## 3 Results and Discussion
### Electrochemical Impedance Spectroscopy Experiments (EIS)  
In order to perform a detailed electrochemical characterization of the cell with the different electrodes, EIS experiments were carried out. From the electrochemical point of view, as reported elsewhere [37; 38; 39], the internal resistances of the cells with different electrodes can be calculated. In Figure 8, the Nyquist diagrams show significant information for the electrochemical processes that occur during the simultaneous wastewater treatment and electricity production.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Electrode** & **OCV** & **Power Output** & **Power Output** & **Power Output** \\  & **(V)** & **(mW)** & **(mW\(\cdot\)g\({}^{-1}\))** & **(mW\(\cdot\)g\({}^{-1}\).L\({}^{-1}\))** \\ \hline WC & 0.66 & 1.12 & 1.5 & 6.00 \\ BC & 0.31 & 0.96 & 0.9 & 3.84 \\ SC & 0.48 & 2.67 & 0.7 & 10.68 \\ \hline \hline \end{tabular}
\end{table}
Table 3: OCV and maximum power output values obtained from the leachate-fed MFC with the three different electrode cases examined.  
Figure 7: Conductivity measurements (**left**) and COD removal values (**right**) for the leachate and the three leachate-fed MFC configurations with different cathode electrodes.  
All three setups depict two distinguished arcs followed by a Warburg element, as seen in other microbial fuel cells [40]. The fitted parameters are explained and reported in earlier publications, and the measurements here held through three electrode setups, as in most cases of fuel cells [32, 33, 41]. Table 4 presents the values of the solution resistance (R\({}_{\text{g}}\)), the biofilm resistance (R\({}_{\text{BF}}\)), the charge transfer resistance (R\({}_{\text{CT}}\)), biofilm capacitance (C\({}_{\text{BF}}\)) and the charge transfer capacitance (C\({}_{\text{CT}}\)), which were calculated from the fitting model. Moreover, the internal resistance (R\({}_{\text{in}}\)) of each case is presented.  
As can be seen from Table 4, the R\({}_{\text{in}}\) of SC electrodes is approximately 40% lower when compared with the R\({}_{\text{in}}\) of the WC electrodes, and 28% lower from the R\({}_{\text{in}}\) of the cell which operated with the BC electrodes. However, from the Nyquist diagrams, it is observed that this difference is attributed to the lower values of biofilm and charge resistances and capacitances of SC electrodes when compared with the respective values of WC and BC electrodes (Table 4). The solution resistance (R\({}_{\text{S}}\)\(\sim\)9.3 \(\Omega\)) is practically the same for all three operation batch modes. This practically denotes that the nature of the electrolyte/leachate does not affect by any means the three setups. On the other hand, the values of R\({}_{\text{BF}}\), R\({}_{\text{CT}}\), C\({}_{\text{BF}}\) and the C\({}_{\text{CT}}\), in the case of SC electrodes, are significantly lower when compared with the respective values of the BC and WC electrodes. This result indicates that the rate of the leachate oxidation is faster in the case of SC electrodes and also that a better biofilm has been developed in the case of SC electrodes [42, 43]. On the contrary, no significant differences on the internal resistances are observed between WC and BC techniques. The above results are in accordance with the polarization experiments, which indicated that the maximum power output was produced with the SC electrodes where the maximum COD removal efficiency also occurred.  
Figure 8: Nyquist diagrams of leachate-fed MFC for three different cathode configurations (WC BC and SC).