A one-atom-thick sheet of carbon, better known as graphene, can split hydrogen a hundred times more efficiently than the best catalysts thanks to strange nano-ridges.
Thanks to tiny ripples on its surface, graphene can split hydrogen a hundred times better than any known chemical catalyst. This ribbed graphene could be used to develop more effective hydrogen fuel cells and make industrial processes more efficient.
Graphene is a carbon layer one atom thick. It is essentially a slice of graphite. Due to the strong mutual carbon bonds, graphite is an extremely non-reactive substance.
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The Dutch-British physicist Andrew Gem from the University of Manchester and his colleagues have, however discovers that graphene, despite its strong bonds, is chemically incredibly reactive. This is because the material is never completely flat. It has small undulations called nanoripples or nanoripples. This allows graphene to split hydrogen as effectively as the best catalysts we have today.
Bulging
To demonstrate this, the researchers produced graphene with as few defects as possible. This is necessary to exclude that the chemical activity is caused by something other than the ridges. They then stretched a sheet of graphene over the top of a microscopic container filled with hydrogen molecules, each consisting of two hydrogen atoms.
The graphene split the hydrogen molecules into individual atoms. They piled up in the bowl. The loose atoms caused pressure to increase and the graphene to bulge out. The researchers measured the degree of bulge to calculate how well the graphene converts hydrogen molecules into hydrogen atoms.
They found that the ability per gram of graphene to split hydrogen was at least a hundred times better than commonly used catalysts such as copper or magnesia. However, if you compare the efficiency of catalysts in relation to their surface area, copper comes out slightly better than graphene.
Ribbing
Geim and his colleagues also compared a nearly perfectly flat sheet of graphene to a sheet on a nano-ribbed silicon surface. They saw that only the surface with the nanoripples seemed to split hydrogen.
According to Geim, the power of the ridges could extend beyond its ability to make graphene split hydrogen more efficiently. It could also mean something for other chemical reactions and for other flat materials. ‘We scientists see two-dimensional materials as beautiful, flat shapes. But ripples bring out a new property in these materials,’ says Geim.
Born in Russia, Dutch-British physicist Andre Geim is the only person in history to win both the Nobel Prize and the Ig Nobel Prize. In 2010, together with his PhD student Konstantin Novoselov, he won the Nobel Prize in Physics for his pioneering research into graphene. Ten years earlier he had already won the playful counterpart of the prize for the levitating a frog. Geim has a thing for animals anyway: in 2001 he made his hamster Tisha co-author of a scientific publication.
Although there have been indications that graphene could be a good catalyst, this research clearly shows that the ridges are the cause, says nanotechnologist Andrew Ferrari from the University of Cambridge. “These direct measurements seem to prove what has hitherto been intuition or an incompletely proven conclusion.”
More durable catalyst
“Most industrial chemical reactions are driven by catalysis, so if we produce catalysts based on pure carbon – which are very, very active, as the researchers argue in this article – it could change a lot of industrial processes,” says chemist Andrew Khlobystov from the University of Nottingham. The splitting of hydrogen, which takes place in hydrogen fuel cells to produce clean electricity, is just one example.
Graphene could also be a much more sustainable choice than current catalysts, which often consist of rare metals, says Khlobystov. And if you embed some of those metals in sheets of graphene, they could do their job even better, he suggests. However, the production of the pure graphene used in these experiments is currently much more expensive than that of conventional metal catalysts, says Khlobystov.