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High thermoelectric power factor organic thin films through combination of nanotube multilayer assembly and electrochemical polymerization Mario Culebras, Chungyeon Cho, Michelle Krecker, Ryan Smith, Yixuan Song, Clara Gomez, Andres Cantarero, and Jaime C. Grunlan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15327 • Publication Date (Web): 27 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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ACS Applied Materials & Interfaces

High Thermoelectric Power Factor Organic Thin Films through Combination of Nanotube Multilayer Assembly and Electrochemical Polymerization Mario Culebrasa, Chungyeon Chob, Michelle Kreckerc, Ryan Smithc, Yixuan Songd, Clara Gomeze, Andres Cantareroa and Jaime C. Grunlanbcd* a

Molecular Science Institute, University of Valencia, PO Box 22085, 46071 Valencia, Spain.

b

Department of Mechanical Engineering, Texas A&M University, College Station, Texas

77843, United States. c

Department of Chemistry, Texas A&M University, College Station, Texas 77843, United

States. d

Department of Materials Science and Engineering, Texas A&M University, College Station,

Texas 77843, United States. e

Materials Science Institute, University of Valencia, Dr. Moliner 50, 46100 Burjasot, Spain.

KEYWORDS: layer-by-layer assembly, thermoelectric, power factor, carbon nanotubes, polymer nanocomposites, electrochemical polymerization *Corresponding Author: E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: In an effort to produce effective thermoelectric nanocomposites with multiwalled carbon nanotubes (MWCNT), layer-by-layer assembly was combined with electrochemical polymerization to create synergy that would produce a high power factor. Nanolayers of MWCNT stabilized with poly(diallyldimethylammonium chloride) or sodium deoxycholate were alternately deposited from water. Poly(3,4-ethylene dioxythiophene) [PEDOT] was then synthesized electrochemically by using this MWCNT-based multilayer thin film as the working electrode. Microscopic images show a homogenous distribution of PEDOT around the MWCNT. The electrical resistance, conductivity (σ) and Seebeck coefficient (S) were measured before and after the PEDOT polymerization. A 30 bilayer MWCNT film (< 1 µm thick) infused with PEDOT is shown to achieve a power factor (PF = S2σ) of 155 µW/m K2, which is the highest value ever reported for a completely organic MWCNT-based material and competitive with lead telluride at room temperature. The ability of this MWCNT-PEDOT film to generate power was demonstrated with a cylindrical thermoelectric generator that produced 5.5 µW with a 30 K temperature differential. This unique nanocomposite, prepared from water with relatively inexpensive ingredients, should open up new opportunities to recycle waste heat in portable/wearable electronics and other applications where low weight and mechanical flexibility are needed.


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Introduction The majority of the energy produced today comes from nonrenewable resources, such as fossil fuels and natural gas, which are known to contribute to environmental problems. To combat these issues, significant research has been devoted to developing alternative energy that is both renewable and clean. Although there are a variety of alternative energy options, they are not very efficient and comprise only a small percentage of global energy production. One area of alternative energy that could help to address this issue is thermoelectric recycling of heat. More than two thirds of the energy produced in the world is lost as waste heat. The second law of thermodynamics precludes the complete conversion of thermal energy to other forms of energy, and the majority of the world’s energy production comes from power plants that convert thermal energy to mechanical or electrical energy. Thermoelectric devices turn heat into electricity and could eventually recover much of the world’s wasted heat. Thermoelectric efficiency is commonly measured by the dimensionless figure of merit ZT:

ZT =

S 2σT

κ

(1)

where S, T, σ and κ are the Seebeck coefficient, absolute temperature, electrical conductivity and thermal conductivity, respectively. For more than 60 years, inorganic materials such as Bi2Te3,1,2 or ternary alloys incorporating Pb, have been the most commonly used thermoelectric materials. In the early years, the figure of merit of these inorganic compounds was around 0.6-0.7. Several decades later, Hicks and Dresselhaus demonstrated a novel way of increasing the thermoelectric efficiency of a semiconductor by reduction of dimensionality.3 Using this idea, semiconductor superlattices, such as Bi2Te3 or Sb2Te3,4,5 and semiconductors alloys (e.g., PbSeTe and SiGe)


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can now achieve a ZT near 2 (~15% efficient at 300 K).6,7 Despite this significant improvement, these inorganic materials have serious drawbacks when it comes to the fabrication of thermoelectric generators, such as scarcity of raw materials, high cost of production, and toxicity.8,9 Organic semiconductors have recently been shown to exhibit reasonable thermoelectric performance,10,11,12 which is one alternative to lead and tellurium-based materials. There are several advantages to organic materials relative to inorganic materials, including low cost of production, abundance of carbon, low thermal conductivity and good mechanical properties (e.g., flexibility). The main problem with intrinsically conductive polymers as thermoelectric materials is their low efficiency (ZT < 0.01).10,11,13 A number of approaches have been used to improve the thermoelectric efficiency of conductive polymers. Optimizing the doping level has become one of the most powerful methods to increase power factor (PF = S2σ), which is a useful parameter to gauge performance when thermal conductivity is similar amongst the materials being compared. In the case of PEDOT, ZT > 0.2 and PF > 300 µW/m K2 was achieved with various dopants.14

The addition of nanostructured fillers to the conducting

polymers has also been used to increase their thermoelectric efficiency. For example, very high power factors have been obtained for polyaniline (PANI)/double-walled carbon nanotube (DWCNT)/Graphene (1750 µW/m K2) and PANI/DWCNT/PEDOT:PSS/Graphene (2710 µW/m K2) multilayer thin films prepared using layer-by-layer (LbL) assembly.15,16 Polymer composites have also been prepared using inorganic semiconductor nanostructures such as Te nanorods17,18,19,20 or even Bi2Te3,21 but the use of inorganic fillers does not solve the problems of high cost and scarcity of raw materials. For this reason, it is more useful to develop high performance thermoelectric materials based on completely organic semiconductors. There are very promising examples of good thermoelectric performance for PEDOT:PSS films and


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PANI/DWCNT/PEDOT:PSS/Graphene multilayer assemblies, but these materials are very thin (60-1000 nm).14,15,16 From the point of view of thermoelectric device fabrication, these thin films are expected to have large internal resistance in the final thermoelectric generator due to the series connections between the legs, which is challenging for energy generation. It is important to produce films with a very low electrical resistance in combination with a large power factor. In principle, it is possible to reduce the resistance just by increasing the thickness of the film, but this is not completely true for polymers. When the thickness of the polymer film increases, the number of defects also increases,22 creating a poor morphology for electrical transport through the material. As a result, the final electrical conductivity sometimes decreases with thickness, which reduces efficiency. The most commonly used method to prepare nanocomposites containing carbon nanotubes (or graphene) and conducting polymers is by dispersing these fillers in a polymer solution.23,24,25 This approach has several challenges, including difficulty stabilizing the nanoparticles in the polymer solution and corresponding problems with homogeneity of the dispersion in the final composite. For these reasons, it is crucial to develop new ways of preparing these kinds of nanocomposites with low electrical resistance. In the present work, multi-walled carbon nanotubes (MWCNT)/PEDOT hybrids films were prepared using a combination of LbL assembly and electrochemical polymerization. This unique process resulted in nanocomposites with very low electrical resistance (1-2 Ω) and good PF (155 µW/m K2), which can be used to fabricate thermoelectric generators with low internal resistance. Layer-by-layer (LbL) assembly has been successfully used to prepare films from a carbon nanotube solutions,26,27,28 while the electrochemical polymerization of PEDOT can produce homogenous films with high electrical conductivity.29,30 In this case, MWCNT-based assemblies were deposited and then PEDOT was


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soaked into the nanoporous thin film and polymerized in-situ. These low resistance films were incorporated into a unique cylindrical thermoelectric module, containing only p-type nanocomposite elements. The module generated more than 5 µW of power with a temperature gradient of only 30oC. The traditional LbL assembly alone suffers from its slow growth behavior (several nm/cycle), requiring a large number of cycles to obtain enough thickness to be a freestanding film. This combination of fabrication techniques (LbL assembly and electrochemical polymerization), along with the use of relatively inexpensive components, reduces processing time and makes LbL deposition more amenable to commercial production of thermoelectric materials. This proof-of-concept demonstrates the opportunity to use organic polymer nanocomposites to generate substantial power that could potentially be used over large areas or to operate health sensors and/or wirelessly transmit data.

Results and Discussion Multi-walled carbon nanotube films were prepared using the layer-by-layer assembly method shown in Figure 1. Multilayer thin films tightly assemble due to the electrostatic interaction between poly(diallyldimethylammonium chloride) and sodium deoxycholate molecules used to stabilize the MWCNT in the aqueous deposition suspensions. In the case of PDDA, the polymer is believed to adsorb to the MWCNT, creating positively-charged nanotubes that are stable in water. In the same way, the interaction between DOC and the surface of MWCNT imparts water stability and a negative charge. Alternately exposing a 179 µm thick PET substrate to these two oppositely-charged aqueous solutions results in the incremental growth of a nanotube-based thin film with high electrical conductivity, as shown in Figure 2.


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Figure 1.

Schematic of the layer-by-layer deposition of multi-walled carbon nanotubes

stabilized by poly(diallyldimethylammonium chloride) and sodium deoxycholate. Figure 2(a) shows the approximately linear growth in thickness and mass deposited during the LbL assembly of MWCNT-PDDA and MWCNT-DOC. The thickness increases from 400 nm at 20 bilayers (BL) to 3.1 µm at 70 BL. The relatively large size and complex shape of the carbon nanotubes results in significant error associated with these values. The change in the slope of growth beyond 40 BL is believed to be the result of greater PDDA and DOC interdiffusion that adds more mass and thickness per bilayer deposited. QCM and profilometry are very different measurement techniques and they are measuring two different properties (mass and thickness, respectively), so the slight offset in their trends are not surprising.


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4.0 3.5

2

Mass deposition (µg/cm )

150 120

3.0 90

2.5 2.0

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1.5 30

1.0 0.5

0

Thickness (µm))

0.0 0

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20

30

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70

σ (S/cm)

Number of bilayers

(a)

90

120

75

100 80

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Resistance (Ω)

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20 20

30

40

50

60

Number of bilayers

70

(b)

Figure 2. Mass deposition and thickness of MWCNT-PDDA/MWCNT-DOC films as a function of the number of bilayers deposited (a). Electrical conductivity and absolute resistance as a function of the number of bilayers deposited (b). The lines in each graph are just to guide the eyes.


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Electrical conductivity and resistance of these MWCNT-based films, as a function of the number of bilayers deposited, are shown in Figure 2(b). The resistance decreases greatly from 20 to 30 BL and then continues to decrease more gradually. In a bulk material, increasing thickness typically decreases electrical resistance, but these multilayered thin films exhibit more complex changes in the electrical behavior. During layer-by-layer deposition, diffusion of DOC and PDDA deeper into the film can increase the proportion of insulating material, which causes an increase in the resistance, although not necessarily proportional to the increase in thickness due to the reduction of the electrical conductivity. For this reason, a maximum in the electrical conductivity appears at 30 BL, where these conditions are optimized. The dense packing of MWCNT, that contributes to relatively high electrical conductivity (Fig. 2(b), is shown in TEM cross sections of a 50 BL MWCNT-PDDA/MWNT-DOC film (Fig. 3). The nanotubes appear more tightly packed closer to the PET substrate, which may indicate greater stabilizer content (DOC and PDDA) near the multilayer film surface.

This is a

contributing factor to the behavior shown in Fig. 2(b) (i.e., there is no clear trend between electrical conductivity and film thickness). Greater packing density with a smaller number of bilayers results in the greater electrical conductivity for the 20 and 30 BL films.


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Figure 3. TEM images of the cross section of a 50 BL MWCNT-PDDA/MWCNT-DOC film.

After depositing the nanotube thin films, 3,4-ethylenedioxythiophene was polymerized into them. More details about the electrochemical polymerization of EDOT is available in the Supporting Information The morphology of these MWCNT-PEDOT thin films was visualized using SEM, as shown in Figure 4. Prior to EDOT infusion and polymerization, the MWCNT assembly exhibits a well-connected network, with a homogeneous distribution of nanotubes (Fig. 4(a)). After 10 minutes of EDOT polymerization, uniform and conformal coverage of the nanotube film is observed (Fig. 4(b)). MWCNT are still clearly visible with this polymerization time, but longer time causes PEDOT to fill interstitial space amongst the nanotubes (Fig. 4(c)), further improving connectivity. Filling open space in these nanotube assemblies with intrinsically conductive polymer was done to improve electrical conductivity and thermoelectric performance. After 60 min of polymerization, all MWCNT are covered and excess PEDOT


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creates a layer with a globular morphology (Fig. 4(d)), which is commonly seen with electrochemical polymerization.,30,29, 31

Figure 4. SEM images of a 50 BL of MWCNT-PDDA/MWCNT-DOC film (a) and the same film after 10 (b), 30 (c) and 60 min (d) of EDOT polymerization.

Electrical conductivity, Seebeck coefficient and power factor were measured (or calculated) as a function of the number of bilayers deposited and PEDOT polymerization time, as shown Figure 5. Conductivity increases several orders of magnitude with the addition of PEDOT to the


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MWCNT-based multilayer thin films. A maximum in electrical conductivity is observed with 30 min of polymerization when the number of bilayers is smaller than 50, but 60 minutes of polymerization is needed to achieve maximum conductivity with 50 or more bilayers. In this case, electrical conductivity is primarily influenced by polymerization time and film thickness. With increased polymerization time (i.e., more PEDOT incorporation), excess PEDOT produces a single layer that decreases conductivity. With greater film thickness, the number of defects increases and, consequently, the electrical conductivity is reduced. For this reason, the electrical conductivity reaches a maximum when the film has an optimal number of MWCNT layers and PEDOT covers the entire nanotube network without significant excess, which happens at 30 BL and with 30 min of polymerization. The maximum electrical conductivity of 2100±100 S/cm is unprecedented for a MWCNT-based composite, which is comparable to films prepared with double-walled nanotubes or highly doped PEDOT.16,32


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Figure 5. (a) Electrical conductivity, (b) resistance, (c) Seebeck coefficient and (d) power factor of MWCNT-PDDA)/MWCNT-DOC multilayers as a function of PEDOT polymerization time.

Figure 5(b) shows electrical resistance of the MWCNT-PDDA/MWCNT-DOC films as a function of PEDOT polymerization time. As expected, resistance decreases with the incorporation of more PEDOT (i.e., greater polymerization time) to the MWCNT network. Resistance below 1 Ω was achieved for some films, which makes them very promising for thermoelectric module fabrication. Unfortunately, Seebeck coefficient tends to decrease with


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polymerization time as well (Fig. 5(c)). This is not an unexpected result because PEDOT has an inherently low Seebeck coefficient (7-10 µV/K)30,29 and greater polymerization time increases the concentration in these nanocomposite films. Power factor is the product of the Seebeck coefficient and electrical conductivity (PF = S2σ), so it reaches a peak value at intermediate film thickness (30 BL) and polymerization time (30 min), as shown in Figure 5(d). The maximum power factor is approximately 150 µW/m K2.

The thermoelectric module shown in Figure 6 was fabricated using the 30 BL MWCNT-based films and 30 min of PEDOT polymerization. All of the thermoelectric films were connected from the hot side to the cold side because the legs have only one type of conduction (p-type). The supplied power was measured as a function of the voltage (Fig. 7(a)) and the load resistance (Fig. 7(b)). The maximum power supplied was 5.5 µW with a load of 40 Ω. As expected, power generation increases with greater temperature differential. The cylindrical generator morphology of this device is ideal for recycling waste heat from pipes. The power achieved here is greater than most other thermoelectric generators produced with organic materials, which range from 30 nW up to~2 µW with ∆T from 10 to 50K.33,34,35,36


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Figure 6. Schematic and photos of the thermoelectric module fabricated from PEDOT-infused MWCNT-based films deposited on PET.

Figure 7. Power from cylindrical thermoelectric generator as a function of (a) voltage and (b) load resistance, with varying temperature differential supplied.


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Conclusions Completely organic thin films with high electrical conductivity and thermoelectric power factor were prepared through a unique combination of multilayer assembly and electrochemical polymerization. Layer-by-layer deposition of multiwalled carbon nanotubes, stabilized with poly(diallyldimethylammonium chloride) or sodium deoxycholate, produced thin films that were infused with 3,4-ethylene dioxythiophene that was polymerized in-situ. The best film, produced with 30 MWCNT-PDDA/MWCNT-DOC bilayers and 30 minutes of polymerization time, achieved electrical conductivity of 2100 S/cm and power factor of 155 µW/m K2. A cylindrical thermoelectric generator was constructed using strips of this p-type material, which achieved a maximum power of 5.5 µW with ∆T=30 K. These values of conductivity, power factor and power generation among the best ever reported for an organic material and provide the basis for practical organic thermoelectric recycling of waste heat from pipes and possibly the human body if applied to fabric substrates.

Experimental Materials

Poly(diallyldimethylammonium chloride) (PDDA), with a molecular weight of 100,000 200,000 g/mol, sodium deoxycholate (DOC) (C24H39NaO4), 3,4-ethylenedioxythiophene (EDOT), lithium perchlorate (LiClO4), (Milwaukee, WI)

and acetonitrile were purchased from SigmaAldrich

All chemicals were used as received.

Multi-walled carbon nanotubes


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(MWCNT) were obtained from Bayer MaterialScience (12–15 nm outer and 4 nm inner wall diameter, 1+ µm length, C ≥ 95 wt%; Leverkusen, Germany). Substrates

Polyethylene terephthalate (PET), with a thickness of 179 µm, (trade name ST 505 by DuPont Teijin, purchased from Tekra Corp, New Berlin, WI) and 175 µm polystyrene (PS) (trade name ST 311125, purchased from Goodfellow, Cambridge, UK), were rinsed with DI water, methanol, DI water, and dried with filtered air before use. Corona treatment (a BD-20C Corona Treater, Electro-Technic Products Inc., Chicago, IL) was used to improve adhesion of the first layer by oxidizing the polymer surface.. Polished Ti/Au crystals with a resonance frequency of 5MHz were purchased from Maxtek, Inc. (Cypress, CA) and used to characterize deposited mass per layer with a quartz crystal microbalance (QCM). Assembly of MWCNT films

0.05 wt % multi-walled carbon nanotubes were dispersed in an aqueous solution of 0.25 wt% PDDA or 1 wt% DOC. Both MWCNT suspensions were bath sonicated for 30 min, followed by 20 min of 15 W tip sonication in an ice water bath, and another 30 min of bath sonication to homogenize. MWCNT dispersions were then centrifuged at 4000 rpm for 20 min and the supernatant was decanted. The pH of the nanotube suspensions, in either PDDA or DOC, was left unaltered at 5.8. Each substrate was immersed into the cationic PDDA-based suspension for 5 min, followed by rinsing and drying, and then dipped into the anionic DOC suspension for another 5 min. This process results in one deposition sequence of a MWCNT-PDDA/MWCNTDOC bilayer (BL). After the initial BL was deposited, all subsquent layers were deposited with 1 min dip times, with rinsing and drying in between. This cycle was repeated to deposit the desired


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number of bilayers. Deposited multilayer films were washed and air-dried overnight and then stored in a desiccator prior to further processing or characterization. Electrochemical deposition of poly (3,4-ethylenedioxythiophene)

A solution of 0.01 M EDOT and 0.1 M LiClO4 was prepared in acetonitrile. The polymerization was carried out in a three-electrode electrochemical cell at 3 mA vs an Ag/AgCl reference electrode in an Epsilon 851 cyclic voltammeter (BASi Instrumentation, West Lafayette, IN) . During the electrochemical polymerization of EDOT, a platinum grid was used as counter electrode and the MWCNT film acted as the working electrode. Thermoelectric module fabrication

For thermoelectric generator fabrication, a PET substrate (8.5 x15 inch2) was first masked using tape to make a striped pattern. The space between the stripes determined the width (0.6 inch in this study) of the MWCNT films. After the MWCNTs deposition, tape stripes were removed from the PET substrate and used as a working electrode during EDOT polymerization. The process was carried out in galvanostatic mode at 3mA/inch for 60 min. In this step, the PEDOT stripes are cut to 1 inch length and electrically connected with silver paste from the bottom to the top (series connection). The thermoelectric fundamental units (30 in this case) were attached to a copper tube, using silicone as adhesive. The fundamental units were connected in groups of three in series, with all groups in parallel. The fabrication steps are summarized in the schematic of Fig 8.


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Figure 8. Schematic of thermoelectric module fabrication. Characterization of film growth and structure

Nanotube assembly thickness on Si-wafers was measured by a profilometer (P-6 Stylus Profiler, KLA-Tencor Corporation, Milpitas, California) and reported values represent an average of at least 10 separate measurements on each film. The mass of the deposited layer on Au/Ti crystals was measured with a Maxtek Research Quartz Crystal Microbalance (Cypress, CA). The quartz crystal was blown with compressed nitrogen gas prior to being left on the microbalance to analyze the mass change of LbL thin films. Morphological characterization was carried out using a Hitachi 4800 S field-emission scanning electron microscope (FE-SEM) at an accelerating voltage of 20 kV and a working distance of 14 mm for palladium-gold coated surfaces. Transmission electron microscope (TEM) samples were prepared by embedding a small piece of coated PET in Durcupan™ ACM resin (SigmaAldrich, Munich, Germany), curing overnight and then cutting cross sections using an Ultra 45°diamond knife (Diatome, Hatfield,PA). Samples were imaged on copper grids using a JEOL JEM-1010, coupled with a digital camera MegaView III at 100 kV.


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Thermoelectric characterization

Sheet resistance of the nanocomposite assemblies on PET was measured using a Lucas Labs (Gilroy, CA) four-point probe system (Gilroy, CA) with 0.4 mm probe tip diameter and 1.0 mm tip spacing. Electrical conductivity was obtained by taking the inverse of the product of the sheet resistance and thickness. For the Seebeck coefficient measurement, a homemade four-probe setup with two T-type thermocouples and two copper wires was used. This apparatus is equipped with a Keithley 2000 Multilmeter, a Lake Shore model 350 temperature controller (Lake Shore Cryotronics, Inc., Westerville, OH) and operated with a LabVIEW interface. Thermoelectric voltage across the films (typical sample size is ~ 8 X 15 mm) on a PET substrate was measured at 10 different temperatures between -10 and +10 K and the Seebeck coefficient was obtained from the slope of the temperature gradient-voltage plot. Reported electrical conductivity and Seebeck coefficient values were the average of 5 measurements on two independent samples (i.e., 10 total measurements). The power supplied was calculated using:

P=

∆V 2 RL

(3)

where ∆V is voltage across the contacts and RL is the load resistance. ASSOCIATED CONTENT Supporting Information Available:


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Includes additional information on the potentiometric curve during the PEDOT synthesis, more SEM images, the cross section of the films after PEDOT synthesis. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Tel: +1 979-845-3027 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources U.S. Air Force Office of Scientific Research (Grant No. FA9550-13-1-0085) Dirección General de Investigación Científica y Técnica (Grant No. CSD2010-0044) Dirección General de Investigación Científica y Técnica (Grant No. MAT2015-63955-R) Spain Ministry of Education FPU training programme

ACKNOWLEDGMENTS M.C., C.G. and A.C. acknowledge financial support from the Dirección General de Investigación Científica y Técnica through Grant CSD2010-0044 of the Programme Consolider Ingenio and grant MAT2015-63955-R. MC would like to acknowledge the Ministry of Education for financial support through the FPU training programme. C.C., M.K., R.S. and J.C.G. gratefully


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acknowledge financial support from the U.S. Air Force Office of Scientific Research (Grant No. FA9550-13-1-0085).


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