Researchers at Manchester University have developed a graphene-based electrical device that converts heat from car exhausts and engine bodies into electrical current.

Known as a ‘ballistic rectifier’, the nano-electronic device has the potential to improve energy efficiency, using the recaptured heat to power things such as air conditioning, or top up a car’s battery. Key to the rectifier’s performance is graphene’s exceptionally high electron mobility, a property which determines how fast an electron can travel in a material and how fast electronic devices can operate. The electron mobility of the ballistic rectifier enables high conversion efficiency from an alternating current to a direct current.

“The working principle of the ballistic rectifier means that it does not require any band gap,” said Manchester’s Prof Aimin Song, one of the research leads and the inventor of the device. “Meanwhile, it has a single-layered planar device structure which is perfect to take advantage of the high electron-mobility to achieve an extremely high operating speed.”

“Unlike conventional rectifiers or diodes, the ballistic rectifier does not have any threshold voltage either, making it perfect for energy harvest as well as microwave/infrared detection.”

According to the researchers, the device is the most sensitive room-temperature rectifier ever made. Conventional devices with similar conversion efficiencies require cryogenically low temperatures, but the ballistic rectifier can operate in the extreme heat of engine exhausts, parts of which can reach 600 degrees Celsius.

“Graphene has exceptional properties,” said Greg Auton, who carried out experiments with the rectifier. “It possesses the longest carrier mean free path of any electronic material at room temperature.”

“Despite this, even the most promising applications to commercialise graphene in the electronics industry do not take advantage of this property. Instead they often try to tackle the the problem that graphene has no band gap.”

The Manchester team claims the technology could also be used to recover wasted heat energy in power plants. Song and his colleagues are now looking to scale up the research by using large wafer-sized graphene and conducting high-frequency experiments.