New bendable supercapacitor developed by UCL researchers. (Credit: UCL)

A new bendable supercapacitor made from graphene, which charges quickly and safely stores a record-high level of energy for use over a long period, has been developed and demonstrated by UCL and Chinese Academy of Sciences researchers.

While still at the proof-of-concept stage, it shows enormous potential as a portable power supply in practical applications including electric vehicles, smart phones and smart wearable technology. The discovery overcomes the issue faced by high-powered, fast-charging supercapacitors — that they usually cannot hold a large amount of energy in a small space.

Dr. Zhuangnan Li (UCL Chemistry) said: “Our new supercapacitor is extremely promising for next-generation energy storage technology as either a replacement for current battery technology, or for use alongside it, to provide the user with more power. We designed materials that would give our supercapacitor a high power-density — which is how fast it can charge or discharge; and a high energy-density — which will determine how long it can run. Normally, you can only have one of these characteristics, but our supercapacitor provides both.”

The supercapacitor can bend to 180 degrees without affecting performance and doesn’t use a liquid electrolyte, which minimizes the risk of explosion. This makes it ideal for integrating into bendy smart phones or wearable electronics.

The new design uses an innovative graphene electrode material with pores that can be changed in size to store charge more efficiently. This tuning maximizes the energy density to 88.1 Wh/l (Watt-hours per liter), which is the highest ever reported energy density for carbon-based supercapacitors. Similar fast-charging commercial technology has an energy density of 5 – 8 Wh/l while traditional slow-charging but long-running lead-acid batteries used in electric vehicles typically have 50 – 90 Wh/l. Although the supercapacitor developed by the team has a comparable energy density to state-of-the-art lead-acid batteries, its power density is two orders of magnitude higher, at over 10,000 Watts per liter.

Professor Ivan Parkin (UCL Chemistry), said: “Successfully storing a huge amount of energy safely in a compact system is a significant step towards improved energy storage technology. We have shown it charges quickly, we can control its output, and it has excellent durability and flexibility, making it ideal for use in miniaturized electronics and electric vehicles. Imagine needing only ten minutes to fully-charge your electric car or a couple of minutes for your phone and it lasts all day.”

The researchers made electrodes from multiple layers of graphene, creating a dense, but porous material capable of trapping charged ions of different sizes. They characterized it using a range of techniques and found it performed best when the pore sizes matched the diameter of the ions in the electrolyte.

The 6 cm x 6 cm supercapacitor was made from two identical electrodes layered either side of a gel-like substance that acted as a chemical medium for the transfer of electrical charge. It was used to power dozens of light-emitting diodes (LEDs) and was found to be highly robust, flexible, and stable. Even when bent at 180 degrees, it performed almost same as when it was flat, and after 5000 cycles, it retained 97.8% of its capacity.