Thermodynamic Properties and Reversible Hydrogenation of LiBH4–Mg2FeH6 Composite Materials

Abstract

Abstract:In previous studies, complex hydrides LiBH 4 and Mg 2 FeH 6 have been reported to undergo simultaneous dehydrogenation when ball-milled as composite materials (1 − x)LiBH 4 + xMg 2 FeH 6 . The simultaneous hydrogen release led to a decrease of the dehydrogenation temperature by as much as 150 K when compared to that of LiBH 4 . It also led to the modified dehydrogenation properties of Mg 2 FeH 6 . The simultaneous dehydrogenation behavior between stoichiometric ratios of LiBH 4 and Mg 2 FeH 6 is not yet understood. Therefore, in the present work, we used the molar ratio x = 0.25, 0.5, and 0.75, and studied the isothermal dehydrogenation processes via pressure-composition-isothermal (PCT) measurements. The results indicated that the same stoichiometric reaction occurred in all of these composite materials, and x = 0.5 was the molar ratio between LiBH 4 and Mg 2 FeH 6 in the reaction. Due to the optimal composition ratio, the composite material exhibited enhanced rehydrogenation and reversibility properties: the temperature and pressure of 673 K and 20 MPa of H 2 , respectively, for the full rehydrogenation of x = 0.5 composite, were much lower than those required for the partial rehydrogenation of LiBH 4 . Moreover, the x = 0.5 composite could be reversibly hydrogenated for more than four cycles without degradation of its H 2 capacity.

Functional Materials Based on Metal Hydrides

“…Chaudhary, Dornheim, Orimo and co-workers investigated a transition metal containing a reactive hydride composite (1 − x)LiBH 4 -xMg 2 FeH 6 , and measured excellent pressure-composition-isothermal (PCT) data [21]. The results indicate that the same stoichiometric reaction (x = 0.5) occurred in all investigated samples with optimal hydrogen storage and reversibility properties.…”

“…Moreover, the x = 0.5 composite could be reversibly hydrogenated for more than four cycles without degradation of the H 2 capacity. The authors conclude that magnesium hydride plays a vital role as an intermediate in the hydrogen release and uptake reactions [21].…”

In Situ Synchrotron X-ray Diffraction Studies of Hydrogen-Desorption Properties of 2LiBH4–Mg2FeH6 Composite

“…To date, the reported reaction mechanism of joint 2LiBH 4 -Mg 2 FeH 6 decomposition has been investigated under isothermal conditions, and the results have been reported by several groups [11,12,[14][15][16][17][18], while the knowledge of the actual reaction path for decomposition under dynamic heating of the mixture, and the present competition between different possible decomposition reactions, is limited. Indeed, the current study's goal lies in investigating the dehydrogenation of the 2LiBH 4 -Mg 2 FeH 6 assemblage mechanism in detail, following the applied reaction conditions such as dehydrogenation under vacuum, reaction with temperature-programmed dehydrogenation (dynamic decomposition), and isothermal dehydrogenation using in situ X-ray diffraction data.…”

“…In a study by Li et Al. on a composite of Mg 2 FeH 6 and LiBH 4 in different compositions (Mg 2 FeH 6 /LiBH 4 molar ratios (X) of 0.25, 0.5, and 0.75), the authors reported X = 0.5 as the correct stoichiometric ratio for Mg 2 FeH 6 and LiBH 4 joint decomposition [18].…”