Radioisotope Thermoelectric Generators (RTGs)
RTGs produce electricity from the difference in temperature between its radioactive heat source and the environment via thermocouples using the Seebeck effect. Plutonium-238 has been used many times as the radioactive heat source. It produces heat while it decays. While useful due to their compact size and reliablity, RTGs are limited by their poor efficiency, constantly decreasing power (as the heat source decays) and low mass-to-power ratio.
Barker, Theodore R., Wilfred L. Kershaw, George S. Stivers, and Jack L. Thomas. "Radioisotope thermoelectric generator." U.S. Patent 3,615,869, issued October 26, 1971.