Microbial fuel cells (MFC’s) are a different kind of fuel cell that utilizes chemical reactions to generate electrical current with the flow of electrons through a circuit. It is similar to the standard fuel cells that contain the same components such as anode, cathode, and permeable membrane. Since MFC’s are biological fuel cells we can arbitrarily name the anode and cathode the anaerobic (deoxygenated) and aerobic (oxygenated) chamber, respectively. The difference in a biological fuel cell is that a living microorganism is placed in the anaerobic chamber that undergoes anaerobic respiration which requires a void of oxygen. The electrons are oxidized and transported from the anaerobic chamber to aerobic chamber. Protons (H+) are produced and exchanged through the permeable membrane to the aerobic chamber. This creates the electrochemical gradient that that allows for the flow of current through the wires. This biological fuel cell serves is an inexpensive science project that can be made for young scientist.
Step 1: Materials Needed
· Chobani Greek Yogurt Vanilla
· Sediment Sample
· 2 Plastic Tuba-wear 1890 mL Containers
· Alligator Clips
· Carbon cloth
· PVC Pipe
· Potassium Chloride
· 16 Gage Copper Wires
· Wire cutters
· Hot Glue Gun
· 0.1 M Phosphate Buffer pH≈6.2
· Electrical Tape
· DI water or Tap Water
· Potassium Ferricyanide
Step 2: MFC Construction
First cut 6.5 cm of PVC with a hacksaw. Sketch an outline of the PVC diameter on both of the plastic containers then cut out using box cutters. Tightly wrap and seal one end of the PVC with plastic wrap and tape to prevent leakage of the hot agar solution when poured into the PVC pipe. To make a salt bridge an agar solution needs to be made. First add 38.5 grams of potassium chloride to an Erlenmeyer flask containing 80mL of DI water. Then swirl the flask and place in the microwave for approximately one minute. Take it out and swirl to see if all of the potassium chloride has dissolved in the solution (wear insulated gloves). Add small amounts of water (≈5mL) and continue to heat up in small intervals until all the salt has dissolved. Immediately after the salt has dissolved add 5 grams of agar to the solution and heat it up in the microwave for about 30 seconds. Once the agar solution is taken out of the microwave swirl then pour it in the PVC pipe until nearly full of solution. It takes about 30-45 minutes for the solution to solidify in the PVC pipe. Once the agar solution solidifies, it can be fed through the two holes of the plastic containers and sealed securely with a hot glue to form water tight seal.
Aerobic & Anaerobic Chamber
Next add the stream sediment (≈500mL) and Chobani’s yogurt (≈907g) inside the anaerobic chamber and mix evenly. The stream sediment and yogurt act as the organisms that are able to produce electricity. Since this a biological fuel cell it requires fuel for the organism to survive and continuously to produce electrons for the constant production of current, thus 15.5 grams of glucose is used in this experiment. The Phosphate Buffer was made from this website http://cshprotocols.cshlp.org/\. In the aerobic chamber, add 0.1M Phosphate Buffer pH≈6.2 (1500mL) and add ≈3.75mL of potassium ferricyanide to the phosphate buffer.
Electrode & Voltmeter
Next the make electrode by using 16 gauge copper wires by stripping the ends of each and threading the individual strands through carbon cloth . Once the electrodes are made submerge them in both containers making sure they soak. The electrodes promote the transfer of electrons from the anaerobic chamber to the aerobic chamber to produce electrical current. Then the other end of the stripped copper wire can be attached to the voltmeter. The black alligator clip is attached to the anaerobic chamber electrode and the red to the aerobic chamber electrode which are attached to the voltmeter.
Step 3: Results
The initial voltage readings the first 18 hours of running the MFC were between to 0.118 to 0.196. The high voltage 0.196 V was reached within 5 hours of starting the MFC then it decreased and stabilized over the next 13 hours from 0.163V to 0.185V. During the next 15 hour run the MFC voltage dramatically increased ranging between 0.466V to 0.505V. Right before voltages were taken for the second run ≈15.5 grams of glucose was added to the anaerobic chamber and stirred evenly throughout the sediment and yogurt.
The addition of glucose definitely helped increase the voltage output after the first voltage data collection. During the first 18 hours, the voltage steadily increased for the first five hours to the max voltage 0.196 then dropped and stabilized around 0.163V to 0.185V. The addition of 15.5 grams of glucose helps facilitate the production of electrons from the bacteria so it could then be transferred from the anaerobic (anode) chamber to the aerobic chamber. In terms of concentration only ≈1.1% of the sample mixture contained glucose which is considerably small in comparison to the total size the entire sample mixture. It is possible that if more glucose was added that the MFC would yield higher voltages.
Some problems encountered during MFC production included movement of the salt bridge when the samples were placed in the container. A piece of rope and electrical tape were placed on each end of the salt bridge to help keep it from sliding out of the PVC pipe. There was also a small amount of diffusion of the anaerobic chamber to the aerobic chamber through the salt bridge. This could have yielded inaccurate results as it caused a small mixing of the two chambers.
Step 4: Conclusion
The MFC construction was a unique project in that we had the free will to build it in any way we thought was best via research conducted before hand. We how microbes are essential for life and we now are utilizing them for more technological advances. Obviously the microbial fuel I produced was on a small scale in terms of size and voltage production but with further research I believe that this technology can be highly efficient in the future and help solve some of the world’s energy crisis.