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Tuesday, May 26, 2015

Graphene Supercapacitor equals Li-ion battery energy density

Scientists in South Korea have developed a graphene supercapacitor that stores as much energy per kilogram as a lithium-ion battery and can be recharged in under four minutes.

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.

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 Wu of Gwangju Institute of Science and Technology, in South Korea, 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.

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