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Turn Mud into Energy With a Microbial Fuel Cell

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Difficulty
Time Required Very Long (1+ months)
Prerequisites Previous experience using a multimeter and being familiar with the physics of electricity is helpful, but not required.
Material Availability The Microbial Fuel Cell Kit needs to be special-ordered from our partner Home Science Tools.
Cost Average ($50 - $100)
Safety Be sure to wear the gloves supplied with the kit when handling the microbial fuel cell's electrodes (its cathode and anode). The electrodes are made of a conductive material called graphite fiber and should not be placed near electronics or power plugs, or have their fibers dispersed in the air.

Abstract

What can you do with a bucket of soil? You could use it to grow some beautiful plants and vegetables—or you could use it to produce electricity! Surprised about that? You actually can power electric devices with just mud! Are you curious about how this works? You need some little helpers in the soil—bacteria—that are able to turn their food sources within the soil into electricity in a device called a microbial fuel cell. But is this possible with any soil and does the soil type matter at all? Find out in this science project!

Objective

Monitor and compare the power output of two microbial fuel cells using two different soils.

Credits

Svenja Lohner, PhD, Science Buddies
Teisha Rowland, PhD, Science Buddies

Cite This Page

MLA Style

Lohner, Svenja, and Teisha Rowland. "Turn Mud into Energy With a Microbial Fuel Cell" Science Buddies. Science Buddies, 16 Oct. 2017. Web. 22 Oct. 2017 <https://www.sciencebuddies.org/science-fair-projects/project-ideas/Energy_p042/energy-power/microbial-fuel-cell-mud?class=9WHmVWEvKjQzKP6vV-TD1hOTxsVhbDihWB76KdUOlW4lc4SIpkZMiYEX95q4A95mmNPXml9NqEY>

APA Style

Lohner, S., & Rowland, T. (2017, October 16). Turn Mud into Energy With a Microbial Fuel Cell. Retrieved October 22, 2017 from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Energy_p042/energy-power/microbial-fuel-cell-mud?class=9WHmVWEvKjQzKP6vV-TD1hOTxsVhbDihWB76KdUOlW4lc4SIpkZMiYEX95q4A95mmNPXml9NqEY

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Last edit date: 2017-10-16

Introduction

To turn soil into a battery, the most important thing you need is lots of little helpers—so-called electrogenic bacteria—that help generate electricity. These bacteria include the Shewanella species, which can be found in almost any soil on Earth and are shown in Figure 1, and the Geobacter species, which prefer living in soil deep underground or even under the ocean, where no oxygen is present.


Figure 1. This is a high-magnification image of Shewanella bacteria, specifically the species S. Oneidensis. The bacteria are the cylindrical-shaped rods scattered in this image. (The other parts of the image are ice pieces that the bacteria were submerged in to take this picture.) (Image credit: PLoS Biology)

What do the bacteria actually do to create power? The soil bacteria eat what is in the soil, such as microscopic nutrients and sugars, and in turn, produce electrons that are released back into the soil. Electrons are subatomic particles that have a negative charge. These electrons can be harnessed and used to create electricity, which is a form of energy. How does the harnessing work? This is done in a device called a microbial fuel cell (MFC). An MFC consists of two electrodes and an area that separates the electrodes (called a membrane). What makes an MFC work is the fact that electricity, in the form of electrons generated by the bacteria, flows into one electrode and leaves the other electrode which creates an electric circuit.

Specifically, in a soil MFC using electrogenic soil bacteria, one electrode (called the anode) is buried down in damp soil. Down there, the bacteria multiply and cover the electrode (creating a biofilm on it), supplying it with a lot of electrons from breaking down organic or inorganic substrates in the soil. At the same time, the other electrode (called the cathode) is placed on the top of the soil, leaving one of its sides completely exposed to the air. Electrons from the anode travel up a wire to the cathode; once there, they react with oxygen (from the air) and protons from the anode (made by the bacteria as it digests nutrients in the soil) to create water. See Figure 2 for a visualization of this process. The more electron-producing, soil-munching bacteria are in the soil, the more electricity the MFC produces.

Diagram of the chemical reactions taking place inside a microbial fuel cell.
Figure 2. This diagram shows the reactions taking place in a microbial fuel cell (or MFC) that make it generate electricity. (Wikimedia Commons, 2010, MFCGuy2010)

To evaluate the overall performance of an MFC, usually their power output is determined. This is done by measuring the voltage across a fixed resistor that you attach to the MFC and from that, power is calculated using Ohm's law as in Equation 1.

Equation 1: where
  • P is the power in watts (W),
  • V is the voltage (V), and
  • R is the resistance in ohms (Ω).

If you use several different resistors, you can generate a power-resistance curve, which allows you to determine the maximum power output of your microbial fuel cell, as explained in more detail in the Procedure. The resistance at which your microbial fuel cell has its maximum power output tells you what the internal resistance of your MFC is. The internal resistance—which includes the resistance of the electrodes, the membrane, and the MFC electrolyte—indicates how much energy is lost during electricity production. It depends on many factors, such as the microbial fuel cell design, electrode spacing, the electrode size and material, or the conductivity of the electrolyte (in this case, your soil). As the bacteria will start generating power from the food present in the soil, the power output will start increasing over time and eventually reach a steady-state.

One big advantage of soil microbial fuel cells is that almost every soil is packed with bacteria that can generate electricity when placed in a microbial fuel cell (MFC). Because such bacteria-laden soil is found almost everywhere on Earth, microbial fuel cells can make clean, renewable electricity nearly anyplace around the globe. As natural resources are being depleted, scientists' attention has shifted to pursuing alternative energy sources, such as MFCs, even more than before. In this science project you will investigate how two different soils perform in a microbial fuel cell. Do you think the soil from your backyard will be able to produce power? And how does this soil compare to another soil, such as soil from a forest or topsoil from a nursery?

Terms and Concepts

  • Electrogenic bacteria
  • Shewenella
  • Geobacter
  • Electron
  • Electricity
  • Microbial fuel cell (MFC)
  • Electrode
  • Anode
  • Biofilm
  • Cathode
  • Cathode
  • Power output
  • Resistor
  • Ohm's law
  • Watts (W)
  • Voltage (V)
  • Ohm (Ω)
  • Internal resistance
  • Alternative energy source

Questions

  • What role do bacteria play in a microbial fuel cell?
  • What does a soil microbial fuel cell setup look like?
  • How can you assess the performance of a microbial fuel cell?
  • Where could microbial fuel cells be useful for electricity generation?
  • How can you improve the power output of a microbial fuel cell?

Bibliography

These resources will give you more information about microbial fuel cells and their applications:

For more information about electronics terms and using a voltmeter/multimeter, use this tutorial:

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Materials and Equipment Product Kit Available

These specialty items can be purchased in a kit from our partner Home Science Tools.

  • Microbial Fuel Cell Kit (1). Includes:
    • Microbial Fuel Cell vessels (2)
    • Anodes (2)
    • Cathodes (2)
    • Hacker boards (2)
    • Capacitors (2)
    • LEDs (2)
    • Digital clock/thermometer to be powered by the microbial fuel cell (1)
    • Set of 7 resistors
    • Alligator clips and jumper wires (2)
    • Digital multimeter (1)
    • Nitrile gloves (1 pair)

You will also need to gather these items (not included in the kit):

  • Optional: Old newspapers to protect your work area
  • Two different soils (about 2 cups each)
    • Soil from just about anywhere works—from a backyard, park, open space, or even a riverbed. Just make sure the soil has not been treated with pesticides and that you have permission to take some of it.
    • Soil can also be purchased from a plant nursery, a hardware store, or online. When you buy your soil, choose organic topsoil that has not been sterilized or treated with pesticides. Do not use soils with little white foam balls, vermiculite pieces, or perlite, that are used to aerate the soil and avoid soils with peat moss.
  • A sieve, plastic strainer or colander to remove large particles from the soil
  • Large mixing bowl that can hold all the soil for one MFC (2)
  • Measuring cups or 100 mL graduated cylinder. A 100 mL graduated cylinder is available from Amazon.com
  • Distilled water (at least 250 mL); this can be purchased at most grocery stores.
  • Paper towel or rag
  • Masking tape
  • Permanent marker
  • Stopwatch
  • Lab notebook
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Experimental Procedure

Microbial Fuel Cells in Science Fairs

Working with microbial fuel cells involves growing soil bacteria. Because of this, many science fairs, including those associated with the International Science and Engineering Fair (ISEF) have requirements which need to be met before you start your project. We recommend you:

  • Check with your teacher or science fair coordinator about any requirements
  • Read the Science Buddies Microorganisms Safety Guide to learn how to safely handle bacteria
  • Fill out a Risk Assessment Form (required by ISEF affiliated science fairs for all MFC projects)

Setting up the Microbial Fuel Cells

You will assemble two microbial fuel cells that each contain a different type of soil. Throughout the experiment you will monitor their power outputs and then compare which soil produces the most electricity. Which soil do you think will work best?

  1. First, watch the video or follow the step-by-step instructions to see how to assemble the microbial fuel cells.
  1. Prepare your two different soil muds. Start preparing your first soil for the first microbial fuel cell.
    1. Place a plastic strainer or colander over a large mixing bowl.
    2. Measure a total of about 2 cups (about 500 milliliters [mL]) of your soil into the strainer. Gently shake the strainer over the bowl so that the soil is strained and any small, hard particles (such as rocks, pebbles, twigs, etcetera) are removed from the soil. You will likely need to be patient; it may take several minutes to strain the soil. When you are done, you should have about 200 g of fine, sifted soil in the bowl.
      1. It is important to remove these particles from the soil because they can aerate the soil and inhibit the desired bacteria from growing (the bacteria do not want to be exposed to oxygen).
    3. Add distilled water and mix it in until your topsoil mud feels like cookie dough. Add more water if the mud is too crumbly, or add more topsoil if the mixture feels too wet.
    4. When you have prepared your soil mud, set it aside and wash your hands.
  2. Carefully take the MFC pieces out of the box and lay them out. Identify the different components and use masking tape and a permanent marker to label both devices with one soil type each (for example "forest soil" or "garden soil").
  3. Put on the gloves that came with the MFCs and start assembling the first microbial fuel cell.
  4. Take out the green and orange wires that came with the MFC electrodes. Bend each wire where the plastic part ends so that each wire is now at a 90° angle (shaped like a capital "L"), as shown in the video above.
  5. Remove the MFC anode from its bag. (The anode is the thinner, black, felt-like circle.)
    1. Safety Note: The MFC's cathode and anode (its electrodes) are made of a conductive material called graphite fiber. Do not put the cathode or anode near electronics or power plugs, and do not disperse the fibers in the air, as the fibers will cause electrical shortages when in contact with electronics.
  6. Straighten the metal part of the green wire and carefully insert it into the anode, as shown in Figure 3. Make sure the wire goes straight and does not poke out on the top or bottom sides of the anode.
Inserting the green wire into the microbial fuel cell's anode.
Figure 3. Insert the metal part of the green wire into the anode circle.
  1. Repeat steps 6 and 7 using the cathode (the thicker, black, felt-like circle) and the orange wire (which is shorter than the green wire).
  2. Take the soil mud of your first soil type that you prepared in step 2 and use it to fill the first vessel up to the line next to the "1" on the plastic vessel (marking 1 centimeter [cm]). Once filled, pat the mud so that its surface is smooth.
    1. Tip: You may want to cover the surface you are working on with old newspapers to prevent mud from getting on it.
    2. When you are finished, rinse the mud off your gloves and dry them (but do not take them off yet).
  3. Put the anode on top of the mud in the vessel, as shown in Figure 4.
    1. The green wire from the anode should be sticking up. The green wire should not be stuck down in the mud.
    2. Gently press the anode flat against the mud so that no air bubbles are under the anode. Note: Removing the air bubbles is important, as the trapped oxygen can prevent the formation of an anaerobic bacterial biofilm and reduce the power output of your microbial fuel cell.
An anode added to a microbial fuel cell.
Figure 4. Place the anode on top of the 1 cm of mud.
  1. Use more soil mud to fill the vessel up to the line next to the "5" mark (marking 5 cm). Once filled, again pat the mud so that its surface is smooth.
    1. Run the green wire along the side of the vessel.
    2. Rinse the mud off your gloves and dry them.
  2. Gently place the cathode on top of the mud and press it as flat as you can, as shown in Figure 5.
    1. The orange wire from the cathode should be sticking out of the top side.
    2. Do not let any mud or liquid cover the top of the cathode.
    3. It is best to arrange the cathode so that its orange wire is about 1–2 cm to the left of the green wire.
    4. Let the mud rest in the vessel for a few minutes. Then carefully pour off any excess liquid.
Microbial fuel cell full of mud with cathode and anode added.
Figure 5. Add the cathode on top of the 5 cm of mud.
  1. Use a clean paper towel or rag to wipe any mud off the vessel's rim. Then take off your gloves.
  2. Take the white plastic lid and pass the wires through the small holes in the lid. Arrange the wires so that the orange wire is on the left and the green wire is on the right when the semicircular indentation on the lid is facing the front. Then carefully snap the lid onto the plastic vessel.
  3. Take out the hacker board (the small green circuit). Attach it into the lid's rectangular indentation.
  4. Locate the "+" and "-" ports (holes) on the hacker board. Plug the cathode's wire (orange) into the "+" port and the anode's wire (green) into the "-" port.
  5. Locate ports 1 and 2 on the hacker board. Plug the blue capacitor (the small, cylindrical item with two longer metal prongs) into these ports. The blue capacitor's longer prong should go into port 1 and the shorter prong into port 2. Note: The orientation of the capacitor is important. If you reverse the short and long prongs, it will not work and might even damage the capacitor. You may need to bend the capacitor's longer end slightly so that the capacitor's prongs fit into the ports well.
  6. Plug the red LED below the capacitor into ports 5 and 6 (ports 3 and 4 will remain empty). The LED's longer prong should go into port 5 and the shorter prong into port 6. Note: The LED will only work if inserted in this orientation. If you accidentally reverse the prongs, the LED will not light up. You may need to bend the LED's longer end slightly so that the LED's prongs fit into the ports well.
  7. Make sure that the wires, capacitor, and LED are all securely in place. The assembled hacker board on the top of the MFC should now look like Figure 6.
Close-up of the top of a completely assembled microbial fuel cell.
Figure 6. When you have finished assembling your MFC and its hacker board, the top should look like the one in this image.
  1. After assembling the first MFC, prepare your second soil type and set up the second microbial fuel cell using your second soil type starting at step 2.
  2. Once both microbial fuel cells are assembled, set the MFCs indoors, at normal room temperature (about 19–25° Celsius [C], or 66–77° Fahrenheit [F]), in a place where they will not be disturbed. The MFCs should remain in the same location the entire time after you set them up because if they are moved, this could disrupt the growth of the bacteria. It should take 3–7 days before the red LEDs on the hacker board start blinking, but you will start taking measurements before that, as described in the next section. Note: Temperature variations can cause changes in the power output of the microbial fuel cells due to different bacteria activities.

Measuring Power Output

You will measure the power output for both of your microbial fuel cells every day. Once the power output seems to have stabilized, you will analyze your data and be able to compare the performance of both of your soils.

  1. One day after setting up your microbial fuel cells, check to see if the LEDs are blinking. Most likely, they will not be, but check to make sure. Watch the LEDs for 2 minutes to see if they are blinking.
    1. If one or both of the LEDs are blinking, time how many seconds apart the blinks for each one are.
      1. To do this, start a stopwatch as soon as you see the LED blink and stop the stopwatch when the LED blinks again.
      2. If the LED is blinking faster than once every 5 seconds, do not time the seconds between blinks, but instead time the blinks per second. Time a 10-second interval and count how many times the LED blinks in this period and then divide this by 10 to get blinks per second.
      3. Repeat step 1.a.i. or 1.a.ii. two more times so you have a total of three counts.
      4. Record your results for each MFC in your lab notebook in a data table like Table 2. (If you counted blinks per second, as in step 1.a.ii., change the heading from "Seconds Between Blinks" to "Blinks per Second.") Calculate the average for your three counts and record that, too.

  Seconds Between Blinks
Day Count #1 Count #2 Count #3 Average
1     
2     
3     
Table 2. Check both MFCs each day to see if the LEDs are blinking. If they are, record how many seconds elapse between the blinks (or how many blinks there are per second) for each, resulting in three separate counts. Record the results for each microbial fuel cell in a data table like this one in your lab notebook.

  1. Next, measure the power output for both of the MFCs using the multimeter that comes with the Microbial Fuel Cell kit. You can either watch the video below or follow the step-by-step instructions. If you need help using a multimeter, consult the Science Buddies reference How to Use a Multimeter, as well as the instructions that came with your multimeter.


    1. To measure the MFC's power output, remove the capacitor and LED from the hacker board. Then remove the orange wire from the "+" port and plug it into port 3. This means that the orange wire should be in port 3, the green wire should still be in the "-" port, and all other ports should be empty.
    2. Place a resistor between ports 5 and 6, as shown in the video above. For the resistors the orientation does not matter.
      1. Several resistors come in the MFC kit. Start with the largest-capacity resistor, which will be a 4.7 kΩ resistor. (Ω, the capital Greek letter Omega, is the symbol for ohms, the unit used to measure resistance. 1 kilo-ohm, or 1 kΩ, is 1000 ohms.)
      2. Resistors' values are labeled using color-coded bands. Use Figure 7 to determine the resistance for each resistor.
        1. Tip: If you want, you can confirm the resistance of any resistor using your multimeter by setting it to measure resistance (usually a "Ω" symbol for ohms) and connecting the multimeter's leads on the wire ends of the resistor.
        Resistor chart
        Figure 7. Use this resistor color chart to determine the resistance for each resistor.

      3. Leave the resistor plugged in for 5 minutes.
    3. After the resistor has been plugged in for 5 minutes, use your multimeter to measure the voltage across the resistor.
      1. Make sure the multimeter's black wire is plugged into the "COM" port and its red wire is plugged into the "VΩMA" port on the multimeter.
      2. Set the multimeter to measure DC voltage. This is marked as "V" with a straight line next to it. Specifically turn the dial to "2000 m."
      3. Clip the multimeter's red lead to the resistor's metal wire that is plugged into port 5. Then clip the multimeter's black lead to the resistor's metal wire that is plugged into port 6, as shown in the video. Read the multimeter's screen to see what the voltage is (in millivolts [mV]).
      4. If the voltage seems to be changing a little, such as decreasing slightly over the period of a few seconds, watch the readings on the multimeter for a few seconds more until they stabilize (and stay the same for a few seconds). Use the stabilized value.
        1. If the readings are still changing after several seconds, or if your readings are 0 mV, make sure all of the wires are correctly and securely plugged into the circuit (both the cathode and anode wires, and the resistor's wires), disconnect the multimeter's leads from the resistor, and come back in another 5 minutes. Then repeat step 2.c.
      5. Record your results for both of the microbial fuel cells in your lab notebook in a data table like Table 3.

      Day and Time:
      Resistance (ohms) Voltage (mV) Power (µW)
      4700   
      2200   
      1000   
      470   
      220   
      100   
      47   
      Table 3. In your lab notebook each day, create a data table like this one for each microbial fuel cell to record your voltage measurements. Do not forget to write down the date and the time you started taking measurements on the top line.

    4. Disconnect the multimeter's clips from the resistor. Remove the resistor.
    5. Repeat steps 2.b.–2.d. until you have tested the MFC with all of the resistors in the kit. Start with the resistor with the largest resistance value and end with the resistor with the smallest resistance value.
  2. Once you have finished taking your voltage measurements for the first MFC, plug the wires, the capacitor, and LED back into the hacker board, as described in steps 16–18 in the Setting Up the Microbial Fuel Cells section, and repeat steps 2 and 3 for the second microbial fuel cell.
  3. Calculate the power output (in microwatts, or µW) for each resistor and both microbial fuel cells. You can calculate this by using a derivation of Ohm's law, as described in the Introduction, Equation 1.
    1. Note: It is important to convert the voltage measurements into power output measurements. The power output depends on the resistors you use, so you cannot determine how well the MFC is performing by just looking at the voltage measurements alone; they need to be converted into power for them to be meaningful.
    2. To use Equation 1, you will need to convert your voltage readings from millivolts (mV) to volts (V). To do this, divide the millivolt values by 1000 to give you volts.
      1. For example, if you had a voltage reading of 45 mV, this would equal 0.045 V.
    3. Using Equation 1, your answer will be in watts (W). Convert watts to microwatts by multiplying your answer by 1,000,000.
    4. Once you have calculated it, record the power for each resistor in the data table (such as Table 3) in your lab notebook.
  4. Determine what the peak power of your MFCs is.
    1. In your data tables for both microbial fuel cells, look at the power produced using each resistor. The peak power is the highest power produced by any of the resistors.
    2. If you want to visualize this, you can plot your data for the day on a graph, putting the resistance of the resistors on the x-axis (horizontal axis) and the power on the y-axis (vertical axis) for both MFCs. A sample graph is shown in Figure 8.
      1. You should see a curve, with the peak power at the top of the curve, as shown in the sample graph.
    3. Make a note in your lab notebook of what the peak power for both MFCs is each day, by circling or highlighting this value in your data table. Do these values change over time? Why do you think might this be the case? Remember that the peak power tells you what the internal resistance of your MFC is.
Sample graph showing resistance vs. power using the microbial fuel cell.
Figure 8. This sample graph shows possible power output data using the resistors with the MFC. In this sample, the peak power was found using a 17nbsp;kΩ resistor, and the peak power is about 45.5 µW.
  1. Repeat steps 1–5 each day until it looks like the power output (the peak power) for both of your microbial fuel cells is stabilizing.
    1. Take these measurements around the same time every day. This will limit variables affecting your results (such as changes in temperature).
    2. For step 1, it should take 3–7 days for the LEDs to start blinking. However, even if the LEDs never blink, you may still be able to do this science project; be sure to continue to take the power output measurements every day.
      1. Tip: See the Frequently Asked Questions section for what to do if the LEDs do not blink, or if they were blinking and unexpectedly stopped blinking.
    3. For step 2, you should see the power output slowly increase.
      1. For each day, make a data table, like Table 3, for each MFC in your lab notebook to record your results and use them to determine the peak power.
    4. After about 7–14 days, the power output should stabilize.
      1. It may stabilize anywhere between 10 μW to 200 μW or more. It will depend on the soil you are using and other factors. Wherever it stabilizes, it should make enough power to blink the LED at least once every 30 seconds.
        1. Tip: If the power output seems low, see the Frequently Asked Questions section for suggestions on what to check and try.
      2. When it stabilizes, the peak power should not change by more than about 10% for at least three days in a row.
        1. Do not worry if your peak power changes by a little more than this. If it has been at least 14 days and when you graph the peak power (as described in step 6.d.iii., following) it looks like it is stabilizing (meaning it is not steadily increasing or steadily decreasing from day to day), then it has probably stabilized enough.
        2. Keeping this in mind, if it still does not look like your peak power is stabilizing, see the Frequently Asked Questions section for suggestions on what to check and try.
      3. Tip: Making graphs of your data as you collect it for both MFCs may help you see if the power output is stabilizing. If you do this, put the date on the x-axis and the power output (peak power) for each day on the y-axis. Does the peak power appear to be stabilizing?
      4. The time between LED blinks should also stabilize.

Analyzing Your Results

  1. Make two graphs of your data for each microbial fuel cell, one showing how the power output changed over time and one showing how the frequency of LED blinks changed over time.
    1. For the graph showing power output over time, put the number of days after setting up the MFC on the x-axis and the peak power output (in µW) on the y-axis.
    2. For the graph showing the frequency of LED blinks over time, put the number of days after setting up the MFC on the x-axis and the blinks per second on the y-axis.
      1. If you recorded the time between blinks in your data table, convert this to blinks per second by taking the data for the average seconds between blinks that you collected each day and calculate what 1 divided by this number is. For example, if your LED blinked an average of once every 15 seconds, 1 divided by 15 is 0.067, which is the number of blinks per second it made.
  2. Analyze your graphs.
    1. What happened in both microbial fuel cells to the power output and frequency of the LED blinks over time? How quickly did these measurements stabilize?
    2. When was the power output and blinking frequency the highest for both of the microbial fuel cells? What was the peak power at this time?
    3. Did one MFC outperform the other? Why do you think this is the case? Think about what differentiates the one soil from the other.
    4. Did you see many differences in how the power output changed versus how the frequency of blinks changed? Why do you think this might be?
  3. Look back into each data table for your voltage sweeps to identify the highlighted resistor values that resulted in the peak power output for each of the MFCs. The resistor you use to find the peak power tells you what the internal resistance of the MFC is. If you have to use different resistors over time to find the peak power, this means that the internal resistance of the MFC is shifting. Do you see a shift happening throughout the experiment? Was there a difference between the two different soils? What does this tell you about the internal resistance within both of the MFCs?

Troubleshooting

For troubleshooting tips, please read our FAQ: Turn Mud into Energy With a Microbial Fuel Cell.

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Variations

Note: Carefully clean your microbial fuel cells before you start a new experiment. We recommend using tap water to rinse the electrodes while gently rubbing them (with gloves on) until the dirty water runs clear. Also, completely rinse out the vessel with tap water. See the Frequently Asked Questions for details.
  • In this science project, the soil bacteria are feeding on the nutrients, as well as on organic and inorganic substrates that are naturally present in the soil. This means the richer the soil is in these compounds, the better for the microbial fuel cell. What happens if you add more food to the MFC? Will the power output of the MFC increase? For more details, see the Science Buddies projects How Do Bacteria Produce Power in a Microbial Fuel Cell? and Powered by Pee: Using Urine in a Microbial Fuel Cell.
  • What is the lifespan of both of the MFCs you tested in this science project? If you want to test a long-term science project, you can try this variation out. The MFCs may produce power for years (as long as the MFCs stays sealed and moist), but exactly how long will they keep reliably producing power? Does the power output slowly decrease over time, or does it abruptly stop? Can you see a decline long before the MFCs completely stop? Does the lifespan change between different soil types? Can you do something to prolong their lifespan?
  • What other factors influence the power output of a microbial fuel cell? Investigate other parameters, such as temperature, soil conductivity, or soil moisture. Do you think one of these factors contributed to the results that you saw for your two different soils? Develop a well-reasoned hypothesis for what you think will happen when this factor is changed, and create a way to test it. Which parameter do you think has the biggest effect on power output? If your MFC is generating between 80 µW and 199 µW, it is doing really well, and if it is generating 200 µW or more, it is doing amazingly well! An example for investigating soil conductivity is given in the Science Buddies science project Spice Up the Power of a Microbial Fuel Cell with a Dash of Salt.
  • Knowing that the bacteria are an important factor in a microbial fuel cell, how do you think their numbers change over the course of the experiment? Will their numbers increase as the power output of the MFC increases? Will you see different bacteria coming up over time? For a more microbiological approach to microbial fuel cells, see the Science Buddies science project How Do Bacteria Produce Power in a Microbial Fuel Cell?.
  • You have shown in this science project that your soil MFC can light up a little LED. What about other electrical devices? Do you think your soil can produce enough electricity to power a little digital clock? Investigate what other electronic device(s) could be powered using the amount of voltage and current produced by your MFCs, then test whether it can power the device(s). One digital clock should be provided in your Microbial Fuel Cell kit. Does it work? How much more voltage and current would you need to power other devices? You can even hook multiple MFCs together to make more power! How many MFCs would you need to power larger electronic devices, such as a television or a computer?

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Frequently Asked Questions (FAQ)

If you are having trouble with this project, please read the FAQ below. You may find the answer to your question.
One easy way to figure out how to troubleshoot your experiment is reviewing the flow chart in Figure 9. You will find more detailed information in the FAQ section below.
flowchart setup microbial fuel cell
Figure 9. Flow chart for troubleshooting the microbial fuel cell experiments.
Q: The LED on the microbial fuel cell has never blinked. What should I do?
A: Even if the LED is not blinking, you should still take and analyze your peak power output measurements and you may be able to continue with the experiment, as described in the Measuring Power Output section in the Procedure. If your peak power output stabilizes around at least 4 μW, then this is enough power for you to continue with the experiment. It is possible that the microbial fuel cell is making enough power for you to continue with the experiment even though the LED is not blinking. There are several reasons why the LED might not be blinking:
  • If the hacker board is not set up correctly, the LED will not blink. You should confirm that everything is set up as described in the Setting Up the Microbial Fuel Cells section of the Procedure.
  • The LED might not have enough power to blink. You will need a peak power output of at least 4 μW before the LED starts to blink, and the initial blinking may only be about once every 30 seconds, so you will need to closely watch the LED to see it blink.
  • Depending on the type of topsoil you are using, it may take about 3–7 days for the LED to start blinking, if everything is properly in place. Some types of topsoil might make the microbial fuel cell take longer to blink than other types of topsoil because it may take longer for enough power to be made to light up the LED. Make sure that you are not using any topsoil with little white foam balls, vermiculite pieces, or perlite, since these can aerate the soil and inhibit the desired bacteria from growing (they do not want to be exposed to oxygen). Also avoid types of topsoil with peat moss or topsoil that has become completely dry.
  • The temperature of the room that the microbial fuel cell is in can significantly affect how well the bacteria grow, which affects whether (and how quickly) the LED blinks. The microbial fuel cell should be kept indoors, at normal room temperatures (about 19–25° Celsius [C], or 66–77° Fahrenheit [F]), in the same location the entire time after you set it up. If the room gets too cold, the bacteria may not grow well. If the microbial fuel cell is moved to a different location, this could disrupt the growth of the bacteria.
  • Even if the hacker board is set up correctly, some of the wires might be loose or may not be making good electrical contacts in the hacker board. You can try taking the wires out of the microbial fuel cell and then putting them back into the correct positions, and/or gently jiggling the wires around in the slots in the hacker board. Also make sure that none of the exposed parts of the wires are touching each other.
  • It is possible that the LED has become damaged. You can check to make sure the LED is working by hooking it up to a battery, as shown in Figure 5 of the science project idea: See the Light by Making a Cell Phone Spectrophotometer.
Q: The peak power output of the microbial fuel cell seems low. What might be the problem and what can I do?
A: If your peak power output stabilizes around at least 4 μW, then this is enough power for you to continue with the experiment, as described in the Measuring Power Output section in the Procedure. It may take about 7–14 days for the peak power output to reach at least 4 μW, depending on the topsoil and other conditions used. If your peak power output does not reach at least 4 μW by 7–14 days after setting up the microbial fuel cell, there are several possible reasons for this:
  • If the hacker board is not set up correctly, it will not produce power. You should confirm that everything is set up as described in the Setting Up the Microbial Fuel Cells section of the Procedure.
  • Even if the hacker board is set up correctly, some of the wires might be loose or may not be making good electrical contacts in the hacker board. You can try taking the wires out of the microbial fuel cell and then putting them back into the correct positions. Also make sure that none of the exposed parts of the wires are touching each other.
  • If it looks like the peak power output is still increasing each day, it may just take a few more days for the microbial fuel cell to reach 4 μW.
  • The soil you are using might not work well in the microbial fuel cell. First be sure that you are using topsoil, and not potting soil. Make sure that you are not using any topsoil with little white styrofoam balls, vermiculite pieces, or perlite, since these can aerate the soil and inhibit the desired bacteria from growing (they do not want to be exposed to oxygen). Also avoid types of topsoil with peat moss or topsoil that has become completely dry.
  • If there are air bubbles trapped in the damp topsoil in the microbial fuel cell, this can prevent the bacteria from growing well because they do not want to be exposed to oxygen. When packing the mud in the microbial fuel cell, pat down the mud and electrodes, as described in the Setting Up the Microbial Fuel Cells section of the Procedure, so that you do not have any trapped air bubbles in the mud. Air bubbles may also be caused by small, hard particles (such as rocks, pebbles, vermiculite, twigs, etcetera) being trapped in the mud, so anything like this should be removed from the topsoil before adding it to the microbial fuel cell.
  • Check if your soil in the microbial fuel cell is still moist. The soil needs to be moist for good electron conductivity and for the bacteria to grow. If your soil is dry, carefully open the microbial fuel cell and mix distilled water into the soil until it is moist again. However, make sure not to disturb the anodic biofilm.
  • If the temperature of the room that the microbial fuel cell is in is very cold, the bacteria may not grow well, which would decrease the peak power output. The microbial fuel cell should be kept indoors, at normal room temperatures (about 19–25° C, or 66–77° F), in the same location the entire time after you set it up. Also, if the microbial fuel cell is moved to a different location (particularly if it is at a different temperature), this could disrupt the growth of the bacteria.

Even if the microbial fuel cell is not making more than 4 μW, this may still be an interesting result, especially if you are testing conditions different from the ones described in the main project idea. Keep in mind that a science project does not have to work as anticipated in order to be real, good science. It is important to communicate (on your Science Fair Project Display Boards, report, or however you are presenting your project) your question, your hypothesis, what you anticipated would happen, then what you actually saw (such as your peak power output measurements over time), your questions about what was happening, and your attempts to troubleshoot. You could also communicate any possibilities of what could be happening and how you would test that (if you do not have enough time to test it now).

Q: Is the voltage made by my microbial fuel cell too low?
A: When you take the voltage measurements with the different resistors each day, the voltage measurements need to be converted into power (in microwatts, or μW). This is described in the Procedure in the section Measuring Power Output. The power output depends on the resistors you use, so you cannot determine how well the microbial fuel cell is performing by just looking at the voltage measurements alone; they need to be converted into power for them to be meaningful. Once you have converted the voltage measurements into power outputs and determined your peak power output, you can determine whether the peak power output seems low or not. If your peak power output stabilizes around at least 4 μW, then this is enough power for you to continue with the experiment, as described in the Measuring Power Output section in the Procedure. It may take about 7–14 days for the peak power output to reach at least 4 μW.
Q: The LED on the microbial fuel cell was blinking, but now it has stopped and this is unexpected. Why did this happen?
A: Even if the LED stops blinking, you should still continue to take and analyze your peak power output measurements and you may be able to still do the experiment, as described in the Measuring Power Output section in the Procedure. If the LED was blinking one day but then stopped blinking the next, there are several possible reasons for this:
  • If the hacker board is not set up correctly, the LED will not blink. You should confirm that everything is still set up correctly, as described in the Setting Up the Microbial Fuel Cells section of the Procedure.
  • Even if the hacker board is set up correctly, some of the wires might be loose or may not be making good electrical contacts in the hacker board. You can try taking the wires out of the microbial fuel cell and then putting them back into the correct positions, and/or gently jiggling the wires around in the slots in the hacker board. Also make sure that none of the exposed parts of the wires are touching each other.
  • The LED may no longer have enough power to blink. You will need a peak power output of at least 4 μW, for the LED to blink, and the blinking may be as infrequent as about once every 30 seconds, so you will need to closely watch the LED to see it blink.
  • If the temperature of the room that the microbial fuel cell is in is very cold, the bacteria may not grow well, which would decrease the peak power output and affect whether the LED blinks. The microbial fuel cell should be kept indoors, at normal room temperatures (about 19–25° C, or 66–77° F), in the same location the entire time after you set it up. Also, if the microbial fuel cell is moved to a different location (particularly if it is at a different temperature), this could disrupt the growth of the bacteria.
  • It is possible that the LED has become damaged. You can check to make sure the LED is working by hooking it up to a battery, as shown in Figure 5 of this science project idea: See the Light by Making a Cell Phone Spectrophotometer.
  • Check if your soil in the microbial fuel cell is still moist. The soil needs to be moist for good electron conductivity and for the bacteria to grow. If your soil is dry, carefully open the microbial fuel cell and mix distilled water into the soil until it is moist again. However, make sure not to disturb the anodic biofilm.

Keep in mind that a science project does not have to work as anticipated in order to be real, good science. You should always record your observations, even unexpected ones, and think of ways to explain why they happened.

Q: The peak power output does not seem to be stabilizing. Should I be concerned?
A: About 7–14 days after setting up the microbial fuel cell, the peak power output should stabilize to around at least 4 μW. For the peak power output to be considered "stabilized," it should not change by more than about 10% for at least three days in a row. But even if your peak power output changes by a little more than this, do not worry; if it has been at least 14 days and the peak power looks like it is stabilizing when you graph it each day (meaning it is not steadily increasing or steadily decreasing from day to day), then it has probably stabilized enough. Keeping this in mind, if it still does not look like the peak power output is stabilizing, this could be because some conditions have changed (such as the temperature has changed or the fuel cell has been moved to a different location). It is very important that conditions are kept as consistent as possible for the peak power output to stabilize and to do the rest of the experiment using the fuel cell, as minor changes in conditions can significantly affect the power output.
Q: What is the purpose of using the different resistors?
A: You need to use multiple resistors so that you can determine what the peak power output of your microbial fuel cell is. The power output depends on which resistors you use, and you need to determine which resistor gives you your peak power output, as described in the Measuring Power Output section of the Procedure. The resistor that gives the peak power output is closest to the internal resistance of the microbial fuel cell, and this depends on the type of topsoil you are using and other factors.
Q: During the voltage sweep, the multimeter readings fluctuate a lot and do not seem to stabilize. Should I be concerned?
A: It is important to leave the resistor plugged in for at least 5 minutes before you take the voltage measurements. It is normal to see the voltage readings decrease slightly over the period of a few seconds. In this case, watch the readings on the multimeter for a few seconds more until they stabilize (and stay the same for a few seconds). Use the stabilized value. If the voltage readings fluctuate randomly over a large voltage range, check your wire connections. If your microbial fuel cells already run for a long time (longer than 4 weeks), there might also be the chance that the internal connections inside the hacker board got worn out and the hacker board is damaged. In this case, you need to replace the hacker board or end your experiment.
Q: Is there a way to clean out the microbial fuel cell without damaging it so that I can use it in another experiment?
A: Yes, the microbial fuel cell can be cleaned out without damaging it. To do this, carefully rinse the vessel and electrodes with tap water. The electrodes should be gently rubbed while rinsing them until the water running off of them is not dirty anymore. Also, to make sure you are starting with a clean slate of bacteria for each test, the vessel and electrodes should be quickly rinsed with 70% isopropyl rubbing alcohol, which should kill the bacteria. To do this, you can put the electrodes in the vessel (using it as a cup) and fill it with just enough alcohol to submerge the electrodes. Get some of the rubbing alcohol on the inside sides of the vessel while you do this, too. Then pour off the rubbing alcohol and briefly rinse the electrodes and vessel with tap water (so the rubbing alcohol does not remain on these parts). Make sure to follow all proper safety precautions when using the rubbing alcohol.

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