Graphene can split hydrogen 100 times better than any known chemical catalyst thanks to tiny ripples on its surface. It could potentially be used to develop more effective hydrogen fuel cells and make many industrial processes more efficient.
A one-atom-thick layer of carbon, graphene is essentially a slice of graphite. The latter is an extremely unreactive compound because of its strong carbon bonds.
However, at the University of Manchester, UK, and his colleagues have found that graphene, despite also having strong bonds, is incredibly chemically reactive. This is because it can鈥檛 be completely flat, instead having small undulations in it called nanoripples. These allow it to split hydrogen as effectively as the best catalysts we have today.
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To show this, the researchers produced graphene with as few defects as possible to rule out the chemical activity coming from some other feature and stretched a sheet of it across the top of a microscopic container filled with hydrogen molecules.
As the graphene split the hydrogen into individual atoms, they built up inside the container, increasing the pressure, and caused the graphene to bulge out. The researchers measured the size of the bulge to calculate the graphene鈥檚 catalytic ability.
They found that its ability per gram to split hydrogen was at least 100 times better than the most widely used catalysts, such as copper or magnesium oxide. However, if you compare the efficiencies of catalysts to their surface area, then copper comes out slightly better than graphene.
Geim and his colleagues also compared an almost perfectly flat sheet of graphene with one on a silicon surface that caused nanoripples and found that only the nanorippled surface showed evidence of splitting hydrogen.
He says this ability to work as an effective catalyst could transfer to other chemical reactions and for other flat materials. 鈥淭wo-dimensional materials in our psyche as scientists鈥� are considered nice, flat shapes, but ripples are bringing up a new property,鈥� says Geim.
While there have been previous indications that perfectly flat graphene might be a good catalyst, this comprehensively shows that the ripples are causing it, says at the University of Cambridge. 鈥淭hese direct measurements seem to prove what was an intuition or not fully proven conclusion until now, that it is the ripples.鈥�
鈥淢ost industrial chemical reactions are driven by catalysis, so if we produce catalysts based on pure carbon 鈥� a very, very active one as they claim in this paper 鈥� then it potentially can change many industrial processes,鈥� says at the University of Nottingham, UK, such as for hydrogen splitting, which is commonly used in hydrogen fuel cells to produce clean electricity.
Graphene could be a much more sustainable choice than current catalysts, too, which are often rare metals, says Khlobystov. And embedding metal active sites in sheets of graphene may make them do the job even better, he suggests. However, it is currently much more expensive to produce the kind of pure graphene used in these experiments than conventional metal catalysts, he says.
Proceedings of the National Academy of Sciences
