In response to the US Department of Energy, which has set benchmarks for storage materials that would make hydrogen a practical fuel for light-duty vehicles, the materials scientist Rouzbeh Shahsavari has used a new computational study that pillared boron nitride and graphene to show that they could be a suitable candidate.
Shahsavari's lab has already shown, using computer models, how resilient pillared graphene structures would be and has now worked boron nitride nanotubes into the mix to model a unique three-dimensional architecture.
The team have created samples of boron nitride nanotubes that seamlessly bonded to grapheme; the pillars in the boron nitride graphene then provide space for hydrogen atoms to be stored. The challenge is to make those atoms enter and stay in sufficient numbers and exit upon demand.
In their latest molecular dynamics simulations, the researchers 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. Their models showed that by adding oxygen or lithium to the materials it was possible to make them even better at binding hydrogen.
Their research has focused 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, holding 11.6 percent of its weight in hydrogen (its gravimetric capacity) and about 60 grams per litre (its volumetric capacity).
According to the researchers it easily beat competing technologies like porous boron nitride, metal oxide frameworks and carbon nanotubes.
The Department of Energy's current target for economic storage media is the ability to store more than 5.5 percent of its weight and 40 grams per litre in hydrogen under moderate conditions. The ultimate targets are 7.5 weight percent and 70 grams per litre.
Shahsavari said hydrogen atoms adsorbed to the undoped pillared boron nitride graphene, thanks to weak van der Waals forces. When the material was doped with oxygen, the atoms bonded strongly with the hybrid and created a better surface for incoming hydrogen, which Shahsavari said would likely be delivered under pressure and would exit when pressure is released.
"Adding oxygen to the substrate gives us good bonding because of the nature of the charges and their interactions," he said. "Oxygen and hydrogen are known to have good chemical affinity."
He said the polarised 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.
The study by Shahsavari and Farzaneh Shayeganfar appears in the American Chemical Society journal Langmuir
