Have you ever thought about microbes generating electricity? Well with the proper organic compounds a
microbe can convert chemical energy into electrical energy. This is referred to as a microbial fuel cell (MFC) which breaks down organic substances to produce electrical energy without emitting carbon dioxide into the environment. Since organic waste can be converted into energy, human waste can be used as a potential energy source producing approximately 34 billion kWh of energy each year. Microbial fuel cells can be fueled by organic compounds in the sediment as well allowing it to create electricity constantly without recharging. MFC is a great alternative fuel because it generates sustainable energy and decreases production costs
Step 1: Components of MFC
A MFC consists of anodic and cathodic chambers each with an electrode inside. The chambers can either be separated by a proton permeable membrane or have a salt bridge for the transportation of ions from the anode to the cathode chamber. Microbe substrates are oxidized anaerobically in the anode chamber creating both electrons and protons. The electrons flow from the anode to the cathode via electrical circuit to generate electricity. Some bacteria may require a mediator to be present to transport electrons from microbial cell to anode because the microbe is incapable of releasing electrons directly to anode. Electrons and protons bind in the cathode to lessen the terminal electron acceptor by separating the microbe from it physically. The protons bind to the oxygen to produce water. The terminal electron acceptor used must be rid of in order to steal electrons for electrical purposes. The power production depends on the concentration of the terminal electron acceptor.
Step 2: Materials
o 2 Styrofoam cups- anodic and cathodic chamber
o 2 straws filled with agar- salt bridges
o 2 graphite rods- electrodes
o 10 grams of yogurt- microbe
o 450 mL of dirty pond water- fuel source
o 450 mL of 0.1 M sodium phosphate buffer
(pH 5.8)- maintain pH of microbial fuel cell
o Methylene blue chloride- mediator
o Potassium ferricyanide- terminal
o Erlenmeyer flask
o Saran wrap
o pH strips
o Deionized (DI) water
o Agarose powder
o Red and black wire
o Voltage meter
Step 3: Method
Take a straw and twist it into both cups (one at a time) to make the holes where the salt bridges will go (2 holes per cup). Cut the straw in half equaling both salt bridges.
Mix the ingredients below in a flask and boil in microwave until completely dissolved to make the agar solution to fill the straws.
o 250 mL deionized water
o 2 grams agarose powder
o 15 grams of salt
Saran wrap and tape one end on both straws so that when pouring agar into the straws it will not come out other end. Tape the straws to the inside of a beaker so they are standing straight up. Use pipette to pour agar into straws and allow solidifying. Takes approximately 20 minutes for agar to become solid.
Put agar filled straws into the holes on the cups and use silicon to seal the straws in place. Allow 10 minutes for silicon to completely dry and seal.
When determining what pH to make the sodium phosphate buffer the pH of the yogurt was taken into account.
o Mix a sample of yogurt with DI water and insert pH strip to obtain the pH which was 5.8.
o Mix 7.9 mL Na2HPO4 + 92.1 mL NaH2PO4 + 900 mL of DI water to make 1 L of sodium phosphate buffer with the pH of 5.8.
Mix 10 grams of yogurt and 450 mL of dirty pond water in the left (anodic) chamber. Pour 450 mL of sodium phosphate buffer into the right (cathodic) chamber.
Attach the red wire to one of the electrodes and insert the electrode into the cathodic chamber. Do the same with
the black wire and the other electrode except insert it into the anodic chamber. Put MFC in tray in case of leakage. Plug both wires into the voltage meter and start collecting data (24 hour run).
Add 5 drops of potassium ferricyanide to the cathodic chamber after the initial run of 24 hours to observe if voltage will increase.
Add 3 drops of methylene blue chloride to the anodic chamber after the second 24 hour run to observe if the voltage will increase even more.
Step 4: Data
The microbial fuel cell was set up to run three test trials each time collecting voltages with different ingredients
incorporated. The voltages were recorded from the voltage meter for the beginning, middle, and end hours of the run. The first run was with the yogurt and pond water mixture in the anodic chamber and the phosphate buffer in the cathodic chamber for which voltages were collected for 24 hours. The highest voltage of 0.061 V was obtained after 8 hours of run with the lowest voltage of 0.021 V after 22 hours and 25 minutes.
The second run with potassium ferricyanide incorporated into the MFC cathode chamber ran for a 24 hour period as well. The addition of potassium ferricyanide resulted in the voltages steadily increasing over time and start to decrease within last hour of run. The highest voltage obtained was 0.143 V after 23 hours and 45 minutes, more than doubling the highest voltage from the initial run. 0.037 V was recorded after 5 minutes of second run being the lowest voltage for that run.
The final run incorporated methylene blue chloride into the anode chamber, noting
that potassium ferricyanide was not taken away. This run was recorded over a 16 hour time period instead of 24 hours due to sharing voltage meters with other microbiologists. This run showed an instant increase in voltage after 10 minutes with a voltage of 0.173 V and stayed constant throughout the 16 hours ending with the highest
voltage of 0.179 V.
Granted if allowed to run for 24 hours like the other two trials the voltage possibly would have started to decrease towards the end hours.
Step 5: Conclusion
Microbial fuel cells are great alternative fuel sources to ensure a sustainable future for the world because they are ecofriendly and need little to no maintenance, reducing production costs. They emit absolutely no carbon dioxide into the environment resulting in less pollution. MFCs are not just limited to generating electricity but can also produce hydrogen as a renewable source, treat wastewater by turning it into reclaimed water, and acting as a BOD sensor to measure organic pollution. While the microbial fuel cell is treating wastewater it can also generate electricity doing more than one job at once. The efficiency of MFCs can be improved by adjusting numerous components utilized. The type of microbe used matters because they have different respiration requirements, the fuel source needs to match the nutritional requirement of the microbe, and how the MFC is arranged are just a few things that determines how well the MFC will generate power. It is just amazing how microbial fuel cells can use toxic matters to produce power while breaking the matter down into nontoxic substances. Once they world comes to term with using microbial fuel cells as an widely used fuel source the better the environment and public health will be.