The nonvolatile products remaining after the thermal decomposition of metal amidoboranes (MAB, M = metal) are amorphous and incompletely characterized, increasing the complexity of devising regeneration strategies for these potential hydrogen storage materials. Utilizing the combined prototype electrostatic ground state search and density-functional theory (PEGS+DFT), we find that potential reaction products ([NHBH2]-, [NBH]-, [N3H2B3H3] -, and polymer-M[NHBH2] anion groups) in the decomposition of LiAB and CaAB are calculated to be significantly endothermic, in contrast to the experimentally measured nearly thermoneutral values [∼-4 kJ/(mol H 2) in LiAB and 3.5 kJ/(mol H2) in CaAB], suggesting that there are alternative products formed. The dianion group [NHBHNHBH 3]2- has recently been suggested to form in the decomposition of a calcium amidoborane complex in solution. In LiAB and CaAB, we use PEGS+DFT to predict intermediate metal-dianion compounds, and the static H2 release enthalpy is 27.4 and 27.3 kJ/(mol H2) in LiAB and CaAB, respectively. Introducing vibrational effects by phonon calculations, the enthalpies are shifted down by a roughly constant amount, ∼25 and ∼22 kJ/(mol H2) at 0 and 300 K. Thus, our theoretical H 2 release enthalpies agree with the experimentally measured nearly thermoneutral data in the decomposition of LiAB and CaAB. This agreement supports the existence of the dianion phases as products in the decomposition of metal amidoboranes. Then, using the dianion compound as an intermediate in the decomposition of MAB, we further study the stability trends of a series of MAB (M = Li, Na, K, Ca).
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films