As governments and industry attempt to curb greenhouse gas emissions to net zero, hydrogen and its potential as a clean fuel is once again reaching the headlines.
So what exactly is hydrogen energy and how could it be used as an alternative to fossil fuels?
Watch the BBC’s energy and environment analyst Roger Harrabin explain.
More on the climate summit:
The COP26 global climate summit in Glasgow in November is seen as crucial if climate change is to be brought under control. Almost 200 countries are being asked for their plans to cut emissions, and it could lead to major changes to our everyday lives.