The idea is not new, the first motor based in compressed air as energy source was invented in the past century or even before. The principle is that any gas could fill in theory space, then like combustion engines it could create piston movement at the cylinder.
That's in general how current models achieve power, although not a lot because the energy density of compressed air is not the highest available. It's enough for transportation in some circumstances.
Only to give an approximation of energy density, compressed air at 4,500 psi is currently below the new developments in supercapacitors (almost with instant electricity charging) using graphene technology.
Anyway, once the air is compressed, the vehicle produces zero pollution because simply exchanges the energy done compressing the air for movement.
In recent analysis it was proposed that this technology can't compare with battery powered electric vehicles, and that was said even without the knowledge of new developments in supercapacitors, surpassing the energy density of lead-acid batteries.
But who knows, any idea that could help in the task of resolving air toxicity and atmospheric damage should be more than welcome!