Nickel-cobalt hydroxide/rGO ternary electrode materials for asymmetric supercapacitor

Supercapacitor is an interesting electrochemical device in energy storage application. Its prominent properties such as low cost, excellent cycling stability, high power density, and fast energy charging have drawn great attention over the last few decades. It mainly can be classified into three types; double layer capacitor; which is carbonbased and stores charge electrostatically, pseudocapacitor; which is mostly conducting polymer and metal oxide-based and stores charge electrochemically, and hybrid capacitor; which is combining both materials and charge storage mechanism of double layer capacitance and pseudo capacitance. Transition metal hydroxides are the most sought-after materials in hybrid supercapacitor but single component hydroxides have poor conductivity and low mass diffusion thus low capacitance. One approach to increase the capacitance is to use binary or ternary hydroxides. Recent studies have also showed that incorporating electronic conductive materials into binary or ternary hydroxides can enhance their conductivity and subsequently improved their capacitance. Graphene or reduced graphene oxide (rGO) is one of the good carbonbased electronic conductive materials. This thesis focuses on the preparation and characterization of Nickel-Cobalt hydroxide/reduced graphene oxide (Ni-Co-OHrGO) ternary electrode materials for supercapacitor. Ni-Co-OH-rGO was prepared by two-electrode electrodeposition in aqueous electrolyte consist of rGO (0.001g mL-1), Ni(NO3)2·6H2O and Co(NO3)2·6H2O (molar ratio of Ni to Co is 1:3, fixed to 0.01M metal ions) with Ni foil as both positive and negative electrode. Images and elemental mappings on scanning electron microscopy (SEM) show that Nickel-Cobalt hydroxides (Ni-Co-OH) and rGO uniformly distributed. Beside the peaks of nickel plate, X-ray diffraction (XRD) patterns of Ni-Co-OH-rGO show broad and undefined peaks, indicating the amorphous nature of Ni-Co double hydroxide. Two peaks (anodic and cathodic) are observed in cyclic voltammograms of Ni-Co-OH show redox reaction thus, this material is reversible. Ni-Co-OH with 1:3 Ni to Co ratio has the highest geometric capacitance of 0.25 F cm-2 at scan rate of 1 mV s-1 which in consistent with computational study where Ni-Co-OH 1:3 has the lowest band gap energy; therefore, it was chosen to be composited with rGO. The presence of rGo has increased the electrochemical performance of Ni-Co-OH; where, Ni-Co-OH-rGO deposited at -1.5 mA cm-2 (denoted as Ni-Co-OH-rGO1.5) possessed the best electrochemical performance and has geometrical capacitance of 0.40 F cm-2 at 1 mV s-1 which is nearly two times as that of Ni-Co-OH thus, chosen as electrode for supercapacitor. Asymmetric supercapacitor (ASC) was fabricated using Ni-Co-OH-rGO1.5 as the cathode and activated carbon cloth as the anode. Galvanostatic charge-discharge (GCD) was performed to Ni-Co-OH-rGO1.5 || AC ASC and it shows that the specific capacitance of the fabricated supercapacitor is 235.7 F g-1 and has relatively good cycling stability as it retained 89% of its initial capacitance after 2000 GCD cycles at current density of 2 A g-1. The maximum energy density and power density of Ni-Co-OH-rGO1.5 || AC ASC is 64.2 W h kg-1 at 135.9 kW kg-1 respectively which is remarkable for application in the field of energy storage such as electronic devices.