Scientists have found a way to store the volatile gas in thin sheets of carbon
Graphene, which is actually extremely thin sheets of car bon, can emerge as a potential material for storing hydrogen energy in fuel cells, a team of researchers from Bangalore has found. The scientists, led by Prof. Chintamani Nagesa Ramachandra Rao of the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) in Bangalore, have found that if two or more graphene sheets -which are one atom thick -are stacked together, they can hold hydrogen up to 5 per cent their weight, taking it closer to the realistic target of 6 weight per cent set by the US Department of Energy. Carbon nanotubes, another promising material, was found to store only 3 weight per cent.
Hydrogen, the cleanest of all fuels, is being touted as the answer to the impending energy crisis because of our fast-depleting fossil fuel reserves. Besides, it can offer a much-needed solution to the overarching problem of climate change by reducing the build-up of heat-trapping gases such as carbon dioxide in the atmosphere. What’s more, it is available in plenty and produces only waterty and produces only water as waste.
However, several stumbling blocks pre vent its safe and eco nomical usage. These include safe storage and retrieval in ade quate quantities. It is said that with the current technoloies, a car would require a fuel tank the size of a double-decker bus for a 500-km trip! The JNCASR work, which appeared recently in the Proceedings of the National Academy of Sciences, seeks to address these problems to a certain extent.
The scientists found that graphene, which won two British chemists the Nobel Prize last year, is capable of storing high quantities of hydrogen safely. “Besides, we have found that it is possible to retrieve the gas completely,” says Prof. Rao. According to co-researcher Umesh Waghmare, certain issues are involved in the storage of hydrogen. Since the element is volatile and can explode, its storage should be safe and stable, and such that it is easily retrievable. Scientists the world over are trying to do this through two methods.
One is by chemically binding hydrogen to a material. This method is safe and stable, but a lot of energy has to be expended to retrieve hydrogen for use, making it an expensive proposition. The other one -the process of physically storing hydrogen in tiny cages of a material -makes it easier to release it, but it is inherently dangerous as the gas may leak. “Our method falls somewhere in between,” says Waghmare. “It is more like our lungs. When hydrogen gets in, the graphene layers expand and when it is taken out, they contract.”
Bangalore scientists also demonstrated that a single sheet of graphene can store hydrogen. In fact, hydrogen storage in a monolayer is up to 8 weight per cent, probably one of the highest levels of hydrogen storage so far de monstrated. “But the problem with the monolayer is that the gas binds to both sides of the sheet, requiring higher energy levels for its retrieval,” says Waghmare. With two or more layers of graphene, retrieval is much easier, even though they store less gas.
Rajeev Ahuja, a physicist at the University of Uppasala in Sweden, who has worked on magnesium hydride as a hydrogen storage material, says the work is significant, the more so because graphene is emerging as a material for future applications in electronics. “It is good basic science, but not from the point of view of application. If you put it in a system, the weight percentage will go down further,” says Ahuja, who has shown that magnesium hydride can store up to 7.7 weight per cent hydrogen. Waghmare admits the storage capacity will go down in real application. But this is only a proof of concept study, he says, and that further research may help improve the storage capacity .
The other problem that has to be addressed is the high temperature required by graphene to release hydrogen. At present, the complete retrieval of hydrogen takes place at 500 degrees Celsius.But the scientists have also shown that ultraviolet radiation or lasers can be used to initiate the release. According to Prof. Rao, non-metallic materials like graphene are ideal for fuel cell applications. “Metallic alloys for hydrogen storage are passé as they become brittle and inefficient on repeated use,” he says.
