Thermoelectric materials are evolving continuously by improving their power conversion performance through three engineering strategies: nanoengineering (nano-grain structure or nanocomposite), defect engineering (doping or intrinsic defects), and structural engineering (alloying, texturing, or all-scale hierarchical structure). Nano compositing is the most uncomplicated and straightforward strategy to develop thermoelectric materials. Thermoelectric composites are known for enhanced power conversion performance via interfacial engineering and intensified mechanical, structural, and thermal stabilities. Thermoelectric-based waste heat recovery requires safer and cheaper materials to replace expensive and toxic Te and Pb-based state-of-the-art thermoelectric materials. Recently, copper-based chalcogenides, especially sulfides, have attracted significant attention due to their inexpensive, earth-abundance, non-toxicity, and good thermoelectric performance. Cu₃SbS₄ is one of such kind, with p-type conductivity and high phase stability for potential medium-temperature applications. However, its low structural complexion of tetragonal crystal structure must be tuned out to reduce thermal conductivity.

In the first study, to develop high-efficiency Cu₃SbS₄, we incorporate InSb nanoinclusions via high-energy ball milling followed by the hot-press densification method. We incorporate InSb nanoinclusions to lower thermal conductivity via phonon scattering while increasing the Seebeck coefficient via carrier energy filtering. The maximum figure-of-merit of ~0.4 at 623 K is obtained in Cu₃SbS₄-3 mol.% InSb nanocomposite, which is ~140% higher than pure Cu₃SbS₄. Both thermal and mechanical stability are improved by boundary hardening and dispersion strengthening. Thus, we deliver a facile nanostructured Cu₃SbS₄ with ex-situ added InSb nanoinclusions as a highly efficient, eco-friendly, phase, thermal, and mechanically stable material for next-generation thermoelectric devices.

In the subsequent study, we studied the semiconductive carbon black nano-inclusion’s influence on Cu₃SbS₄ thermoelectric performance. Adding amorphous carbon nano-inclusions in Cu₃SbS₄ causes a reduction in the thermal conductivity by phonon scattering and an improvement in the Seebeck coefficient by carrier energy filtering mechanisms. The maximum figure of merit of 0.51 is obtained for a 3 mol.% carbon nano-inclusion sample at 623 K. Additionally, enhanced thermal stability and mechanical stability (hardness) with increased carbon nano-inclusion concentration is observed.

In this following study, the effect of a multi-walled carbon nanotube (MWCNT) on the thermoelectric parameters of Cu₃SbS₄ is studied. Adding the optimum MWCNT nano-inclusions in Cu₃SbS₄ simultaneously reduced the lattice thermal conductivity by strong phonon scattering and enhanced the Seebeck coefficient by a carrier energy filtering mechanism. This synergistic optimization produced the maximum dimensionless figure of merit (zT) of 0.43 for 3 mol.% MWCNT nano-inclusion composite sample is ~70% higher than the pristine Cu₃SbS₄ at 623 K. Additionally, enhancement in mechanical stability with increasing nano-inclusion concentration is observed. In conclusion, the nanocomposite approach has been proven to be a promising way to improve the thermoelectric properties of Cu₃SbS₄-based materials. The TE performance results of nanocomposites strongly agree that the nanostructuring-assisted lower thermal conductivity and the enhanced Seebeck coefficient than the micron-sized pristine matrix, effectively improving the overall thermoelectric performance. Grain boundary hardening and dispersion strengthening mechanisms help improve mechanical and thermal stability in the composites. Moreover, our detailed studies demonstrate that adding nano-inclusions in Cu₃SbS₄ can produce efficient, non-toxic, and inexpensive state-of-the-art thermoelectric devices.

Famatinite (Cu₃SbS₄, p-type) and chalcopyrite (CuFeS₂, n-type) are well-recognized minerals for their earth-abundance, non-toxicity, good stability, and medium-temperature thermoelectric performance. Therefore, we fabricated a conventional segmented thermoelectric generator (TEG) made of famatinite and chalcopyrite legs for the first time. We used facile and inexpensive ball milling followed by a hot-pressing technique to synthesize high-purity, polycrystalline, and fine-grained materials. Thermoelectric characterization demonstrated the maximum figure of merit (zT) of 0.32 in Cu₃SbS₄ and 0.35 in CuFeS₂ materials at 723 K. The computational simulation of constructing the device verified uniform thermal gradients in the legs and maximum power at the hot-side temperature of 723 K. The fabricated 14-leg TEG device demonstrated the maximum output power of 5 mW and power density of 50 μWcm^-2 at the hot-side temperature of 723 K. Our fabricated TEG device also highlighted strong performance stability and durability at working intermediate temperature. For the first time, we demonstrated an inexpensive and non-toxic segmented TEG made of copper-based sulfide materials. We opened a new direction to fabricate facile TEGs for medium-temperature applications.