It Pays to Be a Cheapskate
Making capacitors from throw-away plastic
Steve Lipka describes the cool ways people on the other side of the globe are using capacitors, tiny battery-like devices that give quick bursts of energy, my first thought is “same old story. The U.S. lags behind the rest of the world again.” Unfortunately, in talking to scientists about alternative energy, I’ve heard this refrain more than a dozen times. He tells me predictably that from camera phones to BMWs to cargo cranes, Europe and Asia lead the world in using capacitors to improve technology and save energy.
But the good news is that Lipka, and by extension the University of Kentucky’s Center for Applied Energy Research (CAER), earned a prominent place on the global stage thanks to a research initiative of one of the world’s leading energy companies, E.ON. In 2007 when E.ON, based in Germany, called for the best energy storage ideas, Lipka competed with scientists from more than 50 universities and institutes from 11 countries. Of the 10 projects chosen, Lipka’s was the only one from the United States, and he netted nearly $1.2 million to expand his research on carbon capacitors.
What set Lipka apart? He’s a cheapskate. He wants to use cheap sources of carbon (like rayon, coal byproducts and plastic bottles) to build the next generation of capacitors for an array of applications, including storing energy from the sun and wind. So here’s what I want to know: Why are capacitors better than batteries? What are capacitors good for? And how do you make a “green” capacitor?
Capacitors vs. Batteries
“We need to talk a little bit about capacitors versus batteries,” Lipka says, leaning against a table in his windowless, second-floor lab at CAER where he’s worked as a senior research associate since 2003. Capacitors and batteries both store ions, negatively or positively charged molecules, he says, “but capacitors beat batteries handsdown when you talk about high-power density—rapid charge and discharge. Capacitors release energy in a quick burst. Batteries, on the other hand, have high energy density—can store a lot of energy and give sustained power over a long period of time—but they’re not good at giving high-power bursts.”
Why? “It all has to do with kinetics—the way the ions move,” he explains. “Batteries store energy chemically. It’s less efficient than electrostatic storage—the way capacitors do it—because the phase changes that battery materials go through happen slowly.
“The life of a battery is very dependent on the way you charge and discharge it. For example, if you’re unlucky you might get three years out of the lead-acid battery in your car. Lucky, five to 10. Now you say, ‘That’s a pretty good length of time.’ The battery works just fine because you are discharging only a small fraction of the total energy it holds. But if you took that brand-new battery, removed all of the energy, then put all of the energy back in, you’d only be able to do that about 500 times; then it would die.
“If you want energy for a sustained period of time, batteries are the way to go. If you want to have energy delivered for a short burst period of time, capacitors are your choice.”
So how do they work? “Both capacitors and batteries have two electrodes in a liquid solution called an electrolyte. One of the electrodes is charged positively and the other negatively, so the positive ions in the solution go to the negatively charged electrode and vice versa.”
Just like AAA- through D-cell batteries, capacitors come in a range of sizes and two basic shapes. The first, Lipka tells me, looks like a jelly roll. The electrode plates are rolled up with a layer of electrolyte. The second, like the lead-acid battery in your car, is “prismatic,” a flapjack-like stack of electrodes and electrolytes.
“I work on asymmetric carbon-based electrochemical capacitors,” Lipka says, expertly navigating that tongue twister.
“People are fascinated by electrochemical capacitors because of their ability to store large amounts of charge,” he says, adding that the amount of charge a capacitor stores is proportional to the surface area. “The theory is, the greater the surface area, the more charge I can store.”
He tells me that the reason carbon makes such good capacitors is that with minimal effort (say, a little steam or an added chemical plus heat) you can create tiny pores, into which ions can fit. He explains that you don’t want pores that are too big or the ions—the charge—isn’t stored efficiently. The same goes with pores that are too small (less than a nanometer).
“Some years ago in Russia, scientists discovered that instead of using two carbon-based electrodes, if you replaced one of those with a battery-type electrode—a metal oxide that uses chemical storage of energy—you could increase the voltage of the capacitor. ‘Asymmetrics’ give you the best of both worlds,” Lipka explains, bringing us back to the first word in the capacitor tongue twister. He says an aqueous carbon-carbon electrode capacitor (with a water-based electrolyte) yields about 1 volt. If you use a lead dioxide-carbon electrode combo, you get 2.1 volts. “We’re trying to find clever ways to increase the energy density of capacitors—we’re shooting for 3.5 volts or more.” There’s a twinkle in his blue eyes as he tells me, “It’s not rocket science. We try to keep things as simple as we can.
“The E.ON grant is funding our work on unique material chemistries no one else has looked at before. One of our asymmetric options is part high-surface-area carbon and part lithium-ion battery.” Lithium-ion batteries (the hottest battery technology out there) are much smaller and six times more powerful than a lead-acid battery.
Lipka is testing these new chemical combinations on a small scale in his lab. (See “Green to Go” to learn how he makes a capacitor from rayon fiber.) By building his tiny test capacitors, Lipka is leading the way to the next generation of products for tomorrow’s hybrid cars and a green-energy-based electric utility grid.
What Are They Good For?
You’d have to be living under a rock not to have noticed the rise of hybrid cars. Toyota’s premier hybrid, the Prius, is what Lipka calls a “full” hybrid. “It’s got an internal combustion engine, an electric motor, and batteries that can be charged and discharged to run that motor.”
But I learn there are other types of hybrids, which use the power of capacitors. “Micro-hybrids are getting attention now in Europe.” (Lipka defines a micro-hybrid as a vehicle in which the electric components do not
contribute to the propulsion drivetrain, but instead act as a generator.) For those who don’t want to trade horsepower for better gas mileage, BMW is working on a micro-hybrid that conserves fuel by coupling a regular internal combustion engine with a large starter motor that’s powered by an electrochemical capacitor.
A system called “auto stop-start” shuts off the engine automatically whenever the car comes to rest and is taken out of gear—for example, at traffic lights—and restarts it the second you hit the clutch pedal or accelerator, saving fuel and reducing emissions.
Lipka says a British study has shown that if every New York City taxicab were replaced with an auto stop-start vehicle, we’d save 10.8 million gallons of gas per year and 105,000 tons of CO2 emissions. “All of the new 1, 3 and 5 series BMWs, as well as the Mini Cooper, are going to use this auto stop-start system, with capacitors supplied by a San Diego company.”
When I ask how the capacitors get their charge, Lipka smiles and says, “That’s the cool part. They call it ‘BERS’—Braking Energy Regeneration System. The energy that you use for braking is recovered and charges up the capacitor again so it can be used the next time the motor needs to be fired up.
“In the Prius, braking charges up the batteries, but the capture efficiency is very low. Batteries can’t capture and store all of that energy because it’s chemical storage of energy versus electrostatic storage.”
Not only are capacitors vastly more efficient at storing this recovered energy, they trump batteries in terms of lifetime. “You could put a capacitor in a vehicle, charge it to its full capability and discharge it fully, and it’ll outlive any car. “And this is the feature that’d be most beneficial in using large capacitor banks to store energy on an electric utility grid. You could have a system that would last 30, 40 or more years.”
Lipka reminds me that our outdated power grid system doesn’t allow us to tap into potential green energy sources. “We’d love to use green, clean energy like solar and wind power, but when you’re at the mercy of Mother Nature, you can’t rely on those sources to produce continual power.” But what if we could harvest those sources, store that energy, and deliver it when there are power outages?
Lipka gives an interesting stat: “Nearly 80 percent of power outages are of 5-second duration or less. You don’t need expensive batteries to give power in those instances.” Quick-burst capacitors would be perfect to cover those outages. They could also provide what Lipka calls “ride-through” power—sustained electricity for critical industries where even a momentary outage can wreak havoc on equipment or shut down an entire assembly line.
Lipka also says great potential lies in linking up batteries and capacitors, and that potential is evidenced by a project he’s working on with funding from the Office of Naval Research. Naval personnel and Marines use a pulse-based communication system. “When you talk or you send or receive information, the system transmits a high-power pulse—and right now a battery sends that pulse.” Like Lipka has explained, and I’m beginning to understand, the chemical energy storage in batteries isn’t geared to quick power bursts, so repeated pulses make the batteries die prematurely. “What we want to do is shrink the size of that battery by coupling it to a capacitor. The capacitor will handle the pulses, and the battery will live a much longer and productive life.”
Green to Go
Both the Office of Naval Research and E.ON projects require Lipka to make the most “energy-dense” carbons he can. He says that means packing as many farads (the measurement of electric charge stored) per gram into his capacitors as possible. “The best commercial carbon materials right now are on the order of 100 farads per gram,” he says, and with a wry smile, continues, “Like everybody else out there, our goal is to blow that out of the water. But the key to our approach is that we want to create carbons based on low-cost starting materials and with low-cost processing methods.”
Lipka’s covered a number of cheap materials so far. He’s made carbon from rayon fiber. “The biggest market for rayon is, of course, clothing. We’ve linked up with the world’s largest producer of rayon fiber, a company in Austria.” Rayon, which is made from cellulose (the structural part of the cell wall in plants), is “green.” Well, actually it’s white. In fact, as he pulls a large bag of rayon out from under the table in his lab and gives me a handful, it looks a lot like wool and feels soft and fuzzy.
Lipka takes rayon through a couple of processing steps to convert it to a porous carbon and then into electrodes. The first steps involve heat. With a handful of rayon, Lipka says, “If we tried to heat up this fuzzy
stuff, it would melt. So first we go through what we call the stabilization/oxidation step at a relatively low temperature—190 to 230 degrees Celsius—that stabilizes the material in an oxygen-rich atmosphere. It starts to turn black at that point.
“The next step is carbonization. The high temperature, around 850ºC, strips away some of the non-carbon atoms and reorients the carbon atoms, a process which improves the conductivity. Then comes the activation step—by exposing the rayon to CO2 or steam at high temperature, we make high-surface area carbon with lots of tiny pores.”
He hands me a baggy filled with pitch-black rayon. Funny, it still looks fuzzy. When I point this out, Lipka says, “Yeah, it retains its fiber form. So we just chop it up, grind it into a powder, add a small amount of conductive graphite, and mix in 3 percent Teflon powder.”
Lipka says the Teflon forms tiny fibrils, like a spider’s web, that hold the material together so he can roll it out in a very thin sheet. (It’s like a much thinner version of aluminum foil that’s matte black.) Then with a tool, not unlike a paper hole punch, he pops out 10 millimeter discs. These tiny black discs, coupled with “current collectors” made from a carbon-filled polymer, and an insulating ring create one electrode.
Lipka says to make a working electrochemical capacitor you’d just repeat this process to make another electrode and then put a separator between them. The separator, made from a tiny disc of shiny white material, also punched out of a big sheet, serves three purposes. It holds the electrolyte (tiny pores soak up the electrolyte like a sponge), provides ionic conduction (allows the free movement of ions between electrodes), and prevents the electrodes from shorting out.
Lipka’s colleague, carbon materials scientist Chris Swartz, sandwiches the electrodes between two larger metal discs welded to wire leads, attaches these leads to an electronic box linked to a computer, and runs a series of tests. This is how Lipka’s team screens materials to determine which cheap sources offer the best power performance. “The rayon material is our best option so far, interestingly,” Lipka says, adding that he’s filed a number of patents on this process and is working on ways to further boost the energy density.
“We’ve achieved 143 farads per gram with activated rayon fiber.” He says that two other labs have achieved 150 farads per gram with carbons produced with exotic metals and a much more expensive activation process.
Lipka’s also turned coal, both bituminous (Kentucky coal) and anthracite (Pennsylvania coal), into a porous carbon for electrodes. And, maybe most exciting of all if you’re worried about all of those plastic drink bottles clogging our landfills, Lipka’s got undergrad students grinding up PET bottles (the most recycled type of plastic bottle) and converting them to high-surface area carbon in a simple two-step process. “We tested the energy storage of these materials in our capacitors and found they hold almost 100 farads per gram. Think of how many of these bottles are out there! It’s a two-hour process to take them from bottle form to activated carbon. That’s really quick compared to how most people are making carbons now—it takes a good part of a day.”
And he’s just starting to work with lignin—the tough stuff that holds up the cell wall of a plant. “This is the stuff the newspaper printers try to take out because it yellows the paper,” Lipka says. “It’s a waste material. Do you think they would pay me to haul it away so I can convert it to useful carbons to store energy? Maybe.
“People are making carbons out of rice hulls, peach pits, coffee beans, all kinds of crazy stuff. There are a lot of carbons out there that are used for other applications that can be tweaked to store charge. I’m just looking for the cheapest and most efficient.”
For Lipka, it’s a world filled with energy-storage potential—he just needs to figure out how to cheaply tap it. More power to him.
Steve Lipka, a senior research associate at the Center for Applied Energy Research, is creating electrochemical capacitors—tiny battery-like devices that give quick bursts of energy—from cheap sources of carbon like plastic bottles. Lipka’s team grinds up PET bottles, the most recycled type of plastic, and turns them into capacitors.
6 Cool Things
to Do with Capacitors
Run car audio systems
One of the first widespread applications for capacitors, automotive aftermarket sound systems use these compact
power sources to run subwoofers that provide thumping base.
Pitch wind turbine blades
As the wind blows in different directions, the wind turbine blade adjusts its pitch to run more efficiently. These pitch
adjustments used to be controlled by lead-acid batteries, but now are performed by banks of highly efficient electrochemical capacitors.
Power LED camera flash
An Australian company is making a capacitor (slightly longer and thinner than a quarter) that fits inside a cell phone to power a tiny LED flash. The better illumination provided by this flash creates higher quality photos with the phone's built-in camera.
Save fuel for shipyard cargo cranes
Rubber tire gantry cranes—the rolling behemoths that raise containers from cargo ships and lower them onto railcars
or tractor trailers—consume a lot of diesel. A Japanese company designed a hybrid crane that uses the kinetic energy of the lowering motion to charge a capacitor to lift the next load. These cranes use half the fuel and have much
lower emissions than traditional cranes.
Save gas with auto stop-start
BMW is leading the world with a hybrid vehicle that couples a regular internal combustion engine with a large starter
motor powered by a capacitor. The auto stop-start system shuts off the engine automatically whenever the car comes
to rest—for example, at traffic lights—and restarts it the second you hit the accelerator, saving fuel and reducing emissions.
Back-up electric power grid
Nearly 80 percent of all power outages are 5 seconds or less in duration. Capacitor banks, which could store wind or solar power, could deliver electricity to efficiently cover these outages. Capacitors can also be used for voltage
stabilization (supplement in times of heavy usage).
CAER carbon materials scientist Chris Swartz starts with current collectors (1) made from a carbon-filled polymer and the rayon-based, activated carbon discs (2). He presses a carbon disc onto the current collector (3) to create one electrode. Then Swartz adds the separator (4).
He injects this tiny white disc with the liquid electrolyte (5), then repeats the process to make another electrode and stacks them to create a capacitor (6). He then sandwiches the capacitor between two larger metal discs welded to wire leads (7), attaches these leads to an electronic box (a potentiostat) linked to a computer (8). By running a series of tests, Steve Lipka’s team can screen materials to determine which offer the best power performance.
CAER’s Illayathambi “Victor” Kunadian (right), a postdoc who earned his Ph.D. in mechanical engineering from UK in 2008, demonstrates how to trim the material for a capacitor to Ifedi Anyaegbunam, a senior from Lafayette High School. Nearly a decade ago, CAER helped start a pre-engineering program at the Lexington high school as the educational outreach component of a National Science Foundation grant. Anyaegbunam is one of two Lafayette students working with Steve Lipka’s electrochemistry team this year.