LiBH4 in Carbon Aerogel Nanoscaffolds - American Chemical Society

Feb 12, 2010 - of Physics, South Dakota School of Mines and Technology, 501 East St. Joseph Street, ... 3011 Malibu Canyon Road, Malibu, California 90...
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J. Phys. Chem. C 2010, 114, 4008–4014

LiBH4 in Carbon Aerogel Nanoscaffolds: An NMR Study of Atomic Motions David T. Shane,† Robert L. Corey,†,‡ Charlie McIntosh,† Laura H. Rayhel,† Robert C. Bowman, Jr.,§ John J. Vajo,| Adam F. Gross,| and Mark S. Conradi*,† Department of Physics, Washington UniVersity, One Brookings DriVe, Saint Louis, Missouri 63130, Department of Physics, South Dakota School of Mines and Technology, 501 East St. Joseph Street, Rapid City, South Dakota 57701, RCB Hydrides LLC, 117 Miami AVenue, Franklin, Ohio 45005, and HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, California 90265 ReceiVed: NoVember 11, 2009; ReVised Manuscript ReceiVed: February 1, 2010

Hydrogen NMR of LiBH4 in the pores of carbon aerogel nanoscaffolds shows the coexistence of motionally narrowed and broad components. The fraction of mobile, diffusing hydrogen, already evident at room temperature, increases continuously with temperature. Thus, a broad distribution of environments is present, as in some ball-milled hydrides. With decreasing pore size from 25 to 13 nm, the narrowed fraction increases, suggesting that the narrow resonance is from the most defective regions, the grain boundaries. The broad component eventually exhibits narrowing in the same temperature window as for bulk material, confirming the bulk-like structure of those regions. Hole-burning measurements reveal magnetization exchange between the broad and narrow resonance lines, confirming the close spatial proximity of the atoms in each line. The solid-solid transition is clearly evident in 7Li line shapes, with a 10-15 °C depression from the bulk. More rapid decay of the quadrupolar satellite signals in spin echoes, compared to the central transition, is due to lithium atoms diffusing between differently oriented nanocrystallites. Our results suggest that crystallites in neighboring pores have similar orientations but are incoherent for diffraction. Remarkably, the T1 data of hydrogen and 7Li are continuous in the vicinity of the transition, in contrast with the bulk T1 data, suggesting that some rapid lithium motion remains below the transition. Introduction In the past, research on hydrogen storage materials focused on interstitial hydrides of transition and rare earth metals.1,2 Many of these systems exhibit good dehydriding and rehydriding reaction kinetics because of (i) high diffusivity of the interstitial H atoms,3 (ii) the comparatively small structural change accompanying the reaction (i.e., from one metallic phase to another), and (iii) the often strong catalytic activity of the surfaces for dissociation of H2 molecules.4 Despite these positive attributes, the metallic hydrides store inadequate amounts of hydrogen (expressed in weight percent) for the envisioned applications in transportation. Thus, interest has focused more recently on the ionic hydrides such as MgH25–7 and complex hydrides such as NaAlH4 and LiBH4.8 This latter material is an ionic solid of Li+ cations and BH4- anions. The strong covalent bonding between B and H in each BH4 unit is evident in vibration (bond stretching) spectra9,10 and in the very slow exchange11 of H atoms between BH4 units even in molten LiBH4. These hydrides are attractive for applications because they consist of abundant and particularly lightweight metal atoms. For example, the reaction

LiBH4 T LiH + B + 3/2H2

(1)

yields 13.6 wt % hydrogen, theoretically. Such high hydrogen content would allow large-scale applications. However, the * Corresponding author. E-mail: [email protected]. † Washington University. ‡ South Dakota School of Mines and Technology. § RCB Hydrides LLC. | HRL Laboratories, LLC.

reaction enthalpy is high,8 so that useful H2 pressures require high temperatures, approximately 400 °C for 1 bar in reaction 1.12,13 The technique of destabilization of the hydride (by stabilizing the dehydrided state by reacting the free B atoms of (1), for example) partially addresses this14,15 but at the cost of reduced hydrogen content per weight. A second serious problem is the slow reaction kinetics of ionic and complex hydrides; none of the above three numbered positive attributes of metallic hydrides are present here. For example, complete rehydriding in bulk LiBH4 according to (1) has only been reported at extreme conditions,8,16 600 °C and 150 or 350 bar. Slow diffusion of hydrogen (and metal atoms, in the case of complex hydrides) is at least partly responsible for slowing the reactions at reasonable conditions. For some systems, mechanical activation (ball-milling) results in enhanced reaction kinetics.6,17–19 The smaller crystallite grain size results in shorter diffusion distances x and correspondingly reduced diffusion times t, following the relation

x2 ) 2Dt

(2)

where D is the temperature dependent diffusion coefficient. In addition, the rate of atomic-level hopping (diffusive) motions is enhanced compared to bulk material. This is the case for ballmilled MgH2, for example, according to hydrogen NMR.20 Similar NMR behavior is present21 in ball-milled NaMgH3, although bulk, polycrystalline material is not available for comparison. This NMR behavior of both ball-milled MgH2 and NaMgH3 includes a motionally narrowed component at a surprisingly low temperature (compared to the onset temperature

10.1021/jp9107365  2010 American Chemical Society Published on Web 02/12/2010

LiBH4 in Carbon Aerogel Nanoscaffolds of narrowing in the bulk) and a continuously increasing fraction of intensity in the narrowed component with increasing temperature. One disadvantage of kinetics improvement through ballmilling is the growth of grains upon repeated dehydriding/ rehydriding cycles.7,22,23 Grain growth may be avoided or reduced by use of an inert scaffold with nanometer dimensions.24–27 The nanoscaffold serves as a structure-directing agent and restricts the crystallite size of the guest hydride. LiBH4 residing in carbon aerogel scaffolds with 13 and 25 nm pores exhibits much faster dehydriding (up to 50 times faster) and rehydriding behavior than bulk material.28 The nanoscaffold-supported LiBH4 exhibits better retention of capacity upon repeated cycling. The equilibrium H2 pressure also appears to be increased, although this is not understood. Finally, we briefly summarize previous NMR results on bulk LiBH4. A solid-solid phase transition29 occurs near 110 °C. The high temperature phase has orientational disorder of the BH4 groups30 and rapid lithium cation diffusion.31 T1 of both 7 Li and hydrogen passes through a T1 minimum in the high temperature phase near 170 °C due to the rapid (109 s-1) lithium hopping.31,32 The hydrogen resonance narrows to nearly zero width from 150 to 220 °C, because of the thermally activated diffusive hopping motions of BH4 anions.11,32 Narrowing of the 11 B resonance at the same temperatures confirms32–34 that the diffusing entity is BH4 anions. NMR of molten LiBH4 rules out H exchange11 between BH4 units at times of order or less than 1 s. Reorientation of the BH4 remains rapid (ns)32,33 at all temperatures above -100 °C. Here, we use hydrogen and 7Li NMR to study LiBH4 in the carbon aerogel nanoscaffolds. Behavior similar to that of ballmilled MgH2 and NaMgH3 is found. Experimental Methods Preparation of the LiBH4-in-aerogel materials at HRL Laboratories has been described elsewhere.28 Briefly, water filled polymer gels were formed from the condensation of resorcinol and formaldehyde. The ratio of organic material to water was varied to control the pore-size distribution. After solvent exchange of acetone for water and drying of the gel, pyrolysis at 800 °C left a nearly all-carbon porous structure. BarrettJoyner-Halenda (BJH) analysis of N2 adsorption isotherms from two materials indicated peaks in the pore-size (diameter) distribution of 13 nm (30 wt % organic recipe) and 25 nm (40 wt % organic recipe). Molten LiBH4 filled the carbon aerogel nanoscaffolds by capillary action. Excess borohydride was removed by scraping the aerogel cubes, and then the cubes were ground to a powder. The material was handled in a N2atmosphere glovebag at Washington University. Samples were flame-sealed into 5 mm OD glass tubes under 0.8 bar N2 or argon gas. For hydrogen (7Li) NMR, the tubes were approximately 18 (4) cm long. For some hydrogen NMR, the samples were loaded into 6 mm OD alumina ceramic tubes 20 cm long and sealed under N2 gas by an O-ring cap, avoiding the reaction35 of molten LiBH4 with glass. There is a small (3-4 at %) content of oxygen atoms in the pyrolyzed framework, according to measurements on a similarly prepared material.36 The concentration of H atoms was determined as 0.0580 ( 0.0004 H atoms per carbon atom, by prompt gamma-ray activation analysis. Thus, the scaffold materials are primarily carbon. The NMR experiments used two identical home-built rf spectrometers with locally written FIDo software. The hydrogen work was at 85.03 MHz in the 2.0 T field of an electromagnet

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Figure 1. Hydrogen NMR spectra for LiBH4 in aerogel with 13 nm pore size. In general, the spectra are superpositions of broad and motionally narrowed components. The narrowed fraction grows smoothly with rising temperature.

with 19F NMR field stabilization. The 7Li NMR was performed at 116.46 MHz in a 7.04 T cryomagnet. Both NMR probes used solenoid rf coils oriented sideways to the static field. A stream of temperature regulated air was used to heat the hydrogen NMR samples; for cooling, cold N2 gas was used. The 7Li samples were heated by a noninductive heater in stationary nitrogen gas. Temperatures were measured to (2 °C with type-T thermocouples located within 2 cm of the samples. NMR spectra were measured by Fourier transformation of the free induction decay (FID) signal following a (typically) 90° rf pulse. For hydrogen, where the broadest lines were encountered, a short 1-2 µs pulse was used to reduce spectral distortion. The FID was typically obscured for ∼3 µs by probe ringing, so this portion of the data was obtained by Gaussian back-extrapolation37 of the uncorrupted data. The 90° pulse times were approximately 10 µs for hydrogen and 11.5 µs for 7Li. For the quadrupolar 7Li nuclear spins, these times were adjusted to give the largest FID amplitude and were nearly equal to the pulse lengths determined for solution-state samples. Thus, our rf pulses appear to be covering the satellite transitions as well as the central transition.38 Occasionally, lithium spectra were obtained from spin echoes. The spin-lattice relaxation times T1 were measured using the saturate-wait-inspect approach. Ten to twenty 90° pulses accomplished saturation for hydrogen; inspection used the FID after a single, final 90° inspection pulse. For 7Li, a long rf pulse (200 µs) saturated through rf field inhomogeneity. Results and Discussion Hydrogen NMR spectra from below room temperature up into the melt for LiBH4 in carbon aerogel with 13 nm mean pore size appear in Figure 1. Even at 21 °C, the spectrum is a superposition of broad and narrow lines, indicating that a fraction of the H atoms (as BH4- anions) move translationally (i.e., diffuse) rapidly enough to narrow the line. Because the rigid lattice (broad component) line width is ∼25 kHz fwhm, the criterion for narrowing38,39 is for the diffusive hopping rate ωH to exceed approximately 105 s-1. In bulk LiBH4, the onset of narrowing occurs32,34 near 150 °C, so some of the BH4 units in the aerogel-supported material diffuse faster already at a much

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Figure 2. Temperature variation of the percentage of motionally narrowed intensity in the hydrogen NMR, for LiBH4 in 13 and 25 nm pore-size aerogels. At every temperature, the narrowed component is larger for the smaller pores.

lower temperature. At decreasing temperatures below 21 °C, the resonance in the aerogels continues to evolve until only a broad component is evident, with width similar to that of bulk LiBH4. For example, for the 13 nm material in Figure 1, the line shape has almost attained its rigid limit by -75 °C. We note that BH4 reorientations remain rapid at this temperature in the bulk32–34 and apparently also in the aerogel. That is, down to -127 °C, our lowest temperature, the line is no broader than for bulk LiBH4 (see below). At temperatures between -25 and 150 °C, the two-component nature of the spectra is sufficiently apparent to allow for simple “by eye” methods of separation. We prefer and have used a fitting of the experimental spectra to a sum of two Gaussians. The relative intensities (spectral areas) of the two components were then computed, as presented in Figure 2 for both 13 and 25 nm aerogels. The fraction of intensity in the narrowed component is equal to the fraction of mobile hydrogen spins, which clearly increases continuously with temperature. In Figure 2, the fraction of the spectral intensity in the narrow component is at every temperature smaller for the 25 nm aerogel than for 13 nm aerogel. Indeed, this is confirmed in Figure 3, where 21 °C spectra are compared for 13 and 25 nm aerogel materials. The pore size and nanocrystallite grain size from diffraction were found to be nearly equal for 25 nm material.28 Thus, the material with the smaller pores has smaller crystallites and a higher fraction of near-grain-boundary sites; experimentally, this corresponds to a larger intensity of the narrowed peak. Hence, it seems that the mobile sites are associated with the defective structures near grain boundaries and LiBH4-aerogel interfaces. In connection with Figures 2 and 3, we note that melting of the LiBH4 in aerogel did not result in irreversible spectral changes. Thus, spectra at 21 °C before and after the material was exposed in the NMR apparatus to 300 °C were nearly identical. We note this result may be expected because the sample preparation involved wicking molten LiBH4 into the aerogel. Of course, this reversibility is a major goal of the inert nanoscaffold approach to controlling grain size. The two-component NMR line shapes were analyzed by fitting to the sum of two Gaussians, as discussed above. Although the broad component appears to be a bit squarer than a Gaussian and spectra that are narrowing are generally

Shane et al.

Figure 3. Comparison of hydrogen NMR at 21 °C for LiBH4 in 25 and 13 nm pore-size aerogels. The peak heights are normalized to be equal, for purposes of display. Clearly, the 13 nm aerogel has a greater fraction of mobile spins. For both pore sizes, the broad components are similar in width to the bulk LiBH4 signal, also at 21 °C.

Figure 4. Line width fwhm (full width at half-maximum) for hydrogen NMR of LiBH4 in 13 nm pore-size aerogel. BC is the broad component, and NC is the narrow component; open symbols are for bulk LiBH4. The BC and bulk line widths are similar for 25-100 °C, and both exhibit similar narrowing above 150 °C.

Lorentzians,38,39 the quality of fits was more than adequate for our purposes. The line widths (fwhm) of the broad and narrow components for LiBH4 in 13 nm aerogel appear in Figure 4, together with line width data32 from bulk LiBH4. Clearly, the broad component has a width similar to the bulk, while the narrow component is much narrower. The difference between the bulk and the aerogel broad component line widths is mostly an artifact of our two-Gaussian fitting, demonstrated by the close correspondence of the bulk and aerogel broad component line shapes at room temperature in Figure 3. We note the good agreement in Figure 4 of the narrowing at high temperatures of the bulk and aerogel broad component signals. This demonstrates that the atomic-scale environments of the regions yielding the broad resonance are bulk-like, so approximately describable as nanocrystallite interiors.

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TABLE 1: Mobile Layer Thicknessa θ T (°C)

θ (nm) for 25 nm pores

θ (nm) for 13 nm pores

21 50 100 150 200

0.33 0.52 0.99 1.27 1.90

0.42 0.57 0.83 1.08 1.52

a Thickness θ from eq 3 using the narrowed fraction fN from Figure 2.

The simplest interpretation of the variation of the fraction of mobile spins (Figure 2) treats the crystallites as spheres of radius R (half of the 13 and 25 nm pore sizes). The outer layer of mean thickness θ, closest to the grain boundary and possibly aerogel wall, has mobile spins; the inner core of radius R - θ has immobile spins. We calculate θ at each temperature from the observed narrowed fraction fN of Figure 2

θ ) R[1 - (1 - fN)1/3]

(3)

The results for both pore sizes appear in Table 1. The mobile layer thicknesses increase steadily with temperature, reflecting the growth of fN in Figure 2. At each temperature, the thickness values for the two pore sizes are approximately equal. This is in accord with the edge regions having similar structures, with the main difference between the systems being simply the size of the crystalline cores (interior regions with immobile spins on the NMR time scale of 10-5 s). We note that the continuous increases with temperature of thickness θ (Table 1) and narrowed fraction (Figure 2) demonstrate a broad distribution of activation energies for the motion of BH4 anions. This is similar to the hydrogen NMR line narrowing data in ball-milled ionic hydrides20,21 and in other ionic solids with nm dimensions.40–42 The spin-lattice relaxation time T1 of hydrogen appears to be essentially equal for the broad and narrow resonances, as presented in Figure 5a. Since it is unlikely that the spins in the different regions have equal intrinsic relaxation times, we propose that some mechanism serves to equilibrate the spin temperatures.39 This should include physical diffusion of H (BH4 groups) in the narrowed regions and spin diffusion (diffusion of longitudinal spin magnetization by spin flip-flop processes driven by dipolar interactions) in the immobile regions. Spin diffusion is the slower of these two processes, because the narrowing criterion is ωHT2RL > 1, where T2RL is the transverse decay time in the absence of narrowing, with flip-flops occurring at a rate of order once per T2RL. Thus, the physical size of the immobile regions must be small enough that spin diffusion can homogenize the spin magnetization within the relaxation time T1. To test this, hole burning was performed. A long, weak 180° rf pulse was used to partially invert the narrow component of LiBH4 in 13 nm aerogel at 21 °C. The recovery of the hole was followed as a function of the time delay (after burning of the hole). Spectra at several recovery times appear in Figure 6a, including a fully relaxed spectrum (2000 ms). Clearly, the hole recovers faster than the overall resonance does, confirming that longitudinal spin magnetization is exchanged between the narrow and broad lines in a time smaller than T1. The amplitude of the narrow component as a function of delay time appears in Figure 6b. The squares represent the data, showing the narrow resonance going from negative amplitude up to its equilibrium (long-time) positive amplitude. The circles represent an expo-

Figure 5. (a) Hydrogen T1 of broad (BC) and narrow (NC) resonance components for LiBH4 in 13 and 25 nm pore-size aerogels. The thin lines represent T1 of bulk LiBH4, for comparison, from refs 32 and 33; the solid-solid transition at 110 °C is evident in the bulk as a discontinuity. (b) T1 of 7Li in the aerogels and bulk data from ref 31.

nential recovery with time constant chosen to equal T1 (as measured separately for the entire resonance, as in Figure 5); the hole recovers faster than time T1. The experimental recovery curve is not a single exponential; instead, the recovery extends over a wide range of times. The data in Figure 6b are halfway recovered at time t ) 10 ms. In the rigid-lattice regions, dipolar interactions drive spin flip-flop events43 every approximately 3T2RL, or about every tff ) 50 µs here. T2RL is the decay time of the FID in the rigidlattice limit. From the relation r2 ) 6Dt for the spreading of spin magnetization in time t, we obtain

r2 t ) tff l2

(4)

Thus, given the l ) 0.4 nm spacing between BH4 units (center to center), the distance covered by a single event, we find (r2)1/2 is approximately 5.6 nm. Compared to the 13 nm diameter of the pores, this is a reasonable radius for the interior regions. The argument here should not be regarded as a measurement

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Figure 6. (a) Hydrogen NMR spectra at 21 °C for LiBH4 in 13 nm aerogel, with variable delay time between hole-burning by a long, weak rf pulse and inspection by a short (broadband) π/2 pulse. (b) Spectral area of the narrow hole as a function of delay time (filled symbols). A single exponential recovery with a time constant equal to T1 is shown for comparison (open symbols). The faster experimental recovery demonstrates exchange of magnetization between the broad and narrow components.

of the mean size of the interiors. Instead, we treat the argument as simply confirming our picture wherein spin diffusion results in equal relaxation times T1 for the broad and narrow components (Figure 5a). Hydrogen T1 data for both 13 and 25 nm aerogels, broad and narrow components, appear in Figure 5a together with bulk data (smooth curves). The bulk data are from 8532 and 90 MHz33 NMR measurements. In the bulk, the sharp solid-solid phase transition29 is evident at 110 °C as a discontinuous jump in T1. Below the transition, T1 relaxation in the bulk has been shown to result from reorientations of the BH4 anions;33,44 lithium and BH4 translational hops are too slow here to be apparent in T1. Above the transition, the relaxation of hydrogen and 7Li in the bulk solid is driven by the rapid lithium motion.31 Indeed, the presence of classical T1 minima in the bulk implies38,39 the lithium ions hop about 109 s-1 (the precession frequency in radians per second) at the temperature of the minima, about 180 °C. The identification of the mobile species as lithium comes

Shane et al. from marked 7Li line narrowing31,32 and is in accord with lithium’s small size compared to BH4. For LiBH4 in aerogel near 175 °C, the hydrogen T1 displays a very similar T1 minimum (nearly the same temperature and minimum value of T1 as in the bulk). Thus, similarly rapid lithium motion is present in the aerogel-supported LiBH4. However, at lower temperatures, the hydrogen T1 in the aerogel materials increases much more slowly than in the bulk; T1 in the aerogel remains a factor of 3-5 smaller than the bulk, even at 85 °C, well below the transition (see 7Li spectra below). The implication is that the aerogel creates disorder, so that some lithium cations continue to execute rapid hopping motion at lower temperatures. We suspect that these are the lithium ions in the grain boundary region, near the LiBH4-aerogel walls. We note that rapid lithium motion has been reported45 in LiBH4 alloyed with LiI, which was interpreted in terms of depression and broadening of the transition. The hydrogen T1 of the 25 nm material is intermediate between the bulk and the 13 nm data. This is in accord with the greater fraction of defective sites in the smaller pore material. At much lower temperatures, T1 of hydrogen passes through a minimum near 160 K (-113 °C). By comparison with bulk data33 in Figure 5a, this is seen to arise from the rate of BH4 reorientations passing through the (angular) precession frequency.44 This minimum is deeper than the high temperature one, because reorientations modulate the strongest spin interactions, the H-H and B-H intramolecular dipolar terms. The 7Li T1 data in Figure 5b are compared to data from bulk31 LiBH4; all of the data are from measurements at 116 MHz. For the aerogel materials, the T1 minimum is deeper, demonstrating that the lithium motion modulates a stronger interaction. Thus, quadrupole interaction with electric field gradients (efg) is present, with the efg arising from the disorder generated by the aerogel framework. We note, for comparison, that the hydrogen T1 discussed above is sensitive to lithium motion only through magnetic dipolar interaction because the proton is spin onehalf and can have no quadrupole moment. However, similar to the hydrogen T1, the 7Li T1 at lower temperatures in the aerogels remains substantially smaller than that in the bulk, suggesting that very rapid lithium motion remains well below the solid-solid transition. It is possible that paramagnetic defects are present, most likely residing on the pyrolyzed aerogel polymer. However, the sharp hydrogen resonances observed (narrow component in Figures 1 and 4) suggest at most a small paramagnetic concentration. The longest hydrogen T1 values (Figure 5a, near 250 °C) are not very different than the bulk T1, suggesting that paramagnetic defects do not play a dominant role in hydrogen T1 relaxation. Given the smaller γ (magnetogyric ratio) of 7Li compared to hydrogen, we believe the paramagnetic effect on the 7Li is even smaller. The solid-solid transition is clearly evident in 7Li spectra, as presented in Figure 7 for 25 nm aerogel material. The broad tail present at 21 °C is resolved at 100 and 200 °C into a Pake powder doublet (with features broadened); it is first evident as shoulders on the resonance tail at 95 °C (an approximately 15 °C depression from the bulk). Scanning calorimetry also found a 10-15 °C depression in the transition temperature in 25 nm aerogel, with latent heat reduced by about 50% (see the Supporting Information of ref 28). The Pake doublet here has an approximate width of about 12 kHz, somewhat smaller than the 20 kHz of the bulk.31,32 These are clearly quadrupolar satellites; their intensity agrees with the expected ratio of intensities39 for spin 3/2 transitions, namely, 3:4:3. The nonva-

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r2 t ) 2 τ l hop

Figure 7. Spectra of 7Li in LiBH4 in 25 nm pore-size aerogel. The broad tails at 21 °C give way to the resolved Pake-powder doublet satellite resonances above the phase transition, at 100 and 200 °C. Shoulders are evident at 95 °C, where the transition is just starting. At 300 °C in the melt, the quadrupolar satellites are absent.

nishing time-average efg reflects the noncubic structure of the high-temperature phase. The resolution of the satellites above the phase transition is due to the faster lithium motion and resulting motional averaging. We point out that some signatures of the solid-solid transition, such as the appearance of well-resolved satellites and the latent heat, appear relatively sharp but depressed 10-15 °C in temperature, while the dynamic signature (here, the lithium motion that drives hydrogen and 7Li T1 relaxation) is qualitatively changed, with very rapid lithium motion extending well below the transition. In particular, we note that the T1 data sets show no discontinuity at the solid-solid transition in the aerogel-supported LiBH4 (Figure 5). Spin echoes on 7Li were formed at 125 °C in 13 nm aerogel using the 90x-τ-90y-τ-echo pulse sequence. This sequence is known to refocus quadrupole-broadened satellite transitions for spin 3/2. Phase cycling (x, jx of the first pulse and add/subtract signal acquisition) was used to avoid contamination from any FID arising from the second pulse. For delay times τ of 0.5 ms and longer, the satellite signal is absent in the echoes, while the central transition remains. This faster decay of the satellite transitions implies a shorter T2 of the satellites; clearly, the source of the extra decay must be quadrupolar. The lithium cations diffuse rapidly in the high-temperature phase, as in the bulk. The time-average efg, which results from the crystallite orientation, produces the Pake powder-average doublet satellite resonances at 100 and 200 °C in Figure 7. Diffusion of lithium cations between differently oriented crystallites results in a slow modulation of the efg and will impair echo refocusing of the satellite transition signals. In the slow motion limit, the satelliteecho decay time will equal the mean time to pass to a crystallite with significantly different orientation. We can estimate the distance traveled by lithium cations between differently oriented crystallites. The fundamental relation for displacement r during a time t for unrestricted diffusion (the aerogels are relatively open structures) in three dimensions is r2 ) 6Dt. For a single hop of length l, with mean hop rate τhop-1, this becomes l2 ) 6Dτhop. Together, these equations yield

(5)

Estimating T2 for the satellite echoes as 0.2 ms, using l ) 0.4 nm from the known LiBH4 high-temperature structure, and estimating τhop as 2 ns from the proximity to the T1 minima of Figure 5, we find the rms value of r to be 126 nm. This is much larger than the X-ray derived28 crystallite size (essentially equal to the pore size, 13 nm). What does this discrepancy indicate? Our estimation of the lithium hopping rate may be in error; certainly, a broad distribution of rates seems to exist, from the slow variation of the T1 data near the minima in Figure 5. Is it possible that the system is in the motionally narrowed limit,39 with motion between differently oriented crystallites occurring faster than the echo decay? No, because well-resolved satellites would not appear in the spectra of Figure 7. Thus, we arrive at one of two conclusions: (1) The lithium ions are nearly confined by the aerogel walls to remain in single pores. Given the relatively open structure of the aerogels in SEM images and the large fraction (70%) of volume occupied28 by the LiBH4, this seems very unlikely. (2) The crystallites in nearby pores have nearly the same orientations; after all, the crystallites in the different pores are well-connected. That is, while the diffraction coherence length is approximately the pore size,28 the distance between crystallites with significantly different orientations is much larger. Thus, most neighboring pores have crystallites of similar orientations. This second picture seems much more likely to us. Conclusions The hydrogen NMR line shape of LiBH4 supported in carbon polymer aerogel nanoscaffolds across a wide temperature range is a superposition of broad and motionally narrowed components. The fraction of intensity in the narrow line is the fraction of hydrogen atoms that are mobile, with motion rates ωH exceeding 105 s-1. This fraction grows continuously with temperature and is already evident at and below room temperature. The continuous evolution with temperature suggests that a continuous distribution of environments is present. Two observations demonstrate that the narrow resonance is associated with grain boundaries and LiBH4-aerogel walls and that the broad resonance arises from less defective, more nearly crystalline regions. First, the fraction of narrowed intensity is larger for 13 nm pore-size aerogel than for 25 nm material. Second, the broad resonance component eventually narrows in the same temperature window as for bulk LiBH4. A simple model of a rigid spherical crystalline core covered by a shell of thickness θ with mobile BH4 units indicates θ increases from low values to nearly 2 nm at 200 °C. Magnetization exchange between the broad and narrow components in hole-burning experiments demonstrates the close proximity of the hydrogen atoms for the two components. The rate of exchange is in accord with an estimate based on spin diffusion and a region about as large as the pore size. This magnetization exchange results in nearly equal hydrogen T1 relaxation times for the two resonances. The solid-solid phase transition for LiBH4 in aerogel is clearly evident in 7Li line shapes; above the phase transition, the central resonance sharpens and quadrupolar satellites become resolved. The transition is relatively sharp but depressed in temperature from the bulk by 10-15 °C, in agreement with earlier scanning calorimetry work. However, T1 of both hydro-

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