The global energy crisis and environmental concerns stemming from fossil fuel combustion necessitate the development of clean energy technologies. Ocean wave energy presents a substantial, clean, and renewable energy source with the potential to meet a significant portion of global energy demand. Triboelectric nanogenerators (TENGs) offer advantages for harnessing wave energy, but their limitations—small, pulsed signals and low power output—require the development of hybrid systems to boost sustainable power generation. The ocean's vast surface area receives substantial solar radiation, highlighting the potential for integrating solar and wave energy harvesting. Existing hybrid systems primarily rely on transparent objects (rain, wind) for triboelectric effects or position photovoltaic cells in shadow-free areas. However, shadows from moving objects significantly reduce photovoltaic cell output, hindering the development of robust triboelectric-photovoltaic hybrid technologies. This research addresses this challenge by introducing a novel shadow-tribo-effect nanogenerator that combines triboelectric and shadow effects to improve energy harvesting from oceans.

Previous research has explored various approaches to harness wave energy using TENGs. Zhong Lin Wang and colleagues demonstrated wave energy harvesting through triboelectric effects at the solid-liquid interface, achieving continuous DC output. Further improvements in current production were achieved by integrating nanoparticle surface modifications with TENGs, resulting in improved energy conversion efficiency. The integration of supercapacitors with TENGs has also been investigated to create self-powered systems for simultaneous energy conversion, storage, and sensing. Supercapacitors, due to their long lifespan and ruggedness, are advantageous for power management in such systems. Molybdenum disulfide (MoS2), with its high specific surface area and favorable ion intercalation properties, has emerged as a preferred electrode material for supercapacitors. However, existing approaches often lack a comprehensive solution to address the detrimental effects of shadows on the overall system efficiency.

This study presents a self-charging power system that integrates a shadow-tribo-effect nanogenerator (S-TENG) and fiber-supercapacitors (F-SCs) with few-layered MoS2 as the active material. The S-TENG is a hybrid energy harvester that simultaneously captures solar and mechanical energy. The system's practical application is demonstrated through an 'energy ball' design. The energy ball consists of an S-TENG-based self-charging power system, a moving aluminum ball (opaque object), and a transparent PET shell. The S-TENG uses an Au/n-Si system (with a 1 cm gap in the middle of the Au film) to collect light energy through the shadow effect and a PDMS film as a friction layer. Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) characterized the Au/n-Si surface properties and work function shift under dark and illuminated conditions. UV-vis spectroscopy studied the transmittance of the PDMS film. The influence of Au film thickness and shadow area ratio on the Au/n-Si system's performance was investigated. The working mechanism of the S-TENG, combining triboelectric and shadow effects, was analyzed through a step-by-step illustration and equivalent circuit model. The output performance of the S-TENG was characterized under various conditions (dark/illumination, different frequencies, light intensities). The F-SCs, with few-layered MoS2, were characterized using X-ray diffractometry (XRD), Raman spectroscopy, scanning electron microscopy (SEM), cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD) analysis to assess their electrochemical performance. Finally, the energy ball's performance was evaluated under simulated ocean conditions (wave machine and lamp), and its application in seawater electrolysis for hydrogen production was demonstrated.

The shadow-tribo-effect nanogenerator (S-TENG) successfully harvested both solar and wave energy. KPFM confirmed a work function shift in the Au/n-Si system under illumination. The presence of a gap in the Au film enhanced the shadow effect, significantly increasing the short-circuit current density (Jsc). The S-TENG generated a DC signal under illumination with a peak Jsc significantly higher than under dark conditions. The output of the S-TENG increased with increasing frequency under dark conditions but remained relatively constant under illumination. Higher light intensity led to increased Jsc. The fiber-supercapacitors (F-SCs) with few-layered MoS2 demonstrated high specific capacitance and excellent cyclic stability. The energy ball efficiently harvested energy from both waves and light, with the shadow effect shortening the charging time of the F-SCs. The system was successfully applied to seawater electrolysis for hydrogen production. A peak power density of 718 μW cm⁻² was achieved by the S-TENG in the energy ball under wave stimulation and light illumination, significantly higher than the 0.28 μW cm⁻² obtained using only the triboelectric effect. The charging time of the F-SCs was reduced from 409.4 s (wave only) to 156.1 s (wave and shadow), highlighting the effectiveness of the shadow-enhanced energy harvesting.

This research successfully demonstrated a novel shadow-tribo-effect nanogenerator for harvesting both wave and solar energy from oceans. The integration of the shadow effect significantly enhanced the energy harvesting efficiency compared to using only the triboelectric effect. The self-charging power system, incorporating high-performance F-SCs, reliably stored the harvested energy. The energy ball design provides a practical and scalable solution for ocean energy harvesting. The application of the system in seawater electrolysis highlights its potential for sustainable hydrogen production. This work offers valuable insights into designing high-performance self-charging power systems for blue energy harvesting and inspires future research on advanced hybrid energy systems.

This study introduced a novel shadow-tribo-effect nanogenerator integrated with fiber-supercapacitors to create a self-charging power system for efficient wave and solar energy harvesting from the ocean. The shadow effect significantly enhanced energy conversion, reducing charging times and increasing power density. The system's successful application in seawater electrolysis demonstrates its potential for clean energy generation. Future research should focus on optimizing the energy ball design for even greater efficiency and exploring its large-scale deployment for practical blue energy harvesting.

The energy efficiency of the energy ball is currently 0.7% under shadow-enhanced conditions. Further optimization of the system components and design is needed to improve energy conversion efficiency. The current study used a simulated ocean environment; future work should focus on real-world testing to evaluate the system's performance under various environmental conditions. The long-term stability of the system in harsh marine conditions remains to be fully assessed. Finally, the scalability and cost-effectiveness of the system for large-scale deployment need further investigation.