Radioisotope thermoelectric generators: Evolution, materials, and future prospects

Radioisotope thermoelectric generators (RTGs) are essential for space exploration, providing reliable, long-term power in environments where solar energy is impractical. This review examines the evolution of RTGs, from the early Systems for Nuclear Auxiliary Power (SNAP) program (1961) to the latest Multi-Mission RTG (MMRTG) and the enhanced MMRTG (eMMRTG) systems. Additionally, it also explores segmentation techniques aimed at optimizing thermoelectric (TE) performance in next-generation RTGs and discusses the potential of miniature RTGs for terrestrial applications. A key focus of this review is the selection of isotopic fuel and advancements in TE materials and devices. Plutonium-238 (Pu-238) remains the primary isotope used in RTGs due to its high power density and long half-life. The development of TE materials has progressed from well-established compounds such as PbTe, (AgSbTe2)0.15(GeTe)0.85 (TAGS), and SiGe—used in existing RTGs—to emerging materials including skutterudites (SKD), Mg3Sb2-Mg3Bi2 alloys, and half-Heusler (HH) compounds. This review also highlights strategies for enhancing thermoelectric performance and improving device fabrication. Despite their proven reliability, RTGs continue to face the challenge of low energy conversion efficiency. This limitation has driven ongoing research into advanced TE materials and technologies, with the goal of improving performance for both space and terrestrial applications.