Researchers develop light-charged supercapacitor for self-powered devices

Researchers and collaborators at the Department of Instrumentation and Applied Physics (IAP), Indian Institute of Science (IISc), have designed a new supercapacitor that can be charged by shining light on it. Such supercapacitors could be used in a variety of devices, including street lights and self-powered electronic devices such as sensors.

Capacitors are electrostatic devices that store energy as charges on two metal plates called electrodes. Supercapacitors are upgraded versions of capacitors; they use electrochemical phenomena to store more energy, explains Abha Misra, Professor at IAP and corresponding author of the study It was published inside Journal of Materials Chemistry A.

New electrodes super capacitor was done zinc oxide (ZnO) nanorods grown directly on fluorine-doped tin oxide (FTO), which is transparent, were synthesized by Pankaj Singh Chauhan, first author and CV Raman postdoctoral researcher in Misra’s group at IISc.

Both ZnO and FTO are semiconductors with proper alignment energy levelsprovides the superior performance of the photo-rechargeable supercapacitor. Being transparent, FTO allows light to fall on the optically active ZnO nanorods, which charges the supercapacitor. Chauhan explains that two electrolytes (a liquid and a semi-solid gel) are used as conductive media between the electrodes.

The charge storage capacity (capacitance) is inversely proportional to the distance between the electrodes.

“The smaller the distance, the higher the capacitance,” Misra explains. In electrostatic capacitors, it’s difficult to maintain a small distance between the electrodes. But in a supercapacitor, the charges on the electrodes attract the oppositely charged ions of the electrolyte, creating a layer of charge just atoms apart—called an electrical double layer, or EDL. This gives supercapacitors their high capacitance.

When the researchers shined ultraviolet (UV) light on their supercapacitors, they noticed that the capacitance increased several times more than previously reported supercapacitors. They also noticed two unusual features. First, while capacitance usually decreases as voltage increases, they found the opposite; the capacitance of their supercapacitors under light illumination actually increased with increasing voltage.

“We call this necking behavior,” says AM Rao, professor and co-author at Clemson University in the US. He explains that this may be due to the high porosity of the electrodes. Second, the energy stored in the supercapacitor usually decreases when charged faster, because the ions in the electrolyte do not move fast enough to respond to the increased charging rate. However, with the liquid electrolyte, the team found that the energy stored in the supercapacitor surprisingly increased when charged quickly under UV light.

Mihir Parekh, a postdoctoral researcher in Rao’s group, developed theoretical models to explain these new observations. He suggests that the findings open the door to developing supercapacitors that are simultaneously fast-charging and energy-dense.

To design their current supercapacitor, the team explored two key ideas. First, the surface area of ​​the electrodes was increased by combining two optically active semiconductor interfaces to maximize interaction with light, leading to higher charge generation. Second, a liquid electrolyte was used to provide an effective EDL. Together, these resulted in superior performance.

“The ideas were simple… but when put together, they worked very well,” Misra explains. He adds that tweaking the supercapacitor’s design could allow it to be charged with visible and infrared light as well. The IISc-Clemson team aims to further explore and better understand the new phenomena observed to design better supercapacitors.

“Supercapacitors have many applications,” Misra explains. For example, they could replace solar cells used in streetlights. Because they have a high power density, they can discharge more quickly than batteries. They could also be used to power chips electronic devices like cell phones.

“We miniaturized the supercapacitors down to the micron scale so that they can be integrated together with these microelectronic chips,” Misra adds.