Development of novel cathode materials and optimization of electrode performance towards scaling-up applications of microbial fuel cells

Open Access
Author:
Zhang, Fang
Graduate Program:
Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 20, 2012
Committee Members:
  • Bruce Ernest Logan, Dissertation Advisor
  • Bruce Ernest Logan, Committee Chair
  • John Michael Regan, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Michael John Janik, Committee Member
Keywords:
  • microbial fuel cells
  • poly(dimethylsiloxane)
  • electrochemical impedance spectroscopy
  • current collector
  • activated carbon
  • set anode potential
  • separator electrode assembly
  • pre-acclimated culture inoculum
Abstract:
Microbial fuel cells (MFCs) represent an emerging approach for bio-electricity production. Mesh current collectors made of stainless steel (SS) can be integrated into MFC cathode structure with a Pt catalyst and a poly(dimethylsiloxane) (PDMS) diffusion layer (DL). It is shown here that the mesh properties of these cathodes can significantly affect performance. Cathodes made from the coarsest mesh (30-mesh) achieved the highest maximum power of 1616±25 mW m–2 (normalized to cathode projected surface area; 47.1±0.7 W m–3 based on liquid volume), while the finest mesh (120-mesh) had the lowest power density (599±57 mW m–2). Electrochemical impedance spectroscopy showed that charge transfer and diffusion resistances decreased with increasing mesh opening size. Oxygen permeability increased with mesh opening size, accounting for the decreased diffusion resistance. At higher current densities, diffusion became a limiting factor, especially for fine mesh with low oxygen transfer coefficients. These results demonstrate the critical nature of the mesh size used for constructing MFC cathodes. PDMS was further investigated as an alternative to Nafion as an air cathode catalyst binder. Cathodes were constructed around either SS mesh or copper mesh using PDMS as both catalyst binder and diffusion layer, and compared to cathodes of the same structure having a Nafion binder. With PDMS binder, copper mesh cathodes produced a maximum power of 1710±1 mW m−2, while SS mesh had a slightly lower power of 1680±12 mW m−2, with both values comparable to those obtained with the Nafion binder. Cathodes with PDMS binder had stable power production of 1510±22 mW m−2 (copper) and 1480±56 mW m−2 (SS) over 15 days at cycle 15, compared to 40% decrease in power with the Nafion binder. Cathodes with PDMS binder had lower total cathode impedance than Nafion. This is due to a large decrease in diffusion resistance, because hydrophobic PDMS effectively prevented catalyst sites from filling up with water, improving oxygen mass transfer. The cost of PDMS is only 0.23% of that of Nafion. These results showed that PDMS is a very effective and low-cost alternative to Nafion binder that will be useful for large scale construction of these cathodes for MFC applications. Activated carbon (AC) air-cathodes are inexpensive and useful alternatives to Pt-catalyzed electrodes in MFCs, but information is needed on their long-term stability for oxygen reduction. AC cathodes were constructed with DLs with two different porosities (30% and 70%) to evaluate the effects of increased oxygen transfer on power. The 70% DL cathode initially produced a maximum power density of 1214±123 mW m−2 (cathode projected surface area; 35±4 W m–3 based on liquid volume), but it decreased by 40% after one year to 734±18 mW m−2. The 30% DL cathode initially produced less power than the 70% DL cathode, but it only decreased by 22% after one year (from 1014±2 mW m−2 to 789±68 mW m−2). Electrochemical tests were used to examine the reasons for the degraded performance. Diffusion resistance in the cathode was found to be the primary component of the internal resistance, and it increased over time. Replacing the cathode after one year completely restored the original power densities. These results suggest that the degradation in cathode performance was due to clogging of the AC micropores. These findings show that AC is a cost-effective material for oxygen reduction that can still produce ~750 mW m−2 after one year. In a separator electrode assembly MFC, oxygen crossover from the cathode raises the anode potential and inhibits current generation by exoelectrogenic bacteria, resulting in difficulties in reactor startup. In order to improve startup performance, MFCs with flat carbon mesh anodes were acclimated at set potentials (–0.2 V or +0.2 V versus standard hydrogen electrode), compared with no set potential control. Performance of these reactors inoculated with wastewater was also compared to those inoculated with cell suspensions from existing MFCs under the same conditions. Anodes inoculated with wastewater and acclimated to –0.2 V produced the highest power (1330±60 mW m–2) but they had the longest startup time (20 days). With inoculation using transferred cell suspensions, consistent and reproducible results in terms of faster startup (10 days) and high power production were obtained. Additional electrochemical analyses confirmed that inoculation with a transferred culture consistently improved anode performance, with the best activity obtained for anodes acclimated at –0.2 V. These results imply that rapid startup of larger-scale reactors will require inoculation with pre-acclimated cultures, and that acclimation at –0.2 V could improve power production compared to a more positive potential (+0.2 V) or a lack of set potential.