With some mud, salt, and water, you can create a closed circuit that generates a current. This is called a microbial fuel cell, a device that uses bacteria to create electrical power by oxidizing simple compounds like glucose or organic matter in wastewater. Given the finite supply of fossil fuels, this biofuel cell is a promising approach for generating power in a renewable, carbon-neutral way. Check out these examples:
The fuel cell works when bacteria attach to the electrode in an anode chamber of a cell that is oxygen-free. Since the bacteria do not have oxygen, they must transfer their electrons somewhere else. The cathode however is exposed to oxygen; thus, the two electrodes are at different potentials and create a bio-barrier or a “fuel-cell.”
1. MAKE THE SALT BRIDGE
Prepare an agar solution according to the instructions on the packaging. Add ½ cup of salt to the water.
Cover one end of the PVC pipe with plastic wrap to contain the prepared agar solution. Place the pipe vertically in a dish, then pour in the solution and allow it to cool (Figure A).
2. OBTAIN A MUD SAMPLE
The mud must come from the benthic zone — anything associated with or occurring on the bottom of a body of water. This is where you will find the electrochemically active anaerobic bacteria. If the sample is being collected from the bottom of a creek, pond, or lake, it should be black in color. Topsoil mixed with distilled water can also work if it contains enough anaerobic bacteria. Place mud in a container and cover.
3. BUILD THE HOUSING
Using a permanent marker, outline a hole on the side of one of your plastic containers large enough to fit a PVC fitting. With a ruler, measure the location of the mark and make a mark at exactly the same location on the side of the second plastic container. Ensure that your marked outlines are exactly opposite and facing each other, then cut out the holes.
Mark the center of each of the two lids for the containers. Drill a small hole in each lid for the copper wire to run through, and an optional hole on one of the containers for the placement of an air pump. Place the PVC fitting into the holes and glue into place. Allow glue to dry (Figure B).
CAUTION: Some adhesives can be irritating to the skin. I wear gloves while handling glue.
4. PREPARE THE ELECTRODES
Strip off each end of the red and black copper wires with a wire stripper and fold one of the ends around a sheet of aluminum mesh. Bind it with the mesh (Figure C) or, optionally, with a paper clip.
Insert the other ends into the pre-drilled holes on the containers and seal with glue.
5. ASSEMBLE THE FUEL CELL
Insert the PVC nipple into the FHT PVC fittings on the containers and hand tighten.
Insert the air pump tube into the pre-drilled hole on the cathode container and seal with glue (optional).
Insert each end of the salt bridge into the pipe connectors and tighten until it is securely in place. Be sure to create a watertight seal (Figure D).
6. FILL THE CONTAINERS
Using gloves, fill the first container halfway with your sludge. Take one of the electrodes and bury it. Remove any air bubbles and continue filling the container. This will be your anode.
Next, fill the second container with distilled water. Add ½ cup of salt and stir. This will be your cathode.
Place your second electrode and seal both containers with their lids. Attach alligator clips to the ends of the protruding copper wires.
Optionally, turn on your aquarium pump to aerate the cathode solution.
Begin to measure voltage output on the multimeter by attaching the respective ends of the alligator clips to the multimeter.
The performance of your biofuel cell can be evaluated by determining its voltage output. Turn on the multimeter and measure the voltage between the anode and cathode. Next, try attaching a resistor and determine the performance of the biofuel cell using its power output, or put an LED light onto the ends of the anode and cathode wire to see if there’s enough energy to illuminate it. Additionally, applying a small voltage to the bacteria produced at the anode can modify the cell. By not using oxygen at the cathode, you should be able to produce pure hydrogen gas. This modified process is known as a microbial electrolysis cell and is based on the idea that fuel cells produce electricity whereas electrolysis produces hydrogen.