Tunable Thermoelectric Properties of Cu2Se/Graphene for Improved Figure of Merit

Copper selenide (Cu2Se) is a promising thermoelectric material for waste heat recovery due to its intrinsically low thermal conductivity. However, its figure of merit (ZT) must be further improved for practical applications. This study explores the addition of trace graphene (0.05-0.2 wt.%) to Cu2Se to tune its thermoelectric properties and enhance ZT. High-purity Cu, Se, and graphene nanoplatelets (GNP) were prepared and mixed through ball milling under an inert atmosphere, followed by hot compaction to yield dense discs of Cu2Se/Gr nanocomposites. Ball milling was done in two steps (Cu2Se for 24 hrs., Cu2Se + Gr for 15 mins) to ensure graphene’s structural integrity is maintained. Microstructural characterization included density measurements, X-ray diffraction (XRD) at room and high temperatures, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Thermoelectric properties were evaluated through four-probe Seebeck coefficient and electrical conductivity measurements, laser flash analysis (LFA), Differential Scanning Calorimetry (DSC), and Hall effect measurements. Graphene incorporation retained a high Seebeck coefficient of ~320 V/K for the 0.15% Gr sample at 560C, while electrical conductivity was decreased from 460 S/cm in the pristine sample to ~166 S/cm for the 0.05% Gr sample, and ~38 S/cm for the 0.15% Gr and 0.2% Gr samples at the same temperature. Cu2Se/Gr nanocomposites reached a power factor (PF) of ~11 W/cmK2 at 420C. Graphene reduced the thermal conductivity from 1.7 W/mK to 1.1 W/mK at 205 °C, leading to a peak in ZT of ~0.5 at 380C. A tradeoff between Seebeck coefficient and thermal conductivity, with electrical conductivity, was observed. Future studies should focus on enhancing the electrical conductivity of Cu2Se/Gr nanocomposites to enhance ZT further.