Thermally reduced graphene oxide using atmospheric pressure tube furnace for microsupercapacitor applications

Abstract

This work is divided into two parts, namely supercapacitor (SC) and microsupercapacitor (µ-SC). In general, the electrochemical properties of thermally reduced graphene oxide (TRGO) were investigated for both SC and µ-SC energy storage applications. The TRGO materials was successfully prepared via a series of techniques which mainly involved oxidation of graphite using Hummer’s method and the reduction of the oxygen functional groups (OFGs) on graphene oxide (GO) using atmosphere pressure chemical vapour deposition (AP-CVD) in argon atmosphere. The structural, morphological and electrical characterization of substrate (Nickel foam (NF) and microscopic glass (MSG)), prepared GO and TRGO were done using X-ray diffraction (XRD), Raman spectrometer, Fourier transform infrared (FT-IR) spectrometer, ultraviolet–visible spectroscopy (UV-Vis), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDS) and four point probe (4PP). In the SC part, TRGO-200 (numeric at the end of TRGO is the reducing temperature (RT)) was directly reduced on NF (current collector) using AP – CVD. This novel method avoided the use of binders such as polyvinylidene difluoride (PVDF) and conductive additives such as carbon nanotube and carbon acetylene black (CAB). This method is also simpler, quicker, cheaper and more effective compared to the conventional electrode preparation route where GO can be thermally reduced separately then prepared into a slurry paste with the aid of PVDF, CAB and drops of N-methyl-2-pyrolidone (NMP). The three electrode electrochemical measurements in 6 M KOH showed a maximum specific capacity of 52.64 mA h g-1 at 0.5 A g-1, while the asymmetric device consisting of TRGO on NF and peanut shell activated carbon (PAC) i.e., TRGO/NF//PAC showed a specific energy and power of 18.72 W h kg-1 and 547.52 W kg-1 respectively at 1 A g-1; and 14.10 W h kg-1 and 2.5 kW kg-1 at 5 Ag-1. The high coulombic efficiency of 99% and capacitance retention of 80% was achieved indicating an outstanding stability, suggesting a significant progress in the fabrication of binder- less and conductive enhancement free electrodes for SC energy storage devices. On the µ-SC part, a novel all – solid – state TRGO µ-SCs fabrication method was demonstrated which was simply prepared by an airbrush spray coater, AP – CVD and a mask – free AxiDraw sketching apparatus. Similar to the SC electrode preparation, this method was also quick, safe, easily scalable and cost effective as compared to the traditional µ-SC preparation method such as screen printing, inkjet printing, laser scribing and photolithography. The structure of TRGO on MSG was analysed using various techniques which indicated a decrease in oxygen functional groups (OFGs), leading to the restacking, and change of colour and transparency of the graphene sheets. The electrochemical performance showed a rectangular shape cyclic voltammetry (CV) depicting outstanding characteristics of electric double layer capacitor (EDLC) behaviour. The µ-SC with 14 digits per unit area (cm-2) showed the highest capacitance over other µ-SCs with various numbers of digits per unit area i.e. 6, 22 and 26 cm-2. This behaviour is mainly attributed to increased electric field strength as compared to the µ-SC with 6 cm-2. When the number of digits per unit area exceeds 14 cm-2 the electric field increase leading to the charge flow leakage within the electrolyte and electrode (short circuit) decreasing the capacitance. In addition, this work clearly shows the importance of OFGs on the electrochemical performance of the SCs and µ-SCs, also narrating that binder-less and conductive additive-free devices can also deliver energy and power comparable to those with binder and conductive enhancement.