## 4 Applied Current CP
### Power Supply Technology of Piezoelectric Generators  
The piezoelectric effect reflects the coupling between the elastic and dielectric properties of a crystal and is divided into two types: positive piezoelectric effect and inverse piezoelectric effect. A piezoelectric nanogenerator (PENG) is based on the positive piezoelectric effect, using it to effectively collect the mechanical energy in the environment to convert it into electrical energy [56]. Wave energy is a widely distributed and high-quality energy source in the marine environment. [PERSON] et al. [57] designed and produced a floating wave-driven PENG using a single set of double crystal P5-1 piezoelectric chips to generate electricity, and the measured output current in the marine environment was eventually stabilized at 0.170 mA when the output voltage was set to 5.0 V (after rectification). The experiments showed that a PENG can use wave energy to achieve a self-sufficient power supply for low-power monitoring equipment at sea, and the device can also be used for array power generation. [PERSON] et al. [58] tested a 3 cm long and 2.5 cm wide, 110 \(\upmu\)m thick, polarized PVDF film with a vibration frequency of 3 Hz and measured an output voltage/current of 2.3 V/0.03 \(\upmu\)A. The simulated accelerated corrosion experiment was carried out in an electrolyte containing NaHSO\({}_{3}\) and NaNO\({}_{3}\), and as the corrosion proceeded, the EIS analysis found that the carbon steel connected to the PVDF film CP system had a lower polarization potential. Macroscopically, only a small number of gray spots appeared on the surface of the carbon steel, with no obvious corrosion, while the control group showed serious corrosion, as seen in Figure 2. It is clear that the PVDF piezoelectric device-driven CP system can indeed protect the metal from corrosion. A PENG has many advantages. Firstly, it can directly generate a suitable voltage, and secondly, unlike electrostatic induction, it does not require an initial voltage. In addition, there are no structural design limitations; therefore, theoretically, its mechanical damping coefficient can be designed to be smaller. PENGs also have the advantages of no electromagnetic interference and long-term piezoelectric oscillator generation [59]. One of the most important challenges in PENG design is the bandwidth of its sensed disturbance frequency. The narrow bandwidth makes it difficult to achieve an effective collection of broadband mechanical energy. The resonant frequency of the piezoelectric member can differ from the design value by 1-5% due to manufacturing and processing differences; therefore, the bandwidth of the piezoelectric member should cover a deviation range of at least 5% of its resonant frequency [60]. PENGs also have other challenges, such as the disturbance of output voltage by external frequencies, the size of the piezoelectric components, and the need for the output current to be connected to the rectifier circuit in order for it to be used as an alternating current. [PERSON] [59] found that the PZT piezoelectric  
Figure 2: Micrographs of the surfaces of carbon steel electrodes after simulated accelerated corrosion: (**a**) with and (**b**) without CP powered by the hybrid NG under the different durations (5, 10, and 15 h) [56]. oscillator open-circuit voltage increased with the oscillator length and excitation frequency when the external disturbance frequency was close to the piezoelectric oscillator resonant frequency, a state which can be obtained under the maximum output voltage and actual sea conditions. If the impact force is too large, it could damage the piezoelectric oscillator, and if the force is too small, it will rebound; therefore, the piezoelectric oscillator should be reasonably designed to obtain the best output voltage and arranged to reduce the energy loss caused by friction [61].