Layers of graphene separated by nanotube pillars of boron nitride may be a suitable material to store hydrogen fuel in cars, according to a new computational study from Rice University published in the journal Langmuir. The material held 14.77 percent of its weight in hydrogen at -321°F, exceeding both current and future benchmarks established by the Department of Energy as part of its efforts to make hydrogen a practical fuel for light-duty vehicles.
DOE’s current target for storage media is the ability to store more than 5.5 percent of its weight and 40 grams per liter in hydrogen under moderate conditions, while the ultimate goals are 7.5 weight percent and 70 grams per liter.
Previous to this study, Rice materials scientists Rouzbeh Shahsavari and Farzaneh Shayeganfar had already determined through computer models how tough and resilient pillared graphene structures would be, and later worked boron nitride nanotubes into the mix to model a unique three-dimensional architecture.
Just as pillars in a building make space between floors for people, pillars in boron nitride graphene make space for hydrogen atoms. The challenge, according to the Rice team, is to make the hydrogen atoms enter and stay in sufficient numbers and exit upon demand.
In its latest molecular dynamics simulations, the research team found that either pillared graphene or pillared boron nitride graphene would offer abundant surface area (about 2,547 square meters per gram) with good recyclable properties under ambient conditions. The models showed adding oxygen or lithium to the materials would make them even better at binding hydrogen.
Shahsavari and Shayeganfar focused simulations on four variants: pillared structures of boron nitride or pillared boron nitride graphene doped with either oxygen or lithium. At room temperature and in ambient pressure, oxygen-doped boron nitride graphene proved the best, according to the researchers, holding 11.6 percent of its weight in hydrogen (its gravimetric capacity) and about 60 grams per liter (its volumetric capacity). The results surpass competing technologies, including porous boron nitride, metal oxide frameworks and carbon nanotubes.
Weak van der Waals forces, Shahsavari says, allowed the undoped pillared boron nitride graphene to absorb the hydrogen atoms. When the material was doped with oxygen, the atoms bonded strongly with the hybrid and created a better surface for incoming hydrogen, a condition which would likely be delivered under pressure and would exit when pressure is released, says Shahsavari. “Adding oxygen to the substrate gives us good bonding because of the nature of the charges and their interactions,” he says. “Oxygen and hydrogen are known to have good chemical affinity.”
He says the polarized nature of the boron nitride where it bonds with the graphene and the electron mobility of the graphene itself make the material highly tunable for applications. “What we’re looking for is the sweet spot,” Shahsavari says, describing the ideal conditions as a balance between the material’s surface area and weight, as well as the operating temperatures and pressures. “This is only practical through computational modeling, because we can test a lot of variations very quickly. It would take experimentalists months to do what takes us only days.”