UBC Theses and Dissertations
Systematic study of separators to improve the performance of passive air-breathing flat-plate microbial fuel cells Kazemi, Sona
Passive air-breathing microbial fuel cells (MFC) are a promising technology for energy recovery from wastewater. The performance of the passive air breathing MFCs is dependent on the separator characteristics, isolating the anaerobic anode from the air-breathing cathode. The separator plays a more important role when the electrodes are placed in close proximity, to reduce the Ohmic resistance. The goal of the present work was to study the separator characteristics and its effect on the performance of passive air-breathing flat-plate MFCs (FPMFCs), through a combination of experimental and theoretical approaches. This was performed through characterization of 8 separators to investigate the ionic resistivity, oxygen and ethanol crossover, and proton transport number. The separators were then examined in three passive air-breathing FPMFCs with different electrode spacing, using three-dimensional graphite felt anodes and platinum-based cathodes. A numerical model was developed based on the mixed potential theory to investigate the sensitivity of the electrode potentials and the power output to the separator characteristics. The separator characterization indicated a greater susceptibility to oxygen and ethanol crossover in diaphragms, compared to the ion-exchange membranes (IEMs). Increasing the electrode spacing was also shown to be desirable for the application of diaphragms, as the anodic mixed potentials were reduced. The peak power density decreased by increasing the mass transfer coefficients of oxygen and ethanol in the separator. The model indicated that this was due to the increased mixed potentials at the anode, caused by the oxygen crossover. The mixed potentials at the cathode did not vary as the ethanol crossover increased, due to the slow kinetics of ethanol oxidation over Pt. The model also indicated that the peak power was affected by the proton transport number of the separator, which affected the cathode pH. The peak power was not sensitive to the resistivity of the separator due to the overshadowing effect of the oxygen crossover. The passive air-breathing FPMFC, using a 6 mm thick graphite felt anode, Pt-based cathode, and Nafion®117 membrane, showed the highest peak power density of ca. 0.52 W/m², which was higher than those reported for the active air flow FPMFCs in the literature.
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