Last month, researchers at Rice University in Texas unveiled a method for making three-dimensional forms made of foamed graphene. The Glasgow team, working independently, used a commercially available graphene foam to make a layered structure with a silver-containing epoxy resin to form supercapacitors capable of storing three times as much power as any similar flexible supercapacitor, they claim.
In a paper in the journal Nano Energy, the team from Glasgow's bendable electronics and sensing technologies (BEST) group, led by Prof Ravinder Dahiya, said that lithium ion batteries may not be suitable for wearable devices because they are inflexible and heavy and their heat may cause injuries. Supercapacitors, which charge and discharge quickly and can be made from more environmentally friendly materials, may be a more promising option, they said.
Dahiya’s team is exploring the emerging technology of hybrid supercapacitor/batteries. These are more stable than traditional supercapacitors, they said, and capable of operating over more than a million charge-discharge cycles without loss of performance.
Their device comprises several layers: a layer made up from 300 individual layers of highly conductive graphene sheet as a current collector, onto which a layer of silver epoxy was bonded, with graphene foam on top of that. An electrolyte of phosphoric acid was dropped onto the foam and a polyester/cellulose sheet was bonded on top of that as an ion permeable membrane. This whole assembly was connected to a commercially available photovoltaic cell. The team has also demonstrated that the capacitor can be charged by a flexible solar powered skin developed by the same group.
The team connected the power pack to a sensor designed to detect sweat pH to test the system, and obtained highly promising results. "We’re very pleased by the progress this new form of solar-powered supercapacitor represents,” Dahiya said. “A flexible, wearable health monitoring system, which only requires exposure to sunlight to charge, has a lot of obvious commercial appeal, but the underlying technology has a great deal of additional potential.
“This research could take the wearable systems for health monitoring to remote parts of the world where solar power is often the most reliable source of energy, and it could also increase the efficiency of hybrid electric vehicles. We’re already looking at further integrating the technology into flexible synthetic skin which we’re developing for use in advanced prosthetics.”