A "digital quantum battery" concept proposed by a physicist at the University of Illinois at Urbana-Champaign could provide a dramatic boost in energy storage capacity--if it meets its theoretical potential once built.
The concept calls for billions of nanoscale capacitors and would rely on quantum effects--the weird phenomena that occur at atomic size scales--to boost energy storage. Conventional capacitors consist of one pair of macroscale conducting plates, or electrodes, separated by an insulating material. Applying a voltage creates an electric field in the insulating material, storing energy. But all such devices can only hold so much charge, beyond which arcing occurs between the electrodes, wasting the stored power.
If capacitors were instead built as nanoscale arrays--crucially, with electrodes spaced at about 10 nanometers (or 100 atoms) apart--quantum effects ought to suppress such arcing. For years researchers have recognized that nanoscale capacitors exhibit unusually large electric fields, suggesting that the tiny scale of the devices was responsible for preventing energy loss. But "people didn't realize that a large electric field means a large energy density, and could be used for energy storage that would far surpass anything we have today," says Alfred Hubler, the Illinois physicist and lead author of a paper outlining the concept, to be published in the journal Complexity.
Hubler claims the resulting power density (the speed at which energy can be stored or released) could be orders of magnitude greater, and the energy density (the amount of energy that can be stored) two to 10 times greater than possible with today's best lithium-ion and other battery technologies.
What's more, digital quantum batteries could be fabricated using existing lithographic chip-manufacturing technologies using cheap, nontoxic materials, such as iron and tungsten, atop a silicon substrate, he says. The resulting devices would, in principal, waste little or no energy as they absorbed and released electrons. Hubler says it may be possible to build a benchtop prototype in one year.
Today, however, digital quantum batteries are merely a patent-pending research concept. Hubler has applied for Defense Advanced Research Projects Agency funding to develop such a prototype, but the concept presents significant challenges. It's not clear that the nanofabricated materials wouldn't break down once loaded with energy, says Joel Schindall, a professor of electrical engineering at MIT.
But Schindall also says the concept has merit. "I'm cautiously intrigued, because he does have some legitimate arguments for the fact that at these quantum dimensions, the energy storage effect is at least predicted to go up considerably," Schindall says. "The first challenge is: are his assumptions correct, or are there some other phenomena that haven't been looked at that get in the way?"
In some ways, the concept represents a variation on existing micro- and nanoelectronic devices. "If you look at it from a digital electronics perspective--it's just a flash drive," says Hubler. "If you look at it from an electrical engineering perspective, you would say these are miniaturized vacuum tubes like in plasma TVs. If you talk to a physicist, this is a network of capacitors."
The digital part of the concept derives from the fact that each nanovacuum tube would be individually addressable. Because of this, the devices could perhaps be used to store data, too.
Other methods exist for boosting the performance of capacitors. Advanced versions, called ultracapacitors, can store significant energy and operate more quickly by increasing the surface area of their electrodes and using an electrolyte. Schindall's group has increased the charge and discharge rates and storage capacity of traditional ultracapacitors by using carbon nanotubes instead of activated carbon on the electrode's surface. In essence, this increases the surface area of the electrode.
The advantages to Schindall's design--increased power output and energy density--could be crucial for applications like soaking up huge pulses of energy rapidly from a field of wind turbines or solar arrays, for example. Plus, his team has actually built a benchtop device. The downside is that the energy density of a given mass of material would still be somewhat lower than that of lithium-ion batteries.
While Hubler hasn't yet built anything, he notes that, in 2005, a group of Korean researchers showed that nanoscale capacitors could be fabricated. Hubler's device would still need billions or even trillions of such devices, however.
"I complete agree we desperately need new ways of storing electric energy," says Schindall. "Though it may be in competition with what I'm doing, I wish him the greatest of success and hope it works."
By David Talbot
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