Synergistic Improvement in Thermoelectric Performance by Forming Environmentally Benign Transition Metal Oxide Composites with Graphite

In pursuit of developing low-cost, environmentally friendly, thermally stable, production-scalable materials for high-temperature thermoelectric power generation, in this paper, we have posited a strategy of synthesizing composite of simple transition metal oxide with graphite. Among the transition metal oxides, TiO2 is considered as one of the promising n-type thermoelectric materials. Although TiO2 exhibits very high Seebeck coefficient, it suffers from poor electrical conductivity, resulting in a poor thermoelectric figure of merit, ZT. In this work, we have synthesized TiO2 nanocomposites with varying graphite (G) concentration using spark plasma sintering (SPS). We have recorded about 1300% improvement in electrical conductivity of TiO2 due to graphite incorporation, which has been further explained with the help of the percolation model. Electrons are expected to percolate through the three-dimensional conductive channel made of graphite flakes in an insulator-like TiO2 matrix. Moreover, the reduced grain size along with homogeneously distributed graphite also aid in lowering the overall thermal conductivity by suppressing the phonon transport of heat. Synergistic improvement in electrical and thermal transport has allowed us to achieve a ZT of 0.12 at 1000 K that is 140% larger than that of pristine TiO2. Our strategy of nanocomposite formation for high temperature thermoelectric applications employing inexpensive and naturally occurring raw materials such as TiO2 and graphite further opens up the possibility of designing low-cost thermoelectric power generator for recycling high-grade waste heat.