Slate: Unexpectedly Amazing Carbon-Based Energy Form
Batteries are terrible. Compared to many other methods of storing energy, especially fossil fuels, batteries aren’t very energy dense—that is, a 1-pound battery stores far less energy than is contained in a pound of gasoline. That wouldn’t be so bad if the energy in a battery were easy to replenish—your Tesla might still go only a couple hundred miles on a single charge, but if you could fully recharge it in five minutes rather than several hours, the low capacity wouldn’t bother you as much.
Scientists have spent decades trying to create the perfect battery—a battery with great energy density or, at least, one that doesn’t take so long to charge. If we could somehow make this perfect battery, pretty much every gadget you use, from your phone to your laptop to your future electric car, would be amazing, or just less annoying than they are today. The perfect battery might also help with some other important stuff: climate change, oil wars, pollution, etc.
One approach for improving the battery is to forget about the battery and instead improve capacitors. A capacitor, like a battery, is a device that stores electrical energy. But capacitors charge and discharge their energy an order of magnitude faster than batteries. So if your phone contained a capacitor rather than a battery, you’d charge it up in a few seconds rather than an hour. But capacitors have a big downside—they’re even less energy dense than batteries. You can’t run a phone off a capacitor unless you wanted a phone bigger than a breadbox.
But what if you could make a dense capacitor, one that stored a lot of energy but also charged and discharged very quickly? Over the past few years, researchers at several companies and institutions around the world have been racing to do just that. They’re in hot pursuit of the perfect “supercapacitor,” a kind of capacitor that stores energy using carbon electrodes that are immersed in an electrolyte solution. Until recently, though, supercapacitors have been expensive to produce, and their energy densities have fallen far short of what’s theoretically possible. One of the most promising ways of creating supercaps uses graphene—a much-celebrated substance composed of a one-atom layer of carbon—but producing graphene cheaply at scale has proved elusive.
Then something unexpectedly amazing happened. Maher El-Kady, a graduate student in chemist Richard Kaner’s lab at UCLA, wondered what would happen if he placed a sheet of graphite oxide—an abundant carbon compound—under a laser. And not just any laser, but a really inexpensive one, something that millions of people around the world already have—a DVD burner containing a technology called LightScribe, which is used for etching labels and designs on your mixtapes. As El-Kady, Kaner, and their colleagues described in a paper published last year in Science, the simple trick produced very high-quality sheets of graphene, very quickly, and at low cost.
El-Kady’s DVD-burning experiment has been characterized as a scientific “accident,” but that description obscures the more interesting story behind it. “Nothing in science is actually an accident—it only looks like that way when you look back,” Kaner says. For many years, students in Kaner’s lab had been experimenting with subjecting various polymers to lasers, including those found in LightScribe drives. El-Kady’s idea of subjecting graphite oxide to the LightScribe was just a lucky continuation of that work. He saw some other students in the lab playing with the laser, so he decided to take a crack at it too. “The appeal of this technique is that anybody could do this—it’s really simple,” says Kaner. “You take a piece of plastic, buy some graphite oxide, stick it in your CD drive and turn it into graphene.” Even more exciting, the technique “makes the most efficient carbon-based supercapacitors that have been made to date.”
How efficient? Kaner points out that the theoretical upper limit for the efficiency of graphene-based capacitors is 550 Farads per gram (a measure of energy storage). Other academic researchers have created supercaps that can store as much as 150 F/g, and Kaner suspects that commercial companies may have done even better. But Kaner and El-Kady’s DVD-laser-produced graphene supercaps go far beyond anything else that has been reported so far. In their Science paper, they reported hitting capacitance rates of up to 276 F/g, close to double what had been previously reported. In another paper published last month in Nature Communications, Kaner and El-Kady described a way to use their DVD burner technique to produce micro-supercapacitors, which can be used to power sensors and other small electronic devices. Those supercapacitors are even more efficient. “With those, we essentially got up to 400 Farads per gram,” Kaner says.
Energy futurists see great potential for such cheap, easy-to-produce, energy-dense supercapacitors. In many applications, these devices could either replace or work alongside batteries to make for more energy-efficient devices. In vehicles, efficient supercaps could be used to save up the kinetic energy your car otherwise loses while braking—i.e., what’s known as “regenerative braking”—and then deliver that power in a burst when you need to accelerate. Several Chinese companies have produced supercap-powered buses. Because supercapacitors charge and discharge rapidly, the buses can be replenished at every bus stop. The quick charge allows the bus to go for a few miles—enough to get to the next stop, where it sips more power.
Kaner says this vision could be more broadly applied to other kinds of vehicles. “The ultimate vision I could see is that even if you had to charge your supercapicator-powered car every 20 miles, you could have a lane on the freeway that was a charging lane,” Kaner says. “As long as you drove in that for a sufficient time, your car gets charged.” Kaner stresses that we’re likely a long way from such a future. Among other obstacles, researchers like him would have to find a way to make graphene even more efficient and producible at large scale. That’s exactly what he’s looking to do next; Kaner and his team have signed a deal with a supercapacitor company to work on ways to commercialize their production technique.
Still, he’s reluctant to put any timeline on when we’ll see such capacitors in products, and he cautions against any immediate great expectations. “I think people are looking for a breakthrough in battery technology, and supercaps offer a lot of promise,” Kaner says. “But when somebody puts out an article with a lot of hype, and then that doesn’t happen in a year, people get frustrated.” So, be warned: Supercapacitors won’t make next year’s gadgets any easier to deal with. But in 5 or 10 years, say, they could change the way the world charges up.