- Email: [email protected]
Powerful curved piezoelectric generator for wearable applications
Nano Energy (]]]]) ], ]]]–]]]
1
Available online at www.sciencedirect.com
3 5 journal homepage: www.elsevier.com/locate/nanoenergy
7 9
RAPID COMMUNICATION
11 13
Q2
15
Powerful curved piezoelectric generator for wearable applications
17 19 Q1
21 23
Woo-Suk Junga, Min-Jae Leea, Min-Gyu Kangb, Hi Gyu Moona, Seok-Jin Yoona, Seung-Hyub Baeka,c,n, Chong-Yun Kanga,d,n a
Electronic Materials Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea b Department of Mechanical Engineering, Virginia Tech, VA 24061, United States c Department of Nanomaterials Science and Technology, University of Science and Technology (UST), Daejeon 305-333, Republic of Korea d KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea
25 27 29 31
Received 24 November 2014; received in revised form 31 December 2014; accepted 22 January 2015
33 35 37 39 41 43 45 47 49 51
KEYWORDS
Abstract
Energy harvester; Piezoelectric; Wearable
With the widespread use of wearable electronics, the flexible piezoelectric energy harvesting devices have been extensively studied to efficiently convert the physical motion of the human body into electrical energy. The major obstacles for realizing a flexible piezoelectric generator include the insufficient output power generation and the poor efficiency at the low-frequency regime. Here, we demonstrate a curved piezoelectric generator favorable for wearable applications, generating a high output power. The curved structure plays a key role to improve the power generation, by effectively distributing the applied force across the piezoelectric layer, as well as to allow operation at the low frequency vibration range. Accordingly, this generator produces 120 V of peak output voltage and 700 mA of peak output current during a cycle. Furthermore, our generator can operate at low frequencies below 50 Hz, generating 55 V of output voltage and 250 mA of output current at 35 Hz, and it even works at frequencies as low as 1 Hz. With this generator, we successfully lit up 476 commercial LED bulbs. In addition, we experimentally demonstrate the possibility that the generator can be used in shoes, watches, and clothes as a power source. Our results will provide a framework to enhance the output power of conventional piezoelectric generators, and open a new avenue for realization of self-powered systems, such as wearable electronic devices. & 2015 Published by Elsevier Ltd.
53 55 Q3
57
n
Corresponding authors at: Electronic Materials Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea. Tel.: +82 2 958 6722(5382); fax: +82 2 958 6720. E-mail addresses: [email protected] (S.-H. Baek), [email protected] (C.-Y. Kang).
59 61
63 65 67
http://dx.doi.org/10.1016/j.nanoen.2015.01.051 2211-2855/& 2015 Published by Elsevier Ltd.
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
69
2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61
W.-S. Jung et al.
Introduction With recent advances in electronic technology, miniaturization, flexibility, and low power consumption have become the developmental trend of electronic devices and portable electronics that are commonly and essentially used in our daily lives, including (among others) smart phones, electronic watches, smart glasses, and wireless headsets.[1–3] However, the use of a battery in such devices has been one of their major implementation problems due to its large size, the insufficient capacity, the danger of explosion, and the inconvenience of recharging.[4,5] For mobile electronics, the integration of a wearable piezoelectric energy harvesting device, where the physical motion of the body can be transformed into electrical power, would be the most promising way to solve these issues, realizing the nature of the sustainable and portable energy source. However, the output power of the energy harvesters still needs to be further improved for such applications. The performance of the piezoelectric energy harvesters is determined mainly by two factors: (1) the electromechanical coupling property of the piezoelectric material, and (2) the device geometry for the efficient conversion of the mechanical energy into electrical energy. The most distinct features of the mechanical motion of the human body are its low frequency nature and the large displacements associated with it. To effectively utilize such mechanical energy, the wearable energy harvester should be equipped with the polymer-based, flexible materials, such as polyvinylidene difluoride (PVDF) [6–8], rather than the rigid ceramic-based materials, such as ZnO, [9,10] BaTiO3, [11,12] and PZT [13–16]. They have been attempted to apply to wearable applications such as a backpack and shoes, using PVDF [17,18]. However, the piezoelectric properties of PVDF are much poorer compared with that of the conventional ceramic-based materials, albeit PVDF is the best piezoelectric material among all polymers, due to its overall performance Q6 characteristics. Therefore, using PVDF requires more critical for the design of the high-performance, wearable piezoelectric energy harvester. The key aspect to the design of the structure of the piezoelectric generators is to maximize the developed stress/strain in the piezoelectric layer due to the externally applied force. This is especially critical for the flexible piezoelectric generator where the strain/stress response may be localized due to the intrinsic softness of the material that can, in turn, limit the amount of electricity generated by the piezoelectric material. Therefore, it is highly desirable to design the structure of the flexible piezoelectric generator in a way that the external force can be well-distributed across the entire piezoelectric layer to maximize the total power generation. Here, we propose a curved, flexible piezoelectric generator to improve the stress/strain distribution across the piezoelectric layer using computational simulations, and to demonstrate the high-power generation of the curved piezoelectric energy harvester. As a model system, we used the PVDF as the piezoelectric layer, integrated on the curved polyimide (PI) platform. The output signal of the generator is rectified by a full-wave bridge diode, and we obtained peak output voltage, output current density, and power density values of 155 V, 700 mA, and 3.9 mW/cm2, respectively, using finger tapping. Moreover, the curved structure enables the generator to operate, harnessing low frequency vibrations below 50 Hz. We implement our curved piezoelectric generator in insoles and a
watch-strap to demonstrate feasibility for wearable applications. In addition, we also examined the generator attaching on elbow to directly harvest electrical energy from body movements and on chest around heart as a heartbeat sensor. In the proposed design, the curved structure plays an important role in enhancing output power because of an effective applied stress distribution. The experimental output performances demonstrated that this work may lead to an innovative way in which wearable technology harvests energy from the physical activity of living creatures.
63 65 67 69 71 73
Working mechanism and experimental
75
Figure 1 shows the structure and working mechanism of the curved piezoelectric generator. It consists of two separate curved piezoelectric generators connected back-to-back, where each generator comprises a curved PI substrate and two piezoelectric materials. As shown in Figure 1(a), piezo 1 and piezo 2 are made of PVDF with electrodes attached on both sides of the curved PI substrate, labeled as ‘curved piezoelectric generator 1’, whereas the ‘curved piezoelectric generator 2’ with piezo 3 and piezo 4 is located on the other side. Here, the curved piezoelectric generator with top/bottom electrodes uses the d31 mode, also extensively used for other piezoelectric applications. In this mode, the direction of the induced electric field is perpendicular to the direction of the applied stress/strain. Therefore, the induced voltage of the curved piezoelectric generator can be computed in accordance to Eq (1), [19] V 3j ¼ σ j g3j Lj ½V
ð1Þ
where σj is the mechanical stress, g3j is the piezoelectric voltage coefficient, and Lj is the distance between electrodes. Notation V3j is the induced voltage in the 3-direction caused by a stress in the j-direction. In case of the curved piezoelectric generator, it uses d31 mode, thus L1 is the thickness of the PVDF and dominant direction is 1 direction (x). Additionally, g3j is defined by Eq. (2), ð2Þ g3j ¼ d 3j =ε0 εT Vm=N where ε0 is the permittivity of free space and εT indicates the permittivity under a constant strain. In the structure of the curved piezoelectric generator, the curved PI substrate plays two important roles. First, it acts as a passive layer to be effectively subjected to the vertical force to the PVDF layer because the PVDF is too thin to receive it as well as has a low Young's modulus. Therefore, the thickness of the PI substrate should be thick enough to shift the neutral plane of the structure out of the piezoelectric layer (Supporting information, Figure S1). However, if it is too thick, it becomes more rigid, so it requires a larger force to generate the electric power. Second, it acts as an active layer that turns the deformed piezoelectric layer into its original shape, much like a spring. Also, it enables the piezoelectric material to be subject to only tensile or compressive stress in a whole volume during pressing and releasing (Supporting information, Figure S1), which results in an enhancement of the output power. This is because the proper thickness of the substrate makes the neutral plane move from PVDF to its inside.
77 79 81 83 85 87 89 91 93 95 97 99 101 103 105 107 109 111 113 115 117 119 121 123
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
Powerful curved piezoelectric generator
3
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19
81
21
83
23
85
25
87
27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61
Figure 1 Structure and working mechanism of the curved piezoelectric generator. (a) Initial state. (b, c) Charge distribution during pressing. (d, e) Charge distribution during force release. (f) 3D schematic view of the curved piezoelectric generator and wire connection of strained piezoelectric generator 1 using bridge diode.
In addition, the curved structure also intensifies the applied stress on the PVDF layer. Normally, when the external force is applied in the middle of a PVDF film without a substrate, it pushes the surface straight down, causing it to locally bend due to its large flexibility. However, the PI substrate enables the PVDF to uniformly increase the applied stress on the entire surface of the material. Using COMSOL simulations, we have compared the output voltages of the curved generator without and with the presence of the 200 mm PI substrate layer (Supporting information, Figure S2). As a result, the generator with a substrate produces a much higher output voltage than the one without the substrate, when the same displacement is experienced in the middle of the generator's surface. This is because the generator without the substrate was subjected to both compressive and tensile stresses at once, thus generating a much smaller potential between the electrodes. In addition, the thicker the PI substrate is, the higher the produced output voltage will be because it enables to receive a higher induced stress on a whole volume of the PVDF (Supporting information, Figure S3). However, the thicker substrate requires larger external forces to deform, thus considering a target application. Furthermore, we compared the curved generator and a flat-surface generator under 1 cm displacement condition (Supporting information, Figure S4). Accordingly, the curved generator produces a higher voltage than the flat type, because the curved structure concentrates the applied stress in the middle of the PVDF. Thus, this analysis indicates that the use of the substrate attached to a piezoelectric film is able to strongly enhance an output power. Figure 1(a)–(e) shows the working mechanism of the curved piezoelectric generator during a single press-and-release cycle.
At the initial state before the mechanical force is applied [Figure 1(a)], there is no electric potential, and thus no charge is transferred. When the external mechanical force is applied on the generator, piezo 1 and piezo 2 with the same poling direction start to be subjected to the compressive and tensile stresses, respectively, resulting in appearance of free charges on the electrodes. Until the piezo 1 touches the piezo 2, free charges on the electrodes of piezo 1 increase and free charges on piezo 2 decrease, as shown in Figure 1(b). Therefore, they produce different output signals each other. This is due to the opposite induced stresses and the same poling direction in piezo 1 and piezo 2. Accordingly, electrons also start to flow from Au1 to Au2 and from Au4 to Au3. When the curved piezoelectric generator 1 touches generator 2, maximum mechanical deformation is elicited [Figure 1(c)], thus generating maximum output power. After this moment, output power slowly reduces, and finally reaches the equilibrium state at full force release. When the applied force is initially released [Figure 1(d)], the pressed arc structures start to bounce back. Unlike the pressing action, piezo 1 and piezo 2 experience the tensile and compressive stresses, respectively, and the electric potentials induced are reversed compared to the press state. Therefore, electrons flow in the reverse directions from Au2 to Au1 and from Au3 to Au4. When the shape of the generator returns to the initial state, it produces the maximum output power again and then decreases, until potentials of piezo1 and piezo 2 disappear. A full cycle is then completed. Similar to the curved piezoelectric generator 1, the curved piezoelectric generator 2, including the piezo 3 and piezo 4, operates along the same working mechanism during the full cycle. Furthermore,
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
89 91 93 95 97 99 101 103 105 107 109 111 113 115 117 119 121 123
4 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29
W.-S. Jung et al.
during periodic pressing-and-releasing cycles, since the curved piezoelectric generator is bent along with the curved PI substrate, it experiences 0.001613% of the identical strain, which can be estimated by neutral plane, distance from the center of PVDF to neutral plane, the length and the radius of the curved generator, and the height of the arc (Supporting information, Figure S5). This applied strain is structurally consistent in periodic multiple cycles. Figure 1(f) shows the wire connection for the curved piezoelectric generator 1, considering current flow. Supporting Figure S6 presents the whole wire connection and measurement configuration in detail. For fabricating the curved piezoelectric genrator, we purchased the PVDF with 0.1 mm of thickness from Fils Co. Ltd., and the PI films with 0.2 mm of thickness from EDS system Inc. Supporting Figure S7 shows the conformation of PVDF polymer chain and a unit cell structure of PVDF crystal. Au electrodes with 200 nm of thickness were deposited on the PVDF surfaces by an electron beam evaporator using on-axis mode with a deposition rate of 0.2 nm/s, 5 10 6 Torr of working pressure, 4.3 kV of power, and 16.4 mA of current. The total size of the curved piezoelectric generator was 7 4 cm2 and its thickness was 0.6 mm. The DPO 4014B Oscilloscope (Tektronix) and current amplifier (Stanford Research SR 570) were utilized for measuring output voltages and output currents, respectively. The performance of the generator in low frequency vibrations was tested under amplitude of 1.2 mm and a velocity of 264 mm/s by the 4809 Vibration Exciter (Brüel & Kjær), the 2718 Power Amplifier (Brüel & Kjær) and the WF 1944B Multifunction Synthesizer (NF Corporation).
31 33
Results and discussion
35
Figure 2 shows the output performances of the curved piezoelectric generator, rectified by a full-wave bridge diode. To obtain enhanced output current, all PVDFs are connected in parallel to increase output current as shown in supporting Figure S6. Using finger tapping, we found that the peak output voltage and peak output current reached 155 V and 700 mA ( 25 mA/cm2 of current density), respectively, as shown in Figure 2(a,b) (see output voltages and currents from piezo 1 to piezo 4 in Supporting information, Figure S8). The instantaneous power density was estimated to be 3.9 mW/cm2, which is the most powerful output to date among all the flexible piezoelectric energy harvesters using PVDF. Some applications require a fast charging and discharging process or reach a higher voltage at a certain time instance. In view of this practical requirement, a capacitor should be chosen first as an energy storage system. Thus, we experimentally investigated the charging energy using three capacitors with different capacitances as shown in Figure 2(c). We found that the 1 mF capacitor was charged to 45 V in 7 s under periodic pressing-and-releasing. In the case of the 10 mF and the 47 mF capacitors, charging to 10 V and to 5 V was achieved in 4 s and 10 s, respectively. Therefore, the curved piezoelectric generator is powerful as an energy harvesting device for power sources that employ mechanical force. With the power output generated from pressingand-releasing motions, we successfully lit up 476 commercial LED bulbs. Figure 2(d) shows the snapshots before and during the moment of LED bulb lit up (Supporting information, movie 1). For practical applications, the device is developed to fully
37 39 41 43 45 47 49 51 53 55 57 59 61
harvest the randomly and naturally available mechanical energy, rather than requiring regular motions executed on purpose. Thus, both the output voltage and current values of the curved piezoelectric generator were verified at low frequencies, below 50 Hz, as shown in Figure 2(e) and (f). Consequently, based on these results, the generator produced 45 V of average output voltage and 225 mA of average output current at 35 Hz, which are the maximum attained values, because 35 Hz represents the generator's natural frequency. In addition, we can control the natural frequency of the curved piezoelectric generator using a load, attaching it onto the top of the curved structure. As a result, even at 1 Hz, we showed the generator with a mass of 18 g as a load produced 9 V of output voltage (Supporting information, Figure S9). However, fluctuations in the output power and the AC output current should be of concern in practical applications. Therefore, we configured a battery management circuit connected to the curved piezoelectric generator as a power-supply system. It consists of a rectifier and a battery management chip. Figure 2(g) shows the battery management circuit for the energy harvesting device, using the bq25504 chip (Texas Instruments) that continuously supplies a constant output voltage of 3.3 V. As shown in Figure 2(h), it can deliver a constant DC output voltage of 3.3 V within 10 s after the generator starts to operate, at a frequency of 35 Hz. This demonstrates that the curved piezoelectric generator is able to powerfully operate at a low frequency, and our system can provide a continuous DC power, adequate to drive various commercial electronic devices. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2015.01.051. Since our generator was aimed towards wearable electronics capable of harvesting electrical energy at low frequencies, such as during human activity and body movements, we chose two applications: the watch and shoes. In order for a device to be wearable, the generator should be integrated without a loss of comfort, or a radical change in design. Therefore, we changed the size of the curved piezoelectric generator that is suitable to be integrated into an application. Figure 3 shows the watch application in which one layer of the curved piezoelectric generator (1.2 16 cm2) was attached onto the watch-strap [Figure 3(a)]. To confirm the principle of energy harvesting from the watch, we suggested its testing for six motions that occur frequently in daily life, namely, twisting (motion 1), bending of the wrist (motion 2), pivoting at the elbow (motion 3), running (motion 3), tapping the watch (motion 5), and grabbing the watch (motion 6) [Figure 3(b)]. As shown in Figure 3(c), most motions produced over 5 V of output voltage, especially, during tapping and grabbing, where the output voltage reached 22 V. Furthermore, the output current of all motions was over 5 mA, and motions 5 and 6 produced 50 mA and 20 mA, respectively (Supporting information, Figure S10). Recently, although smart watches have been released with new designs and functions, it still requires a large capacity battery or a secondary power source to prolong its operating time. We expect that this approach will be utilized for smart watches to increase their operating time. Figure 4 shows a shoe application in which the curved piezoelectric generator (7 4 cm2) is attached on the insole. For the rectification, the output signals were connected to a fullwave bridge diode chip, which is attached on the insole. We evaluated its output power while a 68 kg human was walking and running. During low frequency walking that is close to
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101 103 105 107 109 111 113 115 117 119 121 123
Powerful curved piezoelectric generator
5
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19
81
21
83
23
85
25
87
27
89
29
91
31
93
33
95
35
97
37
99
39
101
41
103
43
105
45
107
47
109
49
111
51
113
53 55 57 59 61
115 Figure 2 Output performance and characteristics of the curved piezoelectric generator. (a, b) Output voltage and current generated by the finger tapping motion (insets: enlarged view). (e) Charging voltage versus time responses of three capacitors with different capacitances. (d) Snapshots of the 476 LEDs before and during the moment of being lit up. (e, f) Plots of output voltage and current produced at, or below 50 Hz. (g) Battery management circuit for energy harvesting device. (h) Plot of the temporal output voltage response of the system that reaches a constant voltage value of 3.3 V.
0.5 Hz we obtained 25 V of average output voltage and 20 mA of average output current. Furthermore, during running, 40 V of average output voltage and 47 mA of average
output current were produced. We expect that it can produce a much higher output power when we use a multi-layered structure and multiple devices in the shoes. We expect that this
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
117 119 121 123
6
W.-S. Jung et al.
1
63
3
65
5
67
7
69
9
71
11
17 19 21
b
40 30 20 10 0 0
5
23
10
15
20
Rectified Open-Circuit Voltage (V)
15
Rectified Open-Circuit Voltage (V)
13
73 50
50
75 40
77 30
79 20
81
10
83
0 0
5
Time (sec)
10
15
20
85
Time (sec)
25
31 33 35 37 39
80 70 60 50 40 30 20 10 0 0
5
10
15
20
Rectified Short-Circuit Current (µA)
29
Rectified Short-Circuit Current (µA)
27
87 90
90
89
80 70
91
60 50
93
40 30
95
20 10
97
0 0
5
Time (sec)
10
15
20
Time (sec)
Figure 3 Watch-strap application. (a) Photos of the curved piezoelectric generator attached on the watch strap. (b) Six motions for energy harvesting from the watch. (c) Output voltage of the device generated by the six motions defined in b.
41
47 49 51 53 55 57 59 61
101 103
43 45
99
105 demonstration can be used for LED lighting during walking at night useful for human safety and for transmitting RF data for exercising. Furthermore, it is possible that our piezoelectric generator can be applied to wearable electronics capable of harvesting energy from human motions, by attaching them onto clothes or on human body parts, such as arms, feet, and the chest. We also experimentally investigated the output voltage and current of the curved piezoelectric generator attached to long-sleeved shirts, around the inner and outer elbows, to allow their evaluation as a power source for wearable sensors and biomedical devices (Supporting information, Figures S11 and S12). We also attached them on a human chest near the heart to sense cardiac impulses and human breathing, as an example of a biomedical application (Supporting information, Figure S13 and movie 2). We expect that the curved piezoelectric generator will effectively extend the operating time of smart watches, supply electrical energy for portable devices, such as a smart phone, and realize self-powered energy systems or sensors in biomedical applications.
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2015.01.051.
Conclusions
107 109 111
In summary, we have developed a curved piezoelectric generator using PVDF that aims to harvest electrical energy from the low-frequency mechanical energy, such as body movements. To enhance output power, we utilized the curved structure for the energy harvesting device. The curved piezoelectric generator produced 3.9 mW/cm2 of instantaneous output power density, and lit up 476 commercial LED bulbs successfully. In addition, it generated 45 V of average output voltage and 225 mA of average output current at 35 Hz, and a mass load enables it to operate at frequencies as low as 1 Hz. Also, we integrated the generator into the battery management circuit that can deliver continuous DC output. Furthermore, we demonstrated that the hybrid generator can be used as a
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
113 115 117 119 121 123
Powerful curved piezoelectric generator
7
1
63
1
3
3
2
4
65
5
67
7
69
9
71
11
73
13
75
5
15
6
77
81
21
83
23
85
25
87
29 31 33 35
30
1
25 20 15 10 5 0 0
2
4
6
8
10
30
2
25 20 15 10 5 0 0
2
39 41 43 45 47 49 51 53 55 57 59 61
30
4
25 20 15 10 5 0 0
2
4
6
6
8
10
30
89
3
25
91
20 15
93
10
95
5 0 0
2
Time (s)
8
10
Re ectified d Open n-Circu uit Volttage (V V)
37
Re ectified d Open n-Circu uit Volttage (V V)
Time (s)
4
30
5
25 20 15 10 5 0 0
2
Time (s)
4
6
4
6
8
10
8
10
30
99
6
25
101
20
103
15 10
105
5
107
0
Time (s)
0
2
4
6
8
10
Time (s)
Figure 4 Piezoelectric generator application for the shoe-insole. (a) Photos of the curved piezoelectric generator inserted in the insole. (b, c) Elicited output voltage and current responses of the insole during walking. (d, e) Elicited output voltage and current responses of the insole during running.
possible power source for a wearable device. To confirm the application of the generator as a wearable device, a curved piezoelectric generator was integrated into a shoe-insole and to a watch strap. Consequently, it was found that, by tapping the watch strap, the insole generator produced 14 V of average output voltage and 18 mA of average output current during running, while the watch generator generated 22 V of average output voltage and 50 mA of average output current. The output power will obviously prolong the operating time of wearable or portable electronics. Thus, our approach will provide a framework to enhance the output power of the
97
Time (s) Re ectified d Open n-Circu uit Volttage (V V)
27
Rectiffied Open-C Circuit Voltage V e (V)
19
Rectiified Open-C O Circuit Voltag V e (V)
79
Rectiified Open-C O Circuit Voltag V e (V)
17
conventional piezoelectric generators, and open a new avenue to realize self-powered systems, such as wearable electronic devices.
109 111 113 115 117
Acknowledgments
119
This work was supported by the Institutional Research Program of the Korea Institute of Science and Technology (2E24881) and KU-KIST Research Program of Korea University (R1309521).
121
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
123
8
W.-S. Jung et al.
1
Appendix A.
3
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.01.051.
5
Supporting information
7
References
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 Q7
45
[1] J.A. Paradiso, T. Starner, IEEE Pervasive Comput. 4 (2005) 18. [2] Y.G. Sun, J.A. Rogers, Adv. Mater. 19 (2007) 1897. [3] Z.L. Wang, R.S. Yang, J. Zhou, Y. Qin, C. Xu, Y.F. Hu, S. Xu, Mater. Sci. Eng. R 70 (2010) 320. [4] Q. Wang, P. Ping, X. Zhao, G. Chu, J. Sun, C. Chen, J. Power Sources 208 (2012) 210. [5] C.Y. Jhu, Y.W. Wang, C.M. Shu, J.C. Chang, H.C. Wu, J. Hazard. Mater. 192 (2011) 99. [6] S.H. Bae, O. Kahya, B.K. Sharma, J. Kwon, H.J. Cho, B. Özyilmaz, J.H. Ahn, ACS Nano 7 (2013) 3130. [7] H. Yu, T. Huang, M. Lu, M. Mao, Q. Zhang, H. Wang, Nanotechnology 24 (2013) 405401. [8] C. Chang, V.H. Tran, J. Wang, Y.K. Fuh, L. Lin, Nano Lett. 10 (2010) 726. [9] M.B. Lee, C.Y. Chen, S. Wang, S.N. Cha, Y.J. Park, J.M. Kim, L.J. Chou, Z.L. Wang, Adv. Mater. 24 (2013) 1759. [10] P. YangX. Xiao, Y. Li, Y. Ding, P. Qing, X. Tan, W. Mai, Z. Lin, W. Wu, T. Li, H. Jin, J. Zhou, C.P. Wong, Z.L. Wang, ACS Nano 7 (2013) 2617. [11] K.I. Park, M.B. Lee, Y. Liu, S. Moon, G.T. Hwang, G. Zhu, J.E. Kim, S.O. Kim, D.K. Kim, Z.L. Wang, K.J. Lee, Adv. Mater. 24 (2012) 2999. [12] K.I. Park, M.B. Lee, Y. Liu, S. Moon, G.T. Hwang, G. Zhu, J.E. Kim, S.O. Kim, D.K. Kim, Z.L. Wang, K.J. Lee, Adv. Mater. 24 (2012) 2937. [13] 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. [14] S.Y. Chung, S. Kim, J.H. Lee, K. Kim, S.W. Kim, C.Y. Kang, S.J. Yoon, Y.S. Kim, Adv. Mater. 24 (2012) 6022. [15] J. Kwon, W. Seoung, B.K. Sharma, S.W. Kim, J.H. Ahn, Energy Environ. Sci. 5 (2012) 8970. [16] W. Wu, S. Bai, M. Yuan, Y. Qin, Z.L. Wang, T. Jing, ACS Nano 6 (2012) 6231. [17] J. Granstrom, J. Feenstra, H.A. Sodano, K. Farinholt, Smart Mater. Struct. 16 (2007) 1810. [18] N.S. Shenck, J.A. Paradiso, IEEE Micro 21 (2002) 30. [19] Frederick A. Leckie, Dominic J. Dal Bello, Strength and Stiffness of Engineering Systems, Springer, Ferroelectric Devices, CRC Press553–554.
47 49 51
Dr. Woo-Suk Jung received the Ph.D. degrees in Department of Electronics from Hoseo Univ. Korea in 2009. Continuously, he has been a post-doctoral research fellow with Electronic Materials Center, Korea Institute of Science and Technology, Seoul, Korea. From 2010 to 2011, he has been a post-doctoral research fellow with the Department of
Mechanical Engineering, University of Toronto. Now his current research interests are development of energy harvesting devices and piezoelectric actuators including design, simulation, and their related circuits.
Dr. Min-Jae Lee received the B.S. degree in the Department of Mechanical System Design Engineering from Seoul National University of Science and Technology, Korea, in 2014. His research interests include the simulation and fabrication of micro- and macro-energy harvesting devices with piezoelectric and triboelectric materials.
55 57 59 61 63 65
Dr. Min-Gyu Kang received the B.S. degree in Department of Electronic Materials Engineering from University of Suwon, Korea, in 2008, and Ph.D. degree in Department of Material Science and Engineering from Korea University, Korea, in 2014. He is currently a post-doctoral research fellow with the Department of Mechanical Engineering, Virginia Tech. His current research interests are in the field of ferroelectric and piezoelectric thin films, and their application.
67 69 71 73
Hi Gyu Moon is studying for his M.S. in the Department of Materials Science and Engineering of Yonsei University. His research interests include the fabrication of metal oxide gas sensors and their electrical characteristics.
75 77 79
Dr. Seok-Jin Yoon received his Ph.D. from his Ph.D. from the Department of Electrical Engineering of Yonsei University in 1992. Now he is a principal research scientist and the head of the Smart Electronic Materials Laboratory in KIST. His research interests include piezoelectric materials, ultrasonic piezoelectric actuators, and gas sensors.
Dr. Seung-Hyub Baek received his B.S. degree from Seoul National University, Seoul, Korea in 2004, and his M.S. and Ph.D. degrees from the University of Wisconsin-Madison, USA in 2007 and 2010, respectively. He served for his postdoctoral research work at the University of Wisconsin-Madison. Since 2011, he has worked for KIST as a senior research scientist. His research interests include piezoelectric MEMS, ferroelectrics, multiferroics, oxide interfacial phenomenon, and thermoelectricity.
81 83 85 87 89 91 93 95
Dr. Chong-Yun Kang received his Ph.D. from the Department of Electrical Engineering of Yonsei University in 2000. Now he is a Principal Research Scientist in KIST from 2000 and a professor of KU-KIST Graduate School of Converging Science and Technology in Korea University from 2012. His research interests include smart materials and devices, expecially, piezoelectric energy harvesting and actuators, electrocaloric effect materials, and nanostructured oxide semiconductor gas sensors.
53
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
97 99 101 103 105
1
Available online at www.sciencedirect.com
3 5 journal homepage: www.elsevier.com/locate/nanoenergy
7 9
RAPID COMMUNICATION
11 13
Q2
15
Powerful curved piezoelectric generator for wearable applications
17 19 Q1
21 23
Woo-Suk Junga, Min-Jae Leea, Min-Gyu Kangb, Hi Gyu Moona, Seok-Jin Yoona, Seung-Hyub Baeka,c,n, Chong-Yun Kanga,d,n a
Electronic Materials Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea b Department of Mechanical Engineering, Virginia Tech, VA 24061, United States c Department of Nanomaterials Science and Technology, University of Science and Technology (UST), Daejeon 305-333, Republic of Korea d KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea
25 27 29 31
Received 24 November 2014; received in revised form 31 December 2014; accepted 22 January 2015
33 35 37 39 41 43 45 47 49 51
KEYWORDS
Abstract
Energy harvester; Piezoelectric; Wearable
With the widespread use of wearable electronics, the flexible piezoelectric energy harvesting devices have been extensively studied to efficiently convert the physical motion of the human body into electrical energy. The major obstacles for realizing a flexible piezoelectric generator include the insufficient output power generation and the poor efficiency at the low-frequency regime. Here, we demonstrate a curved piezoelectric generator favorable for wearable applications, generating a high output power. The curved structure plays a key role to improve the power generation, by effectively distributing the applied force across the piezoelectric layer, as well as to allow operation at the low frequency vibration range. Accordingly, this generator produces 120 V of peak output voltage and 700 mA of peak output current during a cycle. Furthermore, our generator can operate at low frequencies below 50 Hz, generating 55 V of output voltage and 250 mA of output current at 35 Hz, and it even works at frequencies as low as 1 Hz. With this generator, we successfully lit up 476 commercial LED bulbs. In addition, we experimentally demonstrate the possibility that the generator can be used in shoes, watches, and clothes as a power source. Our results will provide a framework to enhance the output power of conventional piezoelectric generators, and open a new avenue for realization of self-powered systems, such as wearable electronic devices. & 2015 Published by Elsevier Ltd.
53 55 Q3
57
n
Corresponding authors at: Electronic Materials Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea. Tel.: +82 2 958 6722(5382); fax: +82 2 958 6720. E-mail addresses: [email protected] (S.-H. Baek), [email protected] (C.-Y. Kang).
59 61
63 65 67
http://dx.doi.org/10.1016/j.nanoen.2015.01.051 2211-2855/& 2015 Published by Elsevier Ltd.
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
69
2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61
W.-S. Jung et al.
Introduction With recent advances in electronic technology, miniaturization, flexibility, and low power consumption have become the developmental trend of electronic devices and portable electronics that are commonly and essentially used in our daily lives, including (among others) smart phones, electronic watches, smart glasses, and wireless headsets.[1–3] However, the use of a battery in such devices has been one of their major implementation problems due to its large size, the insufficient capacity, the danger of explosion, and the inconvenience of recharging.[4,5] For mobile electronics, the integration of a wearable piezoelectric energy harvesting device, where the physical motion of the body can be transformed into electrical power, would be the most promising way to solve these issues, realizing the nature of the sustainable and portable energy source. However, the output power of the energy harvesters still needs to be further improved for such applications. The performance of the piezoelectric energy harvesters is determined mainly by two factors: (1) the electromechanical coupling property of the piezoelectric material, and (2) the device geometry for the efficient conversion of the mechanical energy into electrical energy. The most distinct features of the mechanical motion of the human body are its low frequency nature and the large displacements associated with it. To effectively utilize such mechanical energy, the wearable energy harvester should be equipped with the polymer-based, flexible materials, such as polyvinylidene difluoride (PVDF) [6–8], rather than the rigid ceramic-based materials, such as ZnO, [9,10] BaTiO3, [11,12] and PZT [13–16]. They have been attempted to apply to wearable applications such as a backpack and shoes, using PVDF [17,18]. However, the piezoelectric properties of PVDF are much poorer compared with that of the conventional ceramic-based materials, albeit PVDF is the best piezoelectric material among all polymers, due to its overall performance Q6 characteristics. Therefore, using PVDF requires more critical for the design of the high-performance, wearable piezoelectric energy harvester. The key aspect to the design of the structure of the piezoelectric generators is to maximize the developed stress/strain in the piezoelectric layer due to the externally applied force. This is especially critical for the flexible piezoelectric generator where the strain/stress response may be localized due to the intrinsic softness of the material that can, in turn, limit the amount of electricity generated by the piezoelectric material. Therefore, it is highly desirable to design the structure of the flexible piezoelectric generator in a way that the external force can be well-distributed across the entire piezoelectric layer to maximize the total power generation. Here, we propose a curved, flexible piezoelectric generator to improve the stress/strain distribution across the piezoelectric layer using computational simulations, and to demonstrate the high-power generation of the curved piezoelectric energy harvester. As a model system, we used the PVDF as the piezoelectric layer, integrated on the curved polyimide (PI) platform. The output signal of the generator is rectified by a full-wave bridge diode, and we obtained peak output voltage, output current density, and power density values of 155 V, 700 mA, and 3.9 mW/cm2, respectively, using finger tapping. Moreover, the curved structure enables the generator to operate, harnessing low frequency vibrations below 50 Hz. We implement our curved piezoelectric generator in insoles and a
watch-strap to demonstrate feasibility for wearable applications. In addition, we also examined the generator attaching on elbow to directly harvest electrical energy from body movements and on chest around heart as a heartbeat sensor. In the proposed design, the curved structure plays an important role in enhancing output power because of an effective applied stress distribution. The experimental output performances demonstrated that this work may lead to an innovative way in which wearable technology harvests energy from the physical activity of living creatures.
63 65 67 69 71 73
Working mechanism and experimental
75
Figure 1 shows the structure and working mechanism of the curved piezoelectric generator. It consists of two separate curved piezoelectric generators connected back-to-back, where each generator comprises a curved PI substrate and two piezoelectric materials. As shown in Figure 1(a), piezo 1 and piezo 2 are made of PVDF with electrodes attached on both sides of the curved PI substrate, labeled as ‘curved piezoelectric generator 1’, whereas the ‘curved piezoelectric generator 2’ with piezo 3 and piezo 4 is located on the other side. Here, the curved piezoelectric generator with top/bottom electrodes uses the d31 mode, also extensively used for other piezoelectric applications. In this mode, the direction of the induced electric field is perpendicular to the direction of the applied stress/strain. Therefore, the induced voltage of the curved piezoelectric generator can be computed in accordance to Eq (1), [19] V 3j ¼ σ j g3j Lj ½V
ð1Þ
where σj is the mechanical stress, g3j is the piezoelectric voltage coefficient, and Lj is the distance between electrodes. Notation V3j is the induced voltage in the 3-direction caused by a stress in the j-direction. In case of the curved piezoelectric generator, it uses d31 mode, thus L1 is the thickness of the PVDF and dominant direction is 1 direction (x). Additionally, g3j is defined by Eq. (2), ð2Þ g3j ¼ d 3j =ε0 εT Vm=N where ε0 is the permittivity of free space and εT indicates the permittivity under a constant strain. In the structure of the curved piezoelectric generator, the curved PI substrate plays two important roles. First, it acts as a passive layer to be effectively subjected to the vertical force to the PVDF layer because the PVDF is too thin to receive it as well as has a low Young's modulus. Therefore, the thickness of the PI substrate should be thick enough to shift the neutral plane of the structure out of the piezoelectric layer (Supporting information, Figure S1). However, if it is too thick, it becomes more rigid, so it requires a larger force to generate the electric power. Second, it acts as an active layer that turns the deformed piezoelectric layer into its original shape, much like a spring. Also, it enables the piezoelectric material to be subject to only tensile or compressive stress in a whole volume during pressing and releasing (Supporting information, Figure S1), which results in an enhancement of the output power. This is because the proper thickness of the substrate makes the neutral plane move from PVDF to its inside.
77 79 81 83 85 87 89 91 93 95 97 99 101 103 105 107 109 111 113 115 117 119 121 123
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
Powerful curved piezoelectric generator
3
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19
81
21
83
23
85
25
87
27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61
Figure 1 Structure and working mechanism of the curved piezoelectric generator. (a) Initial state. (b, c) Charge distribution during pressing. (d, e) Charge distribution during force release. (f) 3D schematic view of the curved piezoelectric generator and wire connection of strained piezoelectric generator 1 using bridge diode.
In addition, the curved structure also intensifies the applied stress on the PVDF layer. Normally, when the external force is applied in the middle of a PVDF film without a substrate, it pushes the surface straight down, causing it to locally bend due to its large flexibility. However, the PI substrate enables the PVDF to uniformly increase the applied stress on the entire surface of the material. Using COMSOL simulations, we have compared the output voltages of the curved generator without and with the presence of the 200 mm PI substrate layer (Supporting information, Figure S2). As a result, the generator with a substrate produces a much higher output voltage than the one without the substrate, when the same displacement is experienced in the middle of the generator's surface. This is because the generator without the substrate was subjected to both compressive and tensile stresses at once, thus generating a much smaller potential between the electrodes. In addition, the thicker the PI substrate is, the higher the produced output voltage will be because it enables to receive a higher induced stress on a whole volume of the PVDF (Supporting information, Figure S3). However, the thicker substrate requires larger external forces to deform, thus considering a target application. Furthermore, we compared the curved generator and a flat-surface generator under 1 cm displacement condition (Supporting information, Figure S4). Accordingly, the curved generator produces a higher voltage than the flat type, because the curved structure concentrates the applied stress in the middle of the PVDF. Thus, this analysis indicates that the use of the substrate attached to a piezoelectric film is able to strongly enhance an output power. Figure 1(a)–(e) shows the working mechanism of the curved piezoelectric generator during a single press-and-release cycle.
At the initial state before the mechanical force is applied [Figure 1(a)], there is no electric potential, and thus no charge is transferred. When the external mechanical force is applied on the generator, piezo 1 and piezo 2 with the same poling direction start to be subjected to the compressive and tensile stresses, respectively, resulting in appearance of free charges on the electrodes. Until the piezo 1 touches the piezo 2, free charges on the electrodes of piezo 1 increase and free charges on piezo 2 decrease, as shown in Figure 1(b). Therefore, they produce different output signals each other. This is due to the opposite induced stresses and the same poling direction in piezo 1 and piezo 2. Accordingly, electrons also start to flow from Au1 to Au2 and from Au4 to Au3. When the curved piezoelectric generator 1 touches generator 2, maximum mechanical deformation is elicited [Figure 1(c)], thus generating maximum output power. After this moment, output power slowly reduces, and finally reaches the equilibrium state at full force release. When the applied force is initially released [Figure 1(d)], the pressed arc structures start to bounce back. Unlike the pressing action, piezo 1 and piezo 2 experience the tensile and compressive stresses, respectively, and the electric potentials induced are reversed compared to the press state. Therefore, electrons flow in the reverse directions from Au2 to Au1 and from Au3 to Au4. When the shape of the generator returns to the initial state, it produces the maximum output power again and then decreases, until potentials of piezo1 and piezo 2 disappear. A full cycle is then completed. Similar to the curved piezoelectric generator 1, the curved piezoelectric generator 2, including the piezo 3 and piezo 4, operates along the same working mechanism during the full cycle. Furthermore,
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
89 91 93 95 97 99 101 103 105 107 109 111 113 115 117 119 121 123
4 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29
W.-S. Jung et al.
during periodic pressing-and-releasing cycles, since the curved piezoelectric generator is bent along with the curved PI substrate, it experiences 0.001613% of the identical strain, which can be estimated by neutral plane, distance from the center of PVDF to neutral plane, the length and the radius of the curved generator, and the height of the arc (Supporting information, Figure S5). This applied strain is structurally consistent in periodic multiple cycles. Figure 1(f) shows the wire connection for the curved piezoelectric generator 1, considering current flow. Supporting Figure S6 presents the whole wire connection and measurement configuration in detail. For fabricating the curved piezoelectric genrator, we purchased the PVDF with 0.1 mm of thickness from Fils Co. Ltd., and the PI films with 0.2 mm of thickness from EDS system Inc. Supporting Figure S7 shows the conformation of PVDF polymer chain and a unit cell structure of PVDF crystal. Au electrodes with 200 nm of thickness were deposited on the PVDF surfaces by an electron beam evaporator using on-axis mode with a deposition rate of 0.2 nm/s, 5 10 6 Torr of working pressure, 4.3 kV of power, and 16.4 mA of current. The total size of the curved piezoelectric generator was 7 4 cm2 and its thickness was 0.6 mm. The DPO 4014B Oscilloscope (Tektronix) and current amplifier (Stanford Research SR 570) were utilized for measuring output voltages and output currents, respectively. The performance of the generator in low frequency vibrations was tested under amplitude of 1.2 mm and a velocity of 264 mm/s by the 4809 Vibration Exciter (Brüel & Kjær), the 2718 Power Amplifier (Brüel & Kjær) and the WF 1944B Multifunction Synthesizer (NF Corporation).
31 33
Results and discussion
35
Figure 2 shows the output performances of the curved piezoelectric generator, rectified by a full-wave bridge diode. To obtain enhanced output current, all PVDFs are connected in parallel to increase output current as shown in supporting Figure S6. Using finger tapping, we found that the peak output voltage and peak output current reached 155 V and 700 mA ( 25 mA/cm2 of current density), respectively, as shown in Figure 2(a,b) (see output voltages and currents from piezo 1 to piezo 4 in Supporting information, Figure S8). The instantaneous power density was estimated to be 3.9 mW/cm2, which is the most powerful output to date among all the flexible piezoelectric energy harvesters using PVDF. Some applications require a fast charging and discharging process or reach a higher voltage at a certain time instance. In view of this practical requirement, a capacitor should be chosen first as an energy storage system. Thus, we experimentally investigated the charging energy using three capacitors with different capacitances as shown in Figure 2(c). We found that the 1 mF capacitor was charged to 45 V in 7 s under periodic pressing-and-releasing. In the case of the 10 mF and the 47 mF capacitors, charging to 10 V and to 5 V was achieved in 4 s and 10 s, respectively. Therefore, the curved piezoelectric generator is powerful as an energy harvesting device for power sources that employ mechanical force. With the power output generated from pressingand-releasing motions, we successfully lit up 476 commercial LED bulbs. Figure 2(d) shows the snapshots before and during the moment of LED bulb lit up (Supporting information, movie 1). For practical applications, the device is developed to fully
37 39 41 43 45 47 49 51 53 55 57 59 61
harvest the randomly and naturally available mechanical energy, rather than requiring regular motions executed on purpose. Thus, both the output voltage and current values of the curved piezoelectric generator were verified at low frequencies, below 50 Hz, as shown in Figure 2(e) and (f). Consequently, based on these results, the generator produced 45 V of average output voltage and 225 mA of average output current at 35 Hz, which are the maximum attained values, because 35 Hz represents the generator's natural frequency. In addition, we can control the natural frequency of the curved piezoelectric generator using a load, attaching it onto the top of the curved structure. As a result, even at 1 Hz, we showed the generator with a mass of 18 g as a load produced 9 V of output voltage (Supporting information, Figure S9). However, fluctuations in the output power and the AC output current should be of concern in practical applications. Therefore, we configured a battery management circuit connected to the curved piezoelectric generator as a power-supply system. It consists of a rectifier and a battery management chip. Figure 2(g) shows the battery management circuit for the energy harvesting device, using the bq25504 chip (Texas Instruments) that continuously supplies a constant output voltage of 3.3 V. As shown in Figure 2(h), it can deliver a constant DC output voltage of 3.3 V within 10 s after the generator starts to operate, at a frequency of 35 Hz. This demonstrates that the curved piezoelectric generator is able to powerfully operate at a low frequency, and our system can provide a continuous DC power, adequate to drive various commercial electronic devices. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2015.01.051. Since our generator was aimed towards wearable electronics capable of harvesting electrical energy at low frequencies, such as during human activity and body movements, we chose two applications: the watch and shoes. In order for a device to be wearable, the generator should be integrated without a loss of comfort, or a radical change in design. Therefore, we changed the size of the curved piezoelectric generator that is suitable to be integrated into an application. Figure 3 shows the watch application in which one layer of the curved piezoelectric generator (1.2 16 cm2) was attached onto the watch-strap [Figure 3(a)]. To confirm the principle of energy harvesting from the watch, we suggested its testing for six motions that occur frequently in daily life, namely, twisting (motion 1), bending of the wrist (motion 2), pivoting at the elbow (motion 3), running (motion 3), tapping the watch (motion 5), and grabbing the watch (motion 6) [Figure 3(b)]. As shown in Figure 3(c), most motions produced over 5 V of output voltage, especially, during tapping and grabbing, where the output voltage reached 22 V. Furthermore, the output current of all motions was over 5 mA, and motions 5 and 6 produced 50 mA and 20 mA, respectively (Supporting information, Figure S10). Recently, although smart watches have been released with new designs and functions, it still requires a large capacity battery or a secondary power source to prolong its operating time. We expect that this approach will be utilized for smart watches to increase their operating time. Figure 4 shows a shoe application in which the curved piezoelectric generator (7 4 cm2) is attached on the insole. For the rectification, the output signals were connected to a fullwave bridge diode chip, which is attached on the insole. We evaluated its output power while a 68 kg human was walking and running. During low frequency walking that is close to
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101 103 105 107 109 111 113 115 117 119 121 123
Powerful curved piezoelectric generator
5
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19
81
21
83
23
85
25
87
27
89
29
91
31
93
33
95
35
97
37
99
39
101
41
103
43
105
45
107
47
109
49
111
51
113
53 55 57 59 61
115 Figure 2 Output performance and characteristics of the curved piezoelectric generator. (a, b) Output voltage and current generated by the finger tapping motion (insets: enlarged view). (e) Charging voltage versus time responses of three capacitors with different capacitances. (d) Snapshots of the 476 LEDs before and during the moment of being lit up. (e, f) Plots of output voltage and current produced at, or below 50 Hz. (g) Battery management circuit for energy harvesting device. (h) Plot of the temporal output voltage response of the system that reaches a constant voltage value of 3.3 V.
0.5 Hz we obtained 25 V of average output voltage and 20 mA of average output current. Furthermore, during running, 40 V of average output voltage and 47 mA of average
output current were produced. We expect that it can produce a much higher output power when we use a multi-layered structure and multiple devices in the shoes. We expect that this
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
117 119 121 123
6
W.-S. Jung et al.
1
63
3
65
5
67
7
69
9
71
11
17 19 21
b
40 30 20 10 0 0
5
23
10
15
20
Rectified Open-Circuit Voltage (V)
15
Rectified Open-Circuit Voltage (V)
13
73 50
50
75 40
77 30
79 20
81
10
83
0 0
5
Time (sec)
10
15
20
85
Time (sec)
25
31 33 35 37 39
80 70 60 50 40 30 20 10 0 0
5
10
15
20
Rectified Short-Circuit Current (µA)
29
Rectified Short-Circuit Current (µA)
27
87 90
90
89
80 70
91
60 50
93
40 30
95
20 10
97
0 0
5
Time (sec)
10
15
20
Time (sec)
Figure 3 Watch-strap application. (a) Photos of the curved piezoelectric generator attached on the watch strap. (b) Six motions for energy harvesting from the watch. (c) Output voltage of the device generated by the six motions defined in b.
41
47 49 51 53 55 57 59 61
101 103
43 45
99
105 demonstration can be used for LED lighting during walking at night useful for human safety and for transmitting RF data for exercising. Furthermore, it is possible that our piezoelectric generator can be applied to wearable electronics capable of harvesting energy from human motions, by attaching them onto clothes or on human body parts, such as arms, feet, and the chest. We also experimentally investigated the output voltage and current of the curved piezoelectric generator attached to long-sleeved shirts, around the inner and outer elbows, to allow their evaluation as a power source for wearable sensors and biomedical devices (Supporting information, Figures S11 and S12). We also attached them on a human chest near the heart to sense cardiac impulses and human breathing, as an example of a biomedical application (Supporting information, Figure S13 and movie 2). We expect that the curved piezoelectric generator will effectively extend the operating time of smart watches, supply electrical energy for portable devices, such as a smart phone, and realize self-powered energy systems or sensors in biomedical applications.
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2015.01.051.
Conclusions
107 109 111
In summary, we have developed a curved piezoelectric generator using PVDF that aims to harvest electrical energy from the low-frequency mechanical energy, such as body movements. To enhance output power, we utilized the curved structure for the energy harvesting device. The curved piezoelectric generator produced 3.9 mW/cm2 of instantaneous output power density, and lit up 476 commercial LED bulbs successfully. In addition, it generated 45 V of average output voltage and 225 mA of average output current at 35 Hz, and a mass load enables it to operate at frequencies as low as 1 Hz. Also, we integrated the generator into the battery management circuit that can deliver continuous DC output. Furthermore, we demonstrated that the hybrid generator can be used as a
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
113 115 117 119 121 123
Powerful curved piezoelectric generator
7
1
63
1
3
3
2
4
65
5
67
7
69
9
71
11
73
13
75
5
15
6
77
81
21
83
23
85
25
87
29 31 33 35
30
1
25 20 15 10 5 0 0
2
4
6
8
10
30
2
25 20 15 10 5 0 0
2
39 41 43 45 47 49 51 53 55 57 59 61
30
4
25 20 15 10 5 0 0
2
4
6
6
8
10
30
89
3
25
91
20 15
93
10
95
5 0 0
2
Time (s)
8
10
Re ectified d Open n-Circu uit Volttage (V V)
37
Re ectified d Open n-Circu uit Volttage (V V)
Time (s)
4
30
5
25 20 15 10 5 0 0
2
Time (s)
4
6
4
6
8
10
8
10
30
99
6
25
101
20
103
15 10
105
5
107
0
Time (s)
0
2
4
6
8
10
Time (s)
Figure 4 Piezoelectric generator application for the shoe-insole. (a) Photos of the curved piezoelectric generator inserted in the insole. (b, c) Elicited output voltage and current responses of the insole during walking. (d, e) Elicited output voltage and current responses of the insole during running.
possible power source for a wearable device. To confirm the application of the generator as a wearable device, a curved piezoelectric generator was integrated into a shoe-insole and to a watch strap. Consequently, it was found that, by tapping the watch strap, the insole generator produced 14 V of average output voltage and 18 mA of average output current during running, while the watch generator generated 22 V of average output voltage and 50 mA of average output current. The output power will obviously prolong the operating time of wearable or portable electronics. Thus, our approach will provide a framework to enhance the output power of the
97
Time (s) Re ectified d Open n-Circu uit Volttage (V V)
27
Rectiffied Open-C Circuit Voltage V e (V)
19
Rectiified Open-C O Circuit Voltag V e (V)
79
Rectiified Open-C O Circuit Voltag V e (V)
17
conventional piezoelectric generators, and open a new avenue to realize self-powered systems, such as wearable electronic devices.
109 111 113 115 117
Acknowledgments
119
This work was supported by the Institutional Research Program of the Korea Institute of Science and Technology (2E24881) and KU-KIST Research Program of Korea University (R1309521).
121
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
123
8
W.-S. Jung et al.
1
Appendix A.
3
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.01.051.
5
Supporting information
7
References
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 Q7
45
[1] J.A. Paradiso, T. Starner, IEEE Pervasive Comput. 4 (2005) 18. [2] Y.G. Sun, J.A. Rogers, Adv. Mater. 19 (2007) 1897. [3] Z.L. Wang, R.S. Yang, J. Zhou, Y. Qin, C. Xu, Y.F. Hu, S. Xu, Mater. Sci. Eng. R 70 (2010) 320. [4] Q. Wang, P. Ping, X. Zhao, G. Chu, J. Sun, C. Chen, J. Power Sources 208 (2012) 210. [5] C.Y. Jhu, Y.W. Wang, C.M. Shu, J.C. Chang, H.C. Wu, J. Hazard. Mater. 192 (2011) 99. [6] S.H. Bae, O. Kahya, B.K. Sharma, J. Kwon, H.J. Cho, B. Özyilmaz, J.H. Ahn, ACS Nano 7 (2013) 3130. [7] H. Yu, T. Huang, M. Lu, M. Mao, Q. Zhang, H. Wang, Nanotechnology 24 (2013) 405401. [8] C. Chang, V.H. Tran, J. Wang, Y.K. Fuh, L. Lin, Nano Lett. 10 (2010) 726. [9] M.B. Lee, C.Y. Chen, S. Wang, S.N. Cha, Y.J. Park, J.M. Kim, L.J. Chou, Z.L. Wang, Adv. Mater. 24 (2013) 1759. [10] P. YangX. Xiao, Y. Li, Y. Ding, P. Qing, X. Tan, W. Mai, Z. Lin, W. Wu, T. Li, H. Jin, J. Zhou, C.P. Wong, Z.L. Wang, ACS Nano 7 (2013) 2617. [11] K.I. Park, M.B. Lee, Y. Liu, S. Moon, G.T. Hwang, G. Zhu, J.E. Kim, S.O. Kim, D.K. Kim, Z.L. Wang, K.J. Lee, Adv. Mater. 24 (2012) 2999. [12] K.I. Park, M.B. Lee, Y. Liu, S. Moon, G.T. Hwang, G. Zhu, J.E. Kim, S.O. Kim, D.K. Kim, Z.L. Wang, K.J. Lee, Adv. Mater. 24 (2012) 2937. [13] 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. [14] S.Y. Chung, S. Kim, J.H. Lee, K. Kim, S.W. Kim, C.Y. Kang, S.J. Yoon, Y.S. Kim, Adv. Mater. 24 (2012) 6022. [15] J. Kwon, W. Seoung, B.K. Sharma, S.W. Kim, J.H. Ahn, Energy Environ. Sci. 5 (2012) 8970. [16] W. Wu, S. Bai, M. Yuan, Y. Qin, Z.L. Wang, T. Jing, ACS Nano 6 (2012) 6231. [17] J. Granstrom, J. Feenstra, H.A. Sodano, K. Farinholt, Smart Mater. Struct. 16 (2007) 1810. [18] N.S. Shenck, J.A. Paradiso, IEEE Micro 21 (2002) 30. [19] Frederick A. Leckie, Dominic J. Dal Bello, Strength and Stiffness of Engineering Systems, Springer, Ferroelectric Devices, CRC Press553–554.
47 49 51
Dr. Woo-Suk Jung received the Ph.D. degrees in Department of Electronics from Hoseo Univ. Korea in 2009. Continuously, he has been a post-doctoral research fellow with Electronic Materials Center, Korea Institute of Science and Technology, Seoul, Korea. From 2010 to 2011, he has been a post-doctoral research fellow with the Department of
Mechanical Engineering, University of Toronto. Now his current research interests are development of energy harvesting devices and piezoelectric actuators including design, simulation, and their related circuits.
Dr. Min-Jae Lee received the B.S. degree in the Department of Mechanical System Design Engineering from Seoul National University of Science and Technology, Korea, in 2014. His research interests include the simulation and fabrication of micro- and macro-energy harvesting devices with piezoelectric and triboelectric materials.
55 57 59 61 63 65
Dr. Min-Gyu Kang received the B.S. degree in Department of Electronic Materials Engineering from University of Suwon, Korea, in 2008, and Ph.D. degree in Department of Material Science and Engineering from Korea University, Korea, in 2014. He is currently a post-doctoral research fellow with the Department of Mechanical Engineering, Virginia Tech. His current research interests are in the field of ferroelectric and piezoelectric thin films, and their application.
67 69 71 73
Hi Gyu Moon is studying for his M.S. in the Department of Materials Science and Engineering of Yonsei University. His research interests include the fabrication of metal oxide gas sensors and their electrical characteristics.
75 77 79
Dr. Seok-Jin Yoon received his Ph.D. from his Ph.D. from the Department of Electrical Engineering of Yonsei University in 1992. Now he is a principal research scientist and the head of the Smart Electronic Materials Laboratory in KIST. His research interests include piezoelectric materials, ultrasonic piezoelectric actuators, and gas sensors.
Dr. Seung-Hyub Baek received his B.S. degree from Seoul National University, Seoul, Korea in 2004, and his M.S. and Ph.D. degrees from the University of Wisconsin-Madison, USA in 2007 and 2010, respectively. He served for his postdoctoral research work at the University of Wisconsin-Madison. Since 2011, he has worked for KIST as a senior research scientist. His research interests include piezoelectric MEMS, ferroelectrics, multiferroics, oxide interfacial phenomenon, and thermoelectricity.
81 83 85 87 89 91 93 95
Dr. Chong-Yun Kang received his Ph.D. from the Department of Electrical Engineering of Yonsei University in 2000. Now he is a Principal Research Scientist in KIST from 2000 and a professor of KU-KIST Graduate School of Converging Science and Technology in Korea University from 2012. His research interests include smart materials and devices, expecially, piezoelectric energy harvesting and actuators, electrocaloric effect materials, and nanostructured oxide semiconductor gas sensors.
53
Please cite this article as: W.-S. Jung, et al., Powerful curved piezoelectric generator for wearable applications, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.051
97 99 101 103 105