Accordingly, a time constant can be calculated by τ0 = 1/(2πf0), where VG-S has a smaller τ0 of 5 ms (39.8 Hz) than 32 ms of VG-M (6.3 Hz). This indicates that VG-S system has a faster ion migration and diffusion, and thus will have a better rate capability, which is supported by the GCD results. The real capacitance plot is shown in Figure , the real capacitances of VG-M and VG-S at 10 mHz are 128 F/g and 17 F/g, respectively. The low frequency value of real capacitance corresponds to pure EDLC, where most of the electrolyte ions can access deeply inside the material. The much higher real capacitance of VG-M reveals that more electron-conductive sites in VG-M are available for electrolyte ions. By concluding the results, it is clear that VG-M has a significantly higher specific capacitance while VG-S performs better in rate capability. The similar electrochemical behaviors of the two materials indicate their same chemical nature. However, the obvious differences between VG-M and VG-S in performance and frequency-dependent behaviors are required to be further analyzed, and the mechanisms behind these features suggestively link to the morphology of the materials. The morphology of the samples was characterized by SEM, AFM and HRTEM. The cross-sectional (Figure ) and surface (Figure ) morphology of GOM, VGM and VG-S thick film on PVDF substrates were obtained from SEM. GOM shows a welldefined laminar cross-sectional feature and a smooth surface morphology. VGM 200 shows a roughly same cross-sectional and surface morphology with an increase of wrinkles. However, VG-S thick film exhibits a completely different morphology. The cross-section of VG-S can be defined as a thick film formed by stacking of fiber-like nanoparticles and the surface of VG-S presents a high degree of roughness and nonuniformity. In order to better understand the morphology of the materials that were used to fabricate the electrodes, VG-M, VG-S and GO (for comparison) were also characterized by SEM (Figure ), and the results were in consistent with those of membranes and thick films. The AFM topography shown in Figure -f provides detailed information of thickness and surface morphology of GO, VG-M and VG-S.