Yep. Energy density is the key. That is the reason that alternate energy sources will have a hard time displacing the internal combustion engine.
Hydrogen production from biomass through the gasification process can run a internal combustion engine now, and back then, go ask those people during WWII who used gascifiers every day during the war.
The USA produced gascifiers for Europe during WWII, it’s not new techknowlogy, been around for awhile.