## 4 Applied Current CP
### Triboelectric Nanogenerator Power Supply Technology  
In 2012, [PERSON] [73] first proposed the triboelectric nanogenerator (TENG) based on the principle of frictional electricity and electrostatic induction. Two friction electrode materials with different electronegativity (in Figure 3) generate induced charges when they are rubbed together. When the two materials are separated, the potential difference between them will be maintained for some time due to the insulating nature of the materials, at which time the metal electrode generates a positive charge due to electrostatic induction. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Method No.** & **Reference** & **Experiment** & **Results** \\ \hline \multirow{8}{*}{Multi-resonance systems} & [PERSON] et al. [64] & 3 \(\times\) 5 array PZT-PENG, adjust the size of the individual cantilever beam to achieve the effect of modal separation so that the PENG has a wider bandwidth. The piezoelectric cantilever beam is fixed at both ends, and multiple array single cantilever beams of different sizes are pasted on both sides of the main beam. By adjusting the size of each cantilever beam and the size of the mass block, the purpose of widening the bandwidth is achieved. & The piezoelectric material of the single-beam piezoelectric cantilever structure is divided into N segments. & The resonant frequencies are tuned \(\pm\)5\%. \\ \hline \multirow{8}{*}{Adaptive mechanical resonance} & [PERSON] et al. [67] & Frequency tuning mechanism. Under the action of axial preloading, additional bending moments are generated to influence the total restoring force and thus change the resonance frequency of oscillating beams. & The adaptive system has a large output at 150–190 Hz. \\  & [PERSON] et al. [68] & Fixed and free piezoelectric vibration collectors are connected by hinges. & The resonant frequencies are tuned \(\pm\)15\%. \\ \hline \multirow{2}{*}{Bistable method} & [PERSON] et al. [69] & A bistable rotating piezoelectric vibration collector introduces two external magnetic forces for bistable and up-conversion, respectively. & The bandwidth is twice as large as that of no bistable energy harvester. \\ \hline \multirow{8}{*}{Nonlinear spread spectrum method} & [PERSON] et al. [70] & A magnet is installed at the end of the piezoelectric vibration collectors, and another magnet is placed on the bottom frame. The piezoelectric vibration collectors flex due to magnetic induction forces. & The bandwidth of 25 Hz (24–49 Hz). \\  & & An amplitude-stiffened Duffing spring. The elastic forces include the linear term caused by small deflection & The resonant frequencies are tuned \(\pm\)20\% \\ \cline{1-1}  & [PERSON] et al. [71] & bending and residual stress in the beam (the micro-member can be ignored) and the nonlinear term caused by tension. & The bandwidth of 35 Hz. \\ \cline{1-1}  & [PERSON] et al. [72] & Ad-leg nonlinear compliant orthoplanar springs and piezoelectric vibration collectors. & The bandwidth of 35 Hz. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of bandwidth outputs of the PENG. This results in a positive charge flowing from the bottom metal electrode to the top metal electrode, which is manifested as a current in the external circuit. This new generator can be used to collect various forms of mechanical energy, such as human motion, mechanical vibration, rotation, wind, acoustic energy, raindrop and wave energy, etc., converting various mechanical motions into electrical signals [74]. Since its invention, the area power density of the TENG has increased from the initial 3.67 mW/m\({}^{2}\) to 313 W/m\({}^{2}\), and the volume power density has reached 490 mW/m\({}^{3}\), with a conversion efficiency of 60%. From the perspective of energy matching, the output power of TENGs can theoretically meet the energy required for metal CP [36]. [PERSON] et al. [75] designed a PVC-GO film/Al single-electrode type TENG.