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Wang et al. Soft Sci 2023;3:34 https://dx.doi.org/10.20517/ss.2023.25 Page 13 of 26

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themselves . Because of the unique structural characteristics and extraordinary mechanical, electrical, and
thermal properties, carbon nanomaterials are used to prepare carbon nanomaterials/inorganic composite
TE materials, aiming to combine the excellent electrical properties and flexibility of carbon nanomaterials
with the high power factor (especially for the Seebeck coefficient) of inorganic TE materials to achieve ideal
TE performances.

In 2013, Bark et al. prepared CNT/Bi Te composites by dispersing Bi Te particles and CNTs in a solvent,
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followed by vacuum filtration . Then, the influence of CNTs on the electrical conductivity and Seebeck
[85]
coefficient of composites were studied. With low CNT contents, the electrical conductivity and Seebeck
coefficient were slightly increased by the p-doping effect of CNTs on Bi Te particles. Moreover, with the
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increased amount of CNTs, the electrical conductivity and Seebeck coefficient of composites were decreased
due to the enhanced development of hindered carrier mobility. The highest power factor for the composite
was about 140 μW/mK , which contained 20 V% CNTs. Similarly, Chen et al. prepared a series of n-type
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flexible SWCNT/Bi Te composite films on PVDF membranes by in situ growth of Bi Te nanosheets on
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SWCNTs assisted by poly(vinylpyrrolidone) (PVP) through a solvothermal method and subsequent
vacuum filtration . The morphology and structure of the SWCNT/Bi Te composite films indicated that
[86]
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Bi Te was successfully grown on the SWCNT network [Figure 10]. When the mass ratio of SWCNTs to
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Bi Te was 1:0.8, the composite film showed a high electrical conductivity of 244.6 S/cm at room
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temperature, and a power factor of 57.8 μW/mK was obtained at 386 K. This work provides a convenient
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way to prepare flexible and n-type TE composite films. Fan et al. first prepared 2D SnSe nanobelts by a
chemical exfoliation process. Then, free-standing SWCNT/SnSe nanobelt composite films were obtained via
a facile solution mixing and subsequent vacuum filtration process . A post-treatment of thermal annealing
[87]
was used to further improve the TE performance of the composite. Finally, a maximum power factor
reached 145 ± 28 μW/mK at room temperature when the sample contained 80 wt% SWCNTs, which is
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much higher than the power factors observed in either individual component. Gao et al. fabricated highly
flexible composite films based on reduced graphene oxide (rGO) and Te NWs by vacuum filtration . Due
[88]
to the combination of the high carrier mobility of Te NWs and the high carrier concentration of rGO, the
electrical conductivity and Seebeck coefficient of the optimized composite film could reach 978 S/m and
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286 μV/K, respectively, resulting in a power factor of 80 μW/mK at 40 C. Xiao et al. prepared flexible n-
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[89]
type rGO/Ag Se NW composite films by vacuum filtration and cold-pressing treatment . The composite
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film showed the highest power factor of 228.88 µW/mK at 331 K for the sample with 0.01 wt% rGO.
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Besides, the low electrical conductivity of conducting metal-organic frameworks (MOF) has limited their
application in TE devices. Then, free-standing SWCNT/MOF [Ni-1,2,5,6,9,10-triphenylenehexathiol
(THT)] composite films were fabricated by the vacuum filtration method . Because of the covalent bond
[90]
between SWCNTs and MOFs, the conductive MOFs were successfully grafted onto the surface of SWCNTs.
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The composite film showed a maximum PF of ~98.1 µW/mK at 4 wt% SWCNT loading, which was
considerably higher than that of the pristine Ni-THT. The room-temperature TE properties of carbon
nanomaterials/inorganic composite films are listed in Table 2.
FLEXIBLE INORGANIC TE MATERIALS
Ag Se-based TE materials
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Although conducting polymer-based and carbon nanomaterial-based composites show good flexibility,
obtaining TE performances comparable to inorganic counterparts is still a significant challenge. Inorganic
semiconductors, such as Bi Te , SnSe, and Ag Se, usually show high TE performance, while the inherent
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rigidity and brittleness have limited their application as flexible TE devices . Ding et al. have developed a
[91]
simple, low-cost process to prepare inorganic semiconductors with high TE properties and excellent
flexibility .
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