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Thousands of hours of calculations on Rice University's two fastest supercomputers found that the optimal architecture for packing hydrogen into "white graphene" involves making skyscraper-like frameworks of vertical columns and one-dimensional floors that are about 5.2 angstroms apart. In this illustration, hydrogen molecules (white) sit between sheet-like floors of graphene (gray) that are supported by boron-nitride pillars (pink and blue). Researchers found that identical structures made wholly of boron-nitride had unprecedented capacity for storing readily available hydrogen. Credit: Lei Tao/Rice University

A team from Rice University has found the optimal architecture for storing hydrogen in “white graphene” nanomaterials.

Hexagonal boron nitride (hBN) or white graphene, consists of “floors” of boron nitride sitting atop one another, held precisely 5.2 angstroms apart by boron nitride pillars, exactly like how carbon atoms in flat sheets of graphene are spaced.

“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,” study's lead author, Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering at Rice, said in a statement.

The researchers used a pair of supercomputers to find the optimal architecture for storing hydrogen in boron nitride. The models showed that pure hBN tube-sheet structures could hold 8 weight percent—a measure of concentration similar to parts per million—of hydrogen.

The researchers previously found that hybrid materials of graphene and boron nitride could hold enough hydrogen to meet U.S. Department of Energy storage targets for light-duty fuel cell vehicles.

“The choice of material is important," Shahsavari said. “Boron nitride has been shown to be better in terms of hydrogen absorption than pure graphene, carbon nanotubes or hybrids of graphene and boron nitride.

“But the spacing and arrangement of hBN sheets and pillars is also critical. So we decided to perform an exhaustive search of all the possible geometries of hBN to see which worked best. We also expanded the calculations to include various temperatures, pressures and dopants, trace elements that can be added to the boron nitride to enhance its hydrogen storage capacity.”

The team used several ab initio tests—computer simulations that used the first principals of physics to make the discovery.

“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,” Shahsavari said.

Boron nitride is a sorbent that holds hydrogen through physical bonds that are weaker than chemical bonds, allowing researchers to get the hydrogen out of storage because sorbent materials tend to discharge more easily than their chemical cousins do. The spacing in the boron nitride was also key in maximizing capacity.

“Without pillars, the sheets sit naturally one atop the other about three angstroms apart, and very few hydrogen atoms can penetrate that space,” Shahsavari said. “When the distance grew to six angstroms or more, the capacity also fell off. At 5.2 angstroms, there is a cooperative attraction from both the ceiling and floor, and the hydrogen tends to clump in the middle. Conversely, models made of purely BN tubes--not sheets--had less storage capacity.”     

Hydrogen is one of the cleanest ways to generate electricity, eliminating the need for fossil fuels.  However, the primary drawbacks of hydrogen are portability, storage and safety.

While large volumes can be stored under high pressure in underground salt domes and specially designed tanks, engineers have been unable to develop small-scale portable tanks.

A 2017 report by market analysts at BCC Research found that global demand for hydrogen storage materials and technologies will likely reach $5.4 billion annually by 2021.

The study was published in Small.

 

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