Porous boron: Heralding hydrogen economy

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  • Published: Oct 1, 2011
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Porous boron: Heralding hydrogen economy

Physical and chemical storage

Researchers in Europe have developed a hydrogen storage material based on porous magnesium borohydride that can safely adsorb large quantities of the gas via both a physical and a chemical mechanism. They used X-ray diffraction, infra-red and Raman spectroscopy to investigate this material.

Observers suggest that hydrogen gas, generated from sustainable resources could provide us with a clean and efficient alternative to the burning of fossil fuels and circumvent the issues associated with renewable energy sources, such as wind, wave and solar that provide intermittent maximal output. The so-called hydrogen economy, as opposed to our current carbon-based economy would, the observers suggest take us from a world in which carbon-containing pollutants, including carbon dioxide, enter the atmosphere at vast annual tonnages and contribute to global warming. Hydrogen is carbon free and if established as a fuel would be either carbon neutral or potentially even carbon negative.

However, there is no simple way to safely store the large quantities of hydrogen gas that would be required of a hydrogen economy. Chemists and materials scientists have been searching for lightweight and stable storage materials that can trap enormous volumes of hydrogen without the explosion risk associated with simply compressing it in "gas" cylinders; besides which the volumetric storage capacity of compressed and liquid hydrogen is rather low, says Yaroslav Filinchuk at the Université Catholique de Louvain, Belgium.

Key to development of a successful hydrogen storage material is therefore the development of a substance that can store hydrogen efficiently and securely with the maximum ratio of hydrogen mass to material volume as possible. Moreover, it must also be able to absorb and release its payload on command and without the need for any extreme reaction conditions.

Double-edged hydrogen storage

Researchers in Europe have developed a hydrogen storage material based on porous magnesium borohydride that can safely adsorb large quantities of the gas. They used infrared spectroscopy and other techniques to investigate this material. The team, led by Filinchuk and Torben Jensen at the University of Aarhus, in Denmark, developed highly porous magnesium borohydride so that it can store large amounts of hydrogen gas via two distinct mechanisms - chemically bonded hydrogen and physically adsorbed.

Borohydrides have been investigated previously as hydrogen storage materials and have various advantages and disadvantages as do porous materials in which hydrogen is adsorbed physically but is not incorporated chemically into the storage medium. The researchers state that their material is the first light-metal hydride that is porous like a metal-organic framework (MOF) and is capable of storing molecular hydrogen. The researchers suggest that their porous magnesium borohydride is rather promising in that it can release hydrogen gas at relatively low temperatures but holds on to the gas at a high proportion of the material's mass, about 3 percent (18% of hydrogen, in total).

The new porous material represents a new, "gamma" form of magnesium borohydride, the alpha and beta forms being previously known to science. The team explains that the gamma form has a pore volume comprising a third of its structure with channels wide enough to allow entry and exit of hydrogen gas and other small gas molecules, such as nitrogen and dichloromethane. The team points out how detailed IR/Raman spectroscopy of the amorphous and crystalline alpha and beta phases show that the various phases are identical at the local level.

When high pressure is applied to the gamma form it is converted to a nested, non-porous framework that has a density 80 percent greater than the gamma form. This fourth, "delta" form is the second densest in hydrogen content and more than twice as dense as liquid hydrogen, moreover pressurising the gamma to the delta form reduces its volume by 44 percent, the biggest volume reduction for a hydride observed so far.

"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," Filinchuk explains. "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."

"The next step of this work is to design 'hybrid hydride materials' with improved gas storage properties and controlled stability," Filinchuk told SpectroscopyNOW.

 



The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 Researchers in Europe have developed a hydrogen storage material based on porous magnesium borohydride that can safely adsorb large quantities of the gas via both a physical and a chemical mechanism. They used infra-red spectroscopy and other techniques to investigate this material.

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