Flexible and Robust Thermoelectric Generators Based on All-Carbon

Jul 12, 2017 - As practical interest in flexible/or wearable power-conversion devices increases, the demand for high-performance alternatives to therm...
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Flexible and Robust Thermoelectric Generators Based on All-Carbon Nanotube Yarn without Metal Electrodes Jaeyoo Choi,†,‡,# Yeonsu Jung,‡,# Seung Jae Yang,∥ Jun Young Oh,∥ Jinwoo Oh,† Kiyoung Jo,† Jeong Gon Son,† Seung Eon Moon,⊥ Chong Rae Park,*,‡ and Heesuk Kim*,†,§ †

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea Carbon Nanomaterials Design Laboratory, Global Research Laboratory, Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea ∥ Department of Applied Organic Materials Engineering, Inha University, Incheon 402-751, Republic of Korea ⊥ Electronics and Telecommunications Research Institute (ETRI), Daejeon 34129, Republic of Korea § Nano-Materials and Engineering, Korea University of Science and Technology (UST), Daejeon 34113, Republic of Korea ‡

S Supporting Information *

ABSTRACT: As practical interest in flexible/or wearable power-conversion devices increases, the demand for high-performance alternatives to thermoelectric (TE) generators based on brittle inorganic materials is growing. Herein, we propose a flexible and ultralight TE generator (TEG) based on carbon nanotube yarn (CNTY) with excellent TE performance. The asprepared CNTY shows a superior electrical conductivity of 3147 S/cm due to increased longitudinal carrier mobility derived from a highly aligned structure. Our TEG is innovative in that the CNTY acts as multifunctions in the same device. The CNTY is alternatively doped into n- and p-types using polyethylenimine and FeCl3, respectively. The highly conductive CNTY between the doped regions is used as electrodes to minimize the circuit resistance, thereby forming an all-carbon TEG without additional metal deposition. A flexible TEG based on 60 pairs of n- and p-doped CNTY shows the maximum power density of 10.85 and 697 μW/g at temperature differences of 5 and 40 K, respectively, which are the highest values among reported TEGs based on flexible materials. We believe that the strategy proposed here to improve the power density of flexible TEG by introducing highly aligned CNTY and designing a device without metal electrodes shows great potential for the flexible/or wearable power-conversion devices. KEYWORDS: carbon nanotube yarn, thermoelectric properties, flexible materials, robust module, energy conversion (PEDOT:PSS).3,6,19 However, flexible TE materials based on PEDOT:PSS are sensitive to humidity in ambient conditions, leading to a limitation for their practical applications. As another possible candidate, carbon nanotubes (CNT) have great potential for use in flexible TE materials because of its high electrical conductivity and controllable Seebeck coefficient (or thermopower).20 In addition, it has excellent mechanical properties such as a low density of ∼1 g/cm3, approximately seven times lower than that of bulk Bi2Te3 (7.86 g/cm3). Its carrier type and carrier concentration can be easily tuned as

T

hermoelectric (TE) materials, which harvest electrical energy directly from temperature gradients, are an emerging technology due to their potential applications for next generation power generators.1 As the practical demand for flexible power-conversion devices increases, the development of high-performance alternatives to brittle inorganic TE materials is essential.2−4 Organic polymers,5−7 nanocarbons,8−12 and their hybrid materials13−18 have been investigated as possible alternatives for flexible TE materials because of their well-known advantages including flexibility, lightweight, low cost, easy processability, and scalability.2,7 Although the TE performance of flexible materials has been significantly enhanced, their high performance is generally related to the extremely high electrical conductivity of organic polymers such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) © 2017 American Chemical Society

Received: March 14, 2017 Accepted: July 12, 2017 Published: July 12, 2017 7608

DOI: 10.1021/acsnano.7b01771 ACS Nano 2017, 11, 7608−7614

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Figure 1. Fabrication of the CNTY and flexible TEG based on CNTY. (a) Schematic illustration of the flexible TEG based on CNTY. (b) FESEM image of the CNTY. (c) HR-TEM image of single strand CNT. (d) Electrical conductivity (S/cm) and specific conductivity (S·cm2/g) of the CNTY and typical conducting materials. The specific conductivity was obtained by dividing the electrical conductivity by density.

flexible materials. Furthermore, the flexible TEG based on 240 pairs of n- and p-doped CNTY can successfully power a red LED using an energy harvesting circuit at ΔT = 50 K.

well by simple chemical doping, thereby allowing for its use in many flexible/or wearable TE applications. Most CNT-based TE generators (TEGs) have been fabricated via alternative stacking/or printing of p- and n-type CNT films/or inks with metal deposition as an electrode between p- and n-type unit.10,21 However, in the case of highly electrically conductive CNT, the contact resistance between metal electrode and CNT is higher than the resistance of metal or CNT itself, thus decreasing the output power density of flexible TEGs. In this study, we demonstrate the robust carbon nanotube yarn (CNTY) with excellent electrical conductivity of 3147 S/cm which is due to increased longitudinal carrier mobility derived from highly aligned structure. On the basis of highly conductive CNTY, we fabricate an all-carbon TEG with superior TE performance. The CNTY was alternatively doped into n- and p-types using polyethylenimine and FeCl3, respectively. The CNTY between the doped regions was used as electrodes to minimize the circuit resistance because it has excellent electrical conductivity. On the basis of mechanical strength and shape advantage of CNTY for modular fabrication, various types of flexible TEGs were prepared. A prototype flexible TEG with 60 pairs of n- and p-doped CNTY shows a maximum power density of 10.85 and 697 μW/g at temperature differences (ΔT) of 5 and 40 K, respectively, which are the highest values among reported TEGs based on

RESULTS AND DISCUSSION Figure 1a shows a schematic of flexible TEG based on CNTY without metal electrodes. The synthesized CNTY was wound onto a flexible supporting unit, and alternatively doped into nand p-types using polyethylenimine (PEI) and FeCl3 solutions, respectively, with undoped material between the two doped regions. The CNTY used in TEG fabrication was continuously produced by direct spinning after synthesis. Thousands of individual double walled-CNTs with a diameter of 5 nm compose a CNTY of 30 μm in diameter as shown in the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Figure 1b,c and Figure S1 in the Supporting Information. Highly integrated CNTYs have a density of ∼1.0 g/cm3 and high specific strength of ∼1 GPa/(g· cm−3) (Figure S2), enough to support a 5 kg weight and preserve its tear resistance after soaking in liquid nitrogen (Video S1). Furthermore, the CNTY has excellent specific electrical conductivity on the order of 103 S·cm2/g, which is comparable to some metals (Figure 1d). The superior electrical conductivity of CNTY could be due to the high degree of internal alignment and high longitudinal 7609

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Figure 2. Controlling the degree of CNT alignment. (a−d) FE-SEM images and (insets) morphological schematics illustrations of c-CNTF (a), a-CNTF (b), r-CNTY (c), and CNTY (d). (e−h) Polarized Raman spectra of c-CNTF (e), a-CNTF (f), r-CNTY (g), and CNTY (h).

Figure 3. Thermoelectric properties of the CNT samples after n- and p-type doping. (a) Electrical conductivity. (b) Thermopower. (c) Power factor of c-CNTF, a-CNTF, r-CNTY, and CNTY.

thermopower is mainly due to an increase of carrier mobility derived from the highly aligned structure. For practical use of CNTY in TE applications, the TE properties of CNTY after n- and p-doping are important (doping conditions are shown in Figure S3).24−26 The methods to dope the CNTs can be classified into two categories: (1) substitutional doping in which carbon atoms are replaced by heteroatoms, and (2) surface transfer charge doping in which dopant molecules adsorbed on the CNT surface induce n- or pdoping. The surface transfer doping is a more effective way to dope the CNTs without disrupting the CNT structure. The adsorption of molecules with electron-donating groups on the CNT surface will lead to n-type doping, whereas molecules with electron-withdrawing groups will result in p-type doping. The typical dopants based on PEI and FeCl3 aqueous solutions were used in this study. The PEI with electron donating groups such as primary-, secondary-, and tertiary amines is useful as an ndopant.24 As a p-dopant, the FeCl3 molecules are effective because of the charge transfer from the valence band of CNT to the FeCl3 molecules.25 When we used dopants based on organic solvents, it was difficult to get the electrode parts between the doped regions because the hydrophobic CNTYs strongly absorb organic solutions. Figure 3 also shows the electrical conductivity, thermopower, and power factor of doped CNT samples (detailed values are given in Tables S2 and S3). The electrical conductivity of a-CNTF increases from

carrier mobility it affords. To demonstrate the effect of CNT alignment on electrical conductivity, we systemically controlled the directional alignment of CNT bundles in four different samples (Figure 2): (a) vacuum-filtered film of commercial CNT (c-CNTF), (b) as-synthesized CNT film (a-CNTF), (c) CNT yarn by rolling up a-CNTF (r-CNTY), and (d) CNTY by direct spinning of a-CNTF (CNTY). Figure 2 shows SEM images of four different samples, which indicate that the CNTY features the best alignment. Additionally, the polarized Raman intensity factor (IG∥/IG⊥, the ratio of G peak intensity parallel to the Raman laser to one in the vertical direction) of CNTY is 3.8, stating that the degree of CNTY alignment here is the highest among the samples.22 Figure 3 summarizes the TE properties of these four samples. As the CNT alignment increases, the electrical conductivity (σ) drastically increases from 876 to 3147 S/cm. The specific electrical conductivity also increases with increasing the CNT alignment as shown in Table S1. In contrast, the thermopower (S) is relatively unchanged, thus significantly increasing the power factor (S2σ). According to the Drude model, the thermopower is inversely proportional to the carrier concentration but is unrelated to the carrier mobility.23 As shown in Hall measurement data of c-CNTF and a-CNTF (Table S2), the carrier mobility increases without a main increase in the carrier concentration when the degree of alignment increases. These results indicate that the enhanced electrical conductivity without a major decrease in the 7610

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Figure 4. Thermoelectric performance of the flexible TEG. (a) Photograph of f-TEG with 60 PN pairs. (b) Photograph of power measurement system. (c) Output voltage density of f-TEG with 60 PN pairs as a function of temperature difference. (d) Output power density of f-TEG with 60 PN pairs as a function of load resistance. (e) Output voltage and power of f-TEG with different numbers of PN pairs at ΔT = 5 K. (f) Average resistance per PN pair and circuit resistance of f-TEG with different numbers of PN pairs at ΔT = 5 K.

1340 to 2272 S/cm after p-doping but slightly decreases after ndoping. Nevertheless, the thermopower of a-CNTF slightly increases after doping. To further clarify this effect, we characterized the carrier properties of doped a-CNTF (Table S2). The a-CNTF shows a slight increase in carrier concentration by p-doping without a critical decrease in carrier mobility, thus leading to enhanced electrical conductivity with slightly increased thermopower by effective doping of a-CNTF. It is interesting to note that doped a-CNTF shows a higher thermopower than the pristine sample despite the increased carrier concentration. This could be explained by the effects of the Fe catalyst used as a seed in the synthesis of a-CNTF. As shown in Figure S4, the amount of Fe in the purified CNTY is 1.7 wt % and Fe can act as an n-dopant (because the CNTY and a-CNTF are synthesized using the same procedure, they contain the same amount of Fe catalyst).27 While the assynthesized CNTY with 12.2 wt % Fe shows p-type characteristics with a thermopower of 4 μV/K, the CNTY synthesized with more Fe catalyst and the CNTY purified with 1 M HCl (the CNTY used in all experiments is purified one) show thermopower values of −31 and 50 μV/K, respectively (Table S4). These indicate that the purified CNTY might show lower thermopower than the perfectly purified CNTY without any residual Fe, because Fe acts as an n-dopant. Therefore, the effective doping of CNTY can optimize the charge carriers and consequently increase the thermopower and electrical conductivity, resulting in significantly enhanced power factors of 2387 and 2456 μW/(m·K2) by p- and n-doping, respectively. These values are some of the highest power factors reported for flexible TE materials such as PEDOT:PSS, CNTs, and their composites.15,17,20 The thermoelectric properties of CNTYs

were characterized as a function of temperature (Figure S5). The electrical conductivity of CNTY decreases with an increase of temperature, indicating the metallic behavior of CNTY. Additionally, the n-doped CNTY shows a conversion from n- to p-type around 480 K, which could be due to dissociation of PEI dopant. The thermoelectric figure-of-merit (ZT) of p- and ndoped CNTY was also estimated to approximately 0.014 at room temperature based on the previously reported in-plane thermal conductivity (51 W/(m·K)) of aligned CNT film.20 For further improvements, therefore, one should decrease the in-plane thermal conductivity of CNTY and increase its power factor by combining other water-processable doping processes.28 Excellent TE properties of flexible materials are essential for high performance TEGs. However, for practical applications, many requirements including modular design, flexibility, portability, and facile fabrication must be considered.29 While many previous reports on flexible TE materials show excellent TE properties, they lack facile module fabrication and performance, and these studies do not move past characterization of the TE properties.10,15,20 Because the flexible CNTY in this study shows not only the requisite mechanical strength but also shape advantages for modular fabrication, we can easily design and fabricate various types of flexible TEGs. As a proof of concept, we fabricated a prototype parallel-connected flexible TEG (f-TEG) using CNTYs (Figure 4a). The alternately n-and p-doped CNTY with undoped regions as electrodes was carefully wound around a polydimethylsiloxane (PDMS) supporting unit of 4 × 10 × 80 mm3 in size without interconnection of the CNTY. The dimensions of the PDMS unit were arbitrarily determined for ease of handling, and thus 7611

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Figure 5. Practical application of flexible TEG. (a) Photographs showing the flexibility of f-TEG. (b) Output voltage of f-TEG obtained from the temperature difference between body heat and atmosphere. (c) Photograph showing a red LED powered using f-TEG at ΔT = 50 K.

(Figure 5b). The temperature difference between human arm and f-TEG at room temperature was measured to approximately 6 K (Figure S7). Although the voltage of f-TEG generated from human heat is insufficient due to a low temperature difference, we can successfully power a red LED using an energy harvesting circuit at ΔT = 50 K (Figure 5c and Figure S8), demonstrating CNTY’s great potential as flexible power conversion devices.

further work is needed to optimize the TE performance of this module in terms of shape and size. The TEG composed of 60 PN pairs made by CNTY with 1.7 m in length was characterized using a handmade instrument which can accurately control the temperature gradients by four commercial Peltier modules (Figure 4b). As shown in Figure 4c,d, the TEG with 60 PN pairs shows an output voltage density of 0.15 and 1.2 V/g and maximum power density of 10.85 and 697 μW/g at temperature differences (ΔT) of 5 and 40 K, respectively. Although these values are lower than those obtained from f-TEGs containing Bi2Te3 TE materials,30,31 they are the highest values among reported f-TEGs based on flexible materials. This excellent TE performance is mainly due to the high electrical conductivity of CNTY and minimized circuit resistance by excluding additional metal deposition steps (Figure S6). Figure S6 shows the circuit resistance, output voltage, and output power of f-TEG composed of 5 PN pairs with and without additional metal electrodes. The f-TEG without metal electrodes was prepared in the same procedure mentioned above. As a control, the f-TEG with metal electrodes was prepared by cutting the alternately n-and pdoped CNTY at the undoped regions and then connecting them with Ag paste. The resistivity of Ag paste was measured to 0.72 mΩ·cm which is lower than that of undoped CNTY electrode (1.12 mΩ·cm). As shown in Figure S6, the circuit resistance of f-TEG with and without Ag paste is 27 and 21 Ω, respectively. Because the same CNTY was used for both devices and the resistivity of Ag paste is lower than that of undoped CNTY, the higher resistance of f-TEG with Ag paste is attributed to the contact resistance between Ag paste and doped CNTY, indicating the decreased circuit resistance by excluding additional contacts between metal and CNTY. More importantly, we can easily enhance the generation power by increasing the CNTY length and the number of PN pairs in series. At ΔT = 5 K, as the number of PN pairs increases from 60 to 240, the maximum power density of the TEG increases from 1.3 to 4.2 μW (Figure 4e). The circuit resistance also linearly increases with an increasing number of PN pairs, while the average resistance per PN pair is nearly constant (Figure 4f). These results demonstrate that we can enhance the generation power of these modules by both minimizing the circuit resistance and expanding the number of PN pairs in series. For practical applications, f-TEG with 240 PN pairs was prepared, featuring high flexibility and mechanical stability (Figure 5a). It can generate an output voltage of approximately 54 mV when applied to the human arm at room temperature

CONCLUSION We have demonstrated the f-TEG based on CNTY with excellent TE performance. The high alignment and optimized doping of CNTY result in enhanced TE properties, indicating its powerful potential as a flexible TE material. The CNTY was alternatively doped into n- and p-types, and the CNTY between doped regions was used as electrodes to minimize the circuit resistance due to its superior electrical conductivity. The f-TEG fabricated with 60 PN pairs shows maximum power density of 10.85 and 697 μW/g at ΔT = 5 and 40 K, respectively, which are the highest values among reported f-TEGs based on flexible materials. Furthermore, the f-TEG with 240 PN pairs can successfully power a red LED at ΔT = 50 K. EXPERIMENTAL SECTION Synthesis of CNTY. CNTY was synthesized by a floating catalyst method as previously reported.32 Ferrocene, thiophene, and methane were used as a catalyst precursor, promoter, and carbon source for CNT synthesis at 1200 °C, respectively. CNTs are highly integrated into aerogel-like forms in a reactor. These aerogel forms can be continuously withdrawn from the reactor at the bottom without length limitations. The CNT film was prepared as an aerogel directly wound on a roller and r-CNTY was obtained by vertically rolling a CNT film. CNTY was directly fabricated on a roller and composed of hundreds of thread passing through a water bath after synthesis. Fabrication of Flexible TEG. The CNTY was carefully wound around polydimethylsiloxane (PDMS) supporting unit having dimensions of 4 × 10 × 80 mm3 without interconnection of CNTY. The CNTY at one side of supporting unit was p-doped and one at the other side was n-doped. For the doping process, the doping conditions were optimized by tuning the dopant concentration (Figure S3). The CNTY was p-doped with a FeCl3 ethanol solution (Sigma-Aldrich, 2 mM) for 30 min, followed by well drying in ambient conditions. Additionally, the CNTY was n-doped with a polyethylenimine ethanol solution (PEI (MW = 600), Sigma-Aldrich, 8 mM) for 30 min. The undoped CNTY regions at the top and bottom of supporting unit were used as electrodes. As a control group, a flexible TEG with additional metal electrodes was prepared. The alternately n-and pdoped CNTY was cut at the undoped regions and then connected again with Ag paste (Figure S6). 7612

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ACS Nano Characterization. The microstructural morphology of the synthesized CNTY was investigated using high-resolution TEM (HR-TEM, JEOL, JEM-2100F) and field-emission SEM (FE-SEM, ZEISS, MERLIN Compact). Raman spectroscopy (RAMANplus, Nanophoton) with a 532 nm laser also was used to determine the crystallinity of the CNTs. The mechanical properties of the CNTY were measured with a tensile stage (Linkam, TST350) at a strain rate of 3 mm·min−1 and gauge length of 10 mm. The TE properties of the as-prepared samples deposited on glass substrates with dimensions of 2 × 1 cm2 were analyzed by measuring electrical conductivity (σ) and thermopower (S) at room temperature. The in-plane resistance (R) and S of the samples were simultaneously measured using a four-point probe TE measurement system (TEP 600, Seepel instrument), and the average values of at least 10 measurements were taken. The σ of CNTY was calculated by a wellknown follow equation. (assuming a spherical CNTY cross-sectional area.) σ=

1 l = ρ RA

ORCID

Seung Jae Yang: 0000-0002-4409-3160 Jun Young Oh: 0000-0003-3018-8246 Jeong Gon Son: 0000-0003-3473-446X Chong Rae Park: 0000-0002-9459-9426 Heesuk Kim: 0000-0002-0898-7781 Author Contributions #

J.C. and Y.J. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the Korea Institute of Science and Technology (KIST) Future Resource Research Program (2E27283) and the financial support from the R&D Convergence Program of NST (National Research Council of Science & Technology) (2N39900).

(1)

Here, R, l, and A are the measured electrical resistance (Ω), length (m), and cross-sectional area (m2) of CNTY, respectively. The diameter of the CNTY was obtained by a micrometer (Mitutoyo, 1 μm in accuracy) with a mean of at least five measurements. For S, the probes measured potential differences arising from temperature differences between the two ends of the sample (0.5, 1.5, and 2.5 °C at one end, and −0.5, − 1.5, and −2.5 °C at the other) and calculated S. These were considered reliable when the linear correlation (R2) of the measured potential differences was higher than 0.999. The carrier concentration and mobility of the samples were determined by Hall measurements (HMS-5000, Ecopia) with a 0.55 T and 1 mA, with a mean of at least 10 measurements. The circuit resistance of module was measured by Keithley 2700 acquirement system. Two ends of a module were connected in series with the measurement system, and the circuit resistance of module was measured when the circuit voltage of 2.1 V was intentionally applied. The power density of the module was measured using a homemade system (Figure 4b). By applying a current, a pair of Peltier modules at the bottom side are cooled and those at the top side are heated. The temperature at each side is controlled by the applied currents and measured by temperature sensors. Heat sinks and fans are equipped to maintain the stable temperature gradient. When the temperature gradient is induced, the open Seebeck voltage of module is measured by the Keithley 2700 acquirement system. To power a red LED, we utilized the EH4295 and EH300A modules (Advanced Linear Devices, INC.) for DC to DC up-converter and energy storage capacitor, respectively (Figure S8). The EH4295 module needs an input minimum of 60 mV and boosts up to 6−12 V. The output voltage of fTEG with 240 PN pairs is approximately 500 mV and up-converted by EH4295 module. The power generated from f-TEG was injected into the inputs of the EH300A module, and then was collected and accumulated in an internal storage capacitor until the operation level was enough to power a red LED.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01771. Figures S1−S8 and Table S1−S4 (PDF) Tear resistance of the CNTY after soaking in liquid nitrogen (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 7613

DOI: 10.1021/acsnano.7b01771 ACS Nano 2017, 11, 7608−7614

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DOI: 10.1021/acsnano.7b01771 ACS Nano 2017, 11, 7608−7614