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Passive air-breathing microbial fuel cells (MFCs) are a promising technology for energy recovery from wastewater and their performance is highly dependent on characteristics of the separator that isolates the anaerobic anode from the air-breathing cathode. The goal of the present work is to systematically study the separator characteristics and its effect on the performance of passive air-breathing flat-plate MFCs (FPMFCs). This was performed through characterization of structure, properties, and performance correlations of eight separators in Part 1 of this work. Eight commercial separators were characterized, in non-inoculated and inoculated setups, and were examined in passive air-breathing FPMFCs with different electrode spacing. The results showed a decrease in the peak power density as the oxygen and ethanol mass transfer coefficients in the separators increased, due to the increase of mixed potentials especially at smaller electrode spacing. Increasing the electrode spacing was therefore desirable for the application of diaphragms. The highest peak power density was measured using Nafion ® 117 with minimal electrode spacing, whereas using Nafion ® 117 or Celgard ® with larger electrode spacing resulted in similar peak powers. Part 2 of this work focuses on numerical modelling of the FPMFCs based on mixed potential theory, implementing the experimental data from Part 1.

BACKGROUND: High cost and ohmic loss are two issues that microbial fuel cells (MFC) face before becoming economically viable. To address the high cost and ohmic loss issues, a flat-plate MFC (FPMFC) configuration applying a passive air-breathing cathode and a three-dimensional anode was introduced. Electricity generation was examined in the FPMFC through operation in the presence and absence of a proton exchange membrane (PEM), and in batch and continuous modes.RESULTS: Continuous operation of the FPMFC in the presence of a PEM favored power generation, mainly due to elimination of oxygen and biomass in the anode. Peak power density of 18 Wm -3 was produced in the presence of a PEM (ohmic resistance 40 cm 2 ), which was more than 5-fold higher than that with J-cloth. During batch operation, the power density increased and reached maximum in the third batch (18 W m -3 at 60 A m -3 ). Greater stability was observed during continuous operation resulting in a 2.5-fold increase in peak power density (44 W m -3 at 146 A m -3 ). CONCLUSION:The passive air-breathing FPMFC showed promising performance, offering a more economically viable configuration than the conventional FPMFCs using active (air, ferricyanide, and ferric iron) cathodes.

This study aims to investigate the effect of the graphite felt (GF) substrate surface treatment, the GF active surface area, and the anode chamber depth on the performance of the passive air breathing flat-plate microbial fuel cell (FPMFC) configuration. Three passive air breathing FPMFCs (depth of anode chamber: 2 mm, 4 mm, and 8 mm) were developed and operated using 1, 2, and 3 packed layers of three-dimensional (3D) graphite felt anodes, respectively, with similar cross sectional (geometric) surface area as the cathode and the membrane. The surface of the GF substrate was treated by soaking in a hot solution of nitric acid prior to inoculation. The 2 mm FPMFC generated a peak power density superior to that previously reported for the same configuration with no GF treatment. The peak power density in the 8 mm and 4 mm FPMFCs with 3 and 2 layers of GF increased by 118 % and 48 %, respectively, compared to the 2 mm FPMFC with 1 layer of GF. By using only 1 layer of GF, the peak power density showed no significant variation with the electrode spacing.