Complex Hydrides For Hydrogen Storage Research Articles

Superhalogens as building blocks of complex hydrides for hydrogen storage

Superhalogens are species whose electron affinity (EA) or vertical detachment energy (VDE) exceeds those of halogens. These species typically consist of a central electropositive atom with electronegative ligands. The EA or VDE of species can be further increased by using superhalogens as ligands, which are termed as hyperhalogens. Having established BH4− as a superhalogen, we have studied BH4−x(BH4)x− (x=1–4) hyperhalogen anions and their Li-complexes LiBH4−x(BH4)x using density functional theory. The VDE of these anions is larger than that of BH4−, which increases with the increase in number of peripheral BH4 moieties (x). The hydrogen storage capacity of LiBH4−x(BH4)x complexes is higher but binding energy is smaller than that of LiBH4, a typical complex hydride. The linear correlation between the dehydrogenation energy of LiBH4−x(BH4)x complexes and the VDE of BH4−x(BH4)x− anions is established. These complexes are found to be thermodynamically stable against dissociation into LiBH4 and borane. This study demonstrates the role of superhalogens in designing new materials for hydrogen storage and should also motivate experimentalists to synthesize LiBH4−x(BH4)x (x=1–4) complexes.

Complex and liquid hydrides for energy storage

The research on complex hydrides for hydrogen storage was initiated by the discovery of Ti as a hydrogen sorption catalyst in NaAlH4 by Boris Bogdanovic in 1996. A large number of new complex hydride materials in various forms and combinations have been synthesized and characterized, and the knowledge regarding the properties of complex hydrides and the synthesis methods has grown enormously since then. A significant portion of the research groups active in the field of complex hydrides is collaborators in the International Energy Agreement Task 32. This paper reports about the important issues in the field of complex hydride research, i.e. the synthesis of borohydrides, the thermodynamics of complex hydrides, the effects of size and confinement, the hydrogen sorption mechanism and the complex hydride composites as well as the properties of liquid complex hydrides. This paper is the result of the collaboration of several groups and is an excellent summary of the recent achievements.

Improvement on Hydrogen Desorption Performance of Calcium Borohydride Diammoniate Doped with Transition Metal Chlorides

Calcium borohydride diammoniate with a molecular formula of Ca(BH4)(2).2NH(3) is a novel complex hydride for hydrogen storage. However, it suffers from high temperature and sluggish kinetics for hydrogen desorption. In this work, the temperature and kinetics for hydrogen desorption of calcium borohydride diammoniate were effectively improved through doping transition metal chlorides (such as CoCl2, NiCl2, and FeCl3) in a closed vessel. Three additives could effectively decrease the temperature for hydrogen desorption of Ca(BH4)(2).2NH3. Among them, CoCl2-doped Ca(BH4)(2).2NH3 could desorb hydrogen even at a low temperature of 170 degrees C, and at 200 degrees C, 7.6 wt % hydrogen with high purity is released, which shows superior performance for hydrogen desorption than that of pristine Ca(BH4)(2).2NH(3.) For CoCl2-doped Ca(BH4)(2).2NH(3), X-ray absorption fine structure (XAFS) revealed that the improvements could be ascribed to the catalytic function of well-dispersed cobalt particles, which results in a much lower activation energy (86.1 kJ/mol) for hydrogen desorption of calcium borohydride diammoniate.

Synthesis and hydrogen storage characteristics of Mg–B–H compounds by a gas–solid reaction

Magnesium borohydride [Mg(BH4)2] is an attractive complex hydride for hydrogen storage. In this study, attempts to synthesize Mg(BH4)2 were carried out by a solid–gas reaction through MgH2 and B2H6 in the absence of a liquid medium. The source of B2H6 was obtained by heating a mixture of NaBH4 and ZnCl2. The profile of pressure versus temperature indicated that the absorption kinetics of B2H6 by MgH2 were slow. Structural analysis confirmed the formation of Mg–B–H compounds. The reaction products presented two-step hydrogen release during heating. A small amount of hydrogen could be released from the as-synthesized Mg–B–H compounds at a low temperature of 215 °C. The slow reaction kinetics were significantly affected by the surface conditions of the MgH2 powders.

Separation and characterization of the active species in Ti-doped NaAlH4

The catalytic mechanism of doped complex hydrides for hydrogen storage remains unconfirmed. Here, we report a simple method to separate the active species of Ti-based catalysts in NaAlH(4) by filtration using tetrahydrofuran (THF) as solvent. The results show that the average particle size of the obtained Al-Ti active species is 30-50 nm.

Trend in Research and Development of Lithium Complex Hydrides for Hydrogen Storage

Hydrogen is in the spotlight as an alternative next generation energy source for the replacement of fossil fuels because it has high specific energy density and emits almost no pollution, with zero <TEX>$CO_2$</TEX> emission. In order to use hydrogen safely, reliable storage and transportation methods are required. Recently, solid hydrogen storage systems using metal hydrides have been under extensive development for application to fuel cell vehicles and fuel cells of MCFC and SOFC. For the practical use of hydrogen on a commercial basis, hydrogen storage materials should satisfy several requirements such as 1) hydrogen storage capacity of more than 6.5wt.% <TEX>$H_2$</TEX>, moderate hydrogen release temperature below <TEX>$100^{\circ}C$</TEX>, 3) cyclic reversibility of hydrogen absorption/desorption, 4) non toxicity and low price. Among the candidate materials, Li based metal hydrides are known to be promising materials with high practical potential in view of the above requirements. This paper reviews the characteristics and recent R&D trends of Li based complex hydrides, Li-alanates, Li-borohydrides, and Li-amides/imides.

Hydrogen release from sodium borohydrides at low temperature by the addition of zinc fluoride

The major challenge of complex hydride for hydrogen storage is to release hydrogen at a moderate temperature with rapid kinetics. This paper reports a new composite system (NaBH 4/ZnF 2) to generate hydrogen at a low temperature below 100 °C. The NaBH 4 and ZnF 2 mixture was prepared by a mechanical milling in a 2:1 M ratio. Dehydrogenation of the mixture during a heating process shows that the initial dehydrogenation temperature was below 100 °C with favorable kinetics. The structural analysis confirmed the formation of NaBF 4 and Zn in the reaction products. The study concerning the reaction mechanism reveals that ZnF 2 plays a role of reagent rather than catalyst. The produced intermediate of NaZnF 3, however, affects the decomposition of the emitted B 2H 6. This work suggests that the extension of the decomposition mechanism of NaBH 4/ZnF 2 composite system would favor the development of new catalysts for complex hydride systems.

ChemInform Abstract: Feasibility Study of the Direct Synthesis of Mg(BH4)2 Complex Hydrides by Mechanical Milling.

AbstractThe attempted synthesis of Mg(BH4)2, a promising complex hydride for hydrogen storage, by planetary ball milling of pure Mg and B powders and subsequent hydrogenation is unsuccessful despite of using optimized ball milling parameters.

Feasibility study of the direct synthesis of Mg(BH 4) 2 complex hydrides by mechanical milling

Magnesium borohydride [Mg(BH 4) 2] is a promising complex hydride for hydrogen storage. This study reports a feasibility study of the synthesis of Mg(BH 4) 2 by planetary ball milling of pure Mg–B powder and then a hydrogenation process under a hydrogen atmosphere. The ball milling parameters were firstly optimized. The Mg–B powders with the smallest average particle size of 1.91 ± 1.61 μm and the smallest average crystallite size of 9.4 ± 1.1 nm were used. However, no successful synthesis of Mg(BH 4) 2 was achieved. The obtained nanocrystalline hydride was MgH 2. It was found that the addition of B into magnesium strongly enhanced the kinetic properties of magnesium hydrogenation. The reduction of crystallite size showed a more obvious effect on improving the absorption kinetic properties than that of the particle size. The powder of the smallest crystallite size after hydrogenation comprised a minute amount of B–H compounds. The formation of Mg(BH 4) 2 from pure elements was difficult because of the barriers against breaking the B–B atomic bonds and the migration of the B atoms.

Direct synthesis of nanocrystalline NaAlH4 complex hydride for hydrogen storage

Nanocrystalline NaAlH4 was directly synthesized by ball milling NaH/Al with TiF3 catalyst under hydrogen pressure of 15–25 bar within 50 h. It is found that the synthesized NaAlH4 exhibits a high reversible hydrogen capacity of 4.7 wt % with fast reaction kinetics. It can absorb about 3.5 wt % hydrogen even at ambient temperature. The Ti–Al–H active species formed during reactive ball milling may act as catalyzing agent for hydrogen dissociation/recombination during in situ hydrogenation process and subsequent hydriding/dehydriding cycle.

Hydrogen–deuterium exchange experiments to probe the decomposition reaction of sodium alanate

NaAlH(4) is the archetypical complex hydride for hydrogen storage. The extraordinary effect of dopants on the sorption kinetics triggered the investigation of this empirical finding. In this paper, a short review of the state of the art is given. To gain further understanding of the mechanisms involved we label the interacting species during the sorption process. This was experimentally realized by hydrogen-deuterium exchange measurements during the decomposition of NaAlH(4) followed by thermogravimetry, Raman spectroscopy and mass spectrometry. By these experiments we are able to obtain specific information on the diffusing species and formation of intermediates. The activation energy of tracer diffusion in NaAlH(4) is found to be 0.28 eV. The results are evidence for a vacancy-mediated desorption process of NaAlH(4).

First-principles investigation of Mg(AlH 4) 2 complex hydride

Within the framework of density functional theory, we investigate the structural properties, electronic structure, and the chemical stability of magnesium alanate (Mg(AlH 4) 2), a complex hydride for hydrogen storage, using the projector augmented plane wave method. The calculated structural properties of magnesium alanate compare well with the recent experiment. A detailed study of the electronic structure and charge density redistribution reveals the orbital hybridization and characteristics of bonding orbitals within the complex hydride. We found an ionic covalent bonding between Al and H in the (AlH 4) − anionic complexes embedded in the matrix of Mg 2+ cations. The calculated reaction energy of Mg(AlH 4) 2 is in good agreement with the experimentally determined value.

Mechano-Chemical Synthesis and Characterization of New Complex Hydrides for Hydrogen Storage

AbstractMechano-chemical synthesis has been employed to prepare new light weight complex borohydrides. The precursor complex borohydrides such as NaBH4 and LiBH4 have been used since these materials posses high hydrogen storage capacity of 13.0 and 19.6 wt.%. This advanced materials based technology will meet the US-DOE grand challenge technical targets. The thermal calorimetric and gravimetric analysis of these complex borohydrides exhibits the hydrogen decomposition temperature (Tdec) of 100–150° C with theoretical capacity of ∼8.0-10.0 wt%. The catalysts (e.g. ZnCl2, TiFx3) doping and destabilization of the borohydride by reacting with binary hydride (MgH2) reveals the enhancement of decomposition kinetics and reversible dehydrogenation-rehydrogenation behavior.

High Throughput Screening of Complex Hydrides for Hydrogen Storage

ABSTRACTThe discovery that dopants, such as Ti, cause NaAlH4 to reversibly desorb H2 at mild conditions has spurred a great deal of research into complex metal hydrides. However, no complex hydride meets the targets for automotive hydrogen storage. Our approach is to accelerate the rate of discovery of improved hydrides and dopants through the combination of Virtual High Throughput Screening (VHTS) and Combinatorial Synthesis and Screening (CSS). Our CSS methods will allow us to screen thousands of samples in a year. These samples will be prepared by ball milling mixtures of hydrides and dopants similar to the established method of preparing Ti doped NaAlH4. VHTS exploits a molecular mechanics method to screen a thousand phases in a month. The combination of combinatorial methods and VHTS will help us discover the most promising complex hydrides for hydrogen storage. We will show the results of our medium throughput CSS and VHTS as applied to the NaAlH4–LiAlH4 – Mg(AlH4)2 mixed alanate compositions.

Light metal hydrides and complex hydrides for hydrogen storage.

The availability of feasible methods for hydrogen storage is one of the key-maybe the key-requirements for the large scale application of PEM fuel cells in cars. There are in principle four different approaches, i.e. cryostorage in liquid form, high pressure storage, storage in the form of a chemical compound which is converted to hydrogen by on-board reforming, or reversible chemical storage in different kinds of storage materials. New developments in the field of chemical storage make such systems attractive compared to the other options. This review will discuss the different possibilities for chemical storage of hydrogen and the focus on the presently most advanced system with respect to storage capacity and kinetics, i.e. catalyzed alanates, especially NaAlH(4).