Hydrogen Sorption Properties of Ca(BH4)2 and Ca(BH4)2+MgH2 composite

Abstract

Light metal borohydrides are considered to be promising materials as solid state hydrogen storage due to their high hydrogen contents. Among them, Ca(BH4)2 has high gravimetric (11.5 wt%) and volumetric (~130 kg/m3) hydrogen content. Ca(BH4)2 releases max. 9.6 wt% hydrogen assuming the following overall decomposition reaction: Ca(BH4)2 → 2/3 CaH2 + 1/ 3 CaB6 + 10/3 H2 However, the exact mechanism of dehydrogenation has not known yet. Several intermediate phases such as CaB2Hx, amorphous phase(s), and CaB12H12 have been observed during the dehydrogenation of Ca(BH4)2. For that reason, we investigated here the dehydrogenation and rehydrogenation reactions of Ca(BH4)2 under precisely controlled experimental conditions (H2 partial pressure and temperatures). First, the dehydrogenation reaction pathway of Ca(BH4)2 has been investigated under an isobaric condition of 1 bar hydrogen. β-Ca(BH4)2 gradually transforms into amorphous phase at the initial decomposition stage. Upon further heating, it decomposes into CaB2Hx or CaB12H12, indicating that there is a subtle competition between two different dehydrogenation routes

ⅰ) Ca(BH4)2 → amorphous phase → CaB2Hx + (8-x)/2 H2 → 2/3 CaH2+ 1/3 CaB6 + 10/3H2 and ⅱ) Ca(BH4)2 → amorphous phases → 1/6CaB12H12 + 5/6 CaH2 + 13/6 H2. Among dehydrogenated phases, CaB2Hx as well as CaB6 + CaH2 mixture can be rehydrogenated at 330℃ under 90 bar hydrogen without any catalytic additive, whereas CaB12H12 hardly rehydrogenated under the same condition. Second, during dehydrogenation of Ca(BH4)2, several intermediate phases such as an amorphous phase(s), CaB2Hx, and CaB12H12 have been observed. Among intermediate phases, CaB12H12 is known as a relatively stable compound. If the formation of the very stable phases CaB12H12 can be avoided during dehydrogenation, Ca(BH4)2 can be fully rehydrogenated from CaH2 and CaB6 under mild conditions. To control the dehydrogenation reaction path and suppress the formation of the stable intermediate compounds, we apply three different hydrogen pressures during heating of Ca(BH4)2. The decomposition reaction of Ca(BH4)2 is sensitive to the H2 back pressure. At 1 bar H2 pressure, Ca(BH4)2 decomposes via two competitive dehydrogenation reaction routes to form CaB2Hx or CaB12H12. At 20 bar hydrogen pressure, amorphous elemental boron is formed as a final dehydrogenation product and the formation of CaB2Hx phases is suppressed. Three possible routes to form CaH2 are also described in the present work. Lastly, the dehydrogenation and rehydrogenation reactions of Ca(BH4)2 + MgH2 composite were investigated under hydrogen back pressures. The dehydrogenation reaction sequence is: ⅰ) 6Ca(BH4)2 + 3MgH2 → Ca4Mg3H14 + 2CaB6 + 20 H2, ⅱ) Ca4Mg3H14 → 4CaH2 + 3 Mg + 3H2. The rehydrogenation reactions takes place in reverse order, and about 60% reversibility was achieved after rehydrogenation for 24 h under 90 bar hydrogen pressure at 350 ℃ even without the help of catalysts. From this study, the dehydrogenation and rehydrogenation reactions of Ca(BH4)2 and Ca(BH4)2+MgH2 composite were clarified under several different hydrogen pressures. The suggested reactions are validated by crystallographic (XRD and in-situ XRD) and spectroscopic (FT-IR, Raman, and NMR) analyses on the various dehydrogenated samples and comparisons with calculation results.