Marine corrosion causes significant economic losses and hinders marine progress. While anti-corrosion coatings and electrochemical cathodic protection (ECP) are common methods, traditional ECP methods like impressed current cathodic protection (ICCP) and sacrificial anode cathodic protection (SACP) have limitations. ICCP requires a continuous external power supply, increasing costs and causing pollution, while SACP involves the loss of more active metals and limits the protected area. The challenge lies in developing a green, low-cost, and efficient energy system to harvest wave energy for effective metal cathodic protection. Ocean waves are an abundant and continuous energy source, but their irregular low-frequency nature makes energy harvesting difficult for traditional electromagnetic generators (EMGs). Triboelectric nanogenerators (TENGs), based on triboelectrification and electrostatic induction, offer a potential solution. TENGs have demonstrated higher energy conversion efficiency in low-frequency irregular vibrations and possess advantages in lightweight, simple preparation, material selection, and reliability, making them suitable for wave energy harvesting and ocean applications. Previous TENG designs mainly focused on single working modes (solid-solid or solid-liquid), limiting their application scenarios. This research introduces a hybrid spherical TENG (S-TENG) with both solid-solid and solid-liquid contact modes to enhance wave energy harvesting and improve the efficiency of cathodic protection.

The literature review extensively covers existing methods for marine corrosion prevention, highlighting the limitations of traditional electrochemical cathodic protection techniques (ICCP and SACP). It then explores the advantages of triboelectric nanogenerators (TENGs) over conventional electromagnetic generators (EMGs) for harvesting low-frequency wave energy. Existing research on TENGs for self-powered cathodic protection is reviewed, categorizing them into solid-solid and solid-liquid contact mode TENGs. The authors point out the limitations of single-mode TENGs and emphasize the novelty of their hybrid approach.

The researchers designed and fabricated a hybrid spherical triboelectric nanogenerator (S-TENG) consisting of an inner solid-solid TENG and an outer solid-liquid TENG. The solid-solid TENG utilizes sponge balls inside a hollow plastic sphere, which sway periodically with wave movement, creating contact-separation between Al foils and PTFE-Cu films on the sponge balls. The solid-liquid TENG employs a single-electrode mode using the contact-separation of water and PTFE. The PTFE film's surface was treated with emery papers to enhance hydrophobicity and increase the output performance, as confirmed by contact angle measurements and output characteristic testing. The S-TENG's output performance was characterized in a wave tank, varying the number of sponge balls to optimize energy generation. A rectifier circuit was used to convert the AC output to DC for cathodic protection. Long-term stability tests were also performed. The cathodic protection performance was evaluated using a three-electrode system, measuring potential shifts of stainless steel (304SS) and organically coated carbon steel (Q235CS) with and without the S-TENG. Tafel plots and electrochemical impedance spectroscopy (EIS) were used to analyze the corrosion behavior. Immersion tests were conducted to assess the long-term corrosion protection provided by the S-TENG in a simulated marine environment.

The hybrid S-TENG design significantly improved energy harvesting efficiency compared to single-mode TENGs. The optimized S-TENG achieved a short-circuit current density of 186 mA m⁻³ and an open-circuit voltage of 88.9 V. Surface treatment of the PTFE film using emery paper increased the contact angle from 108° to 132°, enhancing hydrophobicity and improving the output performance. Increasing the number of sponge balls in the S-TENG increased both short-circuit current and open-circuit voltage, with six sponge balls providing optimal performance. The rectified output current of the S-TENG reached approximately 220 µA. Stability tests demonstrated consistent performance over 400,000 cycles. The S-TENG effectively provided cathodic protection for 304SS and Q235CS, with significant negative potential shifts observed. For 304SS, the potential shift was approximately 410 mV, while for Q235CS with a 25 µm waterborne acrylic coating, the shift was approximately 930 mV. Tafel polarization curves confirmed the enhanced corrosion resistance with a significant decrease in corrosion current density. EIS analysis showed a much smaller arc in the Nyquist plot and a lower charge transfer resistance (Rct) for the protected metals, indicating faster electron transfer. Immersion tests showed significantly reduced corrosion on Q235CS with S-TENG compared to unprotected samples.

The results demonstrate the effectiveness of the hybrid S-TENG as a self-powered cathodic protection system for metals in marine environments. The enhanced energy harvesting capability of the S-TENG, due to its dual working modes and optimized design, provides sufficient power to drive the cathodic protection process. The surface treatment of the PTFE film further contributes to the improved performance by enhancing hydrophobicity and reducing water adsorption. The significant potential shifts observed for both 304SS and Q235CS confirm the effective cathodic protection provided by the system. The Tafel and EIS analysis corroborate these findings, highlighting the accelerated electron transfer and enhanced corrosion resistance. The study successfully addresses the limitations of traditional ECP methods by providing a sustainable, low-cost, and environmentally friendly alternative. The linear relationship between potential shift and the area of protected metal allows for scaling the S-TENG to protect larger surfaces.

This study successfully demonstrated a novel hybrid spherical triboelectric nanogenerator (S-TENG) for self-powered cathodic protection of metals in marine environments. The dual working mode design and optimized surface treatment significantly improved energy harvesting efficiency and cathodic protection performance. This eco-friendly approach offers a promising alternative to traditional methods, addressing their limitations in cost, sustainability, and scalability. Future research could focus on further optimizing the S-TENG design, exploring different materials and configurations, and investigating its long-term performance in real-world marine conditions.

The study primarily focused on laboratory-scale experiments using simulated seawater. The long-term durability and reliability of the S-TENG in harsh marine environments need further investigation. The scaling-up of the S-TENG for large-scale applications also requires further research and development. The study primarily focused on specific types of steel; further investigation on other metals and coatings is warranted.