**3. Materials and Methods**

#### *3.1. Preparation of a Ni–Co Cathode*

The electrochemical deposition technique was applied to obtain Ni–Co alloy. A copper mesh was used as the electrode to apply the catalyst (i.e., Ni–Co alloy). The electrolyte for alloying mainly consisted of a mixture of NiSO4 and CoSO4 [7,76]. The electrochemical deposition was carried out at temperatures of 293–323 K, at a current density of 1–3 A·dm−2, and at pH 2.0–5.5 [7]. Prior to the application process of electrolytic alloy, the copper mesh was first washed with 25% aqueous KOH to ensure its adequate wettability. The next step involved its washing in acetic acid and then in alcohol [22].

On the basis of previous studies [7], alloys containing 15, 25, 50, and 75% content of Co were selected for measurement. The temperature, pH, and current density of electrochemical deposition (to obtain different contents of Ni and Co in the alloys) were selected experimentally, but according to the previously proposed methodology for obtaining the alloy [7]. For further measurements, the Ni–Co electrodes with different contents of Co were selected by the XRD method. During electrochemical deposition, for all of the planned alloys (15, 25, 50, and 75% of Co) the ten different alloys with similar concentrations of Co were obtained according to a previously developed methodology [7,76]. Based on the research results (Figures 1–4), for further research, the following alloy samples were selected: 7 (15% of Co; Figure 1), 5 (25% of Co; Figure 2), 2 (50% of Co; Figure 3), and 1 (75% of Co; Figure 4).

The selection was carried out using a single-crystal X-ray diffractometer (Xcalibur, Oxford Diffraction, UK).

### *3.2. Selection of the Ni–Co Electrodes for Measurement*

First, measurements of the stationary potential of the electrodes were performed in order to assess the oxidation activity of the Ni–Co electrodes (with different contents of Ni and Co). Since the electrode is constantly oxidized during the operation of MFC, it was necessary to pre-oxidize the electrodes, otherwise, the electrodes would change their catalytic properties (along with the level of oxidation of the electrode surface) throughout their operation of MFC. The oxidation temperature of all of the Ni–Co alloy samples was 673 K. The oxidation times were 1, 2, 4, 6, 8, and 10 h. Next, we measured the influence of anodic charge on the catalytic activity of the Ni–Co alloy. These measurements were carried out in a glass half-cell (250 mL3) with the use of a potentiostat (Figure 18; 5). An aqueous solution of KOH (2 M) was used as the electrolyte. A saturated calomel electrode (SCE) was used as the reference electrode [7,22,77].

The KS 520/14 silt furnace (ELIOG Industrieofenbau GmbH, Römhild, Germany) was used for electrode oxidation. The experiments of the influence of anodic charge on the catalytic activity of the Ni–Co alloy were conducted using an AMEL System 500 potentiostat (Figure 18; 6) (Amel S.l.r., Milano, Italy). The potentiostat was controlled by a computer using CorrWare software (Scribner Associates Inc., Southern Pines, NC, USA).
