# Novel Microfluidic Septum to Optimize Energy Recovery in Single-Chamber Microbial Fuel Cells
## 3 Results
### Perturbation Phase with Variable Sodium Acetate Concentration  
As devices working in real scenarios rarely receive a controlled amount of carbon energy source, part of this work aimed to assess how cells featuring an IMS performed during periods of reduced nutrient availability. Since the overall devices' performances were strictly correlated with the metabolic activity of electroactive bacteria constituting the biofilm on the anode surface, it was possible to demonstrate the correlation between the maximum output potential generated by MFCs and the concentration of carbon energy source dissolved in the electrolyte [31; 43]. To this end, over sets of consecutive refills, all the cells received electrolyte medium with sodium acetate molarity which reduced from 12 mM down to 2 mM. During this process, the composition of the electrolyte medium remained unchanged, except for the ammonium chloride concentration, which was reduced accordingly to preserve the same mass ratio as sodium acetate.  
From the measured output potentials, it was possible to extract the maximum values reached by the cells after each refill. To reduce fluctuations associated with each cell, the measured maxima were averaged over each a-SCMFC triplet. Figure 4 represents the average peak maxima observed at different concentrations of sodium acetate in the electrolyte refill. It is possible to observe how the IMS cells systematically provided a higher output potential compared to the control cells. Indeed, at the standard 12 mM sodium acetate concentration, the IMS cells featured, on average, a maximum peak potential of (2.8 \(\pm\) 0.2) mV, compared to the control cells at (0.68 \(\pm\) 0.07) mV. Even at sodium acetate concentrations as low as 2 mM, the IMS cells provided average peak maxima at (0.60 \(\pm\) 0.07) mV, higher than the control cells at (0.22 \(\pm\) 0.02) mV.  
Finally, it was possible to observe a cell's behavior when the sodium acetate concentration returned to 12 mM (empty-area symbols in Figure 4). In both cells, the observed trend was confirmed, by being able to return to output potentials compatible with the initial values of (2.7 \(\pm\) 0.4) mV and (0.69 \(\pm\) 0.08) mV for the IMS and control cells, respectively.  
Figure 3: (**A**) Cell output potential measured after two months of activity during the standard working period. Each blue triangle represents one replenishment with electrolyte solution (12 mM sodium acetate concentration). It is possible to observe how the IMS cells provided peaks broadened in time compared to the control cells. (**B**) Comparison of cell output potential measured after two months (**left**) and one year (**right**) of activity. Each blue triangle represents one replenishment with standard electrolyte solution (12 mM sodium acetate concentration).  
To fully assess the a-SCMFCs' performances, the energy recovery parameter previously defined in Equation (1) was also considered [6; 33; 34; 35; 36; 37]. Thus, instead of only focusing on the maximum output potential of the MFC, the \(E_{rec}\) parameter considers the power output from the a-SCMFCs. Figure 5 represents the average energy recovery calculated at different concentrations of sodium acetate in the electrolyte refill. These average values were obtained by aggregating measurements from a-SCMFCs, as previously carried out for peak maxima analysis. Considering the standard 12 mM sodium acetate concentration, the IMS cells featured an average energy recovery of (37 \(\pm\) 1) J/m\({}^{3}\), one order of magnitude higher than that reached by the control cells (3.0 \(\pm\) 0.3) J/m\({}^{3}\). Also, for the minimum sodium acetate concentration (2 mM), the IMS cells outperformed the control cells, providing (0.8 \(\pm\) 0.2) J/m\({}^{3}\) and (0.18 \(\pm\) 0.03) J/m\({}^{3}\), respectively. It was then possible to verify how IMS cells provided higher \(E_{rec}\) compared to the control cells, thus providing an optimization of the a-SCFMC design.