Metal halide doped metal borohydrides for hydrogen storage: The case of Ca(BH4)2–CaX2 (X = F, Cl) mixture

We have explored metal halide doping in metal borohydrides in order to modify hydrogen desorption/absorption properties of such high-capacity solid-state hydrogen storage materials. The specific application here is 10 mol% addition of CaX2 (X = F, Cl) to Ca(BH4)2. The materials are analyzed using in-situ X-ray diffraction, differential scanning calorimetry, thermogravimetry, and IR spectroscopy, and the experimental results are compared against theoretical predictions from first-principles. Interestingly, in a fully hydrogenated state, CaCl2 dissolves into Ca(BH4)2 whereas CaF2 exists as a separate phase. During the course of dehydrogenation, CaH2–CaF2 solid solution, CaHCl, and a new Ca–H–Cl compound are observed. In-situ X-ray diffraction study reveals that CaX2 interacts with Ca(BH4)2 in the early stage of decomposition, which could facilitate a direct decomposition of Ca(BH4)2 into CaH2 and CaB6 without forming intermediate phases such as CaB2Hx which seem to be thermodynamically in close competition with the formation of CaH2 and CaB6. Our first-principles calculation estimates that the decrease in the decomposition temperature due to the CaH2–CaX2 interaction would be less than 10 °C, and therefore the major contribution of CaX2 is to change the dehydrogenation pathway rather than the overall thermodynamics.

Research highlights▶ Metal halides, CaF2 and CaCl2, can change dehydrogenation pathway of Ca(BH4)2. ▶ CaCl2 forms a solid solution with Ca(BH4)2. ▶ CaF2 does not interact with Ca(BH4)2. ▶ Metal halide and CaH2 interaction changes thermodynamics of Ca(BH4)2.