no boiling required just something beyond absolute zero, of course, higher temperature means more heat content so more collision per second, and more wattage. a steaming cup of coffee could be used to provide a small electrical current, even room temperature water as well
This sort of thing in homologue could also be used to harvest the energy of brownian motion, and perhaps even RMS velocity of gas phase matter:
https://www.technologyreview.com/s/427140/graphene-battery-t...
so far ive not seen anything thats will couple with quantum vacuum [zero point energy] that is purported to exist.