batteries requiring frequent charging and replacements are replaced by alternative solutions. One promising way to do so is energy harvesting through which the energy is converted from ambient energy to electrical energy, and thus can be used to power the low power electronics. A variety of approaches to converting the source energy to the electricity have been developed, which are essentially energy harvesting processes, such as triboelectric, [5][6][7][8][9] piezoelectric [10][11][12][13] and electrostatic, [14][15][16][17][18][19] pyroelectric, [20][21][22] and electromagnetic [23][24][25][26] methods. Among the various sources of energy, mechanical energy is of interest, and it presents in an ambient environment in forms of vibrations and shocks, flow of fluids, and motions of the human body etc. Such ubiquitous mechanical energy can be converted to the electrical charges by utilizing the transduction mechanism of piezoelectric effect within the piezoelectric materials. [27,28] The converted electrical energy can be further applied to power the low-power electronics such as small sensor networks and systems, [13,29,30] wearable and implantable medical devices. [31][32][33] With the tendency of modern electronics becoming smaller in size, more flexible in texture, and closer to the human body, bulky batteries will be replaced in the future. Therefore, piezoelectric energy harvesting devices are the cherry-picking due to their advantages such as broad material selections [34][35][36][37][38][39][40][41] and capability of biomechanical energy conversion. [42][43][44] In addition, by utilizing the same transduction mechanism due to the piezoelectric effect, the devices are applicable in sensing applications as well. [32,[45][46][47][48] Among various piezoelectric materials, piezoelectric polymers are widely used in applications where flexibility is a priority. Especially, one of the piezoelectric polymers, poly vinylidene fluoride (PVDF) and its co-polymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), possessing excellent flexibility and biocompatibility, are favorable for biomedical applications. [4,32,33,44,[49][50][51] Typically, the piezoelectric polymers have relatively low piezoelectric coefficients (d 33 in tens of pico coulombs per Newton (pC/N)) compared to the piezoelectric ceramics (d 33 in hundreds pC/N). [52] A variety of approaches to improving the piezoelectric polymers' performance have been developed, such as co-polymerization, [53][54][55] additives of filler with larger piezoelectric coefficients, [56][57][58] modifications to processing methods (drawing, [59] annealing [60][61][62] and poling [62,63] ), Converting mechanical energy from either the ambient environment or the human body motions to the useful electrical energy will revolutionize power solutions for flexible electronics. Here, a hybrid energy harvesting strategy is reported, which combines porous polymeric piezoelectric film with an electrostatic layer as an integration for converting the mechanical energy into electrical energy. The piezoelectric materials through engineered microstructures are developed to enhance energy generation due to the higher compressibility and larger surface contact area. The electrostatic effect from the charged layer further contributes to the generation of electrical charges. By directly coating the stretchable carbon nanotubes onto the elastomers, more intimate integration of the hybrid energy harvesters enables the designs for complex electronics. Such flexible hybrid piezoelectric-electrostatic device exhibits superior energy harvesting performance with a voltage output of 1.95 V, which improves 30% and 100% compared to the electrostatic and piezoelectric alone device, respectively. Experiments are also performed to demonstrate the implementation of the hybrid device's energy conversion to power small electronics and recognition of different body motions. Such hybrid strategy provides a new solution toward future energy revolution for flexible electronics.

Wearable and implantable devices are changing our lives. Fitness trackers motivate people's workouts and healthy lifestyles; [1,2] athlete-tracking systems help professional sports clubs quantify players' performance; [3] implantable medical devices become increasingly prevalent treatment and diagnostic tools in medical practices. [4] Meanwhile, the power supply components in these electronics are becoming smaller in size and running for a longer time. The bulky and rigid traditional www.advmatinterfaces.de micro structuring (generation of microporosity [49,64,65] ), and nanostructuring (by electrospinning [66][67][68] or template wetting [69][70][71] ).

In addition to improving the piezoelectric material itself, researchers also have proposed strategies of combining different energy harvesting mechanisms [72][73][74][75][76][77] for an enhancement of overall energy harvesting performance, such as a combination of piezoelectric and electrostatic mechanisms. [78][79][80] The electret-based electrostatic energy harvester is one strategy of realizing electrostatic energy harvesting, which contains an electret material with quasi-permanent electric charges in its capacitor structure. [81] Such electrostatic energy harvester does not need any initial electrical energy, and its structure deformation directly induces an electrical output. The basic principle for such electrostatic energy harvesting is based on the fact that the charges on the electret are equal to the sum of charges on two electrodes. Therefore, a relative movement of one electrode compared to the electret/electrode combo leads to a reorganization of charges between the electrodes. The electrostatic energy harvesting mechanism has been widely used in microelectro-mechanical system (MEMS). [80,82,83] Reduced graphene oxide (rGO) carries neat negative charges [84][85][86] because of their numerous oxygen-containing groups, therefore, it shows the potential usage as an electret material in electrostatic energy harvesting practice.

Herein, we designed a flexible hybrid energy harvesting/ sensing device possessing a significant enhancement in energy conversion due to the combination of the two transduction mechanisms. This multilayer device was fabricated and assembled by a porous piezoelectric P(VDF-TrFE) film and an electrostatic film of polydimethylsiloxane (PDMS)-rGO sandwiched by two encapsulation layers with single wall carbon nanotubes (SWCNT) electrodes patterned PDMS. The engineered polymeric microstructures were developed to enhance energy generation due to the higher compressibility and larger surface contact area, which improved voltage output by 65% compared to the similar device with solid polymer structure. The rGO carrying neat negative charges were dispersed in the PDMS matrix to introduce the electrostatic energy harvesting effect. By comparing with the electrical outputs generated by the piezoelectric and electrostatic energy harvesting devices, the output from hybrid device were enhanced by 30% and 100%, respectively. Moreover, to demonstrate the hybrid device's energy harvesting and sensing capability, experiments such as lighting up LEDs, charging capacitor, and recognition of different body motions were then performed. Such hybrid device provided a practical power solution for flexible electronics.

An overview of the flexible hybrid energy harvesting device is shown in Figure 1. It consists of a piezoelectric layer of porous P(VDF-TrFE) film, an electrostatic layer of PDMS-rGO film and two encapsulation layers of PDMS patterned with SWCNT electrodes. In this work, we engineered the microstructures of the piezoelectric polymer of P(VDF-TrFE) with high β phase crystallization to enhance piezoelectric performance. Specifically, the porous structure of the piezoelectric film exhibits higher compressibility and stress concentration, resulting in a higher piezoelectric response by comparison with non-porous structures. Such porous morphologies within the material were developed through a phase separation method by controlling the humidity conditions during the material solidification process. [87] Essentially, the water droplets entered the polymer-rich P(VDF-TrFE) solution matrix, and the porous structures were formed after the evaporation of solvent. Next, we added an electrostatic energy harvesting layer by spin coating of rGOdispersed PDMS resin, and a charged PDMS-rGO film was formed. In addition, a facile fabrication technique through the vacuum filtration was developed to form highly compliant SWCNT mats. By leveraging the interplay between conduction by network percolation and the surface adhesion of the energy harvester hybrid device motion sensor www.advmatinterfaces.de elastomer of PDMS, the patterned SWCNT electrodes were transferred onto the PDMS layers directly through contact pressing. Then the separate layers of piezoelectric, electrostatic films and patterned electrode layers were assembled together as an integrated multilayer hybrid device (Figure S1, Supporting Information). Such fabrication method was an efficient approach and allowed for layer designs individually, and thus prevented interference from the layer-by-layer fabrication approach. By utilizing the functional films of porous P(VDF-TrFE) and PDMS-rGO, the combination of both piezoelectric and electrostatic effects led to an overall improvement in energy generation. The developed hybrid device was further demonstrated in energy harvesting and sensing applications, such as powering small electronics, and sensing the human body motions.

Characterization of the materials was performed by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The SEM images in Figure 2A,B show the cross sections of P(VDF-TrFE) layer and PDMS-rGO layer, respectively. An average pore size of 10 µm in diameter was determined within the P(VDF-TrFE) layer, and the uniform dispersion of rGO nano particles was also obtained in the PDMS matrix. During the porous P(VDF-TrFE) film fabrication, we also observed that the N,N-Dimethylformamide (DMF) solvent could partially or even fully dissolve the bottom solid P(VDF-TrFE) layer depending on the time of humidity control conditions. The thicknesses of different layers of PDMS, porous P(VDF-TrFE), and PDMS-rGO were measured. In Figure 2C, it was found that the electrostatic layer of 150 µm thickness was roughly 2.5 times as thick as the piezoelectric layer of 60 µm thickness, and the total thickness of the hybrid device was ≈800 µm. The diffraction spectrum of P(VDF-TrFE) film before and after annealing and poling treatments is shown in Figure 2D. The annealed and poled porous P(VDF-TrFE) shows the enhanced β phase formation with the pure characteristic β phase peaks of 2θ = 19.8°, [84,[88][89][90] 35°, [88,91] 41°[ 91,92] and a crystallinity of 84.4% (also see Fourier-transform infrared spectroscopy (FTIR) spectrum in Figure S2 (Supporting Information) for the enhancement details of β phase before and after treatments). It is worth mentioning that P(VDF-TrFE) shows the lower angle of relevant β phase peaks compared with β-PVDF (JCPDS 38-1638) due to the enlarged inter-chain lattice spacing arising from the incorporation of another large fluorine atom in the TrFE unit. [92] Some other factors such as treatments [90,93] and percentage in the copolymer [94] can also shift the peak angles.

Three energy harvesting devices were developed and tested to compare their energy harvesting performances. The hybrid device, which contained a poled and annealed porous P(VDF-TrFE) layer and an electrostatic PDMS-rGO layer, exhibited both piezoelectric and electrostatic effects (Figure 3A left). Two control groups (Figure 3A middle and right) were further designed as follows: 1) Control 1 device consisted of the unpoled and unannealed porous P(VDF-TrFE) and PDMS-rGO films, where the electrostatic layer dominated the overall energy generation; 2) Control 2 device was made by a single layer of poled and annealed porous P(VDF-TrFE) film, where energy was generated due to the piezoelectric effect alone.

The energy harvesting performance of devices was evaluated by subjecting them to dynamic normal forces at 1 Hz, 10 N magnitude. A shaker testing platform was built to exert forces to the testing device mounted on the vertical wall of a fixture frame. The generated real-time open-circuit voltage outputs from the energy harvesting devices were collected and recorded by an oscilloscope. (See Figure S3 and Video S1, Supporting Information for details). Based on the results in Figure 3B, the hybrid device generated 1.95 V peak-to-peak voltage output (V pp ), which was the maximum among the three devices. This voltage output was 30% higher than 1.54 V pp generated by control 1 device and double the value of 0.97 V pp generated by control 2 device. Such enhancement can be explained by the contribution from two transduction mechanisms of piezoelectric and electrostatic effects (Figure S4, Supporting Information). When the hybrid device was subjected to a compressive force, an equal number of opposite charges was generated in the P(VDF-TrFE) layer due to the piezoelectric effect. Especially, the presence of pores within the polymer structure leads to a higher piezoelectric charge constant due to the increased compressibility, and thus contributes to an enhanced piezoelectric energy harvesting performance. [64,95] Meanwhile, the deformation of the polymer film changed the distance of top and bottom electrode layers, and induced the reorganization of charges between electrodes causing the electrostatic energy harvesting process. [81] To evaluate the performance of porous polymeric film, two hybrid energy harvesting devices were fabricated with porous and solid piezoelectric polymers. The two devices were tested under the same mechanical input of 1 Hz 10 N. It shows in Figure 3C that the solid P(VDF-TrFE)-based device generated an output of 1.18 V pp , which was 40% less than the output (1.95 V pp ) from a porous film-based energy harvesting device. The mechanism can be explained that the strategy of using the porous piezoelectric polymer induced more compressibility and high stress concentration under the same applied mechanical load, and thus resulted in a higher piezoelectric output. Therefore, those results (Figure 3) demonstrated the two advantages of our developed hybrid energy harvesting device: 1) the hybrid energy harvesting strategies outperformed any single transduction mechanism-based devices; and 2) the engineered porous polymeric material generated significantly higher electrical output by comparing with the solid polymer. Moreover, the content of rGO in PDMS-rGO film (0.8 wt.% for current conventional tests) also affects the energy harvesting performance (see Figure S5, Supporting Information): the higher the rGO concentration, the greater the open-circuit output voltage can be generated. www.advmatinterfaces.de

The energy performance of the hybrid device was characterized by testing it under different conditions, such as various input force magnitudes and multiple input frequencies. Figure 4A depicts the voltage output generated by the hybrid device to a variety of magnitudes of applied forces, while the frequency was fixed at 1 Hz. By changing the forces from 5 to 25 N, the peak-to-peak voltages generated from the hybrid device were 1.00 V, 1.95 V, 3.08 V, 4.11 V, and 5.17 V, respectively. A linearity between generated voltage and force magnitude was observed (R 2 = 0.999). On the other hand, the mechanical input frequency also affected the energy generation from hybrid device (Figure 4B). We fixed the magnitude of the force at 10 N and obtained the voltage outputs of 1.27 V, 1.95 V, 2.71 V, 3.21 V, and 3.51 V corresponding to the increasing frequencies from 0.5 to 2.5 Hz. Here, within the lower frequency range, we also observed a linear relation between the generated voltage and the input frequency (R 2 = 0.999). Based on these two sets of experimental results, we conclude that the larger mechanical input (either by increasing the amplitude or the frequency) contributes to a higher electrical energy generation. The measured open-circuit peak-to-peak voltage denotes the maximum voltage an energy harvesting device can generate, showing the energy conversion property of the device. Experiments of determining the optimal load resistance where the maximum power can be delivered were also performed by connecting the resistors of varying resistances, see Figure S6 (Supporting Information) for details.

Next, the cycling stability of the hybrid device was evaluated. By using the same shaker-based testing platform, the energy harvesting device was subjected to a dynamic force (2 Hz, 10 N) continuously over 2 × 10 4 cycles. As shown in Figure 4C, no degradation of electrical output was observed, and it demonstrated an excellent stable performance of the hybrid energy harvesting device. Considering a practical scenario in applications where the hybrid device might undergo complex mechanical loads, different types of deformations, such as the bending and stretching tests, were then performed. Experiments were designed and performed in a modified shaker testing platform, where we installed two grippers to the shaker stinger and the static fixture platform to bend and stretch the testing device horizontally (Figure S7, Supporting Information). The testing device was clamped by the grippers and kept flat and unstretched prior to the test. The sinusoidal movement of the shaker stinger bent the device in the first half cycle and stretched it the second half cycle. In Figure 4D, a much lower electrical energy was generated. It was due to the transduction mechanism of the hybrid energy harvesting device mainly coming from the compressive deformation. Therefore, those results indicate that the potential applications of our hybrid device should mainly consider the working scenarios where normal forces are applied for practical applications.

Next, we further demonstrated the hybrid device for energy harvesting and sensing applications. First, we used the hybrid device to power LEDs and charge a capacitor. For practical use, electronics use direct current (DC) instead of alternating current (AC) due to the efficient power management. Therefore, the device-generated AC was converted to a DC via a full-bridge rectifier. Figure 5A and Video S2 (Supporting Information) showed that five green LEDs in a series connection were lit up by gently finger tapping the energy harvesting device, where each LED required a 2.2 V forward voltage rating. The charging tests were also performed by connecting capacitors of varying capacitance (1, 4.7, and 10 µF) in the same circuit. We applied a mechanical input (10 Hz, 4 N) via the shaker platform to the energy harvesting device. Similarly, the AC voltage generated by converting the mechanical energy was converted to DC voltage and then stored in the capacitor (Figure 5B). The charging process took the hybrid device 40, 40, and 90 sec to reach 0.15, 0.15, and 0.16 V. Although the corresponding average power was still in nano Watts (P = CV 2 /2t) due to the high internal impedances of the energy harvesting device, strategies could be further pursued to achieve significantly higher electrical outputs. For examples, ways to optimize the performance of the devices can be further increasing the nanoparticles of rGO within the electrostatic layer, adding multiple layers of piezoelectric films, decreasing the internal impedances (e.g., making the device much thinner).

The sensing capability of the hybrid device was further tested. the sensing mechanism is the same as the working principle of energy harvesting of the devices. In sensing experiments, the volunteer was asked to perform several body motions by wearing the hybrid device at the different body positions, and the converted electrical signals from the sensing device were recorded. As shown in Figure 6, three motions were performed, and the corresponding electrical generations were distinguishable with different curve patterns. This is due to the fact that the muscle activities of different body motions generate dissimilar patterns and distinct amplitudes of the force. In Figure 6A and Video S3 (Supporting Information), the actions of index finger flexion and extension had the largest voltage outputs among the three tests. The output curve contained one positive and two negative peaks with small amplitudes. Since the hybrid device fully covered the middle joint of the index finger and formed a curved surface, the flexion and extension motions directly added the pressing and releasing actions to the device, resulting in the material's strain and thus outstanding outputs. The obtained double negative peaks may come from the folding and buckling of the curve surface. In Figure 6B and Video S4 (Supporting Information), the hand grasping and releasing action generated a complex output pattern with quadruple peaks and outstanding negative-peak pattern. With the hybrid device attached to the palm and located near the largest joints of the fingers, the mechanical energy came from multiple fingers flexion and extension when palm bending and stretching. Therefore, such motions deformed the sensing device in a complex way, such as uneven bending and buckling. In Figure 6C and Video S5 (Supporting Information), the sensing signals generated from flexion and extension of the elbow showed two distinct peaks of opposite values. Due to the 60° bending range and a relatively flat contact area, the device was simply bent and released, and the gentle pressing and releasing actions were added. Therefore, a lower sensing signal and a concise pattern were obtained. The consecutive negative and positive peaks appeared due to the extension-flexion-pausing motions series.

We have developed a multilayer hybrid energy harvesting/ sensing device by combining the porous piezoelectric polymeric film with an electrostatic layer. The electrical energy has been harvested by two transduction mechanisms due to the piezoelectric and electrostatic effects of the materials. The flexible P(VDF-TrFE) polymers with the engineered porous structures perform as piezoelectric energy harvesting, while the nanoparticles of rGO within the PDMS matrix are formed as a highly charged layer for electrostatic energy harvesting. The energy generation from the hybrid device has been improved 100% by comparing with a piezoelectric polymer-based device and 30% in comparison with an electrostatic energy harvesting device. By designing the porous structures within the piezoelectric polymeric materials, 65% improvement of the voltage generation has been obtained compared to the solid polymer-based device. Overall, the device is fabricated based on biocompatible materials of P(VDF-TrFE) [49,96,97] as well as encapsulation layers of PDMS, [98][99][100] and thus the biocompatible energy harvester has the potential to be used in wearable and implantable applications, such as wearable health monitors, or a power supply for self-charging devices, such as supercapacitors, with nontoxic and biofriendly requirements. Based on the cycling results, the device is flexible, robust, and exhibits excellent repeated flexure exposure to the external environment, therefore, it provides promising applications in wearable devices. For example, the hybrid energy harvesting device could be integrated into fabric or clothing directly for powering small electronics by converting the biomechanical energy from the human body into electrical energy. Moreover, sensing results show the capability of the device to identify different body motions based on the different electrical signals. Integrated with the flexibility and biocompatibility, such hybrid sensor shows the great potential in biomedical sensing applications, such as cardiac sensors. In summary, this work reveals new materials and structural design strategies, and the hybrid piezoelectric-electrostatic energy harvesting/sensing device provides a new solution toward future energy revolution for flexible electronics.

The P(VDF-TrFE) solution was prepared by dissolving P(VDF-TrFE) powders (Arkema 70/30 mol) in the N,N-Dimethylformamide (DMF) solvent (Sigma-Aldrich) with 15 wt.% for over 8 h under magnetic stirring condition. A solid P(VDF-TrFE) film was first made by spin coating of 1 mL P(VDF-TrFE) solution on the glass slide at 1000 rpm for 30 sec and then heated in oven at the temperature of 50 °C for 10 min. Next, the spin-coating process was repeated by directly coating a 1.5 mL of P(VDF-TrFE) solution on top of the formed first solid P(VDF-TrFE) layer at the same speed for the same amount of time. A porous P(VDF-TrFE) film was formed by leaving the solution in a relative humidity of 90% environment. To further increase P(VDF-TrFE) crystallinity, the as-fabricated porous film was annealed in the oven at the temperature of 135 °C for 2 h. Then a totally hour-long electrical poling process was performed under an applied electrical field of 80 V µm -1 , during which the sample was kept at 100 °C for the first half hour and room temperature for the second half.

Next, Sylgard 184 silicone elastomer base (Dow Chemical Company) was mixed with the curing agent at a 10:1 ratio to form PDMS resin. To prepare the electrostatic layer of PDMS-rGO, 0.8 wt.% of rGO powders (Sigma-Aldrich) were mixed with the prepared PDMS resin, and super sonicated for 1 h. The PDMS-rGO film was then formed by spin coating the mixed resin at 700 rpm for 30 sec directly onto the electrode layer. To prepare SWCNT electrodes, 5 mg SWCNT powders mixing with 100 mL 1 wt.% sodium dodecyl sulfate solution (Sigma-Aldrich) and a few drops of Isopropanol (Sigma-Aldrich) were ultrasonicated for 30 min. Then the SWCNT percolating mat was developed through a vacuum filtration method by using the PTFE filter (47 nm pore diameter). Meanwhile, a PDMS substrate was coated onto a glass slide (500 rpm for 30 sec) and cured in the oven at 60 °C for 50 min. Next, SWCNT electrodes were transferred onto the partially solidified PDMS layer directly by contact pressing method. A designated pattern of electrodes was fabricated by using the Myler film as a mask.

The multi-layer hybrid device was fabricated by sandwiching the piezoelectric layer of porous P(VDF-TrFE) film, the electrostatic layer of PDMS-rGO film, and the two electrode layers of SWCNT on PDMS film. We added a heavy weight on the top of the device overnight. Since the top and bottom SWCNT-coated PDMS layers were not fully cured right after the assembling process, they further helped the overall encapsulation of the device during their solidification process. Electrostatic energy harvesting devices (refer to control 1 device) with different rGO weight percentages were fabricated and tested (Figure S5, Supporting Information), and 0.8 wt.% of rGO was used in this work due to the excellent energy harvesting performance and readily experiment practice.

Material Characterization Methods: The porous structure of P(VDF-TrFE) film and rGO dispersion within the PDMS matrix were observed by using scanning electron microscopy (SEM). The cross section of the hybrid device was obtained by the freeze fracture technique in liquid nitrogen, and the SEM imaging of each layer was performed on a JEOL JSM-9700F. The phase identification of the P(VDF-TrFE) material was measured by XRD and FTIR. The fabricated polymeric thin films with/without annealing and poling treatments were tested. The XRD measurement was carried out using a PANalytical Empyrean X-ray diffractometer with Cu-Mo radiation in the 2θ range from 15° to 55°. The FTIR measurement was performed via Agilent Cary 620 FTIR Imaging Microscope with Cary 670 Bench system.

Hybrid Device Testing Approaches: Two shaker-based testing setups were designed to perform the conventional and bending/stretching tests, respectively (see Figures S3 and S7, Supporting Information). The conventional testing setup consisted of a shaker (Modal Shop), and a fixture frame (Newport). The input excitation was provided by a function generator (Keysight 33 500 B) with a power amplifier (Modal Shop). The real-time electrical output generated by the hybrid energy harvester was collected by an oscilloscope (Keysight DSOX1204A). A plywood head (cross section area of 2.1 cm × 2.4 cm) was connected with the load www.advmatinterfaces.de cell (PCB) onto the shaker stinger, and the real-time applied force was measured simultaneously via a data acquisition (DAQ) card (National Instruments) controlled by the LabView software. The bending/stretching experimental method was similar using a modified shaker testing setup. Specifically, two grippers were used to clamp the testing device horizontally, one of which was installed onto the shaker stinger and the other on the fixture frame. The device maintained a flat and unstretched status prior to the test. Given the sinusoidal motion from the shaker platform, the device was bent during the first half of the cycle and stretched in the second half cycle. An oscilloscope was used to collect the voltage output by the hybrid device simultaneously. The experiments involving human subjects have been performed with the full, informed consent of the volunteer, who is also the author of the manuscript.