Graphene now appears to be a promising base material for capturing hydrogen, according to researchers at the National Institute of Standards and Technology (NIST) and the University of Pennsylvania.
Their research suggests stacks of graphene layers could potentially store hydrogen safely for use in fuel cells and other applications.
Graphene does not store hydrogen well in its original form, according to a team of scientists studying it at the NIST Center for Neutron Research. But if oxidised graphene sheets are stacked atop one another, connected by molecules that both link the layers to one another and maintain space between them, the resulting graphene-oxide framework (GOF) can accumulate hydrogen in greater quantities.
Inspired to create GOFs by the metal-organic frameworks that are also under scrutiny for hydrogen storage, the team is just beginning to uncover the new structures’ properties.
NIST theorist Taner Yildirim said: ’No one else has ever made GOFs, to the best of our knowledge. What we have found so far, though, indicates GOFs can hold at least a hundred times more hydrogen molecules than ordinary graphene oxide does. The easy synthesis, low cost and non-toxicity of graphene make this material a promising candidate for gas storage applications.’
The GOFs can retain one per cent of their weight in hydrogen at a temperature of 77 degrees Kelvin and ordinary atmospheric pressure — roughly comparable to the 1.2 per cent that some well-studied metal-organic frameworks can hold, Yildirim said.
Another of the team’s potentially useful discoveries is the unusual relationship that GOFs exhibit between temperature and hydrogen absorption. In most storage materials, the lower the temperature, the more hydrogen uptake normally occurs. However, the team discovered that GOFs behave quite differently.
Although a GOF can absorb hydrogen, it does not take in significant amounts at below 50 Kelvin (-223°C). Moreover, it does not release any hydrogen below this blocking temperature, suggesting that, with further research, GOFs might be used both to store hydrogen and to release it when it is needed — a fundamental requirement in fuel-cell applications.
Some of the GOFs’ capabilities are due to the linking molecules themselves. The molecules the team used are all benzene-boronic acids that interact strongly with hydrogen in their own right. But by keeping several angstroms of space between the graphene layers they also increase the available surface area of each layer, giving it more spots for the hydrogen to latch on.