Improved High-Temperature Thermoelectric Properties of Dual-Doped Ca₃Co₄O₉

Layered structured Ca₃Co₄O₉ has displayed great potential for thermoelectric (TE) renewable energy applications, as it is nontoxic and contains abundantly available constituent elements. In this work, we study the crystal structure and high-temperature TE properties of Ca_3-2 Na₂ Co_4- Mo O₉ (0 ≤ y ≤ 0.10) polycrystalline materials. Powder X-ray diffraction (XRD) analysis shows that all samples are single-phase samples and without any noticeable amount of the secondary phase. X-ray photoelectron spectroscopic (XPS) measurements depict the presence of a mixture of Co³⁺ and Co⁴⁺ valence states in these materials. The Seebeck coefficient (S) of dual-doped materials is significantly enhanced, and electrical resistivities (ρ) and thermal conductivities (κ) are decreased compared to the pristine compound. The maximum thermoelectric power factor (PF = S ²/ρ) and dimensionless figure of merit (zT) obtained for the y = 0.025 sample at 1000 K temperature are ∼3.2 × 10^-4 W m^-1 K^-2 and 0.27, respectively. The zT value for Ca_2.95Na_0.05Co_3.975Mo_0.025O₉ is about 2.5 times higher than that of the parent Ca₃Co₄O₉ compound. These results demonstrate that dual doping of Na and Mo cations is a promising strategy for improving the high-temperature thermoelectric properties of Ca₃Co₄O₉.