Exploring the possibility of enhancing the figure-of-merit ( $>$ 2) of Na$_{0.74}$CoO$_{2}$: A combined experimental and theoretical study

Search of new thermoelectric (TE) materials with high figure-of-merit (ZT) is always inspired the researcher in TE field. Here, we present a combined experimental and theoretical study of TE properties on Na0.74CoO2 compound in high temperature region. The experimental Seebeck coefficient (S) is found to vary from 64 to 118 μV/K in the temperature range 300−620 K. The observed value of thermal conductivity (κ) is ~2.2 W/m-K at 300 K. In the temperature region 300−430 K, the value of κ increases up to ~2.6 W/m-K and then decreases slowly till 620 K with the corresponding value of ~2.4 W/m-K. We have also carried out the theoretical calculations and the best matching between experimental and calculated values of transport properties are observed in spin-polarized calculation within DFT + U by chosen U = 4 eV. The maximum calculated value of ZT is found to be ~0.67 at 1200 K for p-type conduction. Our computational study suggests that the possibility of n-type behaviour of the compound which can lead to a large value of ZT at higher temperature region. Electron doping of ~5.1 × 1020 cm−3 is expected to give rise the high ZT value of ~2.7 at 1200 K. Using these temperature dependent ZT values, we have calculated the maximum possible values of efficiency (η) of thermoelectric generator (TEG) made by p and n-type Na0.74CoO2. The present study suggests that one can get the efficiency of a TE cell as high as ~11% when the cold and hot end temperature are fixed at 300 and 1200 K, respectively. Such high values of ZT and efficiency suggest that Na0.74CoO2 can be used as a potential candidate for high temperature TE applications.