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Wang et al. Soft Sci 2023;3:34 https://dx.doi.org/10.20517/ss.2023.25 Page 11 of 26

[70]
dispersion mediums . The composite films with the layered morphology revealed dramatically improved
2
TE performances with a power factor of 21.7 ± 0.8 μW/mK for the sample containing 60 wt% SWCNTs
using an SDBS dispersion medium. Also, PEDOT-tosylate (PEDOT-Tos)/acidified-SWCNT composites
with greatly intensified TE performance are fabricated by combining chemical oxidative polymerization,
acid treatment of SWCNTs, and vacuum filtration . Due to the formation of a highly conductive network
[71]
between the PEDOT-Tos polymer chains and SWCNTs, along with the acid doping process, the composites
showed an electrical conductivity as high as 4,731.6 ± 42.3 S/cm, which contributed to the improvement of
the TE performance. Xiang et al. reported the TE properties of polyaniline (PANi)/GN nanocomposites
prepared by in situ polymerization of aniline monomer in the presence of GN, following a vacuum filtration
[24]
process . The addition of GN improved the electrical conductivity of the nanocomposite, and the Seebeck
coefficient changed with the initial concentration of aniline in the solution and the protonation of PANi.
Therefore, the ZT value of the nanocomposite was two orders of magnitude higher than either of the
constituents.

To sum up, the conducting polymer/carbon nanoparticle composite films usually show significantly
enhanced TE properties, and the enhancement is mainly ascribed to the strong π-π interaction between
conducting polymers and carbon nanoparticles. Other methods also show similar results; for example,
Zhang et al. prepared PEDOT:PSS/SWCNTs composite films by a doctor-blade process, followed by a
DMSO doping treatment, and the composite film with 74 wt% SWCNT showed a power factor of
2[72]
300 μW/mK ; Wang et al. prepared PANI/GN composite films by a combination of in situ polymerization
and drop-casting processes, and more ordered regions were formed in the PANI/ GN composite due to the
strengthened π-π conjugation interactions, leading to a maximum power factor of 55 μW/mK . However,
2[56]
the preparation process of these methods usually includes a wet-chemical or post-treatment process and is
not convenient compared with the vacuum filtration methods.

CARBON NANOPARTICLE-BASED TE MATERIALS
Pure carbon nanoparticle TE materials
Carbon nanoparticles, especially GN and CNTs, show good film-forming properties because of their 2D and
1D nanostructures. Besides, carbon nanoparticles usually present good electrical conductivities. Therefore,
GN and CNTs fabricate free-standing TE films by a vacuum filtration method. For example, p-type and
n-type CNT bucky papers were prepared by the acid treatment or reduction through PEI of vacuum-
filtrated CNT films . The acid-treated SWCNTs generated a positive thermopower of 60 μV/K at 380 K.
[73]
The PEI-SWCNT caused a negative thermopower of -60 μV/K at 380 K. Then, a TE module composed of
four p-n layers was fabricated. It showed a voltage of 7 mV under a temperature gradient of 50 K. Kim et al.
[74]
used a hybrid filler of GN SWCNTs to prepare a TE device with p-type TE film and n-type bucky paper .
The p-type TE film had a composition ratio of carbon filler (GN:SWCNT, 8:2), PVDF, and IL of 1:1:1. And
n-type TE bucky paper was made with a filtration method with GN and SWCNTs followed by a PEI
solution treatment. The thermocouple composed of 22 p-n legs showed an output power value of 14.9 nW
with a temperature difference of 10 K.

Usually, the as-grown SWCNTs possess different chirality, and the unseparated SWCNTs contained ~2/3 of
semiconducting and ~1/3 of metallic nanotubes . The electronic and bandgap structures of metallic
[75]
SWCNTs and semiconducting SWCNTs are very different [76-78] , leading to various electronic transport
properties and TE performance for SWCNT- and CNT-based composites. Thus, it is vital to investigate the
electronic and TE properties of separated CNTs. Using an aqueous two-phase extraction method,
Tambasov et al. prepared semiconducting and metallic SWCNT dispersions from commercialized
SWCNTs [Figure 9]. Then, using vacuum filtration, thin films based on unseparated, semiconducting,
[79]