Unraveling the contributions of internal resistance components in two-chamber microbial fuel cells using the electrode potential slope analysis

Two-chamber (H-cell) bioelectrochemical systems (BESs) are one of the most widely used devices in bioelectrochemistry studies, but comparisons of performance have been challenged by differences in reactor architecture. Here we showed that the reactor solution resistance should be calculated using only the tube cross-sectional area and length rather than the electrode spacing (i.e. the solution resistance in the chambers is negligible). This approach was demonstrated using two H-cells (tube cross sectional areas of 4.5 cm² or 1.2 cm²) and variable electrode areas. The solution resistance (R_Ω) calculated from the tube diameter and length was consistent with that measured using electrochemical impedance spectroscopy (EIS), and was ∼80% of the total internal resistance with electrode areas larger than the tube area. Anode resistance and total power were similar (R_An = 26 ± 1 Ω to R_An = 34 ± 1 Ω, 0.37 ± 0.00 mW) when the anode was larger than the tube area (4.5 cm²), but the resistance increased (R_An = 431 ± 229 Ω) when the anode size (0.8 cm²) was smaller than the tube. Power and cathode resistance changed more with the size of the cathode (R_Cat = 61 ± 5 Ω at 9.6 cm², to R_Cat = 73 ± 7 Ω at 4.9 cm²), with very high resistance when the cathode was smaller than the tube diameter (R_Cat = 1246 ± 805 Ω for 0.8 cm² cathode). These results demonstrated that internal resistances are relatively insensitive to electrode sizes when the electrodes are larger than the tube area. While it has not been common to report tube geometry in H-cell studies, these dimensions will need to be reported in future studies in order to make practical comparisons of performance among these types of reactors.