Many people do not give enough thought to capacitors. For that matter, most people don’t think about capacitors at all, or even know whether they’re animal, vegetable or mineral. And that’s a shame, because capacitors and, in particular, supercapacitors (just think of them as capacitors on steroids), have an essential role in the quest for net zero global carbon emissions.
Capacitors are devices that store electrical energy, like a familiar electrochemical battery — but without dependence on environmentally problematic materials like lead, lithium, cobalt and nickel. Then, too, they charge and discharge more quickly than batteries and offer vastly more cycles before they wear out. It’s not all wine and roses, though: capacitors have much lower energy density than lithium-ion batteries, so for a given size they hold less energy. And the stored energy dissipates more quickly, so they are best used in applications requiring only short-term storage.
But there are plenty of those. Unless you live in a yurt off the grid you’re surrounded by capacitors, as virtually all electronic devices require them. Standard capacitors are effectively commodities, manufactured by the gazillions and selling for pennies a piece. Supercapacitors, more complicated devices that offer higher energy density and more rapid charge-discharge rates, are still relatively expensive. But prices are dropping as the technology is tweaked and production ramps up.
“Everybody has grand goals for electric vehicles, and there’s a huge requirement for capacitance for that technology alone,” notes Dennis Zogbi, an industry analyst specializing in passive electronic components, i.e., capacitors, resistors, diodes and the like. He sees companies scrambling to hire engineers with capacitor expertise who have spent years overshadowed by sexier semiconductor and microprocessor development. “All these guys who were old and in the way are in demand now. Suddenly those are the guys who can build the voltage.”
Farads and Picofarads
Time out for a no-tears science lesson. Capacitors harness the same force that makes a balloon cling to the wall after you rub it on your sweater; unlike batteries, which store energy electrochemically, capacitors do it electrostatically. Michael Faraday, the British polymath who pioneered the science of electricity in the early 19th century, is credited with inventing capacitors, and in his honor the charge a capacitor can hold is measured in farads. But Faraday stood on the shoulders of others: German and Dutch inventors first built glass capacitors in the 1740s, which were known as Leyden Jars. Benjamin Franklin demonstrated that a flat piece of glass could substitute for an entire jar; indeed, the first flat capacitor was called the Franklin Square.
All capacitors consist of two metal plates, or conductors, one connected to a power source, the other to ground, with an insulating material, or dialectric, between them. The dialectric can be made of most any non-conducting material — think paper, glass, rubber, plastic, even soap and detergents . The charge in the capacitor builds up when electrons flow to the first plate and are hindered from flowing immediately to the second one by the dialectric. The discharge occurs when the plates can hold no more electrons and are provided an exit path — like when you push the shutter button on your camera and the flash fires.
Ceramic capacitors, which look like colored pills with two wires attached, are the most common, used by the score in everything from radios to microwave ovens. Electrolytic capacitors, which look like small tin cans and are most often used to filter out noise and damp voltage ripples in the current supplied to electronic devices, are almost as ubiquitous.
Supercapacitors were invented by General Electric in 1957, but were really only industrialized in the last 20 years. In a normal capacitor the distance between plates is from 10 to 100 microns (millionths of a meter), while in a supercapacitor that distance is just one-thousandth of a micron. And that smaller distance permits the creation of a larger electric field, so more energy storage. In addition, the carbon-coated plates on supercapacitors increase the available surface area for storage capacity by up to 100,000 times. Supercapacitors can store and discharge energy very rapidly, and are already used in applications from wind farms to electric light rail.
A few automakers, among them Mazda and PSA Peugeot-Citroen, have used supercapacitors in gasoline-powered cars, but there will be a much larger role for them in electric vehicles. Supercapacitors can’t replace lithium-ion batteries in EVs. But they can augment them, capturing energy through regenerative breaking and providing the rocket-like acceleration that EV owners covet. To run accessories like air conditioning, most EVs still carry a conventional 12-volt lead-acid battery, which will likely be replaced by supercapacitors.
A variant on the standard supercapacitor, the double-layer carbon supercapacitor, was first made by Maxwell Technologies. “The cool thing about double-layer carbon is its organic,” explains Zogbi. “We can convert organic material — macadamia nuts, peach pits — into dialectrics. We can take refuse from one supply chain and turn it into an energy device.”
On and Off the Grid
Will the EV makers make use of supercapacitors in their vehicles? Elon Musk, for one, has been uncharacteristically unforthcoming on the subject. Today, the largest market for supercapacitors is not in automobiles at all, but in electricity storage connected to the grid — a space Tesla also plays in with its Powerwall system, which uses lithium-ion batteries to store energy from residential solar installations.
“Supercapacitors are not quite ready for the automotive world,” explained Todd Pistorese, founder and chief technology officer of SuperCap Energy, which sells supercapacitor storage systems, primarily to utilities and telecoms. Compared to lithium-ion batteries, “there’s no thermal runaway and much longer cycle life,” he said. “They’re pretty bullet proof.”
SuperCap claims 100 times more charge-discharge cycles than lithium-ion batteries and 500 times more than lead-acid, no degradation over time, and far more rapid charging. Its supercapacitors can be used for off-grid cellular towers, which commonly employ lead-acid batteries that must be replaced every three to five years; FAA weather cameras, which also must operate off the grid; and solar and wind installations. “The sweet spot for supercapacitors is lots of cycles, full charge and full discharge every day,” said Pistorese.
The supercapacitors in SuperCap’s systems are 94 percent graphene, a structural variant of carbon that is the strongest, thinnest and most conductive material known to man. But the company doesn’t manufacture supercapacitors itself; it buys them from a number of vendors, who in turn buy them from China — by far the largest producer. “China has the corner on graphene,” said Pistorese.
Changing the Curve
So will supercapacitors provide China with a hammerlock on yet another keystone technology? Maybe not. Skeleton Technologies, an Estonian company with manufacturing in Germany, has developed a proprietary material it calls curved graphene that has a three-dimensional shape offering more energy-absorbing surface area than conventional flat graphene. It’s a bit of a contradiction in terms, because graphene is by definition effectively two-dimensional, just one atom thick. Best to visualize curved graphene as like a flat piece of paper that has been crumpled.
Skeleton claims the material takes up less space, weighs less, costs less and is simpler to make than graphene. The company’s German production facility, already the largest supercapacitor factory in Europe, has begun shipping curved graphene supercapacitors, with volume sales expected next year.
“We are based on material innovation,” said Sebastian Pohlmann,” Skeleton’s vice president of automotive and business development. “For the last 20 years, everybody was working with the same materials. Now we are on the market with new materials and better performance. We can actually replace the standard carbon materials with our curved graphene with no price increase.”
The advantages don’t end there: Curved graphene uses no rare earths (virtually a Chinese monopoly) and can be recycled more readily than graphene.
Skeleton’s conventional supercapacitors are currently used all over the place — in light rail, hydrogen fuel cell vehicles and electricity grid storage, even in experimental fusion reactors. Actually, Skeleton prefers the term “ultracapacitor,” but that’s a distinction without a difference.
In electric vehicles, Pohlmann said that supercapacitors could help make energy storage smaller, lighter and safer, and curved graphene ultracapacitors even more so because they are already much smaller and carry more punch in a smaller form factor. Supercapacitors still cost more than lithium-ion batteries measured in terms of storage density, but the cost is likely to come down rapidly as production increases.
“Supercapacitors will not reach the same price as lithium-ion, but they don’t have to,” Pohlmann says. “They just have to become a bit cheaper than they are now. They’re not chosen on price per kilowatt-hour as are batteries, but price per solution. We are now at a similar place to where lithium was in [the] early 2000s,” he suggests. And “unlike lithium batteries in [the] early 2000s, people actually believe it.”