[McKay (OP)]

Sorry about the vague title - the system doesnt allow it to be very long.
The expanded question: It is possible to collect radiowaves and microwaves with a simple antenna and a diode to produce electric current, right? What if we lived in an such a cold environment where the background thermal radiation was microwaves, instead of infrared - I see no reason (other than than it being a perpetual motion of the second kind) why the background microwaves couldnt be harvested for electricity. Please explain.
In extension - what, fundamentally, disallows for a similar collection of infrared? Other than there being no diodes that work fast enough for the higher frequency of infrared, but that seems more like technical limit, not a fundamental.

[Kryptid]

Under the right circumstances, yes. It probably wouldn't be very efficient, however. It's possible to generate an electric current across temperature gradients with thermocouples. If you had a region with a given temperature caused by the presence of infrared or microwave radiation and then another region that was colder, you could use a thermocouple there.

[chiralSPO]

Other than there being no diodes that work fast enough for the higher frequency of infrared

There are plenty of diodes or photovoltaics that absorb IR and turn it into useable electricity. The problem is that each IR photon carries relatively little energy compared to visible light, and a significant amount of the energy can just be wasted as heat...

[evan_au]

By manipulating the crystal structure of silicon solar cells, it is possible to make them more sensitive to visible or infra-red radiation. Layering these on top of each other collects energy from both visible and infra-red parts of the spectrum.
By using rare-earth ions, infra-red radiation can be converted into visible light; see:
https://en.wikipedia.org/wiki/Solar_cell#Upconversion_and_Downconversion

Or you could just use the infra-red light directly:
https://en.wikipedia.org/wiki/Solar_power_tower

[Yahya]

the electricity is tiny because radio waves spread with expansion..