Stacked layers of oxidized graphene could be used to store hydrogen fuel for cars and other applications. So say researchers in the US who have made graphene-oxide frameworks (GOFs) that can hold roughly 1% of their weight in hydrogen. This value is 100 times better than graphene oxide itself and compares well with MOF-5 (the most studied metal-organic framework to date for hydrogen storage), which absorbs about 1.3 wt%.

Vehicles and other systems powered by hydrogen have the advantage of emitting only water as a waste product. An important challenge, however, is storing enough hydrogen on board a car to give it a range comparable to a vehicle powered by fossil-fuels. If hydrogen is stored as a compressed gas, it takes up far too much space – and liquefying hydrogen is expensive in terms of both cost and energy.

One promising solution to this problem is to exploit the fact that many solid materials will absorb large amounts of hydrogen. Graphene oxide is a sheet of carbon and oxygen just one atom thick, and hydrogen can be stored between the layers in stacks of this lightweight material. The challenge is to get the spacing between layers just right to reach maximum storage capacity.

Connector molecules

Now, Taner Yildirim and colleagues at NIST and the University of Pennsylvania have boosted the storage capacity of graphene oxide by using organic “connector molecules” to separate individual layers by 1.1 nm. This is three times more than the inter-plane distance in bare graphite – which comprises stacked layers of graphene.

"Being able to control this width is important for a number of applications, including hydrogen storage," explains team member Jacob Burress. He says that the interlayer spacing can be controlled to optimize hydrogen adsorption. The idea is to have pores that are small enough to maximize the interaction between hydrogen and the surface of the frameworks, but at the same time large enough to hold two layers of adsorbed hydrogen.

The team took its inspiration from work already done on metal-organic frameworks (MOFs), widely studied materials for hydrogen storage. Here, inorganic nodes are connected by organic struts using well established chemistry techniques. In the new work, the metal oxides are replaced with graphene oxide and the struts with diboronic acid “pillars”.

Future optimization

The GOFs can store roughly 1 wt% of hydrogen at 77 K and 1 bar. "This is less than one fifth that the 'ideal' GOF structure can hold, according to state-of-the-art computer simulations," says team member Wei Zhou. "Based on our adsorption simulations, the ideal GOF structure can adsorb hydrogen up to 6 wt% at 77 K and atmospheric pressure, suggesting that our GOF materials could be significantly optimized in the future."

As important as its hydrogen-storage properties are, the fact that graphene-oxide production can easily be scaled-up to industrial quantities is a big advantage too. What's more, it is inexpensive and thought to be safe for people and the environment.

The team also discovered that the hydrogen-adsorption kinetics of GOFs are different compared with other materials. At lower temperatures, there is little adsorption and hardly any hydrogen gas is released either. This means that the material can be loaded with gas at higher temperatures and then cooled below this blocking temperature to hold the hydrogen in place. Gas will not be released until the sample is allowed to warm up. Ideally, this blocking temperature needs to be as close to room temperature as possible for practical applications.

Drug delievery

"We expect to see more work on graphene oxide where it is linked by many different connectors for a variety of chemistry and physics applications," Burress tells physicsworld.com. "We anticipate these materials to be very useful not only for hydrogen storage but for other gases such as ammonia and carbon dioxide as well." He also hinted at medical applications: "Once the graphene-oxide layers are separated by sufficiently large distances, one could also imagine adding some biomolecules for drug delivery".

The researchers now hope to look into possible electronic applications for the GOFs because they may be useful as conducting materials for fuel cells or batteries. Another possibility is to use the GOFs as sensors, where gas adsorption leads to a measurable change in the material's electronic properties.

The next immediate step is to optimize hydrogen-storage capacity, says the team. This could be achieved in a number of ways: including removing unreacted hydroxyl groups to increase the useable surface area; and optimizing the linkers in terms of concentrations and chemistry.

"We also want to understand the nature of hydrogen-adsorption kinetics and how we can use it to our advantage!" says Burress. "This is just the beginning of new research and there are many new experimental avenues to follow."

The research was presented last week at the March Meeting of the American Physical Society.