Can supercapacitors replace batteries?
The electric car depends on batteries, and before EVs become a large chunk of our automotive fleet, there are probably going to be some changes.
Right now, Elon Musk is betting he can produce millions of small lithium-ion batteries not much bigger than the ones you put in your flashlight and string them together to power a $35,000 Tesla Model E over a range of 200 miles at speeds of up to 70-80 mph. The Model E also will also need an infrastructure of roadside “filling stations” and home chargers, although the best superchargers still take more than 20 minutes to achieve 80 percent capacity.
But there is another way to store electricity, long familiar to the designers of electrical circuits. It’s the capacitor, a device that stores a small current by static electricity rather than a chemical reaction. Capacitors sit in all of your electrical devices, from radio circuits to the most sophisticated laptops, and are essential to providing the steady electric current needed to run such electronics. But what if the concept of capacitors could be scaled up to the point where they could help power something as big as an electric vehicle? Granted, it’s a long, long way from the 1.5-volt capacitor in your iPad and powering a 4,500-pound Tesla along the Interstate, but researchers are out there probing and are already thinking in terms of a breakthrough.
Right now there’s a huge separation between the things that batteries can do and the things that capacitors can do. In a way they are complementary — the strengths of one are the weaknesses of the other. But researchers are working toward a convergence — or perhaps just a way of using them in tandem.
A battery employs chemistry by splitting ions in the electrolyte so that the negative ones gather on the cathode and the positive on the anode, building up a voltage potential. When they are connected externally an electric current flows. Batteries have a lot of energy density. They can store electricity up into the megawatt range and release a flow of electricity over long periods of time. The process can also be reversed, but, because the reaction is (once again) chemical, it can take a long time.
Capacitors store electrons as static electricity. A thundercloud is a great big capacitor with zillions of electrons clinging to the almost infinite surface area of individual raindrops. And as everyone knows, this huge stored capacity can be released in a “bolt of lightening.” Capacitors can be recharged almost instantly but also they release their energy almost instantly, rather than the even flow of a battery. One of their major uses is in flash photography. But their capacity for storing power is also limited. On a pound-for-pound basis, the best capacitors can only store one-fifth to one-tenth the equivalent of a chemical battery. On the other hand, batteries can start to wear out after five years, while supercapacitors last at least three times as long.
Back in the 1950s, engineers at General Electric, and later at Standard Oil, invented what have come to be called “supercapacitors.” Basically, a supercapacitor changes the surface material and adds another layer of insulating dielectric in order to increase storage capacity. Surface area is the key and engineers discovered that powdery, activated charcoal vastly increased the capacity of the storage plates. Dielectrics were also reduced to ultra-thin layers of carbon, paper or plastic, since the closer the plates can be brought together, the more intense the charge. Since then they have begun experimenting with graphene and other advanced materials that may be able to increase surface area by orders of magnitude. All of this means that much more electricity can be stored in a much smaller space.
But the problem of low energy density remains. Even supercapacitors can only operate at about 2.5 volts, which means they must be strung together in series in vast numbers in order to reach voltage levels required to power something like an electric car. This creates problems in maintaining voltage balance. Still, some supercapacitors are already being employed in gas-electric hybrids and electric buses in order to store the power siphoned off from braking.
Researchers in the field now see some possibility for convergence. Most exhilarating is the idea that the frame of the car itself could be transformed into a supercapacitor. Last month, researchers from Vanderbilt University published an online paper entitled, “A Multifunctional Load-Bearing Solid-State Supercapacitor,” in which they suggested that load-bearing materials such as the chassis of a car or even the walls of your house could be transformed into supercapacitors to store massive amounts of electricity on-site. Combined with advances in evening the flow of electrons from supercapacitors, this opens up whole new avenues of approach to the electric car.
All of these developments are a long way off, of course. Still, supercapacitors support the possibility of pulling out of your driveway in the morning and returning at night in your EV without needing to gas up with foreign oil at your nearest filling station.