by Keith Schubert

The following biological fuel cell was built at the Desert Studies Center at Zzyzx in 2010 as part of NASA Ames' Spaceward Bound.

Making the Fuel Cell

• Make a salt bridge. This will take 15-30 minutes to make and one to two days to dry.
• Setup the cells. This will take 30-60 minutes to make and one day to dry.
• Make the electrodes. This will take anywhere from half an hour to three hours to make and one day to dry.
• Assemble the fuel cell. This will take about 30 minutes.
• Our results results.
• Our pictures.
• Other site links.

Salt Bridge

This is an economic alternative to a PEM (proton exchange membrane). A PEM costs hundreds of dollars for a small sheet and our salt bridge costs less than a buck and can be made from household products. If you use a PEM you still need the tube to connect the two chambers.

• Salt
• Gelatin
• Hot Water
• 2" Pipe (not metal, such as PVC or ABS plastic typically found at home improvement or hardware stores)
• Mesh (Drywall seam mesh tape works great)
• Glue stick for hot glue gun (only need a few drops)
• Plastic wrap

• Saw (hacksaw works well)
• Hot glue gun
• Sand paper
• Rubber band
• bowl

1. Cut about a 3" portion of the 2" pipe. Clean the cut with sandpaper. We only need a 3" portion, the rest of the pipe is no longer needed. We will refer to the 3" section as the pipe for the rest of this section.
2. Sand the inside of the pipe so that it is rough, this helps the gelatin to stick.
3. Wrap one end of the pipe and secure with the rubber band.
4. Fill the pipe half way with gelatin, then fill the rest with salt. Pour it out into the bowl and then mix thoroughly. Do not add water.
5. Cut about a foot of the mesh tape and hot glue it inside the tube such that it goes across the center of the inside of the pipe. The mesh is here to support the gelatin, so make sure it crosses enough to provide support.
6. Pour the gelatin salt powder mixture back into the pipe.
7. Put the pipe in the bowl (provides support and it is easier to clean), then slowly add hot water. Only add enough water to wet the entire mixture, you are not dissolving the powder you are making a solid block. The result should be thick like cake batter.
8. Let it harden over a couple days. It will be reasonably dry in a day, but rock hard in two. After it is reasonably dry you can remove the plastic wrap to help it dry faster. Check the resistance when it is done, the less the better. We got around 5 ohms.

Cell Setup

We will be making two cells, one for the aerobic and one for the anaerobic side.

• 4" pipe (not metal, PVC or ABS pipes that are typically found at a home improvement or hardware store work great. I used pipes typically used for toilets - it seemed appropriate. All the pipe parts below should be of the same type.)
• Two 4" end caps
• two 4" pipe tees, two slip connections, one threaded (test tee H x H x Cleanout)
• two threaded cleanout plugs.
• two 4x2 ABS Flush Bushing Spg x Hub (4" to 2" reducer)
• pipe cement

• Saw (hacksaw works well)
• Sandpaper
• Drill

1. Cut two portion of the 4" pipe around 5" in length. Clean the cut ends with sandpaper. We need this to cap off one end of the tee, and provide a little volume. One piece is for each cell
2. Apply cement in an even ring around the outside of one end of one of the pipes. It should be roughly the width of the brush in the pipe cement can and within half an inch of the end. Don't put too much on, a thin even coat is all you need. Do the same thing to the inside of an endcap. Insert the cemented pipe end into the cemented endcap. Repeat on the other pipe piece and endcap.
3. Apply cement as above to the other end of one pipe and to the inside of one of the slip-fit portions of one tee. Insert them together. Repeat for the other pipe and the other tee.
4. Apply cement to the outside of one of the flush bushings (4" to 2" reducer) and the inside of the remaining slip-fit connection on a tee. Insert the flush bushing into the tee. Repeat for the other flush bushing and tee.
5. Put the cleanout plugs in the threaded cleanouts on the tees so you don't lose them.
6. Drill a small hole (to run wires through) on each tee. I suggest putting it near the threaded portion (but not in the cap as removing and reattaching the cap would twist the wires, or in the threaded portion as it would block the cap.) and close to the reducer as the cells will be connected by the reducers and this will put the wires close to each other.
7. Let dry for a day.

Electrode Construction

• obtain graphite rods. I suggest using graphite from carpenter's pencils. You can buy just the graphite rods for mechanical carpenter's pencils online, or buy the regular pencils at a home improvement store or hardware store and then strip the wood with a knife of dikes (diagonal cutters). Don't use regular mechanical pencil graphite as they break easy and have a small surface area.
• Lay about five rods next to each other to create a large flat surface.
• On one end only wrap thin wire around several times to hold them together. Use conductive epoxy to secure the wires to the graphite. Alternately you could solder the wires to the graphite. This is a pain as graphite does not solder well and thus requires a lot of solder. Sadly, this is what I did because conductive epoxy has to be ordered over the internet and I started my planning for this a couple days before I left. Oh well.
• Strip both ends of a wire with plastic coating about a 1/4". Attach one end of the wire to the metal bonded on the electrode. You can use conductive epoxy or solder.
• Cover all the metal part that are touching the graphite with either silicon caulk or hot glue.
• Make a bunch (you need at least two, but more surface will give more current).
• Let it dry a day for caulk or several minutes for hot glue (I love hot glue).

Assembling the Fuel Cell

• Salt bridge
• Two cells
• TFE paste
• Glue stick for hot glue gun (only need a few drops)

• Hot glue gun
• Shovel
• bucket with bacterial mud. Such mud can be found in any pond or stream, just get the real stinky stuff a few inches down on the bottom of the water. Alternately you can use a bacterial septic tank treatment and mix it with dirt and water. I think using naturally occurring mud is more useful as it provides an opportunity to talk about how bacteria is used to break down waste materials in nature.

• Stand one of the cells you made on its end cap and fill it up to the cleanout with bacterial mud.
• Insert all but one electrode into the mud and thread the wires through the hole you drilled. Seal with hot glue.
• Stand the other cell on its end cap and fill to cleanout with water (preferably from the same place you got the mud).
• Put the remaining electrode in the water and thread the wire through the hole you drilled. Seal with hot glue.
• Put TFE paste in a ring around the outside of the salt bridge near the end. Insert it into the 2" flush bushing on the water fuel cell (TFE paste will stop leaks but allow you to remove the salt bridge at a later time). You put it in the water one first so you can lay it on its side to attach the mud one without leaks. The mud won't spill when placed on its side because it is thick.
• Put the cleanout plug in the water tee.
• Put TFE paste on the other end of the salt bridge. Place the fuel cells on their sides and insert the salt bridge into the 2" flush bushing on the mud cell. You now have a single biological fuel cell.
• Open the cleanout plug on the water tee and fill the remainder with water. Leave the plug off as this side is the aerobic part. Only use the plug for transporting.
• Fill remaining portion of the mud (anaerobic) fuel cell with mud and close the cleanout plug. This one is not supposed to get air so leave it closed.

Results and things to do

• Measure the current and voltage at least once a day for a couple weeks. The values will change as the bacteria grow on the electrodes in the mud.
• Try solving some questions, such as what is the fastest way to get the bacteria up to speed producing electricity? Is it good to just connect the wires for a week or two and then start using it? What food does the bacteria like most? How much methane is produced?
• Make a bubbler to remove methane from the anaerobic chamber.<\li>
• Make an automated feeder (plastic bottle filled with waste vegetation chopped up in water) to slowly drip food into the mud.<\li>

Pictures

• Pictures from Keith Schubert.
• Pictures from Ernesto Gomez.
• Pictures from someone else.
• video from someone else.
• Pictures from Andrew Duncan.
• slides from Heather Smith.

Links

• Penn State's Make a Microbial Fuel Cell Page
• Microbial Fuel Cells
• Bio Batteries and Fuel Cells
• Nanobiocatalysis For Biofuel Cells