Melting of Magnesium Borohydride under High Hydrogen
Pressure: Thermodynamic Stability and Effects of Nanoconfinement
Posted on 2020-06-22 - 19:10
The thermodynamic
stability and melting point of magnesium borohydride
were probed under hydrogen pressures up to 1000 bar (100 MPa) and
temperatures up to 400 °C. At 400 °C, Mg(BH4)2 was found to be chemically stable between 700 and 1000 bar
H2, whereas under 350 bar H2 or lower pressures,
the bulk material partially decomposed into MgH2 and MgB12H12. The melting point of solvent-free Mg(BH4)2 was estimated to be 367–375 °C,
which was above previously reported values by 40–90 °C.
Our results indicated that a high hydrogen backpressure is needed
to prevent the decomposition of Mg(BH4)2 before
measuring the melting point and that molten Mg(BH4)2 can exist as a stable liquid phase between 367 and 400 °C
under hydrogen overpressures of 700 bar or above. The occurrence of
a pure molten Mg(BH4)2 phase enabled efficient
melt-infiltration of Mg(BH4)2 into the pores
of porous templated carbons (CMK-3 and CMK-8) and graphene aerogels.
Both transmission electron microscopy and small-angle X-ray scattering
confirmed efficient incorporation of the borohydride into the carbon
pores. The Mg(BH4)2@carbon samples exhibited
comparable hydrogen capacities to bulk Mg(BH4)2 upon desorption up to 390 °C based on the mass of the active
component; the onset of hydrogen release was reduced by 15–25
°C compared to the bulk. Importantly, melt-infiltration under
hydrogen pressure was shown to be an efficient way to introduce metal
borohydrides into the pores of carbon-based materials, helping to
prevent particle agglomeration and formation of stable closo-polyborate byproducts.
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White, James
L.; Strange, Nicholas A.; Sugar, Joshua D.; Snider, Jonathan L.; Schneemann, Andreas; Lipton, Andrew S.; et al. (2020). Melting of Magnesium Borohydride under High Hydrogen
Pressure: Thermodynamic Stability and Effects of Nanoconfinement. ACS Publications. Collection. https://doi.org/10.1021/acs.chemmater.0c01050
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AUTHORS (9)
JW
James
L. White
NS
Nicholas A. Strange
JS
Joshua D. Sugar
JS
Jonathan L. Snider
AS
Andreas Schneemann
AL
Andrew S. Lipton
MT
Michael F. Toney
MA
Mark D. Allendorf
VS
Vitalie Stavila
KEYWORDS
carbon-based materialsmagnesium borohydridehydrogen overpressureshydrogen backpressurepolyborate byproductshydrogen pressure1000 bar H 22 phasehydrogen releasemetal borohydridesparticle agglomerationtemplated carbonsMgH 2MgB 12 H 12bulk materialThermodynamic Stability350 bar H 2carbon poresMagnesium Borohydridesmall-angle X-raytransmission electron microscopyhydrogen pressuresHigh Hydrogen Pressurecarbon samplesCMK -3hydrogen capacitiesgraphene aerogels700 bar