New graphene-based supercapacitors rival lead-acid batteries

August 5, 2013

SEM image of graphene/ionic liquid hybrid film (credit: Yufei Wang)

Monash University researchers have developed a completely new strategy to engineer graphene-based supercapacitors (SC), making them viable for widespread use in renewable energy storage, portable electronics and electric vehicles.

SCs are generally made of highly porous carbon impregnated with a liquid electrolyte to transport the electrical charge. Known for their almost indefinite lifespan and the ability to re-charge in seconds, the drawback of existing SCs is their low energy-storage-to-volume ratio — known as energy density.

Low energy density of five to eight Watt-hours per liter means SCs are unfeasibly large or must be recharged frequently.

Professor Dan Li’s team has created an SC with energy density of 60 Watt-hours per liter (0.06 Watt-hours  cm-3) — comparable to lead-acid batteries and around 12 times higher than commercially available SCs.

KurzweilAI reported on February 27, 2013 that UCLA scientists had achieved 2.1 mWh cm-3 energy density (max), or 0.0021 Wh cm-3, so the Monash team claim they have achieved ~29 times higher energy density. In April, researchers at the University of Illinois at Urbana-Champaign also announced new microbatteries that they claimed have “2 times greater energy density compared with other 3D microbatteries,” but did not provide a volumetric value. We will attempt to sort these comparisons out further.

“It has long been a challenge to make SCs smaller, lighter and compact to meet the increasingly demanding needs of many commercial uses,” Professor Li said.

Graphene, which is formed when graphite is broken down into layers one atom thick, is very strong, chemically stable and an excellent conductor of electricity.

To make their uniquely compact electrode, Professor Li’s team exploited an adaptive graphene gel film they had developed previously. They used liquid electrolytes — generally the conductor in traditional SCs — to control the spacing between graphene sheets on the sub-nanometer scale. In this way the liquid electrolyte played a dual role: maintaining the minute space between the graphene sheets and conducting electricity.

Unlike in traditional “hard” porous carbon, where space is wasted with unnecessarily large pores, density is maximized without compromising porosity in Professor Li’s electrode.

To create their material, the research team used a method similar to that used in traditional paper making, meaning the process could be easily and cost-effectively scaled up for industrial use.

“We have created a macroscopic graphene material that is a step beyond what has been achieved previously. It is almost at the stage of moving from the lab to commercial development,” Professor Li said.

The work was supported by the Australian Research Council.