Thermoelectric for Bulk Materials

Energy conservation is crucial in the modern world, as invaluable as human life. Our energy dependence has consistently grown, making it a vital resource. Despite technological advancements in energy conversion, energy consumption has increased dramatically. If this trend continues, future energy demands will be impossible to meet, especially with a growing global population. Furthermore, combustion engines only utilize about 30% of their total energy, wasting the remaining 70% as heat released into the atmosphere, which contributes to global warming. The technology that converts the untapped heat energy to electricity is thermoelectricity which is based on Seebeck effect. Our research primarily focuses on developing thermoelectric materials, including topological insulators, transition metal dichalcogenides (TMDCs), layered chalcogenides, Zintl alloys, silicide-based alloys, perovskite oxides, and polymers, for room to mid-temperature applications.

Fig 1. (a) Crystal structure of Fe-substituted Cu2SnS3 (b) scheme of mass fluctuation (c) band diagram (d) phonon scattering and (e) Umklapp scattering

In this regard, we have fabricated the less toxic Fe-substituted Cu₂SnS₃ and the highest power factor of ∼514 μW/mK2 via band modulation and simultaneously reduced thermal conductivity by mass fluctuation scattering, which boosts the figure of merit of ~0.5 at 753 K (Appl. Phys. Lett. 126, 074104 (2025))  is obtained in Cu₂Sn₀.₇Fe₀.₃S₃ sample. Further, we have synthesized p-type Bi₂Se₃ with Ga and Mn substitution demonstrating the decoupling of Seebeck coefficient and electrical conductivity through band modification and enhanced phonon scattering resulting in low thermal conductivity of 0.5 W·m⁻¹·K⁻¹ at 303 K (Chem. Commun., 2023,59, 8119-8122). We have prepared n-type polycrystalline Bi₂₋ₓGaₓGe₂ₓ/₃Se₃ via vacuum melting approach and obtained a high zT of 0.7 at 303 K. The synergistic effect of multi-scale phonon scattering mechanisms through the dissemination of strain field raised from various defects and tuning the band structure resulted in high thermoelectric performance (Small 2024).

Fig 2. (a) Crystal structure of MnSi/CNF (b) Hole scattering in MoS2 (c) Scheme of energy filtering effect (d) Relation between S and κ (e) ZT of Bi1.95Ga0.05Ge0.033Se3 sample.

Further, we have developed a Mg₃₋ₓZnₓSb₂ solid solution and obtained low lattice thermal conductivity of 0.46 W·m⁻¹·K⁻¹ via combined scattering effect of dislocations, lattice strain, and interfaces. A high zT of 0.25 at 753 K was obtained from the synergistic combination of low thermal conductivity and increased power factor (Appl. Phys. Lett. 124, 031601 (2024)). In addition, we have prepared HMS/CNF hybrid composites via vacuum melting, with controlled carrier concentration and electrical conductivity due to the interfacial energy filtering effect and declined lattice thermal conductivity owing to interfaces, grain boundaries, and dislocations leading to the high magnitude of strain resulted in high zT of 0.64 at 803 K (Appl. Phys. Lett. 125, 171603 (2024)). We have fabricated hole (Sr) and electron (Hf) doped LaCoO₃ by solid-state synthesis and obtained a low thermal conductivity of 0.5 W·m⁻¹·K⁻¹. The spin state blockade observed in the electrical resistivity and low lattice thermal conductivity revealed that spin state transition drives the thermoelectric response in Mott insulator LaCoO₃(Appl. Phys. Lett. 125, 034101 (2024)).

In addition, we have achieved ultra-low lattice thermal conductivity of 0.1 W·m⁻¹·K⁻¹ in perovskite structured strongly correlated electron system LaCo₀.₉₅In₀.₀₅O₃ which originates from large mass fluctuation and carrier-mediated lattice softening (Phys. Chem. Chem. Phys., 2023,25, 12914-12922). We have achieved a distinct ambipolar Seebeck coefficient of -20 μV·K⁻¹ with MoS₂ as the hot junction, and 69.5 μV·K⁻¹ with MoS₂-MoO₂ as the hot junction, in the radially transformed growth of MoS₂​ to MoS₂-MoO₂ using the two-zone chemical vapor transport (CVT) technique (J. Phys. Chem. Lett. 2024, 15, 49, 12060–12067). Recently, we fabricated the MoS₂-rGO hybrid via layer-by-layer stacking through hydrothermal method, where π–π interactions at the interface of MoS₂-rGO scatter low charge carriers without affecting mobility, simultaneously decoupling the Seebeck coefficient and electrical conductivity and obtained high power factor of 15.5 nW·m⁻¹·K⁻² at 325 K (ACS Applied Energy Materials 2025).

Wearable Thermoelectric

With the increasing demand for sustainable and autonomous energy sources, wearable thermoelectric generators have emerged as a promising solution for powering wearable electronics. These devices utilize the Seebeck effect, wherein a temperature gradient across a thermoelectric material induces an electrical voltage, enabling the direct conversion of body heat or ambient thermal energy into electricity. Unlike conventional batteries, wearable thermoelectric generators offer continuous, maintenance-free energy generation, eliminating the need for frequent recharging or replacement.  They have an extremely long lifespan because they are free of any moving parts and are not influenced by motion or environmental factors. Our laboratory is engaged in the development of materials for flexible fabric-based wearable thermoelectric generators designed for near-room temperature energy conversion. The primary focus of our research is addressing the challenges related to traditional thermoelectric modules, including material scarcity and toxicity, device rigidity and bulkiness, and the complexities of manufacturing processes. In our research, we have excluded the highly toxic tellurium-based materials and low-stable polymer-based materials by replacing them with several inorganic materials like MoS₂, MnO₂, MoO₃, Ag₂Se, Ag₂S, and so.

In recognition of the necessity for large-scale production in real-time applications, we have selected in-situ hydrothermal/solvothermal growth of the thermoelectric material on conductive fabrics as our preferred approach over advanced coating techniques. This binder-free growth technique enhances the material's charge transport capabilities and facilitates the fabrication of highly wearable-based thermoelectric generators (WTEGs). The initial stage of research involves optimizing the growth of the thermoelectric material on conductive fabric, conducting structural and morphological analyses, and adjusting the thermoelectric properties of the materials through various strategies such as doping, band engineering, energy filtering effects, and many more.

Fig 3. Device structure and fabrication

Further the real-time power conversion ability of the material is analyzed by fabricating a wearable TEG. In this regard, we have investigated the in-plane TE properties of MnO₂/MoS₂/CF and achieved a superior power factor of 548.7 nW·m⁻¹·K⁻² which is 49.5 % higher than that of the pristine MoS₂/CF. In addition, we demonstrated that the 4-pair modules systematically improved the device performance, and the open circuit voltage and output power generated for WTEG comprising of 4 -n/p pairs is measured to be 1.2 mV and 1 nW was achieved via interface-induced energy filtering effect. Here. first-ever report of silver fabric based textrodes leading to enhanced output power (1 nW) at ΔT = 20 K. (Carbon 2024, 218, 118609).

 Similarly, we have grown MoS₂ nanostructures on carbon fabric (CF) by binder-free one-step hydrothermal method. The design and operating condition of the MoS₂/MoO₃ device is optimized, and a maximum open circuit voltage of 6.4 mV is obtained from the fabricated device. (J. Alloys and Compounds 2024, 1002, 175168).

Fig 4. The thermally oxidised MoS2/MoO3 Nanostructure on Carbon fabric for Wearable energy harvesting

Further, we have fabricated TE legs using the Ag₂Se-CF (p-type) and Ag₂S-CF (n-type), and the fabricated legs exhibited an output voltage of ∼20 mV to ∼86.65 mV at a temperature gradient (ΔT) of 3–8 K (J. Colloid Interface Sci. 2023 436-447). Recently, we have prepared and investigated the flexible Ag₂₋ₓSnₓS on carbon fabric for enhanced wearable thermoelectric generators via interfacial engineering. The WTEG device fabricated using 3-pair modules produced an output voltage ranging from 0.09 to 1.5 mV across a temperature gradient of 3 to 8 K. (J. Colloid Interface Sci. 2025 422-434). Additionally, we have been focusing on strategies to improve the output of the fabricated device by optimizing the design and structure of W-TEG for the irresistible development of wearable thermoelectrics.