# Novel Microfluidic Septum to Optimize Energy Recovery in Single-Chamber Microbial Fuel Cells
## 3 Results
### Electrochemical Characterizations  
To assess in more detail the electrical performance of the a-SCMFCs, Linear Sweep Voltammetry (LSV) characterizations were performed. Figure 6 compares the polarization curves of one representative a-SCMFC from each triplet obtained after 8, 10, and 12 months  
Figure 4: Average peak maxima as function of sodium acetate concentration (expressed in mM). The empty-area symbols correspond to peaks associated with 12 mM refills performed at the end of this perturbation phase, with the aim of validating the initial recorded response. Figure 5: Average energy recovery as function of sodium acetate concentration (expressed in mM). IMS cells provided better electrical performances compared to control cells. of activity. All current densities were calculated by normalizing the measured current to 5.76 cm\({}^{2}\), corresponding to the geometric area of the anode electrodes. It is interesting to observe that the IMS cells provided a more stable response, in terms of smaller variations over time, compared to the control cells. This result agrees with what previously observed from output voltage monitoring (Figure 3B). In more detail, comparing Figure 6A-C, it was possible to appreciate that the open-circuit potential (OCP) for the IMS cells experienced less variability than the OCP for the control cells, which significantly decreased over time. A similar trend can be observed for the short-circuit currents. Finally, the linear region of the polarization curves allowed us to estimate an internal resistance of approximately 1 k\(\Omega\) for the IMS cells and 500 \(\Omega\) for the control cells. Electrochemical Impedance Spectroscopy (EIS) was performed to gain a better understanding of the impedances associated with interfaces arising inside the a-SCMFs. Based on the observed Nyquist plots, the impedance spectra could be modeled considering equivalent circuits previously described in the literature [44,45]. Figure 7 compares the Nyquist plots related to one representative a-SCMFC from each triplet. In particular, at the highest frequencies, it was possible to observe an ohmic (real) resistance representing impedances arising from electrodes, electrical contacts, and the electrolyte bulk. To focus on the features related to the electrochemical interfaces, plots were offset to suppress the contribution of this ohmic resistance. As frequency decreased, the appearance of a distorted arc could be attributed to interfacial interactions at the electrode/microbial biofilm/electrolyte interface. Such impedance can be modeled as a parallel between its charge transfer resistance (R\({}_{\text{ct}}\)) and a double-layer capacitance induced by the accumulation of charges. The distortion in the arc indicates a non-ideal capacitance behavior, likely due to the complex morphology and the porosity of electrodes [46,47]. In the equivalent circuit model, this corresponds to a constant-phase element. Focusing on this arc region of the Nyquist plot, over time, both the IMS and control  
Figure 6: LSV analysis was performed on two representative cells at (**A**) 8 months, (**B**) 10 months, and (**C**) 12 months from the start of experimental activity. Red straight and dash lines represent IMS cells, while black straight and dash lines represent control cells. For both cells, solid lines represent the polarization curves (left vertical axis), while dashed lines correspond to the power curves (right vertical axis). cells demonstrated a similar increase in charge transfer resistance. Specifically, the IMS cells experienced an \(\mathrm{R_{ct}}\) increase from (17 \(\pm\) 3) \(\Omega\) (at 8 months) to (51 \(\pm\) 6) \(\Omega\) (at 17 months), and the control cells from (19.2 \(\pm\) 0.6) \(\Omega\) (at 8 months) to (46 \(\pm\) 2) \(\Omega\) (at 17 months). Finally, at lower frequencies, diffusion limitations became the dominant impedance contribution, and the presence of the IMS was more evident.