Ragaiy Zidan

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(4), but also provides an unambiguous understanding of how the catalysts work. Here we show that the stability of NaAlH(4) originates with the charge transfer from Na to the AlH(4) moiety, resulting in an ionic bond between Na(+) and AlH(4)(-) and a covalent bond between Al and H. Interaction of NaAlH(4) with an electronegative substrate such as carbon fullerene or nanotube affects the ability of Na to donate its charge to AlH(4), consequently weakening the Al-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H(2) to reverse the dehydrogenation reaction. In addition, based on our experimental observations and theoretical calculations it appears the curvature of the carbon nanostructure plays a role in the catalytic process. Ab initio molecular dynamics simulation further reveals the time evolution of the charge transfer process.

Synthesis and Characterization of a Lithium-Doped Fullerane (Lix-C60-Hy) for Reversible Hydrogen Storage

Herein, we present a lithium-doped fullerane (Li(x)-C(60)-H(y)) that is capable of reversibly storing hydrogen through chemisorption at elevated temperatures and pressures. This system is unique in that hydrogen is closely associated with lithium and carbon upon rehydrogenation of the material and that the weight percent of H(2) stored in the material is intimately linked to the stoichiometric ratio of Li:C(60) in the material. Characterization of the material (IR, Raman, UV-vis, XRD, LDI-TOF-MS, and NMR) indicates that a lithium-doped fullerane is formed upon rehydrogenation in which the active hydrogen storage material is similar to a hydrogenated fullerene. Under optimized conditions, a lithium-doped fullerane with a Li:C(60) mole ratio of 6:1 can reversibly desorb up to 5 wt % H(2) with an onset temperature of ~270 °C, which is significantly less than the desorption temperature of hydrogenated fullerenes (C(60)H(x)) and pure lithium hydride (decomposition temperature 500-600 and 670 °C respectively). However, our Li(x)-C(60)-H(y) system does not suffer from the same drawbacks as typical hydrogenated fullerenes (high desorption T and release of hydrocarbons) because the fullerene cage remains mostly intact and is only slightly modified during multiple hydrogen desorption/absorption cycles. We also observed a reversible phase transition of C(60) in the material from face-centered cubic to body-centered cubic at high levels of hydrogenation.

NOVEL CATALYTIC EFFECTS OF FULLERENE FOR LIBH4 HYDROGEN UPTAKE AND RELEASE

The addition of catalysts to complex hydrides is aimed at enhancing the hydrogen absorption desorption properties. Here we show that the addition of carbon nanostructure C60 to LiBH4 has a remarkable catalytic effect, enhancing the uptake and release of hydrogen. A fullerene-LiBH4 composite demonstrates catalytic properties with not only lowered hydrogen desorption temperatures but also regenerative rehydrogenation at a relatively low temperature of 350 degrees C. This catalytic effect probably originates from C60 interfering with the charge transfer from Li to the BH4 moiety, resulting in a minimized ionic bond between Li+ and BH4(-), and a weakened covalent bond between B and H. Interaction of LiBH4 with an electronegative substrate such as carbon fullerene affects the ability of Li to donate its charge to BH4, consequently weakening the B-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H2. Degradation of cycling capacity is observed and is probably due to the formation of diboranes or other irreversible intermediates.

Hydriding characteristics of NaMgH2F with preliminary technical and cost evaluation of magnesium-based metal hydride materials for concentrating solar power thermal storage

A simplified techno-economic model has been used as a screening tool to explore the factors that have the largest impact on the costs of using metal hydrides for concentrating solar thermal storage. The installed costs of a number of paired metal hydride concentrating solar thermal storage systems were assessed. These comprised of magnesium-based (MgH2, Mg2FeH6, NaMgH3, NaMgH2F) high-temperature metal hydrides (HTMH) for solar thermal storage and Ti1.2Mn1.8H3.0 as the low-temperature metal hydride (LTMH) for hydrogen storage. A factored method approach was used for a 200 MWel power plant operating at a plant capacity factor (PCF) of 50% with 7 hours of thermal storage capacity at full-load. In addition, the hydrogen desorption properties of NaMgH2F have been measured for the first time. It has a practical hydrogen capacity of 2.5 wt% (2.95 wt% theoretical) and desorbs hydrogen in a single-step process above 478 °C and in a two-step process below 478 °C. In both cases the final decomposition products are NaMgF3, Na and Mg. Only the single-step desorption is suitable for concentrating solar thermal storage applications and has an enthalpy of 96.8 kJ mol−1 H2 at the midpoint of the hydrogen desorption plateau. The techno-economic model showed that the cost of the LTMH, Ti1.2Mn1.8H3.0, is the most significant component of the system and that its cost can be reduced by increasing the operating temperature and enthalpy of hydrogen absorption in the HTMH that, in turn, reduces the quantity of hydrogen required in the system for an equivalent electrical output. The result is that, despite the fact that the theoretical thermal storage capacity of NaMgH2F (1416 kJ kg−1) is substantially lower than the theoretical values for MgH2 (2814 kJ kg−1), Mg2FeH6 (2090 kJ kg−1) and NaMgH3 (1721 kJ kg−1), its higher enthalpy and operating temperature leads to the lowest installed cost of the systems considered. A further decrease in cost could be achieved by utilizing metal hydrides with yet higher enthalpies and operating temperatures or by finding a lower cost option for the LTMH.

Synthesis, characterization, and reversible hydrogen sorption study of sodium-doped fullerene

Herein is presented a novel, straightforward route to the synthesis of an alkali metal-doped fullerene as well as a detailed account of its reversible and enhanced hydrogen sorption properties in comparison to pure C60. This work demonstrates that a reaction of sodium hydride with fullerene (C60) results in the formation of a sodium-doped fullerene capable of reversible hydrogen sorption via a chemisorption mechanism. This material not only demonstrated reversible hydrogen storage over several cycles, it also showed the ability to reabsorb over three times the amount of hydrogen (relative to the hydrogen content of NaH) under optimized conditions. The sodium-doped fullerene was hydrogenated on a pressure composition temperature (PCT) instrument at 275 °C while under 100 bar of hydrogen pressure. The hydrogen desorption behavior of this sodium-doped fullerene hydride was observed over a temperature range up to 375 °C on the PCT and up to 550 °C on the thermogravimetric analysis (TGA). Powder x-ray diffraction verifies the identity of this material as being Na6C60. Characterization of this material by thermal decomposition analysis (e.g. PCT and TGA methods), as well as FT-IR and mass spectrometry, indicates that the hydrogen sorption activity of this material is due to the reversible formation of a hydrogenated fullerene (fullerane). However, the reversible formation of fullerane was found to be greatly enhanced by the presence of sodium. It was also demonstrated that the addition of a catalytic amount of titanium (via TiO2 or Ti(OBu)4) further enhances the hydrogen sorption process of the sodium-doped fullerene material.

Study of chemically synthesized MgMgH2 for hydrogen storage

Abstract A detailed study was carried out to determine the basis for the improved characteristics of a MgMgH 2 system that was previously reported by Bogdanovic and co-workers in this journal. The hydriding-dehydriding kinetics of this chemically synthesized system were investigated. A scanning electron microscope was used to examine the surface morphology of chemically hydrided samples. The surface of chemically prepared samples appeared to be covered with microspheroidal beads ranging in radius between 0.5 and 0.05 μm formed in a fractal-like configuration. Theoretical analysis indicates that the morphology of the chemically prepared samples could be responsible for rapid hydriding-dehydriding. The kinetic enhancement is believed to be partially due to the substantial increase in the surface area and partially due to the fast diffusion into the smaller particles. The effect of the addition of nickel to the surface was also investigated.

Investigation of hydrogen induced fluorescence in C60 and its potential use in luminescence down shifting applications

Herein the photophysical properties of hydrogenated fullerenes (fulleranes) synthesized by direct hydrogenation utilizing hydrogen pressure (100 bar) and elevated temperatures (350 °C) are compared to the fulleranes C60H18 and C60H36 synthesized by amine reduction and the Birch reduction, respectively. Through spectroscopic measurements and density functional theory (DFT) calculations of the HOMO-LUMO gaps of C60Hx (0 ≤ x ≤ 60), we show that hydrogenation significantly affects the electronic structure of C60 by decreasing conjugation and increasing sp3 hybridization. This results in a blue shift of the emission maximum as the number of hydrogen atoms attached to C60 increases. Correlations in the emission spectra of C60Hx produced by direct hydrogenation and by chemical methods also support the hypothesis of the formation of C60H18 and C60H36 during direct hydrogenation with emission maxima of 435 and 550 nm respectively. We also demonstrate that photophysical tunability, stability, and solubility of C60Hx in a variety of organic solvents make them easily adaptable for application as luminescent down-shifters in heads-up displays, light-emitting diodes, and luminescent solar concentrators. The utilizization of carbon based materials in these applications can potentially offer advantages over commonly utilized transition metal based quantum dot chromophores. We therefore propose that the controlled modification of C60 provides an excellent platform for evaluating how individual chemical and structural changes affect the photophysical properties of a well-defined carbon nanostructure.

Development and Characterization of Novel Complex Hydrides Synthesized via Molten State Processing

This study developed novel hydrides for hydrogen storage through a novel synthesis technique utilizing high hydrogen overpressure at elevated temperatures denoted as Molten State Processing, MSP. The ultimate goal is to produce materials that have high hydrogen capacity, are stable after cycling and possess favorable thermodynamic and kinetic characteristics compatible with onboard hydrogen storage for automotive applications. In order to achieve these goals the MSP Process was developed and used to modify and form new complex hydride compounds with desired characteristics. This synthesis technique holds the potential of fusing different known complex hydrides at elevated temperatures and pressures to form new complexes having different sorption and thermodynamic properties. The new complex hydrides produced by this method were identified through structural determination and thermodynamic characterization in order to achieve a more fundamental understanding of their formation and dissociation mechanisms.

Spectroscopic Evidence for Atmospheric Stabilization of Aluminum Borohydride in Polydimethylsiloxane Grease

Raman and infrared vibrational spectroscopy were used to confirm the presence of aluminum borohydride dissolved in a commercial polydimethylsiloxane vacuum grease at room temperature. Spectroscopic evidence for an adduct between the aluminum borohydride and polydimethylsiloxane is also presented. Once dissolved in the polydimethylsiloxane grease, the aluminum borohydride was stabilized with respect to its usual pyrophoric reactivity in wet or dry air.