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

This instructable provides detailed directions for building your own microbial fuel cell using Jello, Saltwater and Septic Tank treatment. It's based on the information provided in Make a Microbial Fuel Cell (MFC) - Part 1

This is a true fuel cell in that both the anaerobic and electrolyte media are replaceable and the salt bridge may be serviced as needed. The fuel cell consumes organic material and produces electricity and methane ( or more properly biogas include CO2 and other gasses ). These are captured in a methane store for further processing....Okay, it's a balloon and I have no idea what you should do with the biogas except be careful and experiment with it.

We'll be using a traditional two cell design with a gelatin/salt bridge. Most commonly agar is used but we'll be trying both traditional unflavored gelatin as well as hide glue. Yes, you can actually use Jello brand flavored gelatin and I would be very interested in your results.

I also intend to experiment with gelatinous nutrient medium in the anode chamber. It seems to me a gel based replaceable anode would be a significant enhancement to the utility of this technology.

The device has two relatively small chambers (approximately 100 mL). In theory microbe population densities are primarily a function of nutrients and not space and we should be able to obtain (more or less) the same charge out of a 100 mL cell as we would from a 1L cell with the same population.

Now let's take a look at the materials we'll be using.

Step 1: Bill of Materials


Drill Press or Drill with bits
Soldering iron with solder
Measuring cups
Skewer ( wood or metal )

2 power tool replacement brushes
I got these from the specialty hardware department at ACE
2 1x1x1 PVC Schedule 40 Tee
3 1" PVC Schedule 40 Connector
1 1" PVC Schedule 40 Slip Cap
2 1" PVC Schedule 40 Plug
1 1x1/2 Slip vs FPT threaded adapter
Titanium fishing lead
Clear plastic wrap
Rubber bands
Salt Bridge Medium
  • Knox Unflavored Gelatin
  • Hide Glue (powdered not liquid)
  • Agar
Household salt or lye
Anaerobic Innoculent
  • Microbe-Lift Septic Tank Treatment - Ace Hardware
Make sure you rurchase an anaerobic treatment, there are a number of treatments including
Rid-X which are not specifically anaerobic. I have not tested these and have no idea what results
their microbes may produce
  • Wastewater primary effluent
  • Fish bearing river, creek, lake or stream sediment
  • Prepared culture such as E. Coli

Step 2: Create the Salt Bridge

The heart, so to speak, of a two chamber MFC lies in the salt bridge or membrane that seperates the cells. Agar is most commonly used but isn't necessarily readily available. However since Agar is a gelatin I thought conventional unflavored gelatin might make a substitute.

This is my least favorite component in this design, although I like its novelty. I would really like to have something at the ends to provide mechanical stability.

To create the salt bridge we will use Knox Unflavored gelatin in place of the more common agar. I have some reservations about the stability of this innovation. My next round will focus on the use of hide glue (dry not liquid) as the binding agent. This will not substantially alter the bridge process.

Start some water to boil, you don't need and actually won't use more than a couple of tablespoons.

Take one of the coupler sections. Place the plastic wrap firmly over one end and secure with a rubber band to form a tight seal. Place the coupler open end up in a bowl.

Take one packet of Knox unflavored gelatin and pour it into the coupling. Fill the rest of the way with salt. Pour this mixture into a dry measuring cup and pour back into the coupling. You'll notice that settling left a little more room. Top off with salt, pour back into the measuring cup and add an extra tablespoon. Mix the dry ingredients thoroughly and pour back into the upright coupler.

When the water is at a strong boil pour a bit into the measuring cup and use this to gently fill the salt bridge. Pour carefully so you don't flush the tube but be generous. The mixture will settle and allow you to "top off" the water, this will disclose air bubbles flushed out and help judge when the bridge is full. Carefully picking it up and examining the bottom can also help make sure that the entire column is saturated.

Allow to stand for a few minutes and pour off any tiny bit of standing water on the top. Then place the bowl in the refrigerator for 3-4 hours.

This is the salt bridge, it should be left in the refrigerator until final assembly.

Step 3: Assemble the Electrodes

It seems the biggest challenge facing the amateur microbiologist or energy, among the challenges facing the amateur microbiologist or energy researcher is the elusive carbon electrode.

Okay, first you will need to use one wire and one carbon brush to make a carbon electrode according to my other instructable Easy Carbon Electrode.

Now taking the DRY sponge cut a strip 3/4" wide and about an inch long. This piece should fit smoothly into the neck opening of the screw top. Now take it back out and using a skewer, nail or other suitable object punch a hole big enough for the wire to pass through.

Using the smallest drill bit you have equal to or greater than the size of the wire drill a small hole in the center of the screw cap. Thread the electrode lead through the sponge and slide the sponge into the the neck of the screw top. Thread the wire through the cap, it should pass more or less freely. Screw the top down lightly and add water to the sponge. It will expand to fill the neck and provide a slight overcover at the base.

This is the anode assembly. It is used for the anaerobic (microbe cell). The small gap will allow the decomposition gasses to escape while the sponge will inhibit the passage of air into the chamber. To add nutrients to the anode chamber simply unscrew the top and gently pour the nutrient medium through the sponge.

For the cathode we are using a different approach. The cathode is inserted into the sponge (actually placed between two pieces of sponge) where air can circulate freely.

To make the cathode assembly remove the coil wire from the remaining brush. Because the brush lead will be fully exposed I did not add a secondary lead or connector although it would certainly do no harm.

From the remaining section of the sponge cut a strip 1" wide by 4" inches long, then cut this into two strips. Please the remaining carbon brush between the two strips so that only the top is exposed. Insert this into one of the 1" connectors. Wet the sponge and it will expand to hold the electrode in place.

Step 4: Assemble the Fuel Cell

Once the salt bridge has cooled and solidified its time to assemble the device. For easier dis-assembly and maintenance I recommend coating the PVC with the graphite powder when assembling.

Take the two remaining couplers and insert the end caps fully into them. Insert these into the base of the tees to provide feet for standing.

Insert the salt bridge into the center tee. When properly seated the tees will meet and the salt bridge connector will not be visible.

Your fuel cell is now complete and ready to be charged.

Step 5: Charging and Operating

Dayyum, that septic tank stuff stinks...You're going to want to do this outside or in a well ventilated area.

Pick on chamber to be the anode (microbe) chamber and one to the cathode chamber.

Fill a measuring cup with about 2 ozs of warm water and add salt to it until no more will dissolve ( a saturated solution ). Stirring and shaking will help. Allow the water to completely cool. Remove the sponge slices from the cathode assembly and make sure they are thoroughly soaked in the salt solution. Pour the rest into the cathode chamber until only the salt slurry remains. Place the carbon electrode on top of one sponge, cover gently with the slurry and then use the remaining slice to complete the sandwich. Place this gently back into its connector and insert into the cathode tube. The tube will overflow as the cathode is inserted, That's okay we want the carbon electrode to come into contact with the air so pour off a bit of water as well.

Fill the anode chamber 1/2-3/4 full of the septic tank treatment. I also added a couple of teaspoons of sugar and some shredded paper to provide carbon and nitrogen. Fill the chamber the rest of the way with pure, dechloronated water. Inser the anode assembly fully into the anode tube.

I began measuring voltage on the open circuit. Within 15 minutes I was generating measurable voltage (82 mA). Once the output voltage seems to stabilize I will measure under load and update.