Hang around the energy storage crowd long enough, and you’ll hear chatter about ultracapacitors. Tesla Motors chief executive Elon Musk has said he believes capacitors will even “supercede” batteries.
What is it that makes ultracapacitors such a promising technology? And if ultracapacitors are so great, why have they lost out to batteries, so far, as the energy storage device of choice for applications like electric cars and the power grid?
Put simply, ultracapacitors are some of the best devices around for delivering a quick surge of power. Because an ultracapacitor stores energy in an electric field, rather than in a chemical reaction, it can survive hundreds of thousands more charge and discharge cycles than a battery can.
A more thorough answer, however, looks at how ultracapacitors compare to capacitors and batteries. From there we’ll walk through some of the inherent strengths and weaknesses of ultracaps, how they can enhance (rather than compete with) batteries, and what the opportunities are to advance ultracapacitor technology.
A basic capacitor consists of two metal plates, or conductors (typically aluminum), separated by an insulator, such as air or a film made of plastic, or ceramic. During charging, electrons accumulate on one conductor, and depart from the other. In effect, a negative charge builds on one side while a positive charge builds on the other.
The negatively charged electrons want to join the depleted (positive) side, but can’t cross over that non-conductive insulator (for the most part, anyway—there is some leakage). This separation of positive and negative charges, which want to balance out, or neutralize, each other, creates what’s called an electric field. Discharging occurs when the electrons are given a path to flow to the other side—in other words, when balance is restored.
Putting the “ultra” in ultracapacitors
Ultracapacitors also have two metal plates, but they are coated with a sponge-like, porous material known as activated carbon. And they’re immersed in an electrolyte made of positive and negative ions dissolved in a solvent. One carbon-coated plate, or electrode, is positive, and the other is negative. During charging, ions from the electrolyte accumulate on the surface of each carbon-coated plate.
Like capacitors, ultracapacitors store energy in an electric field, which is created between two oppositely charged particles when they are separated. Recall that in an ultracapacitor, we have this electrolyte, in which an equal number of positive and negative ions are uniformly dispersed. And remember that in a capacitor, negative charge builds on one side and positive charge builds on the other. Similarly, in an ultracapacitor, when voltage is applied across the two metal plates (i.e. during charging), a charge still builds on the two electrodes—one positive, one negative. This then causes each electrode to attract ions of the opposite charge.
But for an ultracapacitor, each carbon electrode ends up having two layers of charge coating its surface (thus, ultracaps are also called “double layer capacitors”), John Kassakian, a professor in MIT’s Laboratory for Electromagnetic and Electronic Systems (LEES), explained to me: “In effect, an ultracapacitor is actually two capacitors in series, one at each electrode.”
Joel Schindall, another professor in MIT’s LEES and associate director of the lab, explained that during discharging, the charge on the plates decreases as electrons flow through an external circuit. “The ions are no longer attracted to the plate as strongly,” he said, “so they break off and once again distribute themselves evenly through the electrolyte.”
The ultracap advantage
Unlike capacitors and ultracapacitors, batteries store energy in a chemical reaction. Ions are actually inserted into the atomic structure of an electrode (in an ultracap, the ions simply cling). This is an important distinction, because storing energy without chemical reactions allows ultracapacitors to charge and discharge much faster than batteries, Schindall explained. And because capacitors don’t suffer the wear and tear caused by chemical reactions, they can also last much longer.
Charge separation is at work in both capacitors and ultracapacitors. But in a capacitor, the separated charges can get no closer than the distance between the two metal plates. They’re awfully close together—on the order of tens of microns—but limited by the thickness of that ceramic or paper film in the middle (one micron is one-thousandth of a millimeter). In an ultracapacitor, the distance between the ions and opposite-charged electrode is so tiny it’s measured in nanometers (one-thousandth of a micron).
Why should we care about such small distances? Turns out the size of the electric field is inversely proportional to the separation distance. The shorter distance between those separated charges in an ultracapacitor translates to a larger electric field—and much more energy storage capacity.
That’s only part of why ultracapacitors can store more energy than regular capacitors. The activated carbon is also key. See, it’s “so spongy,” according to Schindall, that it affords a surface area 10,000 to 100,000 times greater than the linear surface area of the naked metal. Put simply, all those nooks and crannies in the surface allow more ions to cling to the electrode.
Surface area makes a huge difference for what’s called capacitance, or the amount of electric charge a device will hold given a certain amount of voltage. Capacitance is the key metric for comparing capacitor performance, and it’s measured in Farads (named, as Lost fans might appreciate, after the chemist and physicist Michael Faraday).
Now, the Farad is such a huge unit of measurement, “it’s like measuring distance in light years,” said Schindall. So it’s much more common to see microfarads (one-millionth of a farad) and even picofarads (one-millionth of a microfarad).
A capacitor the size of a D-cell battery, for example, has a capacitance of only about 20 microfarads. But a similarly sized ultracapacitor has a capacitance of 300 Farads. That means, at the same voltage, the ultracapacitor could in theory store up to 15 million times more energy than the capacitor.
Here is where we run into some of the challenges with ultracapacitors, however. A typical 20-microfarad capacitor would be able to handle as much as 300 volts, while an ultracap would be rated at only 2.7 volts. At a higher voltage, the electrolyte starts to break down. So realistically we’re talking about an ultracapacitor storing about 1,500 times the energy of a comparably sized capacitor, said Schindall.
Ultracaps and batteries as partners
Despite offering a huge leap over regular capacitors, ultracaps still lag behind batteries when it comes to energy storage capacity. Ultracapacitors (which are also more expensive per energy unit than batteries), can store only about 5 percent of the energy of comparable lithium-ion batteries. And that, said Schindall, is a “fatal flaw” for many applications.
It would be technically possible, for example, to use ultracaps instead of lithium-ion batteries in cell phones, with some serious benefits: You would never have to replace the ultracapacitor, said Schindall, and the phone would recharge very quickly. But the phone wouldn’t stay charged for very long at all with today’s ultracapacitors—perhaps as little as 90 minutes, or five hours max, Schindall said.
Ultracapacitors are very effective, however, at accepting or delivering a sudden surge of energy, and that makes them a good partner for lithium-ion batteries, Schindall explained. In an electric car, for example, an ultracapacitor could provide the power needed for acceleration, while a battery provides range and recharges the ultracap between surges.
Think of it this way: The ultracapacitor is like a small bucket with a big spout. Water can flow in or out very fast, but there’s not very much of it. The battery is like a big bucket with a tiny spout. It can hold much more water, but it takes a long time to fill and drain it. The small bucket can provide a brief “power surge” (“lots of water” in this analogy), and then refill gradually from the big bucket, Schindall explained.
Putting ultracaps to work
Already, Schindall believes some electric vehicle manufacturers are using ultracapacitors for acceleration. The devices also appear in hundreds of other applications, from cell phone base stations to alarm clocks (as backup power) to audio systems.
For most music, Schindall explained, a high-end audio system with big speakers might do just fine with a 1-watt amplifier. “But then the kettle drum comes in,” demanding a sudden power surge of 1-kilowatt. One solution, Schindall said, is to build a 1-watt supply, plus an ultracapacitor to handle the peak.
Ultracapacitors hold promise for a similar job on the electric grid. Today, transmission lines operate below full capacity (often somewhere above 90 percent), said Schindall, in order to leave a buffer for power surges. Banks of ultracapacitors could be set up to absorb power surges, enabling transmission lines to run closer to 100 percent capacity.
It might not seem like much, especially considering that it would take warehouse-sized banks for ultracaps to do the job. But installing ultracapacitors to handle the peaks would actually be much cheaper, Schindall said, than adding even 5 percent more capacity with new transmission lines.
In cars, ultracapacitors could play a role in the growing market for “microhybrids,” which cut the engine during idling. In these “start-stop” systems, Schindall explained in an email, “The ultracapacitor would provide power during the stop (lights, radio, air conditioner, etc.).” It would also provide power for the restart, and then be “recharged during the next interval of travel.”
How to build better ultracapacitors
There are two basic ways to improve the performance of ultracapacitors: increase the surface area of the plate coating, and increase the maximum amount of voltage that the device can handle.
Recall old Faraday again. Capacitance, measured in Farads, is how much electric energy our device will hold given a certain voltage. Increase the voltage, and you can increase the amount of energy our device holds (energy is equal to half the capacitance, multiplied by voltage squared).
Schindall is tackling the surface area challenge using carbon nanotubes (more like a shag carpet or paintbrush than the sponge-like activated carbon). Other researchers, he noted, are working with graphene or better activated carbon. In addition to boosting the surface area, carbon nanotubes and graphene can also “withstand a somewhat higher voltage” than activated carbon, said Schindall.
The voltage challenge, meanwhile “seems to be a tougher road,” he said. Researchers are experimenting with ionic liquid electrolytes (all ion, no solvent, behaves like a liquid), which under the right conditions can operate at up to three times the voltage of conventional electrolytes.
But ionic liquids are “fussy,” Schindall said. “They don’t like being liquids,” and tend to freeze below room temperature. They’re also expensive, and they have higher resistance than conventional electrolytes, which means you can’t get energy out as fast. The maximum power—one of ultracaps’ key advantages—is decreased. As Schindall put it, “There’s always a tradeoff.”