Session S39: Focus Session: Hydrogen Storage IV

Thermodynamic and Vibrational Properties of LaTM$_{5}$ (TM = Co, Ni) Hydrides from Density Functional Theory

Thermodynamic and vibrational properties of La(TM)$_{5}$H$_{n}$ (with TM one of the magnetic transition metals Co or Ni) and their antecedent intermetallics are discussed. Enthalpies of formation, $\Delta $H, are computed with the plane wave density functional method implemented in the Vienna Ab Initio Simulation Package (VASP). All electron projector-augmented wave potentials based upon the generalized gradient approximation are used for the elemental constituents. With suitable supercells, the zero point and finite temperature contributions to $\Delta $H are computed with the direct phonon method using VASP as the computational engine. Phonon dispersion curves and total phonon density of states are examined for soft modes in each compound and important vibrational modes are identified. The computed vibrational spectra for LaCo$_{5}$ and LaCo$_{5}$H$_{4}$ reveal new information on their crystal structures.   

Hydrogen Bonding in CaSiH(D)$_{1+x}$: Is there Covalent Character?

We report here our neutron powder diffraction and neutron vibrational spectroscopy study of CaSiH(D)$_{1+x}$ along with first-principles calculations, which reveal the hydrogen structural arrangements and bonding in this novel alloy hydride. Both structural and spectroscopic results show that, for $x>$0, H(D) atoms start occupying a Ca$_{3}$Si interstitial site. The corresponding Si-H(D) bond length is determined to be 1.82~{\AA}, fully 0.24~{\AA} larger than predicted by theory. Here we discuss in detail our neutron spectroscopic measurements, which are also generally at odds with strongly covalent Si-H bonding in CaSiH$_{1+x}$ that such calculations suggest. These results may have implications for a number of ongoing studies of metal-hydrogen systems destabilized by Si alloying.   

Slow H hopping motions in MgH$_{2}$ and alanates

Unlike the interstitial (metallic) metal-hydrides, ionic and/or complex hydrides such as MgH$_{2}$ and NaAlH$_{4}$ have very slow rates of H atomic hopping. Because the rates are too small ($<$10$^{5}$ s$^{-1})$ for motional narrowing of the dipolar-broadened H NMR lines, we report here the rates of motion from T$_{1D}$ slow-motion measurements. The activation energy for H motion in MgH$_{2}$ has been determined to be 1.45 eV. In undoped NaAlH$_{4}$, the rate of motion is also thermally activated. In ScCl$_{3}$-doped NaAlH$_{4}$, much faster motion (shorter T$_{1D})$ is found, even at low-temperature. At low-T, the role of rotating AlH$_{6}$ groups formed by partial dehydriding is suspected.   

Structure and Bonding in Destabilized Metal Hydrides for Hydrogen Storage.

Light-metal hydrides possess high hydrogen-storage capacities ($>$ 5 wt.{\%}), but their utility is generally compromised by high thermal stability, rather slow absorption kinetics, and/or problems with reversibility for hydrogen absorption/desorption cycling. There has been great emphasis, particularly in recent years, on attempts to destabilize and otherwise improve the properties of these hydrides by alloying with Si and other elements. We describe here the study of lithium and calcium hydrides alloyed with Si and Ge using ball-milling techniques. The details of the structure and bonding of the Li/Si/H(D), Li/Ge/H(D) and Ca/Si/H(D) systems have been revealed through a combination of neutron and x-ray diffraction, neutron spectroscopy and first-principles calculations. We report the discovery of several new hydride phases, the nature of Si-H bonding in these hydride systems and the effects of amorphization in the Ca/Si/H alloys. The implications of our results for future investigations will be discussed.   

Neutron Scattering And thermodynamic Investigations of Hydrogen Adsorbed on or within Nanomaterials

Nanometer scale materials offer neutron scatters significant opportunities to investigate the adsorption or entrainment properties of hydrogen bearing molecular gases and liquids. We report our latest investigations of combined thermodynamic, computation, neutron diffraction and inelastic scattering (INS) studies of the structure and dynamics of H2 films adsorbed on MgO (100) surfaces and entrained within an oriented, ordered- hexagonal array of cylindrical tunnels within an alumina or amorphous carbon matrix. By combining the INS data with our neutron diffraction results using D2 on the same materials and with computational efforts we propose adsorption behavior that accounts for our findings. Finally, we indicate what other opportunities exist for future experiments in these areas.   

A Study of Magnesium Hydride Thin Film Phase Transition Kinetics Using \textsl{In-Situ} Hydriding/Dehydriding

Magnesium is an attractive material for hydrogen storage because it stores an appreciable amount of hydrogen (7.6 wt.\%) as magnesium hydride (MgH$_{2}$), is abundant in the earth's crust and is relatively inexpensive. Understanding of the structural changes and associated kinetics for the magnesium/magnesium hydride phase transition is crucial to engineering practical metal hydride hydrogen storage materials involving magnesium. A thin film architecture allows us to deposit and analyze precisely controlled structures in order to gain insight into the kinetic mechanisms present in the phase change. Using UHV sputter deposition onto a variety of substrates we have grown Mg thin films with varying degrees of structural texture and orientation. Using x-ray diffraction with \textsl{in-situ} sample heating we see evidence for a solid phase epitaxial (SPE) regrowth mechanism for the Mg regrowing from the MgH$_{2}$ in epitaxial Mg thin films and observe kinetic differences for the discharging of films with different Mg orientations (Mg c-axis in/out of the sample plane). We also determined the crystallographic orientation correlation for the Mg to MgH$_{2}$ transition in our epitaxial thin films. Here we also present our recent work examining and analyzing the kinetics for sample \textsl{charging} utilizing a variety of methods.   

Cohesion of BaReH$_9$ and BaMnH$_9$: Density Functional Calculations and Prediction of (MnH$_9$)$^{2-}$ Salts

Density functional calculations are used to calculate the structural and electronic properties of BaReH$_9$ and to analyze the bonding in this compound. This compound has an exceptionally high H to metal ratio of 4.5. The high coordination of Re in BaReH$_9$ is due to bonding between Re 5d states and states of d-like symmetry formed from combinations of H s orbitals in the H$_9$ cage. This explains the structure of the material, its short bond lengths and other physical properties, such as the high band gap. We compare with results for hypothetical BaMnH$_9 $, which we find to have similar bonding and cohesion to the Re compound. This suggests that it may be possible to synthesize (MnH$_9$)$^{2-}$ salts. Depending on the particular cation, such salts may have exceptionally high hydrogen contents, in excess of 10 weight \%.   

Thermodynamic considerations in the synthesis of complex metal hydrides via mechanicosynthetic techniques

Complex metal hydrides have been synthesized for hydrogen storage through a new synthetic technique utilizing high hydrogen overpressure at elevated temperatures (molten state processing). This synthesis technique holds the potential of fusing different known complex hydrides at elevated temperatures and pressures to form new species with enhanced hydrogen storage properties. Formation of these compounds is driven by thermodynamic and kinetic considerations. Novel synthetic complexes were structurally characterized and there hydrogen desorption properties were investigated. The effectiveness of the molten state process will be compared with mechanicosynthetic ball milling.   

Combinatorial thin film deposition and infrared emission characterization of hydrogen storage materials

Optimal hydrogen sorption/desorption behavior (temperature, pressure, kinetics) depends on a composition and a microstructural state. Combinatorial thin films provide a wide range of continuously changing compositions and microstructures (amorphous, nanocrystalline, single crystal, multiphases) on a single substrate. In this paper we report preparation and characterization of two systems, Fe$_{2}$Ti-FeTi$_{2}$ and MgNi-Mg on silicon wafers with Pd overlayers. The specimens were prepared by a shutter-controlled multilayer e-beam deposition. After-deposition annealing can create a variety of microstructural states. Both as-deposited and annealed films were fully characterized by SEM, x-ray, and selectively by TEM. Hydrogenation of the films was monitored with an infrared (IR) camera. Changes in the IR emissivity in response to the film hydrogenation and phase transition behavior will be discussed.   

Hydrogen Fueling via Guanidine

Three related materials, ammonia (NH3), urea (OCN2H4), and guanidine (CN3H5) are practicable hydrogen-based fuels$^{1}$ that could be produced in the giga-tonne quantities required from air, water and renewable energy. NH3 has long been established as a fuel for internal combustion engines and can be cracked to H2 for use in fuelcells, but is a gas at STP and extremely toxic, so general use is problematic. Urea and guanidine can easily be converted to NH3 and CO2 by addition of hot water from oxidation of NH3. Both are solids at STP, non-toxic, non-explosive and commonly shipped in plastic bags. The energy density in kWhr/L of guanidine is 4.7 compared with 3.0 for urea, 3.5 for liquid NH3, and 0.8 for H gas in 10,000 psi tanks. The specific energies in kWhr/kg for these materials are respectively 3.58, 2.35, 5.2, and (including the tank) 1.8. Guanidine melts at 50 C and is infinitely soluble in both ethanol and water. 1) http://www.energy.iastate.edu/renewable/biomass/AmmoniaMtg06.html   

Bi-Liquid Hydrogen Generation Using Familiar Materials

Greater acceptance of Fuel Cell Power Systems has been greatly constrained due to the lack of a low cost, energy dense, and convenient hydrogen source to fuel these systems. This talk will present a novel bi-liquid approach to resolving current impediments to mobile hydrogen production, and how current R{\&}D is applicable to this bi-liquid approach. The implications of this bi-liquid fueling concept on other primary Fuel Cell subsystems, and an approach to commercial implementation will also be presented. The closing remarks will additionally identify benefits to the nation beyond those normally envisioned in the promise of a hydrogen based economy.   

Hydrogen clathrate hydrates as a potential hydrogen storage material

Recent synthetic activities involving hydrogen clathrate hydrates raised the prospect of utilizing them as an alternative storage material for hydrogen fuel. The current work is a starting point for future studies of hydrogen occupancy of hydrogen clathrate hydrate and its stability. We present studies of the structural and thermal properties of a hydrogen molecule dissolved in liquid water and their possible implication for the hydrogen storage in clathrate hydrates. The radial distribution function, coordination number and coordination number distribution are calculated using different representations of the interatomic forces within molecular dynamics, Monte Carlo and ab initio molecular dynamics simulation frameworks. Although structural details differ in the radial distribution functions generated from the different force fields, all approaches agree that the average and most probable number of water molecules occupying the inner hydration sphere around hydrogen is 16. Furthermore, we estimate the hydrogen hydration free energy. In addition, we will present the quantum mechanical studies of the hydrogen occupancy in single clathrate hydrate cages.   

Clathrate hydrates studied by diffraction and vibrational spectroscopy.

Clathrate hydrate structures are a potentially viable method for hydrogen storage (Mao and Mao 2004). For simple hydrogen-water clathrates, low temperatures ($<$150 K) or high pressures ($>$2 kbar) are needed for stability. We investigated, using inelastic neutron spectroscopy, the hydrogen storage character of a clathrate of hydrogen with the addition of tetrahydrofuran as a promoter molecule. The addition of tetrahydrofuran allows the formation of the clathrate structure at elevated temperature and decreased pressure as compared to the hydrogen clathrate (Lee, et al. 2005). In addition we have examined the higher pressure clathrate forms at lower temperatures. High pressure diamond anvil work has allowed Raman and x-ray spectroscopy on novel clathrate environments. Analysis these model compounds will assist in future investigations to additional clathrate compounds. \newline Lee, Huen, et al. ``Tuning Clathrate Hydrates for Hydrogen Storage.'' \textit{Nature} 434 (April 2005): 743-746. Mao, Wendy, and Ho-kwang Mao. ``Hydrogen Storage in Molecular Compounds.'' \textit{Proceedings of the National Academy of Sciences} 101, no. 3 (2004): 708-710.