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Porosity Modulated High-Performance Piezoelectric Based on Organic/Inorganic Nanomaterials for Self-Powered Structural Health Monitoring Md Masud Rana, Asif Abdullah Khan, Guangguang Huang,* Nanqin Mei, Resul Saritas, Boyu Wen, Steven Zhang, Peter Voss, Eihab Abdel-Rahman, Zoya Leonenko, Shariful Islam, and Dayan Ban*

Cite This: ACS Appl. Mater. Interfaces 2020, 12, 47503−47512 Read Online

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ABSTRACT: In the modern era, structural health monitoring (SHM) is critically important and indispensable in the aerospace industry as an effective measure to enhance the safety and consistency of aircraft structures by deploying a reliable sensor network. The deployment of built- in sensor networks enables uninterrupted structural integrity monitoring of an aircraft, providing crucial information on operation condition, deformation, and potential damage to the structure. Sustainable and durable piezoelectric (PENGs) with good flexibility, high performance, and superior reliability are promising candidates for powering wireless sensor networks, particularly for aerospace SHM applications. This research demonstrates a self-powered wireless sensing system based on a porous polyvinylidene fluoride (PVDF)-based PENG, which is prominently anticipated for developing auto-operated sensor networks. Our reported porous PVDF film is made from a flexible piezoelectric polymer (PVDF) and inorganic (ZnO) nanoparticles. The fabricated porous PVDF-based PENG demonstrates ∼11 times and ∼8 times enhancement of output current and voltage, respectively, compared to a pure PVDF-based PENG. The porous PVDF-based PENG can produce a peak-to-peak short-circuit current of 22 μA, a peak-to-peak open-circuit voltage of 84.5 V, a peak output power of 0.46 mW P =×VIoc sc , and a peak output power density ( 22) of 41.02 μW/cm2 (P/A). By harnessing energy from minute vibrations, the fabricated porous PVDF-based PENG device (area of A = 11.33 cm2) can generate sufficient electrical energy to power up a customized wireless sensing and communication unit and transfer sensor data every ∼4 min. The PENG can generate sufficient electrical energy from an automobile car vibration, which reflects the scenario of potential real-life SHM systems. We anticipate that this high-performance porous PVDF-based PENG can act as a reliable power source for the sensor networks in aircraft, which minimizes potential safety risks. KEYWORDS: piezoelectric nanogenerator, organic/inorganic nanomaterials, , wireless sensing network, structural health monitoring Downloaded via UNIV OF WATERLOO on February 21, 2021 at 09:53:19 (UTC).

1. INTRODUCTION raindrops, ocean waves, and so forth. These technologies

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 8 9,10 Self-powered structural health monitoring (SHM) can be used include but are not limited to triboelectric or piezoelectric to monitor the in-service conditions of aerospace systems.1 nanogenerators (PENGs) and devices based on electro- 11−15 16−18 Such an SHM system can overcome the failure modes of magnetic and electrostatic methods. Triboelectric traditional deficient time-based, high-cost scheduled main- nanogenerators demonstrate adequate energy conversion tenance and thus enhance the safety, consistency, and efficiency, high output voltage, flexible material selection, − efficiency of military and civilian aircraft.2 Currently, wired lightweight, and low cost.8,19 31 However, lack of durability sensor networks are still the industry standard for aircraft and compactness are two important bottlenecks limiting their 3,4 SHM. Nevertheless, the installation of the wired network can SHM applications, particularly for aircraft. On the contrary, fi be an error-prone process requiring a signi cant workforce and PENGs exhibit mechanical robustness, excellent environmental costs. Alternatively, a wireless sensor network system can effectively eliminate wiring problems.5 For such a wireless system, a reliable and long-lasting power supply becomes Received: July 16, 2020 critical. One emerging and promising technology is to use an Accepted: September 24, 2020 energy-harvesting device, which harvests energy from the Published: September 24, 2020 ambient environment and makes the system self-operating.6,7 To date, numerous approaches have been adopted to harvest widespread ambient energies, such as from vibration, wind,

© 2020 American Chemical Society https://dx.doi.org/10.1021/acsami.0c12874 47503 ACS Appl. Mater. Interfaces 2020, 12, 47503−47512 ACS Applied Materials & Interfaces www.acsami.org Research Article adaptability, and better sensitivity, showing a good fit for SHM based PENG, the I and V are enhanced by ∼11 times and − sc oc applications.32 35 Numerous materials have been used to ∼8 times, respectively, and are better than those of many fabricate PENGs such as inorganic lead zirconate titanate36,37 previously reported PVDF composite-based nanogenerators (BaTiO ),38,39 ZnO, Na/KNbO , and ZnSnO (Table S2). The fabricated porous PVDF-based PENG can 3− 3 3 nanoparticles (NPs),40 43 which have large piezoelectric produce sufficient electrical energy to drive a wireless sensing coefficients and high-energy conversion efficiencies. Organic unit or to light up light-emitting diodes (LEDs). This porous piezoelectric polymers, such as polyvinylidene fluoride PVDF-based PENG is successfully integrated with a custom- (PVDF),44 and their copolymers hexafluoropropylene [P- made Bluetooth compatible wireless sensing unit and can send (VDF-HFP)],45 trifluoroethylene [P(VDF-TrFE)],46 and poly- out mimic sensor signals to a remote receiver every ∼4 min. (vinyl acetate)47 have also attracted significant attention The PENG can also generate sufficient electrical energy from because of their high flexibility, biocompatibility, simple the vibration of an automobile car, which reflects the scenario material synthesis process, and the presence of energy-efficient of potential real-life SHM systems. The results demonstrate the β-phase.48 However, intrinsic PVDF-based PENGs yield lower great potential of porous PVDF-based PENGs for practical electrical energy outputs compared to their inorganic counter- applications. parts. To address these issues, numerous approaches have been used, such as incorporating nanofillers of piezoelectric 2. RESULTS AND DISCUSSION materials or replicating micro/ to further − The self-powered wireless SHM system consists of an energy enhance the .49 57 By using ZnO generation unit (the porous PVDF-based PENG device), an templates, Cha et al. fabricated a nanoporous PVDF array energy management circuit, and a radio-frequency (RF) data which can generate a piezoelectric output voltage of 2.6 V and transmission module. In the sketched SHM shown in Figure output current of 0.6 μA under an ultrasonic excitation power 1a, the porous PVDF-based PENG device is placed between of 100 dB at 100 Hz.58 Similarly, anodic aluminum oxide template (200 nm pore diameter)-assisted PVDF-TrFE nanowire arrays46 exhibited a peak output voltage and a current of 3 V and 5.5 nA, respectively, with a strain rate of 0.1% s−1; PVDF-TrFE nanotubes delivered a voltage output of 4.8 V and a power output of 2.2 μW/cm2 at a dynamic compression pressure of 0.075 MPa at 1 Hz which is 36 times higher than that of their bulk PVDF-TrFE counterparts.59 The reason for this high output is mainly attributed to the efficient strain confinement in the direction of applied mechanical excitation, as a result of the alteration of the microstructure of these ferroelectric polymers. The abovementioned PENGs have demonstrated an excellent output performance; however, there remains a distinct gap to deploy them as a sustainable compact power source in real-life applications. Mao et al. ffi Figure 1. Schematic of the wireless sensing system of aircraft SHM. developed a simple yet e cient approach to fabricate (a) Schematic illustration of the functional components of PENGs, mesoporous PVDF PENGs by the etching of embedded which is mainly composed of a porous PVDF-based PENG device ZnO NPs from the PVDF polymer and raised the output unit and an unit; (b) enlarged view of the integrated voltage and current to 11 V and 9.8 μA at 40 Hz due to its circuit; and (c) as-fabricated porous PVDF-based PENG device. higher energy conversion efficiency.60 Such type of porous PVDF-based PENGs can be used as implantable power sources two metal sheets to reflect the scenario of a PENG operating for biomedical applications,61,62 and are promising for next- inside a mechanical joint. The device is composed of a porous generation self-powered . Two main problems limit PVDF piezoelectric thin film (∼50 μm in thickness) which is the use of mesoporous PVDF PENGs in many piezoelectric sandwiched between two copper and encapsulated applications. First, due to the improper etching of the NPs with polyester substrates. For wiring purposes, very thin and from the PVDF, it creates an electrical breakdown during the flexible copper wires (∼100 μm of diameter) are connected to high voltage poling to align the PVDF dipoles and cannot both electrodes. Finally, the layered structure of polyester/ reach their saturated polarization. Second, the presence of copper/porous-PVDF film/copper/polyester is compressed moistures, defects, and pinholes may create short circuit tightly by a commercial thermal laminator to eliminate any between the electrodes. air gaps by confirming uniform adhesion between each layer. A In this paper, we present high-performance porous PVDF- custom-made wireless sensing circuit, as shown in Figure 1b, is based PENGs based on a one-step etching of ZnO NPs, and placed inside a central groove of the metal sheets (Figure 1a), with this high-energy conversion efficiency successfully which includes functions of energy management, storage, demonstrated a self-powered wireless sensing platform. We signal conditioning, and wireless data transmission. also develop a model to explain the experimentally observed The high-performance porous PVDF-based PENG, used as a performance enhancement in the fabricated porous PVDF- sustainable power supply for the SHM system, is based on a based PENG. Experimental measurements show that with mesoporous thin-film structure. As will be revealed shortly, the applied mechanical stress of only 2 kPa (corresponding proof built-in pores in the porous PVDF film substantially elevate mass of 138 g), the device can produce a peak-to-peak short- stress distribution inside the film and thus boost strain-induced μ fi circuit current (Isc)of22 A, an open-circuit voltage (Voc)of piezopotential. PVDF powder is rst dissolved in N,N- 84.5 V, the peak power of 0.46 mW, and areal peak power dimethylformamide (N,N-DMF) solvent and then mixed density of 41.02 μW/cm2. Compared to a similar pure PVDF- with ZnO NPs. The mass ratio between the ZnO NPs (35−

47504 https://dx.doi.org/10.1021/acsami.0c12874 ACS Appl. Mater. Interfaces 2020, 12, 47503−47512 ACS Applied Materials & Interfaces www.acsami.org Research Article

Figure 2. Material characterization of the as-fabricated porous PVDF-based PENG device: SEM images of (a) distributed ZnO NPs on the surface of the PVDF matrix before etching (prepared from a mixture of 50 wt % mass fraction of PVDF and ZnO); (b) top-view SEM image of the porous PVDF film after the etching of the ZnO NPs (the inset is the enlarged view of pore); (c) elemental composition mapping of carbon (C), fluorine (F), zinc (Zn), chlorine (Cl) in the porous PVDF thin film; (d) surface morphology characterized by the AFM; and (e) Fourier transform infrared (FTIR) spectrum of the porous PVDF film to confirm the β-phase formation. (f) XRD patterns of pure PVDF, PVDF with ZnO NPs, and porous PVDF-based thin films.

45 nm of diameter) and the PVDF solution is adjusted, removing ZnO NPs by HCl etching. We envision that during yielding different porosities of the films. To maximize the the etching process, hydrochloric acid (HCl) starts to react uniform distribution of ZnO NPs within the PVDF polymer first with the inorganic ZnO NPs on the surface and then matrix, the solution is stirred on a hot plate, followed by gradually infiltrates to the bulk of the PVDF film. As ZnO NPs extended ultrasonication for half an hour. The whole process of are distributed throughout the film, pores are not only formed porous PVDF thin-film preparation and porous PVDF-based on the surface but throughout the whole film, and the size of PENG device fabrication has been described in detail in the the pores should be larger than the actual NP size. It is seen Experimental Section (4.1 and 4.2) and illustrated in Figure that the pore sizes are 60 nm with those of the clustered ZnO S1a−h. The schematic illustration of a fabricated porous NPs, whereas the average NP sizes are 35−45 nm. The ZnO PVDF-based PENG device with proper packaging and NPs are interconnected inside the film, allowing complete electrical connection is shown in Figure 1c. By adopting the removal via HCl etching. Figure 2C explains the energy- aforementioned thin-film synthesis approach, a scalable and dispersive X-ray spectroscopy (EDS) element mapping of the industry-viable porous PVDF film (area of 15 cm × 15 cm) is top surface porous film. These results confirm that the also fabricated and demonstrated in Figure S1i,j. percentage of detected Zn is negligible compared to the Figure S2a shows the top surface scanning electron percentage of C and F elements detected on the top surface of microscopy (SEM) image of a pure PVDF film, indicating the porous film. The corresponding individual element that the surface topography is homogeneous without wrinkles, mapping of the porous PVDF thin film, as shown in Figure grains, voids, cracks, and deformation. Figure 2a shows the top S3a−e, for a specific region of interest confirms the percentage surface SEM image of a thin film prepared from the mixture of of detected elements in the porous film. The top surface and 50 wt % mass fraction of PVDF and ZnO. The ZnO NPs are cross-sectional SEM images of the porous PVDF film along distributed into the PVDF matrix which shows that the ZnO with the corresponding EDS spectrum, shown in Figure S4a− NPs are accumulated in the PVDF film like a cluster, which is d, further confirm the dominating peak of C and F compared ff mainly due to their di erent molecular weight (MW)(MW of to Zn and Cl. The (AFM) image in ZnO NPs are higher than that of the PVDF molecule). The Figure 2d is a three-dimensional surface topology of the porous main purposes of introducing ZnO NPs to the PVDF matrix PVDF film. The surface roughness of the top surface of the are (1) to create porosity (by HCl etching) in the PVDF film porous PVDF film is confirmed and measured to be ∼100 nm for manipulating its mechanical property and (2) to promote (Figure S5a,b). the formation of a porous PVDF film piezoelectric β-phase In addition to creating porosity, promoting the β-phase through the dipolar interaction between ZnO’sZn2+ cations crystallinity of the PVDF film is crucial as it possesses the ’ − and PVDF sCF2 anion groups. Moreover, ZnO NPs have highest spontaneous polarization than the other polymorphic α γ δ fi β several unique advantages over inorganic (e.g., SiO2)or phases of the PVDF ( , , and ). To con rm the -phase organic (e.g., polystyrene) NPs for the fabrication of similar formation of the porous PVDF thin film, FTIR spectrum porous nanostructures which include cost-effectiveness, non- analysis in the wavenumber range of 400−1000 cm−1 is toxicity, good scalability, and facile removal by acidic performed. The characteristic peaks of the β-phase at 431 and − solution.63 67 Similar phenomena are observed inside the 840 cm−1 can be observed in the FTIR spectrum (Figure composite film for both pure PVDF and ZnO NP-mixed 2e).68,69 The formation of the β-phase in the PVDF matrix is PVDF, which is distinguishable, as revealed by cross-sectional assisted through the interactions between the dipoles of PVDF SEM images (Figure S2b,c). The SEM image of Figure 2b and the surface charges on ZnO NPs. The positively charged shows the top surface of a porous PVDF thin film after Zn2+ cations (0001 surfaces) and O2− terminated anions (0001̅

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− + surfaces) interact with the CF2 or CH2 groups of the PVDF that have negative and positive charge densities, respectively, resulting in the β-phase nucleation. Upon applying a high electric poling field, the dipoles of the PVDF are aligned in the direction of the field. The X-ray diffraction (XRD) pattern of three different films of pure PVDF, PVDF with ZnO NPs, and porous PVDF are shown in Figure 2f. These results demonstrate that the piezoelectric β-phase is well-preserved for both pure PVDF and porous PVDF along with other characteristic peaks of PVDF and some unknown peaks. Whereas the XRD pattern of the PVDF film combined with ZnO shows the dominating ZnO characteristic peaks compared to the PVDF peaks. This is because the XRD for the organic ZnO is much more profound than the polymer PVDF. It is noteworthy to mention that these results also confirmed the etching of ZnO from the film, helping to enhance the mechanical properties, which is more suitable for Figure 3. Piezopotential distribution in the porous PVDF film with PENG device fabrication. varying porosity under a compressive force of 2 GPa (a) with surface The fabricated porous PVDF-based PENG device is pores only, (b) with only inner pores, and (c) nonporous PVDF. The integrated into the SHM system and evaluated. An electro- same scale bar for (a−c). (d) Comparison of piezopotential as a dynamic shaker is employed for mechanical excitation. The function of porosity for surface pores and inner pores. (e) Strain porous PVDF-based PENG device is placed between the distribution of 50% porous PVDF structure ensembled with both shaker hammer and a block of stainless steel (138 g), inner and surface pores. (f) Piezopotential distribution of 50% porous producing piezoelectric output upon the application and PVDF structure ensembled with both inner and surface pores. releasing of mechanical stress. The porous PVDF-based PENG device and the weight system can be treated as a free raised by 13.73% (46.6−53 V) when the porosity increases vibration spring-mass system with damping. The from 0 to 7.85% if pores are inside (Figure 3d) (detailed generation mechanisms from the poled porous PVDF-based calculations are provided in Table S1). PENG device is demonstrated in Figure S6a−e. In the It is also explored that the porous PVDF film will remarkably beginning, the net dipole moment inside the film is zero boost the piezoelectric potential by forming stress concen- with the absence of externally applied force (Figure S6a). tration centers around the pores. To verify this phenomenon, Then, dipoles are aligned by applying a high electric field of strain distribution with lateral bending force is compared (120 V/μm) for 2−3 h to the direction of the electric field between pure PVDF, PVDF with inner pores, and surface (Figure S6b). After that, when a vertical compressive force is pores, as shown in Figure S7a−c. It is clearly shown from the applied on the device, the net polarization within the PENG figures that under the lateral bending force of 2 GPa, the device will change, thus resulting in a piezoelectric potential induced displacement of the three different models of the same (Figure S6c). This forces free electrons to move from one film thickness of 100 nm is drastically different. The to another. After releasing the force, the piezoelectric deformation of the porous PVDF film is higher than that of potential diminishes and electrons move back (Figure S6d). the pure PVDF film. Finally, the PENG film shows an additional damping cycle, It is also obvious that the pore position and size influence which leads to the observation of an alternative output current the mechanical stress distribution, which results in large pulse due to the formation of porous structures (Figure S6e). deformation for the surface pore structure than the inner pore To understand the porosity effect on material piezoelectric structure and so as the potential with lateral bending force. properties, a PENG model based on finite element simulations As shown in Figure S7a−c, the local strain around each pore (COMSOL Multiphysics 5.3) is developed. A PVDF piezo- has a much higher magnitude than the bulk strain in the pure electric polymer is used to study the simple cases of a porous PVDF. This localized strain of higher magnitude induces structure, where a square (100 nm × 100 nm) two- higher potential, which adds up and enhances the bulk dimensional model is employed. Three device models are piezopotential. Similarly, the simulated piezopotential results simulated and compared, including a nonporous (pure) PVDF, are shown in Figure S8a−d, when lateral bending force is a porous PVDF with surface pores, and a porous PVDF with applied to the side surface of the PVDF structure, and inner pores only, with a pore size of 5 nm in radius and described briefly in the Supporting Information (Note S1). As different porosity percentages. To study the piezoelectricity of expected, the output potential increases linearly with the the porous structure, two independent scenariosa compres- applied force, but an important point to be noted is that the sive force and laterally bending force of 2 GPa are uniformly potential for the equal amount of force is smaller for lateral applied on the side and top surfaces, respectively. Figure 3a−c bending force compared to vertical compressive force. This is shows the simulation results of the effect of porosity on clarified by the fact that the piezoelectric coefficient along the piezopotential under compressive force, which demonstrated vertical axis of PVDF has the highest value. Consequently, a clearly that pores modify the relative magnitude of the compressive force applied along the c-axis (vertically) will piezopotential. Accordingly, the piezopotential increased produce higher output piezopotential compared to lateral proportionally with the degree of porosity, which is bending force. Then, the strain distribution phenomenon for represented by the brightness of color grading. The potential vertical compressive force is compared between pure PVDF, is raised by 5.79% (46.6−49.3 V) when the porosity increases PVDF with inner pores, and with surface pores in Figure S9a− from 0 to 3.9% if pores are on the surface, and the potential is c and systematically investigated (Supporting Information,

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Figure 4. Measured electrical output performance of the porous PVDF-based PENG device: (a) measured open-circuit voltage of the porous PVDF thin films of almost identical thickness when the ZnO mass fraction increased from 0 wt % (pure PVDF) to 60 wt %, with a frequency of 30 Hz. (b) Peak-to-peak output voltage at 30 Hz and 2G acceleration when the device was pressed by a standard mass of 138 g. (c) Peak-to-peak output current at 30 Hz and 2G acceleration when the device was pressed by a standard mass of 138 g. (d) Polarity switch testing of short-circuit current to confirm piezoelectricity. (e) Polarity switch testing of open-circuit voltage to confirm piezoelectricity. (f) Stability test of the porous PVDF-based PENG device for 36,000 cycles (at 30 Hz for 20 min).

Note S2). These results conclude that the local strain Further increase of ZnO causes a decrease in the total distribution around the pores, with compressive force and amount of PVDF per unit volume in the mixture, which results lateral bending force, leads to a much higher piezopotential in lowering the β-phase. Therefore, the porous PVDF film enhancement. The strain-induced piezopotential benefits from made from the mixture of 50 wt % of ZnO possessed the the stress concentration behavior through enhanced piezo- highest β-phase quantity, exhibiting the highest output electric polarization and is crucial for determining the performance, which is attributed to enhanced strained-induced 66 fi piezopotential distribution. piezopotential due to ampli ed mechanical displacement inside fi Based on the above discussion, we build a device model the porous lm, as revealed in the modeling. The measured similar to the fabricated device in which pores are distributed peak-to-peak output open-circuit voltage (Voc) and short- both on the surface and in the bulk. The simulation result of 50 circuit current (Isc) at 30 Hz frequency are about 84.5 V and μ 2 wt % of porous PVDF films is compared to that of pure PVDF 22 A, respectively, from an active device area of 11.3 cm for a films. Figure 3e,f shows the stress distribution and output 50 wt % ZnO@PVDF device (Figure 4b,c). This porous PVDF fi composite-based PENG shows a drastic increase in the output voltage of the porous PVDF lms under a uniaxial compressive ∼ ∼ μ − stress of 800 kPa, whereas Figure S10a,b shows the result of current and voltage performance of 11 times [ 22 A(p p)] and ∼8 times [∼84.5 V (p−p)], respectively, compared to pure PVDF. It is evident that the induced displacement and a pure PVDF-based PENG (ZnO NP mass ratio: 0 wt %). It is output of the two PENG models for the same film thickness noteworthy to mention that in the experimental studies, there are remarkably different; the porous PVDF film experiences are other factors in porous PVDF PENG, for example, a higher much larger deformation and thus produces higher voltage amount of the β-phase formation and higher porosity due to output compared to the pure PVDF. the etching of randomly distributed ZnO NPs, which further This phenomenon can be attributed to the position of pores enhance the PENG output. None of these factors was and size influence. Finally, we experimentally investigate the ff considered in the simulation studies. As a result, the PENG e ect of porosity on practical PENG device performance. voltage output is enhanced by ∼8 times which is much higher fi ff PVDF thin lms of di erent porosities are prepared from the than the predicted simulation results. It also shows better ff mixture of ZnO NPs with di erent mass ratios (from 0 to 60 performance compared to many previously reported PVDF wt %) (Figure S10c). As shown in Figure 4a, the device output composite-based nanogenerators (Table S2). We have also − − voltage increases from 18 V (p p) to 84.5 V (p p) when the studied the electrical characterization of the film that contains ZnO NP mass ratio increases from 0 to 50 wt %. However, a 50 wt % of ZnO NPs inside the PVDF matrix in both porous further increase in the ZnO mass ratios (60 wt %) results in a and nonporous conditions (Figure S11a,b). It is seen from the decrease of the output voltage to 60 V (p−p). This is because result that the output voltage and current both are enhanced the porosity-dependent output is related to the amount of β- due to the porosity of the film. A polarity-switching test is phase of PVDF, which is increased with the increase of ZnO carried out. When the connection is reversed, the reversal in mass ratio to PVDF from 10 to 50%. the open-circuit voltage (Figure 4d) and the short-circuit

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Figure 5. Demonstration of high output capability and application prospect of the porous PVDF-based PENG device. (a) Comparison of the output voltage at different frequencies of 10−50 Hz for the device with 50 wt % of ZnO-PVDF composition. (b) Comparison of output current at different frequencies of 10−50 Hz for the device with 50 wt % of ZnO-PVDF composition. (c) Measured output voltage across the various commercial capacitors of 1, 2.2, 4.7, 10, 47, and 100 μF were charged by the 50 wt % ZnO-PVDF PENG device at 30 Hz. (d) Measured peak output power and peak output power density at various loading conditions of the PENG at 30 Hz. (e) Measured output voltage across the input and output capacitor of the EMM, when the PENG is excited with the linear motor shaking at 30 Hz. (f) Wireless signal reception by a Bluetooth device (). (g) Whole sensing system illustration with the aid of a block diagram. current (Figure 4e) revealed identical amplitude with reversed capacitance (1.0, 2.2, 4.7, 10, 47, and 100 μF). It takes 140 s to polarization (Video S1), which confirms the inherent piezo- charge the 100 μF capacitors to 3 V. The output power from electricity originated from the high-performance porous PVDF the porous PVDF-based PENG device is measured as a PENG.60 The fabricated PENG is capable of sustaining over a function of load resistance (Figure 5d). The peak output 2 × long time under a constant oscillation without notable power (Pout = IL RL) (where IL is the load current and RL is degradation in the output signal for 36,000 cycles (30 Hz for the load resistance) is measured to be 78 μW at a load 20 min) (Figure 4f), showing excellent long-term stability. resistance of 7 MΩ, corresponding to a peak power density of The open-circuit voltage (Figure 5a) and short-circuit 12 μW/cm2. This is sufficient for driving the entire wireless current (Figure 5b) of the PENG device are measured within SHM system. A two-stage charging system, in which the input a frequency range from 10 to 50 Hz by turning the frequency capacitor is 1 μF and the output capacitor is 220 μF, is ffi of the VR9500 shaker. The piezoelectric output voltage (Voc) employed to improve electrical energy collection e ciency. peaks at 30 Hz, corresponding to the resonant frequency of the The porous PVDF-based PENG device produces electrical porous PVDF-based PENG device.60 The harnessed energy is energy by harvesting from the vibration of stored in commercial capacitors through a full-wave bridge the VR9500 shaker operated at 30 Hz. Figure 5e shows the circuit, and the charging characteristics are plotted in Figure charging/discharging characteristics of the input (black curve) 5c. As expected, the charging time gradually increases with the and output (red curve) capacitors.

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The whole operation of the custom-designed wireless 3. CONCLUSIONS sensing node is described in detail in Figure 5g. Herein, the In conclusion, we successfully demonstrate a high-performance electrical output from the porous PVDF-based PENG is used porosity modulated robust PENG device that can generate for two purposes: for sensing and powering up the data electrical energy by reaping mechanical vibration energy. The transmission unit. developed PENG exhibits a high output current of 22 μA(p− The alternating electrical output collected from the porous p), a voltage of 84.5 V (p−p), a peak output power of 0.46 PVDF-based PENG device is first rectified by a bridge rectifier mW, and a power density of 41.02 μW/cm2 from a single unit and then fed to the energy management module (EMM). device of area 11.33 cm2, outperforming other recently As a result, the voltage of the input capacitor drops down to reported NGs (Table S2). The control of porosity empowers a regulated voltage of ∼2−3 V. The output capacitor (220 μF) the adaptable mechanical property of the PENG device, which is then disconnected from the input capacitor by the MOSFET is critical for harvesting ambient mechanical energy and switches of the buck converter. The input capacitor is charged developing practical self-powered electronic systems. This very ff up again to continue this charging−discharging cycle. simple and e ective fabrication method also enhances the − formation of piezoelectric β-phase with an applied electric field The charging discharging cycle continues until the output μ capacitor is charged up to ∼3.1 V by when the output of 120 V/ m instead of a large mechanical strain. The as- capacitor releases its stored energy to drive the wireless data fabricated PENG allows forced oscillation to operate in a wide range of frequencies (10−50 Hz) apart from its resonant transmission system. frequency with promising outputs, which unlocks another For sensing purposes, the alternating output from the PENG route toward broad-band energy harnessing. The PENG can is fed to the RF module via an impedance matching network, ffi fi also generate su cient electrical energy by harnessing which contains a diode and an operational ampli er, as shown automobile vibrations. With these outstanding performances in Figure 5g. The RSL-10 system on chip of the RF module is from PENG discussed above, we successfully demonstrate ∼ programmed to operate for a preset 1 s/data transmission several auspicious applications such as self-powered SHM for cycle, during which the measured sensor signal from the PENG remote sensing which is promising at inconsistent environ- device is digitized and transmitted wirelessly to remote ments for modern IoT-based applications and charging receivers (mobile phones). A discharging level controller of commercial capacitors that can power LED devices. It is the output capacitor based on a delay circuit is introduced as expected that the achieved noteworthy evolution of porous well to control the data transmission frequency. The whole PVDF film-based PENG provides a promising solution for system including the rectifier, the EMM, the RF module, and scavenging energy from the surroundings of the real world, the impedance matching unit are integrated on a circular which reveals new prospects for environment-friendly self- printed circuit board of a diameter of 3 cm, as shown in Figure powered technology. 1b. The full operation of energy harvesting, energy-storing, data collecting, and wireless transmitting is demonstrated and 4. EXPERIMENTAL SECTION 4.1. Fabrication of the Porous PVDF Thin Film. PVDF powder recorded (Video S2). This application scenario is demon- ° strated in Figure 5f where the Bluetooth receiver of the (Sigma-Aldrich) is dissolved in N,N-DMF solvent (10 wt %) at 70 C for 24 h. Then, ZnO NPs (35−45 nm, US Research Nanomaterials, smartphone is receiving and decoding the mimic sensor signals Inc.) are mixed within the PVDF matrix solution, and the mass ratio simultaneously. The practical setup including all necessary between ZnO NPs and PVDF is adjusted to create different equipment for the measuring purpose is shown in Figure porosities. This solution is stirred for another 4 h on a hot plate S13.63 with a temperature of 45 °C followed by extended ultrasonication for To demonstrate versatile applicability, the PENG is used in 30 min to maximize the uniform distribution of ZnO NPs within the harnessing vibration from an automobile car engine (the car in PVDF polymer matrix. The suspension then drops cast onto a Si wafer, and degassing is performed for 30 min at 65 °C in a vacuum is the park mode while the engine is turned on, as shown in the oven purged with N2 to suppress potential bubble formation during Video S3). Figure S12a represents the measured output voltage the curing process. After that, annealing is carried out in the same from the PENG mounted on the car engine during the off and vacuum oven at a slightly elevated temperature (75 °C) for another running condition of the engine. The output peak-to-peak 45 min. The films are then peeled off from the Si substrate with care voltage of 8.5 V was measured at the running condition, while and immersed in a 37 wt % HCl solution for 4 h to completely etch off ZnO NPs from the PVDF matrix. After HCl etching, the films are there was no voltage recorded at the off condition. washed by deionized water, dried with N2 gas, and put in a vacuum Corresponding fast Fourier transform (Figure S12b) of the oven overnight at 60 °C for better drying. Finally, porous PVDF thin output voltage was performed, which reveals the major films are ready for device fabrication. contribution of the device output produced from the vibration 4.2. PENG Fabrication. A high-voltage electrical poling of the components of approximately 29 Hz, which is close to the porous PVDF film is performed with an electric field of 120 V/μm for − − fi resonance frequency (30 Hz) of the PENG. The deviation in 5 6 h with a DC voltage of 0 6 kV. The lms are stable throughout ff the entire poling process. No short circuit or noticeable voltage the FFT signal obtained by the PENG from di erent structural fluctuation is detected up to the maximum voltage of 6 kV. Then, the vibration conditions is critical to monitor structural health. poled porous PVDF composite films are inserted between two copper Therefore, the PENG has great potential to SHM applications electrodes. For the characterization purpose, the electrical con- with a wide range of vibrational energy harnessing capabilities. nections are made from both the top and bottom electrodes by very thin and flexible copper conductors. Finally, the layered structure of The harnessed energy from the engine vibration can charge a 1 fi fi μF commercial capacitor of up to 2.2 V within 60 s, as shown polyester/copper/porous PVDF lm/copper/polyester lm is in- serted and passed through a commercial thermal laminator tightly to in Figure S12c. The scenario of energy storing in the eliminate any air gaps to secure uniform adhesion. commercial capacitor is demonstrated in Figure S12d and 4.3. Characterization and Measurements. A JSM-7200F field- recorded (Video S3). emission scanning electron microscope is used to characterize the

47509 https://dx.doi.org/10.1021/acsami.0c12874 ACS Appl. Mater. Interfaces 2020, 12, 47503−47512 ACS Applied Materials & Interfaces www.acsami.org Research Article morphological and structural properties of the PVDF thin films. FTIR of Waterloo, Waterloo, Ontario N2L 3G1, Canada; spectroscopy is performed by Nicolet iS50 to confirm the piezo- orcid.org/0000-0002-8026-9821 β fi electric -phase formation inside the porous PVDF lm by measuring Asif Abdullah Khan − Waterloo Institute for Nanotechnology characteristic absorbance peaks in a wavenumber ranging from 400 to − and Department of Electrical and Computer Engineering, 1000 cm 1. AFM images are captured by using JPK Nanowizard II, fi University of Waterloo, Waterloo, Ontario N2L 3G1, Canada con gured in the intermittent-contact mode (scan rate 0.3 Hz). An − & electrodynamic shaker (VR9500, Lab works Inc.) is used to Nanqin Mei Department of Physics Astronomy, University investigate the electrical output performance of PENGs, which is of Waterloo, Waterloo, Ontario N2L 3G1, Canada controlled by a power amplifier and a controller unit. To record the Resul Saritas − Department of Systems Design Engineering, electrical output from the PENGs, a digital oscilloscope (Tektronix University of Waterloo, Waterloo, Ontario N2L 3G1, Canada 2004 C) and a low-noise current preamplifier (model SR 570, Boyu Wen − Waterloo Institute for Nanotechnology and Stanford Research System Inc.) are used. Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada ■ ASSOCIATED CONTENT Steven Zhang − Department of Electrical and Computer *sı Supporting Information Engineering, University of Waterloo, Waterloo, Ontario N2L The Supporting Information is available free of charge at 3G1, Canada https://pubs.acs.org/doi/10.1021/acsami.0c12874. Peter Voss − Shimco North America Inc., Cambridge, Ontario Identical polarity switching test of output current (MP4) N3E 0A7, Canada Full operation of energy harvesting, energy-storing, data Eihab Abdel-Rahman − Department of Systems Design collecting, and wireless transmitting (MP4) Engineering, University of Waterloo, Waterloo, Ontario N2L Scenario of energy storing in the commercial capacitor 3G1, Canada − & (MP4) Zoya Leonenko Department of Physics Astronomy, Step-by-step fabrication process of the porous PVDF University of Waterloo, Waterloo, Ontario N2L 3G1, Canada − thin film; characterization of the pure and porous PVDF Shariful Islam Shimco North America Inc., Cambridge, film; element mapping of the porous PVDF film; Ontario N3E 0A7, Canada selected area of top surface SEM of the nanoporous Complete contact information is available at: PVDF film and the EDS spectrum; characterization of https://pubs.acs.org/10.1021/acsami.0c12874 the porous PVDF film; energy generation mechanism from the PENG by considering distributed stress on the Notes fi lm; strain distribution in the porous PVDF with varying The authors declare no competing financial interest. porosity under a lateral bending force of 2 GPa; piezopotential distribution in the porous PVDF model ■ ACKNOWLEDGMENTS with varying porosity percentages under lateral bending fi force; strain distribution in the porous PVDF with This research is nancially supported by the Natural Sciences varying porosity under a vertical compressive force of 2 and Engineering Research Council of Canada (NSERC) (grant GPa; stress and potential distribution for the pure PVDF no.: CRDPJ 514858-17), Ontario Centers of Excellence film, prepared solution of PVDF-ZnO with a ZnO mass (OCE) (grant no.: VIP II-28314), University of Waterloo ratio of 0−60 wt %; comparison of the electrical output (grant no: 10001-10643), and Shimco North America Inc., performance for 50 wt % of ZnO NP-based PVDF Cambridge, Ontario, Canada. The authors thank Giga-to-Nano porous and nonporous films; application of the PENG Electronics (G2N) laboratory for the experiment facilities. for SHM; general setup of the piezoelectric character- ization system; comparison of percentage increases of ■ REFERENCES piezopotential with porosity; and performance compar- (1) Qing, X.; Li, W.; Wang, Y.; Sun, H. Piezoelectric Transducer- ison of various PVDF-based PENGs based on different Based Structural Health Monitoring for Aircraft Applications. Sensors chemical composition and experimental conditions such 2019, 19, 545. as the amount of force applied, operation mode, device (2) Alibadi, M. H. F.; Khodaei, Z. S. Structural Health Monitoring for area, and output load resistance (PDF) Advanced Composite Structures; World Scientific Publishing Europe Ltd.: London, U.K., 2018. (3) Staszewski, W. J.; Mahzan, S.; Traynor, R. Health Monitoring of ■ AUTHOR INFORMATION − Aerospace Composite Structures Active and Passive Approach. 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