Graduate Thesis Or Dissertation
 

Cathode development and reactor design for scaling-up microbial fuel cells

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  • Developing new wastewater treatment technologies which will off-set the high-energy cost associated with treatment is necessary to maintain both water and energy security. Microbial fuel cell technology represents one such option. However, there are still many obstacles to overcome before practical application of this technology can be realized. Improving cathode and reactor design while lowering cost and increasing performance will remove two major obstacles to scale-up and move MFC technology one-step closer to practical application. Metal supporting materials are increasingly being used as base materials for microbial fuel cell (MFC) cathodes. However, the potential for corrosion may limit their use as base materials of MFCs during scale-up and long-term operation. In the first study included in this dissertation, the electrochemical performance, power generation in MFCs, hydrostatic pressure tolerance, and stability of activated carbon (catalyst) cathodes with carbon cloth or different size metal mesh as base materials were investigated. Electrochemical testing results showed that the finest stainless steel mesh (250x250 openings per inch) outperformed carbon cloth cathodes by 10-40% at current densities ranging from 6 to 11.2 A m⁻² over the typical cathode operating range of 0.1 V to 0 V. When tested in MFCs, however, carbon cloth based cathodes out performed all stainless steel mesh cathodes by as much as 34%, reaching 1.72 W m⁻²; probably due to the corrosion and salt build-up on the surface of the stainless steel mesh cathodes. Carbon cloth cathodes also maintained high static pressure heads of 1.9 m. The high electrochemical performance, hydrostatic pressure tolerance, and corrosion resistance of carbon cloth suggest that carbon fiber based materials may be more suitable than metal based materials for use as MFC cathodes base material for some applications. Replacing precious metal catalysts by inexpensive activated carbon (AC) is a breakthrough in microbial fuel cell (MFC) cathode fabrication. In the next study covered by this dissertation, activated carbon powders made from bamboo, peat, coal, coconut, and hardwood sources were characterized in terms of their surface area, pore size distribution, surface chemistry, and conductivity to better understand the linkages between the physical and chemical properties of AC and their electrochemical performance with carbon cloth as the base material. The bamboo-based AC demonstrates the highest potential for use as a catalyst for carbon cloth based cathode, reaching -10.6 A m⁻² and 11.27 A m⁻² at 0V with loading of 25mg cm⁻² and 50mg cm⁻², respectively. The maximum power density reached 3.3 W m⁻² in CEA-MFCs and 2.6 W m⁻² in cube-MFCs, respectively. These power densities are much higher than that typically reported for single-chamber MFCs with activated carbon catalysts. The higher proportion of micorpore surface area/volume, higher conductivity, and lower C/O ratio may all contribute to the higher performance. These results demonstrate that activated carbon/carbon cloth cathodes are capable of achieving high performance with very low potential for corrosion, making them more suitable for use in scaling-up MFCs. To further advance MFC technology toward scale-up, developing more efficient reactors that maintain performance during scale-up is paramount for practical application to be realized. When MFCs are scaled up, however, increasing reactor size has typically resulted in decreased power density. Furthermore, the voltage output of a single MFC is normally less than 0.8 V, often less than 0.3 V at maximum power output, which greatly limits the application of MFCs. In the next study, a novel scaled-up MFC configuration that contains multiple cloth electrode assemblies in which the MFCs were internally connected in series (iCiS CEA-MFC) was developed. The iCiS CEA-MFC, equivalent to 3 CEA-MFCs, produced a high voltage output over 1.8 V and a maximum power density of 3.5 W m⁻² using cathodes containing activated carbon as the catalyst. This power density is 6% higher than that reported for a similar smaller CEA-MFC, indicating that power can be maintained during scale-up with a greater than 33-fold increase in total cathode surface area and greater than 20-fold increase in reactor volume. The maximum power density occurred at an HRT of 80 min and an acetate concentration of 5.9 gL⁻¹, which is significantly higher than that typically reported for MFCs. High stability was also demonstrated based on the performance of the iCiS CEA-MFC containing activated carbon/carbon cloth cathodes over a period of one year of operation. The high power and stability is likely due, in part, to a more efficient means of current collection caused by the internal series connection, which avoids the use of expensive current collectors. These results clearly demonstrate the great potential of this MFC design for further scaling-up. However, serial electrical connection of MFCs can result in unbalanced voltage between individual MFCs, which can lead to voltage reversal, causing decreased voltage and power output and electrode material deterioration. In the final study, voltage reversal in newly designed iCiS-MFC stacks with metal mesh or carbon cloth as the cathode base material is examined. Serious corrosion was observed in the MFC stacks with the stainless steel cathode base material, which may have been caused and further worsened by repeated voltage reversal. Higher power output and stability was observed in the MFC stack using carbon cloth as the cathode base material. Conditions related to MFC continuous operation including pump stoppage, gas build-up within the reactor, and rapid decreases in external resistance at high current density, were also examined to determine their relation to voltage reversal and MFC performance. Although negative MFC voltages occurred in some MFCs and the total reactor voltage decreased 67 to 85% under these operational conditions, full recovery following voltage reversal was observed after normal operating conditions were restored in the MFC stacks. These results indicate that voltage reversal can be avoided through proper operation and design of MFC stacks and in the event voltage reversal occurs, full recovery is possible with the iCiS CEA-MFC.
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