>>16724595like the piezoelectric effect, there is a thermoelectric effect, and in a similar manner to a piezo, not only does a strain/temperature gradient cause a potential to develop across an object, so too does an applied voltage cause a strain/thermal gradient to develop
in a radioisotope generator, you wrap a chunk of plutonium that's red hot from its own radioactivity and wrap it in thermoelectric material with heat sinks and radiators on the outside to provide a cold reservoir, since the bigger the thermal gradient the bigger the potential difference