Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L39: Focus Session: Hydrogen Storage II |
Hide Abstracts |
Sponsoring Units: FIAP DMP Chair: Terrence Udovic, NIST Center for Neutron Research Room: Colorado Convention Center 502 |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L39.00001: NMR Studies of the Li-Mg-N-H Phases. Robert Bowman, J. W. Reiter, J. G. Kulleck, S.-J. Hwang, Weifang Luo Solid state NMR including magic-angle-spinning (MAS) and cross-polarization (CP) MAS experiments have been used to characterize various amide and imide phases containing Li and/or Mg. MAS-NMR spectra for the $^{1}$H, $^{6}$Li, $^{7}$Li, and $^{15}$N nuclei have been obtained to improve understanding on formation, processing, and degradation behavior. Only limited information could be obtained from the proton and $^{7}$Li MAS-NMR spectra to due large dipolar interactions and small chemical shifts. However, more success was obtained from the $^{6}$Li and $^{15}$N nuclei although their very long spin-lattice relaxation times did impact signal acquisition times. For example, three distinct $^{6}$Li peaks were resolved from LiNH$_{2}$ phases that were clearly separated from the LiH secondary phase in these samples. While the $^{15}$N spectra for LiNH$_{2}$ phase in isotopically enriched samples exhibited only a single peak at least three distinct $^{15}$N peaks were observed from the similarly enriched Mg amide samples. These differences will be related to crystal structures. The NMR spectra also revealed very little motion in these hydrides upon to nearly 500 K. [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L39.00002: Electronic Structure and Energetics of the Quaternary Hydride \textbf{Li}$_{4}$\textbf{BN}$_{3}$\textbf{H}$_{10}$ Jan Herbst, Louis Hector Jr. Li$_{4}$BN$_{3}$H$_{10}$ has been synthesized recently from LiNH$_{2}$/LiBH$_{4}$ mixtures and its crystal structure determined. We have calculated the electronic structure of this complex hydride and investigated its thermodynamic stability and decomposition energetics. We find that its enthalpy of formation is --708 kJ/mole with respect to the elemental constituents and --6 kJ/mole relative to a 3:1 molar LiNH$_{2}$/LiBH$_{4}$ mixture, in qualitative agreement with experiment. Reaction enthalpies computed for several decomposition pathways suggest \begin{center} Li$_{4}$BN$_{3}$H$_{10} \quad \to $ Li$_{3}$BN$_{2}+\textstyle{1 \over 2}$Li$_{2}$NH + $\textstyle{1 \over 2}$NH$_{3}$ + 4H$_{2}$ \end{center} as the likely dehydriding route. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L39.00003: Tetragonal I4$_{1}$/amd Crystal Structure of Li$_{3}$BN$_{2}$ from Dehydrogenated Li-B-N-H Frederick Pinkerton, Jan Herbst We have determined the crystal structure of Li$_{3}$BN$_{2}$ formed by dehydrogenation of LiB$_{0.33}$N$_{0.67}$H$_{2.67}$ from powder x-ray diffraction (XRD) data using the Rietveld method. XRD measurements indicate unambiguously that this Li$_{3}$BN$_{2}$ polymorph is distinct from any of the previously reported Li$_{3}$BN$_{2}$ phases. We find a body-centered tetragonal I4$_{1}$/amd structure (space group No. 141 in the \textit{International Tables for X-ray Crystallography}) with a = 6.60 {\AA} and c = 10.35 {\AA}. The structure features tightly coordinated, nearly linear N--B--N units with 1.3 - 1.4 {\AA} B--N bond lengths suggesting covalently bonded (BN$_{2})^{-3}$ anions. In situ temperature-dependent XRD showed that the body-centered tetragonal Li$_{3}$BN$_{2}$ phase was present both at elevated temperature during dehydrogenation and after cooling to room temperature. We also describe the results of first principles theoretical modeling of the body-centered tetragonal Li$_{3}$BN$_{2}$ polymorph as well as the tetragonal P4$_{2}$2$_{1}$2 and monoclinic P2$_{1}$/c Li$_{3}$BN$_{2}$ structures previously reported. We obtained excellent agreement between the theoretically calculated I4$_{1}$/amd Li$_{3}$BN$_{2}$ lattice constants and atomic positions and those obtained experimentally from XRD. The approximate enthalpy of formation of the I4$_{1}$/amd Li$_{3}$BN$_{2}$ phase is $\Delta {\rm H}$ = --495 kJ/mole-formula unit. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L39.00004: Ab-initio Investigations of Li and Mg Amide-Imide Systems for Hydrogen Storage Takao Tsumuraya, Tatsuya Shishidou, Tamio Oguchi Reversible hydrogen storage in light-element materials has been recognized as one of the most practical approaches for on-board application. Lithium nitride Li$_3$N can reversibly store large amount of hydrogen in the two-step reversible reaction composed of lithium amide LiNH$_2$ and imide Li$_2$NH[1]. Quite recently, in an effort to reach further performance, several types of magnesium substitutions in Li-N-H system have been investigated. For instance, Leng $et$ $al$. have examined a composite material made by ball milling of 3:8 molar mixture of magnesium amide Mg(NH$_2$)$_2$ and lithium hydride LiH[2]. The hydrogenating and dehydrogenating reaction mechanism and fundamental properties of these hydrides still remain as a matter to be investigated. In particular, crystal structures of some metel imides such as Li$_2$NH, MgNH and Li$_2$Mg(NH)$_2$ are not fully determined yet. In this paper, we discuss structural stability and heats of formation of these hydrides from first-principles calculations based on the all-electron FLAPW method. [1] P. Chen Z. Xiong, J. Luo, J. Lin and K.L. Tan, Nature 420, 302 (2002). [2] H. Y. Leng, T. Ichikawa, S. Hino, N. Hanada, S. Isobe and H. Fujii, J. Phys. Chem. B 108, 8763 (2004). [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L39.00005: First-principles investigation of the Li-Mg-N-H system Zhu Ma, Yan Wang, Mei-Yin Chou The Li-Mg-N-H system has been identified as a promising hydrogen storage material due to its moderate operation conditions as well as the high capacity and reversibility. The Li-Mg mixed imide is reported to have disordered cation or cation-vacancy arrangements at room temperature and above. We present our first-principles investigation to study the crystal structure of Li$_2$Mg(NH)$_2$ using total energy calculations within the density functional theory. A series of ordered low-energy configurations are identified. Specific local orderings are found in the cation-vacancy arrangement, shedding light on the experimental disordered structure models. A possible ordered phase at lower temperature is proposed based on our calculation. Furthermore, the reaction energetics and phase stability involved in this system are discussed. [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L39.00006: Ab-initio kinetics and thermodynamics studies of ammonia-borane for hydrogen storage Caetano R. Miranda, Gerbrand Ceder Ammonia-borane (BH$_{3}$NH$_{3})$ is a promising chemical hydrogen storage material given its high gravimetry and volumetric properties. However, the ammonia-borane (AB) thermal hydrogen release is not very efficient, being mainly limited by the kinetics of hydrogenation. Using ab initio calculations, we have investigated the thermodynamics and kinetics of hydrogen release on AB by calculating the free energies of the H$_{2}$ release reactions for different possible decomposition products. Our results indicate that AB regeneration through the ammonia-borane polymeric and borazine-cyclotriborazane cycles is very unlikely due to the strong exothermic character of the reactions. The kinetics of hydrogen release is further investigated with the recently developed metadynamics method. This method allows us to calculate the multidimensional free energy surface of hydrogen release on AB. Our simulations reveal the atomistic mechanism of hydrogenation and provide the free energies barriers and transition states involved in inter and intramolecule H$_{2}$ release on AB. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L39.00007: pardInvestigation of the Direct Hydrogenation of Aluminum to Alane in Supercritical Fluids Craig Jensen, Sean McGrady, Reyna Ayabe, Ben Reddy Alane, AlH$_{3}$ has many of the properties that are requisite for materials to be considered viable for onboard hydrogen storage applications. Most notibly, it contains 10.1 wt{\%} hydrogen and undergoes dehydrogenation at appreciable rates at temperatures below 100$^{\circ}$C. However, the very low, $\ge $ 6 kJ/mol, enthalpy of dehydrogenation of AlH$_{3}$ prohibits subsequent re-hydrogenation through standard gas-solid techniques except at very high pressures or very low temperatures. The extremely low solubility of gaseous H$_{2}$ in conventional organic solvents also vitiates a solution-based approach. Re-hydrogenation of Al using a supercritical fluid potentially offers a workable approach since the fluid can act as a solvent, at the same time remaining completely miscible with permanent gases like hydrogen. Recently, it has been found that mixtures of NaH and Al can be hydrogenated to sodium alanate, NaAlH$_{4}$ under modest pressures and temperatures in supercritical fluids. We have now extended these studies to the hydrogenation of Al to AlH$_{3}$. The results of these studies and experimental details will be reported. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L39.00008: Stability Studies of Aluminum Hydride Xia Tang, Bruce Laube, Donald Anton, Son-Jong Hwang, Robert Bowman Aluminum hydride has attracted research attention recently as a promising hydrogen storage material due to its high gravimetric, volumetric storage capacity and very low enthalpy. AlH3 forms several phases, all of which are sensitive to moisture. In this study, the discharge kinetics of a stabilized form of alpha aluminum hydride newly synthesized was evaluated. Its desorption kinetics were measured in the temperature range of 60-120$^{\circ}$C at one atmosphere of hydrogen pressure. The material was stable at ambient temperature and no significant dehydrogenation was observed at 60$^{\circ}$C after 70 hours. Approximately 10 wt\% hydrogen was rapidly (quantify in wt\%/min.) released at 100$^{\circ}$C with no additional catalization. The activation energy for desorption was measured at 97.0 KJ/mole H2. The surface and bulk characterization methods Auger, SEM, XRD, and solid state NMR were used to investigate the mechanism of stabilization. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L39.00009: Atomic Simulations of Alane Phase Transformations and Dehydrogenation Mechanisms Susanne Opalka, Paul Saxe, Ole Martin Lovvik Density functional theory atomic ground state, molecular dynamics, and direct method lattice dynamic simulations were used to mechanistically probe phase transformations between the various crystallographically refined $\alpha $, $\alpha $'$_{, }\beta $, and $\gamma $ AlH$_{3}$ phases. Lattice dynamic predictions of the AlH$_{3}$ structures provided an ideal test case for systematically accessing the accuracy of the vibrational thermodynamic property contributions with the harmonic approximation. The predicted transformation pathways involved coordinated tilting and rotation mechanisms, similar to that observed in perovskite structures. Further simulations were conducted to elucidate the mechanism for $\alpha $ AlH$_{3}$ phase decomposition to the Al and H$_{2}$ products and to identify probable barriers to reversible rehydrogenation. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L39.00010: Trapped H$_{2}$ in AlH$_{3}$ Mark Conradi, Lasitha Senadheera, Erik Carl, T.M. Ivancic, R.C. Bowman, Jr., S.J. Hwang, T.J. Udovic Trapped molecular hydrogen has been discussed for years in H-storage systems such as NaAlH$_{4}$. Here we report proton NMR and neutron vibration spectroscopy (NVS) evidence for H$_{2}$ in AlH$_{3}$ samples. In static sample NMR, a sharp line appears on top of the broad AlH$_{3}$ solid signal. MAS further sharpens this line and identifies it as H$_{2}$ by its chemical shift. Upon cooling, the line broadens and disappears near 20K, confirming the H$_{2}$ identification. NVS reports energy-gain peaks at the H$_{2}$ rotational energy (J=1 to 0). [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L39.00011: Inelastic Neutron Scattering Investigation of Ti-doped NaAlH$_{4}$ Monika Hartl, Alice Acatrinei, Luke Daemen Complex hydrides (i.e. alanates (AlH$_{4})^{-}$ or borates (BH$_{4})^{-})$ are widely investigated as hydrogen storage materials. They have lower formation energy than simple metal hydrides and usually higher hydrogen to metal ratio. However, kinetics and performance still represent the main challenge for the actual application of these materials as hydrogen storage materials. The use of transition metal dopants such as Ti, Fe, Zr can improve the hydrogen exchange capability and hydrogen storage capability of a complex metal hydride significantly. However, a satisfactory explanation how and why certain dopants work best with certain complex metal hydrides has not yet been given. We choose sodium aluminium hydride NaAlH$_{4}$ doped with various amount of titanium (precursor: TiCl$_{4})$ for our research on the mechanism of doping. Incoherent inelastic neutron spectroscopy is a well-suited tool to look at hydride (H$^{-})$ in the material and the changes of the hydride in the material upon addition of dopants. Possible changes in the lattice of the ``host material'' NaAlH$_{4}$ are observed by X-Ray diffraction. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L39.00012: Hydrogen-related defects and the role of Ti in NaAlH4 Amra Peles, Chris Van de Walle Titanium-doped sodium alanate is a promising storage material; however, the mechanism of the enhancement in (de)hydrogenation rates induced by Ti has remained unresolved. We performed a comprehensive investigation of hydrogen vacancies and interstitials, which play an important role in the (de)hydrogenation processes. Interestingly, these highly mobile defects cause large rearrangements of the surrounding lattice, and they are always charged; their formation energy therefore depends on the Fermi level. Our investigations show that the Ti-induced modification of the Fermi level increases the defect concentrations, thus explaining the improved kinetics. These novel insights may prompt a reexamination of the role of transition-metal impurities in alanates and related materials, and lead to the design of storage materials with improved characteristics. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L39.00013: Dehydrogenation of NaAlH$_{4}$ from First-principles Molecular Dynamics Brandon Wood, Nicola Marzari Key among the materials challenges facing a hydrogen economy is the discovery of lightweight materials for reversible hydrogen storage in the solid state. Although chemical and metal hydrides have been under intense investigation, progress has been curtailed by a lack of understanding of the reaction paths for hydrogenation and dehydrogenation in such materials. We present here our first-principles molecular dynamics results for NaAlH$_{4}$, one of the promising and most extensively studied candidates for hydrogen storage. We analyze proton transport in the presence of a variety of low-energy defects and surfaces, and we discuss possible candidates for the key mechanisms of dehydrogenation in the material. The results are presented in terms of the structural phase transition to $\alpha$-Na$_{3}$AlH$_{6}$. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L39.00014: Thermodynamic properties of LiAlH4 from first-principle calculations Xuezhi Ke, Changfeng Chen The potential hydrogen-storage materials of LiAlH4, and Li3AlH6 have been studied by using density functional theory (at GGA level), and harmonic phonon approximation. The thermodynamic properties of these materials have been studied in detail. We found that the decomposition of LiAlH4 is not reversible, which may indicate that the direct synthesis of LiAlH4 may be not possible. The calculations indicate that Li3AlH6 can be used as a hydrogen-storage material under certain conditions. In addition, the phase diagram of these materials will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L39.00015: Hydrogen adsorption studies in micro-size cobalt dots A. L. Cabrera, C. P. Romero, J. I. Avila, E. Cisternas, G. B. Cabrera, K. Temst, M. J. Van Bael Hydrogen desorption curves were obtained from a sample composed of square arrangement of Co dots with average diameter of 4.4 microns, separated by a distance of 11.6 microns. A macroscopic sample of Co dots grown on a 2.5x2.5 cm Si substrate was made by standard lithographic techniques and used in these experiments. Thermal programmed desorption (TPD) was performed under ultra-high vacuum conditions. Hydrogen TPD curves were obtained from a 1x1 cm sample of Co dots, Co films and Co foils for comparison. The hydrogen TPD curves peaked at 425 K and have decreasing intensity from the Co foils to the Co dots and to the Co films. A desorption energy of 27 Kcal/mol was obtained for the Co dots suggesting that hydrogen is adsorbed on an hcp or fcc hollow site of the Co dot crystalline structure. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700