Micro-supercapacitors created from graphene ink

Korean researchers have found an inexpensive way to fabricate tiny energy storage devices that can effectively power flexible and wearable skin sensors along with other electronic devices, paving the way towards remote medical monitoring and wearable devices. Their findings have been published in the journal Nano Energy.

“Demand for remote diagnosis and wearable devices is rapidly increasing, and thus many scientists are focusing their research efforts on developing various electronic skin devices, which requires extremely tiny and flexible energy devices as a power source,” noted materials scientist Sungwon Lee from Korea’s Daegu Gyeongbuk Institute of Science and Technology (DGIST). These so-called ‘micro-supercapacitors’ have short charging and discharging times compared to batteries, but they can’t store as much energy.

Graphene is a promising material for improving the devices’ energy storage, as graphene electrodes are highly porous and so provide a larger surface area for the necessary electrostatic reactions to occur. Another way to improve micro-supercapacitor performance is by fabricating electrodes with interlocking teeth, like those of two combs, increasing the amount of energy that can be stored. But this process is expensive and doesn’t work on flexible, temperature-sensitive substrates.

The obvious solution would be spraying of graphene onto a flexible substrate, but vertical spraying leads to electrodes that aren’t very porous and that have compact layers, giving them poor performance. This led Lee and colleagues from DGIST, working with the Korea Institute of Geoscience and Mineral Resources as well as Dongguk University, to try spraying graphene ink onto flexible substrates at a specific angle and temperature, fabricating a paper-thin micro-supercapacitor with interlocking electrodes and impressive performance. The trick, the researchers found, was to spray 10 mL of graphene ink at a 45° angle and 80°C temperature onto a flexible substrate. This led to the formation of porous, multi-layered electrodes.

The team’s resulting micro-supercapacitor is 23 µm thin — 10 times thinner than paper — and retains its mechanical stability after 10,000 bends. It can store around 8.4 microfarads of charge per square centimetre (two times more than the highest value reported to date) and has a power density of 1.13 kW/kg (four times higher than that of Li-ion batteries). The team demonstrated that the micro-supercapacitor could be used in wearable devices that adhere to the skin.

“Our work shows that it’s possible to reduce the thickness of micro-supercapacitors for use in flexible devices, without degrading their performance,” Lee said. The researchers now aim to improve the micro-supercapacitor’s storage capacity and energy consumption to make it feasible for use in real-world electronic skin devices.

Image caption: Professor Sungwon Lee (left) holds up a hand with the tiny micro-supercapacitor stuck on the skin near his wrist. The device was developed together with Dr Koteeswara Nandanapalli (right) and others.

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