An Investigation into the Hydrogen Storage Characteristics of Ca(BH₄)₂/LiNH₂ and Ca(BH₄)₂/NaNH₂: Evidence of Intramolecular Destabilization

We report a study of the hydrogen storage properties of materials that result from ball milling Ca­(BH₄)₂ and MNH₂ (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(BH₄)/LiNH₂ system produces a single crystalline compound assigned to LiCa­(BH₄)₂(NH₂). In contrast, ball milling of the Ca­(BH₄)/NaNH₂ system leads to a mixture of NaBH₄ and Ca­(NH₂)₂ produced by a metathesis reaction and another phase we assign to NaCa­(BH₄)₂(NH₂). Hydrogen desorption from the LiCa­(BH₄)₂(NH₂) compound starts around 150 °C, which is more than 160 °C lower than that from pure Ca­(BH₄)₂. 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 NH₃ released is lower when the desorption reaction is performed in a closed vessel. There is no evidence for diborane (B₂H₆) release from LiCa­(BH₄)₂(NH₂), but traces of other volatile boron–nitrogen species (B₂N₂H₄ and BN₃H₃) 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(BH₄)₂(NH₂) relative to Ca­(BH₄)₂ is believed to originate from intramolecular destabilization