First-Principles Prediction of Intermediate Products in the Decomposition of Metal Amidoboranes

Abstract

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 H2) in LiAB and 3.5 kJ/(mol H2) in CaAB], suggesting that there are alternative products formed. The dianion group [NHBHNHBH3]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 H2 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).

Crystal Structures, Phase Stabilities, and Hydrogen Storage Properties of Metal Amidoboranes

“…A DFT study of intermediates in MAB reaction pathways is forthcoming. 47 We plot the trends in thermodynamics (Figure 3) using the data from Table 4. From Figure 3, we can see that the trends of decomposition enthalpy of these MAB are the same with (the solid line at 300 K) or without (the dashed line of static energy) phonon vibrational contributions.…”

Theoretical Prediction of Metastable Intermediates in the Decomposition of Mg(BH4)2

“…To make the [B 3 H 8 ] − phase competitive in energy with [B 12 H 12 ] 2– , the total energy of the PEGS + DFT predicted Mg(B 3 H 8 ) 2 structure would have to be lowered by ∼260 kJ/(mol fu) (or ∼2.7 eV/fu). Even though the PEGS + DFT method may not predict the true ground state, based on extensive prior experience, ,,− , the predicted ground state energies are never in error by such a large amount . Thus, our calculations indicate that the Mg(B 3 H 8 ) 2 phase is highly unstable and certainly cannot be a thermodynamic reaction product.…”

“…Density functional theory (DFT) has been used to study thermodynamic properties of many H 2 storage materials and yields accurate results. ,− We use DFT to systematically study the structural and thermodynamic properties of a large number of possible intermediates [Mg(B 3 H 8 ) 2 , MgB 3 H 7 , Mg 3 (B 3 H 6 ) 2 , Mg(B 2 H 7 ) 2 , MgB n H n ( n = 6, 7, 8, 9, 10, 11), and Mg(B 11 H 14 ) 2 ] in the decomposition of Mg(BH 4 ) 2 . Since the crystal structures of these hypothetical intermediates have not been determined from experiments, we use a computational approach to predict these unknown low energy crystal structures, the Monte Carlo based prototype electrostatic ground state (PEGS) method .…”

“…Since the crystal structures of these hypothetical intermediates have not been determined from experiments, we use a computational approach to predict these unknown low energy crystal structures, the Monte Carlo based prototype electrostatic ground state (PEGS) method . PEGS has successfully predicted crystal structures of complex hydrides containing anion groups such as [BH 4 ] − (refs and ), [NH] 2– (ref ), [B 2 H n ] 2– (ref ), [NH 2 BH 3 ] − (ref ), [B 12 H 12 ] 2– (ref ), and even the hypophosphite anion [BH 3 PH 2 BH 3 ] − (ref ).…”

Rubidium hydrazinidoborane: Synthesis, characterization and hydrogen release properties

“…This observation indicates differences between N2H4− of HB and N2H3(Rb)− of RbHB in terms of electronic structure and binding strength. This is typical of a derivative where there are weaker interactions of H δ+ and H δ− between molecules (because of longer distance due to the big Rb + cation) [42,43].…”