Researchers at MIT have made pure, dense, thin films of carbon nanotubes that show promise as electrodes for higher-capacity batteries and supercapacitors. Dispensing with the additives previously used to hold such films together improved their electrical properties, including the ability to carry and store a large amount of charge.
Carbon nanotubes can carry and store more charge than other forms of carbon, in part because their nanoscale structure gives them a very large surface area. But conventional methods for making them into films leave significant gaps between individual nanotubes or require binding materials to hold them together. Both approaches reduce the films’ conductivity–the ability to convey charge–and capacitance–the ability to store it.
The MIT group, led by chemical-engineering professor Paula Hammond and mechanical-engineering professor Yang Shao-Horn, made the new nanotube films using a technique called layer-by-layer assembly. First, the group creates water solutions of two kinds of nanotubes: one type has positively charged molecules bound to them, and the other has negatively charged molecules. The researchers then alternately dip a very thin substrate, such as a silicon wafer, into the two solutions. Because of the differences in their charge, the nanotubes are attracted to each other and hold together without the help of any glues. And nanotubes of similar charge repel each other while in solution, so they form thin, uniform layers with no clumping.
The resulting films can then be detached from the substrate and baked in a cloud of hydrogen to burn off the charged molecules, leaving behind a pure mat of carbon nanotubes. The films are about 70 percent nanotubes; the rest is empty space, pores that could be used to store lithium or liquid electrolytes in future battery electrodes. The films “can store a lot of energy and discharge it rapidly,” says Hammond. The capacitance of the MIT films–that is, their ability to store electrical charge–is one of the highest ever measured for carbon-nanotube films, says Shao-Horn. This means that they could serve as electrodes for batteries and supercapacitors that charge quickly, have a high power output, and have a long life.
The MIT group is not the first to use the layering technique to create nanotube films. But previously, researchers using the method layered a positively charged polymer with negatively charged nanotubes, resulting in films that were only half nanotubes. No polymer can equal the electrical conductivity of carbon nanotubes, so these films’ electrical properties weren’t as impressive as those of Hammond and Shao-Horn. Others have made films by growing the nanotubes from the substrate up, but the resulting forest of vertically aligned nanotubes is insufficiently dense.
“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.