## 3 Results and Discussion
The longer process time is needed for stabilizing the micro-sized architecture with a hollow \(\alpha\)-MnO\({}_{2}\) structure. The featured microstructure is believed to provide large surface areas and more active sites that are accessible for electrochemical reactions, which will be proven in the following electrochemical experiments. The variation of shapes and morphologies of produced samples were correlated to the change in BET surface areas, average pore sizes at maximum volume distribution, and the total pore volumes. The BET results for each sample are summarized in Table 1. Samples B, C, and D show a decreased trend in terms of BET surface area as a result of the elongation of nanoparticles from the spherical shape to the nanorod. Sample E, formed with long nanofibres, shows the lowest BET surface area and pore volume. These results indicate that both the surface features and the shape of particles determine BET surface area. Figure 5 displays the plots of pore surface distribution versus pore diameters. It can be seen that Sample B presents a sharp pore size distribution with respect to the pore surface area, while Samples C and D show a wide distribution of surface areas of various pore sizes. This indicates that the hollow microstructures displayed by Samples C and D provide different pore structures with different sizes. Moreover, Sample E discloses a steady decrease in pore surface area as pore size increases, implying an absence of a nano-sized pore distribution in such a structure. As we will see later, all those differences can affect the electrochemical capacitive performance. For investigating the material's capacitance in looking at the supercapacitor applications, the cyclic voltammograms (CVs) of Samples B, C, D, and E, were recorded at a potential scan rate of 0.1 V/s in a 0.1 M Na\({}_{2}\)SO\({}_{4}\) aqueous solution and are shown in Figure 6, respectively. The loading of \(\alpha\)-MnO\({}_{2}\) on the working electrode was 0.4 mg/cm\({}^{2}\). The CVs of Samples B, C, and D display a well-defined rectangular shape, indicating a typical capacitive behaviour. A steady increase in current densities as the synthesis reaction duration increases can be observed. The highest current density is obtained for Sample D, probably due to the uniform distribution of hollow nanorods decorated with fine nano-features on the surface. The current density of Sample E is lower than that of Samples B, C, and D, which is due to the potentially reduced active sites in Sample E. Since the capacitance of electrode materials is proportional to the current density, Sample E thus has a lower electrochemical performance compared to the other samples. The electrochemical capacitance of \(\alpha\)-MnO\({}_{2}\) samples is decreased in the following order: Sample D > Sample C > Sample B > Sample E. As we stated earlier, even though Sample D shows a slightly lower BET surface area than Sample B, its hollow microstructure assembled with an array of nanorods provides some intensive active sites that are easy to access electrochemically. Since Sample B has a relatively dense microstructure, the exposure of active sites is limited, resulting in a lower capacitance. The decline in capacitance that occurs as synthesis reaction time decreases indicates that the well-developed morphology and shape are key factors for determining capacitive performance. For investigating the effect of potential scan rates on material's capacitance, the specific capacitance vs. the scan rate for the best performing sample (Sample D) was plotted and shown in Figure 7. It can be observed that the specific capacitance is progressively lost when cycling at higher rates, indicating a limitation of power capability. On the other hand, their specific capacitance decreases at higher rates, meaning that the kinetics is slow. However, we expect that the results in this paper will provide practical information and methods for shape- and morphology-controlled synthesis for nanostructured functional materials in supercapacitor applications.