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107. Hydrogen cycling in γ-Mg(BH4)2 with cobalt-based additives.

Author

Zavorotynska, O., Saldan, I., Hino, S., Humphries, T. D., Deledda, S., and Hauback, B. C.

Abstract

Magnesium borohydride (Mg(BH4)2) is an attractive candidate as a hydrogen storage material due to its high hydrogen content and predicted favorable thermodynamics. In this work we demonstrate reversible hydrogen desorption in partially decomposed Mg(BH4)2 which was ball milled together with 2 mol% Co-based additives. Powder X-ray diffraction and infrared spectroscopy showed that after partial decomposition at 285 °C, amorphous boron-hydride compounds were formed. Rehydrogenation at equivalent temperatures and hydrogen pressures of 120 bar yielded the formation of crystalline Mg(BH4)2 in the first cycle, and amorphous Mg(BH4)2 with other boron–hydrogen compounds upon the third H2 absorption. Reversibility was observed in the samples with and without Co-based additives, although the additives enhanced hydrogen desorption kinetics in the first cycle. X-ray absorption spectroscopy at Co K-edge revealed that all the additives, apart from Co2B, reacted during the first desorption to form new stable species. [ABSTRACT FROM AUTHOR]

Published
2015
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108. The crystal structure of Zr2NiD4.5

Author

Sørby, M. H., primary, Gunnæs, A. E., additional, Løvvik, O. M., additional, Brinks, H. W., additional, Fjellvåg, H., additional, and Hauback, B. C., additional

Published
2006
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139. Synchrotron Diffraction Studies of Hydrogen Absorption/Desorption on CaH2+ MgB2Reactive Hydride Composite Mixed With Fluorinated Compounds

Author

Suarez-Alcantara, K., Sørby, M. H., Pistidda, C., Karimi, F., Saldan, I., Hauback, B. C., Klassen, T., and Dornheim, M.

Abstract

The reactive hydride composites 9CaH2+ CaF2+ 10MgB2, 10Ca(BH4)2+ 9MgH2+ MgF2, and 9Ca(BH4)2+ Ca(BF4)2+ 10MgH2were prepared by ball milling. Their properties toward hydrogen absorption/desorption were tested by means of manometric measurements. The highest hydrogen storage capacity was obtained for 9CaH2+ CaF2+ 10MgB2(7.6 wt %) at the first cycle. The effects of CaF2, MgF2, or Ca(BF4)2on the dehydrogenation reaction were studied by means of in situ synchrotron radiation powder X-ray diffraction (in situ SR-PXD) and differential scanning calorimetry (DSC). The high resolution SR-PXD technique was used to confirm the formation of hydrogenated products and side products in the 9CaH2+ CaF2+ 10MgB2reactive hydride composites. These studies indicate the formation of a complex mixture of phases.

Published
2015
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141. Hydrogen–fluorine exchange in NaBH4–NaBF4.

Author

Rude, L. H., Filsø, U., D'Anna, V., Spyratou, A., Richter, B., Hino, S., Zavorotynska, O., Baricco, M., Sørby, M. H., Hauback, B. C., Hagemann, H., Besenbacher, F., Skibsted, J., and Jensen, T. R.

Abstract

Hydrogen–fluorine exchange in the NaBH4–NaBF4 system is investigated using a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed using in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF2H2 in the temperature range T = 200 to 215 °C. Combined use of solid-state 19F MAS NMR, FT-IR and DFT calculations supports the formation of a BF2H2− complex ion, reproducing the observation of a 19F chemical shift at −144.2 ppm, which is different from that of NaBF4 at −159.2 ppm, along with the new absorption bands observed in the IR spectra. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at ∼310 °C. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF and BF3 or amorphous products such as closo-boranes, e.g. Na2B12H12. The NaBH4–NaBF4 composite decomposes at lower temperatures (300 °C) compared to NaBH4 (476 °C), as observed by thermogravimetric analysis. NaBH4–NaBF4 (1 : 0.5) preserves 30% of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8% for NaBH4 as measured using Sievert’s method under identical conditions, but more than 50% using prolonged hydrogen absorption time. The reversible hydrogen storage capacity tends to decrease possibly due to the formation of NaF and Na2B12H12. On the other hand, the additive sodium fluoride appears to facilitate hydrogen uptake, prevent foaming, phase segregation and loss of material from the sample container for samples of NaBH4–NaF. [ABSTRACT FROM AUTHOR]

Published
2013
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142. The identification of a hitherto unknown intermediate phase CaB2Hxfrom decomposition of Ca(BH4)2.

Author

Riktor, M. D., Sørby, M. H., Chopek, K., Fichtner, M., and Hauback, B. C.

Abstract

Ca(BH4)2is a promising material for hydrogen storage due to its high gravimetric capacity and expected suitable thermodynamic properties. Experimental reports indicate a more complex reaction pathway than the previously proposed route, including formation of unknown intermediate phases. In this work, the reaction scheme of a mixture of γ- and β-Ca(BH4)2was investigated using temperature programmed desorption and high-resolution synchrotron radiation powder X-ray diffraction. An unknown intermediate Ca–B–H-containing phase has been identified as a CaB2Hxcompound where xmost probably equals 2. A structure model is proposed. [ABSTRACT FROM AUTHOR]

Published
2009
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143. The crystal structure of Zr2NiD4.5.

Author

Sørby, M. H., Gunnæs, A. E., Løvvik, O. M., Brinks, H. W., Fjellvåg, H., and Hauback, B. C.

Subjects
DEUTERIUM, SYNCHROTRON radiation, NEUTRON diffraction, ELECTRON diffraction, TETRAHEDRAL coordinates, CRYSTALLOGRAPHY
Abstract

The crystal structure of Zr2NiD4.5 has been determined by a combination of synchrotron radiation powder X-ray diffraction, electron diffraction and powder neutron diffraction data. Deuterium ordering results in a triclinic supercell given by asuper = 6.81560 (7), bsuper = 8.85137 (9), csuper = 8.88007 (10) Å, αsuper = 79.8337 (8), βsuper = 90.0987 (9), γsuper = 90.3634 (9)°, which relates to the non-super unit cell as asuper = − a, bsuper = − b −  c, csuper = − b +  c. The centrosymmetric and fully ordered deuterium sublattice was determined by simulated annealing and Rietveld refinement. Deuterium was found to occupy three types of tetrahedral sites: two that are coordinated by four Zr atoms and one that is coordinated by three Zr atoms and one Ni atom. All D—D distances are longer than 2 Å. The feasibility of the crystal structure was supported by density functional theory calculations. [ABSTRACT FROM AUTHOR]

Published
2006
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144. Electron-microscopy studies of NaAlH4 with TiF3 additive: hydrogen-cycling effects.

Author

Andrei, C. M., Walmsley, J.C., Brinks, H. W., Holmestad, R., Srinivasan, S. S., Jensen, C. M., and Hauback, B. C.

Subjects
ELECTRON microscopy, SODIUM compounds, HYDROGEN, MICROSTRUCTURE, MATERIALS, MATERIALS science, PHYSICAL & theoretical chemistry, PHYSICS, PHYSICAL sciences
Abstract

NaAlH4 is a promising candidate material for hydrogen storage. Ti additives are effective in reducing the reaction temperatures and improving kinetics. In this work, the microstructure of NaAlH4 with 2% TiF3 has been studied in different conditions using a combination of transmission electron microscopy and scanning electron microscopy, both with energy-dispersive spectroscopic X-ray analysis. The effect of the additive on particle and grain size was examined after the initial ball-milling process and after 15 cycles. The additive has an uneven distribution in the sample after ball milling. Selected-area diffraction and high-resolution imaging confirmed the presence of TiF3. This phase accounts for most of the Ti in the material at this stage and showed limited mixing with the alanate. The grain size within particles for TiF3 is larger than for the alanate particles. Diffraction from the latter was dominated by metallic aluminium. After cycling, the TiF3 has decomposed and energy-dispersive spectroscopic X-ray analysis maps showed some combination of Ti with the alanate phase. There is no significant change in the measurable grain size of the Al-containing alanate particles between the ball-milled and the 15-cycled samples, but more cycles result in agglomeration of the material. [ABSTRACT FROM AUTHOR]

Published
2005
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145. Structural Changes Observed during the Reversible Hydrogenation of Mg(BH4)2with Ni-Based Additives

Author

Saldan, I., Hino, S., Humphries, T. D., Zavorotynska, O., Chong, M., Jensen, C. M., Deledda, S., and Hauback, B. C.

Abstract

The decomposition and rehydrogenation of γ-Mg(BH4)2ball milled together with 2 mol % of Ni-based additives, Ninano, NiCl2, NiF2, and Ni3B, has been investigated during one hydrogen desorption–absorption cycle. Under the applied ball-milling conditions, no mechanochemical reactions between γ-Mg(BH4)2and Niaddwere observed. Hydrogen desorption carried out at temperatures of 220–264 °C resulted for all samples in partial decomposition of Mg(BH4)2and formation of amorphous phases, as seen by powder X-ray diffraction (PXD). PXD analysis after rehydrogenation at temperatures of 210–262 °C and at pressures between 100 and 155 bar revealed increased fractions of crystalline β-Mg(BH4)2, indicating a partial reversibility of the composite powders. The highest amount of [BH4]−is formed in the composite containing Ni3B. Analysis by X-ray absorption spectroscopy performed after ball milling, after desorption, and after absorption shows that the Ni3B additive remains unaffected, whereas NiCl2and NiF2additives react with Mg(BH4)2during the hydrogen desorption–absorption and form compounds with a local structure very similar to that of amorphous Ni3B. Multinuclear NMR spectroscopy confirms the partial reversibility of the system as well as the formation of [B10H10]2–during hydrogen absorption. The presence of [BnHn]2–(n= 10, 12) was also detected by infrared (IR) spectroscopy of the dehydrogenated and rehydrogenated samples. The IR measurements give no clear indication that ions containing B–H–B bridged hydrogen groups were formed during the H-sorption cycle.

Published
2014
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146. Electronic structure and stability of complex hydrides Mg2MHx(M= Fe, Co)

Author

Shelyapina, M., Pinyugzhanin, V., Skryabina, N., and Hauback, B.

Abstract

This paper reports on the results of theoretical investigations carried out for the hydrides Mg2FeH6and Mg2CoH5and the mixed hydride Mg2(FeH6)0.5(CoH5)0.5in terms of the full-potential linearized augmented plane wave (FLAPW) method. It has been shown that the partial substitution of the Co atoms for the Fe atoms leads to a slight increase in the stability of the hydride, but, at the same time, makes it impossible to increase the stability of the alloy. The high stability of the hydrides under investigation has been explained by the strong bonding between atoms of the transition metal and hydrogen.

Published
2013
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147. MgH2in Carbon Scaffolds: A Combined Experimental and Theoretical Investigation

Author

Vajeeston, P., Sartori, S., Ravindran, P., Knudsen, K. D., Hauback, B., and Fjellvåg, H.

Abstract

Understanding the thermodynamics of metal hydrides is crucial in order to employ them for reversible hydrogen storage. The use of a supporting nanoporous matrix for embedding the hydride can improve the kinetics of metal hydride reactions and even change the overall reaction path. In this study, density functional theory calculations were performed to understand the size effect of MgH2particles and changes in the physical and chemical properties of these nano-objects embedded in an amorphous carbon matrix. A stable amorphous carbon structure was successfully generated with two different approaches and further used as a template to construct the scaffold for nanophases of MgH2. Using five different structure models, we have studied the physical and chemical changes of the nano-MgH2in this carbon scaffold. In addition, from small-angle neutron scattering studies, we could demonstrate experimentally that it is possible to incorporate such ultrasmall objects into the carbon scaffolds.

Published
2012
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148. Hindered Rotational Energy Barriers of BH4–Tetrahedra in β-Mg(BH4)2from Quasielastic Neutron Scattering and DFT Calculations

Author

Blanchard, D., Maronsson, J. B., Riktor, M. D., Kheres, J., Sveinbjörnsson, D., Gil Bardají, E., Léon, A., Juranyi, F., Wuttke, J., Lefmann, K., Hauback, B. C., Fichtner, M., and Vegge, T.

Abstract

In this work, hindered rotations of the BH4–tetrahedra in Mg(BH4)2were studied by quasielastic neutron scattering, using two instruments with different energy resolution, in combination with density functional theory (DFT) calculations. Two thermally activated reorientations of the BH4–units, around the 2-fold (C2) and 3-fold (C3) axes were observed at temperatures from 120 to 440 K. The experimentally obtained activation energies (EaC2= 39 and 76 meV and EaC3= 214 meV) and mean residence times between reorientational jumps are comparable with the energy barriers obtained from DFT calculations. A linear dependency of the energy barriers for rotations around the C2axis parallel to the Mg–Mg axis with the distance between these two axes was revealed by the DFT calculations. At the lowest temperature (120 K) only 15% of the BH4–units undergo rotational motion and from comparison with DFT results it is expectedly the BH4–units with the boron atom closest to the Mg–Mg axis, although dynamics related to local disorder existing at the boundary of the antiphase domains or to the presence of solvent in the sample cannot be strictly excluded. No long-range diffusion events were observed.

Published
2012
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