**1. Introduction**

Microbial fuel cells (MFCs) are a technology of electric energy generation using organic matter contained in wastewater [1,2]. Thus, this technology allows the production of energy from waste products (e.g., wastewater, also industry wastewater) [1,3–9]. Moreover, MFCs allow for wastewater pretreatment, e.g., reduction of the chemical oxygen demand (COD) concentration. The first observations of electricity production by bacteria were conducted by Potter [10]. However, greater progress in the development of this technology was only achieved in the 1960s [11–13]. Because of the increasing pollution of the environment, research on MFC technology resumed in the 1990s [14–16], but real development of these technology has only happened in recent years [1–7,17–25].

MFC forms a bio-electrochemical system in which bacteria oxidize organic matter by acting as a biocatalyst. Therefore, organic matter constitutes the fuel applied for the production of electricity [1–4,12,22]. This system includes electrodes in the electrode chambers, usually separated by a proton exchange membrane (PEM) [26]. On the anode, bacteria oxidize organic matter to produce e<sup>−</sup> electrons and H<sup>+</sup> ions (as well as an additional volume of CO2), whereas H2O is generated on the cathode through the reaction of e<sup>−</sup> ions combined with H<sup>+</sup> and O2 [1–4,22]. This is a result of cathode aeration in the cathode chamber. Among many other genera, the *Shewanella* spp., *Pseudomonas* spp., or *Geobacter* spp. bacteria are capable of generating electrons [21,27–34]. Studies have demonstrated that

the highest current values in MFC are generated by using multicultural microorganisms. They have higher efficiency in relation to microorganisms accumulated in monocultures, as a result of the competition between the bacterial cultures [35]. Research into the peak limits of the current density and power level of MFC is still at an experimental stage. The limitation of the maximum power density or current density is the result of the low rate of reactions on electrodes [1–4,36,37]. The rate of the processes taking place on the electrodes is primarily influenced by the type of applied catalyst. Since the role of the anode catalyst is taken over by microorganisms, it is important to look for an adequate cathode catalyst. Because of the excellent catalytic properties, platinum is most commonly used as the catalyst [38,39]. Unfortunately, it is also characterized by high price. Therefore, it is necessary to look for other catalysts that do not contain precious metals [40–47]. As electrodes (also as catalysts) do not contain precious metals, different electrode materials (carbon fiber brush, carbon felt, carbon foam, carbon cloth, graphite paper, and others) were analyzed in various works [48–56]. Of these materials, the most efficient anode material is carbon felt. The lowest parameters during MFC operation were obtained using carbon cloth as electrodes. However, it was found that it was also possible to use metal electrodes (also as metal electrodes with metal catalysts) as the cathodes of microbial fuel cells [1,7,8,22,41,57,58]. Nickel is also characterized by good catalytic properties. However, pure nickel (Raney Ni, the most commonly used form of nickel catalyst) is difficult to use, e.g., it should never be exposed to air. Even after preparation, Raney Ni still contains small amounts of hydrogen gas and may spontaneously ignite when exposed to air. Therefore, Ni (mainly as Raney Ni) is supplied as an aqueous suspension [59]. Therefore, nickel alloys should be easier (also safer) to use while maintaining good catalytic properties. Additionally, other metals and metal alloys are used as catalysts. One of them is cobalt and its alloys. These materials have also been used or analyzed during research as a catalytic material [60–65].

The description of the current density applies the Butler–Volmer exponential function. However, this function only leads to an output in the form of a theoretical value, which usually deviates significantly from the values that are obtained experimentally in comparable circumstances [38,66]. Therefore, it is necessary to conduct experimental research concerned with the selection of a catalyst suitable for a specific substance (in this case a substance employed as the waste material) that further constitutes the fuel applied in MFC [1,2,38,48–55,66]. The selection of a catalyst has an effect on the final cost of electricity production and the expenses associated with pre-treatment of wastewater for the needs of MFC. Because of the large amount of wastewater, it is necessary to provide its treatment, which incurs huge expenses [67]. However, the energy potential of wastewater allows it to be considered in the role of a potential source of energy in future applications, primarily as fuel for MFCs. MFCs are understood as an element that can support the traditional wastewater treatment techniques, as its role is primarily concerned with reducing COD concentration.

In this work the possibility of using a nickel–cobalt alloy (Ni–Co) in MFC was analyzed. The analysis concerned the use of Ni–Co alloy as a cathode catalyst for electricity production and COD reduction.

#### **2. Results and Discussion**

Figures 1–4 show the concentrations of Ni and Co in the alloy samples obtained by the method of electrochemical deposition for planned 15, 25, 50, and 75% concentrations of Co.

Figures 1–4 allow visualization and understanding that it is necessary not only to preserve the deposition parameters, but also to sort and select the resulting alloys. This necessity arises from the fact that despite maintaining constant deposition parameters, different alloy compositions were obtained in corresponding cases.

For further research, the following Ni–Co 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).

**Figure 1.** Concentration of Ni and Co in the alloy samples obtained by electrochemical deposition for planned 15% concentration of Co.

**Figure 2.** Concentration of Ni and Co in the alloy samples obtained by electrochemical deposition for planned 25% concentration of Co.

**Figure 3.** Concentration of Ni and Co in the alloy samples obtained by electrochemical deposition for planned 50% concentration of Co.

Ni–Co alloys that contained exactly 15, 25, 50, and 75% of Co were selected. These alloys were oxidized for 1, 2, 4, 6, 8, and 10 h. Next, measurements were carried out according to the methodology presented in Section 3.2. Before oxidation, all samples were gray, but after oxidation over 4, 6, and 8 h, the surfaces of the samples were multi-colored, while after oxidation over 10 h, the color of the sample surfaces changed to black.

**Figure 4.** Concentration of Ni and Co in the alloy samples obtained by electrochemical deposition for planned 75% concentration of Co.

Figures 5 and 6 show stabilization of the electroless potential in the glass half-cell, with the Ni–Co work electrode (oxidized for 1 and 2 h) in alkaline electrolyte (aqueous solution of KOH). 7,8 show alloy oxidized for 4 and 6 h. 9,10 show alloys oxidized for 8 and 10 h.

**Figure 5.** The electroless potential of Ni–Co electrodes (oxidized for 1 h).

**Figure 6.** The electroless potential of Ni–Co electrodes (oxidized for 2 h).

**Figure 7.** The electroless potential of Ni–Co electrodes (oxidized for 4 h).

**Figure 8.** The electroless potential of Ni–Co electrodes (oxidized for 6 h).

**Figure 9.** The electroless potential of Ni–Co electrodes (oxidized for 8 h).

**Figure 10.** The electroless potential of Ni–Co electrodes (oxidized for 10 h).

The data (Figures 5–10) show that during each measurement, the electroless potential was the highest for the Ni–Co catalyst with 15% of Co. Moreover, the highest electroless potential was obtained for Ni–Co electrodes oxidized for 8 h (Figure 9).

The analysis of the samples also demonstrated that oxides of lower order, NiO and CoO, were generated during oxidation lasting from 1 to 8 h. However, during oxidation taking over 10 h, NiCo2O4 and Co3O4 oxides were formed, which decreased the electroless potential (Figure 10) [68–74]. Therefore, the next measurements, including the influence of anodic charge on the catalytic activity of the Ni–Co catalyst (described in Section 3.2), were performed for the alloy samples oxidized for 8 h (Figures 11–14). Colored lines (1–4) in Figures 11–14 refer to the subsequent anodic charges.

**Figure 11.** Voltage of a half-cell with Ni–Co electrode (alloy with 15% of Co); influence of anodic charge on the catalytic activity of Ni–Co catalyst (lines 1–4 show subsequent anodic charges).

**Figure 12.** Voltage of a half-cell with Ni–Co electrode (alloy with 25% of Co); influence of anodic charge on the catalytic activity of Ni–Co catalyst (lines 1–4 show subsequent anodic charges).

**Figure 13.** Voltage of a half-cell with Ni–Co electrode (alloy with 50% of Co); influence of anodic charge on the catalytic activity of Ni–Co catalyst (lines 1–4 show subsequent anodic charges).

**Figure 14.** Voltage of a half-cell with Ni–Co electrode (alloy with 75% of Co); influence of anodic charge on the catalytic activity of Ni–Co catalyst (lines 1–4 show subsequent anodic charges).

In the next step, the MFCs (described in Section 3.3) with different cathodes (with different contents of Co, and after oxidation of electrodes for 1–10 h) were built. Table 1 shows the maximum power and average cell voltage obtained in MFCs, with Ni–Co cathodes oxidized for 1–8 h, without anodic charge before using the electrodes (cathodes).


**Table 1.** Maximum power and average cell voltage obtained in microbial fuel cells (MFCs), with Ni–Co cathodes oxidized for 1–8 h, and without anodic charge before using the electrodes (cathodes).

The highest parameters (power and cell voltage) for the MFC with electrodes oxidized for 8 h were obtained. Thus, these electrodes were chosen for further measurements of the MFCs (with electrodes after the anodic charge). Table 1 shows the maximum power and average cell voltage obtained in MFCs, with Ni–Co cathodes (15% of Co), and oxidized for 8 h, with anodic charge before using the electrodes (cathodes).

The cell voltage of the third anodic charging of the electrode was also highest for the same catalyst (15% Co) (Figures 11–14, Table 1). Such parameters (Figures 5–14, Tables 1 and 2) show that the Ni–Co electrode with 15% of Co showed high performance. Figure 15 shows the cell voltage and Figure 16 shows the power curves during the MFC with the highest parameter electrode (Ni–Co cathode with 15% of Co, and after triple the anodic charge) operation.

**Table 2.** Maximum power and average cell voltage obtained in MFCs, with Ni–Co cathodes oxidized for 8 h, and with anodic charge before using the electrodes (cathodes).


**Figure 15.** Cell voltage of the MFC in time: red line—with Ni–Co cathode (15% Co), black line—carbon cloth cathode.

**Figure 16.** Power curves during the MFC operation: red line—with Ni–Co cathode (15% Co), black line—carbon cloth cathode.

Figure 17 shows the COD reduction in time in the three reactors without aeration, with aeration, and during the operation of MFC with the highest parameter electrode (Ni–Co cathode with 15% of Co, and after triple the anodic charge) and with the carbon cloth electrode.

**Figure 17.** Chemical oxygen demand (COD) reduction in time in the three reactors: without aeration, with aeration, and during the MFC operation (with Ni–Co and carbon cloth electrodes).

On the basis of the results of the measurements of the electrode potential, we can state that among all analyzed Ni and Co concentrations, the highest value was obtained for oxidized alloys over 8 h at 673 K. In addition, for all oxidation times, the highest values of electrode potential were obtained by an alloy containing 15% Co. A similar outcome was obtained when the electrode potential was performed after anodic charging. In each case, the alloy with the composition comprising 15% Co had the higher catalytic activity of the analyzed alloys. In the case of triple anode charging using this alloy (15% Co), the most favorable potential curve was derived. Therefore, the alloy containing 15% Co, after 8 h of oxidation and three times anode loading, was selected for further measurements by application of MFC. MFC (third reactor, see Section 3.3, Figure 18) was investigated in a setup combined with a Ni–Co cathode and for comparison with a cathode made of carbon cloth. During the MFC operation, the levels of COD reduction to the effectiveness of 90% were recorded: 15 days for a Ni–Co catalyst, and 18 days for the carbon cloth catalyst. In comparison to the data regarding the reactor with aeration (Figure 15, blue line), this time was longer by 3 days for the carbon cloth catalyst (Figure 17, black line), and the same for a Ni–Co catalyst (Figure 17, red line). However, the characteristics of the COD decline curve were comparably less favorable when MFC was applied, as using the aeration taking 5 days resulted in an 84.8% decrease in the initial COD level (Figure 17, blue line). We should note that similar results were obtained for the Cu–B catalyst in the conditions when similar wastewater parameters

were employed; however, the use of Ni–Co alloy as a catalyst provided slightly better parameters (shorter COD reduction time) compared to that based on the Cu–B alloy [8,22,75].

**Figure 18.** Scheme of the measurements: 1—copper mesh (electrode) to deposit the catalyst, 2—electrolysis bath to deposit the catalyst, 3—electrolyte preparation, 4—furnace for electrode (catalyst) oxidation, 5—glass cell for electroless potential analysis and for the anodic charge analysis, 6—potentiostat, 7—third reactor (MFC), 6—second reactor (with aeration), 9—first reactor (without aeration), 10—multimeter, 11—colorimeter, 12—computer.

The measurement of the cell potential demonstrated higher potential during MFC operation of the Ni–Co catalyst than for the case when the carbon cloth was employed (Figure 15). During MFC operation, 5.63 mW of power (with the carbon cloth cathode) and 7.19 mW of power (with the Ni–Co cathode) were obtained (Figure 16). The analysis of the current density (based on data from power measurements and the area of electrodes) showed that the current density for the Ni–Co catalyst reached a maximum of 0.47 mA·cm−2, and 0.23 mA·cm−<sup>2</sup> for carbon cloth. The maximum power obtained during the operation of MFC with Ni–Co alloy, was slightly higher (by 1.08 mW) compared to the maximum power obtained during the operation of MFC with a Cu–B cathode (6.11 mW) [22,75].
