Vacuum filtration method towards flexible thermoelectric films
Abstract
Thermoelectric (TE) conversion technology can directly exploit the temperature difference of several Kelvin between the human body and the environment to generate electricity, which provides a self-powered solution for wearable electronics. Flexible TE materials are increasingly being developed through various methods, among which the vacuum filtration method stands out for its unique advantages, attracting the favor of researchers. It has been proven to construct flexible TE thin films with excellent performance effectively. This paper presents a comprehensive overview and survey of the advances of the vacuum filtration method in producing flexible TE thin films. The materials covered in this study include conducting polymer-based materials, carbon nanoparticle-based materials, inorganic materials, two-dimensional materials, and ternary composites. Finally, we explore potential research outlooks and the significance of flexible films, which are at the forefront of research in TE materials science.
Keywords
INTRODUCTION
As humans enter the Internet of Things era, autonomous sensors play essential roles in various fields, such as biometric monitoring, medical monitoring, and industrial and environmental controls. Moreover, there is an unprecedented demand for portable, reliable, ultra-thin, and sustainable wearable electronic power devices. The application of conventional batteries is limited because of their frequent replacement/recharge and extra maintenance. Therefore, developing a maintenance-free and self-powered sensor system is critically important. Thermoelectric (TE) conversion technology can convert heat from the body or the environment into electricity, providing a viable self-powered solution for flexible electronics[1-3]. Conventional inorganic bulk TE materials, such as Bi2Ti3 and PbTe, usually show high TE performance; however, brittle and rigid properties of inorganic materials and the complicated process have limited their applicability, particularly when in contact with curved or irregular heat sources[4,5]. In this situation, film-based flexible TE materials and generators show significant advantages and application prospects since their conformability enables effective contact with all kinds of curved or irregular heat sources to maximize heat harvesting. Besides, a thin-film thermoelectric generator (TEG) may use fewer materials compared to a bulk TEG and provide easy integration with integrated circuits[6,7]. Figure 1 shows the different structures of traditional inorganic TEGs with a cross-plane structure in Figure 1A and thin-film TEGs with an in-plane structure in Figure 1B. Traditional inorganic TEGs are composed of bulk P/N legs tiled in two dimensions over a ceramic substrate patterned with electrical contacts, in which the temperature gradient across the material is perpendicular to the substrate. Unlike cross-plane structures, the temperature gradient across the material is parallel to the substrate for the in-plane structure. In this structure, the temperature gradient across the TE legs is usually minimal; creative strategies, such as corrugated, origami, and rolled designs[8,9], can convert the 2D arrays of thin-film TE legs into 3D architectures. Thus, the temperature gradient will parallel the substrate to obtain a more significant thermal gradient. A flexible TEG has the advantages of
Figure 1. Schematic diagram of traditional inorganic TEGs with a cross-plane structure (A) and thin-film TEGs (B) with an in-plane structure (TH, hot-side temperature, TC, cool-side temperature). TEG: Thermoelectric generator.
Research on flexible TE technology generally consists of two strategies. One is to directly use organic TE materials with good flexibility and plasticity, and the other is to integrate brittle inorganic TE materials into the flexible substrate.[10-13]. The former has lower electrical performance, resulting in output voltage and power much inferior to inorganic materials; the latter is difficult to produce ultra-thin flexible devices due to its complex structure and technology. The discovery of silver sulfide–based ductile semiconductors has extensively promoted the development of flexible inorganic TE materials[14-16]. Significantly, Yang et al. reported high-performance p-type ductile TE materials of AgCu(Se, S, Te) pseudo ternary solid solutions, with a ZT value of 0.45 at 300 K[14]. They developed ultra-thin flexible TE devices with a conventional
The preparation process of flexible TE films is crucial in constructing wearable TE devices. Inorganic plastic semiconductors are prepared first by traditional melting and annealing methods, and then the prepared inorganic material is cut into thin sheets. In addition, Jin et al. prepared high-performance Bi2Te3-single-walled carbon nanotube (SWCNT) composite flexible self-supported TE thin films with carbon nanotubes (CNTs) as the skeleton using independently improved magnetron sputtering deposition equipment[21]. Vacuum filtration[22-24], vapor deposition[25-28], printing[29-31], and coating[32-34] methods effectively prepare flexible TE films, utilizing substrates such as polyvinylidene fluoride (PVDF), cotton, polyimide, etc. Compared with other preparation methods, vacuum filtration is simple, effective, and economical. The detailed process includes the following steps: (1) connection of filter flask to a vacuum pump;
Unlike the recently published reviews on flexible TE materials, this paper focuses on the research progress of vacuum filtration for preparing flexible TE films. The structure of this review is outlined in Figure 2. The advantages and a range of representative examples of the vacuum filtration method in producing flexible TE thin films, including conducting polymer-based, carbon nanoparticle-based, inorganic, two-dimensional materials, and ternary composites, will be discussed. Furthermore, the present challenges and future development opportunities of this method are forecasted. Given the effectiveness of this method and its broad relevance to other fields of research, we will do our best to guide interested readers to this field, which can be applied and promoted to new heights in the future.
Figure 2. Schematic outline of this review. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid); TE: thermoelectric.
CONDUCTING POLYMER-BASED TE MATERIALS
PEDOT-based TE materials
Polymer semiconductors, as TE materials, offer unique advantages, such as high electrical conductivity, low inherent thermal conductivity, cost-effectiveness, mass production using simple synthesis methods, and the ability to deposit over large areas. Especially, PEDOT, which forms a complex with PSS, is considered the most promising conductive polymer for TE. Previously, PEDOT:PSS TE thin films were prepared mainly based on drip or spin coating on a glass substrate. Considering the future need for large-scale preparation and flexible wearable applications, it is necessary to develop new preparation methods to obtain large-size flexible thin films. In 2015, Mengistie et al. prepared free-standing, flexible smooth PEDOT:PSS bulky papers through vacuum-assisted filtration[23] [Figure 3A]. In vacuum filtration, the liquid is sucked into the filtration membrane by the pump, resulting in a higher filtration rate than gravity filtration. After vacuum drying, the bulky paper can easily peel off the Anodisc filter membrane.
Figure 3. (A) Photos of Flexible PEDOT:PSS bulky papers; (B) S(2p) XPS spectra of PEDOT:PSS bulky papers: pristine, treated with EG, and treated with FA. Reproduced with permission from Ref[23]. Copyright© 2015. American Chemical Society. EG: Ethylene glycol; PEDOT:PSS: poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid).
Apart from being a counterion, PSS is also used as a charge compensator and template for the polymerization of PEDOT in PEDOT:PSS, making it readily dispersible in water. Two PSS types are present in PEDOT:PSS solutions: “main PSS” (complexed to PEDOT grains) and “free PSS” (act as surfactants to reduce intermolecular interactions between two PEDOT grains). However, in excess, PSS is an insulator (non-ionizing dopant), which is the main reason for the low conductivity of commercial PEDOT. Selective removal of PSS is one of the mechanisms to improve electrical conductivity. Numerous approaches have been reported for enhancing the TE performance of PEDOT:PSS, such as post-treatment methods based on doping and de-doping, including overcoating, dipping, or vapor-treatment ways, which may affect the oxidation level and electron transport properties that are strongly associated with TE performance.
In the above experiment by Mengistie et al., they used methanol, formic acid (FA), ethylene glycol (EG), and polyethylene glycol (PEG) to treat PEDOT:PSS, respectively[23]. EG and PEG200 were mixed with PEDOT:PSS before being filtered under pressure. The difference lies in the treatment of annealed bulky papers, which were treated with methanol and FA. Surface PSS was relatively reduced as excess PSS was flushed away by methanol and FA treatment [Figure 3B]. For EG- and PEG-treated papers, no PSS is removed from the surface. Instead, the PSS layer is depleted from the surface, which becomes rich in PEDOT. The amount of PSS removed/exhausted from the PEDOT:PSS paper is consistent with the respective conductivity enhancement. The average conductivities of EG-, PEG-, methanol-, and FA-treated papers are 640, 800, 1,300, and 1,900 S/cm, respectively. Removing PSS, especially from the final film, is essential for improving electrical conductivity.
The current post-treatment technology has low practicability due to various problems, including difficulty in mass production, lengthy and complicated processes, swelling and falling of polymers on the film surface during the dipping process, etc. To solve this problem, Lee et al. reported a solution treatment method using ultrafiltration to remove excess PSS molecules directly from PEDOT:PSS dispersions without
Furthermore, Xiong et al. developed one-step direct dilution to achieve stable, simple, and fast integration of PEDOT:PSS film preparation and solvent treatment[36]. Due to the PSS partly dissolved in the organic solvent, the dilution process caused by PSS initially scattered away from the PEDOT-rich nanoparticle aggregation significantly. The PEDOT:PSS film on PVDF was thin and flexible, so researchers could cut it into any desired form and paste it onto a transparent PET substratum. This approach is expected to affect the electrical and TE properties positively because it avoids the waste of pristine PEDOT:PSS, the limitation of solvent types, the use of sophisticated instruments, and complex processes. The obtained PEDOT:PSS thin films have high electrical conductivity of about 1,500 S/cm and a considerable ZT value of 0.1 with facile treatment for the high or low-boiling-point solvents.
Previous works mainly used commercial PEDOT:PSS to prepare free-standing PEDOT-based films. However, compared with the EDOT monomer, the cost of commercial PEDOT:PSS is too high, which is unsuitable for application in the TE field. Song et al. filtered and obtained a flexible, independent PEDOT nanofiber film through a simple self-assembled micellar soft template method [Figure 4A-D][37]. The processing method perfectly solves the problem that PEDOT is difficult to handle due to its insoluble and refractory properties. By incorporating SWCNTs into PEDOT nanofibers, the PEDOT nanofiber/SWCNTs showed a power factor of 14.4 μW/mK2. Ni et al. successfully used a modified self-assembled micellar
Figure 4. (A) Schematic illustration of the preparation process for synthesizing free-standing 1D PEDOT nanofiber films by a simple self-assembled micellar soft-template approach combined with a vacuum-assisted filtration method; (B) Photographs of free-standing and flexible PEDOT nanofiber films; (C) SEM and (D) TEM images of PEDOT nanofibers. Reproduced with permission from Ref[37]. Copyright© 2021. Elsevier; (E) A schematic plot of the prototype power generator; (F) TE voltage generated by the prepared generator versus ΔT. Reproduced with permission from Ref[38]. Copyright© 2019. Elsevier. PEDOT: Poly(3,4-ethylenedioxythiophene); SEM: scanning electron microscope; TEM: transmission electron microscopes.
PEDOT/inorganic composite TE materials
Despite the high electrical conductivity of PEDOT, its low Seebeck coefficient, typically ranging from 15 to 20 µV/K[10,39,40], has become the main obstructive factor for its TE performance. Incorporating inorganic semiconductors with a high Seebeck coefficient or power factor into PEDOT seems to be a good strategy, which can fully use the advantages of PEDOT and inorganic components. Furthermore, PEDOT-based composite films with high quality can be obtained by dispersing inorganic materials into PEDOT:PSS solution, followed by a simple and effective vacuum filtration method.
Inorganic semiconductor materials, such as Te-, CuSe-, Bi-Te-based alloys, etc., are usually the first choice to construct PEDOT-based composites. Song et al. have conducted excellent work in this field. In 2017, they prepared PEDOT:PSS-functionalized Te (PF-Te) nanorods. The PEDOT:PSS layer provided excellent dispersibility in the PEDOT:PSS solution, and then PEDOT:PSS/PF-Te composite films were obtained through a vacuum filtration method using a 0.22 µm PVDF porous membrane [Figure 5][41]. Because of the high Seebeck coefficient of Te nanorods and an energy filtering effect between the PEDOT:PSS layer and Te, the Seebeck coefficient of the composites was enhanced from 15.6 to 51.6 µV/K. The highest power factor reached 51.4 µW/mK2 for the sample containing 70 wt% PF-Te nanorods. Also, flexible PEDOT:PSS/Ag2Te nanocomposite films on the PES substrate were prepared through vacuum filtration and cold pressing processes using PF-Te nanorods as the templates[42]. An optimal PEDOT:PSS/Ag2Te nanocomposite film exhibited a power factor of 143.3 μW/mK2 at room temperature. In 2019, they prepared PEDOT:PSS-coated CuxSey (PC-CuxSey) NWs through a wet-chemical method, and then they fabricated PEDOT:PSS/CuxSey nanocomposite films on flexible nylon membranes by vacuum filtration and cold-press processes
Figure 5. Schematic illustration of the flexible PEDOT:PSS/PF-Te composite film fabrication on PVDF via a filtration process. Reproduced with permission from Ref[41]. Copyright© 2017. Elsevier. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid); PF-Te: PEDOT:PSS-functionalized Te; PVDF: polyvinylidene fluoride.
Figure 6. The flexibility of a PEDOT:PSS/CuxSey nanocomposite film and its TE properties. Reproduced with permission from Ref[43]. Copyright© 2021. American Chemical Society. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid); TE: thermoelectric.
Except for the PEDOT:PSS solution, PEDOT NWs synthesized by a self-assembled micellar soft-template method also show good film-forming properties due to their 1D nanostructures. For example, Tian et al. used Bi2Te3 powders and Bi2Te3 NWs as inorganic fillers to prepare PEDOT NW/Bi2Te3 nanocomposite films by a vacuum filtration method[51]. The PEDOT NW/Bi2Te3 NW nanocomposite showed a higher power factor of 9.06 µW/mK2 when compared to PEDOT NW/Bi2Te3 powders, indicating that incorporating one-dimensional NWs into the polymer matrix is more effective in improving the TE properties of conducting polymers. Liu et al. prepared PEDOT NW/Te NW nanocomposite films through a similar method, and the Seebeck coefficient of the composite increased from 10.08 µV/K to 89.52 µV/K with 90 wt% Te content[52].
Table 1 summarizes the room-temperature TE properties of PEDOT/inorganic composite films. Overall, the PEDOT-based composites prepared through the vacuum filtration method mentioned above help to improve the TE properties of PEDOT. The obtained TE performances are comparable to those of PEDOT/inorganic composite films prepared from other methods, such as PEDOT:PSS/Bi2Te3 NW composite films obtained from a drop-casting method of a mixed solution with a power factor of 10.6 μW/mK2[53]; and PEDOT:PSS/SnS prepared by drop coating with a power factor of 43.11 μW/mK2[54]. However, the general problem is that most of the Seebeck coefficients of composites are in the range of 10-25 μV/K, which is much lower than that of the original intention of designing (the Seebeck coefficient of pure PEDOT is about 15 μV/K). The main reason is the big difference in structure and properties between polymer and inorganic materials. Composites prepared by traditional physical mixing methods usually suffer from the problem of weak interaction between two phases, poor compatibility, low interface stability, mismatched electrical properties, and easy-to-appear phase separation. Therefore, optimizing the preparation method by various physical or chemical processes to improve the interaction between conducting polymers and the inorganic filling phases in the composite and improve the compatibility and interfacial stability between the two steps will be an effective way to enhance the TE properties of conducting polymer/inorganic nanocomposites.
Room-temperature TE properties of PEDOT/inorganic composite films
| Materials | Preparation method | S (μV/K) | σ (S/cm) | PF (μW/mK2) | ZT | Ref. |
| PEDOT:PSS/PF-Te | Wet-chemical process | ~ | ~ | 51.4 | 0.076 | [41] |
| PEDOT:PSS/Ag2Te | Wet-chemical process | -62.3 | 369.3 | 143.3 | ~ | [42] |
| PEDOT:PSS/CuxSey | Wet-chemical process, cold pressing | 50.8 | 1,047.1 | 270.3 | ~ | [43] |
| PEDOT:PSS/Cu2Se | Wet-chemical process, hot pressing | 78.2 | 470 | 287.4 | ~ | [44] |
| PEDOT:PSS/Bi-Te-based alloy | Lithium intercalation | 20.7 | 1,223.8 | 52.3 | ~ | [45] |
| PEDOT:PSS/Cu2S | Cold pressing | ~ | ~ | 111.54 (393 K) | ~ | [46] |
| PEDOT:PSS/SnSe | Vacuum filtration | ~ | ~ | 24.42 (353 K) | [47] | |
| PEDOT:PSS/MoS2 | Liquid-phase exfoliate | 19.5 | 1,250 | 45.6 | 0.04 | [48] |
| PEDOT:PSS/SiC-NWs | Vacuum filtration | 20.3 | 3,113 | 128.3 | 0.17 | [49] |
| PEDOT:PSS/SiO2 | Vacuum filtration | 24.2 | 1,131.9 | 66.29 | ~ | [50] |
| PEDOT NWs/ Bi2Te3 NWs | Wet-chemical process | 10.8 | 776.52 | 9.06 | ~ | [51] |
| PEDOT NWs/Te NWs | Wet-chemical process | 89.52 | 72.41 | 58.03 | ~ | [52] |
Conducting polymer/carbon nanoparticle composite TE materials
Carbon nanoparticles, including graphene (GN) and CNTs, have been considered as promising candidates to prepare conducting polymer-based composite materials through the vacuum filtration method owing to their high electrical conductivities and decent Seebeck coefficient, which can remarkably improve the TE performance of composite[55-57]. On the one hand, their large active conjugated fused aromatic ring systems and the large specific surface areas can promote interfacial contacts between carbon nanoparticles and conducting polymers significantly, thus resulting in a synergistic effect of the components. On the other hand, the high thermal conductivities of the carbon nanoparticles can be mitigated by the conducting polymers (usually with thermal conductivities of ~ < 1 W/mK) wrapping or connection. Therefore, conducting polymer/carbon nanoparticle TE composites has received increasing attention.
Early in 2015, Xiong et al. fabricated highly conductive PEDOT:PSS/graphene composite films via vacuum filtration, and the sample containing 3 wt% graphene in N,N-Dimethylformamide (DMF) showed a power factor of 38.6 μW/mK2. Subsequently, a hydrazine treatment was used to further optimize the TE performance of the composite, and finally, an estimated ZT value of 0.05 was obtained at room temperature [Figure 7][58]. In another work, they fabricated PEDOT:PSS/SWCNT composite films using a similar process, and the composites showed an increasing trend with the increase of SWCNT content[59]. An optimized power factor of 105 μW/mK2 was obtained at 60 wt% SWCNTs. And despite the high intrinsic thermal conductivity (~1,000 W/mK[60,61]) of SWCNTs, the composite retained a low polymer-like thermal conductivity around 0.15 to 0.36 W/mK due to less favorable paths for thermal energy transport caused by the PEDOT:PSS connection between tube-tube junctions; therefore, a maximum ZT value of 0.12 was obtained. Also, the TE properties of some other PEDOT:PSS/SWCNTs and PEDOT:PSS/GN composite films fabricated by a similar vacuum filtration process with different post-treatment techniques were reported[62-65]. For example, Deng et al. prepared free-standing PEDOT:PSS/SWCNT composite films with an ionic liquid (IL) treatment using a vacuum filtration method [Figure 8][64]. The ion-exchange effect and promotion of SWCNT dispersion by the IL realized a synergistic boost of electrical conductivity and a Seebeck coefficient. The maximum power factor reached 182.7 ± 9.2 μW/mK2 at room temperature. Another fantastic property of CNTs is that their conduction type can be easily converted from p- to n-type through the n-type doping method by different reducing agents[66,67]. Then Song et al. prepared flexible and lightweight TEGs by vacuum filtrated PEDOT:PSS/SWCNTs as p-type materials and polyethyleneimine-doped SWCNTs as n-type materials, respectively[68].
Figure 7. (A) Schematic illustration of the preparation and hydrazine-treatment process of the PEDOT:PSS/graphene composite films; (B) Photographs of dispersion of PEDOT:PSS, graphene, and PEDOT:PSS/graphene in DMF after sonication; (C) Photos of PEDOT:PSS, graphene, and PEDOT:PSS/graphene films. Reproduced with permission from Ref[58]. Copyright© 2015. American Chemical Society. DMF: N,N-Dimethylformamide; PEDOT:PSS: poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid).
Figure 8. (A) Schematic illustration of the preparation procedures of the PEDOT:SWCNT composites with the IL; (B-E) Dependences of TE performance on IL content for PEDOT:PSS/SWCNT composites with different SWCNT contents: (B) 30 wt% SWCNT; (C) 60 wt% SWCNT; and (D) 90 wt% SWCNT; (E) dependence of TE performance on SWCNT content for the PEDOT:PSS/SWCNT-IL50 composite. Reproduced with permission from Ref[64]. Copyright© 2021. American Chemical Society. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid); SWCNT: single-walled carbon nanotube; TE: thermoelectric.
Liang et al. have researched the TE properties of conducting polymer/carbon nanoparticle composite prepared by combining chemical oxidative polymerization and vacuum filtration. For example, a large-area stretchable, super flexible, and mechanically stable PPy/SWCNT film with a diameter of ~18 cm was achieved by an in situ chemical oxidative polymerization of pyrrole monomer in the SWCNT solution, followed by vacuum filtration[69]. Further, they designed a unique layered morphology containing nanosheets of SWCNTs sandwiched by PPy NWs via physical mixing and vacuum filtration in different dispersion mediums[70]. The composite films with the layered morphology revealed dramatically improved TE performances with a power factor of 21.7 ± 0.8 μW/mK2 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[71]. Due to the formation of a highly conductive network 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 process[24]. 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
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
Usually, the as-grown SWCNTs possess different chirality, and the unseparated SWCNTs contained ~2/3 of semiconducting and ~1/3 of metallic nanotubes[75]. The electronic and bandgap structures of metallic 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[79] [Figure 9]. Then, using vacuum filtration, thin films based on unseparated, semiconducting, and metallic SWCNTs were formed from the appropriate distributions. The unseparated SWCNTs showed the highest electrical conductivity, and the semiconducting SWCNTs showed the highest Seebeck coefficient of ~80 μV/K with a low electrical conductivity at room temperature. Finally, the power factors for metallic SWCNTs and unseparated SWCNT films were correspondingly 47 and 213 μW/mK2.
Figure 9. Schematic diagram of the formation process of thin SWCNT films on glass substrates. Reproduced with permission from Ref[79]. Copyright© 2019. Elsevier. PEG: Polyethylene glycol; SWCNT: single-walled carbon nanotube.
Wu et al. used multi-walled CNTs (MWCNTs) to prepare the CNT films by the CVD method and vacuum filtration, and then the films were piled up to fabricate anisotropic 3D CNT sponges with different CNT film layers[80]. The anisotropic TE properties were studied, and due to the small size of pores and the low thermal conductivity of CNTs along the radial direction, the cross-plane value of ZT (ZT⊥) is much larger than that along the in-plane direction (ZT//). This significant anisotropic thermal conduction of CNT films is a primary factor that restrains the TE conversion using the Seebeck effect since the temperature gradient is challenging to maintain in the current flow direction[81,82]. So, Matsumoto et al. introduced polystyrene particles as orientation aligners during the film formation process by vacuum filtration to control the orientation of the CNTs[82]. Through anisotropy control, the through-plane thermal and electrical conductivity of SWCNT films increased due to the increased SWCNTs oriented in the through-plane direction. Consequently, the sizeable in-plane anisotropy was decreased for thermal and electrical conductivity. The GN paper, composed of a bilayer and a few layers of GN, was fabricated through an electrochemical exfoliation and vacuum filtration process by Gee et al.[83]. Then, the anisotropic thermal conductivity of the GN paper was investigated. Due to the highly oriented order of the basal plane rather than the horizontal direction, the in-plane thermal conductivity measured by the TE method was
Carbon nanomaterials/inorganic composite TE materials
Although inorganic TE materials usually show a high power factor, their flexibility could be better. Even by reducing dimensionality and size, it is hard for inorganic TE materials to fabricate flexible films by themselves[84]. 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/Bi2Te3 composites by dispersing Bi2Te3 particles and CNTs in a solvent, followed by vacuum filtration[85]. Then, the influence of CNTs on the electrical conductivity and Seebeck 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 Bi2Te3 particles. Moreover, with the 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/mK2, which contained 20 V% CNTs. Similarly, Chen et al. prepared a series of n-type flexible SWCNT/Bi2Te3 composite films on PVDF membranes by in situ growth of Bi2Te3 nanosheets on SWCNTs assisted by poly(vinylpyrrolidone) (PVP) through a solvothermal method and subsequent vacuum filtration[86]. The morphology and structure of the SWCNT/Bi2Te3 composite films indicated that Bi2Te3 was successfully grown on the SWCNT network [Figure 10]. When the mass ratio of SWCNTs to
Figure 10. (A-F) SEM images of SWCNT/Bi2Te3 composite films, corresponding to mass ratios of the SWCNTs to Bi2Te3-0.6/0.8/1/5/10/15; (G-J) HRTEM of SWCNTs@Bi2Te3-5; (K) XRD patterns of a series of SWCNT@Bi2Te3 films, pristine Bi2Te3 power, and pristine SWCNT film. Reproduced with permission from Ref[86]. Copyright© 2021. American Chemical Society. HRTEM: High-resolution transmission electron microscopy; SEM: scanning electron microscope; SWCNT: single-walled carbon nanotube; XRD: X-ray diffraction analysis.
Room-temperature TE properties of carbon nanomaterials/inorganic composite films
| Materials | Preparation method | S (μV/K) | σ (S/cm) | PF (μW/mK2) | ZT | Ref. |
| Bi2Te3/ MWCNTs | Ball-milling | ~-128 | ~80 | 140 | ~ | [85] |
| Bi2Te3/ SWCNTs | Solvothermal method | ~36.3 | 244.6 | 32.3 | ~ | [86] |
| SnSe/ SWCNTs | Lithium intercalation, thermal annealing | 49 | 588 ± 12 | 145 ± 28 | 0.003 | [87] |
| rGO/Te NWs | Solvothermal method | 9.78 | 286 | 80 (313 K) | ~ | [88] |
| rGO/Ag2Se NWs | Wet-chemical approach, cold pressing | -111.72 | 183.37 | 228.88 (331 K) | ~ | [89] |
| MOFs/ SWCNTs | Vacuum filtration | ~ | ~ | 98.1 | 0.037 | [90] |
FLEXIBLE INORGANIC TE MATERIALS
Ag2Se-based TE materials
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 Bi2Te3, SnSe, and Ag2Se, usually show high TE performance, while the inherent rigidity and brittleness have limited their application as flexible TE devices[91]. Ding et al. have developed a simple, low-cost process to prepare inorganic semiconductors with high TE properties and excellent flexibility[92].
A series of pioneering works were reported on the optimization of the process, analysis of microstructures, and transmission mechanisms on TE properties of Ag2Se-based materials. In 2019, researchers first synthesized Ag2Se NWs, then the NWs were vacuum-filtrated onto a nylon membrane and finally hot pressed at 200 °C and 24 MPa[92]. The Ag2Se film displayed a power factor of 987.4 ± 104.1 μW/mK2 at 300 K with an estimated ZT value of 0.6 [Figure 11]. The high TE performance was mainly attributed to the particular Ag2Se film. The Ag2Se film showed excellent flexibility, and 93% of the original electrical conductivity was retained after 1,000 bending cycles around an 8-mm diameter rod. This excellent flexibility came from the synergetic effect of the nylon membrane and the Ag2Se film intertwined with numerous high aspect-ratio Ag2Se grains. Suitable element doping treatment improves the TE performance of Ag2Se[93]. Therefore, they studied the influence of different elements (including Ga, S) doping on the TE properties of flexible Ag2Se films[94,95].
Figure 11. (A) XRD pattern of the Ag2Se film; (B) Low magnification FESEM image of Ag2Se film; (C) High magnification FESEM image of Ag2Se film; (D) Overview high-angle annular dark field-STEM image; (E) Typical STEM image; (F) FFT image corresponding to (E); (G) Temperature dependence of Seebeck coefficient, electrical conductivity, and power factor for the Ag2Se film. Reproduced with permission from Ref[92]. Copyright© 2019. Springer Nature. FESEM: Field emission scanning electron microscopy; FFT: fast Fourier transform; STEM: scanning transmission electron microscopy; XRD: X-ray diffraction analysis.
To solve the high porosity and control the growth direction of the as-prepared Ag2Se film, they optimized the synthesis temperature of Ag2Se NWs from room temperature to 40 °C. They kept the other process conditions the same as Refs[92,96]. The morphology synthesized Ag2Se changed from uniform NWs to multisized nanostructures, leading to a more compact film (~90% relative density) after hot pressing with a suppressed growth direction along the (00l). Finally, the power factor of the optimized Ag2Se film increased to 1,882 μW/mK2 at 300 K, which is nearly twice as large as the previous report. However, the high density limited the flexibility of the film, and the power factor decreased by 16.1% after bending 1,500 times due to the break of some weak-bonded Ag2Se grain boundaries. To solve the problem, they introduced PVP as a binder to hinge rigid Ag2Se micro grains to improve the flexibility and take full advantage of the intrinsic low thermal conductivity of PVP to increase the ZT value of the Ag2Se film[97]. The microstructure characterizations indicated that the PVP was mainly located at the wall of micropores, and most Ag2Se grains have coherent interfaces [Figure 12]. Then, an optimal film exhibited a power factor as high as
Figure 12. (A-D) Cross-sectional microstructure characterization of the PVP/Ag2Se composite films: (A) a typical cross-sectional TEM image indicating the existence of pores surrounded by several grains (grains A-G); (B) the enlarged image of the red square marked in (A) showing a coherent interface and a PVP layer; (C) the enlarged image of the green square marked in (A) containing four Ag2Se grains coated with PVP; (D) an HRTEM image of the white square marked in (C) containing a coherent interface, with a corresponding FFT image given in the inset; Temperature dependent (E) Seebeck coefficient, electrical conductivity and power factor; and (F) carrier mobility and concentration of the composite film; (G-I) Flexibility test of the composite film: (G) The electrical conductivity and Seebeck coefficient of the composite film as a function of bending times with a bending radius of 4 mm; (H) the degradation of power factor of the composite films as a function of bending times with a bending radius of 4 mm; (I) the stress-strain curve of the composite films (P0: hot-pressed Ag2Se films; P1: hot-pressed PVP/Ag2Se films). Reproduced with permission from Ref[97]. Copyright© 2021. Elsevier. FFT: Fast Fourier transform; HRTEM: high-resolution transmission electron microscopy; PVP: poly(vinylpyrrolidone); TEM: transmission electron microscopes.
Room-temperature TE properties of the Ag2Se-based flexible film
| Materials | Preparation method | S (μV/K) | σ (S/cm) | PF (μW/mK2) | ZT | Ref. |
| Ag2Se | Wet-chemical process, hot pressing | -140 | 497 | 987 | 0.6 | [92] |
| Ag1.98Ga0.02Se | Wet-chemical process, hot pressing | -115 | ~880 | 1,162 | ~ | [94] |
| Ag2S1-ySey | Wet-chemical process, hot pressing | -106 | 849 | 954.7 | ~ | [95] |
| Ag2Se | Optimized wet chemical method, hot pressing | -143 | 919 | 1,880 | 0.8 | [96] |
| PVP coated Ag2Se | In-situ synthesis, hot pressing | -143 | 929 | 1,910 | 1.1 | [97] |
| Ag2Se | A one-pot synthesis method, hot pressing | -150 | 908 | 2,043 | ~ | [98] |
| Ag/Ag2Se | Wet-chemical process, hot pressing | -68.5 | 3,958 | 1,860.6 | ~ | [99] |
| Ag/Ag2Se | Microwave-assisted synthesis method | -89.6 | 3,030 | 2,431 | > 0.55 | [100] |
| Ag/Ag2Se | Wet-chemical process, hot pressing | -91.5 | 2,720.1 | 2,277.3 | 0.71 | [101] |
| BC/Ag2Se | Wet-chemical process, blending + hot pressing | -60 | 810 | 291 | ~ | [102] |
| PVDF/Ag2Se | Wet-chemical process, blending | -95.9 | 205.52 | 189.02 | ~ | [103] |
| Ag2Se | Chemical template method | -82.33 | 385.42 | 261.26 | ~ | [104] |
Other inorganic TE materials
Besides Ag2Se-based materials, researchers fully utilize the convenience and film-forming aiding properties of the vacuum filtration method. Inorganic TE materials, such as Cu-Te, Cu-Se, Ag2Te, Bi2Te3, and other inorganic semiconductors, are used to prepare flexible TE films using vacuum filtration. For example, Zhou et al. prepared a free-standing flexible copper telluride NW/PVDF (Cu1.75Te NWs/PVDF = 2:1) thin film using a five-step vacuum filtration process [Figure 13][105]. By burying the Cu1.75Te NWs into the PVDF matrix, the flexible fabric exhibits a Seebeck coefficient and electric conductivity of 9.6 μV/K and 2,490 S/cm at room temperature, respectively, resulting in a power factor of 23 μW/mK2. Pammi et al. reported Cu2-xSe NW/PVDF composite flexible thin films, which achieved a power factor of 105.32 μW/mK2[106]. Notably, the mechanical hot-pressing transfer process followed by vacuum filtration was found to have increased conductance due to a reduction in the junction resistance and to an interconnection density between NWs and densification compared to various fabrication methods. CuI/nylon composite films with a Seebeck coefficient as high as 600 µV/K were also prepared[107]. Although the low electrical conductivity limited its TE performance, the quick voltage response under temperature gradient showed the potential application in wearable thermal sensors.
Figure 13. Schematic diagram of the preparation process for the flexible Cu1.75Te NW/PVDF TE fabric, including (A) vacuum filtration; (B) mechanical pressing; (C) annealing; (D) drop-coating PVDF solution upon the film; and (E) peeling off the NW/PVDF film; (F) The roll-up photograph of the fabricated Cu1.75Te NW/PVDF film. Reproduced with permission from Ref[105]. Copyright© 2015. American Chemical Society. NW: Nanowire; PVDF: polyvinylidene fluoride; TE: thermoelectric.
To improve the electrical conductivity of and maintain the initial morphology of the Ag2Te NWs, Zeng
Room-temperature TE properties of other inorganic TE materials
| Materials | Preparation method | S (μV/K) | σ (S/cm) | PF (μW/mK2) | ZT | Ref. |
| Cu1.75Te NWs/PVDF | Mechanical Press, drop-coating | 9.6 | 2,490 | 23 | ~ | [105] |
| Cu2-xSe NW/PVDF | Drop casting, cold pressing | 14.16 | 5,578.2 | 111.84 | ~0.04 | [106] |
| CuI/Nylon | Wet-chemical process, hot pressing | 600 | 0.09 | 3.03 | ~ | [107] |
| Ag2Te NWs | Hydrothermal | -99.48 | 153.35 | 151.76 | ~ | [108] |
| Room-temperature welding | -154.96 | 149.82 | 359.76 | ~ | ||
| AgxTe NWs | Hydrothermal, in situ chemical transformation, | -50.4 | 192.5 | 48.9 | ~ | [109] |
| CNF/Bi0.5Sb1.5Te3 | Vacuum filtration | 154 | 493 | 1,169.2 | ~ | [110] |
| CNF/Bi2Se0.3Te2.7 | -130 | 532 | 899.08 | ~ | ||
| Bi2Te3 and Sb2Te3 modified paper | Vacuum filtration | 142 | ~ | ~ | ~ | [111] |
| Se/Ag2S | Wet-chemical process, hot pressing | -81.4 | 743 | 492.6 | > 0.26 | [112] |
TWO-DIMENSIONAL TE MATERIALS
Two-dimensional materials have unique state densities with confined electrons and holes, making them promising TE materials. Due to its high in-plane carrier mobility (200-500 cm2/Vs) and low thermal conductivity (0.1-1 W/mK), MoS2 has attracted wide attention. In 2016, Wang et al. prepared exfoliated bulk MoS2 by lithium intercalation and obtained the restacked MoS2 thin film using a filtration technique[113]. The maximum Seebeck coefficient reached 93.5 μV/K; the highest ZT = 0.01 was calculated. Also, this method could be imitated on other transitional metal dichalcogenides. Piao et al. synthesized 1T phase WS2 nanosheets through hydrothermal methods and fabricated flexible WS2 thin films by vacuum filtration with an electrical conductivity of 45 S/cm and a Seebeck coefficient of 30 μV/K[114]. After thermal annealing, the electrical conductivity was enhanced to 120 S/cm while not lowering the Seebeck coefficient, resulting in an optimized power factor of 9.40 μW/mK2. Then, they reported a bottom-up hydrothermal route to prepare 1T-WS2/SWCNT hybrid composites with 3D architecture in which 1D SWCNT nanotubes and 2D WS2 nanosheets interweaving[115]. Enhanced electrical conductivity and Seebeck coefficients were obtained by introducing SWCNTs into 1T-WS2. Flexible thin films with an impressive power factor of
MXenes have attracted widespread attention as two-dimensional materials containing transition metal carbides and nitrides. In 2017, Kim et al. employed vacuum-assisted filtration to prepare three free-standing MXenes (Mo2CTx, Mo2TiC2Tx, and Mo2Ti2C3Tx) films [Figure 14A][22], which exhibit n-type Seebeck coefficients and high electrical conductivity upon heating to 800 K. For the Mo2TiC2Tx MXene, the power factor reached 3.09 × 10-4 W/mK2 at 803 K. Then, Ding et al. reported that a practical heterostructure design could effectively improve the TE performance of MXenes [Figure 14B][116]. Layered composite films with 2D-3D sandwich structures consisting of 2D MXenes and 1D SWCNTs were prepared by vacuum-assisted filtration. The Ti3C2Tx-SWCNTs-Ti3C2Tx film displayed considerable electrical conductivity of 750.9 S/cm and an enhanced Seebeck coefficient of -32.2 μV/K. Compared to Ti3C2Tx/SWCNT films with interlaced structures, the Ti3C2Tx-SWCNTs-Ti3C2Tx film shows a higher power factor, 25 times higher than that of
Figure 14. (A) TE properties of two-dimensional molybdenum-based MXenes. Reproduced with permission from Ref[22]. Copyright© 2017. American Chemical Society; (B) The schematic structure diagrams of Ti3C2Tx/SWCNT, Ti3C2Tx-SWCNT-Ti3C2Tx, and SWCNT-Ti3C2Tx-SWCNT films. Reproduced with permission from Ref[116]. Copyright© 2020. John Wiley and Sons; (C) Schematic demonstration of the preparation process of Bi2Te2.7Se0.3/Ti3C2Tx composites. Reproduced with permission from Ref[117]. Copyright© 2022. Elsevier. TE: Thermoelectric; SWCNT: single-walled carbon nanotube.
Room-temperature TE properties of 2D materials
| Materials | Preparation method | S (μV/K) | σ (S/cm) | PF (μW/mK2) | ZT | Ref. |
| MoS2 | Lithium intercalation, | 93.5 | 17.5 | 13 | 0.01 | [113] |
| WS2 | Lithium intercalation | 45 | 30 | 4.05 | ~ | [114] |
| WS2/SWCNT | Hydrothermal | 43.2 | 318 | 61.7 | ~ | [115] |
| Mo2TiC2Tx | Wet-chemical process | -47.3 | 1,380 | 309 (803K) | ~ | [22] |
| Ti3C2Tx/SWCNTs/Ti3C2Tx | Lithium intercalation, layer-by-layer stacking | -32.2 | 750.9 | 77.9 | ~ | [116] |
| Bi2Te2.7Se0.3/Ti3C2Tx | Physical mixing, hot pressing | -148 | 830 | 1,818 | 1.2 | [117] |
TERNARY COMPOSITE TE MATERIALS
Introducing carbon materials with high electrical conductivity into inorganic TE materials fillers with high Seebeck coefficients and combining a conducting polymer matrix with low thermal conductivity to prepare ternary composites shall be an effective strategy to achieve excellent TE properties. In such cases, the interfacial contact and interaction between components will directly affect the film formation and properties. The direct vacuum filtration method has been proven to be a good choice. By vacuum filtration, Liu et al. prepared flexible SWCNT/SnSe/PEDOT:PSS nanocomposite films on a nylon membrane
Figure 15. (A) Schematic illustrations for exfoliation of SnSe particles into nanosheets; (B) Fabrication of flexible SWCNT/SnSe/PEDOT:PSS nanocomposite films; (C) Digital photos for fabrication of the free-standing, flexible SWCNT/SnSe/PEDOT:PSS composite films. Reproduced with permission from Ref[118]. Copyright© 2020. John Wiley and Sons. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid); SWCNT: single-walled carbon nanotube.
As mentioned above, Ag2Se is a particularly excellent inorganic TE material. To further improve the TE performance of Ag2Se, Lu et al. have conducted a series of studies on Ag2Se-based ternary composite materials by vacuum filtration. By adjusting the molar ratio of Cu2+:Ag+:Se2- in the precursor, they prepared n-type Ag2Se/Ag/CuAgSe ternary films with a recorded power factor of 1,593.9 μW/mK2[122]. The bending test shows that the hybrid film has excellent flexibility, and the power factor only decreases by 10% after bending 1,000 times along the rod with a radius of 4 mm. Optimized carrier transport and the interfacial energy filtering effect of the composite contribute to the ultrahigh power factor. They also prepared PEDOT/Ag2Se/CuAgSe (1,603 μW/mK2[123]), Ag2Se/Ag/PEDOT (1,442 μW/mK2[124]), Ag2Se/Se/Polypyrrole (2,240 μW/mK2[125]) composite films for flexible TEGs. These results are several orders of magnitudes higher than those of Ag2Se-based composite films prepared from other methods, such as Ag2Se NW/PEDOT:PSS composite films prepared from a drop-casting method with a maximum room-temperature power factor of 178.59 µW/mK2[126]; and Ag2Se NW/PEDOT:PSS composite film prepared from a digital light processing-based 3D printing method with a power factor of 81.94 µW/mK2[127], indicating the significant advantages of vacuum filtration method for the preparation of flexible TE films.
CONCLUSION AND OUTLOOK
Recent years have seen significant progress in using vacuum filtration to advance flexible TE films scientifically and technologically. The current review article highlights the construction of conducting polymer-based, carbon nanoparticle-based, inorganic, two-dimensional materials, and ternary composites, respectively. Despite the recent success in this research area, many critical issues must be addressed.
(1) Currently, the filter membranes used in the filtration method are insulating. On the one hand, they are embedded in the TE film as a flexible skeleton. On the other hand, their insulation affects the overall TE performance, considering that TE materials are the properties of bulk materials. In the future, we could design a filter membrane with high conductivity to provide electrical conductivity further to enhance TE performance while achieving flexible support. Even once the organic TE filter membrane is developed, the composite TE film can be directly prepared through vacuum filtration.
(2) Although some materials can detangle from the filter to form self-supporting films, this is limited to thick films at the micron scale. Is it possible to transfer nano-thickness films such as PEDOT:PSS from the filter membrane substrate to organic optoelectronic devices in the future? Most PEDOT:PSS hole transport layers in organic optoelectronic devices are prepared by spin coating or drip coating methods, which are still difficult to remove PSS. Since the filtration method we discussed above can effectively filter out much
(3) The future of wearable applications will require flexibility, stretchability, and anti-fatigue stability. TE films prepared based on this method have achieved the goal of flexibility, but there are few studies on stretchability, fatigue stability, etc. In the future, improving both the substrate and the material may be possible, such as finding a stretchable substrate or further enhancing the binding force between the substrate and the film. Only in this way can the material prepared by the vacuum filtration method realize the leap from flexibility to stretchability and be closer to practical application.
(4) The integrated use of multifunctional equipment is the future development trend of intelligent equipment. For example, combining TE devices and supercapacitors can enable the integration of functions from power generation to power storage. The combination of TE and electrochromic devices allows the device to achieve the role of color change in the environment without a power supply. Suppose the process of preparing TE flexible films by vacuum filtration can be extended to the preparation process of other functional films. The difficulty and cost of preparing multifunctional integrated devices will be significantly simplified in that case. There have been reports on the preparation of flexible supercapacitor films by the vacuum filtration method, and we expect that there will be reports on the application of multifunctional integrated devices based on the extraction filtration method shortly.
DECLARATIONS
Authors’ contributions
Initiated the idea: Jiang Q, Song H
Conducted the literature review: Jiang Q, Wang C
Outlined the manuscript structure: Song H, Jiang Q
Wrote the manuscript draft: Song H, Jiang Q, Wang C, Liu Q
Designed and formatted the figures: Song H, Wang C, Liu Q
Reviewed and revised the manuscript: Song H, Jiang Q
All authors have read the manuscript and approved the final version.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the Natural Nature Science Foundation of China (52203221); the Natural Science Foundation of Guangdong Province (2022A1515010063); SAFEA: the High-End Foreign Experts Project (G2022163014L); the Science and Technology Bureau Project of Jiaxing (Grant No. 2020AY10010), and the Research Training Program for College Students of Jiaxing University in 2022 (CD 8517221295).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2023.
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Wang, C.; Liu, Q.; Song, H.; Jiang, Q. Vacuum filtration method towards flexible thermoelectric films. Soft Sci. 2023, 3, 34. http://dx.doi.org/10.20517/ss.2023.25
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