Triboelectric Nanogenerators Based on Organic–Inorganic Hybrid Nanomaterial
Basic Concept of TENGs and Its Operating Principle
Founded on the omnipresent but detrimental contact-electrification effect TENGs are originally developed for scavenging the mechanical energy from impacts, sliding, and rotations [63-64], producing higher output voltages than other harvesters. Tribo- electrification/contact electrification creates static polarized charges on two material surfaces, whereas electrostatic induction on the electrodes by the tribo-charges converts the applied mechanical energy to electrical energy by changing the separating distance and hence creating potential differences . According to the lump circuit model of TENG, it is a variable capacitor-type voltage generator in which the device output is associated with the separation distance between the electrodes (Figure 9.8c). TENGs offer advantages such as more flexible material choices, easy fabrication, lightweight, and low cost. In 2012, Prof. Wang’s group first introduced the TENG model in energy-harvesting device fabrication, exhibiting unprecedented energy generation potentialities at low frequency. As demonstrated in Figure 9.8a, the TENG device structure is composed of Au/Kapton/Air gap/PET/Au. Periodic contact- separation between PET and Kapton can generate sufficient output power. From there onward, significant research progress has been made and a maximum output power density of 500 W/m2 has been reported recenty .
Depending on the of the polarization change and electrode configuration, four different operation modes of the TENG have been proposed  including vertical contact-separation (CS) mode, lateral-sliding (LS) mode, single-electrode (SE) mode, and freestanding triboelectric-layer (FT) mode, as shown in Figure 9.9. These can scavenge almost all types of mechanical energy from the environment. The vertical CS mode uses relative motion perpendicular to the interface and the potential change between electrodes, and thus external current flow is dictated by the gap distance between material surfaces. The LS mode uses the relative displacement in the direction parallel to the interface, and it can be implemented in a compact package via rotation-induced sliding.
FIGURE 9.8 Theoretical models of TENG. (a) Schematic illustration of the first TENG and its operating cycle. Reproduced with permission . Copyright 2012. Elsevier, (b) The displacement current model of a contact-separation-mode TENG. (c) The equivalent electrical circuit model of TENG. Reproduced with permission . Copyright 2019, Wiley-VCH.
The SE mode takes the ground as the reference electrode and is versatile in harvesting energy from a freely moving object without attaching an electric conductor, such as a hand typing, human walking, and moving transportation. The FT-layer mode is developed upon the SE mode, but instead of using the ground as the reference electrode, it uses a pair of symmetric electrodes and electrical output is induced from asymmetric charge distribution as the freely moving object changes its position . One thing worth noting is that the practical application of TENG is not limited to one single mode but relies more on the conjunction or hybridization of different modes to harness their full advantages.
Material Design Criteria and Techniques for Performance Enhancement
Based on the electron affinity of different materials, the triboelectric series has been created to quantify the figure of merits of TENG design. The displacement current model, the capacitive model, and the figure of merits of TENG all suggest that its
FIGURE 9.9 Four working modes of TENG. (a) Vertical contact-separation (CS) mode, (b) Lateral-sliding (LS) mode, (c) Single-electrode (SE) mode, (d) Freestanding triboelectric layer (FT) mode. Reproduced with permission . Copyright 2019, Wiley-VCH.
output current and voltage are proportional to the triboelectric SCD and that the output power is proportional to the square of the SCD . That is why the underlying mechanism concerning the origin of the triboelectric charge still requires further investigation.
The improvement of SCD can be classified into three major approaches: material composition modification, enhancement of effective contact area, and adjustment of environmental conditions. The material modification strategy can be further divided into chemical surface functionalization and bulk composition manipulation. In chemical surface functionalization, the triboelectric material is modified by changing the functional groups exposed on the surface so that its charge capture capability is enhanced [66-68]. For example, Wang et al. demonstrated the use of self-assembled monolayers, thiols, and silanes to modify the surfaces of the conductive material Au and dielectric material Si02, respectively . The results show' that the output of the Au-based TENG is enhanced by the largest scale when the more triboelectrically positive function group, amine (-NH,), is introduced on the Au surface, w'hile its performance deteriorates when the triboelectrically negative group (-C1) is used. This approach eludes the change in bulk material and their properties and still possesses long-term stability with experimental validation.
Secondly, the SCD can be improved by increasing the surface contact area through surface engineering. The active contact area of two solid materials is generally small due to surface roughness, and thus by simply improving the contact effectiveness, the total amount of triboelectric charges will increase. Some forthright and widespread approaches such as surface texturing and nanostructure preparation can be adopted through lithography-assisted nanofabrication techniques, which elevate the SCD to several times higher for the same material.
The third approach is the control and tuning of environmental conditions such as temperature and pressure. Lu et al. showed that the performance of a PTFE-based TENG and its electrical output decreased with increasing temperature in the range of -20 to 20 °C, remained stable from 20 °C to 100 °C, and then dropped subsequently . This temperature-dependent change in output charge density is attributed to the change in material permittivity and temperature-induced surface defects such as surface oxidation or defluorination. Wang el al. enhanced the triboelectric charge density of a basic Cu-PTFE-based TENG to a record-high value of 660 pC/m2 by simply operating the device in a high vacuum to prevent air breakdown, which is the biggest performance-limiting factor of TENG [68-70]. Besides the aforementioned three approaches, high dielectric constant materials such as BaTiO, and SrTiO, have been widely used as fillers (P(VDF-TrFE)) as the matrix material for charge-attracting, and high-dielectric barium titanate (BTO) NPs for charge-trapping to enhance TENG performance [71-73]. Adding a carbon nanotube (CNT) charge transport layer between the triboelectric and dielectric material can effectively increase triboelectric charge density by facilitating the charge accumulation process .
Therefore, the quest for new device design, material innovation, and underlying fundamental physics investigation by considering the ambient parameters, such as temperature, humidity, ambient pressure, is important to realize an era of self- powered TENG-based micro/nanosystems.
An all-in-one or multifunctional triboelectric nanogenerator (MTENG), which can simultaneously act as a sensor and as an energy harvester to operate a whole radiofrequency (RF) transmitter and signal processor unit, is developed and integrated with a self-powered wireless sensing and monitoring system . The long-term reliability of the MTENG output and the RF transmission capability are also tested without any interruption for ~38,000 cycles. Each TENG unit consists of a nano- structured Al foil and polytetrafluoroethylene (PTFE) as triboelectrically positive and negative layers, respectively.
For producing a nanostructured PTFE surface, 10-nm gold (Au) is deposited on the PTFE surface by e-beam evaporation and the shadowing effect of the thin Au NPs is employed in the following dry etch process. PTFE nanostructures on the surface are shown in Figure 9. lOi-ii.
The self-powered wireless sensing and monitoring system contains two units; the top unit contains the TENG units and the bottom unit contains energy and control circuits. As shown in Figure 9.10a, the mechanical structure is made of three aluminum plates of 6.5 cm x 6.5 cm x 0.5 cm. The device is sandwiched between the top and middle plate, while the bottom unit remains immovable due to the fixed aluminum blocks in order to carry and protect a current rectification unit, an energy management module (EMM), and an RF module. The top TENG unit is used for sensing purposes and the rest of the units are used for harvesting mechanical energy. Then the whole device is encapsulated to the top unit of the spring-assisted structure as illustrated in Figure 9. lOb-c.
The working principle of each TENG unit is demonstrated in Figure 9.11 a. Herein, at first, the contact between the top Al electrode and the PTFE surface creates positive triboelectric charges on the top electrode and negative charges on the PTFE surface (state i). Then the separation between the top electrode and the PTFE film produces a difference in electric potential between the two electrodes, which drives the flow of free electrons from the bottom Al electrode to the top one (state ii). The current continues until the physical separation reaches the maximum (state iii).
FIGURE 9.10 Structure design of the MTENG. (a) Schematic illustration of the functional components of MTENG. which is mainly composed of a TENG unit and an integrated circuit unit, (i-ii) SEM images of the nanostructured PTFE and Al surfaces. Photographs of (b) an as-fabricated MTENG before encapsulation with the acrylic and (c) an as-fabricated MTENG after encapsulation with the acrylic. Reproduced with permission . Copyright 2019, Elsevier.
When the top A1 electrode and the PTFE surface get closer to each other, the free electrons flow from the top electrode back to the bottom one, thus generating a reverse current (state iv). As shown in Figure 9.1 lb the peak-to-peak output voltage from the top TENG unit is -700 V and the maximum peak output voltage reaches ~400 V. To theoretically validate the result, finite element simulations are performed using COMSOL (Figure 9.1 lc). Based on the electron affinity of PTFE (-190 nC/J) and the applied mechanical force (7 N corresponding to the potential energy of -0.035 J), a maximum surface charge density (MSCD) -6.65 pC/m2 is expected.
The MTENG device exhibits a peak output voltage of -400 V, corresponding to a SCD of -3.75 pC/m2, which is -56% of the theoretical MSCD (-6.65 pC/m2). It was previously reported that triboelectric materials cannot attain the MSCD due to the limitations imposed by air breakdown, thermal fluctuations, and humidity in the environment .
Then the output current from the device is measured by connecting all the TENG units in parallel, and after rectification, the average output current reaches -300 pA with normal hand pressing (Figure 9.lid). It can be seen from the output current signal that the rectified output current displays a higher peak followed by a lower peak in each cycle. The higher peak is from the pressing motion while the lower one is from the releasing motion. Subsequently, different resistors are used to investigate the reliance of the output electric power of MTENG on the external load. The corresponding instantaneous output power as a function of the load resistance (P = I2R) is presented in Figure 9.1 le. The maximum output power of -10 mW and the corresponding power density of -4 W/m2 are achieved at a load resistance of 1 M
The effect of the vibration frequency of the linear shaker on the output performance of the MTENG is also investigated. An iron mass of 0.5 kg is attached to the spring-supported top plate of the MTENG and the combined output current is measured with a constant acceleration of 1 G. With 5-mm peak-to-peak vibration displacement from the linear shaker at 10 Hz, the combined output current from the devices is -30 pA. The output current drops as the frequency increases from 10 Hz to 60 Hz (Figure 9.1 If). The displacement profile of the linear shaker with the same acceleration condition is shown in the inset (i) of Figure 9.1 If. The correlation between the short-circuit current and the displacement implies that the amplitude of vibration plays a critical role in TENG output performance.
Through the innovative structural design of the MTENG and the correlation of output with the ambient vibration frequency, it can be utilized as a vibration sensor and an energy harvester unit. The EMM unit collects, stores, and manages the generated electrical energy. An RF wireless module is then powered to transmit the vibration signal to multiple receivers simultaneously, which holds many promising applications, especially in structural health monitoring, automobile engine vibration monitoring, and biomechanical applications.
Though TENGs have higher output energy compared to PENGs, the harvested energy by TENGs may be insufficient for some high-power applications under
FIGURE 9.11 Theoretical simulation and output performance of the multifunctional TENG (a) Schematic diagram showing the working principle of the MTENG. (b) Simulated potential distribution of the MTENG at four different displacement conditions (i-iv) by COMSOL software. (c)-(d) Measured output voltage and rectified short-circuit current of the MTENG with a frequency of -5 Hz. (e) The measured output power of the MTENG with a frequency of ~5 Hz and applied force of ~7 N. (0 Comparison of the rectified output current at different frequency excitation of a linear motor (Inset (i) showing the displacement variation of the linear motor with different frequencies. Reproduced with permission . Copyright 2019, Elsevier.
certain scenarios. The output power of the state-of-art energy harvesters based on a single mechanism (i.e., piezoelectric or triboelectric) is still quite low. limiting the range of their applications. Therefore, it is highly desirable to employ multiple mechanisms not only to miniaturize NGs but also at the same time to obtain high output performance for powering the devices. Some such examples are PENG and SC-based hybrid energy cells, PENG and biochemical cell-based hybrid cells, PENG/TENG- based hybrid cells, and a combination of all. In the following section, different combinations of hybrid NGs will be discussed.