Investigation On The Synthesis And Hydrogen Desorption Performance Of Calcium Alanates

In recent years, light metal complex hydrides like NaAlH4 have been investigated intensely as a new type of hydrogen storage materials just beacase of their high hydrogen storage capacity.The main strategies of hydrogen storage research are screening out new materials with more excellent properties and optimizing the existing materials. Based on the summary of researches about the light metal complex hydrides in the world, this paper elucidated two ways of preparing Ca(AlH4)2 using different raw materials by mechanical ball-milling, their structural changes and dehydrogenation behaviors were characterized by X-ray Differaction(XRD), Infra Spectrum(IR) and Differential Scanning Calorimetry-Mass Spectrum-thermogravimetry synchronized(DSC-TG-MS). At the same time, we investigated the influence of fluoride doping (TiF3, FeF3) on the hydrogen storage properties of Ca(AlH4)2, and explored new types of metal complex hydrides with multi-cation.Investigation shows that it requires at least 48 h for ball-milling of commercial NaAlH4 and CaCl2 in order to synthesize Ca(AlH4)2, the amout of hydrogen released in the first two steps reaches 5.2 wt.%H2 when heating to 250℃. The other way of preparing Ca(AlH4)2 using NaH/Al needs to fulfill in two steps. First NaAlH4 is prepared with NaH/Al and small amount of CeAl4, this process need 90 h; the second step is ball-milling the prepared NaAlH4 and CaCl2 for 20 h. The results reveal that 5.53 wt.%H2 is released within 200℃, which behaves better performance. Analysis provides that the activation energy of Ca(AlH4)2 decompositon differ for the two ways, the later is lower than the former value.Investigation of fluoride doping on the hydrogen storage behavior of Ca(AlH4)2 includes TiF3 and FeF3 doping. The results show that 10 wt.%TiF3 doping shows good dehydriding performance in the first two-step decomposition reactions, with decomposition temperature dropped by 32℃and 48℃respectively, whereas the first decomposition reaction proceeded in the doping process.10 wt.%FeF3 doping Ca(AlH4)2 exhibits the most excellent dehydriding performance. The decomposition temperature for the second step declines 50℃, and the activation energy decreases to 88.3 kJ/mol. We conducted the investigation of the mechanism of catalyzation, and find that the TiF3 and FeF3 have both participated in the decomposition process of Ca(AlH4)2, which leads to the generation of complex intermediate and transition metals Ti and Fe, and boosts the thermodynamics and kinetics of dehydrogenation. However, the reversibility of the catalyzed Ca(AlH4)2 system is still a problem.Investigation of the multi-cation effects on the Ca(AlH4)2 system brings on the discover of three new complex hydrides:LiCa(AlH4)3, NaCaAlH6 and LiCaAlH6. The structure and dehydrogenation behaviors of the three hydrides were characterized respectively. LiCa(AlH4)3 is firstly decomposed into Li3AIH6 and CaAlHj.the interaction of the two intermediates is too weak that the reversibility of this system is inaccessible. NaCaAlH6 is firstly decomposed into NaAlH4 and CaH2, the amount of hydrogen released when heating to 450℃reaches 4.5 wt.%. The following decomposition has no meaning due to the chemical stability of CaH2. LiCaAlH6 is firstly decomposed into CaAlH5 and LiH, the following decomposition is too difficult to clarify, the amount of hydrogen released when heating to 450℃reaches 4.65 wt.%. The calculation results show that activation energy of LiCaAlH6 is rather high that the kinetics of it is very poor.