Cellulose Acetate Thermoelectric Papers - ACS

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Carbon Nanotube/Cellulose Acetate Thermoelectric Papers Jun-Hyun Mo, Jae-Yeop Kim, Young Hun Kang, Song Yun Cho, and Kwang-Suk Jang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03670 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Sustainable Chemistry & Engineering

Carbon Nanotube/Cellulose Acetate Thermoelectric Papers

Jun-Hyun Mo,† Jae-Yeop Kim,† Young Hun Kang,‡ Song Yun Cho*,‡ and KwangSuk Jang*,†,§

†Department

of Applied Chemistry, Hanyang University, 55 Hanyangdeahak-ro, Sangnok-gu,

Ansan, Gyeonggi-do 15588, Republic of Korea. E-mail: [email protected] ‡Division

of Advanced Materials, Korea Research Institute of Chemical Technology, 141

Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea. E-mail: [email protected] §Department

of Chemical and Molecular Engineering, Hanyang University, 55 Hanyangdeahak-

ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea

Corresponding Authors *E-mail: [email protected] (K.S.J.) *E-mail: [email protected] (S.Y.C.)

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ABSTRACT: Free-standing single-walled carbon nanotube (SWNCT)/cellulose acetate composite films were fabricated by a simple bar-coating method. As a paste solvent, acetone, a low-boiling-point solvent, was used. By simple brushing of polyethylenimine/ethanol solution, ntype thermoelectric composite films could be obtained. The optimal p-type and n-type power factors of the free-standing thermoelectric films were 1.41 ± 0.22 and 0.516 ± 0.172 μW cm-1 K-2, respectively, at room temperature. We also fabricated the organic thermoelectric generator from the free-standing composite films and confirmed their electrical power generation capability. By applying the temperature difference of 10 K, the output power of 2.28 μW was obtained. KEYWORDS: organic thermoelectric materials, organic thermoelectric generators, thermoelectric papers, bar-coating, carbon nanotubes


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During the past decades, researchers have significantly studied organic thermoelectric materials for their use in flexible and low-cost energy harvesting devices.1-19 By using the Seebeck effect, thermoelectric generators harvest energy from heat. To evaluate the performance of thermoelectric materials, their Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) should be measured to extract ZT, the dimensionless figure of merit (S2σT/κ). The power factor (S2σ) is also used to estimate of the thermoelectric performance of organic thermoelectric films. In recent years, organic thermoelectric thin films with excellent thermoelectric power factors have been reported.1-7 It was reported that the poly(3,4-ethylenedioxythiophene) (PEDOT) thin films doped with tosylate exhibit power factors up to 3.24 µW cm-1 K-2.1 It was reported that PEDOT thin films doped with poly(styrenesulfonate) (PSS) exhibit power factors up to ~5 µW cm-1 K-2. 2 It was reported that PEDOT thin films with power factors up to 1.27 µW cm-1 K-2 can be obtained with precise controlling of the oxidation level.3 Such improvement in the thermoelectric power factor is very promising for applications of organic thermoelectric materials. However, use of polymer thermoelectric thin films with a thickness less than 1 µm might have limitations for device applications, owing to the high internal resistance of the thermoelectric generators. Organic thermoelectric materials can be fabricated into diverse shapes and dimensions for efficient device applications using various solution processes. Recently, free-standing thermoelectric polymer films with a thickness over 10 µm have been reported.8-11 It was reported that formic acidtreated PEDOT:PSS films with a thickness of ~100 µm exhibit a power factor of 0.806 µW cm-1 K-2.8 To improve the thermoelectric performance of the polymers, carbon nanotubes (CNTs) have been added to polymer matrices as a filler.12-17 In general, CNTs exhibit much higher electrical conductivities than conjugated polymers. Because single-walled CNTs (SWCNTs) exhibit better thermoelectric performance than multi-walled CNTs,12-17 hybridization of conjugated polymers



with SWCNTs is thought to be an effective means to enhance the thermoelectric performance. Recently, free-standing thermoelectric CNT/polymer composite films with thickness more than 10 µm have also been reported.18,19 It was reported that the SWCNT/PEDOT:PSS/polyvinyl acetate nanocomposite films prepared with thickness in the range of 20 to 53 µm exhibit power factors up to ~1.6 µW cm-1 K-2.18 In general, CNT/polymer composites show p-type Seebeck coefficients. For fabrication of highperformance organic thermoelectric devices, both p-type and n-type thermoelectric materials should be used. Because CNTs can be doped with effective electron donor molecules such as polyethylenimine (PEI) to obtain n-type characteristics,20-23 n-type thermoelectric composites can be fabricated by using n-type conjugated polymers or electrically insulating polymers as a polymer matrix. Piao et al. reported that free-standing SWCNT/polyvinyl alcohol (electrically insulating polymer) composite films with thickness in the range of 150 to 200 µm exhibit a p-type power factor of ~0.001 µW cm-1 K-2.24 PEI-doped free-standing composite films have been reported to show an n-type thermoelectric power factor of ~0.0004 µW cm-1 K-2. The energy harvesting device was fabricated using 5 pairs of p-type and n-type CNT/polyvinyl alcohol composite films,24 and the optimal output power obtained from a temperature difference of 50 K was 4.5 nW. For applications, both the p- and the n-type power factors of the CNT/electrically insulating polymer nanocomposite film need to be enhanced further. We report a facile method for fabrication of free-standing SWCNT/insulating polymer composite films and their n-type doping for obtaining high-performance organic thermoelectric generators in this study. Free-standing p-type SWCNT/cellulose acetate composite films with a thickness of ~30 µm were prepared by simple bar-coating of the acetone-based paste. For the n-type doping of


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SWCNTs, the PEI/ethanol solution was just brushed on the composite films. The thermoelectric composite papers exhibited optimal p-type and n-type thermoelectric power factors of 1.41 ± 0.22 and 0.516 ± 0.172 μW cm-1 K-2 at room temperature, respectively. The free-standing thermoelectric composite films, i.e., thermoelectric papers, can be cut, folded, and pasted for fabricating efficient thermoelectric generators. We fabricated a paper-based thermoelectric generator by using the thermoelectric paper strips with alternating p-type and n-type sections and studied the capability of the electrical power generation. The bar-coating process was used for facile fabrication of free-standing SWCNT/electrically insulating polymer composite films. To obtain homogeneous films with a thickness more than 10 µm, we used a common low-boiling-point solvent, acetone, as a paste solvent. Cellulose acetate was used as the polymer matrix, because it is well dissolved in acetone and is known to form freestanding thick films, such as laboratory membrane filters, easily. Because the boiling point of acetone is 56 °C, the bar-coated films were dried within 30 min, forming homogeneous SWCNT/cellulose acetate composite films with a thickness of ~30 µm. When using a solvent with a higher boiling point as a paste solvent for fabrication of CNT/polymer composite films, more than a few hours may be required for the drying process. During solvent drying, the paste become concentrated and the dispersion state of the CNTs would change. In this concentrated paste, the CNTs would agglomerate, inducing a non-homogeneous morphology in the final composite films.15 We fabricated SWCNT/cellulose acetate composite films with SWCNT content from 10 to 90 wt% (see the Experimental section in the Supporting Information). For comparison, SWCNT films were also prepared by the same process. The thermoelectric performances of the films were evaluated



(see the Experimental section in the Supporting Information). Because the film with a SWCNT content of 0 wt%, i.e. the cellulose acetate film, is electrically insulating, its Seebeck coefficient cannot be measured. The temperature difference (ΔT) was controlled to be 1 to 8 K. For the measurement of the Seebeck coefficients, ΔV was measured 20 times at each ΔT point and the average ΔV value was obtained. The Seebeck coefficients were obtained from the slope of the ΔVΔT curves.13-17 The representative ΔVΔT curve is given in the Supporting Information (Figure S1). Figure 1 shows the SWCNT content-dependent Seebeck coefficient, electrical conductivity, and power factor of the prepared films. The thermoelectric properties of the composite films are mainly affected by the characteristics of the SWCNTs and their dispersion state in the composites, because cellulose acetate is electrically insulating. Because SWCNTs are p-type, the thermoelectric films have p-type Seebeck coefficients. However, the relationship of the Seebeck coefficient and the SWCNT content is not clear. The composite films with SWCNT content ranging from 30 to 90 wt% exhibit Seebeck coefficient in the range of 37.9 to 45.5 µV K-1, which is comparable to that of the SWCNT films, 45.7 µV K-1. The composite films with SWCNT contents of 10 and 20 wt% exhibit higher Seebeck coefficients, 65.2 and 56.3 µV K-1, respectively. Although the reason for this is unclear, it might be related to the relatively well-dispersed state of CNTs at lower SWCNT contents and/or the energy filtering effect at the CNT-cellulose acetate interface, where carriers with higher energy might pass preferentially, resulting to increasing the Seebeck coefficient.25,26 We found that the SWCNT content is related more to the electrical conductivity of the thermoelectric films. The electrical conductivity of the composite films increases exponentially, as the SWCNT content increases. The SWCNT content in the composite films is strongly related to the number of junctions between CNT bundles, which are the electrical pathways. It was reported that the electric conduction of SWCNT films follows a three-


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dimensional variable-range hopping model, while that of SWCNT/electrically insulating polymer composite films follows a fluctuation-induced tunneling model.27 In the composite films, the polymer part acts as a barrier to hopping between the CNT bundles. Interestingly, the electrical conductivity of the composite films with a SWCNT content of 90 wt%, 873 ± 100 S cm-1, is much higher than that of the SWCNT films, 549 ± 31 S cm-1. This might be related to the morphology and the dispersion state of SWCNT bundles in the films. The scanning electron microscopy (SEM) images of the SWCNT/cellulose acetate composite film with SWCNT contents of 90 and 100 wt% were obtained (Figure 2). In both the films, the networks of CNT bundles are clearly visible. The CNT bundles with smaller diameters of approximately 20–30 nm and their secondary aggregates are observed in the film with SWCNT content of 90 wt%. Whereas, the number of secondary aggregates of the CNT bundles is relatively less and their diameter is much smaller at 100 wt% SWCNT content. The parallel contact between the axial-direction surfaces of the CNT bundles might reduce the overall inter-bundle resistance in the composite film. The overall conductivity of the CNT/insulating polymer composite films is determined by the resistance between the CNT bundles.12,14 The secondary aggregates of CNT bundles, forming a parallel contact, have also been observed in vacuum-filtrated single- and few-walled CNT films with very high electrical conductivities of 2136 and 3920 S cm-1, respectively.13 The SEM images of the thermoelectric SWCNT/cellulose acetate films with the CNT contents of 10, 30, 50, and 70 wt% were also obtained (Figure S2). In the composites with the CNT content of 10 wt%, the SWCNT bundles were wrapped well with cellulose acetate chains, barriers to hopping between the CNT bundles. In the SWCNT/cellulose acetate composites with the CNT content of 10 and 30 wt%, there are CNT bundles with diameters of approximately 20–30 nm and their secondary aggregates are not observed. In the SWCNT/cellulose acetate composite films with the SWCNT content of 50 and 70



wt%, the cellulose acetate chains are not clearly visible. By increasing the SWCNT content from 50 to 90 wt%, the number and diameter of secondary aggregates of the CNT bundles increases. The high SWCNT content and presence of cellulose acetate might be related with the formation of the secondary aggregates of the CNT bundles. The power factors of the thermoelectric films were calculated using the Seebeck coefficient and electrical conductivity. The power factor of the composite films increases exponentially as the SWCNT content increases in the range of 10 to 90 wt%. This increase originates from the higher electrical conductivity of the composite films at higher SWCNT contents. As a result, the power factor of the composites with a SWCNT content of 90 wt%, 1.41 ± 0.22 μW cm-1 K-2, is much higher than that of the SWCNT films, 1.15 ± 0.05 μW cm-1 K-2.


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Figure 1. Dependence of (a) Seebeck coefficient, (b) electrical conductivity, and (c) power factor of SWCNT/cellulose acetate thermoelectric films on the SWCNT content.



Figure 2. SEM images of the (a) SWCNT (90 wt%)/cellulose acetate composite film and (b) SWCNT film.

For the fabrication of high-performance organic thermoelectric devices, both p-type and n-type elements should be used. For n-type doping, the PEI/ethanol solution was simply brushed on the surface of the composite films. Figure 3 shows the dependence of Seebeck coefficient, electrical


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conductivity, and power factor of the PEI-treated SWCNT/cellulose acetate thermoelectric films with the SWCNT content of 90 wt% on the dopant concentration. The composite films treated with 0 to 10 wt% PEI solution exhibited n-type characteristics. The PEI solutions with concentrations over 10 wt% cannot be used because the viscosity was too high for the brushing process. The composite films exhibit the optimal Seebeck coefficient at a PEI concentration of 6 wt%. The electrical conductivity of the composite films increases as the PEI concentration is increased from 2 to 8 wt%. The concentration of n-type carriers in the thermoelectric films may increase significantly by increasing the PEI concentration from 2 to 8 wt%. By increasing the PEI concentration further, the electrical conductivity decreases. This may be attributed to the presence of excessive amounts of PEI molecules in the 10 wt% PEI/ethanol solution-treated composite films. As a result, the n-type power factor of the composite films exhibits a maximum at a PEI concentration of 6 wt%. Thus, by using a simple brushing process, SWCNT/cellulose acetate composite films with an excellent n-type power factor of 0.516 ± 0.172 μW cm-1 K-2 could be obtained. The dopant molecules on surface of the SWCNT bundles act as excellent electron donors.20-23 At the low concentration of PEI, there might be remaining p-type CNT bundles. At the PEI concentration where the amount of p-type CNT bundles will be negligible, the n-type Seebeck coefficient will be optimized.17,20-23 As the PEI concentration increases further, the n-type carrier concentration increases, the Seebeck coefficient decreases, and the electrical conductivity increases.17,20-23 However, here, a significant statement cannot be made because the error ranges in the thermoelectric properties are too large. By brushing of the PEI solution on the composite films with a thickness of ~30 µm, the remaining amount of PEI molecules in the composite films cannot be controlled precisely. The SEM image of the 6 wt% PEI/ethanol solution-treated SWCNT (90 wt%)/cellulose acetate composite film is shown in Figure S3. The CNT bundles and their



secondary aggregates were also observed. In addition, there are spherical particles with a diameter of ~30 nm. The particles on the surface of the film might be the aggregates of excessive PEI

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The thermoelectric film with a SWCNT content of 90 wt% could be easily peeled off owing to poor adhesion. The free-standing thermoelectric composite films, i.e., thermoelectric papers, could be cut, folded and pasted for fabricating efficient thermoelectric generators (Figure 4). The thermoelectric paper with a thickness of ~30 µm was cut to strips with a width of ~2 mm and a length of 6 cm. The 6 strips were pasted and folded repeatedly, coiling the paper substrate. The strips in the paper-based thermoelectric generator were connected in series using silver paste. The untreated thermoelectric paper exhibits p-type characteristics. To make alternating p-type and ntype sections in the paper-based thermoelectric generator, a 6 wt% PEI/ethanol solution was brushed on one face of the paper device. As a result, the paper-based thermoelectric generator with a width of 15 mm and a length of 7 cm consists of alternating non-doped and PEI-doped strip regions with 15 n-type and 16 p-type components (Figure 5a). Figure 5b shows the power versus current curve and the voltage versus current curve of the prepared device. We applied the temperature difference of 10 K between the two sides of the device. The paper-based energy harvesting device exhibits the maximum power of 2.28 μW. For practical use of these paper-based devices, the energy harvesting performance still needs to be enhanced. Therefore, this study demonstrates a simple device structure; because the thermoelectric papers have diversity in shape and dimensions, the performance of the paper-based thermoelectric generators may be enhanced by optimizing the device structures. For example, the more extended thermoelectric generator will exhibit the higher output power. It is expected that the paper-based thermoelectric generators can be rolled or folded for optimizing the device structure. Thermoelectric properties of the thermoelectric papers were measured at room temperature. In general, organic thermoelectric materials are thought to be applicable at low temperatures below 323 K. Study on the heat resistance and stability by repeated use of the thermoelectric papers will be our future work for the



application. To estimate ZT values, thermal conductivities of the thermoelectric papers were measured thorough the in-plane direction. The thermal conductivities of the 30.2 µm thick thermoelectric paper with the 90 wt% SWCNT content and the 30.5 µm thick 6 wt% PEI/ethanol solution-treated thermoelectric paper with the 90 wt% SWCNT content were measured to be 37.5 and 38.5 W m-1 K-1, respectively. The ZT values of the p-type and n-type thermoelectric papers are calculated to be ~0.0011 and ~0.00040. The measured ZT values are comparable to the ZT values of the CNT/polyaniline composites and polyaniline/3D CNT network composites, ~0.0007 and 0.0022, respectively.28,29


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Figure 4. Photo images representing the fabrication process of the paper-based organic thermoelectric generator.



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Figure 5. (a) Photo image and (b) voltage versus current curve and power versus current curve of the organic thermoelectric generator.

In summary, we report the fabrication of thermoelectric SWCNT/cellulose acetate papers and their n-type doping. The thermoelectric papers with a SWCNT content of 90 wt% exhibited an optimal p-type power factor of 1.41 ± 0.22 μW cm-1 K-2 at room temperature. The thermoelectric papers brushed with 6 wt% PEI/ethanol solution exhibited an optimal n-type power factor of 0.516 ± 0.172 μW cm-1 K-2 at room temperature. The thermoelectric papers could be cut, folded, and pasted to obtain an efficient device structure for organic thermoelectric generators. Using a simple brushing process, selective n-type doping could be achieved for forming alternating p-type and ntype sections in series. The capability of the paper-based energy harvesting device was demonstrated. The maximum output power of the device was 2.28 μW. Our results suggest that the thermoelectric paper is one of the most promising candidates for flexible and low-cost energy harvesting devices.


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■ ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Experimental; representative ΔVΔT curve; SEM images of the composite films; schematic drawing of setting the ΔT in the device (PDF)

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (K.S.J.) *E-mail: [email protected] (S.Y.C.) Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03930409) and the R&D Convergence Program of NST (National Research Council of Science & Technology).



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The paper-based thermoelectric generator was demonstrated with the carbon nanotube/cellulose acetate thermoelectric papers.


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Cellulose Acetate Thermoelectric Papers - ACS

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