This “fractal-like structure greatly increases the surface area,” says Joel Schindall, associate director of MIT’s Laboratory for Electromagnetic and Electronic Systems, who was not involved in the work.
In a paper published online this week in the journal Nature Nanotechnology, the Maryland group describes making 125-micrometer-wide arrays, each containing one million nanocapacitors. The surface area of each array is 250 times greater than that of a conventional capacitor of comparable size. The arrays’ storage capacity is about 100 microfarads per square centimeter.
But surface area isn’t the only determinant of energy density. The Maryland group’s nanocapacitors also benefit from the very small spacing between their electrodes, and the work is unique in this respect, says Robert Hebner, director of the Center for Electromechanics at the University of Texas at Austin. Hebner was not involved in the Maryland research.
If the electrodes are far apart, the like charges on their surfaces strongly repel each other. When the electrodes are placed closer together, the negative and positive charges on either side balance out these repulsive forces, and more total charge can be stored in a given area. The total thickness of each nanocapacitor is just 25 nanometers, and the charges can pack very close together. “It’s impressive,” says Hebner. “I hope they can scale it up.”
So far, the nanocapacitor arrays can’t store much total energy because they’re so small. “Instead of making these little dots, we want to make a large area that contains billions of nanocapacitors to store large amounts of energy,” says Lee. Both he and Rubloff say that scaling up to a practical level is not trivial, but the pair is working together to make larger arrays. “There are many scale-up issues,” says Rubloff. “We’ll look at how large we can make these and still have all of them work.”
Even if this problem is solved, they’ll still have to make sure that they can effectively connect multiple arrays to one another. But Hebner says that this problem is not intractable, and he points to devices on the market, including sensitive magnetic detectors, that successfully overcome similar connectivity issues.
One advantage of the new fabrication method is that the nanopore dimensions and the respective thicknesses of the electrode and insulator can be carefully controlled. “Regularity and uniformity are central to scaling nanotechnologies up to something manufacturable and commercializable,” says Rubloff. “There are still major hurdles, but we’re trying to decide how to commercialize this–there’s definitely a thirst to do so.”