Stretchy supercapacitors power wearable electronics

This stretchy supercapacitor made from graphene could spur the development of wearable electronics. Credit: Xiaodong Chen, Ph.D.

This stretchy supercapacitor made from graphene could spur the development of wearable electronics.
Credit: Xiaodong Chen, Ph.D.

A future of soft robots that wash your dishes or smart T-shirts that power your cell phone may depend on the development of stretchy power sources. But traditional batteries are thick and rigid — not ideal properties for materials that would be used in tiny malleable devices. In a step toward wearable electronics, a team of researchers has produced a stretchy micro-supercapacitor using ribbons of graphene.

The researchers will present their work today at the 252nd National Meeting & Exposition of the American Chemical Society (ACS). ACS, the world’s largest scientific society, is holding the meeting here through Thursday. It features more than 9,000 presentations on a wide range of science topics.

“Most power sources, such as phone batteries, are not stretchable. They are very rigid,” says Xiaodong Chen, Ph.D. “My team has made stretchable electrodes, and we have integrated them into a supercapacitor, which is an energy storage device that powers electronic gadgets.”

Supercapacitors, developed in the 1950s, have a higher power density and longer life cycle than standard capacitors or batteries. And as devices have shrunk, so too have supercapacitors, bringing into the fore a generation of two-dimensional micro-supercapacitors that are integrated into cell phones, computers and other devices. However, these supercapacitors have remained rigid, and are thus a poor fit for soft materials that need to have the ability to elongate.

In this study, Chen of Nanyang Technological University, Singapore, and his team sought to develop a micro-supercapacitor from graphene. This carbon sheet is renowned for its thinness, strength and conductivity. “Graphene can be flexible and foldable, but it cannot be stretched,” he says. To fix that, Chen’s team took a cue from skin. Skin has a wave-like microstructure, Chen says. “We started to think of how we could make graphene more like a wave.”

The researchers’ first step was to make graphene micro-ribbons. Most graphene is produced with physical methods — like shaving the tip of a pencil — but Chen uses chemistry to build his material. “We have more control over the graphene’s structure and thickness that way,” he explains. “It’s very difficult to control that with the physical approach. Thickness can really affect the conductivity of the electrodes and how much energy the supercapacitor overall can hold.”

The next step was to create the stretchable polymer chip with a series of pyramidal ridges. The researchers placed the graphene ribbons across the ridges, creating the wave-like structure. The design allowed the material to stretch without the graphene electrodes of the superconductor detaching, cracking or deforming. In addition, the team developed kirigami structures, which are variations of origami folds, to make the supercapacitors 500 percent more flexible without decaying their electrochemical performance. As a final test, Chen has powered an LCD from a calculator with the stretchy graphene-based micro-supercapacitor. Similarly, such stretchy supercapacitors can be used in pressure or chemical sensors.

In future experiments, the researchers hope to increase the electrode’s surface area so it can hold even more energy. The current version only stores enough energy to power LCD devices for a minute, he says.

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New electrode gives micro-supercapacitor macro storage capacity

© Anaïs Ferris – LAAS-CNRS Image obtained from scanning tunneling microscopy on a porous 3D-gold structure.

Anaïs Ferris – LAAS-CNRS
Image obtained from scanning tunneling microscopy on a porous 3D-gold structure.

Micro-supercapacitors are a promising alternative to micro-batteries because of their high power and long lifetime. They have been in development for about a decade but until now they have stored considerably less energy than micro-batteries, which has limited their application. Now researchers in the Laboratoire d’analyse et d’architecture des systèmes (LAAS-CNRS)1 in Toulouse and the INRS2 in Quebec have developed an electrode material that means electrochemical capacitors produce results similar to batteries, yet retain their particular advantages.

This work was published on September 30, 2015 in Advanced Materials.

With the development of on-board electronic systems3 and wireless technologies, the miniaturization of energy storage devices has become necessary. Micro-batteries are very widespread and store a large quantity of energy due to their chemical properties. However, they are affected by temperature variations and suffer from low electric power and limited lifetime (often around a few hundred charge/discharge cycles). By contrast, micro-supercapacitors have high power and theoretically infinite lifetime, but only store a low amount of energy.

Micro-supercapacitors have been the subject of an increasing amount of research over the last ten years, but no concrete applications have come from it. Their lower energy density, i.e. the amount of energy that they can store in a given volume or surface area, has meant that they were not able to power sensors or microelectronic components. Researchers in the Intégration de systèmes de gestion de l’énergie team at LAAS-CNRS, in collaboration with the INRS of Quebec, have succeeded in removing this limitation by combining the best of micro-supercapacitors and micro-batteries.

They have developed an electrode material whose energy density exceeds all the systems available to date.

The electrode is made of an extremely porous gold structure into which ruthenium oxide has been inserted. It is synthesized using an electrochemical process. These expensive materials can be used here because the components are tiny: of the order of square millimeters. This electrode was used to make a micro-supercapacitor with energy density 0.5 J/cm², which is about 1000 times greater than existing micro-supercapacitors, and very similar to the density characteristics of current Li-ion micro-batteries.

With this new energy density, their long lifetime, high power and tolerance to temperature variations, these micro-supercapacitors could finally be used in wearable, intelligent, on-board microsystems.

 
 

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Scientists create tiny bendy power supply for even smaller portable electronics

13-aug-bendy-super-capacitor-360

Scientists have created a powerful micro-supercapacitor, just nanometres thick, that could help electronics companies develop mobile phones and cameras that are smaller, lighter and thinner than ever before.

The tiny power supply measures less than half a centimetre across and is made from a flexible material, opening up the possibility for wearable electronics.

The research is published in the Royal Society of Chemistry journal Energy & Environmental Science.

A bottleneck in making portable electronic devices like mobile phones even smaller is reducing the size and increasing the flexibility of the power supplies in electronic circuits. Supercapacitors are attractive power supplies because they can store almost as much energy as a battery, with the advantage of high-speed energy discharge. Supercapacitor electrodes are normally made from carbon or conducting polymers, but these can be relatively costly.

A team led by Professor Oliver G Schmidt at the Leibniz Institute for Solid State and Materials Research in Dresden (IFW-Dresden) examined the use of manganese dioxide as an alternative electrode material, which is more environmentally friendly and less expensive than the standard materials. Manganese dioxide is not a natural choice for an electrode material because it isn’t very electrically conductive, nor is it naturally flexible or strong. However, the scientists overcame this by vaporising the manganese dioxide using an electron beam and then allowing the gaseous atoms to precipitate into thin, bendy films. They incorporated very thin layers of gold into the films to improve the electrical conductivity of the material.

Tests on the new micro-supercapacitor showed that the tiny, bendy power supply can store more energy and provide more power per unit volume than state-of-the-art supercapacitors.

Dr Chenglin Yan, leader of the research group at IFW-Dresden, said: “Supercapacitors, as a new class of energy device, can store high energy and provide high power, bridging the gap between rechargeable batteries and conventional capacitors. So we thought a micro-supercapacitor would be an important development in the rapid advance of portable consumer electronics, which need small lightweight, flexible micro-scale power sources.

“The device could be applied to many miniaturised technologies, including implantable medical devices and active radio frequency identification (RFID) tags for self-powered miniaturised devices.”

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