How to Make Cathodes with a Diffusion Layer for Single-Chamber
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Microbial Fuel Cells
Joshua Middaugh 6/5/06
Updated by Shaoan Cheng and Wenzong Liu, 5/12/08
Introduction
The u of a cathode with a diffusion layer in a single-chamber microbial fuel cell (MFC) can significantly improve the performance of the MFC by increasing its coulombic efficiency and maximum power density while simultaneously decreasing its loss of water. The coulombic efficiency is incread by the incread power output and by the incread operation time of the MFC. The operation time is likely incread due a reduction in the amount of oxygen flow to the anode chamber where bacteria preferentially u oxygen directly as an electron acceptor instead of using oxygen indirectly via the anode of the MFC. Oxygen flow, however, is still necessary for the catalyst-side, which is opposite the air-expod side, of the cathode. Thus, an optimum thickness for the diffusion layer exists and is the subject of a paper by Cheng et al (2006) The instructions below describe how to make 12 cm 2 circula
r cathodes with a polytetrafluoroethylene (PTFE) diffusion layer of optimum thickness. This process takes veral hours due to a number of steps that require wait time.
Pt/C Catalyst Layer
PTFE Diffusion Layers
AIR SIDE
SOLUTION SIDE
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Carbon Cloth
Carbon Ba Layer
Figure 1: Exaggerated side-view of a cathode with a four-coating PTFE diffusion layer
Materials
• Carbon Cloth with 30% by Weight PTFE Wet-Proofing (E-TEK B-1/B/30WP) • Carbon Black Powder (Cabot Vulcan XC-72)
• 40% by Weight PTFE Solution (diluted from 60% solution)
• 60% by Weight PTFE Solution (Aldrich PTFE 60 wt. % dispersion in water)
zigzag什么意思• 10% by Weight Platinum on Carbon Powder (ETEK C1-10 10% Pt on Vulcan XC-72) • 5% by Weight Nafion® Solution (Aldrich Nafion® perfluorinated ion-exchange resin) • Pure Iso-propanol (99%) (Alfa Aesar) • De-ionized (DI) Water
• Solid glass beads, 3mm in diameter (Propper Manufacturing Co. Inc.)
Equipment
• Metric Ruler
Corrections by Rachel Wagner 9/30/08
• Sharp Utility Knife
express card
• Electronic Balance (Resolution in milligrams) • Flat Spatula
• 1000 μL Micropipettor with Several Pipette Tips • Piece of Cardboard (Approximately 1 ft. × 1 ft.) • Small Paintbrush (1/4 in. Wide Tip) • Furnace (Capable of Heating at 370 ºC) • High-Temperature Fiberglass Gloves • High-Temperature Ceramic Plate • 3.8 cm Diameter Punch • Small Mallet or Hammer • Wooden Cutting Board
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• Plastic sample vial with cap. 8mL, • Touch mini Vortexer.
Applying the Carbon Ba Layer
privateequityNOTE: Inhomogeneities and variations in carbon black and PTFE content can lead to variations in performance from cathode to cathode. Thus, it is recommended that care be taken to ensure that as homogeneous a coating as possible is applied with as little as possible loss of the carbon black ba coat mixture.
1) Cut a piece of carbon cloth, using the ruler and utility knife, to make 2 cathodes (4 cm × 8 cm).
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2) Measure 1.56 mg of carbon black for every 1 cm 2
of cathode surface area using the electronic balance.可操作性
EXAMPLE: A two-cathode piece of carbon cloth, having an area of 32 cm 2
, would require 1.56
mg/cm 2
× 32 cm 2
= 50 mg of carbon black.
3) Measure, using the micropipettor, 12 µL of 40% PTFE solution for every 1 mg of carbon black ud in the step above.
EXAMPLE: A two-cathode piece of carbon cloth, using 50 mg of carbon black, would require 50 mg × 12 µL/mg = 600 µL of 40% PTFE solution.
4) Put carbon black into a plastic sample vial, add 6-8 glass beads and the PTFE solution, cap the
vial, vortex for 20 conds.
5) Coat all carbon suspension onto the one side of carbon cloth using small paintbrush, gently paint
to prevent getting the carbon suspension into another side of carbon cloth.
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Figure 2: Glass beads(left), paintbrush(center), and plastic vial(right).
6) Allow the coating to air-dry on the piece of cardboard for at least 2 hours. The drying process can
be expedited using a hair dryer.
CAUTION: Before using the furnace, note that hot surfaces and radiating heat when operating the furnace can cau rious injury to expod body parts. Be sure to exerci care and to wear high-temperature gloves when handling this equipment.
7) Place the piece of carbon cloth on a high-temperature ceramic plate in a pre-heated furnace at 370
ºC for about 20-30 minutes.
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Figure 3: Three two-cathode pieces of carbon cloth, ready to be placed in the furnace.
Figure 4: The furnace ud to heat the pieces of carbon cloth.
8) Remove the ceramic plate and carbon cloth allowing them to cool to room temperature on
high-temperature tiles before handling. Alternatively, the items may be cooled, albeit more
slowly, by turning off the furnace and leaving the furnace door open slightly.
NOTE: Curling of the carbon cloth after it has cooled, as en in Figure 4, is normal and
becomes more pronounced as PTFE diffusion layers are added.
Figure 5: two-cathode piece of carbon cloth that curled after cooling from 370 ºC to room temperature.
Applying the Diffusion Layer
1) Shake the 60% PTFE solution well since this solution is actually a suspension of PTFE particles
that needs to be well mixed in order to ensure evenly distributed coatings.
2) Apply one coat of 60% PTFE solution in a uniform fashion to the previously coated side of the
carbon cloth using the small paintbrush.
3) Smooth out any bubbles and remove any chunks of PTFE using the paintbrush.
4) Allow the PTFE coating to air-dry for at least 5-10 minutes. The coating should turn white when
dry.
Figure 6: two-cathode pieces of carbon cloth with white PTFE layer after air-drying.
Figure 7: two-cathode pieces of carbon cloth with black PTFE layer after heat treating in furnace.
5) Place the piece of carbon cloth on a high-temperature ceramic plate in a pre-heated furnace at 370 ºC for about 10-15 minutes.
6) Remove the ceramic plate and carbon cloth allowing them to cool to room
temperature on high-temperature tiles before handling. The PTFE coating should turn glossy black.
7) Repeat steps 1 through 5 above three more times to add and heat a total of 4 PTFE coatings. This number of coatings provides the optimum thickness for the diffusion layer.
8) Punch out the circular cathodes using the small hammer to hit the metal punch through the carbon cloth on top of a wooden cutting board. The wooden board prerves the sharp edge of punch since impact of the punch with a hard surface dulls this edge, or cut out cathode to right size using a scissors.
Figure 8: A circular 12 cm 2
cathode that has been cut out of a two-cathode piece of carbon cloth.
