## 3 Results and Discussion  
The morphology of as-prepared Co\({}_{3}\)O\({}_{4}\) was investigated by SEM. Figure 1 presents SEM images of the precursor (Figure 1a,b) and the calcined Co\({}_{3}\)O\({}_{4}\) (Figure 1c,d). Surprisingly, the Co\({}_{3}\)O\({}_{4}\) effectively inherited the morphology of the precursor (negligible size contraction and few broken pieces) and exhibits uniform spherical structures. Furthermore, the magnified image (Figure 1d) shows that the spheres tend to interconnect with one another and clearly confirms the hollow nature, as well as the thin shell thickness of the Co\({}_{3}\)O\({}_{4}\) from the view of a broken piece. The diameter of the spheres lies between 500 and 600 nm. In addition, the shell is assembled from numerous nanoparticles, constructing a highly porous architecture. This structure results in more active sites on the surface and easier transportation of ions, leading to better electrochemical performance. The XRD patterns seen in Figure 2a, where the eight identified diffraction peaks located at 19\({}^{\circ}\), 31.3, 36.9, 38.6, 44.9, 55.8, 59.5, and 65.4\({}^{\circ}\) are indexed to the (111), (220), (311), (222), (400), (422), (511), and (440) planes respectively, are perfectly consistent with the cubic phase of Co\({}_{3}\)O\({}_{4}\) (JCPDS card No. 65-3103) [33]. The element compositions were confirmed by EDX, as depicted in Figure 2b. The peaks correspond to the elements Co and O, respectively. No other impurity element was observed. Both XRD and EDX characterizations reveal the high purity of the product. The XPS was used to analyze the surface component and valence state of the as-obtained Co\({}_{3}\)O\({}_{4}\). In the Co 2p core-level XPS spectrum seen in Figure 3a, the Co 2p was deconvoluted into two doublets. The peaks at 779.4 and 794.4 eV are related to Co\({}^{3+}\), and the peaks at 780.4 and 795.7 eV are assigned to Co\({}^{2+}\)[34]. In the high-resolution XPS spectrum of O 1s seen in Figure 3b, the O 1s can be fit into three different peaks at binding energies of 529.9, 530.9, and 532 eV, which are referenced to the lattice oxygen, the OH\({}^{-}\) species absorbed onto surface, and the multiplicity of physical and chemical absorbed water near surface, respectively [35]. This analysis of XPS further verifies the successful synthesis of the Co\({}_{3}\)O\({}_{4}\). Figure 1: SEM images of (**a**) low- and (**b**) high-magnification of the precursor; (**c**) low- and (**d**) high-magnification of the calcined Co\({}_{3}\)O\({}_{4}\). The electrochemical performance of as-formed Co\({}_{3}\)O\({}_{4}\) as an electrode material for supercapacitors was evaluated systematically. Figure 4a shows CV curves of the Co\({}_{3}\)O\({}_{4}\) at various scan rates within a potential window of 0-0.55 V. As expected, the CV curve shape is totally different from those of electric double-layer capacitors (which have an almost rectangular shape), which reveals that the capacitance characteristics of the Co\({}_{3}\)O\({}_{4}\) are those of typical pseudocapacitive capacitance. Additionally, two distinct pairs of redox peaks are visible in the CV curves. The first redox couple is ascribed to the faradic redox reaction of Co\({}^{2+}\)/Co\({}^{3+}\), expressed as Equation (3) [29; 30; 36]:  
\[\mathrm{Co_{3}O_{4}+OH^{-}+H_{2}O\leftrightarrow 3 CoOOH+e^{-}}, \tag{3}\]  
and the second redox couple corresponds to the conversion of Co\({}^{3+}\)/Co\({}^{4+}\), expressed as Equation (4) [29; 30; 36]:  
\[\mathrm{CoOOH+OH^{-}\leftrightarrow CoO_{2}+H_{2}O+e^{-}}. \tag{4}\]  
When the scan rate increases from 5 to 100 mV/s, the peak current increases enormously. Furthermore, the anodic and cathodic peaks shift to higher and to lower potentials, suggesting classical pseudocapacitive behavior. The CV curve of the Co\({}_{3}\)O\({}_{4}\) at 100 mV/s retains a redox shape similar to the original shape at 5 mV/s. It implies that the Co\({}_{3}\)O\({}_{4}\) possesses an excellent rate capability, which is further confirmed by the calculated specific capacitances.