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Turn Mud into Energy with a Microbial Fuel Cell — and a Dash of Salt

Difficulty
Time Required Very Long (1+ months)
Prerequisites Having used a voltmeter/multimeter before is helpful, but not required.
Material Availability The microbial fuel cell kit needs to be special ordered from the Science Buddies Store.
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, power plugs, or have their fibers dispersed in the air. The fibers will cause electrical shortages when in contact with electronics.

Abstract

Generating power from mud sounds like science fiction, but it is actually real science, and a promising source of alternative energy. Topsoil is packed with bacteria that generate electricity when placed in a microbial fuel cell. Because such bacteria-laden soil is found almost everywhere on Earth, microbial fuel cells can make clean, renewable electricity nearly anyplace around the globe. They are an up-and-coming technology that scientists and engineers are working on making even more efficient. In this science project, you will experiment with a real microbial fuel cell, investigating how to make it produce more power by adding a few dashes of salt.

Objective

Investigate how adding salt to a microbial fuel cell changes its power output.

Credits

Teisha Rowland, PhD, Science Buddies

Thanks to Bob Rowland, ColdQuanta Inc., for assistance with testing this project, and to Ben Finio, PhD, and Howard Eglowstein, Science Buddies, and Keegan Cooke, Executive Director at Keego Technologies LLC for their feedback.

This project was adapted from Keego Technologies LLC.

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Cite This Page

MLA Style

Science Buddies Staff. "Turn Mud into Energy with a Microbial Fuel Cell — and a Dash of Salt" Science Buddies. Science Buddies, 24 Oct. 2014. Web. 27 Nov. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p071.shtml?from=Blog>

APA Style

Science Buddies Staff. (2014, October 24). Turn Mud into Energy with a Microbial Fuel Cell — and a Dash of Salt. Retrieved November 27, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p071.shtml?from=Blog

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Last edit date: 2014-10-24

Introduction

In the early 1900s, scientists showed that microbes (microscopic organisms, including bacteria) could make electricity, which is the basis of microbial fuel cell (or MFC) technology. While only a limited number of scientists researched this technology early on, more recently, as natural resources are depleted, scientists' attention has shifted to pursuing alternative energy sources, such as MFCs.

A microbial fuel cell, also known as a biological fuel cell, is a device that can use microbial interactions to generate electricity. It is a renewable, clean source of energy, and thus quite appealing. An MFC has an anode, a cathode, and an area that separates the two (called a membrane). Anodes and cathodes are both electrodes. An electrode is something that conducts electricity, with electricity either flowing into, or out of, it. An anode specifically has electricity flowing into it, whereas a cathode has electricity flowing out of it. So, for an MFC to function, electricity must be made to flow into the anode and then leave from the cathode. How is this accomplished?

To answer this question, we will look at MFCs that use microbes from the soil to generate electricity. When you think of electricity, and how it can be made naturally, you may think of lightning and electric eels, though you probably do not think about microbes! But some soil microbes, specifically soil bacteria, can help generate electricity, too. These bacteria, known as electrogenic bacteria, include the Shewanella species, which can be found in almost any soil on Earth, and the Geobacter species, which prefer living in soil deep underground or even under the ocean, where no oxygen is present. How can these bacteria help make electricity? 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 electric charge. These electrons can be harnessed and used to create electricity, which is a form of energy caused by charged particles (such as electrons) flowing from a power source (such as a battery) to other components in the circuit — for example, a light bulb. If a light bulb is connected to a circuit that is made correctly and has enough electricity flowing through it, the light bulb will light up.

In an MFC using these soil bacteria, the anode is buried in the damp soil. Down there, the bacteria multiply and cover the anode (creating a biofilm on it), supplying it with lots of electrons. At the same time, electrons are taken away from the cathode. How does this happen? While the anode is buried in the soil, the cathode sits on 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 and, once there, they react with oxygen (from the air) and hydrogen (produced by the bacteria as it digests the nutrients in the soil) to create water. (The anode is buried deep enough, where there is no oxygen, so this reaction could not take place right next to the anode.) See Figure 1 below 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 1. This diagram shows the reactions taking place in a microbial fuel cell (or MFC) that make it generate electricity. (Wikimedia Commons, 2010, MFCGuy2010)

While scientists and engineers know how to make MFCs, they are still trying to figure out what conditions MFCs need to work the best. For example, if we add something to the soil, would it make the MFC produce electricity better, increasing the amount of power it makes? In this electricity and electronics science project, you will investigate how adding salt to an MFC changes its electrical power output. Why salt? Salt, or sodium chloride (NaCl), can be dissolved in a liquid-rich solution, such as wet soil, to create an electrolyte. An electrolyte is a liquid-rich medium that has ions, which are atoms or molecules that have an electric charge. (In the salt electrolyte, there is a positively charged sodium ion and a negatively charged chlorine ion.) Consequently, if electrodes are put into an electrolyte, it can conduct electricity. (In terms of electricity, how well something can conduct electricity is related to its resistance; if something has low resistance, then electricity should easily flow through it, and it can be said to conduct electricity well — and vice versa.) If you add salt to the MFC, how will this affect the MFC's power output? You would expect an initial increase in the power output (as the resistance of the MFC decreases and its conductivity increases), but what would happen if you kept adding salt? Would the MFC continue to produce more and more power, or would it crash after a while because, perhaps, the bacteria will have died from the excess of salt? What is the ideal amount of salt to add to improve the MFC's power output?

Terms and Concepts

  • Microbes
  • Microbial fuel cell
  • Electrode
  • Anode
  • Cathode
  • Bacteria
  • Electrons
  • Electricity
  • Power output
  • Electrolyte
  • Ions
  • Electrical resistance
  • Resistor
  • Ohms
  • Voltage
  • Watts

Questions

  • How does an MFC work?
  • What different types of MFCs are there?
  • How do soil bacteria help make electricity in an MFC?
  • How much salt do you think you will need to add to the MFC to maximize its power output? What do you think will happen if you add more salt than this?

Bibliography

These resources will give you more information about microbial fuel cells, electricity, and renewable energy:

This project idea was adapted from Keego Technologies LLC, and additional resources can be found through the company's website:

For more information about electronics terms and using a voltmeter/multimeter, use these primers:

  • Science Buddies Staff. (n.d.). Electronics primer: Introduction. Retrieved December 7, 2012, from Electronics Primer: Introduction
  • Science Buddies Staff. (n.d.). Electronics primer: Using a multimeter. Retrieved December 7, 2012, from Multimeter Tutorial

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Materials and Equipment Product Kit Available

These specialty items can be purchsed from the Science Buddies Store:

  • Microbial Fuel Cell kit (1). Includes:
    • Microbial Fuel Cell vessel
    • Anode
    • Cathode
    • Hacker board with capacitors, an LED, and a digitial clock/thermometer to be powered by the microbial fuel cell
    • Resistors (7)
    • Digital multimeter
    • Nitrile gloves (1 pair)

You will also need to gather these items:

  • Topsoil (about 2 cups)
    • Topsoil from just about anywhere works — from a backyard, park, open space, or even a riverbed. Just make sure the topsoil has not been treated with pesticides and that you have permission to take some of it.
    • Topsoil can also be purchased from a plant nursery or online. Here are two that will work:
      • Scotts® Premium Humus & Manure Topsoil, available at Amazon.com
      • Scotts® Premium Topsoil, available at Amazon.com
    • Do not use any topsoil with little white StyrofoamTM balls, vermiculite pieces, or perlite, since these can aerate the soil and inhibit bacteria from growing that do not want to be exposed to oxygen. Also avoid topsoils with peat moss.
  • Measuring cups or 100 mL graduated cylinder. A 100 mL graduated cylinder is available online at Amazon.com.
  • Large mixing bowl
  • A plastic strainer or colander. This is for removing large particles from the topsoil to prevent them from aerating the soil and inhibiting bacteria from growing.
  • Optional: Old newspapers
  • Paper towel or rag
  • Stopwatch
  • Measuring teaspoon
  • Table salt (sodium chloride, NaCl)
  • Lab notebook

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies does participate in affiliate programs with Amazon.comsciencebuddies, Carolina Biological, and AquaPhoenix Education. Proceeds from the affiliate programs help support Science Buddies, a 501( c ) 3 public charity. If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

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Experimental Procedure

Setting Up the Microbial Fuel Cell

The first thing you need to do is assemble your microbial fuel cell (MFC).

  1. First watch the video below to see how to assemble your MFC.

This video shows how to assemble your microbial fuel cell (or MFC). Read the steps below for how to assemble the hacker board
This video shows how to assemble your microbial fuel cell (or MFC). Read the steps below for how to assemble the hacker board http://www.youtube.com/watch?v=RPnJ0OGjCDM
  1. Prepare your topsoil mud.
    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 topsoil into the strainer. Gently shake the strainer over the bowl so that the topsoil is strained and any small, hard particles (such as rocks, pebbles, twigs, etc.) 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, the soil in the bowl should be very fine.
      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.
      1. If you are using Scotts Premium Topsoil, you will likely need about ½ cup (120 mL) of water.
    4. When you have prepared your topsoil 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.
  3. Put on the gloves that came with the MFC.
  4. Take out the green and orange wires that came with the MFC. 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 Figure 3, below.
Wires needed for a microbial fuel cell.
Figure 3. Bend the orange and green wires so they are each at a 90 ° angle where the plastic and metal parts meet, as shown here.
  1. 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.
  2. Straighten the metal part of the green wire and carefully insert it into the anode, as shown in Figure 4, below. Make sure the wire goes straight and does not poke out on the top or bottom sides of the anode, as shown in Figure 5, below.
Inserting the green wire into the microbial fuel cell's anode.
Figure 4. Insert the metal part of the green wire into the anode circle.


Assembled microbial fuel cell anode.
Figure 5. Completely insert the metal part of the green wire into the anode, making sure the wire does not poke out above or below the anode.
  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). The assembled cathode should look like the one in Figure 6, below.
Assembled microbial fuel cell cathode.
Figure 6. Assemble the cathode with the orange wire just as you assembled the anode with the green wire. This is what a complete cathode should look like.
  1. Take the topsoil mud that you prepared in step 2 and use it to fill the 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, as shown in Figure 7, below.
    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).
A microbial fuel cell vessel filled with 1 cm soil.
Figure 7. Fill the microbial fuel cell vessel with 1 cm soil, up to the "1" mark, and then pat the mud to make the surface smooth.
  1. Put the anode on top of the mud in the vessel, as shown in Figure 8, below.
    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.
An anode added to a microbial fuel cell.
Figure 8. Place the anode on top of the 1 cm of mud.
  1. Use more topsoil mud to fill the vessel up to the line next to the "5" mark (marking 5 cm), as shown in Figure 9, below. 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.
An anode in a microbial fuel cell with soil added to it.
Figure 9. Place soil on top of the anode up to the 5 cm mark.
  1. Gently place the cathode on top of the mud and press it as flat as you can, as shown in Figure 10, below.
    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 10. 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, as shown in Figure 11, below.
Microbial fuel cell with lid attached and wires sticking out.
Figure 11. Snap the lid in place, making sure the orange wire is on the left and the green is on the right when facing the semicircular indentation.
  1. Take out the hacker board (the small green circuit). Attach it into the lid's rectangular indentation.
  2. Locate the "+" and "-" ports (i.e., holes) on the hacker board. Plug the cathode's wire (orange) into the "+" port and the anode's wire (green) into the "-" port, as shown in Figure 12, below.
Microbial fuel cell showing circuit with wires attached.
Figure 12. Insert the orange wire into the "+" port and the green wire into the "-" port.
  1. Locate ports 1 and 2 on the hacker board. Plug the blue capacitor (the small, cylinder-shaped 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, as shown in Figure 13, below.
    1. Note: You may need to bend the capacitor's longer end slightly so that the capacitor's prongs fit into the ports well.
Microbial fuel cell lid with capacitor attached.
Figure 13. Insert the blue capacitor's longer prong into port 1 and its shorter prong into port 2.
  1. 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.
    1. Note: You may need to bend the LED's longer end slightly so that the LED's prongs fit into the ports well.
  2. Make sure that the wires, capacitor, and LED are all securely in place. The top of the MFC should look like Figure 14, below.
Close-up of the top of a completely assembled microbial fuel cell.
Figure 14. When you have finished assembling your MFC and its hacker board, the top should look like the one in this image.
  1. Set the MFC indoors, at normal room temperature (about 19 to 25° Celsius [C], or 66 to 77° Fahrenheit [F]), in a place where it will not be disturbed. The MFC should remain in the same location the entire time after you set it up because if it is moved this could disrupt the growth of the bacteria. It should take between three and ten days before the red LED on the hacker board starts blinking, but you will start taking measurements before that, as described in the next section.

Measuring Power Output and Adding Salt

You will measure the power output of your MFC every day. Once the power output seems to have stabilized, you will add some salt. The salt should increase the power output of the MFC. Once the power output stabilizes again, you will add more salt. You will keep doing this until it appears that the addition of salt no longer increases the power output. How much salt do you think it will take?

  1. One day after setting up your MFC, check to see if the LED is blinking. Most likely, it will not be, but check to make sure. Watch the LED for 2 minutes to see if it is blinking.
    1. If the LED is blinking, time how many seconds apart the blinks 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 made three counts total.
      4. Record your results in your lab notebook in a data table like Table 1 below. (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 1. Each day check the MFC to see if the LED is blinking. If it is, record how many seconds elapse between the blinks (or how many blinks there are per second), making three separate counts. Record the results in a data table like this one in your lab notebook.
  1. Next measure the power output of the MFC using the multimeter that comes with the Microbial Fuel Cell kit. If you need help using a multimeter, consult the Science Buddies' Multimeter Tutorial, 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.
      1. Several resistors come in the MFC kit. Start with the largest-capacity resistor, which will probably be a 4.67 K Ω 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 the pictures in the booklet that comes with the kit 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.
      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 (other multimeters may mark it with "DCV"). 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. 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 in your lab notebook in a data table like Table 2 below.

      Day and Time:
      Resistance (ohms) Voltage (mV) Power (μW)
      4670   
      2190   
      1000   
      470   
      220   
      100   
      47   
      Table 2. In your lab notebook each day, create a data table like this one 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, plug the capacitor and LED back into the hacker board, as described in steps 15–18 in the "Setting Up the Microbial Fuel Cell" section above.
  3. Calculate the power output (in microwatts, or μW) for each resistor. You can calculate this by using a derivation of Ohm's law shown as Equation 1 below.
    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 dividing your answer by 1,000,000.

      Equation 1:

      • P is the power in watts (W).
      • V is the voltage (V).
      • R is the resistance in ohms(Ω).
    4. Once you have calculated it, record the power for each resistor in the data table (such as Table 2 above) in your lab notebook.
  4. Determine what the peak power of your MFC is.
    1. In the second data table in your lab notebook, 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). A sample graph is shown in Figure 15, below.
      1. You should see a curve, with the peak power being 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 is each day, by circling or highlighting this value in your data table.
    4. You can investigate peak power more in the Make It Your Own section. Although it will not be explored in this science project, you might like to know that the peak power tells you what the internal resistance of your MFC is. The resistor that gives the peak power is closest to the internal resistance of the MFC. This may change a little over time.
Sample graph showing resistance vs. power using the microbial fuel cell.
Figure 15. This sample graph shows possible power output data using the resistors with the MFC. In this sample, the peak power was found using a 1 K ohm 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) 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 3–10 days for the LED to start blinking. However, even if the LED never blinks, 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 LED does not blink, or if it was 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 2, above, in your lab notebook to record your results and use them to determine the peak power.
    4. After about 10–12 days, the power output should stabilize.
      1. It may stabilize anywhere between 10 μW to 200 μW or more. A lot depends on the topsoil 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 0.5 μW for at least 3 days in a row.
        1. Do not worry if your peak power changes by a little more than this (such as by 1 μW or 2 μW). If it has been at least 10 days and when you graph the peak power (as described in step 6.d.iii., below) it looks like it is stabilizing (e.g., 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 a graph of your data as you collect it 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.
  2. Once it appears that the power output has stabilized, carefully open up the MFC and mix in ¼ teaspoon (tsp.) of salt throughout the mud.
    1. Take your measurements for the day as usual before adding the salt.
    2. Measure out ¼ tsp. of salt and leave it nearby.
    3. Unplug the anode and cathode from the hacker board and carefully remove the lid.
    4. Put your gloves on and gently lift up the cathode, being careful not to get any mud on top of the cathode.
      1. Safety Note: The MFC's electrodes are made of a conductive material called graphite fiber. Do not put the cathodes or anodes 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.
    5. Sprinkle some of the salt on top of the mud, mix it in, and then dig around inside of the mud, adding salt and mixing it in as you go. You want to mix the salt as evenly into the mud as possible, but do not disturb the anode - leave it in place (as well as its wire and the mud below the anode).
      1. Note: Disrupting the anode could damage the developing biofilm and interfere with your results.
    6. Rinse the mud off your gloves and dry them.
    7. Assemble the MFC exactly as you put it together before, following the instructions from the "Setting Up the Microbial Fuel Cell" section above to make sure that the wires are twisted together properly and everything is reconnected to the hacker board correctly.
      1. Specifically, this will be following steps 11–18 from the previous section.
      2. Do not get any mud on the top of the cathode. If you do, carefully wipe it off, being careful not to grind it into the cathode.
  3. Starting the day after you add the salt, repeat steps 1–5 each day until it looks like the power output (the peak power) is stabilizing again (as described in step 6).
    1. Take these measurements at the same time every day.
    2. On the day after adding the salt, you should see an increase in power output. The power output may then slowly decrease over time until it stabilizes again.
    3. For each day, make a data table like Table 2 above in your lab notebook to record your results and use them to determine the peak power. Note: It is possible that the resistor you use to determine the peak power will change slightly. Make a note of this in your lab notebook if it happens.
    4. About four to five days after adding the salt, the power output should stabilize again.
      1. When the power output is stabilized, the peak power should not change by more than about 0.5 µW for at least three days in a row.
      2. Do not worry if your peak power changes by a little more than this. If it has been about five days after adding the salt and the peak power is not steadily decreasing each day, then it has probably stabilized enough.
      3. Tip: Making a graph of your data as you collect it may help you see if the power output is stabilizing. If you do this, put the days on the x-axis and the power output (peak power) on the y-axis. Does the peak power appear to be stabilizing?
      4. The time between LED blinks should also stabilize.
  4. Repeat steps 7 and 8, adding ¼ tsp. of salt each time the MFC stabilizes, until adding salt no longer results in an increase in power output.
    1. Depending on the type of topsoil you are using and other factors, this may take a total of about ¾ tsp. of salt.
    2. If you have added a total of 1 tsp. of salt and still see an increase in power output after adding the fourth quarter teaspoon of salt, try adding ½ tsp. salt in the future (instead of ¼ tsp. salt).
  5. After you have determined that adding salt no longer increases the MFC's power output, add ¼ tsp. of salt one last time and see if this changes the power output (repeat steps 7 and 8).
    1. Continue monitoring the MFC's power output for at least seven days after adding salt this last time.
    2. Did the power output remain stable, or did it decrease? If it decreased, did it then stabilize after decreasing, or did it continue decreasing?

Analyzing Your Results

  1. Make two graphs of your data, 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. Locate the days that you added salt. What happened to the power output and frequency of the LED blinks the day after adding salt? How quickly did these measurements stabilize? When they stabilized, were they higher or lower than they were originally, before adding salt?
    2. When was the power output and blinking frequency the highest? What was the peak power at this time? How much salt had been added to the MFC by this point?
    3. What happened to the power output and frequency of LED blinks when you added the last bit of salt? Why do you think this is? What do you think would happen if you added even more salt? Why?
    4. Based on your results, what is the ideal amount of salt to add to your topsoil mud for maximum power output? Why do you think so?
    5. 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? Which measurement do you think is more accurate? Why? If you want to explore this relationship further, see the Make It Your Own section.

Troubleshooting

For troubleshooting tips, please read our FAQ: Turn Mud into Energy with a Microbial Fuel Cell — and a Dash of Salt.

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Variations

  • In this science project you measured both LED blinks per second and power output using the MFC, but you did not extensively analyze how those two types of measurements correlate with each other. Go through some of your data and create a graph showing power output (in µW) on the x-axis and blinks per second on the y-axis. How does the power output correlate with the blinks per second? Is there a linear relationship, or something else? Are there some points at which the relationship does not seem to work, such as at very low or very high power outputs? Why do you think this is?
    • If you want a more challenging way to explore this relationship further, try using a power supply with the MFC hacker board and two multimeters. Have someone very experienced with electronics help you hook up the power supply with the hacker board and the multimeters, arranging it so that one multimeter measures the voltage from the power supply and the second multimeter measures the current. Knowing the voltage and current, you can use Equation 2 below to calculate the power going into the hacker board. What is the relationship between the frequency of LED blinks and the power supplied? Is there a minimum amount of voltage needed to make the LED blink? Why do you think this is? How does this data correlate with the data you collected using the MFC to power the hacker board?
      • Safety Note: Adult supervision is recommended with use of the power supply, as it can cause serious harm to the hacker board and user if used incorrectly.
      • Note To prevent damaging the hacker board, it is recommended to test it with no greater than 0.75 V.
      • Tip: The hacker board has a voltage-boosting chip (the tiny black rectangle in between the circular testing pads). This chip "up-converts" the voltage the hacker board receives into short bursts to blink the LED. Additional information on the chip can be found at Digi-Key® Corporation.

Equation 2:

P = I V
  • P is the power in watts (W).
  • V is the voltage (V).
  • I is the current in amperes (amps, or A).
  • The MFC in the Science Buddies kit normally powers a red blinking LED light, but it could power other electronic devices instead, if it produces enough power for them. Investigate what other electronic device(s) could be powered using the amount of voltage and current produced by your MFC, then test whether it can power the device(s). Does it work? How much more voltage and current would you need to power other devices? Knowing that you can 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?
  • In this science project you used different resistors to figure out the peak power made by the MFC, but you were limited by the resistors supplied in the MFC kit. You could repeat this project, using additional resistors to more closely figure out what the peak power is. For example, if you used the 100-ohm, 220-ohm, and 470-ohm resistors from the kit and found the peak power when using the 220-ohm resistor, this tells you that the peak power is somewhere between 100 ohms and 470 ohms, but it may not be exactly at 220 ohms — this is just the closest resistor you had available. You could use additional resistors that fall within this range (100 ohms to 470 ohms) to more closely find the peak power. On the other hand, if you found that the peak power could be produced using resistors greater than the ones supplied in the Science Buddies MFC kit (such as shown in Figure 3 in the Procedure section, in sample graph #2), you can use larger resistors to test this. Is the actual peak power significantly different than what it was using the resistors in the kit? (If you find that the peak is shifting over time, you could investigate the next Variation below.)
    • To use additional resistors, you can either find an electronics specialist who might let you use some of his or her resistors, or you can purchase them online as a kit from SparkFun Electronics® or individually at a company such as Digi-Key. (The type of resistors you want are "Through Hole Resistors," with power of 0.25W and a composition of metal film. Note: If you order resistors from Digi-Key, some may need to be purchased in large quantities — check the minimum quantity needed after selecting the resistor you want to purchase.)
    • Alternatively, you can try to use the resistors in the kit to narrow down the peak power. Connecting the resistors in series (connecting the resistors to each other) will increase the total resistance, but this can be challenging to do successfully. Connecting the resistors in parallel (turning on two resistors) will decrease the resistance. If you want to try this, research more on how to do it.
    • As yet another alternative, you can use a device called a potentiometer to narrow down the peak power (although fixed resistors may work better with the MFC). Look into potentiometers to figure out how to use one this way.
  • 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. If you saw this happening, why do you think this is? Do you see a correlation with anything, such as a shift after adding more salt or a change in the power output? Hint: Re-read the Introduction and research internal resistance and electrolytes to try and figure this out. You may want to use additional resistors to narrow down the internal resistance of the MFC, as described in the previous Variation above.
  • For additional ideas focusing on the biology aspects of the microbial fuel cell, see the Science Buddies science project Powered by Pee: Using Urine in a Microbial Fuel Cell and its Make It Your Own section.

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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.
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 and Adding Salt" 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 Cell" 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 3–10 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. From our experience, using the Scotts Premium Topsoil—which is recommended in the Materials section—results in the microbial fuel cell starting to blink after about 4–5 days. 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.
  • 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 this 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 and Adding Salt" section in the Procedure. It may take about 10–12 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 10–12 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 Cell" 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. From our experience, the Scotts Premium Topsoil—which is recommended in the Materials section—works well under our testing conditions and the microbial fuel cell produces at least 4 µW after about four days or earlier, but other types of topsoil may behave differently. 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 Cell" 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, etc.) being trapped in the mud, and so anything like this should be removed from the topsoil before adding it to the microbial fuel cell.
  • 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 and Adding Salt." 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 and Adding Salt" section in the Procedure. It may take about 10–12 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 and Adding Salt" 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 Cell" 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 4µW, 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.
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 10–12 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 0.5 µW for at least three days in a row. But even if your peak power output changes by a little more than this (such as by 1 µW or even 2 µW), do not worry; if it has been at least 10 days and the peak power looks like it is stabilizing when you graph it each day (i.e., 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 (i.e., 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: After I set up the microbial fuel cell, the peak power output was increasing, and then decreased before I added any salt. Should I be concerned?
A: Within the first 10–12 days of setting up the microbial fuel cell it is normal to see some fluctuations in the peak power output. This may include seeing the peak power output increasing, and then decreasing a little. After about 10–12 days the peak power output should appear to stabilize. If this does not appear to happen, refer to the other relevant questions in this FAQ.
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 and Adding Salt" 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: Why do I need to wait until the peak power output has stabilized before adding the salt?
A: If you do not wait for the peak power output to stabilize before adding salt then you will not know how the addition of salt changed the peak power output because it would have still been fluctuating. You would not be able to know for certain whether any changes in peak power output you see, after adding the salt, would then be due to normal fluctuations of the microbial fuel cell or actually to the addition of the salt.
Q: Why is salt added to the microbial fuel cell?
A: As discussed in the Background, when the salt is dissolved in the damp soil, it creates an electrolyte, which is a liquid-rich medium that has an electric charge. Consequently, if electrodes are put into an electrolyte, it can conduct electricity. This means that initially when salt is added to the microbial fuel cell, there should be an increase in the peak power output, but eventually if too much salt is added it may kill the bacteria and decrease the peak power output.
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.

Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

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    Examples

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    Bad Question I don't understand the instructions. Help!
    Good Question I am purchasing my materials. Can I substitute a 1N34 diode for the 1N25 diode called for in the material list?
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