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Abstract:

The ultimate electronic energy-storage device would store plenty of energy but also charge up rapidly and provide powerful bursts when needed. Sadly, today's devices can only do one or the other: capacitors provide high power, while batteries offer high storage.

Now researchers at the University of Maryland have developed a kind of capacitor that brings these qualities together. The research is in its early stages, and the device will have to be scaled up to be practical, but initial results show that it can store 100 times more energy than previous devices of its kind. Ultimately, such devices could store surges of energy from renewable sources, like wind, and feed that energy to the electrical grid when needed. They could also power electric cars that recharge in the amount of time that it takes to fill a gas tank, instead of the six to eight hours that it takes them to recharge today.

There are many different kinds of batteries and capacitors, but in general, batteries can store large amounts of energy yet tend to charge up slowly and wear out quickly. Capacitors, meanwhile, have longer lifetimes and can rapidly discharge, but they store far less total energy. Electrochemists and engineers have been working to solve this energy-storage problem by boosting batteries' power and increasing capacitors' storage capacity.

Sang Bok Lee, a chemistry professor, and Gary Rubloff, a professor of engineering and director of the Maryland NanoCenter, created nanostructured arrays of electrostatic capacitors. Electrostatic capacitors are the simplest kind of electronic-energy-storage device, says Rubloff. They store electrical charge on the surface of two metal electrodes separated by an insulating material; their storage capacity is directly proportional to the surface area of these sandwich-like electrodes. The Maryland researchers boosted the storage capacity of their capacitors by using nanofabrication to increase their total surface area. Their electrodes work in the same way as ones found in conventional capacitors, but instead of being flat, they are tubular and tucked deep inside nanopores.

The fabrication process begins with a glass plate coated with aluminum. Pores are etched into the plate by treating it with acid and applying a voltage. It's possible to make very regular arrays of tiny but deep pores, each as small as 50 nanometers in diameter and up to 30 micrometers deep, by carefully controlling the reaction conditions. The process is similar to one used to make memory chips. "Next you deposit a very thin layer of metal, then a thin layer of insulator, then another thin layer of metal into these pores," says Rubloff. These three layers act as the nanocapacitors' electrodes and insulating layer. A layer of aluminum sits on top of the device and serves as one electrical contact; the other contact is made with an underlying aluminum layer.

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."