Next-gen energy storage devices: Ultra-small, smart

Schematic diagram of the device. Photo by special arrangement

Scientists at the Indian Institute of Science (IISc) have designed a novel ultra-micro supercapacitor — a tiny device capable of storing an enormous amount of electric charge.

In the age of the Internet of Things (IoT), artificial intelligence (AI) and machine learning (ML) based models, edge computing devices play a critical role. These are hardware devices that perform data processing and analysis on the edge of a network, closer to the source of the data. Thus, they would also require a continuous supply of energy to power them.

Edge computing devices can be as simple as a sensor, a gateway, or a small server. They can be deployed in remote and rugged environments where cloud computing is not feasible or desirable. They are used to overcome the limitations of traditional cloud computing, where data is transmitted to a centralised data centre for processing and analysis.

Imagine an edge computing device deployed in a busy traffic intersection or in a remote forest to gather data. They need to operate consuming energy over a long duration without an opportunity to be charged frequently.

A supercapacitor is an energy storage device with a unique mechanism to store energy in the form of electrochemical energy, similar to a battery with high-power density, rapid charge/discharge rates, and long cycling life.

Prof Abha Misra and her colleagues at the IISc have come up with much smaller, smarter, more efficient and more compact supercapacitors that can potentially be used in many devices ranging from streetlights to consumer electronics, electric cars and medical devices. The study was published in ACS Energy Letters in February this year.

Holding charge for long

Supercapacitors find widespread application in various fields such as consumer electronics, sensors, voltage stabilising systems, and inverter batteries. The structure of a supercapacitor comprises two electrodes composed of porous materials that are electrochemically active and possess a larger surface area. These electrodes are separated by a membrane that allows the passage of ions. The supercapacitor stores charge by the accumulation of positive and negative ionic charges on the two electrodes. The ion-permeable membrane acts as a barrier layer that prevents the combination of these charges.

Typically, most electronics and some of the edge computing devices are powered by batteries. However, they lose charge over time and have a finite life span. On the other hand, supercapacitors can store electric charge for a longer duration, perhaps even up to a decade. Supercapacitors can be thought of as a hybrid of a battery and a capacitor.

IISc scientists have developed a new type of ultra-small energy storage device using two-dimensional (2D) molybdenum disulphide (MoS2) and graphene-based electrodes. They have used field effect transistors (FETs) as the charge collectors instead of metallic electrodes which are typically used in capacitors.

Abha Misra notes that current capacitors use metal oxide-based electrodes and are limited by poor electron mobility. Hence they explored building hybrid FETs by alternating a few atoms thick layers of molybdenum disulphate and graphene to enhance electron mobility. They then used a solid gel electrolyte between the two FET electrodes to build a solid-state supercapacitor, which is built on a silicon base.

The scientists claim that since the new supercapacitor is so ultra-small that it can’t be seen even under a microscope. And it has to be fabricated with very high precision. After the fabrication, they tested it under certain conditions. It was found that the capacitance increases by 3000%, which is way higher than the MoS2 capacitor without graphene under similar conditions.

“The design is the critical part because you are integrating two systems,” says Misra in a release shared by IISc. The two systems are the two FET electrodes and the gel electrolyte. These have different charge capacities thus making the integration challenging.

The study shows that this technology has great potential for ultrahigh charge storage capability. As a next step, the scientists plan to explore replacing MoS2 with other materials. They will also explore the variations in the capacitance of the supercapacitors.

With the increasing applications of edge computing devices, an energy storage device as efficient as this is only making things better. Given their sizes, the scientists note that these have the potential to be integrated into any miniaturised system by on-chip integration.

Notably, this is first of a kind on-chip ultra-micro electrochemical capacitor that also has the smallest electrode than others reported in studies. With a very high-efficiency capacitance compared to its size, this promises to open up the next generation of ultra-small charge storage devices.