What is it?
The concept is similar to hybrids: an electric motor would drive the car much of the time. In this case, the motor would be charged by something under the hood called a fuel-cell stack, which converts hydrogen and oxygen into electricity that flows to the battery. The on-board fuel would be hydrogen.
What's good about it?
Many things. Hydrogen is widely available, in natural gas and water, for instance. The only tailpipe emission is water. Pound for pound, hydrogen fuel has more inherent energy than gasoline, which could mean higher mileage: A prototype Honda fuel-cell vehicle gets the equivalent of nearly 70 miles per gallon.
What's bad about it?
While it can be extracted from water, the cheapest source of hydrogen is natural gas, an unrenewable hydrocarbon. There's no distribution system or standardized method of storage, which is crucial since hydrogen fuel is a gas that must be kept under high pressure.
Where would it be most useful?
Fuel cells make sense for most types of vehicles. One enduring challenge is "cold start"— the ability to power up at temps as low 30 below zero Fahrenheit—which means fuel cells are ill-suited for the coldest climates. That may be resolved by the time other technology matures.
How much will it cost?
If mass-produced and widely distributed like gasoline, the cost of hydrogen fuel could be equivalent to $2 per gallon or less. Plus, fuel cells are about three times as efficient as gas engines, which means better mileage. But building an infrastructure to deliver hydrogen would cost billions, which would certainly add to consumers' cost.
When's it coming?
Could still be 10 or 20 years away. There are major technical hurdles in terms of producing the fuel, distributing it widely, and storing it in cars.
What's taking so long?
Researchers are still searching for cost effective ways to produce the fuel, transport it, and store it in a car. An even bigger problem is building a hydrogen infrastructure comparable to a gas station on every corner, which would cost billions and require the unprecedented cooperation of automakers, energy companies, and the government.
Who's doing it?
Most of the big automakers have fuel-cell programs. GM has recruited ordinary consumers to test a fleet of 100 fuel cell vehicles on the east and west coasts. Honda plans to lease a fuel-cell car, for about $600 per month, to a few consumers in 2008. Others could announce similar programs.
Could it be a silver bullet?
Maybe. If the technology matures, costs fall, and hydrogen fuel becomes widely available, it would solve several problems: Hydrogen could come from renewable sources and generates no tailpipe emissions. And theoretically, it would be affordable—maybe even cheap.