Microbial fuel cells (MFCs) are sustainable bioenergy machineries that utilize organic materials to produce electricity. MFCs typically consist of two electrodes, an anode and cathode which are divided by a cation membrane. In the anodic chamber the catalyst and fuel source are derived from the same thing, urine. The anodic chamber has the capability to get very basic while the cathode compartment can become slightly acidic so in order to maintain a balanced pH throughout both compartments, a phosphate buffer is required. The metabolic processes the microorganism undergoes yields electrons and protons from the oxidization of a carbon source. Electrons are propagated on the anode and transmitted to an external circuit creating a voltage. Electrons continue to travel onward to the cathode associating with the protons that were exchanged through the permeable membrane. Water is formed as a result of protons binding with the terminal electron acceptor which is typically oxygen in the cathode. Oxygen is unfavorable in the anode compartment because it is a terminal electron acceptor and interferes with the electron flow throughout the external circuit. This compels the anode to acquire an oxygen free atmosphere.
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Step 1: Materials
1. (2) 450 mL Tupperware bowls
2. (2) 5” 16 gauge primary wires
3. filter paper
4. 50 mL urine
5. Sterile urine cup
6. 1 mL potassium ferricyanide (K3[Fe(CN)6])
7. 700 mL of 0.1 M sodium phosphate buffer (pH: 5.8)
- 7.9 mL Na2HPO4
- 92.1 mL Na2H2PO4
- 900 mL of deionized water
8. Alligator wires
9. Voltage meter
10. Wire cutter/stripper
11. Box cutter or sharp knife
Step 2: Constructing MFC
- Preparing phosphate buffer
- To prepare 1 L of 0.1 M sodium phosphate buffer with a pH of 5.8 7.9 mL of Na2HPO4 was mixed with 92.1 mL of Na2H2PO4. In order to obtain the desired amount of buffer of 1 L the two solvents had to be diluted with 900 mL of deionized H2O. Thoroughly shake the solution to incorporate all of the components .
- Assembling the fuel cell
- Use wire cutter/stripper to cut 16 gauge wires into two 5’’ in length.
Once you cut the wires, use the same device to strip approximately ½ inch off of both ends of each wire.
Due to the microbe being a bodily fluid we will need a closed anodic chamber. Take a lid of one Tupperware container and cut a circle in the middle of the lid which allows the electrode to fit through the hole.
In the middle of one side of the lid closest to the raised edge that secures the lid with the bowl carefully cut a rectangular area that allows the salt bridge to comfortably rest in the opening.
Once the lid is cut, measure out 350 mL of phosphate buffer and pour into the anode. Measure another 350 mL of phosphate buffer this time emptying the buffer into the cathode.
Insert one electrode into the chamber containing urine and buffer through the hole in the middle of the lid and clip the end of the exposed wires with the black alligator clips. Carefully clip one end of the remaining electrode with the red alligator clips and submerge the other end and into the cathode chamber. *make sure that the electrodes do not come in contact with the salt bridge.
Form a salt bridge between both halves of the fuel cell using filter paper. Insert filter paper into the hole carved in step 4 for the anodic chamber and place in the other end of the paper into the cathode chamber.
Measure out 1 mL of (K3[Fe(CN)6]) and pour into cathode and mix phosphate buffer and potassium ferricyanide together.
Measure 50 mL of urine from sterile urine cup and dispense into anode. Gently stir components to integrate components. Secure anodic chamber with lid making sure the cut made in step 4 is placed on the side facing the cathode.
Place voltage meter next to fuel cell and clip the remaining end of the red alligator wire to the red wire of the voltage meter. Connect the black alligator wire with the black wire from the voltage meter. *Assure that the black and red wires do not touch.
Once connected, power on voltage meter and record initial value, recording values periodically over the course of 24 hours.
Step 3: Results
Voltage values recorded ranged during the first nine hours ranged between 0.182 V at 0.8 h and 0.112 V at 9.2 h. The initial recording value was -0.231 V [table 1]. After the first nine hours the MFC began to become more constant in the voltages generated which were between 0.100 V and 0.091 V for the remainder of the 24 hours. The lowest voltage recorded was 0.063 at 21.1 h [table 1].
Values produced by the MFC were low when compared to the experiment performed at University of Western England (UWE). This could be a result of the components compromised of the cathode and anode. Urine contains bacteria but depending on the source the sample was collected the types and abundance may vary greatly amongst people. The absence of a specific microbe could have been a determining factor in the lower voltage values. The components of the apparatus were sufficient in transferring protons across the salt bridge and electrons between chambers.
If I construct the experiment in the future I plan to conduct the same protocol for 24 hours and then add an additional microbe and determine if there are any differences in the voltage values.
Step 4: Conclusions
Electricity constructed from a microbial fuel cell can be generated by a variety of microbes and fuel sources. The main objective of this experiment was to determine if urine utilized in an MFC would harvest electricity. Urine proved to be a successful microbe and fuel source producing electricity using an MFC. When constructing urine powered MFCs on a larger scale, urine could yield higher electric voltages which could positively impact the environment and the wastewater treatment industry. MFCs can produce renewable energy resources and reduce the amounts of pollution and CO2 released into the environment. Microbial fuel cells can be applied to various aspects in the environment besides generating electricity when catalyst types are altered which yield differing results. MFCs are capable of treating wastewater and transforming it into water that can be reused for numerous purposes which is an amazing feat. Through conducting this experiment I learned that although urine may be considered waste and useless it can contains some beneficial purposes which encourage a more sustainable environment.