Calculations for O-doped PGBN yielded gravimetric hydrogen uptake capacities greater than 11.6 wt% at room temperature. The researchers attributed this increased value to the pillared morphology, which improves mechanical properties and increase porosity, as well as the high binding energy between oxygen and GBN. At -196 ˚C, the material held 14.77% of its weight in hydrogen.

Left: Hydrogen (green) molecule distribution for O-doped pillared graphene/boron nitride (PGBN). Right: Gravimetric hydrogen uptake of O-doped PGBN. Credit: ACS, Shayeganfar and Shahsavari (2016). Click to enlarge.

The Department of Energy’s current target for economic storage media is the ability to store more than 5.5% of its weight and 40 grams per liter in hydrogen under moderate conditions. The ultimate targets are 7.5% percent and 70 grams per liter.

This work demonstrated the novel hybrid carbon/BNNT nanostructure as an excellent candidate for hydrogen storage, owing to the combination of the electron mobility of graphene and the polarized nature of BN at heterojunction, which enhance hydrogen uptake capacity, providing ample opportunities to further tune this hybrid material.

—Shayeganfar and Shahsavari (2016)

Shahsavari’s lab 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. (Samples of boron nitride nanotubes seamlessly bonded to graphene have been made.)

Just as pillars in a building make space between floors for people, pillars in boron nitride graphene make space for hydrogen atoms. The challenge is to make them enter and stay in sufficient numbers and exit upon demand.

In their 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 adding oxygen or lithium to the materials would make them even better at binding hydrogen.

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. Oxygen and hydrogen are known to have good chemical affinity.

—Rouzbeh Shahsavari

Shahsavari said 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.

He said the structures should be robust enough to easily surpass the Department of Energy requirement that a hydrogen fuel tank be able to withstand 1,500 charge-discharge cycles.

Overlay map showing gravimetric uptake of hydrogen storage devices calculated using two different computational tools: Molecular Dynamic and Monte-Carlo simulation. Bracketed numbers are literature references cited in the paper. Credit: ACS, Shayeganfar and Shahsavari (2016). Click to enlarge.

Shayeganfar, a former visiting scholar at Rice, is an instructor at Shahid Rajaee Teacher Training University in Tehran, Iran.

The researchers used the BlueBioU supercomputer and the National Science Foundation-supported DAVinCI supercomputer, which are both administered by Rice’s Center for Research Computing and were procured in partnership with Rice’s Ken Kennedy Institute for Information Technology.

Resources

• Farzaneh Shayeganfar and Rouzbeh Shahsavari (2016) “Oxygen and Lithium Doped Hybrid Boron-Nitride/Carbon Networks for Hydrogen Storage” Langmuir doi: 10.1021/acs.langmuir.6b02997