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University develops optimised hydrogen storage

Hexagonal boron nitride, as known as 'white graphene', arranged in the pillar and sheet nanostructure, with hydrogen atoms in between (Lei Tao/Rice University)
Hexagonal boron nitride, as known as 'white graphene', arranged in the pillar and sheet nanostructure, with hydrogen atoms in between (Lei Tao/Rice University)

Researchers working at Rice University in Texas, US have published a paper outlining a structure of ‘white graphene’ nanomaterials that can hold an unprecedented amount of hydrogen. The gas is being looked at as a possible replacement for fossil fuels as it only releases water when burnt.

A nanostructure resembling a skyscraper with ‘floors’ of Hexagonal boron nitride held apart by pillars of the same material has been identified as an optimal container for hydrogen. These ‘floors’ are held 5.2 angstroms apart, one angstrom being equal to one ten billionth of a metre.

Hexagonal boron nitride consists of atom-thick sheets of boron and nitrogen, and is sometimes called ‘white graphene’ because the atoms are spaced exactly like carbon atoms in flat sheets of graphene. Boron nitride holds hydrogen through physical bonds instead of stronger chemical bonds, making it easier to get the gas out of the material.

A major advantage of hydrogen as a fuel is its energy-to-mass ratio, which is far higher than that of fossil fuels, say the researchers. But its potential has been held back by storage difficulties and safety concerns (hydrogen is extremely flammable). While large volumes of the gas can be stored in big high pressure tanks, smaller quantities that you would use to power a car have proven difficult to contain effectively.

“The motivation is to create an efficient material that can take up and hold a lot of hydrogen — both by volume and weight — and that can quickly and easily release that hydrogen when it’s needed,” said the study’s lead author, Rouzbeh Shahsavari, assistant professor of civil and environmental engineering at Rice.

The structure was found after months of calculations by two of Rice’s fastest supercomputers. Shahsavari and graduate student Shuo Zhao set up dozens of ‘ab initio’ tests, computer simulations that use first principles of physics. According to Shahsavari the approach was computationally intense but offered the most precision.

“We conducted nearly 4,000 ab initio calculations to try and find that sweet spot where the material and geometry go hand in hand and really work together to optimize hydrogen storage.”

He added that naturally occurring wrinkles in the structure could provide advantages in terms of safety: “The] wrinkles can provide toughness. If the material is placed under load or impact, that buckled shape can unbuckle easily without breaking.”

The article by Shahsavari and Zhao, ‘Merger of Energetic Affinity and Optimal Geometry Provides New Class of Boron Nitride Based Sorbents with Unprecedented Hydrogen Storage Capacity’, was published in the journal Small 8 March.

See the Rice University press release here.

Hexagonal boron nitride, as known as 'white graphene', arranged in the pillar and sheet nanostructure, with hydrogen atoms in between (Lei Tao/Rice University)