Hydrogen-powered cars are a promising innovation, but there are still plenty of hurdles ahead before the technology becomes fully sustainable, affordable, and popular.

One of the promising aspects of hydrogen is its abundance. There is a lot of hydrogen in our world. It’s comprised simply of one proton and one electron. In total, hydrogen accounts for an estimated 75 percent of the known universe. On top of this, the U.S. is creating more than 9 million tons of hydrogen per year.

Since 1839, automakers have been producing hydrogen fuel cells, taking in oxygen and hydrogen to power a car, emitting only clean water vapor through the process. Currently, the main form of production consists of natural gas steam reformation: using high temperatures and pressures to break natural gas into hyrdrogen and carbon monoxide molecules. Alternatively, hydrogen producers are relying on electrolysis: using electricity to split water into hydrogen atoms and oxygen. Both processes have some pretty major caveats though. Steam reformation produces carbon monoxide, which is toxic to humans. Electrolysis needs an energy source to power that process, usually relying on coal power. Once hydrogen is made, it has to be stored, cooled, and transported-all of which require more fossil fuels in our current mode of production.

Still, there’s reason to be optimistic. In the U.S., the government is partnering with private companies to build more hydrogen fuel stations for consumers. California has an ambitious goal of having 15 percent of all cars in the state be zero-emission vehicles by 2025. To accomplish that, the state needs more fueling stations and investment-both of which are in the works.