# Novel Microfluidic Septum to Optimize Energy Recovery in Single-Chamber Microbial Fuel Cells
## 2 Materials and Methods  
### Design and Fabrication of a-SCMFCs  
The structural components of the asymmetric, square, single-chamber MFCs, with open-air cathode configuration [31], were fabricated via 3D printing technology (Stratasys OBJET 30 with VeroWhite material). To ensure complete solvent removal, 3D-printed components were kept overnight in an oven at 110 \({}^{\circ}\)C. The a-SCMFCs were composed of three separate components kept together by screws and rubber sealing: an anodic compartment (air-tight), an intermediate spacing (with a window and inlet/outlet holes), and a (open-air) cathode compartment (Figure 1A,B). The available geometric area occupied by the electrodes in both the anode and cathode compartments was equal to 5.76 cm\({}^{2}\), and the inner volume occupied by the electrolyte was 12.5 mL.  
Figure 1: (**A**) Cross-sectional view of an a-SCMFC featuring the intermediate microfluidic septum proposed in this work. (**B**) Picture of an IMS cell connected to the experimental apparatus. (**C**) Intermediate spacing including the IMS proposed in this work. (**D**) Comparison with the flow field (without IMS) modeled in previous studies. The image of the flow field was adapted and re-printed with the permission of John Wiley and Sons from the article: [32].  
The intermediate spacing component allowed us to perform cell refills through its top and bottom apertures. Moreover, the window present in such a spacing component permitted the insertion of the intermediate microfluidic septum (Figure 1A,C). The novel design of the IMS was based on knowledge acquired from previous studies [31; 32] which modeled and studied the microfluidic flow field arising inside cells under normal operative conditions (Figure 1D). For the MFCs in this study, the IMS was designed to occupy the entire window present in the intermediate spacing, except for a narrow pass-through opening with a geometric area of 0.36 cm\({}^{2}\). With the IMS, the inner volume available for the electrolyte was reduced to 6.6 mL. The opening was oriented so as to connect both the inlet and outlet with a straight line, following the trajectory around which the flow field normally develops during electrolyte replenishment [31; 32], as highlighted in Figure 1D.  
To assess the contribution provided by the IMS, this experiment consisted of a triplet of cells featuring the IMS (here named IMS 1, IMS 2, and IMS 3) and a triplet of standard a-SCMFCs without the IMS as the controls (referred to as Control 1, Control 2, and Control 3). During the experiment, all IMS and control cells were simultaneously refilled using the same electrolyte solution.  
As the anode, as-received carbon felt (FuelCell Store, Bryan, TX, USA) was used by cutting it into 30 \(\times\) 30 mm\({}^{2}\) squares. Inside the cells, the anodes were held in place and electrically contacted by a 3D-printed frame threaded with titanium wire (Goodfellow Cambridge Limited, Huntingdon, UK). As the cathode electrode, a commercial AvCarb Gas Diffusion System (AvCarb Material Solutions, Lowell, MA, USA) was employed, composed of carbon paper, a poly-tetra-fluoro-ethylene (PTFE) treatment on the air-facing side, and micro-porous layer surface coating facing the electrolyte side. To promote the oxygen reduction reaction, a catalyst paste, to be spread on the cathodic micro-porous layer, was prepared according to previous works [38; 39; 40]. This paste was based on Platinum (10 wt% Pt on carbon, from Sigma Aldrich (St. Louis, MO, USA), final Pt loading 0.5 mg/cm\({}^{2}\)) and Nafion (5 wt% Nafion, from Sigma Aldrich, final concentration 3 mg/cm\({}^{2}\)), acting as a binder.