High volumetric hydrogen density phases of magnesium borohydride at high-pressure:A first-principles study

The previously proposed theoretical and experimental structures, bond characterization, and compressibility of Mg（BH4）2 in a pressure range from 0 to 10 GPa are studied by ab initio density-functional calculations. It is found that the ambient pressure phases of meta-stable I41/amd and unstable P-3ml proposed recently are extra stable and cannot decompose under high pressure. Enthalpy calculation indicates that the ground state of F222 structure proposed by Zhou et al. [2009 Phys. Rev. B 79 212102] will transfer to I41/amd at 0.7 GPa, and then to a P-3ml structure at 6.3 GPa. The experimental P6122 structure （a-phase） transfers to I41/amd at 1.2 GPa. Furthermore, both I41/arnd and P-3ml can exist as high volumetric hydrogen density phases at low pressure. Their theoretical volumetric hydrogen densities reach 146.351 g H2/L and 134.028 g H2/L at ambient pressure, respectively. The calculated phonon dispersion curve shows that the I41/amd phase is dynamically stable in a pressure range from 0 to 4 CPa and the P-3ral phase is stable at pressures higher than 1 GPa. So the I41/arnd phase may be synthesized under high pressure and retained to ambient pressure. Energy band structures show that they are both always ionic crystalline and insulating with a band-gap of about 5 eV in this pressure range. In addition, they each have an anisotropic compressibility. The c axis of these structures is easy to compress. Especially, the c axis and volume of P-3ml phase are extraordinarily compressible, showing that compression along the e axis can increase the volumetric hydrogen content for both I41/amd and P-3ml structures.

Theoretical Studies on Dehydrogenation Reactions in Mg2（BH4）2（NH2）2 Compounds

Borohydrides have been recently hightlighted as prospective new materials due to their high gravimetric capacities for hydrogen storage. It is, therefore, important to under- stand the underlying dehydrogenation mechanisms for further development of these ma- terials. We present a systematic theoretical investigation on the dehydrogenation mecha- nisms of the Mg2（BH4）2（NH2）2 compounds. We found that dehydrogenation takes place most likely via the intermolecular process, which is favorable both kinetically and thermo- dynamically in comparison with that of the intramolecular process. The dehydrogenation of Mg2（BH4）2（NH2）2 initially takes place via the direct combination of the hydridie H in BH4 and the protie H in NH2-, followed by the formation of Mg-H and subsequent ionic recombination of Mg-Hδ-…Hδ＋-N.

Effect of MWCNTs Additive on Desorption Properties of Zn(BH_4)_2 Composite Prepared by Mechanical Alloying

In this study, the effect of multi-walled carbon nanotubes （MWCNTs） additive on the dehydriding properties of the Zn（BH4）2/NaCl composite prepared by high energy ball milling were investigated. X-ray diffraction （XRD）, Fourier transform infrared spectroscopy （FTIR） results demonstrated that Zn（BH4）2 was produced from mechanochemical reaction between ZnCl2 and NaBH4. Compared with the undoped sample, 10 wt% MWCNTs effectively lowered the decomposition temperature of Zn（BH4）2 by 15 ℃. The complex released 3.6 wt% hydrogen within 250 s at 100 ℃ and totally released 4.5 wt% hydrogen within 2500 s, indicating it has a considerable potential as a hydrogen storage material.