Efficient DMSO-Vapor Annealing for Enhancing Thermoelectric

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Applications of Polymer, Composite, and Coating Materials

Efficient DMSO-Vapor Annealing for Enhancing Thermoelectric Performance of PEDOT:PSS-Based Aerogel Xiaodong Wang, Peipei Liu, Qinglin Jiang, Weiqiang Zhou, Jingkun Xu, Jing Liu, Yanhua Jia, Xuemin Duan, Youfa Liu, Yukou Du, and Fengxing Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19168 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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ACS Applied Materials & Interfaces

Efficient DMSO-Vapor Annealing for Enhancing Thermoelectric Performance of PEDOT:PSS-Based Aerogel Xiaodong Wang,†,‡ Peipei Liu,† Qinglin Jiang,† Weiqiang Zhou,† Jingkun Xu,†,ǁ,* Jing Liu,† Yanhua Jia,† Xuemin Duan, † Youfa Liu,§ Yukou Du, § and Fengxing Jiang,†, ǁ,* † Department

of Physics, Jiangxi Science and Technology Normal University,

Nanchang 330013, China. ‡

Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an

710054, China. §

College of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou 215123, P.R. China. ǁ

School of Chemistry and Molecular Engineering, Qingdao University of Science and

Technology, Qingdao 266042, Shandong, China.

KEYWORDS: PEDOT:PSS, Te nanowire, aerogel film, vapor annealing, thermoelectric performance

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ABSTRACT: Conducting polymer-based composite aerogel film is desired to be used as thermoelectric (TE) materials due to its good flexibility and ultralow thermal conductivity. Here, we proposed a simple freeze drying method to fabricate freestanding PEDOT:PSS-based aerogel films without any crosslinker addition. The evolution of morphology and TE performance were systemically investigated with various organic solvent addition. Furthermore, a series of the PEDOT:PSS/Te-NWs composite aerogel films was prepared, and the relationship between the structure and the charge transport mechanism of the binary complex system were explored based on series and parallel models. Finally, an efficient DMSO-vapor annealing was employed to further optimize the TE performance of PEDOT:PSS/Te-NWs composite aerogel films. The ZT value was estimated to be 2.0×10-2 at room temperature. Based on the flexibility and highly enhanced TE performance, a prototype TE generator consisting of p-type PEDOT:PSS/Te-NWs aerogel films and n-type carbon nanotube fibers as legs has been fabricated with an acceptable output power of 1.28 W at the temperature gradient of 60 K, which could be potentially applied in wearable electronics.

INTRODUCTION Since the discovery of aerogel, considerable attention has been focused on the preparation and various applications of aerogel materials.1-4 Aerogel materials possess distinguished properties such as light weight, large surface areas, and high porosity, making them viable candidates for use in energy storage, catalysis, sensing applications and so on.5-6 Their intrinsic thermally insulating property has also triggered increasing

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interest in TE materials and devices that have the capability for directly converting heat energy into electric energy without any moving part and working fluids. Conducting polymer (CP) aerogels are expected to have potential applications in energy related systems.7-9 In thermoelectrics, great attention have been devoted to microporous polymer-based aerogel materials with ultra-low thermal conductivity (), yet those that require to adjust the electron structure and tune the Fermi level because the majority of organic materials are intrinsic low electrical conductivity () and low Seebeck coefficient (S), which possibly lead to a highly enhanced TE figure of merit ZT (ZT=S2··T/).10-13 To find an effective strategy to obtain a large S and  with a low thermal conductivity () simultaneously is the key challenge for the development of organic TE aerogel materials. Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), as one of the most popular and excellent CPs, has been widely investigated for organic TE films with a high ZT value (> 0.1).14-16 Due to the unique structure, PEDOT:PSS is a good candidate for conductive organic aerogel.17,18 It is noted that CP aerogels are usually prepared by two strategies. One is the use of an insulating aerogel skeleton, which results in a very low  value.19 The other is the introduction of crosslinking agent or dopant in CPs to form a three-dimensional (3D) network.20 Nevertheless, the  of aerogel materials prepared by this method are also significantly influenced by insulating agents. Zhang et al.21 fabricated PEDOT:PSS aerogels by liquid phase polymerization with Fe3+ as oxidant and CO2 supercritical drying method. The Fe3+ plays a significant role to make PSS chains to crosslink by electrostatic interaction and form a 3D network

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structure. Gordon22 and co-workers prepared PEDOT:PSS aerogel by the facile freeze drying method and obtained an enhanced  by one order of magnitude after ethylene glycol (EG) post-treatment. However, the PEDOT:PSS aerogel suffered from a low S of ~15 V K-1 and TE power factor (P) of 6.8 W m-1 K-2. Compared to highly conductive PEDOT:PSS thin-films, these as-prepared PEDOT:PSS aerogel have a low

 and S values, which are desired to be improved further for a good TE material in despite of low  value. As we know, organic post-treatment have been widely used to improve the  of PEDOT:PSS films (>1000 S cm-1).8,

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Recently, solvent vapor

annealing has been developed and confirmed to be an efficient method on improvement of performance for organic solar cells24-27 and TE materials28-30. Compared to solvent immersion, vapor annealing is more suitable for PEDOT:PSS-based aerogel materials. Firstly, aerogel with porous network is favorable to facilitate the diffusion of solvent molecule and keep good morphology structure. Secondly, vapor annealing treatment is convenient to control the content of solvent and avoid the difficulty in drying for aerogel materials. Furthermore, it is known that the S of PEDOT:PSS can be effectively improved by combining with high S value of inorganic materials.31-34 Considering the porous network, a high performance of nanowires should be a perfect choice for PEDOT:PSSbased TE composite aerogel. Nanowires can not only maintain a good threedimensional (3D) network structure with enhanced mechanical property, but also provide effective electron transport path to optimize TE performance of aerogels. Recently, the multi-walled carbon nanotube (MWCNT) and Ag nanowires are used to

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construct

a

3D

network

with

PEDOT:PSS,

they

compose

a

ternary

PEDOT:PSS/MWCNT/Ag (PCA) composite aerogel film by freeze drying method resulting in a maximum ZT of 7.56 × 10-3.35 Although the TE performance of PEDOT:PSS-based aerogel film have been significantly improved, the compromise between  and S is still difficult to achieve a high ZT value due to their opposite dependence on the charge carrier density.36 A new method and theory should be developed further to look for a better compromise and achieve high TE performance for PEDOT:PSS-based composite aerogels. In this work, we selected PEDOT:PSS and tellurium nanowires (Te-NWs) to compose a 3D composite aerogel film (PEDOT:PSS/Te-NWs) for TE materials by a facile vacuum freeze drying method. The main feature involves the incorporation of high conductive PEDOT:PSS and Te-NWs with a large S as well as a low  based on porous network structure for aerogels. Meanwhile, various solvents as additive and DMSO-vapor annealing (VA-d) method were used to optimize the TE performance of PEDOT:PSS-based composite aerogel films. With the help of n-type carbon nanotube fibers (CNFs), a prototype TE generator has been fabricated by consisting of 6 pairs of p-n legs. This work could provide some basic insight for the development of high performance of aerogel thermoelectric materials. EXPERIMENTAL SECTION Materials

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PEDOT:PSS (Clevios PH 1000) aqueous solution with the mass content of 1.3 wt.% were purchased from HC Stark (Germany). Sodium tellurite (Na2TeO3), poly(vinyl pyrrolidone) (PVP), hydrazine hydrate (N2H4), ammonia solution (NH3H2O), and acetone were obtained from J&K Scientific Ltd and used to fabricate tellurium nanowires (Te-NWs). Several kinds of organic solvent including ethylene glycol (EG), isopropanol (IPA), dimethylformamide (DMF), N-methyl pyrrolidone (NMP), methanol (MeOH), ethanol (EtOH), and dimethyl sulfoxide (DMSO) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical grade and directly used without further purification. Preparation of PEDOT:PSS Aerogel Films PEDOT:PSS aerogel samples were prepared by vacuum freeze drying method according to the previous report.37 Firstly, a certain amount of pristine PEDOT:PSS aqueous solution was added with different solvent (5 vol.%) including H2O, NMP, EG, DMSO, MeOH, EtOH, DMF, and IPA, marked as PP-H2O, PP-NMP, PP-EG, PPDMSO, PP-MeOH, PP-EtOH, PP-DMF, and PP-IPA, respectively. Secondly, all the samples were soaked in liquid nitrogen for about 10 min, resulting in a frozen hydrogel. Then, put into the vacuum freeze dryer with the temperature set to -40~-60 oC and apply a sufficiently strong vacuum to promote sublimation of the ice crystals by air. After about 24 h, the aerogel samples were obtained with a porous three-dimensional (3D) crosslinking network. Finally, the PEDOT:PSS aerogel films were prepared by pressing the 3D aerogel samples.

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Fabrication of PEDOT:PSS/Te-NWs (PPT) Aerogel Films Te-NWs were synthesized by the hydrothermal method according to the precious literature, and the detailed preparation process was presented in the Supporting Information.38 Different amounts of the previously obtained Te-NWs were added into 1.0 mL PEDOT:PSS solution, and a homogeneous dispersion was obtained during the stirring process. The PEDOT:PSS/Te-NWs (PPTx) composite aerogel were fabricated by a similar process liking PEDOT:PSS aerogel films. For PPTx, the subscript of x is the content of Te-NWs (0~100%). DMSO-Vapor Annealing (VA-d) of PPT Aerogel Films Firstly, the PPT composite aerogel network prepared by freeze drying method was placed in a Teflon-lined stainless steel autoclave containing a certain amount of DMSO solvent, and the sample was separated from the solvent by the as-prepared scaffold. Then, the sealed container was placed in a 150 oC oven for 30 min. Finally, the DMSOvapor annealing PPT (VA-d-PPT) composite aerogel film was dried in a vacuum oven. The schematic illustration of the formation of the PEDOT:PSS and its Te-NWs composite aerogels was clearly shown in (Scheme 1), which the aerogel composite presents a 3D porous network. The as-prepared composite aerogels were suspended and placed in a 150 oC autoclave for the DMSO-vapor annealing (VA-d) process.

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Scheme 1. Schematic illustration of the preparation of the PEDOT:PSS and its Te-NWs aerogel composite film with DMSO-vapor annealing (VA-d) treatment. Assembly of the TE Generator The protype TE generator was assembled by alternately connecting p-type PEDOT:PSS-based aerogel films and the n-type carbon nanotube fibers (CNFs) in serises. The connections were linked by the help of silver paste. The output performance of the protype TE generator was performed at the temperature gradient (T) from 0 K to 60 K. RESULTS AND DISCUSSION The microstructure investigation of PEDOT:PSS aerogel films with different solvents as additives were examined by scanning electron microscope (SEM) and the results are shown in Supporting Information (SI) Figure S1 and Figure S2 with the corresponding digital pictures. One can see that the PEDOT:PSS with NMP (PP-NMP) presents a relatively smooth surface, and a regular orientation stripes micro-surface can be obviously seen from SEM image in Figure S1. This is mainly because that the strong polarity of NMP solvent can effectively induce the molecular conformation changes of 8


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PEDOT:PSS, promoting the formation of physical crosslinks in the PEDOT:PSS network. For the detailed process of the formation of the PEDOT:PSS aerogel network and the comparison illustration were discussed in the SI. The porous three-dimensional (3D) solid skeleton prepared by freeze drying method makes aerogel film remarkably different from that of thin films by directly film-forming methods. Additionally, the asprepared PEDOT:PSS aerogel films possess good mechanical flexibility as shown in Figure S3. The TE performance of PEDOT:PSS aerogel film depends on the bending degree were discussed below.

Figure 1. SEM images of (a) PP-NMP, (b) PPT30, and (c) PPT90 aerogel composite films; (d) XPS spectra of PEDOT:PSS, PP-NMP, PPT30 and VA-d-PPT30 aerogel films; (e) XPS spectra of pure Te-NWs (black line) and PPT30 aerogel (red line) films.

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The as-prepared Te-NWs were characterized by Transmission Electron Microscope (TEM) shown in Figure S4. The results show that Te-NWs present a uniform size and large aspect ratio with 162 nm in diameter, indicating a well onedimensional (1D) nanostructure. The SEM images of the PEDOT:PSS/Te-NWs (PPT) composite aerogels with different contents of Te-NWs are displayed in Figure 1a~c. Compared with PP-NMP aerogel films in Figure 1a, the significantly linear structure corresponding to the Te-NWs can be clearly seen in Figure 1b. When the content is low, the Te-NWs are mainly wrapped by PEDOT:PSS, so the linear stripes will appear macroscopically. The interfacial adhesion in the PPT composite system is mainly caused by the van der Waals forces between Te atom layer and π-interaction of PEDOT:PSS chains, which is consistent with the previous report.39 The template effect of Te-NWs also be benefit for the formation of the linear morphology, giving rise to a high degree of preferred orientation compared with that of pure PEDPT:PSS. This kind of organization may promote the charge carriers transport in the complicated aerogel system.40 When a further increase of Te-NWs content to 90% in Figure 1c, some of the Te-NWs are wrapped by the relatively low content of PEDOT:PSS. However, a large number of Te-NWs are agglomerated and the increased porosity can be observed, leading to a decreased continuity of 3D aerogel solid skeleton. Additionally, Figure S5 shows the XRD spectra of PPT aerogels, indicating the successful composite of PEDOT:PSS and Te-NWs. The XPS spectra in Figure 1d presents the contrast curves of the S 2p of PEDOT:PSS, suggesting the chemical composition of as-prepared aerogel composite films. The most 10


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obviously different lies in the integral area of characteristic peaks between 162~166 eV, indicative of both the NMP addition and VA-d can influence the relative content of PEDOT to PSS (thiophene/sulfonate, RT/S) in the corresponding films.41,42 It is known that the pristine PEDOT:PSS solution is full of PSS chains, the RT/S is calculated to be 1:2.5. One can find that this value increases to 1:1.3 after the addition of NMP. This is attributed to the screen effect of NMP solvent, inducing the conformation arrangement of PEDOT:PSS molecule.43-45 When 30% Te-NWs added, the RT/S of the composite aerogels shows a slight decrease from 1:1.13 to 1:1.14, indicating that the addition of Te-NWs does not obviously affect the removal of PSS. It is worth to be noted that the RT/S of VA-d-PPT presents a significant decrease to 1:0.9. Considering the porous structure of aerogels, the increased surface area may produce larger area (inner surface and outer surface) for the extraction of non-conducting PSS component to enhance the possibility of solvent effects. This result is consistent with the previous reports.28,29 Additionally, it can be seen from Figure 1e that the Te-NW is easy to be oxidized, which results in the degradation of their properties. However, according to the XPS peak (Figure 1e) of Te 3d in the PPT30 aerogel composites, the typical oxidation peak of Te (Teox) at 585.6 eV (3d5/2) and 576.2 eV (3d3/2) present a significantly lower intensity compared to pure Te-NWs, which indicates that PEDOT:PSS effectively wrap Te-NWs and plays a significant role in preventing them from oxidation.46 However, not all TeNWs have been fully wrapped in PEDOT:PSS to isolate air. More specifically, the TeNWs on the outer surface of aerogel were wrapped with a thinner PEDOT:PSS layer.

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That is understandable that the PEDOT:PSS still plays a significant role in retarding oxidation of Te-NWs. TE Performance Effect of Solvent Addition on TE Performance of PEDOT:PSS Aerogel Films

Figure 2. The electrical conductivity () (a), Seebeck coefficient (S) (b), and power factor (P) (c) of PEDOT:PSS aerogel films with different chemical addition, (the black circle represents the compressed samples and the red box represents un-compression 12


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samples). The effect of solvents on the TE performance of PEDOT:PSS aerogel films has been investigated before and after compression. PEDOT:PSS aerogel film without any organic solvent addition have a large number of pore and discontinuous surface, which seriously affect their electron transport property. As shown in Figure 2, the overall TE performance of the aerogel films before compression are significantly lower than those after compression. This can be attributed to that aerogel films after compression become more denser with a decreased thickness (Table S2), leading to an improved TE performance. Hence, we compressed the aerogel samples for further use. The electrical conductivity () and Seebeck coefficient (S) of PEDOT:PSS aerogel films without solvent addition are 0.5±0.2 S cm-1 and 28.0±2.1 V K-1, respectively. These results are consistent with previous report.35 Noted that the effect of different solvents addition on the TE enhancement of PEDOT:PSS aerogel films is significantly different. Although the PP-EG exhibits the highest enhancement in  compared to other solvents, the increase in P is limited by the decrease of S. Among all the solvents used in this work, the PP-NMP aerogel film achieves a high  of 35.0 S cm-1 and S of 18.8 V K-1, resulting in the largest P of 1.24 W m-1 K-2, which is higher than that of pristine PEDOT:PSS. Also, this  value is almost two orders of magnitude higher than that of the PEDOT:PSS hydrogel (0.46 S cm−1) with the addition of 0.1 M H2SO4 reported previously.47 Furthermore, two different content 1vol.% and 10 vol.% of NMP solvent were added into the PEDOT:PSS for TE performance comparison in Figure S6. The results show that 5 vol.% NMP presents a higher P than that of 1 vol.% and 10 vol.% 13



NMP addition. Such a significant enhancement for P can be ascribed to two aspects. On the one hand, the high polarity and high dielectric constant of NMP is able to produce screen effect to induce the conformation arrangement from benzoid to quinoid structure, and enhance their inter-chain interaction to in a 3D network with physical crosslinks. Similar screen effect on PEDOT:PSS has been reported by Ouyang et al.48 On the other hand, the melting points of the solvents play a positive effect on the morphology of PEDOT:PSS aerogel network. Only the melting point of NMP matches well with the temperature of the freeze dryer, thus producing a porous structure with uniform size and a certain orientation on the surface. This kind of macroscopic orientation arrangement prove by SEM analysis in Figure S1 and Figure S2 is beneficial for the charge carrier transport. Therefore, NMP is regarded as solvent additive for the preparation of PEDOT:PSS/Te-NWs aerogel films.

Figure 3. The bending test of PEDOT:PSS aerogel films with the corresponding bended modes (inset). The good mechanical flexibility of the aerogel composite films is crucial for a 14


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wearable TE generator in order to endure the diversity actions and guarantee the stable energy output. The bending test of the aerogel films were investigated, as shown in Figure 3 with the corresponding bended modes (insets) and in Figure S3. These results suggest that the  of PEDOT:PSS aerogel films does not changed significantly during the reciprocating bended process from 0o to 180o. While Seebeck coefficient only presents a small increase. This proves that the as-prepared PEDOT:PSS aerogel films has a good flexibility, similar to the report by Madeleine and co-workers.49 TE Performance of PPT Aerogel Films

Figure 4. (a) TE performance of (a) PPT-H2O and (b) PPT-NMP aerogel composite films depend on the contents of Te-NWs. The organic/inorganic composites have been demonstrated to be an efficient route 15



to achieve an enhancement of TE performance.50-54 Figure 4 displays the TE performance of PPT-H2O and PPT-NMP with different contents of Te-NWs. For the PPT-NMP composite aerogel film, the S increases from 18.8 V K-1 to 682 V K-1, while the  decreases from 35.0 to 0.002 S cm-1 with varying the contents of Te-NWs from 0 to 100%. The decrease in  is mainly due to the inherent poor electron transport performance of Te-NWs (~0.002 S cm-1). Although the change of  shows a decreasing trend, it still remains above 10 S cm-1 at the Te-NWs content lower than 40%. Fortunately, the variation in S and  of PPT produce a beneficial for the optimization of P. Upon the addition of 30% Te-NWs, the PPT-NMP30 reaches its maximum value of 3.6 W m-1 K-2, which is almost 3 times higher than that of the PP-NMP aerogel film and almost 20 times higher than that of PP-H2O aerogel film. The comparison of the TE performance of PEDOT:PSS and PEDOT:PSS-based aerogel composites has been shown in Table S3. Furthermore, the high S (49.2 V K-1) of PPT30 is comparable to that of the aforementioned PCA composite aerogel film (61.3 V K-1)35 probably owing to the interfacial energy filtering effect.55 A potential energy barrier may be formed at the interfaces between the nano-filler of Te-NWs and the conductive PEDOT:PSS matrix, which can hinder the transport of low-energy carriers and only allow the highenergy carriers to pass through.56 However, according to the variation in S, it is possible for the improvement of TE performance of composite aerogel films could be caused by the interfacial energy filtering effect involving the following rules:57,58 (1) there must be created an intimate contacts between organic (PEDOT:PSS) and inorganic (Te-NWs) for constructing the

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energy-filtering interfaces; (2) an energy barrier formed at the interfaces effectively regulates the charge carriers by scattering; (3) energy filtering effect often occurs at a low content nanomaterials filled into the conductive matrix and S generally shows an initial increase and then decrease trend for composites. There exists another explanation on the enhancement of S of binary composite aerogel films by series and parallel models proposed firstly by Gelbstein59 on the investigation of Sn/SnTe system. It has been widely used to interpret the increase in S.60,61 Here, we introduce the model to investigate the effect of composite structural on the TE performance of composite aerogel films.

Figure 5. The definition formulae of S and  of PPT composite aerogel films under the series (a) and parallel (b) connected the models with the schematic diagram of corresponding charge carrier transport mechanism (c) and (d). As described in Figure 5a and b, these equations of series and parallel models could be used to analyze the measured data of  and S with the neglect of the energy filtering effect in the composite.62 Where the subscript P and Te are stand for the corresponding 17



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values for PEDOT:PSS and Te-NWs, respectively. And the corresponding TE performance calculated by these Equations are listed in Table 1. It should be noted that both of the  and S were measured in the in-plane (lateral) direction. CPs generally show significant anisotropic behavior depending on in-plane (∥) and out-of-plane ( ⊥)

direction of the film. Currently, it is a big challenge for us to measure the  ∥ of

PEDOT:PSS-based aerogel films. Although there may be a big difference, we still estimated the ZT values of PEDOT:PSS, PEDOT:PSS/Te-NWs, and Te-NWs aerogel films to be 0.14, 0.168, and 0.266W m-1 K-1 with the thermal anisotropy factor of  ∥ / ⊥ =1.40  0.22 reported by Pipe et al. for PEDOT:PSS film.14 These PEDOT:PSSbased composite aerogels show significantly lower 

∥

values than those of

PEDOT:PSS films (0.42 W m-1 K-1) prepared by common methods such as dipping, spin-coating and so on. In the series model in Figure 5c, PEDOT:PSS and Te-NWs are connected head with tail to form a series carrier transmission channel, in which the carriers can transport directly between two phases. In parallel mode in Figure 5d, PEDOT:PSS and Te-NWs crosslink by parallel structure so that the carrier transmission satisfies the parallel circuit law, implying a fast carrier transmission occurring in this channel.59 It can be seen that the measured  and S lie between the results of series and parallel models, indicating a complex mixture connected model with series and parallel models. Table 1. The in-plane , S and ∥ of PEDOT:PSS and Te-NWs aerogel films.

σ (S cm-1)

PP-H2O

PP-NMP

Te-NWs

aerogel film

aerogel film

aerogel film

0.5

35

0.002

18


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S (μV K-1)

18.8

28

685

∥ (W m-1 K-1)

0.14

0.168

0.266

Further, we propose the modified transport model combined of the series and parallel described as Equation (1) to explore the composite mechanism of binary complex system for the PEDOT:PSS/Te-NWs composite aerogel films without considering the effect of energy filtering.60 𝑆=

[𝑆𝑃 ∙ 𝑇𝑒 ∙ 𝑥 + 𝑆𝑇𝑒 ∙ 𝑃 ∙ (1 ― 𝑥)]

𝑇𝑒 ∙ 𝑥 + 𝑃 ∙ (1 ― 𝑥)

𝑦+

[𝑆𝑃 ∙ 𝜎𝑃 ∙ 𝑥 + 𝑆𝑇𝑒 ∙ 𝜎𝑇𝑒 ∙ (1 ― 𝑥)] 𝜎𝑃 ∙ 𝑥 + 𝜎𝑇𝑒 ∙ (1 ― 𝑥)

(1 ― 𝑦)

（1）

where, y represents the content of the series model, and its variation range is from 0 to 1. As shown in Figure 6, the S values of PPT aerogel composite films are compared with that of the scale calculation of different series and parallel models under the modified formula of the composite model. One can see that the calculated S gradually increases for PPT-H2O and PPT-NMP as the increase of y from 0 to 1. This implies the S increases with the increase in the ratio of series models. It suggests that the S of binary composite is determined by the average entropy of carrier transport.60 In the parallel model, a higher electron transport property contributes more to the overall  of composites resulting in apparent influence on S. Noted that a more prominent phenomenon can be observed for the PPT-NMP compared to PPT-H2O. This is attributed to the 60 times enhancement of  for PPT-NMP. By contrast, the carriers with series model pass through two phases in turn, resulting in the equivalent contribution to S. Therefore, a dominant series mode is desired to occur in composite due to a larger S. Compared to energy filtering effect, the binary composite model may present more accurate and better explanation for the enhancement of TE performance.60 The results from experimental test for binary complex system are in consistence of the 19



calculated ratio of 2:8 for the series and parallel model. It means that a dominant 80% parallel model have achieved in the as-prepared PPT composite aerogel system resulting in a three times enhancement of P (3.6 W m-1 K-2) for PPT30. It is probably due to the inherent film-forming property of PEDOT:PSS during the gelation process with the PEDOT:PSS-wrapped Te-NWs.

Figure 6. Calculated S by using various ratios of the series and parallel models along with the experimentally measured S for (a) PPT-H2O and (b) PPT-NMP aerogel composite films depending on the contents of Te-NWs. TE Performance of VA-d-PPT Aerogel Films

Figure 7. TE performance of PPT30 aerogel composite films depend on the time of vapor phase annealing. 20


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As we know, there is large amount of PSS in pristine PEDOT:PSS for charge balance, and the excess PSS has a negative impact on charge transport among PEDOT. Many efforts have been devoted into the removal of PSS from solid PEDOT:PSS. Part of non-conductive PSS chains can be removed by DMSO post-treatment, resulting in a significant improvement of  without the almost unchanged S.33, 63 Considering the 3D porous network, the vapor phase annealing with DMSO (VA-d) as an efficient treatment was employed for the optimization of TE performance for PEDOT:PSS/TeNWs composite aerogel films due to easy diffusion of DMSO vapor.29 As shown in Figure 6, the VA-d process results in a significant increase in  from 14.8 S cm-1 to 126 S cm-1 with the delay of exposed time. This indicates that an obvious change has been occurred for the conformation of PEDOT:PSS leading in an efficient carrier transport route.64 It is consistent with the XPS analysis in Figure 1e and previous reports.29, 65-66 Noted that the S also presents the apparent decrease from 49.2 μV K-1 to 24.1 μV K-1 as the increase in exposed time. However, a compromise result has been achieved with an enhanced P value of 11.3 μW m-1 K-2 for 20 min treatment. To estimate ZT value, the electronic thermal conductivity (e) were calculated according to the WiedemannFranz law defined in Equation (2)

e=LT

(2)

where, L and T are the Lorenz number (2.44×10-8 W  K-2) and the absolute temperature. For organic or porous material, the e plays the predominant influence on the thermal conductivity () of material. Based on the measured  of 119 S cm-1, the e of PPT30 after VA-d treatment is estimated to be 8.6×10-2 W m-1 K-1 at room

21



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temperature (298 K), which is 32.3% of in-plane thermal conductivity (∥) in Table 1. This indicates that the lattice L of 0.188 W m-1 K-1 is dominant in 

∥

. For

nanocomposites, the L can be effectively reduced by nanointerface between PEDOT:PSS and Te-NWs which scatter phonons more than electrons. Therefore, the low  ∥ of the aerogel composite films is greatly beneficial for high TE performance materials.67 Finally, the optimized ZT of PEDOT:PSS/Te-NWs with 30 wt.% Te-NWs after 20 min VA-d treatment is 2.0 × 10-2, which is higher than PCA aerogel composite films (7.56 × 10-3) in Table S3. Output Performance of the TE Generator

Figure 8. (a) The output voltage and output current of the TE generators consisted by p-type PEDOT:PSS-based aerogel films and n-type CNFs depending on the hot side temperature with the temperature difference from 0 K to 100 K. The inset presents a scheme of the TE generator consisting of 6 pairs of p-type PEDOT:PSS-based aerogel and n-type CNFs as legs. (b) The relationship between output power, output voltage and output current corresponding to the temperature gradient ΔT=100 K. With the help of n-type carbon nanotube fibers (CNFs), we compared the output

22


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performance of the TE generator by the VA-d-PPT30 and PP-NMP films, named TEG1 and TEG2 respectively. Figure 8a presents the output voltage and output current of the TE generators as a function of the hot side temperature with different temperature gradient from 0 K to 60 K. The inset displays a scheme of the TE generator consisting of 6 pairs of p-type PEDOT:PSS-based aerogel and n-type CNT fibers as legs. It should be noted that the n-type CNT fiber used in this work was prepared with a high S of 55 V K-1 based on previous report.68 The output voltage of an assembled TE generator can be calculated based on the equation (3).69

Voc=(n×Sp-type -n×Sn-type)×△T

(3)

where n is the number of the p-n pair, △T is the temperature gradient, Sp-type and Sn-type are the S of the p-type PP-NMP or VA-d-PPT30 and n-type CNFs, respectively. The maximum output power (Pmax) is often obtained when the external resistance matched the internal resistance of module and estimated by the equation (4). Pmax=Voc2/(4Rint)

(4)

where Voc is the output power of the TE generator at a temperature gradient and Rint is the total internal resistance. A high Pmax requires a high Voc and a low Rint simultaneously. The internal resistance of the as-fabricated two TE generator were measured to be 252 Ω (TEG1) and 192 Ω (TEG2). According to the open-circuit voltage of TEG1 (26.3 mV) and TEG2 (31.2 mV) shown in Figure 8b, the Pmax of TEG1 has achieved a as high as 1.28 W value, which is about two times higher than that of TEG1 at △T=60 K. 23



CONCLUSION A series of aerogel composite films were fabricated by incorporating highly conductive PEDOT:PSS and Te-NWs with high Seebeck coefficient through freeze drying method. The effect of different organic solvents as additives has been investigated on the TE performance of PEDOT:PSS aerogel films. The addition of NMP results in a porous network of PEDOT:PSS with smooth surface and good flexibility as well as a large TE power factor of 1.24 mW m-1 K-2. However, the PP-NMP still suffers from low  and S. The composite aerogel films combined of PEDOT:PSS and 30 wt.% Te-NWs shows an enhanced power factor of 3.6 W m-1 K-2. A 80% parallel connected model is found to improve carrier transport property in PEDOT:PSS/Te-NWs composite aerogel film. The vapor phase annealing with DMSO further achieves a high TE power factor of 11.3 μW m-1 K-2 and ZT of 2.0×10-2 estimated with a low in-plane thermal conductivity for PEDOT:PSS/Te-NWs films. This stragtegy proposes a facile and effective route to fabricate a flexible free-standing aerogel film with high TE performance. A prototype TE generator consisting of p-type PEDOT:PSS/Te-NWs aerogel films and n-type carbon nanotube fibers as legs was fabricated and achieved a maximum output power of 1.28 W at T = 60 K. This work provides an efficient pathway for the optimization of TE performance for composite aerogel films with high power factor and low thermal conductivity. ASSOCIATED CONTENT Supporting Information 24


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Further characterization of the final aerogel samples is available free of charge on the ACS Publications website at http://pubs.acs.org. Experimental details, SEM, TEM, and XRD (PDF) AUTHOR INFORMATION Corresponding Author *The corresponding authors, email: [email protected] (J. Xu); [email protected] (F. Jiang) Conflicts of interest There are no conflicts to declare. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Natural Science Foundation of China (51762018, 51572117, 51863009, 51762020, and 51662012), Innovation Driven "5511" Project of Jiangxi Province (20165BCB18016), the Natural Science Foundation of Jiangxi Province (20181ACB20010 and 20171ACB20026), and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology). Notes 25



Page 26 of 38

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Harvesting. Chem. Eng. J. 2017, 320, 201-210, DOI: 10.1016/j.cej.2017.03.023.

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Efficient DMSO-Vapor Annealing for Enhancing Thermoelectric

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