Improved High-Temperature Thermoelectric Properties of Dual-Doped Ca 3 Co 4 O 9 .

Layered structured Ca 3 Co 4 O 9 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 y Na 2 y Co 4- y Mo y O 9 (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 Co 3+ and Co 4+ 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 2.95 Na 0.05 Co 3.975 Mo 0.025 O 9 is about 2.5 times higher than that of the parent Ca 3 Co 4 O 9 compound. These results demonstrate that dual doping of Na and Mo cations is a promising strategy for improving the high-temperature thermoelectric properties of Ca 3 Co 4 O 9 .