NOVEL CATALYTIC EFFECTS OF FULLERENE FOR LIBH4 HYDROGEN UPTAKE AND RELEASE

Our recent novel finding, involving a synergistic experiment and first-principles theory, shows that carbon nanostructures can be used as catalysts for hydrogen uptake/release in aluminum based complex metal hydrides (sodium alanate) and also provides an unambiguous understanding of how the catalysts work. Here we show that the same concepts can be applied to boron based complex hydride such as lithium borohydride, LiBH{sub 4}. Taking into account electronegativity and curvature effect a fullerene-LiBH{sub 4} composite demonstrates catalytic properties with not only lowered hydrogen desorption temperatures, but regenerative rehydriding at relatively lower temperature of 350 C. This catalytic effect likely originates from interfering with the charge transfer from Li to the BH4 moiety, resulting in an ionic bond between Li{sup +} and BH{sub 4}{sup -}, and a covalent bond between B and H. Interaction of LiBH{sub 4} with an electronegative substrate such as carbon fullerene affects the ability of Li to donate its charge to BH{sub 4}, consequently weakening the B-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H{sub 2} to reverse the dehydrogenation reaction. Degradation of cycling capacity is observed and is attributed to forming irreversible intermediates or diboranes

Temperature-Programmed Desorption: Principles, Instrument Design, and Demonstration With NaAlH4

This article is a brief introduction to temperature-programmed desorption (TPD), an analytical technique devised to analyze, in this case, materials for their potential as hydrogen storage materials. The principles and requirements of TPD are explained and the different components of a generic TPD apparatus are described. The construction of a modified TPD instrument from commercially available components is reported together with the control and acquisition technique used to create a TPD spectrum. The chemical and instrumental parameters to be considered in a typical TPD experiment and the analytical utility of the technique are demonstrated by the dehydrogenation of titanium-doped NaAlH{sub 4} by means of thermally programmed desorption

CARBON NANOMATERIALS AS CATALYSTS FOR HYDROGEN UPTAKE AND RELEASE IN NAALH4

A synergistic approach involving experiment and first-principles theory not only shows that carbon nanostructures can be used as catalysts for hydrogen uptake and release in complex metal hydrides such as sodium alanate, NaAlH{sub 4}, but also provides an unambiguous understanding of how the catalysts work. The stability of NaAlH{sub 4} originates from the charge transfer from Na to the AlH{sub 4} moiety, resulting in an ionic bond between Na{sup +} and AlH{sub 4}{sup -} and a covalent bond between Al and H. Interaction of NaAlH{sub 4} with an electro-negative substrate such as carbon fullerene or nanotube affects the ability of Na to donate its charge to AlH{sub 4}, consequently weakening the Al-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H{sub 2} to reverse the dehydrogenation reaction. Ab initio molecular dynamics simulation further reveals the time evolution of the charge transfer process with hydrogen desorption occurring when the charge transfer is complete

ALUMINUM HYDRIDE: A REVERSIBLE MATERIAL FOR HYDROGEN STORAGE

Hydrogen storage is one of the greatest challenges for implementing the ever sought hydrogen economy. Here we report a novel cycle to reversibly form high density hydrogen storage materials such as aluminium hydride. Aluminium hydride (AlH{sub 3}, alane) has a hydrogen storage capacity of 10.1 wt% H{sub 2}, 149 kg H{sub 2}/m{sup 3} volumetric density and can be discharged at low temperatures (< 100 C). However, alane has been precluded from use in hydrogen storage systems because of the lack of practical regeneration methods; the direct hydrogenation of aluminium to form AlH{sub 3} requires over 10{sup 5} bars of hydrogen pressure at room temperature and there are no cost effective synthetic means. Here we show an unprecedented reversible cycle to form alane electrochemically, using alkali alanates (e.g. NaAlH{sub 4}, LiAlH{sub 4}) in aprotic solvents. To complete the cycle, the starting alanates can be regenerated by direct hydrogenation of the dehydrided alane and the alkali hydride being the other compound formed in the electrochemical cell. The process of forming NaAlH{sub 4} from NaH and Al is well established in both solid state and solution reactions. The use of adducting Lewis bases is an essential part of this cycle, in the isolation of alane from the mixtures of the electrochemical cell. Alane is isolated as the triethylamine (TEA) adduct and converted to pure, unsolvated alane by heating under vacuum

INVESTIGATION OF THE THERMODYNAMICS GOVERNING METAL HYDRIDE SYNTHESIS IN THE MOLTEN STATE PROCESS.

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 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. We report on investigations of the thermodynamics. Novel synthetic complexes were formed, structurally characterized, and their hydrogen desorption properties investigated. The effectiveness of the molten state process is compared with mechanicosynthetic ball milling

HIGH TEMPERATURE PRESSURE PROCESSING OF MIXED ALANATE COMPOUNDS

Mixtures of light-weight elements and hydrides were investigated to increase the understanding of the chemical reactions that take place between various materials. This report details investigations we have made into mixtures that include NaAlH{sub 4}, LiAlH{sub 4}, MgH{sub 2}, Mg{sub 2}NiH{sub 4}, alkali(ne) hydrides, and early third row transition metals (V, Cr, Mn). Experimental parameters such as stoichiometry, heat from ball milling versus hand milling, and varying the temperature of high pressure molten state processing were studied to examine the effects of these parameters on the reactions of the complex metal hydrides

Hydrogen Motion in Magnesium Hydride by NMR

In coarse-grained MgH2, the diffusive motion of hydrogen remains too slow (<10^5 hops s^−1) to narrow the H NMR line up to 400 °C. Slow-motion dipolar relaxation time T1D measurements reveal the motion, with hopping rate ωH from 0.1 to 430 s^−1 over the range of 260 to 400 °C, the first direct measurement of H hopping in MgH2. The ωH data are described by an activation energy of 1.72 eV (166 kJ/mol) and attempt frequency of 2.5 × 10^15 s^−1. In ball-milled MgH2 with 0.5 mol % added Nb2O5 catalyst, line-narrowing is evident already at 50 °C. The line shape shows distinct broad and narrow components corresponding to immobile and mobile H, respectively. The fraction of mobile H grows continuously with temperature, reaching ∼30% at 400 °C. This demonstrates that this material’s superior reaction kinetics are due to an increased rate of H motion, in addition to the shorter diffusion paths from ball-milling. In ball-milled MgH2 without additives, the line-narrowed component is weaker and is due, at least in part, to trapped H2 gas. The spin−lattice relaxation rates T1^−1 of all materials are compared, with ball-milling markedly increasing T1^−1. The weak temperature dependence of T1^−1 suggests a mechanism with paramagnetic relaxation centers arising from the mechanical milling

Synthesis, Characterization, and Atomistic Modeling of Stabilized Highly Pyrophoric Al(BH_4)_3 via the Formation of the Hypersalt K[Al(BH_4)_4]

The recent discovery of a new class of negative ions called hyperhalogens allows us to characterize this complex as belonging to a unique class of materials called hypersalts. Hyperhalogen materials are important while serving as the building blocks for the development of new materials having enhanced magnetic or oxidative properties. One prime example of a hydperhalogen is the Al(BH_4)_4^– anion. Aluminum borohydride (17 wt % H) in itself is a volatile, pyrophoric compound that has a tendency to release diborane at room temperature, making its handling difficult and very undesirable for use in practical applications. Here we report that the combination of Al(BH_4)_3 with the alkaline metal borohydride KBH_4 results in the formation of a new compound KAl(BH_4)_4 which is a white solid that exhibits remarkable thermal stability up to 154 °C and has the typical makeup of a hypersalt material. Using a variety of characterization tools and theoretical calculations, we study and analyze the physical characteristics of this compound and show its potential for stabilizing high hydrogen capacity, energetic materials

NMR Studies of the Hydrogen Storage Compound NaMgH_3

Hydrogen and ^(23)Na NMR were performed to 400 °C on NaMgH3 powder produced by reactive ball-milling of NaH and MgH2. The H resonance shows narrowing already at 100 °C as a narrow line superimposed on the broad, rigid-lattice signal. With increasing temperature, the fraction of spins in the narrow component grows smoothly, approaching 100% near 275 °C. This heterogeneous narrowing suggests a wide distribution of H motion rates. After annealing at 400 °C, the narrow component intensity at temperatures below 200 °C was substantially reduced and both H and ^(23)Na relaxation rates 1/T_1 were decreased. Thus, it appears that the high rates of H motion, particularly on first heating, are due to regions with poorly organized crystal structure. If this disorder could be maintained, this might be an avenue toward improved reaction kinetics of this or other hydrides. In the annealed sample, the activation energy for H diffusion is approximately 95 kJ/mol