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

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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Last edit date: 2013-03-22

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.

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:

• Logan, B. E. (n.d.). Research - bioenergy: Microbial fuel cells. Department of Civil and Environmental Engineering, The Pennsylvania State University. Retrieved December 6, 2012, from http://www.engr.psu.edu/ce/enve/logan/bioenergy/research_mfc.htm
• The Physics Classroom. (n.d.). Current electricity - chapter outline. Retrieved December 7, 2012, from http://www.physicsclassroom.com/class/circuits/
• Pacific Gas and Electric Company (PG&E). (n.d.). About solar and renewables. Retrieved December 6, 2012, from http://www.pge.com/myhome/saveenergymoney/solarenergy/about/

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

• Keego Technologies Staff. (2012, April 26). New tutorial video. Retrieved December 6, 2012, from http://news.keegotech.com/2012/04/26/new-tutorial-video/

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 http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-intro.shtml
• Science Buddies Staff. (n.d.). Electronics primer: Using a multimeter. Retrieved December 7, 2012, from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-multimeter.shtml

Materials and Equipment

• 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 Styrofoam^TM 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
• Microbial fuel cell (MFC) kit available at the Science Buddies Store.
• Optional: Old newspapers
• Paper towel or rag
• Stopwatch
• Voltmeter/Multimeter. Must be able to measure 10 mV; for example, this one at Amazon.com.
• Measuring teaspoon
• Table salt (sodium chloride, NaCl)
• Lab notebook

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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.

2. Prepare your topsoil mud.
   1. Pour about 2 cups (about 500 milliliters [mL]) of your topsoil into a large mixing bowl.
   2. Add water 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.
   3. When you have prepared your topsoil mud, set it aside and wash your hands.
3. Carefully take the MFC pieces out of the box and lay them out. Identify the different components.
4. Put on the gloves that came with the MFC.
5. Remove the MFC cathode from its bag. (The cathode is the black, felt-like circle with the red wire sticking out.)
   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. Take the cathode's red wire and put it through the hole in the "donut disk" (the transparent disk).
   1. Arrange the donut disk so that its tab is facing up and away from the cathode.
7. Then put the cathode's red wire through one of the holes in the "dome lid" (the white, dome-shaped plastic part with two holes in it).
   1. Gently pull the red wire so that its rubber bumper is up against the hole in the dome lid. Do not worry about it staying in place or completely sealing the hole yet.
8. Remove the MFC anode from its bag. (The anode is the black, felt-like circle with the green wire sticking out.)
9. Repeat steps 6 and 7 using the anode's wire instead of the cathode's wire.
   1. In step 7 for the anode, put the anode's wire through the other hole in the dome lid.
10. Once the wires are in place, with each one sticking out of the dome lid, twist the wires together between the dome lid and the donut disk one full turn.
    1. This can be a bit tricky, but just take your time turning the wires together.
    2. Doing this should help keep the wires in place, but do not worry if they get moved around in the next steps. You will adjust everything a little in step 17.
    3. Set this anode-cathode assembly aside for now.
11. Put the ruler sticker on the MFC vessel (the plastic jar). Do this by lining up the top of the sticker with the rounded top edge of the vessel.
12. 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 ruler (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.
13. Take the anode-cathode assembly you set aside in step 10 and put the anode on top of the mud in the vessel.
    1. The green wire from the anode should be sticking out of the top side of the anode. 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.
14. Use more topsoil mud to fill the vessel up to the line next to the "4" on the ruler sticker (marking 4 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.
15. Gently press the cathode flat against the mud.
    1. The red 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. Let the mud rest in the vessel for a few minutes. Then carefully pour off any excess liquid.
16. Use a clean paper towel or rag to wipe any mud off the vessel's rim.
17. Set the donut disk on top of the vessel and place the dome lid on top of that.
    1. Adjust the wires if needed so that they are loosely twisted together between the donut disk and dome lid.
    2. Gently pull on the ends of the wires sticking out of the dome lid so that their rubber bumpers are up against their holes in the dome lid.
18. Put the "sealing ring" (the white plastic ring) around the dome lid and screw the ring in place.
19. Take out the hacker board (the small green circuit). Locate the "+" and "-" ports on the top.
20. Attach the hacker board to the top of the dome lid using the piece of Velcro® included in the kit. Attach the hacker board between the wires, with the "+" near the cathode (red) wire and the "-" near the anode (green) wire.
21. On the hacker board, plug the cathode (red) wire into the "+" port and plug the anode (green) wire into the "-" port.
22. Locate ports 1 and 2 on the hacker board. Place the jumper (the small, black, rectangular-shaped item with two small metal prongs) so that its prongs plug into these ports.
23. Plug the capacitor (the small, cylinder-shaped item with two longer metal prongs) into ports 3 and 4, just below the jumper. The capacitor's longer prong should go into port 3 and the shorter prong into port 4.
    1. Note: You may need to bend the capacitor's longer end slightly so that the capacitor's prongs fit into the ports well.
24. Plug the red LED right below the capacitor into ports 5 and 6. 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.
25. Make sure that the wires, jumper, capacitor, and LED are all securely in place. The top of the MFC should look like Figure 2 below.

26. Set the MFC indoors, in a place where it will not be disturbed. 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 two 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 five 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.
      3. Repeat step 1ai or 1aii 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 1aii, change the heading from "Seconds Between Blinks" to "Blinks per Second.") Calculate the average for your three counts and record that, too.

2. Next measure the power output of the MFC using a voltmeter/multimeter. If you need help using a voltmeter/multimeter, consult the Science Buddies' Electronics Primer: Using a Multimeter, as well as the instruction manual that came with your voltmeter/multimeter.
   1. To measure the MFC's power output, first remove the jumper, capacitor, and LED from the hacker board. Note: Leave the cathode and anode wires plugged in to the hacker board - only ports 1-6 should be empty. Doing this switches the hacker board to open circuit mode.
   2. Leave the MFC like this for 30 minutes.
   3. Place a resistor between ports 1 and 4.
      1. Several resistors come in the MFC kit. Start with the largest-capacity resistor, which will probably be a 1 K ohm (1000 ohms) resistor. (Ohm, also written as Ω, is the unit that resistance is measured in. 1 K ohm is also called a kilohm.)
      2. Use the pictures in the hacker board booklet that comes with the kit to determine the resistance for each resistor.
         • Tip: If you want, you can confirm the resistance of any resistor using your voltmeter/multimeter by setting it to measure resistance (usually a "Ω" symbol for ohms) and placing the voltmeter/multimeter's leads on the wire ends of the resistor.
      3. Leave the resistor plugged in for five minutes.
   4. After the resistor has been plugged in for five minutes, use your voltmeter/multimeter to measure the voltage across the two test pads (the small metal circles on the bottom of the hacker board).
      1. Set the voltmeter/multimeter to measure DC voltage. (On a voltmeter/multimeter, this is usually marked as "DCV" or "V" with a straight line above it.)
      2. Hold the voltmeter/multimeter's red lead on the test pad with the "+" next to it and hold the voltmeter/multimeter's black lead on the test pad with the "-" next to it.
      3. If the voltage seems to be changing a little, such as decreasing slightly during a few seconds, watch the readings on the voltmeter/multimeter for a few seconds more until they stabilize (and stay the same for a few seconds). Use the stabilized value.
      4. Record your results in your lab notebook in a data table like Table 2 below.
      
      

      Day and Time:
      Resistance (ohms)	Voltage (V)	Power (µW)
      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.
   5. Remove the resistor.
   6. Repeat steps 2c to 2e until you have tested the MFC with all of the resistors in the kit.
      1. Start with the largest-capacity resistor and end with the smallest-capacity one.
3. Once you have finished taking your voltage measurements, plug the jumper, capacitor, and LED back into the hacker board, as described in steps 22-25 in the "Setting Up the Microbial Fuel Cell" section above.
4. 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. Using Equation 1, your answer will be in watts. Convert watts to microwatts by dividing your answer by 1,000,000.

      Equation 1:

      P = V² / R

      P is the power in watts (W). V is the voltage (V). R is the resistance in ohms (Ω).

   2. Once you have calculated it, record the power for each resistor in the data table (such as Table 2 above) in your lab notebook.
5. 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). Sample graphs are shown in Figure 3 below.
      1. You may see a curve, with the peak power being at the top of the curve, as shown in sample graph #1 in Figure 3 below.
      2. It is possible that instead of a curve you will just see the power increasing with resistance, creating a nearly straight line sloping upward, as shown in sample graph #2. This means that finding the peak power may require using larger resistors. But do not worry if this is the case - for this science project, just use the power produced by the largest-capacity resistor as your peak power (it should be the highest power produced by any of the resistors).
   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.

6. 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 three to ten days for the LED to start 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 to determine the peak power.
      2. The peak power will probably be found with the same resistor each day.
   4. After about ten to 12 days, the power output should stabilize.
      1. It may stabilize anywhere between 5 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.
      2. When it stabilizes, the peak power should not change by more than about 0.5 uW for at least three days in a row.
      3. Do not worry if your peak power changes by a little more than this. If it has been about ten to 12 days and the peak power is not steadily increasing each day, then it has probably stabilized enough.
      4. 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?
      5. The time between LED blinks should also stabilize.
7. 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, unscrew the sealing ring, remove the dome lid, and carefully take off the donut disk, being careful to untangle and leave the anode and cathode attached to the MFC.
   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 6-7, 9-10, and 15-25 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.
8. 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.
   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 to determine the peak power.
      1. It is possible which 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.
9. 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).
10. 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. To determine blinks per second, take 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.

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:

• In this science project you tested how the addition of salt affects the power output of the MFC, but other factors may affect and improve the power output as well. Investigate other factors, such as temperature or the addition of other substances like sugar. Develop a well-reasoned hypothesis for what you think will happen when this factor is changed and create a way to test it. How does changing other factors affect the MFC's power output? Can you change something that greatly improves the MFC's power output? If your MFC is generating between 80 µW and 199 µW, it is doing really well, and if it is generating more than 200 µW, it is doing amazingly well!
  • Hint: You may want to look into what the soil bacteria (discussed in the Introduction) like to eat and the environments they thrive in.
• How well do different topsoils work in the MFC? Try collecting some different types of soils from different locations (such as from a yard or park compared to the bottom of a stream), or purchasing different soils. Test the soils, one at a time, over a set period of time. Which soil is best, resulting in the MFC generating the most power? Which soil is worst? If multiple soils work well, what do they have in common? What factors do you think help make a soil "good" or "bad" for an MFC?
  • Note: Carefully clean the MFC between testing different soils. To do this, 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.
  • Note: If you use a soil with perlite, Styrofoam^TM balls, or vermiculite pieces, strain these particles out before using the soil, as they may damage the MFC or severely inhibit bacterial growth.
• 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.
• What is the lifespan of the MFC you tested in this science project? If you want to test a long-term science project, you can try this out. The MFC may produce power for years (as long as the MFC stays sealed and moist). But exactly how long will it 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 MFC completely stops? Can you do something to make it have a longer lifespan?
• How does adding salt to the MFC, as you did in this science project, compare to adding nothing to the MFC and just letting it continue to run as you originally set it up? To try this out, you can purchase a second MFC as a control to run at the same time as the one that you add salt to, or alternatively you can run the original MFC again after your salt trial but only add topsoil to it the second time. How does the power output of the MFC that has no salt added compare to the one that you added salt to?
  • Note: Carefully clean the MFC between testing different soils. 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.

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