Investigating the Stability of Cu₂Se Superionic Thermoelectric Material in Air Atmosphere

Copper selenide (Cu₂Se) has garnered significant attention as an exceptional thermoelectric material due to its high thermoelectric figure of merit (ZT values > 2). This remarkable efficiency makes it a strong candidate for various applications. However, the practical deployment of thermoelectrics often requires operation in an oxygen-containing atmosphere, which poses a significant challenge for Cu₂Se due to its environmental instability. This work investigates the environmental behavior of high-purity Cu₂Se, which was synthesized via a direct high-temperature reaction and spark plasma sintering (SPS). Our Temperature-Programmed Oxidation (TPO) studies determined that the onset of oxidation occurs at a temperature as low as 623 K. Further analysis using SEM-EDS confirmed the formation of copper oxides, Cu₂O and CuO. Critically, thermogravimetric analysis (TGA) revealed that the SeO₂ formation and sublimation process is an equally profound degradation mechanism, alongside copper oxidation, particularly within the optimal 673-973 K temperature range. Complementary XRD studies of samples annealed in air underscore this severe material degradation, which is especially devastating between 873 and 973 K. Ironically, this is the precise temperature window where Cu₂Se's highest ZT values have been reported. Our findings demonstrate that the direct application of Cu₂Se in air is impractical, highlighting the urgent need for developing robust protective layers to unlock its full potential.