Global warming is a hot topic. Scientists tell us that our hunger for energy may be heating up the planet, but we remain unconvinced. We're loath to accept the fact that our desire for mobility, is to a large extent, fueling the warming. Most of us are taken aback when we learn that an average car produces six tons of the greenhouse gas carbon dioxide annually. With over two hundred million vehicles on our nation’s highways, the amount of CO2 pumped into our atmosphere each year is staggering. While the automobile is certainly not the only source of man-made CO2, it definitely is a major contributor. It’s no wonder that both scientists and senators are anxious to find an emission-free vehicle.
Solutions to the problem are many and varied. At the heart of many of the schemes are non-polluting, renewable energy sources. Of all these sources, hydrogen is being touted as the fuel of the future. Hydrogen is both the simplest and most plentiful element in the universe. High in energy content, hydrogen produces virtually no pollution when burned. In fact, when hydrogen is combined with oxygen, only water and heat are produced. It is envisioned that hydrogen powered devices called fuel cells will allow the pollution-free production of electrical energy that may be used to power a car or even light your home.
A fuel cell is an electrochemical device that uses hydrogen and oxygen to create electricity. If pure hydrogen is used as fuel, the fuel cell emits only water and heat as byproducts. Fuel cells have the obvious advantages of not producing greenhouse gases that contribute to global warming and none of the air pollutants that create smog and health problems. Furthermore, fuel cells are significantly more efficient than power producing technologies that rely on fossil fuels such as oil, coal, and natural gas.
The fuel cell’s operation is based on the reversibility of an electrochemical process, first witnessed in 1839 by British physicist William Grove. While performing electrolysis experiments, Grove observed that following the separation of water into hydrogen and oxygen by electrolysis, a potential difference remained across the platinum electrodes he was using even after the power supply had been removed.
Despite its simplicity, abundance, and cleanliness, hydrogen doesn't usually occur as a gas on Earth—it is most often combined with other elements. However, hydrogen can be made by separating it from chemical compounds by applying heat, a process known as "reforming." Currently, most hydrogen is made this way from natural gas. Through the process of electrolysis, an electrical current can also be used to separate water into its components of oxygen and hydrogen.
But therein lies the rub. To produce the energy needed for either reforming or electrolysis, conventional means of power production must be used. But this negates the reason for switching to fuel cells in the first place. The hope is that power from environmentally friendly sources such as wind, hydro, tidal, geothermal, or solar power may be used to produce hydrogen.
This is already happening in Iceland where hydro and geothermal power is used to electrolyze water. There you can find the world’s first hydrogen filling station sitting on the side of a highway at Reykjavik city limits. In September, 2003 the first city buses fueled up on hydrogen produced at the station through electrolysis.v
Will hydrogen be the fuel of the future? Only time will tell.
The following experiments will help you understand how sunlight may be used to split water into hydrogen and oxygen and how a fuel cell produces electrical energy through the recombination of these gases. Although all the experiments relate to the operation of a fuel cell car, they may be used as stand alone experiments and demonstrations. We hope you and your students enjoy this interactive introduction to hydrogen power.
1. Solar Energy
Key Concept: Radiant energy may be transformed into other forms
Example 1: The Radiometer
A radiometer is a device that converts radiant energy into rotational kinetic energy. It consists of four vanes, silvered on one side and blackened on the other. When light falls on the vanes they rotate. Contrary to popular belief, this motion is not produced by radiation pressure. So what does produce the rotation? (Hint: Which ways do the vanes rotate?)
The vanes are mounted in a glass bulb that is evacuated to a low pressure. The black surfaces absorb more radiant energy, and hence are warmer than the silvered surfaces. Molecules colliding with the black surfaces gain more energy than those rebounding from the silvered surfaces. The more energetic molecules transfer more momentum to the vanes than do the molecules bouncing off the silvered sides. The overall effect: a net push in the direction of the silver side of the vanes.
Place a radiometer in sunlight or light from a lamp. What do you observe? Which way are the vanes rotating? Can you get the vanes to move in the opposite direction?
Example 2: The Photocell
Photocells (solar cells) transform light energy into electrical energy. They are most effective in bright light that shines directly onto the cell. Students will experiment with different intensities and angles of incidence, and observe how these different arrangements affect the reading on a galvanometer and the motion of a motor.
Connect the photocell’s two leads to an electrical meter such as a galvanometer or milliammeter. Hold the front side of the photocell toward the sun or light from a bright light bulb. What do you observe? (If nothing happens, reverse the leads on the meter). If you are using a light bulb as a light source, vary the distance between the light bulb and the photocell. What happens as the distance between the photocell and the light bulb increases? Decreases? For a given distance from the photocell, how does tilting the photocell affect the meter reading? How do you explain this?
Remove the photocell’s leads from the meter and connect them to a small motor. Describe what happens when the photocell is illuminated by a light bulb. If the motor shaft doesn’t begin to spin what might you do to remedy the situation? Once you have the motor operating, move the photocell closer, then farther away from the light bulb. What effect does distance between light source and photocell have on the motor speed? Now tilt the photocell so that it doesn’t point directly at the light source. Describe how this affects the operation of the motor.
2. Electrical Current
Key Concept: An electrical current consists of moving charges.
When charged particles such as electrons move, they create a magnetic field. This field can be detected with a magnetic compass. Students will arrange a current-carrying wire near a compass to show that when the circuit is complete, a magnetic field results and shows evidence of moving charges. This concept is important in understanding the importance of removing electrons from the hydrogen atoms to create an electric current. Note: This is a short circuit, and the wire will become hot after only a short time. Remind students to disconnect the wire from the battery after they make their observations.
Place a length of insulated wire over the needle of a compass making certain that the wire is touching the cover of the compass and that it is aligned parallel to the needle. Watch the needle carefully as you attach the ends of the wire to the positive and negative terminals of a standard 1.5 v cell. What do you observe?
Repeat the procedure, but this time reverse the leads to the battery. What happens this time? What hypothesis can you formulate to explain your observations? Cite evidence that something is moving in the wire in a particular direction.
Key Concept: Electrolysis is the process of decomposing a substance (e.g. water) into positive and negative ions using electricity.
It’s a simple matter to use electrolysis to produce hydrogen gas. The following activities guide students through the basic electrolysis of water, yielding gaseous hydrogen and chlorine. The primary reaction of interest is:
2H2O(l) + 2Cl-(aq) ⇌ Cl2(g) + H2(g) + 2OH-(aq)
Sodium ions and hydroxide ions combine in solution to form sodium hydroxide. (Fig.2)
While gas is evolving at the two aluminum electrodes, submerge a test tube in the tray of water (see Fig.2). After completely filling the test tube with water, carefully pull the closed end of the test tube out of the water while keeping the opening of the test tube submerged. Move the upwards-pointing test tube over the cathode. The cathode is the electrode connected to the power supply’s negative terminal.
Try to catch all the bubbles rising from the cathode in the test tube. Wait until the test tube is filled about one-third with gas. Lift the test tube out of the water while keeping the opening pointed downwards. Now quickly and carefully move the test tube towards a lighted candle and hold it over the flame. What do you observe? (The hydrogen will explode, producing a "pop.") Safety notes: the test for hydrogen should be done by an adult. Chlorine gas is dangerous if inhaled. Perform electrolysis in a well-ventilated area.
Students will then power their electrolysis apparatus with a photocell, as might be done in a solar powered fuel cell car or at a solar powered hydrogen fuel station.
Electrolysis using a battery or power supply (Fig.1)
First dissolve a teaspoon of salt in a tank or tray of water. Attach two 1cm x 6cm strips of aluminum foil to the ends of two connecting wires. (Fig.1) Now place the two aluminum electrodes in the saltwater solution. Make certain that the two strips are not touching. Connect the other ends of wires to a 3-v to 6-v DC power supply. (This may be accomplished with either two to four 1.5 v batteries connected in series or a DC power supply.)
Watch the two electrodes closely once the connections to the power source have been made. What do you observe? What is the source of the gases that are forming on the electrodes? Do you know what these gases are? Are the gases produced at the same rate at each electrode?
Electrolysis using a photocell (Fig.3)
Remove the two connecting wires from the power source. Connect the two leads from the photocell to the two connecting wires. Illuminate the photocell with light from the sun or a lamp. What do you observe? How does the use of the photocell affect the rate of gas production? How does the intensity of the light striking the photocell affect the rate of gas production?
4. Constructing a Simple Fuel Cell
Key Concept: A reverse electrolysis reaction may be used to produce a potential difference.
By introducing a few new materials, you can build a simple fuel cell. The process begins with electrolysis, as before. Hydrogen gas collects on the electrodes and the voltage source is removed. The platinum serves as a catalyst allowing the recombination of hydrogen and chlorine into hydrochloric acid. The process, known as heterogeneous catalysis, is the basis of this type of fuel cell. The resulting potential difference (voltage) can deflect a voltmeter or light an LED for a few seconds.
A form of fuel cell, referred to as a gas battery, may be easily constructed using two nickel electrodes or two platinum electrodes, salt water, a beaker or glass, a DC 4.5 to 6 volt power supply, a voltmeter, and some connecting wires with alligator clips.
The voltmeter should be set to the 0 – 20 DC range before completing the circuit. When the circuit is closed, electrolysis will begin and the voltmeter will read between 4 and 6 v. Shortly after completing the circuit, you should observe the production of gas at the electrodes. Hydrogen gas will collect at one electrode, chlorine gas at the other. In fact, you may be able to smell the chlorine when you are close to the beaker.
After gas covers both electrodes, carefully remove the power supply from the circuit. The goal is to retain as many bubbles on the electrodes as possible. When the circuit is once again closed, the voltmeter will still indicate a reading of between 1 and 1.5-v. With the battery removed from the circuit, a reaction now occurs at the surfaces of the platinum electrodes that produces a potential difference. How long does a measurable potential difference exist?
Repeat the experiment using other types of electrodes. Compare the magnitude and duration of the potential difference produced by the different metals.
Obtain a red Light Emitting Diode (LED) and repeat the electrolysis with new salt solution and, if needed, new electrodes. After gas covers both electrodes, remove the power supply from the circuit and replace it with a red LED. Does the LED light? If so, for how long? Try lighting LEDs of different colors. You have to connect two or more fuel cells in series to light the LED. (A note from Mark: To do this activity you'll need to hook up two or more fuel cells in series to light the LED. After your students do the first activity, have them get together in groups to make the series circuit.)
5. Gas Laws
Key Concept: If the pressure is kept constant, the volume of a gas increases, or decreases, in direct proportion to the increase, or decrease, in absolute temperature. This basic law of gases is known as Charles’ Law.
Teacher Demonstration: Decreasing the volume of a gas with liquid nitrogen (CAUTION! Please use proper safety precautions when using liquid nitrogen!)
Place an inflated balloon in a container of liquid nitrogen. A decrease in molecular kinetic energy causes the pressure exerted by the air in the balloon to decrease dramatically. As a result, the balloon shrivels up into what resembles a wrinkled pancake. Try placing additional balloons into the container. You’ll be amazed at the number of super-cooled balloons you can fit into a small volume.
Using forceps, remove a balloon from the liquid nitrogen. If the balloon is translucent, hold it up to a bright light source. You will see your condensed breath sloshing around at the bottom of the balloon. You must make this observation immediately after removing the balloon from the liquid nitrogen, for the liquid air quickly evaporates.
Once again, use forceps to remove the balloons one at a time from the liquid nitrogen and place them on the table. Heat transferred from the table to the air in the balloon causes the pressure inside the balloon to increase. Watching the balloons return to their original shape and size is great fun. Uneven thawing of the rubber will sometimes produce a tear, so be prepared for an occasional pop!
The decrease in volume with temperature will allow the storage of a great deal of air in a small volume. This principle is used to store and transport a variety of gases, including hydrogen for use in fuel cells.
Key Concept: Efficiency is the ratio of energy output to energy input, stated as a percentage. The efficiency of a machine is always less than 100% because of friction and other factors.
This is a simple experiment for measuring efficiencies of a mechanical system and a simple electrical system. Students can analyze the energy losses in these systems and predict what factors might affect the efficiency of a fuel cell.
Example 1: The efficiency of a simple machine
Set up an inclined plane by placing one end of a board on a stack of books (see figure above). Slowly pull a dynamics cart up the incline using a Newton spring scale. Observe and record the reading on the scale as well as the length of inclined plane. Calculate the work done in pulling the cart up the incline by finding the product of the force and distance. This will yield the energy input.
To find the energy output, weigh the cart by suspending it from the scale. Also measure the height of the upper end of the incline. The work done lifting the cart directly to the top of the incline is found by multiplying the weight of the cart by the height of the incline. This equals the useful energy output.
The efficiency of the incline plane is found by dividing the useful energy output by the energy input and multiplying by 100%:
efficiency = useful energy output / energy input x100%
Example 2: The efficiency of the Genecon Hand-Crank Generator
Connect the leads of two Genecon generators together. As you turn the handle of one of the Genecons, the handle of the second Genecon will also turn. In this arrangement, the first Genecon is functioning as a generator, the second Genecon, a motor.
Disconnect the wires and line up the handles on the two Genecons. For example, they both could point straight up. After reconnecting the wires, turn the handle of one Genecon 10 times while a second person simultaneously counts the number of times the second handle turns. The ratio of the number of turns of the “motor” to the number of turns of the “generator” times 100% equals the efficiency of the two-Genecon system.
Key Concept: A catalyst is a substance that increases the rate of a chemical reaction without being used up or changed itself. Catalysts are used in a wide range of chemical processes. These include metallurgy, petroleum cracking, and organic synthesis. The catalytic converter, perhaps the best known example of a catalytic device, has greatly reduced the harmful emissions associated with the combustion of gasoline. In the fuel cell, catalysts aid in the combination of hydrogen and oxygen.
Examples of the use of catalysts to speed up chemical reactions abound. The two demonstrations that follow were selected for their simplicity.
Place a sugar cube in a clothespin or lab forceps. Insert the sugar cube in a candle flame. Observe that the sugar cube browns and melts, but does not catch fire. Now rub a tiny bit of wood ash on the sugar and once again hold the sugar cube in the flame. It will immediately flame up and continue to burn.
The ash serves as a catalyst. An analysis of the post-combustion remains would indicate that the ash remains unchanged. The ash is needed to initiate combustion, but does not take part in the reaction.
Hydrogen peroxide is an unstable chemical compound that readily breaks down into water and oxygen. The reaction is accelerated by exposure to light. For that reason, hydrogen peroxide is stored in opaque bottles.
The break down of hydrogen peroxide can be greatly accelerated by using manganese dioxide as a catalyst. Have students watch a sample of hydrogen peroxide as the catalyst is added to the liquid. The increase in oxygen production will be dramatic. Also have students note that the manganese dioxide does not appear to change. This of course is the defining characteristic of a catalyst.
How the Fuel Cell Transfers Energy
Hydrogen flows through channels in flow field plates to the anode where the platinum catalyst promotes its separation into protons and electrons. Hydrogen can be supplied to a fuel cell directly or may be obtained from natural gas, methanol or petroleum using a fuel processor, which converts the hydrocarbons into hydrogen and carbon dioxide through a catalytic chemical reaction.
Membrane Electrode Assembly
Each membrane electrode assembly consists of two electrodes (the anode and the cathode) with a very thin layer of catalyst, bonded to either side of a proton exchange membrane.
Air flows through the channels in flow field plates to the cathode. The hydrogen protons that migrate through the proton exchange membrane combine with oxygen in air and electrons returning from the external circuit to form pure water and heat. The air stream also removes the water created as a by-product of the electrochemical process.
Flow Field Plates
Gases (hydrogen and air) are supplied to the electrodes of the membrane electrode assembly through channels formed in flow field plates.
Fuel Cell Stack
To obtain the desired amount of electrical power, individual fuel cells are combined to form a fuel cell stack. Increasing the number of cells in a stack increases the voltage, while increasing the surface area of the cells increases the current.
Mark Sulek joined Ford Motor Company in 1989 as a Research Scientist. In 1993 Mark moved to the Alternative Power Source Technology Department where he supervised and contributed to the installation of the first fuel cell test facility at Ford Motor Company. The hydrogen fueling station was the first in North America to fuel Hydrogen Fuel Cell vehicles with either liquid or gaseous hydrogen. He played a major role in the design studies that eventually lead to the development of the drivable P2000 fuel cell vehicle and was awarded the Henry Ford Technical Achievement Award. Mark’s current position involves bringing Ford’s fuel cell message to the public through ride and drive events. Mark’s help in providing detail, background, and student activities to this issue of CoolStuff was tremendously valuable. Our thanks go to him and his associates who actually worked out these student activities while also holding down their regular duties at Ford.