Energy is required to compress the air. That probably comes from fossil fuels.
It requires a tremendous amount of energy to compress air....of all the equipment in my shop, I pay more for the air compressor than anything else. And that’s only at about 100 psi. A SCUBA tank requires a High Pressure Air Compressor (HPAC), anything in excess of 1000 psi, and would likely be a 5K psi compressor. Although it’s a cute idea, it would be far less efficient than good ole’ gasoline.