- Email: [email protected]
An integral p-n connected all-graphene fiber boosting wearable thermoelectric energy harvesting
Composites Communications 16 (2019) 79–83
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Short Communication
An integral p-n connected all-graphene fiber boosting wearable thermoelectric energy harvesting
T
Yuancheng Lina,1, Jing Liua,1, Xiaodong Wanga,1, Jingkun Xua,b, Peipei Liua,∗, Guangmin Nieb, Cheng Liua, Fengxing Jianga,∗∗ a b
Department of Physics, Jiangxi Science and Technology Normal University, Nanchang, 330013, PR China School of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, 266042, Shandong, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Thermoelectrics Wearable electron Fiber generator
Fiber-based thermoelectric (TE) generators have arose significant attention in recent years for portable energy supply devices. However, it is still a great challenge to fabricate integral p-n type connected graphene fibers (GFs) at present. Herein, we prepared p-type GFs by a simple process of chemical reduction. An integral p-n connected all-graphene fiber without additional adhesive was obtained with discontinuous doping with electrondonating polyethyleneimine ethoxylated (PEIE). The integral p-n connected GFs displayed a steady electrical conductivity, excellent flexibility and acceptable tensile properties. Furthermore, a flexible wearable TE generator was fabricated through integrating as-prepared integral p-n connected GFs into flexible polydimethylsiloxane (PDMS) substrate, exhibiting a stable output power at the temperature difference between human body and air temperature, which may provide a good reference for fiber-based wearable TE energy supply.
In recent years, development of electronic devices and intellectual products tends to be flexible, wearable and miniaturized [1]. The demand for flexible power-supply systems of portable electronic devices has rapidly increased in the process. Compared with films and bulks, fibers are more suitable to achieve wearable energy supply system because of its unique structure features [2]. Wearable TE devices have attracted great attention in recent decades due to the characteristics mainly involving simple structure, and sustainable energy supply [3,4]. TE materials which are able to convert heat into electricity with a temperature gradient between human body and air provide a good way for the power supply of wearable electronic devices and smart textiles [5–7]. An indicator to measure the thermoelectric performance is a dimensionless figure of merit (ZT), specifically, ZT=(S2·σ·T)/κ [8], where σ, S, T, and κ are the electrical conductivity, Seebeck coefficient, absolute temperature, and thermal conductivity, respectively. It proves indispensable for an excellent thermoelectric material to have a higher Seebeck coefficient and remarkable electrical conductivity as well as inferior thermal conductivity [9]. Graphene fibers have been extensive researched in sensing devices [10], energy harvesters [11] and supercapacitor [12] on account of its superior mechanical properties [13,14] and high electrical conductivity
[15,16]. In contrast with traditional wet- or dry-spinning techniques, self-assembly in tube module is a more accessible method to prepare graphene fibers. Furthermore, the semiconductor behavior of graphene is easily to be converted from p-type to n-type via doping of electrondonating molecules [17], providing us an inspiration to prepare an integral p-n connected graphene fiber. Fiber-based TE generator is a promising candidate for wearable energy supplying system. Currently Liu et al. [18] reported highly a fiber-based TE generator consisting of p-type highly conductive PEDOT:PSS hydrogel fibers and n-type CNT fibers, presenting an particularly high power density as 481.17 μW cm−2 with small effective area at 60 K temperature difference, which was a good reference for assembling fiber-based TE generator. Lan et al. assembled a TE generator based on p-type PEDOT:PSS conductive fabrics and n-type CNT fibers, which achieved a large output power as 375 μW with 8 pairs of p-n legs under 60 K temperature difference [19]. Yet the additional conductive adhesive employed to maintain a good ohmic contact between p-n legs would not only increase the complexity of the assembling process, but also severely constrain the mechanical stability of the device. In this work, we obtained an integral p-n connected all-graphene fiber without any additional conductive adhesive and assembled a
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (P. Liu), [email protected] (F. Jiang). 1 Yuancheng Lin, Jing Liu, and Xiaodong Wang contributed equally. ∗∗
https://doi.org/10.1016/j.coco.2019.09.002 Received 16 June 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 Available online 06 September 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
Composites Communications 16 (2019) 79–83
Y. Lin, et al.
Fig. 1. Schematic illustration of the fabrication process of an integral p-n connected all-graphene fiber.
wearable fiber-based TE energy harvesting device as shown in Fig. 1. Graphene oxide (GO) dispersion was prepared by oxidation of natural graphite powder via modified Hummers’ method [20]. Homogeneously mixed GO and ascorbic acid (AA) hybrid solution was sealed in PTFE tube. The GO fiber (noted as GF) was kept at 80 °C for 2 h. The GF was further reduced with N2H4·H2O to achieve a higher TE performance, which was noted as p-GF. Then to obtain an integral p-n connected allgraphene fiber, as-prepared GF was rolled onto a plastic rod and partially immersed into PEIE solution, which was noted as p-n-GF. Finally, the integral p-n connected all-graphene fiber was integrated into polydimethylsiloxane (PDMS) to achieve a flexible fiber-based TE device. Detailed experimental process was presented in supporting information. The length and diameter of GFs dried in air have obvious shrinkage because of the evaporation of water as shown in Fig. S1. During the chemical reduction process, GFs were obtained from self-assembly process induced by powerful π-π stacking between graphene sheets and effect of residual hydrophilic oxygenated groups on reduced GO sheets [21]. In order to better display this process, the digital photos of GO dispersion before and after chemical reduction were shown in Fig. S2. Based on scanning electron microscope (SEM) images of p- (Fig. 2A and C) and n-GF (Fig. 2B and D) prepared in PTFE tube with 1 mm diameter, the diameter of these two kinds of GF was basically the same, which was calculated as 88.8 ± 1.2 μm and 90.1 ± 0.6 μm, respectively. As shown in Fig. 2C, the morphology of the p-GFs presented a distinct wrinkle of graphene, which subsequently decreased after treatment with PEIE solution as shown in Fig. 2D, indicating PEIE molecules might be absorbed in the surface of GF. Furthermore, we found that the diameter of PTFE tube has an obvious effect on the dimeter of as-prepared fibers. As shown in Fig. S3, the dimeter of p-GF prepared in PTFE with 0.5 mm and 2 mm diameter was calculated as 57.9 μm and 229.3 ± 2.9 μm, respectively, indicating that a larger diameter of PTFE tube would result in a larger diameter of GF. This trend for n-GF was also confirmed as shown in Fig. S4. Raman spectra of GFs, p-GFs and n-GFs were presented in Fig. 3A. The typical D and G bands locate at 1305 cm−1 and 1584 cm−1. The G
band was attributed to sp2 carbon atom, while the D band was mainly caused by structural defects or partially disordered graphitic domains [22]. Intensity ratio of D to G peaks (ID/IG) was a quantitative indicator for the level of structural defects. After treated with N2H4·H2O or PEIE, ID/IG both decreased from 1.68 to 1.56 and 1.42 respectively, indicating that the conjugated network in graphene was restored to some extent during the reduction process [23]. Tensile properties are also key parameters for wearable energy supply. The bi-linear strain-stress curves of as-prepared GFs were shown in Fig. 3B, which is a characteristic of linear strain-hardening behavior [24]. The Young's modulus of GF was calculated as 18.4 GPa. After treated with N2H4·H2O, the Young's modulus increased about 40% to 25.8 GPa for p-GF, which might be ascribed to the reductant-induced enhancement of π-π stacking interactions between graphene sheets [25]. For n-GF, Young's modulus decreased to 12.8 GPa compared to that of GF. We assumed that as a hyperbranched polymer, PEIE molecules absorbed in the surface of GF might act as a plasticizer, which suppressed the elastic deformation and boosted the plastic deformation of GF, consequently leading to the decrease of Young's modulus. Furthermore, classical linear hardening parameter (H), final tensile stress (σm), the yield stress (σy) and the plastic strain (D) of as-prepared graphene fibers were presented in Table 1. Fig. S5 presented the TE performance of GF prepared under 80 °C for varied period of time, from which one can see that reaction time post negligible effects on the TE performance of GF. We choose 2 h as reaction time. The ratio of GO and AA may also affect the TE performance of GFs. As shown in Fig. S6, σ decreased slowly with the increase of GO in hybrid solution, meanwhile S firstly increased from 17.4 μV K−1 to 21.4 μV K−1 when the weight content of GO reached 20 wt%, and then displayed an obvious dropping trend. Therefore, GFs were prepared with GO and AA hybrid solution containing 20 wt% of GO heated under 80 °C for 2 h and we used this GF for further treatment. As-prepared GFs were treated with N2H4·H2O to achieve better TE performance. We found that after treated with N2H4·H2O, the electrical conductivity of GF increased from 11.6 S cm−1 to 19.3 S cm−1, which might be mainly ascribed to the reestablishment of the conjugated 80
Composites Communications 16 (2019) 79–83
Y. Lin, et al.
Fig. 2. SEM images of p-GFs (A) and its surface morphology (C). SEM images of n-GFs (B) and its surface morphology(D).
Fig. 3. (A) Raman spectra of GFs, p-GFs and n-GFs. (B) The typical stress–strain curves of GFs, p-GFs, n-GFs and p-n-GFs. (C) The stability for TE performance of p-GFs. (D) Seebeck coefficient, electrical conductivity and power factor of n-GFs with treatment time of PEIE solution. 81
Composites Communications 16 (2019) 79–83
Y. Lin, et al.
Table 1 Mechanical performance of Different type Graphene Fibers. Types
Young's Modulus (GPa)
σm (MPa)
σy (MPa)
D (%)
H (GPa)
GFs p-GFs n-GFs p-n-GFs
18.4 25.8 12.8 18.0
105.4 99.5 91.4 84.2
78.2 88.9 77.5 75.2
0.77 0.96 0.40 0.98
3.5 1.1 3.4 0.9
graphene network after the reduction of N2H4·H2O as we previously confirmed in Raman spectra [26]. With slight decrease of Seebeck coefficient (4.82%), an obvious enhancement of power factor was achieved from 0.53 μW m−1 K−2 to 0.78 μW m−1 K−2. Furthermore, the air-stability of this p-GF was presented in Fig. 3C, illustrating a significant increase of electrical conductivity with almost constant Seebeck coefficient. Besides, we have concluded a series of tensile strength and electrical conductivity of graphene fibers made via different methods such as wet-spinning, hydrothermal treatment and scrolling methods. As shown in Fig. S7, one can see that the graphene fibers obtained via wet-spinning both have a higher mechanical property and electrical conductivity. But self-assembly in tube mould is a more accessible method to prepare graphene fiber compared with wetspinning technique. Moreover, it is difficult to obtain highly conductive graphene fiber with high tensile strength via strolling method. The graphene fibers prepared in this work achieved an acceptable balance between electrical conductivity and tensile strength. n-GFs were obtained via immersing GFs into PEIE solution. We first examined the effect of the weight content of PEIE solution on the TE performance of n-GFs. As shown in Fig. S8, electrical conductivity presented an obvious increase trend meanwhile Seebeck presented an opposite trend as the immerse treatment processing. The optimal weight content of PEIE solution was 7.5 wt%. Then we treated GFs with 7.5 wt% PEIE solution for different period of time. As shown in Fig. 3D, the variation trend of electrical conductivity, Seebeck coefficient and power factor was similar to that in Fig. S8. As the immerse treatment processing, the fiber presented a more dominant n-type characteristic, i.e., a larger negative Seebeck coefficient with the decrease of electrical conductivity. As confirm in our XPS results (Fig. S9), one can see that the n-GFs showed distinguished N1s signal, indicating a large amount of nitrogen occurred on the surface of GO fiber after treated with PEIE solution, indicating that PEIE coated on the surface of fiber. PEIE possessed a large amount of electron-donating amino groups. The lone-pair of electrons in nitrogen transferred to GO fiber, prompting the upward shift of Fermi level to valence band, consequently leading to the transition of semiconducting behavior from p-type to n-type. Furthermore, although the conjugated network of graphene was restored to some extend as confirmed in Raman spectra, the intrinsic insulative nature of PEIE hindered the transport of charge carriers between graphene layers, which served as a more dominant factor for the decrease of electrical conductivity [27]. The power factor of n-GFs was achieved as 0.23 μW m−1 K−2 when the GFs were treated with 7.5 wt% for 4 h. The thermal conductivity (κ) of p-GF and n-GF with negligible difference was measured as 0.26 W m−1 K−1 and 0.24 W m−1 K−1, from which the ZT was calculated as 2.04 × 10−6 and 0.96 × 10−6. In addition, the electrical conductivity of graphene-based samples was summarized in Table S1. Our work was higher than most same type of hydrogel and aerogel but lower than samples which fabricated from modification. To learn the progress made in comparison with other thermoelectric fibers, we concluded the electrical conductivity and Seebeck coefficient in Table S2. We assembled a wearable TE energy harvesting device via integrating a p-n-GF consisting of 20 pairs p-n legs into flexible PDMS substrate. Mechanical reliability is an important indicator for wearable device. We tested the electrical resistance variation of this fiber-based device for about 1000 times of bending-releasing circle. The initial
Fig. 4. Digital photographs of TE generators (A). The output voltage and output power of p-n connected TE fibers (B) at various temperature differences ranging from 5 to 70 K. The stability with time for output performance of TE generators (C).
electrical resistance was measured as about 75 KΩ. As shown in Fig. 4A, the electrical resistance maintained almost the same with about less than 5% variation, indicating a good mechanical reliability of this device. The output voltage and power of this fiber-based device consisting of 4 pairs of p-n legs were calculated under the temperature gradient ranging from 0 to 70 K. The maximum output power (Pmax) can be calculated as Pmax = Voc2/(4Rint), where Voc and Rint were open-circuit voltage and total internal resistance of this device under a certain temperature difference. As shown in Fig. 4B, one can see that the Voc 82
Composites Communications 16 (2019) 79–83
Y. Lin, et al.
and Pmax were calculated 0.75 mV and 124.1 nW per p-n leg under 70 K temperature difference. Furthermore, Fig. 4C recorded a practical measurement results of the output voltage and power for several minutes at temperature difference between human body and air temperature (ΔT = 10 ± 0.5 K), indicating a stable TE energy harvesting performance. This work may provide reference for wearable energy harvesting fields based on graphene fibers.
[5]
[6]
[7]
Conclusions
[8]
In summary, p-type graphene fiber was successfully obtained via self-assembly of graphene dispersion assistant with ascorbic acid with weight content from 70% to 90% in tube mould. TE performance of GF can be elevated via N2H4·H2O treatment, which was ascribed to the restoration of conjugated network during chemical reduction process. An integral p-n connected graphene fiber without any conductive adhesive was fabricated via discontinuous doping with electron-donating PEIE. Furthermore, the p-n-GFs were weaved into flexible PDMS substrate to fabricate a wearable TE generator, of which the output power was measured to about ~1.3 pW at the temperature difference between human body and air temperature. This work provides a new kind of technical support for a facile preparation method of obtaining integral p-n connected TE fibers with reliable good stretch performance.
[9]
[10] [11]
[12] [13] [14] [15]
[16]
Acknowledgements
[17]
This work was financially supported by the National Natural Science Foundation of China (51762018, 51572117, and 51863009), the Innovation Driven ‘‘5511’’ Project of Jiangxi Province (20165BCB18016), the Natural Science Foundation of Jiangxi Province (20181ACB20010).
[18]
[19]
[20]
Appendix A. Supplementary data [21]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.coco.2019.09.002.
[22]
[23]
References
[24]
[1] Z. Bao, X. Chen, Flexible and stretchable devices, Adv. Mater. 28 (2016) 4177–4179. [2] L. Zhang, S. Lin, T. Hua, B. Huang, S. Liu, X. Tao, Fiber-based thermoelectric generators: materials, device structures, fabrication, characterization, and applications, Adv Energy Mater 8 (2018) 1700524. [3] Y. Chen, Y. Zhao, Z. Liang, Solution processed organic thermoelectrics: towards flexible thermoelectric modules, Energy Environ. Sci. 8 (2015) 401–422. [4] M. Ito, T. Koizumi, H. Kojima, T. Saito, M. Nakamura, From materials to device
[25]
[26] [27]
83
design of a thermoelectric fabric for wearable energy harvesters, J. Mater. Chem. A. 5 (2017) 12068–12072. Y. Jia, L. Shen, J. Liu, W. Zhou, Y. Du, J. Xu, et al., An efficient PEDOT-coated textile for wearable thermoelectric generators and strain sensors, J. Mater. Chem. C 7 (2019) 3496–3502. X. Wang, P. Liu, Q. Jiang, W. Zhou, J. Xu, J. Liu, et al., Efficient DMSO-vapor annealing for enhancing thermoelectric performance of PEDOT:PSS-based aerogel, ACS Appl. Mater. Interfaces 11 (2018) 2408–2417. C. Li, F. Jiang, C. Liu, P. Liu, J. Xu, Present and future thermoelectric materials toward wearable energy harvesting, Appl Mater Today 15 (2019) 543–557. Y. Xue, C. Gao, L. Liang, X. Wang, G. Chen, Nanostructure controlled construction of high-performance thermoelectric materials of polymers and their composites, J. Mater. Chem. A. 6 (2018) 22381–22390. T. Wang, C. Liu, F. Jiang, Z. Xu, X. Wang, X. Li, et al., Solution-processed twodimensional layered heterostructure thin-film with optimized thermoelectric performance, Phys. Chem. Chem. Phys. 19 (2017) 17560–17567. B. Fang, D. Chang, Z. Xu, C. Gao, A review on graphene fibers: expectations, advances, and prospects, Adv. Mater. (2019) 1902664. W. Ma, Y. Liu, S. Yan, T. Miao, S. Shi, M. Yang, et al., Systematic characterization of transport and thermoelectric properties of a macroscopic graphene fiber, Nano Res 9 (2016) 3536–3546. Z. Lu, J. Foroughi, C. Wang, H. Long, G. Wallace, Superelastic hybrid CNT/graphene fibers for wearable energy storage, Adv Energy Mater 8 (2018) 1702047. Y. Liu, Z. Xu, J. Zhan, P. Li, C. Gao, Superb electrically conductive graphene fibers via doping strategy, Adv. Mater. 28 (2016) 7941–7947. B. Fang, Y. Xiao, Z. Xu, D. Chang, B. Wang, W. Gao, et al., Handedness-controlled and solvent-driven actuators with twisted fibers, Mater Horiz 6 (2019) 1207–1214. B. Fang, J. Xi, Y. Liu, F. Guo, Z. Xu, W. Gao, et al., Wrinkle-stabilized metal-graphene hybrid fibers with zero temperature coefficient of resistance, Nanoscale 9 (2017) 12178–12188. T. Xu, Z. Zhang, L. Qu, Graphene-based fibers: recent advances in preparation and application, Adv. Mater. (2019) 1901979. M. Cha, W. Song, Y. Kim, D. Jung, M. Jung, S. Lee, et al., Long-term air-stable n-type doped graphene by multiple lamination with polyethyleneimine, RSC Adv. 4 (2014) 37849. J. Liu, Y. Jia, Q. Jiang, F. Jiang, C. Li, X. Wang, et al., Highly conductive hydrogel polymer fibers toward promising wearable thermoelectric energy harvesting, ACS Appl. Mater. Interfaces 10 (2018) 44033–44040. X. Lan, T. Wang, C. Liu, P. Liu, J. Xu, X. Liu, et al., A high performance all-organic thermoelectric fiber generator towards promising wearable electron, Compos. Sci. Technol. 182 (2019) 107767. D. Marcano, D. Kosynkin, J. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, et al., Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806–4814. Y. Xu, K. Sheng, C. Li, G. Shi, Self-assembled graphene hydrogel via a one-step hydrothermal process, ACS Nano 4 (2010) 4324–4330. K. Wasalathilake, D. Galpaya, G. Ayoko, C. Yan, Understanding the structureproperty relationships in hydrothermally reduced graphene oxide hydrogels, Carbon 137 (2018) 282–290. S. Stankovich, D. Dikin, R. Piner, K. Kohlhaas, A. Kleinhammes, Y. Jia, et al., Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565. P. Ladevèze, J. Pelle, Mastering Calculations in Linear and Nonlinear Mechanics, Springer, New York, 2005. K. Sheng, Y. Xu, C. Li, G. Shi, High-performance self-assembled graphene hydrogels prepared by chemical reduction of graphene oxide, New Carbon Mater. 26 (2011) 9–15. S. Pei, H. Cheng, The reduction of graphene oxide, Carbon 50 (2012) 3210–3228. C. Yu, A. Murali, K. Choi, Y. Ryu, Air-stable fabric thermoelectric modules made of N- and P-type carbon nanotubes, Energy Environ. Sci. 5 (2012) 9481–9486.
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Short Communication
An integral p-n connected all-graphene fiber boosting wearable thermoelectric energy harvesting
T
Yuancheng Lina,1, Jing Liua,1, Xiaodong Wanga,1, Jingkun Xua,b, Peipei Liua,∗, Guangmin Nieb, Cheng Liua, Fengxing Jianga,∗∗ a b
Department of Physics, Jiangxi Science and Technology Normal University, Nanchang, 330013, PR China School of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, 266042, Shandong, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Thermoelectrics Wearable electron Fiber generator
Fiber-based thermoelectric (TE) generators have arose significant attention in recent years for portable energy supply devices. However, it is still a great challenge to fabricate integral p-n type connected graphene fibers (GFs) at present. Herein, we prepared p-type GFs by a simple process of chemical reduction. An integral p-n connected all-graphene fiber without additional adhesive was obtained with discontinuous doping with electrondonating polyethyleneimine ethoxylated (PEIE). The integral p-n connected GFs displayed a steady electrical conductivity, excellent flexibility and acceptable tensile properties. Furthermore, a flexible wearable TE generator was fabricated through integrating as-prepared integral p-n connected GFs into flexible polydimethylsiloxane (PDMS) substrate, exhibiting a stable output power at the temperature difference between human body and air temperature, which may provide a good reference for fiber-based wearable TE energy supply.
In recent years, development of electronic devices and intellectual products tends to be flexible, wearable and miniaturized [1]. The demand for flexible power-supply systems of portable electronic devices has rapidly increased in the process. Compared with films and bulks, fibers are more suitable to achieve wearable energy supply system because of its unique structure features [2]. Wearable TE devices have attracted great attention in recent decades due to the characteristics mainly involving simple structure, and sustainable energy supply [3,4]. TE materials which are able to convert heat into electricity with a temperature gradient between human body and air provide a good way for the power supply of wearable electronic devices and smart textiles [5–7]. An indicator to measure the thermoelectric performance is a dimensionless figure of merit (ZT), specifically, ZT=(S2·σ·T)/κ [8], where σ, S, T, and κ are the electrical conductivity, Seebeck coefficient, absolute temperature, and thermal conductivity, respectively. It proves indispensable for an excellent thermoelectric material to have a higher Seebeck coefficient and remarkable electrical conductivity as well as inferior thermal conductivity [9]. Graphene fibers have been extensive researched in sensing devices [10], energy harvesters [11] and supercapacitor [12] on account of its superior mechanical properties [13,14] and high electrical conductivity
[15,16]. In contrast with traditional wet- or dry-spinning techniques, self-assembly in tube module is a more accessible method to prepare graphene fibers. Furthermore, the semiconductor behavior of graphene is easily to be converted from p-type to n-type via doping of electrondonating molecules [17], providing us an inspiration to prepare an integral p-n connected graphene fiber. Fiber-based TE generator is a promising candidate for wearable energy supplying system. Currently Liu et al. [18] reported highly a fiber-based TE generator consisting of p-type highly conductive PEDOT:PSS hydrogel fibers and n-type CNT fibers, presenting an particularly high power density as 481.17 μW cm−2 with small effective area at 60 K temperature difference, which was a good reference for assembling fiber-based TE generator. Lan et al. assembled a TE generator based on p-type PEDOT:PSS conductive fabrics and n-type CNT fibers, which achieved a large output power as 375 μW with 8 pairs of p-n legs under 60 K temperature difference [19]. Yet the additional conductive adhesive employed to maintain a good ohmic contact between p-n legs would not only increase the complexity of the assembling process, but also severely constrain the mechanical stability of the device. In this work, we obtained an integral p-n connected all-graphene fiber without any additional conductive adhesive and assembled a
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (P. Liu), [email protected] (F. Jiang). 1 Yuancheng Lin, Jing Liu, and Xiaodong Wang contributed equally. ∗∗
https://doi.org/10.1016/j.coco.2019.09.002 Received 16 June 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 Available online 06 September 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
Composites Communications 16 (2019) 79–83
Y. Lin, et al.
Fig. 1. Schematic illustration of the fabrication process of an integral p-n connected all-graphene fiber.
wearable fiber-based TE energy harvesting device as shown in Fig. 1. Graphene oxide (GO) dispersion was prepared by oxidation of natural graphite powder via modified Hummers’ method [20]. Homogeneously mixed GO and ascorbic acid (AA) hybrid solution was sealed in PTFE tube. The GO fiber (noted as GF) was kept at 80 °C for 2 h. The GF was further reduced with N2H4·H2O to achieve a higher TE performance, which was noted as p-GF. Then to obtain an integral p-n connected allgraphene fiber, as-prepared GF was rolled onto a plastic rod and partially immersed into PEIE solution, which was noted as p-n-GF. Finally, the integral p-n connected all-graphene fiber was integrated into polydimethylsiloxane (PDMS) to achieve a flexible fiber-based TE device. Detailed experimental process was presented in supporting information. The length and diameter of GFs dried in air have obvious shrinkage because of the evaporation of water as shown in Fig. S1. During the chemical reduction process, GFs were obtained from self-assembly process induced by powerful π-π stacking between graphene sheets and effect of residual hydrophilic oxygenated groups on reduced GO sheets [21]. In order to better display this process, the digital photos of GO dispersion before and after chemical reduction were shown in Fig. S2. Based on scanning electron microscope (SEM) images of p- (Fig. 2A and C) and n-GF (Fig. 2B and D) prepared in PTFE tube with 1 mm diameter, the diameter of these two kinds of GF was basically the same, which was calculated as 88.8 ± 1.2 μm and 90.1 ± 0.6 μm, respectively. As shown in Fig. 2C, the morphology of the p-GFs presented a distinct wrinkle of graphene, which subsequently decreased after treatment with PEIE solution as shown in Fig. 2D, indicating PEIE molecules might be absorbed in the surface of GF. Furthermore, we found that the diameter of PTFE tube has an obvious effect on the dimeter of as-prepared fibers. As shown in Fig. S3, the dimeter of p-GF prepared in PTFE with 0.5 mm and 2 mm diameter was calculated as 57.9 μm and 229.3 ± 2.9 μm, respectively, indicating that a larger diameter of PTFE tube would result in a larger diameter of GF. This trend for n-GF was also confirmed as shown in Fig. S4. Raman spectra of GFs, p-GFs and n-GFs were presented in Fig. 3A. The typical D and G bands locate at 1305 cm−1 and 1584 cm−1. The G
band was attributed to sp2 carbon atom, while the D band was mainly caused by structural defects or partially disordered graphitic domains [22]. Intensity ratio of D to G peaks (ID/IG) was a quantitative indicator for the level of structural defects. After treated with N2H4·H2O or PEIE, ID/IG both decreased from 1.68 to 1.56 and 1.42 respectively, indicating that the conjugated network in graphene was restored to some extent during the reduction process [23]. Tensile properties are also key parameters for wearable energy supply. The bi-linear strain-stress curves of as-prepared GFs were shown in Fig. 3B, which is a characteristic of linear strain-hardening behavior [24]. The Young's modulus of GF was calculated as 18.4 GPa. After treated with N2H4·H2O, the Young's modulus increased about 40% to 25.8 GPa for p-GF, which might be ascribed to the reductant-induced enhancement of π-π stacking interactions between graphene sheets [25]. For n-GF, Young's modulus decreased to 12.8 GPa compared to that of GF. We assumed that as a hyperbranched polymer, PEIE molecules absorbed in the surface of GF might act as a plasticizer, which suppressed the elastic deformation and boosted the plastic deformation of GF, consequently leading to the decrease of Young's modulus. Furthermore, classical linear hardening parameter (H), final tensile stress (σm), the yield stress (σy) and the plastic strain (D) of as-prepared graphene fibers were presented in Table 1. Fig. S5 presented the TE performance of GF prepared under 80 °C for varied period of time, from which one can see that reaction time post negligible effects on the TE performance of GF. We choose 2 h as reaction time. The ratio of GO and AA may also affect the TE performance of GFs. As shown in Fig. S6, σ decreased slowly with the increase of GO in hybrid solution, meanwhile S firstly increased from 17.4 μV K−1 to 21.4 μV K−1 when the weight content of GO reached 20 wt%, and then displayed an obvious dropping trend. Therefore, GFs were prepared with GO and AA hybrid solution containing 20 wt% of GO heated under 80 °C for 2 h and we used this GF for further treatment. As-prepared GFs were treated with N2H4·H2O to achieve better TE performance. We found that after treated with N2H4·H2O, the electrical conductivity of GF increased from 11.6 S cm−1 to 19.3 S cm−1, which might be mainly ascribed to the reestablishment of the conjugated 80
Composites Communications 16 (2019) 79–83
Y. Lin, et al.
Fig. 2. SEM images of p-GFs (A) and its surface morphology (C). SEM images of n-GFs (B) and its surface morphology(D).
Fig. 3. (A) Raman spectra of GFs, p-GFs and n-GFs. (B) The typical stress–strain curves of GFs, p-GFs, n-GFs and p-n-GFs. (C) The stability for TE performance of p-GFs. (D) Seebeck coefficient, electrical conductivity and power factor of n-GFs with treatment time of PEIE solution. 81
Composites Communications 16 (2019) 79–83
Y. Lin, et al.
Table 1 Mechanical performance of Different type Graphene Fibers. Types
Young's Modulus (GPa)
σm (MPa)
σy (MPa)
D (%)
H (GPa)
GFs p-GFs n-GFs p-n-GFs
18.4 25.8 12.8 18.0
105.4 99.5 91.4 84.2
78.2 88.9 77.5 75.2
0.77 0.96 0.40 0.98
3.5 1.1 3.4 0.9
graphene network after the reduction of N2H4·H2O as we previously confirmed in Raman spectra [26]. With slight decrease of Seebeck coefficient (4.82%), an obvious enhancement of power factor was achieved from 0.53 μW m−1 K−2 to 0.78 μW m−1 K−2. Furthermore, the air-stability of this p-GF was presented in Fig. 3C, illustrating a significant increase of electrical conductivity with almost constant Seebeck coefficient. Besides, we have concluded a series of tensile strength and electrical conductivity of graphene fibers made via different methods such as wet-spinning, hydrothermal treatment and scrolling methods. As shown in Fig. S7, one can see that the graphene fibers obtained via wet-spinning both have a higher mechanical property and electrical conductivity. But self-assembly in tube mould is a more accessible method to prepare graphene fiber compared with wetspinning technique. Moreover, it is difficult to obtain highly conductive graphene fiber with high tensile strength via strolling method. The graphene fibers prepared in this work achieved an acceptable balance between electrical conductivity and tensile strength. n-GFs were obtained via immersing GFs into PEIE solution. We first examined the effect of the weight content of PEIE solution on the TE performance of n-GFs. As shown in Fig. S8, electrical conductivity presented an obvious increase trend meanwhile Seebeck presented an opposite trend as the immerse treatment processing. The optimal weight content of PEIE solution was 7.5 wt%. Then we treated GFs with 7.5 wt% PEIE solution for different period of time. As shown in Fig. 3D, the variation trend of electrical conductivity, Seebeck coefficient and power factor was similar to that in Fig. S8. As the immerse treatment processing, the fiber presented a more dominant n-type characteristic, i.e., a larger negative Seebeck coefficient with the decrease of electrical conductivity. As confirm in our XPS results (Fig. S9), one can see that the n-GFs showed distinguished N1s signal, indicating a large amount of nitrogen occurred on the surface of GO fiber after treated with PEIE solution, indicating that PEIE coated on the surface of fiber. PEIE possessed a large amount of electron-donating amino groups. The lone-pair of electrons in nitrogen transferred to GO fiber, prompting the upward shift of Fermi level to valence band, consequently leading to the transition of semiconducting behavior from p-type to n-type. Furthermore, although the conjugated network of graphene was restored to some extend as confirmed in Raman spectra, the intrinsic insulative nature of PEIE hindered the transport of charge carriers between graphene layers, which served as a more dominant factor for the decrease of electrical conductivity [27]. The power factor of n-GFs was achieved as 0.23 μW m−1 K−2 when the GFs were treated with 7.5 wt% for 4 h. The thermal conductivity (κ) of p-GF and n-GF with negligible difference was measured as 0.26 W m−1 K−1 and 0.24 W m−1 K−1, from which the ZT was calculated as 2.04 × 10−6 and 0.96 × 10−6. In addition, the electrical conductivity of graphene-based samples was summarized in Table S1. Our work was higher than most same type of hydrogel and aerogel but lower than samples which fabricated from modification. To learn the progress made in comparison with other thermoelectric fibers, we concluded the electrical conductivity and Seebeck coefficient in Table S2. We assembled a wearable TE energy harvesting device via integrating a p-n-GF consisting of 20 pairs p-n legs into flexible PDMS substrate. Mechanical reliability is an important indicator for wearable device. We tested the electrical resistance variation of this fiber-based device for about 1000 times of bending-releasing circle. The initial
Fig. 4. Digital photographs of TE generators (A). The output voltage and output power of p-n connected TE fibers (B) at various temperature differences ranging from 5 to 70 K. The stability with time for output performance of TE generators (C).
electrical resistance was measured as about 75 KΩ. As shown in Fig. 4A, the electrical resistance maintained almost the same with about less than 5% variation, indicating a good mechanical reliability of this device. The output voltage and power of this fiber-based device consisting of 4 pairs of p-n legs were calculated under the temperature gradient ranging from 0 to 70 K. The maximum output power (Pmax) can be calculated as Pmax = Voc2/(4Rint), where Voc and Rint were open-circuit voltage and total internal resistance of this device under a certain temperature difference. As shown in Fig. 4B, one can see that the Voc 82
Composites Communications 16 (2019) 79–83
Y. Lin, et al.
and Pmax were calculated 0.75 mV and 124.1 nW per p-n leg under 70 K temperature difference. Furthermore, Fig. 4C recorded a practical measurement results of the output voltage and power for several minutes at temperature difference between human body and air temperature (ΔT = 10 ± 0.5 K), indicating a stable TE energy harvesting performance. This work may provide reference for wearable energy harvesting fields based on graphene fibers.
[5]
[6]
[7]
Conclusions
[8]
In summary, p-type graphene fiber was successfully obtained via self-assembly of graphene dispersion assistant with ascorbic acid with weight content from 70% to 90% in tube mould. TE performance of GF can be elevated via N2H4·H2O treatment, which was ascribed to the restoration of conjugated network during chemical reduction process. An integral p-n connected graphene fiber without any conductive adhesive was fabricated via discontinuous doping with electron-donating PEIE. Furthermore, the p-n-GFs were weaved into flexible PDMS substrate to fabricate a wearable TE generator, of which the output power was measured to about ~1.3 pW at the temperature difference between human body and air temperature. This work provides a new kind of technical support for a facile preparation method of obtaining integral p-n connected TE fibers with reliable good stretch performance.
[9]
[10] [11]
[12] [13] [14] [15]
[16]
Acknowledgements
[17]
This work was financially supported by the National Natural Science Foundation of China (51762018, 51572117, and 51863009), the Innovation Driven ‘‘5511’’ Project of Jiangxi Province (20165BCB18016), the Natural Science Foundation of Jiangxi Province (20181ACB20010).
[18]
[19]
[20]
Appendix A. Supplementary data [21]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.coco.2019.09.002.
[22]
[23]
References
[24]
[1] Z. Bao, X. Chen, Flexible and stretchable devices, Adv. Mater. 28 (2016) 4177–4179. [2] L. Zhang, S. Lin, T. Hua, B. Huang, S. Liu, X. Tao, Fiber-based thermoelectric generators: materials, device structures, fabrication, characterization, and applications, Adv Energy Mater 8 (2018) 1700524. [3] Y. Chen, Y. Zhao, Z. Liang, Solution processed organic thermoelectrics: towards flexible thermoelectric modules, Energy Environ. Sci. 8 (2015) 401–422. [4] M. Ito, T. Koizumi, H. Kojima, T. Saito, M. Nakamura, From materials to device
[25]
[26] [27]
83
design of a thermoelectric fabric for wearable energy harvesters, J. Mater. Chem. A. 5 (2017) 12068–12072. Y. Jia, L. Shen, J. Liu, W. Zhou, Y. Du, J. Xu, et al., An efficient PEDOT-coated textile for wearable thermoelectric generators and strain sensors, J. Mater. Chem. C 7 (2019) 3496–3502. X. Wang, P. Liu, Q. Jiang, W. Zhou, J. Xu, J. Liu, et al., Efficient DMSO-vapor annealing for enhancing thermoelectric performance of PEDOT:PSS-based aerogel, ACS Appl. Mater. Interfaces 11 (2018) 2408–2417. C. Li, F. Jiang, C. Liu, P. Liu, J. Xu, Present and future thermoelectric materials toward wearable energy harvesting, Appl Mater Today 15 (2019) 543–557. Y. Xue, C. Gao, L. Liang, X. Wang, G. Chen, Nanostructure controlled construction of high-performance thermoelectric materials of polymers and their composites, J. Mater. Chem. A. 6 (2018) 22381–22390. T. Wang, C. Liu, F. Jiang, Z. Xu, X. Wang, X. Li, et al., Solution-processed twodimensional layered heterostructure thin-film with optimized thermoelectric performance, Phys. Chem. Chem. Phys. 19 (2017) 17560–17567. B. Fang, D. Chang, Z. Xu, C. Gao, A review on graphene fibers: expectations, advances, and prospects, Adv. Mater. (2019) 1902664. W. Ma, Y. Liu, S. Yan, T. Miao, S. Shi, M. Yang, et al., Systematic characterization of transport and thermoelectric properties of a macroscopic graphene fiber, Nano Res 9 (2016) 3536–3546. Z. Lu, J. Foroughi, C. Wang, H. Long, G. Wallace, Superelastic hybrid CNT/graphene fibers for wearable energy storage, Adv Energy Mater 8 (2018) 1702047. Y. Liu, Z. Xu, J. Zhan, P. Li, C. Gao, Superb electrically conductive graphene fibers via doping strategy, Adv. Mater. 28 (2016) 7941–7947. B. Fang, Y. Xiao, Z. Xu, D. Chang, B. Wang, W. Gao, et al., Handedness-controlled and solvent-driven actuators with twisted fibers, Mater Horiz 6 (2019) 1207–1214. B. Fang, J. Xi, Y. Liu, F. Guo, Z. Xu, W. Gao, et al., Wrinkle-stabilized metal-graphene hybrid fibers with zero temperature coefficient of resistance, Nanoscale 9 (2017) 12178–12188. T. Xu, Z. Zhang, L. Qu, Graphene-based fibers: recent advances in preparation and application, Adv. Mater. (2019) 1901979. M. Cha, W. Song, Y. Kim, D. Jung, M. Jung, S. Lee, et al., Long-term air-stable n-type doped graphene by multiple lamination with polyethyleneimine, RSC Adv. 4 (2014) 37849. J. Liu, Y. Jia, Q. Jiang, F. Jiang, C. Li, X. Wang, et al., Highly conductive hydrogel polymer fibers toward promising wearable thermoelectric energy harvesting, ACS Appl. Mater. Interfaces 10 (2018) 44033–44040. X. Lan, T. Wang, C. Liu, P. Liu, J. Xu, X. Liu, et al., A high performance all-organic thermoelectric fiber generator towards promising wearable electron, Compos. Sci. Technol. 182 (2019) 107767. D. Marcano, D. Kosynkin, J. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, et al., Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806–4814. Y. Xu, K. Sheng, C. Li, G. Shi, Self-assembled graphene hydrogel via a one-step hydrothermal process, ACS Nano 4 (2010) 4324–4330. K. Wasalathilake, D. Galpaya, G. Ayoko, C. Yan, Understanding the structureproperty relationships in hydrothermally reduced graphene oxide hydrogels, Carbon 137 (2018) 282–290. S. Stankovich, D. Dikin, R. Piner, K. Kohlhaas, A. Kleinhammes, Y. Jia, et al., Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565. P. Ladevèze, J. Pelle, Mastering Calculations in Linear and Nonlinear Mechanics, Springer, New York, 2005. K. Sheng, Y. Xu, C. Li, G. Shi, High-performance self-assembled graphene hydrogels prepared by chemical reduction of graphene oxide, New Carbon Mater. 26 (2011) 9–15. S. Pei, H. Cheng, The reduction of graphene oxide, Carbon 50 (2012) 3210–3228. C. Yu, A. Murali, K. Choi, Y. Ryu, Air-stable fabric thermoelectric modules made of N- and P-type carbon nanotubes, Energy Environ. Sci. 5 (2012) 9481–9486.