Piezoelectric and Triboelectric Dual Effects in Mechanical-Energy

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Piezoelectric and Triboelectric Dual Effects in Mechanical Energy Harvesting Using BaTiO3/Polydimethylsiloxane Composite Film Guoquan Suo, Yanhao Yu, Zhiyi Zhang, Shifa Wang, Ping Zhao, Jianye Li, and Xudong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11108 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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ACS Applied Materials & Interfaces

Piezoelectric and Triboelectric Dual Effects in Mechanical Energy Harvesting Using BaTiO3/Polydimethylsiloxane Composite Film Guoquan Suo1,2, Yanhao Yu1, Zhiyi Zhang1, Shifa Wang3, Ping Zhao3,Jianye Li*2, Xudong Wang*1 1. Department of Material Sciences and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA 2. University of Science and Technology Beijing, 100083, China 3. Mechanical and Industrial Engineering, University of Minnesota Duluth, Duluth, Minnesota 55812, USA KEYWORDS: Piezoelectric; Triboelectric; Nanogenerator; Mechanical energy harvesting; Nanocomposite

ABSTRACT: Piezoelectric and triboelectric nanogenerators have been developed as rising energy harvesting devices in the past few years to effectively convert mechanical energy into electricity. Here, a novel hybrid piezo/triboelectric nanogenerator based on BaTiO3 NPs/PDMS composite film was developed in a simple and low cost way. The effect of BTO content and polarization degree on the output performance was systematically studied. The device with 20 wt% BTO in PDMS and 100 µm thickness film shows the highest output power. We also

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designed three measurement modes to record hybrid, tribo, piezo output separately with simple structure, which has only two electrodes. The hybrid output performance is higher than that of tribo and piezo. This work will provide not only a new way to enhance output power of nanogenerators, but also new opportunities for developing built-in power source in self-powered electronics.

1. Introduction Development of sustainable and miniaturized power sources are of critical importance for portable electronics and implantable medical devices [1-4]. As one of the most abundant and accessible energy source, mechanical movements serve as an attractive platform for developing such power sources. In 2007, piezoelectric nanogenerator (PENG) emerged as a promising technology for generating electricity from ambient vibration relying on the electromechanical coupling effect in piezoelectric materials.[5] By using nanoscale piezoelectric materials or composites, PENGs can effectively harvest mechanical energy from various sources such as acoustic waves, fluid flow and body movements.[6-8] Recently, triboelectric nanogenerator (TENG) is rising as a new pathway for efficiently harvesting mechanical energy on the basis of the triboelectrification and electrostatic effects.[9] Compared to PENG, TENG significantly promoted the power output from sub-miliwatt to watt level, which largely broadened the application potential in powering electronic devices. Practical demonstrations of powering commercial electronics (e.g. cell phones, alarms and light bulbs) have been reported with different TENG designs [10-13]. Current endeavors in the improvement of TENG performance are predominately focused on the optimization of device structures [14-16]. Efforts were also shown to hybridize TENG with other energy harvesting techniques, such as solar cells and


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electromagnetic induction generators, to further enhance the energy output and expand the application scope. [17-20]. Although piezoelectricity and triboelectricity are two different mechanisms, they share many operational features in common for mechanical-to-electrical energy conversion. For example, they both respond well to many mechanical energy sources, such as deflection, compressing, and vibration. Furthermore, their different mechanisms suggest that their electricity generation processes will not overlap with each other, since TENG only requires the function of material surface while the bulk part would be responsible for the piezoelectric component. Therefore, these two effects may be combined together for the same type of mechanical energy harvesting with integrated electrical energy output. Some pioneer works have illustrated the feasibility of combining the triboelectric and piezoelectric effects for mechanical energy harvesting by specially design different components to promote the piezoelectric and triboelectric effects in one integrated system [21-24]. In this paper, we show that the piezoelectric and triboelectric effects can generally coexist in one material component and interact with each other to alter the electric output. A composite film was developed comprising ferroelectric barium titanate (BTO) nanoparticles (NPs) in a polydimethylsiloxane (PDMS) polymer matrix. The electrical output of the composite film was found closely related to the poling condition of BTO and the mass ratio of BTO in PDMS. The individual contributions of piezoelectricity and triboelectricity were distinguished and systematically studied. This work confirms that electric signals from the piezoelectric and triboelectric mechanisms could be added up in one single mechanical displacement step, opening a new route for developing a high-performance mechanical energy harvesting devices. This work also suggests that the triboelectric effect could be easily induced and alter the electrical response when pure piezoelectric effect is of interests.


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2. Experimental Section Fabrication of BTO/PDMS composite films: BTO NPs with an average size of 200 nm were obtained from US Research Nanomaterials, Inc. BTO NPs were dispersed in a mixture of liquid PDMS elastomer and cross-linker with a ratio of 10:1. The weight ratio between BTO NP and PDMS was varied from 5% to 30%. The BTO/PDMS composite mixture was magnetically stirred for 30min followed by ultrasonic bath for 1h. After degasing in vacuum for 20 min, the mixture was casted onto a sandpaper and cured at 80 °C for 2 h. The film thickness was adjusted by the amount of casting mixture. Finally, the composite film was peeled off from sandpaper. For poling, the composite film was sandwiched between two Cu electrodes and poled at room temperature under +12MV/m or -12MV/m for 12 h in air. Material Characterizations: SEM (Zeiss Leo 1530 field-emission microscope) was utilized to observe the morphologies of the composite film. The crystalline structure of BaTiO3 NPs was characterized by the Bruker D8 Discovery with Cu Kα radiation. A modified Sawyer-Tower circuit was adopted to acquire the polarization P vs electric field E loop. Device Assembly and Electrical Characterization: All the composite films were cut into a size of 1 cm × 1 cm and attached to the center of an ITO/PET substrate (2 cm × 5 cm), which was considered as the top electrode. A copper film with the same size as the composite film was attached to a PET substrate as the counter electrode. The Cu surface was separated from the composite film by two pieces of glass spacers (1 mm in thickness). The Cu and ITO electrodes were connected to an external circuit for signal measurement. During the measurement, the PET/ITO top side was pressed toward the Cu electrode by a computer-controlled electric shaker at a frequency of 20 Hz. The bottom electrode was anchored on the table surface to stabilize the system. The cantilever-type device was fabricated from a 20wt% BTO:PDMS film with a size of


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3 cm × 1 cm. he bottom side of the film was attached to an ITO/PET substrate. The topside of the film was partially covered with a thin ITO/PET film with a size of 2 cm × 1 cm. A Cu film electrode with the size of 3 cm × 1 cm tape was attached to the top ITO/PET film through a 1 mm-thick glass spacer. The Cu film was electrically connected to the top ITO electrode because both served as the top electrode. The top ITO-covered end of the composite beam was anchored to a stationary stage forming a cantilever resonator. Periodic force with compute controlled actuating distance was applied toward the free-standing end at a frequency of 10 Hz. The voltage outputs were recorded using an Agilent DSO1012A oscilloscope with an internal impedance of 1MΩ. The current outputs and charge transfer were measured using an Autolab PGSTAT302N station. 3. Results and Discussion The piezoelectric composite film was fabricated by mixing BTO NPs in a PDMS matrix (see experimental section for detailed fabrication procedures). The concentration of BTO was varied from 5 wt% to 30 wt%. All the as-received composite films had a thickness of 200 µm and exhibited excellent flexibility (Fig. 1a). The BTO NPs were ~200 nm in diameter. They were uniformly distributed in the PDMS matrix across the entire film volume, as shown by the crosssectional SEM image in Fig. 1b. X-Ray diffraction (XRD) spectrum of the composite film revealed six characteristic diffraction peaks ((1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0), (2 1 1)), which corresponded well to the ferroelectric tetragonal phase of BTO. In order to confirm the ferroelectricity of the composite film, the polarization-electric field (PE) hysteresis loop was measured from a 100 µm-thick, 20 wt% BTO:PDMS composite film. As shown in Fig. 1d, clear ferroelectricity was observed, where the remnant polarization of the film was ~0.015C/m2. This


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value was comparable to those of bulk BTO thin films (~0.026C/m2) [25], suggesting the appreciable ferroelectricity of the composite film. The effect of ferroelectric polarization to the electrical output of the composite film was investigated first. A 200 µm thick and contained 20 wt% BTO:PDMS film was selected for the study. Prior to the measurement, the composite film was poled under an electric field of 12 MV/m for 12 hours. A schematic diagram of the device for the measurement was shown in the supplementary material (Fig. S3). Output voltage (V), short-circuit current density (Jsc) and transferred charges (∆Qsc) were then measured when the film was under a periodic compressive force at a frequency of 20 Hz, as shown in Fig. 2a-c, respectively. The voltage was measured by oscilloscope with an internal impedance of 1MΩ. There certainly was some current flow through the measurement circuit as we measured the voltage. This is the reason that the voltage shows both positive and negative values if only triboelectric effect was measured. In general, the positively poled composite film yielded the highest electric output and the reversely poled film had the lowest output. The specific values were listed in Table 1. In an ideal ferroelectric material, unpoled material typically yields the lowest or no piezoelectricity due to the randomly oriented domains that cancelled out each other. Reversed poling will generate opposite output polarization compared to the positive poling situation, while the amplitude may be identical. The electric signals obtained from our composite film were clearly not this case. All the signals exhibited the same polarization upon compression (see the enlarged voltage pulse profile in Fig. 2d). The observation of the difference in height of positive peaks and negative peaks can be explained by the fact that pressing is imposed by the external vibrator while the releasing is caused by the resilience of PDMS itself. Therefore, it is very likely that releasing corresponds to a slower process and thus a smaller but wider current signal [26]. Significant output was obtained


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from the unpoled film, and the amplitude was in the middle of forward and reversely poled films. These unique features suggested that the output might comprise two components, one is related to the ferroelectric polarization (piezoelectricity) and one is independent to the ferroelectric polarization (triboelectricity). The electricity from the unpoled samples may primarily from the triboelectric effect. To understand the combined effect of piezoelectricity and triboelectricity in the composite film, the stepwise operation mechanism under the short-circuit mode is schematically illustrated in Fig. 3. Using the simple vertical contact/releasing mode as an example, the BTO:PDMS composite film was fixed to a PET/ITO back electrode, and its front was facing a Cu electrode that was normally separated. Upon the contact between the composite film and Cu electrode under an external force, electrons were transferred from Cu to PDMS surface as a consequence of the large difference of their electron affinities. Meanwhile, opposite charges were induced on the backside of the composite film, resulting in a net current flow from ITO to Cu electrode (step i). This is known as the triboelectric current pulse (It). When the external force persisted, (which was a typical case) the composite film was compressed and produced piezoelectric polarization (step ii), resulting an additional current pulse (Ip). Because the polarization direction is determined by the ferroelectric poling direction, the poling direction could either enhance or suppress the total current flow (Itotal = It+ Ip). When the external force was removed, the compressive strain was first released and the piezoelectric polarization disappeared (step iii). Recovery of the piezo-induced charge resulted in a reverse current pulse (-Ip). When the composite film became separated from the Cu electrode again, triboelectric-induced charge flew back making addition contribution to the reverse current pulse (-It, step iv). It should be noted


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that voltage would also exhibit the same trend as current because in the NG case, voltage is a function of the total amount of the induced charge. To further evaluate the principle of combining both effects and the role of ferroelectric polarization, BTO/PDMS composite films with a series of BTO NP mass ratio (0-30% BTO) were prepared. Their V and Jsc and ∆Qsc were measured at a frequency of 20 Hz. Since all the signals exhibited the same trend, here we only selected to show the voltage output as an example for discussion. The current and charge information are included in the supplementary material (Fig. S1). The typical voltage pulse curves measured from a 200 µm composite films with different BTO mass ratio were shown and compared in Fig. 4a-c, for the cases of forward poling, unpoled, and reverse poling, respectively. The average peak voltage for all the three cases were summarized and plotted as a function of BTO weight ratio in Fig. 4d. First, it can be clearly seen that most the BTO/PDMS composite films exhibited higher output than pure PDMS films. Because PDMS only produced triboelectric signal, the enhanced output by adding BTO NPs evidenced the contribution of piezoelectricity. Furthermore, for the unpoled film, introducing BTO NPs also showed significantly positive effect toward the output. This could be attributed to the increase of dielectric constant of the composite film, which enhanced the charge density on the PDMS surfaces [27]. Optimal BTO concentration was found to be ~20 wt%. Further increase the amount of BTO NP ratio might lead to higher level of NP agglomeration and made more difficult to align the ferroelectric polarization under electrical poling. The BTO ratio-related output change was the most significant in the forwardly poled films. This observation reflects the positive piezoelectricity add-up from favorably aligned BTO NPs. On the other hand, the voltage output decreased upon adding BTO NPs initially when the film was reversely poled because the piezoelectricity and triboelectricity were cancelling out from each other. The less-significant but


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still-increasing output from the reversely poled films might be a result of incomplete poling and raised dielectric constant. Considering the degree of polarization can be affected by the film thickness, we decreased the thickness of the composite films to 100 µm and compared the output with those from 200 µmthick films. The average peak voltages of all BTO ratios under three poling conditions were shown in Fig. 4e. Corresponding voltage profiles were included in supplementary materials (Fig. S2). The voltage output of the 100µm-thick film was clearly higher than that of 200 µm films in general, while a similar trend was observed as a function of BTO ratio from the forward and unpoled films. Notably that the reversely poled film exhibited a monotonic decrease as the BTO ratio increased. This further confirmed that as a higher degree of polarization was achieved, the piezoelectricity overwhelmed the triboelectricity and dominated the overall output signal. It should be noted that under practical testing or application conditions, the piezoelectric and triboelectric processes occur nearly simultaneous and their contributions are indistinguishable. As shown in our tests, the difference was only reflected by the overall intensity of the electric signal peaks. In order to identify the piezoelectric and triboelectric contribution in a mechanical energy conversion process, we designed a cantilever-type resonator using the composite film what is partially covered by electrodes. As schematically shown in Fig. 5a, the bottom side of a composite film strip (1 cm x 3 cm) was fully attached to a PET/ITO electrode, while the topside only had 2/3 area covered by a PET/ITO electrode. An additional Cu electrode with the same size as the composite film was attached on the top PET surface and connected to the top ITO electrode. The end of the cantilever with top ITO electrode was anchored at the edge of a stationary block for the characterization. An external force with finely-controlled strength was applied to the free-standing end. In this design, the triboelectricity was produced by contacting


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the Cu electrode and exposed composite film surface but not deflecting the cantilever. Pure piezoelectricity was generated from the deflection of the cantilever without using the top electrode. The combined contribution was obtained by pushing the Cu electrode to deflect the cantilever beam. Operation details are included the supplementary materials S4.Figure 5b shows the output voltage from the three modes. The average peak values of voltage of hybrid, tribo and piezo were ~3.3V, ~2.2V and ~1.1V, respectively. The output of the hybrid mode was nearly the sum of both triboelectric and piezoelectric outputs, confirming the add-up effect. Although the piezoelectric effect and triboelectric response were not the same as the compressing mode, they are most suitable to distinguish two effects and show the different outputs separately. Through this design, the detailed peak features could also be distinguished between different modes. Figure 5c shows one pulse of voltage signal of the three modes. Because triboelectric output was a result of releasing induced charge upon contact, its peak was sharp and narrow. Since piezoelectricity was a result of strain, it could be generated during the entire deflection process, and thus a relative broad peak was received. Obviously, the hybrid peak exhibited the features of both types of peaks. ∆Qsc data also confirmed the total charge transfer was almost identical to the sum of charge transfer from the two individual effects. Combination of the piezoelectric and triboelectric effects was expected to improve the total energy generation capability. To demonstrate this potential, the cantilever was connected to a capacitor (2.2 µF) through a full-wave bridge circuit. Figure 5d shows the charging process of the capacitor under different operating modes. The hybrid mode reached its saturation voltage (~1.44V) most rapidly, which was also the highest among the three. 4. Conclusion


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In summary, through the development of BTO NP:PDMS composite film, we demonstrated a combine effect of piezoelectricity and triboelectricity for mechanical-to-electrical energy conversion in a single material component. We systematically studied who the electric output of the composite film was related to the ferroelectric polarization direction and BTO ratio in PDMS matrix. The BTO/PDMS composite film with 20wt% mass ratio of BTO NPs exhibited the highest performance due to the variation of ferroelectric polarization strength and dielectric constant. By implementing a cantilever-type resonator with a special electrode design, we clearly differentiated the contribution of triboelectricity and piezoelectricity. Our research revealed that appropriately align the piezoelectric polarization with the triboelectric charge exchange direction could produce a net gain of the overall electric output. Because both piezoelectricity and triboelectricity could be generated by subjecting a dielectric material to external forces, these two effects could be seamlessly integrated into one functional unit that would potentially exhibit higher mechanical-to-electrical energy conversion efficacy compared to using either individual piezoelectric or triboelectric effect. This discovery may eventually lead to a more efficient nanogenerator design with further enhanced power output.


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Figures and Captions:

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2θ(degree) Electric field (MV/m) Figure1. a) Photo image of the prepared BTO-PDMS composite film. b) Cross-sectional SEM image of the composite film. c) X-ray diffraction pattern of BaTiO3 NPs. d) Polarization-electric field hysteresis loop measured on the 200 µm 20wt% BTO:PDMS composite film at room temperature.


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Figure2. Comparison of the voltage (a) short-circuit current (b) and transferred charges (c) of the 200µm composite film based generator with three polarization condition under a periodic compressive force at a frequency of 20 Hz. (d).Enlarged one cycle voltage pulse from (a). (e) Enlarged one cycle short-circuit current pulse from (b). (f) Enlarged one cycle pulse of transferred charges from (c).


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Table1. The specific average values of voltage、short-circuit current and transferred charges of the 200µm composite film based generator with three polarization condition. Forward poling

Without poling

Reverse poling

Voltage(V)

13.5±1.7

9.2±1.0

5.8±0.4

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17.2±2.1

12.1±0.4

7.4±0.1

ΔQSC(µC/m2)

69.2±0.1

55.4±0.2

48.8±0.2


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Figure3. The stepwise operation mechanism of the combined effect of piezoelectricity and triboelectricity.


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Figure4. Dependence of voltage on different BTO mass ration in PDMS of the 200 µm film based generator with forward poling (a)without poling (b) and reverse poling condition(c).The average peak voltage for the three poling condition of the 200µm (d) and 100µm (e) film based generator with different mass ration of BTO.


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Figure5. (a). Schematic set-up and working mechanism to identify the piezoelectric and triboelectric contribution. (b). Output voltage under the three testing modes: hybrid, triboelectric and piezoelectric. (c). One pulse of voltage signal of the three modes. (d). Voltages measured across a 2.2 µF capacitor when charged under the three modes.


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Supporting Information. Current and transferred charges results of 200µm films with different BTO mass ratio; Voltage of 100µm films with different BTO mass ration description with different poling condition. AUTHOR INFORMATION Corresponding Author *(X. D. Wang) Email:[email protected]. *(J. Y. Li) Email: [email protected] Funding Sources This work is supported by the National Science Foundation (NSF) under award # CMMI1148919.  Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Science Foundation (NSF) under award # CMMI1148919, G.Suo thanks the China Scholarship Council for financial support. REFERENCES [1] Patolsky, F.; Timko, B. P.; Yu, G.; Fang, Y.; Greytak, A. B.; Zheng, G.; Lieber, C. M. Detection, Stimulation, and Inhibition of Neuronal Signals with High-density Nanowire Transistor Arrays. Science, 2006, 313, 1100-1104. [2] Tian, B.; Cohen-Karni, T.; Qing, Q.; Duan, X.; Xie, P.; Lieber, C. M. Three-Dimensional, Flexible Nanoscale Field-Effect Transistors as Localized Bioprobes. Science, 2010, 329, 830834.


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Table of Contents Graphic


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Piezoelectric and Triboelectric Dual Effects in Mechanical-Energy

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