J. Phys. Chem. C 2008, 112, 2743-2749
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Formation of Ca(BH4)2 from Hydrogenation of CaH2+MgB2 Composite Gagik Barkhordarian,*,† Torben R. Jensen,‡ Stefania Doppiu,§ Ulrike Bo1 senberg,† Andreas Borgschulte,| Robin Gremaud,⊥ Yngve Cerenius,# Martin Dornheim,† Thomas Klassen,† and Ru1 diger Bormann† Institute for Materials Research, GKSS Research Center Geesthacht GmbH, D-21502 Geesthacht, Germany, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, UniVersity of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark, Institute for Metallic Materials, P.O. Box 270116, D-01171 Dresden, Germany, Materials Science and Technology Laboratory 138, Hydrogen & Energy, U ¨ berlandstrasse 129, CH-8600 Du¨bendorf, Switzerland, Condensed Matter Group, de Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, and MAXLAB, Lund UniVersity, S-22100 Lund, Sweden ReceiVed: August 7, 2007; In Final Form: NoVember 8, 2007
The hydrogenation of the CaH2+MgB2 composite and the dehydrogenation of the resulting products are investigated in detail by in situ time-resolved synchrotron radiation powder X-ray diffraction, high-pressure differential scanning calorimetry, infrared, and thermovolumetric measurements. It is demonstrated that a Ca(BH4)2+MgH2 composite is formed by hydrogenating a CaH2+MgB2 composite, at 350 °C and 140 bar of hydrogen. Two phases of Ca(BH4)2 were characterized: R- and β-Ca(BH4)2. R-Ca(BH4)2 transforms to β-Ca(BH4)2 at about 130 °C. Under the conditions used in the present study, β-Ca(BH4)2 decomposes first to CaH2, Ca3Mg4H14, Mg, B (or MgB2 depending on experimental conditions), and hydrogen at 360 °C, before complete decomposition to CaH2, Mg, B (or MgB2), and hydrogen at 400 °C. During hydrogenation under 140 bar of hydrogen, β-Ca(BH4)2 is formed at 250 °C, and R-Ca(BH4)2 is formed when the sample is cooled to less than 130 °C. Ti isopropoxide improves the kinetics of the reactions, during both hydrogenation and dehydrogenation. The dehydrogenation temperature decreases to 250 °C, with 1 wt % of this additive, and hydrogenation starts already at 200 °C. We propose that the improved kinetics of the above reactions with MgB2 (compared to pure boron) can be explained by the different boron bonding within the crystal structure of MgB2 and pure boron.
1. Introduction Considering the limited resources of fossil energy, new energy concepts are essential for the future industrial society. New energy concepts should address three main issues: (1) the source of energy, (2) the storage of energy, and (3) the transformation of the energy. In this respect, hydrogen is an ideal solution to the problem of energy storage. However, to be able to use hydrogen as an energy storage material, it should be possible to efficiently store hydrogen as well. Research over the past decades has shown that this task is extremely challenging, specially for mobile applications, where there is a need for both high-energy density and high-energy efficiency. Besides the more conventional ways of storing hydrogen in gas or liquid form, which fail at the energy density or energy efficiency criterion, respectively, a viable alternative for hydrogen storage is provided by metal hydrides. High gravimetric and/or volumetric hydrogen densities can be stored in metal hydrides. In addition, by variation of the composition of the hydride or the components in the corresponding reactions, it is principally possible to tune the energy efficiency of the systems by tuning * Corresponding author. Address: Institute for Materials Research, GKSS Research Center Geesthacht GmbH, D-21502 Geesthacht, Germany. Email:
[email protected]. † GKSS Research Center Geesthacht GmbH. ‡ University of Aarhus. § Institute for Metallic Materials. | Materials Science and Technology Laboratory 138. ⊥ Condensed Matter Group. # Lund University.
the reaction enthalpy. These factors make metal hydrides very promising materials for mobile hydrogen storage applications. Since no single metal hydride could be found that satisfies both the energy density and efficiency criteria, research has now been concentrated on more complex systems with several interacting components (the so-called reactive hydride composites (RHCs)). Despite the lower total capacity of these systems compared to the component of the system with highest capacity, by careful selection of the components, it is possible to obtain a relatively high total hydrogen capacity with this approach. The latest progress in this respect has been the discovery of a unique kinetic property of MgB2,1-4 through which highcapacity light metal complex borohydrides can be hydrogenated with low reaction enthalpy. This kinetic property for the first time provides systems that can operate in the optimal thermodynamic range for hydrogen storage (high-energy efficiency) and, at the same time, contain high gravimetric hydrogen capacity (high-energy density). In our previous communication we reported on our initial investigation on hydrogenation of CaH2+MgB2, by which Ca(BH4)2+MgH2 is formed and 8.3 wt % hydrogen can be stored.2 On the basis of the stability of the formed complex borohydrides,5 the corresponding reaction enthalpy is estimated to be the lowest compared to other systems (LiBH 4+MgH2 and NaBH4+MgH2), making it very promising for hydrogen storage. Continuing the line of our previous study, in the present work we investigated the hydrogenation and dehydrogenation reactions of CaH2+MgB2 and Ca(BH4)2+MgH2, respectively, in
10.1021/jp076325k CCC: $40.75 © 2008 American Chemical Society Published on Web 01/31/2008
2744 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Barkhordarian et al.
more detail. In particular, we used in situ time-resolved synchrotron radiation powder X-ray diffraction (SR-PXD), highpressure differential scanning calorimetry (HP-DSC), infrared (IR), and titration measurements to examine the evolution of the reactions from structural, energetic, and kinetic points of view. On the basis of the results, the possible origin of the above-mentioned kinetic advantage of MgB2 is discussed. Further, one additive is proposed that improves the hydrogenation and dehydrogenation reaction kinetics. 2. Experimental Section CaH2 (98%, Mg
