of water on the etched Al substrate. (g) SEM image of the etched Al substrate modified with low surface energy material. (h) High‐magnification SEM image of the modified Al substrate. (i) Image of the contact angle of water on the modified Al substrate. (j) Measured contact angles of three different Al surfaces. (k) Photograph of the water drops on the superhydrophobic surface.
Source: Reproduced with permission from Zhao et al. [61]. Copyright 2016, American Chemical Society.
2.3.2.4 Nanoparticle and Nanowire
Nanoscale materials including nanoparticles and nanowires have attracted broad interest due to their unique chemical and physical characteristics. Unlike bulk materials, reducing the size of materials to the nanoscale could make them exhibit high reactivity. Nanoparticles (NPs) with high surface energy can be easily re‐formed into functional materials, and nanowires (NWs) offer high surface area and restrain the mechanical degradation [66]. These nanoscale materials have been widely used in applications of energy collecting and storing devices.
NWs with high surface area that restrain the mechanical degradation could be used to optimize the performance of TENGs. Jiang et al. developed a Ag nanowire‐based TENG, where Ag nanowires were used as both triboelectric layers and electrodes. Ag nanowires were fabricated through a polyol synthesis method [67]. Figure 2.8a,b shows that the diameter and length of the nanowire are about 70 nm and 10 μm, respectively. Photographic paper was used as the substrate to fix the Ag nanowire slurry, resulting in a metal electrode with excellent conductivity (Figure 2.8c). The Ag nanowires closely accumulate on the photographic paper to form the Ag nanowires membrane, as shown in Figure 2.8d. Cheon et al. fabricated PVDF–Ag NW composite by the electrospinning method. The electrostatic interactions between the Ag NWs and the dipoles of the PVDF chains could promote β‐phase crystal formation of PVDF, which can be further used to increase the output performance of the TENGs [68]. From a triboelectric perspective, NPs could increase the surface roughness and dielectric property of friction films, leading to enhanced performances of TENGs. Jiang et al. reported a Ag nanoparticle‐based TENG, where Ag nanoparticles were used as both triboelectric layers and electrodes [69]. Ag nanoparticles with a diameter of about 50 nm were fabricated through an ice−water bath method as shown in Figure 2.8e. The Ag nanoparticles were stuck on the photographic paper to form the Ag membrane, increasing the mechanical stability of nanoparticles, as shown in Figure 2.8f. It is found that the membrane could form positive charges when it made contact with a FEP film in the TENG.
Figure 2.8 Ag nanoparticles and Ag nanowires. (a) SEM image of the Ag nanowires. (b) High‐magnification SEM image of Ag nanowires. (c) A photograph of the electrode based on Ag nanowires. (d) The SEM image of the electrode.
Source: Reproduced with permission from Jiang et al. [67]. Copyright 2018, American Chemical Society.
(e) SEM image of the low‐density Ag nanoparticles on Al foils. (f) The membrane prepared by Ag nanoparticles on a photographic paper.
Source: Reproduced with permission from Jiang et al. [69]. Copyright 2017, American Chemical Society.
NPs can be used to increase the surface roughness of triboelectric layers. Lee et al. fabricated textile electrodes with nanostructured geometries, where Al NPs were grown by using thermal evaporation [70]. When the Al NPs were contacted with the PDMS, triboelectric charges could be generated due to their different triboelectric series. It is found that Al NPs could remarkably increase the output voltage of the TENG. Chun et al. developed an Au NP‐embedded mesoporous TENG, where Au NPs were embedded into the pores of the PDMS. It is found that the contact between Au NPs and PDMS could enhance the surface potential energy, resulting in high output performance of TNEG [71]. Zhang et al. explored the effect of the output performance of TENGs based on Cu NP‐embedded films and ZnO NP‐embedded films, respectively. The results showed that Cu NPs can effectively increase the output performance, but ZnO NPs hardly do that [72].
2.3.3 Performance
Currently, mechanical stability and high electrical output are two main performances considered by numerous researchers for TENGs. It is found that the favorable mechanical property could increase the electrical property of the devices, especially in WD‐TENGs.
2.3.3.1 Mechanical Behavior
Many flexible structures, which can be bent, folded, and twisted, were applied in the WD‐TENGs. Taneda et al. explored diversiform flapping modes, such as one‐node, imperfect‐node, and two‐node flutters. In addition, some researchers found the distinct dynamic states by using one‐dimensional filaments [73]. Wang et al. developed a TEG for harvesting wind energy based on the flow‐driven vibration of a Kapton film, and analyzed vibrating modes of the film [33]. It is found that an oscillating flow, which is known as Karman vortex shedding, was formed when wind flowed past a bluff body. Figure 2.9a shows that vortices are formed behind the bluff body and separated periodically from either side of the body. Thus, a flexible structure attached on the back of the body could periodically vibrate when wind flowed past this structure. Figure 2.9b shows the model of the designed structure, where a bluff body is fixed on the medium position of the acrylic tube and used to fix one side of the Kapton film. The tube could import air flow in a single direction, resulting in a preliminary adjustment of the flow. The air flow could be further separated by the bluff body and vibrate the free side of the film, leading to periodic contact between the film and the walls of the acrylic tube. The frequency of Karman vortex shedding can be expressed by the Strouhal number (St):
(2.3)
where V is the flow speed of the inlet port, fs is the vortex shedding frequency, and D is the height of the perpendicular edge.
Figure 2.9 Schematic illustrations and simulation of the vibration. (a) The vortex shedding effect. (b) Schematic diagram of the device. (c) The theoretical simulation of the film. (d) The fourth vibration mode in the theoretical simulation and the real deformation of the device.
Source: Reproduced with permission from Wang et al. [33]. Copyright 2015, John Wiley and Sons.
The constraint mode analysis was studied via finite element method to optimize the vibration mode of the device. Figure 2.9c shows six modes under different frequencies. The working frequency of the TEG is about 155 Hz, which coincides with the simulated fourth order mode, as shown in Figure 2.9d. Bae et al. explored dynamic characteristics of flutters in the flutter‐driven TENG, where the woven flag displayed a vibrating node [37]. The fluttering amplitude was very small above the node, but the amplitude below the node increased with increasing distance. By studying different regimes of dynamic interactions, such as single‐contact, double‐contact, and chaotic modes, the fluttering performance could be optimized to increase the performance of the TENGs. Zhang et al. used a high‐speed camera to capture the dynamic process of the contact‐separation