New porous form of magnesium borohydride shows 2nd highest density among all known hydrides for hydrogen storage
|A highly porous form of Mg(BH4)2 (Mg green, BH4 blue, unit cells shown in red) reversibly absorbs H2, N2, and CH2Cl2. Source: Filinchuk et al. Click to enlarge.|
A team led by Yaroslav Filinchuk at the Université Catholique de Louvain, Belgium, and Torben R. Jensen at the University of Aarhus in Denmark report on a new highly porous form of magnesium borohydride in the journal Angewandte Chemie. This material can store hydrogen in two ways: chemically bound and physically adsorbed.
The first reported light-metal hydride that is porous like a metal–organic framework and is capable of storing molecular hydrogen, this new form of magnesium borohydride has a large permanent porosity and can also reversibly adsorb nitrogen and small molecules such as dichloromethane.
Magnesium borohydride (Mg(BH4)2) is one of the most promising materials for chemical hydrogen storage because it releases hydrogen at relatively low temperatures and can hold a high proportion by weight (about 15%) of hydrogen. Two forms of this compound, α and β, were previously known. The researchers made a third form, designated the γ form.
Its pore volume represents about 33% of the structure, and its channels are wide enough to take up and store small gas molecules, such as nitrogen, dichloromethane, and most importantly hydrogen.
Under high pressure this material converts into a nested, non-porous framework with a density that is nearly 80% higher. This makes the δ form the second densest in hydrogen content and more than twice as dense as liquid hydrogen. Furthermore, this conversion results in a 44 % reduction in volume, which is the largest contraction yet observed for a hydride.
δ-Mg- (BH4)2 possesses no empty voids and has the second highest volumetric hydrogen density (147 g H2/L at ambient conditions) among all known hydrides; this value is slightly below Mg2FeH6 with the hydrogen density of 150 g H2/L. We note that the latter compound has a much lower gravimetric hydrogen density of 5.5%, compared to 14.9 wt% in Mg-(BH4)2. The second highest volumetric hydrogen density in borohydrides, 127 g H2/L, is recorded for the toxic Be(BH4)2, which has an extreme gravimetric hydrogen density of 20.7 wt %.—Filinchuk et al.
A combination of the chemical (through covalent bonding) and physical (through adsorption in the pores) storage of hydrogen seems to be difficult in practical applications. However, this research has a broader impact, as it reveals a new class of hydride-based porous solids for storage and separation of various gases.—Yaroslav Filinchuk
Yaroslav Filinchuk, Bo Richter, Torben R. Jensen, Vladimir Dmitriev, Dmitry Chernyshov, and Hans Hageman (2011) Porous and Dense Magnesium Borohydride Frameworks: Synthesis, Stability, and Reversible Absorption of Guest Species. DOI: 10.1002/anie.201100675