BOSTON—The next electric car you buy might have a little extra zip. That’s because researchers have developed a new electric storage material that’s among the best at holding large amounts of charge as well as charging and discharging in just seconds, they report this week. Moreover, because the starting materials for making it are commercially available and relatively cheap, it may prove more useful than higher performance—yet more exotic—materials currently under development. That could eventually allow automakers to build faster charging electric cars with a longer driving range than any on the road today.
The new material, called a covalent organic framework (COF), is a highly porous crystal. It’s used to store electricity in the heart of devices called supercapacitors, which are widely used in everything from cars to computers. In their simplest form, supercapacitors consist simply of two metal electrodes separated by a conducting liquid, or electrolyte. To charge the device, you apply a voltage between the two electrodes. That causes oppositely charged ions to snuggle up to the surface of the electrodes, where they remain even after the voltage is turned off. When the supercapacitor is discharging, electrons flow from the negatively charged electrode to the positive one, doing work along the way.
Because the transfer of electric charges happens so fast, supercapacitors can be charged and discharged in seconds, compared with the hours it takes for batteries. That’s made them ideal for applications such as regenerative braking systems in electric cars, which use the energy in braking to generate an electric current that is stored instantly.
The trouble is that the storage capacity of supercapacitors is limited by the surface area of the electrodes, which is far less than the volume-based storage of a battery. Not surprisingly, companies have sought to increase the surface area of their electrodes by making them out of porous, conductive materials like activated carbons, which now dominate the market. Of course, they are always looking to do better.
One solution is materials with very high surface areas, such as carbon nanotubes and graphene. Both of these are made from single layers of carbon atoms, and have been used to make the highest capacity supercapacitors to date. But the materials themselves remain expensive and relatively difficult to produce in the volumes that would be needed for large-scale applications. Another electrode-building material is redox-active molecules, which readily absorb electrons and later give them back up. But redox-active materials have their own challenges. Some fall apart after electrons cycle on and off a few times, and others aren’t porous enough for making good supercapacitors.
William Dichtel, a chemist at Cornell University, showed 2 years ago that COFs can do better. Dichtel and colleagues reported making the first-ever redox-active COF, assembled from organic building blocks 2,6-diaminoanthraquinone (DAAQ) and 1,3,5-triformylphluroglucinol (TFP). Under the right conditions, Dichtel’s team found that DAAQ and TFP spontaneously assemble themselves into large hexagonal rings with single holes in the center. What’s more, the hexagons link together like sheets of tiles on a bathroom floor. Additional sheets form on top of the first with all the holes lining up. Ultimately, the material becomes a regular crystal of tiled and stacked hexagons shot through with tiny pores, giving them a surface area similar to activated carbons.
But because redox-active COFs have the ability to absorb electrons as well, they have the potential to make better supercapacitor electrodes. Earlier this year, the researchers reported that when they grew their material as thin sheets atop a gold electrode, the COF had a capacity of about 160 farads per gram (F/g) of material. That wasn’t yet as good as the best commercial supercapacitors. The problem was the COFs themselves weren’t very conductive, even though they could charge and discharge quickly and hold an impressive 12 electrons per hexagonal tile. The lack of conductivity meant that the electrons in the upper portion of any COF more than 200 nanometers thick wouldn’t be able to make it to the electrode. “There was no way to get the charges out of the thicker films,” Dichtel says.
Until now. At the meeting of the American Chemical Society here this week, Dichtel reported that he and his team got over their size hurdle by coating their thick DAAQ-TFT COFs with a thin layer of the conducting polymer poly 3,4-ethylenedioxythiophene, known as PEDOT. The result was that all the stored charges could zip through the PEDOT into the underlying gold electrode, giving them a capacitance of 350 F/g, higher than any supercapacitor on the market today. That’s still well below the 3300 F/g numbers reported for a carbon nanotube–based device. But because the organic building blocks are readily available, new COF-based supercapacitors could have an easier path to market.
“It’s neat stuff,” says George Whitesides, a chemist at Harvard University. Whitesides cautions that it’s still early days for COF-based supercapacitors, as they must be proven robust enough to handle automotive applications, among others. But Dichtel notes that his materials have already withstood thousands of charge-discharge cycles without showing any signs of degradation. As well, Dichtel says, there are lots of other redox active molecules that can be used to make COFs, so there’s hope for doing even better. “We are just at the very beginning of this,” he says. Already, they’re doing pretty well.