Thermoelectric (TE) devices can be used reversely as a heat pump for refrigeration (heat management) and mostly oxide materials are used in these devices. Metal oxide-based TE materials possess several advantages compared to metal chalcogenides. These materials can be employed, since they are environment-friendly and cost-effective. TE performance can be easily tuned and this can be achieved through composition and structure. In the present work, Na 0.77 CoO 2 /nano Co 3 O 4 composites are chosen. Although Na x CoO 2 possesses superior TE performance to other oxide-based materials, its application has been limited due to its instability in air. It decomposes into the insulating Co(OH) 2 and/or CoCO 3 by absorbing moisture and/or CO 2 from the ambient environment. Upon heating, both Co(OH) 2 and CoCO 3 decompose further into the insulating Co 3 O 4 and CoO. Therefore, it is important to explore the chemical stability of Na x CoO 2 . This study analyses the reasons for the chemical instability of Na x CoO 2 without impairing its TE properties. Na 0.77 CoO 2 was prepared by the solid-state synthesis method and the single phase was observed in XRD. Again XRD was taken after 15 days, to analyse the stability of the prepared sample. The peaks corresponding to Na 0 . 77 CoO 2 disappeared. The reasons for this instability were analysed and possible solutions are suggested.