Peptide has recently been demonstrated as a sustainable and smart material for piezoelectric energy conversion. Although the power output was improved compared to other biomaterials, the use of a piezoelectric device alone can only capture the energy from the minute deformation in materials. In comparison, the triboelectric effect can convert mechanical energy from large motion. Consequently, utilizing both piezoelectric and triboelectric effects is of significant research interest due to their complementary energy conversion mechanisms. Here we demonstrated a hybrid nanogenerator that combined a peptide-based piezoelectric nanogenerator with a single-electrode triboelectric nanogenerator. Our device structure enabled the voltage and current outputs of each individual type of nanogenerator to be superposed in the hybrid nanogenerator, producing overall constructive outputs. The design of our device also enabled a simplified configuration of hybrid nanogenerator. This study is important not only for the enhancement of peptide-based piezoelectric device but also for the future design of hybrid piezoelectric and triboelectric nanogenerators.
Piezoelectric bio-inspired materials have emerged as promising candidates for electromechanical energy conversion thanks to their biocompatibility, versatility, and mild synthesis processes.1–3 Among them, diphenylalanine (FF) peptide has attracted increasing research interest due to its strong piezoelectric coefficient and the ability to produce parallel electrical dipoles.2,4,5 Although the nanogenerator (NG) fabricated from FF peptide showed significant improvement in performance compared to other bio-inspired materials, new techniques to enhance the output are needed to expand its applications.
Due to their complementary effects in mechanical energy harvesting, piezoelectric nanogenerators (PENGs) have been proposed to be hybridized with triboelectric nanogenerators (TENGs).6–9 While PENG converts the deformation of the piezoelectric material into electricity, TENG converts relative motion into electricity through contact electrification and electrostatic induction.10 The available ambient mechanical energy can be provided to the NG in two forms, i.e., relative motion and material deformation, so it is advantageous to combine a TENG and PENG. Among the possible operation modes of TENG, the two-electrode modes were most often used to demonstrate hybrid devices.6,9,11 These TENG modes require two separate conductive electrodes to be stacked with the PENG. However, in applications where the thickness of the hybrid NG is restricted, it can be more advantageous to use the single-electrode mode of TENG in combination with the PENG due to the reduced number of layers and their structural similarity.7
Here we investigated the output enhancement of an FF-based PENG through hybridization with a single-electrode TENG. The piezoelectric FF microrod array was fabricated using our recently developed method. The integration with a single-electrode TENG and the resultant electricity generation process were studied. Our work can serve as a guideline for the future hybrid design of a PENG and TENG.
First, an FF microrod array was fabricated using our previously reported epitaxial growth process.2,12 Briefly, the crystalline seed film with vertically aligned domains was obtained by placing an amorphous FF gel film in vigorously circulated moist air. This step was followed by putting the seed film in a saturated FF water solution at 55 °C for epitaxial growth. Throughout this process, an electric field was applied to obtain uniform polarization. The size of the obtained microrod array is 1.25 × 1.25 cm2. Figures 1(a) and 1(b) show the Scanning Electron Microscope (SEM) pictures of the obtained vertical microrod array on a gold-coated silicon substrate. The piezoelectricity of the microrods was confirmed by Piezoresponse Force Microscopy (PFM) (Asylum MFP 3D). The PFM probe (ASYELEC-01, Asylum Research) was vibrated at 20 kHz, far from contact resonance to avoid signal amplification. The applied voltage to the probe was swept from 2 V to 10 V as it scanned on the tip of the microrod. The corresponding probe vibration amplitudes were recorded in Figure 1(c). The effective piezoelectric coefficient d33 of the microrods can be estimated from the slope of the fitted line as about 11.4 pm/V, verifying the good piezoelectricity of the obtained FF array.
The schematic and photo of the hybrid NG are shown in Figures 2(a) and 2(b). The FF-based PENG was fabricated by putting another 1.25 × 1.25 cm2 gold-coated silicon substrate on the top of the microrod array. This top electrode was connected to the external circuit by attaching an aluminum foil on the top side. The TENG materials selected in this study were polyethylene terephthalate (PET) (purchased from McMaster) and Kapton (purchased from DuPont) due to their stable output and distinct charge affinity. The single-electrode TENG was assembled by laminating a pristine PET film on the aluminum foil on the top of the PENG, while the ground is connected to the bottom electrode of the PENG, as shown in Figure 2(a). Thus effectively no additional conductive electrodes needed to be fabricated for the single-electrode TENG because it utilized the already available ones of the PENG, simplifying the configuration of the hybrid NG. The PENG, together with the lower part of the TENG, was mounted on the bottom of a plastic enclosure. The top part of the single-electrode TENG, which is a standalone pristine Kapton film with no wire connection, was attached to the movable top part of the enclosure to form a compact setup for characterization.
The energy conversion process is explained in Figures 2(c)–2(g). Because Kapton and PET have different charge affinity, upon contact Kapton gains a negative charge and PET gains a positive charge,10,13 Figure 2(c). When the two surfaces first separate, current flows from the top to the bottom electrode to balance the charge (Figure 2(d)). This current is positive according to the connection of the measuring device. As Kapton moves back to contact but with no pressing force, current flows from the bottom to the top electrode (Figure 2(e)) and is therefore negative. As the linear actuator continues to apply the pressing force to the NG, the Kapton film side compresses the piezoelectric FF microrods and current continues to flow to the top electrode due to the electrical dipole created in the piezoelectric FF microrod under pressure. When the pressing force is released, the FF microrods are no longer compressed and the current flows back to the bottom electrode (Figure 2(g)). The Kapton film is then separated from the PET, as in Figure 2(d) and current continues to flow back to the bottom electrode to complete one cycle. As a result, the hybrid device can harvest energy from both the significant movement of the top layer of the TENG and minute deformation in piezoelectric materials caused by the pressing force.
Measurements were conducted for the piezoelectric and triboelectric output separately in Figure 3 to discern the constructive output in the subsequent hybrid NG. The independent piezoelectric output was obtained by keeping the Kapton film side pressing on the PET side with a force larger than zero, which prevented the separation of the two layers of the TENG. This process is represented by the steps in Figures 2(f) and 2(g). The triboelectric output was independently obtained by moving the Kapton film to just in contact with the PET film with an insignificant pressing force and then separating them. This process is similar to the steps in Figures 3(a)–3(d). Voc and Isc from the piezoelectric nanogenerator were about 0.7 V and 30 nA, respectively, under an applied force of 50 N. The average charge, calculated by the area under the current peak, was 406 pC. These results were consistent with the reported output of FF-based NG.2 Voc and Isc from the triboelectric nanogenerator were about 1.6 V and 20 nA, respectively, with the average charge of 875 pC. The lower peak current but higher charge of the TENG part was due to the relatively slow motion of the Kapton film to reduce the impact on the FF microrod array upon contact. The current direction and voltage polarity of the triboelectric output were in the same direction as the piezoelectric output. Two outputs were thus expected to be superimposed constructively as both contact and pressing are performed.
The piezoelectric and triboelectric power generation processes were then combined to demonstrate the enhancement of the FF-based NG. A 4-step force profile shown in Figure 4(a) was applied to the hybrid NG to provide a press/release cycle after the contact between the Kapton and PET surfaces. The initial force was about 10 N, which was determined experimentally to be just enough to resist the spring force to keep the Kapton and PET films in contact, while not producing significant pressure on the piezoelectric FF microrod array. Then an additional 50 N force was applied to the FF PENG to deform the piezoelectric FF microrods, with a total applied force of 60 N. The pressing force was then released, but the contact between the Kapton and the PET film was still maintained. Finally, the polymer films were separated as no force was applied. The output open-circuit voltage and short-circuit current were recorded during the force application process as shown in Figures 4(b) and 4(c). Figure 4(b) shows a constructive step as the FF-based PENG is pressed, with total output voltage up to 2.2 V compared to 0.7 V or 1.6 V if only either PENG or TENG operates, respectively. The voltage output has a rectangular waveform instead of peaks because the insulating property of FF crystals as well as the high internal resistance of the measuring electrometer prevented the unintentional discharge. Figure 4(c) shows four current peaks in one force application cycle which are also in the constructive direction as the force is applied or withdrawn. The directions of the output current were consistent with the process explained in Figures 2(d)–2(g), verifying the constructive energy conversion mechanism of the hybrid NG.
In conclusion, we have demonstrated a hybrid NG structure that utilized a single-electrode TENG to provide constructive additional voltage and current outputs to the FF-based PENG. The design of our hybrid NG was simple because the addition of a single-electrode TENG to the existing FF-based PENG did not require additional conductive electrodes. Although Kapton and PET were selected in this study, the TENG materials can still be optimized to best suit a specific application. Since the Kapton film was not connected to the circuit, it could be designed to be a part of the environment of the hybrid NG, which could simplify the fabrication process and reduce the dimension of the hybrid NG. Our study can serve as a guideline for the design of a future FF-based device as well as future hybridization between a PENG and TENG.
We sincerely thank the support from NSF (No. ECCS-1150147) and the 3M company. The electron microscopy images were obtained in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. Part of the device fabrication was performed in the Minnesota Nano Center, a part of the NSF-funded National Nanotechnology Coordinated Infrastructure.