Nanocarbon-induced 3d–π hybridization for optimizing thermoelectric performance in Ca2.96Co4O9 composites

Abstract

Cobalt-based oxides as excellent p-type thermoelectric materials have attracted widespread attention due to their outstanding high-temperature stability and broad operating temperatures. However, the low electrical conductivity is a limiting factor for the thermoelectric performance of Ca₃Co₄O₉. Herein, the effect of nanocarbons on charge carriers in Ca_2.96Co₄O₉-based composites was considered. The incorporation of nanocarbons into the Ca_2.96Co₄O₉ matrix led to an increase in charge carrier concentration from 5.2 × 10¹⁹ to 9.4 × 10¹⁹ cm⁻³ at room temperature. Furthermore, the electronic interaction mechanism between nanocarbons and the Ca_2.96Co₄O₉ matrix was governed by 3d–π orbital hybridization, which concurrently enhanced the electrical conductivity from 99.7 to 109.1 S cm⁻¹ and raised the Seebeck coefficient from 182.5 to 204.8 µV K⁻¹ at 873 K. Nanocarbons simultaneously achieved a reduction in lattice thermal conductivity at low temperatures, which was attributed to enhanced phonon scattering at both point defects and grain boundaries. As a result, a large ZT value of 0.28 for Ca_2.96Co₄O₉–rGO composites was obtained at 873 K. The hybridization engineering strategy for optimizing electrical properties provided a viable route to improve the thermoelectric properties of oxide composites.