Active Diffusion Controlled Dual Stability in Thermoelectrics for Sustainable Heat Harvesting

Thermoelectric technology offers a promising pathway toward global sustainability by harvesting waste heat. However, long‐term stability is hindered by inevitable elemental diffusion, degrading both the thermoelectric junction and material properties, which prevents the realization of power generation applications. Here, dual and superior stability is achieved in high‐performance Mg3(Bi,Sb)2, surpassing prior studies that focus on either junction or material stability. By introducing an Mg layer at the junction, detrimental Mg diffusion is suppressed and compensate for Mg loss in the material, effectively stabilizing both junctions and materials for over 100 days. As a result, a thermoelectric module with 30‐day‐aged Mg3(Bi,Sb)2 is able to maintain an outstanding power density of 0.45 W cm−2 and remarkable conversion efficiency of 8.6%, demonstrating unprecedented stability. These findings provide new insights into thermoelectric junction engineering, shifting from interface optimization to comprehensive stabilization, advancing the practical viability of thermoelectric energy harvesting for renewable and waste heat applications.