Fuel cells work in the opposite way of electrolysis. In electrolysis of water, hydrogen and oxygen are generated when you supply electricity to the cell. In the fuel cell, when you have supply of hydrogen and oxygen, electricity is generated. The combustion of hydrogen is just water vapor and this is non-polluting.
The proton exchange membrane fuel cell (PEMFC) uses one of the simplest reactions of any fuel cell. The anode, which is the negative post of the fuel cell, has several jobs:
• It conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit. It has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst.
• The cathode, is the positive post of the fuel cell. This has channels etched into it that distribute the oxygen to the surface of the catalyst. It also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water.
• The electrolyte is the proton exchange membrane. This specially treated material, which looks something like ordinary kitchen plastic wrap, only conducts positively charged ions. The membrane blocks electrons.
• The catalyst is a special material that facilitates the reaction of oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM.
There are other types of fuel-cell technologies being developed for possible commercial use:
• Alkaline fuel cell (AFC), has been used in the U.S. space program since the 1960s. The AFC is very susceptible to contamination, so it requires pure hydrogen and oxygen. It is very expensive, so this type of fuel cell is unlikely to be commercialized.
• Phosphoric-acid fuel cell (PAFC), has potential for use in small stationary power-generation systems. It operates at a higher temperature than PEM fuel cells, so it has a longer warm-up time. This makes it unsuitable for use in cars.
• Solid oxide fuel cell (SOFC), are best suited for large-scale stationary power generators that could provide electricity for factories or towns. This type of fuel cell operates at very high temperatures (around 1,832 F, 1,000 C). This high temperature makes reliability a problem, but it also has an advantage: The steam produced by the fuel cell can be channeled into turbines to generate more electricity. This improves the overall efficiency of the system.
• Molten carbonate fuel cell (MCFC), are also best suited for large stationary power generators. They operate at 1,112 F (600 C), so they also generate steam that can be used to generate more power. They have a lower operating temperature than the SOFC, which means they don't need such exotic materials. This makes the design a little less expensive.
Fuel-cell-powered vehicles should start to replace gas and diesel-engine vehicles in the near future. A fuel-cell vehicle will be very similar to an electric vehicle but with a fuel cell and reformer instead of batteries. You will probably fill your fuel-cell vehicle up with methanol, although some companies are working on gasoline reformers. Other companies hope to do away with the reformer completely by designing advanced storage devices for hydrogen.