The January 2024 issue of IEEE Spectrum is here!
Graphene and other 2D materials yield truly microscopic-size energy storage Double Layer Capacitors
Researchers have made a micrometer-scale supercapacitor by using flakes of graphene and molybdenum disulfide as the electrodes [pictured], which they covered with a gel electrolyte.
As smart devices, wearable sensors, IoT technologies, and implantable electronics shrink in size, so too should the energy-storage devices they rely on. Supercapacitors—with high capacity for energy storage but also a capability to handle rapid charge-discharge cycles that would founder a conventional chemical battery—have in recent years shrunk to “micro” size. Now researchers in India report the tiniest microsupercapacitor yet, using the two-dimensional materials graphene and molybdenum disulfide (MoS2).
This newest microsupercapacitor in fact lives up to its prefix, says Abha Misra, a professor of instrumentation and applied physics at the Indian Institute of Science, in Bengaluru. While previous-generation devices, despite their micrometer-size billing, measure closer to the millimeter scale, “we’ve achieved three orders of magnitude reduction in dimensions,” she says. “We’ve really gone to the micrometer scale.”
Supercapacitors are a hybrid between a battery and a capacitor. Capacitors store energy by accumulating charge on two conductive surfaces separated by a thin insulating material. Batteries, meanwhile, convert chemical energy to electrical through electrochemical reactions.
Like batteries, supercapacitors are made of two electrodes—usually made of a carbon material in a supercapacitor—impregnated with a liquid electrolyte that allows ions to flow through. When voltage is applied, ions from the electrolyte move onto oppositely charged electrode surfaces. Charge builds up at the interface between the electrode and the electrolyte, forming what’s called an “electric double-layer.” This lets them store and deliver large amounts of energy quickly. Besides their quick charging and high power, they can also last much longer than batteries.
To make tiny supercapacitors for small electronics and sensors, researchers have used graphene in various forms for the electrodes. “People usually make a graphene ink and spray-coat it on electrodes,” Misra says. This process typically yields small millimeter-scale supercapacitors. But the device’s features are often hard to control, while the spray-on application of the graphene produces a random structure, which limits capacitance, she says.
Misra and her colleagues made their ultramicro-supercapacitor with multilayer electrodes. Each electrode is composed of a few flakes of 2D MoS2 topped with a few flakes of graphene. Once the researchers make the layered electrodes on a silicon dioxide substrate, they cover them with a gel electrolyte. The resulting device has a capacitance of 1.8 millifarad per square centimeter (mF/cm2).
The advantage of the 2D materials is that they are semiconductors, she says. So each electrode is essentially a field-effect transistor. When the researchers apply a gate voltage from under the silicon dioxide, electrons are drawn to the surfaces of the materials. This draws ions into the space between the MoS2 and graphene sheets. So now, she explains, an electric double-layer is formed not only on the electrode-electrolyte interface, it also forms between the electrode layers. This makes the capacitance shoot up 30 times, to 54 mF/cm2.
Others have reported higher capacitance values for supercapacitor devices. But for its true micrometer size, the new device shows an exceptionally high capacitance, says Misra. Plus, the devices provide the option to more easily integrate with electronic chips given their use of a gel electrolyte instead of a liquid, she says.
The team now plans to make devices using other 2D materials to see if they can boost capacitance further. They reported their discovery in a recent issue of the journal ACS Energy Letters.
Electric Double Layer Capacitor Prachi Patel is a freelance journalist based in Pittsburgh. She writes about energy, biotechnology, materials science, nanotechnology, and computing.