A P.E.M. fuel cell has two sides separated by a thin substance or membrane. Hydrogen gas is forced through one side where it comes in contact with a reactive material containing the metal platinum.

　　The membrane separates the electrons（电子) from the protons（质子) in the hydrogen atoms（原子). The protons pass through it to the other side of the fuel cell. But the electrons are captured to do work: like powering a motor.

　　Oxygen from the air is forced into the other side of the fuel cell. There, the gas meets the protons that have passed through the membrane. They combine to form water and heat.

　　A single fuel cell does not produce a lot of electricity. But when many fuel cells are combined, they can produce enough electricity to power a vehicle. The product of the chemical reaction that powers fuel cells is water. This makes fuel cells a very clean technology.

　　But fuel cell cars have been slow to develop because of many technical problems that have to be solved. For example, it is unclear how long the membranes in P.E.M. fuel cells will last.

　　Also, fuel cells need water for their chemical reactions. They must be designed to start easily in low temperatures and in dry climates. And smaller, less costly fuel cells must be designed before they can truly take the place of gasoline engines.

　　There are more models of fuel cell vehicles being tested than ever before. The threat of climate change and the high cost of oil has increased interest in these vehicles that do not cause pollution. But car buyers should not expect to see fuel cell cars at their local dealer any time soon. Automakers say they will not build many fuel cell cars until hydrogen fueling and service stations are widely available. And that will cost billions of dollars and years of effort.

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