Implementation of Sediment Microbial Fuel Cells in the ...



Implementation of Sediment Microbial Fuel Cells in the High School Science Classroom

Sina Kirk

Cheney High School

Cheney, WA

A.J. Herres

Pre-service

Lewis & Clark State College

Lewiston, ID

Washington State University Mentors

Dr. Haluk Beyenal

Chemical and Bioengineering

&

Matt Shim

July 2010

Table of Contents

Page

Project Summary

Overview of project……………………………………….....………………… 3

Intended audience……………………………………………………………... 3

Estimated duration…………………………………………………………...... 3

Introduction…………………………………………………………………………… 4

Rationale for module…………………………………………………………………... 4

Science………………………………………………………………………………… 5

Engineering……………………………………………………………………………. 5

Goals…………………………………………………………………………………… 5

Activity #1: An Introduction to Microbial Fuel Cells.….…………………………….. 7

Activity #2: Electrical Circuits and Electrochemistry …………….………………….. 20

Activity #3: Construction and Operation of the SMFC ……………………………….. 26

Activity #4: Manipulation of a Variable in the SMFC ………….…………………..… 31

References……………………………………………………………………………… 34

PROJECT SUMMARY:

Overview of Project

In this project students will build a sediment microbial fuel cell and measure the electrical potential generated. This project is designed to give students an opportunity to apply engineering concepts within a physical science curriculum. Through the implementation of the microbial fuel cell as a learning module, students will develop an understanding of the conservation, transference, and transformation of energy within a system as well as the chemical reactions that take place. The sustainability of the sediment microbial fuel cell (SMFC) and its potential applications will also be discussed.

Once students are familiar with the form and function of the SMFC, they will choose a single variable to manipulate and study the effects of that variable on the electrical potential generated. Students will graph their data and summarize their findings in a report that will be presented to the class.

Intended Audience

This module is intended for a 9th grade honors physical science class but could be easily modified for use in a biology or environmental science class. It is recommended that students have at least a semester of chemistry and are familiar with concepts such as atomic structure, the periodic table, and atomic bonding. Additionally, students should be familiar with the scientific method and be able to conduct a controlled experiment.

Estimated Duration

This module is estimated to take between two and three weeks to complete. This includes approximately seven, 55-minute class periods for construction, discussion, and presentations. The remaining days will be used to take measurements and record data. This can typically be accomplished in less than 10 minutes each day.

INTRODUCTION

This module is designed to engage students in hands-on laboratory activities that promote inquiry and scientific thinking through real-world applications. Through investigation of the SMFC, students will learn about oxidation –reduction reactions as they occur in cellular respiration and how they can generate electricity. Students will also examine the flow of energy and matter through the system. As an extension, students will choose a variable to manipulate in an attempt to increase the electrical potential produced. Examples of variables would be: the surface area of the electrodes, their location in the cell or the material from which they are made; the volume or source of the water; and the temperature of the system. These are just some examples but students may choose to manipulate other variables if it is feasible to do so.

This module includes 4 activities beginning with a PowerPoint to familiarize the students with the components of an MFC and give them a basic knowledge of the mechanisms involved and ending with an unguided inquiry activity. At the conclusion, students will present their finding to the class in a brief presentation.

It is recommended that students work together in cooperative learning groups of three in order to complete the activities presented in this module. However you may choose to have larger or smaller groups depending on the time and resources available.

RATIONALE FOR MODULE

According to the National Science Foundation (2009), the United States ranks behind nearly every industrialized nation in the ratio of natural science and engineering degrees to the total number of degrees awarded. This statistic shows the need for high school science teachers to develop strategies for improving the outlook of careers in science and engineering. This module integrates the state science standards with the excitement of cutting-edge-research in engineering in an attempt to increase student interest in the fields of natural science and engineering. The SMFC is an ideal tool because it allows for multiple concepts, such as electrochemistry, microbiology, and biochemical engineering, to be addressed through hands-on activities. This will provide a concrete model for learning abstract ideas.

SCIENCE

The SMFC transforms chemical energy stored in the bonds of organic matter into electrical energy. Microorganisms consume organic matter contained in the river water. This organic matter is broken down by the organisms through the process of cellular respiration. As microbes breathe, organic compounds are oxidized and electrons are released. The electrons travel to the anode and are then transferred through an external circuit to the cathode. This flow of electrons creates electrical energy. When the electrons reach the cathode, they are accepted by oxygen molecules that are pumped in at the site. In order for a SMFC to function, the anode is buried in a layer of mud to create an anaerobic environment, thereby separating the sites where oxidation and reduction occur. By doing so, we can use the external circuit as a means to measure, use and/or store the energy that is being transformed. If the anode is exposed to oxygen both the release (oxidation) and acceptance (reduction) of electrons will occur at the anode and no useable electrical potential will be generated.

ENGINEERING

The ability for the teacher to develop questions and tasks using the Sediment Microbial Fuel Cell utilizes the essence of engineering. Students will learn that the basics of engineering are to take simple systems and known concepts and creatively use these to solve interesting tasks or problems. The final activity in our module uses the SMFC and asks for creativity in creation of useful applications for the SMFC. This will expose the students to the interesting and challenging world of biological and chemical engineering.

GOALS

This module addresses the following Washington State Grade Level Standards:

PS2G: Chemical reactions change the arrangement of atoms in the molecules of substances. Chemical reactions release or acquire energy from their surroundings and result in the formation of new substances.

PS2I: The rate of a physical or chemical change may be affected by factors such as temperature, surface area, and pressure.

PS3A: Although energy can be transferred from one object to another and can be transformed from one form of energy to another form, the total energy in a closed system is constant and can neither be created nor destroyed.

LS1B: The gradual combustion of carbon-containing compounds within cells, called cellular respiration, provides the energy source of living organisms; the combustion of carbon by burning of fossil fuels provides the primary energy source for most of modern society.

LS2A: Matter cycles and energy flows through living and nonliving components in ecosystems. The transfer of matter and energy is important for maintaining the health and sustainability of an ecosystem.

LS2F: The concept of sustainable development supports adoption of policies that enable people to obtain the resources they need today without limiting the ability of future generations to meet their own needs. Sustainable processes include substituting renewable for nonrenewable resources, recycling, and using fewer resources.

SYSD: Systems can be changing or in equilibrium

Activity 1- An Introduction to Microbial Fuel Cells

This is a PowerPoint-guided lecture intended to familiarize students with the concepts behind a fuel cell and how they are applied specifically to an SMFC. It begins with a brief description of what a fuel cell is and how it differs from a battery. It then describes the components of a traditional fuel cell and how they are modeled in an SMFC.

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So, we have to have a transfer of electrons. This occurs when one substance wants to lose electrons and another substance wants to gain electrons. When these two substances come in contact with one another, electrons are transferred.

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The electrodes are necessary because the electrons cannot go directly from the solution to the wire, they need a solid surface to serve as an intermediate. The electrodes are usually made of metal but can be made of any substance that will conduct electricity. The tube of electrolyte solution (commonly referred to as a salt bridge) completes the circuit. Without the salt bridge, the solution on the left would develop a net positive charge and the solution on the right would develop a net negative charge. The cell will not operate this way. The salt bridge allows ions to flow between the two solutions so that the solutions remain neutral and the cell will operate.

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The mud serves to separate the anode chamber from the cathode chamber. The water is the electrolyte solution that allows the completion of the circuit.

[pic] The chemicals come from decaying plant and animal matter in the river itself and as a result of runoff. They are mostly organic chemicals which consist mainly of carbon, hydrogen, and oxygen.

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Activity 2 – Electrical Circuits and Electrochemistry

Lecture notes and information:

There are many complex components to electrical circuits, but there are also simple fundamentals to understand. To begin with we'll first consider a simple example: the headlight circuit of an automobile. This circuit consists of a battery, a switch, the headlamps and the wires that connect them in a closed path.

Chemical reactions in the battery create a flow of electrons through the circuit. These electrons (energy) flow from the negative post of the battery, through the wires (conductors), to the headlamps, through the switch, and back into the positive post of the battery. The material in the headlights, usually special tungsten wires, can withstand high temperatures. Since tungstun is a poorer conductor than copper, collisions occur between the atoms of the tungsten and the wires heat up. This heat produces light from the glow of the wires.

In order to better understand an electrical circuit, we can compare it to a Fluid-flow analogy. The battery is like the pump and charge is like the fluid. Conductors are like frictionless pipes through which the fluid flows. Electrical current is comparable to the flow rate of the fluid. Voltage corresponds to the pressure difference between points in the fluid circuit. The resistance found in the headlamps is like the constriction in the fluid system, creating turbulence and conversion of energy to heat. Realize that current is a measure of the flow of a charge through the cross section of a circuit element, whereas voltage is measured across the ends of a circuit or between any other two points in the circuit. This will become very important with understanding the voltage potential of the SMFC.

When charge moves through circuit elements, energy can be transferred. In our simple headlamp circuit, stored chemical energy is supplied by the battery and absorbed by the headlamps where it converts to heat and light. The voltage associated with a circuit element is the energy transferred per unit of charge that flows through the element. Voltage can also be defined as the difference in energy potential between two substances based on their ability to give or receive electrons, or also as the amount of electricity in the form of electrons passing through a substance such as a conductor. The units of voltage are volts (V).

Electrical current is measured in units called amperes (A) or amps and are defined as the time rate of flow of electrical charge through a conductor or circuit element. Generally, a rate of flow measured in milliamps is a relatively low amount of electrons passing through the circuit. A rate of flow measured in amps uses more energy and realize that a household outlet will usually have a 15 or 20 amp breaker or fuse. If more than this number of amps is pulled through the system, the breaker will trip or the fuse will break. A normal space heater draws about 13 amps on its high setting and this is why they don't make space heaters higher than 1500 watts for a standard household outlet.

Let's get a better understanding of current. Current isn't a stuff. Electric current is the flow of a stuff. So what's the name of the stuff that flows during an electrical current? The flowing stuff is called “Charge.” The quantity of charge is measured in Coulombs and the word “ampere” means the same thing as “one coulomb of charge flowing per second.” If we were talking about water, Coulombs would be like gallons, and amperes would be like gallons-per-second.

Watts are closely related to amperes. They are the name of an electrical flow, but what is flowing? Energy! A “watt” is just a way of saying “quantity of electrical energy flowing per second.” This quantity can be found by multiplying the Amps by the Voltage (W=A*V) or another way to look at it is the amount of push (voltage) on the charge (amps).

Resistance is another significant concept to understand about electricity. Resistance is basically the amount of friction in the electrical system. The copper wires are used in most electrical systems because they are nearly frictionless. It is the device we are trying to power that has a variable amount of resistance. The headlamps in your car or lightbulb in your house gets bright and hot because of the incredible amount of friction used in the element. All components in an electrical circuit have a measureable amount of friction which is their resistance, measured in Ohms (Ω).

Now that we have the basics of electricity, it is important to know there are two types of charge flow, direct current (DC) and alternating current (AC). DC is the unidirectional flow of electric charge. This type of current is produced by sources such as batteries, solar cells, and capacitors. The electric charge flows in a constant direction, distinguishing it from AC. In alternating current, the movement of electric charge consistently reverses directions. This is the form of power from a household outlet and the waveform of an AC power circuit is usually a sine wave.

Our SMFC uses direct current so we will learn more about this form of charge flow. Most hand-held devices use some sort of battery, and usually more than one battery. We can group the batteries together serially to form higher voltages in which each voltage adds to the previous voltage, or in parallel to form high currents where each current adds to the previous current. In a series arrangement, all of the positive ends connect together in the circuit and all the negative ends connect together as well. In a parallel arrangement, the negative end of one battery is connected to the positive end of another battery and this occurs for all the batteries in the circuit.

Capacitors are very similar to batteries, but instead of containing chemicals that create a flow of electrons, they are two conductors, separated by an insulator. By adding electrical energy to a capacitor we can “Charge” it and by releasing the energy we “Discharge” it. Capacitors have the ability to discharge very rapidly, unlike a battery. The flash on a camera uses a capacitor. The batteries charge the capacitor and then the energy is discharged all at once in the form of the flash. You can charge a capacitor simply by wiring it up into an electric circuit. When you turn on the power, an electric charge gradually builds up on the plates. One plate gains a positive charge and the other plate gains an equal and opposite (negative) charge. If you disconnect the power, the capacitor keeps hold of its charge (though it may slowly leak away over time). But if you connect the capacitor to a second circuit containing something like an electric motor or a flash bulb, charge will flow from the capacitor through the motor or lamp until there's none remaining on the plates.

Guided Classroom Discussion

Write each of the bolded terms out where the students can all see them. Ask the students for any prior knowledge or definitions they may be able to give about any of the terms. Spend time clearly defining and giving examples of each of the terms and concepts.

Hands-On Activity

|BATTERIES |[pic] |

|THE LEMON BATTERY[pic] | |

|Materials: | |

|- a lemon | |

|- a strip of copper | |

|- a strip of zinc | |

|- a voltmeter | |

|- two cables with alligator clips | |

|- a thermometer or clock with an LCD | |

|display | |

 

Roll the lemon firmly with the palm of your hand on a tabletop or other hard surface in order to break up some of the small sacks of juice within the lemon. Insert the two metal strips deeply into the lemon, being careful that the strips not touch each other. Using the voltmeter, measure the voltage produced between the two strips (figure 3). It should show to be about one volt.

It would be nice to be able to illuminate a light bulb using your new lemon powered battery, but unfortunately it is not strong enough. If you were to try to light a bulb using this setup, the voltage across the strips would fall immediately to zero. Given this, if you want to demonstrate that the current produced by this battery is capable of powering something, try with a small device that uses an LCD display. A clock or a thermometer usually works well. An LCD display consumes an extremely small amount of current and your lemon battery is able to adequately drive this type of device. Remove any conventional battery that is in your clock or thermometer and power it with your lemon battery. You should see the device recommence functioning normally. If not, try swapping the polarity of the electricity from your lemon battery. This system allows you to demonstrate that the battery is producing energy even if you don't have a voltmeter.

How does this battery work? The Copper (Cu) atoms attract electrons more than do the Zinc (Zn) atoms. If you place a piece of copper and a piece of zinc in contact with each other, many electrons will pass from the zinc to the copper. As they concentrate on the copper, the electrons repel each other. When the force of repulsion between electrons and the force of attraction of electrons to the copper become equalized, the flow of electrons stops. Unfortunately there is no way to take advantage of this behavior to produce electricity because the flow of charges stops almost immediately. On the other hand, if you bathe the two strips in a conductive solution, and connect them externally with a wire, the reactions between the electrodes and the solution furnish the circuit with charges continually. In this way, the process that produces the electrical energy continues and becomes useful.

As a conductive solution, you can use any electrolyte, whether it be an acid, base or salt solution. The lemon battery works well because the lemon juice is acidic. Try the same setup with other types of solutions. As you may know, other fruits and vegetables also contain juices rich in ions and are therefore good electrical conductors. You are not then, limited to using lemons in this type of battery, but can make batteries out of every type of fruit or vegetable that you wish.

Like any battery, this type of battery has a limited life. The electrodes undergo chemical reactions that block the flow of electricity. The electromotive force diminishes and the battery stops working. Usually, what happens is the production of hydrogen at the copper electrode and the zinc electrode acquires deposits of oxides that act as a barrier between the metal and the electrolyte. This is referred to as the electrodes being polarized. To achieve a longer life and higher voltages and current flows, it is necessary to use electrolytes better suited for the purpose. Commercial batteries, apart from their normal electrolyte, contain chemicals with an affinity for hydrogen which combine with the hydrogen before it can polarize the electrodes.

Knowledge Check

1. What does a switch do in a circuit?

2. What happens in the filament of a light bulb to cause light and heat?

3. Which direction do electrons flow in a battery-operated circuit?

4. The difference in energy potential between two points in a circuit is known as ________.

5. An ampere is defined as:

6. Is 20 amps a small amount of current or a large amount with regard to household usage?

7. How is a Watt similar to an Ampere?

8. __________ is the term to describe friction in an electrical circuit.

9. State two differences between AC voltage and DC voltage

10. Explain what is meant by a serial arrangement and what is meant by a parallel arrangement.

11. Explain how a capacitor is charged. Also, how does a capacitor differ from a battery?

ACTIVITY # 3 Construction and Operation of the SMFC

Purpose:

Students will construct a laboratory sediment microbial fuel cell.

Prerequisite Knowledge:

Students should have a basic knowledge of MFC’s and their components, learned in Activity #1.

Materials list to build the electrodes

A. (1) 12”x12”x1/4” graphite plate

B. Conductive epoxy

C. RTV silicone adhesive and sealant

D. Approximately 10ft of 20 gauge insulated copper electrical wire

E. Drill with a 3/32” drill bit

F. Hand saw with a metal-cutting blade

Assembling the electrodes

1. Use the saw to cut the graphite plate in half.

2. Drill (2) holes, spaced ½” apart and ½” deep, into the

shorter side of each of the two graphite plates.

3. Cut the copper wire into 4 pieces approximately 2.5ft long and strip 5/8” of the insulation off each end. Twist two wires around each other so they stay together to form one unit pair. Do this again for the second pair of wires.

4. Measure out equal parts of the conductive epoxy, a ½” line of epoxy should be enough. Then mix the two parts together.

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5. Immediately coat each wire end in the conductive epoxy and insert that end into the hole drilled in the graphite plate. Do this for each of the 4 holes and 4 wire ends. This will leave the opposite 4 ends uncovered and these bare ends are where you will connect the alligator clips for measuring voltage potential and charging capacitors.

6. Once each electrode is assembled, place them both in an area that will remain undisturbed for 20+ hours for the epoxy to cure.

7. The following day, cover the epoxy and wire-to-graphite site with RTV silicon sealant. It is important that the connection is fully covered so that no water or sediment contacts it.

Materials to build the SMFC

A. (1) 5.5 Gallon aquarium tank

B. (2) 15” Bubble wands

C. (1) 1-2 Port aquarium air pump

D. (1) 2-4 Port Aquarium gang valve

E. approximately 6 ft of silicon or rubber tubing

F. Dental floss or fishing line

G. Duct tape

H. River sediment

I. River water

Assembling the SMFC

1. Cover the bottom of the aquarium with a layer of sediment at least 1” thick.

2. Place one of the electrodes on the layer of sediment and position the wire so that it comes up

and out of the top of the tank.

3. Cover the electrode in sediment to a depth of at least 1”. The electrode should be completely surrounded with and packed into the mud so that it has no

access to the air.

4. Attach the bubble wands to the sides of the aquarium, one on each side. They should be positioned about 1.5 to 2 inches above the sediment.

5. Attach an 18” length of tubing to one end of

each bubble wand. Attach the other ends to the anti-siphon that came with the bubble wand.

6. Attach a 4” length of tubing from each siphon to the outlet ports on the gang valve.

7. With the remaining length of tubing (approx. 24”), attach the gang valve to the air pump.

8. Cut the dental floss or fishing line into 2 pieces approximately 18” in length. Take one of the pieces of floss and using duct tape, attach one of the ends to the outside of the aquarium near the top. This should be done approximately 4” from the corner on the long side of the tank. Then take the other end of the floss and tape it in the same location but on the opposite side of the tank. This should create a support to suspend the other electrode in the water.

9. Take the second piece of floss and do the same but tape it about 4” from the other side of the tank. This should give you 2 parallel supports about 8” apart.

10. Gently fill the tank with river water. If you dump it all in, you may erode the sediment covering the buried electrode and expose it to air. So, pour it in slowly and gently. Fill the aquarium to the bottom of the black rim.

11. Center the remaining electrode over the two supports and gently lower it down onto them. Position the wire so that it hangs over the edge and outside of the aquarium.

12. Wrap a piece of tape around each of the wires from the tank and label the wires from the buried electrode as the anode and the wires from the suspended electrode as the cathode.

Operation of the SMFC

1. Begin by measuring the potential of the SMFC by running an open circuit (the wires are not connected between the anode and cathode) for 5 days. Connect the black test probe of the multimeter to the reference electrode and then independently measure the voltage of the anode and cathode. Record the measurements each day. Also use a thermo-coupler or thermometer to read and record the temperature of the SMFC water.

2. Using the recorded data, graph the results of the potential for each electrode.

As the voltage from the anode, buried in the sediment, begins to approach -300 to -400 mV, give classtime to discussion/ideas with the students on possible applications of the SMFC. Have a couple of examples of items that can be powered by low voltage and current requirements, LED's, a small motor, a hand-held radio, etc. We picked up a digital thermometer/humidity display that normally ran off a single AAA battery. This is the proper time to introduce capacitors to the students. They will unlikely find any items that can power on with less than 600-700 mV. Most devices operate at 1.5V and higher, so a way is needed to increase the overall potential of from the SMFC. This can be accomplished by connecting the SMFC to 3 or 4 super capacitors in a parallel arrangement to charge each one to the maximum potential of the SMFC, a value of 0-800mV. Once the super capacitors are charged, connecting them in a serial arrangement will multiply the voltage of the single capacitor by the number of capacitors arranged. 2 F or 4 F capacitors work well for storage capacity, enough to power many devices for a useable period of time.

ACTIVITY # 4 Manipulation of a Variable in the SMFC

Purpose:

The purpose of this activity is for students to design an experiment with the goal of increasing or decreasing the charge potential of the system. This activity is unguided inquiry where the students develop a problem, hypothesis, equipment list, procedure and data table. The students must then analyze the data and write up a conclusion of their findings.

Prerequisite Knowledge

Students will have learned how to use all materials and equipment in previous labs.

Equipment

See equipment list for Activities #3.

Procedure

Part A: Predicting and Planning an Investigation

You are given the task of affecting the voltage potential of the Sediment Microbial Fuel Cell.

1. In your science notebook, state the outcome you are attempting to change in this activity.

2. Develop a hypothesis that states what you predict the outcome of the experiment is and a reason for your prediction. Use the IF, THEN, BECAUSE format.

3. Work as a group to determine which materials you will need for this experiment. Keep in mind that you will only have 1 sediment microbial fuel cell to use for your experiment.

4. Write a detailed, numbered procedure list that includes how often you will record measurements, who will read, and who will record.

5. Be sure to include a data table for recording your results.

6. Your problem, hypothesis, materials list, procedure and data table must be submitted for review and accepted by your teacher before beginning the investigation.

Part B: Conducting your investigation

After receiving your teacher’s approval for your experiment, you may begin to conduct the investigation. Be sure to record daily observations and any variation in your procedure or original expectations. You will have 4 days to perform your experiment from start to finish.

Conclusion

In your conclusion, restate the problem you were trying to solve, whether or not the hypothesis was correct, and evidence from the experiment that supports your conclusion. Explain any problems or issues you encountered while conducting the experiment and why or why not your hypothesis may have been correct/incorrect.

Assessment

A Grading rubric is provided as follows:

References

Carboni, G. (1998). Experiments in Electrochemistry. Retrieved July 27, 2010, from

Dewan, A. & Beyenal, H. Microbial Fuel Cells Education Module. Washington State University, Pullman. 2007.

Hambley, Allan R. (2005) Electrical Engineering: Principles and Applications. Pearson Custom Publishing. (pp. 8-24)

How are Watts, Ohms, Amps and Volts Related. Accessed July 20, 2010 at

National Science Foundation . Reasons for International Changes in the Ratio of Natural Science and Engineering Degrees to the College Age Population. Accessed July 7, 2010 at

Online Grade Level Standards & Resources. Washington State Office of Superintendent of Public Instruction. Accessed July 6, 2010 at

Watts, Amps and Volts and How to Understand Electricity. Hubpages. Accessed July 20, 2010 at

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