Researchers at the Indian Institute of Science (IISc) Department of Instrumentation and Applied Physics (IAP) and associates created a new supercapacitor that can be charged by shining light on it. Supercapacitors of this type have multiple applications, such as self-powered electronic devices like sensors and streetlights. The study has been published in the Journal of Materials Chemistry A.
Photo-chargeable supercapacitor showed 3000% rise in capacitance under light compared to the dark. A novel necking behavior is discovered upon illumination. Image Credit: Indian Institute of Science
Electrostatic devices, such as capacitors, store energy as charges between two metal plates, known as electrodes. According to Abha Misra, a Professor at IAP and the study's corresponding author, supercapacitors are enhanced versions of capacitors that use electrochemical phenomena to store more energy.
Zinc oxide (ZnO) nanorods grown directly on transparent fluorine-doped tin oxide (FTO) served as the electrodes for the new supercapacitor. Pankaj Singh Chauhan, the first author and CV Raman postdoctoral fellow in Misra's group at IISc, synthesized it. Since ZnO and FTO are semiconductors with properly aligned energy levels, the photo-rechargeable supercapacitor can perform better. FTO is transparent and light can pass through to the optically active ZnO nanorods, causing the supercapacitor to charge. Chauhan clarifies that the conducting medium between the electrodes was made up of two electrolytes: a liquid and a semi-solid gel.
The distance between the electrodes has an inverse relationship with the capacitance, or ability to store charges.
As the distance becomes very small, the capacitance shoots up,” Misra explains.
Abha Misra, Professor and Study Corresponding Author, Department of Instrumentation and Applied Physics, Indian Institute of Science
It is challenging to keep the distance between electrodes in electrostatic capacitors small. However, in a supercapacitor, the charges on the electrodes draw the oppositely charged ions in the electrolyte, forming a charge layer that is only a few atoms apart and known as an electric double layer, or EDL. As a result, supercapacitors have a high capacitance.
Image Credit: Mrs_ya/Shutterstock.com
The researchers observed a significant increase in capacitance, several times higher than previously reported supercapacitors, when they exposed their supercapacitors to ultraviolet (UV) light. They also observed two peculiar characteristics. First, they discovered that contrary to popular belief, capacitance increased with voltage when the supercapacitor was exposed to illumination.
We call it the necking behavior.
AM Rao, Professor and Study Co-Author, Clemson University
Rao clarifies that the high porosity of the electrodes could be the cause of this. Secondly, because the ions in the electrolyte do not move quickly enough to react to the increased charging rate, the supercapacitor's stored energy usually decreases as it is charged more quickly. The researchers discovered, however, that when using a liquid electrolyte and fast charging under UV light, the supercapacitor’s stored energy unexpectedly increased.
Postdoctoral researcher Mihir Parekh in Rao's group created theoretical models to explain these novel observations. According to Rao, the results pave the way for the development of supercapacitors that are simultaneously energy-dense and fast-charging.
In designing their current supercapacitor, the group considered two main concepts. To maximize interaction with light and increase charge generation, two optically active semiconductor interfaces were combined to increase the electrodes' surface area. Secondly, a liquid electrolyte was employed to guarantee an efficient EDL. When combined, these produced better work.
“The ideas were simple … but when combined together, they worked very well,” explained Misra.
Misra continues by saying that the supercapacitor’s design can be changed to allow for both visible and infrared light charging. To create better supercapacitors, the IISc-Clemson team wants to investigate and comprehend the new phenomena.
Supercapacitors have lots of applications.
Abha Misra, Professor and Study Corresponding Author, Department of Instrumentation and Applied Physics, Indian Institute of Science
For instance, supercapacitors might take the place of the solar cells found in streetlights. Supercapacitors can release charge faster than batteries because of high power density. Moreover, chips in electronic gadgets like cell phones can be powered by supercapacitors.
“We have miniaturized supercapacitors to the micron scale so that they can be integrated along with these microelectronic chips,” added Misra.
Journal Reference:
Parekh, M., et al. (2024) Influence of electrolyte on the photo-charging capability of a ZnO–FTO supercapacitor. Journal of Materials Chemistry A. doi.org/10.1039/D4TA04702H