Electron Spin Resonance Investigation of Hydrogen Absorption in Ball

Mar 18, 2009 - Nanostructured hydrogenated graphite (CnanoHx) was synthesized here from graphite by ball-milling under a hydrogen (H2) atmosphere...
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J. Phys. Chem. C 2009, 113, 5409–5416

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Electron Spin Resonance Investigation of Hydrogen Absorption in Ball-Milled Graphite Christopher I. Smith,†,‡ Hiroki Miyaoka,§ Takayuki Ichikawa,§,| Martin O. Jones,† Jeffrey Harmer,⊥ Wataru Ishida,| Peter P. Edwards,*,† Yoshitsugu Kojima,§,| and Hironobu Fuji§ Department of Chemistry, Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom, ISIS Facility, Rutherford Appleton Laboratory, Chilton OX11 0QX, United Kingdom, Institute for AdVanced Materials Research, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8530, Japan, Department of Quantum Matter, ADSM, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8530, Japan, and Department of Chemistry, Centre for AdVanced Electron Spin Resonance, UniVersity of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom ReceiVed: NoVember 10, 2008; ReVised Manuscript ReceiVed: January 29, 2009

Nanostructured hydrogenated graphite (CnanoHx) was synthesized here from graphite by ball-milling under a hydrogen (H2) atmosphere. X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and transmission electron microscopy (TEM) show that ball milling results in the rupture of graphene sheets creating active defects and allowing hydrogen to be dissociated and then chemisorbed. Most likely small quantities of iron particles incorporated during the milling process act as a catalyst. CnanoHx thus possesses unique characteristic hydrogenated states present in nanometer scale particles, unlike standard hydride materials. Electron spin resonance (ESR) spectroscopy resolves two distinct paramagnetic components. The first is assigned to (intrinsic) delocalized π-electrons in the graphene sheets (g ∼ 2.008) which disappears after approximately 32 h milling. The second ESR component grows in intensity with milling time and is assigned to localized electrons (g ∼ 2.003) with a concentration of 3 × 1020 spins per gram after 80 h milling. HYSCORE spectroscopy reveals proton hyperfine couplings, and variable temperature CW ESR spectra demonstrate an unexpected magnetic ordering at low temperatures (∼10 K). Formation of CnanoHx thus consists of weakly coupled localized electrons with wave functions that extends over small graphitic moieties. The radical centers do not physisorb a large quantity of H2 molecules under conditions required for practical H2 storage materials and are a product of the milling process and chemisorption of hydrogen. Introduction Since Dillon et al. reported the potential of single-walled carbon nanotubes (CNTs) as a hydrogen storage material in 1997,1 various carbon-based materials have been the subject of intense investigation. In these materials, there are two sorption processes for hydrogen; physisorption, involving relatively weak van der Waals interactions between molecular hydrogen and carbon,2-11 and chemisorption, resulting from the dissociation of H2 and the concomitant formation of carbon-hydrogen chemical bonds.12 For physisorbed hydrogen, a lowering of the ambient temperature below the boiling point of liquid nitrogen is necessary to obtain high (>5 wt %) hydrogen capacities in nanostructured carbons with high surface areas (CNTs, carbon nanofiber, activated carbon, etc.), as the trapping energy for the physisorption of H2 is estimated to be only a few kJ/mol-1.2 [5 wt% corresponds to approximately 5 hydrogen atoms for every 8 carbon atoms.] Therefore, these carbon materials have been deemed unsuitable for practical hydrogen storage media.13 In contrast, ball milled, hydrogenated nanostructured graphite (CnanoHx) is able to store large amounts of hydrogen due to the * To whom correspondence should be addressed. Tel: +44 1865 272646. Fax: +44 1865 272656. E-mail: [email protected]. † Inorganic Chemistry Laboratory, University of Oxford. ‡ Rutherford Appleton Laboratory. § Institute for Advanced Materials Research, Hiroshima University. | ADSM, Hiroshima University. ⊥ Centre for Advanced Electron Spin Resonance, University of Oxford.

Figure 1. The progress of carbon-based hydrogen stores, to date, together with the targets set by the IEA, DOE, and NEDO; the regions of physisorption and chemisorption reflect the differing processes of hydrogen sorption.20

process of chemisorption.14-19 In Figure 1, we outline the current progress and shortfalls of carbon-based hydrogen stores relative to the properties pertaining to an ideal hydrogen storage material as indicated by DOE, IEA, and NEDO (representing the respective materials “target” values for various national and international agencies). CnanoHx was synthesized here from graphite by mechanical ball-milling under a hydrogen atmosphere for up to 80 h. The CnanoHx material requires heating to more than 700 °C for hydrogen release, and additionally, hydrocarbons such as methane (CH4) or ethane (C2H6) are also desorbed. During the

10.1021/jp809902r CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

5410 J. Phys. Chem. C, Vol. 113, No. 14, 2009 ball-milling process, it is expected that active edges or defects are generated in the graphene sheets of graphite as they are mechanically broken down to small fragments with nanoscale dimensions and a large amount of hydrogen would be able to be chemisorbed, in the form of stable C-H bonds, at these active sites. These C-H bonds, which are incorporated in -CH, -CH2, or -CH3 functional groups, have been examined by neutron scattering measurements21,22 and IR absorption spectroscopy.23 However, it has not been possible to develop a complete model of the hydrogen absorption process from these data, and it is hoped that the hydrogen absorption properties of the CnanoHx material would be further clarified by the direct observation of the dangling bonds at graphene edges or defects created during the ball milling process. If paramagnetic centers are induced by the fragmentation of C-C chemical bonds in this product by milling and subsequent hydrogen sorption, characteristic electron spin resonance (ESR) signals should be observed. ESR spectroscopy is unquestionably a powerful technique to interrogate unpaired electron spins in paramagnetic materials and has been applied for example to pure graphite,9,24 amorphous and nanoparticle carbon,25,26 carbon nanotubes,27 activated carbon fibers,28 carbon nanofoams,29 and semiconducting graphene nanoribbons.30 From an analysis of the ESR signals, the concentration of unpaired electrons, their chemical distinction and molecular structure, and the reactivity of such radicals may be clarified and investigated. In carbon based materials, ESR is able to conveniently discriminate between localized and itinerant electrons. In this paper, various CnanoHx products are synthesized by ball-milling for different milling times to investigate the hydrogen absorption process. The products were then investigated using both structural (XRD, FTIR, TEM, elemental analysis, TDMS) and ESR techniques. This allowed for a quantitative analysis on the effects of defects on the hydrogen storage capacity. Experimental Details The hydrogenated graphite (CnanoHx) was synthesized from graphite powder (99.999%, STREM CHEMICALS) by a planetary (rotating) ball-mill apparatus (Fritsch, P7) under hydrogen (H2) atmosphere. 300 mg of graphite, and 20 steel balls with 7 mm in diameter or 20 ZrO2 balls with 8 mm in diameter were put into a milling vessel with an inner volume of about 30 cm3 made of Cr steel. The ball-milling was performed under 1.0 MPa H2 pressure at room temperature for a variety of milling times, (1, 4, 8, 32, or 80 h) and the CnanoHx products made by steel and ZrO2 balls are denoted as CnanoHx (steel) and CnanoHx (ZrO2), respectively. All of the samples were handled in a glovebox (Miwa MFG, MP-P60W) filled with purified Ar gas (>99.9999%) to avoid pre- and postsynthesis oxidation (O2 and H2O content 99.9999%) in order to estimate the amount of hydrogen and iron derived from the milling process present in the products. All samples were protected from atmospheric oxidation by tin (Sn) foil, which was applied to the sample prior to its leaving the glovebox. The powder samples were placed into quartz tubes (ID 4 mm), and these tubes were sealed with vacuum grease in an argon filled glovebox. The morphology and microstructure of the synthesized CnanoHx materials was studied using transmission electron microscopy (JEOL 3000F TEM operating at 300kV) in the Department of Materials, University of Oxford. X-band ESR measurements were made on a Bruker E680 spectrometer (pulse) and a Bruker EMX (CW), both of which were equipped with a helium gas-flow cryostat from Oxford Inc. Samples were loaded into quartz ESR tubes (ID 3 mm), evacuated to approximately 10-5 mbar via an Ultra-Torr Swagelok fitting connected to a vacuum line and then sealed using a gas torch. CW ESR spectra were measured with a modulation amplitude of 0.05-0.5 mT and a modulation frequency of 100 kHz. Signals were carefully checked to ensure that they were not saturated in the temperature range 5-295 K and a microwave power of 8 × 10-3 mW (44 dB) was determined to be suitable. Echo-detected ESR spectra were recorded by integrating the echo intensity created with the microwave (mw) pulse sequence π/2-τ-π-τ-echo with mw pulse lengths tπ/2/tπ ) 16/32 ns, and a τ ) 160 ns. The first derivative of this spectrum was calculated numerically. Phase memory relaxation time traces (T2M) were measured by incrementing τ in steps of 8 ns. Spin-lattice relation time traces (T1) were measured with an inversion recovery sequence π-T-π/2-τ-π-τ-echo with T stepped out in 16 ns steps. HYSCORE spectra were measured with a repetition time of 1 ms using the sequence π/2-τ-π/2-t1-π-t2-π/2-τ-echo. The mw pulse lengths were tπ/2 ) 16 ns and tπ ) 16 ns, starting times t10 ) t20 ) 64 ns, and a time increment of ∆t ) 16 ns (data matrix 128 × 128). Results and Discussion X-Ray Diffraction (XRD). In Figure 2 we show the XRD profiles of the CnanoHx (steel) products milled for 1, 4, 8, 32, and 80 h using steel balls. The XRD profiles of the CnanoHx

Hydrogen Absorption in Ball-Milled Graphite

Figure 3. TEM of CnanoHx product milled for 8 h. The length of the crystalline needle shown is approximately 25 nm.

(ZrO2) milled for 80 h using ZrO2 balls, and that of graphite (PDF #65-6212) are also shown. After 1 h milling, the diffraction peaks arising from the (002) plane of graphite clearly remain, although it is noticed that other peaks are markedly suppressed. The intensity of the (002) diffraction peak gradually decreases with increasing milling time, and completely disappears after 32 h milling, suggesting that the graphite has been broken down to nanoscale fragments. In the case of the 80 h milled CnanoHx (ZrO2) product, nanostructures are also generated after 80 h milling. Additionally, the formation of a small peak at approximately 2θ ) 45° is observed in the CnanoHx (steel) sample milled for 80 h, which is assigned to crystalline iron carbide (Fe3C), as iron is mixed into CnanoHx (steel) from the steel balls and vessel during the ball-milling process.16 Transmission Electron Microscopy (TEM). TEM images of the CnanoHx product milled for 8 h were obtained (Figure 3). Here it can be clearly seen that, while the majority of the graphite has been pulverized to a material with an amorphous texture, some crystalline graphite still exists. The fragment shown in Figure 3 is approximately 25nm in length. This shows that as the milling time is increased, the graphite structure is being progressively broken down to give nanosized fragments, also in accordance with the previous work by Orimo et al.,19 which showed the crystallite size of CnanoHx milled for 80 h (steel) to be approximately 4nm. This data agrees with the XRD profile for the same sample (Figure 2). Analysis of the d-spacings on the highlighted area confirms this to be graphite and not iron impurities from the steel balls used in the milling process. Elemental Analysis. The hydrogen capacities in the series of CnanoHx (steel) and 80 h milled CnanoHx (ZrO2) products measured by CHN elemental analysis are shown in Figure 4. The amount of iron contaminant from the steel balls and the milling vessel during the milling process is also shown, assuming that the elemental analysis residue corresponds to the quantity of iron present in the products. As shown in Figure 4, the hydrogen capacity increases with an increase in the milling time. After 80 h milling, the amount of hydrogen in CnanoHx (steel) has risen up to approximately 6.0 mass %, which is similar to previous results.12 These data indicate that the amount of hydrogen absorbed in the product strongly depends on the milling time, and thus that the increased hydrogen capacity is due to the increase in the number of edges and defects induced by the structural destruction during ball-milling. A fuller discussion on the nature and origin of these defects is given later. For the product milled for 80 h, the amount of the iron is

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Figure 4. Hydrogen and iron capacities for the CnanoHx (steel) products estimated from elemental analysis by using the oxygen-combustion method, where it is assumed that the residue of each CHN analysis corresponds to iron. The hydrogen and the iron capacities of the 80 h milled CnanoHx (ZrO2) product are also shown

Figure 5. TDMS profiles of hydrogen from the CnanoHx (steel balls) milled for 1, 4, 8, 32, and 80 h.

about 15.0 mass %, which is consistent with the quantity of iron measured by energy dispersive spectroscopy, although the 80 h milled CnanoHx (ZrO2) product possesses only a small quantity of iron ( 50 K) gives x ∼ 0.25 (Figure 9B).

Figure 9. X-band CW ESR spectra measured between 5 to 294K, signal intensities were computed from the double-integral. (A, B) CnanoHx (steel) sample milled for 1 h. (A) ESR spectra vs temperature and a typical simulation at 50 K showing the total and the inhomogeneous contribution. Unmilled graphite is shown at the top. (B) Signal intensity vs temperature, the data deviates slight from a Curie behavior and was modeled assuming conduction and localized spin contributions (dotted trace). Inset: amplification of the region from 50 to 294 K. (C, D) CnanoHx (ZrO2) sample milled for 80 h. Spectral shape and position are independent of temperature and the signal intensity shows an approximate Curie behavior above 20 K before a cusp around 10 K and a decrease in intensity at lower temperatures.

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Smith et al.

Figure 10. HYSCORE spectrum measurements at 20K showing (A) CnanoHx milled for 8 h (steel balls) and (B) for 80 h (ZrO2). Similar couplings are observed in both samples with a significantly greater signal-to-noise ratio in the 80 h sample.

Clearly the 1 h sample has a heterogeneous structure, and there is no evidence of low temperature magnetic ordering. Further characterization of the inhomogeneously broadened line was afforded by direct measurement of the electron relaxation times, T1 (spin-lattice) and T2M (phase memory time), using an inversion recovery and primary echo sequence, respectively (see Supporting Information). For all samples the T2M time was approximately independent of temperature and of the order of 1 µs. The times T1, around 1.5 µs at 295 K and 2.5 µs at 10 K, are very short as compared to typical isolated organic radicals in low concentration. This fast T1 relaxation time stems from a high local concentration of unpaired electrons and the associated time-dependent magnetic field fluctuations. Both T1 and T2M times are similar to those of carbon nanotubes.26 A dominant relaxation mechanism, involving iron incorporated into the sample as a result of the ball milling seems unlikely, since the 80 h milled CnanoHx (ZrO2) sample also has short relaxation times (T2M ∼ 1 µs and T1 ∼ 2 µs), with a much reduced iron content (Figure 4). The relaxation times T1 and T2M of the inhomogeneous line is relatively independent of the total spin concentration (Figure 8), further supporting the idea of high local concentrations or clusters of unpaired electrons. It is worth noting that short electron relaxation times of unpaired electrons in carbon SWNT have been attributed to interactions with small magnetic clusters (catalytic particles) imbedded in the carbon framework.27 In the CnanoHx material ferromagnetic particles from the milling certainly inhomogeneously broaden the resonance (Figure 9), but we do not expect the T2/T1 lifetimes of the localized radicals to be dramatically shortened. In addition, the number of paramagnetic centers per carbon atom in the two materials is considerably different, typically ∼10-5 in SWNT39 and ∼10-2 in CnanoHx after 80 h milling. Information on the magnetic nuclei coupled to the paramagnetic centers was obtained with hyperfine sublevel correlation (HYSCORE)40 spectroscopy carried out at 20 K on the CnanoHx sample milled for 8 h (steel balls) and 80 h (ZrO2 balls) and is shown in Figure 10. HYSCORE is a high-resolution 2-dimensional technique used here to measure small hyperfine couplings not resolved in the CW ESR spectrum. Panels A and B in Figure 10 both display intense signals along the proton antidiagonal line (free proton frequency 15 MHz), which correspond to proton hyperfine couplings in the range |A(1H) | ∼ 3-11 MHz (0.11-0.39 mT) for (A), and proton hyperfine couplings around 29 MHz (104 mT) for (B), implying radicals with nearby protons. The nonzero isotropic component of the hyperfine coupling results from a Fermi contact interaction, which

indicates an electronic connection between the unpaired electron and the proton(s), rather than a pure through space dipole-dipole interaction. If we assume the ESR signals result from π radicals, then we can use the proton hyperfine couplings to estimate the spin populations Fπ at the adjacent carbon atoms with the McConnell equation,41 aH(R) ) QHCHFπ, where QHCH ) - 2.7 mT, and aH(R) is the proton hyperfine coupling in mT. Using our hyperfine couplings, and assuming a negative sign, we get carbon spin densities between 5-15%. As the spin density sums to 1, this implies a radical of the order of 5-20 HR-C units and thus a structure (graphene sheet) comparable in size to (for example) a phenalenyl radical molecule. A single carbon centered radical with bonded hydrogen atoms like a methyl radical can be ruled out since in this case typically A(1H) ∼ 67 MHz (2.4 mT), a hyperfine coupling much bigger than observed in the HYSCORE spectrum and would result in an observable splitting in the CnanoHx CW ESR spectra (line width 1.5 mT), contrary to observation. HYSCORE data from the 80 h milled sample (Figure 10B) reveals proton hyperfine couplings around 29 MHz, a value comparable to those of conjugated aliphatic radicals.42 In addition, HYSCORE 13C signals (in natural abundance) at low frequencies in Figure 10A indicate 13C hyperfine couplings in the range |A(13C)| < 4 MHz (0.14 mT), which if purely an isotropic coupling, equates to very small spin densities of F13C < 0.1%. The fact that a 13C signal is detected in natural abundance material indicates many carbon atoms contribute. The proton magnetic interactions complement the FT-IR data which show the presence of -CH, -CH2, and -CH3 groups (Figure 6). Nature of the Radicals. It is known that the starting graphite material, before milling, contains delocalized spins, with a concentration of approximately ∼1017 spins/mgram (approximately 2 × 10-3 unpaired electrons per carbon atom). After 1 h milling under a H2 atmosphere the CnanoHx concentration is reduced to ∼1015 spins/mgram. This implies that the paramagnetic states are being quenched by incorporation of hydrogen atoms into the graphite particles, preferentially at the radical sites. A decrease in the ESR signal has been reported upon hydrogenation of carbon nanotubes,28 mechanically milled graphite particles, and in polycrystalline silicon, as a result of a decrease in the density of dangling bonds. With further milling time there is a steady increase in the CnanoHx radical concentration with the largest change between 32 and 80 h. These radicals are, therefore, relatively stable, and the samples show only a slow degradation of the ESR signal when exposed to oxygen.

Hydrogen Absorption in Ball-Milled Graphite The hyperfine couplings derived from the HYSCORE data show that the (unpaired) spin density, and thus wave function of the radical is spread over small graphitic moieties (sizes comparable to anthracene for example. Most likely the paramagnetic states are created from the cleavage of graphene sheets, due to the milling process, resulting in so-called zigzag and armchair edges, having bonded hydrogen atoms in CH, CH2, and CH3 groups.30,35,43 There is also a population of radicals associated with aliphatic structures. Incorporation of hydrogen results in saturated and unsaturated carbon bonds, and stabilizes highly reactive dangling bonds. These structural types are believed to create nonbonding π-electron states, which accommodate a high unpaired spin concentration. In the 80 h sample with a ratio H/C ∼ 1.5 there are, on average, approximately 1 × 10-2 paramagnetic centers per carbon atom (5 × 1020 spins/gram). The large spatial but still localized wave functions of the radicals allow exchange interactions of varying strengths sufficient to induce magnetic order (short-range spin order) at low temperatures, as demonstrated by the 80 h sample, with the highest spin concentration. This ordering process could potentially be described in terms of a spin glass (proof of this from SQUID data is complicated by the iron particles). Similar looking anomalous ESR susceptibility versus temperature data in pristine SWNT have also been reported, and proposed to result from the formation of superconducting domains around 12K.42,44 Here the intensity of a narrow ESR component (0.5 mT) is reduced at the expense of an increase in intensity of a broad ESR component. This is clearly not the case in CnanoHx. Other possible mechanisms, leading to π-radicals in carbon based materials26 to consider, are carbon adatom defects occurring on graphene planes and bridging C-C bonds,45 sterically protected carbon radicals immobilized in aromatic sp2 bonded carbon structures,46 and radicals based on sp2 carbon vacancies in graphene sheets, resulting from the removal of a carbon atom from a six membered ring, followed by a concurrent relaxation of surrounding carbon atoms.47 The exact catalytic role of the iron particles on the milling process in an H2 atmosphere is not well understood, and it is possible that in the absence of catalytic iron particles that activated radical centers may be formed that are able to sorb H2, but not active enough to break the H2 bond. It is unlikely that the iron impurities, within the samples, result directly in an appreciable radical concentration, as the 80 h (ZrO2) sample with a low iron content has the highest unpaired spin concentration (the iron content is not zero since the ball mill itself contains iron). Conclusion From our experimental results, it is observed that the hydrogen absorption capacity is enhanced with increased destruction of the graphite structure, typically down to the nanometer scale regime, by ball-milling. X-band CW and pulse ESR measurements show that as the milling time is increased, the graphitic network with intrinsic delocalized conduction electrons is being broken down to nanoscale fragment structures, containing localized radicals. It is considered that electronically active carbon sites arising from the edges and defects in the nanostructured graphite are induced by ball-milling, and hydrogen molecules are then dissociated and chemisorbed at the active carbon sites in a stable hydrogenated state. This process is presumably enhanced by the catalytic activity of a small quantity of iron particles, incorporated during the milling process. These particles account for the secondary dependence of hydrogen storage capacity on iron content, due to formation of catalytic amounts of Fe3C. Most likely, the localized ESR

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5415 signal predominantly originates from nonbonding π-electrons in nanosize graphene moieties, where carbon defects and carbon atoms, at the edges in zigzag and armchair configurations, have bonded hydrogen atoms in CH, CH2, and CH3 groups. The wave functions of these localized radicals are sufficiently extended, and the local spin concentrations high enough, to enable magnetic ordering at low temperatures. The combination of infrared spectroscopy and X-ray diffraction indicate that the hydrogen is primarily chemisorbed to the nanoscale carbon. Thermal desorption mass spectrometry measurements show that hydrogen is only evolved between 400 and 700 °C, far in excess of that expected for physisorbed species. This is also much higher than that expected for FeH, which has an enthalpy of formation of 7.9 kJ/mol,48 equivalent to a decomposition temperature of approximately 30 °C. A comparison of the hydrogen storage capacity of materials, milled at 80hrs with Fe and ZrO2 balls, with samples milled for 32 h with Fe balls (Figure 4), indicates that hydrogen capacity increases as a function of milling time above any increase which may be attributed to increasing Fe content. The high hydrogen capacity of our samples arises as a result of the chemisorption of hydrogen to nanoscale graphite, rather than from interactions with impurity iron. In summary, the radical centers in CnanoHx are not able to physisorb a large quantity of H2 molecules under conditions required for practical H2 storage materials, and are a product of the milling process and chemisorption of hydrogen. Acknowledgment. The authors thank Dr. S. Haigh and Professor A. Kirkland at the Department of Materials, Oxford University for transmission electron microscopy. C.I.S. acknowledges the Engineering and Physical Research Council (EPSRC) and the Science and Technology Facilities Council (STFC) for funding. The authors also acknowledge the support of the Centre for Advanced Electron Spin Resonance (CAESR). This work was partially supported by the project “Advanced Fundamental Research Project on Hydrogen Storage Materials” of the New Energy and Industrial Technology Development Organization (NEDO) and Research Fellowships of the Japan Society for the Promotion of Science for young Scientists (JSPS). Supporting Information Available: Figure S1 shows the X-band CW ESR spectra for CnanoHx samples milled for 1, 4, 8, 32, and 80 h under a hydrogen atmosphere (steel balls), and Figure S2 depicts the X-band pulse data showing T1 (A/C) and T2M (B,D) data between 10 and 294 K. Figure S3 shows a simulation of the CW ESR spectrum measured at 20 K for the sample milled for 1 h under hydrogen, modeled with a Dysonian line shape function. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (2) Zuttel, A.; Nutzenadel, C.; Sudan, P.; Mauron, P.; Emmenegger, C.; Rentsch, S.; Schlapbach, L.; Weidenkaff, A.; Kiyobayashi, T. J. Alloys Compd. 2002, 330, 676. (3) Dillon, A. C.; Heben, M. J. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 133. (4) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. Science 1999, 285, 91. (5) Park, C.; Anderson, P. E.; Chambers, A.; Tan, C. D.; Hidalgo, R.; Rodriguez, N. M. J. Phys. Chem. B 1999, 103, 10572. (6) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127. (7) Hynek, S.; Fuller, W.; Bentley, J. Int. J. Hydrogen Energy 1997, 22, 601.

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