Alanates

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Alexandre Alexakis - One of the best experts on this subject based on the ideXlab platform.

K. J. Gross - One of the best experts on this subject based on the ideXlab platform.

  • thermal properties characterization of sodium Alanates
    Journal of Alloys and Compounds, 2005
    Co-Authors: Daniel E. Dedrick, Michael P. Kanouff, B C Replogle, K. J. Gross
    Abstract:

    High energy density hydrogen storage is a critical technology requirement in a hydrogen-based energy infrastructure. Although there are no current storage methods that meet desired energy density goals for vehicular hydrogen storage, complex metal hydride based systems are among the most promising. These materials form compounds with hydrogen under appropriate conditions and release hydrogen by thermal decomposition. The complex hydride, sodium alanate, is particularly useful due to its favorable reversibility. Thermal properties characterization of sodium alanate has been performed at Sandia National Laboratories to gain a detailed understanding of how complex hydrides will behave in a storage system. Thermal properties were investigated using the thermal probe method (ASTM D5334). Custom test hardware was designed and built to accommodate the complex decomposition and recombination of sodium alanate. Thermal conductivity and thermal wall resistance were determined by utilizing analytical and numerical data analysis methods. The thermal conductivity of sodium alanate was found to vary by more than 90% with changes in phase composition and hydrogen gas pressures between 1 and 100 atm. The quality of thermal contact between the alanate and the vessel wall was characterized numerically for various pressures and phase compositions. The contact resistance is high for all states, indicating poor contact between the material and the vessel wall.

  • the effects of titanium precursors on hydriding properties of Alanates
    Journal of Alloys and Compounds, 2003
    Co-Authors: K. J. Gross, E H Majzoub, S Spangler
    Abstract:

    An overview is presented of recent advances in the development of new and improved Alanates for applications and in the fundamental understanding of how Ti-doping enhances hydrogen absorption. Sample materials were produced using approaches based on direct-synthesis and dry Ti-doping methods. It is desirable to introduce Ti through non-reactive processes to avoid the hydrogen capacity loss that occurs through the formation of inactive byproducts (for example Na–halide from the decomposition of Ti–halides and Na–oxides from the decomposition of Ti–alkoxides). We show, for the first time, that Alanates can be Ti-doped using TiH2 or through indirect-doping by pre-reacting TiCl2 with LiH. Both methods result in enhanced kinetics. However, improved rates were achieved only after a prolonged activation period of about a 10 cycles, suggesting that cycling leads to Ti diffusion and substitution into the alanate lattice which provides the mechanism through which Ti-doping enhances kinetics. Thus, the reactive decomposition of Ti–halide and alkoxide precursors in the doping process serves an important but not necessarily required function.

  • catalyzed Alanates for hydrogen storage
    Journal of Alloys and Compounds, 2002
    Co-Authors: K. J. Gross, G J Thomas, Craig M. Jensen
    Abstract:

    The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the Alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH4⇔1/3Na3AlH6+2/3Al+H2⇔NaH+Al+3/2H2. Initial work resulted in improved catalysts, advanced methods of preparation, and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties, as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions were observed real-time through in situ X-ray powder diffraction. These measurements demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verified the segregation of aluminum to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 bar at 33°C, which suggest that, thermodynamically, this material is ideally suited to on-board hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at temperatures approaching that found in the operation of PEM fuel cells (125–165°C) were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase the capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti–Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications.

  • dynamic in situ x ray diffraction of catalyzed Alanates
    Journal of Alloys and Compounds, 2002
    Co-Authors: K. J. Gross, G Sandrock, G J Thomas
    Abstract:

    The discovery that hydrogen can be reversible absorbed and desorbed from NaAlH{sub 4} by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. NaAlH{sub 4} releases hydrogen through the following set of decomposition reactions: NaAlH{sub 4} {r_arrow} 1/3({alpha}-Na{sub 3}AlH{sub 6}) + 2/3Al + H{sub 2} {r_arrow} NaH + Al + 3/2H{sub 2}. These decomposition reactions as well as the reverse recombination reactions were directly observed using time-resolved in-situ x-ray powder diffraction. These measurements were performed under conditions similar to those found in PEM fuel cell operations (hydrogen absorption: 50--70 C, 10--15 bar Hz, hydrogen resorption: 80--110 C, 5--100 mbar H{sub 2}). Catalyst doping was found to dramatically improve kinetics under these conditions. In this study, the alanate was doped with a catalyst by dry ball-milling NaAlH{sub 4} with 2 mol.% solid TiCl{sub 3}. X-ray diffraction clearly showed that TiCl{sub 3} reacts with NaAlH{sub 4} to form NaCl during the doping process. Partial desorption of NaAlH{sub 4} was even observed to occur during the catalyst doping process.

  • Catalyzed Alanates for hydrogen storage
    Journal of Alloys and Compounds, 2002
    Co-Authors: K. J. Gross, G J Thomas, Craig M. Jensen
    Abstract:

    The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the Alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH 4⇔1/3Na 3AlH 6+2/3Al+ H 2⇔NaH+Al+3/2H 2. Initial work resulted in improved catalysts, advanced methods of preparation, and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties, as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions were observed real-time through in situ X-ray powder diffraction. These measurements demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verified the segregation of aluminum to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 bar at 33°C, which suggest that, thermodynamically, this material is ideally suited to on-board hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at temperatures approaching that found in the operation of PEM fuel cells (125-165°C) were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase the capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti-Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications. © 2002 Elsevier Science B.V. All rights reserved.

Maximilian Fichtner - One of the best experts on this subject based on the ideXlab platform.

  • M alanate—a material for reversible hydrogen storage?
    2020
    Co-Authors: Maximilian Fichtner, Olaf Fuhr, Oliver Kircher
    Abstract:

    Magnesium alanate was synthesized in a metathesis reaction of magnesium chloride and sodium alanate followed by purification. The material obtained was sufficiently pure and it was investigated by X-ray diffraction (XRD) and by thermogravimetry (TG) and mass spectrometry (MS) of the evolved gas, respectively. Thermal analysis showed a decomposition with a release of hydrogen proceeding in two major steps. Measured in vacuum, the peak decomposition temperature of the first step was found to be 163 8C and the residue at 200 8C consisted of MgH and Al which continues to release hydrogen and transforms into an Al Mg /Al mixture at higher temperatures. 2 32 In the first decomposition step 6.6 wt.% of hydrogen was released. To enhance the kinetics of the decomposition, magnesium alanate was doped with a titanium based promoter and ball milled for up to 100 min, resulting in a significantly reduced peak decomposition temperature. First results on determining thermodynamic properties indicate equilibrium desorption pressures in the range of AB 5 compounds. In total, the promising properties determined so far have inspired further investigations on the thermodynamics, kinetics and reabsorption properties of the compound.  2002 Elsevier Science B.V. All rights reserved.

  • wide line solid state nmr characterizations of sodium Alanates
    Journal of Physical Chemistry C, 2009
    Co-Authors: M H W Verkuijlen, Maximilian Fichtner, Jan P M Van Bentum, Wiebke Lohstroh, A P M Kentgens
    Abstract:

    Pure NaAlH4, TiCl3-doped NaAlH4, and pure Na3AlH6 were characterized using 1H, 23Na, and 27Al solid-state NMR. The signal intensities and linewidths of 1H NMR spectra using several spin echo sequences and backprediction of a single pulse experiment were compared to find the optimal experiment to measure wide-line NMR spectra of the Alanates. Second moment calculations using the Van Vleck equations compared with fits of the dipolar coupling line broadening confirm that NaAlH4 has a rigid crystal lattice. On the other hand, for Na3AlH6, a narrowing of the proton and aluminum lineshape was observed, indicating a fast rotational motion of AlH6 clusters at room temperature. A broadening of the 1H and 27Al linewidth was observed upon lowering the temperature. This process is successfully described using thermally activated rotational jumps of AlH6 clusters assuming a fast rotational motion around one single C4 axis and a slower rotation around the other two C4 axes with an activation barrier of Ea = 25 kJ/mol a...

  • Thermal coupling of a high temperature PEM fuel cell with a complex hydride tank
    International Journal of Hydrogen Energy, 2009
    Co-Authors: P. Pfeifer, Clemens Wall, O. Jensen, Horst Hahn, Maximilian Fichtner
    Abstract:

    Abstract Sodium alanate doped with cerium catalyst has been proven to have fast kinetics for hydrogen ab- and de-sorption as well as a high gravimetric storage density around 5 wt%. The kinetics of hydrogen sorption can be improved by preparing the alanate as nanocrystalline material. However, the second decomposition step, i.e. the decomposition of the hexahydride to sodium hydride and aluminium which refers to 1.8 wt% hydrogen is supposed to happen above 110 °C. The discharge of the material is thus limited by the level of heat supplied to the hydride storage tank. Therefore, we evaluated the possibilities of a thermal coupling of a high temperature PEM fuel cell operating at 160–200 °C. The starting temperatures and temperature hold-times before starting fuel cell operation, the heat transfer characteristics of the hydride storage tanks, system temperature, fuel cell electrical power (including efficiency) as well as alanate kinetics were varied by system modelling with gPROMS®. The kinetics of the hydride decomposition was found to have a major influence on the performance of the system. A cumulative output of 0.8 kWh was reached in a test run.

  • Investigation of the Nature of a Ti−Al Cluster Formed upon Cycling under Hydrogen in Na Alanate Doped with a Ti-Based Precursor
    Journal of Physical Chemistry C, 2008
    Co-Authors: Aline Leon, Galina Yalovega, Alexander V. Soldatov, Maximilian Fichtner
    Abstract:

    The hydrogen storage material of Ti-doped Na alanate has been studied by using both experimental and theoretical Ti K-edge X-ray absorption near edge structure (XANES) analysis. The results suggest...

  • nanocrystalline Alanates phase transformations and catalysts
    Journal of Alloys and Compounds, 2005
    Co-Authors: Maximilian Fichtner, Oliver Kircher, Patrizia Canton, Aline Leon
    Abstract:

    Abstract X-ray absorption in combination with powder X-ray diffraction and surface analytical studies were performed in order to investigate the chemical state, local order and spatial distribution of Ti in sodium alanate doped with TiCl3 and small Ti clusters. Kinetic investigations of the different hydrogenation and dehydrogenation steps of doped NaAlH4 indicate a nucleation and growth mechanism as rate limiting step in the transformation of the material. The results support a mechanism where Ti has an accelerating effect on the phase transformation, probably by introducing non-thermal defects into the material.

Craig M. Jensen - One of the best experts on this subject based on the ideXlab platform.

  • Hydrogen bonding in sodium alanate: a muon spin rotation study.
    Physical Review Letters, 2008
    Co-Authors: Ryosuke Kadono, Koichiro Shimomura, K.h. Satoh, S. Takeshita, A. Koda, Kusuo Nishiyama, Etsuo Akiba, R. Ayabe, Kuba M, Craig M. Jensen
    Abstract:

    We have detected the occurrence of hydrogen bonding involving an interstitial positive muon situated between hydrogen atoms of two independent alanate anions in sodium alanate (NaA1H 4 ). Ti doping, which is known to dramatically improve the hydrogen cycling performance of NaA1H 4 , reduces the kinetic barrier of the transition of the muon from the muon-dialanate state to a mobile interstitial state. This observation strongly suggests that hydrogen bonding is the primary bottleneck for hydrogen release or uptake in sodium alanate, which might be common to other complex hydrides.

  • dynamics of defects in Alanates
    Journal of Alloys and Compounds, 2007
    Co-Authors: R Cantelli, Craig M. Jensen, O Palumbo, A Paolone, M T Kuba, R. Ayabe
    Abstract:

    Abstract Anelastic spectroscopy experiments on the dehydrogenation process of undoped and catalysed NaAlH 4 revealed the formation of a highly mobile species during decomposition. This species, as indicated also by isotope effect measurements, was identified as a hydrogen containing defect complex. In particular, the most likely defect complex may be of type AlH x , in which fast local dynamics of H-vacancies can occur. The formation of such defects begins at temperatures much lower in Ti-doped than in undoped samples. The catalyst atoms decrease the energy barrier to be overcome by H to break the bond, thus enhancing the kinetics of the chemical reactions and decreasing the temperature at which the dehydrogenation processes take place.

  • Structure and hydrogen dynamics of pure and Ti-doped sodium alanate
    Physical Review B, 2004
    Co-Authors: Jorge Íñiguez, Taner Yildirim, Terrence J. Udovic, M. Sulic, Craig M. Jensen
    Abstract:

    We have studied the structure, energetics, and dynamics of pure and Ti-doped sodium alanate sNaAlH4d, focusing on the possibility of substitutional Ti doping in the bulk. Our ab initio calculations reproduce well the measured neutron inelastic scattering spectrum, which exhibits surprisingly strong and sharp two-phonon features. The calculations also reveal that substitutional Ti doping is energetically possible, and imply that Ti prefers to substitute for Na and is a powerful hydrogen attractor that facilitates multiple Al-H bond breaking. Our results hint at ways of improving the hydrogen dynamics and storage capacity of the Alanates.

  • catalyzed Alanates for hydrogen storage
    Journal of Alloys and Compounds, 2002
    Co-Authors: K. J. Gross, G J Thomas, Craig M. Jensen
    Abstract:

    The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the Alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH4⇔1/3Na3AlH6+2/3Al+H2⇔NaH+Al+3/2H2. Initial work resulted in improved catalysts, advanced methods of preparation, and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties, as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions were observed real-time through in situ X-ray powder diffraction. These measurements demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verified the segregation of aluminum to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 bar at 33°C, which suggest that, thermodynamically, this material is ideally suited to on-board hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at temperatures approaching that found in the operation of PEM fuel cells (125–165°C) were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase the capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti–Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications.

  • Catalyzed Alanates for hydrogen storage
    Journal of Alloys and Compounds, 2002
    Co-Authors: K. J. Gross, G J Thomas, Craig M. Jensen
    Abstract:

    The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the Alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH 4⇔1/3Na 3AlH 6+2/3Al+ H 2⇔NaH+Al+3/2H 2. Initial work resulted in improved catalysts, advanced methods of preparation, and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties, as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions were observed real-time through in situ X-ray powder diffraction. These measurements demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verified the segregation of aluminum to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 bar at 33°C, which suggest that, thermodynamically, this material is ideally suited to on-board hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at temperatures approaching that found in the operation of PEM fuel cells (125-165°C) were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase the capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti-Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications. © 2002 Elsevier Science B.V. All rights reserved.

David Grassi - One of the best experts on this subject based on the ideXlab platform.

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