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

Abstract

Metal amidoboranes, M­(NH₂BH₃)_<i>n</i> (M = alkali metal or alkaline-earth metal), are candidates for on-board hydrogen storage materials with high gravimetric capacity, low H₂ release temperature, and the ability to suppress toxic borazine emission. We have used a first-principles density functional theory (DFT) combination with Monte Carlo method to search for crystal structures for a wide array of metal amidoboranes (M = Li, Na, K, Be, Mg, Ca, Sr, and Sc). In cases where the experimental structures are known, the DFT energies of the theoretically predicted LiNH₂BH₃, NaNH₂BH₃, KNH₂BH₃, and Ca­(NH₂BH₃)₂ structures are degenerate with the DFT energies computed for the experimental structures [to within 4 kJ/(mol f.u.)], confirming the accuracy of our approach. On the basis of the decomposition reaction pathway, M­(NH₂BH₃)_<i>n</i> → MH_<i>n</i> + <i>n</i>BN + 2<i>n</i>H₂, we compute the H₂ release reaction enthalpies and show that the stability of metal amidoboranes obeys the following trend: The metal amidoborane becomes more stable (the decomposition reaction becomes less exothermic) as the metal cation becomes more electropositive, that is, as the metal cation goes down in the periodic table along a given column or as the metal moves to the left along a given row. The only exception to this rule is Mg­(NH₂BH₃)₂, which is more stable than Ca­(NH₂BH₃)₂. Introducing vibrational entropy effects does not change this exceptional behavior of Mg amidoborane: the phonon contribution serves to shift all reaction enthalpies down by a roughly constant amount, ∼22 kJ/(mol H₂) at <i>T</i> = 300 K