K.Walz, MATC
Version 3.0 April 2006
Building a Fuel Cell - An Electrifying Experience
Introduction:
Fuel cells are devices that convert chemical energy into electrical energy. Fuel cells and batteries are both electrochemical devices that operate on the principles of oxidation-reduction reactions.
The main difference between batteries and fuel cells is that while both are energy conversion devices, batteries also perform the function of energystorage. Batteries contain all of the reactants (and products) stored inside of a sealed cell. As a result, a battery will eventually "go dead" once the reactants have been consumed, and the battery is then either thrown away (if it is a primary cell) or must be recharged (if it is a secondary cell). Fuel cells store neither reactants nor products. In a fuel cell, reactants are continuously provided to the cell from an external supply. As long as there is a flow of chemicals into the fuel cell, it will continue to provide electricity – the fuel cell will never “go dead”.
There are several different types of fuel cells in use today employing various types of redox chemistries. The proton exchange membrane fuel cell (PEMFC) is one of the most promising fuel cell designs and employs one of the simplest oxidation-reduction systems. In a PEMFC, hydrogen is oxidized at the anode, and oxygen is reduced at the cathode. A schematic of a fuel cell is shown below in Figure 1.
The Proton Exchange Membrane (PEM) is a special polymer designed to transport positively charged hydrogen ions (protons) from the anode side of the fuel cell to the cathode side. The PEM is capable of conducting charged protons, but does not conduct electrons. The electrons lost in the oxidation reactions are forced to travel out of the fuel cell through an external circuit.
The fuel cell also requires a catalyst to provide a surface that facilitates the oxidation and reduction reactions occurring within the cell. The catalyst is usually fabricated from platinum, which is doped or deposited as a thin layer on a support made of carbon or other suitably conductive material.
A single hydrogen fuel cell produces between 0.7 and 1.0 volts when in operation. To obtain higher voltages for demanding applications, multiple cells are connected in series to create a fuel cell “stack”. A fuel cell stack is similar in concept to a 9 volt battery that is formed from six 1.5 Volt cells that are packaged together. A fuel cell stack is shown below in Figure 2.
Many scientists, engineers, and investors believe that in the near future fuel cells could compete with many other energy conversion devices, including the gas turbine in your city's power plant, the gasoline engine in your car, and the battery in your laptop computer. In order to succeed in these applications, fuel cells must be able to not only convert chemical energy into electricity, but also do it at a fast enough rate to provide large amounts of power.
The power delivered by any electrical source is simply the amount of work provided in a given amount of time (P=w/t). Power may also be calculated as the product of the voltage and the current (P=IV). In this lab, you will construct a hydrogen powered fuel cell and measure the voltage, current, and maximum power that it can provide.
Figure 1. Fuel Cell Schematic Figure 2. A Fuel Cell Stack Figure 3. Fuel cell stacks in
a Hydrogen Vehicle.
A potentiometer is an electrical device that can be used as a variable resistor. You are probably already familiar with this type of device because they are frequently used as light switch dimmer controllers and as the volume control knobs on home stereos. To make a potentiometer, a resistive strip is enclosed inside a metal or plastic housing, along with a mechanism for moving or rotating a conductive "wiper" across the length of that resistive strip. Terminals on the outside of the potentiometer allow the user to attached electrical leads to one side of the resistive strip and to the central axis of the wiper. The wiper is mounted on a rotary shaft controlled by the user. Rotating the shaft alters the length of the resistive strip being used, and thus changes the resistance of the potentiometer. A typical potentiometer can provide resistances ranging from about 15 to 500,000 Ohms.
In this experiment, you will use a potentiometer to vary the resistive load applied to your fuel cell. By varying the resistance, you can control the amount of current that flows in a circuit and measure the corresponding fuel cell voltage. From this data, you will determine the maximum power provided by the cell. An illustration of a rotary potentiometer is shown below along with pictures of actual devices.
Materials Required:
Two Multimeters (One for voltage, another for current) Wood splint
Six Electrical leads (banana plug and alligator ends) Sodium borohydride (NaBH4)
Potentiometer (~15 – 500 k) Distilled Water
Two Pt plated wire mesh electrodes (16 or 40 mesh Nichrome V)Nafion membrane (1 ½” x 1 ½”)
Two 10 mL syringes Forceps or dissecting probe
Vacuum or stopcock grease Hot plate or cigarette lighter
50 or 100 mL graduated cylinder
Prelab - Answer the following questions in your lab notebook:
1. Provide a definition, the symbol, and the metric unit(s) for each of the following scientific quantities: Charge, Voltage, Current, Resistance, Power
2. Sodium borohydride (NaBH4)reacts with water to yield sodium borate (NaBO2) and hydrogen gas. Write a balanced chemical reaction for this process.
3. Write the half reactions that occur at both the anode and the cathode of fuel cell operating on H2 fuel. Using the half reaction method, balance the overall redox reaction and determine the standard cell potential, Eo, for your fuel cell.
4. In a few sentences, briefly describe any environmental or performance advantages that hydrogen powered fuel cells might have over internal combustion engines as power sources for motor vehicles.
5. In a few sentences, briefly describe any limitations or disadvantages that might prevent fuel cells from being developed and implemented on a commercial scale.
Procedure - Part I: Assembling the fuel cell
1. Remove the pistons from the syringes and set them aside – they will not be used in this experiment.
2. Heat the metal forceps or dissecting probe on a hot plate or using a cigarette lighter. Use the hot tip to gently puncture a small “vent hole” near the upper part of each syringe barrel as shown in Figure 1. Use a wire cutter to snip off a few millimeters from the syringe tip, and increase the diameter of the opening.
3. Obtain two Pt coated electrodes and determine which side has been plated with the Platinum. (The Pt coat will appear darker than the uncoated Nicrome mesh)
4. Place the Platinum side of one of the electrodes over the flange, taking care to center the syringe barrel within the Pt coated area. While holding the electrode in place, gently bend the uncoated part of each electrode away from the horizontal, as shown in Figure 2. This will help prevent electrical shorts and facilitate making electrical connections to the cell. Repeat with the second electrode.
5. Using a wooden splint, apply a thin layer of grease to the flange of one syringe. Place the Platinum side of the electrode over the flange, taking care to center the syringe barrel within the Pt coated area. Inspect the grease to make sure it penetrates through the opposite side of the electrode. If necessary, apply additional grease to obtain a thin smooth, thin layer of uniform thickness.
6. The first syringe/electrode assembly (the anode side) can be temporarily placed in the mouth of a graduated cylinder to continue with the procedure. Prepare the second syringe/electrode assembly (the cathode side) as you did with the first.
7. Center and place the Nafion membrane over the first syringe/electrode assembly as it is supported in the graduated cylinder. Then place the second syringe/electrode assembly on the opposite side of the membrane to complete the construction of the fuel cell. Use a pair of free alligator clamps to pinch the two sides together, creating a water tight seal between the syringes and membrane, as shown in Figure 3.
8. With the fuel cell supported in the graduated cylinder, add 1-3 mL of water to the upper syringe (the anode side) as shown in Figure 3. Inspect the seal between the syringe, the anode, and the membrane to insure that there are not any leaks.
9. If a leak is found, dry the cell, disassemble, and add additional grease as necessary. Reseal the cell, and check if the leak has been stopped. Once you obtain a leak proof cell, you may continue with Part II.
Figure 1. Syringe PreparationFigure 2. Bending ElectrodesFigure 3. Completed Cell
Procedure: Part II – Operating and characterizing the fuel cell
1. Obtain a small amount of sodium borohydride, NaBH4 and mix it with 3-5 mL of water in a small beaker. You should quickly observe the generation of hydrogen gas that occurs. Use an eye dropper to add this solution to the anode of your fuel cell (the upper syringe). If at any time your fuel cell runs out of hydrogen you can replenish it in this fashion.
2. Obtain a multimeter to measure the voltage of the fuel cell. Set the meter to read DC volts. Attach the leads to the “VDC” and the “common” terminals on the meter. The common lead should be attached to the anode of the fuel cell (the upper syringe) and the VDC to the cathode (the lower syringe). Record the open circuit voltage, Voc, of the cell when no current is being delivered. Record this as your first data point in the table provided. Leave the voltmeter connected and in place for the duration of the experiment.
3. Obtain a potentiometer to measure the current supplied by your fuel cell. Turn the shaft on the potentiometer counter-clockwise until it stops. This is the maximum resistance setting.
4. Obtain a second multimeter and set the meter to read DC Amps. Attach a lead from the cathode of your fuel cell (the lower syringe) to the “mA” terminal on the ammeter. Attach another lead in series from the “common” terminal to the center “wiper” terminal on the potentiometer. Finally, attach a third lead from the left side terminal on the potentiometer to the anode of your fuel cell (the upper syringe). The circuit should like that shown below.
5. With the potentiometer turned completely counter-clockwise, the voltage displayed on the first multimeter should be close to the value that you recorded for the open circuit voltage. The current displayed on the second multimeter should be very small (most likely too small to measure).
6. While observing the voltage on the first multimeter, slowly turn the potentiometer clockwise. As current begins to flow through the potentiometer the voltage supplied by the fuel cell will drop. Continue adjusting the potentiometer until you are able to able to measure a small current (~0.01 mA). Record both the voltage and the current in your data table.
7. Adjust the potentiometer until the voltage is decreased by about 0.050 volts. Record the new voltage and current. Repeat this process until the voltage of the fuel cell drops to ~0.075 Volts.
8. Remove the potentiometer from the circuit and attach the lead from the left potentiometer terminal directly to the lead from the center “wiper” terminal. The voltage supplied by the fuel cell should drop to ~ 0.000 V and the current should increase up to a maximum. Record the voltage and the short circuit current, Isc, in your data table.
Data Analysis:
1. Using Excel, create a spreadsheet containing columns of Voltage (in Volts) and Current (in mA) for your fuel cell. Add a third column, and calculate the Instantaneous Power (in mW) for each of the data points you collected.
2. Plot a graph of Voltage vs. Current for your fuel cell. Add a secondary y-axis and plot the Instantaneous Power versus Current on the same graph.
3. Send your instructor an email, and attach an electronic copy of your completed spreadsheet to go along with the hardcopy that you submit with this lab report.
Post lab - Answer the following questions in your lab notebook.
1. Did your measured open circuit voltage match the cell potential you predicted in the prelab? If not, what might explain this difference?
2. Remove the batteries from a common portable electrical device (personal stereo, digital camera, electric razor, laptop computer, etc.) and determine its voltage requirement. How many fuel cells of the type you constructed would be required to provide this voltage? How would you connect them together?
3. What was the maximum power provided by your fuel cell? Look up the power rating for another common household appliance (toaster, hair dryer, etc.) How many fuel cells of the type you created would be required to operate this appliance?
4. What feature(s), characteristic(s), or properties most limited the maximum power of your fuel cell? Describe 3 ways in which you could alter the fuel cell design to improve the power output.
5. The following I-V curve was obtained for a one square cm photovoltaic “solar” cell purchased from a local electronics supplier. How does the I-V curve for your fuel cell compare with that for the photovoltaic cell? Which provides the highest maximum power? Discuss any other differences that you observe.
6. Assume that you are an R&D engineer for a manufacturer of portable laptop computers. The companies marketing team has identified lithium-ion batteries, photovoltaic cells, and fuel-cells as possible power supplies for this application. Write a short paragraph discussing the pros and cons of each technology and issues that would need to be addressed for each. Which technology would you supportfor applications in the near term (1-3 years), long term (8-10 years), and distant future (>15 years)?