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.
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.
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.
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.
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.
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.