## 2 Sample Preparation and Testing Techniques  
The initial raw materials for the synthesis of carbon composites were selected from agricultural processing waste-dry corn stalks and sugar beet pulp. The carbon composite synthesis was carried out according to the diagrams shown in Figure 2. The raw material was first washed in distilled water at room temperature to a visually clear efflux and dried in an oven in an air atmosphere of 100-110 \({}^{\circ}\)C until the mass of the raw material stopped changing. Next, a portion of the raw material was prepared directly for pyrolysis, and another portion was pre-modified with ferric chloride. Dried and crushed beet pulp and corn stalks were filled with an aqueous solution of ferric chloride FeCl\({}_{3}\) using a salt to raw material ratio = 1:10. Raw materials prepared in this manner were stored at room temperature for 48 h, and then dried at 100-110 \({}^{\circ}\)C to achieve a constant weight. The next step was pyrolysis, which was carried out in a tubular stainless-steel reactor. The process was carried out at 700 \({}^{\circ}\)C for 90 min in an argon atmosphere. The resulting AC was scrubbed three times with hot distilled water. Such samples will be referred one-step synthesis samples (Figure 2). The papers concerning the modification of raw materials with KOH, for Instance [34], have shown the effectiveness of two-step carbonisation-activation in AC material produc  
Figure 2: Block diagrams showing one- and two-step organic raw materials-based AC production. tion. This idea was also used to obtain AC materials with ferric chloride modifier. The raw materials for modification were corn stalks and beet pulp carbonised in an inert atmosphere at 400 \({}^{\circ}\)C for 90 min. The carbonised corn stalks and beet pulp were soaked in an aqueous ferric chloride solution, held for 24 h, and then dried in an oven at the temperature of 100-110 \({}^{\circ}\)C. In the next step, we subjected the ferric chloride-modified raw material to pyrolysis at 700 \({}^{\circ}\)C in an argon atmosphere for 90 min. The resulting AC material was scrubbed three times with hot distilled water. The AC material was then dried until the carbon mass stopped changing. Such samples will be referred to two-step synthesis samples (Figure 2). A scanning electron microscope REMMA-102-02 (SELMI, Sumy, Ukraine) with a low-vacuum chamber and an energy-dispersive microanalysis system was used to obtain images of synthesised carbon composites. Magnetic measurements were carried out using a vibration magnetometer based on the method described in [35]. The magnetometer was calibrated by the comparison method. Pure non-porous nickel with a density of \(\rho\) = 8.9 g/cm\({}^{3}\) was used as a standard. The magnetisation curves of the samples under study were recorded in magnetic fields from \(-\)300 kA/m to +300 kA/m. The content of the magnetic phase in the synthesised composites was calculated from the measurement of the specific saturation magnetisation. For the correct determination of the saturation magnetisation, the study should be performed in strong magnetic fields sufficient for complete saturation. In our case, a magnetic field of 800 kA/m was used. The parameters of the porous structure of the studied samples were estimated by the isothermal processes of nitrogen adsorption/desorption at its boiling point (T = 77 K). The adsorption/desorption isotherms were obtained using a NOVAttouch LX2 ([PERSON], Boynton Beach, FL, USA). All synthesised samples were degassed in a vacuum at 473 K before measuring the isotherms, and the duration of degassing was 12 h.  
X-ray diffractograms of the AC materials were obtained using a DRON-3 diffractometer (NPO Bureau5, Saint Petersburg, Russia) in Cu K\(\alpha\) radiation (\(\lambda\) = 0.1542 nm), monochromatized by reflection from the (002) plane of the pyrographic monocrystal. Diffractograms were measured in continuous detector scanning mode at a rate of 2 degrees/min.