Researchers have long sought to boost energy storage in ultracapacitors by improving electrode design. Schindall and his colleagues are trying to make electrodes coated with carbon nanotubes, which have a greater surface area than activated carbon and are excellent conductors. Other research groups are using better charge-storing materials, such as manganese oxide and conducting polymers.
The new electrode combines the advantages of these two methods. First, the researchers grow an array of carbon nanotubes on a foil made from the metal tantalum, which is commonly used in capacitors. Then they grow 100-nanometer-wide flower-shaped nanoparticles directly on the array. The nanotubes grow more or less vertically, but they’re not very stiff and tend to fall across each other. The nanoflowers grow mostly at the junctions of multiple nanotubes and have a large surface area (236 square meters per gram) compared with typical particles of manganese oxide.
“Each manganese oxide nanoflower is connected directly with the tantalum foil by two or more electron superhighways, the carbon nanotubes,” says Gaoping Cao, Zhang’s coleader on the project. “This superior conducting network allows for efficient charge transport.” When current flows through the tantalum foil, charges quickly get transferred to and stored in the manganese oxide: the electrode stores twice as much charge as the same volume of activated carbon. The nanotubes’ high conductivity could also give them a greater power output than current ultracapacitors have, the researchers say.
“The way of growing manganese oxide on carbon nanotube arrays is new and has produced beautiful structures,” says Yury Gogotsi, a materials-science and engineering professor at Drexel University. Gogotsi says that combining the high conductivity of the carbon nanotubes with the charge-storage capacity of manganese oxide is an attractive approach. But, he adds, “it is not practical for large volume, such as automotive applications, because the use of carbon nanotube arrays and tantalum foil makes them expensive.”
Indeed, says Schindall, cost could be the main barrier to ultracapacitors with nanostructured electrodes. “They’ve found a way to grow these structures,” he says, “but now they’ve got to be able to grow them densely enough and economically enough to be practical.”