A hybrid triboelectric nanogenerator for enhancing corrosion prevention of metal in marine environment

The study addresses corrosion of metals in marine environments, a major economic and safety issue for maritime infrastructure. Conventional cathodic protection approaches—impressed current cathodic protection (ICCP) and sacrificial anode cathodic protection (SACP)—suffer from drawbacks including continuous external power needs, cost, environmental impacts, sacrificial metal consumption, and limited protected areas. Ocean waves provide abundant low-frequency energy; however, traditional electromagnetic generators are inefficient at such low frequencies. Triboelectric nanogenerators (TENGs) offer high conversion efficiency at low frequencies, light weight, and simple fabrication, and have been explored as power sources for ICCP. Prior work largely used single-mode TENGs (solid-solid or solid-liquid), limiting application scenarios. This work proposes a hybrid spherical TENG (S-TENG) combining solid-solid and solid-liquid modes to harvest wave energy more effectively for self-powered cathodic protection of 304 stainless steel (304SS) and Q235 carbon steel (Q235CS) in simulated marine conditions.

The authors review cathodic protection methods (ICCP and SACP) and describe their limitations in marine environments. They highlight TENGs as promising for low-frequency, irregular mechanical energy harvesting, with advantages over electromagnetic generators. TENGs have been applied to self-powered anticorrosion and ICCP, typically in either solid-solid or solid-liquid modes. Prior studies reported arrays of liquid-solid TENGs for wave energy and cathodic protection, paper-based TENGs, underwater wave energy harvesters, and hybrid systems. The authors note that single-mode TENG designs restrict application scope and that combining modes could improve energy utilization by leveraging both internal and external surfaces of a spherical device.

Device design and fabrication: The hybrid spherical TENG (S-TENG) comprises an inner solid-solid TENG and an outer solid-liquid TENG integrated into a hollow plastic sphere (diameter ~16 cm; device volume ~2144.66 cm³). Inner solid-solid component: multiple sponge balls (diameter 40 mm) with copper foil tape electrodes covered by PTFE films contact and separate from aluminum (Al) foil electrodes affixed to the inner surface of the sphere. A central wooden pendulum induces periodic motion of sponge balls with wave-induced oscillations, enhancing contact-separation events. Outer solid-liquid component: the sphere's outer surface is patterned with Cu electrodes covered by PTFE films operating in single-electrode mode, generating charge via contact-separation with water during wave motion. The joint between hemispheres is sealed with PTFE sealing tape and transparent adhesive to prevent water ingress. Surface treatment: PTFE films (thickness 100 µm) were polished with emery papers to increase roughness and hydrophobicity. SEM characterized surface morphology; contact angle measurements assessed hydrophobicity. Electrical characterization: A wave tank simulated ocean motion. Output short-circuit current (Isc) and open-circuit voltage (Voc) were measured (Keithley 6514, LabVIEW data acquisition). A rectifier circuit converted AC outputs for storage; charging tests used capacitors from 1 µF to 47 µF. Cathodic protection evaluation: In 3.5 wt% NaCl solution, open-circuit potential (OCP) changes of 304SS and Q235CS (with waterborne acrylic coatings of varied thickness for Q235CS) were measured using a three-electrode setup (working: metal sample; counter: Pt; reference: saturated calomel electrode). Stability was evaluated over extended operation (up to 2 h continuous protection; device stability over 400,000 cycles). Area-dependence of protection was tested by varying exposed metal area. Tafel polarization (sweep 0.167 mV s⁻1, ±120 mV vs OCP) and EIS (10 mV amplitude; 10⁻2–10⁵ Hz; ZView 3.1 fitting) characterized corrosion kinetics and equivalent circuits. Materials: PTFE films, conductive Cu foil tape (65 µm), sponge balls, bamboo pendulum, hollow plastic shells, Al foil (15 µm), NaCl (GR ≥ 99.5%), waterborne acrylic coatings. Sample preparation included mechanical polishing (emery grades 400–2000) and Al2O3 slurries (1.0 µm, 0.3 µm), cleaning, and coating application with 24 h cure.

• The hybrid S-TENG effectively harvests wave energy via combined solid-solid and solid-liquid triboelectrification, utilizing both inner and outer spherical surfaces.
• Electrical output: Inner solid-solid TENG Isc increased from 95.8 µA (4 sponge balls) to 174 µA (6 balls); Voc from 9.6 V to 14.6 V. Outer solid-liquid TENG produced ~150 µA Isc and ~60 V Voc. Combined S-TENG yielded Isc from 237 µA (4 balls) to 399 µA (6 balls) and Voc from 73.6 V to 88.9 V. Rectified current was ~220 µA. A short circuit current density of 186 mA m⁻³ and open-circuit voltage of 88.9 V were achieved.
• Surface treatment: Emery-polished PTFE increased contact angle from 108° to 132°, improving hydrophobicity and output (higher Isc and Voc vs untreated PTFE), consistent with Wenzel's model.
• Stability and storage: The S-TENG maintained stable output over 400,000 cycles, with enhanced output due to charge accumulation on PTFE. Capacitors (1–47 µF) were charged to 15 V, with charging time increasing from ~2 s (1 µF) to ~7 min (47 µF).
• Cathodic protection performance (304SS): OCP in 3.5 wt% NaCl was ~-0.21 V (vs SCE) without TENG. Coupling to solid-solid or solid-liquid TENG shifted potentials to ~-0.47 V and ~-0.52 V, respectively; coupling to S-TENG shifted to ~-0.62 V, providing more effective protection. The negative shift increased with immersion time from 0.34 V initially to ~0.49 V, stabilizing after ~7 days. Over 2 h continuous operation, potential remained at ~-0.62 V. Tafel analysis showed corrosion potential shifting from ~-0.23 V to ~-0.63 to -0.64 V (vs SCE) with S-TENG, and corrosion current density increased (text indicates acceleration of electrochemical reactions via electron injection). EIS showed reduced low-frequency impedance and changes in Nyquist/Bode plots consistent with facilitated charge transfer under S-TENG.
• Area scaling (304SS): Protection potential shift decreased linearly with exposed area: y = 0.41 − 0.057x (V), where x is area (cm²). For effective protection (≥260 mV shift), the present device (volume ~2144.66 cm³) effectively protects ~1.77 cm² of 304SS; larger areas require proportionally larger S-TENGs.
• Cathodic protection performance (Q235CS with coating): Effective protection range (-0.77 to -1.1 V vs SCE) was met for waterborne acrylic coating thickness r = 25 µm when coupled with S-TENG. The S-TENG effectively protected 7.07 cm² of coated Q235CS. The shift in protection potential also exhibited a linear relationship with area: y = 0.97 − 0.057x (V).
• Visual immersion tests (Q235CS): In 3.5 wt% NaCl, samples without S-TENG developed extensive rust over 6 h, whereas S-TENG-coupled samples showed markedly fewer corrosion spots, indicating reduced corrosion rate.

Combining solid-solid and solid-liquid TENG modes in a spherical architecture increases energy harvesting from low-frequency wave motion by exploiting both internal and external contact interfaces. The enhanced electrical output translates directly to improved cathodic polarization of metals, evidenced by larger negative potential shifts for both 304SS and coated Q235CS, stability over extended operation, and visible mitigation of rust formation. Surface roughening of PTFE improves hydrophobicity and contact efficacy, boosting output and enabling more reliable operation in aqueous environments. The linear dependence of protection potential shift on exposed area for both alloys provides a practical scaling law for designing S-TENG size relative to the protected surface area. Electrochemical analyses (Tafel, EIS) indicate that electron injection from the S-TENG modifies corrosion kinetics, shifting corrosion potentials negatively and reducing impedance at low frequencies consistent with enhanced charge transfer. Overall, the S-TENG offers a low-cost, environmentally friendly, self-powered alternative to conventional ICCP for marine corrosion protection, particularly suited to low-frequency, irregular wave energy where electromagnetic generators are inefficient.

The study presents a hybrid spherical TENG that integrates solid-solid and solid-liquid contact modes to efficiently harvest wave energy for self-powered cathodic protection in marine environments. The device achieves up to ~399 µA Isc and ~88.9 V Voc, stable operation over 400,000 cycles, and effective energy storage. It significantly improves corrosion protection for 304SS and coated Q235CS, with predictable scaling of protection efficacy with metal surface area. This demonstrates a viable, green alternative to traditional power sources for ICCP. Future work could focus on scaling up device arrays to protect larger surface areas, optimizing mechanical design and materials for harsh marine conditions, integrating power management for continuous protection under varying sea states, and performing long-term field trials in real seawater environments to validate durability and performance.

Protection area is limited by device scale: with a device volume of ~2144.66 cm³, effective protection of 304SS is ~1.77 cm² (≥260 mV shift), necessitating larger S-TENGs for larger structures. The optimal number of internal sponge balls is constrained by the sphere’s internal volume. Cathodic protection performance for Q235CS depends on coating thickness (e.g., effective at r = 25 µm). Tests were conducted in simulated marine conditions (3.5 wt% NaCl solution and wave tank), and field performance in actual ocean environments was not assessed.