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A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application
Nano Energy (2015) 13, 298–305
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application Min Zhanga,1, Tao Gaoa,1, Jianshu Wanga, Jianjun Liaoa, Yingqiang Qiua, Quan Yanga, Hao Xuea,n, Zhan Shia, Yang Zhaoc, Zhaoxian Xionga, Lifu Chena,b,nn a
Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China b Key Laboratory of High Performance Ceramic Fibers (Xiamen University), Ministry of Education, Xiamen 361005, China c Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen 361005, China Received 15 December 2014; received in revised form 14 January 2015; accepted 27 February 2015 Available online 13 March 2015
KEYWORDS
Abstract
Piezoelectric composite fiber; Fabric nanogenerator; Mechanical energy harvesting; Interdigited electrodes.
Wearable nanogenerators are vital important for wearable devices and portable electronic devices. Here we report a flexible hybrid piezoelectric fiber based two-dimensional fabric nanogenerator which can be promising to be easily integrated with clothing and convert the mechanical energy of human body motion into electric energy. The hybrid piezoelectric fiber comprised aligned BaTiO3 nanowires and PVC polymer. The PVC polymer made the fiber be enough flexible for performing the woven process and the aligned BaTiO3 nanowires enhanced the piezoelectric properties as active materials. The metal copper wires and cotton threads were woven into the fabric to construct the nanogenerator with interdigited electrodes. By attaching the fabric nanogenerator on an elbow pad which was bended by human arms, the nanogenerator can generate 1.9 V output voltage and 24 nA output current and the output are large enough to power a LCD. & 2015 Elsevier Ltd. All rights reserved.
n
Corresponding author. Corresponding authors at: Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China. E-mail addresses: [email protected] (H. Xue), [email protected] (L. Chen). 1 These authors contributed to the work equally. nn
http://dx.doi.org/10.1016/j.nanoen.2015.02.034 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
Introduction The waste energy harvesting from the ambient environment and biomechanical movement has been considered as an attractive alternative over traditional rechargeable batteries for providing electrical power to low-energy devices such as body worn
A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application sensors and wearable consumer electronics. Piezoelectric nanostructures have attracted extensive attention because they can provide a practical way to scavenge mechanical energy from the environment. They can also be put into use as self-powered sensing devices. a wide variety of mechanical energy have been successfully demonstrated to produce electricity, such as wind [1–4], water waves [5–8], vibration, [9,10] ultrasonic waves, [11,12] and human body movement, [13–15] and various piezoelectric materials, such as ZnO [11,16–19], CdS [20], InN [21], ZnSnO3 [22,23], NaNbO3 [24], KNbO3 [25], PZT [26–29], PMN-PT [30], BaTiO3 [31–34], NaxK1 xNbO3 [35], yBa(ZrxTi1 x)O3-(1-y) (BaxCa1 x)TiO [36,37], GaN [38] and PVDF [12,39,40] have been employed for fabricating nanogenerators. In the various forms of energy harvesting, the wearable generator may be most attractive, since it is feasible and compatible way to power the portable electronic devices, biomedical sensors, and so on. However, it should be concerned that inorganic piezoelectric materials are usually very brittle, and can only work in the case of small level of strain. These shortcoming makes it hard to be integrated on a large scale wearable devices which mostly need to adapt to curved surface and large level of strain. Fiber-based electric power generators are highly promising as they are light and comfortable to wearers. By the electrospinning technique, a number of piezoelectric materials, such as Pb[ZrxTi1x]O3[26, 4142], NaxK1 xNbO3 [35], yBa(ZrxTi1 x)O3 (1y)(BaxCa1 x) TiO3 [36,37] have been developed to be fabricated into nanofibers or nonwovens which were used to prepare wearable nanogenerators, but they are still needed to be imbedded into polymer to overcome the brittleness. Up to now, most wearable nanogenerators were embedded inside a fabric or attached on the skin of human body to perform the energy harvesting process. For wearable application, energy harvesting device should provide the comfort and natural feel to the wearer, so the better manner of the device should be integrated in the textile as part even whole of the cloth. Ideally, if the energy harvesters were implemented as textile-fiber structures, such fibers would provide perfect building elements for a smart shirt, as they could be naturally integrated into fabrics during the weaving process without affecting comfort, flexibility, air permeability and maneuverability. Wang et al. have reported generators based on textile fibers which produce power output by the friction motion [3] and electrostatic effect [43] of the fibers. In order to provide more energy harvesting solutions, we have explored the possibility of flexible and durable piezoelectric generators based on simple procedure and low cost strategies. Here, we developed a twodimensional fabric-like nanogenerator based on three kinds of fibers including BaTiO3 nanowire-PVC hybrid piezoelectric fibers, conventional cotton threads and copper wires. The piezoelectric properties of the BaTiO3 nanowire-PVC fibers were effectively improved by the inorganic BaTiO3 active nanowires. The details of results and analysis have been reported by our previous research works [44]. The copper wires served as the electrodes which seems to be of interdigited structure, while the cotton threads were employed as spacer to avoid the shorting of the electrodes as well as to enhance the durability of whole device. This structure makes the electrodes be naturally integrated in the fabric so that the entire device look no different from the conventional fabrics. This fiber based 2-D fabric nanogenerator with excellent strength and super flexibility is capable of converting mechanical energy of human
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body motion into electrical energy. The device has been demonstrated to power a LCD display.
Experimental The fabrication of highly o0014 oriented BaTiO3 nanowire-PVC microfibers The fabrication process of BaTiO3-polymer fibers have been introduced and discussed in detail in our previous work [44]. In order to present the clear structure and fabricating process of fabric nanogenerator based on the BaTiO3 nanowires-polymer fibers, here we briefly described it again. First, BaTiO3 nanowires were prepared by the topochemical method [44,45]. The mixture of K2CO3 and TiO2 in the molar ratio of 1:3 was heated at 1000 1C for 18 h. Then, the as-synthesized product was washed with deionized water to remove the K2CO3 and dried at 80 1C for 8 h and meanwhile the intermediate products of K2Ti4O9 nanowires were obtained. BaTiO3 nanowires were finally obtained by hydrothermal ion-exchange conversion of the K2Ti4O9 nanowires in 0.2 M Ba(OH)2 solution at 100–180 1C for 12 h. The obtained BaTiO3 nanowires and polyvinyl chloride (PVC) powder according to the weight ratio of 1:2 were dispersed in dimethylacetamide (DMA) and were stirred by magnetic stirrers for 1 h. The BaTiO3 nanowire-PVC composite fibers were fabricated by the spinning method. The sketch of the spinning equipment are shown in Fig. S1. The mixed slurry were carried in the container and the slurry were extruded through a spinneret which has a single hole of 250 μm, the polymer fiber was coagulated in the coagulation bath containing DMF/distilled water at room temperature. Drawing was performed in boiling water through the speed control of the two rollers. Finally, consecutive BaTiO3 nanowirepolymer fibers as fine as 60–70 μm were fabricated.
The fabrication of flexible fabric nanogenerator (FNG) The BaTiO3 nanowire-PVC fibers were used to fabricate flexible fabric nanogenerators. The source materials for the FNG are the BaTiO3 nanowires-polymer microfiber in the warp direction, the copper wires as electrodes in the weft direction and the cotton threads as the insulating spacers of the electrodes. The fabrics were fabricated via hand knitting using above source materials, then the fabrics was fixed on a PET plastic substrate. It is noted that only the four sides of the fabric is fixed on the substrate and the main part of the fabrics maintain a natural contact with the substrate. Two Cu wires which were used as extraction electrodes were also fixed on the substrate and connected the Cu wires in the fabrics to form interdigitated electrodes configuration. Then, the BaTiO3 nanowire-polymer microfiber in the fabrics were poled by applying an electric field of 4 kV/mm across the interdigited electrodes at a temperature of about 70 1C for 20 h. As comparison, a referential device, in which the simple PVC polymer fibers replaced the BaTiO3 nanowires-PVC fibers, was also fabricated using the same process.
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Characterization and application The morphology and structure of the samples was observed by SEM (Hitachi SU70) and TEM (Joel 2100). The tensile strength of the BaTiO3 nanowire-PVC composite fiber were measured by universal materials tester (Galdabini SUN 2500). The fabric piezoelectric nanogenerator was demonstrated to harvesting the energy output of human body's movement. The fabric nanogenerator was attached on an elbow pad and converted the mechanical energy of human body motion into electrical power. The output voltage and current were measured by Keithley source meter 2400. The nanogenerator were connected with LCD to demonstrate the application of powering an electronic device. An 80 MΩ resistance as external load was connected to the NG to evaluate the power output of the nanogenerator.
Result and discussion The fabrication and structure of the FNG The FNG were fabricated using BaTiO3 nanowire-PVC piezoelectric fibers. The BaTiO3 nanowire-PVC piezoelectric fibers were fabricated by spinning method and the details have been described in our previous work [44]. The structure and morphology of the hybrid fibers are shown in Fig. S2. In Ref. [44] and supplementary file, the structure and advantages of the fibers have been discussed in detail. The BaTiO3 nanowires-polymer fiber is very flexible, robust and even stretchable. These merits can overcome the brittleness of conventional piezoelectric ceramic fibers and will greatly benefit the fabrication of wearable piezoelectric nanodevices. The tensile strength of 54 MPa (Seen in Fig. S3) of the composite fiber provided a guarantee for subsequent fabricating and working processes of the fabric nanogenerator. Up to now, the most way to integrate a piezoelectric material into the wearable device is to embed the piezoelectric materials into polymer matrix or to transfer the brittle nanowires onto a flexible substrate. For energy harvesting from human movement, the fiber based electrical power generators are highly desirable as they are light, comfortable and look similar to the conventional fabrics. The combination of piezoelectric materials in fabrics provides a facile route for the
M. Zhang et al. fabrication of flexible piezoelectric generators. The fabric structures can be designed so as to offer piezoelectric output. The fabric piezoelectric generator were prepared with commodity cotton threads spacers between the copper wires electrodes in the weft direction and the BaTiO3 nanowire-PVC fibers in the warp direction. Fig. 1 shows the photo and schematic of the FNG. In this work, the sample was fabricated through a simple hand-knitting process.
Proposed power generation mechanism of the fiberbased generators Now we briefly discuss the power generation mechanism. Significant factor in piezoelectricity is the relationship between the applied mechanical stress and the generated charge, defined as a piezoelectric charge constant dij, where the first i and second j subscripts represent the directions of the poled dipoles and applied force, respectively [46,47]. The d31 and d33 modes are widely used for piezoelectric applications, in which the two modes are differentiated according to whether the direction of the generated electric signals is perpendicular (d31) or parallel (d33) to the applied stress/strain direction. Furthermore, the d33 constant is approximately twice larger than d31. The open-circuit voltage (V3j) generated when the piezomaterials are deformed by mechanical strain (εxx) can be expressed as Z g3 j εð1ÞEd1j ð1Þ V out ¼ where g3j is the piezoelectric voltage constant (g3j = d3j/εT, εT denotes the permittivity under a constant strain), ε(l) is the strain, E is the elasticity modulus and the integral of lj is the distance between the electrodes. Since the d33 and g33 constants are much larger than d31 and g31, respectively, constructing a d33 working mode should be helpful to produce higher outputs for generators. Fig. 2 illustrates the operating mechanism of the FNG. As shown in Fig. 2(a), the copper wires-A which were woven into the fabric play the role of the internal electrodes and the copper wires-B were set as extraction electrodes, so both copper wires A and B together construct a structure of interdigited electrodes (IDEs). The cotton threads between
Fig. 1 The photo and schematic of the fabric nanogenerator (FNG). (a) The photo of the FNG, (b) The structure of the FNG.
A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application
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Fig. 2 Schematics of the operating mechanism of the FNG. (a) The structure of the FNG, (b) The FNG were deformed by the mechanical bending along the direction of piezoelectric fibers, (c) the FNG under unbending status, (d) The FNG after poling process; the dipole direction is along the piezoelectric fiber, (e) under the mechanical bending, the enhanced dipole result in that the positive and negative piezopotentials are produced at adjacent electrodes which lead to the carriers flow, (f) under unbending status, the potential fade away and the carriers flow back.
the copper wires-A play the role of spacer which avoid shorting of the adjacent electrodes. Consequently, employing IDEs as electrodes in the FNG gives a tool for utilizing the d33 mode, thus obtaining the realization of a high-output of nanogenerator. For this FNG with interdigited electrodes, in Eq. (1), the direction of l referred is perpendicular to the finger shape electrodes. The integral length is the distance of the adjacent electrodes. The piezoelectric voltage constant g33 is proportional to the piezoelectric coupling coefficient d33. The working mechanism of fabric nonogenerator with IDEs can be described by the piezoelectric effect between each pair of adjacent electrodes (Fig. 2(b)–(f)). When the device is bended, the piezoelectric fiber in the fabrics should be in tension status, then the direction of the applied force and the direction of the produced dipoles are the same, hence making the active mode as d33. When a high voltage were applied to the IDEs ( the copper wires woven into the fabric) at high temperature (about 70 1C ) in advance, the high electric field along the piezoelectric fiber
would polarize the active piezoelectric materials and produce the dipole component (yellow arrows in Fig. 2(d)) along the direction of the piezoelectric fiber. Once the mechanical bending were applied onto the nanogenerator as shown in Fig. 3(b)–(c), the strain enhances the dipole component which generates the positive and negative piezopotentials at the adjacent electrodes and the piezopotentials induce the flowing of the charge carrier in the external load, as shown in Fig. 2(d)–(e). Subsequently, when the bending of the nanogenerator is released, the dipole component will return to the initial state, the piezopotential will fade away and the charge carriers then flow back and produce the reverse output signals, as shown in Fig. 2(e)–(f).
Application of FNG on the energy harvesting of human body motion To evaluate the output performance, the applications of this piezoelectric generator for energy harvesting of biomechanical
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Fig. 3 The energy harvesting of human body motion by FNG. (a) The FNG attached on an elbow pad which was bend by human arm, (b) A LCD display driven by FNG which converted the biomechanical energy into electric energy, (c) The voltage output of the FNG, (d) the current output of the FNG.
Fig. 4 Switching evaluation of open-circuit voltage and short-circuit current measured from FNG in the forward (a) and reverse (b) connections, indicating that the output signals are based on piezoelectric response.
energy from human motions have been demonstrated. This FNG device was aimed to wearable electronics capable of harvesting energy from human activity, so it was attached onto an elbow
pad which was bended by human arm to evaluate its performance. In this demonstration, the circuit section of the device were protected to prevent direct contact with the skin, which
A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application could bring electrical noise. In the expanding application in the future, if such a fabric structure is used to make shirt capable of generating electricity, the isolation between the electrical circuit and human skin is also required. In the demonstration of this work, the FNG was attached on an elbow pad, as shown in Fig. 3a. In keeping with the bending of human arm, the FNG produce voltage and current outputs which reached up to 1.9 V and 24 nA, respectively, as shown in Fig. 3c and d. The output is sufficient to light a LCD (Seen in Fig. 3b and supplementary video S1). Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2015.02.034. To verify the origin of the nanogenerator's output, a switching test were performed and the testing results were shown in Fig. 4. The opposite sequence of peak occurrence with time according to the forward or reverse connections proved that the output signals originate from the piezoelectricity of BaTiO3 nanowires-polymer fibers. Furthermore, the output voltage of a reference device without piezoelectric active fibers was tested to evaluate the contribution of friction and capacitance effects from materials other than piezoelectric fibers. The results are shown in Fig. 5. The signals are much lower of about 0.02 V compared to the nanogenerator's output voltage of 1.9 V. Accordingly, it is confirmed that the output of nanogenerators mainly comes from the piezoelectric effect of BaTiO3 nanowires-polymer fibers.
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To evaluate the output power of the FNG, an 80 MΩ resistance as external load was connected to the FNG. When the FNG were bended by the human motion, the output power of the FNG was calculated according to following Equation [26, 36]: Z 1 U2 ðtÞ P¼ dt ð2Þ T R where U(t) is the real-time voltage on external load, R is the impedance of the external load, and T is the period of bending and releasing. The testing results indicated that the outpower of the FNG reached 10.02 nW. The results in detail are shown in Fig. S4. In many of previously reported applications, usually there was a direct impact which was applied onto the device to produce electrical energy output. But common human movement rarely produces a direct impact force and more movement were in the form of flexion of arthrosis or the extension and contraction of muscles, so the structure and working mode of the devices which are suitable for these form of human movement is a significant issue for wearable device. The output of this design is not larger than those which have been reported because the optimization of the nanogenerators is not specified in this work and the contact between the copper wires and the piezoelectric fibers is not sufficient close, but the structure of fabric may be more meaningful with great potential as a
Fig. 5 The comparison between the output signals of FNG device and referential device (in which PVC fibers replace the piezoelectric BaTiO3-PVC fibers). (a) Schematics of the structure of FNG and referential device; (b(i)) The open-circuit voltage of referential device, The insets show the magnified output voltage generated by the referential device, (b(ii)) the open-circuit voltage of FNG; (c(i)) The short-circuit current of referential device, The insets show the magnified output current generated by the referential device, (c(ii)) the short-circuit current of FNG.
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nanogenerator in the future. The low textile density of our sample, which were fabricated with a simple hand knitting method, will influence the output of the device due to the insufficient contact between the piezoelectric active fibers and the Cu electrodes. On the other hand, we only fabricated a fabric based nanogenerator with a small size to demonstrate the energy harvesting of human body motion, but the piezoelectric active material and the electrode integrated fabric structure can be used to prepare the whole cloth which can convert the mechanical energy of various human movements into electrical power. Thus, it is expected that the mature textile technology and large scale of device will effectively enhance the output levels of the nanogenerator. From the feasibility of the application, this fabric structure nanogenerators would have a significant potential application.
Conclusion In summary, we have designed, fabricated and characterized a novel fabric nanogenerator consisting of BaTiO3 nanowire-PVC fibers fabric integrated with conducting wires electrodes and insulating spacer cotton threads. The FNG can convert biomechanical motions energy into electricity utilizing the piezoelectric effect. We have demonstrated the FNG that show a high performance and feasibility as wearable power source. By attaching the FNG on the human arm, the open-circuit output voltage, short-circuit current and power on an 80 MΩ external load reached 1.9 V, 24 nA and 10.02 nW, respectively. This all fiber piezoelectric fabric is not very different from any other conventional textile material and it can be easily integrated with cloth to convert the mechanical energy of human motion by natural manner. We believe that these piezoelectric fabrics provides an effective option for the development of high performance energy-harvesting devices.
Acknowledgment This research work was supported by the National Natural Science Foundation of China (no. 51202204), the Fundamental Research Funds for the Central Universities of China (No. 2010121052).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen. 2015.02.034.
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A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application [40] J. Chang, M. Dommer, C. Chang, L. Lin, Nano Energy 1 (2012) 356–371. [41] W. Wu, S. Bai, M. Yuan, Y. Qin, Z.L. Wang, T. Jing, Acs Nano 6 (2012) 6231–6235. [42] L. Gu, N. Cui, L. Cheng, Q. Xu, S. Bai, M. Yuan, W. Wu, J. Liu, Y. Zhao, F. Ma, Y. Qin, Z.L. Wang, Nano Lett. 13 (2012) 91–94. [43] J. Zhong, Y. Zhang, Q. Zhong, Q. Hu, B. Hu, Z.L. Wang, J. Zhou, Acs Nano 8 (2014) 6273–6280. [44] M. Zhang, T. Gao, J. Wang, J. Liao, Y. Qiu, H. Xue, Z. Shi, Z. Xiong, L. Chen, Nano Energy 11 (2015) 510–517. [45] N.Z. Bao, L.M. Shen, A. Gupta, A. Tatarenko, G. Srinivasan, K. Yanagisawa, Appl. Phys. Lett. 94 (2009) 253109. [46] Y.B. Jeon, R. Sood, J.h. Jeong, S.G. Kim, Sens. Actuators A: Phys. 122 (2005) 16–22. [47] A.E. Cohen, R.R. Kunz, Sens. Actuators B: Chem. 62 (2000) 23–29. Min Zhang received her B.S. (2012) in Materials Science and Engineering from Xiamen University. Now she is a M.S. student in college of materials of Xiamen University. Her research focuses on fabrication of piezoelectric nanodevices.
Tao Gao received his B.S. (2013) in Materials Science and Engineering from China University of Geosciences. Now he is a M.S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices.
Jianshu Wang received his B.S. (2013) in Materials Science and Engineering from Fuzhou University. Now he is a M. S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices.
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Quan Yang received his B.S. (2014) in Materials Science and Engineering from China University of Mining & Technology. Now he is a M.S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices, triboelectric nanogenerator.
Hao Xue received his B.S. (2001) in Materials Science and Engineering and Ph.D. (2006) in Materials Science and Engineering from Tsinghua University. From 2012 to 2013, he worked as a visiting scholar and Postdoc at University of Pittsburgh. Currently, he is an associate professor at Department of Materials Science and Engineering, Xiamen University. His research interests include piezoelectric mate rials and devices, functional nanodevice. Zhan Shi received his B.S. (2002) in Materials Science and Engineering and Ph.D. (2007) in Materials Science and Engineering from Tsinghua University. Currently, he is an associate professor at Department of Materials Science and Engineering, Xiamen University. His research interests include piezoelectric materials and devices, magnetoelectric materials and functional nanodevice.
Yang Zhao received his B.S. (2003) in Mechanical Manufacture and Automation and Ph.D. (2009) in Mechanical Manufacture and Automation from Jilin University. Currently, he is an assistant professor at Department of Mechanical and Electrical Engineering, Xiamen University. His research interests include smart materials, flexible electronic devices.
Jianjun Liao received his B.S. (2013) in Materials Science and Engineering from Central South University. Now he is a M. S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices.
Zhaoxian Xiong received his B. S. (1985) and M. S. (1988) from Xi'an Jiaotong University in electronic engineering and received his Ph. D. (1999) from The Hong Kong Polytechnic University in physics. He now is a professor in Materials Science and Engineering at Xiamen University. His research interests include microwave ceramics and devices, piezoelectric materials and photoluminescence materials.
Yingqiang Qiu received his B.S. (2012) in Materials Science and Engineering from Fuzhou University. Now he is a M.S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices.
Lifu Chen received his B. S. (1985) and M. S. (1987) from Dalian University of Technology in materials science and engineering and received his Ph. D. (1992) from The University of Leeds in materials science. He now is a professor in Materials Science and Engineering at Xiamen University. His research interests include high performance ceramics, ceramic fiber and composite materials.
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RAPID COMMUNICATION
A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application Min Zhanga,1, Tao Gaoa,1, Jianshu Wanga, Jianjun Liaoa, Yingqiang Qiua, Quan Yanga, Hao Xuea,n, Zhan Shia, Yang Zhaoc, Zhaoxian Xionga, Lifu Chena,b,nn a
Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China b Key Laboratory of High Performance Ceramic Fibers (Xiamen University), Ministry of Education, Xiamen 361005, China c Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen 361005, China Received 15 December 2014; received in revised form 14 January 2015; accepted 27 February 2015 Available online 13 March 2015
KEYWORDS
Abstract
Piezoelectric composite fiber; Fabric nanogenerator; Mechanical energy harvesting; Interdigited electrodes.
Wearable nanogenerators are vital important for wearable devices and portable electronic devices. Here we report a flexible hybrid piezoelectric fiber based two-dimensional fabric nanogenerator which can be promising to be easily integrated with clothing and convert the mechanical energy of human body motion into electric energy. The hybrid piezoelectric fiber comprised aligned BaTiO3 nanowires and PVC polymer. The PVC polymer made the fiber be enough flexible for performing the woven process and the aligned BaTiO3 nanowires enhanced the piezoelectric properties as active materials. The metal copper wires and cotton threads were woven into the fabric to construct the nanogenerator with interdigited electrodes. By attaching the fabric nanogenerator on an elbow pad which was bended by human arms, the nanogenerator can generate 1.9 V output voltage and 24 nA output current and the output are large enough to power a LCD. & 2015 Elsevier Ltd. All rights reserved.
n
Corresponding author. Corresponding authors at: Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China. E-mail addresses: [email protected] (H. Xue), [email protected] (L. Chen). 1 These authors contributed to the work equally. nn
http://dx.doi.org/10.1016/j.nanoen.2015.02.034 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
Introduction The waste energy harvesting from the ambient environment and biomechanical movement has been considered as an attractive alternative over traditional rechargeable batteries for providing electrical power to low-energy devices such as body worn
A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application sensors and wearable consumer electronics. Piezoelectric nanostructures have attracted extensive attention because they can provide a practical way to scavenge mechanical energy from the environment. They can also be put into use as self-powered sensing devices. a wide variety of mechanical energy have been successfully demonstrated to produce electricity, such as wind [1–4], water waves [5–8], vibration, [9,10] ultrasonic waves, [11,12] and human body movement, [13–15] and various piezoelectric materials, such as ZnO [11,16–19], CdS [20], InN [21], ZnSnO3 [22,23], NaNbO3 [24], KNbO3 [25], PZT [26–29], PMN-PT [30], BaTiO3 [31–34], NaxK1 xNbO3 [35], yBa(ZrxTi1 x)O3-(1-y) (BaxCa1 x)TiO [36,37], GaN [38] and PVDF [12,39,40] have been employed for fabricating nanogenerators. In the various forms of energy harvesting, the wearable generator may be most attractive, since it is feasible and compatible way to power the portable electronic devices, biomedical sensors, and so on. However, it should be concerned that inorganic piezoelectric materials are usually very brittle, and can only work in the case of small level of strain. These shortcoming makes it hard to be integrated on a large scale wearable devices which mostly need to adapt to curved surface and large level of strain. Fiber-based electric power generators are highly promising as they are light and comfortable to wearers. By the electrospinning technique, a number of piezoelectric materials, such as Pb[ZrxTi1x]O3[26, 4142], NaxK1 xNbO3 [35], yBa(ZrxTi1 x)O3 (1y)(BaxCa1 x) TiO3 [36,37] have been developed to be fabricated into nanofibers or nonwovens which were used to prepare wearable nanogenerators, but they are still needed to be imbedded into polymer to overcome the brittleness. Up to now, most wearable nanogenerators were embedded inside a fabric or attached on the skin of human body to perform the energy harvesting process. For wearable application, energy harvesting device should provide the comfort and natural feel to the wearer, so the better manner of the device should be integrated in the textile as part even whole of the cloth. Ideally, if the energy harvesters were implemented as textile-fiber structures, such fibers would provide perfect building elements for a smart shirt, as they could be naturally integrated into fabrics during the weaving process without affecting comfort, flexibility, air permeability and maneuverability. Wang et al. have reported generators based on textile fibers which produce power output by the friction motion [3] and electrostatic effect [43] of the fibers. In order to provide more energy harvesting solutions, we have explored the possibility of flexible and durable piezoelectric generators based on simple procedure and low cost strategies. Here, we developed a twodimensional fabric-like nanogenerator based on three kinds of fibers including BaTiO3 nanowire-PVC hybrid piezoelectric fibers, conventional cotton threads and copper wires. The piezoelectric properties of the BaTiO3 nanowire-PVC fibers were effectively improved by the inorganic BaTiO3 active nanowires. The details of results and analysis have been reported by our previous research works [44]. The copper wires served as the electrodes which seems to be of interdigited structure, while the cotton threads were employed as spacer to avoid the shorting of the electrodes as well as to enhance the durability of whole device. This structure makes the electrodes be naturally integrated in the fabric so that the entire device look no different from the conventional fabrics. This fiber based 2-D fabric nanogenerator with excellent strength and super flexibility is capable of converting mechanical energy of human
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body motion into electrical energy. The device has been demonstrated to power a LCD display.
Experimental The fabrication of highly o0014 oriented BaTiO3 nanowire-PVC microfibers The fabrication process of BaTiO3-polymer fibers have been introduced and discussed in detail in our previous work [44]. In order to present the clear structure and fabricating process of fabric nanogenerator based on the BaTiO3 nanowires-polymer fibers, here we briefly described it again. First, BaTiO3 nanowires were prepared by the topochemical method [44,45]. The mixture of K2CO3 and TiO2 in the molar ratio of 1:3 was heated at 1000 1C for 18 h. Then, the as-synthesized product was washed with deionized water to remove the K2CO3 and dried at 80 1C for 8 h and meanwhile the intermediate products of K2Ti4O9 nanowires were obtained. BaTiO3 nanowires were finally obtained by hydrothermal ion-exchange conversion of the K2Ti4O9 nanowires in 0.2 M Ba(OH)2 solution at 100–180 1C for 12 h. The obtained BaTiO3 nanowires and polyvinyl chloride (PVC) powder according to the weight ratio of 1:2 were dispersed in dimethylacetamide (DMA) and were stirred by magnetic stirrers for 1 h. The BaTiO3 nanowire-PVC composite fibers were fabricated by the spinning method. The sketch of the spinning equipment are shown in Fig. S1. The mixed slurry were carried in the container and the slurry were extruded through a spinneret which has a single hole of 250 μm, the polymer fiber was coagulated in the coagulation bath containing DMF/distilled water at room temperature. Drawing was performed in boiling water through the speed control of the two rollers. Finally, consecutive BaTiO3 nanowirepolymer fibers as fine as 60–70 μm were fabricated.
The fabrication of flexible fabric nanogenerator (FNG) The BaTiO3 nanowire-PVC fibers were used to fabricate flexible fabric nanogenerators. The source materials for the FNG are the BaTiO3 nanowires-polymer microfiber in the warp direction, the copper wires as electrodes in the weft direction and the cotton threads as the insulating spacers of the electrodes. The fabrics were fabricated via hand knitting using above source materials, then the fabrics was fixed on a PET plastic substrate. It is noted that only the four sides of the fabric is fixed on the substrate and the main part of the fabrics maintain a natural contact with the substrate. Two Cu wires which were used as extraction electrodes were also fixed on the substrate and connected the Cu wires in the fabrics to form interdigitated electrodes configuration. Then, the BaTiO3 nanowire-polymer microfiber in the fabrics were poled by applying an electric field of 4 kV/mm across the interdigited electrodes at a temperature of about 70 1C for 20 h. As comparison, a referential device, in which the simple PVC polymer fibers replaced the BaTiO3 nanowires-PVC fibers, was also fabricated using the same process.
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Characterization and application The morphology and structure of the samples was observed by SEM (Hitachi SU70) and TEM (Joel 2100). The tensile strength of the BaTiO3 nanowire-PVC composite fiber were measured by universal materials tester (Galdabini SUN 2500). The fabric piezoelectric nanogenerator was demonstrated to harvesting the energy output of human body's movement. The fabric nanogenerator was attached on an elbow pad and converted the mechanical energy of human body motion into electrical power. The output voltage and current were measured by Keithley source meter 2400. The nanogenerator were connected with LCD to demonstrate the application of powering an electronic device. An 80 MΩ resistance as external load was connected to the NG to evaluate the power output of the nanogenerator.
Result and discussion The fabrication and structure of the FNG The FNG were fabricated using BaTiO3 nanowire-PVC piezoelectric fibers. The BaTiO3 nanowire-PVC piezoelectric fibers were fabricated by spinning method and the details have been described in our previous work [44]. The structure and morphology of the hybrid fibers are shown in Fig. S2. In Ref. [44] and supplementary file, the structure and advantages of the fibers have been discussed in detail. The BaTiO3 nanowires-polymer fiber is very flexible, robust and even stretchable. These merits can overcome the brittleness of conventional piezoelectric ceramic fibers and will greatly benefit the fabrication of wearable piezoelectric nanodevices. The tensile strength of 54 MPa (Seen in Fig. S3) of the composite fiber provided a guarantee for subsequent fabricating and working processes of the fabric nanogenerator. Up to now, the most way to integrate a piezoelectric material into the wearable device is to embed the piezoelectric materials into polymer matrix or to transfer the brittle nanowires onto a flexible substrate. For energy harvesting from human movement, the fiber based electrical power generators are highly desirable as they are light, comfortable and look similar to the conventional fabrics. The combination of piezoelectric materials in fabrics provides a facile route for the
M. Zhang et al. fabrication of flexible piezoelectric generators. The fabric structures can be designed so as to offer piezoelectric output. The fabric piezoelectric generator were prepared with commodity cotton threads spacers between the copper wires electrodes in the weft direction and the BaTiO3 nanowire-PVC fibers in the warp direction. Fig. 1 shows the photo and schematic of the FNG. In this work, the sample was fabricated through a simple hand-knitting process.
Proposed power generation mechanism of the fiberbased generators Now we briefly discuss the power generation mechanism. Significant factor in piezoelectricity is the relationship between the applied mechanical stress and the generated charge, defined as a piezoelectric charge constant dij, where the first i and second j subscripts represent the directions of the poled dipoles and applied force, respectively [46,47]. The d31 and d33 modes are widely used for piezoelectric applications, in which the two modes are differentiated according to whether the direction of the generated electric signals is perpendicular (d31) or parallel (d33) to the applied stress/strain direction. Furthermore, the d33 constant is approximately twice larger than d31. The open-circuit voltage (V3j) generated when the piezomaterials are deformed by mechanical strain (εxx) can be expressed as Z g3 j εð1ÞEd1j ð1Þ V out ¼ where g3j is the piezoelectric voltage constant (g3j = d3j/εT, εT denotes the permittivity under a constant strain), ε(l) is the strain, E is the elasticity modulus and the integral of lj is the distance between the electrodes. Since the d33 and g33 constants are much larger than d31 and g31, respectively, constructing a d33 working mode should be helpful to produce higher outputs for generators. Fig. 2 illustrates the operating mechanism of the FNG. As shown in Fig. 2(a), the copper wires-A which were woven into the fabric play the role of the internal electrodes and the copper wires-B were set as extraction electrodes, so both copper wires A and B together construct a structure of interdigited electrodes (IDEs). The cotton threads between
Fig. 1 The photo and schematic of the fabric nanogenerator (FNG). (a) The photo of the FNG, (b) The structure of the FNG.
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Fig. 2 Schematics of the operating mechanism of the FNG. (a) The structure of the FNG, (b) The FNG were deformed by the mechanical bending along the direction of piezoelectric fibers, (c) the FNG under unbending status, (d) The FNG after poling process; the dipole direction is along the piezoelectric fiber, (e) under the mechanical bending, the enhanced dipole result in that the positive and negative piezopotentials are produced at adjacent electrodes which lead to the carriers flow, (f) under unbending status, the potential fade away and the carriers flow back.
the copper wires-A play the role of spacer which avoid shorting of the adjacent electrodes. Consequently, employing IDEs as electrodes in the FNG gives a tool for utilizing the d33 mode, thus obtaining the realization of a high-output of nanogenerator. For this FNG with interdigited electrodes, in Eq. (1), the direction of l referred is perpendicular to the finger shape electrodes. The integral length is the distance of the adjacent electrodes. The piezoelectric voltage constant g33 is proportional to the piezoelectric coupling coefficient d33. The working mechanism of fabric nonogenerator with IDEs can be described by the piezoelectric effect between each pair of adjacent electrodes (Fig. 2(b)–(f)). When the device is bended, the piezoelectric fiber in the fabrics should be in tension status, then the direction of the applied force and the direction of the produced dipoles are the same, hence making the active mode as d33. When a high voltage were applied to the IDEs ( the copper wires woven into the fabric) at high temperature (about 70 1C ) in advance, the high electric field along the piezoelectric fiber
would polarize the active piezoelectric materials and produce the dipole component (yellow arrows in Fig. 2(d)) along the direction of the piezoelectric fiber. Once the mechanical bending were applied onto the nanogenerator as shown in Fig. 3(b)–(c), the strain enhances the dipole component which generates the positive and negative piezopotentials at the adjacent electrodes and the piezopotentials induce the flowing of the charge carrier in the external load, as shown in Fig. 2(d)–(e). Subsequently, when the bending of the nanogenerator is released, the dipole component will return to the initial state, the piezopotential will fade away and the charge carriers then flow back and produce the reverse output signals, as shown in Fig. 2(e)–(f).
Application of FNG on the energy harvesting of human body motion To evaluate the output performance, the applications of this piezoelectric generator for energy harvesting of biomechanical
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Fig. 3 The energy harvesting of human body motion by FNG. (a) The FNG attached on an elbow pad which was bend by human arm, (b) A LCD display driven by FNG which converted the biomechanical energy into electric energy, (c) The voltage output of the FNG, (d) the current output of the FNG.
Fig. 4 Switching evaluation of open-circuit voltage and short-circuit current measured from FNG in the forward (a) and reverse (b) connections, indicating that the output signals are based on piezoelectric response.
energy from human motions have been demonstrated. This FNG device was aimed to wearable electronics capable of harvesting energy from human activity, so it was attached onto an elbow
pad which was bended by human arm to evaluate its performance. In this demonstration, the circuit section of the device were protected to prevent direct contact with the skin, which
A hybrid fibers based wearable fabric piezoelectric nanogenerator for energy harvesting application could bring electrical noise. In the expanding application in the future, if such a fabric structure is used to make shirt capable of generating electricity, the isolation between the electrical circuit and human skin is also required. In the demonstration of this work, the FNG was attached on an elbow pad, as shown in Fig. 3a. In keeping with the bending of human arm, the FNG produce voltage and current outputs which reached up to 1.9 V and 24 nA, respectively, as shown in Fig. 3c and d. The output is sufficient to light a LCD (Seen in Fig. 3b and supplementary video S1). Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2015.02.034. To verify the origin of the nanogenerator's output, a switching test were performed and the testing results were shown in Fig. 4. The opposite sequence of peak occurrence with time according to the forward or reverse connections proved that the output signals originate from the piezoelectricity of BaTiO3 nanowires-polymer fibers. Furthermore, the output voltage of a reference device without piezoelectric active fibers was tested to evaluate the contribution of friction and capacitance effects from materials other than piezoelectric fibers. The results are shown in Fig. 5. The signals are much lower of about 0.02 V compared to the nanogenerator's output voltage of 1.9 V. Accordingly, it is confirmed that the output of nanogenerators mainly comes from the piezoelectric effect of BaTiO3 nanowires-polymer fibers.
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To evaluate the output power of the FNG, an 80 MΩ resistance as external load was connected to the FNG. When the FNG were bended by the human motion, the output power of the FNG was calculated according to following Equation [26, 36]: Z 1 U2 ðtÞ P¼ dt ð2Þ T R where U(t) is the real-time voltage on external load, R is the impedance of the external load, and T is the period of bending and releasing. The testing results indicated that the outpower of the FNG reached 10.02 nW. The results in detail are shown in Fig. S4. In many of previously reported applications, usually there was a direct impact which was applied onto the device to produce electrical energy output. But common human movement rarely produces a direct impact force and more movement were in the form of flexion of arthrosis or the extension and contraction of muscles, so the structure and working mode of the devices which are suitable for these form of human movement is a significant issue for wearable device. The output of this design is not larger than those which have been reported because the optimization of the nanogenerators is not specified in this work and the contact between the copper wires and the piezoelectric fibers is not sufficient close, but the structure of fabric may be more meaningful with great potential as a
Fig. 5 The comparison between the output signals of FNG device and referential device (in which PVC fibers replace the piezoelectric BaTiO3-PVC fibers). (a) Schematics of the structure of FNG and referential device; (b(i)) The open-circuit voltage of referential device, The insets show the magnified output voltage generated by the referential device, (b(ii)) the open-circuit voltage of FNG; (c(i)) The short-circuit current of referential device, The insets show the magnified output current generated by the referential device, (c(ii)) the short-circuit current of FNG.
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nanogenerator in the future. The low textile density of our sample, which were fabricated with a simple hand knitting method, will influence the output of the device due to the insufficient contact between the piezoelectric active fibers and the Cu electrodes. On the other hand, we only fabricated a fabric based nanogenerator with a small size to demonstrate the energy harvesting of human body motion, but the piezoelectric active material and the electrode integrated fabric structure can be used to prepare the whole cloth which can convert the mechanical energy of various human movements into electrical power. Thus, it is expected that the mature textile technology and large scale of device will effectively enhance the output levels of the nanogenerator. From the feasibility of the application, this fabric structure nanogenerators would have a significant potential application.
Conclusion In summary, we have designed, fabricated and characterized a novel fabric nanogenerator consisting of BaTiO3 nanowire-PVC fibers fabric integrated with conducting wires electrodes and insulating spacer cotton threads. The FNG can convert biomechanical motions energy into electricity utilizing the piezoelectric effect. We have demonstrated the FNG that show a high performance and feasibility as wearable power source. By attaching the FNG on the human arm, the open-circuit output voltage, short-circuit current and power on an 80 MΩ external load reached 1.9 V, 24 nA and 10.02 nW, respectively. This all fiber piezoelectric fabric is not very different from any other conventional textile material and it can be easily integrated with cloth to convert the mechanical energy of human motion by natural manner. We believe that these piezoelectric fabrics provides an effective option for the development of high performance energy-harvesting devices.
Acknowledgment This research work was supported by the National Natural Science Foundation of China (no. 51202204), the Fundamental Research Funds for the Central Universities of China (No. 2010121052).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen. 2015.02.034.
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Tao Gao received his B.S. (2013) in Materials Science and Engineering from China University of Geosciences. Now he is a M.S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices.
Jianshu Wang received his B.S. (2013) in Materials Science and Engineering from Fuzhou University. Now he is a M. S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices.
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Quan Yang received his B.S. (2014) in Materials Science and Engineering from China University of Mining & Technology. Now he is a M.S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices, triboelectric nanogenerator.
Hao Xue received his B.S. (2001) in Materials Science and Engineering and Ph.D. (2006) in Materials Science and Engineering from Tsinghua University. From 2012 to 2013, he worked as a visiting scholar and Postdoc at University of Pittsburgh. Currently, he is an associate professor at Department of Materials Science and Engineering, Xiamen University. His research interests include piezoelectric mate rials and devices, functional nanodevice. Zhan Shi received his B.S. (2002) in Materials Science and Engineering and Ph.D. (2007) in Materials Science and Engineering from Tsinghua University. Currently, he is an associate professor at Department of Materials Science and Engineering, Xiamen University. His research interests include piezoelectric materials and devices, magnetoelectric materials and functional nanodevice.
Yang Zhao received his B.S. (2003) in Mechanical Manufacture and Automation and Ph.D. (2009) in Mechanical Manufacture and Automation from Jilin University. Currently, he is an assistant professor at Department of Mechanical and Electrical Engineering, Xiamen University. His research interests include smart materials, flexible electronic devices.
Jianjun Liao received his B.S. (2013) in Materials Science and Engineering from Central South University. Now he is a M. S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices.
Zhaoxian Xiong received his B. S. (1985) and M. S. (1988) from Xi'an Jiaotong University in electronic engineering and received his Ph. D. (1999) from The Hong Kong Polytechnic University in physics. He now is a professor in Materials Science and Engineering at Xiamen University. His research interests include microwave ceramics and devices, piezoelectric materials and photoluminescence materials.
Yingqiang Qiu received his B.S. (2012) in Materials Science and Engineering from Fuzhou University. Now he is a M.S. student in college of materials of Xiamen University. His research focuses on fabrication of piezoelectric nanodevices.
Lifu Chen received his B. S. (1985) and M. S. (1987) from Dalian University of Technology in materials science and engineering and received his Ph. D. (1992) from The University of Leeds in materials science. He now is a professor in Materials Science and Engineering at Xiamen University. His research interests include high performance ceramics, ceramic fiber and composite materials.