Scavenging Biomechanical Energy Using High-Performance, Flexible

Apr 17, 2017 - The piezoelectric output performance of the CF was investigated as a function of the weight ratio of the BTO NCs in the polymer matrix,...
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Research Article pubs.acs.org/journal/ascecg

Scavenging Biomechanical Energy Using High-Performance, Flexible BaTiO3 Nanocube/PDMS Composite Films Nagamalleswara Rao Alluri,† Arunkumar Chandrasekhar,‡ Venkateswaran Vivekananthan,‡ Yuvasree Purusothaman,‡ Sophia Selvarajan,§ Ji Hyun Jeong,† and Sang-Jae Kim*,‡ †

School of Applied Energy Systems, Jeju National University, Engineering Building No. 4, D-130, Ara-1-Dong, Jeju-Si, Jeju-Do Jeju-690756, South Korea ‡ Nanomaterials & System Lab, Department of Mechatronics Engineering, Jeju National University, Engineering Building No. 4, D-130, Ara-1-Dong, Jeju-Si, Jeju-Do Jeju-690756, South Korea § Department of Advanced Convergence Science & Technology, Jeju National University, Ara-1-Dong, Jeju-Si, Jeju-Do Jeju-690756, South Korea S Supporting Information *

ABSTRACT: Highly flexible, biocompatible, large-scale production of BaTiO3 nanocube (BTO NC)/poly(dimethylsiloxane) (PDMS) composite films (CFs) prepared via a simple, cost-effective solution casting technique are reported for the first time for high-performance piezoelectric nanogenerators (PNGs). The crystalline BTO NCs were synthesized via a simple low-temperature molten salt method. The piezoelectric output performance of the CF was investigated as a function of the weight ratio of the BTO NCs in the polymer matrix, electrical poling, constant mechanical loading, and low-frequency biomechanical energy harvesting. The composite PNG (CPNG) with 15 wt % of BTO NCs displayed an excellent peak-to-peak voltage (Vpp) of 126.3 V and current density (J) of 77.6 μA/cm2 and generated a maximum instantaneous areal power density of 7 mW/cm2 at 100 MΩ at the low input mechanical pressure of 988.2 Pa. The generated output was sufficient to drive commercial light-emitting diodes and low-powered consumer electronic devices. Next, the CPNG was tested to harness waste biomechanical energy in our daily life; it generated a Vpp of 29 V (human hand palm force) and 55.9 V (human foot stress). The proposed device was lightweight, flexible, eco-friendly, cost-effective, and a potential candidate to generate high electrical output at low mechanical pressure. KEYWORDS: BaTiO3 nanocubes, Biomechanical energy, Composite film, Nanogenerator, Piezoelectricity

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INTRODUCTION Pollution, global warming, and carbon emissions are increasing dramatically due to the continuous use and greater consumption of natural resources for energy generation.1 This needs to be controlled for a better society. However, reducing the world energy crisis (i.e., power supply and demand) is a major challenge.2 This can be achieved somewhat by the use of unconventional energy harvesting approaches such solar,3 thermal,4 piezoelectric,5 triboelectric,6 and pyroelectric.7 Another advantage of these technologies is their dual functionality; i.e., they can function as portable independent energy harvesting sources to drive low-powered electronic devices and can also work as self-powered sensors to measure various physical8/chemical9/biological10 inputs in a precise manner under different environmental conditions. Several of these technologies suffer from issues relating to device design limitations, development of highly efficient nanomaterials/ composite films (CFs), power management circuits, unstable output behavior, and packaging issues. Many researchers believe that the piezoelectric nanogenerator (PNG)5,8−11 is an efficient unconventional approach for harvesting waste © 2017 American Chemical Society

mechanical energy such as from human body movements (walking, running, and human finger movements), ocean waves, wind/water flow, and low-frequency mechanical vibrations.12 Traditional nanogenerators have been successfully used to power low-powered electronic devices such as light-emitting diodes (LEDs), liquid crystal displays (LCDs), and sensors.8−12 Wang introduced the new technique in 2006, harnessing mechanical energy using nanosized ZnO.5 Over the past decade, extensive research has improved the generated output power using innovative device designs and various inorganic materials. Similarly, flexible PNGs were developed using piezoelectric organic polymers such as poly(vinylidene fluoride) (PVDF)13 and its copolymers.14 This type of PNG generates a lower electrical output response than high-performance-based PNGs using inorganic nanoparticles (NPs) having a high piezoelectric coefficient (d33) due to the lower d33 of polymers. The present trend focusing on the development of smart PNGs Received: January 12, 2017 Revised: March 22, 2017 Published: April 17, 2017 4730

DOI: 10.1021/acssuschemeng.7b00117 ACS Sustainable Chem. Eng. 2017, 5, 4730−4738

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ACS Sustainable Chemistry & Engineering using smart composite materials,9,12,15 i.e., substitution of high d33-based inorganic NPs as fillers in an organic polymer, seeks to improve the output power of PNGs at low input mechanical energy.16,17 The use of composite materials for PNGs has many advantages such as cost-effectiveness, easy processing, ecofriendliness, flexibility, lower internal leakage current, adaptability for large mechanical forces, ease of large-scale fabrication, and the possibility of providing a higher output power compared with that from traditional PNGs using inorganic nanostructures alone.18 Composite structures for PNGs are more common and facilitate different device designs to achieve better output with good reliability and stability. For example, Zheng et al.19 developed the porous cellulose nanofibril (CNF)/poly(dimethylsiloxane) (PDMS) aerogel film. Siddiqui et al.20 fabricated a highly durable, stable composite piezoelectric nanogenerator (CPNG) using nanocomposite nanofibers (NFs) of P(VDF-TrFE) and BT NPs. Mao et al.21 designed sponge-like piezoelectric polymer films for integratable PNGs and self-powered electronic systems. Park et al.15 initially demonstrated the feasibility of a CPNG using the BaTiO3 nanoparticle (BTO NP)/multiwalled carbon nanotube (MWCNT)/PDMS composite and extended the design to other systems using zinc oxide (ZnO), lead zirconate titanate (PZT), and alkaline niobate NPs (KNN) with reasonable output power.15 Alluri et al. developed the Ba(ZrTi)O3 nanocube (NC)/PVDF composite,12 BTO NP/ Ca-alginate spherical beads,22 and linear17 and wavy-patterned composite worms9 having reasonable electrical outputs; devices made with them were used to measure water velocity from an outlet pipe, finger gestures, and pH values. Kim et al.23 designed the Na0.47K0.47Li0.06NbO3 NCs-PDMS CF system that generated an output voltage of 48 V and current density (J) of 0.43 μA/cm2 under a vertical compressive force of 19.7 N at a frequency of 3 Hz. Lin et al.24 studied the BTO NT/PDMS composite; the PNG made with it generated electrical outputs of 5.5 V and 350 nA. The performance of a composite materialbased PNG depends on the selection of the organic polymer, high-d33-based inorganic NPs, weight ratio of the NPs, thickness of the composite layer, and surface morphology of the particles (NCs, nanotubes (NTs), nanofibers (NFs)). In certain cases, polymers such as PVDF, P(VDF-HFP), and P(VDF-TrFE) that have been used for the development of composite materials play a dual role such as providing a supporting layer to hold the NPs and being an active layer to contribute to the electrical output.12,18,25 Shin et al.18,25 developed a BTO NP/P(VDF-HFP) CF that had a high electrical response; they stated that the role of the P(VDFHFP) was as a cementing layer (not an additive component to BTO). The electrical output contribution from the P(VDFHFP) was very small compared with that of the CF, and the authors noted that P(VDF-HFP) has an α-phase that remains nonpiezoelectric even after poling. Alluri et al.12 designed Ba(ZrTi)O3 NC/PVDF CFs (≤100 μm thick) and studied the role of Zr doping into BaTiO3 nanocubes (BTO NCs) for energy harvesting. In this case, PVDF had a dual functionality, i.e., as a supporting and an active layer, because it contained the electro-active polar β-phase. Therefore, identification of the role(s) of PVDF in PVDF/inorganic NP CFs is crucial; however, this requires careful measurements. In contrast, PDMS, polycarbonate, poly(methyl methacrylate), and a few other polymers function only as a supporting cross-linker to hold the NPs; there is no ambiguity concerning the role of these polymers with respect to any improvement in

piezoelectric properties and electrical energy response.9,17,23,24 The following composites have generated low electrical outputs: BTO NT/PDMS (5.5 V, 350 nA),24 BTO nanowire (NW)/PDMS (7 V, 360 nA),26 BTO NP-MWCNT/PDMS (3.2 V, 350 nA),15 BTO NC/PVDF (7.99 V, 1.01 μA),12 virustemplated BTO nanostructure/PDMS (6 V, 300 nA),27 BTO NF/PDMS (2.67 V, 261 nA),28 and BTO NP/bacterial cellulose-based (14 V, 150 nA/cm2).29 Notably, the BTO NP/P(VDF-HFP) composite (110 V, 22 μA) exhibited a high electrical output.25 Clearly, extensive research is necessary to identify cost-effective fabrication techniques, shorten processing times, and enhance the piezoelectric output of BTO materials in a suitable polymer matrix under the application of low mechanical forces. This could be realized by increasing the thickness of the CF, optimizing the weight ratio of the NPs in the polymer matrix, orienting the electric dipole via a suitable electrical poling process, and altering the surface morphology and interfacial interactions between the NPs and the polymer. All of these changes would reduce the dielectric constant and improve the d33 of the CF, thereby leading to enhanced electrical output response of CF-based PNGs. This type of PNG is reliable and suitable for remote area locations for harvesting waste mechanical energy, i.e., biomechanical energy and that from wind/ocean waves and tiny vibrations. In this study, highly crystalline BTO NCs were synthesized using a low-temperature molten salt method. Flexible and biocompatible CFs were developed using a simple, low-cost solution casting technique to form a PDMS matrix containing BTO NCs (BTP-CFs). BTP-CF-based PNGs were fabricated and electrically poled at 8 kV for 24 h at room temperature. The piezoelectric performance of all of the PNG devices was investigated by applying a constant mechanical load of 2 N or a pressure of 988.2 Pa. The CPNG device containing 15 wt % of BTO NCs generated a maximum peak-to-peak voltage (Vpp) and current density (J) of 126.3 V and 77.6 μA/cm2, respectively. The role of the PDMS in energy generation, rolling capability, and thickness of the BTP-CF was demonstrated. Additionally, a weight ratio analysis of BTO NPs in the PDMS matrix was performed to study the load resistance, charging capability of commercial capacitors using the CPNG device output, and stability and power-up ability toward five commercial green LEDs. Finally, a CPNG device containing 15 wt % of BTO NCs was used to harness biomechanical energy from human hand and foot movements. Application of human foot stress to a CPNG device generated a higher piezoelectric potential than with human hand force. All of the results demonstrated that the CPNG device is a potential candidate for recovering waste mechanical energy (human body movements, wind/water flow motions, ocean waves, and other mechanical vibrations) available in the environment.

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EXPERIMENTAL SECTION

Synthesis of BaTiO3 Nanocubes (BTO NCs). Highly oriented and crystalline BTO NCs were synthesized using a low-cost, lowtemperature molten salt method.12 The detailed synthetic procedure is schematically given in Figure S1 and discussed herein. The initial precursors were separated into two parts. The first part was the reactants, i.e., BaCO3 (99.95%) and TiO2 (98%), that were used according to their atomic weight ratio to prepare a target stoichiometric BTO perovskite composition. The second part was the eutectic mixture composed of 50:50 mol % of NaCl:KCl flux with extra salt (1 g of Na2SO4) acting as a medium. The reactants and eutectic mixture were thoroughly ground in a mortar and pestle using ethanol for 30 min. The homogeneous powder was placed in an

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Figure 1. (a) Fabrication of a piezoelectric BaTiO3 nanocube (BTO NC)/poly(dimethylsiloxane) (PDMS) composite films (CFs) (BTP-CFs) using the solution casting technique. The inset photographs show the transparency of the BTP-CFs fabricated with various weight ratios of BTO NCs in the PDMS matrix. (b, c) Photographs showing the flexibility and rolling capabilities of the BTP-CFs. alumina boat and fired in an open-tube furnace at 750 °C for 3 h. During the heat treatment process, the eutectic mixture assisted the growth of the NC microstructure and provided homogeneous nucleation. Next, the chloride ions were removed from the obtained product using multiple washings with hot deionized water. The materials were then dried in an oven at 60 °C overnight to form the pure single-phase perovskite BTO NCs. These were used to fabricate piezoelectric CFs. Fabrication of Piezoelectric BTO NC/PDMS Composite Films (BTP-CFs). The 5 wt % BTP-CF was fabricated using the solution casting method (Figure 1a) as follows. A homogeneous transparent solution (10 g) was prepared using PDMS:hardener at a 10:1 ratio; BTO NCs (0.5 g) were then dispersed in the solution under magnetic stirring at 1000 rpm. A white, homogeneous, nontransparent solution was obtained after 30 min of stirring and was poured into a smooth glass Petri dish, followed by heating at 70 °C for 30 min in a hot-air oven. A whitish-colored BTP-CF was formed and peeled off from the glass Petri dish. The CPNG device was fabricated using the required dimensions (4.6 cm × 4.4 cm) of the as-fabricated BTP-CF. A similar protocol was followed to prepare the pure PDMS film and other BTPCFs with the weight ratios of 10, 15, 20, and 25 wt %. Fabrication of CPNG Devices. The CPNG device was fabricated using the required dimensions (4.6 cm × 4.4 cm) of the as-fabricated BTP-CF. To prepare the 5 wt % CPNG device, as-fabricated BTP-CF (5 wt %) was sliced and sandwiched between flexible top Al/Kapton films and bottom Al/Kapton films using the hot-pressing technique for 30 min. Two Cu wires were attached to the top and bottom of the Al electrode using silver paste; these were used to collect the generated charge carriers through the external circuit. To protect the asfabricated CPNG from external physical damage, the whole device was encapsulated with a thin, pure PDMS layer. After that, the CPNG devices were electrically poled at 8 kV for 24 h at room temperature to provide a permanent electric dipole orientation. A similar fabrication procedure was used to develop the pure PDMS-based device and other CPNG devices with the weight ratios of 10, 15, 20, and 25 wt %. Measurement System. The surface morphology, average size of the BTO NCs, and thickness of the CPNG device were analyzed by field-emission scanning electron microscopy (FE-SEM; model Supra55vp, Zeiss). The dynamic symmetry and active modes of the BTO NCs and the BTP-CFs were investigated using Raman spectroscopy

(model HR Evaluation, LabRAM) with a source wavelength of 514 nm, applied power of 10 mW, and a grating of 600 grooves/mm. The crystalline phase analysis of the as-prepared samples was determined by X-ray diffraction (XRD; Rigaku, Cu Kα radiation) with an input voltage of 40 kV and 40 mA current. The output voltage and current piezoelectric performance was measured using an electrometer (model 6514, Keithley) and current preamplifier (model SR570, Stanford Research Systems). Here, the constant mechanical input force was generated by a linear motor (model HF01-37, LinMot) having a shaft mass of 2 kg and operated using a predefined software program. Load resistance and capacitance analyses of a CPNG device were evaluated at constant mechanical pressure.

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RESULTS AND DISCUSSION The BTP-CFs composed of different weight ratios of BTO NCs (5, 10, 15, 20, and 25 wt %) in the PDMS matrix were fabricated using a simple, low-cost solution casting method (Figure 1a). The detailed fabrication procedure of the BTP-CFs is discussed in detail in the Experimental Section. A pure PDMS film possesses a low relative permittivity (K ≈3) and good flexibility and transparency. It is thus highly suitable as a supporting polymer matrix to cross-link (or immobilize) NP fillers. The substitution of piezoelectric BTO NCs as a filler material into the PDMS matrix would improve the relative permittivity of the PDMS matrix due to the higher relative permittivity of the BTO NCs. However, Figure 1a shows that the transparency of the PDMS film was lost with increasing weight percentage of BTO NCs in the PDMS matrix. Figure 1b and c illustrates the flexibility and rolling capability of the asfabricated BTP-CF films; these results indicate that the piezoelectric BTP-CF is suitable for flexible energy harvesting applications, such as portable and wearable self-powered sensors. The piezoelectric phase of the BTO NCs and BTPCFs were characterized by XRD and Raman spectroscopy. Figure 2a shows the XRD spectra of the as-fabricated BTO NCs, pure PDMS film, and BTP-CF (15 wt %). The XRD spectra of the BTO NCs indicated a single-phase perovskite 4732

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Figure 2. (a) X-ray diffraction analysis of the as-synthesized (i) BTO NCs, (ii) PDMS film, and (iii) BTP-CF (15 wt %). (b) Raman spectroscopic analysis of the as-synthesized (i) BTO NCs, (ii) PDMS film, and (iii) BTP-CF (15 wt %). (c, d) Field-emission scanning electron microscopy (FESEM) images showing the highly crystalline BTO NCs at different magnifications at the 1 μm and 200 nm scales.

PDMS has characteristic peaks at 494 and 713 cm−1 related to symmetrical Si−O−Si stretching and symmetrical Si−C stretching, respectively, and the peak at 1121 cm−1 corresponds to the supporting glass substrate (Figure 2b (ii)). The Raman spectrum of a BTO−CF consisted of the major active modes of the BTO NC tetragonal phase and the minor PDMS characteristic peaks, which were completely diffused into the BTO peaks. The phase shift to lower wavelength observed for the BTP-CFs relative to the pure BTO NCs pattern represents stress-induced effects. The phase information from the Raman spectra and the XRD patterns were in good agreement and confirmed the existence of noncentrosymmetric regions centered on titanium (Ti) atoms. The surface morphologies of the BTO NCs, PDMS film, and BTP-CF (15 wt %) were characterized and analyzed by FESEM. Figure 2c shows the uniform and high-quality crystalline BTO NC surface morphology at the 1 -μm scale, synthesized via the molten salt method. Figure 2d shows the magnified image of BTO NCs at the 200 nm scale that reveals the formation of BTO NCs at sizes ranging from 100 to 400 nm. The surface morphologies of the as-fabricated PDMS and BTPCF (15 wt %) are shown in Figure 3a and b. The pure PDMS film had a smooth surface at the 2 μm scale (Figure 3a). FESEM images of BTP-CF (15 wt %) at the 2 μm and 200 nm scales (Figure 3b) reveal the uniform distribution of BTO NCs in the PDMS matrix. A fabricated flexible piezoelectric BTP-CF

structure (Figure 2a (i)). The calculated average crystallite size and lattice strains were 27.51 nm and 3.86 × 10−3, respectively. The splitting of the diffraction peak at 45° corresponds to the indexed planes (002)/(200); the other major peaks are associated with the tetragonal crystal structure of the BTO lattice.2 This phase structure is more favorable toward high piezoelectric and ferroelectric properties compared with the nonpiezoelectric cubic phase of the BTO lattice. This XRD pattern was in good agreement with the International Centre for Diffraction Data (ICDD) pattern no. 98.001-3771. Figure 2a (ii) shows the XRD pattern of pure PDMS where the broad peak at 11.96° represents the amorphous phase of the polymer matrix. Figure 2a (iii) shows the XRD pattern of the BTP-CF (15 wt %), which reveals the amorphous phase of the PDMS and the piezoelectric phase of the BTO NCs. Here, the insulating PDMS matrix prevented the piezoelectric phase of the BTO NCs from changing, so that the as-fabricated BTPCFs could function as a flexible, piezoelectric active layer for the conversion of mechanical to electrical energy. Raman spectroscopy was used to resolve the dynamic symmetry and lattice vibrations of the BTO NCs and for phase identification of the BTO NCs and BTP-CFs (Figure 2b). No active modes are available for the cubic BTO lattice. However, the tetragonal BTO phase12 has active modes that are related to the octahedral sites of TiO6 centers; these modes correspond to the peaks at 312, 523, and 720 cm−1 shown in Figure 2b (i). 4733

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Figure 3. (a) FE-SEM image showing the smooth surface morphology of the pure PDMS film at the 2 μm scale. (b) FE-SEM image of BTP-CF (15 wt %) showing the uniform distribution of BTO NCs in the PDMS matrix at the 2 μm and 200 nm scales. (c) Schematic drawing of the CPNG device for harnessing mechanical energy. (d) Cross-sectional FE-SEM image of the CPNG device at the 100 μm scale; the inset is a photograph of the CPNG device (4.4 cm × 4.6 cm) without the PDMS packaging layer.

JSC (peak‑to‑peak) = 77.69 μA/cm2) than the pure PDMS-based device. There was no electric dipole orientation in the pure PDMS matrix, and PDMS acted as an insulator, thus having zero electrical output. This clearly confirmed that PDMS acted as a supporting cross-linker to hold the BTO NCs in place and did not contribute to the conversion of the mechanical energy into electrical energy. The CPNG device had a high output performance due to the piezoelectric effect of the BTO NCs. It is established that d33 of BTO (100−190 pC/N for bulk BTO film) varies with shape, particle size, doping, electrical poling, and input mechanical force. The working mechanism of a CPNG device for the generation of electrical energy depends on the generation and collection of free charge carriers on the top and bottom surfaces of the BTP-CFs and its Al electrodes. Under the no-force condition, the CPNG device generates zero electrical output due to the zero net dipole moment. The constant load (988.2 Pa) acting perpendicular to the CPNG device generated a piezoelectric potential. This was because the stress-induced poling maximized the number of electric dipoles aligned in one particular direction and led to a higher net dipole moment. The VOC of a CPNG device depends on d33 and can be evaluated as follows:12

is useful to harness waste mechanical energy such as that from human body movements, wind/water flow, and any mechanical vibrations. This is now possible with a CPNG device using BTP-CFs. In this work, a CPNG device was fabricated having dimensions of 4.4 cm × 4.6 cm, and its electrical output behavior using the linear motor force and biomechanical energy (hand and foot) was quantified. The detailed fabrication procedure of the CPNG device is given in the Experimental Section, and the device configuration is schematically shown in Figure 3c. Figure 3d shows a cross-sectional FE-SEM image of the CPNG device without the PDMS packaging layer at the 100 μm scale. The image clearly shows that no air gap formed between the layers during the fabrication of the CPNG device. The inset of Figure 3d shows a digital photograph of the asfabricated CPNG device made with the 15 wt % BTP-CF. The energy harvesting capability of CPNG devices with different weight ratios of BTO NCs in the PDMS matrix was assessed under a constant mechanical pressure (988.2 Pa) generated from the linear motor at a fixed acceleration of 1 m/ s2. Prior to testing, all of the CPNG devices were electrically poled at 8 kV for 24 h at room temperature, which provided a permanent polarization via electrical stress on the electric dipoles existing in the BTP-CFs. Such poling aligned the maximum number of dipoles in one particular direction. Figure 4a and b compares the open-circuit voltage (VOC) and shortcircuit current density (JSC) of pure PDMS and a CPNG device (15 wt %) under the constant mechanical load of 988.14 Pa. It clearly shows that the CPNG device (15 wt %) generated a higher electrical output response (VOC (peak‑to‑peak) = 126.3 V,

VOC = g33σYt

(1)

g33 = d33/ε0K

(2)

where t is the thickness of the CPNG device, Y is Young’s modulus of the BTP-CF, and σ is the strain acting perpendicular to the device. The piezoelectric voltage constant 4734

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Figure 4. Electrical response of the various piezoelectric nanogenerators. (a, b) Comparison of the current density−voltage (J−V) responses of the pure PDMS and CPNG (15 wt %) devices under the 2 N load (988.14 Pa). (c) Comparison of the J−V responses of the CPNG devices as a function of the BTO NC weight ratio under the low mechanical pressure of 988.14 Pa. (d) Demonstration of a real-time application using the CPNG device output to drive five green light-emitting diodes (LEDs).

(g33) is directly proportional to d33 and inversely proportional to the relative permittivity (K) of the BTP-CF. The maximum output of a CPNG device under constant mechanical load will be realized at an optimal weight ratio of the BTO NCs in the PDMS matrix. A weight ratio analysis of the electrical response was performed on CPNG devices containing 0, 5, 10, 15, 20, and 25 wt % of BTO NCs in the PDMS matrix (Figure 4c). The properties of the device made with the 5 wt % BTP-CF (25 V, 20 μA/cm2) improved (to 50 V, 50 μA/cm2) for the 10 wt % case and reached the maximum output (126.3 V, 77.6 μA/ cm2) at the 15 wt % composition under the constant mechanical load of 988.2 Pa. The 20 and 25 wt % BTP-CF cases had lower outputs than the CPNG device (15 wt %) as shown in Figure 4c. These findings suggest that particle agglomeration may have occurred at the higher weight ratios of the BTO NCs in the PDMS matrix. In such a situation, an applied force may not be distributed uniformly over all of the particles, and the orientations of the electrical dipoles may partially cancel each other. This would create a net dipole momentum lower than that for the 15 wt % BTP-CF device. Thus, 15 wt % BTO NCs in the PDMS matrix was the optimal weight ratio to achieve the maximum electrical output (Figure S2). The optimized weight percentage (15 wt %) is similar to previous findings. The generated output from the CPNG device (15 wt %) was sufficient to drive commercially available low-power LEDs as shown in Figure 4d. Under the no-force

condition, LEDs are in the OFF state, but the application of a constant load on the CPNG device illuminated five green LEDs simultaneously in the absence of any storage capacitor or additional battery energy. Our as-fabricated BTP-CFs and their nanogenerators pave the way for unconventional energy harvesting approaches and may be suitable to harness waste mechanical energy from the environment, such as from human body movements, wind/water motion, and any kind of mechanical vibration. Further studies such as load resistance analysis, power density calculations, stability testing, charging capability of commercial capacitors, and energy storage of various capacitors were carried out using the CPNG device (15 wt %) under the constant mechanical load of 988.2 Pa. The CPNG device output increased as the load resistance was increased from 500 Ω to 3 GΩ as shown in Figure 5a. The areal power density of the CPNG device under a 2 N applied load was evaluated as follows:2 PA = V 2/(R × A)

(3)

where V is the generated output voltage from the CPNG device, R is the load resistance, and A is the active device area of the CPNG (4.4 × 4.6 cm2). The areal power density (≈7 mW/ cm2) for the CPNG device was a maximum at a load resistance of 100 MΩ (Figure 5a). This indicated that 100 MΩ was the load-matching resistance for achieving the maximum output 4735

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Figure 5. (a) Load resistance analysis and power density calculations for the CPNG device (15 wt %) under a 2 N applied mechanical load (or 988.14 Pa). (b) Stability test analysis of the CPNG device at the 988.2 Pa (or 2 N) mechanical loading. (c) Charging and discharging behavior of a commercial capacitor (0.22 μF) using the CPNG device output via a full-wave bridge rectifier. (d) Comparison of the stored electrical energy for various commercial capacitors (0.22, 1, and 4 μF) using the CPNG device output under a constant mechanical load (988.2 Pa).

hand/foot motions are low frequency (≤10 Hz) movements, and the generating force is variable. The above-mentioned CPNG device output with respect to the low mechanical pressure (988.2 Pa), output dependency on the weight ratio of the BTO NCs in the PDMS matrix, charging and stability analyses, ability to drive commercial LEDs, and real-time biomechanical harvesting suggests that the proposed CPNG device could be used in remote areas to generate eco-friendly power and be used for various self-powered sensor applications.1−16

power required for real-time applications. Next, an endurance test was carried out to establish the stability of the CPNG device over the time interval of 1600 s under a 2 N applied load. The absence of change in the output voltage of the device over the time interval (Figure 5b) indicated good stability of the device. Charging analysis of a commercial capacitor (0.22 μF) was also conducted by passing the CPNG device output through a full-wave bridge rectifier. Figure 5c shows the charging and discharging curves of the 0.22 μF capacitor under the 2 N applied load. The capacitor stored a maximum voltage of 4.86 V during the 160 s time interval at a continuous input load of 2 N; the voltage started to decrease in the discharge process of the capacitor whenever the force acting on the CPNG device was removed. The charging and discharging cycle of the commercial capacitor was repeated to establish the cyclic stability as shown in Figure 5c. The storage capability of a capacitor using the CPNG device output was analyzed by increasing the capacity from 0.22 to 1 and 4 μF (Figure 5d). The circuit for the experimental demonstration and the charging curves of the various capacitors are given in Figure S3 (Supporting Information). The 1.24 μJ of energy stored in the 4 μF commercial capacitor when a 2 N load acted on the CPNG device increased to 1.42 μJ at 1 μF and 2.59 μJ for the 0.22 μF case. The fabricated CPNG device is targeted at harvesting waste biomechanical energy such as that from natural human body motions. Figure 6a shows the real-time experimental demonstration of a CPNG device to recover lowfrequency biomechanical energy. The CPNG device generated a Vpp of ≈29 V under human hand palm force and 55.9 V under the human foot press/release conditions (Figure 6b). Human

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CONCLUSIONS In summary, high-quality piezoelectric BTO NCs, synthesized using the low-temperature molten salt method, were used to fabricate efficient, flexible BTP-CFs by a simple, cost-effective solution casting technique. The performance of the CPNG device was studied as a function of the weight ratio of the BTO NCs in the PDMS matrix by electrical poling (8 kV for 24 h) under a constant low mechanical pressure (988.2 Pa), various resistance/capacitance loadings in a stability test, and under a biomechanical hand/foot stress in real-time. The optimal device was highly sensitive and generated a high Vpp (126.3 V) and high J (77.6 μA/cm2), delivering a maximum areal power density of 7 mW/cm2 at a 100 MΩ load resistance upon application of the low mechanical pressure of 988.2 Pa. A commercial capacitor (0.22 μF) stored 2.59 μJ of energy during a 160 s time interval. We also demonstrated that the generated CPNG device output was sufficient to drive five green lowpower commercial LEDs without using any storage capacitor or external circuits to replace the battery energy, which would 4736

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Figure 6. Real-time experimental demonstration using CPNG device to harness low-frequency waste biomechanical energy. (a) Photographs of the human hand and foot release and press conditions acting on the CPNG device. (b) Comparison of the open-circuit voltage (VOC) when the human hand and foot release/press force acted on the CPNG device.

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increase the cost factor. The CPNG device was also highly sensitive to biomechanical energy (hand and foot stress) and generated a maximum output voltage of 55.9 V during foot stress. This demonstrated that the CPNG could use normal human body physical motions and may be a reliable alternative unconventional energy harvesting approach to partially solve the world energy crisis. The proposed method for fabricating the CFs is cost-effective, eco-friendly, and suitable for largescale production. The fabricated optimal CPNG device is highly capable of harnessing waste mechanical energy found throughout society.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nagamalleswara Rao Alluri: 0000-0001-7997-8260 Arunkumar Chandrasekhar: 0000-0002-4561-0975 Venkateswaran Vivekananthan: 0000-0003-1756-6548 Sang-Jae Kim: 0000-0002-5066-2622 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) funded by the Korean government Grant No. 2016R1A2B2013831.

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00117. Synthesis of BTO NCs using the molten salt approach. Weight ratio dependence analysis of CPNG devices under constant mechanical load. Charging analysis using commercial capacitors. (PDF)

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