Highly Efficient Flexible n‐Type Thermoelectric Films Formed by Encapsulation of Bi₂Se₃‐MWCNT Hybrid Networks in Polyvinyl Alcohol

Abstract

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.

p-Type PVA/MWCNT-Sb2Te3 Composites for Application in Different Types of Flexible Thermoelectric Generators in Combination with n-Type PVA/MWCNT-Bi2Se3 Composites

“…The thermoelectrical measurements were carried out at room temperature (300 K) under ambient conditions using a home-made device, allowing for the bending of the samples which were calibrated by Standard Reference Material 3451 (NIST, Gaithersburg, MD, USA) for a low-temperature Seebeck coefficient, as described elsewhere [ 10 ]. The room-temperature electrical characterization was performed using the Keithley 6430 Sub-Femtoamp Remote Source Meter (Cleveland, OH, USA).…”

“…The room-temperature electrical characterization was performed using the Keithley 6430 Sub-Femtoamp Remote Source Meter (Cleveland, OH, USA). The low-temperature resistance measurements were performed using the thermal transport option of the physical property measurement system DynaCool9T (Quantum Design, San Diego, CA, USA) and MultiVu software, as described elsewhere [ 10 ]. During the bending experiments, the resistance at a specific bending radius was calculated from a voltage setting (always 0.1 V ± 1 mV) and measured current (±100 pA).…”

“…[ 7 ], the study of their applicability in flexible thermoelectrics is limited to a few reports [ 8 , 9 ]. Previous research showed that despite the solubility of PVA in water, the PVA-based n-type thermoelectric composites show stable reversible behavior under relative humidity (RH) levels up to 60% [ 10 ]. As it is known, RH levels between 30 and 60% are optimal for health, work performance, and a lower risk of infection for humans [ 11 ], and are generally recommended for air-conditioned buildings.…”

“…Due to the direct electrical contact between the graphene or carbon nanotubes and inorganic nanostructures, established during the synthesis, such hybrid nanostructured materials become flexible, while retaining a high electrical conductance and Seebeck coefficients comparable with their bulk inorganic counterparts [ 16 ]. Recently, our group reported the application of such n-type multiwall carbon nanotube-bismuth selenide (MWCNT-Bi 2 Se 3 ) hybrid networks as a filler for n-type PVA-based thermoelectric composites [ 10 , 17 ]. These composites reached PF values significantly exceeding the PFs reported by Pires et al for n-type PVA/Bi 2 Te 3 composites [ 8 ], while requiring three times less (15–30 wt.% vs. 60–90 wt.%) filler material.…”

Synthesis and Properties of Bismuth Selenide Based Nanolaminates for Application in Thermoelectrics

“…The sample size was 15 mm × 5 mm, with the 7 mm distance between the inner electrodes, as described elsewhere. [ 40 ] For determination of Seebeck coefficient, the temperature difference between the sides of the thin films did not exceed 3% from the measurement temperature.…”