Hydrogen is, at least in theory, a fuel free of carbon emissions. Cars that run on battery-like hydrogen fuel cells produce only pure water as their waste product. But hydrogen has downsides. Most of the hydrogen used today comes from splitting natural gas, which releases carbon. The lightweight gas is also difficult to transport.

Now researchers in China have come up with a way to produce hydrogen on-demand that addresses both issues. The technique, reported in the Journal of Renewable and Sustainable Energy, could lead to easy, efficient generation of hydrogen fuel onboard a fuel cell vehicle.

Hydrogen has been slow to take off as a fuel. The first fuel cell cars came on the market in 2015. Today there are only 12,000 around the world. Part of the reason behind the sluggishness of the hydrogen economy is that the highly reactive gas is costly and energy-intensive to make, and difficult to store.

To be a truly clean fuel, hydrogen needs to come from non-fossil fuel sources. It can be made from water using electrolysis, but that requires electrical energy, which today mainly comes from fossil fuels.

The research team used an alloy of gallium, indium, tin and bismuth. They placed the alloy along with a thin aluminum plate in a salt-water solution. As soon as the materials come in contact, bubbles of hydrogen rise up, which the researchers piped to a fuel cell. The hydrogen generation speeds up when the researchers increase the temperature and dilute the salt solution.

There’s still work to be done before this technique can find its way into fuel cell vehicles though. For practical use, the researchers write, it will be important to figure out a way to recycle and reuse the alloy. At the end of the hydrogen-production reaction, the salt solution can be filtered out to leave behind a mixture of the alloy and aluminum hydroxide. But separating the aluminum hydroxide is challenging and inefficient. They are now working on developing a better solution for recycling.

“The merit of this method is that it could realize real-time and on-demand hydrogen production,” said Jing Liu, a professor at the Chinese Academy of Sciences and Tsinghua University, in a press release. “It may offer a possibility for a green and sustainable energy era.”