Hydrogen Fuel cells are the answer.
In spite of decades of R&D and pilot projects the market has
decisively said otherwise for most applications, particularly automobiles.
Current fueling stations can be changed over easily
Takes a
lot of capital to do so since hydrogen tankage, compressors, and ancillary equipment are very different beasts than gasoline and diesel equivalents. Given the immense pressures involved and the difficulty containing incredibly-volatile hydrogen this equipment is markedly more expensive and requires considerable safety engineering.
the engines are really just a small conversion from gas/diesel engines
While one can convert typical internal combustion engine designs to run on hydrogen, the issue is limitations on storage due to hydrogen's inherently low density.
Compress hydrogen to a modest 700 bar and you're going to be lucky to realize some 50 miles or so of range with an ICE. The immense pressure demands shapes in the form of spheres or domed cylinders which are appreciably less space-efficient within the confines of automobile packaging than the arbitrary shapes of liquid fuel tanks with negligible pressure requirements.
Step up the game with spicier cryogenic liquid hydrogen and you can get something close to standard ICE vehicle range. But you
for sure can't just fill it up and park it because that -243C / 30K liquid hydrogen desperately wishes to again be a gas due to the ~+270K positive temperature gradient that surrounds it and it's not possible to build a tank that's both strong enough to gracefully handle thermal expansion of liquid H2 trying to become gaseous H2 while also being light enough for mobility. So you're going to have to manage it something like a rocket on a launch pad - consume it promptly, allow it to boil off, or defuel before temperatures rise above a threshold.
Thus industry's focus on low-temperature hydrogen fuel cells which are considerably more efficient than an ICE and can achieve acceptable range with compressed hydrogen. But those applications have been slow-developing and mostly limited to larger vehicles where
packaging isn't as big of a concern - trains, busses, drayage/OTR trucks.
