Studying the Seebeck coefficient and exploring the possibility of enhancing ZT upto 1.8 for NaCo$_2$O$_4$ in high temperature region

Abstract: Here, we have studied the temperature dependent Seebeck coefficient (S) of the NaCo$_2$O$_4$ (NCO) by using experimental and computational methods. The range of experimentally obtained S is $\sim$55 to 103 $\mu$V/K in the temperature range of 300-600 K, which confirms the p-type behaviour of NCO. The electronic structure of this compound is obtained via DFT+U formalism. The band dispersion and partial density of states confirms the magnetic and half metallic nature. Furthermore, in the transport properties, the obtained S using a U = 4 eV gives the best match with experimental data. The temperature and chemical potential dependent S$^{2}$$\sigma$$/$$\tau$ is calculated using the obtained electronic transport properties, in which the maximum value obtained for p(n)-type doping is $\sim$22(61)$\times$10$^1$$^4$ $\mu$WK$^{-2}$cm$^{-1}$s$^{-1}$. The possibility of enhancing the ZT is identified, and it is calculated in temperature range 300-1200 K. The maximum calculated value of ZT is 0.64 for p-type and 1.8 for n-type doping at 1200 K. The calculated carrier concentration obtained for p(n) type doping at 1200 K is $\sim$1.17 (1.6)$\times$10$^2$$^2$ cm$^-$$^3$. This study suggests that the careful doping of p and n type can enhance the applicability of this compound in thermoelectric for high temperature application.