This cool project was something we experimented with in my Microbiology class! We were instructed to do a little background research, check out some online designs, and design our own microbial fuel cell. Then we tested our projects!
I was originally going to test the amount of energy produced from three different sources around my campus - mud from the stream where there is little to no sunlight, the pond mud where many geese call home (excess of poop in this area), and the ditch where there is still water with LOTS of bubbling mud! However, I ran out of time and energy so I had to choose between the three locations. For this project, I chose the most promising of the three locations - the bubbling muck!
Step 1: Organize Your Materials!
Several different papers and instructables inspired me to combine experiments and materials to create my own microbial fuel cell.
Here are the supplies I used:
-2 plastic bottles with screw-on caps (One will be the anode chamber, the other will be the cathode chamber)
-Baggies (to collect and store the mud)
-Muck from a muddy location (you want muck from beneath the surface where anaerobic bacteria live)
-7.0pH Phosphate Buffer (80mL)
-Methylene Blue (I only needed one drop into the anode chamber which is approximately 50uL)
-Small cap (to measure glucose)
-Graduated Cylinder (to measure Phosphate Buffer)
-Heatplate with magnetic stirring capabilities
-Drill (to make holes in bottles)
-Screwdriver (to manipulate holes in bottles)
-Hot glue/Silicone glue
-Salt (I used almost one whole cup)
-Beaker (for boiling salt bridge)
-Small piece of rope (this will act as the salt bridge between the two bottles)
-Scissors (to cut wire)
-Carbon material (this will be the electrode)
-Small piece of plastic tubing (passageway for oxygen to cathode chamber)
Step 2: Make Your Salt Bridge!
Salt bridges can be made with agar, rope, and filter paper. For my experiment, I chose to use rope. I cut down the length to just enough to connect the two bottles together. You do not want a really long salt bridge because it will take too long for the solution to soak through and build the circuit between the anode and cathode.
I boiled almost a cup of salt in a beaker of water and added the length of rope into the beaker. I let it soak and boil for at least half an hour. While I let the rope soak, I drilled holes into the caps of the bottles and the side of the bottles as equally as possible for the salt bridge to be attached.
After boiling, I pushed the ends of the rope through each of the holes in the plastic bottles to connect the two together and see if the rope was the right length. When I felt satisfied with the length of rope, I wrapped it in electrical tape and then duct tape to prevent leakage. I stuck the ends of the rope back into the bottles and then hot glued the remaining opening of the holes. However, the hot glue did not work as well as planned, so I had to use silicone to completely lock in the water so there would be no leakage when I added my cathode liquid.
Step 3: Prepare Your Electrodes!
The electrodes I decided to use were leftover fabric pieces of carbon. I poked holes into the carbon electrodes and pushed the end of the copper wire that was exposed through the hole. To keep the electrode attached, I folded the wire over. I strung the copper wire through the drilled holes in the caps. One electrode will be submerged in the anode mud and one electrode will be submerged in the cathode.
Step 4: Preparing the Anode and Cathode Chambers
Once every other aspect has been prepared, you can begin making the anode and cathode chambers.
-Pour in the muck you collected
-Add 80mL Phosphate Buffer (I used 7.0pH)
-Add two capfuls of Glucose (I started with one, but after a day and a half I added a second and it seemed to help when the voltage values went into the negatives)
-If after a day and a half the values do not look normal, add one drop (50uL) of Methylene Blue to see if the values would stabilize in any way
-Glue tubing to side of bottle with tubing pointed upward (this will be for oxygen)
-Pour in the remaining warm water from the boiling of the salt bridge step
-Add a stir bar
-Set up the fuel cell so the cathode sits on a hot plate and the magnetic stir bar can stir the contents (added this step after the first 1.5 days)
Step 5: Monitor the Voltage of the Contraption With a Voltmeter
Insert the electrodes into their respective chambers, submerging them in its contents. Screw on the caps and connect alligator clips to the copper wires. Make sure the voltmeter is connect correctly to those clips. The red wire should lead to the cathode and the black wire should lead to the anode.
Monitor the voltage produced by the contraption after the first 24 hours then on your own time following that. Make adjustments as necessary.
Step 6: Results
Here are some of the voltages obtained over time (in V):
-0.867 (added another cap of glucose, added stir bar to cathode, added one drop of Methylene Blue to anode)