Georgia Tech team develops high-capacity supercapacitor using cobalt oxide nanonets; design approach suited for other supercap and battery application

Specific capacitances of CFP-supported Co3O4 nanonet (mass loading of 0.4, 0.7, and 1.4 mg/cm2) and nanocube electrodes (a mass loading of 0.7 mg/cm2) at different current densities. Credit: ACSA, Yang et al. Click to enlarge.

A team at Georgia Tech has developed a high-capacity supercapacitor based on a hierarchical network architecture consisting of a cobalt oxide (Co₃O₄) nanowire network (nanonet) coated on a carbon fiber paper (CFP). With this tailored architecture, the electrode shows ideal capacitive behavior and large specific capacitance (1124 F/g) at high charge/discharge rate (25.34 A/g), still retaining 94% of the capacitance at a much lower rate of 0.25 A/g.

In a paper published in the ACS journal Nano Letters, the team attributed the much-improved capacity, rate capability, and cycling stability to the unique hierarchical network structures, which improves electron/ion transport, enhances the kinetics of redox reactions, and facilitates facile stress relaxation during cycling.

It has been well established that electrodes with proper nanostructures may enhance not only power density (or rate capability) but also cycling stability. While a wide variety of nanostructures have been created and tested, it still represents a grand challenge to identify the most promising structure or architecture that dramatically enhance the capacity while maintaining the excellent rate capability and charge−discharge cycling life. For example, cobalt oxides with a broad range of morphologies were successfully fabricated, including three-dimensional (3D) oval-shaped microparticles, 2D nanosheets, and 1D needlelike nanorods. It was demonstrated that these porous Co3O4 structures exhibit enhanced initial specific capacity (∼111 F/g), yet the rate capability and capacitance retention are still unsatisfactory.

Nanonet, a random network of nanotubes, nanowires, or nanofibers, has been recently proposed as an advantageous architecture for transparent electrodes in optoelectronic devices due primarily to high transparency, better network conductance and fault tolerance. Considering the similar requisite characteristics of electrodes in optoelectronic devices and in supercapacitors, we introduced oxide thin films with nanonet structures into supercapacitors in order to effectively enhance the specific capacitance and charge−discharge kinetics because the improved electron and ion percolation may enhance ionic and electronic transport through the electrode system.

...In this report, we present our findings on fabrication of a cobalt oxide nanonet thin film supported on a conductive carbon fiber network paper and demonstrate that such hierarchical nano/micro network architectures dramatically enhanced redox kinetics at high charge/discharge rates while maintaining electrochemical and structural stability.

—Yang et al.

They deposited Co₃O₄ with various morphologies directly onto CFP using a hydrothermal synthesis route; this deposition technique produced thin, uniform coatings of Co₃O₄ on CFP, thereby retaining the network structure of the CFP. The researchers found that the directly grown thin films can ensure good mechanical adhesion and electrical connection to the carbon fiber paper, avoiding the use of polymer binders and conducting additives (carbon or metal).

The Co₃O₄ nanonet, similar to the carbon fiber network, creates an electron and ion percolation path with high fault tolerance as well as numerous suitable pores for efficient ion access; these, the authors found, are more advantageous than the properties of Co₃O₄ nanocube coatings.

To test the CFP-supported Co₃O₄ nanonet and nanocube electrodes for electrical energy storage, they performed cyclic voltammetry measurements at different potential scan rates. They also characterized the charge−discharge behaviors of the cobalt oxide nanonet and nanocube electrodes under galvanostatic conditions.

The nanonet electrode yielded substantially higher specific capacitances than nanocube electrode. At a mass loading of 0.4 mg/cm², the specific capacitance for the nanonet electrode was 1190 F/g at 0.25 A/g, which was more than twice that of the nanocube electrode (540 F/g at 0.28 A/g) and much higher than the values reported in the literatures. Also, the capacitance of the nanocube electrode dropped to 255 F/g when the current density was increased to 14.07A/g.

However, the specific capacitance of the nanonet electrode was not kinetically limited and remained relatively constant at very high current densities, for example, reaching 1124 F/g at 25.34 A/g, which was ∼94.4% of the value at 0.25A/g. The excellent rate capability is superior to those of Co₃O₄ electrodes ever reported, even better than that of the Co₃O₄−Ag hybrid electrode...The results suggest that this hierarchical architecture is flexible in areal capacity and is ideally suited for fast and efficient energy storage.

...With this tailored architecture, the electrode shows high specific capacitance while maintaining an ideal capacitive behavior: high rate capability and excellent cycling stability, making it one of the best electrode systems for high-performance, lightweight supercapacitors. Further, the electrode design concept can be readily applied to other electrode materials (e.g., MnO₂ and V₂O₅) for supercapacitor and battery applications.

—Yang et al.

Source: Green Car Congress