Bendable energy storage device based on graphene-based ultracapacitor

Peng ZHAO

Since Andre Geim and Konstantin Novoselov, the winners of the Nobel Prize in Physics 2010, succeeded in producing, isolating, identifying and characterizing graphene from original graphite, a variety of applications based on graphene structure and its relative properties has arisen. This article will introduce a novel energy storage device (ESD) in which graphene membranes act as a key element. Basically, there are two frequently used methods to store electrical energy today: chemically and statically. The common rechargeable battery belongs to the former one, while capacitance class devices store electricity statically. In essence, a graphene-based power source is a kind of ultracapacitor. Compared with other capacitor energy storage devices, a graphene-based ultracapacitor boasts excellent electrical conductivity, mechanical flexibility, high energy density and a better life cycle.

 

Before we discuss the schematic representation of this functional material, the certain properties of graphene which contribute to the performance of this graphene-based ESD should be illustrated. Since the combination between high conductivity and mechanical bendability is always regarded as a great challenge in flexible power source design and fabrication, scientists and engineers need to find a new material that can solve this problem. Like other carbon material, graphene sheets have superior stability in chemical reaction and a wide range of electrochemical potential, which results in the fact that graphene materials have huge potency in battery business. However, in addition to the common properties mentioned above, the most critical nature for graphene nanosheets is the high surface area to volume ratio, which means a pretty high value for the surface area(S) and thickness (d) ratio. From the basic equation of capacitance  , it is easy to conclude if bendable devices are feasible and an ultracapacitor with high power density could be made. As a matter of fact, experiment results have shown—due to a high breaking strength of graphene sheets, which may be estimated to be 200 times greater than steel—that a flexible paper capacitor is theoretically realizable. Besides, the remarkable electronic transport properties of graphene offer a good compatibility with cathode material, and the integration of current collector and conducting agents becomes possible.

 

Fig.1 Graphene paper with  cathode

 

For the fabrication of this graphene-based ultracapacitor, we can get a general description from Fig. 1 above about graphene nanosheets, the fundamental structure for this ESD. First, many previous experiments use graphite dispersion to get rough graphene material, and then a filtration process will be followed so as to fabricate pure graphene sheets.  is chosen for the material of cathode, since it can grow from graphene paper by a laser deposition process in a vacuum chamber. On the basis of the preparation of the material mentioned above, the assembled schematic structure of the ultracapacitor is shown in Fig.2, where graphene papers are used as cathode and anode, which are separated by the partition with a certain dielectric constant.

Fig.2 Graphene-based ultracapacitor

 

The performance of this novel energy storage device was tested using a charge-discharge experiment. Respectively, the curves that reflect the change of capacity are shown in Fig.3. As a reference, a /Al capacitor in the same condition was also tested. The profiles indicated that during the voltage sweep from 1.7V~3.8V, the indicative /Graphene capacitor shows a much higher current density than the reference one, while keeping the same recharging rate.

 

Fig.3 Charge-discharge profiles for /Graphene capacitor and /Al capacitor

 

In summary, this article demonstrates a novel bendable energy storage device based on graphene material, and compares its mechanical and electrochemical performance with the conventional ESD, which exhibits a promising application prospect.

 

Reference:

 

1. Meryl D. Stoller, Sungjin Park, Yanwu Zhu, Jinho An & Rodney Ruoff, Graphene-Based Ultracapacitors. Nano letters, 10,(2008)
2. Hyeokjo Gwon, Hyun-Suk Kim, Dong-Hwa Seo, Yun Chang Park & Yun-Sung Lee, Flexible energy storage devices based on graphene paper. Science, 4,(2011)