Synthesis of calcium alanate and its dehydriding performance enhanced by FeF3 doping

Ca(AlH4)2 was synthesized by ball-milling the mixture of NaAlH4 and CaCl2 in a molar ratio of 2:1 and under a hydrogen atmosphere of 1 MPa. The results indicate that the reactants have entirely transformed to Ca(AlH4)2 with a byproduct of NaCl after ball-milling for 48 h. Investigations of dehydriding behavior of the as-prepared Ca(AlH4)2 sample show that approximately 5.2 wt.% of hydrogen is desorbed during the first two dehydrogenation reactions of Ca(AlH4)2, which exhibit an exothermic event at 148 °C and an endothermic event at 267 °C, respectively. The high temperature dehydrogenation at 267 °C mainly concentrates on the thermolysis of CaAlH5 intermediate. FeF3-doped Ca(AlH4)2 system represents an improved dehydriding performance, the dehydrogenation temperature of CaAlH5 intermediate is decreased about 43 °C. After FeF3 doping, the apparent activation energy of CaAlH5 is reduced from 153.4 kJ/mol (undoped) to 88.3 kJ/mol (doped), it renders a possibility to realize the rehydrogenation of CaAlH5. The catalytic effect is attributed to a fluorine transfer reaction that occurred to generate CaF2 and Fe catalysts.

Research highlights▶ The synthetic process and characterization of Ca(AlH4)2 is illustrated. ▶ After FeF3 doping the dehydrogenation temperature of its CaAlH5 intermediate is decreased about 43 °C. ▶ After FeF3 doping the apparent activation energy of CaAlH5 is reduced from 153.4 kJ/mol (undoped FeF3) to 88.3 kJ/mol (doped FeF3). ▶ The mechanism of FeF3 doping for the enhanced dehydrogenation performance of Ca(AlH4)2 system include the reaction of CaAlH5 and FeF3, and the catalyst effects rendered by the generated CaF2 and transitional metal Fe catalyst.