Stretchable piezoelectric nanocomposite generator
© The Author(s) 2016
Received: 17 March 2016
Accepted: 12 April 2016
Published: 3 June 2016
Piezoelectric energy conversion that generate electric energy from ambient mechanical and vibrational movements is promising energy harvesting technology because it can use more accessible energy resources than other renewable natural energy. In particular, flexible and stretchable piezoelectric energy harvesters which can harvest the tiny biomechanical motions inside human body into electricity properly facilitate not only the self-powered energy system for flexible and wearable electronics but also sensitive piezoelectric sensors for motion detectors and in vivo diagnosis kits. Since the piezoelectric ZnO nanowires (NWs)-based energy harvesters (nanogenerators) were proposed in 2006, many researchers have attempted the nanogenerator by using the various fabrication process such as nanowire growth, electrospinning, and transfer techniques with piezoelectric materials including polyvinylidene fluoride (PVDF) polymer and perovskite ceramics. In 2012, the composite-based nanogenerators were developed using simple, low-cost, and scalable methods to overcome the significant issues with previously-reported energy harvester, such as insufficient output performance and size limitation. This review paper provides a brief overview of flexible and stretchable piezoelectric nanocomposite generator for realizing the self-powered energy system with development history, power performance, and applications.
Attractive approaches based on energy harvesting technology that convert ambient energy resources such as thermal, solar and mechanical energies into electrical energy have been recently studied to realize the demonstration of self-powered energy system in portable devices without external power sources like batteries [1–4]. Among these sustainable energy resources, the mechanical energy is easily accessible compared to outdoor renewable energy in anytime and anywhere (even inside human body) of our daily [5–8]. To convert the mechanical energy resources (e.g., pressure, bending, stretching and vibrational motions) to electricity, the piezoelectric energy conversion are proposed and investigated by many research groups [3, 9, 10]. In particular, the flexible energy harvester called a nanogenerator which can generate electrical energy from not only mechanical energy but also tiny biomechanical energy (e.g., heartbeat, muscle motions, and eye blinking) have attracted attention in response to the demands of infinite self-powered sources for operating flexible and wearable electronic systems .
Because polymer materials have the naturally flexible properties and mechanical stabilities, the piezoelectric polymers as polyvinylidene fluoride (PVDF) have been used to fabricate polymeric flexible nanogenerators (Fig. 1b) [16–18]. Figure 1b-i presents the piezoelectric PVDF nanofibers placed on a working substrate using a unique direct-write technique. The PVDF based nanogenerator generates electrical outputs of about 5–30 mV and 0.5–3 nA when the substrate is deformed by stretching and releasing motions [16, 17]. Cha et al.  proposed the polymeric nanogenerator made of porous PVDF by employing a novel template-assisted method (Fig. 1b-ii). This sonic wave driven energy harvester produces higher output performance compared to dense PVDF based nanogenerators due to the effective nanoporous structure.
In 2010, there have been new approaches to use perovskite-structured ceramic [PbZrxTi1−xO3 (PZT) and BaTiO3] thin films with inherently high piezoelectricity for higher energy conversion efficiency (Fig. 1c) [19–22]. The PZT and BaTiO3 thin films on bulk substrate are transferred onto flexible PI substrates by adopting unique transferring techniques after high temperature crystallization process (Fig. 1c-i, ii). The perovskite thin film-based nanogenerators show higher power density compared with other flexible piezoelectric devices with the similar device structure. Qi et al.  presented the energy harvester of wavy piezoelectric PZT ribbons, that can scavenge stretching motions to generate.
Recently, the research on large-area, low-cost, mechanically-stable, and high-output nanocomposite generator has been invented by using simply casting piezoelectric nanocomposites onto flexible plastic substrates at low temperature (Fig. 1d) [23–27]. In particular, several research groups have developed lead-free and high-performance flexible energy harvesters using bio-eco-compatible piezoelectric ceramics such as BaTiO3 , (K, Na)NbO3 [24, 25], and Li-doped (K, Na)NbO3 . Subsequently, a new concept of ultra-stretchable elastic-composite generator, was also developed by employing the Ecoflex silicone rubber-based piezoelectric composites and long silver nanowire-based stretchable electrodes . This review paper highlights a brief overview of flexible and stretchable piezoelectric nanocomposite generator for realizing the self-powered energy system, summarizing development history, power performance, and energy applications. These technologies provide novel solutions to overcome the challenging issues about nanogenerators, such as insufficient output signals and size limitation for the power sources of commercial electronics.
2.1 BaTiO3 nanoparticles-based flexible nanocomposite generator
2.2 NCG device based on various piezoelectric particles
Since the first demonstration of BaTiO3 NPs-based NCG device, many research groups proposed other type of composite-based nanogenerators composed of various piezoelectric materials such as ZnO, PZT, and alkaline niobate particles using the NCG technique [26, 27, 29], due to the advantage of highly-efficient and large-area energy harvesters with simple and low-cost process. By using ZnO NPs of the higher piezoelectric performance compared to ZnO nanowire, the novel flexible nanogenerator composed of ZnO NPs and multiwall-carbon nanotubes (MW-CNTs) was demonstrated, as shown in Fig. 3b . The energy harvester generated output voltage of 0.4 V and current pulse of 50 nA during the repeat finger gestures with the frequency of 1 Hz. The voltage and current signals reached up to the 7.5 V and 2.5 μA, respectively, under hammer knocking on the energy device. The inherently excellent piezoelectric PZT particles were also adopted to fabricate NCG device (Fig. 3c) . The NCG device with PZT particles and multiwall (MW)-CNTs showed the distinguishable improvement in output performance of NCG device (3 × 3 cm2): the converted output voltage and current signals are ~10 V and 1.3 μA, respectively. Although this nanogenerator provided the methodology for high-output flexible energy harvesting device, PZT-based device has critical environmental drawbacks due to Pb-related problems. One of the most attractive lead-free piezoelectric materials is alkaline niobate which provides similar piezoelectric properties compared to PZT material. Jeong et al.  constructed the lead-free NCG device made of outstanding piezoelectric and bio-ecofriendly (K,Na)NbO3-LiNbO3 (KNLN) nanoparticles and well-dispersible copper nanorods (NRs) fillers (Fig. 3d). The KNLN-based NCG device not only creates high output with 12 V and 1.2 μA but also shows excellent stability and durability without any degeneration under the repeated bending cycles. By employing the bar-coating method, a large-area NCG device of 30 × 30 cm2 was demonstrated, as shown in Fig. 3e . A large-scale NCG device harvested high output signals, output voltage of ~100 V and current of ~10 μA.
2.3 Piezoelectric nanowire/tubes-based NCG device
2.4 NCG device by adopting unusual structure and Bioinspired approach
2.5 Hyper-stretchable composite energy harvester
2.6 Energy applications by flexible and stretchable NCG devices
The developments of flexible and stretchable energy harvesting device through piezoelectric materials-polymer composites have enabled the self-powered energy system with high performance and improved mechanical stability. Simple and low-cost fabrication processes such as spin-casing, die-casting, and bar-coating allow the large-area energy harvester. First, the typical piezoelectric nanomaterial (e.g., BaTiO3 NPs) was utilized to form the piezoelectric composites; subsequently, the inherently excellent piezoelectric PZT material-based high-output NCG device were developed. Recently, the many researchers have investigated the new approaches for employing the alkaline niobate-based high piezoelectric ceramics and unusual structured BaTiO3 due to additional motivations for environment-friendly and biocompatible energy harvesters. Moreover, the ultra-stretchable NCG device were demonstrated via hyper-stretchable electrodes of Ag NWs percolation network in response to the demand of energy harvesting devices closely in contact with the curvy surfaces. The fabricated NCG device can produce the power from biomechanical movements and sufficiently operate the commercial electronic devices. The NCG technology will be expected to improve further by adopting the controllable lead-free piezoelectric ceramic materials [such as Ba(Zr,Ti)O3 (BZT), (Bi, Nd)Ti3O12, and SrBi2Ta2O9] and the enhaced core–shell structured nanomaterials. Furthermore, the novel printing technique with p-NC will open a key role in the development of the commercially available self-powered energy system. The piezoelectric rubbers will facilitate solutions to develop the self-powered road-system consisting of energy generation source, highly-sensitive sensor, and wireless transmitter for military and wearable applications. In addition, these unique approaches for flexible energy harvesters shall support energy sources of various state-of-a-art flexible and future electronic systems [37–56].
KIP and CKJ wrote the manuscript and KJL guided manuscript preparation. All authors read and approved the final manuscript.
Prof. Kwi-Il Park is currently assistant professor in the Department of Energy Engineering at Gyeongnam National University of Science and Technology (GNTECH). Prof. Park received his Ph.D. in Materials Science and Engineering (MSE) at Korea Advanced Institute of Science and Technology (KAIST). During his Ph. D. at KAIST, he studied flexible energy harvesting technology using inorganic-based piezoelectric materials by. He was previously a senior researcher at Agency for Defense Development from 2014 to 2015. His research interests include the synthesis of high performance piezoelectric nanomaterials and development of flexible/stretchable energy harvester based on inorganic piezo-materials.
Dr. Chang Kyu Jeong received his Ph.D. degree in Materials Science and Engineering from KAIST in 2016 and B.S. degree from Hanyang University in 2011. He is currently working as a research fellow in KAIST Institute for the NanoCentury (KINC). His research topics focus on mechanical energy harvesting, soft/biomaterials, surface chemistry, and nanomaterial syntheses.
Prof. Keon Jae Lee received his Ph.D. in Materials Science and Engineering (MSE) at University of Illinois, Urbana-Champaign (UIUC). During his Ph.D. at UIUC, he involved in the first co-invention of “Flexible Single-crystalline Inorganic Electronics”, using top-down semiconductors and soft lithographic transfer. Since 2009, he has been a professor in MSE at KAIST. His current research topics are self-powered flexible electronic systems including energy harvesting/storage devices, LEDs, large-scale integration (LSI), high-density memory and laser material interaction for in vivo biomedical and flexible applications.
This work was supported by Gyeongnam National University of Science and Technology (GNTECH) Grant 2015. This study was also supported by Global Frontier R&D Program on Center for Integrated Smart Sensors (Grant No. CISS-2012M3A6A6054193)
The authors declare that they have no competing interests.
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