The approach could make SCs more viable for widespread use in renewable energy storage, portable electronics and electric vehicles. A paper on the work is published in Science.

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 density. With a low energy density of 5-8 Wh/liter, SCs are unfeasibly large or must be re-charged frequently.

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

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 the 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.

Porous yet densely packed carbon electrodes with high ion-accessible surface area and low ion transport resistance are crucial to the realization of high-density electrochemical capacitive energy storage but have proved to be very challenging to produce. Taking advantage of chemically converted graphene’s intrinsic micro-corrugated two-dimensional configuration and self-assembly behavior, we show that such materials can be readily formed by capillary compression of adaptive graphene gel films in the presence of a nonvolatile liquid electrolyte. This simple soft approach enables sub-nanometer scale integration of graphene sheets with electrolytes to form highly compact carbon electrodes with a continuous ion transport network. Electrochemical capacitors based on the resulting films can obtain volumetric energy densities approaching 60 watt-hours per liter.

—Yang et al.

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

The work was supported by the Australian Research Council.

Resources

• Xiaowei Yang, Chi Cheng, Yufei Wang, Ling Qiu, and Dan Li (2013) Liquid-Mediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage. Science 341 (6145), 534-537 doi: 10.1126/science.1239089