Supercapacitors have received a considerable amount of attention recently in research fields related to energy storage devices. These devices have numerous advantages, such as a short charge-discharge time, a high-power density, long-term cycling ability, and chemical stability. However, they have lower capacitance and energy densities than batteries. Therefore, research on supercapacitors continues in an effort to improve the capacitance and energy density.
Electro-deposition is a simple method with which to deposit metal oxides or hydroxides. It is associated with low costs, short experimental times, and easy control of the thickness and weight of the deposited metal oxides or hydroxides.
In this work, Ti-mesh, graphene nanosheet (GNS), and functionalized graphene (fGNS) substrates were used in supercapacitors. fGNS has many functional groups with C-O bonds as compared to GNSs. The C-O bonds can increase the adsorption sites of metal oxides or hydroxides through an electro-deposition method. Co(OH)2 was deposited at 8.1, 9.0 and 9.5 mg onto Ti mesh, GNS and fGNS substrates at a deposition voltage of ?1.0 V, respectively. From the SEM images, Co(OH)2 was deposited onto the surface of an fGNS substrate, which has a thinner cross-section and thicker sheets of Co(OH)2 than Ti mesh and GNS substrates. The density of the Co(OH)2 is also enhanced. Moreover, the different electro-deposition voltage permits a controllable thickness of the Co(OH)2 film with deposition potentials ranging from ?1.4 to ?0.75 V under potentiostat conditions. When the electro-deposition voltage is applied in a lower direction, the thickness of the Co(OH)2 reaches its maximum values of 33.96, 22.89. and 16.65 ㎛ with Co(OH)2-Ti, Co(OH)2-GNS and Co(OH)2-fGNS electrodes deposited at ?1.4 V, respectively. The capacitances of supercapacitors deposited at various electro-deposition voltages were measured by the CV and CD methods. Co(OH)2-Ti, Co(OH)2-GNS and Co(OH)2-fGNS electrodes deposited at ?1.4 V are the most valuable composites, showing values of 1290, 1758.5 and 3636.5 mF/cm2 at a cyclic voltammetry (CV) of 5 mV/s. Among the prepared electrodes, the Co(OH)2-fGNS electrode is utilized to fabricate a hybrid capacitor because the cycle retention is 90.5 % at an electro-deposition voltage of ?1.0 V. The characteristics of the hybrid capacitor are also evaluated by electrochemical methods. A hybrid capacitor that combines the characteristics of an EDLC and a pseudocapacitor is regarded as a feasible means of improving low-energy densities in the field of energy storage systems. A hybrid capacitor consists of activated carbon (AC) and Co(OH)2-fGNS depending on the type of beaker cell. The electrochemical behavior shows a value of 854 mF/cm2 at a CV of 5 mV/s, with cycle stability of 89.7 %. The energy density is 78 Wh/kg and power density is 1137 W/kg.