Boosting Supercapacitor Performance of Carbon Fibres Using Electrochemically Reduced Graphene Oxide Additives. (Oral; The Main Work Has Been Published Online On Physical Chemistry Chemical Physics Lately)

A kind of hybrid fibres as supercapacitor electrodes based on carbon fibres and graphene additives by a synchronous electrochemical reduction and deposition method was demonstrated. The electrochemically reduced graphene oxide coated carbon fibres (ERGO@CF) with only a small amount addition (~1 wt. %) of ERGO displayed excellent electrochemical performance in both aqueous and gel electrolytes. The superior capacitive performance could be attributed to the synergistic effect between wrinkled and porous ERGO sheets and highly conductive CFs.

Fig. 1(A) Schematic diagram of ERGO@CF. (B) Digital photograph of a home-made setup for continuous preparation of long ERGO@CFs. (C) (D) SEM image of ERGO@CF with gully-like porous structure of ERGO on the surface of CF.

In the electrochemically deposition process, GO sheets were gradually coated on the surface of CFs, meanwhile most of oxygen containing functional groups on GO sheets were removed, indicating that GO sheets were electrochemically reduced. The chemical structures of GO and ERGO were verified by Raman spectra and X-ray photoelectron spectroscopy (XPS).

The specific capacitance of an optimized single hybrid ERGO@CF fibre was measured to be as high as 22.6 µF cm^-1 (10.3 µF cm^-2) in 1 M Na₂SO₄ aqueous electrolyte at a scan rate of 10 mV s^-1, which was around three orders of magnitude enhanced compared to pure CF. The hybrid fibre electrodes showed excellent capacitive stability after thousands of charge-discharge cycles.

Fig. 2(A) Cyclic voltammetry (CV) curves of a single ERGO@CF at different scan rates in aqueous electrolyte. (B) Galvanostatic charge-discharge curves of ERGO@CF at different current densities in aqueous electrolyte. (C) Ragone plot of a solid supercapacitor indicates that the energy density does not decrease significantly when the power density increases.

Solid supercapacitor that combined two ~250 hybrid ERGO@CF fibres together coated with gel electrolyte (PVA-H₃PO₄) was studied to possess a specific capacitance of up to 13.5 mF cm^-1 (307 mF cm^-2) at a current density of 0.05 mA cm^-1. Meanwhile, the energy density did not decrease notably as the power density increased. The maximum power density of such a solid supercapacitor could reach 0.74 mW cm^-1 (8.5 mW cm^-2) and the maximum energy density was measured to be 1.9 µWh cm^-1 (21.4 µWh cm^-2), representing a relatively high values compared with similar fibre-like ones.

This fibre electrode material also offered advantages such as easy operation, mass production capability, mechanical flexibility and robustness, and could be easily mass-produced with the possibility to overcome the major hurdle in the development of large scale applications of graphene.