Researchers at the Department of Instrumentation and Applied Physics (IAP), Indian Institute of Science (IISc), have designed a novel ultramicro graphene supercapacitor using gate field induced FET transistor technology, a tiny device capable of storing an enormous amount of electric charge.
It is also much smaller and more compact than existing supercapacitors and can potentially be used in many devices ranging from streetlights to consumer electronics, electric cars and medical devices.
Most of these devices are currently powered by batteries. However, over time, these batteries lose their ability to store charge and therefore have a limited shelf-life. Capacitors, on the other hand, can store electric charge for much longer, by virtue of their design.
For example, a capacitor operating at 5 volts will continue to operate at the same voltage even after a decade. But unlike batteries, they cannot discharge energy constantly—to power a mobile phone, for example.
Supercapacitors, on the other hand, combine the best of both batteries and capacitors—they can store as well as release large amounts of energy, and are therefore highly sought-after for next-generation electronic devices.
In the current study, published in ACS Energy Letters, the researchers fabricated their supercapacitor using Field Effect Transistors or FETs as the charge collectors, instead of the metallic electrodes that are used in existing capacitors. “Using FET as an electrode for supercapacitors is something new for tuning charge in a capacitor,” says Abha Misra, Professor at IAP and corresponding author of the study.
Current capacitors typically use metal oxide-based electrodes, but they are limited by poor electron mobility. Therefore, Misra and her team decided to build hybrid FETs consisting of alternating few-atoms-thick layers of molybdenum disulfide (MoS2) and graphene—to increase electron mobility—which are then connected to gold contacts. A solid gel electrolyte is used between the two FET electrodes to build a solid-state supercapacitor. The entire structure is built on a silicon dioxide/silicon base.
“The design is the critical part, because you are integrating two systems,” says Misra. The two systems are the two FET electrodes and the gel electrolyte, an ionic medium, which have different charge capacities. Vinod Panwar, Ph.D. student at IAP and one of the lead authors, adds that it was challenging to fabricate the device to get all the ideal characteristics of the transistor right. Since these supercapacitors are very small, they cannot be seen without a microscope, and the fabrication process requires high precision and hand-eye coordination.
Once the supercapacitor was fabricated, the researchers measured the electrochemical capacitance or charge-holding capacity of the device by applying various voltages. They found that under certain conditions, the capacitance increased by 3000%. By contrast, a capacitor containing just MoS2 without graphene showed only an 18% enhancement in capacitance under the same conditions.
In the future, the researchers are planning to explore if replacing MoS2 with other materials can increase the capacitance of their supercapacitor even more. They add that their supercapacitor is fully functional and can be deployed in energy-storage devices like electric car batteries or any miniaturized system by on-chip integration. They are also planning to apply for a patent on the supercapacitor.
Abstract
On-chip microscopic energy systems have revolutionized device design for miniaturized energy storage systems. Many atomically thin materials have provided a unique opportunity to develop highly efficient small-scale devices. We report an ultramicro-electrochemical capacitor with two-dimensional (2D) molybdenum disulphide (MoS2) and graphene-based electrodes. Due to the tunable density of states, 2D MoS2 provides electric field-induced doping and, combined with a graphene interface, leads to a high carrier mobility. The fabricated solid-state energy storage device is obtained using a gel electrolyte that provides an electrochemical capacitance of 1.8 mF/cm2. An extraordinary enhancement of ∼3000% in electrochemical capacitance (55 mF/cm2from 1.8 mF/cm2, measured from a cyclic voltammetry curve) is observed upon application of back-gate field of −25 V, which is more than the enhancement (18%) observed in a MoS2 electrochemical capacitor (0.95 mF/cm2 from 0.8 mF/cm2) without graphene, whereas the galvanic charge–discharge measurements analysis shows ∼1677% enhancement under the application of −25 V back-gate voltage. Thus, the electric field-induced doping in 2D MoS2, in addition to a high charge carrier mobility due to the graphene, plays a crucial role in an extraordinary large energy storage in the ultramicro-electrochemical capacitor. We also evaluated the capacitance response using an AC signal superimposed with the DC bias to investigate the influence of polarization potential on the electrolyte. The study provides a benchmark development of an ultramicro-electrochemical capacitor for ultrahigh charge storage capability.
