DEVELOPMENT AND OPTIMIZATION OF ACTIVATED CARBON AIR CATHODE TOWARDS SCALE UP APPLICATIONS OF MICROBIAL FUEL CELLS

Open Access
Author:
Yang, Wulin
Graduate Program:
Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 15, 2016
Committee Members:
  • Bruce E. Logan, Dissertation Advisor
  • Bruce E. Logan, Committee Chair
  • John M. Regan, Committee Member
  • Michael A. Hickner, Committee Member
  • Manish Kumar, Outside Member
Keywords:
  • Activated carbon
  • Air cathode
  • Microbial fuel cell
  • Oxygen reduction
  • Membrane
  • Phase inversion
Abstract:
Microbial fuel cells (MFCs) are emerging technologies that produce bio-electricity from inorganic and organic wastes. Air cathodes used in MFCs need to have high catalytic activity for oxygen reduction, but they must also be easy to manufacture, inexpensive, and watertight. Several different approaches were used here to improve air cathode performance, and reduce water leakage. A low cost poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) phase inversion coating was developed as a cathode diffusion layer (DL) to improve power. A maximum power density of 1430 ± 90 mW m–2 was achieved at a PVDF-HFP loading of 7.1 mg cm–2 (4:1 polymer:carbon black), with activated carbon as the oxygen reduction cathode catalyst. This power density was 31% higher than that obtained with a more conventional platinum (Pt) catalyst on a carbon cloth (Pt/C) cathode with a poly(tetrafluoroethylene) (PTFE) diffusion layer (1090 ± 30 mW m–2). The improved performance was due in part to the larger oxygen mass transfer coefficient of 3 × 10–3 cm s–1 for the PVDF-HFP coated cathode, compared to 1.7 × 10–3 cm s–1 for the carbon cloth/PTFE-based cathode. The diffusion layer was resistant to electrolyte leakage up to water column heights of 41 ± 0.5 cm (7.1 mg cm–2 loading of 4:1 polymer:carbon black) to 70 ± 5 cm (14.3 mg cm–2 loading of 4:1 polymer:carbon black). This new type of PVDF-HFP/carbon black diffusion layer could reduce the cost of manufacturing cathodes for MFCs. A simple one-step, phase inversion process was used to construct an inexpensive cathode using a poly(vinylidene fluoride) (PVDF) binder and an activated carbon catalyst. The phase inversion process enabled cathode preparation at room temperatures, without the need for additional heat treatment, and it produced for the first time a cathode that did not require a separate diffusion layer to prevent water leakage. MFCs using this new type of cathode produced a maximum power density of 1470 ± 50 mW m–2 with acetate as a substrate (28 mL reactor), and 230 ± 10 mW m–2 with domestic wastewater (130 mL reactor). These power densities were similar to those obtained using cathodes made using more expensive materials or more complex procedures, such as cathodes with a polytetrafluoroethylene (PTFE) binder and a poly(dimethylsiloxane) (PDMS) diffusion layer, or a Pt catalyst. Even though the PVDF cathodes did not have a diffusion layer, they withstood up to 1.22 ± 0.04 m of water head (~12 kPa) without leakage, compared to 0.18 ± 0.02 m for cathodes made using PTFE binder and PDMS diffusion layer. The cost of PVDF and activated carbon ($3 m–2) was less than that of the stainless steel mesh current collector ($12 m–2). PVDF-based AC cathodes therefore are inexpensive, have excellent performance in terms of power and water leakage, and they can be easily manufactured using a single phase inversion process at room temperature. Applications of MFCs will require cathodes that are highly resistant to water leakage. This can be accomplished by using a better diffusion layer even with cathodes relatively resistant to water leakage, but the materials must be inexpensive. To improve the resistance of the cathode to leakage, a hydrophobic PVDF membrane synthesized using a simple phase inversion process was examined as an additional low cost ($0.9/m2), carbon black free DL that could prevent water leakage at high pressure heads compared to a more expensive PTFE/carbon black DL ($11/m2). The power density produced with a PVDF (20% ,w/v) DL membrane of 1400 ± 7 mW/m2 was similar to that obtained using a wipe DL [cloth coated with poly(dimethylsiloxane)]. Water head tolerance reached 1.9 m (~19 kPa) with no mesh supporter, and 2.1 m (~21 kPa, maximum testing pressure) with a mesh supporter, compared to 0.2 ± 0.05 m for the wipe DL. The elimination of carbon black from the DL greatly simplified the fabrication procedure and further reduced overall cathode costs. While the PVDF membrane was shown to be a useful DL, a method was needed to integrate the DL into the cathode structure. A hot pressing method was used to bind the PVDF DL onto the air side of the activated carbon cathode, and additional catalyst layers were added to improve performance. Cathodes pressed at 60 °C produced a 16% higher maximum power density of 1630 ± 10 mW m–2 than non-pressed controls (1400 ± 7 mW m–2). Cathode performance was further increased to 1850 ± 90 mW m–2 by catalyst stacking, through the addition of an extra catalyst layer (CL), which better utilized the available surface area of the stainless steel mesh (SS) current collector. The use of one stainless steel current collector and two catalyst layers (SS/2CLs) produced more positive cathode potentials compared to other designs (SS/CL or 2SS/2CL). Low materials costs and high power production for MFCs using these cathodes could enable more cost effective power production using MFCs. To improve oxygen reduction kinetics of the cathode catalyst, an iron-nitrogen-carbon co-catalyst was incorporated into activated carbon (Fe-N-C/AC), resulting in a nearly four electron transfer, compared to a two-electron transfer for plain activated carbon (AC). With acetate as the fuel in 200 mM phosphate buffer solution, the maximum power density was 4.7 ± 0.2 W m–2, which is higher than any previous report for an air-cathode MFC. With domestic wastewater as a fuel, MFCs with the Fe-N-C/AC cathode produced up to 0.8 ± 0.03 W m–2, which was twice that obtained with a platinum-catalyzed cathode. The use of this Fe-N-C/AC catalyst can therefore substantially increase power production, and enable broader applications of MFCs for renewable electricity generation using waste materials. Long-term operation of MFCs can result in substantial degradation of AC air-cathode performance. In order to examine a possible role in fouling from natural organic matter in water, cathodes were exposed to high concentrations of humic acids. Cathodes treated with 100 mg L–1 of humic acids did not exhibit any significant change in performance. Exposure to 1000 mg L–1 of humic acids, which increased the mass of the cathodes by ~5% (14 ± 2 mg of organic matter per cathode), decreased the maximum power density of the MFCs by 14% (from 1310 ± 30 mW m–2 to 1130 ± 30 mW m–2). Total cathode resistance increased by 30% (from 57 Ω to 74 Ω) for cathodes treated with 1000 mg L–1 of humic acids, primarily due to an increase in the diffusion resistance (from 32 Ω to 50 Ω), based on resistance components measured using electrochemical impendance spectroscopy. The adsorption of the humic acids decreased the total surface area by 12% (from 520 m2 g–1 to 460 m2 g–1), suggesting that the main impact of the adsorption of organic matter was due to pore blockage. Minimization of external mass transfer resistances using a rotating disk electrode reduced the impact of organic matter adsorption to only a 5% reduction in current, indicating about half the impact of the humics was associated with external mass transfer resistance and the remainder was due to internal resistances. Rinsing the cathodes with deionized water did not restore cathode performance. These results demonstrated that humic acids could contribute to cathode fouling, but the extent of power reduction was relatively small in comparison to the large mass of humics adsorbed. Other factors, such as microbially produced biopolymers, or precipitation of inorganic chemicals in the water, are therefore likely more important contributors to long term fouling of MFC cathodes.