Web Release Date: February 23,
Hydrogen Storage Properties in (LiNH2)2-LiBH4-(MgH2)X Mixtures (X = 0.0-1.0)
and
Ford Motor Company, Research and Advanced Engineering, MD 1170/RIC, P.O. Box 2053, Dearborn, Michigan 48121, and Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208
Received: October 11, 2007
Abstract:
We have recently reported the synthesis and properties of a novel hydrogen storage composition comprised
of a 2:1:1 molar ratio of three hydride compounds: lithium amide (LiNH2), lithium borohydride (LiBH4),
and magnesium hydride (MgH2). This new ternary mixture possesses improved hydrogen (de)sorption attributes
(relative to the individual compounds and their binary mixtures), including facile low-temperature kinetics,
ammonia attenuation, and partial reversibility. Comprehensive characterization studies of its reaction pathway
revealed that these favorable hydrogen storage properties are accomplished through a complex multistep
hydrogen release process. Here, we expound on our previous findings and determine the impact of MgH2
content on the resulting hydrogen storage properties by examining a series of (LiNH2)2-LiBH4-(MgH2)X
reactant mixtures (i.e., 2:1:X molar ratio) where X = 0, 0.15, 0.25, 0.40, 0.50, 0.75, and 1.0. Specifically, we
characterize each starting composition (after ball-milling) using powder X-ray diffraction (PXRD) and infrared
spectroscopy (IR) and find that addition of MgH2 facilitates a spontaneous milling-induced reaction, introducing
new species (Mg(NH2)2 and LiH) into the hydride composition. We additionally measure the relative hydrogen
and ammonia release amounts for each mixture using temperature-programmed desorption mass spectrometry
(TPD-MS) and find that ammonia liberation is suppressed for increasing values of X (<0.1 wt % NH3 for
X = 1). Kinetic hydrogen desorption data reveal a low-temperature reaction step (centered at ~160 
C) for
all MgH2-containing samples which grows in intensity for larger values of X (up to ~4.0 wt % H2 for X =
1). Finally, we characterize desorbed samples to investigate the dependence of X (MgH2 amount) on the
resulting distribution of observed product phases. These data are used to understand how MgH2 contributes
to and impacts the low- and high-temperature hydrogen release events through comparing theoretical (based
on the previously proposed reaction set) and observed desorption data for these reactions.
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