Change in the wt percent of hydrogen in few-layer graphene sample created via exfoliation (EG) and arc evaporation of graphite under hydrogen (HGH). (Inset) The evolution of hydrogen as recorded by gas chromatograph. Credit: Subrahmanyam et al.
A group of researchers from the Jawaharlal Nehru Center for Advanced Scientific Research in Bangalore, India say they have come across a new approach for using graphene for hydrogen storage. They say in a paper published in the Proceedings of the National Academy of Sciences they have been able to create samples containing up to 5 wt percent hydrogen, which they say can be completely released through heating or by irradiating with a laser or UV light source. For comparison purposes, the maximum amount of hydrogen that can be contained in graphene is 7.7 wt percent.
This isn’t the first time researchers have looked at graphene. Much of this work has been done in the context of trying to find some sort of suitable solid body for hydrogen storage. Previously, some investigators began thinking about carbon nanotubes. Some storage effects were achieved, but overall the results have been disappointing.
Other research also has been done at Columbia University using single-layer graphene showing that hydrogenation can occur and be reversed through a photothermal heating process, but apparently the amount of hydrogen that is stored in the single layer was not measured (the work was focused on methods to manipulate the charge transport properties of the graphene).
The JNCASR group, led by C.N.R. Rao, looked at additional research that suggested that hydrogen loading might be better accomplished through the use of multiple layers of graphene, and decided to do some detailed studies in this area.
In brief, the group used two methods to form few-layer graphene samples: exfoliation of graphite oxide (forming 6–7 layers) and arc evaporation of graphite under hydrogen (forming 2–3 layers). The researchers hydrogenated both samples (using Birch reduction), and both samples displayed a hydrogen content of approximately 5 wt percent.
They found that the hydrogen-containing graphene is stable at room temperature “and can be stored over long periods.”
When the samples are heated, the hydrogen begins to be released around 200°C and is totally released at 500°C. As mentioned above, they also used laser and UV irradiation to break the C–H bonds and free the hydrogen.
The group feels this storage system may have potential applications, and that a better storage system may be achievable. The authors note, “Although Birch reduction enabled us to incorporate 5 wt percent of hydrogen in few-layer graphenes, it may be possible to carry out hydrogenation more effectively by other methods.” They also report they have achieved 3 wt percent storage using graphene nanoribbons, which also fully releases its hydrogen at 500°C.