Researchers at the California NanoSystems Institute (CNSI) at UCLA have been hotly pursuing the ability to apply graphene to the electrodes of supercapacitors. While their efforts have shown progress—improving energy density for a supercapacitor to almost 40 Watt-hours per kilogram from the industry average for a standard supercapacitor of 28 Wh/kg—it apparently hasn’t provided a big enough boost for supercapacitor manufacturers to walk away from the much cheaper activated carbon.
This has not deterred the team at CNSI from continuing to work with graphene and supercapacitors. In fact, they have recently employed the ubiquitous manganese dioxide used in alkaline batteries to create a hybrid material that they believe should boost the commercial prospects for 2-D materials in supercapacitors.
The story of graphene in supercapacitors can be represented by the old adage: its greatest strength is its greatest weakness. Of course, the name of the game in supercapacitor energy density is surface area. The greater the surface area, the greater number of ions you can store on the electrodes. While graphene has a theoretical surface area of 2630 square meters per gram, this density is only possible with a single, standalone graphene sheet.
But you can’t actually use a standalone sheet for the electrode of a supercapacitor because it will result in a very low volumetric capacitance. To get to a real-world device, you have to stack the sheets on top of each other. When you do this, the surface area is reduced. That’s why the best real-world electrode surface area achieved with graphene on the electrodes of a supercapacitor is around 1520 square meters per gram, or roughly the same that is possible with the crushed coconuts of activated carbon, or used cigarette butts.
So, while the 2-D characteristic of graphene may limit its usable surface area for supercapacitors, it does offer a way to make supercapacitors with small dimensions, something that would be impossible with activated carbon.
It is this strength that the CNSI researchers are aiming to exploit in their supercapacitor, which is small enough to be used as a wearable or implantable device. Their creation, which they describe in the Proceedings of the National Academy of Sciences, is only one-fifth the thickness of a sheet of paper, but the researchers claim it can hold twice as much charge as a typical thin-film lithium ion battery.
“Let’s say you wanted to put a small amount of electrical current into an adhesive bandage for drug release or healing assistance technology,” said Richard Kaner, a professor at UCLA, in a press release. “The microsupercapacitor is so thin you could put it inside the bandage to supply the current. You could also recharge it quickly and use it for a very long time.”
The CNSI team used a particular type of graphene (dubbed laser-scribed graphene, or LSG) that holds an electrical charge for a long time, is highly conductive, and charges very rapidly. They combined the LSG with molybdneum disulfide and then combined that 2-D hybrid with manganese dioxide because it holds a lot of charge and is inexpensive and abundant.
In their tests with the devices, the CNSI researchers were able to charge the supercapacitor with a solar cell and then have it power an LED light throughout the night on that one charge.
“The LSG–manganese-dioxide capacitors can store as much electrical charge as a lead acid battery, yet can be recharged in seconds, and they store about six times the capacity of state-of-the-art commercially available supercapacitors,” Kaner said. “This scalable approach for fabricating compact, reliable, energy-dense supercapacitors shows a great deal of promise in real-world applications, and we’re very excited about the possibilities for greatly improving personal electronics technology in the near future.”
The supercapacitors developed at CNSI appear to be wrestling for the same piece of application real estate as advances made at Rice University. Earlier this year, Rice researchers reported on work in which they used laser-induced graphene, or LIG, to make flexible supercapacitors. Based on the level of interest in this area, I would say they both have it right.