## 3 Results and Discussions
The isotherms of composites synthesised in two steps are characterised by the presence of a hysteresis loop, which can be classified as type H4. This behaviour of the isotherm is typical for porous solids with slit-like pores [45]. Figure 4: X-ray diffractogram of beet pulp carbon composite using a one-step synthesis (**c**) and using a two-step synthesis (**d**), corn carbon composite using a one-step synthesis (**e**) and using a two-step synthesis (**f**). For comparison, the lines of Fe\({}_{3}\)O\({}_{4}\) magnetite (**a**) and polycrystalline graphite (**b**) are shown. To calculate the parameters of the porous structure of the synthesised composites, the obtained isotherms were analysed using the Quantachrome TouchWin version 1.22 software. To do this, we used the multipoint BET method [45], which approximates the experimental data with a straight line in the range of relative pressures \(\mathrm{P}/\mathrm{P}_{0}=0.05\div 0.35\). The calculation data are given in Table 1.  
As can be seen from Table 1, the two-step synthesis results in an increase in the specific surface area and specific volume by about half, regardless of the type of initial raw material. At the same time, the average pore size remains virtually unchanged. Magnetic measurements have shown that the synthesised nanocomposite samples have magnetic hysteresis. The corresponding hysteresis loops are shown in Figure 6.  
The coercive force \(\mathrm{H_{c}}\) and specific saturation magnetisation \(\mathrm{\sigma_{s}}\) were calculated from the hysteresis loops. These data are shown in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline
**Type of Nanocomposite** & **S, m\({}^{2}\)/g** & **V, cm\({}^{3}\)/g** & **<d>, nm** \\ \hline AC from beet pulp, 1-step synthesis & 359.6 & 0.167 & 1.86 \\ \hline AC from beet pulp, 2-step synthesis & 738.2 & 0.361 & 1.96 \\ \hline AC from corn, 1-step synthesis & 413.1 & 0.269 & 2.6 \\ \hline AC from corn, 2-step synthesis & 889.8 & 0.591 & 2.66 \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of the porous structure of synthesised nanocomposites. Figure 5: Nitrogen adsorption/desorption isotherms 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 red curves correspond to adsorption and the blue curves to desorption of nitrogen. Based on the results of measuring the saturation specific magnetisation of synthesised nanocomposites, the content of magnetic moment carrier particles, magnetite, can be determined. For the calculation, it is necessary to know the value of the saturation specific magnetisation of magnetite particles. The saturation specific magnetisation of massive magnetite is 92 A\(\cdot\)m\({}^{2}\cdot\)kg\({}^{-1}\)[46]. The value of the specific saturation magnetisation of magnetite particles required for the calculation was chosen taking into account the phenomenon of a decrease in the magnetisation of small particles with a decrease in their size [47]. According to the data on the dependence of coercive force on particle diameter in Fe\({}_{3}\)O\({}_{4}\) dispersions [48; 49], for the measured values of coercive force, the size of magnetic moment carriers is on average 50 nm for the sample of the composite synthesised from beet pulp in one step, 30 nm for a sample of the composite synthesised from corn stalks in one step, and 29 nm for the composite synthesised from corn stalks in two steps. Accepting from the above sources that the indicated particle sizes correspond to an average value of the specific saturation magnetisation of 80 A\(\cdot\)m\({}^{2}\cdot\)kg\({}^{-1}\), the percentage of magnetite in the composites was determined (Table 2). Galvanostatic charge/discharge cycles were carried out on the manufactured symmetric supercapacitors. Figure 7 shows a typical charge-discharge curve for supercapacitors.