Solid State NMR Characterization of Complex Metal Hydrides systems for Hydrogen Storage Applications

Solid state NMR is widely applied in studies of solid state chemistries for hydrogen storage reactions. Use of ^(11)B MAS NMR in studies of metal borohydrides (BH_4) is mainly focused, revisiting the issue of dodecaborane formation and observation of ^(11)B{^1H} Nuclear Overhauser Effect

Hydrogenation of Magnesium Nickel Boride for Reversible Hydrogen Storage

We report that a ternary magnesium nickel boride (MgNi_(2.5)B_2) mixed with LiH and MgH_2 can be hydrogenated reversibly forming LiBH_4 and Mg_2NiH_4 at temperatures below 300 °C. The ternary boride was prepared by sintering a mechanically milled mixture of MgB_2 and Ni precursors at 975 °C under inert atmosphere. Hydrogenation of the ternary, milled with LiH and MgH_2, was performed under 100 to 160 bar H_2 at temperatures up to 350 °C. Analysis using X-ray diffraction, Fourier transform infrared, and ^(11)B magic angle spinning NMR confirmed that the ternary boride was hydrogenated forming borohydride anions. The reaction was reversible with hydrogenation kinetics that improved over three cycles. This work suggests that there may be other ternary or higher order boride phases useful for reversible hydrogen storage

Deuterium site occupancy and phase boundaries in ZrNiDx (0.87<=x<=3.0)

ZrNiDx samples with compositions between x=0.87 and x=3.0 were investigated by 2H magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR), powder x-ray diffraction (XRD), neutron vibrational spectroscopy (NVS), and neutron powder diffraction (NPD). The rigid-lattice MAS-NMR spectrum for a ZrNiD0.88 sample in the triclinic beta phase shows a single phase with two well-resolved resonances at +11.5 and −1.7 ppm, indicating that two inequivalent D sites are occupied, as was observed previously in ZrNiD1.0. For ZrNiD0.88, the ratio of spectral intensities of the two lines is 1:0.76, indicating that the D site corresponding to the +11.5 ppm line has the lower site energy and is fully occupied. Similarly, the neutron vibrational spectra for ZrNiD0.88 clearly confirm that at least two sites are occupied. For ZrNiD1.0, XRD indicates that ~5% of the metal atoms are in the gamma phase, corresponding to an upper composition for the beta phase of x=0.90±0.04, consistent with the MAS-NMR and neutron vibrational spectra indicating that x=0.88 is single phase. The MAS-NMR and NVS of ZrNiD1.87 indicate a mixed-phase sample (beta+gamma) and clearly show that the two inequivalent sites observed at x=0.88 cannot be attributed to the sites normally occupied in the gamma phase. For ZrNiD2.75, NPD results indicate a gamma-phase boundary of x=2.86±0.03 at 300 K, increasing to 2.93±0.02 at 180 K and below, in general agreement with the phase boundary estimated from the NVS and MAS-NMR spectra of ZrNiD1.87. Rigid-lattice 2H MAS-NMR spectra of ZrNiD2.75 and ZrNiD2.99 show a ratio of spectral intensities of 1.8±0.1:1 and 2.1±0.1:1 (Zr3Ni:Zr3Ni2), respectively, indicating complete occupancy of the lower-energy Zr3Ni2 site, consistent with the NPD results. For each composition, the correlation time for deuterium hopping was determined at the temperature where resolved peaks in the MAS-NMR spectrum coalesce due to motion between inequivalent D sites. The measured correlation times are consistent with previously determined motional parameters for ZrNiHx

Stability and Reversibility of Lithium Borohydrides Doped by Metal Halides and Hydrides

In an effort to develop reversible metal borohydrides with high hydrogen storage capacities and low dehydriding temperature, doping LiBH4 with various metal halides and hydrides has been conducted. Several metal halides such as TiCl3, TiF3, and ZnF2 effectively reduced the dehydriding temperature through a cation exchange interaction. Some of the halide doped LiBH4 are partially reversible. The LiBH4 + 0.1TiF3 desorbed 3.5 wt % and 8.5 wt % hydrogen at 150 and 450 °C, respectively, with subsequent reabsorption of 6 wt % hydrogen at 500 °C and 70 bar observed. XRD and NMR analysis of the rehydrided samples confirmed the reformation of LiBH4. The existence of the (B12H12)−2 species in dehydrided and rehydrided samples gives insight into the resultant partial reversibility. A number of other halides, MgF2, MgCl2, CaCl2, SrCl2, and FeCl3, did not reduce the dehydriding temperature of LiBH4 significantly. XRD and TGA-RGA analyses indicated that an increasing proportion of halides such as TiCl3, TiF3, and ZnCl2 from 0.1 to 0.5 mol makes lithium borohydrides less stable and volatile. Although the less stable borohydrides such as LiBH4 + 0.5TiCl3, LiBH4 + 0.5TiF3, and LiBH4 + 0.5ZnCl2 release hydrogen at room temperature, they are not reversible due to unrecoverable boron loss caused by diborane emission. In most cases, doping that produced less stable borohydrides also reduced the reversible hydrogen uptake. It was also observed that halide doping changed the melting points and reduced air sensitivity of lithium borohydrides

LiSc(BH_4)_4 as a Hydrogen Storage Material: Multinuclear High-Resolution Solid-State NMR and First-Principles Density Functional Theory Studies

A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH_4)_4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH_4 and ScCl_3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH_4)_4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH_4)_3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I (#82) is predicted to be the lowest energy state for LiSc(BH_4)_4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB_2 and [B_(12)H_(12)]^(−2) anion containing (e.g., Li_2B_(12)H_(12)) along with residual LiBH_4. Reaction of a desorbed LiSc(BH_4)_4 + 4LiCl mixture (from 4LiBH_4/ScCl_3 sample) with hydrogen gas at 70 bar resulted only in an increase in the contents of Li_2B_(12)H_(12) and LiBH_4. Full reversibility to reform the LiSc(BH_4)_4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications

Crystal structure of Li_2B_(12)H_(12): a possible intermediate species in the decomposition of LiBH_4

The crystal structure of solvent-free Li_2B_(12)H_(12) has been determined by powder X-ray diffraction and confirmed by a combination of neutron vibrational spectroscopy and first-principles calculations. This compound is a possible intermediate in the dehydrogenation of LiBH_4, and its structural characterization is crucial for understanding the decomposition and regeneration of LiBH_4. Our results reveal that the structure of Li_2B_(12)H_(12) differs from other known alkali-metal (K, Rb, and Cs) derivatives

Sum rules and three point functions

Sum rules constraining the R-current spectral densities are derived holographically for the case of D3-branes, M2-branes and M5-branes all at finite chemical potentials. In each of the cases the sum rule relates a certain integral of the spectral density over the frequency to terms which depend both on long distance physics, hydrodynamics and short distance physics of the theory. The terms which which depend on the short distance physics result from the presence of certain chiral primaries in the OPE of two R-currents which are turned on at finite chemical potential. Since these sum rules contain information of the OPE they provide an alternate method to obtain the structure constants of the two R-currents and the chiral primary. As a consistency check we show that the 3 point function derived from the sum rule precisely matches with that obtained using Witten diagrams.Comment: 41 page

Study of aluminoborane compound AlB_4H_(11) for hydrogen storage

Aluminoborane compounds AlB_4H_(11), AlB_5H_(12), and AlB_6H_(13) were reported by Himpsl and Bond in 1981, but they have eluded the attention of the worldwide hydrogen storage research community for more than a quarter of a century. These aluminoborane compounds have very attractive properties for hydrogen storage: high hydrogen capacity (i.e., 13.5, 12.9, and 12.4 wt % H, respectively) and attractive hydrogen desorption temperature (i.e., AlB_4H_(11) decomposes at ~125 °C). We have synthesized AlB_4H_(11) and studied its thermal desorption behavior using temperature-programmed desorption with mass spectrometry, gas volumetric (Sieverts) measurement, infrared (IR) spectroscopy, and solid state nuclear magnetic resonance (NMR). Rehydrogenation of hydrogen-desorbed products was performed and encouraging evidence of at least partial reversibility for hydrogenation at relatively mild conditions is observed. Our chemical analysis indicates that the formula for the compound is closer to AlB_4H_(12) than AlB_4H_(11)

Deuterium Exchange Dynamics in Zr_2NiD_(4.8) Studied by ^2H MAS NMR Spectroscopy

Variable temperature (VT) ^2H magic angle spinning (MAS) spectroscopy was employed to measure deuterium diffusion behavior in the Zr_2NiD_(4.8) phase. ^2H MAS NMR spectrum at ∼190 K provides with well-resolved 4 different site occupancies which can be assigned based on the crystal structure (16k (Zr_2Ni_2), 32m (Zr_3Ni), Zr_4 (16/ and 4b)). As the temperature rises, the ^2H peaks sensitively reflect the exchange behavior among the sites with evident change at around 230 K and reaching a uniform distribution of site occupancies, indistinguishable in NMR timescale, above 245 K. This behavior is reflected by the collapse of the ^2H MAS spectrum into a single peak. From analyses of VT MAS NMR spectra, we were able to extract multiple hopping rates and activation energies among face sharing interstices: for example, 32m ↔ 16/ hopping shows _(τc)=2.8×10^(-4)s at 245 K and E_a = 62.2 kJ/mol