“I see particular importance of these findings for supercapacitors, because all-nanotube materials can potentially store a greater amount of charge,” says Nicholas Kotov, a professor of chemical engineering and materials science at the University of Michigan.
In addition to their high capacitance, the nanotube films have other advantages as electrode materials, says Shao-Horn. Conventional high-energy-density electrodes are made of carbon powder held together with a binder. But particles of the binder in the surface of the electrode reduce its active area and make it difficult to modify. With carbon nanotubes, says Shao-Horn, “you have systematic control of surface chemistry.” Adding charged molecules to the electrodes’ surface, for example, could increase their capacitance and energy density.
“Many researchers are pursuing thin films of carbon nanotubes for diverse applications in electronics, energy storage, and other areas,” says John Rogers, a professor of materials science and engineering at the University of Illinois at Champaign-Urbana. The MIT group is primarily focused on developing the films for electrochemical applications like batteries, but the layering technique is versatile. By varying the pH of the nanotube solutions and the number of layers in the films, it’s possible to tailor the films’ electrical properties. This is “an attractive feature of this approach,” says Rogers. The technique could be used to make nanotube films for flexible electronics, for example. Kotov also sees other potential uses of the nanotube films. When immersed in liquid, the films swell. “This will be useful, because it changes both the conductivity and capacity of the material, which opens up a lot of prospects for sensing applications and smart coatings,” says Kotov.
The layer-by-layer method is time consuming, however. Typical electrodes are 10 to 100 micrometers thick; those that the MIT group has made so far are only about 1 micrometer thick. But Hammond, a pioneer in layer-by-layer assembly of polymers, has developed a layer-by-layer spraying technique that should be adaptable to nanotubes. “This reduces the time it takes by an order of magnitude, which will be necessary for commercial development,” says Shao-Horn.