Cathode/Catholyte Impacts on Power Density and Anode Performance in Microbial Fuel Cells

Open Access
- Author:
- Lawson, Kathryn Elizabeth
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- May 01, 2020
- Committee Members:
- Bruce Logan, Thesis Advisor/Co-Advisor
John Michael Regan, Thesis Advisor/Co-Advisor
Christopher A Gorski, Committee Member
Patrick Joseph Fox, Program Head/Chair - Keywords:
- microbial fuel cell
MFC
bioenergy
bioelectrochemistry - Abstract:
- Microbial fuel cells (MFCs) are biological batteries that have the potential to turn wastewater treatment into an energy-generating process, though more research is needed for them to be feasible on a large scale. Standard cube-reactor shaped MFCs were tested with different cathode chamber conditions (ferricyanide catholytes or air cathodes), and an electrode area-normalized MFC comparison method called the electrode potential slope (EPS) method was used to quantify cell potentials and electrode resistances. Reactors with a ferricyanide catholyte and brush electrodes (FC-B) had the highest maximum power density of 2.46 ± 0.26 Wm-2. Maximum power densities were lower for MFCs with ferricyanide catholytes using flat carbon paper cathodes with a stirred catholyte (FC-F-S, 1.98 ± 0.28 Wm-2), and the same configuration without stirring (FC-F, 1.76 ± 0.12 Wm-2). Air cathode MFCs with a 70% porosity diffusion layer (A-70) had higher maximum power densities (1.33 ± 0.14 Wm-2) than those with a 30% porosity diffusion layer (A-30, 0.97 ± 0.07 Wm-2), but both of these produced lower maximum power densities than the ferricyanide cathode MFCs. Although the ferricyanide MFCs had higher maximum power densities, using the EPS analysis showed that total electrode resistances were lower for the air cathode MFCs than the ferricyanide MFCs. The sum of the cathode and catholyte resistances in the two types of flat cathode ferricyanide reactors were 36 ± 0 mΩ m¬2 (FC-F-S) and 46 ± 5 mΩ m¬2 (FC-F) compared to 20 ± 0 mΩ m¬2 (A-70) and 28 ± 6 mΩ m¬2 (A-30) for the air cathode MFCs. However, the sum of cathode and catholyte resistances for the brush cathode ferricyanide reactor, FC-B, (17 ± 1 mΩ m¬2) was slightly lower than the air cathode reactors, possibly due to improved mass transfer due to increased surface from the 3D brush that was not included in the projected area-based power normalization. These results show that the use of a flat electrode with ferricyanide does not necessarily provide the lowest cathode/catholyte resistances for MFCs. Unlike the MFC cathodes, the bioanodes of all the reactors performed similarly. The anode resistances appeared to be uninfluenced by the choice of cathode or catholyte in any of these configurations (FC-B, 25 ± 3 mΩ m¬2; FC-F-S, 22 ± 2 mΩ m¬2; FC-F, 22 ± 2 mΩ m¬2; A-70, 17 ± 1 mΩ m¬2; A-30, 21 ± 1 mΩ m¬2), which suggests that anode performance was independent of cathode performance and did not contribute to differences in overall reactor performance. Using the EPS method, it was shown that the primary benefit of ferricyanide towards higher maximum power is the higher theoretical open circuit potential (361 mV) as compared to the more commonly utilized two-electron transfer oxygen reduction reaction (267-337 mV). If improvements in air cathodes can be made to increase the utilization of four-electron transfer reactions (815 mV), which have a much higher thermodynamic potential than ferricyanide, it follows that reactor maximum power densities will also be higher with the improved air cathodes than with ferricyanide.