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Wednesday, March 5, 2008

MIT Builds Efficient Nanowire Storage to Replace Car Batteries

Could the ultracapacitor replace lithium ion in hybrids and plug-in vehicles? Our senior automotive editor already thinks the science adds up, but it’s in a tiny box at a messy lab that the future of automotive efficiency is taking a surprising turn toward extending range and battery life.
CAMBRIDGE, Mass. — Sometimes the cliché fits: It looks like a bomb went off—not necessarily in this lab, but somewhere, with the aftermath seemingly carted here. The gutted remains of a sedan, its engine exposed, the seats ripped out of the frame, sits encased in cables. At other workstations the focus is a single part—an isolated camshaft, an alternator hooked up to test apparatus. It would be easy to misinterpret this place and think that researchers at MIT’s Lab for Electromagnetic and Electronic Systems (LEES) are either piecing back together some shattered car or entering the Automotive X Prize. In fact, each of these experiments has different methodologies, but many have the same goal: automotive efficiency, by any means necessary.

The wired car, for example, is an effort to test more detailed diagnostic systems, with sensors that detect changes in the system’s electrical signature—and maybe even warn you before the starter motor fails. And the modifications made to the alternator would let it run at 30 percent greater efficiency, with a smoother electrical system translating to about 1 mpg in improved mileage. Researchers estimate that the increased cost for the manufacturer would be about $5.

One of the most promising experiments here is tucked away in what appears to be the messiest part of the entire lab, a small room littered with hand tools and testing gear. Joel Schindall, the associate director of LEES, pulls a tray out of a cabinet and flips it open. Inside are four black squares, like overturned tiles from a Magnetic Poetry set. If my job was to clean out this lab, I would probably take one look at these unassuming little things and fling the entire tray into the nearest trash can. Because unless they’re under an electron microscope, vertically aligned carbon nanotube arrays don’t look like much.

The point of these particular arrays is to capture ions and eventually give traditional rechargeable batteries a run for their money. The focus of Schindall’s research is ultracapacitors, which store drastically less energy than a battery but have essentially none of the drawbacks. In any capacitor, there’s no battery memory caused by partial discharging and no reduction in capacity with each recharge. “They never wear out, they have no electrolyte, they don’t have any chemistry taking place in them,” Schindall says. “It’s just an electric field that stores the energy. So you can recharge a capacitor a gazillion times. It’s very efficient—just the internal resistance of the wires.” The ions cling electrostatically to materials in a capacitor, which also allows for much quicker charge times. And by avoiding the chemical reaction that drives traditional batteries, there’s no real danger of a capacitor suddenly overloading—or exploding like a laptop’s lithium-ion battery pack. (For more on how this technology works, read senior automotive editor Mike Allen’s new take on why ultracapacitors could replace batteries in hybrid cars.)

The problem with capacitors—and the reason they’ve taken such a back seat to batteries since they were first stumbled upon in the ’60s—is capacity. Even ultracapacitors can manage only a fraction of the power of a lead-acid or lithium-ion battery. So the recipe for a better ultracapacitor is more surface area. Researchers have already expanded capacity with the addition of activated carbon coatings, which are porous enough to provide an effective surface area that’s 10,000 times greater than the materials previously used to gather ions. Around four years ago, Schindall was reading about various experiments that utilized nanowire arrays, when he experienced—though no scientist, Schindall included, would ever actually put it this way—the proverbial “eureka” moment.

By replacing the porous activated carbon used in ultracapacitors with tightly bunched nanotubes, Schindall believed that the ion-collecting surface area could be increased by as much as five. Since current ultracapacitors can store around 5 percent of the energy in an equivalent-size battery, the addition of nanowires could bring this up to 25 percent. “And you can also operate [the ultracapacitor] at a higher voltage with the nanotubes, and that’s about another factor of two in energy,” he says. “We are hopeful—we haven’t proven it—that we can get up somewhere between 25 and 50 percent of a battery’s energy. At that point, it becomes a compelling device for many applications.”

Those applications could include not only electric vehicles, where the benefits of unlimited charge cycles and less overload-prone storage are clear, but in hybrid cars as well. The math gets a little complicated here, but Schindall says that even standard ultracapacitors, with their relatively paltry 5 percent storage, are potential competitors for the pack in his Toyota Prius. “In order to prolong the life of the battery in my car, they only use it over the middle 10 to 15 percent of its range,” he says. “So actually I’m only using perhaps 15 percent of the capacity. With an ultracapacitor you can use it all, or almost all. So the difference between 5 percent and 15 percent is not nearly as severe.”

According to Schindall, ultracapacitors would also outlive the car, possibly solving the complicated warranty issues surrounding hybrids and, whenever they’re finally released commercially, plug-in hybrids. If nanotube ultracapacitors can reach that 25 or 50 percent mark, then they could not only compete with the batteries currently used by Toyota, but thanks to their ability to discharge without risk, they could provide even longer ranges. “I try to contain myself, because it hasn’t been proven yet, but it could be a real paradigm change,” Schindall says.

The process of creating the nanowire arrays is relatively straightforward—a tiny piece of conductive substrate is coated with a catalyst, and then placed in a vacuum chamber. The chamber is then filled with carbon gas, and the square is heated until a black, sootlike coating appears. After about 10 minutes, the tile is complete, and the nanowires are fully grown. The challenge has been in reaching the theoretical capacity that Schindall’s team originally calculated. So far, the nanotubes can match the energy storage of standard ultracapacitors, but the goal remains to boost that capacity by a factor of five or even 10. “A couple of years ago, we thought we were six months to a year away. And that time has come and gone,” he says.

The next step for this project is to create test cells about the size of watch batteries to be distributed to existing ultracapacitor manufacturers. The team will also release its latest results, but by allowing companies to independently verify that data, Schindall believes it could demonstrate the commercial viability of the nanotube approach. He hopes to have those test cells ready within a year, or possibly as soon as a few months. Still, it could take years for ultracapacitors of any kind to reach the kind of production volume and capacity necessary to rival batteries in the marketplace. So for now, these nano-dusted squares are going back in their tray and back on the shelf to fight for energy storage supremacy another day.

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