Except the companies, including General Motors, Honda, Toyota, Daimler and Hyundai, persisted.

After many years and billions of dollars of research and development, hydrogen cars are headed to the showrooms.

Hyundai has been leasing the hydrogen-powered Tucson sport utility, which it describes as the world’s first mass-produced fuel cell car, since June, for a $2,999 down payment, and $499 a month. (That includes the hydrogen. A lease on a gas-powered Tucson is about half as much.) This week, Toyota is introducing a sedan called Mirai, which means “future” in Japanese.

“It’s a no-brainer that I think the next evolution is to go to fuel-cell based technologies,” said Nihar Patel, the vice president for North American business strategy at Toyota, at a conference here last week.

The Mirai will go on sale in California this year for $57,500 — cheaper than the Tesla Model S.

California is spending millions of dollars to build hydrogen fueling stations, aiming to increase the network from nine today to 50 by the end of next year, mostly around Los Angeles and the San Francisco Bay Area. Japan and Germany, two other early markets for hydrogen cars, are building a similar number of stations.

“We really believe that we’re at a turning point here,” Mr. Patel said.

The combustion of one gallon of gasoline releases almost 20 pounds of carbon dioxide. In 2012, some 1.8 billion tons of carbon dioxide were discharged by cars and trucks in the United States, or more than a quarter of the nation’s greenhouse gas emissions. Concerns about climate change are intensifying discussions about alternatives to gasoline and diesel engines.