Carbon nanotube based composites for electricity storage in supercapacitors

Abstract

In the context of fossil-fuel shortage and climate change, the production, conversion, storage and distribution of energy have become the focus of today's world. Supercapacitors, with their unique energy and power density specifications, cover the application gap between batteries and conventional capacitors and hence making valuable contributions in energy storage and distribution. Caron nanotubes (CNTs), with their unique aspect ratio and other distinctive physical, electrochemical and electronic properties have been chosen to enhance traditional electrode materials for supercapacitors, i.e. conducting polymer and transition metal oxides. Polypyrrole/CNTs (PPy/CNTs), polyaniline/CNTs (PAni/CNTS) and manganese oxides/CNTs (MnOx/CNTs) nanocomposites have been synthesised through chemical redox reaction in aqueous solutions. The nanocomposites have been characterised with scanning electron microscopy (SEM), transition electron microscopy (TEM), BET nitrogen surface adsorption, X-Ray diffraction (XRD), thermogravimetric analysis (TGA), infrared and X-ray photoelectron spectroscopy (XPS) to examine and to select the appropriate candidates as electrode materials. Electrochemical characterisations, i.e. cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), have been conducted with the selected nano-composites in a classic three-electrode compartment cell. Desirable capacitive behaviour, with long-term cycling stability, has been identified within appropriate potential windows for each of the nanocomposites. Asymmetric and symmetric supercapacitor prototypes have been constructed with the nanocomposites synthesised and characterised in this work. Carbon materials, due to their higher hydrogen overpotential in aqueous systems, have been proved to be good negative electrode materials in this study. Excellent specific capacitances of 1.2 F cm-2, 0.83 F cm-2 and 0.96 F cm-2 have been achieved with PAni/CNTs, PPy/CNTS and MnOx/CNTs electrodes respectively. Supercapacitor-stacks with multiplied cell voltage have been constructed with both symmetric and asymmetric prototype cells. Therefore, it has been confirmed that desirable cell voltage and capacitance can be achieved by connecting appropriate individual cells in parallel and in series to cater the requirements of the end-users. Last but not least, the prototype cells have been fitted with equivalent circuits to gain an insight into the resistive and capacitive contributions from each component. Suggestion for improvement has been given based on the simulation results of the prototype cells