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 Ca3-2yNa2yCo4-yMoyO9 (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 (rho) and thermal conductivities (kappa) are decreased compared to the pristine compound. The maximum thermoelectric power factor (PF = S-2/rho) and dimensionless figure of merit (zT) obtained for the y = 0.025 sample at 1000 K temperature are similar to 3.2 x 10(-4 )W m(-1) K-2 and 0.27, respectively. The zT value for Ca2.95Na0.05Co3.975M0.025O9 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.