## 2 Materials and Methods  
### Sample Preparation Techniques  
Nickel chloride was used as an activator for the synthesis of Ni/C NC from natural materials [16]. The synthesis was carried out in one stage by pyrolysis with steam-gas activation. It was noted that nickel chloride not only provided the formation of magnetic moment carriers in carbon, but also contributed to the development of carbon material surface and reduction of the amount of impurities present in the initial raw materials.  
However, the use of other activators in the synthesis of activated carbon is often more effective using the two-stage synthesis method. This method is most commonly used in KOH or NaOH activation [20; 21] and consists in the preliminary carbonization of the initial raw material at 300-500 \({}^{\circ}\)C; then, the char is mixed with alkali in a certain proportion, placed in an oven and activated in an inert atmosphere at 600-1000 \({}^{\circ}\)C. As the result, a highly porous activated carbon is obtained, which is both an effective sorbent and active material of supercapacitor electrodes. In [22], the one- and two-stage synthesis was performed using FeCl\({}_{3}\). It was shown that, as a result of both types of synthesis, Fe/C NC was obtained, in which iron ions were in the state of Fe\({}_{3}\)O\({}_{4}\) magnetite. However, the two-stage synthesis gives a significantly larger specific surface area and smaller magnetite nanoparticles compared to these parameters for the sample obtained by the one-stage synthesis.  
Based on the results of [22], the synthesis of Ni/C NC was performed by the aforementioned one- and two-stage methods using nickel chloride NiCl\({}_{2}\) (anhydrous, powder, 99.99% trace metals basis) as an activator. The wheat straw granules were used as the initial raw material (cf. Figure 1).  
At the first stage, wheat straw granules were carbonized in an inert atmosphere (argon) for 90 min at 400 \({}^{\circ}\)C. As the result, the weight of the obtained char was determined to be \(\approx\)50% of the initial weight of the initial raw material.  
Based on these data, the samples were prepared for further activation:  
1. The initial wheat straw granules were poured over with an aqueous solution of NiCl\({}_{2}\) with the salt/raw material ratio 1:10, as had been carried out successfully for other natural precursors in the work [16] (sample NC1_10);
2. Since there is a loss of \(\approx\)50% of the initial weight of the raw material in the process of preliminary carbonization, the char of wheat straw granules was poured over with an aqueous solution of NiCl\({}_{2}\) with the salt/char ratio 1:5, which in terms of weight of the initial raw material would correspond to the same concentration as in the sample NC1_10 (sample NC2);
3. To compare the effect of a higher concentration of the activator (as in the sample NC2), the initial granules of wheat straw were poured over with an aqueous solution of NiCl\({}_{2}\) with the salt/raw material ratio 1:5 (sample NC1_5).  
The materials prepared in this way were kept at room temperature for 48 h and then dried at 110 \({}^{\circ}\)C until a constant weight. NC was synthesized at 700 \({}^{\circ}\)C and kept at this temperature for 90 min with additional steam-gas activation, which was provided by the supply of water aerosol in argon atmosphere from an ultrasonic aerosol generator to the reactor during carbonization. The gaseous by-products were withdrawn out of the reactor with Ar flow through a hydro-seal. The argon overpressure in the reactor was kept at the level of 1 kPa and the gas flow rate was in the range 2-10 l/min. As-obtained composites were boiled in distilled water for 30 min in a flask with a reflux condenser. Then, they were dried until the constant weight at 100 \({}^{\circ}\)C. For further experimental studies, the composites were milled mechanically in a porcelain mortar.  
Figure 1: Granules of wheat straw.