## 3 Results and Discussion
### NR and MV Electrochemical Characterization  
Figures 2A and 2B, respectively, show the electrochemical behavior of NR and MV in LP medium in the presence of glycerol (1 g\(\cdot\)L\({}^{-1}\)) at pH 7.0. Figure 1: (**A**) Power and polarization curves of the glycerol-fed MFC with _C. pasteurianum_ DSM 525 as biocatalyst (\(\mathrm{MFC_{DSM}}\)—blue line) and without the biocatalyst (abiotic control—black line). (**B**) EIS spectra obtained for \(\mathrm{MFC_{DSM}}\) (blue dots) and the abiotic control (black dots) in 1.0 g\(\cdot\)L\({}^{-1}\) glycerol at OCP. NR is a phenazine-type molecule bearing a nitrogen-heterocyclic ring in its structure [22]. As mentioned earlier, many microorganisms lack redox-active proteins for DET on their cell surfaces, but they can perform MET by producing their own mediators, such as pyocyanin [11]. Alternatively, molecules of this class, which have well-known redox properties, can be added as mediators in BESs [17]. Figure 2A displays the CV curves obtained for 100.0 uM NR at different scan rates. A redox couple centered around \(-0.47\) and \(-0.53\) V vs. Ag/AgCl (sat) emerged. [PERSON] et al. (2016) studied the formation of poly(neutral red) films and reported a pair of redox peaks centered at \(-0.6\) and \(-0.5\) V vs. SCE, in phosphate buffer, which they attributed to NR-leuco-NR reduction-oxidation processes [23]. Furthermore, CV measurements at various scan rates ranging from 1 to 50 mV\(\cdot\)s\({}^{-1}\) provided insights into the electrochemical behavior of NR. I\({}_{qp}\) was linearly related with the square root of the scan rate, described by the equation \(I_{ap}=5.3290\times 10^{-4}\
u-7.4421\times 10^{-4}\) (R\({}^{2}\) = 0.9514). Similarly, I\({}_{cp}\) increased linearly with the square root of the scan rate, following the equation \(I_{cp}=-5.1927\times 10^{-4}\
u-8.1165\times 10^{-4}\) (R\({}^{2}\) = 0.9409). This response, shown in the graph inserted in Figure 2A, indicated a diffusion-controlled process [24]. Overall, the electrochemical behavior of NR in LP medium closely resembled the way it behaves in supporting electrolytes such as phosphate buffer solutions [23] and MOPS buffer with 10 mM MgCl\({}_{2}\) and 100 mM glucose [22]. _MV_, a viologen-based mediator, plays a significant role as an electron relay. It exhibits rapid electrochemical response at negative potentials, which allows it to be applied in diverse areas such as batteries and as a redox mediator in numerous enzymatic reactions [14; 25]. Moreover, in other applications, _MV_ has been shown to redirect the electron flow from _Clostritium acetobutylicum_ by controlling the NADH/NAD\({}^{+}\) balance [26]. _MV_ displays three main oxidation states [27]. Figure 2B depicts the CVs recorded for 200 uM _MV_ at scan rates ranging from 1 to 50 mV\(\cdot\)s\({}^{-1}\). The CV curves exhibited two redox pairs: one at \(-0.41\) and \(-0.48\) V vs. Ag/AgCl\({}_{\text{(sat)}}\), denoted 1 and 1\({}^{\prime}\), respectively, and another at \(-0.65\) and \(-0.83\) V vs. Ag/AgCl\({}_{\text{(sat)}}\), denoted 2 and 2\({}^{\prime}\), respectively. According to [PERSON] and [PERSON], reduction peak 1\({}^{\prime}\) corresponds to \(MV^{+}\) formation, which is consistent with reaction 1. Reduction peak 2\({}^{\prime}\) refers to formation of the neutral molecule in \(MV^{0}\) solution (reaction 2). The oxidation peaks labelled 2 and 1 correspond to reactions 3 and 4, respectively [27]  
\[\text{Reaction 1}:\ MV^{2+}+e^{-}\to MV^{+}\ (peak\ 1^{\prime})\] \[\text{Reaction 2}:\ MV^{+}+e^{-}\to MV^{0}\ (peak\ 2^{\prime})\] \[\text{Reaction 3}:\ MV^{0}\to MV^{+}+e^{-}(peak\ 2)\] \[\text{Reaction 4}:\ MV^{+}\to MV^{2+}+e^{-}(peak\ 1)\]  
Figure 2: Cyclic voltammograms (CVs) of (**A**) 100 uM NR and (**B**) 200 uM MV in LP medium at pH 7.0 (25 \(\pm\) 2 \({}^{\circ}\)C). CVs recorded as a function of the scan rate. Inserted graph: anodic (\(I_{qp}\)) and cathodic (\(I_{qp}\)) current peaks plotted as a function of the square root of the scan rate.