expanded graphite composites with improved power factor

Materials Chemistry and Physics 167 (2015) 315e319

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

A simple strategy to fabricate polyaniline/expanded graphite composites with improved power factor Cheng Pan a, Lei Zhang a, Zeng Pan a, Mingxi Chen a, Yue Liu a, Guanbo Huang a, Heya Na a, Wei Wang b, Haixia Qiu a, *, Jianping Gao a, c, ** a b c

School of Science, Tianjin University, Tianjin 300072, PR China School of Chemical Engineering, Tianjin University, Tianjin 300072, PR China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


PANI intercalates into the EG sheets and forms a sandwich structure.
Ultrasonic mixing PANI with EG method is simple and effective.
Power factor of PANI/EG is higher than some PANI based composites.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 May 2015 Received in revised form 30 September 2015 Accepted 23 October 2015 Available online 31 October 2015

Polyaniline/expanded graphite (PANI/EG) composites with enhanced thermoelectric properties have been successfully synthesized by a simple ultrasonic mixing method with different EG contents. Their structure and morphology were investigated by XRD, SEM, and FTIR. Besides, thermal stability was investigated by TG analysis, which indicated that the PANI/EG composites exhibit better thermal stability than pure PANI. The PANI/EG composites show an interesting structure: PANI intercalates into the EG sheets and forms a sandwich structure. The thermoelectric properties of the samples were measured at room temperature. With the EG content increasing, the electrical conductivity and Seebeck coefficient were improved. As a result, a remarkably improved thermoelectric power factor was achieved. This work demonstrates a simple and effective method for improving the thermoelectric properties of conducting polymers. © 2015 Elsevier B.V. All rights reserved.

Keywords: Composite materials Polymers Electrical conductivity Thermoelectric effects

1. Introduction Thermoelectric (TE) materials, widely used in Peltier coolers and

* Corresponding author. ** Corresponding author. School of Science, Tianjin University, Tianjin 300072, PR China. E-mail addresses: [email protected] (H. Qiu), [email protected] (J. Gao). http://dx.doi.org/10.1016/j.matchemphys.2015.10.050 0254-0584/© 2015 Elsevier B.V. All rights reserved.

thermoelectric power generators, have drawn a lot of attention in recent years [1]. TE materials have been promising candidates to replace traditional fossil energy due to their great advantage in the conversion between thermal energy and electrical energy [2]. The energy conversion efficiency of TE materials is evaluated by a dimensionless figure of merit ZT ¼ S2sT/k, where S (in V K1), s (in S m1), T (in K), and k (in W m1 K1) are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Obviously, for a high-performance TE

C. Pan et al. / Materials Chemistry and Physics 167 (2015) 315e319

material, excellent Seebeck coefficient, high electrical conductivity and low thermal conductivity are required [3]. However, S, s and k are interdependent-changing one alters the others, making optimization extremely difficult [4]. Fortunately, when variation of thermal conductivity is secondary to modulation in electrical conductivity, power factor (PF), PF ¼ S2s in W m1 K2, is employed to evaluate TE property of a material [5]. Especially for polymer TE materials, due to the low value of k, TE properties are often assessed with PF [6]. Inorganic semiconductors and metal alloys are the mostly reported TE materials mainly due to their high ZT and PF values, such as Bi2Te3 [7e10], BiCuSeO [11], BieSbeTe bulk alloy [12]. However, they also have shortcomings: firstly, inorganic TE materials usually contain toxic and rare elements such as Bi, Te, Sb [13]. Secondly, processes to fabricate inorganic materials like melt-spinning, ball milling and hot pressing are high cost and need a long payback time. Thirdly, most inorganic materials are expensive, heavy and brittle. However, organic materials are abundant, light-weight, flexible and low-cost [5,14]. Therefore, they are regarded as promising candidates for TE materials and are attracting more and more attention. The majority of organic TE materials are based on conductive polymers like polyaniline (PANI), polypyrrole (PPY), and poly(3-hexylthiophene) (P3HT) [15e18]. Among these conductive polymers, PANI has attracted special attention because of its unique electronic, easy synthesis, and good environment stability [4]. Different methods have been employed to improve the TE properties of PANI, including doping it or blending it with different kinds of filling materials to prepare functional PANI composites [19]. Zhang et al. showed that the multi-walled carbon nanotube (MWCNT)/PANI composite fabricated by cryogenic grinding is an effective way of improving the TE properties of PANI [1]. PANI/ graphite composites synthesized by mechanical ball milling exhibited higher electrical conductivity and Seebeck coefficient than pure PANI, and the TE power factor was remarkably improved [20]. The improvement in the properties of TE materials follows the known percolation law: once a critical volume fraction of filling materials is reached, dramatic enhancement of TE properties can be achieved [14]. Expanded graphite (EG), an excellent conductive material, is composed of stacks of nanosheets that may vary from 100 to 400 nm and has good affinity for polymer [21], so it can be good filling materials to fabricate functional polymer composites. The PANI/EG composites synthesized by emulsion polymerization show good electrical property [22]. In the present paper, we use a simple and effective method to fabricate PAN/EG composites and then investigate their TE properties.

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50wt%EG 45wt%EG 35wt%EG 25wt%EG 15wt%EG PANI

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2θ(°) Fig. 1. X-ray diffraction patterns of the PANI and PANI/EG composites with different contents of EG.

60  C in a vacuum to obtain the dark-green PANI. 2.3. Preparation of expanded graphite EG was obtained by thermal treatment of expendable graphite at 800  C for 1 min in the muffle furnace. A mixture of 0.5 g EG and 30 mL ethanol was treated using an ultrasonic cleaner for 7 h in a beaker, and the final EG was collected by filtration and then dried at 60  C for 12 h. 2.4. Preparation of PANI/EG composites EG and dark-green PANI were weighed and added to 20 mL ethyl alcohol in a beaker, then the mixture was treated using an ultrasonic cleaner for 2 h. Once the PANI/EG composite was homogeneously mixed, the blend was filtrated and then dried under vacuum at 60  C overnight. The dry composite was put in a glass mortar and mechanically ground. In order to measure the TE properties of the PANI/EG composite, the powdered composite was molded into pellets using a steel die of 13 mm diameter in a hydraulic press under a pressure of 12 MPa for 3 min. 2.5. Characterization

2. Experiential 2.1. Materials Expandable graphite was obtained from Huadong Graphite Co. Aniline, hydrochloric acid (HCl), ethyl alcohol, ammonium persulfate ((NH4)2S2O8) were all from Tianjin Chemical Reagent Co. All Chemicals were used as received without any further purification. 2.2. Synthesis of PANI Aniline (0.466 g) was mixed with 20 mL of 1 M HCl solution in a vessel with stirring at 0  C for 20 min to form a solution. Weighed (NH4)2S2O8 (1.141 g) was dissolved in 20 mL deionized water to form a solution which was then transferred to the dropping funnel and slowly added to the aniline solution. The mixture reacted for 10 h at 0  C, and then the product was filtered, washed with distilled water and ethyl alcohol for several times, and dried at

The X-ray diffraction (XRD) patterns of the samples were measured using an X-ray diffractometer (BDX3300) with a reference target, Cu Ka radiation (l ¼ 0.154 nm) voltage, 30 kV, and current, 30 mA. And the samples were measured from 10 to 90 with steps of 4 /min. Fourier transform infrared spectroscopy (FTIR) of the samples was measured with a PerkinElmer Paragon1000 FTIR spectrometer in the range of 500e4000 cm1. The structure of the PANI/EG composites was observed by scanning electron microscope (SEM, JEOL-6700F ESEM, Japan). Their thermogravimetric diagrams (TG) were measured with an analyzer

H N

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Cl

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H N n

Fig. 2. Chemical structure of PANI emeraldine hydrochloride salt.

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(Rigaku-TD-TDA, Japan). Electrical conductivity of the sample was measured using a four point configuration at room temperature. The home-made devices TJU-EC2002 was used to obtain the Seebeck coefficient. The sample was pressed tightly between two copper bars. The hot end copper bar was heated up steadily by a small heater and temperature gradient (△T) was established along the sample. The temperature difference between the copper bars was determined with thermocouples. Meanwhile the resulting voltage drop (△V) at different values of △T was measured. The Seebeck coefficient was calculated from the voltage △V generated by △T using S ¼ △V/△T.

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3. Results and discussion

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Wavenumber (cm-1) Fig. 3. FTIR spectra of PANI and PANI/EG composites with various EG contents.

Fig. 1 shows the X-ray diffraction patterns of the as-prepared PANI and PANI/EG composites. The XRD peaks of PANI were observed at 2q ¼ 14.98 , 20.05 and 25.58 corresponding to (011), (020) and (200) reflections of PANI in its emeraldine salt form, respectively [1]. Meanwhile, two diffraction peaks at 2q ¼ 14.98 and 25.58 assigned to the repeat unit of the PANI chain and the periodicity parallel to the polymer backbone chain, respectively [23]. These obvious peaks indicate that PANI shown in Fig. 2 has been successfully fabricated [14]. In addition, there are significant increases in peak intensity at 26.04 and 54.76 with the increasing

Fig. 4. SEM images of (a) PANI, (b) EG and (cef) PANI/EG composites.

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EG is thermally stable and has low weigh loss. 3.1. Thermoelectric performance In order to characterize the TE performance of the composites, the Seebeck coefficient and electrical conductivity were

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Electrical conductivity (S/cm)

EG content. At the same time, the peak at 2q ¼ 14.98 was slightly decreased. The FTIR spectra of pure PANI and PANI/EG composites with the increasing of EG content from 15 wt% to 50 wt% are shown in Fig. 3. The strong bands at 1559 cm1 and 1480 cm1 can be attributed to the C]C stretching of quinoid rings and C]C stretching of benzenoid, which are important characteristics of PANI. The appearance of absorption peaks at 1297 cm1, 1107 cm1, and 801 cm1 can be assigned to the CeN stretching vibration in the aromatic region, C]N stretching vibration, and CeH out-of-plane bending vibration, respectively. These peaks confirm the successful synthesis of PANI in its emeraldine hydrochloride salt form [1]. Compared with pure PANI, the peaks of the composites show a blue-shift. As can be seen from Fig. 3, the band at 1559 cm1 shifted to 1575 cm1 for 50 wt% the PANI/EG composite. The blue-shift may be due to interactions between PANI and EG, such as the conjugated structure and the multiple pep interactions between the backbone of PANI and EG [24]. Thus, it can be inferred that addition of EG will contribute to the increase in electrical conductivity of the PANI/EG composites. Typical SEM micrographs of PANI, EG and PANI/EG composites powder are shown in Fig. 4. Fig. 3 FTIR spectra of PANI and PANI/EG composites with various EG contents. It can be seen from Fig. 4(a), the pure PANI is granular agglomerate and exhibits an irregular sphere-like structure with a diameter of about 1 mm. In Fig. 4(b), large and smooth EG sheets can be observed. There exist big enough interspaces between two layers that can be occupied by PANI (Fig. 4(c)e(d)). As displayed in Fig. 4(e)e(f), sandwich-like [25] structures of EG sheets and PANI can be obtained. Intercalation of PANI into the EG layers may be caused by the strong pep stacking interaction between the PANI ring and the EG basal planes (Fig. 4(e)-(f)). The SEM photos demonstrated that the ultrasonic mixing is a simple and effective way to composite PANI with EG layers. It believes that the special architecture will influence TE properties of PANI/EG composites. TG analysis is a useful method to evaluate the thermal stability of materials. It performed under nitrogen flow from 30  C to 800  C and the result is displayed in Fig. 5. The TG curves of pure PANI and PANI/EG composites show a similar shape, indicating that these composites undergo a similar degradation procedure. All samples showed a gradual weight loss at around 100  C due to the deintercalation of H2O adsorbed on the samples. In addition, there is an accelerated weight loss staring from 230  C, which may be due to PANI degradation [4]. Obviously, the higher the content of EG is, the lower weight loss are the composite powders. This is because that

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Temperature (°C) Fig. 5. TGA curves of PANI and PANI/EG composites.

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EG content (%) Fig. 6. (a) Electrical conductivity, (b) Seebeck coefficient and (c) PF of the PANI/EG composites with different EG contents measured at room temperature.

C. Pan et al. / Materials Chemistry and Physics 167 (2015) 315e319

systematically measured at room temperature and PF was calculated. Fig. 6(a) shows the electrical conductivity of PANI and PANI/ EG composites with different EG contents. The electrical conductivity of the composites increased dramatically from 0.23 S cm1 to a maximum value of 116 S cm1 when the weight percentage of EG reached to 50%. This increase may be due to the following reason: On the one hand, the electrical conductivity of EG is higher than PANI; on the other hand, when PANI particles are embedded into the interspace of EG layers, the multiple pep interactions between the backbone of PANI and EG would increase the carrier mobility and thus enhance the electrical conductivity of PANI/EG composites [26]. Since the electron directional movement in the EG layers and the electron transition between the layers are main ways to explain the conduction mechanism of EG, the existence of PANI conjugated chains plays a part as bridged linkage and provides effective conductive paths to make electron transfer easier, not only along the EG layers, but also between the EG layers, which may increase the carrier mobility [22]. The synergistic effect of these parts lead to the dramatic increase of the conductivity of EG/PANI composites. The Seebeck coefficient values of the PANI/EG composites are shown in Fig. 6(b). All the samples display a positive Seebeck coefficient, indicating p-type conduction. With the EG content increasing, the Seebeck coefficient has a slight increase from 8.1 to 14.5 mV K1. The calculated PF is shown in Fig. 6(c). It increased with the increase of EG content, which is beneficial from the enhancement of the electrical conductivity and Seebeck coefficient. The maximum of 2.43 mW m1 K2 is achieved for PANI/EG composite at room temperature, which is 1620 times higher than that of PANI (0.0015 mW m1 K2). The value is also higher than those of other reported PANI based composites like PANI/(MWCNT) composites (power factor is 0.11 mW m1 K2) with 30 wt% MWCNT [1], the CNT network/PANI (power factor is 2.19 mW m1 K2) with 44 wt% CNT [27]. Moreover, the ultrasonic mixing method is simple and low-cost, besides, EG is much cheaper than CNT or MWCNT. These results demonstrate that ultrasonic mixing PANI with EG is an effective way to improve the TE properties of PANI. 4. Conclusions In this work, we have developed a simple strategy to conveniently prepare PANI/EG composites with obviously enhanced TE power factor by forming a sandwich-like structure. The multiple pep interactions between the backbone of PANI and EG increase the carrier mobility and thus enhance the electrical conductivity of PANI/EG composites. Namely, the sandwich-like structure induced improvement of the carrier mobility. The maximum power factor value of 2.43 mW m1 K2 was obtained for the PANI/EG composite

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with 50wt% EG. This study suggests a simple method to improve thermoelectric power factor of PANI. Acknowledgments This work was supported by the National Science Foundation of China (51202158, 21074089 and 21276181). References [1] Q.L. Zhang, W.J. Wang, J.L. Li, J.J. Zhu, L.J. Wang, M.F. Zhu, W. Jiang, J. Mater. Chem. A 1 (2013) 12109e12114. [2] C.Z. Meng, C.H. Liu, S.S. Fan, Adv. Mater 22 (2010) 535e539. [3] L.Y. Wang, F.Z. Liu, C. Jin, T.R. Zhang, Q.J. Yin, RSC Adv. 4 (2014) 46187e46193. [4] Y. Du, S.Z. Shen, W.D. Yang, R. Donelson, K.F. Cai, P.S. Casey, Synth. Met. 161 (2012) 2688e2692. [5] Q. Zhang, Y.M. Sun, W. Xu, D.B. Zhu, Adv. Mater 20 (2014) 6829e6851. [6] K.L. Xu, G.M. Chen, D. Qiu, J. Mater. Chem. A 1 (2013) 12395e12399. [7] P. Sundarraj, D. Maity, S.S. Roy, R.A. Taylor, RSC Adv. 4 (2014) 46860e46874. [8] D. Madan, Z.Q. Wang, A. Chen, R.C. Juang, J. Keist, P.K. Wright, J.W. Evans, ACS Appl. Mater. Interfaces 4 (2012) 6117e6124. [9] M.M. Rashid, K.H. Cho, G.S. Chung, Appl. Surf. Sci. 279 (2013) 23e30. [10] P. Anandan, M. Omprakash, M. Azhagurajan, M. Arivanandhan, D.R. Babu, T. Koyama, Y. Hayakawaa, CrystEngComm 16 (2014) 7956e7962. [11] Y. Liu, L.D. Zhao, Y.C. Liu, J.L. Lan, W. Xu, F. Li, B.P. Zhang, D. Berardan, N. Dragoe, Y.H. Lin, C.W. Nan, J.F. Li, H.M. Zhu, J. Am. Chem. Soc. 133 (2011) 20112e20115. [12] B. Poudel, Q. Hao, Y. Ma, Y.C. Lan, A. Minnich, B. Yu, X. Yan, D.Z. Wang, A. Muto, D. Vashaee, X.Y. Chen, J. Liu, M.S. Dresselhaus, G. Chen, Z.F. Ren, Science 320 (2008) 634e638. [13] D.Y. Chung, C. Uher, M.G. Kanatzidis, Chem. Mater 24 (2012) 1854e1863. [14] B. Abada, I. Alda, P. Diaz-Chao, H. Kawakami, A. Almarza, D. Amantiac, D. Gutierrez, L. Aubouy, M. MartinGonzalez, J. Mater. Chem. A 1 (2013) 10450e10457. ~ i, R.Y. Rozen, [15] C. Bounioux, P.D. Chao, M.C. Quiles, M.S.M. Gonz
alez, A.R. Gon C. Müller, Energy Environ. Sci. 6 (2013) 918e925. [16] N. Mateeva, H. Niculescu, J. Schlenoff, L.R. Testardi, J. Appl. Phys. 83 (1998) 3111e3117. [17] Z. Golsanamlou, M.B. Tagani, H.R. Soleimani, Phys. Chem. Chem. Phys. 17 (2015) 13466e13471. [18] M. Culebras, B. Uriol, C.M. Gomez, A. Cantarero, Phys. Chem. Chem. Phys. 17 (2015) 15140e15145. [19] Y. Du, S.Z. Shen, K.F. Cai, P.S. Casey, Prog. Polym. Sci. 37 (2012) 820e841. [20] L. Wang, D.G. Wang, G.M. Zhu, J.Q. Li, F. Pan, Mater. Lett. 65 (2011) 1086e1088. [21] R. Sengupta, M. Bhattacharya, S. Bandyopadhyay, A.K. Bhowmick, Prog. Polym. Sci. 36 (2011) 638e670. [22] C. Xiang, L.C. Li, S.Y. Jin, B.Q. Zhang, H.S. Qian, G.X. Tong, Mater. Lett. 64 (2010) 1313e1315. [23] Q. Yao, Q. Wang, L.M. Wang, L.D. Chen, Energy Environ. Sci. 7 (2014) 3801e3807. [24] J.K. Avlyanov, Synth. Met. 102 (1999) 1272e1273. [25] Q.T. Qu, S.B. Yang, X.L. Feng, Adv. Mater 23 (2011) 5574e5580. [26] Y. Zhao, G.S. Tang, Z.Z. Yu, J.S. Qi, Carbon 50 (2012) 3064e3073. [27] J.K. Chen, X.C. Gui, Z.W. Wang, Z. Li, R. Xiang, K.L. Wang, D.H. Wu, X.G. Xia, Y.F. Zhou, Q. Wang, Z.K. Tang, L.D. Chen, ACS Appl. Mater. Interfaces 4 (2012) 81e86.