An Introduction to Sediment Microbial Fuel Cells: Can Electricity really be Dirt Cheap?

Transcription

1 An Introduction to Sediment Microbial Fuel Cells: Can Electricity really be Dirt Cheap? Norlinda Connolly Mansfield High School Mansfield, WA & Patrick Yecha Lyle High School Lyle, WA Washington State University Mentor Dr. Haluk Beyenal Chemical and Bioengineering WSU & Benjamin Lantz Graduate Assistant & Alim Dewan Graduate Assistant July, 2009 The project herein was supported by the National Science Foundation Grant Award No. EEC : Dr. Richard L. Zollars, Principal Investigator. This module was developed by the authors and does not necessarily represent an official endorsement by the National Science Foundation. 1 P a g e

2 Table of Contents Page Project Summary.. 3 Intended Audience.. 3 Estimated Duration... 3 Introduction.. 3 Rationale for Module. 3 Science. 4 Engineering.. 4 Goals.. 4 Materials Needed. 4 Pre-Requisite Knowledge 5 Pre-Gauge 6 Pre-Gauge Answers.. 7 Activity 1: Using a Multi-meter. 8 Activity 2: Cellular Respiration.. 12 Activity 3: Can Electricity really be Dirt Cheap?.. 16 Conclusion.. 25 Glossary. 26 References P a g e

3 Project Summary Project Overview The lessons in this module are aimed at 9 th or 10 th grade Biology students. The module is also appropriate for an Environmental Science class when introducing the Alternative Energy unit. Students will first be introduced to the available types of fuel cell. A lesson on electrochemistry of batteries is also reviewed before delving into Sediment Microbial Fuel Cells. A lecture on cellular respiration is also necessary in understanding how electricity is generated by microorganisms. The culminating project is a experiment in which the students design and construct a Sediment Microbial Fuel Cell that will generate the greatest potential is an unguided inquiry where the students develop a problem, hypothesis, equipment list, procedures and data table. The students must then analyze the data and write up a conclusion of their findings. Intended Audience This module is intended for high school students and teachers with some background in electrochemistry, microbiology and biology. The background information included is sufficient in helping teachers deliver the content. The activities prior to the culminating project provide reviews for the underlying concepts that are important to the understanding of the Sediment Microbial Fuel Cell. The equipment needed for this module can be prepared by the student with teacher s guidance. With the exception of a few, most of the supplies are readily available. Estimated Duration This module should take three days of introductory activities, followed by 20 days of approximately ten minutes each day for the students to measure the cell potential. This will be followed by one to two days for students to use the electricity gathered to power the devices they have selected. Introduction The module is designed to motivate and engage students in exploring the idea of generating electricity from river sediment. Students will discover how cellular respiration carried out by the microbes in the sediment can lead to electron production that can be captured by electrodes. As these electrons flow through the circuit as electrical current, they can be used to power electronic devices. The module includes an activity on how to use a multi-meter and capacitors since students will be using multi-meter and capacitors in measuring the performance of the sediment fuel cell. Students will design their own Sediment Microbial Fuel Cell with the goal of generating the greatest cell potential. The Environmental Science can study the relationship of water quality to fuel cell performance. A chemistry class can use the fuel cell oxidation and reduction reactions as part of the red-ox reaction in cellular respiration. Rationale for Module The goal of this module is to germinate interest in engineering among high school students through hands-on learning. This microbial fuel cell module is a vehicle that can be used to deliver the concepts of electrochemistry, respiration and microbiology. 3 P a g e

4 Students will realize the relevance of biological concepts such as cellular respiration as it applies to the Sediment Microbial Fuel Cell. Students will use engineering to construct a sediment fuel cell that would produce the highest cell potential. They will consider various factors that may influence the performance of their fuel cell, form a hypothesis and test their hypothesis. Science Electrochemistry cannot be discussed in any detail without including the concept of oxidation and reduction reactions, often the method by which chemical energy is changed to electrical energy. A microbial fuel cell (MFC), like a battery is an electrochemical device that generates electricity directly from organic chemicals using microorganisms. In simpler terms, the microorganisms consume organic compounds to produce energy for their own survival, and we collect the electrons that they produce as electricity. A MFC also has two electrodes, anode (-) and cathode(+). When the microorganisms carry out respiration, organic compounds are oxidized and electrons are released and transferred to the anode. Then, the electrons are transferred through an external circuit to the cathode which can be used to power electronic devices. The protons that are produced during oxidation are diffused from the anode to the cathode through the proton exchange membrane to complete the circuit. Oxygen is pumped into the cathode chamber where it accepts electrons from the anode through the external circuit. By separating the oxidation and reduction environments, using a proton exchange membrane MFC enables the electrons to be transferred through an external circuit. In the anode chamber: oxidation of organic compound(example: glucose) C 6 H 12 O 6 + 6H 2 O 6CO H e - In the cathode chamber: reduction of oxygen(electron acceptor) 6O H e - 12H 2 O Engineering As a culminating project, teams of students are to design and build a microbial fuel cell that can be used to generate electricity from the microbes found in the river sediment. The goal is to build a fuel cell that would produce the most power in 30 hours while keeping the cost low. This project is also an example of chemical and biological engineering through which students are discovering that an electrical current can be borrowed from microbes in the sediment that would otherwise be unused. Goals - Upon the completion of this module, students will be able to - Construct a sediment fuel cell that could yield the highest potential - Explain the role of cellular respiration in microbial fuel cell - Explain the role of redox reactions as it pertains to the electrical current production in the microbial fuel cell - Identify different sources of alternative energies currently available for use by humans 4 P a g e

5 Materials needed (throughout the whole project) - Multi-meter - 9V Batteries - 1.5V Batteries - Reference Electrode - Graphite - Insulated Copper Wire - Conductive Epoxy - Non-conductive Epoxy (Silicon) - Beads (Black, Blue, and Pink/Red) - Toothpicks - Handouts (as needed per lesson) - Power Drill with Drill Bits - Metal File - Sandpaper - Wire Stripper - Electrical tape Volt Capacitor (we used 4 Farad, but that is based on project needs) - Resistors - LED Bulbs Pre-requisite knowledge Review with students if necessary prior to starting the module. How to use a multi-meter? Students should learn how to use a multi-meter. They should learn the following: - The black wire should ALWAYS be connected to the COM - The other side of the black test probe should ALWAYS be connected to the negative (anode) side of the battery - When measuring current: o The red wire should be connected to the Ω side of the battery (cathode). Students should be aware that they should only use the ones marked with == - When measuring voltage: o The red wire should be connected to the V The other side of the red test probe should be connected to the positive side of the battery (cathode). Students should be aware that they should only use the ones marked with == 5 P a g e

6 Name: Date: Period: Sediment Microbial Fuel Cells (SFMC s) Pre-Gauge 1) What is sustainability? 2) What is the difference between energy and power? 3) What is the difference between a series circuit and a parallel circuit? 4) What is the charge given off by the anode? a. Positive b. Negative c. Neutral 5) What is the charge given off by the cathode? a. Positive b. Negative c. Neutral 6) What are some sources of alternative energy? 7) The bacteria that are used by the SMFC s go through the process of: a. Photosynthesis b. Cationization c. Anionization d. Cellular Respiration 8) What are some common uses of electricity? 6 P a g e

7 Answers to Pre-Gauge 1) Sustainability, in a broad sense, is the capacity to endure. In ecology the word describes how biological systems remain diverse and productive over time. For humans it is the potential for long-term maintenance of wellbeing, which in turn depends on the wellbeing of the natural world and the responsible use of natural resources. (Wikipedia) 2) Energy is the ability to do work. Ex: the more energy a device has, the more work it can do. 3) Power is work over time. The power is how much work can be done in a unit time. 4) When a simple series is connected, a single pathway is formed through which current flows. A parallel circuit, forms branches, each of which is a separate path for the flow of electrons. Both series and parallel connection have their own distinctive characteristics. 5) B 6) A 7) Some sources of alternative energy are: wind power, water power, solar power, fuel cells, biofuels, thermal energy, etc. 8) D 9) Most of what we have in our house is powered by electricity, such as stoves, blenders, dishwashers, etc. Electricity is also used in other machines such as cars assembly lines. There are multiple uses for electricity, the most common being powering appliances and machines that we use every day. 7 P a g e

8 Name: Date: Period: Purpose of Part 1: ACTIVITY #1: USING A MULTIMETER To learn how to use a multi-meter to measure to things that a battery releases: voltage and current. Materials/Equipment for Part 1: Multi-meter 9 V battery Electrical tape (if needed) Directions/Procedures for Part 1: How to use a multi-meter INSTRUCTIONS FOR MEASURINGVOLTAGE: Black Test Probe: Plug into the black terminal on multi-meter marked COM Red Test Probe: Plug the red probe into the red voltage socket marked V or V/Ω Turn the dial to the V== segment. You may have several numbers to choose from (2, 20, or 200 for example). These are all voltage ranges. A maximum of 2 Volts, 20 volts, and 200 volts. Choose the one that fits the battery. Remember you are using a 9 Volt battery. - Take the black test probe and attach it to the negative (--) side of the battery - Take the red test probe and attach it to the positive (+) side of the battery If you do not get a reading ask your teacher for help. What voltage is the multi-meter reading? Look at the voltage on the side of the battery. What is the voltage? What would cause the actual voltage to be less than the voltage reading on the side of the battery? If you were measuring the voltage of an AA battery what would you need to set the multi-meter to? 8 P a g e

9 INSTRUCTIONS FOR MEASURING CURRENT When you connect the probes do NOT leave them attached for more than 5 seconds. This draws energy from the battery. Plug the red test probe into the Red 20A socket. Current is measured in amps. Turn the multi-meter to the 20A == setting. DO NOT turn to any of the amp setting that have this sign on it (~) - Take the black test probe and attach it to the negative (--) side of the battery - Take the red test probe and attach it to the positive (+) side of the battery. What current are you reading on the multi-meter? Check your answers with your teacher: True or False: To measure voltage of a battery the multi-meter should be turned to V~. If false change the answer so it is true: True or False: To measure the current of a battery the multi-meter should be turned to A~. If false change the answer so it is true: Answer the following questions by checking the appropriate box: Black Test Probe This test probe plugs into the V/Ω socket. This test probe plugs into the A socket This test probe is ALWAYS plugged into the COM socket To measure voltage this test probe must Red Test Probe 9 P a g e

10 be plugged into the V== socket To measure current this test probe must be plugged into the 20A socket This test probe touches the positive (+) side of the battery This test probe touches the negative (--) side of the battery Purpose for Part 2: The purpose of this activity is to learn what good conductors are and what good insulators are. In addition, to give you some practice measuring voltage and current. Finally, to learn how to connect the wires from a battery to a light bulb (or other object) to power the devise. Equipment/Materials for Part 2: Battery Multi-meter Three wires with alligator clips Plastic pen Wood pencil Rubber eraser Graphite pencil lead Glass stirring rod Aluminum Wire Copper Wire Any other types of metal (copper strips, etc.) 12 V / 4 W light bulbs purchased at Home Depot (see picture) Directions/Procedures for Part 2: Conductors versus Insulators: A conductor allows energy to pass through it quickly. An insulator causes energy to pass through it slowly, if at all. Construct a set-up like the diagram below: - Light blub Battery + Pencil, glass, aluminum foil, etc. will fill in the gap here Put each item into the space between the battery and multi-meter then fill in the table: Material Description of Light Voltage Current 10 P a g e

11 Aluminum Wire Copper Wire Glass stirring rod Graphite (pencil lead) Nail Plastic Pen Rubber eraser Wood Intensity What types of materials make good conductors? What types of materials make good insulators? Write the materials from your list in order from best conductor to best insulator in the space below: Best Conductor: Best Insulator: Explain how you came up with the order for your Best Conductor to Best Insulator list: Look at the intensity difference between the graphite and the nail. Identify which object lights the object better and explain why in the space below: 11 P a g e

12 Activity 2: Cellular Respiration Power Point Presentation on cellular respiration as it relates to Microbial Fuel Cell, followed by Cellular Respiration-Equation Model activity. Purpose The purpose of this power point presentation is to demonstrate how the breaking down of organic compounds during cellular respiration can generate electrical current in a microbial fuel cell. Prerequisite It is recommended that the teacher review with student about atoms and compounds. When chemical bonds are broken, energy is released. Energy cannot be created or destroyed, but it can be transformed. Instructional Strategies Teacher should refer to background information on Microbial Fuel Cell in the Appendix prior to the power point presentation. After the power point presentation, teacher should go over the directions of the Equation Model activity After the power point presentation, students should be familiar with the process of cellular respirations. As a review idea, students will go through the building process of the cellular respiration equation. As told to students: Simply put, respiration is the release of energy from energy-storing compounds. Fire example: Light a piece of paper on fire and explain that chemically, the process of respiration is the same as burning a piece of paper. The reactants and products are the same. FIRE: Oxygen + Fuel Carbon Dioxide + Water + Energy (Heat & Light) RESPIRATION: 6O2 + C6H12O6 6CO2 + 6 H2O + Energy (ATP, Heat) The students will now build the models to show them the inputs and outputs of the cellular respiration equation. Each student will be given a kit containing a total of 36 beads and numerous toothpicks: - The black beads (6) represent atoms of carbon. - The pink beads (18) represent atoms of oxygen. - The blue beads (12) represent atoms of hydrogen. - The toothpicks represent covalent bonds. The point of this activity is to have students build the structure of glucose and six oxygen molecules. Once they have completed this, the teacher will come around, sign off on it, and mix up the molecules. The students will then take the exact same pieces and build the products from the reaction between glucose and oxygen. After completing this part, the students should recognize that the 12 P a g e

13 reactants (atoms) that go into the equation are the same elements that come out, just in a different form. After the students have completed the activity, they will be given a worksheet as homework that asks questions having to do with the whole process of cellular respiration. The questions will be due the next day as homework, and as a review to help them understand the processes behind what drives microbial fuel cells. 13 P a g e

14 Name: Date: Period: Cellular Respiration: Equation Models Instructions for Model Building 1. Obtain a molecular model building kit. 2. As you examine the contents of the kit, you should notice that you have a total of 36 beads and numerous toothpicks: - The black beads (6) represent atoms of carbon. - The pink beads (18) represent atoms of oxygen. - The blue beads (12) represent atoms of hydrogen. - The toothpicks represent covalent bonds. 3. Using the appropriate beads and connectors (broken toothpicks), construct a glucose molecule (seen above) and six diatomic oxygen molecules. 4. After you have completed your glucose and oxygen molecules I will examine your models and sign off your lab sheet. After you have been signed off, disconnect your models and see how many carbon dioxide (CO 2 ) and water (H 2 O) molecules you can make using the same beads. You may or may not use the same amount of bonding toothpicks. Record the number of water and carbon dioxide molecules you made in the spaces listed below. 5. Disconnect all models and place all of the building materials back into the cups. They will be collected at the end of class. Teacher Signature: Number of Carbon Dioxide Molecules: Number of Water Molecules: Final Respiration Equation: C 6 H 12 O 6 + O CO 2 + H 2 O + 14 P a g e

15 Name: Date: Period: Cellular Respiration Guided Reading 1) The energy in food molecules, such as glucose, is converted into. 2) Where does glycolysis take place? 3) Briefly describe the reaction that occurs during glycolysis. 4) Where does the Krebs cycle take place? 5) Why is the Krebs cycle also known as the citric acid cycle? 6) What process occurs when oxygen is not present for cellular respiration? 7) What type of fermentation takes place in your muscles when you exercise vigorously? 15 P a g e

16 Activity #3 - Can Electricity be Dirt Cheap? Introduction In this lesson students will learn to build a sediment MFC. A sediment MFC uses electricigenic microorganisms such as Geobacter or Rhodoferax (found in river or marine sediment) to oxidize organic compounds to CO 2 while transferring electrons to solid electrodes. The anode is buried in the mud sediment while the cathode is immersed in the water above the sediment. The anode does not have to be immersed in the water all the time. A cation exchange membrane is not necessary in sediment MFC because of the decreasing oxygen gradient. This lesson will take several days to complete: Day 1: A. Preparing the electrodes Day 2: B. Designing a structure to hold the electrodes in the river. Day 3: C. Deployment of sediment MFC Day 4- Day 24: Measure and record the reading of potential from anode and cathode against the reference electrode for 20 days. Day 1 1) Make anode and cathode from a graphite plate (GraphiteStore.com). Cut the anode and cathode into 6 X 4.5. The thickness of the graphite was 3/8. 2) Smooth the edge of the electrodes using a file and sandpaper. Then, clean the debris off the surface of the electrodes using a shop vacuum. 3) Drill a small hole (2 mm diameter) 1 cm deep, on each shorter side of the electrodes. These holes should be slightly larger in width than the diameter of the copper wire. Vacuum the graphite debris out of the holes. 4) Cut the copper wire about 1 foot long for each electrode (*). Strip about 0.5 inch of insulation of the wire. Twist the stripped part of wire and dip it in a small amount of conductive epoxy. Fill each hole on the electrode with conductive epoxy. Let the epoxy dry approximately 24 hours. When the conductive epoxy is dry, seal the outside of the hole with non-conductive epoxy (silicon) about 30 minutes. 16 P a g e

17 * Measure the length of the distance of the river bank to the site where the sediment MFC will be deployed. You will need this length of copper wire plus another foot. It is better to have a little extra than not enough. It is a good idea to check out the location where you will deploy the sediment MFC so that you know how much wire you need. 17 P a g e

18 Sediment microbial fuel cell Cathode chamber Anode chamber Day 2 Design a structure that will hold the cathode. There are many possibilities when it comes to a structure to secure electrodes, and possible sources can be wood, PVC pipe, or iron. Remind the class that the anode needs to be buried in the sediment about 3-6 inches deep. The anode has to stay above the sediment in the water. The cathode does need to be fully submerged in the water but not in the sediment. The structure needs to be able to withstand the current/flow of the water source, especially when it rains. The cathode is secured to the tripod shaped structure made of wood about 2 feet tall by means of zip-ties. The stripped ends of the wires are kept dry and secured to a structure on the river bank. Mark the anode and cathode wires so that you can connect them correctly to the multi-meter when measuring voltage/ potential. The structure that we used can be seen below. 18 P a g e

19 Day 3 - Day 11 1) Measure the potential of the SMFC by running open circuit for 9 days. (* The potential should increase exponentially and then reach a plateau by about the ninth day.) 2) Measure the anode and cathode potential (voltage) against a saturated calomel electrode (reference electrode) using a digital multi-meter each day. 3) Record and graph the MFC s potential. Discussion/Brainstorm session with students: What device can be powered by the SMFC? Since the goal is to run a device using the potential generated by the SMFC, students should look for a device with a low voltage and current requirement that can be powered by the SMFC. The red LED has a voltage/current requirement of V/ 0.02 A. Other studies on SMFC have shown that the potential produced is V. Connecting several SMFC s in a series does not increase overall cell potential because the electrodes are immersed in the same electrolyte (river water), forming a short circuit. In order to light up the LED, we need to increase the overall potential of the SMFC. The cell 19 P a g e

20 potential can be increased by connecting the SMFC to 3 super capacitors (4 Farad/2.5 V) in parallel arrangement. 2 Farad super capacitors can also be used. Capacitors are used to store charges/electrons which later can be discharged to light the LED. Super capacitors have the minimum amount of charge leakage compared to regular capacitors. By connecting the super capacitors in parallel, to the SMFC, each capacitor will store V. When three capacitors are connected in a series, to the LED the overall potential discharged is V, sufficient to light the LED. 20 P a g e

21 Day 12 What is a capacitor and how does it work? Name: Date: Period: The capacitor is an invention that was used to store up an electrical charge, and then discharge it into a circuit. This can be used to smooth out electrical impulses, or turn a constant electrical flow into a series of impulses. Capacitor Circuit In an electronic circuit, a capacitor is shown like this: When you connect a capacitor to a battery, here's what happens: The plate on the capacitor that attaches to the negative terminal of the battery accepts electrons that the battery is producing. The plate on the capacitor that attaches to the positive terminal of the battery loses electrons to the battery. Once it's charged, the capacitor has the same voltage as the battery (1.5 volts on the battery means 1.5 volts on the capacitor). For a small capacitor, the capacity is small. But large capacitors can hold quite a bit of charge. You can find capacitors as big as soda cans that hold enough charge to light a flashlight bulb for a minute or more. 21 P a g e

22 Activity A Build a circuit consisting of a 1.5 volt of battery and a multi-meter. Measure the voltage of the battery. volts. Connect the capacitor to the battery. After 5 minutes, replace the capacitor and measure and record the voltage of the battery with a multi-meter. volts Replace the multi-meter with the capacitor. After another 5 minutes measure and record the voltage of the battery again. volts Repeat step 2 and 3 but instead of measuring and recording the voltage of the battery, measure and record the voltage of the capacitor. volts After another 5 minutes. volts What can you conclude from Activity A? 22 P a g e

23 Activity B Hook the capacitor as in the diagram below. What happens to the light bulb as the circuit is closed? What happens after a few seconds? Remove the battery from the circuit and replace it with a wire. What happens to the light bulb now? Write a statement to explain your observations in Activity B-1 & P a g e

24 Day 13 Connect the super capacitors in parallel arrangement to the SMFC. Every 2 minutes, using the multi-meter take the potential stored in the capacitors. Record the potential for 1 hour. Once the recordings are completed, leave the capacitors attached to SMFC to charge to the maximum capacity of the system. Have the student s graph their data using excel as a homework assignment. Day 14 Measure the potential stored in the capacitors using the multi-meter. If the potential does not change and it has reached the maximum potential SMFC (from the open circuit experiment), it shows that the capacitors are fully charged. Disconnect the capacitors from the SMFC. The following can be done in the classroom/lab if it is more convenient. Connect the capacitors in a series arrangement to the red LED and a resistor. To figure out the appropriate resistor, use the following formula: R= (V S -V LED ) I LED For example: I LED = 0.02 A V LED = 2.03 V V S = V I LED and V LED are the Red LED rating. V S is the overall potential discharged/stored by the capacitors as it is connected in a series. R= = 7.19 ohm (the closest resistor is 10 ohm) Connect the 10 ohm Resistor to the capacitors (in a series) and to the red LED. Measure and record how long the LED stayed lit up. Take the devices that the students have decided on earlier (see Day 3-12) and allow them to try and power the device using the available voltage and current. Make sure students have calculated the necessary resistance for the device they chose (if they don t, the device may be destroyed by an overflow of electric current). 24 P a g e

25 Conclusion: Once you have completed the Sediment Microbial Fuel Cell, discuss with students all of the possibilities that can be had from using this as an alternative energy source. Be sure to discuss the limitations of a SMFC (such as power, needs to be near water, water speed, etc.), as well as possible ideas for deployment. Encourage students to do their own research on SMFC s. With the graphite plates, make sure they are cleared of any biofilm on them, and then store for the next year. If the teacher chooses, they can give the pre-gauge assessment again to check for understanding. They may also create a test, or give a written assignment. 25 P a g e

26 Glossary Anode: an electrode through which electric current flows into a polarized electrical device. Cathode: an electrode through which electric current flows out of a polarized electrical device. Capacitor: a device used to store an electrical charge, measured in farads. Cellular Respiration: a process through which sugars are oxidized and form carbon dioxide and water and energy. Electricigenic: the ability of bacteria to be able to produce an electrical current. Electrode: an electrical conductor used to make contact with a nonmetallic part of a circuit. Electric Current: the rate of flow of an electric charge. Farad: the SI unit of capacitance, it is the charge in coulombs a capacitor will accept for the potential across it to change 1 volt. Light Emitting Diode (LED): an electronic light source powered by a semi-conductor diode. Ohm: the resistance between two points of a conductor. Super capacitor: a high density capacitor with the least amount of charge leakage. Voltage: an electrical potential difference between two positions. 26 P a g e

27 References Dewan, A. & Beyenal, H. Microbial Fuel Cells Education Module. Washington State University, Pullman How Capacitors Work. Howstuffworks.com. July 24, Morgan, B. & Dotson, D. The Power of Fruit: A Study in Electrochemistry. Washington State University. Pullman, Microbial Fuel Cells. Microbialfuelcell.org. July 24, Power System: How do batteries work? Northwestern University. July 24, Sediment Battery Preparation. Geobacter.com. July 24, P a g e