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Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

Clean Energy Research Center, University of South Florida, 4202 E. Fowler Avenue, ENB 118, Tampa, FL 33620, USA; Mechanical Engineering, University of South Florida, 4202 E. Fowler Avenue, ENB 118, Tampa, FL 33620, USA; Electrical Engineering, University of South Florida, 4202 E. Fowler Avenue, ENB 118, Tampa, FL 33620, USA; Chemical and Biomedical Engineering, University of South Florida, 4202 E. Fowler Avenue, ENB 118, Tampa, FL 33620, USA; QuantumSphere Inc., 2905 Tech Center Drive, Santa Ana, CA 92705, USA; Department of Physics, College of Engineering, Architecture and Physical Sciences, Tuskegee University, 1200 W. Montgomery Rd, Tuskegee, AL 36088, USA
International Journal of Hydrogen Energy 01/2009; DOI: 10.1016/j.ijhydene.2009.07.065

ABSTRACT In this article, we investigate the ternary LiNH2–MgH2–LiBH4 hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH2:MgH2:LiBH4 is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH2 are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH2, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH2 crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.

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    ABSTRACT: A density functional theory study with the generalized gradient approximation (GGA) and projected augmented wave (PAW) method is performed for the hydrogen storage properties of the complex multinary storage Li–Mg–B–N–H system. Using ab initio methods, stability of the structures at finite temperatures is confirmed via. phonon spectrum calculations. Thermodynamic properties such as heat of reaction, and Gibbs energy for each reactant and product in the reaction steps in different temperature zones are calculated. It is found that reversibility occurs in the temperature range of 160–225 °C with approximately 4.38 wt % hydrogen storage capacity. The enthalpy of reversible re-/de-hydrogenation is found to be 55.17 kJ/mol H2, which is supported by experimental data. The total hydrogen storage capacity of this material is calculated to be 8.76 wt% from the desorption behavior observed at different temperatures up to 350 °C. These theoretically established reactions are validated with the suggested mechanism from experimental observations for the dehydrogenation reaction of this Li–Mg–B–N–H multinary system. These efforts are expected to contribute toward identification of suitable hydrogen storage materials.
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