Conventional (e.g. MgH 2 ) and complex hydrides (e.g. alanates, borohydrides, and amides) are the two primary classes of solid-state hydrogen-storage materials. [1][2][3] Many of these "high-density" hydrides have the potential to store large amounts of hydrogen by weight (up to 18.5 wt % for LiBH 4 ) and/or volume (up to 112 g L À1 for MgH 2 ), values that are comparable to the hydrogen content of gasoline (15.8 wt %, 112 g L À1 ). However, all known hydrides are inadequate for mobile storage applications due to one or more of the following limitations: a) unfavorable thermodynamics (they require high temperatures to release hydrogen [4] ), b) poor kinetics (low rates of hydrogen release and uptake), c) decomposition pathways involving the release of undesirable by-products (e.g. ammonia), and/or d) an inability to reabsorb hydrogen at modest temperatures and pressures (i.e. "irreversibility").In spite of these drawbacks, renewed interest in complex hydrides has been stimulated recently by substantial improvements in their kinetics and reversibility [5,6] provided by catalytic doping (e.g. TiCl 3 -doped NaAlH 4 ), [7,8] and by thermodynamic enhancements achieved through reactive binary mixtures [9] such as LiNH 2 /MgH 2 , [10,11] LiBH 4 /MgH 2 , [12] and LiNH 2 /LiBH 4 . [13,14] These compositions, previously termed "reactive hydride composites", [15] represent the state-of-the-art in hydrogen-storage materials; compared to their constituent compounds, they exhibit improved thermodynamic properties, higher hydrogen purity, and, in some cases, reversibility. The desorption behavior of these previously studied composites is illustrated in Figure 1 a. It is evident from the hydrogen desorption profile (top panel) that the composites generally desorb hydrogen at significantly lower temperatures than their individual components. For example, the lowest temperature reaction, which involves a Figure 1. a) Hydrogen (top) and ammonia (bottom) kinetic desorption data as a function of temperature (5 8C min À1 to 550 8C) for the ternary composition (blue trace) and its unary and binary constituents. Hydrogen desorption is measured in weight percent (wt %) to 1 bar whereas relative ammonia release is measured as partial pressure (torr) in a flow-through set-up (100 sccm Ar). b) Ternary phase space defined by unary compounds (nodes), LiBH 4 (pink), MgH 2 (purple), and LiNH 2 (orange) and the binary mixtures (edges), LiBH 4 /MgH 2 (gray), MgH 2 / LiNH 2 (green), and LiNH 2 /LiBH 4 (red). The present ternary composition, which is a 2:1:1 mixture of LiNH 2 , LiBH 4 , and MgH 2 , and previously investigated binaries, are identified.

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Conventional (e.g. MgH 2 ) and complex hydrides (e.g. alanates, borohydrides, and amides) are the two primary classes of solid-state hydrogen-storage materials. [1][2][3] Many of these "high-density" hydrides have the potential to store large amounts of hydrogen by weight (up to 18.5 wt % for LiBH 4 ) and/or volume (up to 112 g L À1 for MgH 2 ), values that are comparable to the hydrogen content of gasoline (15.8 wt %, 112 g L À1 ). However, all known hydrides are inadequate for mobile storage applications due to one or more of the following limitations: a) unfavorable thermodynamics (they require high temperatures to release hydrogen [4] ), b) poor kinetics (low rates of hydrogen release and uptake), c) decomposition pathways involving the release of undesirable by-products (e.g. ammonia), and/or d) an inability to reabsorb hydrogen at modest temperatures and pressures (i.e. "irreversibility").In spite of these drawbacks, renewed interest in complex hydrides has been stimulated recently by substantial improvements in their kinetics and reversibility [5,6] provided by catalytic doping (e.g. TiCl 3 -doped NaAlH 4 ), [7,8] and by thermodynamic enhancements achieved through reactive binary mixtures [9] such as LiNH 2 /MgH 2 , [10,11] LiBH 4 /MgH 2 , [12] and LiNH 2 /LiBH 4 . [13,14] These compositions, previously termed "reactive hydride composites", [15] represent the state-of-the-art in hydrogen-storage materials; compared to their constituent compounds, they exhibit improved thermodynamic properties, higher hydrogen purity, and, in some cases, reversibility. The desorption behavior of these previously studied composites is illustrated in Figure 1 a. It is evident from the hydrogen desorption profile (top panel) that the composites generally desorb hydrogen at significantly lower temperatures than their individual components. For example, the lowest temperature reaction, which involves a Figure 1. a) Hydrogen (top) and ammonia (bottom) kinetic desorption data as a function of temperature (5 8C min À1 to 550 8C) for the ternary composition (blue trace) and its unary and binary constituents. Hydrogen desorption is measured in weight percent (wt %) to 1 bar whereas relative ammonia release is measured as partial pressure (torr) in a flow-through set-up (100 sccm Ar). b) Ternary phase space defined by unary compounds (nodes), LiBH 4 (pink), MgH 2 (purple), and LiNH 2 (orange) and the binary mixtures (edges), LiBH 4 /MgH 2 (gray), MgH 2 / LiNH 2 (green), and LiNH 2 /LiBH 4 (red). The present ternary composition, which is a 2:1:1 mixture of LiNH 2 , LiBH 4 , and MgH 2 , and previously investigated binaries, are identified.

We recently reported (Yang, J.; et al. Angew. Chem., Int. Ed. 2008, 47, 882) a novel hydrogen storage composite involving a 2:1:1 LiNH2:LiBH4:MgH2 ratio. On the basis of in-depth experimental and computational analysis, this composite was found to release hydrogen via a complex multistep reaction cascade, which seeded the products of a subsequent reversible hydrogen storage reaction. This so-called autocatalytic reaction sequence was found to result in favorable kinetics, ammonia attenuation, and partial low-temperature reversibility. Here, we extend our original study by examining the effects of reactant stoichiometry on the ensuing hydrogen storage desorption pathway and properties. In particular, we examine four (LiNH2) X −(LiBH4) Y −(MgH2) Z composites, where X:Y:Z = 2:1:2, 1:1:1, 2:0.5:1, and 2:1:1 (original stoichiometry). For each sample, we characterize the postmilled mixtures using powder X-ray diffraction (PXRD) and infrared spectroscopy (IR) analyses and observe differences in the relative extent of two spontaneous milling-induced reactions. Variable-temperature hydrogen desorption data subsequently reveal that all composites exhibit a hydrogen release event at rather low temperature, liberating between 2.3 (1:1:1) and 3.6 (2:0.5:1) wt % by 200 °C. At higher temperatures (200−370 °C), the hydrogen release profiles differ considerably between composites and release a total of 5.7 (1:1:1) to 8.6 (2:0.5:1) wt %. Utilizing variable-temperature IR and PXRD data coupled with first-principles calculations, we propose a reaction pathway that is consistent with the observed phase progression and hydrogen desorption properties. From these data, we conclude that premilled reactant stoichiometry has a profound impact on reaction kinetics and high-temperature reaction evolution because of reactant availability. From this enhanced understanding of the desorption process, we recommend and test a stoichiometrically optimal ratio (3:1:1.5) which releases a total of 9.1 wt % hydrogen. Finally, we assess the reversibility (at 180 °C) of the four primary composites over two desorption cycles and find that only the 2:1:1 and 2:0.5:1 are reversible (3.5 wt % for 2:0.5:1).