Professor Jie Tang, Group Leader of the 1D Nanomaterials Research Group of the Materials Processing Unit, Japan National Institute for Materials Science, and Mr. Qian Cheng, a doctoral student and NIMS Junior Researcher in the same Group, have succeeded in dramatically increasing the energy density of supercapacitors, which are used to store electrical energy. Supercapacitors can charge and discharge hundreds of times faster than batteries but have had lower energy densities. Supercapacitors with high energy densities would enable batteries to be replaced in many applications.
In order to achieve a revolutionary increase in density of energy storage, Professor Tang and her team, in collaboration with Professor Lu-Chang Qin of the University of North Carolina at Chapel Hill in the United States, have designed and developed a graphene-based composite structure, in which graphene is used as the base material of the capacitor electrodes and carbon nanotubes (CNT) are inserted between the graphene sheets. In this structure graphene offers a far larger specific surface area (2630 m2/g) than the conventional materials and the CNTs function as spacers as well as conducting paths to enable adsorption of a larger quantity of electrolyte ions on the graphene surface. With this graphene-CNT composite as the capacitor electrodes, Professor Tang has obtained a high energy density of 62.8 Wh/kg and output power density of 58.5 kW/kg using organic electrolyte. By using an ionic liquid as the electrolyte, they have achieved an energy density of 155.6 Wh/kg, which is comparable to that of nickel metal hydride batteries.
Among the many industrial applications of capacitors, the new capacitors developed in this research offer promises as power sources for electric and hybrid vehicles, which require high energy density. As the current production processes are also inexpensive and can be scaled up, large expectations are placed on practical applications.
The researchers developing a new electrode in which graphene nanosheets are stacked in a layered structure with carbon nanotubes sandwiched between the graphene layers.