Make a Microbial Fuel Cell (MFC) - Part 1

In response to numerous questions about what happens to the collected algae this instructable should help someone to build a microbial fuel cell (MFC) with household items and materials. As its name suggests, an MFC uses microbes to catalyze electricity-producing reactions.

This instructable is based on work done by Bruce Logan and his team at Penn State University and on the microbial fuel cells built by Abbie Groff, a student at Conestoga Valley High School in Lancaster, PA. The research she performed with her MFCs helped her win the Grand Champion Award at the 2005 Lancaster County Science Fair.

Now to be completely honest the fuel cell we will build is not "purely" an algae fuel cell, it is a microbial fuel cell that uses anaerobic bacteria to decompose organic matter, in our case dead algae.

The fuel cell will consume the algae (or other organic material) with two significant by-products, electricity (always useful) and methane gas. In a production system the methane must be captured for further use for instance as fuel for a steam powered generator which processes its exhaust plume through an algae based (or other) exhaust gas scrubber.

In this design its very cleverly captured in the sealed anode and if you come up with some clever way to do something with it, I'd be very pleased to hear it.

Step 1: Bill of Materials

Tools:

Drill or drill press
Razor knife or scissors
Hot glue gun
Funnel - Optional for filling bottles
Soldering iron
Lead Solder
Materials and where I got them. I have no relationship with any of these stores or products. They are inexpensive and should be commonly available:

• Two heavy duty plastic bottles with sealable lids. Wide mouth bottles are best choice for ease of inserting broad surface area electrodes.
• Low power aquarium air pump

I recommend the Hagen Elite 799 available at PetSmart for around $6. Rated at 1.8W.

• 1 x 2" PVC pipe 1-2 feet long
• 2 x 1 quart mason jars or similar
• 1 x 2" PVC Nipple or Sprinkler system riser
• 4 x PVC slip caps. You can probably get away with two, I use two for storing the salt bridge
• 2 x 1in square Silkscreen material or other nonconductive mesh

Cut to 2 circles that match the outdiameter of the PVC nipple

• Agar - 100g per Liter of water
• Salt - 400g per Liter of water ( 14 oz/Qt ).
• Carbon cloth or carbon paper

Carbon cloth of various types can be purchased online. These folks http://www.uscomposites.com/carbonpage.html offer it for around $40/yd. I'm using plain old fashioned carbon paper with wire mesh support. Eventually I plan to experiment with alternative designs that use carbon filtration technologies.

• Nitrogen for removing oxygen from water. This can be readily obtained from several sources locally and online.
• Welding supply places will have Nitrogen and Argon in tanks for various sizes. Around $35 for a small tank here.
• Local tire dealers. Local tire dealers often replace air in tires with nitrogen (for a fee). They will happily fill up your inner tube or inexpensive pressure tank (for a fee).
• Bacteria for a MFC can be obtained from several sources.
• Shultz Liquid Plant food

Most likely, wastewater or anaerobic sediments will initially contain enough organic matter to serve as food for the bacteria, but this will eventually run out. Algae will provide the food source (substrate) used to maintain the MFC. The liquid plant food and soil water will provide trace minerals and nutriets.

Step 2: Preparing the Electrolyte

Okay so here's the first tricky bit. We're going to make an algar/salt solution and use it to fill the salt bridge. Now the really clever bit is figuring out how to keep the salt bridge upright while the agar cools.

Agar can be obtained from health food stores and high end supermarkets with health food departments. The clerks will be able to help you if you ask if they carry 'the dietary supplement agar and calcium supplements with calcium carbonate because a friend recommended them" instead "Hey, you guys got any stuff for a microbial reactor salt bridge?"

Preparing Tablet or Powdered Agar:

Dissolve 1 bar, 10 tablets or 7 grams of powdered agar in 1/2 quart of the soilwater we prepared in the last step. Warm the agar in a hot water bath by placing it in a glass bottle. Heat water on the stove until just about boiling, remove from heat and place the agar bottle in the water. The microwave is another good way. Simply heat up a bowl of water and place the bottle with agar in the center.

As it warms the agar will begin to melt, warm the agar until it becomes liquid. Add the salt and mix well to dissolve the salt in the agar. Remove from the heat

Dissolving salt:

• Finely ground salt such as canners salt or table salt dissolves much faster than coarsely ground salt (rock salt).
• It is not clear that all salt added must be dissolved if a solution is to obtain maximum electron exchange
• Salt dissolves much faster in hot water than in cold water.
• Salt dissolves much slower as the salt concentration increases. The last bit of salt may take a long ime to dissolve.
• Agitation greatly increases the rate at which salt dissolves
• A layer of salt on the bottom may take days to dissolve if left undisturbed

Step 3: Making the Salt Bridge

Now that we've prepared the agar/salt electrolyte solution we're going to fill the salt bridge with it. Here's what I recommend.

Get a short glass large enough to hold the bridge PVC upright. Cut a piece of cardboard large enough to span the glass and at least double the size of the bridge pipe. Cut a circular hole in the center that matches the outer diameter of the bridge.

Place a PVC nipple on one end of the pipe. Insert the pipe though the hole in the cardboard and place in the glass so that it is standing on the flat end of the nipple. This should hold the bridge upright while the agar cools and solidifies.

Now carefully take the melted agar/saline solution and using the funnel fill the salt bridge to the top. I found that wrapping a wash cloth around the base of the pipe where it exits the cardboard made overflow much easier to deal with when filling the bridge. Carefully place a second PVC cap on the end to prevent accidental spillage and allow to cool.

Once the agar has cooled and solidified remove it from the assembly. It may be stored in the refrigerator.

Now using a drill or drill press with a large diameter bit drill out the end of two PVC slip caps leaving a short collar around the end. When the slip is placed over the end of the salt bridge it should hold a circular piece of non-conductive/non-reactive mesh such as plastic silk screen material firmly in place.

Step 4: Obtaining Culture Media

As I mentioned in the introduction this is not truly an algae based fuel cell. This fuel cell relies upon algae to provide biomass feedstock for the production of electricity, methane and CO2. The fuel cell itself is driven by bacteria adapted to live in sediment where no oxygen is found. These anaerobic bacteria can be obtained from several sources.

One of the recommendations from the folks at Penn State was to obtain some primary affuent (waste water) from the local water purification plant. But that was just too icky.

Anaerobic bacteria are also found in the benthic zone. According to Wikipedia:

The benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers. Organisms living in this zone are called benthos. They generally live in close relationship with the substrate bottom; many such organisms are permanently attached to the bottom. The superficial layer of the soil lining the given body of water is an integral part of the benthic zone, as it influences greatly the biological activity which takes place there. Examples of contact soil layers include sand bottoms, rock outcrops, coral, and bay mud.

Therefore much like we obtained our algae culture from natural sources we will obtain culture amples from available natural resources.

So go to a local lake or creek, a swamp is good if you're lucky enough to have one. If you fish or you are have a friend who fishes ask them where the best catfish or suckers are to be found. Tell them you're looking for bottom feeders which indicates a rich organic base.

Basically you're going to wade into the water until you're pretty deep in bottom muck, optimally the water will cover the tube when it is buried to obtain the sample. Push or drive the tube into the bottom to a depth of at least 6" inches or more. If the tube is not fully submerged when the sample is taken fill it before inserting and "top it off" with lake, pond or running water take water from near the bottom if possible.

Tilt the pipe over and place a slip cap over the mud end. Top off the pipe with ambient water and place a slip cap over the other end. Keep the pipe closed until you are ready to use it ( step 7 I think ). The bacteria of interest are anaerobic which means we want to expose it to oxygen as LITTLE as possible.

Once again wikipedia provides:

Anaerobic is a technical word which literally means without air (where "air" is generally used to mean oxygen), as opposed to aerobic.

In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen. An anaerobic adhesive is a bonding agent that does not cure in the presence of air.

Take each of the mason jars and scoop down deep in the bed, possibly scooping down several times. Fill each jar with deep mud. This should generally insure that you get a good healthy sample of bacteria along with sufficient additional material to create high quaility nutrient media.

Try to disturb the water and bed as little as possible fill the Mason jars with ambient water. This will capture trace elements which may be present and beneficial to our bacteria.

Leave everything alone until we're ready to use them.

So now we're going to make a salt bridge.

Step 5: Constructing the Electrodes

Layers of copper/galvanized steel mesh
carbon paper

Copper Mesh Swatches from TWP Inc.

Almost any conductive metal mesh such as window screen mesh. I'm particularly excited about investigating the role of galvanized steel and zinc mesh. The most efficient MFC electrodes use a very expensive platinum or titanium catalyst. However zinc is so closely associated with effective energy transfer that it be very interesting indeed.

In this case both electrodes will be constructed using the same technology, layers of carbon paper interlaced with layers of wire mesh. The carbon paper layers are slightly smaller in size than the mesh layers. Copper wire is soldered to the exposed wire mesh around the edge.

Wire mesh layers should be placed on the outside to contain the carbon paper and maintain integrity of the electrodes in the mud or sand. The assembly should be tight to ensure good connectivity between the carbon and mesh layers plus circulation of media and water.

Apply solder liberally around the outside edge to ensure the best connectivity between the layers.

Once the electrodes have been assembled the exposed metal mesh should be protected with a good coating of hot glue.

Now we will insert the electrodes into the caps for later assembly into the MFC.

Drill a small hole in the center of one of the caps and a larger 1/4" hole off to the side. Pass the electrode wire through the small hole leaving the electrode to dangle where it will be approximately centered in the jar. Seal the wire hole with hot glue.

Push a length of air through the hole long enough to reach to the bottom of the cathode jar. Insert the air stone into the tube and seal. Seal the hole around the tube with hot glue or epoxy.

This assembly is called the cathode top and electrode.

Take the lid for the other jar. Pass the wire from the second electrode through that, also placed so that it hang approximately in the middle of the jar. The bacteria culture fed by the vinegar should thrive in the de-gassed water and it should not be necessary to bury the electrode in the mud.

Now that the sub-assemblies are complete we are ready to put our MFC together and fire it up.

Step 6: Assembling the Reactor - Part I

Now its time to bring it all home.

Cut a hole in each of the reactor bottles at the same height equal to the outside diameter of the slip caps. Holding the bridge pipe upright remove the temporary slip cap. Cut a circle of silkscreen material (pantyhose might work) or other plastic mesh to the diameter of the pipe. Place the silkscreen over the agar. Now get the slip caps we drilled out when we created the salt bridge and verify that they hold the agar/electrolyte. Place glue on the pipe and seat the slip cap.

Depending on the screen material used to hold the agar/electrolyte solution in place it it probably possible to use the slip bushing (pictured). This would be secured to the side of the bottle and the saltr bridge inserted into with a sleeve of retaining mesh held in place by the bushing. The bushing is held in place with hot glue and the salt bridge secured with it.

Now we're going to prepare a conductive solution for the cathode chamber. Again using the solubility of salt maxes out at 400g/L we're going to prepare enough solution fill one bottle. Heating the water and other suggestions that were used during creating the salt bridge also apply here. If heat is used allow the water to cool completely before proceeding.

Step 7: Assembling the Reactor - Part II Assembling the Cathode

In this design the two chambers are not particularly distinct. At the moment I am inclined to think that they should be approximately equal but that may be nothing other than my own compulsion to symmetry.

Pick one chamber for the cathode (saltwater/electrolyte chamber) and one chamber for the anode (mud/culture chamber).

Prepare enough salt solution to fill the cathode chamber following the procedure outlined in step 3 without the agar. When the solution has cooled fill the cathode chamber with the electrolyte.

Now place the cathode cap containing the air hose, bubbler stone and electrode on the cathode. Approximately center the electrode in the jar and close the lid fully then open a small amount to allow air to vent.

Next we will populate the anode chamber with bacteria and culture media which should produce electricity.

Step 8: Assembling the Reactor - Part III Assembling the Anode

Now we turn our attention back to the we obtained earlier. The collected water will be used as culture media and the mud in the pipe will be used to provide the culture.

We'll be diverging from Abbie's design a bit. Where Abbie used the actual mud we're going to try to extract a sample and culture the bacterium.

I have been advised by Dr. Logan at Penn State that they do not de-gas the influent since the bacteria will handle it due to the low solubility of oxygen. The material in italics may be bypassed but I left it in case anyone ever wants to de-gasify water for some reason.

"'To do that we're going to de-gas the water using nitrogen, argon or other inert gas. Strictly speaking you don't have to do this but it will give our bacteria a better chance to breed.

We're going to do it slowly. Since the options for obtaining gas range from inner tube to pressure tank I give you the following simple instruction. Connect a section of aquarium air tube to the end of a hose that is connected to the gas reserve. Hopefully you have some way to regulate the gas. If you're using an inner tube I suggest filling a balloon (insert it over the valve and push the valve in) and using that to dispense the gas by inserting the air hose into the end and letting the gas slowly out of the balloon. Yeah, I know, it ain't easy being that cheap. Get over it or spend some money for a tank with a valve.

Now having arranged by some mechanism to regulate the gas flow of the nitrogen into the air tube attach the bubbler stone to it. Fill the anode chamber (one bottle) with the culture media water. Agitate well. Insert the bubbler stone into it and slowly bubble nitrogen through the solution.

How much nitrogen to bubble and for how long? A very, very good question and as soon as I have an answer I'll update this. I've asked the folks at Penn State so its not impossible this will be updated. Try to stretch it out, say 15 minutes maybe stirring occassionally.

If you've got the nitrogen do both bottles since we'll be topping off the culture media.

Once you've de-gassified the water or not its time to introduce the culture (mud).

This is also the time to put in some algae sludge if you have some.

See I said it was an algae fuel cell....

Grab the sample pipe and place the mud end in the anode chamber preferably under water before removing the slip cap. You might need to push it off with a long screwdriver or kitchen fork. Pull the slip cap off of the other end and slowly begin pushing mud out with a dowel or broom handle. You want to push out the bottom 2-3 inches of mud then take out the pipe. This should give you a solid culture of primarily anaerobic bacteria. Top off the culture media if needed from the second jar and add 2 drops of vinegar for each quart of culture media in the jar to supplement the natural nutrients.

Now place the anode cap and electrode in the anode jar and seal the edge with hot glue. The anode should be air tight and is not designed to be recharged at this time. Once the food is exhausted the bacteria will die. This will be indicated by a drop off in the output voltages.

Because the MFC cannot be recharged it is properly called a battery. To rectify this (get, a little electrical joke...very little) use a container which gives a solid, air tight seal and add a feeder hole and tube to it. Remember to seal the feeder tube when not in use and preferably flush with nitrogen.

Step 9: Operating Your MFC

Now if you attach an external circuit through a resistor you should begin getting voltage out of your MFC. The voltage will increase the bacterial population grows and then begin to decrease.

If you don't get voltage there could be any number of good reasons, including the culture. Try again, make sure you get a good seal on the anode.

Electrode construction and connectivity is also important.

For more information and design alternative on Microbial fuel cells I recommend

Dr. Bruce Logan's excellent work

Abigail Groff's outstanding page on her award winning project. Without her work this instructable would not be done.

And don't forget the folks at MicrobialFuelCell.org

I hope you enjoyed this instructable, I will be updating and revising as time goes on.