With the rapid development of science and technology, industrialization and the rapid development of information technology, computers, mobile phones, cameras and other electronic products has become a necessity in life. This requires the electronic equipment energy storage system must have a long time power supply capacity, can make the electronic equipment from the power line constraints, as easy to use the mobile device. Super capacitor is a new type of energy storage device, with high capacity, high power density, high charge and discharge speed and so on.

The advent of graphene flexible supercapacitors will subvert the entire industry

Flexible supercapacitors are a classification of supercapacitors. Super capacitor is composed of electrode material, current collector, diaphragm, electrolyte, and flexible super capacitor is a flexible substrate, electrode material, solid electrolyte composition. Wherein the electrode material can simultaneously function as a storage energy and a current collector, and the solid electrolyte can simultaneously function as an electrolyte and a separator. Compared with traditional supercapacitors, flexible supercapacitors have the following advantages: the use of stable performance of the electrode material to improve the safety; ultra-thin electrode material and streamlined assembly process, greatly reducing the size of the entire device smaller, lightweight ; Electrode materials and electrolyte materials with less, reducing production costs, and safety and environmental protection.

1, flexible super capacitor works

Flexible supercapacitor and super capacitor works the same, can be divided into double electric layer energy storage mechanism, pseudo-capacitance energy storage mechanism and composite energy storage mechanism:

(1) electric double layer storage mechanism is the use of electrode materials and electrolyte contact surface storage charge, the formation of two charge layer, the whole process does not occur chemical reaction, only the ion adsorption desorption.

(2) pseudo-capacitance energy storage mechanism is the use of electrode materials in the active material surface reversible redox reaction storage charge, belonging to the Faraday reaction process.

(3) composite energy storage mechanism refers to the entire reaction process at the same time there double electric layer energy storage mechanism and pseudo-capacitance energy storage mechanism.

For example, in the process of double-layer energy storage, only the charge and desorption of the charge and the high cycle life of the electrode material are high, but the surface area of the stored charge is limited and the capacitance value is low. The pseudo-capacitor energy storage process can obtain high capacitance , But because of the irreversibility of the redox reaction, the cycle life is low. Two mechanisms of synergies, to play their own advantages, to make up for their own shortcomings, the super-capacitor electrochemical performance fully play out.

2, graphene-based flexible supercapacitor

(1) based on graphene flexible supercapacitor

The graphene is a honeycomb-shaped two-dimensional crystal structure which is densely packed with sp2-hybrid carbon atoms. Since its inception, due to its high specific surface area, excellent electrical properties and stable chemical properties and other characteristics, in the field of super capacitor concern.

The advent of graphene flexible supercapacitors will subvert the entire industry

Stoller et al. Used KOH chemically modified graphene as the electrode material to verify the feasibility of graphene application in the field of supercapacitor electrode materials. Since then, there has been an endless stream of research on graphene as an electrode material for supercapacitors. As shown in Fig. 1, the graphene flexible supercapacitor has a different compositional form.

Chen et al. Injected the oxidized graphene suspension into a glass tube and, after reduction, obtained a graphene fiber having a similar shape to the glass tube. The resulting supercapacitor has good electrochemical performance and flexibility.

Zhao and other pyrrole monomer added to the oxide graphene suspension, after polymerization and reduction, to obtain a good elasticity of graphene / polypyrrole three-dimensional structure. The assembled flexible supercapacitor has a good compressibility.

El-Kady et al. Used a DVD-driven laser to reduce graphene oxide as an electrode material to produce a flexible supercapacitor having a specific capacitance of 4 mF / cm2 and excellent deformation properties.

The advent of graphene flexible supercapacitors will subvert the entire industry

Due to the small size of the plate-like flexible supercapacitor, the resistance itself is small during the deformation and is therefore more susceptible to deformation. (Poly (ethylene terephthalate), PET; polydimethylsiloxane, PDMS; polyethylene, PE, and so on. Matte cloth and filter paper) and assembled with colloidal electrolyte into a flexible supercapacitor with a "sandwich"structure.

Depending on the nature of the flexible substrate, different deformation performance tests are performed on the capacitor, such as bending, stretching, folding, or any deformation (Figure 2). Test results show that after a variety of deformation of the capacitor can still maintain a stable capacitance performance, and can withstand hundreds of times deformation, with good deformation stability. In the actual situation, dynamic deformation is more common, and flexible supercapacitor in the deformation process can still maintain a stable electrochemical performance, that is, with excellent dynamic deformation performance.

The advent of graphene flexible supercapacitors will subvert the entire industry

As shown in Fig. 3, Li et al. Extended the deformation type to dynamic tensile deformation, transferred the carbon nanotubes to the PDMS substrate, and tested the change in electrochemical performance at different strain frequencies (up to 4.46% / s).

Zang and so on to make full use of graphene mesh film can be closely integrated with the substrate characteristics, to obtain pre-stretched fold PDMS as the base, graphene mesh film as the electrode material of the dynamic tensile (bending) super capacitor. Dynamic stretching (bending) frequency up to 60% / s. The results show that there is no obvious performance damage during the dynamic stretching (bending) process, and the dynamic deformation performance is very good.

(2) Flexible supercapacitors based on graphene composites

Although the carbon material has excellent performance of the electric double layer capacitor, the surface area of the carbon material is limited and the ability to store the charge is limited. In order to further enhance the energy storage capacity of flexible supercapacitor, the pseudo-capacitance material is introduced to obtain the composite material of graphene and pseudostructure material. In the composite material, graphene as both electric layer storage energy, but also as a pseudo-capacitance material support skeleton and conductive channel. The combination of the electric double layer and the pseudostructure allows the two energy storage mechanisms to work together to achieve optimum performance.

Polymeric conductive polymer is a simple and superior performance of the pseudo-capacitance materials, such as polyaniline, polypyrrole and so on. Polyaniline is a typical conductive polymer, with high conductivity, unique doping mechanism and good environmental stability and other characteristics, and easy to obtain raw materials, low cost. The preparation method of the conductive material is simple, including in situ polymerization, electric polymerization, solution method and so on.

Zhang et al. In situ polymerization of polyaniline on graphene oxide to obtain a mass ratio of 480F / g. Cong and so on in the one-step preparation of the reduction of graphene on the load of polyaniline, to obtain flexible, lightweight, high conductivity of the composite electrode material, the mass ratio of up to 763F / g. Zang et al. Used electrochemical polymerization in the stone

The surface area ratio of the device is increased from 2mF / cm2 to 23mF / cm2, and it still has good flexibility after load.

Graphene and conductive polymer compound, you can greatly improve the capacitance value. In addition, the polymer can enhance the adhesion between graphene and the substrate, but also improve the ability of flexible deformation, in the field of flexible supercapacitor has a good application prospects.

3, conclusions and prospects

In summary, because graphene has a unique structure and electrochemical properties, and can be combined with the pseudo-capacitance material, is a potential flexible super capacitor electrode material. The graphene in the flexible supercapacitor can achieve many types of static deformation and high frequency dynamic deformation, and the electrochemical performance in the process of deformation is stable, thus greatly promoting the field of flexible supercapacitor. I believe that with the improvement of graphene performance and supporting micro-processing technology sophisticated, graphene flexible super capacitor will be more and more broad application space development.