Abstract
In previous studies, complex hydrides LiBH4 and Mg2FeH6 have been reported to undergo simultaneous dehydrogenation when ball-milled as composite materials (1 − x)LiBH4 + xMg2FeH6. The simultaneous hydrogen release led to a decrease of the dehydrogenation temperature by as much as 150 K when compared to that of LiBH4. It also led to the modified dehydrogenation properties of Mg2FeH6. The simultaneous dehydrogenation behavior between stoichiometric ratios of LiBH4 and Mg2FeH6 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 LiBH4 and Mg2FeH6 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 H2, respectively, for the full rehydrogenation of x = 0.5 composite, were much lower than those required for the partial rehydrogenation of LiBH4. Moreover, the x = 0.5 composite could be reversibly hydrogenated for more than four cycles without degradation of its H2 capacity.
Highlights
Boron-based complex hydrides MBH4 (M = Li, Na, and K), which consist of an M+ cation and a [BH4 ]− complex anion, have high gravimetric H2 densities (7.7–18.4 mass %); these materials have the potential to be used as hydrogen storage materials [1,2,3]
We considered that the stoichiometric reaction was hidden in the continuous hydrogen releasing events in the dynamic thermogravimetry–mass spectrometry (TG–MS) measurements, where the thermodynamic and kinetic factors both affected the dehydrogenation process
Both LiBH4 and Mg2 FeH6 were thermodynamically destabilized by this reaction. x = 0.5 is the optimal ratio for the composite material, which is the reacting ratio of the two complex hydrides in the stoichiometric reaction
Summary
Boron-based complex hydrides MBH4 (M = Li, Na, and K), which consist of an M+ cation and a [BH4 ]− complex anion, have high gravimetric H2 densities (7.7–18.4 mass %); these materials have the potential to be used as hydrogen storage materials [1,2,3]. MBH4 are thermodynamically stable and the hydrogenation is difficult to achieve under mild temperatures and pressures. When undergoing the following Reaction (1), the dehydrogenation temperature of LiBH4 at 0.1 MPa H2 has been estimated to be 683 K on the basis of enthalpy ∆H and entropy ∆S changes of 66.6 kJ/(mole of H2 ) and 97.4 J/K(mole of H2 ), respectively [4]. Significant dehydrogenation only occurs at temperatures greater than 700 K. Partial rehydrogenation of LiBH4 requires a much higher pressure and temperature of 35 MPa and 873 K.
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