*3.3. Measurements of Electricity Production and COD Reduction during the Operation of MFC (with Ni–Co Cathode)*

The following phase involved the analysis of the Ni–Co alloy in the function of the cathode catalyst in MFC. The research included measurements of COD decline and electric energy production during the operation of the microbial fuel cell.

Waste material was applied to serve as fuel for MFC with Ni–Co cathodes. For this purpose, wastewater (WW) derived from the municipal wastewater treatment plant (WWTP) was used. Table 3 contains the COD concentration and pH of the WW applied in the measurements.

**Table 3.** Parameters of wastewater (WW) (from the wastewater treatment plant (WWTP)) used in measurements of electricity production and COD reduction.


To assess the effectiveness of COD reduction in the microbial fuel cell, the following were compared: COD reduction time in MFC, COD reduction time in WW without interference, and COD reduction time accompanying WW aeration. To this end, three reactors were applied: one without aeration (Figure 18; 9), one with aeration (Figure 18; 8), and one containing MFC (Figure 18; 7). Figure 18 shows the scheme of the measurements.

Electrical parameters of the MFC were measured in parallel (at the same time) with the COD reduction [7,22,78]. All reactors had the same dimensions, and therefore the same volume of WW (15 L) [7,22]. The reactors worked at the same time, and were filled with the same WW. Measurements of COD were performed in all reactors until obtaining a 90% reduction of COD [20,78]. In the reactor with aeration, an interface between the WW and air occurred only at the WW surface [5,20,75]. For the aeration of the WW, the air pump was used (air pump capacity: 270 L·h<sup>−</sup>1) [7,22]. The MFC included a carbon cloth anode, a Ni–Co cathode, and a Nafion PEM. After the measurements of the MFC with the Ni–Co cathode, measurements of the MFC with the carbon cloth cathode were also made. In both cases (for measurements of the MFC), the surface area of the anode was 20 cm2, and the surface area of the cathode was 15 cm2. The cathode was immersed in 0.1 N aqueous solution KOH and aerated (air pump capacity: 10 L·h<sup>−</sup>1) [22]. The electrodes of the MFC were constantly connected with a 10 <sup>Ω</sup> resistor [5,20]. Microorganisms were acclimated for 5 days [1,2,22,78].

Nafion PF 117 (183 μm thick) (The Chemours Company, Wilmington, DE, USA) was used as the PEM. A Zortrax M200 printer (Zortrax S.A, Olsztyn, Poland) was used to print the housing of the cathode (Figure 18; 7). To prepare the 3D object (cathode housing) for printing, Z-Suite software (Zortrax S.A, Olsztyn, Poland) was used. A Hanna HI 83224 colorimeter (HANNA Instruments, Woonsocket, RI, USA) was applied for the COD measurement in WW. A Fluke 8840A multimeter (Fluke Corporation, Everett, WA, USA) was used for the electrical measurements.

#### **4. Conclusions**

This paper reports the results of a study concerned with methodology applicable for the production and selection of Ni–Co alloy as a cathode catalyst for MFC fed by municipal WW. Following that, measurements were performed with the application of the resulting alloys. As was demonstrated by the research, the most favorable catalytic parameters were obtained when a Ni–Co alloy containing 15% Co was applied, which was oxidized over 8 h at 673 K. This alloy demonstrated the highest value of the electroless potential (Figures 5–10) and the cell potential after charging as an anode (Figures 11–14). For this reason, this alloy was applied as a cathode catalyst (Figure 18; 7) in further measurements using MFC.

The measurements concerned with COD reduction also demonstrated that in each of the analyzed cases (without aeration, with aeration, and using MFC), the assumed level of reduction was 90%. The time of COD reduction using MFC with Ni–Co cathode (15% Co) to the assumed level of 90% was 15 days (Figure 15). The time obtained was shorter (by 3 days) than the time of COD reduction using MFC with a carbon cloth electrode, and also shorter than when an electrode with a Cu–B catalyst was applied [22,75].

During MFC (with Ni–Co cathode, 15% Co) operation, a current density of 0.47 mA·cm−<sup>2</sup> and a power of 7.19 mW were recorded (Figure 17). For comparison, in MFC with the carbon cloth cathode, a current density of 0.23 mA·cm−<sup>2</sup> and a power of 5.63 mW were recorded.

The maximum power obtained during the operation of MFC with Ni–Co alloy was slightly higher (by 1.56 mW) than the maximum power obtained during the operation of MFC with the carbon cloth cathode (Figure 16), and slightly higher (by 1.08 mW) than the maximum power obtained during the operation of MFC with the Cu–B cathode [22,75].

For these reasons, this article demonstrated the applicability of a Ni–Co alloy (containing 15% of Co) as a cathode catalyst in a microbiological fuel cell, which is based on the supply of municipal WW. At the same time, the higher catalytic activity of a Ni–Co catalyst (containing 15% of Co) was demonstrated compared to a carbon cloth, as well as compared to a Cu–B catalyst.

**Author Contributions:** Data curation, P.P.W. and B.W.; investigation, P.P.W. and B.W.; methodology, P.P.W.; writing—Review and editing, P.P.W. and B.W.; supervision, P.P.W. and B.W.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
