While the promise of FCVs is tantalizing, the challenges leave some critics doubting they will ever become practical.

Hydrogen production. Hydrogen can be made from water, but it takes a lot of electricity. And a big question is how the electricity will be generated. Using conventional powerplants is appealing in California and other states that generate electricity from relatively clean energy sources such as natural gas, and nuclear, water, and wind power. But in much of the country, electricity is generated by burning coal, which produces more carbon dioxide overall than burning gasoline.

The government is funding the development of renewable sources such as solar power. But its limitations will most likely leave this dream decades away.

Currently, hydrogen is primarily produced from natural gas. Some experts say the country already has the capacity to fuel one million FCVs using this method. Because of the efficiency of FCVs compared with gas engines, it would save energy overall, according to a 2005 study by Air Improvement Resource, an engineering company, the Argonne National Laboratory, and GM.

But natural gas is not a renewable resource, and the U.S. currently imports 16 percent of its supply. And natural-gas prices have doubled since 2002, which affects the price of hydrogen. Without subsidies, hydrogen today would sell for about $15 to $20 for the equivalent of a gallon of gasoline, according to Ed Kiczek, senior manager at Air Products and Chemicals, an industrial gas supplier. But with large volumes, he says, prices should drop to about $3.50 to $6. The government is targeting a price of about $2 to $3.

But even at $6, the cost per mile looks competitive; 10 cents vs. 13 cents for an equivalent-size gasoline-powered car. Honda claims its new FCX, for example, gets the equivalent of about 61 mpg.

Range and storage issues. Hydrogen is the lightest element, and so it carries very little energy. At a pressure of about 5,000 psi, even with a large hydrogen tank, a typical FCV has a driving range of less than 300 miles. GM and other automakers are working to extend the range by using tanks that hold hydrogen at 10,000 psi.

Few hydrogen stations. Today there are only 44 hydrogen stations in the U.S., according to the National Hydrogen Association, an industry trade group. And only a few are open to the public.

To kick-start the hydrogen infrastructure, California, Illinois, New York, and Texas, plus British Columbia, have launched initiatives to build "hydrogen highways" that would support FCVs. They will initially be centered in major metro areas and will extend out from there.

Joan Ogden, director of the Sustainable Transportation Energy Pathways project at UC Davis, estimates it will cost $12 billion to $20 billion between 2012 and 2025 to build enough stations along major interstate highways to allow FCVs to travel across the country.

Energy companies are optimistic that once universal codes and standards are adopted, stations can be built quickly. "Our intention is that we'd have a sufficient number of stations to support a rollout of fuel-cell cars between now and 2020," says David Austgen, a general manager at Shell Hydrogen.

Vehicle costs. Experts say the price of FCVs would have to be comparable with that of conventional cars before consumers would buy them. Bill Reinert, national manager of Toyota's Advanced Technologies Group, expects that improvements in materials and production will bring costs in line.

An example of that is Honda's second-generation FCX, which will cost less than half to produce than the first one did, says Chris Naughton, a Honda representative. Still, the first FCVs may sell at luxury prices.

Safety concerns. Some people worry about driving a car with a tank of pressurized, flammable hydrogen gas. But experts claim that with proper care, hydrogen could be at least as safe as gasoline. In testing, the storage tanks are subjected to drop tests, vibration, fires, and bullets, and so far there have been no red flags, says Sunita Satyapal, a team leader for hydrogen storage at the Department of Energy.

Hydrogen "diffuses to the point that it's hard to have a safety issue," says Keith Wipke, a senior engineer at the National Renewable Energy Laboratory. For example, hydrogen won't pool underneath a car after an accident, as does gasoline.

Still, leaks can occur. And if hydrogen gas accumulates in a closed space, it could be ignited. Some experts have proposed that hydrogen detectors be developed to warn people of leaks.