I've heard of cars powered by liquid nitrogen, since it boils at ambient temperature (even if the weather is below freezing), you can use it to expand a piston. The energy comes from the ambient environment.

Thermal equilibrium with outer space is about 4 Kelvin (due to background radiation). That's really cold. If we could make a large radiator exposed to open sky at night could we use it to produce liquid nitrogen? Not exactly, because the air itself can emit radiation. This is the greenhouse effect.

But would it be possible to coat the radiator with quantum dots that preferentially emit thermal radiation at a frequency not absorbed by the atmosphere?

If it works, this would be a completely passive system capable of producing fuel, and might also make cryonics and systems using superconductors more economical, such as long distance power transfer and grid scale energy storage in magnetic fields.