Abundant, cheap and clean-burning, hydrogen has long been described as the fuel of the future.
That future has never quite materialised, however, due to hydrogen's disadvantages. It's difficult to transport, it can make metal brittle and it's 20 times more explosive than petrol.
But in recent years, "green hydrogen" — hydrogen made without fossil fuels — has been identified as the clean energy source that could help bring the world to net-zero emissions.
Billions of dollars of investment capital and taxpayer support has flowed into the industry, and company share prices have soared.
This has accelerated in recent months, driven by the rising adoption of zero-emission vehicles, a deadline set by many countries to go carbon-free by 2050 and US President Joe Biden's support for clean energy.
The European Union plans to scale up renewable hydrogen projects and invest a cumulative amount of 470 billion euros ($740 billion) by 2050.