# Research Progress in Fluid Energy Collection Based on Friction Nanogenerators
## 2 Recent Developments in Triboelectric Nanogenerator-Based Fluid Energy Harvesting Systems
### Wave-Based Energy Harvesting Device
#### 2.2.4 Bionic Structure Energy Harvesting Device  
One of the treasures that [PERSON] has given to mankind is life, and human beings continue to explore. Biomimicry has injected life into the development of science. Inspired by the lotus petal opening and closing movement, a FL-TENG consisting of flower-petal sub-TENGs and flower-core sub-TENGs was proposed by [PERSON], as shown in Figure 11a. The FL-TENG periphery consists of six flower-petal sub-TENGs and flower-core sub-TENGs at the center of the flower, all of which work in contact-separation mode. When the water wave is excited, FL-TENG produces folding and telescopic motions, and the \"flowering\" and \"folding\" movements convert kinetic energy into electrical energy. The six flower-petal sub-TENGs mainly collect the kinetic energy of horizontal motion with two degrees of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**TENG** & \begin{tabular}{c} **Operating** \\ **Frequency** \\ \end{tabular} & \begin{tabular}{c} **Short-** \\ **Circuit** \\ **Current** \\ \end{tabular} & \
\begin{tabular}{c} **Open-** \\ **Circuit** \\ **Voltage** \\ \end{tabular} &
\begin{tabular}{c} **Power/Power** \\ **Density** \\ \end{tabular} & **Ref.** \\ \hline Arctic-TENG & 0.2 Hz & 32 \(\upmu\)A & 1000 V & 21.4 W/m\({}^{3}\) & [66] \\ iTEHG & 0.67\(\sim\)1.47 Hz & 2 \(\upmu\)A & 200 V & 7.25 \(\upmu\)W/cm\({}^{3}\) & [67] \\ AR-TENG & & 125 \(\upmu\)A & 70 V & 12.5 mW & [68] \\ TS-TENG & 2 Hz & 0.3 \(\upmu\)A & 84 V & 0.21 mW/m\({}^{2}\) & [69] \\ PS-TENG & 0.5 Hz & 2.77 \(\upmu\)A & 3072 V & 14.71 W/m\({}^{3}\) & [70] \\ SC-TENG & 1 Hz & 16 \(\upmu\)A & 1284 V & 8.3 mW & [71] \\ SSR-TENG & 1 Hz & 10 \(\upmu\)A & 120 V & 0.14 mW & [72] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Performance of energy harvesting devices based on pendulum structure in recent years. freedom and the kinetic energy of rotational motion with three degrees of freedom, while the core of core sub-TENGs collects the translational kinetic energy in the vertical direction of one degree of freedom [73]. Inspired by kelp that drifts with the waves, [PERSON] et al. studied a kind of TENG (kelp-forest-like TENG) that mimics the movement of kelp and its structure is shown in Figure 11b. At a wave oscillation frequency of 1 Hz, a single kelp-forest-like TNG cell can deliver an _Isc_ of 10 \(\upmu\)A and a _Voc_ of 260 V, which is sufficient to drive at least 60 LEDs with a power density of 25 \(\upmu\)W/cm\({}^{2}\)[74]. As shown in Figure 11c, [PERSON] et al. first proposed a sealed bionic fishtail structure TENG (SBF-TENG) based on anticorrosive coatings. The SBF-TENG generates an electric potential through the contact separation mode of the rubber layer with the PU coating, as shown in Figure 11d. Both the rubber layer and PU coating have good sealing, stability, and fatigue resistance. SBF-TENG can also be used as a sensitive solution or seawater salinity sensor, and the charge provided by SBF-TENG can be used to build a cathodic protection system and improve the lifetime of metals in the ocean or in solution [75]. In recent years, the performance of some energy harvesting devices based on biomimetic structures has been summarized in Table 8.  
Figure 11: (a) FL-TENG shape and structure diagram. Reprinted with permission from Ref. [73]. Copyright 2021, Elsevier Ltd. (b) kelp-forest-like TENG seaweed-like TENG structure. Reprinted with permission from Ref. [74]. Copyright 2018, Elsevier Ltd. (c) SBF-TENG structure diagram and (d) working principle. (i) Initial state; (ii) Squeeze occurs and charges are generated on the rubber; (iii) Release the squeeze, reduce the charge on the rubber, and reverse the direction of the current; (iv) Squeeze the lower rubber, gradually completing charge accumulation on its surface; (v) Squeeze to the maximum limit, and charge accumulation is completed; (vi) Release the squeeze and restore the device to its original charge balance state. Reprinted with permission from Ref. [75]. Copyright 2023, Elsevier Ltd.