Inorganic Hybrid Nanostructured Materials for

445

12 Organic/Inorganic Hybrid Nanostructured Materials for Thermoelectric Energy Conversion Yucheng Lan 1 , Xiaoming Wang 2 , Chundong Wang 3 , and Mona Zebarjadi 4 1 Morgan State University, Department of Physic and Engineering Physics, 1700 East Cold Spring Lane, Baltimore, MD 21251, USA 2 University of Toledo, Department of Physics and Astronomy, 2801 West Bancroft, Toledo, OH 43606, USA 3 Huazhong University of Science and Technology, School of Optical and Electronic Information, No. 1037, LuoYu Road, Wuhan 430074, China 4 Virginia University, Department of Materials Science and Engineering, School of Engineering, 395 McCormick Road, Charlottesville, VA 22904, USA

12.1 Introduction Thermoelectric (TE) materials can directly convert heat into electricity. A TE junction can generate a voltage gradient via the Seebeck effect when there is a temperature gradient, discovered by Thomas Johann Seebeck in 1821 [1, 2]. Likewise, an electrical current flowing across a TE junction can produce a temperature gradient across the junction (the Peltier effect), discovered by Jean Charles Athanase Peltier in 1834 [3]. Solid-state TE devices can be used in a wide range of applications, such as temperature sensing, waste heat recovery, air conditioning, and refrigeration [4–13]. TE devices have attracted extensive interests for several decades because of their unique features such as no moving parts, quiet operation, low environmental impact, and high reliability [4, 6–10, 12]. The efficiency of TE materials is determined by a dimensionless figure of merit (ZT), defined as [4, 7, 14, 15] ZT = (S2 𝜎∕𝜅)T

(12.1)

where S, 𝜎, 𝜅, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature at which the properties are measured, respectively. The efficiency of a TE device is an increasing function of the TE’s figure of merit, ZT. In the case of power generators, the device efficiency can be written as √ Th − Tc 1 + ZT − 1 (12.2) 𝜖= √ T Th 1 + ZT + Tc h

Functional Organic and Hybrid Nanostructured Materials: Fabrication, Properties, and Applications, First Edition. Edited by Quan Li. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA.

446

12 Organic/Inorganic Hybrid Nanostructured Materials for Thermoelectric Energy Conversion

and for air-conditioning and refrigeration, the cooling efficiency of devices is √ T 1 + ZT − Th Tc c 𝜂= (12.3) √ Th − Tc 1 + ZT + 1 where Th and Tc are the hot-end and cold-end temperature of the TE materials respectively. T is the average of Tc and Th . Therefore, it is important to use materials with a high ZT value for practical applications. TE devices are comprised of a series of p–n thermoelectric couples, as shown in Figure 12.1. The p–n elements are connected electrically in series Heat absorbed

Substrates

Thermoelectric elements

+Current Metal interconnects

Extenal electrical connection

Heat rejected

Heat absorption

h+ Heat flow

p

e–

n

Heat rejection

Figure 12.1 Thermoelectric module showing the direction of charge flow on both cooling and power generation. (Snyder and Toberer 2008 [13]. Reproduced with permission of Nature Publishing Group.)

12.1 Introduction

Figure 12.2 Various TE materials used in thermoelectric research and development. (Goldsmid and Douglas 1954 [16]. Reproduced with permission of IOP Publishing.)

8%

4%

12%

17%

18%

PbTe Bi2Te3 SiGe Skutterudite 19% Clathrate Half-Heusler alloy Other emerging inorganic materials Conducting polymers CNT/graphene–polymer composite

2% 2% 18%

and thermally in parallel. Since the 1950s, TE properties of narrow-bandgap semiconductors have been widely investigated. Some of these TE semiconductors have been applied in space applications and in the auto industry. Examples include skutterudites, telluride-based materials (PbTe, Bi2 Te3 , etc.), rare earth chalcogenides (La3−x Te4 , etc.), copper ion liquid-like materials (e.g., Cu2 Se), SiGe alloys, half-Heusler alloys, and clathrates. Recently, organic TE materials and hybrid nanostructured TE composites have been investigated. Figure 12.2 illustrates the percentage contribution of these materials to TE technologies. 12.1.1

Inorganic Thermoelectric Materials

Since the discovery of thermoelectricity, metals and metallic alloys were the first to be investigated. Because of their low ZT values (ZT≪1), these materials can only be used in thermocouples to measure temperature and radiant energy [6]. The field of thermoelectricity experienced a wave of optimism in the late 1950s [16–18]. Many TE materials, especially narrow-bandgap semiconductors, had been investigated [5]. The TE materials exhibited ZT approximately 0.5 early on, and values of ZT approximately 1 were achieved after about a decade of research, as shown in Figure 12.3. Various inorganic TE materials have been reviewed [5, 13, 19]. However, ZTs of these materials were limited to about unity until late 1990s. With the low ZT values, TE device efficiency was too low for TE devices to compete with present air-conditioners and electric generators. These TE materials have only been used for small-scale solid-state cooling/heating and direct thermal to electrical power production. For example, Bi2 Te3 and SiGe alloys with ZT approximately 1 had been used commercially in low-power cooling, such as beverage coolers and laser diode coolers, and low-power generators in space missions. Figure 12.4 shows the ZTs of several conventional TE bulk materials developed during the 1950s–1990s. These materials had been used for TE power generation and cooling, covering the working temperature ranging from 0 to 1000 ∘ C. However, the low ZT limits their device efficiency. In the early 1990s, nanostructuring concept was introduced in individual nanostructures [20, 21], and ZT was enhanced significantly. The idea has been extended to nanostructured composite bulk materials since 2008 [22], such as

447


2.5 Hierarchical PbTe

2.0 R.I. PbTe

Nano-PbTe

Cooling

1.5 ZTmax

Nano-Bi2Te3

Power generation

PbTe

Bi2Te3

SiGe

SiGe

Bi2Te3

PbTe

0.5

Nano-SiGe

Zn4Sb3

Bi2Te3

1.0

Hicks and dresselhaus

MnTe ZnSb

0.0 1940

1950

1960

1970

1980 Year

1990

2000

2010

Figure 12.3 Evolution of the maximum ZT over time. (Reprinted with permission from Ref. [17]. Copyright 2013, Nature Publishing Group.) n-Type zT

p-Type zT

1.4

1.4

1.2 PbTe

zT

0.8

CeFe4Sb12

0.8

0.6

0.6

0.4

0.4

0.2

0.2

Yb14MnSb11

Sb2Te3

1.0

CoSb3

0

PbTe

SiGe

0 0

(a)

TAGS

1.2

SiGe

Bi2Te3

1.0 zT

448

200

400

600

Temperature (°C)

800

1,000

0

(b)

200

400

600

800

1,000

Temperature (°C)

Figure 12.4 Figure of merit (ZT) of the state-of-the-art commercial TE bulk materials without nanostructering. (Snyder and Toberer 2008 [13]. Reproduced with permission of Nature Publishing Group.)

Bi2 Te3 nanocomposites [23] and SiGe alloys [24, 25]. Strategies for enhancing the TE response of TE materials via nanostructuring have been reviewed recently [18, 22, 26–35]. These approaches culminated in the record ZT approximately 2.2 in PbTe nanostructures with multiscale hierarchical structures [18, 36]. Figure 12.3 shows the enhanced ZT values of some nanostructured TE bulk nanocomposites. These TE materials can be used to harness waste heat and provide highly efficient electricity through the Seebeck effect. TE nanocomposites are one kind of novel nanostructured TE materials using commercially available TE materials (such as PbTe and Bi2 Te3 ) and new TE materials with the aim of reducing the thermal conductivity and increasing the power factor through reducing crystalline size. The effects of confinement and

12.1 Introduction

scattering on the electrical/thermal conductivity and Seebeck coefficient have been studied. Interested readers are referred to published review literature and references therein [18, 22, 28–31]. 12.1.2

Organic Thermoelectric Materials

The first conductive polymer, doped polyacetylene (PA), was discovered in 1977 [37, 38]. Intrinsic PA is nonmetallic and changes to a metallic state after I2 doping, and its conductivity increases with the doping level. Since then, a number of conductive organic polymers have been synthesized and applied in batteries, light-emitting diodes, and so on, because of their advanced mechanical, optical, and electrical properties, such as polypyrrole [39–42], polyphenylene [43], and poly(alkyl thiophene) [44]. The TE properties of these conductive organic polymers were first investigated in the late 1980s [45, 46]. Park et al. [45, 46] measured the thermoelectric power of PA films doped with transition-metal halides in 1984. Jeszka et al. [47] measured the Seebeck coefficient of reticulate-doped polymer bulk samples prepared from polymer solutions, such as polyethylene/1 wt% tetrathiafulvalene, in 1993. The measured Seebeck efficiencies of these metallic polymers were not higher than 100 μV K–1 at room temperature. Table 12.1 shows the chemical formulas of several conductive polymers commonly used in thermoelectricity, such as PA, polyaniline (PANI), poly(alkyl thiophene) (PTH), poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), and poly(2,7-carbazolyenevinylene). Besides these TE polymers, other organic TE polymers, such as PEDOT–poly(styrene sulfonate) (PEDOT:PSS), polycarbazole (PC), polyphenylenevinylene (PPV) and their derivatives, have been investigated as TE materials. Detailed chemical structures of these organic TE polymers can be found in some review literature [19, 33, 48–52]. The key concepts of organic TE materials have been discussed elsewhere [50, 53], and state-of-the-art organic TE materials have been reviewed recently [51, 54, 55]. In contrast to inorganic TE materials in which the thermal conductivity 𝜅 generally increases with increasing electrical conductivity 𝜎, the thermal conductivity of organic TE polymers can be constant while electrical conductivity 𝜎 is increased by increasing the doping levels. Therefore, ZT of organic TE materials can be improved effectively by increasing the electrical conductivity 𝜎. PA is the first reported organic TE material. Its chemical structure is shown in Table 12.1. It was shown that the electrical conductivity of PA doped with iodine [56–58] and metal halides [45, 46, 58–60] can be continuously varied over several orders of magnitude [38]. The doped PA can have very high electric conductivity values (1–3 × 104 S cm–1 [56]). Despite that, the measured ZT values of doped PA are small due to their relatively low Seebeck coefficient (≤100 μV K–1 ) and their large thermal conductivity. Experimental studies have shown that the application of PA is significantly limited due to its oxidation instability. To date, PANI has been the most studied conductive polymer because of its high conductivity, excellent stability, as well as easy preparation and processing. Its thermal conductivity is about 0.7 W m–1 K–1 for highly doped PANI and does not depend on the kind of dopants [48]. Intrinsic PANI has low ZT values

449

Poly(2,7carbazolylenevinylene)

Polythiophene (PTH)

H13C6

S

S

O

n C6H13

C6H13

N

n

Poly(paraphenylene)

Polyparaphene vinylene

Polypyrrole (PPy) O

Poly(3,4-ethylenedioxythiophene) (PEDOT)

Polymer material

Polyaniline (PANI)

Chemical structure

Polyacetylene (PA)

Polymer material

Table 12.1 Chemical structures of a few typical conductive polymers used as TE materials.

H N

N H

n

N H

Chemical structure H N

n

12.1 Introduction

(ZT approximately 10−5 at room temperature) [61]. Its chemical structure is illustrated in Table 12.1. Doped PANI is a good TE material. PANI has been doped by phosphoric acid [62], hydrochloric acid [61, 63–65], camphor sulfonic acid [48, 61, 66–68], and other organic acids [61, 63, 69]. The electrical conductivity is 10–200 S cm–1 depending on the doping level and the type of dopant. The Seebeck coefficient of the material decreases with increasing doping concentration (at low doping concentration) (≤100 μV K–1 ) while its electrical conductivity increases. A ZT value of 0.01 was achieved at room temperature by optimizing the doping level [48, 63, 69]. More details of the preparation, doping, and TE properties of PANI can be found in Ref. [70]. PEDOT is the second most studied conductive polymer after PANI because of its relatively high electrical conductivity, excellent stability, low thermal conductivity, low density, easy handling, and environmental stability. The chemical structure of PEDOT is illustrated in Table 12.1. PEDOT possesses a bandgap of 0.9 eV. The basic TE properties of PEDOT-based materials are listed in Table 12.2. The electrical conductivity of the organic TE materials is high and thermal conductivity is low, benefiting TE properties. However, their Seebeck coefficient and power factor (PF) are lower. The Seebeck coefficient of PEDOT is 100 and 140 μV K–1 for p-type and n-type, respectively. When PEDOT is properly doped, its Seebeck coefficient can be as high as 4000 μV K–1 at room temperature [76]. The PF of most PEDOT TE materials is in the range 10−6 –10−10 W m–1 K–2 , which is three orders of magnitude lower than that of typical inorganic TE materials [22]. Power factors of PEDOT nanowires are higher than those of PEDOT films [77]. A ZT value of 0.1 can be obtained nowadays. More details of PEDOT-based materials can be found in the literature [53, 71, 78]. The major drawback of PEDOT is its low electrical conductivity. The electrical conductivity of PEDOT can be increased by incorporating conducting species. PEDOT:tosylate (PEDOT:Tos) has been prepared directly by mixing the PEDOT monomers and oxidative solutions of iron(III) tris-p-toluenesulfonate. Bubnova et al. [75] optimized the power factor of the Table 12.2 TE properties of PEDOT materials.

Materials

Thermal conductivity (W m–1 K–1 )

Electrical conductivity (S cm–1 )

Seebeck (𝛍–1 K–1 )

PEDOT

—

3.2–18.3

33–57

—

[71]

PEDOT:PSS

—

220–298

12.5–14.2

0.006

[72]

—

298.5

12.7

0.007

[73]

0.32–0.34

330–570

13.5–14.6

0.009

[49]

∼0.23

∼900

∼60

0.28

[74]

∼0.24

∼900

∼70

0.42

[74]

0.37

67

220

0.25

[75]

PEDOT:Tos

ZT

References

451

452


conducting polymer PEDOT:Tos by controlling its oxidation level through using tetrakis(dimethylamino)ethylene as a de-doping agent. With a low intrinsic thermal conductivity of conducting polymers of 0.35 W m–1 K–1 , ZT reached 0.25 at room temperature at 22% oxidation level in PEDOT:Tos with a very high power factor of 324 μW m−1 K−2 . PEDOT has limited solvent solubility, somewhat restricting its application. PEDOT has been doped by poly(styrene sulfonate) (PSS) to form PEDOT:PSS to overcome this limitation. PEDOT:PSS is a promising organic TE material because of its stability in air [79], very high electrical conductivity (measured over 3000 S cm–1 [80]), and low intrinsic thermal conductivity (0.2 W m–1 K–1 [49, 81, 82]). However, the power factor S2 𝜎 of PEDOT:PSS is very low (of the order of 1–10 μW m–1 K–2 ) due to its low Seebeck efficient S at high carrier concentrations [71, 72]. Detailed TE properties of doped PEDOT:PSS with additives have been reviewed [71]. Various research groups have achieved ZT = 0.007 [49, 72, 73]. Kim et al. [74] recently treated PEDOT:PSS with ethylene glycol (EG) and dimethylsulfoxide (DMSO) to enhance its electrical conductivity. A maximum ZT value of 0.42 was obtained for DMSO-mixed PEDOT:PSS at room temperature and 0.28 for EG-mixed PEDOT:PSS. A ZT = 1 may be possible for PEDOT-based TE materials in the near future [71]. TE properties of carbon nanotubes and graphene have also been investigated [83–85]. Their thermoelectric power was first measured on multiwalled carbon nanotubes (MWCNTs) [84] and then on single-walled carbon nanotubes (SWCNTs) [85], and was found to be extremely sensitive to the CNT preparation history and alignment [86]. Up to now, TE properties of CNTs [87–89] and graphene [90, 91] have been fully investigated, and their ZTs were found to be very low (∼10 × 10−4 ). It was predicted that the ZT of graphene nanoribbons can be as high as 3.25 at 800 K [92], or even 4.0 at room temperature [91]. Many conducting polymers besides PA, PANI, and PEDOT are potential TE materials. Up to now, PPy [39, 61], poly(3-methylthiophene) [93], polythieno[3,2-b]thiophene (PTT) [94], polythiophene [93, 95], poly(metal 1,1,2,2-ethenetetrathiolate) [54], and poly(2,7-carbazole) (PC) [96] have been reported as TE materials. PPy is more stable than PA while having a relatively low electrical conductivity. PC and its derivatives have excellent stability, being good TE candidates [96] with an electrical conductivity of 500 S cm–1 , Seebeck coefficient of 70 μV K–1 , and a power factor of 19 μW m–1 K–2 . These organic polymer-based TE materials and their TE properties have been reviewed recently [51, 52, 97, 98]. The Seebeck coefficient of organic TE materials has been theoretically explained [99, 100]. Their ZTs have been increased by several orders of magnitude in the past decades. These organic TE materials have been fabricated into TE devices [101] as flexible TE power generators. The manufacturing techniques are inexpensive [54, 75, 102]. A power output of 1.2 μV cm–2 was reported under a temperature gradient of 30 K. With the development of flexible devices and requirements of green energy, flexible organic TE devices have a great potential to generate electricity from human bodies. Compared to inorganic TE materials, organic TE polymers possess some advantageous properties, such as low cost due to plenty of carbon sources, excellent mechanical flexibility, low mass density, low thermal conductivity,

12.1 Introduction

120 Organic thermoelectric materials Number of publications

100 80 60 40

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

2005

2004

2003

2002

2001

0

2000

20

Year

Figure 12.5 Thomson Reuters Web of Science publication report for the topic “organic thermoelectric” from 2000 to 2015. (Lu et al. 2016 [100]. Reproduced with permission of Royal Society of Chemistry.

easy synthesis in general, and easy processabilty into varied forms. Therefore, organic TE polymers have attracted a great deal of attention. Figure 12.5 shows the Thomson Reuters Web of Science publication report on the topic “organic thermoelectric” for the last 16 years. 12.1.3

Hybrid Thermoelectric Nanostructured Composites

Inorganic TE materials show higher performance but are brittle and expensive. Their poor processability limits their wide applications. Organic TE materials are mechanically flexible and economical, whereas their TE performance is not excellent. Therefore, many researchers have proposed a fabrication of nanostructured polymer/inorganic TE composites to integrate the excellent TE performance of inorganic TE materials with the flexibility/inexpensiveness of organic TE materials. Up to now, a large amount of experimental data has been accumulated on nanostructured polymer/inorganic TE composites. It is expected that the novel hybrid organic/inorganic TE nanocomposites will be inexpensive and flexible while possess high TE performances. ZT of the novel hybrid composites should be enhanced significantly because of the low thermoelectric conductivity of organic TE materials and high Seebeck coefficient of inorganic TE materials. When organic and inorganic TE materials are physically mixed as hybrid nanocomposites, the electrical conductivity 𝜎 hybrid and the Seebeck coefficient Shybrid of the hybrid nanocomposites can be estimated by the following formulas [103]: 𝜎hybrid = (1 − x)𝜎organic + x𝜎inorgnanic (1 − x)Sorganic 𝜎organic + xSinorganic 𝜎inorganic Shybrid = (1 − x)𝜎organic + x𝜎inorganic

(12.4) (12.5)

453

454


where Sorganic , 𝜎 organic , Sinorganic , 𝜎 inorganic are the Seebeck coefficient and electrical conductivity of the organic and inorganic constituents respectively, which may be assumed to be constant under no band-bending. x is the content of the inorganic constituent in hybrid inorganic/organic nanocomposites. Therefore, the TE performance of physically mixed hybrid composites is usually superior to that of their organic counterparts but cannot match that of the inorganic counterparts. For example, Bi2 Te3 nanocomposites fabricated from ball-milled nanoparticles (with clean surfaces) yielded a ZT = 1.4 at 100 ∘ C [23]. However, Bi2 Te3 nanocomposites fabricated from solution-grown bismuth telluride nanostructures (naturally coated by organic materials in solutions) yielded a much lower ZT (0.03–0.2) [104, 105] because of poor electric conductivity and thermal power caused by organic residues on inorganic nanostructure surfaces; although their thermal conductivity was reduced down to 0.5 W m–1 K–1 . The TE performance of hybrid TE composites can be enhanced to be better than that of their inorganic counterparts by other means, such as energy filtering. This concept was originally proposed for TE superlattices to scatter low-energy carriers substantially [106]. Recently, this concept was extended to three-dimensional bulk nanocomposites where boundary interfaces play the major role of energy filtering [107, 108]. Such hybrid composites can be fabricated by in situ polymerization. Organic/inorganic interfaces in in situ polymerized hybrid nanocomposites act as energy filters [109]. The Seebeck efficient can be enhanced while the electrical conductivity is increased and thermal conductivity is maintained low, resulting in a higher ZT than that of their inorganic counterparts. The purpose of this chapter is to review the progress in research on hybrid organic/inorganic TE nanostructured bulk composites. Conducting polymers such as PANI, PTH, poly(3,4-ethylenedioxythiophene), PA, PPy, PC, and PPV are used as matrices of these hybrid nanocomposites. Bi, Te, and Bi2 Te3 nanostructures are employed as the inorganic counterparts to be mixed with the conductive TE polymers because of their high power factor at room temperature, facile synthesis, and solution-processed dispersion. The hybrid TE nanostructured composites reviewed here are fabricated by physical mixing, solution mixing, or in situ polymerization [97, 98].

12.2 Organic/Inorganic Thermoelectric Nanostructured Materials Embedding inorganic TE nanostructures into polymer matrices is one of the most promising approaches to combine the advantages of organic TE materials (low thermal conductivity, flexibility, solution processability) and inorganic TE materials (excellent TE properties) [53]. The shortcomings of both organic TE materials (low Seebeck coefficient and power factor) and inorganic TE materials (brittleness) can be overcome. The TE properties of these hybrid composites are better than those of pure organic polymers [51, 97, 110]. Figure 12.6 illustrates how such hybrid nanocomposites are fabricated. The details of the fabrication method can be found in the literature [97, 98].

12.2 Organic/Inorganic Thermoelectric Nanostructured Materials

+

Monomer

Inorganic particles

Nanocomposite

Figure 12.6 Formation of TE nanocomposites from inorganic–organic materials. (Yang et al. 2013 [110]. Reproduced with permission of John Wiley & Sons.)

Depending upon the nature of inorganic and organic components, TE hybrid nanocomposites can be classified into two categories: one in which the inorganic particles are embedded in organic matrices (organic counterparts are the majority or the host), and the other in which organic TE materials are dispersed among inorganic nanoparticles (inorganic counterparts are the host). In each case, the nanocomposites are physically mixed and compacted, or the inorganic components are chemically encapsulated by their organic counterparts and compacted into hybrid nanocomposites. It was reported that inorganic TE nanomaterials can be synthesized from chemical solutions [35] and compacted into bulk materials. The produced nanocomposites can be categorized as the second type of hybrid nanocomposites (in which inorganic part is the matrix). Organic TE materials are deposited on inorganic nanoparticle surfaces. The thermal conductivity of the compacted hybrid nanocomposites is thus reduced, and therefore higher ZT values are expected. Unfortunately, the addition of organic materials also reduces the electrical conductivity, and therefore the power factor, resulting in lower ZT values. Because of low TE performances of the second kind of hybrid TE nanocomposites, here we do not discuss the case in which inorganic component is the host (majority) while focus on the first case in which the organic component is the major constituent in the hybrid composites. Inorganic TE nanomaterials with better TE performance are embedded in organic TE matrices to increase TE performance of the hybrid nanocomposites. Up to now, various inorganic/organic TE nanostructured composites have been fabricated, including films (such as PEDOT-PSS/Ca2 Co4 O4 composite films [111], PANI/Bi2 Te3 nanocomposite films [112], PANI/Bi films [113], PEDOT:PSS/Te composite films [114], MWCNT/poly(vinylidene fluoride) films [115], PEDOT:PSS/CNT films [116]), and nanocomposite bulks. Table 12.3 lists the TE properties of some hybrid TE nanocomposites. More details of the reported hybrid TE nanocomposites are discussed below. Hybrid inorganic/organic materials can be screen-printed as flexible TE modules [117] to generate electricity from the temperature difference between human body and ambient air. Therefore, these hybrid TE nanostructures can be employed as green power sources in wearable devices. 12.2.1

PEDOT Hybrid Nanocomposites

The conductive PEDOT polymer has attracted attention because of its high electrical conductivity, high optical transparency, environmental stability, low mass density, good flexibility, low thermal conductivity, excellent thermal stability, and

455

456


Table 12.3 Hybrid TE nancomposites at a glance. TE characterization at RTa)

Nanocomposite

Polymer

PANI

Thermal conductivity (W m–1 K–1 )

Electrical conductivity (S m–1 )

Seebeck coefficient (𝛍V K–1 )

ZT

References

SWCNT (41.4 wt%)

—

1.25 × 104

40

0.004

[112]

Bi0.5 Se1.5 Te3 (93–99 wt%)

—

—

15

—

[113]

Bi2 Te3

—

∼2

∼130

0.009 (350 K)

[114]b)

Bi2 Te3

—

∼2

∼−50

—

[115]

CNTs (∼80 wt%)

0.5

∼5 000

∼30

—

[116]

Inorganic particle

PbTe

—

0.019

626

—

[117]

Graphite (50 wt%) [118]

1.20 (393 K)

12 000 (393 K)

18.66 (393 K)

1.37 × 10−3 (393 K)

[119]

Bi nanoparticles

—

0.1–10

12–30

—

[120]b)

CNTs

039–0.5

30–90

12–28

—

[116]

SWCNT

0.5–1.0

10–125

11–40

—

[112]

CNT network

0.29

4 035

23.3

2.2 × 10−3

[121]

MWCNT

0.27

1 410

79.8

0.01

[122]

∼0.558

6 000–15 000

60–150

0.08

[76]b)

0.4

∼40 000

21–25

0.02

[118]

Te nanorods

0.22–0.30

1930

163

∼ 0.10

[123]b)

Ca3 Co4 O9 (50%)

—

7 500

18

—

[124]b)

PEDOT:PSS Bi2 Te3 SWCNT (35 wt%)

PbTe

—

0.6

1 205

—

[125]

Te

0.22–0.30

1 930

163

∼0.1

[123]

PVAc

CNT (20 wt%)

0.34

4 800

80

0.006

[126]

P3HT

Bi2 Te3

—

450

118

—

[109]

PANI, polyaniline; PEDOT:PSS, poly(3,4-ethylenedioxythiophene) (PEDOT) doped by poly(styrenesulphonate) (PSS); PVAc, poly(vinyl acetate); P3HT, poly(3-hexylthiophene). a) Other temperature is listed in brackets. b) Films.

easy handling. Most hybrid nanocomposites are fabricated from PEDOT and doped PEDOT. Up to now, various TE inorganic materials have been embedded into the PEDOT matrix to fabricate PEDOT hybrid nanostructured composites, such as PEDOT/PbTe [119], PEDOT:PSS/Te [114, 120], PEDOT:PSS/Bi2 Te3 [76, 118], PEDOT:PSS/Ca3 Co4 O9 [111], and PEDOT:PSS/graphite [121]. Wang et al. [119] added PbTe nanoparticles (50 nm in diameter) into PEDOT polymerization media and cold-pressed the resulting powder into pellets at

0.7

–4500

0.6

–4000

0.5

–3500 –3000

0.4

–2500

0.3

α (μV K–1)

σ (S m–1)


–2000 0.2 –1500 0.1 –1000 0.0 0

10

20

30

40

50

PbTe content (wt%)

Figure 12.7 Electrical conductivity and Seebeck coefficient of PEDOT composite pellets with different PbTe contents. (Wang et al. 2011 [119]. Reproduced with permission of American Chemical Society.)

10 MPa. Figure 12.7 shows the room-temperature electrical conductivity and Seebeck coefficient of the cold-pressed composites as a function of the PbTe content. The electrical conductivity of the pure PEDOT is very low (0.064 S m–1 ). When the PbTe content increases to 43.9 wt%, the electrical conductivity of the composite increases to 0.616 S m–1 (an order of magnitude higher, due to the PbTe particles with higher electrical conductivity), but it is lower than that of PbTe (8.2 S m–1 ). The Seebeck coefficient decreases with increasing PbTe content. As the PbTe content is increased, the Seebeck coefficient decreases from 4088 μV K–1 (pure PEDOT) to 1205 μV K–1 (a sample with 43.9 wt% PbTe) because of the low positive Seebeck coefficient of pure PbTe. A preferable way to improve the TE properties of PEDOT:PSS/inorganic nanocomposites is to choose inorganic materials with high Seebeck coefficients because PEDOT:PSS has a relatively high electrical conductivity and low thermal conductivity. See et al. [114] reported PEDOT:PSS/Te nanorod nanocomposites. These solution-processed nanocomposites exhibited the electrical behavior of PEDOT:PSS and nonfunctionalized Te nanorods but retained a low thermal conductivity comparable to that of PEDOT:PSS, and a much higher power factor (70.9 μW m–1 K–2 ) than those of the individual constituents (0.05 μW m–1 K–2 for PEDOT:PSS and 2.7 μW m–1 K–2 for Te). The Seebeck coefficient of the hybrid nanocomposites was 40% of that of Te nanorods and 8.6 times higher than that of PEDOT:PSS. This combination of an excellent power factor S2 𝜎 (70 μW m–1 K–2 ) and low thermal conductivity (0.2 W m–1 K–1 ) yielded a ZT of 0.1 at room temperature. This ZT value was much higher than that of PEDOT:PSS (ZT max = 6 × 10−5 ) and Te nanorods (ZT max = 4 × 10−4 ). The power factor of the hybrid nanocomposites was also superior to that of each component, while the thermal conductivity of the organic counterpart (PEDOT:PSS)

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was retained, resulting in an enhanced ZT. The improved power factor and ZT in PEDOT:PSS/Te nanocomposites were attributed to the energy-filtering effect at the Te nanorod surfaces passivated with PEDOT:PSS [114]. Zhang et al. [76] impregnated Bi2 Te3 nanoparticles into PEDOT:PSS emulsions to fabricate nanostructured TE composites. They obtained a maximum power factor of 130 μW–1 m–1 K–2 . The estimation of ZT for 10% PEDOT:PSS and 90% p-type Bi2 Te3 was 0.08, about 1/10 of Bi2 Te3 and higher than that of PEDOT:PSS. Du et al. [118] reported Bi2 Te3 nanosheet/PEDOT:PSS composite films prepared by a drop-casting technique. The composite films containing 4 wt% Bi2 Te3 nanosheets showed an electrical conductivity of 1295 S cm–1 , which was higher than that of PEDOT:PSS films (754 S cm–1 ) prepared under the same condition and that of Bi2 Te3 bulk material (850–1250 S cm–1 ). The obtained composite films also showed a high power factor of 32 μW m–1 K–2 . Zaia et al. [120] produced tellurium nanowire/PEDOT:PSS nanocomposites containing Cu1.75 Te nano-islands using a facile aqueous-based technique. The power factor of the nanocomposites was enhanced by 22% through modulating carrier scattering. Liu et al. [111] mechanically dispersed Ca3 Co4 O9 powders in PEDOT:PSS solutions and cast the resultant mixtures on polypropylene substrates as hybrid nanostructured composites. The Seebeck coefficient was improved by increasing the Ca3 Co4 O9 content in the hybrid nanocomposite films. The highest Seebeck coefficient was enhanced by 24.8% compared with that of a free-standing PEDOT-PSS film. However, the electrical conductivity of the composites decreased significantly with increasing inorganic nanomaterials, leading to a remarkable reduction in the power factor. Culebras et al. [121] prepared homogeneous PEDOT:PSS/graphite composites by solvent evaporation on glass substrates at different graphite contents. The thermal stability was not greatly improved upon graphite addition. The Seebeck coefficient was almost same after the addition of 80 wt% graphite (12–16 μV K–1 ). The electrical conductivity increased sharply upon the addition of expanded graphite, and the thermoelectric power factor of these composites was dominated by the electrical conductivity, as shown in Figure 12.8. The electrical conductivity and power factor increased up to 200 S cm–1 and 5 μW m–1 K–2 , respectively, when the graphite content was 80 wt%. The Seebeck coefficient was about 15 μV K–1 , and did not change depending on the graphite content. ZT was 2.3 × 10−4 for PEDOT:PSS with 50 wt% expanded graphite, while it was 5.74 × 10−6 for the PEDOT:PSS. The addition of expanded graphite to the PEDOT:PSS matrix increased the ZT value by three orders of magnitude. 12.2.2

PANI Hybrid Nanostructured Composites PANI is thermally stable up to 250 ∘ C [122], and its electrical conductivity is 6.28 × 10−9 S m–1 without doping while 4.60 × 10−5 S m–1 by doping with 4% HBr [123]. PANI has been widely mixed with inorganic TE nanomaterials for making hybrid TE nanocomposites. Up to now, PANI have been mixed with metal oxides [124], Bi [113], Bi2 Te3 and its alloys [112, 125, 126], NaFe4 P12 [128], carbon nanotubes [129, 130], PbTe [131] and graphite [132].

12.2 Organic/Inorganic Thermoelectric Nanostructured Materials 6

PF (μW m–1 K–2)

σ (S cm–1)

200 150 100 50

4

2

0 0 0

(a)

30

60

wt% expanded graphite

90

0

(b)

30

60

90

wt% expanded graphite

Figure 12.8 (a) Electrical conductivity and (b) power factor (PF) of PEDOT:PSS/expanded graphite composites. (Spitalsky et al. 2010 [127]. Reproduced with permission of Elsevier.)

PANI was incorporated into a V2 O5 ⋅nH2 O xerogel by in situ oxidative polymerization/intercalation [124]. The electrical conductivity of freshly prepared samples was 10−4 –10−1 S cm–1 at room temperature and depended on the degree of polymerization inside the layers and invariably increased upon aging. The Seebeck coefficient varied from −30 to 200 μV K–1 depending on the polymer content and the degree of polymerization. Anno et al. [113] prepared PANI/Bi nanoparticle composites using a planetary ball-milling technique. PANI solutions and Bi powders were ball-milled, cast onto quartz substrates, and dried in air. Bi nanoparticles ranged in size from several tens of nanometers to a few hundred nanometers and were dispersed in the PANI matrix. The Seebeck coefficient increased by three times at room temperature, from 10 to 30 μV K–1 after the Bi nanoparticles were added. The electrical conductivity was reduced by 10 times at room temperature. Zhao et al. [125] mechanically blended 1–7 wt% HClO4 -doped PANI additives with Bi0.5 Sb1.5 Te3 powder and cold-pressed at 1 GPa into bulk nanostructured composites. The Seebeck coefficient of the produced nanocomposites was 10% lower than that of Bi0.5 Sb1.5 Te3 , and the electric conductivity decreased to 30–70% of that of Bi0.5 Sb1.5 Te3 bulk material, leading to a remarkable decrease in power factor and ZT. The reduced ZT was still much higher than that of PANI. Li et al. [126] mechanically ball-milled hydrothermally synthesized Bi2 Te3 nanoparticles with HCl-doped PANI in ethyl alcohol, dried under vacuum, and cold-pressed into tablets. The Seebeck coefficient of −429.8 μV K–1 was observed at 300 K, and the electrical conductivity of the composites was almost the same as that of PANI. Toshima et al. [112] physically mixed Bi2 Te3 nanoparticles in PANI solutions, cast on glass plates, and dried as nanocomposite films. The obtained hybrid films had the same order of electrical conductivity and one order higher Seebeck coefficient compared to PANI materials. The power factor of the hybrid films was about 50 times higher than that of pure PANI. Liu et al. [128] prepared PANI/NaFe4 P12 whisker nanocomposites and PANI/NaFe4 P12 nanowire composites using an in situ compounding method. The electrical conductivity of the PANI/NaFe4 P12 whisker composites was higher than that of PANI. The electrical conductivity of the PANI/NaFe4 P12

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nanowire nanocomposites was lower than that of PANI. The thermopower of the PANI/NaFe4 P12 nanowire was slightly larger than that of PANI. Wang et al. [131] fabricated PANI/PbTe nanostructured composites through an interfacial polymerization method at room temperature. PbTe nanoparticles were coated by PANI, which were then cold-pressed into pellets for electrical conductivity and Seebeck coefficient measurements. Electrical conductivity of the nanocomposites was 10−4 –10 S m–1 and the Seebeck coefficient was 150–600 μV K–1 , depending on experimental conditions. Wang et al. [132] mechanically ball-milled HClO4 -doped PANI with graphite and cold-pressed at 20 MPa. The thermal conductivity (𝜅) increased slightly with increasing graphite content, while the electric conductivity 𝜎 and the Seebeck coefficient S increased remarkably, enhancing the figure of merit ZT to 1.37 × 10−3 at 393 K for the composites with a graphite concentration of 50 wt%. The ZT was at least 10 000 times greater than that of the HClO4 -doped PANI without graphite (1.13 × 10−7 ). The thermal conductivity of the composites increased from 0.29 W K–1 m–2 for a sample without graphite to 1.20 W K–1 m–2 for a sample with a graphite content of 50 wt%. The maximum thermal conductivity was obtained for a sample with 50 wt% graphite, which was about four times higher than that of pure PANI without graphite. The electrical conductivity increased dramatically from 1.23 × 102 to 1.2 × 104 S cm–1 with the increase in graphite content and reached 1.0 × 104 S cm–1 for a sample with a graphite concentration of 50 wt% at room temperature. The Seebeck coefficient of the composites increased from −0.82 to 18.66 μV K–1 at 393 K with the increase of graphite content from 0 to 50 wt%. The increase of the Seebeck coefficient was attributed to the large number of nano-layered interfaces between the HClO4 -doped PANI and graphite phases.

12.2.3

CNT/Polymer Nanostructured Composites

CNTs are widely recognized as one of the most effective fillers to enhance the electrical conductivity of polymer matrices because of their extremely high charge transport [127]. Generally, CNTs show a low Seebeck coefficient, 20–40 μV K–1 , and high thermal conductivity, 10–3000 W m–1 K–1 . The ZT value of CNTs is normally in the range 10−3 –10−2 , indicating that CNTs are not suitable for TE applications. However, ZT > 2 was theoretically predicted in CNTs. Therefore, many groups have been working on the TE behavior of CNTs. It was reported that the experimental ZT values of CNT bulky papers was significantly enhanced to 0.4 by Ar plasma treatments [133]. The electrical conductivity of CNTs is high. Therefore, CNTs have also been integrated into TE polymers as an electrically conductive filler to increase the electric conductivity of hybrid nanocomposites. Polymer/CNT nanostructured composites can be produced by solution processing, mechanical mixing, melt mixing, and in situ polymerization [127]. The electrical conductivity of the thus produced TE nanocomposites is usually significantly enhanced.


The electrical and thermal conductivities of CNT composites containing different types of CNTs (SWCNTs, double-walled CNTs, and MWCNTs) were investigated [134]. CNTs, can form conductive pathways inside hybrid composites to increase the electrical conductivity significantly, while incorporation of CNTs into polymers results in a slight enhancement of the thermal conductivity. The details are discussed below. 12.2.3.1

CNT/PVAc Composites

100

4000

80

0

3000

5

10 15 CNT wt%

60

20

40 2000 20

1000

σ S 0

5

10 CNT wt%

15

0 20

Thermopower, S(μV K–1)

5000

0

(a)

4 3 2 1 0 –1 –2

logσ(S m–1)

Electrical conductivity, σ(S m–1)

6000

Thermal conductivity (W m–1 K–1)

Yu et al. [135] incorporated CNTs into poly(vinyl acetate) (PVAc) and dried into polymer/CNT hybrid composites. Figure 12.9 shows the thermal conductivity of the hybrid nanocomposites with different CNT concentrations. The thermal conductivity varied only slightly with the CNT concentration, only by 1.5 times for the 20 wt% sample compared to that of the 0 wt% sample. The thermopower was more or less constant (40–50 μV K–1 ). This value was close to that of metallic CNTs. Despite the slight increase in the thermal conductivity and thermopower, the electrical conductivity increased rapidly with CNT concentration, typically obeying a power law as a function of the conductive CNT fraction and fitting very well to a three-parameter power curve over the entire CNT concentration. At 20 wt%, the electrical conductivity was 4800 S m–1 . ZT of the composite with 20 wt% CNT was calculated to be about 0.006 at 300 K. It was assumed that the dramatic increase in the electrical conductivity and nearly constant thermal conductivity and thermopower with CNT concentration came from the thermally disconnected while electrically connected junctions that are present in the CNT networks inside the composites. Scanning electron microscopy (SEM) studies indicated that CNTs formed a network in the composites, as shown in Figure 12.10. These CNTs are thermally disconnected but electrically connected. The CNT network dramatically increased the electrical conductivity of the composites, while thermal conductivity and thermopower remained relatively insensitive to the CNT concentration. Therefore, CNTs can tune the properties of the nanocomposites for obtaining a higher thermoelectric figure of merit. 0.4

0.3

0.2

0.1

0.0

(b)

0

5

10 CNT wt%

15

20

Figure 12.9 (a) Electrical conductivity (indicated by red circles)/thermopower (indicated by blue squares) and (b) thermal conductivities of CNT/PVAc composites at room temperature when the CNT concentrations are 0, 0.5, 1, 2, 3, 4, 5, 10, and 20 wt%, respectively. (Yu et al. 2008 [135]. Reproduced with permission of American Chemical Society.)

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CNT

Polymer

12.2.3.2

Figure 12.10 Schematic of the CNT/PVAc nanocomposites. The CNTs form a three-dimensional network along the surfaces of the spherical PVAc particles. (Yu et al. 2008 [135]. Reproduced with permission of American Chemical Society.)

CNT/PANI Nanostructured Composites

Yao et al. [130] prepared hybrid nanocomposites containing CNTs and ordered PANI through a polymerization reaction. The SWCNT/PANI nanocomposites showed both higher electrical conductivity and Seebeck coefficient as compared to those of pure PANI. The electrical conductivity and Seebeck coefficient of the nanocomposites were enhanced to 1.25 × 104 S m–1 and 40 μV K–1 , respectively. The maximum power factor was up to 2 × 10−5 W m–1 K–2 , which is more than two orders of magnitude higher than that of pure PANI. The maximum ZT was estimated to be 0.004 at room temperature for a composite with 41 wt% SWCNTs. It was assumed that the strong π–π interactions between PANI and the nanotubes resulted in a more ordered molecular structure and enhanced the carrier mobility in the composites than in pure PANI, dramatically improving both the electrical conductivity and Seebeck coefficient of the PANI-based hybrid composites. The thermal conductivity of the composites did not change much and still kept very low values, which was attributed to the phonon scattering effect of the nano-interfaces produced by the SWCNT/PANI nanostructures. Meng et al. [129] coated PANI onto random and oriented CNT sheets to fabricate CNT nanocomposites. Figure 12.13a shows a typical SEM image of PANI-coated CNT arrays. The transmission electron microscopy (TEM) images (Figure 12.13b) further demonstrated the nanostructure of individual CNTs coated with PANI. PANI chains grew on the outer walls of the CNTs. π-Bonded surfaces of the CNTs should interact strongly with the conjugated structure of PANI. The Seebeck coefficients of PANI and the CNT array were 2.7 and 25.5 μV K–1 at 300 K, respectively. After PANI coating, the CNT/PANI nanocomposites possessed markedly higher Seebeck coefficients than pure CNTs or PANI (Figure 12.13c). The enhancement for the CNT array/PANI nanocomposites was 29% compared to pure CNTs. The Seebeck coefficient of random CNT/PANI nanocomposites was enhanced by 20% at room temperature. These increases of the Seebeck coefficient were attributed to energy-filtering effects at PANI/CNT interfaces. The thermal conductivity of these CNT composites was 0.4–0.5 W m–1 K–1 when the PANI content was 10–100%. Zhang et al. [136] coated porous PANI onto MWCNTs and molded the obtained powder into pellets under 15 MPa at 80 ∘ C. The Seebeck coefficient was enhanced by thermal annealing. The porosity of PANI/MWCNT composites at a higher MWNT fraction resulted in a higher ZT. The optimized samples showed an electrical conductivity of 14.1 S cm–1 , Seebeck coefficient of 79.8 V K–1 , and thermal conductivity of 0.27 W m–1 K–1 , resulting in the highest figure of merit (ZT) of 0.01 at a very small filler loading (