<i>In Operando</i> X-ray Diffraction Study of Thermoelectric Cu₂Se

Cu2Se is a mixed ionic–electronic conductor with outstanding thermoelectric properties originally envisioned for space missions. Applications were discontinued due to material instability, where elemental Cu grows at the electrode interfaces during operation. Here, we show that when Cu2Se is operating in air, an oxide surface layer forms in addition to the deposition of elemental Cu. In operando X-ray scattering and electrical resistivity measurements quantify Cu+ migration through the thermoelectric leg from the refinement of atomic occupancies and phase composition analysis. Maximum entropy electron density analysis provides experimental evidence that actual Cu+ migration pathways under thermal and electrical gradients differ from predictions under equilibrium conditions. The presence of O2 is found to cause additional ion-migration toward the surface due to the formation of Cu2O, which quantitatively can be related to the changing stoichiometry of the material during operation. Analysis of the thermoelectric properties after the operando stability test shows no significant deterioration despite the formation of elemental Cu and Cu2O. Thermoelectric properties are typically measured in vacuum or inert atmosphere, but it is important to also characterize thermoelectric materials in oxygen containing atmospheres relevant for applications. The study provides unique atomistic insight into ion diffusion and materials degradation under thermoelectric operating conditions.