Green hydrogen
Hydrogen as a fuel has a lot of energy per kilo. Unfortunately, since it is the lightest gas, it does not have a lot of energy per litre. Carrying it around in vehicles requires high pressure, liquefaction, or adsorption onto substrates. However it does have a very big advantage that the only end-product of energy production, i.e ‘tailpipe gas’, is water. It is flexible: it can be burnt, or produce electricity directly in a fuel cell. Also, its main rival technology is the electric battery, and these also do not store as much energy per kilo. For these reasons it has garnered a lot of attention as a solution for mobile energy. It may well be needed for industry, shipping and possibly heavy lorries.
Hydrogen is not found unbound anywhere in nature, it is too reactive (although this has recently come into question). Therefore it must be produced from something else.