Published: February 10, 2011 r2011 American Chemical Society 3762 dx.doi.org/10.1021/jp108236e | J. Phys. Chem. C 2011, 115, 3762–3768 ARTICLE pubs.acs.org/JPCC Ca(BH 4 ) 2 -MgF 2 Reversible Hydrogen Storage: Reaction Mechanisms and Kinetic Properties Rapee Gosalawit-Utke,* ,† Karina Suarez, † Jose M. Bellosta von Colbe, † Ulrike B€ osenberg, † Torben R. Jensen, ‡ Yngve Cerenius, § Christian Bonatto Minella, † Claudio Pistidda, † Gagik Barkhordarian, † Matthias Schulze, || Thomas Klassen, † R€ udiger Bormann, † and Martin Dornheim † † Institute of Materials Research, Materials Technology, Helmholtz-Zentrum Geesthacht, D-21502 Geesthacht, Germany ‡ Center for Energy Materials, iNANO and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark § MaXLAB, Lund University, S-22100 Lund, Sweden ) Institute of Material Technology, Helmut-Schmidt-University, University of the Federal Armed Forces Hamburg, Holstenhofweg 85, D-22043 Hamburg, Germany ABSTRACT: A composite of Ca(BH 4 ) 2 -MgF 2 is proposed as a reversible hydrogen storage system. The dehydrogenation and rehydrogenation reaction mechanisms are investigated by in situ time-resolved synchrotron radiation powder X-ray diffraction (SR-PXD) and Raman spectroscopy. The forma- tion of an intermediate phase (CaF 2-x H x ) is observed during rehydrogenation. The hydrogen content of 4.3 wt % is obtained within 4 h during the first dehydrogenation at isothermal and isobaric conditions of 330 °C and 0.5 bar H 2 , respectively. The cycling efficiency is evaluated by three release and uptake cycles together with absorbed hydrogen content in the range 5.1-5.8 wt % after 2.5 h (T = 330 °C and p(H 2 ) = 130 bar). The kinetic properties on the basis of hydrogen absorption are comparable for all cycles. As compared to pure Ca(BH 4 ) 2 and Ca(BH 4 ) 2 -MgH 2 com- posite, Ca(BH 4 ) 2 -MgF 2 composite reveals the kinetic destabilization and the reproducibility of hydrogen storage capacities during cycling, respectively. 1. INTRODUCTION Owing to the guidelines defined by the U.S. Department of Energy (DOE), hydrogen storage systems with a target of 6 wt % hydrogen content and desorption of hydrogen at 1 bar below 85 °C, corresponding to a formation enthalpy of ∼ -47 kJ/mol of H 2 , are required for fuel cell powered vehicles to be able to replace petroleum-fueled vehicles on a large scale. 1,2 Metal hydrides, e.g., alanates, 3 amides, 4 and boro- hydrides, 5 are the most promising candidates for hydrogen storage systems due to their safety, the fact that they are light- weight, and their compact size as well as their highest storage capacity by volume as compared to a compressed or liquid hydrogen system. One of the most interesting compounds with a high hydrogen storage density is Ca(BH 4 ) 2 containing theoretically 11.6 wt % H 2 capacity. As compared to LiBH 4 , although the hydrogen storage capacity of Ca(BH 4 ) 2 is less than that of LiBH 4 , it still has a high enough theoretical capacity (11.6 wt %) to reach the target of DOE (9 wt %), and has a lower dehydrogenation temperature than LiBH 4 . 6 Calcium borohydride can be pre- pared by several reactions, for example, (i) calcium hydride 7 or alkoxide 8 with diborane; (ii) reaction in tetrahydrofuran (THF), 9 forming Ca(BH 4 ) 2 -THF, which is commercially available. Regarding the structure of known Ca(BH 4 ) 2 poly- morphs, the Fddd structure of R-Ca(BH 4 ) 2 was presented in 2006; 10 thereafter, an alternative structure of F2dd symmetry for R-Ca(BH 4 ) 2 was proposed. 11,12 In the case of β-Ca(BH 4 ) 2 , several structures were revealed such as P4 2 /m, P4 2 , and P4 symmetries. 13-15 Interestingly, it was found that R-Ca(BH 4 ) 2 transformed to an R 0 -Ca(BH 4 ) 2 (I42d symmetry) at elevated tem- perature, before transforming completely to β-Ca(BH 4 ) 2 . 11,15 Moreover, a γ-Ca(BH 4 ) 2 with Pbca structure was also exposed as a dynamically stable and nonexperimental phase. 11 The primary and most likely hydrogen desorption pathways of Ca(BH 4 ) 2 are Received: August 30, 2010 Revised: January 10, 2011