New Miami, OH, United States
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Shane D.T.,Washington University in St. Louis | Corey R.L.,South Dakota School of Mines and Technology | Rayhel L.H.,Washington University in St. Louis | Wellons M.,Savannah River National Laboratory | And 5 more authors.
Journal of Physical Chemistry C | Year: 2010

LiBH4 doped with 1.6 mol % well-dispersed C60 is studied with solid-state nuclear magnetic resonance (NMR). Variable-temperature hydrogen NMR shows large changes between the data upon first heating and after exposure to 300 °C. After heating, a large fraction on the order of 50% of the hydrogen signal appears in a motionally narrowed peak, similar to a previous report of LiBH4 in a porous carbon aerogel nanoscaffold. Magic-angle spinning (MAS) NMR of 13C in a 13C-enriched sample finds the C60 has reacted already in the as-mixed (unheated) material. Dehydriding and rehydriding result in further 13C spectral changes, with nearly all intensity being found in a broad peak corresponding to aromatic carbons. It thus appears that the previously reported improved dehydriding and rehydriding kinetics of this material at least partially result from in situ formation of a carbon framework. The method may offer a new route to dispersal of hydrides in carbon support structures. © 2010 American Chemical Society.

Sorte E.G.,Washington University in St. Louis | Corey R.L.,Washington University in St. Louis | Corey R.L.,South Dakota School of Mines and Technology | Bowman Jr. R.C.,RCB Hydrides LLC | And 3 more authors.
Journal of Physical Chemistry C | Year: 2012

Hydrogen and 23Na NMR were used to probe diffusive motions of the ions in several NaH powders. In three NaH samples, the H resonance is a superposition of broad and narrow components, reflecting the presence of relatively immobile and mobile H, respectively. The fraction of mobile H grows from 23 to 250 °C; this pattern has been observed previously in other ionic hydrides. By 300 °C, the formerly broad hydrogen component has itself motionally narrowed. In these samples, the observation of a smaller amount of 23Na line narrowing by 300 °C indicates that only the H - ions are mobile at 300 °C, leaving 23Na- 23Na dipole interactions unaveraged. A deep minimum in the hydrogen rotating-frame relaxation time T 1ρ is observed near 325 °C, as expected from the onset of motional averaging. In a fourth sample, the entire H line is narrowed already by 150 °C. In this sample, the 23Na is also partially narrowed at this temperature and by 175-225 °C, further narrowing of the 23Na resonance indicates that now both ions are in rapid motion. In all the samples, the spin-lattice relaxation times T 1 for hydrogen and sodium decrease monotonically with temperature, in qualitative accord with relaxation by physical diffusion of spin magnetization to relaxation centers. © 2012 American Chemical Society.

Li W.,HRL Laboratories | Vajo J.J.,HRL Laboratories | Cumberland R.W.,HRL Laboratories | Liu P.,HRL Laboratories | And 4 more authors.
Journal of Physical Chemistry Letters | Year: 2010

We report that a ternary magnesium nickel boride (MgNi 2.5B 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 11B 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. © 2009 American Chemical Society.

Kim C.,California Institute of TechnologyCA | Kim C.,Hannam University | Hwang S.-J.,California Institute of TechnologyCA | Bowman R.C.,RCB Hydrides LLC | And 2 more authors.
Journal of Alloys and Compounds | Year: 2015

Variable temperature (VT) 2H magic angle spinning (MAS) NMR spectroscopy was employed to measure deuterium diffusion behavior in the Zr2NiD4.8 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 (Zr2Ni2), 32m (Zr3Ni), Zr4 (16l 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 → 16l hopping shows τc=2.8×10-4s at 245 K and Ea = 62.2 kJ/mol. © 2014 Elsevier B.V. All rights reserved.

Ivancic T.M.,Washington University | Hwang S.-J.,California Institute of Technology | Bowman Jr. R.C.,RCB Hydrides LLC | Birkmire D.S.,University of Hawaii at Manoa | And 3 more authors.
Journal of Physical Chemistry Letters | Year: 2010

The NaAlH4 system remains the archetype of hydrogen storage solids. However, the detailed mechanisms of the hydrogen reactions of NaAlH 4 remain unknown. We report 27Al in situ nuclear magnetic resonance (NMR) spectroscopic data revealing an Al-bearing mobile species that could provide the large scale metal-atom transport needed for rehydriding. This new species forms under reaction conditions but can be captured at ambient temperature using excess H2 pressure. The NMR measurements demonstrate that the species is highly mobile (at 20°C) and carries both Al and H atoms. On the basis of the 27Al shift (close to NaAlH 4) and the disorder evident in X-ray diffraction, the species is identified as highly defective NaAlH4, likely having a large AlH 3 vacancy concentration. © 2010 American Chemical Society.

Shane D.T.,Washington University in St. Louis | Corey R.L.,Washington University in St. Louis | Corey R.L.,South Dakota School of Mines and Technology | McIntosh C.,Washington University in St. Louis | And 5 more authors.
Journal of Physical Chemistry C | Year: 2010

Hydrogen NMR of LiBH 4 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 T 1 data of hydrogen and 7Li are continuous in the vicinity of the transition, in contrast with the bulk T 1 data, suggesting that some rapid lithium motion remains below the transition. © 2010 American Chemical Society.

Sorte E.G.,University of Washington | Emery S.B.,University of Washington | Majzoub E.H.,University of Missouri-St. Louis | Ellis-Caleo T.,University of Washington | And 5 more authors.
Journal of Physical Chemistry C | Year: 2014

1H and 27Al NMR is used to reveal the motions of AlH4 anions in KAlH4. Line-narrowing from rotations and from translational diffusion is observed in the NMR of both nuclei. Unlike the anions in NaAlH4 and LiAlH4 that are not rotating on the NMR time scale at room temperature, the KAlH4 anions are already rotating rapidly at 23 C. Based on the onset of rotation-induced line narrowing, the 1H T1 minimum, and the low- temperature hydrogen T1ρ minimum associated with reorientations, the rotational activation energy Erot,act = 0.28 eV is determined. Similarly, we use the onset temperature of translational motion-induced line narrowing and the high-temperature T1ρ minimum to determine the diffusion activation energy Ediff,act = 0.70 eV. Lack of sharp structure in the first-order quadrupole pattern and the absence of second-order quadrupole structure in the 27Al NMR data suggest asymmetry (η ≠ 0) and/or variations in the anion electric field gradients from structural disorder. © 2014 American Chemical Society.

Luo S.,University of Vermont | Flanagan T.B.,University of Vermont | Bowman Jr. R.C.,RCB Hydrides LLC
Journal of Alloys and Compounds | Year: 2013

Hydrogen absorption isotherms (273-333 K) have been measured for closely stoichiometric, annealed, un-activated LaNi5, i.e., a virgin un-activated LaNi5 sample was employed at each temperature. This is the first time that such isotherms have been measured for annealed, un-activated LaNi5 at such low temperatures ≤298 K. After measurement of the initial absorption isotherms, the second absorption and desorption isotherms have been measured at each temperature. Van't Hoff plots have been made from these absorption and desorption plateau pressure data and, from these, ΔHplat values have been obtained for the initial and subsequent absorption and desorption reactions. The |ΔHplat| determined from the van't Hoff plot is smaller for the initial isotherm for the annealed, un-activated form of LaNi5 than it is for activated LaNi5. © 2013 Elsevier B.V. All rights reserved.

Sorte E.G.,Washington University in St. Louis | Majzoub E.H.,University of Missouri-St. Louis | Ellis-Caleo T.,Washington University in St. Louis | Hammann B.A.,Washington University in St. Louis | And 4 more authors.
Journal of Physical Chemistry C | Year: 2013

The presence of approximately 10 mol % NaOH mixed with NaH powder is shown to result in much more rapid hydrogen motion in the NaH above 150 C, as indicated by the onset of hydrogen NMR line narrowing at this temperature. A similar result appears for air-exposed NaH due to the formation of hydroxide from atmospheric H2O. The NMR line narrowing is too rapid, as a function of temperature, to be described by thermal activation; rather, it is suggestive of a phase transition. Indeed, differential scanning calorimetry finds, after an initial thermal cycle, a reversible thermal anomaly indicating a phase transition near 150 C. Powder X-ray diffraction with an excess of NaOH displays a remarkable lattice expansion of the NaH in the temperature range of 100-240 C where the (200), (220), and (311) reflections indicate a volume lattice expansion of up to 11%; the expansion is reversible upon cycling. The data thus point to a reversible phase transition in which NaOH enters the NaH structure above 150 C and exits the NaH below that temperature. First-principles calculations using OH-substituted NaH supercells indicate significant solubility in the NaH lattice and find a transition temperature and enthalpy change that are in approximate agreement with differential scanning calorimetry (DSC) measurements, confirming the presence of a phase transition. © 2013 American Chemical Society.

Sorte E.G.,Washington University in St. Louis | Bowman Jr. R.C.,RCB Hydrides LLC | Majzoub E.H.,University of Missouri-St. Louis | Verkuijlen M.H.W.,Radboud University Nijmegen | And 2 more authors.
Journal of Physical Chemistry C | Year: 2013

Sodium alanate (NaAlH4) is the archetypical complex (ionic-covalent) hydride compound for hydrogen storage applications. However, the details of the reactions of this compound upon hydrogen cycling remain unclear. Recently, a new mobile aluminum-containing species termed S105 was discovered in the hydrogen chemistry of sodium alanate using in situ NMR with S105 a likely facilitator of Al-atom transport in NaAlH4. Here, we find that hydroxide impurities play a crucial role in the formation of the mobile species. Specifically, in bare NaAlH4, S105 is present after melting (under sufficient hydrogen pressure to block the dehydriding reaction) only in samples either exposed to H2O vapor or mixed with metal hydroxides. We find that the 27Al line position of S105 is close to that of NaAlH4 (after correcting for second-order quadrupole effects), indicating that S105 involves very mobile AlH4 - tetrahedra (rotationally and translationally). We propose that hydroxide impurities promote fast diffusion of nearby AlH4 - units, similar to enhanced motions seen in NaH; the hydroxides also react with NaAlH4 to form NaH and subsequently produce Na3AlH 6, which is always found to accompany S105. Our measurements reveal that the only chemical components of S105-containing alanate apart from hydroxides are NaAlH4 and Na3AlH6. Presence of the S105 species in NaAlH4 samples also leads to faster dehydriding in hot, undoped NaAlH4 solid, pointing to an enhancement of the hydrogen reaction kinetics by S105. © 2013 American Chemical Society.

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