Abstract
The objective of this study was to apply three-dimensional x-ray microtomographic imaging to understanding morphologies in the diphasic destabilized hydride system: MgH2 and LiBH4. Each of the single phase hydrides as well as two-phase mixtures at LiBH4:MgH2 ratios of 1:3, 1:1, and 2:1 were prepared by high energy ball milling for 5 minutes (with and without 4 mol % TiCl3 catalyst additions). Samples were imaged using computed microtomography in order to (i) establish measurement conditions leading to maximum absorption contrast between the two phases and (ii) determine interfacial volume. The optimal energy for measurement was determined to be 15 keV (having 18% transmission for the MgH2 phase and above 90% transmission for the LiBH4 phase). This work also focused on the determination of interfacial volume. Results showed that interfacial volume for each of the single phase systems, LiBH4 and MgH2, did not change much with catalysis using 4 mol % TiCl3. However, for the mixed composite system, interphase boundary volume was always higher in the catalyzed system; increasing from 15% to 33% in the 1:3 system, from 11% to 20% in the 1:1 system, and 2% to 14% in the 2:1 system. The parameters studied are expected to govern mass transport (i.e., diffusion) and ultimately lead to microstructure-based improvements on H2 desorption and uptake rates.
Highlights
Many light metal hydrides have been considered for hydrogen storage applications
It is clear to see that the phase contrast between LiBH4 and MgH2 (18% transmission at 15 keV) gives rise to LiBH4 particles which appear lighter than the darker MgH2 ones
~50 μm–100 μm in size and the MgH2 particles are slightly larger at ~100 μm–150 μm
Summary
The light metal hydride which has demonstrated a higher hydrogen gravimetric capacity at 9 wt % hydrogen is LiBH4. This destabilization reaction was first demonstrated by Vajo et al in 2005—who reduced the enthalpy of the dehydrogenation reaction by 25 kJ/mol H2 over the pure LiBH4 [1,2]. By extrapolation, this data implies a lowering in the reaction temperature at 1 bar of H2 pressure from 400 °C in pure. Further thermodynamic predictions show that other M(BH4)x compounds
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