Investigation of different graphite precursors for graphene oxide supercapacitors.

Abstract

The impact of non-renewable energy sources has had adverse effects on the environment resulting in climate change. Many countries have undertaken a call for renewable energy sources as a cleaner and more sustainable alternative. With the change towards renewable energy sources, storing this energy has become a challenge due to its intermittent nature. An energy storage device that could help solve the above problem is a supercapacitor due to its high power density, long cycle life, and high rate capability, which are desirable characteristics for energy storage devices. Supercapacitors downfall is their low energy density. Improvement in the electrode material of the supercapacitor may help address the low energy density issue. A promising candidate is graphene oxide (GO). GO has shown notable potential as electrode material in past research due to the pseudocapacitance effect. The primary precursor for the synthesis of GO is graphite. Varying graphite precursors may yield GO with different properties. In this research, graphite precursors with different characteristics were investigated to determine the effect they have on GO supercapacitor energy storage capabilities. Eight graphite precursors were used to synthesise GO. The samples were characterised using High-Resolution Transmission Electron Microscopy (HRTEM), Scanning Electron Microscopy (SEM), Elemental Analysis, Fourier Transform Infrared (FTIR) Spectroscopy and Raman Spectroscopy. GO supercapacitors were fabricated using GO as an active electrode material, stainless steel plates as current collectors, and phosphoric acid (H3PO4) hydrogel polymer as electrolyte and separator. The electrochemical testing conducted on GO electrode material were Cyclic Voltammetry (CV) at different scan rates to determine specific capacitance and energy density. It was found that increasing flake size of natural graphite precursors produced GO with higher specific capacitance with an implicit limiting point. With the lack of peaks between the voltage limits of the CV curves for GO produced from natural graphite precursors, this indicated that the pseudocapacitance effect from oxygen functional groups is insignificant to the overall specific capacitance for these samples. These results led to further research into other possible factors that can be playing a role in GO’s high specific capacitance. It was observed that GO produced from the smallest flake size (0.045 mm) synthetic graphite precursor had the highest specific capacitance compared to GO produced from natural graphite precursor of all flake sizes investigated in this research. Firstly, the synthetic GO sample produced from smallest flake size had higher crystallinity compared to natural GO samples which were estimated using Raman spectroscopy. Secondly, high oxygen content shown in elemental analysis and peaks observed between voltage limits of CV curves provided a high pseudocapacitance effect that is significant to the overall capacitance. Thirdly, low amount of defects determined from low ID/IG (Intensity of D band/Intensity of G band) ratio in Raman spectrum may enable the ions from the electrolyte to move through the GO structure efficiently. It is the combination of these characteristics that attribute to GO produced from smallest flake size (0.045 mm) synthetic graphite precursor that improved energy storage capabilities and be an excellent electrode material to use in supercapacitors.