“Green Hydrogen” is made by using electricity to split water into hydrogen and oxygen. There’re no carbon emissions in that process, but to be truly “green” the electricity must come from a carbon free source like wind, solar, nuclear, or hydroelectric.
The process of electricity to hydrogen to compressed hydrogen to fuel cell to electricity is about half as energy efficient as electricity to li ion battery to electricity. As a form of electricity storage green hydrogen is significantly less efficient than batteries.
Green hydrogen only makes sense as a fuel in situations where batteries are not feasible.
And right now making green hydrogen at all does not make sense because if you build a new low carbon source of electricity it will make a larger impact if you use it to displace fossil fuel based electricity generation rather than using it to create green hydrogen.