Flexible asymmetric supercapacitors with high energy and high power density in aqueous electrolytes

Abstract

Supercapacitors with both high energy and high power densities are critical for many practical applications. In this paper, we discuss the design and demonstrate the fabrication of flexible asymmetric supercapacitors based on nanocomposite electrodes of MnO(2), activated carbon, carbon nanotubes and graphene. The combined unique properties of each of these components enable highly flexible and mechanically strong films that can serve as electrodes directly without using any current collectors or binders. Using these flexible electrodes and a roll-up approach, asymmetric supercapacitors with 2 V working voltage were successfully fabricated. The fabricated device showed excellent rate capability, with 78% of the original capacitance retained when the scan rate was increased from 2 mV s(-1) to 500 mV s(-1). Owing to the unique composite structure, these supercapacitors were able to deliver high energy density (24 W h kg(-1)) under high power density (7.8 kW kg(-1)) conditions. These features could enable supercapacitor based energy storage systems to be very attractive for a variety of critical applications, such as the power sources in hybrid electric vehicles and the back-up powers for wind and solar energy, where both high energy density and high power density are required.

Carbon Nanomaterials for Flexible Energy Storage

“…This can be achieved by vacuum filtration of the stable suspension containing both CNTs and AC particles. [15,38,66] Experimental results also demonstrated that the flexible composite electrodes have much better performance compared with other electrodes made with traditional approaches. Flexible and binder-free CNTs/AC electrodes containing 95 wt% of AC were able to reach specific capacitance of 268 F/g at 200 mV/s in 6 M KOH.…”

“…In a recent study, Cheng et al developed a strategy to fabricate flexible electrodes with high specific capacitance at thick MnO 2 coatings by enabling the synergistic effects from graphene and CNTs. [14,15] In that work, functionalized CNTs were used to link the pre-synthesized graphene/MnO 2 pieces (Figure 7). The introduction of highly conductive CNTs brings about remarkable conductivity (∼ 5 / ) and mechanical strength (Young's modulus of 2.3 GPa and tensile strength of 48 MPa) to the freestanding electrodes.…”

“…[88] In a recent study, Cheng et al fabricated flexible ASC that have even a higher voltage of 2 V in an aqueous electrolyte. [15] These devices were fabricated using a flexible graphene/MnO 2 /CNTs film as the positive electrode and AC/CNTs film as the negative electrode without using any current collectors through the roll-up approach ( Figure 8). As has been discussed above, these electrodes have superior flexibility and outstanding mechanical strength owing to the interconnected CNTs networks.…”

“…Therefore, these devices were able to deliver much higher energy density under high-power conditions compared with other systems, reaching 24 Wh/kg at 7.8 kW/kg. [15] In a similar study, Shao et al used porous graphene/MnO 2 nanorod and graphene/Ag thin films that were both prepared using the vacuum filtration method as positive and negative electrode, respectively, in their fabrication of flexible ASC. This study demonstrated that Ag nanoparticles is very important in improving the performance of the graphene [15] negative electrode because they can repair defects in the graphene surface and enhance their electronic conductivities.…”

“…One reason is that heavy metallic current collectors, conducting additive and binder are usually not necessary for flexible electrodes, thus could reduce the total weight of the device. [14,15] The removal of metallic current collectors also has benefits including improved cycling stability and corrosion resistivity. Additionally, flexible electrodes offer better manufacturability since they can be folded, twisted or cut to fit any desired spaces.…”