Characterization and Performance of Activated Carbon Catalysts and Polymer Membrane Layers for Microbial Fuel Cell Cathodes and an Analysis of Power Overshoot

Open Access
Author:
Watson, Valerie J
Graduate Program:
Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 22, 2013
Committee Members:
  • Bruce Ernest Logan, Dissertation Advisor
  • John Michael Regan, Committee Member
  • Fred Scott Cannon, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Peggy Ann Johnson, Committee Member
Keywords:
  • Microbial Fuel Cells
  • Activated Carbon
  • Oxygen Reduction
  • Membrane-Cathode Assembly
Abstract:
Microbial fuel cells (MFCs) are a promising technology for treatment of wastewater streams in combination with electricity production. Commercialization and implementation of MFCs could eliminate the large energy consumption common in traditional wastewater treatment and allow for the utilization of this untapped renewable energy source. Polarization curves from microbial fuel cells (MFCs) often show unexpectedly large drops in voltage with increased current densities, leading to a phenomenon in the power density curve referred to as “power overshoot”. Linear sweep voltammetry (LSV, 1 mV s−1) and variable external resistances (at fixed intervals of 20 min) over a single fed-batch cycle in an MFC both resulted in power overshoot in power density curves due to anode potentials. Increasing the anode enrichment time from 30 days to 100 days did not eliminate overshoot, suggesting that insufficient enrichment of the anode biofilm was not the primary cause. Running the reactor at a fixed resistance for a full fed-batch cycle (~1 to 2 days), however, completely eliminated the overshoot. These results show that acclimation at low fixed resistances are needed to stabilize current generation by bacteria in MFCs, and that even relatively slow LSV scan rates and long times between switching circuit loads during a fed-batch cycle may produce inaccurate polarization and power density results for these biological systems. Membrane separators reduce oxygen flux from the cathode into the anolyte in MFCs, but water accumulation and pH gradients between the separator and cathode reduces performance. To avoid these problems, air cathodes were spray-coated (water-facing side) with anion exchange, cation exchange, and neutral polymer coatings of different thicknesses to incorporate the separator into the cathode structure. The anion exchange polymer coating resulted in greater power density (1167 ± 135 mW m−2) than a cation exchange coating (439 ± 2 mW m−2). This power output was similar to that produced by a Nafion-coated cathode (1114 ± 174 mW m−2), and slightly lower than the uncoated cathode (1384 ± 82 mW m−2). Thicker coatings reduced oxygen diffusion into the electrolyte and increased coulombic efficiency (CE = 56 – 64%) relative to an uncoated cathode (29 ± 8%), but decreased power production (255–574 mW m−2). Electrochemical characterization of the cathodes using abiotic anodes in separate reactors showed that the cathodes with the lowest charge transfer resistance and the highest oxygen reduction activity produced the most power in MFC tests. The results using hydrophilic cathode separator layers revealed a tradeoff between power and CE. Cathodes coated with a thin coating of anion exchange polymer showed the most promise for controlling oxygen transfer while minimally affecting power production. Platinum is commonly used as the catalyst in MFC cathodes, but platinum is an expensive and limited resource. Activated carbon (AC) is a promising material for the replacement of platinum catalysts because it is inexpensive and can be made from renewable waste sources, but its catalytic performance in neutral solutions used in MFCs in not well understood. Commercially available AC powders made from different precursor materials (coal, peat, coconut shell, hardwood, and phenolic resin) were evaluated as oxygen reduction catalysts, and tested as cathode catalysts in MFCs. Carbons were characterized in terms of surface chemistry, specific surface area, and pore volume distribution, and kinetic activities were compared to carbon black and platinum catalysts using a rotating disk electrode (RDE). Cathodes using the coal–derived AC had the highest maximum power densities in MFCs (1620 ± 10 mW m–2) even though this AC had only average catalytic activity, measured by reduction onset potential (Eonset = 0.09 V), and selectivity, based on number of electrons transferred (n = 2.4). This coal–based AC also had the lowest specific surface area (550 m2 g–1) among the ACs tested. Peat–based AC performed similarly in MFC tests (1610 ± 100 mW m–2) but had the best catalyst performance (Eonset = 0.17 V, n = 3.6) in RDE tests and a lower than average specific surface area (810 m2 g–1). Hardwood based AC had the highest number of acidic surface functional groups and a higher specific surface area (1010 m2 g–1), but it had the poorest performance in MFC and catalyst tests (630 ± 10 mW m–2, Eonset = –0.01V, n = 2.1). There was a strong inverse relationship between onset potential and the quantity of strong acid (pKa < 8) functional groups, and a larger fraction of microporosity was negatively correlated with power production in MFCs. These results showed that surface area alone was a poor predictor of catalyst performance, and that a high quantity of acidic surface functional groups was detrimental to oxygen reduction and cathode performance. Four of the commercially available AC powders (peat, coconut shell, coal, and hardwood) were treated with ammonia gas at 700 °C in order to improve their performance as oxygen reduction catalysts. Ammonia treatment resulted in a decrease in oxygen (by 29 – 58%) and an increase in nitrogen content (total abundance up to 1.8 atomic %) on the carbon surfaces, which also resulted in an increase in the basicity of the bituminous, peat, and hardwood ACs. The kinetic activity and selectivity of ammonia–treated carbons were evaluated using a rotating ring disk electrode (RRDE), and compared to untreated ACs and platinum. All of the ammonia–treated ACs exhibited better catalytic performance than their untreated precursors, with the bituminous (treated, Eonset = 0.12 V, n = 3.9; untreated, Eonset = 0.08 V, n = 3.6) and hardwood (treated, Eonset = 0.03 V, n = 3.3; untreated, Eonset = –0.04 V, n =3.0) based samples showing the most improvement. These ACs were tested in MFC cathodes made by sandwiching the AC catalyst and polytetrafluoroethylene (PTFE) binder mixture between two current collectors, one coated with polydimethylsiloxane (PDMS) diffusion layers on the air–side, and the second one on the solution–side used to improve power. Cathodes made from the ammonia–treated coal-based AC had the one of the highest maximum power densities (2450 ± 40 mW m–2). Even though the ammonia–treated peat–based AC had the greatest ORR activity in RRDE testing, the untreated sample had higher power production in the MFC tests (2360 ± 230 mW m–2). The treated coconut and hardwood derived ACs outperformed the untreated precursor ACs in both electrochemical and MFC testing. These results show that reduction in oxygen abundance and increase in nitrogen functionalities on the surface of ACs can increase the catalytic performance for oxygen reduction in neutral media.