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Thermoelectric properties of graphene nanosheets-modified polyaniline hybrid nanocomposites by an in situ chemical polymerization
Materials Chemistry and Physics 138 (2013) 238e244
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Thermoelectric properties of graphene nanosheets-modified polyaniline hybrid nanocomposites by an in situ chemical polymerization Yan Lu, Ying Song*, Fuping Wang* School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, PR China
h i g h l i g h t s < PANi adsorbed on the surface of the GNs possesses more extended structure. < Electrical conductivity and Seebeck coefficient of PANi/GNs composites are superior to those of PANi. < Thermal conductivity of the composites still keeps relatively low values.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 June 2012 Received in revised form 8 November 2012 Accepted 18 November 2012
A hybrid material of polyaniline protonated with hydrochloric acid and conductive graphene nanosheets (PANi/GNs) has been prepared by an in situ chemical polymerization method. The interactions between PANi and GNs in the hybrid composites are investigated by utilizing XRD, FT-IR, UVevis and Raman. It is found that the PANi are adsorbed on the surface of the GNs, and the morphology of PANi transforms from twist structure to extended structure after the GNs are introduced. The thermoelectric (TE) properties of PANi/GNs composites have been investigated in the range from 323 K to 453 K. The electrical conductivity and the Seebeck coefficient of PANi/GNs composites are obviously higher than those of the PANi, while the thermal conductivity of the composites still keeps relatively low values even with high GNs content, resulting in the increase in dimensionless figure of merit (ZT). A highest ZT value of 1.95 103 has been obtained for the composite containing 30 wt % GNs at 453 K, which is about 70 times higher than that obtained from the PANi. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Polymers Electrical conductivity Thermal conductivity Thermoelectric effects
1. Introduction Thermoelectric (TE) materials make conversion between thermal and electrical energy without moving mechanical components, which are of great interest for applications as power generators and cooling systems. TE conversion efficiency is determined by the dimensionless figure of merit ZT ¼ S2sT/k, where S is Seebeck coefficient, s is electrical conductivity, k is thermal conductivity and T is absolute temperature. A material with favorable TE property is desired for an excellent electrical conductivity, a large Seebeck coefficient and a low thermal conductivity [1]. Up to now, the high performance TE materials have been mainly focused on inorganic semiconductors [2,3]. Both theoretical and experimental data have been confirmed that low dimensional materials could significantly enhance ZT value originating from
* Corresponding authors. Tel./fax: þ86 451 86418409. E-mail addresses: [email protected] (Y. Lu), [email protected] (Y. Song), [email protected] (F. Wang). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.052
electron confinement, which increases S2s owing to the high density of states induced near the Fermi level EF, and decreases k because the phonon boundary scattering is enhanced in the nanostructures [4,5]. Recently, increasing attention has been paid to conjugated conducting polymers as potential thermoelectric material, such as polyaniline, polypyrrole, polythiophene and their derivatives [6]. These materials usually have much lower thermal conductivity compared with inorganic materials, and their nanostructures are easy to obtain, which are beneficial to the enhancement of thermoelectric properties. Moreover, the conducting polymers would also be of great interest in some microscale applications for different geometry of cooling and power generation due to their lightweight, flexible, and easy to process. The advantages of polyaniline (PANi) compared to other conducting polymers are its high and tunable electrical properties, good environmental stability as well as low cost. The composites consisting of PANi and inorganic nanostructures may have better TE properties compared to pure PANi due to a synergistic effect [7,8]. Although the ZT value of PANi is still relatively low compared to the inorganic semiconductors TE materials, the composites containing
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carbon-based filler (e.g. graphite, carbon black, carbon nanotubes, graphene and graphene nanosheets) are attracting more and more attention due to their combination of the excellent electronic and mechanical properties of carbon-based material with low thermal conductivity of the PANi [9,10]. Among these carbon-filler materials, graphene would be preferred over other conventional nanofillers, which has been widely applied to improve the performance of conducting polymer composites [11e13]. Graphene nanosheets (GNs) consisting of one to a few graphene layers (w10 nm) have similar properties to the graphene and can improve electrical conductivity and mechanical property of polymer at substantially smaller loadings than graphite or expanded graphite [14,15], thus they are more suitable for application to large scale production. Recently, PANi/GNs composites have been intensively studied for potential applications in various devices such as energy storage, batteries and supercapacitors etc. [16]. There are few reports on the TE properties of the PANi/GNs composite. Most recently, Yang et al. found that the electrical conductivity as well as Seebeck coefficient simultaneously increased in both pellets and films for PANi/GNs composites at room temperature [17]. However, the effect of GNs contents on the thermal conductivity of the composites has not been reported in the case. In situ polymerization stands out as one of the most facile method for tailoring the properties of polymer-based composites. Usually, PANi/GNs composites were fabricated by reduction based on the in situ growth of PANi on the graphene oxide interface [18,19]. In addition, a number of approaches have been explored to fabricate PANi/GNs composites including microwave solvothermal synthesis [20], electrochemical synthesis [21], polymerized ionic liquid [22] and so on. However, these processes are not suitable for applications on a large scale due to using organic solvents or expensive additive. Hence, the fabrication of this kind of composite with multiple functions by using simple and mild methods remains challenging. The conventional chemical oxidative polymerization of aniline is initiated by adding an oxidant in a strong acidic solution at temperature between 5 and 0 C [23]. However, interfacial polymerization and rapid mixing methods provide new routes to synthesize nanostructured polymer with rapid mixing at room temperatures [24]. Based on the methods mentioned above, we present here a facile approach to fabricate PANi/GNs composites by the aid of ultrasonic and rapid mixing methods at room temperatures. This process is a mild and economic method for producing the PANi/GNs composites on a large scale without using organic solvents or expensive additives. In this work, GNs as template are used for polymerization of aniline to produce PANi/GNs composites under mild conditions. The composites are characterized by XRD, FT-IR, UVevis and Raman spectra to investigate the interaction between PANi and GNs in detail. The thermoelectric properties of the PANi/GNs composites after being cold pressing are investigated over the temperature range from 323 K to 453 K. Furthermore, effects of GNs content on the thermal conductivity of the PANi/GNs composites are discussed. 2. Experimental
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Ltd, China, respectively. All other reagents were at least of analytical grade and used without further purification. Deionized water was used in all the process of aqueous solution preparations and washing. The PANi/GNs composites were prepared using the in situ chemical polymerization method. GNs and aniline monomer (1.84 ml) were first added to 50 ml HCl (1 mol L1) and dispersed by applying the ultrasonic wave at 200 W for 2 h. The oxidant APS (4.56 g) dissolved in 50 ml HCl (1 mol L1) was added rapidly and stirred continuously at room temperature for 1 h. Then the reaction mixture was kept at room temperature for 24 h to generate PANie GNs composites. The composites were filtered and washed repetitively with HCl (1 mol L1), ethanol and deionized water. The GNs in the composites was varied as 0.05:1, 0.15:1, 0.2:1 and 0.3:1 according to the nominal weight percentage of GNs to the PANi, and the composites obtained were labeled as PANie5GNs, PANie15GNs, PANie20GNs and PANie30GNs, respectively. The obtained composites after drying at 60 C under vacuum were compressed into the blocks with a diameter around 13 mm under the pressure of 20 MPa. Then the obtained compact pellets were cut carefully with dimensions of 10 mm 3 mm 3 mm. The electrical conductivity and Seebeck coefficient were measured on the pressed surface of the pellets. The same powder was weighed and compressed into the blocks with a diameter around 12.7 mm and thickness of 2 mm under the same pressure and processing time for thermal diffusivity measurements. Polyaniline protonated with hydrochloric acid without GNs (recorded as PANi) was prepared under the same conditions for comparison. 2.2. Characterization X-ray diffraction patterns (D/max-RB) of the samples were carried out using a standard diffractometer with Cu Ka radiation at a scanning rate of 5 min1. Transmission electron microscope (TEM) measurement was conducted on A JEOL 2010 FEG microscope at 300 keV. For TEM observation, specimens were prepared by sonicating traces of the GNs or PANieGNs powder in ethanol for 30 min. Then the mixing solution was kept stationary and dropped onto the carbon-coated copper grid using a needle. Fourier transforms infrared (FT-IR) spectra were recorded on a Varian 3100 by using pressed KBr plates. Raman spectra were performed using a Raman spectrometer (Renishaw Raman System 2000) with 458 nm excitation to study the fine structure of the specimen. The photo-absorption property was recorded with a diffuse reflectance UVevisible spectrometer to barium sulfate reference (UV-2400, Shimadzu, Japan). The electrical conductivity s (T) and Seebeck coefficient S (T) were measured simultaneously in helium atmosphere over the temperature range of 323 Ke453 K by using a standard four-probe method on ULVAC ZEM-3 system. The thermal conductivity k ¼ DCpd was calculated from the specific heat capacity (Cp), the thermal diffusivity (D) and the density (d), in which Cp was carried out on a TA Q200 DSC Instruments at a heating rate of 5 C min1 under nitrogen flow, D was measured by the laser flash diffusivity method using the laser flash technique (Netzsch LFA 457), and d was determined by the Archimedes method.
2.1. Materials and synthesis 3. Results and discussions Graphene nanosheet powder with purity >99.85 wt % was purchased from Xiamen Knano graphite technology Co., Ltd, China. It showed sheet morphology with about 1e30 mm in diameter and 5e15 nm in thickness, and the aspect ratio of graphene nanosheets was between 1300 and 6000. The aniline and the oxidant ammonium peroxydisulfate (APS) were purchased from Aladdin Chemistry Co. Ltd, China and Sinopharm Chemical Reagent Beijing Co.,
3.1. Structural characterization Fig. 1(a) shows the X-ray diffraction patterns of PANi and PANi/ GNs composites. The PANi consists of three main reflection peaks at 2q ¼ 15 , 20 and 25 , which is indicative of crystalline regions dispersed in an amorphous medium [25,26]. The broad peak at
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Fig. 1. (a) XRD patterns of PANi and PANi/GNs hybrid composites (b) UVevis spectra of PANi and PANi/GNs hybrid composites (c) FT-IR spectra of PANi and PANi/GNs hybrid composites (d) Raman spectra of PANi, GNs and PANi/GNs hybrid composites.
2q ¼ 15 is assigned to the repeat unit of the polyaniline chain. The peak centered at 2q ¼ 20 is ascribed to periodicity perpendicular to the polymer chain. The peak at 2q ¼ 25 is due to periodicity parallel to the polymer chain corresponding to the face to face interchain stacking between benzene rings [27]. It can be seen that the peak of 2q ¼ 25 appears stronger than that of 2q ¼ 20 , which is similar to that of highly doped emeraldine salt [28]. For PANi/GNs composites, the peaks at 2q ¼ 15 and 2q ¼ 20 are similar to those of the pure PANi, indicating the unit structures of the polyaniline chain are unchanged after being loaded on the GNs, whereas the loading GNs makes the peak 2q ¼ 25 narrower and shifts from 2q ¼ 25 to 26.5 as a result of stacking interactions between GNs and benzene rings of PANi. In addition, the intensity of the peak at 2q ¼ 26.5 enhanced with the increasing GNs loading amount, which represents the graphite plane is composed of well-ordered graphene [13]. However, for the PANie30GNs, the peaks at 2q ¼ 15 and 2q ¼ 20 become broader and weaker than those of the PANie20GNs and PANi. It is indicated that the excess GNs will hinder the growth of crystallization of the PANi chains and form barriers to carrier transport in the PANi chains, which is in agreement with the electrical conductivity of PANie30GNs [29]. Fig. 1(b) compares the UVevis spectra of PANi and PANi/GNs composites to investigate the interaction between PANi and GNs. The PANi sample exhibits three absorption peaks at 330 nm, 630 nm and 420 nm, which are assigned to the pep transition of benzene rings and quinone rings, as well as the polaron band transition caused by protonation, respectively [30]. The two peaks at 330 nm and 630 nm in the PANi/GNs composites present blue shifts and dramatic decreases in peak intensity, indicating there exists an interaction scattering between PANi and GNs particles. A shift of the peak at 330 nm toward lower wavelength is also observed for the PANie30GNs composite, which indicates that the higher loading GNs would weaken the pep transition of benzene
rings. These behaviors suggest that the degree of charge delocalization becomes lower as the loading amount of GNs increases [31]. The FT-IR spectra of the PANi/GNs composites with various GNs loading amount are shown in Fig. 1(c). It can be seen that the spectra of PANi/GNs composite are very similar to that of PANi. The main peaks at 1568 and 1480 cm1 can be assigned to the stretching vibrations of quinone and benzene rings, respectively. The peaks at 1292 and 1236 cm1 correspond to the CeN stretching vibration. The peaks at 1131 and 800 cm1 are attributed to in-plane bending of CeH and the out-of-plane bending of CeH, respectively [18]. These suggest that PANi is formed in the emeraldine salt during polymerization. Although the ratio of intensity between quinoid ring and benzenoid ring has no significant change for PANi/GNs composite with different GNs contents, the peaks of PANi presents a blue shift, indicating that the pbonded surface of GNs interacts with the conjugated structure of PANi. The Raman spectra of the samples are presented in Fig. 1(d) to provide further evidence of the chemical bonding between PANi and GNs. The spectrum of GNs shows strong peaks at 1367 cm1 and 1580 cm1, which are assigned to the D band (defects or edge areas) and G band (the vibration of sp2 hybridized carbon), respectively [32]. There are only two peaks at 1352 cm1 and 1584 cm1 in the spectrum of PANi, i.e. CeNþ (polaron) stretching of the bipolaron structure and CeC stretching of the benzenoid rings [33], and the two peaks are of the similar intensity. However, for PANie30GNs, it is found that the peak at 1584 cm1 moves toward the lower wavenumbers to 1573 cm1 compared to PANi, which originates from the additional pep interactions between PANi and GNs [10]. In addition, the intensities of the peaks increase after being loaded GNs, which is attributed to the site selective interactions between the quinoid rings and GNs [34].
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3.2. Morphologies
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nanorods are connected randomly with the homogeneous nucleation in the PANi/GNs composites.
Fig. 2 displays the typical TEM images of (a) GNs (b) PANi (c) PANie20GNs (d) PANie30GNs. The single layer graphene nanosheet is almost transparent and very smooth after being treated further with ultrasonic wave as shown in Fig. 2(a). This reveals that the graphene nanosheets can be torn into sheets with a large width-to-thickness aspect ratio (w5000). Such nanoscale dispersion facilitates the formation of the electrical conductive network in the polymer matrix. The selected area electron diffraction pattern in inset of Fig. 2(a) also shows that the GNs have a good crystalline character. Fig. 2(b) shows the twisted nanorods with a mean diameter of 50 nm after the rapid mixing of monomer and oxidant. For the PANi/GNs composites, a mixture structure with PANi nanorods intercalated between the layers or adsorbed on the surface of the GNs are shown in Fig. 2(c, d). It is seen that the larger surface areas of GNs are covered with PANi because GNs act as a support to supply active sites for the nucleation of PANi during the in situ polymerization. It is also found that the morphology of PANi transforms from twist structure to extended structure after being loaded GNs (marked by the white arrows), suggesting strong interactions between PANi and GNs. The GNs coated with PANi may greatly affect the electrical properties of the composites, which will be discussed below. Based on the analysis of TEM results, a formation mechanism of PANi/GNs composites is speculated in Fig. 3. As a support material, GNs could provide more active sites for nucleation of PANi as well as excellent channels of electronic transmission. The most active nucleation sites are generated on the GNs surface through heterogeneous nucleation at the beginning of the polymerization process. These active sites are beneficial to the subsequent growth of polyaniline owing to minimizing the interfacial energy barrier between the GNs surface and PANi. The PANi nanorods are further generated by subsequent growth of PANi on the initially formed PANi/GNs structures during polymerization. As a result, PANi
3.3. Thermoelectric properties 3.3.1. Electrical conductivity The electrical conductivity as a function of temperature for PANi and PANi/GNs samples is studied as shown in Fig. 4(a). The electrical conductivities of all the samples degrade with increasing temperature over the measured temperature, as indicative of a metallic-like transport behavior (ds/dT < 0). An enhanced electrical conductivity with increasing temperature, however, has been observed in PANi/graphite composites [35]. Such an opposite trend may be originated from different additives, leading to different transport behavior. The electrical conductivity of the PANi decreases from 500 S m1 at 323 K to 425 S m1 at 453 K. In this case, polarons as the main carriers can be transferred along the intrachains or hopping between interchains [36]. Meanwhile, the charge carriers can be delocalized completely in higher temperature region [37], which implies metallic-like conduction may play a major role during carrier transport. This behavior is mainly attributed to GNs coated with the extended PANi nanorods, which would make the polaron mobility easier, and the presence of GNs can also bridge the carrier transport between interchain through tunneling mechanism. Hence, all the PANi/GNs samples show a metallic-like character. Note that a trend toward remarkable increase in electrical conductivities of the composites when GNs content varies from 5 wt % to 20 wt %, then decrease with the further increase of the GNs content. A maximum value of 5400 S m1 for PANie20GNs at 323 K is obtained, which is about 11 times higher than that of the PANi at the same temperature. The enhancement of the electrical conductivity comes mainly from the high carrier mobility of GNs [13,17]. The GNs can bridge the carrier transport by means of the pep interactions with PANi, which may further enhance the
Fig. 2. TEM images of (a) GNs (b) PANi (c) PANie20GNs (d) PANie30GNs.
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Fig. 3. Schematic illustration of nucleation and growth mechanism of PANieGNs composites.
carrier mobility, and thus increase the electrical conductivity of PANi/GNs hybrid composites. However, the electrical conductivity decreases slowly with further increasing GNs content more than 20 wt %. The reasons are as follows: (1) The excess GNs will hinder the growth of crystallization and form barriers to carrier transport in the PANi chains, which is consistent with the XRD results in Fig. 1(a). (2) The sample with higher amount of GNs, for example PANie30GNs, more resistive contacts induce longer hopping lengths for charge carriers, and the charge carriers can only be transferred by tunneling between contacting nanorods in bulk materials. The carrier mobility decreases in the highly disordered system, and thus lowers the electrical conductivity. (3) The protonation degree of the PANi decrease in the higher content of GNs due to the part adsorption Hþ onto GNs, which increases hopping distance and weakens the hopping mechanism due to phonon-assisted tunneling between electronic localized states centered at different positions, resulting in the decrease in electrical conductivity [38]. In order to understand the conduction mechanism of PANi/GNs, we adopted variable range hopping (VRH) model to fit the data of
PANi/GNs. In the VRH model, the temperature dependence of conductivity follows the relation: s ¼ s0exp[(T0/T)g], where T0 is the Mott characteristic temperature and s0 is the conductivity at T ¼ N, and g ¼ 1/4 for three dimensions, 1/3 for two dimensions and 1/2 for one dimension [39]. The s (T) follows well with the VRH model to describe the transport of the carrier from the plots of ln(s/ s0) vs. T1/4 as shown in Fig. 4(b). It is found that these plots exhibit straight lines behavior for all the samples in the range of 323 K < T < 435 K, suggesting that the temperature dependences of the electrical conductivity are consistent with the characteristics of variable range hopping conduction.
Fig. 4. (a) Electrical conductivity s of PANi and PANi/GNs hybrid composites as functions of temperature, and (b) ln(s) T1/4 of PANi and PANi/GNs hybrid composites.
Fig. 5. (a) Seebeck coefficient S and (b) power factor S2s as functions of temperature for PANi and PANi/GNs hybrid composites.
3.3.2. Seebeck coefficient The temperature dependence of the Seebeck coefficient (S) for the composites is shown in Fig. 5(a). The S values of all the samples are positive over the whole temperature range, indicating p-type conduction. All the composites have higher Seebeck coefficient compared to pure PANi, and the S reaches a maximum value of
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40 mV K1 at 453 K for PANie15GNs. Generally, these two parameters, i.e. electrical conductivity and the Seebeck coefficient, can not increase simultaneously due to the inverse relation to the gap between narrow transport level (ET) and Fermi level (EF) of the material [40]. However, both the Seebeck coefficient and electrical conductivity of PANi/GNs are simultaneously increased for the samples containing GNs less than 15 wt %, indicating that the conduction mechanism could not be explained by a conventional model based on the band theory or the electron-phonon scattering [9]. We use Mott formula to understand the nature of the Seebeck coefficient, which can be expressed as follows:
S ¼
Ce p2 k2B T vln mðεÞ þ vε n 3e ε¼εF
(1)
where n, m(ε), ce and kB are carrier concentration, energy correlated carrier mobility, specific heat and Boltzmann constant, respectively. Both carrier concentration and carrier mobility will affect the Seebeck coefficient. When the GNs content is less than 15 wt %, the carrier mobility plays a major role because the carrier concentration remains basically unchanged as the GNs content is increasing [13,17]. The S value of composites is to a large extent determined by the last term of the formula and proportional to the energy correlated carrier mobility. Thus the S value of PANie15GNs is higher than that of PANie5GNs. However, the S value of composites begins to decrease when the GNs content exceeds 15 wt%, and this decrease is not continuous. The reason is that the twisted chains increase in PANi/GNs composites with increasing GNs content and then produce more conjugated defects [41], which will decrease the carrier mobility and lead to the decrease of the S value. In addition, it could be observed that the S value of the PANie30GNs is higher than that of PANie20GNs. The Seebeck coefficient shows an inverse tendency compared to that of electrical conductivity, which is in agreement with that reported in the literature [42]. There are several reasons for the tendency. On the one hand, excess GNs will hinder the growth of crystallization in the PANi chains, which enhances electron energy filtering effects by blocking or scattering the transport of some carriers along the GNs surface [9], thus enhancing the Seebeck coefficient and reducing the electrical conductivity. On the other hand, when the GNs content exceeds a certain value, carrier concentration is decreased due to forming more disordered PANi. S value of composites is to a large extent determined by the former term of the formula and inversely proportional to the carrier concentration, which is similar to previous literature on PEDOT: PSS-graphene [13]. Thus, the S value of the PANie30GNs is higher than that of the PANie20GNs. The power factor (S2s) is calculated based on the electrical conductivity and Seebeck coefficient, as shown in Fig. 5(b). The power factor is enhanced gradually with increasing GNs content. The maximum power factor is up to 2.6 106 W m1 K2 for PANie30GNs, which is almost 260 times higher than that of PANi. 3.3.3. Thermal conductivity The temperature dependence of the thermal conductivities for the PANi/GNs composites is shown in Fig. 6. The thermal conductivity of PANi is up to 0.2 W m1 K1 over the temperature range of 323 Ke453 K, and no further varies with the temperature. The thermal conductivity of PANi/GNs composites enhances from 0.2 to 0.7 W m1 K1 with increasing GNs content despite of GNs having a rather higher theoretical thermal conductivity, which is still one order of magnitude lower than that of inorganic counterparts. Such a low thermal conductivity is mainly attributed to the poor thermal conductivity of PANi and phonon scattering from the substantial grain boundaries between the
Fig. 6. Thermal conductivity k of PANi and PANi/GNs hybrid composites as functions of temperature.
PANi and GNs in bulk materials [43], which is advantageous for improving its TE properties. According to the WiedemanneFranz law, the total thermal conductivity comprises of electrical component (ke) and phonon component (kp). kp ¼ 1/3cvl where v is the phonon velocity, l is the mean free path and c is the specific heat capacity. This relationship is suitable to different type of heat transfer, such as phonon and electronic etc. As for the PANi, the phonon as main thermal carrier will determine the total thermal conductivity because the electrical conductivity is low. J. Jin et al. have studied the thermal conductivity of different scales for PANi. They found that there is no correlation between thermal and electrical conductivity for microscale PANi [44]. Here, no significant change of thermal conductivity is found with the increasing temperature. The possible reasons are as follows: the phonon heat capacity and mean free path are constant in the bulk [44]. In the present PANi/GNs composites, heat capacity is no longer changes at high temperature region, and the opportunities for collision of activated thermal carriers remarkably increased, leading to gradual decrease in mean free path. Thus thermal conductivity gradually decreases with increasing temperature. 3.3.4. Dimensionless figure of merit The ZT values as function of temperature are presented in Fig. 7. The ZT values of all the samples increase monotonically with increasing temperature from 323 K to 453 K. Both the electrical conductivity (s) and the Seebeck coefficient (S) of PANi/GNs are obviously higher than those of the PANi, and thermal conductivity of PANi/GNs composites still keeps relatively low values even with high GNs content, resulting in the increase of ZT values. The highest
Fig. 7. The figure of merit ZT of PANi and PANi/GNs hybrid composites as functions of temperature.
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ZT of 1.95 103 is obtained for PANie30GNs sample at 453 K, which is about 70 times higher than that of the PANi. This indicates that the enhancement of electrical conductivity should be the principal way to improve the TE properties of PANi. Moreover, the present ZT of PANie30GNs is higher than that reported by L. Wang et al. for PANi-50 wt % graphite system (ZTmax w1.37 103 at 393 K), and the content of GNs used in our work is much lower than that of the graphite in their work [35]. This suggests that GNs as electric filler is better than graphite to enhance the TE of PANi, and PANi/GNs composites possess better TE properties at high temperature. 4. Conclusions In this work, we synthesized PANi/GNs composites using a facile in situ chemical polymerization method at room temperature, which is a mild and low cost method to produce the PANi/GNs composites on a large scale. The PANi in the composites possesses more extended structure, and thus results in a high mobility due to the interactions between PANi and GNs. It was found that both the electrical conductivity (s) and the Seebeck coefficient (S) of PANi/ GNs are obviously higher than those of the pure PANi. The thermal conductivity of PANi/GNs composites still keeps relatively low values even with high GNs content. A highest dimensionless figure of merit ZT of 1.95 103 is obtained for PANie30GNs composite at 453 K, which is about 70 times higher than that of PANi. This suggests that the enhancement of electrical conductivity may be the principal way to improve the TE properties of PANi, and PANi/ GNs composites possesses better TE properties at high temperature. The ZT value would be further improved by adjusting the content of GNs with higher electrical conductivity. Acknowledgment This work was financially supported by the National Nature Science Foundation of China (Grant No. 50772026). References [1] M. Zebarjadi, K. Esfarjani, M. Dresselhaus, Z. Ren, G. Chen, Energy Environ. Sci. 5 (2012) 5147e5162. [2] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 413 (2001) 597e602. [3] T.M. Tritt, M. Subramanian, MRS Bull. 31 (2006) 188e198.
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Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Thermoelectric properties of graphene nanosheets-modified polyaniline hybrid nanocomposites by an in situ chemical polymerization Yan Lu, Ying Song*, Fuping Wang* School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, PR China
h i g h l i g h t s < PANi adsorbed on the surface of the GNs possesses more extended structure. < Electrical conductivity and Seebeck coefficient of PANi/GNs composites are superior to those of PANi. < Thermal conductivity of the composites still keeps relatively low values.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 June 2012 Received in revised form 8 November 2012 Accepted 18 November 2012
A hybrid material of polyaniline protonated with hydrochloric acid and conductive graphene nanosheets (PANi/GNs) has been prepared by an in situ chemical polymerization method. The interactions between PANi and GNs in the hybrid composites are investigated by utilizing XRD, FT-IR, UVevis and Raman. It is found that the PANi are adsorbed on the surface of the GNs, and the morphology of PANi transforms from twist structure to extended structure after the GNs are introduced. The thermoelectric (TE) properties of PANi/GNs composites have been investigated in the range from 323 K to 453 K. The electrical conductivity and the Seebeck coefficient of PANi/GNs composites are obviously higher than those of the PANi, while the thermal conductivity of the composites still keeps relatively low values even with high GNs content, resulting in the increase in dimensionless figure of merit (ZT). A highest ZT value of 1.95 103 has been obtained for the composite containing 30 wt % GNs at 453 K, which is about 70 times higher than that obtained from the PANi. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Polymers Electrical conductivity Thermal conductivity Thermoelectric effects
1. Introduction Thermoelectric (TE) materials make conversion between thermal and electrical energy without moving mechanical components, which are of great interest for applications as power generators and cooling systems. TE conversion efficiency is determined by the dimensionless figure of merit ZT ¼ S2sT/k, where S is Seebeck coefficient, s is electrical conductivity, k is thermal conductivity and T is absolute temperature. A material with favorable TE property is desired for an excellent electrical conductivity, a large Seebeck coefficient and a low thermal conductivity [1]. Up to now, the high performance TE materials have been mainly focused on inorganic semiconductors [2,3]. Both theoretical and experimental data have been confirmed that low dimensional materials could significantly enhance ZT value originating from
* Corresponding authors. Tel./fax: þ86 451 86418409. E-mail addresses: [email protected] (Y. Lu), [email protected] (Y. Song), [email protected] (F. Wang). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.052
electron confinement, which increases S2s owing to the high density of states induced near the Fermi level EF, and decreases k because the phonon boundary scattering is enhanced in the nanostructures [4,5]. Recently, increasing attention has been paid to conjugated conducting polymers as potential thermoelectric material, such as polyaniline, polypyrrole, polythiophene and their derivatives [6]. These materials usually have much lower thermal conductivity compared with inorganic materials, and their nanostructures are easy to obtain, which are beneficial to the enhancement of thermoelectric properties. Moreover, the conducting polymers would also be of great interest in some microscale applications for different geometry of cooling and power generation due to their lightweight, flexible, and easy to process. The advantages of polyaniline (PANi) compared to other conducting polymers are its high and tunable electrical properties, good environmental stability as well as low cost. The composites consisting of PANi and inorganic nanostructures may have better TE properties compared to pure PANi due to a synergistic effect [7,8]. Although the ZT value of PANi is still relatively low compared to the inorganic semiconductors TE materials, the composites containing
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carbon-based filler (e.g. graphite, carbon black, carbon nanotubes, graphene and graphene nanosheets) are attracting more and more attention due to their combination of the excellent electronic and mechanical properties of carbon-based material with low thermal conductivity of the PANi [9,10]. Among these carbon-filler materials, graphene would be preferred over other conventional nanofillers, which has been widely applied to improve the performance of conducting polymer composites [11e13]. Graphene nanosheets (GNs) consisting of one to a few graphene layers (w10 nm) have similar properties to the graphene and can improve electrical conductivity and mechanical property of polymer at substantially smaller loadings than graphite or expanded graphite [14,15], thus they are more suitable for application to large scale production. Recently, PANi/GNs composites have been intensively studied for potential applications in various devices such as energy storage, batteries and supercapacitors etc. [16]. There are few reports on the TE properties of the PANi/GNs composite. Most recently, Yang et al. found that the electrical conductivity as well as Seebeck coefficient simultaneously increased in both pellets and films for PANi/GNs composites at room temperature [17]. However, the effect of GNs contents on the thermal conductivity of the composites has not been reported in the case. In situ polymerization stands out as one of the most facile method for tailoring the properties of polymer-based composites. Usually, PANi/GNs composites were fabricated by reduction based on the in situ growth of PANi on the graphene oxide interface [18,19]. In addition, a number of approaches have been explored to fabricate PANi/GNs composites including microwave solvothermal synthesis [20], electrochemical synthesis [21], polymerized ionic liquid [22] and so on. However, these processes are not suitable for applications on a large scale due to using organic solvents or expensive additive. Hence, the fabrication of this kind of composite with multiple functions by using simple and mild methods remains challenging. The conventional chemical oxidative polymerization of aniline is initiated by adding an oxidant in a strong acidic solution at temperature between 5 and 0 C [23]. However, interfacial polymerization and rapid mixing methods provide new routes to synthesize nanostructured polymer with rapid mixing at room temperatures [24]. Based on the methods mentioned above, we present here a facile approach to fabricate PANi/GNs composites by the aid of ultrasonic and rapid mixing methods at room temperatures. This process is a mild and economic method for producing the PANi/GNs composites on a large scale without using organic solvents or expensive additives. In this work, GNs as template are used for polymerization of aniline to produce PANi/GNs composites under mild conditions. The composites are characterized by XRD, FT-IR, UVevis and Raman spectra to investigate the interaction between PANi and GNs in detail. The thermoelectric properties of the PANi/GNs composites after being cold pressing are investigated over the temperature range from 323 K to 453 K. Furthermore, effects of GNs content on the thermal conductivity of the PANi/GNs composites are discussed. 2. Experimental
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Ltd, China, respectively. All other reagents were at least of analytical grade and used without further purification. Deionized water was used in all the process of aqueous solution preparations and washing. The PANi/GNs composites were prepared using the in situ chemical polymerization method. GNs and aniline monomer (1.84 ml) were first added to 50 ml HCl (1 mol L1) and dispersed by applying the ultrasonic wave at 200 W for 2 h. The oxidant APS (4.56 g) dissolved in 50 ml HCl (1 mol L1) was added rapidly and stirred continuously at room temperature for 1 h. Then the reaction mixture was kept at room temperature for 24 h to generate PANie GNs composites. The composites were filtered and washed repetitively with HCl (1 mol L1), ethanol and deionized water. The GNs in the composites was varied as 0.05:1, 0.15:1, 0.2:1 and 0.3:1 according to the nominal weight percentage of GNs to the PANi, and the composites obtained were labeled as PANie5GNs, PANie15GNs, PANie20GNs and PANie30GNs, respectively. The obtained composites after drying at 60 C under vacuum were compressed into the blocks with a diameter around 13 mm under the pressure of 20 MPa. Then the obtained compact pellets were cut carefully with dimensions of 10 mm 3 mm 3 mm. The electrical conductivity and Seebeck coefficient were measured on the pressed surface of the pellets. The same powder was weighed and compressed into the blocks with a diameter around 12.7 mm and thickness of 2 mm under the same pressure and processing time for thermal diffusivity measurements. Polyaniline protonated with hydrochloric acid without GNs (recorded as PANi) was prepared under the same conditions for comparison. 2.2. Characterization X-ray diffraction patterns (D/max-RB) of the samples were carried out using a standard diffractometer with Cu Ka radiation at a scanning rate of 5 min1. Transmission electron microscope (TEM) measurement was conducted on A JEOL 2010 FEG microscope at 300 keV. For TEM observation, specimens were prepared by sonicating traces of the GNs or PANieGNs powder in ethanol for 30 min. Then the mixing solution was kept stationary and dropped onto the carbon-coated copper grid using a needle. Fourier transforms infrared (FT-IR) spectra were recorded on a Varian 3100 by using pressed KBr plates. Raman spectra were performed using a Raman spectrometer (Renishaw Raman System 2000) with 458 nm excitation to study the fine structure of the specimen. The photo-absorption property was recorded with a diffuse reflectance UVevisible spectrometer to barium sulfate reference (UV-2400, Shimadzu, Japan). The electrical conductivity s (T) and Seebeck coefficient S (T) were measured simultaneously in helium atmosphere over the temperature range of 323 Ke453 K by using a standard four-probe method on ULVAC ZEM-3 system. The thermal conductivity k ¼ DCpd was calculated from the specific heat capacity (Cp), the thermal diffusivity (D) and the density (d), in which Cp was carried out on a TA Q200 DSC Instruments at a heating rate of 5 C min1 under nitrogen flow, D was measured by the laser flash diffusivity method using the laser flash technique (Netzsch LFA 457), and d was determined by the Archimedes method.
2.1. Materials and synthesis 3. Results and discussions Graphene nanosheet powder with purity >99.85 wt % was purchased from Xiamen Knano graphite technology Co., Ltd, China. It showed sheet morphology with about 1e30 mm in diameter and 5e15 nm in thickness, and the aspect ratio of graphene nanosheets was between 1300 and 6000. The aniline and the oxidant ammonium peroxydisulfate (APS) were purchased from Aladdin Chemistry Co. Ltd, China and Sinopharm Chemical Reagent Beijing Co.,
3.1. Structural characterization Fig. 1(a) shows the X-ray diffraction patterns of PANi and PANi/ GNs composites. The PANi consists of three main reflection peaks at 2q ¼ 15 , 20 and 25 , which is indicative of crystalline regions dispersed in an amorphous medium [25,26]. The broad peak at
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Fig. 1. (a) XRD patterns of PANi and PANi/GNs hybrid composites (b) UVevis spectra of PANi and PANi/GNs hybrid composites (c) FT-IR spectra of PANi and PANi/GNs hybrid composites (d) Raman spectra of PANi, GNs and PANi/GNs hybrid composites.
2q ¼ 15 is assigned to the repeat unit of the polyaniline chain. The peak centered at 2q ¼ 20 is ascribed to periodicity perpendicular to the polymer chain. The peak at 2q ¼ 25 is due to periodicity parallel to the polymer chain corresponding to the face to face interchain stacking between benzene rings [27]. It can be seen that the peak of 2q ¼ 25 appears stronger than that of 2q ¼ 20 , which is similar to that of highly doped emeraldine salt [28]. For PANi/GNs composites, the peaks at 2q ¼ 15 and 2q ¼ 20 are similar to those of the pure PANi, indicating the unit structures of the polyaniline chain are unchanged after being loaded on the GNs, whereas the loading GNs makes the peak 2q ¼ 25 narrower and shifts from 2q ¼ 25 to 26.5 as a result of stacking interactions between GNs and benzene rings of PANi. In addition, the intensity of the peak at 2q ¼ 26.5 enhanced with the increasing GNs loading amount, which represents the graphite plane is composed of well-ordered graphene [13]. However, for the PANie30GNs, the peaks at 2q ¼ 15 and 2q ¼ 20 become broader and weaker than those of the PANie20GNs and PANi. It is indicated that the excess GNs will hinder the growth of crystallization of the PANi chains and form barriers to carrier transport in the PANi chains, which is in agreement with the electrical conductivity of PANie30GNs [29]. Fig. 1(b) compares the UVevis spectra of PANi and PANi/GNs composites to investigate the interaction between PANi and GNs. The PANi sample exhibits three absorption peaks at 330 nm, 630 nm and 420 nm, which are assigned to the pep transition of benzene rings and quinone rings, as well as the polaron band transition caused by protonation, respectively [30]. The two peaks at 330 nm and 630 nm in the PANi/GNs composites present blue shifts and dramatic decreases in peak intensity, indicating there exists an interaction scattering between PANi and GNs particles. A shift of the peak at 330 nm toward lower wavelength is also observed for the PANie30GNs composite, which indicates that the higher loading GNs would weaken the pep transition of benzene
rings. These behaviors suggest that the degree of charge delocalization becomes lower as the loading amount of GNs increases [31]. The FT-IR spectra of the PANi/GNs composites with various GNs loading amount are shown in Fig. 1(c). It can be seen that the spectra of PANi/GNs composite are very similar to that of PANi. The main peaks at 1568 and 1480 cm1 can be assigned to the stretching vibrations of quinone and benzene rings, respectively. The peaks at 1292 and 1236 cm1 correspond to the CeN stretching vibration. The peaks at 1131 and 800 cm1 are attributed to in-plane bending of CeH and the out-of-plane bending of CeH, respectively [18]. These suggest that PANi is formed in the emeraldine salt during polymerization. Although the ratio of intensity between quinoid ring and benzenoid ring has no significant change for PANi/GNs composite with different GNs contents, the peaks of PANi presents a blue shift, indicating that the pbonded surface of GNs interacts with the conjugated structure of PANi. The Raman spectra of the samples are presented in Fig. 1(d) to provide further evidence of the chemical bonding between PANi and GNs. The spectrum of GNs shows strong peaks at 1367 cm1 and 1580 cm1, which are assigned to the D band (defects or edge areas) and G band (the vibration of sp2 hybridized carbon), respectively [32]. There are only two peaks at 1352 cm1 and 1584 cm1 in the spectrum of PANi, i.e. CeNþ (polaron) stretching of the bipolaron structure and CeC stretching of the benzenoid rings [33], and the two peaks are of the similar intensity. However, for PANie30GNs, it is found that the peak at 1584 cm1 moves toward the lower wavenumbers to 1573 cm1 compared to PANi, which originates from the additional pep interactions between PANi and GNs [10]. In addition, the intensities of the peaks increase after being loaded GNs, which is attributed to the site selective interactions between the quinoid rings and GNs [34].
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3.2. Morphologies
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nanorods are connected randomly with the homogeneous nucleation in the PANi/GNs composites.
Fig. 2 displays the typical TEM images of (a) GNs (b) PANi (c) PANie20GNs (d) PANie30GNs. The single layer graphene nanosheet is almost transparent and very smooth after being treated further with ultrasonic wave as shown in Fig. 2(a). This reveals that the graphene nanosheets can be torn into sheets with a large width-to-thickness aspect ratio (w5000). Such nanoscale dispersion facilitates the formation of the electrical conductive network in the polymer matrix. The selected area electron diffraction pattern in inset of Fig. 2(a) also shows that the GNs have a good crystalline character. Fig. 2(b) shows the twisted nanorods with a mean diameter of 50 nm after the rapid mixing of monomer and oxidant. For the PANi/GNs composites, a mixture structure with PANi nanorods intercalated between the layers or adsorbed on the surface of the GNs are shown in Fig. 2(c, d). It is seen that the larger surface areas of GNs are covered with PANi because GNs act as a support to supply active sites for the nucleation of PANi during the in situ polymerization. It is also found that the morphology of PANi transforms from twist structure to extended structure after being loaded GNs (marked by the white arrows), suggesting strong interactions between PANi and GNs. The GNs coated with PANi may greatly affect the electrical properties of the composites, which will be discussed below. Based on the analysis of TEM results, a formation mechanism of PANi/GNs composites is speculated in Fig. 3. As a support material, GNs could provide more active sites for nucleation of PANi as well as excellent channels of electronic transmission. The most active nucleation sites are generated on the GNs surface through heterogeneous nucleation at the beginning of the polymerization process. These active sites are beneficial to the subsequent growth of polyaniline owing to minimizing the interfacial energy barrier between the GNs surface and PANi. The PANi nanorods are further generated by subsequent growth of PANi on the initially formed PANi/GNs structures during polymerization. As a result, PANi
3.3. Thermoelectric properties 3.3.1. Electrical conductivity The electrical conductivity as a function of temperature for PANi and PANi/GNs samples is studied as shown in Fig. 4(a). The electrical conductivities of all the samples degrade with increasing temperature over the measured temperature, as indicative of a metallic-like transport behavior (ds/dT < 0). An enhanced electrical conductivity with increasing temperature, however, has been observed in PANi/graphite composites [35]. Such an opposite trend may be originated from different additives, leading to different transport behavior. The electrical conductivity of the PANi decreases from 500 S m1 at 323 K to 425 S m1 at 453 K. In this case, polarons as the main carriers can be transferred along the intrachains or hopping between interchains [36]. Meanwhile, the charge carriers can be delocalized completely in higher temperature region [37], which implies metallic-like conduction may play a major role during carrier transport. This behavior is mainly attributed to GNs coated with the extended PANi nanorods, which would make the polaron mobility easier, and the presence of GNs can also bridge the carrier transport between interchain through tunneling mechanism. Hence, all the PANi/GNs samples show a metallic-like character. Note that a trend toward remarkable increase in electrical conductivities of the composites when GNs content varies from 5 wt % to 20 wt %, then decrease with the further increase of the GNs content. A maximum value of 5400 S m1 for PANie20GNs at 323 K is obtained, which is about 11 times higher than that of the PANi at the same temperature. The enhancement of the electrical conductivity comes mainly from the high carrier mobility of GNs [13,17]. The GNs can bridge the carrier transport by means of the pep interactions with PANi, which may further enhance the
Fig. 2. TEM images of (a) GNs (b) PANi (c) PANie20GNs (d) PANie30GNs.
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Fig. 3. Schematic illustration of nucleation and growth mechanism of PANieGNs composites.
carrier mobility, and thus increase the electrical conductivity of PANi/GNs hybrid composites. However, the electrical conductivity decreases slowly with further increasing GNs content more than 20 wt %. The reasons are as follows: (1) The excess GNs will hinder the growth of crystallization and form barriers to carrier transport in the PANi chains, which is consistent with the XRD results in Fig. 1(a). (2) The sample with higher amount of GNs, for example PANie30GNs, more resistive contacts induce longer hopping lengths for charge carriers, and the charge carriers can only be transferred by tunneling between contacting nanorods in bulk materials. The carrier mobility decreases in the highly disordered system, and thus lowers the electrical conductivity. (3) The protonation degree of the PANi decrease in the higher content of GNs due to the part adsorption Hþ onto GNs, which increases hopping distance and weakens the hopping mechanism due to phonon-assisted tunneling between electronic localized states centered at different positions, resulting in the decrease in electrical conductivity [38]. In order to understand the conduction mechanism of PANi/GNs, we adopted variable range hopping (VRH) model to fit the data of
PANi/GNs. In the VRH model, the temperature dependence of conductivity follows the relation: s ¼ s0exp[(T0/T)g], where T0 is the Mott characteristic temperature and s0 is the conductivity at T ¼ N, and g ¼ 1/4 for three dimensions, 1/3 for two dimensions and 1/2 for one dimension [39]. The s (T) follows well with the VRH model to describe the transport of the carrier from the plots of ln(s/ s0) vs. T1/4 as shown in Fig. 4(b). It is found that these plots exhibit straight lines behavior for all the samples in the range of 323 K < T < 435 K, suggesting that the temperature dependences of the electrical conductivity are consistent with the characteristics of variable range hopping conduction.
Fig. 4. (a) Electrical conductivity s of PANi and PANi/GNs hybrid composites as functions of temperature, and (b) ln(s) T1/4 of PANi and PANi/GNs hybrid composites.
Fig. 5. (a) Seebeck coefficient S and (b) power factor S2s as functions of temperature for PANi and PANi/GNs hybrid composites.
3.3.2. Seebeck coefficient The temperature dependence of the Seebeck coefficient (S) for the composites is shown in Fig. 5(a). The S values of all the samples are positive over the whole temperature range, indicating p-type conduction. All the composites have higher Seebeck coefficient compared to pure PANi, and the S reaches a maximum value of
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40 mV K1 at 453 K for PANie15GNs. Generally, these two parameters, i.e. electrical conductivity and the Seebeck coefficient, can not increase simultaneously due to the inverse relation to the gap between narrow transport level (ET) and Fermi level (EF) of the material [40]. However, both the Seebeck coefficient and electrical conductivity of PANi/GNs are simultaneously increased for the samples containing GNs less than 15 wt %, indicating that the conduction mechanism could not be explained by a conventional model based on the band theory or the electron-phonon scattering [9]. We use Mott formula to understand the nature of the Seebeck coefficient, which can be expressed as follows:
S ¼
Ce p2 k2B T vln mðεÞ þ vε n 3e ε¼εF
(1)
where n, m(ε), ce and kB are carrier concentration, energy correlated carrier mobility, specific heat and Boltzmann constant, respectively. Both carrier concentration and carrier mobility will affect the Seebeck coefficient. When the GNs content is less than 15 wt %, the carrier mobility plays a major role because the carrier concentration remains basically unchanged as the GNs content is increasing [13,17]. The S value of composites is to a large extent determined by the last term of the formula and proportional to the energy correlated carrier mobility. Thus the S value of PANie15GNs is higher than that of PANie5GNs. However, the S value of composites begins to decrease when the GNs content exceeds 15 wt%, and this decrease is not continuous. The reason is that the twisted chains increase in PANi/GNs composites with increasing GNs content and then produce more conjugated defects [41], which will decrease the carrier mobility and lead to the decrease of the S value. In addition, it could be observed that the S value of the PANie30GNs is higher than that of PANie20GNs. The Seebeck coefficient shows an inverse tendency compared to that of electrical conductivity, which is in agreement with that reported in the literature [42]. There are several reasons for the tendency. On the one hand, excess GNs will hinder the growth of crystallization in the PANi chains, which enhances electron energy filtering effects by blocking or scattering the transport of some carriers along the GNs surface [9], thus enhancing the Seebeck coefficient and reducing the electrical conductivity. On the other hand, when the GNs content exceeds a certain value, carrier concentration is decreased due to forming more disordered PANi. S value of composites is to a large extent determined by the former term of the formula and inversely proportional to the carrier concentration, which is similar to previous literature on PEDOT: PSS-graphene [13]. Thus, the S value of the PANie30GNs is higher than that of the PANie20GNs. The power factor (S2s) is calculated based on the electrical conductivity and Seebeck coefficient, as shown in Fig. 5(b). The power factor is enhanced gradually with increasing GNs content. The maximum power factor is up to 2.6 106 W m1 K2 for PANie30GNs, which is almost 260 times higher than that of PANi. 3.3.3. Thermal conductivity The temperature dependence of the thermal conductivities for the PANi/GNs composites is shown in Fig. 6. The thermal conductivity of PANi is up to 0.2 W m1 K1 over the temperature range of 323 Ke453 K, and no further varies with the temperature. The thermal conductivity of PANi/GNs composites enhances from 0.2 to 0.7 W m1 K1 with increasing GNs content despite of GNs having a rather higher theoretical thermal conductivity, which is still one order of magnitude lower than that of inorganic counterparts. Such a low thermal conductivity is mainly attributed to the poor thermal conductivity of PANi and phonon scattering from the substantial grain boundaries between the
Fig. 6. Thermal conductivity k of PANi and PANi/GNs hybrid composites as functions of temperature.
PANi and GNs in bulk materials [43], which is advantageous for improving its TE properties. According to the WiedemanneFranz law, the total thermal conductivity comprises of electrical component (ke) and phonon component (kp). kp ¼ 1/3cvl where v is the phonon velocity, l is the mean free path and c is the specific heat capacity. This relationship is suitable to different type of heat transfer, such as phonon and electronic etc. As for the PANi, the phonon as main thermal carrier will determine the total thermal conductivity because the electrical conductivity is low. J. Jin et al. have studied the thermal conductivity of different scales for PANi. They found that there is no correlation between thermal and electrical conductivity for microscale PANi [44]. Here, no significant change of thermal conductivity is found with the increasing temperature. The possible reasons are as follows: the phonon heat capacity and mean free path are constant in the bulk [44]. In the present PANi/GNs composites, heat capacity is no longer changes at high temperature region, and the opportunities for collision of activated thermal carriers remarkably increased, leading to gradual decrease in mean free path. Thus thermal conductivity gradually decreases with increasing temperature. 3.3.4. Dimensionless figure of merit The ZT values as function of temperature are presented in Fig. 7. The ZT values of all the samples increase monotonically with increasing temperature from 323 K to 453 K. Both the electrical conductivity (s) and the Seebeck coefficient (S) of PANi/GNs are obviously higher than those of the PANi, and thermal conductivity of PANi/GNs composites still keeps relatively low values even with high GNs content, resulting in the increase of ZT values. The highest
Fig. 7. The figure of merit ZT of PANi and PANi/GNs hybrid composites as functions of temperature.
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ZT of 1.95 103 is obtained for PANie30GNs sample at 453 K, which is about 70 times higher than that of the PANi. This indicates that the enhancement of electrical conductivity should be the principal way to improve the TE properties of PANi. Moreover, the present ZT of PANie30GNs is higher than that reported by L. Wang et al. for PANi-50 wt % graphite system (ZTmax w1.37 103 at 393 K), and the content of GNs used in our work is much lower than that of the graphite in their work [35]. This suggests that GNs as electric filler is better than graphite to enhance the TE of PANi, and PANi/GNs composites possess better TE properties at high temperature. 4. Conclusions In this work, we synthesized PANi/GNs composites using a facile in situ chemical polymerization method at room temperature, which is a mild and low cost method to produce the PANi/GNs composites on a large scale. The PANi in the composites possesses more extended structure, and thus results in a high mobility due to the interactions between PANi and GNs. It was found that both the electrical conductivity (s) and the Seebeck coefficient (S) of PANi/ GNs are obviously higher than those of the pure PANi. The thermal conductivity of PANi/GNs composites still keeps relatively low values even with high GNs content. A highest dimensionless figure of merit ZT of 1.95 103 is obtained for PANie30GNs composite at 453 K, which is about 70 times higher than that of PANi. This suggests that the enhancement of electrical conductivity may be the principal way to improve the TE properties of PANi, and PANi/ GNs composites possesses better TE properties at high temperature. The ZT value would be further improved by adjusting the content of GNs with higher electrical conductivity. Acknowledgment This work was financially supported by the National Nature Science Foundation of China (Grant No. 50772026). References [1] M. Zebarjadi, K. Esfarjani, M. Dresselhaus, Z. Ren, G. Chen, Energy Environ. Sci. 5 (2012) 5147e5162. [2] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 413 (2001) 597e602. [3] T.M. Tritt, M. Subramanian, MRS Bull. 31 (2006) 188e198.
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