Thermoelectric Properties and Performance of n-Type and p-Type Graphite Intercalation Compounds

Abstract

n-Type alkali metal-graphite intercalation compounds (GICs) and p-type metal chloride-GICs were prepared from commercially available graphite sheets, namely PGS^® and GRAFOIL^®. Their thermoelectric properties, electrical conductivity, thermal conductivity, and Seebeck coefficient were measured, and their thermoelectric performance was estimated in terms of the figure of merit and power factor. The electrical conductivity (10³ S cm⁻¹ to 10⁴ S cm⁻¹) and thermal conductivity (40 W m⁻¹ K⁻¹ to 200 W m⁻¹ K⁻¹) of these GICs are much higher than those of other, conventional thermoelectric materials, whereas their absolute Seebeck coefficients (±30 μV K⁻¹) are lower. Therefore, although the figures of merit of the GICs are somewhat lower (∼10⁵ K⁻¹) than those of other thermoelectric materials, their power factors (∼10³ W m⁻¹ K⁻²) are sufficiently high. The thermoelectric properties of these GICs mainly depend on the host graphite type and slightly on the intercalated species. However, the Seebeck coefficient is independent of both, and the thermoelectric performance of the GICs is strongly governed by their high electrical conductivity. The power factors of almost all the GICs prepared from PGS were greater than 10⁻³ W m⁻¹ K⁻², which is a critical threshold value for use of thermoelectric materials in practical applications. Furthermore, the dilute K-GIC with stage-7 structure had a large Seebeck coefficient (−58 μV Κ⁻¹), which improved the power factor to more than 10⁻² W m⁻¹ K⁻². Considering the advantages of GICs, this study confirms their significant potential as thermoelectric materials.