An Investigation into the Hydrogen Storage Characteristics
of Ca(BH<sub>4</sub>)<sub>2</sub>/LiNH<sub>2</sub> and Ca(BH<sub>4</sub>)<sub>2</sub>/NaNH<sub>2</sub>: Evidence of Intramolecular Destabilization
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Abstract
We report a study of the hydrogen
storage properties of materials that result from ball milling Ca(BH<sub>4</sub>)<sub>2</sub> and MNH<sub>2</sub> (M = Li or Na) in a 1:1
molar ratio. The reaction products were examined experimentally by
powder X-ray diffraction, thermogravimetric analysis and differential
scanning calorimetry (TGA/DSC), simultaneous thermogravimetric modulated
beam mass spectrometry (STMBMS), and temperature-programmed desorption
(TPD). The Ca(BH<sub>4</sub>)/LiNH<sub>2</sub> system produces a single
crystalline compound assigned to LiCa(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>). In contrast, ball milling of the Ca(BH<sub>4</sub>)/NaNH<sub>2</sub> system leads to a mixture of NaBH<sub>4</sub> and Ca(NH<sub>2</sub>)<sub>2</sub> produced by a metathesis reaction and another
phase we assign to NaCa(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>). Hydrogen desorption from the LiCa(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>) compound starts around 150 °C, which is more than 160
°C lower than that from pure Ca(BH<sub>4</sub>)<sub>2</sub>.
Hydrogen is the major gaseous species released from these materials;
however various amounts of ammonia form as well. A comparison of the
TGA/DSC, STMBMS, and TPD data suggests that the amount of NH<sub>3</sub> released is lower when the desorption reaction is performed in a
closed vessel. There is no evidence for diborane (B<sub>2</sub>H<sub>6</sub>) release from LiCa(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>), but traces of other volatile boron–nitrogen species (B<sub>2</sub>N<sub>2</sub>H<sub>4</sub> and BN<sub>3</sub>H<sub>3</sub>) are observed at 0.3 mol % of hydrogen released. Theoretical investigations
of the possible crystal structures and detailed phase diagrams of
the Li–Ca–B–N–H system were conducted
using the prototype electrostatic ground state (PEGS) method and multiple
gas canonical linear programming (MGCLP) approaches. The theory is
in qualitative agreement with the experiments and explains how ammonia
desorption in a closed volume can be suppressed. The reduced hydrogen
desorption temperature of LiCa(BH<sub>4</sub>)<sub>2</sub>(NH<sub>2</sub>) relative to Ca(BH<sub>4</sub>)<sub>2</sub> is believed to
originate from intramolecular destabilization