A new type of ultra-tiny supercapacitor demonstrates remarkable energy storage capabilities and a potential revolution in device power supplies. Researchers have developed an ultra-tiny supercapacitor that exceeds all currently commercially available models in terms of storage capacity and compactness. Its design combines field-effect transistors with layers of molybdenum disulfide and graphene to increase capacitance by an impressive 3,000% under certain conditions.
Researchers from the Department of Instrumental and Applied Physics (IAP) of the Indian Institute of Science (IISc) have designed a new type of ultra-micro supercapacitor, a tiny device capable of storing large amounts of electric charge. It is smaller and more compact than existing supercapacitors and could be used in devices ranging from street lights to consumer electronics, electric vehicles and medical equipment.
Currently, most of these devices are battery powered. However, over time, these batteries lose their ability to store charge and therefore have a limited shelf life. Capacitors, by virtue of their design, can store charge for longer periods of time. For example, a capacitor operating at 5 volts will still be operating at the same voltage ten years later. But unlike batteries, supercapacitors cannot be continuously discharged, such as to power a cell phone.
Supercapacitors, on the other hand, combine the advantages of batteries and capacitors and can both store and release large amounts of energy, making them popular in next-generation electronic devices.
In the study, recently published in ACSE Energy Letters, the researchers created their supercapacitor using field-effect transistors (FETs) as charge collectors instead of the metal electrodes used in existing capacitors. "Using field-effect transistors as electrodes in supercapacitors is a new way to tune the charge of the capacitor," said Abha Misra, a professor at IAP and corresponding author of the study.
Innovations in Capacitor Design
Current capacitors typically use metal oxide-based electrodes, but they are limited by low electron mobility. So Misra and her team decided to create hybrid field-effect transistors made of alternating a few-atom-thick layers of molybdenum disulfide (MoS2) and graphene to increase electron mobility, then connected with gold contacts. A solid gel electrolyte is used between two FET electrodes to build a solid-state supercapacitor. The entire structure is built on a silica/silicon substrate.
"Design is the key part because you are integrating two systems, which are two field-effect transistor electrodes and a gel electrolyte (an ionic medium), which have different charge capacities," Misra said. Vinod Panwar, one of the study's lead authors and a doctoral student at IAP, added that fabricating such a device to get all the desirable properties of a transistor is challenging. Because these supercapacitors are so small, they cannot be seen without a microscope, and the fabrication process requires high precision and hand-eye coordination.
Performance and future plans
After the supercapacitor was created, the researchers measured the device's electrochemical capacitance, or charge-holding ability, by applying various voltages. They found that capacity increased by 3,000% under certain conditions. In comparison, a capacitor containing only MoS2 without graphene only increased its capacity by 18% under the same conditions.
In the future, the researchers plan to explore whether replacing MoS2 with other materials can further improve the storage capacity of supercapacitors. They added that their supercapacitor is fully functional and can be used in energy storage devices such as electric vehicle batteries or in any miniaturized system through on-chip integration. They also plan to patent the supercapacitor.