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Superior Reversible Hydrogen Storage of the LiBH4 + MgH2 System Enabled by High-Energy Ball Milling with In-Situ Aerosol Spraying
Title
Superior Reversible Hydrogen Storage of the LiBH4 + MgH2 System Enabled by High-Energy Ball Milling with In-Situ Aerosol Spraying
Creator
Ding, Zhao
Date
2019
Description
The prospect of LiBH4 + MgH2 mixture has been limited by its sluggish kinetics, despite its excellent hydrogen storage capacity theoretically....
Show moreThe prospect of LiBH4 + MgH2 mixture has been limited by its sluggish kinetics, despite its excellent hydrogen storage capacity theoretically. We have designed a novel process termed as high-energy ball milling of MgH2 at ambient temperature along with aerosol spraying of LiBH4 dissolved in tetrahydrofuran (THF) solution (BMAS) to improve the thermodynamic and kinetic performance of LiBH4 + MgH2 hydrogen storage materials. Through this BMAS process, we have demonstrated that, for the first time, the reaction between LiBH4 + MgH2 can take place near ambient temperature, and the in-situ formation of LiH and MgB2 during BMAS is achieved through a new reaction pathway in which nano-LiBH4 decomposes to Li2B12H12 first and the newly formed Li2B12H12 reacts with MgH2 to form LiH and MgB2.Using the newly designed automated BMAS apparatus, we have successfully produced a BMAS mixture containing 1 mole of MgH2 + 0.5 mole of LiBH4, i.e., with 25% LiBH4 in the mixture for the stoichiometric reaction. The BMAS powder with 25% LiBH4 can release and absorb ~5.7 wt.% H2 at 265 oC, which is the highest one ever reported for the LiBH4 + MgH2 system at temperature ≤ 265 oC. It is found that the unusually high reversible hydrogen storage is accomplished through two parallel reaction pathways. One is nano-LiBH4 decomposes to form Li2B12H12 and H2 first and then Li2B12H12 reacts with MgH2 to form MgB2, LiH and H2. The other is nano-MgH2 decomposes to form Mg and H2 first and then Mg reacts with LiBH4 to form MgB2, LiH and H2. These reaction pathways become possible because of the presence of nano-LiBH4 and nano-MgH2 and their intimate mixing, enabled by the BMAS process. We have also revealed that the solid-state dehydrogenation kinetics of the BMAS powder with 25% LiBH4 at 265 oC is nucleation-and-growth controlled. The rate-limiting step for dehydrogenation via the two parallel reaction pathways has been identified through examination of the elementary reactions as the nucleation and growth of reaction products LiH and MgB2. Given the significantly improved hydrogen storage capacity for the LiBH4 + MgH2 system obtained via BMAS, investigation on increasing the LiBH4 content in the BMAS powder from 25% to 50% is performed. It is shown that Mg(BH4)2 can be produced during the BMAS process and it contributes to H2 release at temperature ≤ 265 oC. Three parallel H2 release mechanisms have been identified from the BMAS powder. These include (i) nano-LiBH4 decomposes to form Li2B12H12 and H2 first and then Li2B12H12 reacts with MgH2 to form MgB2, LiH and H2, (ii) nano-Mg(BH4)2 decomposes to form MgH2, B and H2, and (iii) nano-MgH2 decomposes to Mg and H2. Together these three mechanisms result in 4.11 wt.% H2 release in the solid state at temperature ≤ 265 oC. Furthermore, the predicted property of Fe3B in absorbing more H2 than releasing it is confirmed experimentally for the first time in this study. Varied models have been identified to describe the kinetic of solid-state dehydrogenation of the BMAS powder with 50% LiBH4 at 265 oC with increasing cycles. Additionally, the geometries of the solid particles involving with the dehydrogenation have also been estimated.
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