(255d) Supercapacitive Behavior of Co(OH)2/Graphene Nano Sheet Composite Prepared By Electrodeposition

Electric double layer capacitor (EDLC) is widely known as one of the most promising devices due to high power density, long cycle life, excellent reversibility and speedy charging time. The positive and negative electrodes of EDLC are made of the same materials like carbon materials that possess a high surface area. Among carbon materials, graphene nano sheet (GNS) is considered to be the most promising materials in various energy devices field due to its large surface area, good electrochemical ability and superior electric conductivity. Intensive studies to improve the energy storage capability of GNS have been conducted by preparing the composites of GNS and other materials such as conducting polymers and metal oxides. In this study, GNS based Co(OH)₂ composite for application to electrode materials of supercapacitor was synthesized by a cathodic deposition method. The prepared composites have been characterized by various analytical tools. The supercapacitive behavior of the composites has been investigated by cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS). The functionalized GNS (fGNS) was prepared to attach the hydrophilic oxygen functional group on GNS surface by acid treatment in prior to the electrodeposition of Co(OH)₂. For comparison, GNS was also employed as a substrate to deposit the active material, Co(OH)₂. A graphite rod and platinum foil as the counter electrode, Ag/AgCl reference electrode and Ti mesh as working electrode were used in 1M KOH aqueous electrolyte and 0.1 M Co(NO₃)₂·6H₂O electrolyte solution. Chronoamperomety was employed to cathodically deposit Co(OH)₂ onto GNS electrode at more negative potential than OCP. After deposition, the SEM image of deposited Co(OH)₂  on GNS shows vertically grown crystal on surface of GNS. The deposition of Co(OH)₂ was also evidenced from Co 2p and O1s XPS results that can identify Co(OH)₂ functional groups. The capacitance of Co(OH)₂/GNS and Co(OH)₂/fGNS prepared here were found to be 1138 and 2148 mF/cm², respectively, which is 20 times greater than that of GNS (65 mF/cm²). An asymmetric coin-cell type supercapacitor with Co(OH)2/fGNS and activated carbon electrodes was fabricated for the performance test.