## 3 Results and Discussions
The specific capacitance of the carbon material was determined from the formula [50]:  
\[\mathrm{C}=\frac{2\mathrm{I}\Delta t}{\mathrm{m}\Delta\mathrm{U}} \tag{4}\]  
where: I is the discharge current, \(\Delta t\) is the discharge time, m is the active mass of one electrode, \(\Delta\mathrm{U}\) is the discharge voltage. As a result of the analysis, it was found that one-step modification yields not very high capacitance values in alkaline electrolyte--for nanocomposites from beet pulp the value is 41 F/g, and for nanocomposites from corn stalks it is 72 F/g. In contrast, a two-step synthesis using pre-carbonised raw materials boosted the specific capacitance of beet pulp nanocomposites to 96 F/g, and that of corn stalk nanocomposites to 95 F/g. Coulombic efficiency in both cases was higher than 95 percent. The current density during the study of charging and discharging processes was 1 A/g. Since composites made in two-step process show higher capacitance, further studies were carried out on the properties of supercapacitors made from these materials. Figure 6: Hysteresis loops of beet pulp carbon composite (**a**) using a one-step synthesis (1) and using a two-step synthesis (2), corn carbon composite (**b**) using a one-step synthesis (1) and using a two-step synthesis (2). The working potential or \"electrochemical window\" of a supercapacitor is determined by the electrochemical stability of the electrolyte. Among other electrochemical methods, the cyclic voltammetry method can be considered the most efficient method for determining the \"electrochemical window\". For smooth electrodes, it is believed that the decomposition processes of electrolyte components start to run intensively when the surface current density is 0.5 mA/cm\({}^{2}\)[51]. In the case of porous electrodes, there is no such criterion, so it is believed that the electrolyte component decomposition processes begin to occur when the current in the system goes up rapidly. The presence of Faraday electrochemical processes in the system is manifested in the form of voltammogram peaks [52]. Figure 8 show cyclic voltammograms of supercapacitors, electrodes of which were manufactured from carbon nanocomposites synthesised in two steps. The tests were carried out for different scanning speeds. As can be seen in both figures, the shape of the obtained curves indicates mainly the capacitance nature of the electrochemical system behaviour with a good ionic response [53]. Increasing the scan rate to 50 mV/s leads to the acceleration of charge-discharge processes, which reduces the availability and movement of electrolyte ions inside the electrode material. At the same time, the effective interaction between the electrolyte ions and the electrode is significantly reduced due to the electrode material resistance, which is reflected in the deviation of the cyclic voltammogram from a rectangular shape [54; 55]. We do not observe clear current peaks in all curves, which indicates the absence of parasitic capacitances and the absence of undesirable electrochemically active impurities in the electrochemical systems of manufactured supercapacitors. The obtained shape of the  
Figure 8: Cyclic voltammograms for supercapacitors made from beet pulp (**a**) and corn stalks (**b**) carbon composites synthesised in two steps. Figure 7: Typical charge-discharge curve for manufactured supercapacitors. curves allows us to conclude that practically only adsorption and desorption of K+ cations and OH- anions occur on the surface of electrodes in the set potential range. To confirm the electrochemical stability of the obtained nanocomposites, long-term tests of the fabricated supercapacitors were carried out. The results of the change in specific capacitance depending on the number of charge-discharge cycles are shown in Figure 9.  
It is known that supercapacitors based on Fe\({}_{3}\)O\({}_{4}\) nanoparticles do not have stable electrochemical parameters at a large number of charge-discharge cycles [56].