Fuel Cells used hydrogen to produce electricity. There were no emissions but water.
(Note: The background section of this page uses most of the same words as the hydrogen page of Terra Futura to save time.)
Hydrogen was discovered in 1766 by British scientist Henry Cavendish. James Dewar, another British scientist, later created liquid hydrogen in 1898. During the early 20th century, hydrogen was used as fuel for zeppelins. Hydrogen was the lifting gas. In those days, airplanes were noisy, so airships were much more common. Then, in 1937, tragedy struck. The German airship, Hindenburg, burned up as it approached Lakehurst, New Jersey. Hydrogen would never be used as lifting gas again. However, hydrogen was very important in the development of quantum mechanics. And it would continue to be used as fuel.
Hydrogen had many uses. It could even be used as fuel in internal combustion engines. In Homogeneous Charge Compression Ignition (HCCI), no nitrogen oxides were produced. However, when using gasoline, there was a higher amount of carbon monoxide and hydrocarbons. Thus, hydrogen was the preferred fuel in HCCI vehicles. Hydrogen had a lot more specific energy than gasoline despite its lower energy density. But the most common use of hydrogen as fuel was in Fuel Cells.
Tech Level: 10
In the third decade of the 21st century, hydrogen fuel cells became commonplace in cars. There was one problem. Before the 2020s, hydrogen had to be pressurized so that it could be pumped into a car. What got hydrogen fuel cells to become commonplace was chicken feathers. This allowed the hydrogen tanks to store more hydrogen with less pressure before being used in fuel cells. With this, hydrogen fuel cell cars became commonplace. There were no emissions but water. One problem remained. How would hydrogen be created? The solution was artificial photosynthesis.