reports about hybridized nanogenerators in other countries such as Korea and Singapore in recent years [20–23].
1.2.1 Hybrid Energy Cells
In 2009, mechanical and solar energies have been reported to be scavenged with a hybrid device by using a piezoelectric nanogenerator at the bottom surface and a dye‐sensitized solar energy harvester at the top surface of the device [7]. The output voltage of the solar energy harvester can be about 0.591 V and the output current density can be about 6.9 μA/cm2. When the piezoelectric nanogenerator is working, the increased output voltage can be about 0.6 V and the output current density still remains at a fixed value, where the enhancement of the voltage is due to the piezoelectric nanogenerator [7]. By integrating a piezoelectric nanogenerator, the used dye‐sensitized solar energy harvester has been also utilized to create a hybrid energy cell for concurrently scavenging solar and mechanical energies [24]. The corresponding output voltage of the solar cell was found to increase from 0.415 to 0.433 V due to the piezoelectric nanogenerator.
From 2011 to 2014, various hybrid energy cells were reported to scavenge multi‐types of energies at the same time for realizing the enhancement of the conversion efficiency of the related energy devices, exhibiting practical applications in sensor networks and electrochemical reactions. In 2013, Yang et al. reported a hybrid energy cell consisting of a TENG and a micropyramid Si solar cell, which can be utilized to scavenge mechanical and solar energies at the same time [25]. As displayed in Figure 1.1a, the working of the TENG is due to the periodical contact and separation between the indium tin oxide (ITO) film and the polydimethylsiloxane (PDMS) nanowires. The fabricated solar cell consists of Si‐based micropyramid p–n junctions, where the used Si pyramids have sizes ranging from 1 to 10 μm, as presented in Figure 1.1b. The inset of the Figure 1.1b shows the used PDMS nanowires. Figure 1.1c presents the J–V curves of the used solar energy harvester with and without the transparent PDMS nanowires on the surface of the device, where the output voltage of the device can be about 0.6 V and the output current density can be about 35 mA/cm2 under simulated sunlight illumination, respectively. After using the PDMS nanowire array on the solar cell, the corresponding conversion efficiency of the fabricated device was decreased from 16% to 14%. Figure 1.1d nanomaterial displays the V–t curves of the hybrid energy cell, indicating that the rectified output voltage signals of TENGs exhibit DC peak characteristics.
Figure 1.1 Hybrid energy cell for scavenging solar and mechanical energies. (a) Schematic diagram of the designed device. (b) Scanning electron microscopy (SEM) image of the Si pyramid structures. The inset displays an SEM image of the PDMS nanowire array. (c) J–V curves of the solar cell covered with and without PTFE(polytetrafluoroethylene)‐PDMS polymers under a light illumination intensity of 100 mW/cm2. (d) V–t curves of the solar cell, hybrid energy cell, and TENG.
Source: Reproduced with permission from Yang et al. [25]. Copyright 2013, American Chemical Society.
Yang et al. also reported many other hybrid energy cells [26–29], including TENGs, piezoelectric nanogenerators, solar cells, thermoelectric nanogenerators, and pyroelectric nanogenerators. All the hybrid energy cells are based on effectively integrating the multimode energy scavenging units into a system to obtain sustainable power supply. The purpose of developing hybrid energy cells is to maximize the energies obtained from our living environment. How to effectively integrate the different energy scavenging units is still a challenge in practical devices. Moreover, the ratios among different energy scavenging abilities also need to be considered to make sure that this integration is useful in the system.
1.2.2 Electromagnetic–Triboelectric Hybridized Nanogenerators
The electromagnetic effect is due to the electromagnetic induction in Faraday's law, where the magnet and the coil have relative movements to induce the voltage/current signals. The working of a TENG is based on the coupling effect between the triboelectrification effect and the electrostatic induction in the periodical mechanical motion process [2]. In 2015, Prof. Ya Yang and coworkers first developed a method to integrate an EMG and a TENG in one device [8], so that the same mechanical motions can generate more electric energy due to the use of two energy scavenging devices. The corresponding energy conversion efficiency from mechanical energy to electric energy can be largely increased. This method has been rapidly extended to scavenge many kinds of other mechanical energies, such as wind energy [30], biomechanical energy [31], rotational energy [11], and so on.
To harvest vibration energy, Wu et al. demonstrated a spring‐based hybridized nanogenerator including an EMG and a TENG [8], as illustrated in Figure 1.2a. In the vibration process, the electromagnetic and triboelectric effects can coexist in the same mechanical motions, which can effectively enhance the energy conversion efficiency. Figure 1.2b displays a photograph of the device, where the TENG can work under the periodical mechanical motions. Moreover, the working of an EMG is based on the relative mechanical motions between the magnet and the coils. Figure 1.2c illustrates the photograph of the vibration part. The pyramid microstructures with average sizes smaller than 10 μm can be found on the surfaces of the PDMS films, as shown in Figure 1.2d. The output voltage of the TENG can be up to 600 V, and the corresponding output current can be about 3.5 μA. Under the same mechanical motions, the output voltage of the EMG can be about 3 V and the output current can be about 1 mA.
Figure 1.2 Electromagnetic–triboelectric hybridized nanogenerator for scavenging vibration energy. (a) Schematic diagram of the electromagnetic–triboelectric hybridized nanogenerator. (b) Photograph of the fabricated device. (c) Photograph of vibration part in the device. (d) SEM of the PDMS surface with micro/nanostructures.
Source: Reproduced with permission from Wu et al. [8]. Copyright 2015, Elsevier.
To scavenge wind energy, Wang et al. designed an electromagnetic–triboelectric hybridized nanogenerator, which is based on a tube structure [12]. As displayed in Figure 1.3a, the Kapton film can vibrate in the tube when wind goes through the tube from the right side of the device to the left side of the device [32]. By utilizing a high‐speed camera, we can find that the effective contact between the two electrodes and the Kapton film for the TENG is mainly at the left side of the acrylic tube (marked with red dot line). There is no contact between the Kapton film and the two electrodes at the right most region of the tube, where most of the vibration energy was wasted due to the required effective contact/separation for the working of the TENGs. By integrating an EMG at the right region of the device in Figure 1.3b, the wind‐driven vibration of the organic film can drive the working of the EMG and generate more electricity [12]. To understand the ratios of the produced energies for the two generators, we can calculate the electricity generated in the time of 10 seconds, where the TENG can give 2.6 mJ while the EMG can give 3.2 mJ.
