During our testing, we occasionally saw spikes of up to 6 volts while adjusting the apparatus. We were not able to stabilize the output to the extent that we were able to stabilize it to approximately 220 millivolts. I recommend experimenting with the size of the containers and the position of the air pump and electrodes.
The goal of the project was to construct a working two-chamber microbial fuel cell with a maximum output of 300 millivolts (mV), or 0.3 volts (V). A microbial fuel cell, or MFC, is a fuel cell in which the naturally occurring electrochemical processes of anaerobic bacteria breaking down food, are harnessed to generate electricity.
The chosen source of bacteria and organic substance in the cell was sludge retrieved from the bottom of Bluff Creek behind Playa Vista Park in Los Angeles, California. This was chosen as the ideal resource for the microbial fuel cell because it lay in still water, which provides a good environment for anaerobic bacteria growth.
- 2 1l Plastic Containers
- Cotton rope
- Aluminum mesh
- Paper clips
- Copper wire
- Alligator clips
- Electrical tape
- Glue gun
- Glue sticks
- Aquarium air pump
- Duct tape
- Hand spade
- Small Pot
- Drill gun
Collect sludge from the bottom of a still creek or pond into a bucket. Such a source will most likely have plenty of anaerobic bacteria.
Drill one hole for copper wire on lids of containers. On one of the two lids, drill one hole for the air pump tube and one small hole for ventilation (this will not be sealed). Drill one hole on one side to both containers for salt bridge.
Prepare the electrodes. Fold aluminum mesh a few times over and bind with large paper clips. Strip ends off of copper wire and attach to both electrodes.
Insert copper wire and air pump tube into drilled holes on lids. Seal with hot glue or caulk.
Prepare the salt bridge. Heat water over stovetop and dissolve in as much salt as possible. Twist a long rope around itself to create a thicker rope. If necessary, cut the rope to approx. 15 cm. Soak the rope in the salt water. Once damp, wrap the rope in one layer of electrical tape and one of duct tape, but keep the ends exposed.
Insert each end of the salt bridge into the drilled holes on the sides of the containers. Seal with hot glue or caulk and extra tape (as needed).
Fill one container almost to the rim with sludge and the other with water.
Submerge the electrodes into the sludge and water. Close the lids of the containers, and make sure the one over the sludge is airtight.
Anaerobic bacteria should be exposed to as little oxygen as possible.
Attach alligator clips to each loose end of copper wire. Clamp the alligator clips onto the voltmeter probes. Make sure the red probe is attached electrically to the electrode in the water. The black probe should connect to the electrode submerged in the sludge.
Turn on the aquarium air pump.
Turn the dial on the voltmeter to 20 VDC (can also be marked by the symbol, ) to measure the force of electricity moving through the circuit in terms of volts. Turn the dial to 2000m to measure it in terms of millivolts, a thousandth of a volt, to observe a more accurate reading.
Turn the dial to 10A. This number is the flow of current the microbial fuel cell is generating, measured in amps.
To calculate the rate of transfer energy as one joule per second, also known as one watt, use the formula below.
watts (W) = amps (I) x volts (V)
The results were significantly more successful than anticipated. The microbial fuel cell generated 221 mV at its peak efficiency when measured with a voltmeter and occasionally peaked at 6 V when we messed with the positioning of the electrodes and water pump tube. Something especially interesting occurred when the air pump was unplugged, which stopped extra oxygen from flowing throughout the container. It was observed that the current generated by the cell rapidly dropped down to a low of 168 mV. The lack of oxygen slowed down the reduction reaction and consequently slowed down the current, as well.
The salt bridge allows for current to flow of hydrogen ions in a circuit. If it did not exist, there would be a pileup of electrons, and the current would stop, rendering the cell useless. The anode of the microbial fuel cell depends on a potential that causes the current to flow, and therefore, the oxidation reaction in its chamber is also dependent on a potential. In this case, the potential element is the cathode in the chamber filled with oxygenated water, because it helps to complete a reduction reaction in that chamber. Oxidation and reduction reactions always have to occur together (called a redox reaction), and in a microbial fuel cell, they are absolutely necessary. Electrons emitted by an oxidation reaction must be accepted by atoms or ions of another substance.
During the first (oxidation) reaction, which takes place in the sludge-filled container on the anode, the bacteria consume glucose for energy and water. They then yield carbon dioxide, positive hydrogen ions, and electrons. The positive hydrogen ions and the electrons are attracted to compounds in the second container and will take part in a reduction reaction. The electrons travel up through the aluminum mesh and copper wire to the cathode in the second container. The positive hydrogen ions make their way across the salt bridge to the second container. This is where the second part of the redox reaction takes place.
During the reduction reaction, the positive hydrogen ions combine with the electrons leftover from the oxidation reaction and oxygen from within the water to yield water. The redox reaction ends here. Therefore, the lifetime of the microbial fuel cell is limited by the lifetime of the bacteria within the sample of sludge.
The salt bridge can also be called a proton exchange membrane, or PEM. It allows for hydrogen protons yielded by the bacteria to pass through.
The aquarium pump provides extra oxygen to the reaction yielding water.
Because the bacteria are not exposed to oxygen, they produce carbon dioxide, protons, and electrons instead of carbon dioxide and water.
Any organic waste material should be compatible with the microbial fuel cell. As long as there is a good enough balance of bacteria and organic material, it should work. This type of cell is a relatively new invention. The very first idea of it was formed in 1911, and the first design was conceived in 1977. There are still ways to go in terms of further developing the technology, and people involved in related projects are still unsure whether it would ever catch on, given its current state of relative inefficiency. However, like any new development, there is only a matter of time before someone is able to improve on its design. Then, it stands as a viable option for energy generation in waste treatment plants.
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