Less is More: A Comprehensive Study on the Effects of the Number of Gas Diffusion Layers on Air–Cathode Microbial Fuel Cells

Air-cathode microbial fuel cells (AC-MFC) use a gas-diffusion-layer (GDL) coating based on polytetrafluoroethylene applied to the cathode to prevent electrolyte leakage. However, this type of GDL can also lead to a decrease in MFC performance due to electron-transfer limitation, mass-transfer limitation or catalyst availability. This study provides a comprehensive understanding of the significance of the GDL coating, demonstrating the interaction between the number of GDL coatings and the external resistance (R_ext) used. An experimental design in 28 mL AC-MFCs was prepared and conducted using two different R_ext (10 and 249 Ω) and four different GDL coatings (1 to 4 layers). The coating effect was not significant when operating with a high R_ext, where the electron transfer was the limiting process. However, when the R_ext was low, the amount of polytetrafluoroethylene limited the cathode performance due to a significant decrease in the Pt availability on the catalytic surface. Thus, GDL-1 with 10 Ω as R_ext reached 0.96 mA/cm², 3-fold higher than that obtained with 249 Ω as R_ext (ca. 0.30 mA/cm²). Besides, the current density did not vary noticeably in the other cathodes with 249 Ω as R_ext. Contrarily, the current density with 10 Ω as R_ext decreased as the number of GDL increased (0.74, 0.57 and 0.37 for GDL-2, GDL-3 and GDL-4 respectively). These values agreed with those of the polarization curve. Furthermore, limitations were also observed in electrochemical impedance spectroscopy measurements: the charge resistance increased with the number of GDL, related to the ease of electron flow. These values were18 Ω, 22 Ω, 53 Ω and 58 Ω for GDL-1, GDL-2, GDL-3 and GDL-4, respectively, for both 10 and 249 Ω cathodes.