## 3 Results
Notably, the obvious increase of peak currents and excellent symmetries in CV curves as well as negligible distortion indicate the rapid and well reversible redox process [30], further possibly resulting in an outstanding rate capability. GCD tests between potential window of 0-0.65 V (vs. Ag/AgCl) were conducted to further investigate the capacitive properties. The discharge curves of Co\({}_{3}\)O\({}_{4}\)-1, Co\({}_{3}\)O\({}_{4}\)-2 and Co\({}_{3}\)O\({}_{4}\)-3 electrodes at representative current densities of 1, 2, 5, 10 and 20 A/g are presented in Figure 4a-c, respectively. It is intuitive that all discharge curves deliver evident plateaus coinciding with the redox reaction, which also confirms the pseudocapacitive behavior of each Co\({}_{3}\)O\({}_{4}\)[25,30]. In addition, the GCD performances comparison at current densities of 1 A/g shown in Figure 4d also confirms the Co\({}_{3}\)O\({}_{4}\)-2 sample offers the largest specific capacitance [29], which is consistent with the aforementioned CV result (Figure 3a). Figure 4: Discharge curves at representative current densities ranging from 1 to 20 A/g: (**a**) Co\({}_{3}\)O\({}_{4}\)-1, (**b**) Co\({}_{3}\)O\({}_{4}\)-2, (**c**) Co\({}_{3}\)O\({}_{4}\)-3; (**d**) discharge curves of Co\({}_{3}\)O\({}_{4}\)-1, Co\({}_{3}\)O\({}_{4}\)-2 and Co\({}_{3}\)O\({}_{4}\)-3 electrodes at current density of 1 A/g. Figure 3: (**a**) Cyclic voltammetry (CV) curves of Co\({}_{3}\)O\({}_{4}\)-1, Co\({}_{3}\)O\({}_{4}\)-2 and Co\({}_{3}\)O\({}_{4}\)-3 electrodes at scan rate of 5 mV/s; (**b**) CV curves of Co\({}_{3}\)O\({}_{4}\)-2 electrode at typical scan rates ranging from 5 to 100 mV/s. The specific capacitances based on GCD measurements are calculated according to Equation (1) for all Co\({}_{3}\)O\({}_{4}\) electrodes, and the results are plotted in Figure 5. The Co\({}_{3}\)O\({}_{4}\)-2 electrode exhibits the specific capacitances of 1832.7, 1760.9, 1640.4, 1526.6 and 1324.5 F/g at 1, 2, 5, 10 and 20 A/g, respectively. Moreover, Figure 5 demonstrates that the boost in current densities leads to a decrease in specific capacitances. Unfortunately, the similar fading tendencies of Co\({}_{3}\)O\({}_{4}\)-1, Co\({}_{3}\)O\({}_{4}\)-2 and Co\({}_{3}\)O\({}_{4}\)-3 are inevitable owing to the correlation between current density and electrolytic ions diffusion (OH-in this study) in electrode material (Co\({}_{3}\)O\({}_{4}\) in this study). Namely, compared with the diffusion at high current density, the OH- ions have relatively sufficient time to transfer at a low current density. The phenomenon gives rise to inner active sites failing to be comprehensively devoted to the redox reaction at high current density. In contrast, both the inner and outer surface can be involved at low current density, thoroughly making a redox reaction proceed and thus contributing to a large specific capacitance [35]. Nevertheless, the Co\({}_{3}\)O\({}_{4}\)-2 electrode can maintain capacitance of 72.3% from 1 to 20 A/g (43.5% of Co\({}_{3}\)O\({}_{4}\)-1, 62.6% of Co\({}_{3}\)O\({}_{4}\)-3). The slight decay provides the Co\({}_{3}\)O\({}_{4}\)-2 bright application with the high power required for energy storage devices. The relieved fading in specific capacitances can be explained by the unique morphology of synthesized Co\({}_{3}\)O\({}_{4}\). During the redox process, the hollow structure can serve as an \"OH- ions buffer reservoir\" [35] to effectively minimize the influence of limited ions diffusion at a high current density on rate performance. In short, the impressive performances in specific capacitance and rate capability make the as-synthesized Co\({}_{3}\)O\({}_{4}\)-2 a promising electrode material in supercapacitors application. Figure 6a illustrates the Nyquist plots of different Co\({}_{3}\)O\({}_{4}\) electrodes from frequency range of 100 mHz to 100 kHz. The plots of Co\({}_{3}\)O\({}_{4}\)-1, Co\({}_{3}\)O\({}_{4}\)-2 and Co\({}_{3}\)O\({}_{4}\)-3 are similar, which consists of a semicircle and a straight line.