Synthesis and characterization of a novel sulphur-reduced graphene oxide/metal oxides composite for supercapacitor applications

Abstract

This study is focussed on synthesis and characterization of a novel sulphur-reduced graphene oxide/metal oxides composite for supercapacitor applications. Carbon-based materials have validated its capability in various applications, especially as electrode materials for energy storage devices hence become an attention-grabbing to the researchers, however, they suffer from low specific energy. Thus, further modification is still required for commercial level applications. Specifically, graphene-based composites materials have displayed immeasurable feasibility since they can be chemically combined with other carbon-based/metal oxides materials and with a range of different elements to form strong covalent bonds which improve their electrochemical properties. Hence, it is essential to study and produce different carbon-based/metal oxides composites materials by optimizing their morphology and surface functionalities so as to enhance their electrochemical performance. This study has established synthesis of sulphur-reduced graphene oxide composites materials which are discussed in different sections in chapter four. The composite materials were synthesized by Hummers, hydrothermal and precipitation methods. Some of the composite materials have gone through freeze drying and annealing process to complete their synthesis. Morphology, structure, elemental mapping/composition and specific surface area/pore size distribution were analysed by field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Brunauer-Emmett-Teller (BET) techniques. The electrochemical performance was evaluated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in three and two electrode configurations. The specific capacity of 113.8 mAh g-1 was obtained for RGO-S at 0.5 A g-1 which is much higher compared to RGO with a specific capacity of 12.5 mAh g-1 at the same specific current. The fabricated hybrid device (RGO-S//C-FP) presented a high specific energy and specific power of 35.2 Wh kg-1 and 375 W kg-1 at 0.5 A g-1 in an working voltage of 1.5 V, respectively. A good cycling stability performance with an energy efficiency of 99% was recorded for the hybrid device for up to 10,000 cycling at 3 A g-1. Also, an optimization was done by varying mass loading of MnO2 (50, 100 and 150 mg) into the pristine sample (RGO-S) and the 100 mg MnO2 was studied as the optimal amount to have good synergy between the RGO-S and MnO2 due to the higher electrochemical performance demonstrated by RGO-S/100 mg MnO2 composite. The RGO-S/100 mg MnO2 composite presented a specific capacitance of 180. 4 F g-1 at 1 A g-1. The assembled device (RGO-S/MnO2//AC-PS) presented an outstanding specific energy of 71.7 Wh kg-1 with its corresponding specific power of 850 W kg-1 at 1 A g-1. An outstanding performance was noted when the device was able to withstand a specific energy of 55.3 Wh kg-1 at a high specific current of 5 A g-1. The capacitance retention of 94.5 % and columbic efficiency of 99.6 % up to 10,000 cycles at 5 A g-1 was recorded. A voltage holding of up to 90 h was attained with an efficiency of 70.5 %. An exceptional self-discharge of about 1.45 V was recorded within the first 10 h, and 1.00 V after 72 h out of the initial potential of 1.7 V. Additionally, an optimized RGO-S/200 mg Co3O4 composite material showed the uppermost specific capacity of 171.8 mAh g-1 at 1 A g-1 and an excellent stability of 99.7 % for over 5000 cycles at 5 A g-1 in three-electrode configuration. The assembled supercapattery device (RGO-S/200 mg Co3O4//AC-PS) demonstrated high specific energy of 45.8 Wh kg-1 and specific power of 725 W kg-1 at 1 A g-1 in an operating potential of 1.45 V. The constructed device retained 83.4 % of its initial capacitance for over 10,000 cycles, with a columbic efficiency of 99.5 % at 8 A g-1. The device maintained an efficiency of 71.6 % over an outstanding floating time of 150 h at 10 A g-1. The outcomes of this study has established an impressive capability of sulphur-reduced graphene oxides composites as a novel electrode materials for supercapacitor applications.