Abstract

Presently, there are deep concerns over the environmental consequences and the consumption of non-renewable energy sources, with the accelerated greenhouse effect which triggered enormous interest in the use of renewable energy sources e.g. solar, hydropower, wind and geothermal. However, the intermittent nature of harvesting renewable energy sources has recently gained considerable attention in the alternative, reliable, cost effective and environmentally friendly energy storage devices. The supercapacitors are considered more efficient electrical energy storage devices than the conventional energy storage systems. This is due to the fact that they are usable in plethoric wide range of devices owing to their high power and high energy density. This is particularly true as they have found usability in portable electronic devices and Electrical Vehicles (EVs) or Hybrid Electrical Vehicles (HEVs) in recent times. In order to make the efficient usage of these stationary energy storage devices, state of the art research on new and advanced electrode material is highly needed. The aim of this research is to investigate the scope of graphenepolyaniline nanocomposite electrodes for light weight, high power density and wider voltagewindow supercapacitor devices. Graphene-polyaniline nanostructured composite films fabricated on both non-conducting quartz and FTO substrates at ambient temperature is reported. The top-quality and well-reproducible nanostructured composites have been derived through the oxidative polymerization of anilinium ion by ammonium persulphate, APS [(NH4)2S2O8] in an acidic aqueous medium using chemical bath deposition (CBD) on pre-deposited graphene films synthesized through vacuum filtration of ultrasonicated turbostratic graphite flakes. These films were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), x-ray diffraction (XRD), UV-vis spectrophotometry, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD). The AFM images, depending on the annealing temperature, show that the films are composed of both nearly cone-shaped and irregular spherical-shaped grains/clusters uniformly distributed with lots of pores on the surface area. Also, the thicknesses, ranging from ~50 – 96 nm of the films were found to be dependent on the annealing temperatures of synthesized thin films. An average rootmean-square (RMS) value of roughness of ~ 103 nm was estimated for polyaniline, graphene and composite films. The SEM analysis corroborates the AFM results as they depict homogenous, nanofibrous and highly porous surface which also increases with annealing temperatures of the films. The XRD analysis confirmed that polyaniline films are structurally amorphous even when xvii annealed in nitrogen atmosphere at high temperature while the graphene and graphene-polyaniline nanocomposite shows sharp and well defined peaks at ~260 corresponding to [0 0 2] plane with differing intensities. Particularly for the composite films, we observe that graphene retained its highly ordered structure when dispersed in polyaniline matrix during polymerization. The optical absorption analysis of polyaniline, graphene and graphene-polyaniline hybrid films revealed that direct optical transition exists in the photon energy range 1.27 – 6.05 eV, 1.13 – 6.30 eV and 1.13 – 4.50 eV with bandgaps 2.30, 1.24 and 2.60 eV respectively. The refractive index has peak at 405 nm, 474 nm and 430 nm in the dispersion region 300 – 1100 nm respectively. The films exhibit high transmittance of about 80% in the visible region of the e-m spectrum. For the electrochemical studies, polyaniline, graphene and graphene-polyaniline hybrid electrodes were tested using CV and GCD techniques in 1 M H2SO4 electrolyte within the potential range -0.1 to +0.8 V vs at 0.5 mA.cm2 current density. We observed through CV, the adverse effects of annealing temperature on the supercapacitive properties of the polyaniline electrodes. This is adequately confirmed by the decrease in the specific capacitances of polyaniline electrodes as annealing temperature increases. Graphene-polyaniline composite electrode revealed reasonably good specific capacitance of 983 F/g than individual electrodes of polyaniline and graphene having 872 and 433 F/g respectively. The specific power, specific energy and columbic efficiency of the composite film is also seen to be enhanced than the pristine materials with values of 5.28 kW/kg, 997 Wh/kg and 64%. The GCD analysis corroborates the CV results. As well, the stability and chargedischarging ability of the electrodes revealed a better profile for the composite film which is as a result of the introduction of graphene preventing polyaniline from mechanical deformation (i.e. shrinkage and breakage). Hence, we found that our system could withstand about 2000 cycles without a significant decrease in the specific capacitance, which adequately clarify highly stable (90%) nature of graphene-PANI composite electrode in energy storage applications.