Nanocapacitors with Big-Energy Storage

Nanopore arrays combine high power and storage capacity.

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.

Nanopore power: Arrays of capacitors built inside nanopores are shown here in a scanning electron micrograph image overlaid with an illustration that shows their design. The pores are etched into an aluminum substrate (dark yellow). The capacitors form two thin layers of metal (blue) separated by a layer of insulating material (light yellow). Credit: A. James Clark School of Engineering, University of Maryland

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.

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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.