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Text from PDF Page: 002lyst, and a cathode catalyst. In a hydrogen fuel cell, hydrogen is fed into the cell and flows over the anode catalyst. When hydrogen mol- ecules hit the anode catalyst, the H2 molecule separates into two hydrogen ions (that is, two protons) and two electrons by the following chemical reaction: +- h2➞2h +2e The hydrogen ions and the electrons part ways at this point. The electrons flow through the wire toward the other side of the fuel cell. Meanwhile, the hydrogen ions are headed toward the other side of the fuel cell, but they get there a different way. The hydro- gen ions pass through the membrane to get to the other side. The membrane is made of a special polymer called Nafion that allows posi- tively charged ions to pass through. oh (adsorbed) + e- + h+ → h2o The most important step in this whole process happens when the electrons flow through the wire from one side of the fuel cell to the other. Remember that an electrical cur- rent is just electrons flowing through a wire. Just like the flow of water in a river can be used to turn a water wheel, the flow of mov- ing electrons can be used to do real work, like turning an electric motor in a car. Some problems with hydrogen For all its efficiency, hydrogen fuel has some serious drawbacks. Hydrogen is a gas, and gases are hard to store. One solution could be a car with a pressurized tank to hold hydrogen, and fuel stations would need pres- More H2 can be made from the CO by the “water-gas shift” reaction: co+h2o➞co2 +h2 but reaction produces carbon dioxide, the same greenhouse gas we’re trying not to make by using hydrogen as a fuel in the first place, plus you also have to worry about traces of CO remaining in the H2, since CO can poison the Pt catalyst used in the fuel cell. Because making hydrogen is still a tricky proposition, many scientists are looking into other fuels that might work just as well or bet- ter in fuel cells. Two alcohols, methanol and ethanol, are showing promise. methanol Methanol is the simplest alcohol, with a formula of CH3OH. It has already been used as a fuel in ordinary internal combustion engines. For example, methanol was once the race car fuel used in the Indianapolis 500, until the race organizers switched to ethanol a few years ago. Methanol is a liquid, so it is easier to store and transport than hydrogen is. You can store it and sell it in the same tanks and pumps that gas stations already have. everything we need to store, ship, and use methanol is already in place, except the cars. even the cars are in the works, as Daimler- Chrysler recently drove an experimental car powered by a methanol fuel cell across the united States. In a fuel cell, methanol would react with oxygen by the following chemical reaction: 2ch3oh+3o2 ➞4h2o+2co2 But wait! Isn’t this reaction producing carbon dioxide? Again, one advantage of alternative fuels is the opportunity to reduce the amount of carbon dioxide we put into the atmosphere. However, it turns out there is more to the story than meets the eye. Methanol is normally made from meth- ane (CH4) in a complicated process with several steps. However, you can also make methanol using a fuel cell. How does this work? Normally, when you run a methanol fuel cell, methanol and oxygen react to make water and carbon dioxide, and in the process an electrical current is generated. However, you can make the whole process run in reverse. By simply running electrical current through the cell, you can make the cell convert carbon dioxide and water vapor from the air thomas Bradley and reid thomas go through the procedure of starting up the fuel cell aircraft during a test flight at the Atlanta dragway. surized tanks, too. Trans- porting the hydrogen fuel isn’t easy, either. Since gases aren’t very dense, a truck wouldn’t be able to haul very much of it at a time. An even bigger problem is making the hydrogen. You don’t often find hydrogen gas on earth. You have to separate hydrogen from some hydrogen- containing compound. For example, you can separate it from water by passing an electric cur- rent through the water in When the hydrogen ions and the elec- trons both get to the other side of the fuel cell, they don’t just reform a hydrogen molecule again. At the cathode catalyst, oxygen from the air gets involved. The cathode catalyst separates oxygen molecules into oxygen atoms. As electrons arrive at the cathode cata- lyst, they increase the negative charge on the oxygen atom, inducing it to pick up hydrogen ions that are arriving via the membrane: When hydrogen ions reach the cathode catalyst, they react with the oxide ions to form water molecules: 1 Chemmatters, DECEMBER 2007 a process called electrolysis (electro referring to electricity, lysis meaning “to break”). 2h2o ➞ 2h2 + o2 As this requires electricity, you have to generate that electricity somehow. Because most of our electricity comes from coal, that doesn’t really help us cut down our use of fossil fuels. Burning coal generates carbon dioxide just like burning gasoline does. Some scientists have proposed sepa- rating hydrogen from methane, the main compound in natural gas, using the following “steam reforming” chemical process: ch4 +h2o➞co+3h2 www.acs.org/chemmatters o2 (g) → 2 o(adsorbed) o(adsorbed) + e- + h+ → oh(adsorbed) COuRTeSY OF GeORGIA TeCH: GARY Meek
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