Capacitors are devices that store electric charge. Electrochemical capacitors – also known as supercapacitors or electric double-layer capacitors – can store much more charge thanks to the double layer formed at an electrolyte-electrode interface when voltage is applied. Although promising energy-storage materials, they still lag behind traditional batteries (which store energy through electrochemical reactions) in terms of energy densities: just 4 to 5 Wh/kg as opposed to 10 to 150 Wh/kg. They do, however, have a much longer shelf- and cycle life than batteries and can deliver large amounts of power much more quickly.

Now, a research team led by Richard Kaner and Maher El-Kady at the University of California, Los Angeles, say they have developed a graphene-based device that combines both the power performance of capacitors with the high energy density of batteries. The researchers have come up with a new process that involves coating an ordinary DVD disc with a film of graphite oxide supported on a sheet of plastic.

Kaner and colleagues begin by reducing the graphite oxide to graphene using a standard “LightScribe” DVD drive head – usually used to optically etch labels and images on DVD media discs. The process can easily be monitored as the golden-brown-coloured graphite oxide turns into black-coloured graphene. The plastic (now coated with graphene) is subsequently peeled off and cut with scissors to make different devices.

Electrochemical capacitors are made by simply “gluing” together two identical pieces of graphene sheet (which can be used as electrodes without the need for any additional binders or additives) with a little polymer gel electrolyte that is placed between them. “We also tested a variety of other electrolytes confirming that the material can be used in a number of device systems for different applications,” said Kaner.

Large surface area

The LightScribe graphene sheets have a large specific surface area of more than 1500 m2/g, which helps to increase their energy-storage capacity, and a high electrical conductivity of more than 1700 S/m. They are also very flexible mechanically and can be bent thousands of times without suffering any damage to their electrical properties. All of these characteristics make them ideal not only for making supercapacitors but also a host of other electronic devices, says El-Kady, who also lectures at Cairo University.

The LightScribe process overcomes another important problem too in that it produces non-stacked graphene sheets. Graphene sheets have a natural tendency to stack during production, something that reduces the overall surface area of the material.

“We believe that our work will help pave the way to making flexible supercapacitors for use in bendable electronic equipment for the upcoming boom in flexible portable electronics,” El-Kady told nanotechweb.org. “Applications include roll-up computer displays, wearable electronics that harvest and store energy produced by body movement, electronic wallpaper and energy-storage systems that can be combined with flexible photovoltaic cells.”

The team now plans to look at scaling up their production technique in a cost-effective manner. “Our initial calculations show that the price of the precursor, graphite oxide and the whole process is completely viable for commercial applications,” added El-Kady.

Details of the current work are published in Science.