(221c) Sorption Kinetics and Catalysis of 1:1 LiNH2:MgH2

High-performance on-board hydrogen storage systems are increasingly recognized as critical to implementation of hydrogen fuel cells as clean, efficient automotive power plants [1]. Among the various storage system types: adsorbent materials, chemical hydrides and metal hydrides, the use of complex hydrides is one of the candidates because of their large gravimetric and volumetric storage capacities [2], on-board reversibility, and indefinite ambient temperature storage duration. The destabilized complex hydride system composed of 1:1 LiNH₂:MgH₂ is one of the leading candidates of hydrogen storage compositions. In 2007, Lu et al. showed that the reaction between MgH₂ and LiNH₂ in a 1:1 molar ratio resulted in a new nitride-binary light metal, LiMgN, for hydrogen storage applications [3]. The dehydrogenation reaction pathway is as follows: LiNH₂+MgH₂→LiMgN+2H₂ This 1:1 molar ratio of MgH₂ to LiNH₂ mixture has a theoretical hydrogen weight capacity of 8.2 wt%. The rehydrogenation and subsequent dehydrogenation process of LiMgN produces LiH, Mg(NH₂)₂, and MgH₂, as seen here: LiMgN+2H₂«LiH+0.5Mg(NH₂)₂+0.5MgH₂. In a study of more than 300 destabilization reactions using first principle density function theory (DFT), Alapati et al. predicted that the reaction of between MgH₂ and LiNH₂ in a 1:1 molar ratio was energetically favorable with an enthalpy of 31.9 kJ/molH₂ [4]. This enthalpy suggests that favorable sorption thermodynamics should be obtainable at temperatures in the range of 160°C to 260°C. Preliminary experiments reported in literature indicate that sorption kinetics are substantially lower than expected in this temperature range despite favorable thermodynamics. Systematic isothermal and isobaric sorption experiments were performed on these materials using a Sievert's apparatus to form a baseline data set by which to compare kinetic results. Various catalytic modifications were performed in attempts to increase the kinetics while lowering the sorption temperatures. In this paper, we investigate the base line sorption kinetics of the 1:1 LiNH₂:MgH₂ composition and the effects of catalyst on these rates, the temperature of initial hydrogen release and the amount of ammonia released from the catalyzed and catalyst-free materials. Possible implications for use of this material for hydrogen storage applications are summarized.

1. Pinkerton, F.E., Decomposition kinetics of lithium amide for hydrogen storage materials. Journal of Alloys and Compounds, 2005. 400(1-2): p. 76-82.

2. Matsumoto, M., et al., Hydrogen desorption reactions of Li-N-H hydrogen storage system: Estimation of activation free energy. Journal of Alloys and Compounds, 2007. 439(1-2): p. 358-362.

3. Lu, J., et al., Potential of Binary Lithium Magnesium Nitride for Hydrogen Storage Applications. The Journal of Physical Chemistry C, 2007. 111(32): p. 12129-12134.

4. Alapati, S.V., J.K. Johnson, and D.S. Sholl, Identification of Destabilized Metal Hydrides for Hydrogen Storage Using First Principles Calculations. The Journal of Physical Chemistry B, 2006. 110(17): p. 8769-8776.