For years, automakers, environmentalists and scientists have promoted hydrogen fuel cells as a breakthrough technology that will eventually enable people to travel without puffing pollutants and greenhouse gases into the atmosphere.

Fuel cells operate by setting off a chemical reaction between hydrogen and oxygen in the air. When they bond, an electric charge and a small amount of water are created. A few hundred cells stacked together generate enough electricity to power a car motor.

And compared with other types of electric cars, which must be recharged by drawing power from some sort of electricity, hydrogen fuel-cell cars do not need recharging and have a longer cruising range. (Not that hydrogen, which is sold as a highly compressed gas provided by industrial suppliers, is easy for any home chemist to produce in sufficient, useful quantities.)

But the hydrogen future has been slow to arrive because of a frustrating predicament: Automakers had little incentive to produce fuel-cell vehicles as long as there were no hydrogen stations to fill them up; energy companies saw no sense in opening stations if there were no cars on the market.

So it took financial backing from the State of California, as well as Toyota, Honda and other automakers, to spur development of hydrogen fueling stations. Twenty are now open to the public, and three more go into service this month. The number should rise to 50 by the end of next year, according to California officials.

California also encourages zero-emission cars by letting them use the swift, high-occupancy vehicle lanes on the famously crowded freeways. The state also offers tax rebates to consumers who buy or lease hydrogen-powered cars, which in the case of the Mirai is worth $5,000 — in addition to a $8,000 federal tax credit.