Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y24: Focus Session: Hydrogen Storage: Atomic and Molecular Motions |
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Sponsoring Units: DMP Chair: Eric Majzoub, University of Missouri--St. Louis Room: 326 |
Friday, March 20, 2009 8:00AM - 8:36AM |
Y24.00001: Atomic Motions in Ionic Hydrides: MgH$_{2}$, NaMg$_{3}$H$_{3}$, and LiBH$_{4}$ Invited Speaker: In hydrogen storage, rapid hydrogen diffusion is a key component for rapid reaction rates of dehydriding and rehydriding. In metallic systems, the light interstitial H atoms typically do display rapid diffusion. However, recent interest has focused on ionic and complex hydrides of light metal-atoms because of their high weight fractions of reversible hydrogen. These ionic complex hydrides generally reveal slow hydrogen diffusion and resultingly slow reaction kinetics. We report here studies of H diffusion using NMR in several such hydrides. In MgH$_{2}$, the rate $\omega _{H}$ of H hopping remains too slow to narrow the H NMR up to 400 $^{\circ}$C. T$_{1D}$ measurements, however, can detect the motion and find an activation energy of 1.72 eV, the first reported direct measurement of diffusion in MgH$_{2}$. In ball-milled (bm) material with Nb$_{2}$O$_{5}$ catalyst additive, a fraction of the resonance intensity is narrowed starting at 50 $^{\circ}$C, with the narrow fraction growing to 30{\%} by 400 $^{\circ}$C. A model for continuous growth of the narrow line, based on a wide distribution of motion rates, is presented. Ball-milling also greatly increases the laboratory-frame relaxation rates, T$_{1}^{-1}$, from paramagnetic defects created by the mechanical process. In bm NaMgH$_{3}$, an even larger fraction of the resonance is motionally-narrowed, growing to nearly 100{\%} by 300 $^{\circ}$C. Clearly, ball-milling has a much more profound effect on ionic hydrides than the simple reduction of grain sizes and diffusion distances. In coarse-grain LiBH$_{4}$ (with 13.8 weight{\%} reversible hydrogen), an orientationally disordered solid phase occurs above 110 $^{\circ}$C. Above the transition, the rate of Li ion diffusion increases remarkably. H diffusion starts to narrow the H NMR line around 170 $^{\circ}$C, continuing to narrow up to the melt near 280 $^{\circ}$C. To distinguish diffusion of (already rapidly rotating) BH$_{4}$ units from H exchange between neighboring BH$_{4}$, the $^{11}$B resonance was studied. The boron line central transition becomes much narrower (400 Hz) than the width (1500 Hz) expected from Van Vleck M$_{2}$ for the case of static boron spins (with rapid Li and H diffusion). Thus, intact BH$_{4}$ units are the diffusing species. Even in molten LiBH$_{4}$, the BH$_{4}$ lifetime is found to be at least 2 seconds from observations of the B-H J-coupling pattern, so it is probably much larger in the solid. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y24.00002: Understanding and Enhancing Hydrogen Diffusion in MgH$_{2}$ and NaMgH$_{3}$ David Sholl, Shiqiang Hao The transport properties of hydrogen in metal hydrides are crucial to the kinetics of H$_{2}$ storage in these materials. We use first-principles calculations to identify the defects that are relevant for H transport in MgH$_{2}$ and NaMgH$_{3}$. In both materials, the physically relevant defects are charged and H diffusion is dominated by mobility of negatively charged interstitial H. Interestingly, the diffusion of these species occurs via concerted mechanisms with low energy barriers. To improve the charged interstitial H diffusivity, a series of transition-metal additives are screened to lower the formation energy of mobile defects. Our results provide a practical way to examine and alter H diffusion in light metal hydrides. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y24.00003: Quasielastic Neutron Scattering of Hydrogen Adsorbed in KC$_{24}$ Justin Purewal, James Keith, Channing Ahn, Brent Fultz, Craig Brown Quasielastic neutron scattering (QENS) and volumetric techniques were used to study the adsorption of H$_{2}$ by the stage-2 potassium graphite intercalation compound KC$_{24}$. A zero-coverage sorption enthalpy of 8.5~kJ/mol was measured from H$_{2}$ isotherms recorded at 77~K and 87~K. The saturation H$_{2}$ adsorption amount at 77~K was 1.2~mass\%, corresponding to a stoichiometry of KC$_{24}$(H$_{2}$)$_{2.0}$. Quasielastic neutron scattering spectra for KC$_{24}$(H$_{2}$)$_{1.0}$ were collected at temperatures between 40~K and 80~K on a chopper spectrometer and a backscattering spectrometer. Two distinct H$_{2}$ diffusion processes were identified with characteristic times of approximately $\tau = 10$~ps and $\tau = 350$~ps at 60~K, respectively. By operating the backscattering spectrometer in fixed window mode, the total elastic scattering of KC$_{24}$(H$_{2}$)$_{1.0}$ was measured as a function of temperature. A sharp decrease in elastic intensity was observed at 35~K due to the onset of quasielastic scattering. This was interpreted as a melting transition of the H$_{2}$ adsorbate in KC$_{24}$. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y24.00004: Novel geometry for simultaneous resistive, Hall and optical measurement of MgH$_{x}$ thin films D.W. Koon, C. C. W. Griffin, J.R. Ares, F. Leardini, C. Sanchez We describe a novel specimen geometry we have used to simultaneously probe optical transmission, sheet resistance and sheet Hall resistance in 100nm Mg films during hydrogen absorption. A Mg-film cloverleaf overlaps four rectangular Pd pads at the corners of a glass slide, a variation on a two-pad geometry used by Ingason and Olafsson for resistive studies of Pd-capped MgH$_{x}$ films [J. Alloys and Compounds \textbf{404- 406} (2005), 469-72.]. Hydrogen diffuses laterally through the Pd pads before entering the magnesium layer from below. The sample holder also includes an LED-photodiode pair for measuring [monochromatic] optical transmission. We show that the simultaneous measurement of these three quantities during the metal-to-insulator transition in a hydriding MgH$_{x}$ film allows for a more complete understanding of the hydriding process in these films. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y24.00005: Proton Tunneling: A Decay Channel of the O-H Stretch Mode in $\mathrm{KTaO}_{3}$ Erik Spahr, Michael Stavola, Lanlin Wen, Lynn Boatner, Leonard Feldman, Norman Tolk, Gunter L\"{u}pke Proton vibrational dynamics play a key role in the important processes of hydrogen diffusion and transport. In particular, perovskite structured proton conductors are an important class of hydrogen transport materials with a wide range of potential applications. We have measured for the first time the vibrational lifetimes of the O-H and O-D stretch modes in the perovskite oxide, $\mathrm{KTaO}_{3}$, by pump-probe infrared spectroscopy. Both stretch modes are exceptionally long lived and exhibit a large ``reverse'' isotope effect, due to a phonon-assisted proton tunneling process, which involves the O-Ta-O bending motion. The excited-state tunneling rate is found to be seven orders of magnitude larger than from the ground state in the proton conducting oxide, $\mathrm{BaCeO}_{3}$ [1]. [1] I. Kuskovsky et al., Phys. Rev. B \textbf{60}, R3713 (1999). [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y24.00006: Reversible Low Temperature Hydrogen Storage Using Ternary Borides Wen Li, John Vajo, Robert Cumberland, Ping Liu, Son-Jong Hwang, Chul Kim, Robert Bowman Among many materials for hydrogen storage, complex borohydride of light metals with high hydrogen capacity, have been studied extensively. However, the thermodynamic and kinetic properties of borohydrides limit their ability to cycle hydrogen reversibly at low temperature. For example, although LiBH$_{4}$ is thermodynamically quite stable, the formation of LiBH$_{4}$ from LiH + B requires elevated temperatures and pressures of up to 600 \r{ }C and 150 bar. Here, we report ternary borides with active boron species that can be hydrogenated forming [BH$_{4}$]$^{-}$ anions at temperatures as low as $\sim $280\r{ }C. These ternary borides were prepared through milling of precursors followed by thermal treatment under inert atmosphere. Samples were then milled with additional binary hydrides before hydrogenation. Analysis using FTIR and $^{11}$B MAS NMR indicated that the ternary borides were hydrogenated to [BH$_{4}$]$^{-}$ species with good kinetics. After hydrogenation, the mixture could be cycled with dehydrogenation occurring in two steps that begin at 280\r{ }C and 345\r{ }C, respectively. Characterization using FTIR, $^{11}$B MAS NMR, and XRD, indicates that the [BH$_{4}$]$^{-}$ anions are consumed in the first dehydrogenation step. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y24.00007: Thermodynamic and kinetic destabilization in LiBH$_{4}$-based hydride systems John Vajo, Wen Li, Ping Liu The LiBH$_{4}$/MgH$_{2}$ destabilized hydride system has attracted considerable attention recently because it is a reversible system, based on the [BH$_{4}$]$^{-}$ anion, with a high hydrogen capacity (11.6 wt {\%} gravimetric). However, hydrogen release during a temperature ramp occurs in two steps with dehydrogenation of MgH$_{2}$ to Mg + H$_{2}$ occurring first followed by reaction of Mg with LiBH$_{4}$ to form LiH + MgB$_{2}$ + H$_{2}$. These two steps occur despite there being a direct reaction that is thermodynamically allowed at lower temperatures. In this talk we describe a LiBH$_{4}$-based hydride system that is kinetically as well as thermodynamically destabilized. In this system, a direct (concerted) dehydrogenation reaction of LiBH$_{4}$ occurs at temperatures lower than the dehydrogenation temperature of any of the components separately. In addition, the system is reversible with an equilibrium hydrogen pressure (based on preliminary measurements) that is $\sim $20X higher than the pressure for the LiBH$_{4}$/MgH$_{2}$ system. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y24.00008: Structure and librational dynamics in borohydrides Monika Hartl, Michael Wolverton, Alice Acatrinei, Abhijit Bhattacharyya, Luke Daemen Borohydrides are candidates for reversible hydrogen storage. The attention accorded to this class of materials is supported by extensive hydrogenation/dehydrogenation thermodynamic measurements. However, the underlying chemical reaction mechanisms remain uncertain. We used neutron diffraction and inelastic neutron scattering, together with a computational approach, to examine the connection between structure and dynamics in several borohydrides and the possible role played by dynamics in the approach to the dehydrogenation transition state. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y24.00009: First-principles Study on the Vibration Modes and Electronic Structure of Alkali and Alkaline-earth Amides and Alanates Takao Tsumuraya, Tatsuya Shishidou, Tamio Oguchi Light alkaline and alkaline-earth metal hydrides such as amides $M$(NH$_2$)$_n$ and alanates $M$(AlH$_4$)$_n$ ($M$=K, Na, Li, Ca, and Mg) have attracted a growing interest as reversible hydrogen storage materials recently because of their innately high hydrogen contents. [1, 2] We study the electronic structure of the amides and alanates with different cations, focusing on the role of cation states from first-principles calculations based on the all-electron FLAPW method. Calculated breathing stretch vibration modes for these compounds are compared with measured infrared and Raman spectra. In the amides, we find a significant tendency such that the breathing stretch vibration frequencies and the structural parameters of NH$_2$ vary in accordance with the ionization energy of cation, which may be explained by the strength in hybridization between cation orbitals and molecular orbitals of (NH$_2$)$^-$. We elucidate the microscopic mechanism of correlations between the breathing stretch vibration frequencies of N-H and structural parameters by analyzing the calculated electronic structure from a view point of the molecular-orbitals. A similar tendency in the alanates is also discussed. [1] P. Chen, Z. Xiong, J. Luo, J. Lin and K.L. Tan, Nature \textbf{420}, 302 (2002). [2] B. Bogdanovi and M. Schwickardi, J. Alloys Compd. \textbf{253-254}, 1 (1997). [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y24.00010: Neutron spectroscopy of $\gamma $-AlH$_{3}$ Alexander Kolesnikov, Jason Graetz, Craig Jensen, Walker Langley, Vladimir Antonov The density of vibrational states, G(E), for $\gamma $-AlH$_{3}$ is measured by inelastic neutron scattering. The obtained spectrum noticeably differs from that of $\alpha $-AlH$_{3}$, because $\alpha $-AlH$_{3}$ has a structure built of corner-sharing AlH$_{6}$ octahedra, while the $\gamma $-AlH$_{3}$ is composed of both corner- and edge-sharing AlH$_{6}$ octahedra. The first acoustic phonon peak in the G(E) of $\gamma $-AlH$_{3}$ appears at a lower energy and the band of translational modes extends to higher energies (55 vs. 42 meV) than in the spectrum of $\alpha $-AlH$_{3}$. The bands of Al-H bending modes are observed at about the same energies of 60--140 meV in $\gamma $-AlH$_{3}$ and $\alpha $-AlH$_{3}$, only the structures of the bands are different. The Al-H stretching modes in the $\gamma $-phase show eight peaks in the range 145 to 265 meV, while in the $\alpha $-phase they exhibits only two peaks at 200 and 235 meV. The G(E) spectrum of $\gamma $-AlH$_{3}$ is in good agreement with recent simulations [Y. Wang \textit{et al., Phys. Rev. B} \textbf{77} (2008) 014101], which took into account the existence of a unique double-bridge bond between certain Al and H atoms. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y24.00011: Cubic Metallic Phase of Aluminum Hydride Showing Improved Hydrogen Desorption Ralph H. Scheicher, Duck Young Kim, Rajeev Ahuja AlH$_3$ is of great interest for hydrogen storage applications, with a particularly attractive feature being its large hydrogen capacity of 10 wt.\%. Here we report the results of our density functional theory study of the dehydrogenation properties in a cubic phase of AlH$_3$. The metallic nature of the electronic structure entails a more favorable hydrogen removal energy which is lowered by 75\% compared to the insulating hexagonal phase. This remarkable reduction in the Al--H bond strength might bear important consequences for feasible applications of AlH$_3$ as an on-board hydrogen storage material for mobile applications. Suggestions are made how the cubic phase could be prepared and stabilized at ambient pressure by off-board quenching. See also: R.\ H.\ Scheicher, D.\ Y.\ Kim, S.\ Leb\`egue, B.\ Arnaud, M.Alouani, and R.\ Ahuja, {\it Appl.\ Phys.\ Lett.} {\bf 92}, 201903 (2008) and D.\ Y.\ Kim, R.\ H.\ Scheicher, and R.\ Ahuja, {\it Phys.\ Rev.\ B} {\bf 78}, 100102(R) (2008). [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y24.00012: Regeneration of Aluminum Hydride studied with Raman Microscopy. David Lacina, Jason Graetz, J.J. Reilly We are interested in developing new methods to form aluminum hydride directly from aluminum powder and hydrogen. Due to the low free energy of formation, aluminum and hydrogen require extremely high pressures to react and form the hydride. It is possible to form alane directly at low pressure when it is catalyzed with a small amount of titanium (2 mol {\%}) and stabilized as an adduct. We have studied the formation of amine-alanes by direct hydrogenation of aluminum and have attempted to understand the mechanisms behind these reversible reactions and the role of the catalyst. We will present the results from our recent survey of possible reactions between aluminum, hydrogen and various amines. We will also present the results of a Raman spectroscopy study of the alane polymorphs at ambient and high pressure and alane amines. [Preview Abstract] |
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