International Journal of Hydrogen Energy, Vol.45, No.3, 2044-2053, 2020
In-situ synthesis of amorphous Mg(BH4)(2) and chloride composite modified by NbF5 for superior reversible hydrogen storage properties
The solvent-free amorphous Mg(BH4)(2) composite was in-situ synthesized by ball milling LiBH4 and MgCl2. It is found that the onset dehydrogenation temperature of the as synthesized composite is 126.9 degrees C, which is roughly 156 degrees C lower than that of pristine Mg(BH4)(2). The activation energy of the amorphous Mg(BH4)(2) and pristine Mg(BH4)(2) for the first dehydrogenation step was calculated as 120.01 kJ/mol and 487.99 kJ/mol, respectively. Hence the kinetics improvement is certified by the lower Ea value of the dehydrogenation process. When adding NbF5 into the composite, the catalyzed composite exhibits better hydrogen storage properties compared to pristine and amorphous Mg(BH4)(2). The catalyzed composite starts to release hydrogen at proximately 120 degrees C with a total capacity of 10.04 wt %. The reversibility of the catalyzed composite is also improved. The capacity of the catalyzed composite at the second cycle is 5.5 wt%. For the third and fourth cycles the catalyzed composite can still liberate 4 wt% H-2. Besides, the onset hydrogen desorption temperature during four cycles are extremely lower than those of pristine and amorphous Mg(BH4)(2). The peaks of the intermediate MgB12H12 is detected by FTIR as the regenerated hydrogenation product in the catalyzed composite. It can be speculated from the detailed analysis that there are mainly three reasons for the improved properties. Firstly, the additive NbF5 is favorable to enhance the hydrogen storage properties by modifying the dehydrogenation path and producing MgF2 and NbB2 as new products. Secondly, the in-situ formation of amorphous Mg(BH4)(2) is likely to improve the dehydrogenation properties of the samples due to its different reactivity comparing to crystal ones. Finally, LiCl can serve as buffer in the composite and thus improve the dehydrogenation properties. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.