Electric generator

Is it possible to build an electric generator no bigger then a grain of rice that can produce 5volts or more?

Sure:
1. A tiny thermalpile (just add heat)
2. Piezo electric (needs mechanical stimulous)
3. A tiny inductor that harvests power from ambient EMF
4. A tiny inductor next to a tiny magnet (agitate the inductor, the magnet or both)
5. Tiny vile containing acid with two tiny probes of dissimular metal (limited power capacity)
6. Tiny atomic battery (might be health concerns, as shielding might not be possible)
7. Two or three SMD LEDs in series, exposed to light.
8. A tiny solar array (challenging to attach electrodes to silicon that small).

But, then, you didn't mention any power requirements or longevity or even if AC or DC is required.

Of course.. Tiny vile containing acid with two tiny probes of dissimular metal. But, then, you didn't mention any power requirements or longevity or even if AC or DC is required. 

Tiny vile containing acid with two tiny probes of dissimular metal. But, then, you didn't mention any power requirements or longevity or even if AC or DC is required. mybkexperience

the tiny vile acid containing having two two tiny probes dissimular metal. but didnt metion any power requirements or longevity or some else AC and DcCommercial kitchen hood installation Los Angeles