## 3 Results
### Electrochemical Measurements  
Figure 6a demonstrates the CV curves of electrodes based on MWCNTs and Mn-0 composites, measured at a potential scan rate of 5 mV/s. The CV curve for the MWCNTs-based electrode is symmetrical and has a shape close to rectangular. This designates that the storage of charge in this case is realized in an electric double layer (EDL). Apparently, a small number of functional groups present on the surface of the initial MWCNTs do not significantly affect the mechanism of charge accumulation. The specific capacitance of the MWCNTs-based electrode at 5 mV/s scan rate is 21.1 F/g (Table 4). The CV curve area of the Mn-0 electrode is significantly higher compared to MWCNTs. This indicates that redox reactions involving manganese oxide and electrolyte ions make the main addition to the capacitance of the Mn-0 composite electrode. The specific capacitance of the Mn-0 composite electrode is 49.2 F/g at 5 mV/s scan rate. Figure 6: CV curves of electrodes based on MWCNTs and Mn-0, measured at a potential scan rate of 5 mV/s (**a**); CV curve of the electrode Mn-580 (**b**); CV curve of the electrode Mn-700 (**c**); CV curves measured at different potential scan rates for electrodes based on MWCNTs (**d**), Mn-580 (**e**) and Mn-700 (**f**). The CV curve of the electrode based on Mn-0 composite has no pronounced peaks, which is characteristic of redox reactions involving magnesium oxides when interacting with an electrolyte [58]. The CV curves of the electrodes based on Mn-580 and Mn-700 composite are also without pronounced redox peaks (Figure 6b,c). The peak on the cathode branch near the potential value of 1 V is probably associated with the onset of water decomposition in the electrolyte. The specific capacitance of this electrode is 61.7 F/g at 5 mV/s scan rate (Table 4). The following redox reactions (Equations (1)-(3)) can occur on the surface of the electrodes obtained on the basis of composites [58]:  
\[\text{Mn}_{2}\text{O}_{3}+3\text{H}_{2}\text{O} \rightarrow 2\text{Mn}\text{O}_{2}+2\text{H}_{3}\text{O}^{+}+2\text{e}^{-} \tag{1}\] \[\text{Mn}_{3}\text{O}_{4}+6\text{H}_{2}\text{O} \rightarrow 3\text{Mn}\text{O}_{2}+4\text{H}_{3}\text{O}^{+}+4\text{e}^{-}\] (2) \[\text{MnO}_{2}+\text{Na}^{+}+\text{H}^{+}+2\text{e}^{-} \leftrightarrow\text{ Mn}\text{O}\text{ON}\text{a}\text{H} \tag{3}\]  
It is probable that the increase in the specific capacitance of the composite due to heat treatment is due to changes in the morphology of the surface, namely, the formation of individual nanoparticles on the surface of the CNTs, which increases the contact area of manganese oxide with the electrolyte compared to the continuous distribution of oxide over the surface of the MWCNTs. Additionally, the rise in the specific capacitance may be a consequence of the crystallization of manganese oxide. The analysis of the MWCNTs-based electrode's rate capability indicates good electrochemical behavior--with an increase in the potential scan rate, a noticeable increase in the area of the CV curves is observed (Figure 6d) and the shape of the curves remains quasi-rectangular. The specific capacitance retains 52% (the \(R\) value in Table 4) with an increase in the scanning speed from 5 to 80 mV/s. This indicates a sufficiently high availability of the CNT surface for electrolyte ions. Electrodes based on Mn-0, Mn-580 and Mn-700 composites do not exhibit sufficiently good electrochemical behavior and have a significantly lower rate capability (Figure 6e,f, Table 4). At a potential scan rate of 80 mV/s, the value of the specific capacitance for the Mn-580 composite practically coincides with the value of the specific capacitance MWCNTs and is slightly inferior to the specific capacitance value of the Mn-0 electrode. This indicates that diffusion processes involving manganese oxide do not have time to proceed at high potential scan rates, and the capacitance of composite electrodes is almost completely determined by the formation of EDL.