A simple trick could improve the ability of advanced ultracapacitors, or supercapacitors, to store charge. The technique, developed by Stanford University researchers, could enable the use of new types of nanostructured electrode materials that store more energy.
While ultracapacitors provide quick bursts of power and can be recharged many more times than batteries without losing their storage capacity, they can store only a tenth as much energy as batteries, which limits their applications. To improve their energy density, researchers have focused on the use of electrode materials with greater surface area—such as graphene and carbon nanotubes—which can hold more charge-carrying ions.
The Stanford team, led by Yi Cui and Zhenan Bao, used composite electrodes made of graphene and manganese oxide. Manganese oxide is considered an attractive electrode material because, “one, manganese is abundant so it’s very low cost,” Cui says. “It also has high theoretical capacity to store ions for supercapacitors.” However, in the past its use has been hindered by its low conductivity, which makes conveying ions in and out of the material difficult.
The researchers dipped the composite electrodes into either a carbon nanotube solution or a conductive polymer solution. The coating improves the electrodes’ conductivity and hence their capacitance—their ability to store charge—by 20 percent and 45 percent respectively. The researchers report their work in a paper that appeared online in the journal Nano Letters.
“This is an important advancement,” says Lu-Chang Qin, a physics professor at the University of North Carolina at Chapel Hill, who has recently developed similar graphene–manganese oxide electrodes. These results “promise hopes for a new generation of supercapacitors,” Qin says. However, he points out that the Stanford team has yet to measure the energy density of its new electrodes. Qin has collaborated with Japanese researchers to make electrodes from carbon nanotube graphene. These have an energy density of 155 watt-hours per kilogram, comparable with that of nickel–metal hydride batteries.
Bor Jang, cofounder of Nanotek Instruments in Dayton, Ohio, which makes graphene electrodes for supercapacitors, says the new electrodes may lack energy density. Besides, he says, “a combination of graphene, MnO2, and a conductive polymer or carbon nanotubes might be overkill.”
Others have obtained much higher capacitance numbers with graphene–metal oxide or conductive polymer electrodes. However, Cui says what’s most exciting about the new work is that such a simple dipping technique can enhance capacitance. He says the technique might be used to improve the conductivity of other electrode materials such as sulfur, silicon, and lithium manganese phosphate, thereby enhancing the performance of lithium-ion batteries. Cui and his colleagues are now working on improving battery electrodes using the new method.