Pressure-Collapsed Amorphous Mg(BH₄)₂: An Ultradense Complex Hydride Showing a Reversible Transition to the Porous Framework

Abstract

Hydrogen-storage properties of complex hydrides depend of their form, such as a polymorphic form or an eutectic mixture. This Paper reports on an easy and reproducible way to synthesize a new stable form of magnesium borohydride by pressureinduced collapse of the porous γ-Mg(BH 4 ) 2 . This amorphous complex hydride was investigated by temperature-programmed synchrotron X-ray diffraction (SXRD), transmission electron microscopy (TEM), thermogravimetric analysis, differential scanning calorimetry analysis, and Raman spectroscopy, and the dynamics of the BH 4 − reorientation was studied by spin−lattice relaxation NMR spectroscopy. No long-range order is observed in the lattice region by Raman spectroscopy, while the internal vibration modes of the BH 4 − groups are the same as in the crystalline state. A hump at 4.9 Å in the SXRD pattern suggests the presence of nearly linear Mg−BH 4 −Mg fragments constituting all the known crystalline polymorphs of Mg(BH 4 ) 2 , which are essentially frameworks built of tetrahedral Mg nodes and linear BH 4 linkers. TEM shows that the pressure-collapsed phase is amorphous down to the nanoscale, but surprisingly, SXRD reveals a transition at ∼90°C from the dense amorphous state (density of 0.98 g/cm 3 ) back to the porous γ phase having only 0.55 g/cm 3 crystal density. The crystallization is slightly exothermic, with the enthalpy of −4.3 kJ/mol. The volumetric hydrogen density of the amorphous form is 145 g/L, one of the highest among hydrides. Remarkably, this form of Mg(BH 4 ) 2 has different reactivity compared to the crystalline forms. The parameters of the reorientational motion of BH 4 groups in the amorphous Mg(BH 4 ) 2 found from NMR measurements differ significantly from those in the known crystalline forms. The behavior of the nuclear spin−lattice relaxation rates can be described in terms of a Gaussian distribution of the activation energies centered on 234 ± 9 meV with the dispersion of 100 ± 10 meV. ■ INTRODUCTIONHigh gravimetric and volumetric hydrogen densities are the main requirements for potential hydrogen-storage materials. Thus, complex hydrides and especially light metal borohydrides are extensively studied for this purpose. 1,2 Magnesium borohydride and its multiple polymorphs are very promising candidates, given its hydrogen desorption reversibility 3 and formation of reversible reactive hydride composites. 4 Magnesium borohydride exists in a form of various polymorphs, 5−10 having framework structures and very different densities. In particular, the recently discovered porous form of Mg(BH 4 ) 2 having 14.9 wt % covalently bound hydrogen can store an additional 3 wt % of physisorbed molecular H 2 at low temperatures. 5 The form of the solid hydride can be determinative for the decomposition reaction pathway and thus for the hydrogen desorption reversibility. For instance, many borohydrides yield upon heating stable [B 12 H 12 ] 2− species that decrease their hydrogen release potential and prevent their reversibility; see a review 11 . This mechanism is one o...

Melting Behavior and Thermolysis of NaBH4−Mg(BH4)2 and NaBH4−Ca(BH4)2 Composites

“…Compound 1 does not show similarities to any of the recently discovered Mg(BH4)2 polymorphs [8,9,12,38]. The compound appears to crystallize from the molten phase, possibly due to excess sodium borohydride.…”

“…Normalized diffracted intensities of selected Bragg peaks of the compounds are extracted as a function of temperature; see Figure 5. The diffraction pattern measured at RT has a broad hump in the background in the range 9 < 2θ < 13°, originating from amorphous Mg(BH4)2 [11,38]. The porous structure of γ-Mg(BH4)2 may have collapsed during the ball-milling, as the characteristic Bragg peaks from γ-Mg(BH4)2 are not observed at RT in the in situ SR-PXD experiment.…”

Combined X-ray and Raman Studies on the Effect of Cobalt Additives on the Decomposition of Magnesium Borohydride

“…The sample was ball-milled before the thermal treatment, following the same procedure adopted for the samples with the additives. The RT pattern of γ-Mg(BH4)2 ( Figure 1) has a high background at small 2θ, which can be assigned to amorphous Mg(BH4)2 formed upon material storage [44] and milling [25]. In the 150-200 °C range, pure γ-Mg(BH4)2 underwent two phase transitions.…”

Isotopic Exchange in Porous and Dense Magnesium Borohydride

“…The pores have a diameter of about 7 Å and are accessible to guest molecules. Other polymorphs of Mg(BH 4 ) 2 , α‐, β‐, β′‐, ε‐, δ‐, ζ‐Mg(BH 4 ) 2 ,5a, 7, 8 and amorphous,8c do not possess this unique high SSA. At about 423–473 K both α‐, and γ‐Mg(BH 4 ) 2 irreversibly transform through several high‐temperature polymorphs to β‐Mg(BH 4 ) 2 , which decomposes above 473 K 5a,b…”