAA) using P-tube irradiation for magnesium, copper, and aluminum analysis and in-core irradiation for nickel, iron, and molybdenum. Surface area and pore size analysis of the nano(22) Li, X.; Zhu, H.; Ci, L.; Xu, C.; Mao, Z.; Wei, B.; Liang, J.; Wu, D. Carbon 2002, 39, 2077. (23) Hou, P.-X.; Yang, Q.-H.; Bai, S.; Xu, S.-T.; Liu, M.; Cheng, H.-M. J. Phys. Chem. B 2002, 106, 963. (24) Krishnakutty, N.; Park, C.; Rodriguez, N. M.; Baker, R. T. K. Catal. Today 1997, 37, 295.

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Figure 1. High-pressure adsorption/desorption apparatus. structured carbon materials were obtained with a Micromeritics ASAP 2000 unit using nitrogen at -196 °C; standard methods were used for BET surface area analysis and pore size distribution analysis. Prior to measurement, the samples were degassed at 150 °C in a vacuum. TEM examinations of the materials were performed in a JEOL 2000EXII instrument (point-to-point resolution of 0.18 nm). Transmission specimens were prepared by ultrasonic dispersion of a small quantity of a given sample in isobutanol and then application of a drop of the suspension onto a holey carbon film. It was possible to obtain images without interference from the underlying substrate by locating sections of the nanofibers that protruded over the edge of the carbon film. High-resolution examinations were carried out on several regions of a given specimen and micrographs were taken of the typical appearance of the various nanofibers. A closed circuit TV system on the TEM allowed images to be viewed on a monitor at magnifications in excess of 12 million times. High-Pressure Sorption Experiments. High-pressure hydrogen uptakes were assessed with a specially designed volumetric adsorption/desorption system (Figure 1). The unit was custom-built to withstand high-pressure conditions up to 69 bar (1000 psia) while maintaining leak-free conditions by using VCR face seal compression fittings (H. E. Lennon). The unit consisted of a 6.1 mL sample cell module connected to a variable volume reservoir; the standard reservoir volume used was 34.1 mL, however, this could be adjusted through the use of an additional compartment. The relative volumes for the sample cell and desorption reservoir were chosen on the basis of an uncertainty analysis from an error propagation calculation (UNCANAL, University of Michigan). The Bellows valves (4BK, H. E. Lennon) were used to seal the high-pressure portion of the apparatus. The outlet of the sample cell was directed to either a vacuum for pretreatment or an in-line two-stage gas pressure regulator to allow flow through the system. Pressure was measured with an ultrahigh purity pressure transducer (Hi-Tech) connected to a DP41-E panel meter (Omega). The accuracy of the pressure reading was 0.1% of full scale or 1 psi. The sample cell consisted of a stainless steel tube sealed with 0.5 µm filtering gasket (H. E. Lennon). High-temperature valves (Kel-F, H. E. Lennon) at the outlet of the sample cell formed a system that could be sealed and detached to facilitate pretreatment. Secondary valves outside the sample cell module were used to maintain “pressure locks” to minimize leakage across valve seats prior to desorption experiments. Additional in-line VCR filters were placed between these double valves in order to exclude nanofiber contamination of the reservoir. Materials for construction of the sample cell module, including valves and fittings, were chosen such that the vessel could be heated to 500 °C using an external source to outgas or activate the samples. A thermocouple welded into a VCR fitting was used to calibrate an external thermocouple; the internal welded thermocouple was removed from the system prior to high-pressure readings in order to eliminate leakage. The volumes of the system, both with and without samples, were calibrated by attaching a reservoir with known volume to the sorption reservoir. Hydrogen compressibility factors were confirmed via desorption experiments with inactive samples; the best fit to these blank experiments was achieved when the Pitzer

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Table 1. Pretreatments Used for the GNF gas

temp (°C)

time

conditions in situ in high-pressure apparatus external treatment, transfer in glovebag in situ in high-pressure apparatus in situ in high-pressure apparatus external treatment, transfer in glovebag external treatment, transfer in glovebag in situ in high-pressure apparatus

PT1

helium

300

12+ h

PT2

air

550

10 min

PT3

air

550

10 min

PT4

air

350

12 h

PT5

argon

950

PT6

hydrogen

800

1h

PT7

hydrogen

700

1h

correlation was used to correct the compressibility factor at intermediate pressures and the compressibility factors reported by Darkrim et al.25 were used to correct pressures at 69 bar. Initial sample masses for the high-pressure experiments ranged from 100 to 200 mg. The different pretreatment protocols are defined in Table 1. Samples pretreated externally were transferred to the adsorption cell in a low-oxygen environment by use of a glovebag and then pretreated in situ with flowing helium at 150 °C. This secondary helium pretreatment was not used in the case of in situ pretreated samples. Typically, a series of experiments on the same sample were performed in an attempt to establish the reproducibility or “cyclability” of the measurement. Prior to a secondary run, the samples were pretreated at 500 °C in a vacuum. After pretreatment and prior to sorption studies, the integrity of the system was checked by charging the system with helium. At the conclusion of each series of desorption measurements, the sample was removed from the vessel and any changes in mass due to pretreatment were determined. Prior to desorption measurements, adsorption consisted of saturation in flowing hydrogen for 10 h at room temperature and 69 bar, a method used in previous studies.21 Hydrogen flow was maintained with the inlet of the in-line two-stage pressure regulator set at 69 bar and the outlet set at 5 psig (1.3 bar). After adsorption, the pressure and temperature conditions were recorded as the sample was sealed. The desorption reservoir was then depressurized and equilibrated, followed by depressurization of the pressure locks. Desorption was measured immediately after depressurization of the pressure locks in order to minimize potential leakage across valves. Room-temperature desorption was measured by allowing the gas in the sample cell to expand into the desorption reservoir for a series of measurements. Each measurement required the pressure to be stable for 15 min before the next depressurization step. This process was repeated until the system pressure reached ∼1 bar. The final pressure was recorded after an additional 1012 h. Desorption measurements from each separate pressure expansion were summed to determine the total amount of hydrogen released. Thus, each intermediate desorption was not necessarily a true equilibrium measure, but the sum of the desorption steps reflects equilibrium. For each measurement, changes in pressure and fluctuations in room temperature were recorded periodically. Helium blanks were used to calibrate the system and to ascertain the effect of expansion on the pressure readings. After the final desorption measurement at 1 bar, the sample cell was heated to 300 °C, based on methods previously developed;21 however, no additional hydrogen desorption was typically observed after heating to this temperature.

Results and Discussion Surface Area of the Graphite Nanofibers. The surface areas of the GNF are highly dependent upon the type of structure, the reaction conditions, and the nature of the catalyst used in the growth process (Table 2) and (25) Darkrim, F.; Vermesse, J.; Malbrunot, P.; Levesque, D. J. Chem. Phys. 1999, 110, 4020.

Table 2. Characterization of Different GNF Materials

carbon fiber

catalyst

GNF-FC-HNO3 Fe:Cu ) 7:3 GNF-F16 6 Cu:Ni ) 2:8 GNF-F16 8 Cu:Ni ) 1:9

BET SA (m2/g) 44.7 242 136

metal content (wt %)

gasification onset (°C) H2 741 630

air 480 510

Fe

Ni

Cu

3.4 BDL 0.77 BDL 2.0 0.42 0.01 2.9 0.36

these data are in accord with those reported previously.26 Comparisons with the surface areas found for other GNFs reported in the literature suggest that this parameter is highly dependent upon specific synthesis conditions. The surface area reported by Ahn et al.15 was an order of magnitude lower than that reported by Park et al.14 Similarly, the GNF generated in this investigation from the decomposition of ethylene over a Cu-Fe (3:7) catalyst had a surface area that was less than half that of the material produced from the decomposition of the same hydrocarbon over a Cu-Ni (2:8) catalyst. One might intuitively assume that a correlation would exist between the surface area as measured by nitrogen adsorption and the capacity of the GNF to adsorb hydrogen. Experimental results, however, do not support the existence of such a relationship. Ahn et al.15 and Strobel et al.16 reported that the GNF materials used in their respective studies exhibited high surface areas but displayed little ability to adsorb hydrogen. This theme was also highlighted by Browning et al.:17 the hydrogen storage levels for the GNF exceeded that expected from the external surface area as measured by nitrogen adsorption by a factor of 3. Clearly, one cannot use the value of the surface area as a criterion for predicting the ability of a given GNF structure to adsorb hydrogen. Structural Characteristics of the Graphite Nanofibers. It has been established that the nature of the catalyst particles plays a key role in determining the structural characteristics of GNF.27 While the diffusion of dissolved carbon through the bulk of the particle is the ratecontrolling step,28 it is the atomic arrangement of the faces that dictates the degree of crystalline perfection and orientation of the precipitated carbon.29,30 In this regard, it should be stressed that copper and nickel form alloys over the entire composition range, whereas copper and iron exhibit only limited miscibility.31 These differences in miscibility manifest themselves in both the reaction pathway as evidenced by the reaction products and the particle morphology. In addition, changes in the structural characteristics of the alloy exert an impact on the structure of nanofibers formed during the carbon deposition step.32,33 Graphite nanofibers grown from the Cu-Fe catalyst (GNF-FC) were found to exhibit a tubular structure (Figure 2), whereas the materials generated from the CuNi catalysts (GNF-F166 and GNF-F168) displayed a herringbone arrangement. It is apparent from the TEM micrographs that the F168 material possessed a higher (26) Baker, R. T. K.; Kim, M. S.; Chambers, A.; Park, C.; Rodriguez, N. M. Catalyst Deactivation: Stud. Surf. Sci. Catal. 1997, 111, 99. (27) Rodriguez, N. M.; Chambers, A.; Baker, R. T. K. Langmuir 1995, 11, 3862. (28) Baker, R. T. K. In Encyclopedia of Materials: Science and Technology; Elsevier Science Ltd.: Amsterdam, 2001; p 932. (29) Yang, R. T.; Chen, J. P. J. Catal. 1985, 115, 52. (30) Baker, R. T. K.; Barber, M. A.; Harris, P. S.; Feates, F. S.; Waite, R. J. J. Catal. 1972, 26, 51. (31) Krishnankutty, N.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1996, 158, 217. (32) Kim, M. S.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1991, 131, 60. (33) Kim, M. S.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1992, 154, 253.

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Figure 3. Effect of synthesis catalyst and pretreatment conditions on the ability of the GNF to desorb hydrogen after different pretreatments (see Table 1 for pretreatment notation).

Figure 2. As-produced GNF-FC showing tubular structure.

degree of crystallinity than the F166 sample. Additional support for this conclusion is provided by the temperatureprogrammed oxidation profiles that show the onset temperature of gasification in air is lower for the F166 material than that of the F168 material. Previous work demonstrated that GNF structures generated from the Cu-Ni (1:9) catalyzed decomposition of ethylene exhibited gasification characteristics that were very similar to those of graphite, whereas materials formed from the corresponding copper-rich alloys possessed a more disordered structure.33 Under these circumstances it follows that the F166 material is expected to be less graphitic than the F168 sample. Hydrogen Storage Measurements. The high-pressure hydrogen adsorption experiments reported in this investigation were designed to minimize possible experimental artifacts that are frequently encountered as a result of leakages, variations in calculation of the compressibility factor, changes in the internal temperature of the sample vessel due to expansion effects, and the effect of room temperature fluctuations on high-pressure measurements, features that were previously discussed by Tibbetts et al.12 In the present work, hydrogen desorption values are reported since such measurements are less vulnerable to possible leakage effects. Hydrogen blanks with inactive materials and helium blanks with activated materials were used in order to confirm the system integrity, calibrate the system, and delineate temperature effects of gas expansion. Typically, over 80% of the desorption occurred upon initial expansion of the sample, and was referenced to a helium blank (assuming that this gas does not adsorb on the carbon materials). This method serves to alleviate the concerns about temperature drift and other experimental artifacts associated with volumetric experiments. The present work exposes the sample to high-pressure hydrogen under flow conditions, in contrast to the static procedure commonly practiced. Relationship between Hydrogen Storage Capacity and Graphite Nanofiber Structure. The various types of GNF resulting from the preparation conditions were used to assess how the structural characteristics of

such materials influence the subsequent hydrogen storage performance. Initially, the hydrogen desorption behavior of F166 [prepared from Cu-Ni (2:8)] was compared to GNF-FC [prepared from Cu-Fe (3:7)] after two pretreatments. Following an in situ pretreatment of flowing helium at 300 °C (PT1), F166 had a 1.5% hydrogen desorption value compared to a level of 0.4-0.5% for the GNF-FC sample series (Figure 3). Argon pretreatment at 950 °C (PT5) resulted in a value of 0.85% for F166 versus 0.40.5% for the GNF-FC. The herringbone structure of the F166 sample is expected to be more favorable for hydrogen storage when compared to the tubular structure of GNFFC (Figure 2). High-resolution examination revealed that relatively few edge regions were exposed to the gas phase in the tubular structure and these materials tend to possess a high degree of crystalline perfection (i.e., they contain very few defect regions). In contrast, the herringbone structure of F166 possesses numerous exposed edge regions and a significant number of defects. It is an interesting exercise to compare the effect of the catalysts used in this work to synthesize GNF materials with those that can be discerned from the literature. The low hydrogen storage levels reported by Ahn et al.15 were all obtained for GNF structures grown from Fe-Cu catalyst particles. On the other hand, the catalysts used to prepare GNF used by Chambers et al.13 and Park et al.14 were not directly disclosed. Recent reports of significant hydrogen storage levels are for GNF materials generated from a Cu-Ni catalyst18,19 or an Fe-Ni-Cu system.17 The precise composition of the Cu-Ni used for the GNF synthesis also plays a critical role determining the structural characteristics of the material and the subsequent hydrogen storage performance. The nanofibers produced from a Cu-Ni (1:9) catalyst showed lower hydrogen storage capacity after two pretreatments (Figure 3). When treated in helium at 300 °C (PT1), the F166 sample (prepared from Cu-Ni ) 2:8) had an uptake of 1.5 wt % compared to a 1.0 wt % value for F168 (prepared from Cu-Ni ) 1:9). After air pretreatment at 550 °C (PT3), the F166 material had a hydrogen uptake of 2.7 wt % compared to a hydrogen uptake of 0.59% for the F168 sample (Figure 3). These findings are consistent with those of Browning et al.,17 who reported that as the nickel content in an Fe-Ni-Cu trimetallic catalyst was progressively raised, so the hydrogen storage capacity of the GNF materials produced from this system increased by a factor of 2.5. A further factor that must be taken into consideration is the fraction of defects that are present in the various GNF structures. The creation of edge dislocations causes a spreading of adjacent layers in the crystal structure,

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Lueking et al. Table 3. Hydrogen Desorbed from Herringbone GNF-F166 Carbon Fibera after Various Pretreatments hydrogen desorbed, wt % pretreatment

cycle 1

cycle 2*

cycle 3

cycle 4

PT1 PT2 PT2 PT2 PT3 PT3 PT4 PT5 PT6 PT7 PT7

1.44 2.31 0.90 0.75 3.43 3.13 3.39 0.91 1.24 3.51 1.83

1.35 1.92 1.87 0.72 3.81 2.82 2.01 0.86 1.08 3.80 2.26

3.69 2.52 2.36

3.71 2.72 2.17

1.75

3.29 1.80

a

Figure 4. Hydrogen storage for GNF-F166 after several different pretreatments (see Table 1 for pretreatment notation). PT2 and PT3 were performed multiple times, with a separate entry on the x-axis denoting a new sample. The fibers retained their hydrogen storage capacity after multiple hydrogen exposures; this cyclability of the fibers is represented along the z-axis.

and these features can become regions for preferential adsorption of hydrogen. One might rationalize the observed improvement in performance of the F166 sample as being due to the presence of a high concentration of defect sites, a feature that was clearly evident in the electron micrographs. Indeed, Fujii and Orimo34 capitalized on this behavior by using a mechanical milling technique to induce defect sites into graphite crystals. In this approach graphite was milled under 1 MPa of hydrogen for up to 80 h. Following this process, the amount of stored hydrogen was found to reach a level of 7.4 wt %. In this system it was necessary to heat the sample up to 675 °C in order to release all the hydrogen. The presence of edge dislocations was deemed to be responsible for trapping hydrogen between the graphite layers. Finally, in situ treatment of the GNF samples in either oxygen or hydrogen was found to result in severe edge attack and these are regions that have the capability of catalyzing the dissociation of hydrogen,17 an aspect that will be discussed later. Effect of Various Pretreatments on Hydrogen Storage in Graphite Nanofibers. Pretreatments of GNF may induce changes in carbon structure and lead to increased hydrogen storage capacity. Park et al.14 attributed the increase in hydrogen storage levels following high-temperature inert gas treatment of herringbone GNF to removal of the oxygen functional groups that blocked access of hydrogen to the internal graphite surfaces. Support for this concept was provided in a related study with SWNT that showed that high-temperature pretreatments removed surface functionalities responsible for the blocking of pores.35 In an attempt to further explore pretreatment effects of the GNF sample, F166 was selected for a detailed study since this material showed the highest potential for hydrogen storage. Common pretreatments that had been used by other workers to treat GNF and/or MWNT in order to enhance hydrogen storage were selected. These procedures included heat treatments in oxidative, reductive, and inert atmospheres. The results of this series of experiments are presented in Figure 4 and Table 3 and are discussed below. The procedure that ultimately resulted in the highest hydrogen desorption was an in situ hydrogen pretreatment (34) Fujii, H.; Orimo, S. Physica B 2003, 328, 77. (35) Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. J. Chem. Phys. 2000, 112, 9590.

Cu:Ni ) 2:8.

of herringbone GNF for 1 h at 700 °C (PT7). The hydrogen desorption levels ranged from 3.3 to 3.8 wt %. Subsequent examination of the samples revealed that this action resulted in significant edge attack of the GNF structures. An analogous in situ treatment of the same material in oxygen at 550 °C for 10 min (PT3) resulted in a subsequent hydrogen uptake of 2.5-3.6%. A similar uptake could be achieved by lowering the oxidation temperature to 350 °C and after a 12-h treatment (PT4) under these conditions the hydrogen uptake ranged from 2.0% to 3.4%. In some respects, the effect of different pretreatments is comparable to variations between samples and cycles. This finding suggests that the major function of all pretreatments is one of activating the sample and the resulting modifications in carbon structure are responsible for the observed improvements in hydrogen storage capacity. Significant differences in hydrogen storage behavior are observed following ex situ and in situ pretreatments of the sample under otherwise identical conditions. Samples treated externally in air at 550 °C (PT2) exhibited hydrogen desorption levels ranging from 0.72 to 2.3 wt %. When the same material was treated in situ under the same conditions, the hydrogen desorption values increased up to 3.6 wt %. Similarly, external pretreatment in hydrogen at 800 °C (PT6) resulted in a hydrogen desorption value of 1.2 wt %, whereas the corresponding in situ treatment gave a desorption level of 3.8 wt %. Previous work by Park et al.36 highlighted the hydrophilic nature of herringbone GNF materials. The molecular diameter of the water molecule (0.265 nm) is smaller than that of hydrogen (0.289 nm), and would likely interfere with hydrogen adsorption. Thus, this difference in performance is thought to be due to the presence of adsorbed water vapor in the GNF structure, which occurs during the transfer process to the sample vessel of the high-pressure storage unit; this will be explored in future work. In the metal hydride literature, the term cyclability is generally used to denote the performance of the material after subsequent hydrogen exposures. The deterioration of hydrogen uptake with cycle is one limiting factor in the application of metal hydrides. In contrast, hydrogen cyclability for GNF was on the order of 10% for PT3 and PT7. The only sample showing any significant deterioration upon repeated hydrogen exposure was GNF-F166 pretreated at 350 °C (PT4). In general, GNF-F166 shows promise for cyclability considerations as the materials retain hydrogen uptake upon repeated exposure. In all cases, there was no significant hydrogen desorption for the heat treatment that followed the final room (36) Park, C.; Engel, E.; Crowe, A.; Gilbert, T. R.; Rodriguez, N. M. Langmuir 2000, 16, 8050.

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Figure 5. TEM analysis of GNF-F166 after pretreatment in air at 550 °C (PT3) and high-pressure hydrogen exposure for (a, top panel) low resolution and (b, bottom panel) high resolution. The pretreatment has led to edge attack (denoted by arrows) and central dislocations (areas in circles).

temperature desorption. This indicates that the measured hydrogen is desorbable under relatively moderate conditions. There may be additional chemisorbed hydrogen present in the GNF that is desorbable at elevated temperatures; however, this chemisorbed hydrogen was not measured in this study due to experimental heating constraints and the fact that hydrogen desorbed at elevated temperatures is not practical for mobile fuel cell applications. Previous work from this laboratory and by others suggests that a threshold temperature must be reached in order to activate the nanofibers. In the present investigation, the pretreatment temperatures were chosen so as to correspond to the onset of gasification of the nanofibers: about 650-750 °C in a hydrogen atmosphere and 480-510 °C in air (Table 2). Several reports, however, have claimed that utilizing relatively low pretreatment temperatures did not improve the ability of nanofibers to store hydrogen.12 On the other hand, it should be noted that both Gupta and Srivastava18,19 and Browning et al.17 were able to achieve significant hydrogen uptakes in GNF materials (10-15 and 4.16 wt %, respectively) after only a 150 °C vacuum pretreatment. We have demonstrated that one can activate the herringbone GNFs by high-temperature in situ pretreatment in either oxygen or hydrogen. The effects of these treatments on the carbon structure were revealed by highresolution TEM examinations. In all cases, low-magnification studies show that the GNF materials are quite clean and no polymeric material has formed on the surfaces during pretreatment or high-pressure hydrogen exposure. Lattice fringe images of the graphite sheets constituting

Figure 6. TEM analysis of GNF-F166 after external pretreatment in hydrogen at 800 °C (PT6) and high-pressure hydrogen exposure for (a, top panel) low resolution and (b, bottom panel) high resolution. The pretreatment has led to extensive edge attack (denoted by arrows) and central dislocations (areas in circles).

the nanofibers are clearly evident at high magnification and are well ordered in arrangement features that are indicative of a highly crystalline material (top panels in Figures 5-7). A striking feature of the TEM micrographs of the samples that had been subjected to high-pressure hydrogen exposure is that both the oxidative and reductive pretreatments resulted in attack of some edge regions (Figures 5-7). The gasification behavior was very heterogeneous in nature in that some regions were extensively attacked, whereas others appeared to be unchanged. Despite disruption of the edge regions it was significant that high-resolution images showed the appearance of lattice fringe images detailing the herringbone arrangement of the graphene layers. It should be stressed, however, that this behavior did not have a direct correlation with the ability of the GNF to adsorb hydrogen. Indeed, the nanofibers treated at 800 °C (PT6) showed the most extensive attack (see Figure 6, bottom panel) but did not display significant hydrogen storage capacity. Close inspection of the hydrogen-pretreated samples revealed the presence of a significant number of dislocations in the central regions or body of the nanofibers. These dislocations are believed to be centers for the preferential storage of hydrogen. These defects are similar in many respects to interlayer voids of the expanded GNF structure

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Figure 7. TEM analysis of GNF-F166 after in situ pretreatment in hydrogen at 700 °C (PT7) and high-pressure hydrogen exposure for (a, top panel) low resolution and (b, bottom panel) high resolution. The pretreatment has led to edge attack (denoted by arrows) and central dislocations (areas in circles).

following exposure to high-pressure hydrogen, shown by Gupta and Srivastava.19 There have been other reports of enhanced hydrogen uptake resulting from various pretreatment of carbon nanomaterials. It is not clear, however, as to whether this phenomenon is due to increased graphitization,22,23,37,38 removal of adsorbed species,14,35 or activation of the carbonaceous solid.21 Recently, Pradhan et al.39 used modeling studies in combination with experimental hydrogen uptake measurements to show that surface roughening increased the hydrogen binding energy, resulting in a more favorable adsorption performance. This concept is similar to the edge attack observed with the present materials achieved with oxidative and reductive pretreatments. Although this behavior appears to contradict the belief that graphitization of the carbon solid increases the potential for hydrogen storage, it is possible that such a high-temperature treatment might merely be removing tightly bound chemical functionalities from the carbon structures. It has been demonstrated that mild oxidation of multiwalled nanotubes opens up the ends of the carbon structure40 as well as purifies single-wall nanotubes by (37) Zhu, H.; Li, X.; Lijie, C.; Xu, C.; Wu, D.; Mao, Z. Mater. Chem. Phys. 2003, 78, 670. (38) Ci, L.; Zhu, H.; Wei, B.; Xu, C.; Wu, D. Appl. Surf. Sci. 2003, 205, 39. (39) Pradham, B. K.; Harutyunyan, A. R.; Stojkovic, D.; Grossman, J. C.; Zhang, P.; Cole, M. W.; Crespi, V.; Goto, H.; Fujiwara, J.; Eklund, P. C. J. Mater. Res. 2002, 17, 2209. (40) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333.

Lueking et al.

removing accumulated amorphous carbon from the surfaces of the material.41 This same treatment of SWNTs was found to enhance the hydrogen uptake 3-fold when compared to similar untreated or annealed samples and was attributed to purification as well as activation of the material.42 On the other hand, severe oxidation of SWNTs achieved by a refluxing action in nitric acid had an adverse effect on the ability of the carbon materials to adsorb hydrogen. Furthermore, this treatment appeared to change the carbon structure as evidenced by Raman spectroscopy.39 When performed under the correct conditions, hydrogasification is also effective in cleaning the carbon nanostructures by selective removal of the more reactive amorphous components. In addition, hydrogen pretreatment has been shown to activate MWNT when conducted in the presence of a residual NiMgO catalyst.21 As was the case for GNF-F166, a high fraction of nickel was present in these latter samples and this metal is known to tunnel and/or channel into the graphite structure when heated in the presence of hydrogen.43-46 Nickel monolayer channeling commences at approximately 700 °C and the rate increases with increasing nickel surface area.46 Careful inspection of the micrographs of the F166 sample after pretreatment in hydrogen shows that the edge attack was not limited to the region in close proximity to the coppernickel particles. Furthermore, it was not possible to discern whether any spreading action of the catalyst occurred along the edges of the nanofiber. One cannot therefore establish that the edge attack in the presence of hydrogen was exclusively due to a catalytic action. Previously, Browning et al.17 attributed a large fraction of the hydrogen storage observed in their GNF materials to chemisorption behavior. Evidence for such a mechanism included the magnitude of hydrogen uptakes and the equilibration time. In addition, calculations showed similarities between the adsorption rates and those found in the literature for hydrogen-deuterium exchange on graphitized carbon black. These workers suggested that the GNF edges were catalytic sites and that the entire surface of the herringbone structures was potentially active. In the present work, the generation of edge defects following either oxygen or hydrogen attack is consistent with this proposed mechanism and provides evidence that additional catalytic sites may be formed after particular pretreatments. It was also proposed that atomic hydrogen could become intercalated into the GNF structure and could facilitate the intercalation of molecular hydrogen. Again, the creation of edge defects arising from certain pretreatments supports this hypothesis and illustrates how the interior structure of the GNF could be opened up by such attack. Conclusions The results of this investigation have demonstrated that the type of structure and pretreatment of the GNF are key features in determining the ability of the materials to store hydrogen. A herringbone GNF structure possessing a relatively short-range crystalline order and a significant number of dislocations was found to exhibit (41) Smith, M. R., Jr.; Hedges, S. W.; LaCount, R.; Kern, D.; Shah, N.; Huffman, G. P.; Bockrath, B. Carbon 2003 (submitted for publication). (42) Smith, M. R., Jr.; Bittner, E. W.; Shi, W.; Johnson, J. K.; Bockrath, B. C. Carbon 2003 (submitted for publication).. (43) Keep, C. W.; Terry, S.; Wells, M. J. Catal. 1980, 66, 451. (44) Baker, R. T. K.; Sherwood, R. D. J. Catal. 1981, 70, 198. (45) Baker, R. T. K.; Sherwood, R. D.; Derouane, E. G. J. Catal. 1982, 75, 382. (46) Goethel, P. J.; Yang, R. T. J. Catal. 1987, 108, 356.

Hydrogen Storage in Graphite Nanofibers

the highest hydrogen adsorption capacity. In a further series of experiments it was demonstrated that it was possible to enhance the hydrogen uptake by activation of GNFs via in situ oxidization or reduction treatments. This behavior was not achieved to the same degree if the pretreatment step was performed prior to transfer of the sample to the storage vessel. It is suggested that the in situ pretreatments effectively remove absorbed moisture from the GNF materials, which is an essential step if one is to achieve significant hydrogen storage levels. Finally,

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the advantages of exposure to high-pressure flowing hydrogen versus a static protocol were discussed.

Acknowledgment. We thank Leah Minc of the Michigan Memorial Phoenix Project for conducting the elemental analysis on the Ford nuclear reactor. This work was funded in part by NSF CTS-0138190. LA0349875