Microstructure and high-temperature thermoelectric properties of CuO and NiO co-substituted NaCo2O4

The partial substitution of Cu for Co in Na(Co1−zNiz)2O4 (0 ≤ z ≤0.15) led to an increase in the electrical conductivity (σ) mainly because of an increase in the hole concentration, density, and grain size of sintered bodies. On the other hand, the partial substitution of Ni for Co in Na(Co1−yCuy)2O4 (0 ≤ y ≤ 0.15) gave rise to a decrease in the electrical conductivity mainly because of a decrease in the density and grain size. In particular, the Cu and/or Ni substitution led to a significant increase in the Seebeck coefficient (α). The power factor (σα2) was substantially improved by the partial co-substitution of Cu and Ni for Co in NaCo2O4. The highest value of power factor (3.27 × 10−3 Wm−1 K−2) was attained for Na(Co0.7Cu0.15Ni0.15)2O4 at 1073 K. It is concluded that the Cu and Ni co-substitution is effective for enhancing high-temperature thermoelectric properties.