Electrochemical studies of separators and activated carbon cathodes in single-chamber, air-cathode microbial fuel cells

Open Access
- Author:
- Wei, Bin
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 10, 2012
- Committee Members:
- Bruce Ernest Logan, Thesis Advisor/Co-Advisor
- Keywords:
- Electrochemical resistances
electrochemical impedance spectroscopy
separator electrode assembly
activated carbon
PTFE binder
air cathode
microbial fuel cells - Abstract:
- Microbial fuel cells (MFCs) are devices that use bacteria to produce electricity which provides an alternative to consumption of fossil fuels as an energy source. To gain an insight into these systems, various efforts worldwide from researchers have been made on the materials, configurations, and other related aspects of MFC construction. Power output is one of the fundamental aspects of MFCs, which can be characterized by varying resistances. In air cathode MFCs, which have an advantage of using passive air instead of forced air flow, the cathode plays an important role in determining power densities. In order to better understand the factors that affect power generation in MFCs, current-voltage curves and electrochemical impedance spectroscopy (EIS) were used to obtain cathode polarization, solution-separator, charge transfer and diffusion resistances of clean and used separators in separator electrode assemblies (SEAs) in single-chamber, air-cathode MFCs. Cathode polarization resistance was reduced at lower cathode potentials. EIS results showed that at a set cathode potential of 0.3 V (versus a Standard Hydrogen Electrode, SHE), diffusion resistance was predominant and it substantially increased when adding multiple separators between the electrodes. At a lower cathode potential of 0.1 V, resistances did not show significant differences in the presence and absence of separators. Used SEAs had increased charge transfer and diffusion resistances when one separator was used, at a cathode potential of 0.1 V. However, charge transfer resistance increased and diffusion resistance did not appreciably increase with four separators in used reactors (after three months of operation). The addition of plastic mesh to compress four separators against the cathode improved the maximum power densities (by 21%), likely due to minimization of the trapped water between the electrodes. Based on these results, it was concluded that multiple wipe separators and biofilm formation had hindered ion diffusion, and that it was important to compress the separator against the cathode. The catalyst is the main component of an air cathode MFC. Activated carbon (AC) is a promising candidate for large scale applications of MFCs as it has considerable catalytic activity for oxygen reduction, and it is commercially available. AC air cathodes were constructed using variable amounts of carbon (43 − 171 mg/cm2) and an inexpensive binder (10 wt% polytetrafluoroethylene, PTFE), and with or without a cloth diffusion layer (DL) that was sealed with PDMS to prevent water leakage. These cathodes were compared to commonly used, but much more expensive materials (carbon cloth, Pt, and Nafion binder) in MFCs, and by using different electrochemical tests. The cathodes with the highest AC loading of 171 mg/cm2, and no diffusion later, produced 1255 ± 75 mW/m2. Slightly higher power densities were initially obtained using 100 mg/cm2 of AC (1310 ± 70 mW/m2) and a PDMS/wipe diffusion layer, although the performance of this cathode decreased to 1050 ± 70 mW/m2 after 1.5 months, and 1010 ± 190 mW/m2 after 5 months. AC loadings of 43 mg/cm2 to 100 mg/cm2 did not appreciably affect performance (with diffusion layers). MFCs with the Pt catalyst and Nafion binder initially produced 1295 ± 13 mW/m2, but decreased to 930 ± 50 mW/m2 (1.5 months) and then 890 ± 20 mW/m2 (5 months). Cathode performance was optimized for all cathodes by using the least amount of PTFE binder (10%, in tests using up to 40%). These results provide a method to construct cathodes for MFCs that use only inexpensive AC and a PTFE, while producing power densities similar to those of Pt/C cathodes. The method used here to make these cathodes will enable further tests on carbon materials in order to optimize and extend the lifetime of AC cathodes in MFCs.