Full to the brim with hydrogen: Porous form of magnesium borohydride can store hydrogen

Hydrogen could be one of the most important fuels in a new energy economy based on renewable resources. However, no ideal hydrogen storage material has yet been found. A team led by Yaroslav Filinchuk at the Université Catholique de Louvain, Belgium, and Torben R. Jensen at the University of Aarhus in Denmark has now introduced 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 perfect hydrogen storage material must store hydrogen efficiently and securely in a small volume, and should release it on demand. It must be rapidly refillable under mild conditions, while being as light and inexpensive as possible. One approach to this is solid-state storage. In such systems, hydrogen can be chemically bound, as in borohydride compounds, or it can be adsorbed as a molecule into a nanoporous material, as in some metal–organic frameworks.

The researchers have now found a material that can do both. It is a new, highly porous form of magnesium borohydride—the first light-metal hydride that is porous like a metal–organic framework and is capable of storing molecular hydrogen.

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, ? and ß, were previously known. The researchers have now made a third form, designated the ? form. Its pore volume comprises 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.

Interestingly, under high pressure this material converts into a nested, non-porous framework with a density that is nearly 80 % higher. This makes the ? 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.

“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,” explains Filinchuk. “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.”

More information: Yaroslav Filinchuk, Porous and Dense Magnesium Borohydride Frameworks: Synthesis, Stability, and Reversible Absorption of Guest Species,

High-energy density magnesium batteries for smart electrical grids

Magnesium-based batteries are, in theory, a very attractive alternative to other batteries.

Magnesium (Mg) is cheap, safe, lightweight, and its compounds are usually non-toxic. Mg is less expensive (metallic lithium [Li] costs about 24 times more than metallic Mg) because Mg is abundant in the Earth’s crust. Mg is safer because it is stable when exposed to the atmosphere. Mg provides a theoretical specific capacity of 2,205 ampere-hours/kilogram, making it an attractive high-energy density battery system.

Furthermore, it provides two electrons per atom and has electrochemical characteristics similar to Li (12 grams-per-Faraday [g/F], compared to 7 g/F for Li or 23 g/F for sodium).

Proper design and architecture should lead to Mg-based batteries with energy densities of 400-1,100 watt-hour per kilogram for an open circuit voltage in the range of 0.8 – 2.1 V, which would make it an attractive candidate for electrical grid energy storage and stationary back-up energy.

To make Mg-based batteries practical, researchers at DOE’s National Energy Technology Laboratory are developing novel alloys of Mg doped with different elements such as calcium, zinc, and yttrium. These alloys are being produced by melting and casting as well as powder metallurgy.

A new displacement reaction hypothesis, based on the reaction of nanostructured transition metal compounds with Mg, has resulted in a thermodynamically favorable reversible displacement reaction of transition metals and Mg-alloys.

Recent accomplishments include a new, intermetallic anode compound formulated by melting/casting and synthesis of a new MgMn1-xFexSiO4/C composite, and other transition metal oxide spinel cathode systems. Mg-based electrolytes and other ionic electrolytes have also been developed and are being tested.

Provided by National Energy Technology Laboratory