Figure 4d shows the Nyquist Selleck Tariquidar plots for the ZnO, pristine Gr, and graphene-ZnO hybrid electrodes. All these plots display a semicircle in the high-frequency region and a straight line in the low-frequency region. The straight line in the low-frequency range is called the Warburg resistance, which is caused by the frequency dependence of ion diffusion/transport from the electrolyte to the ATR inhibitor electrode surfaces . The arc for the very high-frequency range corresponded to the charge transfer limiting
process and was ascribed to the double-layer capacitance in parallel with the charge transfer resistance (Rct) at the contact interface between the electrode and electrolyte solution . The Rct can be directly measured from the Nyquist plots as the semicircular arc diameter. The Rct for the graphene-ZnO hybrid electrode is 3.5 Ω, which is substantially smaller
than those of pristine ZnO (26.4 Ω) and Gr (8.2 Ω) electrodes, indicating the better conductivity of the graphene-ZnO hybrid electrode. It indicated the incorporation selleck inhibitor of ZnO nanorods into the graphene nanosheets, resulting in an improved charge transfer performance for the electrode. Figure 5 showed the effects of ZnO amount on electrochemical properties. It can be seen that increasing the ZnO content can improve the electrochemical properties of graphene-ZnO hybrid. However, the electrochemical properties of graphene-ZnO hybrid decreased when the ZnO content is excess 60%. The reason is due to the poor conductivity of ZnO. Figure 5 Effects of ZnO amount on electrochemical properties. To test their feasibility for application as an energy storage device, solid-state symmetrical supercapacitors based on graphene-ZnO hybrid were fabricated by sandwiching H2SO4-PVA-based solid-state electrolyte between two pieces of graphene-ZnO electrodes (Figure 6a). CV curves of the solid-state supercapacitor device
measured at various scan rates are collected in Figure 6b. All the CV curves exhibit a rectangular-like shape, which reveals the ideal capacitive behavior and fast charge–discharge behavior. Figure 6c shows the galvanostatic charge–discharge curves of the solid-state supercapacitor device collected at different current densities. The discharge curves of this Anacetrapib device are relatively symmetrical with its corresponding charge counterparts, confirming the good capacitive behavior and fast charge–discharge behavior of the fabricated supercapacitor device. The specific capacitance for the electrodes can be obtained from charge–discharge data according to Equation 2 (2) where C (F g−1) is the specific capacitance, I (A) is the constant discharging current, ∆t (s) is the discharging time, ∆V (V) is the potential window, and m (g) is the mass loading of the active material in the working. The specific capacitances of the graphene-ZnO hybrid electrode are 196, 115, and 102 F g−1 at the current densities of 0.8, 2.5, and 4.0 mA cm−2, respectively.