Highly Conductive Hydrogel Polymer Fibers toward Promising

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Highly Conductive Hydrogel Polymer Fibers toward Promising Wearable Thermoelectric Energy Harvesting Jing Liu,† Yanhua Jia,† Qinglin Jiang,† Fengxing Jiang,*,† Changcun Li,† Xiaodong Wang,† Peng Liu,† Peipei Liu,†,∥ Fei Hu,*,‡ Yukou Du,§ and Jingkun Xu*,†,∥

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†

Jiangxi Engineering Laboratory of Waterborne Coatings, Jiangxi Science and Technology Normal University, Nanchang 330013, People’s Republic of China ‡ School of Materials Science and Energy Engineering, Foshan University, Foshan 528000, People’s Republic of China § College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of China ∥ School of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, Shandong People’s Republic of China S Supporting Information *

ABSTRACT: The requirement of a portable electron is functioning as a driving force for a wearable energy instrument. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), as one of the most promising organic electron materials, has been widely studied in energy conversion devices. However, the efforts for PEDOT:PSS fibers are insufficient to boost the development of wearable thermoelectric energy harvesting. Here, a highly conductive p-type PEDOT:PSS fiber was produced by gelation process, which was 3 orders of magnitude higher than that of previous hydrogel fibers. Surprisingly, a posttreatment with organic solvents such as ethylene glycol and dimethyl sulfoxide tripled their electrical conductivity with an only 5% decreased Seebeck coefficient, consequently leading to an optimized thermoelectric power factor. Furthermore, we assembled a p−n-type thermoelectric device connecting five pairs of p-type PEDOT:PSS fibers and n-type carbon nanotube fibers. This fiber-based device displayed an acceptable output voltage of 20.7 mV and a power density of 481.2 μW·cm−2 with a temperature difference of ∼60 K, which might pave the way for the development of organic thermoelectric fibers for wearable energy harvesting. KEYWORDS: PEDOT:PSS, gelation, thermoelectric fiber, energy harvesting, fiber device

1. INTRODUCTION Nowadays, the great development of technique has been achieved in stretchable and flexible smart electronics. Yet their market application has been severely confined by an extra energy system in view of its bulky, heavy, and nondeformable natures.1−4 Converting thermal5−9 and mechanical10,11 energy from the body, as well as solar energy,12,13 to electricity has been considered a promising way to breakthrough this shackle. Besides, textile woven form fibers are much more suitable for wearing and energy harvesting compared with a conventional and bulk energy device.3,14,15 Thermoelectric (TE) materials are capable of converting heat into electricity if there is only a © XXXX American Chemical Society

small temperature gradient between human body and ambient conditions.16−18 TE performance is defined by ZT = σS2T/κ, where S, σ, κ, and T are Seebeck coefficient, electrical conductivity, total thermal conductivity, and absolute temperature, respectively. A parameter named as power factor (PF = σS2) is usually used to weight the TE performance when it comes to organic TE materials due to their inherent low thermal conductivity.19 It is readily apparent that a good Received: September 4, 2018 Accepted: November 26, 2018

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DOI: 10.1021/acsami.8b15332 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

lium bromide ([BMIM]Br, 99%) were purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd. Anhydrous ethanol (EtOH) was obtained from Sinopharm Chemical Reagent Co., Ltd. 2.2. Fabrication of PEDOT:PSS Fibers. We prepared PEDOT:PSS fiber based on a previous report.31 In brief, a certain volume of PEDOT:PSS suspension (Clevios, PH1000) containing 0.05 M H2SO4 was sealed in a poly(tetrafluoroethylene) capillary (inner diameter = 1.0 mm), which was kept at 90 °C for 3.0 h. To optimize the synthesis procedure, vacuum evaporation was employed to condense the solid content of PEDOT:PSS in solution. After concentration, 5.0 vol % DMSO was added into the as-condensed solution following ultrasonic treatement for 10 min. After the formation of hydrogel, the as-obtained fibers were released into IPA, EtOH, or acetone bath from capillary by pumping air. They were cleaned by deionized water repeatedly and dried under 60 °C in vacuum for 1.0 h. Then the resultant fibers were immersed in EG, DMSO, and [BMIM]BF4. Finally, the as-prepared fibers were washed with deionized water again for several times and dried at 80 °C for 30 min. 2.3. Assembly of Thermoelectric Fiber Device. Five pairs of as-prepared p-type EG-treated PEDOT:PSS and n-type CNT fibers were connected in series. Then those two types of TE fiber were linked by silver paste. The experimental detail information for the preparation of n-type CNT fibers can be obtained in the Supporting Information. 2.4. Measurement and Instrumentation. Scanning electron microscope (SEM) images were obtained by JSM-7500F instrument (JEOL, Japan). Raman spectra were recorded by HR-800 (Horiba Jobin Yvon, France) at an excitation length of 523 nm. Electron spin resonance (ESR) was performed on a JES FA200 apparatus (JEOL, Japan). The surface composition of PEDOT:PSS fibers was analyzed by Escalab 250Xi X-ray photoelectron spectroscope (XPS, Thermo Fisher Scientific, USA). Keithley 2700 equipment was employed to detect the electrical conductivity and the sheet resistance Rs of the fibers as well as the device via a standard four-point-probe technique. Rs was calculated as Rs = 2πrR/L, where L, R, and r represented the length, resistance, and radius of a fiber, respectively. We measured the thermopower of fibers and output voltage of the device by a Keithley 2700 and 2401 system. The temperature difference between both ends of the fibers was achieved by a heat resistor on the heat side and controlled by regulated direct current. The definition of the Seebeck coefficient, S, is estimated as S = −ΔV/ΔT, where ΔV and ΔT represented the induced voltage and the in-plane temperature gradient of fibers, respectively.

organic TE material should possess a high TE power factor consisting of large electrical conductivity and Seebeck coefficient. As one of the most prospective organic TE materials, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is much more superior than the majority of inorganic materials for obtaining a wearable energy harvesting device due to its nontoxicity and flexibility along with easy processability.6,20−22 Presently, wet-spinning is one of the most commonly used techniques to obtain PEDOT:PSS fibers.23−25 Zhou et al.57 reported the fabrication of PEDOT:PSS fibers by wet-spinning with assistance of a hot-drawing technique. After being treated with ethylene glycol (EG), the fiber presented a pronounced high electrical conductivity of 2804 S·cm−1. In spite of outstanding electrical conductivity with a good mechanical property, high production cost and complex preparation craft hinder its way for extensive application. Moreover, PEDOT:PSS-based fibers were developed via the surface modification of commercially available fibers or fabric textiles.26−28 For example, Kirihara and his co-workers fabricated PEDOT:PSS-coated cotton cellulose fabric, of which the conductivity was only 10 S·cm−1.29 This dyeing or surface modification with PEDOT:PSS is a facile method to obtain conductive fibers or fabric textiles, but the productions often suffer from their low electrical conductivity. Currently, PEDOT:PSS-based hydrogel fiber has been obtained and has gotten increasing attention for stress sensor and energy storage thanks to their unique structural features. Lee and his co-workers developed a highly stretchable PEDOT:PSS−PAAm hybrid hydrogel, which was proven to possess a potential electromechanical sensor.30 Interestingly, PEDOT:PSS could achieve arbitrary shape via gelation process with assistance from sulfuric acid according to Shi and his coworkers,31 which is an accessible approach to obtain PEDOT:PSS fiber based on a capillary. Porous structure for as-fabricated fibers occurred after freeze-drying, resulting in the high volumetric capacitance. Yet on the basis of current hydrogel, the electrical conductivity of PEDOT:PSS fibers was insufficient for wearable TE energy harvesting. In this work, we successfully obtained highly conductive PEDOT:PSS hydrogel fibers by gelation process with a posttreatment of organic solvents. Aiming to gain a more continuously conductive fiber, the effects of sulfuric acid concentration, additives, and coagulation bath with various organic solvents were investigated on the gelation process of PEDOT:PSS. Furthermore, a post-treatment was chosen to optimize the TE performance of as-prepared PEDOT:PSS fibers. Scanning electron microscope, X-ray photoelectron spectroscopy, Raman spectra, and electron spin resonance were used to identify the effect of the micromorphology and structure of fibers on TE properties. Finally, a prototype TE fiber generator was assembled based on p-type PEDOT:PSS fibers and n-type carbon nanotube (CNT) fibers to demonstrate its feasibility in wearable energy harvesting.

3. RESULTS AND DISCUSSION 3.1. Characterization. PEDOT:PSS could gelate with a certain H2SO4 solution according to a previous report.31 Unfortunately, the effect of H2SO4 concentration has been not investigated systematically. Therefore, then, we systematically determined the effect of the amount of H2SO4 on the gelation process of PEDOT:PSS. We chose the various concentrations of H2SO4 solution (c0) and added each into 1.0 mL of pristine PEDOT:PSS aqueous solution, which led to the increased volume of total solution meaning the reduced solid content of PEDOT:PSS. Figure 1 presented the final H2SO4 concentration (c1) in PEDOT:PSS/H2SO4 mixed solution. We found that PEDOT:PSS enables gelation when the H2SO4 concentration is lower than 0.05 mol/L in mixed solution (green shaded region in Figure 1), while it is impossible to form PEDOT:PSS hydrogel with a larger H2SO4 concentration (gray shaded region in Figure 1). Tables S1 and S2 record the detailed data in the Supporting Information. This suggests that the contents of H2SO4 affect the gelation process out of the enhancement in hydrophobic attraction and π−π packing as well as the hydrogen bond among PEDOT:PSS grains. Also,

2. EXPERIMENTAL SECTION 2.1. Materials. A poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) aqueous solution (Clevios PH1000) was purchased from HC Stark. Acetone was acquired from J&K Scientific Ltd. Sulfuric acid (H2SO4) and isopropanol (IPA) were obtained from Xilong Chemical Co. Ltd. Ethylene glycol (EG), dimethyl sulfoxide (DMSO), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4, 99%), and 1-butyl-3-methylimidazoB


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Figure 1. Added H2SO4 concentration (c0) dependent on the calculated H2SO4 concentration (c1) in PEDOT:PSS/H2SO4 mixed solution with 1.0 mL of pristine PEDOT:PSS aqueous solution. Shaded regions present that PEDOT:PSS is able (green) and unable (gray) to gel with various H2SO4 contents.

Figure 2. (a) Digital photographs of as-fabricated PEDOT:PSS fiber. SEM images of PEDOT:PSS fibers (b) before and (c) after EG posttreatment.

be seen in Figure 2b that the as-fabricated PEDOT:PSS fiber has an average diameter of 142.5 ± 42.2 μm. After EG posttreatment, the diameter of the PEDOT:PSS fiber has a slight decrease to 141.8 ± 36.2 μm probably owing to the removal of PSS from the surface of the PEDOT:PSS fiber by EG. Note that there is a relatively large error of diameter for the asformed fiber due to the uniformity of shrinkage during gelation process. As shown in Figure 2b,c, the surface of the fibers becomes clearer and more trace can be seen after EG posttreatment probably due to the reduction of PSS. Panels a and b of Figure S3 show more SEM images of PEDOT:PSS fibers before and after EG post-treatment. As we know, the posttreatment has a large effect on the thermoelectric performance of PEDOT:PSS films due to its changes of composition and structure. The chemical composition of the as-fabricated PEDOT:PSS hydrogel fibers was determined by XPS spectra, as shown in Figure 3a,b. Because of different chemical environments of sulfur atom in the thiophene ring of PEDOT and the sulfonic acid group of PSS, electrons in the S(2p) spin−orbital had different binding energies.38 The S(2p) doublet at higher binding energy (168 and 169.2 eV) was assigned to sulfur atom in PSS, and the doublet at lower binding energy (164 and 165.3 eV) was assigned to that in thiophene.38−41 The strength of the S(2p) peak from PEDOT in PEDOT:PSS fiber was observed to be much lower than that in the treated fiber. The ratio between the intensity of two peaks at higher and lower binding energy could effectively reflect the composite changes of PEDOT to PSS. The ratios between PEDOT and PSS were calculated to be 1:1.07, 1:1.75, and 1:2.35 for PEDOT:PSS fiber before and after EG treatment as well as pristine PEDOT:PSS without any additives, respectively. The ratio of PEDOT/PSS for fibers was higher than that of pristine PEDOT:PSS thin film without any additives, which was ascribed to the configurational change induced by sulfuric acid similar to previous reports.31,44,45 After treatment with EG, the ratio of PEDOT/PSS was further elevated to 1:1.07, indicating the removal of PSS chains from PEDOT:PSS. This posttreatment process came from the decrease in the Coulombic interaction between conductive PEDOT and insulating PSS chains by a screening effect of EG,38,42,43 resulting in the

we found that a highly concentrated H2SO4 led to gelation during a short time. More digital photographs of PEDOT:PSS hydrogel containing a series of amounts of sulfuric acid can be seen in Figure S1. According to the report by Yao et al.31 the gelation of PEDOT:PSS is attributed to the enhancement in π−π stacking interaction and hydrophobic attraction between PEDOT chains induced by H2SO4. Beyond that, we propose that PEDOT:PSS gel particles possessed a PEDOT-rich core and PSS-rich shell on the basis of the structure of PEDOT:PSS, which can be ascribed to the interconnection via hydrogen bonds.32 Through the introduction of sulfuric acid, additional H+ would strengthen the hydrogen bonds throughout the whole hybrid solution until almost all particles were blended into one bulk. As we know, organic solvents and ionic liquid as additives have a positive influence on the thermoelectric performance of PEDOT:PSS.33−35 Figure S2 shows digital pictures of PEDOT:PSS gelation with 5.0 vol % DMSO (a), EG (b), [BMIM]BF4 (c), and [BMIM]Br (d) added in 1.0 mL of PEDOT:PSS aqueous solution containing 0.05 mol/L H2SO4, respectively. One can see that EG and ionic liquid would hinder the gelation process of PEDOT:PSS, while the solution with DMSO formed hydrogel well. Therefore, DMSO as additive was chosen to prepare the PEDOT:PSS hydrogel. Presently, the PEDOT:PSS hydrogel fibers were fabricated by gelation and the freeze-dry technique leading to a porous structure.31 However, for a TE material, this loose structure probably related to the low solid contents of PEDOT:PSS in solution has a negative impact on the electrical conductivity.36,37 To avoid the occurrence of unnecessary porosity, the original PEDOT:PSS volume was condensed to obtain a higher mass concentration via a vacuum evaporation at 55 °C. After gelation, the as-formed PEDOT:PSS fibers were too brittle to keep the complete fiber shape when released into deionized water (DI) or onto a glass substrate. We tried to replace DI water for the other organic solvents of EtOH, acetone, and IPA as coagulation bath; the PEDOT:PSS fibers became compact enough to maintain the as-formed shape. Figure 2a presented the digital photograph of as-fabricated PEDOT:PSS fibers with a large length more than 20 cm. It can C


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Figure 3. (a) S(2p) signal of XPS spectra for PEDOT:PSSfibers before and after EG post-treatment. (b) S(2p) signal of XPS spectra for EG treated PEDOT:PSS fibers. (c) Raman and (d) ESR spectra of PEDOT:PSS fibers before and after EG post-treatment.

Figure 4. (a) Sheet resistance and electric conductivity and (b) Seebeck coefficient and power factor of PEDOT:PSS fiber before and after EG post-treatment.

enrichment of the conductive PEDOT region. Moreover, the XPS analysis was also conducted on fibers treated with DMSO as shown in Figure S4b. The ratio of PEDOT/PSS for DMSOtreated fibers was calculated as 1:1.08, which was similar to that of EG-treated fibers. Panels c and d of Figure 3 show the Raman spectroscopy and ESR characterization for EG posttreated PEDOT:PSS fibers. One can see that there is no obvious change, suggesting a similar oxide level before and after EG -treatment. 3.2. Thermoelectric Properties. Figure S5 exhibits an unconspicuous difference in the TE performance of asfabricated PEDOT:PSS fibers with and without DMSO as additive when the solid content of PEDOT:PSS is 13.6 mg in 1.0 mL of aqueous solution. EtOH, acetone, or IPA as

coagulation bath has a similar effect on the TE performance of PEDOT:PSS fibers as shown in Figure S6. Note that when the solid content of PEDOT:PSS was increased, one can see the decrease in electrical conductivity of as-fabricated fibers due to the increase in the diameter of the PEDOT:PSS fiber with similar S values, which subsequently generated a poor power factor. Therefore, we chose the EG-treated PEDOT:PSS hydrogel fibers to investigate the TE performance. Panels a and b of Figure 4 illustrated the Rs, σ, S, and σS2 as a function of EG post-treatment time. The σ of PEDOT:PSS fibers without any treatment was measured to be 58.6 S·cm−1, which was 3 orders of magnitude higher than the PEDOT:PSS hydrogel reported by Yao et al.31 It is assumed that the structure of PEDOT:PSS obtained from shrinking at room D


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ACS Applied Materials & Interfaces temperature is more compact than those synthesized from freeze-drying, which means the strategy in this work would produce a more continuous electrically conductive grid. The S of PEDOT:PSS fibers is estimated to be 17.5 μV·K−1. It is worth mentioning that, after the EG post-treatment for 45 min, the σ of PEDOT:PSS fibers almost tripled as 172.5 S·cm−1, while its S only decreased by 5% as 14.8 μV·K−1. The electrical conductivity of PEDOT:PSS is strongly dependent on the ratio of PEDOT/PSS, morphology, and molecular configuration, which have been demonstrated by XPS. Solid PEDOT:PSS generally consists of an insulating PSS-rich shell wrapping a conductive PEDOT-rich core, consequently generating a poor electrical conductivity. First, the introduction of sulfuric acid makes a configurational change to conductive PEDOT chains.31 We found that the electrical conductivity of fibers increased to be 58.6 S·cm−1, which was over 50 times higher than that of pristine PEDOT:PSS thin films. Second, the further EG treatment effectively removed the insulating PSS chains in the surface of fibers, subsequently leading to the exposure and contact of conductive PEDOT chains.23,43 Furthermore, EG post-treatment under elevated temperature for several minutes was also applied on PEDOT:PSS fibers. It should be mentioned that, under higher treatment temperature, the time necessary to reach the climax of the power factor was shortened, yet there is no remarkable difference among all the optimal power factors in Figure S7. Also, the σ of PEDOT:PSS-based fibers were summarized in Table 1.The

Figure 5. Thermoelectric performance of the PEDOT:PSS fiber treated with (a) DMSO and (b) [BMIM]BF4 for several minutes and dried in 120 °C for 30 min.

Table 1. Electrical Conductivity of PEDOT:PSS Samples Prepared by Different Methods sample a

PEDOT:PSS-coated Spandex PEDOT:PSS-coated polyester fabrica PEDOT:PSS-coated cotton cellulose fibersa PEDOT:PSS fiberb PEDOT:PSS fiberb PEDOT:PSS fiberb PEDOT:PSS fiberb PEDOT:PSS−PAAmc PEDOT:PSSc PEDOT:PSSc

σ (S cm−1)

ref

2 1.5 10 467 264 56 2804 0.01 8.8 172.5

56 26 27 23 25 24 57 30 31 this work

which was mainly due to these organic solvents being effectively able to remove the insulating PSS chains. To make further investigation of the conductive mechanism changes initiated by EG post-treatment, the dependence of electrical conductivity on temperature ranging from 200 to 300 K was performed for the fibers with and without the EG posttreatment. As shown in Figure 6a,b, the electrical conductivities of both PEDOT:PSS fibers decrease with the drop of temperature, indicating a typical behavior of semiconducting materials. Furthermore, the one-dimensional (1D) variable range hopping (VRH) was employed to explore the conductive mechanism,47,48 σ = σ0 exp[(T0/T)1/2], where σ0 is a constant and T0 stands for the energy barrier between localized states. The T0 values of PEDOT:PSS fibers before and after EG posttreatment were calculated to be 259.0 and 90.4 K respectively. It implies the decrease in carrier hopping barrier due to the reduction of isolated PSS promotion and the increase in the electrical conductivity of the as-fabricated fibers EG treatment. 3.3. Output Properties of the Fiber Device. As shown in Figure 7a, we assembled a p−n TE fiber device consisting of post-treated p-type PEDOT:PSS fibers and n-type CNT fibers as legs. As we know, human body temperature is stable at 37 °C in normal condition and outdoor temperature may vary from −20 to 35 °C. We calculated the TE conversion property of this fiber device with various ΔT from 0 to 60 K. Figure 7b displayed the output voltage and power density of the fiber TE module with five couples of legs as a function of temperature gradient. The effective device area was calculated as n × (Ap‑type + An‑type), where n, Ap‑type, and An‑type represent the number of thermal couples and the cross-section areas of p-type and ntype legs, respectively.49 According to the measured diameter

Surface modification. bWet-spinning. cHydrogel fiber.

a

electrical conductivity in this work was much higher than that of the fiber fabricated from surface modification or PEDOT:PSS hydrogel fiber, but it was slightly lower than that of the fiber prepared via wet-spinning. In addition, DMSO and [BMIM]BF4 were also employed to treat PEDOT:PSS fibers, as shown in Figure 5a,b. Actually, there is a difference among the TE performance of PEDOT:PSS hydrogel fiber treated with DMSO, EG, and [BMIM]BF4. We found that the thermoelectric performance of fibers was barely elevated after being treated with [BMIM]BF4, being mainly ascribed to ionic liquids hardly inducing the conformation changes of conductive PEDOT chains when the PEDOT:PSS was solidified.46 The effect of DMSO and EG on the thermoelectric performance of fibers was obviously superior to that of the ionic liquid. According to our experimental results, the optimal power factor for PEDOT:PSS hydrogel fiber treated with EG, DMSO, and [BMIM]BF4 was calculated as 4.77, 3.79, and 2.26 μW m−1·K−2, respectively. The treatment effect of EG and DMSO was approximate, E


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Figure 6. (a) Temperature-dependent electrical conductivity and (b) analysis of the temperature dependence of the electrical conductivity with the 1D VRH model of PEDOT:PSS fiber before and after EG post-treatment.

Figure 7. (a) Schematic diagram for thermoelectric device and (b) the output voltage and power density of five couples of legs at various temperature differences ranging from 5 to 60 K. The inset in panel b is a scheme for the fiber device consisting of five pairs of p−n legs consisting of EG-treated p-type PEDOT:PSS fibers and n-type CNT fibers.

of fibers, we calculated the average effective device area. Compared with a tradition thin-film-based device with ca. 0.625 mm2 per couple,50 a fiber device can easily achieve a small effective device area with 0.018 mm2 per couple, which would dramatically make a promotion to the power density of the whole device. The internal electrical resistance is the sum of the resistance of the individual elements in this device.51 This device in our work consisted of five pairs of legs, and its internal resistance was 246.6 Ω. In an ideal case, the output voltage of this device was calculated as Voc = (nSPEDOT:PSS − nSn‑type CNT) ΔΤ, where n was the number of p- or n-type legs for the device, ΔT was the temperature difference between the cold and hot sides of the device, and SPEDOT:PSS and Sn‑type CNT were the Seebeck coefficients of the PEDOT:PSS and n-type CNT fibers, respectively. Voc for this device was calculated to be about 10.7 mV at ΔT = 30 K, which was slightly inferior to the measured value. It was noteworthy that the TE energy harvesting device would reach its peak output power on the condition of the internal load resistance matching the external.52−55 The maximum output power can be estimated by the equation Pmax = Voc2/(4Rint), where Voc is the output power of the device at a certain temperature gradient and Rint is the total internal resistance. As ΔT increasing, the open-circuit voltage gradually increased from 1.81 to 20.69 mV and the power density from 3.69 to 481.17 μW·cm−2. A higher power density represents a higher utility rate of space of TE materials with such geometrical shape to generate the same or more

power. This is of great significance to achieve a wearable energy harvesting system.

4. CONCLUSIONS In summary, PEDOT:PSS fiber with elevated thermoelectric performance was successfully fabricated via gelation process with a certain addition of sulfuric acid. The sulfuric acid content and additives have a significant effect on the gelation process of PEDOT:PSS. PEDOT:PSS solution cannot form hydrogel with a sulfuric acid concentration more than 0.05 mol/L in mixed solution. Compared with EG and ionic liquid, DMSO has no influence on the gelation process of PEDOT:PSS. The solvents of EtOH, acetone, or IPA as coagulation bath can keep the fiber shape better than water. EG and DMSO post-treatments result in a higher TE performance of PEEDOT:PSS hydrogel fibers than [BMIM]BF4. In addition, a p−n fiber TE device was assembled to make a quantitative evaluation on thermal harvesting efficiency. The device presented an acceptable output voltage of 20.69 mV and notably a high power density as 481.17 μW·cm−2 due to a small effective device area when the temperature gradient reached 60 K. This work provides illumination for a much more accessible preparation method to obtain PEDOT:PSS hydrogel fibers with high electrical conductivity as well as the development of a fiber-based thermoelectric energy harvesting system. F


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15332. Photographs of the PEDOT:PSS hydrogel fabricated from different formulas, thermoelectric performance varied for different fabrication and post-treatment conditions, XPS of spin-cast PEDOT:PSS thin film, and fabrication of n-type CNT (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F. Jiang). *E-mail: [email protected] (F. Hu). *E-mail: [email protected] (J. Xu). ORCID

Fengxing Jiang: 0000-0001-8907-9445 Jingkun Xu: 0000-0003-3492-5450 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the financial support of the National Natural Science Foundation of China (Grant Nos. 51762018, 51572117, and 51863009), the Innovation Driven “5511” Project of Jiangxi Province (Grant No. 20165BCB18016), and the Natural Science Foundation of Jiangxi Province (Grant No. 20181ACB20010).

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REFERENCES

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Highly Conductive Hydrogel Polymer Fibers toward Promising

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