This research is devoted to the fabrication of polyvinyl alcohol (PVOH) based n-type thermoelectric composites with innovative hybrid bismuth selenide-multiwalled carbon nanotube (Bi2Se3-MWCNT) fillers for application in flexible thermoelectric devices. Hybrid fillers were synthesized by direct deposition of Bi2Se3 on multiwalled carbon nanotubes using a physical vapor deposition method, thus ensuring direct electrical contact between the carbon nanotubes and Bi2Se3. The Seebeck coefficient of prepared PVOH/Bi2Se3-MWCNT composites was found to be comparable with that for the Bi2Se3 thin films, reaching −100 µV·K−1 for the composite with 30 wt.% filler, and fluctuations of the resistance of these composites did not exceed 1% during 100 repetitive bending cycles down to 10 mm radius, indicating the good mechanical durability of these composites and proving their high potential for application in flexible thermoelectrics. In addition, other properties of the fabricated composites that are important for the use of polymer-based materials such as thermal stability, storage modulus and linear coefficient of thermal expansion were found to be improved in comparison with the neat PVOH.

where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. Having both n-and p-type TE materials is an essential requirement for building an efficient TE device. [3] So far the use of organic-inorganic nanocomposites, where electrically conductive p-or n-type polymer is used as a matrix for supporting a network of carbon nanotubes (CNTs) and/or inorganic TE nanostructures as flakes, nanoplates, or nanowires has shown the greatest potential for engineering efficient flexible TE devices. [4] CNTs have been reported to be one of the most promising components for producing p-type flexible TE nanocomposites. [5][6][7][8][9] However, application of CNTs in n-type TE composites is complicated due to necessity of CNTs pretreatment (annealing under high vacuum) for oxygen desorption and transition of their conductivity type from p-type (arising from extrinsically induced by the adsorbed oxygen hole-like concentration [10] ) to n-type, and further isolation from ambient environment (air) to prevent oxygen adsorption. This significantly complicates the practical use of n-type CNTs in ambient conditions (air). Another drawback of n-type conductive polymers and their based composites is poor stability in air, resulting in decrease of electrical conductivity by orders of magnitude when exposed to air even for a short period of time. [10] The use of organic-inorganic nanocomposites has shown the greatest potential for engineering efficient flexible thermoelectric devices. In this work, a novel approach of encapsulation of as-grown bismuth selenide-multiwalled carbon nanotubes (Bi 2 Se 3 -MWCNT) hybrid network in polyvinyl alcohol for fabrication of n-type flexible thermoelectric films is demonstrated as a successful alternative to the mechanically mixed counterparts. The developed stable flexible n-type thermoelectric material has a Seebeck coefficient and power factor at room temperature as high as −85 μV K −1 and 0.4 μW m −1 K −2 , and figure-of-merit, exceeding the value shown by the mixed counterpart by ≈2 orders of magnitude, while requiring 3-4 times less inorganic material in comparison with mixed composites. Charge carrier transport mechanisms and contribution of Bi 2 Se 3 and MWCNT components of not encapsulated and encapsulated hybrid networks to the total Seebeck coefficient, electrical conductance, and power factor are studied. In addition, the fabricated flexible thermoelectric films show good environmental stability at relative humidity levels up to 60%, as well as great mechanical and electrical stability with the increase of resistance within 0.5% and deviations of the Seebeck coefficient within 2% from the initial value during the 100 repetitive bending cycles.

This work is devoted to the fabrication of p-type polyvinyl alcohol (PVA)-based flexible thermoelectric composites using multiwall carbon nanotubes-antimony telluride (MWCNT-Sb2Te3) hybrid filler, the study of the thermoelectrical and mechanical properties of these composites, and the application of these composites in two types (planar and radial) of thermoelectric generators (TEG) in combination with the previously reported PVA/MWCNT-Bi2Se3 flexible thermoelectric composites. While the power factors of PVA/MWCNT-Sb2Te3 and PVA/MWCNT-Bi2Se3 composites with 15 wt.% filler were found to be similar, the PVA/MWCNT-Sb2Te3 composite with 25 wt.% filler showed a ~2 times higher power factor in comparison with the PVA/MWCNT-Bi2Se3 composites with 30 wt.% filler, which is attributed to its reduced electrical resistivity. In addition, developed PVA/MWCNT-Sb2Te3 composites showed a superior mechanical, electrical, and thermoelectric stability during 100 consequent bending cycles down to a 3 mm radius, with insignificant fluctuations of the resistance within 0.01% of the initial resistance value of the not bent sample. Demonstrated for the first time, 2-leg TEGs composed from p-type PVA/MWCNT-Sb2Te3 and n-type PVA/MWCNT-Bi2Se3 composites showed a stable performance under different external loads and showed their potential for applications involving low temperature gradients and power requirements in the range of nW.

P-type multiwalled carbon nanotubes (MWCNTs), as well as heterostructures fabricated by direct deposition of inorganic thermoelectric materials as antimony and bismuth chalcogenides on MWCNT networks are known as perspective materials for application in flexible thermoelectric polymer-based composites. In this work, the electrical response of three types of Sb2Te3-MWCNT heterostructures-based flexible films—free standing on a flexible substrate, encapsulated in polydimethylsiloxane (PDMS), and mixed in polyvinyl alcohol (PVA) is studied in comparison with the flexible films prepared by the same methods using bare MWCNTs. The electrical conductance of these films when each side of it was subsequently subjected to compressive and tensile stress during the film bending down to a 3 mm radius is investigated in relation to the distribution gradient of Sb2Te3-MWCNT heterostructures or bare MWCNTs within the film. It is found that all investigated Sb2Te3-MWCNT films exhibit a reversible increase in the conductance in response to the compressive stress of the film side with the highest filler concentration and its decrease in response to the tensile stress. In contrast, free-standing and encapsulated bare MWCNT networks with uniform distribution of nanotubes showed a decrease in the conductance irrelevant to the bending direction. In turn, the samples with the gradient distribution of the MWCNTs, prepared by mixing the MWCNTs with PVA, revealed behavior that is similar to the Sb2Te3-MWCNT heterostructures-based films. The analysis of the processes impacting the changes in the conductance of the Sb2Te3-MWCNT heterostructures and bare MWCNTs is performed. The proposed in this work bending method can be applied for the control of the uniformity of distribution of components in heterostructures and fillers in polymer-based composites.