On the rehydrogenation of decomposed Ca(BH 4 ) 2 M.D. Riktor a , M.H. Sørby a,⇑ , J. Muller a , E.G. Bardají b , M. Fichtner b , B.C. Hauback a a Physics Department, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway b Institute of Nanotechnology, Karlsruhe Institute of Technology, P.O. Box 3640, D-76021 Karlsruhe, Germany article info Article history: Received 19 December 2014 Received in revised form 23 January 2015 Accepted 27 January 2015 Available online 3 February 2015 Keywords: Hydrogen storage Calcium borohydride Hydrogenation Powder X-ray diffraction IR spectroscopy abstract Ca(BH 4 ) 2 with 11.6 mass% hydrogen is considered a promising material for hydrogen storage applications due to its high gravimetric and volumetric capacity as well as predicted thermodynamic properties suit- able for hydrogen storage purposes. However, the material displays poor reversibility, and a deeper understanding of decomposition pathway and rehydrogenation mechanisms are needed to solve this issue. In this work the rehydrogenation properties of phases present in different samples of decomposed Ca(BH 4 ) 2 were investigated. The dehydrogenated and rehydrogenated samples were characterized by Powder X-ray Diffraction (PXD) and infrared spectroscopy (IR). The results show that at least one amor- phous phase is hydrogenated to form Ca(BH 4 ) 2 at relatively moderate conditions (100 bar, 305 °C) with- out addition of a catalyst. Bragg peaks from the earlier reported intermediate phase, CaB 2 H x , disappear during rehydrogenation. However, it cannot be confirmed from the results presented here if this is due to rehydrogenation or if CaB 2 H x is consumed in a reaction with phases formed during rehydrogenation of the amorphous phase(s). Infrared spectra show presence of amorphous decomposition products differ- ent from the earlier identified decomposition products CaH 2 , CaB 6 and CaB 12 H 12 . These phases cannot be identified due to lack of matching literature spectra, but quantitative analysis indicates presence of amorphous products less rich in boron than CaB 6 and CaB 12 H 12 . Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction A major challenge for the introduction of hydrogen as an energy carrier is safe and efficient storage of hydrogen. Storage in solid compounds based on lightweight elements is considered the only solution able to meet the long-term goals with respect to gravimet- ric hydrogen capacity [1]. Complex hydrides based on boron (boro- hydrides) exhibit the highest gravimetric capacity for hydrogen and are therefore obvious candidates to meet these requirements. In addition to the gravimetric capacity the thermodynamic properties are of high importance. Interest in Ca(BH 4 ) 2 as a hydro- gen storage material began in 2006 when Miwa et al. [2] calculated the dehydrogenation enthalpy for Ca(BH 4 ) 2 and predicted a decomposition reaction according to reaction R1 (below) at favor- able conditions. Later theoretical work suggests also a second plau- sible decomposition pathway with comparable enthalpy (R2) [3,4]. CaðBH 4 Þ 2 ! 2 3 CaH 2 þ 1 3 CaB 6 þ 10 3 H 2 DH r 32 kJ=mol H 2 ðR1Þ CaðBH 4 Þ 2 ! 5 6 CaH 2 þ 1 6 CaB 12 H 12 þ 13 6 H 2 DH r  35 kJ=mol H 2 ðR2Þ Experimental investigations of the phase transformations and the decomposition route of Ca(BH 4 ) 2 show a more complex behav- ior than the proposed reactions (R1) and (R2), including polymor- phic phase transitions and formation of unknown intermediate phases [5–14]. Furthermore, the boron containing decomposition products tend to form in an amorphous state, and the identification has turned out to be a challenge. All studies agree on the formation of CaH 2 as a final product, but concerning the boron containing phases a variety of results exists. At present, experimentally observed phases in decomposed Ca(BH 4 ) 2 include a crystalline CaB 2 H x -phase [10,11,14], amorphous or nano-crystalline CaB 6 [10,11,14–16], CaB 12 H 12 [4,10,11,16] and elemental boron [10,16]. A general challenge for the use of borohydrides as hydrogen storage materials is the limited reversibility. Several authors have studied dehydrogenation and rehydrogenation properties and the effect of different additives [4,16–20]. Rönnebro et al. have formed Ca(BH 4 ) 2 from CaB 6 and CaH 2 at 400–440 °C and 700 bar H 2 pres- sure, obtaining a yield of approx. 60% when TiCl 3 + Pd or RuCl 3 was added [20]. Kim et al. have reported on 60% reversibility of decomposed Ca(BH 4 ) 2 at 90 bar and 350 °C when NbF 5 was used as an additive [18]. The products of the dehydrogenation reaction, and thus the starting point for the rehydrogenation reaction were, however, not clearly identified. From X-ray diffraction a CaH 2x F x solid solution phase was found to be the main decomposition http://dx.doi.org/10.1016/j.jallcom.2015.01.243 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: magnuss@ife.no (M.H. Sørby). Journal of Alloys and Compounds 632 (2015) 800–804 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom