Energy &Environmental Sciencersc.li/ees

ISSN 1754-5706

COMMUNICATIONSunho Jeong, Youngmin Choi, Su Yeon Lee et al. High-performance piezoelectric nanogenerators based on chemically-reinforced composites

Volume 11 Number 6 June 2018 Pages 1345–1642

This journal is©The Royal Society of Chemistry 2018 Energy Environ. Sci., 2018, 11, 1425--1430 | 1425

Cite this: Energy Environ. Sci.,

2018, 11, 1425

High-performance piezoelectric nanogeneratorsbased on chemically-reinforced composites†

Eun Jung Lee,a Tae Yun Kim,b Sang-Woo Kim, b Sunho Jeong, *a

Youngmin Choi*a and Su Yeon Lee *a

High-performance flexible piezoelectric nanogenerators (PNGs)

based on composite thin films comprising amine-functionalized

lead zirconate titanate (PZT) nanoparticles (PZT-NH2 NPs) and a

thermoplastic triblock copolymer grafted with maleic anhydride are

fabricated. The chemically reinforced composite contains a stable

dispersion of PZT NPs within the polymer matrix with enhanced

stress applied to the PZT NPs. Without additional dispersants, the

uniform distribution of PZT-NH2 NPs in the polymer composite

improves the piezoelectric power generation compared to that of a

PNG device using pristine PZT NPs. This unique composite behavior

allows the PZT-NH2 NP-based flexible PNG to exhibit a high output

voltage of 65 V and current of 1.6 lA without time-dependent

degradation. This alternating energy from the PNG can be used to

charge a capacitor and operate light-emitting diodes through a full

bridge rectifier. Furthermore, the proposed PNG is demonstrated

as a promising energy harvester for potential applications in self-

powered systems.

Introduction

Piezoelectric materials have recently attracted great attention asactive materials in piezoelectric energy harvesters, self-poweredsensors, and actuators.1–4 Among these devices, energy harvestersthat can effectively generate electricity without restriction are ofparticular interest because of their promising capacity to convertresources of irregular mechanical energy into electrical energy.5–7

Moreover, piezoelectric energy harvesters based on flexiblesubstrates can harvest electricity from small biomechanicalenergy resources to provide power for portable and personalelectronics.8–12 To date, many piezoelectric materials, including

ZnO nanowires,13,14 GaN nanowires,15 piezoelectric ceramics,16–18

and poly(vinylidene fluoride) (PVDF),19,20 have been synthesizedand exploited to fabricate piezoelectric nanogenerators (PNG).PNGs produced with these materials show remarkably improvedoutput performance. Many researchers have attempted to achievehigh-performance PNG devices using various fabricationprocesses such as densely aligned nanowire growth, transfertechniques, electrospinning, and compositing methods.10,21–23

Composite-based PNG devices incorporating piezoelectricnanoparticles (NPs) into soft polymer matrices offer uniqueadvantages over other approaches because the fabrication processis simple and scalable, using low-cost materials and attainingimproved mechanical durability and high flexibility undermechanical deformation. Park et al. fabricated a PNG device bycompositing PbZrxTi1�xO3 (PZT) NPs with poly(dimethylsiloxane)(PDMS) and demonstrated its enhanced output performance.24

a Division of Advanced Materials, Korea Research Institute of Chemical Technology

(KRICT), 141 Gajeongro, Daejeon 34114, Republic of Korea.

E-mail: sjeong@krict.re.kr, youngmin@krict.re.kr, sylee@krict.re.krb School of Advanced Materials Science and Engineering, Sungkyunkwan University

(SKKU), Suwon 440-746, Republic of Korea

† Electronic supplementary information (ESI) available: Detailed description ofthe experimental procedures and characterization. See DOI: 10.1039/c8ee00014j

Received 3rd January 2018,Accepted 6th March 2018

DOI: 10.1039/c8ee00014j

rsc.li/ees

Broader contextPiezoelectric nanogenerators (PNGs) based on piezoelectric composites areemerging as promising energy harvesters for scavenging mechanical energyand converting it into electrical energy. A uniform dispersion of piezo-electric particles in composite films is essential for fabricating composite-based PNGs with a high output as it ensures well-distributed piezopotentialwithin the active energy-harvesting composite layer. However, owing to theunresolved issues of uniformity and reproducibility with regard to flexiblePNGs, composite-based PNGs do not fully satisfy the energy requirementsof wearable electronics. In this study, we rationally designed piezoelectricnanoparticles–polymer composites to overcome the practical limitations ofexisting PNGs. Our work represents the first approach to PNG fabricationvia chemical reinforcement of composite systems. The PNGs were fabri-cated using amine-modified lead zirconate titanate (PZT) nanoparticlescomposited with triblock copolymers grafted with maleic anhydride.Benefiting from the unique composites, our composite system showsexcellent dispersion of the PZT nanoparticles in the polymer matriceswithout the use of dispersing agents as well as high performance involtage and current generation under applied flexural stresses. Our studyopens a new route in the development of high-performance PNGs, andprovides an insight into energy sources for low-power-consumptionportable devices, biomechanical sensors, and wearable electronics.

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Lee and coworkers prepared a flexible PNG based on compositefilms of Pb-free piezoelectric NPs, carbon nanomaterials,and PDMS; the PNG was demonstrated to operate commercialelectronic devices.10 In addition to the piezoelectric materialproperties, the uniform distribution of piezoelectric particleswithin the polymer matrix is also critical to achieve highoutput power generation. To create homogeneous dispersions,carbon nanomaterials and metal nanorods have been used asdispersants for composite systems.10,25 Although the reportedflexible energy harvesters show the capacity to realize uniformdistributions and high performances, proposed PNG devicesutilizing dispersion enhancers continue to show problems andlimitations.

Here we propose a high-performance flexible PNG devicebased on a chemically strengthened system compositing amine-functionalized PZT NPs (PZT-NH2 NPs) with a thermoplastictriblock copolymer without additional dispersion enhancers.The piezoelectric PZT NPs are surface-functionalized with aminegroups, ensuring covalent coupling with maleic anhydride mole-cules in the polymer matrix. The uniform distribution of thePZT-NH2 NPs in the polymer composite improves the piezo-electric power generation compared to that of a PNG devicecontaining pristine PZT NPs. The fabricated PZT-NH2-basedflexible PNG device based on this unique composite producesopen-circuit voltages reaching 65 V and short-circuit currentsreaching 1.6 mA during periodic bending and unbendingmotions. The power generated by a single device is sufficientto operate a commercial LED without rectification or storagemodules. Furthermore, the PNG device was combined withcapacitors to build power modules for self-powered systems.This achievement is unprecedented in the rational optimizationof composite-based PNG devices and represents the firstapproach utilizing chemically reinforced composites systems.

Results and discussion

Fig. 1a shows a schematic image of the PZT NP/elastomercomposite-based PNG; the detailed fabrication process is describedin the Experimental section. To prepare well-dispersed compositematerials for a high-performance energy harvester, we used amine-functionalized PZT NPs as energy generation sources. As shownin Fig. 1b, direct coupling between the polymer matrix andthe PZT NPs offers many advantages because the PZT NPs aretrapped within the polymer network. The integration of the PZTNPs into composite films improves the distribution uniformityof NPs in the polymer matrix, which enhances the stressesapplied to the NPs during PNG deformation. Enhanced stressesincrease the output performance of the flexible composite-basedPNG. To prove this hypothesized mechanism, we functionalizedthe surface of the PZT NPs via electrostatic interactions betweenthe PZT NPs and poly(ethyleneimine) (PEI) under basic condi-tions. A scanning electron microscope (SEM) image of the PZTNPs used is shown in Fig. S1a in the ESI;† the NPs exhibitirregular shapes and the average size of B500 nm. The X-raydiffraction (XRD) results reveal excellent crystallinity of the

tetragonal-phase PZT NPs; this phase exhibits good piezoelectricproperties (Fig. S1b in the ESI†). In the XRD pattern, thediffraction peaks for tetragonal phase of PZT NP appeared tobe more distinctive according to the ratio of Zr/Ti = 52/48. Asshown in Fig. S1c (ESI†), the PZT NPs exhibited a negative zetapotential over �5 mV in an aqueous medium with a neutral pH.The isoelectric point (IEP), at which the surface zeta potential isshifted to zero, is reported at pH between 5 and 7 for PZT.26 Thiscorresponds well to the measured IEP value (5.8) for the PZTNPs. PEI is a cationic polymer with some capacity to preserve itssurface charge, even in a slightly basic aqueous medium. Byadding an aqueous PEI solution to an aqueous dispersion of PZTNPs at basic pH, the surface charge of the PZT NPs wassignificantly shifted by over 40 mV at a neutral pH (Fig. S1c,ESI†). After the excess PEI was removed by centrifugation, thesurface zeta potentials of the PZT-NH2 NPs were measured.

For a high-performance PNG based on composite materials, thedispersive stability of the piezoelectric NPs within the composite isthe most important factor under high solid-loading conditions. Toensure a stable dispersion, a styrene-b-ethylene/butylene-b-styrenetriblock copolymer (SEBS, denoted as SS in this study) and SEBSgrafted with maleic anhydride (SEBS-g-MA, denoted as SM in thisstudy) were used as the polymer matrix. As shown in Fig. 1c, thePZT-NH2 is well distributed in the thermoplastic SEBS-g-MA (SM)matrix, attributed to the reaction of the amine functional group(–NH2) of PZT with the maleic anhydride in SM. The chemicalreaction between anhydride groups on SM polymer and aminegroups on PEI was confirmed by Fourier-transform infrared (FTIR)spectra. Fig. S1d (ESI†) compares FTIR spectra of composite filmsannealed at 25 and 80 1C. The proposed chemical reaction at 80 1Cgives rise to an amide bond, which was observed by C–N stretchpeaks in the range of 950–1250 cm�1. Another C–N stretch peak,known as aromatic amine, was also observed in the range of1250–1330 cm�1.27 Compared to previously reported composite-based energy harvesters, no metal dispersion-stabilizing fillerthat could deteriorate the poling effect was incorporated in the

Fig. 1 Schematics of the (a) prepared PZT-NH2 NP-based PNG deviceand (b) PZT-NH2 NP-to-polymer interaction. (c) Cross-sectional SEMimages of composite film comprising PZT-NH2 NPs and polymer matrix.(d) Photograph of flexible PZT-NH2 NP-based PNG device.

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composite films. The flexible PZT-NH2/SM composite filmsadhered well to Al-coated plastic substrates. The completedflexible PZT-NH2 NP-based PNG device can be bent by humanfingers, as shown in Fig. 1d, indicating high flexibility and goodstability during deformation.

To elucidate the role of the amine functionalization ofthe PZT NPs in producing high performance in the PNGs, wecharacterized the performances of PNG devices with and with-out functionalization. The thickness of all PNGs in this studyranges from 200 to 250 mm, so that the mechanical neutralplane is located at the bottom substrate. The output perfor-mance of PNGs with different thickness can be varied due tothe movement of the neutral plane. The generated outputvoltages and currents of the PNG devices were measured underperiodic bending and unbending processes. Fig. 2a presentsthe output voltages and currents from the non-poled PNG,poled PNG prepared with pristine PZT NPs-SM/SS composites,poled PNG prepared using PZT-NH2 NPs-SM composites, andpoled PNG prepared with PZT-NH2 NPs-SM/SS composites. Asshown in Fig. 2a-i, no remarkable signals are generated by thenon-poled device. In addition, very low values for the voltageand current under periodic bending motions were observedfrom some control experiments (Fig. S2 in the ESI†), whichshows that neither triboelectric nor electrostatic effects contribute

to the output signals of the PNG device. In the compositesprepared using pristine PZT NPs, high aggregation and poordispersion are observed in the NPs (Fig. S3a in the ESI†). Inaddition, the pristine PZT NPs have a lower piezoelectriccoefficient than the PZT-NH2 NPs, as determined from theslope of the piezoresponse–applied voltage curve (Fig. S3b inthe ESI†). Therefore, the poled PNG device prepared withpristine PZT NPs-SM/SS composites exhibits a low outputvoltage and current (Fig. 2a-ii, Fig. S3c and d, ESI†).

Another effect of amine functionalization on the outputperformance of PNG is enhancement of the stress applied tothe PZT NPs by chemical bonding between the NPs and polymermatrix. As shown in the high-resolution transmission electronmicroscopy (HRTEM) images and energy-dispersive X-ray spectro-scopy (EDX) maps shown in Fig. 2b and c, the PZT-NH2 NPs arewell distributed in the SM/SS polymer matrix with strong chemicallinkages between the amine groups and maleic anhydride. Asshown in Fig. 2a, the remarkable output performance of thepoled PNG device prepared using PZT-NH2 NPs-SM/SS is clearin comparison with that of the poled PNG device prepared withPZT-NH2 NPs-SM composites. The output voltage and currentof 65 V and 1.6 mA, respectively, are measured from the SM/SS-based PNG; those of 12 V and 0.3 mA, respectively, are obtainedfrom the –SM-based PNG. The generated output voltage value ishigher than that of the previously reported composite-basedflexible PNGs.24,25,28–30

The generation mechanism of the electrical energy from thePZT NP-based PNGs is schematically illustrated in Fig. S4 ofthe ESI.† The randomly distributed dipole moments within thecomposite films can be aligned along the poling directionbelow the Curie temperature. When the PNGs is subjected tothe mechanical stress by bending motions, the strong chemicallinkages between the particles and polymer matrix exception-ally enhance the stress applied to PZT-NH2 NPs and the piezo-electric potential is generated between two electrodes, whichleads to the movement of the electrons from bottom to topelectrode to balance the potential difference (Fig. S4a, ESI†). Asthe PNG is released, the induced piezopotential disappear andthe accumulated electrons on top electrode flow back to bottomelectrode (Fig. S4b, ESI†). The enhanced output performance ofthe SM/SS-based PNG can be attributed to the mechanicalproperties of the composites films. In the PZT-NH2 NPs-SMcomposite, strong chemical linkages between the particles andpolymer matrix increase the Young’s modulus relative to that ofthe SM/SS mixture because of the lack of chemical compatibilitybetween the amine-functionalized PZT NPs and SS polymer matrix(Fig. S5a, ESI†); therefore, the output performance of the SM-basedPNG is decreased because the stress applied to the PZT-NH2 NPs inSM is decreased. The concentration ratios of the SM to SS solutionsalso affected the output voltages of the PNGs. The highest outputvoltage was observed in the PNG produced using a polymersolution with equal parts SM and SS (Fig. S5b, ESI†).

The poling process aligns the piezoelectric dipoles in the PZTNPs in the same direction; this enhances the electrical outputproperties of the PNG devices. Fig. 2d shows the generatedoutput voltage and current signals from the PZT-NH2 NP-based

Fig. 2 (a) Output voltages and currents generated from (i) non-poledPNG device, (ii) poled PNG device prepared with pristine PZT NPs-SM/SScomposites, (iii) poled PNG device prepared with PZT-NH2 NPs-SMcomposites, (iv) poled PNG device prepared with PZT-NH2 NPs-SM/SScomposites. (b and c) HRTEM image and compositional distributionanalysis for PZT-NH2 NP-based composite films. (d) Output voltage andcurrent generated from the PZT-NH2 NP-based PNG devices as a functionof poling voltage from 300 to 700 V. (e) Variation of output voltage andcurrent obtained from PNG devices with different weight fractions ofPZT-NH2 NPs.

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PNG devices polarized by varying poling voltages from 300 to700 V. The low output signals from the non-poled PNG deviceare increased to approximately 65 V and 1.6 mA when the deviceis poled at 500 V. To determine the optimal concentration ofPZT-NH2 NPs in the polymer matrix for high-performanceenergy harvesters, PNG devices with various ratios of PZT NPsand polymers were fabricated. As shown in Fig. 2e, the generatedoutput voltage and current of the PNGs are gradually increasedwith increases in the density of the piezoelectric PZT NPs from0 to 30 wt% in the composite films. The increased voltage mayarise from the increased polarization from the significantchanges in the overall dielectric constant within the PZTNP–polymer composite films. However, the piezoelectric voltageand current output from the PNGs are decreased for increases inthe PZT NP concentration 430 wt% because of the degradedelectromechanical coupling effects from the overly high dielectricconstants of the composite films.31,32

To confirm that the measured output performances of theflexible PNG are generated by the piezoelectric properties of thePZT-based composite films, we conducted switching polaritytests (Fig. 3a and b).12 When the PNG device is forward-connected to a measurement instrument, it generates a positivevoltage and current in the bent state (Fig. 3a-i and ii). However,the polarity of the output signal is inverted when the connec-tion is switched (Fig. 3b-i and ii). The output voltage andcurrent generated from the PNG devices show reduced lifetimes

(full widths at half maximum) of 5.2 ms and 4.8 ms, respec-tively, shorter than that of the composite film containingpristine PZT NPs. This enhancement is most likely caused bythe improved mechanical properties of the composite materialvia the chemical bonding between the PZT-NH2 NPs andpolymer matrix. In addition, the strain-dependent output per-formance of the PZT-based PNG device was evaluated underperiodic bending motions with various bending radii and strainrates. For a constant strain rate, the amplitude of the outputvoltage increases with decreases in the bending radius becausethe piezoelectric potential within the composite film is enhancedby larger bending strains (Fig. S6a, ESI†). When the PZT-basedPNG device is bent quickly to a fixed bending radius, a greateroutput voltage is obtained than that produced by the slowbending of the PNG device (Fig. S6b, ESI†). To evaluate thepractical applicability of the flexible energy harvester, mechan-ical durability tests were conducted. Voltage measurements overmore than 5000 bending cycles in 700 s with bending to thedisplacement of 10 mm at the strain rate of 6 cm s�1 arepresented in Fig. 3c. The generated voltages from the PNG deviceof approximately 65 V show no degradation, indicating electro-mechanical stability throughout the 5000 cycles.

To investigate the instantaneous power, the voltage andcurrent signals are measured from a flexible energy harvesterwith diverse external-load resistors ranging from 100 kO to1 GO, as shown in Fig. 4a. With increasing resistance, theoutput voltage signals are increased gradually; the outputcurrent signals exhibit the opposite trend. The instantaneousoutput power is calculated by multiplying the maximum outputvoltage and current. Fig. 4b shows the instantaneous power ofthe PZT-NH2 NP-based PNG across various resistive loads,yielding the maximum power of B26 mW at the resistanceof B50 MO. To develop PZT-NH2 NP-based PNGs as energysources for low-power-consumption electronic devices, the out-put power from the piezoelectric energy harvesters should beused to charge energy storage devices. We utilized four diodesas a bridge rectifier circuit and electrolytic capacitors connectedin series to store the electrical energy from the energy harvesters.The rectified output power was fed directly to the capacitors(Fig. S7a, ESI†). The inset of Fig. 4c shows the equivalent circuitdiagram: the capacitor can be charged by the rectified outputvoltage generated by the PNG under periodic bending motionswith the displacement of 10 mm at the strain rate of 6 cm s�1

and frequency of 5 Hz. Three capacitors with different capaci-tance values are successfully charged, as shown in Fig. 4c. The2.2 mF capacitor is charged by our energy harvester to 4 V in100 s under periodic bending and unbending. Similarly, chargesof 2 V in 300 s and 1 V in 400 s for the 22 mF and 47 mF capacitorsare obtained, respectively. Furthermore, by the continuousbending and unbending movement of our PNG connected inseries to six 22 mF capacitors for 20 min, the total usable chargedvoltage reaches B10.35 V (Fig. S7b, ESI†). As shown in Fig. 4d,the electrical energy produced from the mechanical energy issuccessfully used to drive four commercial light-emitting diodes(LEDs) aligned in series by connecting the six charged capacitorsin series.

Fig. 3 (a) The measured output voltage and current signals of the PNGdevice in forward connection during periodic bending motions. (b) Theopen-circuit voltage and short-circuit current signals generated in thereverse connection. (c) The durability test for the PZT-NH2 NP-based PNGdevices under 5000 cycles of bending motions.

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To demonstrate the use of the electrical energy generated fromthe PZT-NH2 NP-based PNG device and its potential application inenergy harvesting technologies, we directly operated a number ofcommercial LEDs without using charged capacitors. As shown inFig. 5a, the PNG is connected to a full-wave bridge rectifier circuitto convert the alternating electric signals into DC-type outputs,and the rectified output is connected to 20 LEDs. Commercialgreen LEDs displaying the letter ‘‘K’’ are operated successfullyupon cyclic bending of our PNG device, indicating that the PNGdirectly power the LEDs without external electrical power sources.In practice, the PNG device could also be applied as a bending-based energy harvester for sustainable power generation or abending-motion sensor. Instantaneous output peaks, measuredfrom the PZT-NH2 NP-based PNG attached to a human elbow

and wrist, are shown in Fig. 5b and c. Consequently, thecomposite-based flexible energy harvester can be utilized forenergy harvesting from biomechanical movements, as well asfor monitoring human motion without external power sources.

Conclusions

In conclusion, we have developed a high-performance PNGdevice based on a chemically reinforced composite system.By incorporating amine-functionalized PZT NPs and a thermo-plastic triblock copolymer grafted with maleic anhydride,homogeneous dispersions with well-distributed PZT NPs wereobtained without using dispersion enhancers. The generatedoutput voltage and current reached 65 V and 1.6 mA, respectively,sufficient to drive commercial LEDs. The PZT-NH2 NP-basedflexible PNG exhibited remarkable stability without degradationover 5000 cycles of mechanical deformation. By using a full-wavebridge rectifier circuit, the alternating output energy generatedfrom the PNG device was stored in capacitors and subsequentlyused to power a commercial LED device. Our composite systemsuccessfully overcomes the dispersion stability-related restric-tions of previous nanogenerators, enabling the fabrication of asimple, robust, and large-scale energy harvester system. Further-more, the proposed approach presents a significant advance incomposite-based PNG research for self-powered flexible energysources to realize wearable electronics and sensors usable forubiquitous wireless communication.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work has been performed as a Basic project (referringto projects performed with the budget directly contributed bythe Government to achieve the purposes of establishment ofGovernment-funded research institutes) and supported by theKOREA RESEARCH INSTITUTE of CHEMICAL TECHNOLOGY(KRICT, SI1802) and the Global Research Laboratory Programof the National Research Foundation (NRF) funded by Ministryof Science, Information and Communication Technologies andFuture Planning (NRF-2015K1A1A2029679).

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Fig. 4 (a) Output voltage and current signals under variable externalresistances ranging from 100 kO to 1 GO. (b) Calculated instantaneousoutput power at different load resistances. (c) The charged voltage in2.2 mF, 22 mF, and 47 mF electrolytic capacitors from continuous bending ofthe flexible PNG device. Inset: Schematic circuit diagram of the full bridgerectifier and electrolytic capacitor. (d) Photographs showing commercialgreen LEDs operated by the energy stored in capacitors charged withelectrical energy generated from the PNG device.

Fig. 5 (a) An array of 20 green LEDs in series simultaneously powereddirectly by the electricity generated from the PNG device. The generatedoutput voltage (open-circuit voltage) from the PNG device attachedto (b) elbow and (c) wrist when subjected to bending.

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