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

Abstract

Metal amidoboranes, M(NH 2 BH 3 ) n (M = alkali metal or alkaline-earth metal), are candidates for on-board hydrogen storage materials with high gravimetric capacity, low H 2 release temperature, and the ability to suppress toxic borazine emission. We have used a firstprinciples 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 LiNH 2 BH 3 , NaNH 2 BH 3 , KNH 2 BH 3 , and Ca(NH 2 BH 3 ) 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(NH 2 BH 3 ) n → MH n + nBN + 2nH 2 , we compute the H 2 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(NH 2 BH 3 ) 2 , which is more stable than Ca(NH 2 BH 3 ) 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 H 2 ) at T = 300 K. ■ INTRODUCTIONAmmonia borane (NH 3 BH 3 or AB) is a chemical hydride and promising candidate for hydrogen storage application due to its high hydrogen gravimetric density (19.6 wt %) and the fact that it is releases ∼12 wt % hydrogen at a moderate temperature (∼150°C). 1−3,5,6 While the high volumetric and gravimetric density of hydrogen is impressive, there are two technical challenges that must be overcome for practical applications:The exothermic H 2 release enthalpy [−20 kJ/(mol H 2 ) 3 for the first mole of H 2 , NH 3 BH 3 → (NH 2 BH 2 ) x + H 2 ], precludes the possibility of on-board regeneration and the formation of even trace quantities of volatile borazine ([N 3 B 3 H 6 ]) limits the fuel cell lifetime. These drawbacks limit the possibilities for AB as a suitable hydrogen storage material for reversible on-board vehicle applications. 4 Recently, many researchers 6−14 have used alkali metal and alkaline-earth metal atoms to modify ammonia borane by replacing one H atom from an NH 3 unit with a metal (M) atom. The resulting metal amidoboranes [M(NH 2 BH 3 ) n or MAB] successfully suppress borazine release and decrease the H 2 release temperature to ∼90°C in LiAB 6,11 and NaAB 6,10 and ∼100°C in CaAB. 11 However, the intermediate and final products in MAB decompostion are still unclear, and it is difficult to rehydrogenate these decomposition products.Hence, it is crucial to understand the thermodynamic properties of these MAB...

Mono- and Bimetalic Amidoboranes

“…A computational modeling of a crystal structure of Mg(AB) 2 was performed [30] but without comparative study with experimental results of Luo et al [29].…”

“…The 10 h lasting thermal treatment caused evolution of 0.83 equivalent of H 2 per AB molecule, which means that the synthesis was not complete. The synthesis can be described by the simplified reaction Equation (30). …”

Efficient Synthesis of an Aluminum Amidoborane Ammoniate

“…Metal amidoboranes (MNH2BH3, MAB) and metal borohydride ammoniates (MBH4·nNH3, MBA) are currently among the most promising candidate materials [1][2][3][4][5][6]. Recent demonstration of the regeneration of ammonia borane derivatives using hydrazine in liquid ammonia point to the feasibility of off-board reversibility [7][8][9].…”

Ammonia borane–metal alanate composites: hydrogen desorption properties and decomposition processes

“…6,7 To overcome these disadvantages, several approaches have been developed, such as infusion of AB in nanoscaffolds, 8 doping with transition metals as catalysts, 9 size and catalytic effects from graphitic carbon nitride, 10 and chemical modification of AB by replacing one of H atoms with an alkali or alkaline earth metal to form metal amidoboranes. 11 In previous reports, many kinds of AB-MH (Metal Hydride) composites, such as AB-LiH, [11][12][13][14][15][16] AB-NaH, were synthesized and their dehydrogenation properties were investigated. Recently, we experimentally verified that AB-MAlH 4 (M = Na, Li) composites, which were prepared based on the indicator we proposed, can suppress the emission of NH 3 , B 2 H 6 and B 3 H 6 N 3 .…”