## 3 Results  
Figure 1 presents the XRD patterns of the calcination products. The identified diffraction peaks of all samples can be assigned to the face centered Co\({}_{3}\)O\({}_{4}\) (JCPDS No. 42-1467, space group: _Fd3m (227)_), suggesting a high purity of the samples [30]. According to the Scherrer equation [31], the crystallite sizes of Co\({}_{3}\)O\({}_{4}\)-1, Co\({}_{3}\)O\({}_{4}\)-2 and Co\({}_{3}\)O\({}_{4}\)-3 are around 23.6, 20.0 and 24.3 nm, respectively. Typical SEM images of ZIF-67 are shown in Figure 2a-c. The ZIF-67 present monodispersity and the average sizes of ZIF-67-1, ZIF-67-2 and ZIF-67-3 are 0.39 \(\pm\) 0.05, 1.98 \(\pm\) 0.18 and 3.28 \(\pm\) 0.37 \(\upmu\)m, respectively. Namely, the average sizes increase with the decrease of 2-methylimidazole concentration. This phenomenon is attributed to the stepwise formation of ZIF-67. The process can be briefly divided into a nucleation phase and growth stage. In the initial nucleation phase, the  
Figure 1: X-ray powder diffraction (XRD) patterns of Co\({}_{3}\)O\({}_{4}\)-1, Co\({}_{3}\)O\({}_{4}\)-2 and Co\({}_{3}\)O\({}_{4}\)-3.  
increased concentration of 2-methylimidazole enhanced the nucleation rate, determining a small size of each unit cell. Subsequently, diffusive particles slowly grew around the cell to form ZIF-67 [32; 33]. After the conversion, the ZIF-67 transferred to hollow Co-LDH with a significant morphology change. Figure 2d-f shows that there were nanosheets vertically growing on the surface of as-formed ZIF-67 and the average sizes shrunk a little bit. Meanwhile, with the continuous dissolution of the inner cobalt, the hollow Co-LDH nanoboxes with different sizes were formed. Hollow Co\({}_{3}\)O\({}_{4}\) were obtained through calcination, as displayed in Figure 2g-i. Beyond expectations, the overall size and hollow feature were well maintained even after heat treatment, except for Co\({}_{3}\)O\({}_{4}\)-1 due to the very thin configuration of the shell. It could be dimly seen that the shell of hollow Co\({}_{3}\)O\({}_{4}\) consisted of small nanoparticles. The CV measurements in a potential range of 0-0.65 V (vs. Ag/AgCl) were first characterized to reveal the supercapacitors behavior of the Co\({}_{3}\)O\({}_{4}\) electrodes. Figure 3a presents CV curves of the Co\({}_{3}\)O\({}_{4}\) with different sizes at a scan rate of 5 mV/s. As expected, all samples deliver a distinct pair of peaks induced by a faradaic redox reaction, which clearly reveals the typical pseudocapacitive characteristics [23; 24]. In particular, the integrated area under CV curve of Co\({}_{3}\)O\({}_{4}\)-2 electrode is the largest among three Co\({}_{3}\)O\({}_{4}\) electrodes, demonstrating that the Co\({}_{3}\)O\({}_{4}\)-2 possesses the largest specific capacitance [34]. The CV curves of the Co\({}_{3}\)O\({}_{4}\)-2 electrode measured at typical scan rates from 5 to 100 mV/s are displayed in Figure 3b. The following classical and desirable pseudocapacitive phenomena were observed: (1) the CV curves are highly symmetrical; (2) the peak currents remarkably increase along with the increase of scan rates; (3) the peak position also changes accompanied by the increase of scan rates. The peaks generated in the anodic sweep shifted to a higher potential value, while the peaks which originated from the cathodic sweep moved towards a lower potential value; (4) no  
Figure 2: Scanning electron microscope (SEM) images of dodecahedral diamond zeolitic imidazolate framework-67 (ZIF-67): (**a**) ZIF-67-1, (**b**) ZIF-67-2, (**c**) ZIF-67-3; hollow Co-layered double hydroxide (Co-LDH): (**d**) Co-LDH-1, (**e**) Co-LDH-2, (**f**) Co-LDH-3; hollow Co\({}_{3}\)O\({}_{4}\): (**g**) Co\({}_{3}\)O\({}_{4}\)-1, (**h**) Co\({}_{3}\)O\({}_{4}\)-2, (**i**) Co\({}_{3}\)O\({}_{4}\)-3.  
significant distortion existed in a comparison of CV curves obtained at 5 and 100 mV/s.