Pressure and Temperature Influence on the Desorption Pathway of the LiBH 4 −MgH 2 Composite System (original) (raw)

Compaction pressure influence on material properties and sorption behaviour of LiBH4–MgH2 composite

International Journal of Hydrogen Energy, 2013

Among different Reactive Hydride Composites (RHCs), the combination of LiBH 4 and MgH 2 is a promising one for hydrogen storage, providing a high reversible storage capacity. During desorption of both LiBH 4 and MgH 2 , the formation of MgB 2 lowers the overall reaction enthalpy. In this work, the material has been compacted to pellets for further improvement of the volumetric hydrogen capacity. The influence of compaction pressure on the apparent density, thermal conductivity and sorption behaviour for the Li-based RHC during cycling has been investigated for the first time. Although LiBH 4 melts during cycling, decrepitation or disaggregation of the pellets is not observed for any of the investigated compaction pressures. However, a strong influence of the compaction pressure on the apparent hydrogen storage capacity is detected. The influence on the reaction kinetics is rather low. To provide explanations for the observed correlations, SEM analysis before and after each sorption step was performed for different compaction pressures. Thus, the low hydrogen sorption in the first cycles and the continuously improving sorption for low pressure compacted pellets with cycling may be explained by some surface observations, along with the form stability of the pellets.

A thermodynamic assessment of LiBH4

Calphad, 2012

(AAM) is copyrighted and published by Elsevier. It is posted here by agreement between Elsevier and the University of Turin. Changes resulting from the publishing process-such as editing, corrections, structural formatting, and other quality control mechanisms-may not be reflected in this version of the text. The definitive version of the text was subsequently published in CALPHAD, 39, 2012, 10.1016/j.calphad.2012.08.005. You may download, copy and otherwise use the AAM for non-commercial purposes provided that your license is limited by the following restrictions: (1) You may use this AAM for non-commercial purposes only under the terms of the CC-BY-NC-ND license. (2) The integrity of the work and identification of the author, copyright owner, and publisher must be preserved in any copy.

Hydrogen reversibility of LiBH4–MgH2–Al composites

Physical Chemistry Chemical Physics, 2014

The detailed mechanism of hydrogen release in LiBH 4 -MgH 2 -Al composites of molar ratios 4 : 1 : 1 and 4 : 1 : 5 are investigated during multiple cycles of hydrogen release and uptake. This study combines information from several methods, i.e., in situ synchrotron radiation powder X-ray diffraction, 11 B magicangle spinning (MAS) NMR, Sievert's measurements, Fourier transform infrared spectroscopy and simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy. The composites of LiBH 4 -MgH 2 -Al are compared with the behavior of the LiBH 4 -Al and LiBH 4 -MgH 2 systems. The decomposition pathway of the LiBH 4 -MgH 2 -Al system is different for the two investigated molar ratios, although it ultimately results in the formation of LiAl, Mg x Al 1Àx B 2 and Li 2 B 12 H 12 in both cases. For the 4 : 1 : 1-molar ratio, Mg 0.9 Al 0.1 and Mg 17 Al 12 are observed as intermediates. However, only Mg is observed as an intermediate in the 4 : 1 : 5-sample, which may be due to an earlier formation of Mg x Al 1Àx B 2 , reflecting the complex chemistry of Al-Mg phases. Hydrogen release and uptake reveals a decrease in the hydrogen storage capacity upon cycling. This loss reflects the formation of Li 2 B 12 H 12 as observed by 11 B NMR and infrared spectroscopy for the cycled samples. Furthermore, it is shown that the Li 2 B 12 H 12 formation can be limited significantly by applying moderate hydrogen partial pressure during decomposition.

High-Pressure Polymorphism as a Step towards Destabilization of LiBH4

Angewandte Chemie International Edition, 2008

Lithium borohydride could be an extremely efficient energy storage system containing 18.5 wt % hydrogen. However, owing to its high thermal stability, it is not yet regarded as a practical H-storage material. More experimental and theoretical efforts are required to improve the hydrogen-storage properties of this compound. Experimental investigations of light metal borohydrides such as LiBH 4 are difficult owing to the weak diffracting power of the light elements for X-ray diffraction and to considerable incoherent scattering by H and high absorption by natural B and Li for neutron diffraction. For these reasons, LiBH 4 has been extensively studied theoretically by "first-principles" methods based on density functional theory (DFT). A large amount of information has been generated, including possible crystal and electronic structures, lattice dynamics, surface properties, decomposition mechanisms, and intermediate products. Surprisingly, theory and experiment agreed only on the symmetry of the room-temperature, ambient-pressure polymorph of lithium borohydride. Despite the fact that the temperature-induced structural transition in LiBH 4 has been known for a long time, the experimental structural data on the high-temperature form have not yet been confirmed by theory. In particular, the presumed hexagonal P6 3 mc structure, first suggested from diffraction experiments, was found to have a relatively high energy and imaginary vibrational frequencies. Other calculations have also shown that the P6 3 mc structure is rather unstable. The same problem holds for the pressure evolution of LiBH 4 ; a phase transition below 5 GPa was identified more than 30 years ago, but there is still no agreement on the structure of the high-pressure phase. Theoretical predictions suggest a cubic NaBH 4 -type structure above 6.2 GPa, a monoclinic P2 1 /c structure at approximately 1 GPa, and a monoclinic Cc structure above 2.2 GPa (3 GPa in reference [4]). However, the most recent experimental study of LiBH 4 at pressures up to 9 GPa concludes that none of these predictions are correct, although the structure of the highpressure polymorph itself could not be identified, owing to experimental limitations. Thus, the first efforts in understanding the material properties, both experimental and theoretical, were discouraging. Both the pressure and temperature evolution of the corresponding structure have found no consistent explanation in the framework of "first-principles" theories.

Hydrogen sorption properties of MgH 2–LiBH 4 composites

Acta Materialia, 2007

A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH 2 + 2LiBH 4 M MgB 2 + 2LiH + 4H 2 was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH 2 + 2LiBH 4 M Mg + 2LiBH 4 + H 2 M MgB 2 + 2LiH + 4H 2 . However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH 4 and MgH 2 are found to form simultaneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250-300°C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used.

Effects of stoichiometry on the H2 storage properties of Mg(NH2)2-LiH-LiBH4 tri-component system

Chemistry, an Asian journal, 2017

Hydrogen desorption pathways and storage properties of 2Mg(NH2)2-3LiH-xLiBH4 samples (x = 0, 1, 2 and 4) were investigated systematically by combination of Pressure-Composition-Isotherm (PCI), Differential scanning calorimetric (DSC) and volumetric release methods. Experimental results show that the desorption peak temperatures of 2Mg(NH2)2-3LiH-xLiBH4 samples are ca. 10-15 oC lower compared to that of 2Mg(NH2)2-3LiH. Especially the 2Mg(NH2)2-3LiH-4LiBH4 composite begins to release hydrogen at 90 oC, exhibiting a superior dehydrogenation performance. All the LiBH4 doped samples can be fully dehydrogenated and re-hydrogenated at the temperature of 143 oC. The high hydrogen pressure region (above 50 bar) of PCI curves for the LiBH4 doped samples rise with the increasing of the LiBH4 amount. LiBH4 changes the desorption pathway of the 2Mg(NH2)2-3LiH sample under 50 bar hydrogen pressure, resulting in the formation of MgNH and molten [LiNH2-2LiBH4], which is different from the dehydroge...