Introduction: Make a Microbial Fuel Cell (MFC) - Part III

This 5 gallon single chamber microbial fuel cell uses inexpensive conductive cloth to provide a silver catalyst and a styrofoam permeable membrane.

Operation of a single chamber MFC is complicated by oxygen poisoning resulting from the conflicting requirements of the anaerobic population and the cathode requirement for dissolved O2. This design attempts to limit oxygen poisoning by using a brush type carbon electrode combined conventional carbon paper to provide a gas barrier without limiting free flow of the microbial medium.

The use of styrofoam as the permeable membrane is novel as far as I can tell.

Conductive cloth is used to provide an inexpensive silver catalyst to promote oxidation at the cathode. We use silver plated conductive cloth from lesseremf but any silver plated material should do. The use of silver as a catalyst is based on the research done for the air cathode in an aluminum air battery and is experimental in an MFC.

As you can see from the picture I got about 90 mV across the open circuit after charging and assembling the cell. The extraction of power from an MFC scales directly in proportion to two things, population density and surface area of the electrode surface at the cathode and anode.
Power will continue to rise as the population density increases.

Step 1: Bill of Materials

This is a simple bill of materials and assembly. Because we are building a single chamber MFC we won't need to create a salt bridge or anything like that.

1 5 gallon plastic pail with lid - I got my mine at Home Depot on sale for $2.97 plus the lid
8 in styrofoam disc - Cut out of packing material or I'm using one I got at Michael's
Use loose styrofoam if you can get it. You want air to pass easily through
Anaerobic Microbial Culture - I'm using "Microbe Lift" brand Septic Tank and Cesspool Treatment
Yeast should also work
1 Package of large aquarium charcoal filter pads. I'm using Fluval brand 4 PLUS filters
Silver coated conductive cloth - I'm using the "Stretchy silver coated sheer nylon weave". This is used as a catalyst to facilitate oxyidation at the air cathode. It also as the electron collection grid.
A couple of wire leads with clips or some way to attach them to the carbon pads.
Two sheets of carbon paper. This is used to get good mechanical connectivity between the pads.

Step 2: Create the Styrofoam Cathode

The styrofoam cathode which sits at the top of the MFC chamber is a gateway that admits air and vents biogas resulting from the microbe action.

As you can see in the picture it is a sandwich. The bottom layer is styrofoam, next is a layer of conductive cloth, then two carbon pads laid side by side. This is followed by another layer of conductive cloth, then the carbon paper. As you can see in the picture the carbon paper is placed with the shiny side facing the conductive cloth.

The electrode assembly is attached to the styrofoam by means of 4 toothpicks and a strip of clear plastic tape.

Step 3: Prepare the Microbial Chamber

Now take the lid and carefully cut out the inner circle. This will be an almost perfect fit for the styrofoam disk.

Next prepare the anode assembly on the bottom of the pail ( see picture ). This is done by laying down an 8x5 inch piece of carbon paper, followed by a 8x5 inch piece of conductive cloth then the two remaining pads from aquarium filter.

This assembly is buoyant and tends to float when the solution is added. I found it necessary to attach this assembly to the bottom of the pail with clear plastic tape. I think super glue and/or a slightly different assembly might be easier.

Okay now we're ready to charge this puppy up and see what happens...

Step 4: Assembling and Operating the MFC

Okay Tesla let's turns microbes into electrons....

Put in the rest of your shredded paper so that the anode is liberally covered.

I went down to Big Lots and picked up a couple of 3L bottles of inexpensive soda to use as microbial media but almost anything can be used. Make yourself a nice 5% sugar water solution or use a mild vinegar solution.

I chose these because I wanted the 3L bottles for the next generation of my Wind Turbine and thought it would make a good sugar based medium. I certainly wasn't going to drink the stuff. Also the acid in the soda will help to break down the paper.

After you've added the soda place the bucket out in the afternoon sun for a couple of hours to vent the gas. The fill the pail the rest of the way with water. The styrofoam will float on the water and press against the lid to obtain a seal (more or less).

Attach the second lead to the carbon electrode on the styrofoam. Float the styrofoam ( carbon side down ) in the water and the two leads around it through the hole in the lid.

Add 1/4 cup or so of Microbe Lift to charge the MFC and then press the lid down firmly.

Simply attach a 220 ohm resistor across the leads and begin measuring voltage.