Graphene supercapacitors
Sheet lightning

Electric cars need better batteries. Graphene may help

Science & technologyMay 21st 2015 edition

SEARCHING for a way to make an electric car that is truly competitive with its petrol and diesel equivalents is frustrating. The lithium-ion batteries used to store the juice which powers such cars are almost cheap enough and almost long-lasting enough to do so, but not quite. And unless they are fitted with petrol-driven generators known as range extenders, electric cars cannot go far without topping up the batteries. Even with the best modern technology, such top-ups take at least 20 minutes. What is needed is a better battery.

Many have tried. Many have failed. Hope, though, springs eternal. The latest attempt involves graphene, the wonder material de nos jours. The work’s progenitor, Lu Wu of Gwangju Institute of Science and Technology, in South Korea, thinks that if his process can be commercialised it might just solve the problem.

Strictly speaking, what Dr Lu and his colleagues are working on is not a battery but a supercapacitor—a device that combines a battery-like electrolyte with the physics of a normal electrical capacitor. In a supercapacitor, as in a normal capacitor, energy is stored on the surfaces of materials in the form of static electricity. But, unlike in a normal capacitor, that static relies, in part, on ions from the electrolyte that are attracted to these surfaces when the supercapacitor is charged up. This reliance on static electricity, rather than on changes in the chemical states of the ions (which is how a battery works), makes the process of storage more rapid. Supercapacitors can thus be charged faster than batteries.

Supercapacitors are not a new idea. But graphene, which is a form of carbon composed of sheets a single atom thick, is especially suitable for making them. Graphene has an area of 2,675 square metres per gram. All of this surface is available for the storage of static electricity. Graphene could therefore be used to make supercapacitors that hold more energy per kilogram than lithium-ion batteries.

Dr Lu’s problem was making graphene in a form that he could use in a supercapacitor—and doing so in a way that might plausibly be industrialised. His solution is, in effect, to blow up pieces of graphite in controlled explosions.

Graphene is to graphite what a single playing card is to a full pack. Strong chemical bonds keep the graphene layers intact, but the individual layers are held to each other only weakly, which is why graphite can be used to make the “lead” in pencils. To make small amounts of graphene, you can peel the layers from the surface of a graphite crystal one at a time, as a dealer might when distributing cards (there are various ways of doing this). To make a lot of it, though, you have to pull the whole crystal apart, as one might scatter a pack across a table.

Dr Lu did this in two stages. First, he exposed powdered graphite to oxygen in a controlled manner to produce a substance called graphite oxide. This is not a true oxide, with a fixed chemical formula. Rather, it is a graphite-like substance that has oxygen-rich clusters of atoms between the graphene layers. This done, he then heated the graphite oxide to 160°C in a vessel which had an internal pressure of a tenth of an atmosphere. The heat caused chemical reactions inside the graphite oxide, and these produced carbon dioxide and steam. The increased internal pressure these gases created, pushing against the reduced external pressure in the vessel, blew the graphite apart into its constituent sheets. Those, after a bit of further treatment to remove surplus oxygen, were then suitable for incorporation into a supercapacitor—which Dr Lu did.

The result, though small, worked well. It stored as much energy per kilogram as a lithium-ion battery and could be recharged in under four minutes. Scaled up to the size needed for a car, the current required to recharge it that quickly would require a pretty robust delivery system. But, though such a system would probably be unsuitable for home use, it could be installed in roadside service stations.

That would get rid of the range problem that electric cars now suffer from. And, if a crude first attempt such as this can match a lithium-ion battery’s energy-storage ability, a refined version would surely exceed it markedly. How long such a supercapacitor would function for remains to be seen. But if it proves as durable as a battery—and if it can be made reliably and cheaply—then it may turn out to be the breakthrough that changes the future of motoring.

This article appeared in the Science & technology section of the print edition under the headline "Sheet lightning"

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