Triboelectric Nanogenerators for Marine Applications: Recent Advances in Energy Harvesting, Monitoring, and Self‐Powered Equipment

Progress in advanced electronics has initiated the investigation of new ways to develop and apply self‐powered smart devices. The concern for meteoric exhausting non‐renewable energy sources has spurred such endeavors. Even so, using external power sources like batteries is problematic due to limited capacity, maintenance inconvenience, replacement, and environmental hazards. Triboelectric nanogenerators (TENGs) capable of converting various forms of mechanical energies into electrical output are gaining popularity. The marine and coastal areas are abundant sources of salvable mechanical energy. TENGs can convert lower‐frequency, ununiform, multidirectional energies into usable electricity. This can solve the device‐powering problem and can generate diverse signals to act as monitoring or sensing platforms themselves. In this review, three main TENG‐based/TENG‐driven application themes are addressed, i.e., energy harvesting, marine environment monitoring, and self‐powered equipment for marine‐related activities. It attempts to emphasize that various design features of TENGs can influence output performance; TENGs can power devices and monitor ocean parameters; TENGs‐integrated modern IoT networking systems can transmit real‐time data. Overall, this review encompasses the fundamental working mechanisms, structure designs, and practical implementation scenarios of recently developed devices in diverse marine applications. Finally, the existing challenges and potential future directions for TENG‐based marine self‐powered electronic systems are discussed.


Introduction
Over the past few decades, real concern has developed regarding the exhaustion of finite energy resources. [1]ver increasing standard of human lives and technology-dependency demand a more stable energy supply in the modern era.Unfortunately, the polluting aftereffects of commonly used fossil fuelbased resources have been devastating and have also been addressed in recent years. [2]Hence, scientists have been looking into various alternatives, preferably renewable energy sources, to restore a satisfactory supply level and provide a clean energy source.Renewable energy can be referred to energy directly produced by the sun (e.g., photo-chemical, photo-electric, thermal, etc.), indirectly driven by the sun (e.g., stored energy in biomass produced by photosynthesis), and natural environmental movement (e.g., geothermal, marine energy, etc.). [3]Lately, marine energy has been gaining popularity as a promising renewable energy source.71% of the earth's surface is covered with ocean, accounting for 97% of the total earth's water. [4,5]Thus, it stands as a viable and enduring option among the various sources of energy.Furthermore, using marine energy promises to promote the "blue economy" concept with the potential to generate an enormous 32 PWh/y. [6]arine environment comprises various elements and given proper advancement, it can be converted into a massive renewable energy source.Despite the challenges posed by its intermittent, irregular, and weaker nature, which require large-scale conversion and storage, the potential of this energy source was recognized long ago.9][10] Now, this energy is being harnessed to contribute an important portion of the global energy demand.Currently, global marine energy capacity stands at 532 MW, and Europe is leading the way with 241 MW. [11] Various monitoring, forecasting, and communicative devices around the ocean provide continuous information important for weather forecasting, transportation, route selection, etc.These equipment are often set up in remote locations where it might be quite challenging to supply power. [12]The rise in marine pollution has made it necessary to have sensors that can monitor and control the environment in real-time.The devices encompass small-scale wireless sensors to large-scale complex observation networks. [13]or example, satellite sensors like, moderate resolution imaging spectroradiometer (MODIS), the geostationary ocean color imager (GOCI), the visible infrared imager radiometer suite (VI-IRS), the ocean and land color imager (OLCI), the Landsat operational land imager (OLI), and the Sentinel-2 multispectral instrument (MSI) are already being employed for ocean monitoring via color data. [14]Remote communication of such devices is cru-cial as a large area needs to come under coverage-these demand consumption of a considerable amount of electrical inputs.Thus, advancement in marine energy harvesting can usher in a way of making the marine environment an autonomous system capable of producing its own energy to drive these devices.In the modern Internet of Things (IoT) era, the idea of a smart ocean has become a reality.The main aspects of marine applications, namely, energy harvesting for rendering devices self-powered, various phenomena monitoring, and developing smart equipment have been depicted in Figure 1.
Typically, wave energies, tidal current, ocean current, offshore wind, ocean thermal energy, mechanical vibration in marine element bodies, salinity, etc., can carry different forms of energy (Figure 2). [3]However, to be practical, the natural forms of marine energies need to be converted into usable electricity.The concept of "Renewable Energy Technologies" primarily deals with this scientific advancement. [3]Triboelectrification-based nanogenerators have been widely acknowledged for harvesting lower amplitude, lower frequency, and randomly directed sources.This type of energy is also named "high entropy energy" (HEE) and ocean waves fall into this category. [15]arious energy harvesting devices have been set up in or around oceans to harness energy that can power electronic devices.Different types of energy harvesters, including electromagnetic (EMG), dielectric, ionic composite, piezoelectric, triboelectric, and hybrid harvesters, have been studied for this purpose. [16]lthough the EMG is the current technique used for energy harvesting from the waves, inefficient performance is noticed in irregular and lower frequency (<5 Hz) waves. [5,15][19] Triboelectricity occurs due to physical contact and subsequent electrostatic charge generation, particularly due to the electronegativity of the participating materials.Contact, separation, or sliding imposed on tribo-dielectric layers by any mechanical phenomena causes a potential difference that results in electron transfer across constituent electrodes via an external circuit.Principally TENGs function based on four major working modes, namely, i) contact separation (CS), ii) lateral sliding (LS), iii) single electrode (SE), and iv) freestanding (FS) modes. [17]The CS mode, fundamentally based on Maxwell's displacement current, can generate electricity due to the electrical pulse triggered by relative movement between electrodes and the dielectric layer. [4]Contact between tribo-positive and tribonegative dielectric layers allows the equal and opposite magnitude of charge density to be generated due to contact electrification induced by the difference in electronegativity between the participants.During vertical separation, a potential difference is thus created, causing the flow of electrons, i.e., electricity, in an alternative manner. [20]Another technique, LS, involves changing contact surface area due to sliding movement and thus disrupting the electrostatic balance.This further introduces a flow of electrons through an external circuit, generating electricity.While these two mechanisms include pairs of displaceable electrodes, SE mode incorporates only one electrode linked with the ground.This allows charge transfer between electrode and ground when the triboelectric materials approach or leave the fixed electrode.FS mode involves keeping the electrode pairs stationary while the triboelectric material moves between them.Induction causes an unbalanced distribution of charge as a result of such movement, and thus, charge transfer occurs between the electrodes. [21]These working modes (Figure 3) based on the TENG principle have been incorporated in various forms for the salvation of wave energy.Despite other ocean energies like vibration, thermal, wind, etc. exist, research on wave energy is much more dominant.The higher energy density of waves in comparison to other forms may be a decisive factor in this regard. [22]t is necessary to integrate TENGs with any marine device since they cannot operate as individual or separate units.Therefore, the structural design must be well-compatible with the host equipment and surroundings. [23]The performance of TENGs is also greatly influenced by their physical shape and structural construction.The design is thus a critical factor in maximizing their performance potential in different environments.For example, the theoretical design of TENGs comprises only two parallel plates (dielectric-to-dielectric or dielectric-to-conductor) configurations.This may limit their optimum performance due to planer configuration and bounded charge density while agitated or stimulated by different marine elements. [20,24]Recent studies have explored rolling structures, liquid-solid concepts, disk rotation, and stacking techniques to maximize performance.Nevertheless, most of the inspiration for making various structural designs came from natural objects.For example, Wang et al. [25] proposed a seaweed-like TENG structure inspired by actual sea plants capable of efficiently converting wave energy into mechanical vibration.Another recent study by Wen et al. [19] exhibited a TENG inspired by flower core and petals.The natural flowering and folding mechanisms were imitated to accommodate the triboelectric mechanism triggered by water wave motion.In most cases, it was found that energy harvesting performance varied based on structural features and designs.Many different structures have been studied for their potential use in marine applications, including rolling sphere TENG (RS-TENG), hierarchically structured TENG (HS-TENG), pendulum structured TENG (P-TENG), cylindrical structured TENG (C-TENG), and springbased TENG (S-TENG).
In recent times, the use of TENG technology in marine applications has become increasingly popular.Triboelectric nanotechnology itself is relatively new and integration with the Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/. [21]Copyright 2021, The Authors, published by Elsevier.
marine environment is even more recent.Wang's group first proposed TENG in 2012 which used displacement current to convert mechanical energy to electrical output. [26]Research on the applicability of TENG in various aspects of the marine environment has been steadily increasing since then.In recent years, a huge number of comprehensive research and review studies encompassing TENGs have been reported for a plethora of applications, like energy harvesting, marine equipment monitoring, and sensing.According to Figure 4, there is a significant increase in studies about strictly integrating TENG technology in marine environments, indicating high interest among scientists.The research trend accounts for a much more rapid increase in recent times, specially after 2019.Figure 4 is based on two of the most prominent databases for scientific research articles, i.e., Web of Science and Scopus.Hence, TENG integration in marine applications is an impactful and extensively researched area.
This review focuses on the incorporation of TENG-based devices and equipment in various marine activities.Exploration is underway to establish the energy available in waves, tides, currents, and wind in coastal areas and oceans as a major renewable energy source.This power can be used to support monitoring and sensing of various ocean elements or phenomena such as waves, underwater, fish behavior, surface fluctuation, vibration, positioning, oil spillage, and ship intensity.The structural and architectural features play an important role in designing sophisticated TENG-integrated equipment for creating a selfsustained smart networking system for marine applications.This review comprehensively explores various ocean energy harvesting mechanisms where TENGs were utilized as primary harvesters.It further attempts to realize TENGs contribution to the marine equipment for monitoring various factors.Here, the authors explicitly concentrated on accumulating the recent research regarding TENG-driven equipment for harvesting energies ma-rine environment can offer, the mechanism by which TENG operates to monitor various factors, and how they are developed as an integral part of such equipment.This article further offers a thorough critical analysis of areas that need more attention to guide future research direction.

TENGs for Marine Energy Harvesting
The unpredictable, complicated, and harsh nature of the ocean has made powering marine devices like buoys, underwater vehicles, sensors, etc., quite challenging. [15]There are various renewable energy sources present in the marine environment that can be utilized to power different equipment.The energy present in ocean water, also known as blue energy, is a significant source.Tidal energy, wave energy, ocean currents, and offshore wind can be efficiently converted into stable electricity output with the help of TENGs.The HEE in micro and nanoscale from ocean waves is considered a futuristic sustainable source of energy. [27,28]esearchers have delved into countless design features in their quest to uncover the ultimate harvesting performance. [16]This section outlines the major architectures explored.

Wave and Ocean Current Energy Harvesters
Among various forms of energy available in the ocean, waves are the most common and easily obtained.It is a form of clean energy induced by the wind on the ocean surface and is considered one of the sources of high-density energy. [29]The availability and persistence make it even more desirable. [1]The blowing force of air can transform the initial small ripples into big, crushing waves given appropriate time and space. [30]The amount of energy dissipated when ocean waves break depends on their size.The wave size is mainly affected by dispersion phenomena that occur due to the combined influence of surface tension and gravity.The wave energy is primarily related to the square of wave amplitude and wave motion period. [31]They can be distinguished from wind energy due to superior continuity and spatial concentration. [32]dditionally, wind strength, wave height, wavelength, and propagation direction are important factors to consider. [33,34]For instance, a 4 feet high 10 s ocean wave can register more than 35 000 horsepower energy per coast mile. [15]Gobato et al. further mentioned a 1-meter-wide ocean wave of 2-meter amplitude, acting for 7-10 s can carry 50-60 KW energy. [35]The key difference between wave and tidal energy is the induction of gravitational force.While ocean waves are dependent on the wind in the area, tidal energy is transferred to the ocean from the sun and moon due to the gravitational rotation of the earth. [36]The gravitational pull induced by the sun (32%) and moon (68%) can amplify the tidal wave amplitude up to 7 m. [31]The ocean current on the other hand, indicates the continuous movement of seawater, caused by various factors like waves, wind, temperature, salinity, events like storms, earthquakes, etc. [37] The kinetic energy carried by ocean waves, current and tidal waves can be harvested as a green source of energy.However, harvesting this energy for an efficient, consistent, and ceaseless power supply is often limited by the ultralow, irregular, and random frequency (typically 0.17-1.25 Hz). [38]pecial design features of the harvesters can provide optimum performance despite these challenges.This is because optimizing design features for specific environments can have a significant impact on the energy harvesting ability. [26]4] The rolling structure involves a TENG platform and rolling balls to achieve contact electrification.Utilizing polymers to develop these devices allows for greater electrical charge movement, while also being more cost-effective and lightweight.For example, an RS-TENG device was developed by Wang et al. [55] to harvest wave energy.Nylon and polyethylene-naphtholate were applied as the rolling ball and fully enclosed spherical TENG inside coating, respectively.The contact electrification mechanism upon rocking has been depicted in Figure 5a.A 6 cm diameter sphere yielded maximum output power and current of 10 mW and 1 μA, respectively.The experiment also revealed at 1.43 Hz, wave energy-driven electricity could power tens of LED lights, and the stored energy from this system could run an electric thermometer.Another recent study by Wang et al. [56] addressed an RS-TENG made of silicon rubber balls and an Al or Cu film-based triboelectric layer (Figure 5b).The lightweight device capable of floating on the ocean surface could harvest energy from exceptionally lower amplitude oscillation (≈1.1 Hz) in its FS mode.Factors like dielectric element size, number and oscillation intensity, etc., could positively impact the output.The maximum output voltage and power were reported as 12.75 V and 455 nW, respectively, for the Cu-based TENG.Another sea-snake-shaped RS-TENG formed of polytetrafluorethylene (PTFE) ball and nylon film-covered acrylic-Cu electrode was developed by Zhang et al.. [29] The spring-loaded flexible dimension allowed the device to bend in tandem with the wave and thus allowed the rolling balls to move across the nylon film, generating contact electrification (Figure 5c).Such a device demonstrated a maximum power generation of 4 μW.
C-TENGs have been reported to harvest low-frequency wave energy.However, it is quite challenging to generate high-intensity electricity due to the unpredictable, irregular, and random nature of the waves.To improve the performance, a frequency multiplying C-TENG was reported by Jung et al. [57] in a recent work.A weight with a specific mass was stored in a cylindrical structure using two repulsive magnets to hold it in place, thereby storing potential energy.Upon exceeding this repulsive magnetic force, the stored energy was released in the form of a highfrequency swing, resulting in kinetic energy.The triboelectrification principle was a soft contact mechanism between Al electrodes and fluorinated ethylene propylene (FEP) (Figure 6(a)).A high 6.67 W m −3 power density at a very low frequency (0.33 Hz) was generated in a 12 m long water tank.Further, TENGs can be attached to the floating bodies in the ocean to harvest wave energy and vibrations conveniently.In a recent study, Zhang et al. [58] presented a multi-tunnel structured TENG (MT-TENG).The FS contact electrification mechanism included PTFE balls and electrodes made of Cu (Figure 6d).The multi-tunneling structure facilitated increased contact area and reduced movement obstruction.With a 10-tunnel TENG system and 2 Hz frequency, the maximum short circuit current and charge transfer were reported to be 4.1 μA and 0.42 μC, respectively.
TENGs can be hybridized with other harvesters like electromagnetic generators (EMG)s or solar cells (SC)s to enhance the performance and efficacy of power generation.Such a power unit was fabricated by Shao et al. [59] that combined the mechanisms of a CS TENG, FS sliding EMG, and waterproof commercial SC.The TENG part, made of PTFE film coated with Cu film and another Al layer, acted as the primary electricity generator.The Reproduced with permission. [55]Copyright 2015, Wiley-VCH GmbH.b) Fabrication information and working mechanism of rolling sphere TENG.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/. [56]Copyright 2022, The Authors, published by MDPI.c) Working mechanism of sea-snake shaped rolling ball TENG.Reproduced with permission. [29]Copyright 2022, Elsevier.b, i-vi) Working mechanism of TENG-EMG-SC hybrid power unit.Reproduced with permission. [59]Copyright 2017, Elsevier.c) Working principle of the hybridized EMG and TENG.Reproduced with permission. [18]Copyright 2016, Wiley-VCH GmbH.d) Schematics of freestanding contact electrification of an MT-TENG to harvest marine energyReproduced under the term of CC-BY licensehttps://creativecommons.org/licenses/by-nc-nd/4.0/. [58]Copyright 2019, Elsevier.b) Schematics and working mechanism of a spring-assisted TENG device.Reproduced with permission. [61]opyright 2017, Elsevier.c) Schematics and working mechanism of a matryoshka doll inspired HS-TENG.Reproduced with permission. [62]Copyright 2019, Elsevier.coordinated movement of a magnet pair in tandem with wave kinetics drove the contact and separation (Figure 6b).Apart from working over a wide frequency range, the device could work under various weather conditions.The device was tested with a very low-frequency range (0.2-2.0 Hz) water wave; a maximum ≈142 V voltage and ≈23.3 μA short circuit current was recorded.The output was sufficient to light LEDs or charge supercapacitors.Water flow energy under the sea surface can be very different and might pose a harsher environment.A waterproof HS-TENG based on TENG and EMG was developed by Guo et al., [18] capable of harvesting energy from the underwater flow.The device composed of an acrylic layer, Cu coils, FEP layer, and magnets worked based on the same and opposite charge generation on Cu and FEP surfaces (due to electro affinity difference) during triboelectrification (Figure 6c).The experiment conducted with a rotation of 1600 rpm (rotation per minute) provided maximum outputs of 2.3 mA and 5 V of short circuit current and open circuit voltage, respectively.
Scientists are also investigating various techniques to enhance energy harvesting, particularly related to the structural designs of TENG devices.TENGs structured like a pendulum can substantially enhance energy harvesting performance by allowing large area to contact instead of point-to-point contact.A study conducted by Zhong et al. [60] reported a stacked structure of P-TENGs for energy harvesting from very low-frequency water waves (<0.5 Hz).A high output charge density, and power density of 4622 μC m −3 and 14.71 W m −3 , respectively were achieved.The TENG unit consisted of a tightly packed disk-track made of copper plate and aluminum foil, combined with an acrylic guide spacer and an electrode panel made of epoxy glass, gold, copper, and PTFE film.The inside part of the guide spacer had an inside arc track allowing the pendulum-like movement of the rolling disk.Thus, the contact friction (Figure 7a) could generate power as the wave clashed with the device.Jiang et al. [61] further reported the fabrication of a TENG assisted by a spring mechanism that enhanced the energy conversion performance by up to 150.3%.Spring can increase productivity as it can store potential energy when mechanically triggered.The device comprised two Cu-PTFE-acrylic blocks connected via a mechanically rigid spring.Contact and separation between Cu electrodes and PTFE film was the driver for generating electricity from low-frequency waves (Figure 7b).With enhanced motor acceleration (10 ms −2 ), the spring-assisted device could generate a maximum of 755.8 V voltage and 65.0 μA electricity, respectively.Another attempt to enhance efficiency at lower-frequency ocean waves involved a nest-assembled HS-TENG structure inspired by a matryoshka.Pang et al. [62] designed an HS-TENG with three acrylic spherical shells of decreasing sizes.Multiple PTFE balls were put in the space between successive shells.It enabled maximum utilization of surface area contact between balls and electrodes inside the structure at multiple levels.The working mechanism has been illustrated in Figure 7c.Maximum 544 μW power was reported from a lower frequency of 2 Hz wave, which was 6.5 times greater than a single ball TENG of a similar size.Such output could power dozens of LEDs and an electronic thermometer.The underlying logic of modifying various structures ensures that the maximum possible output can be generated with the least input.Table 1 summarizes the input and outputs of the different structural designs of TENGs.They have been categorized according to their working mechanisms.
It is apparent from Table 1 that, typically, CS-structured TENGs are used more frequently as they offer superior performance.The benefits include a larger contact area, simpler structure, and longer durability. [43,76]CS-TENGs have also been reported to have higher electrical output compared to SE-TENGs. [63]FS-TENGs  [81] Copyright 2021, The Authors, published by MDPI.b) Schematics of energy harvesting mechanism induced by both vertical and horizontal vibrations.Reproduced with permission. [88]Copyright 2018, Wiley-VCH GmbH.c) Working principle of cotton-FEP TENG for harnessing energy from wind blow and water flow.Reproduced with permission. [89]Copyright 2022, Elsevier.
are better for rolling structures because the dielectric ball-shaped element can move freely causing a maximum amount of contact with the dielectric layers. [77]As a result, the frequency of generating potential difference significantly increases to generate electricity.On the contrary, LS mode is prone to high wear and friction of the constituent triboelectric layers.However, scientists are looking into different solutions to preserve materials, like segmenting triboelectric layers, using rabbit fur in between triboelectric layers and electrodes, etc. [78,79]

Marine Vibration Energy Harvesters
Various environmental loads can cause vibration in the marine environment. [80]For instance, fluids flowing through marine pipelines can induce vibration via vortex shedding phenomena. [81]Typically, vibration is not favorable for engineering situations because it can cause elements to become loose, fractured, or damaged.Unfortunately, it's challenging to control or minimize marine vibration because of irregular hydrodynamics, deformations, and self-excited non-linear responses. [80]Varieties of structures in and around the marine environment fall victim to different intensities of vibration by the factors like current, wind, dynamic wave, ice, earthquake, etc. [82][83][84][85][86] Hence, monitoring the structural health of these marine equipment is necessary.Generally, expensive, sophisticated sensors are installed to carry out the tasks.These sensors demand an external power supply and result in considerable energy loss due to long-distance connections. [81]The generated vibration in these elements can be a good source of a self-powering system to carry out ceaseless monitoring.Typically, vibration can occur in two major ways, harmonious and non-harmonious (random).Spring and elastomer-associated TENGs have been employed to work with harmonious vibration, while point, planer, or curved surface contact-based TENGs corresponded to non-harmonious vibrations. [87]Piezoelectric nanogenerators (PENGs) and TENGs are two major techniques to scavenge mechanical vibration energy.Nevertheless, TENGs are much more suitable for handling lower frequency, nonlinear energies like the vibration of water bodies.
Li et al. [81] recently designed a waterproof TENG based on the CS principle to scavenge vibration energy that could power a structural monitoring sensor.The device contained a couple of dielectric films connected by a spring system to store energy.PTFE and nylons were used as dielectric materials, and Cu foils as conductive electrodes.Vibration-induced oscillation motion caused contact and separation in four stages, i.e., full contact, separation, maximal separation, and approach to generating alternating current (AC) (Figure 8a).With optimum installation and positioning, the maximum peak voltage and average power output were reported to be 2.1 V and 0.028 μW, respectively.Xu et al. [88] presented a spring-integrated TENG that could harvest vibration energy triggered by ocean waves.A CS mechanism could be imposed via the movement of structured helical coils of a spring in both horizontal and vertical directions (Figure 8b).Elastomeric layers formed of silicone rubber and CNTs acted as conductive electrodes (strain ≈133%).Due to triboelectrification and electrostatic induction phenomenon, up to 30 V peak voltage was generated at maximum vibration frequency.

Off-Shore Wind Energy
Off-shore wind energy refers to harnessing wind power over a body of water, typically an ocean, through the installation of wind farms.Generally, wind velocity is higher around water bodies compared to lands due to less friction caused by relatively smoother water surfaces.Thus, one of the most impactful and rapidly growing sources of marine energy is the off-shore wind. [90]The year 2021 was marked as the most fruitful year for the global offshore wind energy industry in recent times.Globally, about 21.1 GW (gigawatts) of new connections were installed in 2021, of which China was the largest contributor (80%).GWEC (Global Wind Energy Council) forecasted, by 2030, this capacity will reach about 316 GW. [91] The attempt of utilizing off-shore wind energy through TENG is becoming more and more prevalent in current times.In a recent study, a wheel-disk structured TENG was developed by Xia et al. [89] with the ability to harness energy from both water flow and wind blow.The simultaneous harvesting from both water and air made it a potent candidate to harness energy from the marine environment.Relative movement between cotton and fluorinated ethylene-propylene (FEP) films (triboelectric layer) driven by wind blow or water flow introduced coupling effect and electrostatic induction to generate electricity.Two major parts, the stator (FEP film pasted on acrylic parts) and rotator (cotton sheet adhered to polymethyl methacrylate (PMMA) disk), worked in a four-step cycle (Figure 8c).The maximum opencircuit voltage of 782 V and short-circuit current of 8.9 μA were generated from 210 rpm rotation.Another Bernoulli effect-based TENG (B-TENG) for harnessing multidirectional lower-velocity wind energy was reported by Chen et al.. [92] The vertical contactpropagation-separation operating principle was adopted via simulation of flapping film pair.Ag was incorporated as an electrode, and PVDF-FEP films as triboelectric layers.Differences in device sizes and unpredictable wind velocities simulated in dynamic interactions via stable, out-of-phase, in-phase, and chaotic flapping modes to imitate real outdoor situations.At an optimum wind speed of 8 m s −1 , the maximum voltage, current and output power were found to be 175 V, 43 μA, and 2.5 mW, respectively.Even at the lowest fluttering velocity of 1.6 m s −1 , the device demonstrated a conversion rate of 3.23%.
Despite having many potential sources of energy, marine energy is often underutilized due to technical limitations and a lack of efficient devices.For a while, Faraday's law-driven EMG technology has been utilized to harvest water energy.Nonetheless, it shows inadequate performance in collecting energy at frequencies below 2 Hz. [93]Since TENGs are capable of scavenging various forms of mechanical energies within a particularly lower frequency range, they are suitable candidates for such operations.Among various parameters, device design is the most crucial to exploit the best performance for ceaseless and efficient power output.Materials used as triboelectrification layers, electrodes, and device flexibility, and durability are other parameters that drive effective energy harnessing.

TENGs for Marine Environmental Monitoring
The ocean is a plentiful resource of energy that can be utilized to power battery-less sensors.These sensors can be employed to monitor marine ecosystems in real-time, observing ocean activity, fish behavior, pipeline leaks, wave behavior, etc.More advanced aspects like hydrologic observation, liquid composition determination and target detection in the form of an electrorecep-tor are also possible. [94]Self-powered marine devices are a growing research phenomenon to forecast weather, pollution, sea life behavior, coastal temperature, wave height, etc. [95] With this inspiration, TENG-based self-powered sensors are reaching a new height that has been depicted in the following section for different aspects of marine ambiance.

Ocean Surface Wave Monitoring
In the marine environment, ocean surface wave monitoring is crucial as it includes many important aspects, like surveillance of the aquatic environment, safe navigation, weather forecasting, contamination intensity monitoring, marine structure monitoring, etc. [96] Besides, in light of the global energy crisis and climate change, numerous countries worldwide are placing greater emphasis on utilizing sustainable, renewable ocean energy that is less harmful to the environment. [22,97]However, deep-sea ecosystems and the biodiversity of the ocean are extremely challenging for long-term monitoring with a self-powered device. [98]Continuous research on the development of self-powered marine sensing tools has revolutionized ocean monitoring since TENG-based harvesters are capable of harvesting energy and powering ocean monitoring sensors.Recently, Wang et al. [99] developed a bionic coral wave sensor (BCWS) based on TENG composed of a fixation mechanism, a buoyancy tray, a counterweight mechanism, and coral tentacles (Figure 9a).CS working mode between FEP and conductive ink electrodes generated electrical signals ranging between 0.3255 to 0.8162 V at 25 to 125 mm corresponding wave heights.The sensor's primary function was to generate data for ocean wave information.Additionally, it could be used in marine engineering construction, resource development, and disaster warning.In another study, Zhang et al. [100] introduced a novel liquid-solid TENG (LS-TENG) made of polytetrafluoroethylene (PTFE) coated steel as dielectric material, uncoated steel electrode, and ocean water as a frictional layer.The device could be used to monitor the water height.Additionally, it demonstrated corrosion resistance functionality that could be used to safeguard the steel body of the ship hull.Here the output transferred charge, short-circuit current and open-circuit voltage were reported as 0.23 μC, 40 nA, and 52 V, respectively.Further, Wang et al. [101] proposed a sandwich-structured TENG (SS-TENG) accompanied by two Al-coated acrylic plates.The buoy contained seven hexagonal SS-TENG units that were arranged in parallel formation with PTFE balls stacked inside (Figure 9b).Ocean waves made a free motion of PTFE balls between Al electrodes, generating 34.65 W m −3 power density and 20.91 μA short circuit current that could support marine navigation sensors and light up 12 W LED in lanes buoy.
Furthermore, aquatic environment contains an abundance of wave amplitudes that can be employed to power TENG sensors for wave monitoring and consequently forecast weather from remote places with the aid of IoT (Internet of Things).Zhao et al. [102] recently introduced an SS-TENG for monitoring centimeter-level wave heights and powering marine sensors.The device comprised seven SS-TENG units containing ten layers of the stackable acrylic frame, Al electrode, and PTFE balls that functioned in FS mode to deliver 61.20 mW peak power at the wave height of 6 cm (Figure 10a).For a similar Figure 9. Application scenario for ocean monitoring.a) Functional unit and application of BCWS-TENG for marine information and disaster warning.Reproduced with permission. [99]Copyright 2021, Wiley-VCH GmbH.b) TENG-based internal structure for marine navigation supporting buoy.Reproduced with permission. [101]Copyright 2021, Elsevier.
purpose, Wen and his coworkers [19] developed a flower-like TENG (FL-TENG) capable of scavenging and analyzing water wave frequency (Figure 10(b)).PTFE, PLA, Fe, and spring steel sheet fabricated FL-TENG generated energy from the wave frequency and height 1.3 Hz and 8 cm, respectively, capable of charging 220 μF capacitor to 1.3 V in 1 min.Earlier, Rodrigues et al. [12] proposed rolling spheres-based anisotropic circular TENG (AC-TENG), unidirectional flat TENG (UF-TENG), and unidirectional lateral TENG (UL-TENG) comprising polylactic acid (PLA) supported silver film coated Nylon 6,6 and PTFE triboelectric pair that worked in CS mode.Different wave amplitudes allowed the device to generate electricity ≈2.4,1.0, and 2.7 μA, respectively (Figure 10c).A floating buoy was used to monitor real sea state wave amplitudes and periods with the device.Similarly, Zhang et al. [103] developed a triboelectric oceanwave spectrum sensor (TOSS) having super sensitivity of 2530 mV mm −1 that analyzed signals to sense marine wave height, period, frequency, velocity, length, and steepness.The TOSS composed of copper and PTFE as dielectric materials, was integrated with a hollow ball buoy.It functioned in sliding mode to scavenge marine wave energy into electrical signals.
Self-powered, self-functional, and wireless ocean surface oscillation monitoring sensors can provide vital marine weather information.Normally, TENGs have been used to analyze the water amplitudes and supply energy to the monitoring sensors.Wang et al. [104] proposed a wave-driven TENG made of ethylene chlorotrifluoroethylene (ECTFE) film and ionic hydrogel elec-trodes.It worked in FS mode to produce an open circuit voltage of 332 V and a power density of 1.85 W m −2 .The device gathered significant surface oscillation information from the boundless ocean to forecast marine meteorology information such as surface water height and wave amplitudes (Figure 11a).It could also be used to measure ocean temperature and humidity.Moreover, Gao et al. [50] developed a rotating gyro-structured TENG combined with an EMG consisting of FEP taped gyro, coil, cylindrical NdFeB magnet, and two electrodes that worked in FS mode to harvest potential energy from the sea wave.The EMG harvested energy could power the global position system (GPS) module.The TENG harvested voltage amplified from 6.5 to 23.76 V with the increasing water crest from 20 to 125 mm.This fluctuation in generated voltage precisely followed the water wave fluctuation (Figure 11b).On the same note, Wang et al. [105] constructed a TENG-based magnetic flap-type difunctional sensor (MFTDS) consisting of a copper electrode, PTFE-coated copper film as a frictional layer, outer magnetic flap, an inner magnetic float, and a conical cavity.The device precisely monitored the pneumatic flow of liquid, and the output voltage sharply followed the liquid level fluctuation from 30 to 130 mm.

Underwater Monitoring
The significance of underwater monitoring in marine environment is becoming essential to analyze and monitor different  [102] Copyright 2022, Elsevier.b) Schematic design of FL-TENG and application in the marine environment.reproducedwith permission. [19]Copyright 2022, Elsevier.c) Design of unidirectional and anisotropic TENG integrated floating buoy.Reproduced with permission. [12]Copyright 2022, Elsevier.b) The output of Gyro-structured TENG-EMG in the different surface oscillation of water.Reproduced with permission. [50]Copyright 2022, American Association for the Advancement of Science.b) Conceptual design of TENG integrated surface buoy for underwater monitoring.Reproduced under Creative Common license https://creativecommons.org/licenses/by-nc-nd/4.0/. [109]Copyright 2022, MDPI.aquatic ecosystems. [106]Self-powered TENGs have been a solution for examining electrical signals and providing energy to underwater monitoring sensors. [95]In line with underwater exploration, Zhang et al. [107] designed a self-powered underwater cable structured TENG to monitor the underwater object's motion trajectory, speed, and dive depth (Figure 12a).The TENG was fabricated by inner single nylon yarn coated with polyvinylidene fluoride-trifluoro ethylene (PVDF-TrFE) and carbon nanotubes (CNT).The outer layer was composed of silicone rubber and nylon coating on the winded silver wire.Thus, the integrated cable network TENGs functioned in CS mode with the help of underwater mechanical motion that generated a power density of ≈95.5 μW m −1 for a 5 cm-long cable.In another study, Feng et al. [108] proposed fabric-based TENG (F-TENG) to sense underwater drowning, and the fabric was manufactured by silk having urethane perfluorooctyl silane (NHCOO-PFOTS).Nylon acted as triboelectric frictional layers to generate 2.08 W m −2 power density.Furthermore, Wang et al. [109] introduced a stackable TENG (S-TENG) (Figure 12b) where each layer was composed of multiple channels having PTFE balls in between Al electrodes that produced the peak power density of 49 W m −3 and lit up 350 LEDs.This study suggested that S-TENG could serve as an effective marine sensor for measuring underwater salinity, temperature, and acidity.Additionally, S-TENG could be utilized in navigation buoys.Further, a flag-shaped TENG was developed by Wang et al. [67] excited by flow-induced vibration in underwater conditions.The purpose of developing this device was to scavenge underwater flow velocity and power electrical appliances.Conductive ink-coated PET membranes and PTFE strips sealed with PTFE tape formed one functional unit.Extremely good and consistent performance under very low-velocity condition was achieved due to air gap, microstructure, and flapping action produced by the flag-like TENG.The CS principle-driven device with six units could produce a peak output 52.3 μW at an extremely low flow velocity of 0.461 m s −1 .

Leakage Monitoring
The safety of the marine environment is crucial for the aquatic ecosystem. [110]This has inspired researchers to create selfpowered sensors that can detect leaks in marine pipelines and other structures, such as offshore platforms, ships, and container bodies.These sensors aim to prevent maritime accidents and pollution.Apart from acting as a sensor itself, the selfpowered TENG-based energy harvesting devices have been used to power other monitoring sensors.Recently, Chang et al. [111] demonstrated a honeycomb structure inspired TENG for detecting ammonia leakage from marine vessels (Figure 13(a)).The device was constructed with PTFE balls containing two copper electrodes and generated a peak power density of 59.783 W m −3 to run the ammonia sensor.The sensor could detect as minimum as 0.2 ppm ammonia leakage from an ocean voyage.Moreover, Zhang et al. [112] reported a bubble motion-based triboelectric sensor (BM-TES), which could provide real-time data regarding pipeline blockage or leakage (Figure 13b).In this experiment, the sensor was composed of ring-shaped copper electrodes and PTFE as frictional layers that worked in CS mode to sense with Reproduced with permission. [111]Copyright 2022, Elsevier.b) Schematical model and voltage signal generation of BM-TES for bubble motionbased leakage and blockage detection.Reproduced with permission. [112]Copyright 2022, Springer Nature.
an accuracy of 10 cm.Similarly, Li et al. [81] also exploited a CS mode TENG to harvest energy 14.0 μW as maximum power output from the vibrating pipes for sensing the structural health of marine pipes.The instrument was prepared with PTFE and nylon as dielectric materials supported by copper foils and acrylic plates.The maximum output power density was reported as 5.56 mW m −2, capable of powering the marine pipeline monitoring sensor.Zhao et al. [113] recently proposed a rope like TENG (R-TENG) capable of monitoring structural failure induced by mechanical loading of any marine body.With increasing strain, the output voltage of TENG also increased linearly.Further, other structural stimuli like bending and pressing could also influence this change in output, which facilitated real-time monitoring of construction health.The unit structure consisted of a latex tube (shell), silicone rubber (core) as dielectric layers, and steel spring as the electrodes.Driven by contact electrification, the TENG could generate sensitivity ranging from 0.84 to 0.038 V N −1 for low force to high force.Additionally, the structure offered higher flexibility, stretchability (up to 140%), and humidity resistance (up to 93%).

Fish Behavior Monitoring
Primitive methods of studying fish behavior were tough jobs.However, with the rise in popularity of TENG-based self-powered wireless sensors, researchers are now exploring their use for developing underwater object monitoring sensors, tracing gadgets, wireless communication, and soft robots. [114]This has become an interesting area of research.Generally, the monitoring sensors can be run by energy from the self-powered TENGs.With the same inspiration, Wang et al. [115] unveiled an antibacterialcoated air sac TENG (AS-TENG) consisting of two Al foils and PTFE film as the dielectric layer.Magnets were used in a silicone sheet that wrapped the electrodes to ensure an efficient CS mechanism (Figure 14a).The peak output power of 0.74 mW was used to support real-time fish kinetics monitoring data transmission sensors.Ma et al. [116] demonstrated fish-scale-like TENG (FSL-TENG) composed of PET film and PTFE sheet that device used to monitor rotational behavior.Jurado et al. [117] revealed water and FEP combined water-dielectric SE mode TENG (WDSE-TENG) that could support the self-powered sensors by releasing maximum electrical output power of 79.18 mW and power density of 0.344 mW cm −2 (Figure 14b).The proposed device was employed to detect fish behavior and analyze water levels and marine weather.

Water Level Monitoring for Ship Draft Detection
In a general sense, ship draft is the distance between the surface of the water to the hull or keel of the ship that indicates the water depth required for the ship to float. [118]It can be quite handy to analyze and monitor water levels to predetermine ship draft.TENGbased self-powered sensors have been reported to carry out this functionality.Recently, Tan et al. [66] developed elliptical cylindrical structured TENG (EC-TENG) made of two coaxial elliptical polylactic acids (PLA) shells and nylon filmed internal FS mode TENG.There were four outer V-shaped CS mode TENGs.Thus, the generated 12 mW was applied for real-time water wave monitoring and the ship draft determination (Figure 15a).Moreover, Li et al. [119] designed a bubble-based TENG (B-TENG) to monitor water level height to assess the real-time ship draft in the deep ocean (Figure 15b).Herein the device functioned while the air bubble moved into the liquid-filled PTFE tube to generate a stable open circuit voltage of 17.5 V by working up to 600 cycles.Xu et al. [120] used FEP and Nylon 6,6 as dielectric frictional layers using a copper electrode and acrylic support to create a CS Figure 14.The application scenario for fish behavior monitoring.a) Conceptual positioning, design, and application of AS-TENG for fish behavior monitoring.Reproduced with permission. [115]Copyright 2022, Wiley-VCH GmbH.b) Circuit model and detection process of WDSE-TENG for fish behavior and water height.Reproduced with permission. [117]Copyright 2020, Elsevier.
mode TENG sensor.The reported sensor having a sensitivity of 4.63 kHz cm −1 , was employed to monitor the water level for measuring ship draft.It generated a peak maximum output voltage of 2200 V.

Tilt Monitoring for Ship Attitude Sensing
As IoT technology progresses, self-powered sensors based on TENG are being used to gather data from moving objects in the marine environment.These sensors can also function by the energy produced from self-powered TENGs. [121,122]Recently, Wang et al. [123] developed a square box-shaped TENG-based sensor to determine the movement of an object intended to be used for marine navigation.The device was made of a cubic box having three PLA (Polylactic acid) rings and steel balls with FEP, which worked in CS mode to generate output voltage 5-8 V (Figure 16a).The ball in the ring was always pulled toward the lowest position by gravity.The ring had 18 channels, each spanning 20 degrees.When the ball contacted and then moved away from specific channels due to a change in the sensor's rotation, the resulting voltage signal could be analyzed to determine the ship's attitude.Moreover, Zhou et al. [124] introduced coordination of displacement and conduction currents based TENG (DcCc-TENG) that consisted of dielectric materials such as FEP tube having water, and the tube was wrapped by copper tape at a certain distance interval.The DcCc-TENG was able to generate a peak power density of 16.6 W m −3 , providing energy to a water-tube-based tilt sensor for monitoring the attitude of a ship in real-time (Figure 16b).Addressing similar functionalities, Wang et al. [125] proposed exceedingly durable, maintenance, and adverse environmental impactfree annular liquid-solid interfacing TENG, which showed effective tilt sensitivity even in low frequency and inclination situations (Figure 16c).Copyright 2022, The Authors, published by Springer Nature.b) Conceptual working mechanism of bubble-like TENG for ship draft detection via water level monitoring.Reproduced with permission. [119]Copyright 2022, Elsevier.

Vibration and Position Monitoring
Marine life is an unlimited source of vibrational energy, which can be utilized to generate electricity and power various marine monitoring devices. [126]TENG-based energy scavenging devices can be employed for analyzing electrical signals and powering monitoring sensors.Zou et al. [87] reported various selfpowered TENG-based harmonic and non-harmonic vibration sensors.Through a combination of coupling effect and electrostatic induction, this device could accurately detect different aspects of vibration, such as amplitude, frequency, acceleration, velocity, and direction.Ren et al. [127] demonstrated a trapezoidal cantilever-structure TENG to monitor low-frequency vibration that was fabricated with flexible FEP and an Al-wrapped PETformed cantilever structure.It functioned in CS mode to generate a power density of 62.2 W m −3 with a vibration frequency of 5 Hz (Figure 17a).Earlier, Chandrasekhar et al. [44] designed a smart buoy hybrid generator (SB-HG) combining TENG and EMG.Here, the tribo-pairs were PDMS and Al.EMG acted while the magnet moved inside the copper coil tube (Figure 17b).Thus, the maximum output TENGs energies were 90 V and 2 mA at a lower 2 Hz vibration.The fabricated sensor was employed with a fishnet or other marine objects to ascertain wave vibration and position tracking.In a more recent study, Du et al. [128] proposed silicone rubber strip-based TENG (SRS-TENG) composed of PLA supported Al electrode pair and a silicone rubber strip participating in CS action to generate a maximum power density of 94.95 W m −3 .The reported SRS-TENG could be used in a ship for realtime vibration sensing.

Lubricating Oil Condition Monitoring
The pollution of marine water is becoming increasingly worrisome due to the presence of lubricating oil, which is a key ingredient in contaminating the aquatic environment.The pollution is caused by the intransigent hydrocarbons carried by these oils. [129]o detect the level of oil in the marine environment, the output from TENG-based sensors can be analyzed.Further, the power generated by TENGs can be utilized to operate sensors that monitor oil levels.TENGs have shown superior performance in detecting water contamination, with a capability of up to 100 ppm.Moreover, TENGs have been employed in detecting oil contaminated by worn debris, with a sensitivity of up to 0.01 wt.%.In this regard, Zhao et al. [130] developed a highly durable oleophobic TENG sensor composed of an Al electrode where PTFE and polyimide (PI) acted as dielectric materials.It was coated with a hydrophobic layer consisting of a mixture of SiO 2 and fluorocarbon (Fc) as well as 1H, 1H, 2H, 2H-Perfluorodecyltriethoxysilane (Fs) to detect water contamination caused by different lubricating oil through FS mode (Figure 18a).The device not only could sense oil contamination but also provided power to thermometers for real-time ocean temperature monitoring.The studied device could measure 0.01 wt% contaminant particles and supply charge and power density of 9.1 μC m −2 and 1.23 mW m −2 , respectively.In a similar study, Zhao et al. [131] reported liquid (oil)solid contact-based TENG that was used for real-time monitoring of lubricating oil conditions where the device precisely measured up to 1 mg mL −1 debris and 0.01 wt % water contamination in the oil.The TENG reported in the study consisted of PTFE  [123] Copyright 2023, Elsevier.b) Self-powered DcCc-TENG driven ship tilt sensor with an alarm system.Reproduced with permission. [124]Copyright 2022, Elsevier.c) Schematics of TENG driven tilt sensor and its application in ship attitude monitoring.Reproduced with permission. [125]Copyright 2022, American Chemical Society.b) Schematics and application of TENG-EMG hybrid device for position tracking by sensing wave vibration.Reproduced with permission. [44]Copyright 2022, The Authors, published by Springer Nature.b) Illustration of MT-TENG unit integrated with floating pipeline in the ocean for oil contamination sensing.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/. [58]Copyright 2022, The Authors, published by MDPI.and copper electrodes, where the PTFE worked with oil in FS mode to generate maximum voltage 0.58, 0.65, and 0.37 V with the flow of polyalphaolefin 6 (PAO-6), paraffin, and rapeseed oils, respectively, for debris fractions of 4, 10, and 4 mg mL −1 .More recently, Zhang et al. [58] designed wave energy harvesting multitunnel TENG (MT-TENG) composed of PTFE balls inside multitunnel copper electrode that also worked in FS mode to generate power peak density of 8.3 W m −3 (Figure 18b).The device was reported to be used for signaling night-time off-shore oil delivery and warning maritime rescue.
In Table 2, some recent studies have been accumulated that showcase the use of TENGs in monitoring various marine elements, either directly or indirectly.
The theme of monitoring the marine environment involves various types of energy-scavenging sensors that use TENG technology and are made from different materials.A significant concern is the use of sustainable, non-polluting, and durable materials for continuous monitoring of marine ecosystems.Typically, TENGs are self-powered, driven by wave amplitudes, oscillations, vibrations, and other factors.Although the devices' monitoring and sensing capabilities are satisfactory, they require significant improvement for commercialization.Moreover, IoT-based data transmission systems hold enormous potential for long-distance communication.In numerous cases, TENGs have complex structures for real-time monitoring, necessitating challenging maintenance and synchronization.Ideally, a simple structure, nonpolluting materials, minimized energy loss, high output performance, a long-distance data transmission module, and selfmaintenance are desired for long-term serviceability with optimum TENG performance.

TENGs for Self-Powered Marine Equipment Development
TENG has been known as an effective device for marine energy harvesting and has excellent energy distribution capabilities.However, this nanogenerator requires an organized and structured network with some essential modules.Additionally, adjusting this device at different levels to perform effectively needs better coordination in self-powered devices.Thus, equipment placement is a crucial factor for effective functioning.By designing TENG devices and properly coordinating and maintaining them the utmost utilization can be achieved and numerous possibilities in TENG-based applications can be unlocked (Figure 19).The TENG network and its applications encompass a wide range of technologies that pertain to the marine environment.Therefore, various marine equipment has been developed with integrated TENGs, and their application in the marine environment has been discussed in the following section. [142,143]

Forecasting in Marine Meteorology
Marine meteorology involves monitoring important marine and coastal parameters, like ocean weather forecasting, humidity, disaster warnings, etc. [27] As high-performance technologies have progressed, it has become a common practice for advanced marine meteorological systems to keep ships, vessels, the marine transport network, and coastal communities alerted with realtime meteorological information updates. [144]Therefore, stable operation, accurate data collection, real-time updating, remote    data processing, and transmission are critical for marine operation and development. [145]Yang et al. [27] developed a selfpowered temperature sensing system integrated wireless transmitting device for meteorological monitoring and forecasting.This device was powered by a barycenter self-adapting TENG (BSA-TENG).This device functioned based on the FS triboelectrification principle whose underlying function was to convert unstable HEE seawater waves into effective rotational energy and, consequently, to electrical output.The efficacy was highlighted in terms of delivering 0.1 mW of peak power with a load resistance of 500 MΩ at a very small working frequency of less than 1 Hz.Zaw et al. [146] focused on how they could efficiently apply TENG in marine weather information tracking and other relevant applications.The proposed wave-powered and solid-liquid contact electrification-driven TENG integrated device could potentially provide important meteorological information measuring wind speed, pressure, flow, and direction by powering an anemometer. [147]It further demonstrated the potentiality of hydrogen synthesis from seawater with additional benefits of being light-weight, flexible, waterproof, corrosion resistant, and high-power density.This innovation helped in meteorological monitoring, ocean water desalination, navigation safety, and so forth.Shi et al. [148] developed a double-layered self-powered water-based TENG (SWTENG) integrated into a network struc-ture for harvesting blue energy.Similar to previous studies, the device also worked based on solid-liquid contact triboelectrification.The SWTENG was deposited on a buoy ball to effectively power up sensors for marine environmental monitoring.Water temperature, pollution levels in the ocean, water height and level, the velocity of water flow, etc., provided various meteorological information, all of which could be generated with the help of TENGs.Zang et al. [29] fabricated the single-segmented TENG containing a rotational shaker that could analyze the real-time situation at the ocean (Figure 20a).A unique design principle was introduced combining surface electrification with rolling spherical motion, which minimized the limitation associated with dielectric shielding and less output performance of a typical TENG.An array of multiple TENG units can be utilized over the surface of ocean water to generate vital information.Spring-assisted construction, air-gap structure, and multiple rolling ball layers design in a single unit could detect motion even under very low wave frequency along with minimizing the effect of water dielectric shielding.Moreover, many informative devices like humidity sensors could be powered.Figure 20b illustrates a WB-TENG developed by Xia et al., [149] that could be employed in the ocean as a part of a water balloon.The structure with a square box and a balloon (NaCl solution inside the PVC film cover) had a copper wire dipped into the NaCl solution.This double-plate shaped structure Reproduced with permission. [29]opyright 2018, Elsevier.b) Structure of WB-TENG floating on the sea.Reproduced under the term of CC-BY license https://creativecommons.org/ licenses/by-nc-nd/4.0/. [87]Copyright 2022, The Authors, published by MDPI.c) Tumbler-shaped triboelectric nanogenerator (TH-TENG) structure.Reproduced with permission. [133]Copyright 2020, Elsevier.d) Device architecture of the rolling TENG.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/. [72]Copyright 2021, The Authors, published by American Chemical Society.
employed CS mode for generating electricity.Due to the elastic nature of the balloon, the device could realize a multi-frequency response.Thus, with 28 times greater charge transfer ability than simple TENG, it could power various interconnected electronics to gather meteorological information.With the commensurate applications of advanced internet of things (IoTs), TENGs can be incorporated in intelligent environmental monitoring, portable energy supply, and miniatured prototype devices. [150,151]For instance, the tumbler-shaped TENG was integrated with IoTs by Zhao et al. [133] This amphibious self-powered device employed an SE working mode of TENG (liquid) principle with a rolling ball inside, enhancing device performance (Figure 20c).Important meteorological data like wind speed, wave height, frequency, etc., were realized.For accurate environmental monitoring in low frequency and irregular disturbance, TENG provides better results in evaluating the solubility of total solid and plotting the hygrothermograph.Chen et al. [72] explained how the performance of TENG-based devices could be improved using PTFE films incorporated with a nanostructured surface in a rolling ball mechanism.Electricity was produced during rolling cycles through the potential difference between electrodes, which was caused by the charge difference between the rolling ball and triboelectric layers.
Their study revealed a positive correlation between TENG output performance and rolling ball radius by systematically investigating the rolling ball TENG system (Figure 20d).The maximum output power was achieved through 45% nano-micro-PTFE concentration, which significantly impacted sensing important meteorological signals for environmental monitoring.
Incorporating TENG with EMG in hybrid devices may bring many benefits in harvesting power from a broad frequency and covering a large area. [18,152]Such a self-powered sensing of EMG/TENG hybrid generator, fabricated by Chen et al., [53] could be utilized to sense data over a place of more than 300m distance from the coastline.The chaotic pendulum-based mechanism was capable of high electromechanical energy conversion and could also generate energy from a very low frequency.The peak energy output of the device demonstrated a power output of 15.21 μW which could light up at least 100 LEDs.Thus, this combination with sensors has the potential to bring unprecedented results in marine monitoring. [93,153,154]

Ocean Buoy
Interconnected systems have been developed in the ocean for efficiently operating and coordinating various marine activities.For example, they support the marine economy (i.e., catching fish, marine transportation, mining resources, and many more) and strengthen security (i.e., defending the ocean border, climate issues, and environmental challenges).Ocean buoys have proven useful for various purposes in marine distributed systems. [109,155]However, typical surface or subsea buoys do not provide enough support in collecting and utilizing data.][158] For instance, TENGbased whisker sensors integrated with buoys have proven to be Figure 21.TENG-integrated marine buoys.a) Floating body TENG integrated buoy structure incorporated with COMSOL softwar.Reproduced with permission. [164]Copyright 2022, Springer.b) Schematics of a TENG functional unit integrated with buoy.Reproduced with permission. [25]Copyright 2021, American Chemical Society.c) Conceptual illustration of TENG-integrated self-powered buoy.Reproduced with permission. [101]Copyright 2021, Elsevier.d) Schematics structure of liquid-solid-contact buoy TENG.Reproduced with permission. [165]Copyright 2018, Wiley-VCH GmbH.e) Multilayered TENG structure design in a buoy and f) A real AS-fabricated TENG unit in size of Φ15 cm × 7 cm Reproduced with permission. [161]Copyright 2019, Elsevier.
effective tactile sensors where obstacle avoidance and mapping of unknown territory are essential. [159]A typical marine buoy consists of various components, such as sensors, navigation beacons, data-collecting tools, and communication devices.However, increasing the number of electrical components could result in higher energy consumption.Therefore, the TENG-based buoy system can actively support fulfilling these specific application requirements. [160]esearchers aimed to develop an intelligent monitoring system for sustainable, continuous, and autonomous operation with higher efficiency and docile mechanism, which supports energy generation from very low frequency with TENG.As a result, Xi et al. [161] developed a self-powered, and high-performance multilayered intelligent buoy (SIBS) based on multilayered CS principled TENG technology that could carry out wireless sensing for versatile energy stages for an infinite lifetime (Figure 21e,f).The power management system of this device easily converted DC voltage of 2.5 V for micro-controller units and other components like the sensors and the transmitters.A superior mean output power density of 13.2 mW m −2 was recorded at a very low wave frequency (2 Hz).It further demonstrated efficient data transmission (19 bytes in every 30 s).Chandrasekhar et al. [44] fabricated a buoy to track the global positioning system (GPS) over long distances (even kilometers from shore) with an EMG-TENG hybrid generator.The TENG functioned based on CS mode within two separate units.The energy harvesting was enhanced by multiplying layers while a controlling device was incorporated to maintain wave motion.A high output energy (20 V/15 mA and 100 V/2 μA) was reported which could be used to process services like GPS tracking and tracing fishnets in marine conditions.
Marine IoTs are evolving to increase protection and collect information from the underwater environment in the ocean.Recently, Wang et al. [25] developed a TENG-integrated self-powered intelligent buoy system (SIBS) having an excellent power management setup (Figure 21b).Based on the CS working principle, it could generate an output voltage of up to 2.5 V for powering a micro-engineered control unit (MCU), various microsensors, and a data transmitter unit.In their earlier study, the same team designed another buoy with a sandwich like TENG, which was subjected to the first-ever model test in an actual ocean-like situation created in a basin. [40,101]Wang et al. [101] developed a selfpowered buoy that was used for providing safety during navigation.The integrated SS-TENG device held PTFE balls in the middle.The balls encountered the top and bottom electrodes when moved vertically.They could also move freely in the left-right direction.Therefore, based on the direction of wave excitation, the device employed a combination of CS and FS mechanisms to generate electricity.Almost 12 W LED bulbs with high brightness powered by TENG helped to support the navigation process (Figure 21c).In a similar study, Chang et al. [162] developed a solid-solid buoy structured TENG device that could harvest energy in both horizontal and vertical directions via a CS mechanism, even in different wave conditions.It contained a doublesided TENG to provide efficient energy conversion securing an optimum output voltage of 1055 V and current of 72 μA.Therefore, it could power LED lamps and small hygrometers.Further, Cestaro et al. [163] developed a model to estimate the voltage distribution of TENG-integrated buoys.This device could effectively monitor voltage generation and store it in the "Particle Swarm Optimization" (PSO) process to establish a strong and efficient energy distribution network.Yu et al. [164] also fabricated a novel TENG buoy (BUOY-41) which was driven directly and incorporated with COMSOL software to analyze the power generation parameters.This device with BUOY-41 could generate motion response over two units analyzed by the hydrodynamic testing tool STAR-CCM+ software (Figure 21a).The TENG attached to the buoy worked in FS mode and the device could reach a peak power density of 7.68 W m -2 .As a result, it could provide endless and autonomous power supply to the devices with proper operational flexibility and a certain endurance time.Li et al. [165] developed a high-performance TENG network with electrification through liquid-solid interface contact (Figure 21d).The TENG network, consolidated in a buoy was able to supply power to almost a hundred LEDs for wireless SOS system for emergency situations in the ocean.All these information denote that TENG, a mechanical energy converter, was used to improve the performance and durability of the buoys.

Marine IoTs
Internet of Things (IoTs) has initiated an intense advancement in science, specially, by exploiting the maximum usage with low-power consuming wireless sensors and transmission tools. [166,167]Internet of underwater things (IoUT) has been useful in connecting underwater and undiscovered elements that are interlinked intellectually with each other.In terms of sensing, communicating, and computing, TENGs are regarded as quite efficient contenders to be a part of IoT application settings and places where cables are unreachable. [168,169]TENG-based sensors are distributed in the ocean and a network structure often connects all of them for data collection, evaluation, and implementation in different application set ups.172][173] Liu et al. [174] recently fabricated a spherical TENG having a helical unit with maximized space utilization of up to 92.5%.The incorporated TENG unit adopted a CS working mechanism and could power not only a water quality measuring unit but also a Bluetooth-operating thermo-hygrometer with a smart alarm sys-tem for distant environmental monitoring.Thus, it contributed to interconnected intelligent system development for ocean IoTs along with efficient carbon neutralization.Zhang et al. [175] developed a TENG-based power generation system capable of charging itself even from low-frequency wave energy by employing a consistent contact and separation action.Polypyrrol (PPy) integrated with conductive polymers in the device enhanced stability and capacitance in the ocean.Through ion transmission, it provided superior power management for sensors and wave-driven electronics.Thus, it helped to keep the devices stable and made the interconnected IoT network stronger.Liu et al. [176] fabricated a direct current TENG that could be rotated by wind forces.It functioned based on the CS mechanism to convert the wind energy into DC electricity with a peak open-circuit voltage of 450 V and a current of 11 μA.One of the key features of the system was its ability to process multiple sea-level data and connect various IoT devices.Li et al. [177] developed a dual-mode AC/DC-TENG to supply energy to particular operation zones.AC and DC currents were generated alternately as the slider material moved in a reciprocating motion.The self-powered device could monitor the condition of the structural state of a construction by continuously producing AC signals within a safe vibration range.Once the vibration crossed the danger vibration threshold, the AC signal was converted to DC, which was set to trigger the interrelated alarm system immediately and thus carry out real-time structural health monitoring (Figure 22a).This system can be adapted to any construction health monitoring in marine environments.
The efficacy of TENG can often be disrupted in an unfavorable environment, such as in high humidity conditions where the device exhibits extreme sensitivity.[180] To address this, a winddirection adjusting flag-type and humidity-resistant TENG was fabricated by Wang et al. [181] capable of wind energy harvesting.Carbon coating along with PET and PTFE membranes was used to design a flag-type TENG that functioned based on FS contact mode.Correspondingly, Zhao [182] tried to improve the power output of such TENG-based devices with surface modification by PTFE.The TENG performed based on both CS and FS modes.They tried to enhance the range and performance of the TENG devices with a peak output of power and energy (increment by 255% and 344% for output voltage and current, respectively).Yet, there were some challenges in the single-flag TENG devices (i.e., an anomaly in water conditions, not uniform performance at ranges).Further, to provide a remedy to the challenges of the single-flag type TENGs, Zou et al. [183] developed the highperformance flag-type TENG (HF-TENG) with carbon coating of PET and PTFE.Based on FS mode, this device performed four times better than the normal and untreated one.Most importantly, this device was a great candidate for power supply to an interconnected network of LEDs, temperature sensors, and capacitors of the devices (Figure 22b).The integration of wireless power generation with radio frequency identification (RFID) has advanced the technology of non-contact energy transmission and communication systems to a new level.For instance, Cao et al. [184] fabricated a TENG-integrated device to produce electricity utilizing a contact and sliding mechanism in a segmented structure.The device could transmit wireless energy to different segments of an interrelated IoT system.The generated current was enough  [177] Copyright 2020, American Chemical Society.b) Application of high-performance flag-type (HF) TENG.Reproduced under the term of CC-BY license https://creativecommons.org/ licenses/by-nc-nd/4.0/. [183]Copyright 2022, The Authors, published by MDPI.c) Rotary electrodeless TENG structure with collectors.Reproduced with permission. [184]Copyright 2018, Wiley-VCH GmbH.d) Design of a wind tunnel and architecture of a flutter-driven TENG with surface characteristics of a flexible flag.Reproduced with permission. [185]Copyright 2014, Springer Nature.e) A blue energy network integrated with TENG.Reproduced with permission. [142]Copyright 2017, Springer Nature.f) TENG integrated wind sensing system Reproduced with permission. [186]Copyright 2018, American Chemical Society.
to power up LEDs at an output voltage of 65 V within a 3 cm gap (Figure 22c).Bae et al. [185] fabricated a flutter-driven TENG device capable of performing contact-based electrification and optimizing the connections among interconnected devices.A reciprocating interaction between the flags and a rigid plate took place in three specific modes, i.e., chaotic, single, and double.The structural dimension was 7.5 cm × 5 cm and it could produce superior electrical performance (60 μA, 200 V, and mean power density of ≈0.86 mW with 150 Hz of frequency) at the wind speed of 15 m s −1 (Figure 22d).Nevertheless, due to not-so-adequate advancements in the technology of harvesting energy in wave farms, energy harvesting was just limited to the slow movements of the waves in random directions and generating a very little amount of energy.Lin et al. [142] introduced an interesting concept in a TENG-based interrelated network that could harvest energy for the connected devices.Motivated by the mechanism of generating energy from human movement and heartbeats, they thought of having a device to produce substantial energy from two things rubbed onto each other.For example, a plastic comb produces static energy when it goes against the wool's contents.They developed a more feasible, efficient, and durable generator that can power the entire network based on freestanding movement of a metallic ball inside dielectric hollow sphere (Figure 22e).In Marine IoT, it has always been a challenge to have a device that is able to perceive the signals and stimuli from the environment and react accordingly.To solve this problem and scarcity of such sensing devices, Wang et al. [186] developed a TENG with an intercon-nected wind sensor that could analyze the direction and details of wind flow.The wind sensor system had an anemometer with TENG working in an FS mode, while another wide vane TENG (v-TENG) worked in SE mode.The introduced device could provide information about the real-time speed of wind (ranging from 2.7 to 8.0 m s −1 ) and direction of it.With an effective soft friction mode, it could also enhance the performance of the TENG (i.e., the sensitivity, resolution, and measurement scale of the device).This IoT-integrated system could further contribute to marine environment monitoring significantly (Figure 22f).Thus, the distributed devices in the network directly or indirectly connected to TENG could conveniently enter the world of IoTs.

Ocean Data Collection
Recording accurate marine information requires significant effort.[189] Detection of environmental factors such as humidity, temperature, and salinity is crucial as they provide important information for ships and the ocean itself.Enhancing mechanical stability and ensuring the waterproofing ability of the encapsulation material of such devices have been essential for charge reformation and smooth  [195] Copyright 2015, American Chemical Society.b) TENG integrated self-powered water quality monitoring device.Reproduced with permission. [51]Copyright 2019, Elsevier.c) Design of Spring-mass-assisted TENG.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/. [87]Copyright 2022, The Authors, published by MDPI.d) Schematics of a TENG-operated hybridized nanogenerator.Reproduced with permission. [196]Copyright 2019, Royal Society of Chemistry.
operations.Moreover, some factors may cause corrosion to the sensors and equipment.Hence, it is important to maintain the chemical stability of the materials.TENG-based vibration sensors can monitor some of these factors before using TENG in the very first place. [190,191]mong different ocean-wave spectrum sensors for precise ocean-wave spectrum detection (i.e., remote sensing through radar, photoelectric devices, liquid level sensors), a highly sensitive and liquid-solid contact-based TENG (LC-TENG) was developed which could monitor the wave heights.Measurement of individual parameters and forecasting environmental risks were the major challenging tasks for LC-TENGs.[194] Zhang et al. [103] developed a self-powered and efficient TENG-based ocean-wave spectrum sensor (TOSS) with tubular TENG and a hollow-ball buoy.It worked in the principle of liquid-solid contact electrification.Apart from measuring multi-directional water waves, the device could further mitigate the negative influence of the waves at the time of its operation.Through the high-sensitivity ability of TOSS, the multiparameters like wave heights, period, frequency, motion, wavelength, and steepness were evaluated in real-time.Jiang et al. [195] addressed the structural optimization part of the TENG devices, where a wavy-structured TENG made of Cu-Kapton-Cu film, two FEP thin films, and a metal ball inside was evaluated.The TENG worked based on collision-driven CS mode and was consolidated in a box-shaped sensor (Figure 23a).The results revealed that the optimum ball size in the TENG devices had an effective impact on output performance.Bai et al. [51] developed a ball-shell structured contact-based TENG having a tandem disk that could evaluate water quality.The TENG was accommodated in a radial grating disk structure.Its power density set a benchmark of 7.3 W m −3 (Figure 23b).Figure 23c demonstrates the internal construction of a spring-loaded TENG with spring-mass having a six-layer and helical structure.This CS-driven device was capable of generating electrical signals with Cu electrodes easily. [87]The TENG-based single active vibration sensor was consolidated in a 3D layered structure for effective sensing performance.Chen et al. [196] developed a rotating circular-shaped hybrid EMG-TENG device that possessed multifunctional potentials with different levels of wireless data collection and transmission ability.The rotator-stator based contact electrification could be achieved by the interaction between PTFE film and Au electrodes.With the addition of a commercial voltage booster, the output voltage could reach up to 153 V at an 80 cm distance (Figure 23d).

Marine Anti Corrosion
Marine corrosion poses a major obstacle to the progress of marine systems as it has the potential to cause significant economic damage.The marine environment is naturally conducive to corrosion, making it a significant challenge to overcome. [197,198]201] Correspondingly, the electrochemical cathodic process of protection is considered one of the most efficient ways to incorporate corrosion prevention in the metallic marine component. [199,201]igure 24.TENG-integrated anti-corrosion devices.a) Schematics of solid-liquid TENG-based PTFE filtration membrane and waterdrop on the membrane.Reproduced with permission. [200]Copyright 2021, Elsevier.b) Cathodic protection of S-TENG in NaCl solution.Reproduced with permission from. [213]Copyright 2022, Springer Nature.c) Image of paper/PVDF-based TENG.Reproduced with permission. [205]Copyright 2016, Royal Society of Chemistry.d) Schematics of coating-based TENG device.Reproduced with permission. [214]Copyright 2021, Springer Nature.e) Potentiometric monitoring of steel electrodes immersed in ocean water.Reproduced with permission. [100]Copyright 2022, American Chemical Society.
With the traditional methods, i.e., impressed current cathodic protection (ICCP) [202] and sacrificial anode cathodic protection (SACP), [203] it is not an easy job to provide a continuous supply of power for sacrificial anodes and the cathodic protections in the deep sea. [204]onsidering the fact of developing a green, cheap, and better energy system, TENG grabbed the attention of new researchers with the dual effect of electrostatic induction and triboelectrification. [205][206][207][208] ICCP system in TENG, as an external power source, can perform in two modes: a) effective packaging for differentiating water through solid-solid CS mode, and b) the liquid-solid CS through triboelectrification of water and polymers. [199,209]Generally, the solid-liquid TENG is quite inferior to the solid-solid ones in terms of effectiveness. [210]Issues such as constant frictions leading to decreased performance and water coming into contact with electrodes resulting in a solidliquid TENG causing short circuits have motivated the fabrication of an enclosed and protected TENG device. [211,212]To this extent, Sun et al. [200] developed a dual liquid-solid working modebased TENG array with PTFE ultrafiltration membrane in the friction layer (Figure 24a).This device showed a great output generation of 2.68 μA as a short current circuit and a voltage of 105 V. Efficient cathodic protection for metal surfaces could be achieved through this innovation.As a solution to all the limitations of the single working mode of TENG, Wu et al. [213] developed a spherical-shaped hybrid TENG (Figure 24b).This could work both in solid-liquid and solid-solid surface contact modes and efficiently collect low-frequency wave energy for metal corrosion.Feng et al. [205] developed a paper-based CS mode-driven TENG with the ability of antifouling and anti-corrosion in marine applications.With an increased charge density of up to 76 μC m −2 , it could also light up around 496 LEDs (Figure 24c).Liu et al. [214] came up with an excellent idea to develop a hydrophobic liquid-solid electrification-driven water-solid (WS-TENG) device that had cathodic protection against corrosion in the marine environment.TENG device was coated with a polyacrylic acid coating that was incorporated with polyacrylate resin (F-PAA) (Figure 24d).As a result, it increased the output performance by around six times.Further, Zhang et al. [100] tried to develop a liquid-solid interface contact-based TENG.This research aimed to reduce the corrosion problem with a seawater-based TENG.It reduced the friction coefficient of the ocean water by at least 43.8% without using any coating with the material (Figure 24e).Thus, TENGs have an immense role in marine anticorrosion applications.

Distress Signal Emitter
TENG-based self-powered hybrid devices are quite effective as distress signal emitters.Typically, distressed signal-emitting devices provide responses with LED lights or through any output to send information to the receiver within a certain periodic interval. [32]Wang et al. [55] developed an entirely enclosed FS rolling-spherical structured TENG that could harvest energy with low-frequency water waves.The structure with 6 cm of diameter set up into the ocean wave could essentially generate a 1 μA of peak current with a certain short-circuit to 10 GΩ and the peak instantaneous power could be up to 10 mW.However, this device could easily light around 10 LEDs and a certain number of capacitors could be charged with it (Figure 25a).A flexible seaweed-like TENG functioning based on liquid-solid contact electrification was developed by Wang et al. [25] This low-cost innovation introduced a battery-free system in IoT.Moreover, a certain number  [55] Copyright 2015, Wiley-VCH GmbH b) Schematics of providing power for a lighthouse model via S-TENGs.Reproduced with permission. [25]Copyright 2021, American Chemical Society.c) Distress signal LED bulbs driven by the hybrid TENG embedded on a life jacket.Reproduced with permission. [215]opyright 2014, Elsevier.d) TENG network-based self-powered ocean emergency SOS emitter (10 cm scale bar).Reproduced with permission. [218]opyright 2014, American Chemical Society.
of connected parallel seaweed-TENG could be used for coastal power stations and other substantial devices (Figure 25b).Another study showed TENG-based devices having 15 cm × 6 cm × 0.8 cm in dimension with 75 μm-thick films of FEP for hydrophobicity, could generate power for almost 48 commercial LEDs with more than 40 V to an open-circuit.TENG-based distress signal emitter could be implemented into the life jackets, therefore, it should be easier to locate the lost person in the open wide ocean. [215,216]Lin et al. [217] developed a contact electrificationdependent TENG consisting of flexible SiO 2 /P(VDF-TrFE) film.It was capable of charging the capacitors and lighting up the LEDs for emitting distress signals integrated into a life vest.Su et al. [215] further developed an all-in-one hybridized TENG that functioned based on the liquid-solid contact principle.This TENG-integrated device with two parts of interfacial electrification-based TENG (IE-TENG) and impact-TENG to generate energy continuously from water waves and water drops (Figure 25c).IE-TENG of FEP could harvest interfacial energy from water and the impact-TENG could scavenge energy for whole cycles of water motion.In a situation of waves with 0.5 m s −1 velocity, 5.1 mA and 4.3 mA were the peak values of current from IE-TENG and impact-TENG, respectively.The network-structured single-electrode TENG device developed by Chen et al. [218] could render an average output of 1.15 MW within 1 km 2 surface area.Moreover, it rendered a better service with SOS signal generating during an emergency at the ocean (Figure 25d).Some more recent studies demonstrating TENG integrated self-power marine systems have been enlisted in Table 3 with important criteria.
TENG has been incorporated into various marine devices employed in widespread practice.Its self-power generation capability is one of the main advantages, as it eliminates the need for external sources.Additionally, it offers a range of functions such as sensitivity, signal generation, data collection, transmission, protection, navigation, and more.Hybrid TENG-integrated devices are also being used momentously to enhance performance in specific marine applications like ocean meteorology, IoTs, data collection and transmission, and marine anti-corrosion.This section of this review discussed some impactful studies concerning TENG-integrated device development along with their features and functionalities.

Conclusion and Outlook
This review comprehensively captures the present state of TENGdriven electronics progress from a marine and coastal set-up perspective.With TENG technology, the potential for marine energy to become a dependable power source and self-sustaining system is highly promising.Despite the advancements made, TENG technology has yet to meet the demands of the commercial world.There are still obstacles that need to be overcome to achieve the desired performance level.Addressing the current limitations and conducting further research will be crucial for the success of this promising area.Particularly, the long-term prospect of this technology, which is aimed at developing a self-sufficient and wireless data transmission device as a part of an interconnected IoT system (Figure 26).Measuring salinity, temperature, and acidity.Monitoring wave height, wave-period, and ocean-wave parameters.TENGs are notably efficient in converting lower-frequency mechanical energy into electrical output, outperforming other energy harvesting technologies.However, their conversion efficacy, output rate, and consistency are still inferior to other techniques like EMG. [227] One of the major reasons for this lower energy conversion rate might be the energy conversion process itself, i.e., surface-to-surface contact.Frequent contact mechanisms might lead to frictional loss. [228]Nevertheless, as a new era of artificial intelligence is approaching, there are more and more inclinations toward developing self-sustainable, automated systems.When considering a truly self-efficient system for an interconnected cluster of marine devices, a consistent power supply system with high output performance is desired. [229,230]Although researchers are exploring strategies such as material selection, increasing charge density, [231] and modifying surface properties [232] to enhance TENG's performance, further research is still needed.In addition, TENG technology still requires improvement in the area of application scalability.Currently, TENGs have demonstrated greater potential for handling smaller-scale motions or vibrations.However, there is still room for improvement in terms of their ability to manage power distribution for larger network structured systems. [233]Therefore, it is quite clear that the focus of upcoming research will be on enhancing TENG's conversion rates and application range.
Further, a major portion of the output energy is depleted due to the complex design, corrosion, and limited contact separation concerting dielectric materials and electrodes. [100]This situation can be improved by modifying TENG structures to have wider contact areas of electrodes and frictional layers.Self-powered underwater TENG-based sensors have limited functionalities due to their lower output signals which can be solved using physically or chemically modified structures.Real-time underwater monitoring needs high modularization, synchronization, and integration. [107]On top of that, the durability and maintenance of the oil or chemical monitoring sensors are not satisfying as per the expected service life. [141]Thus, new materials selection and simple energy scavenging TENGs can seamlessly support the oil and hazardous chemicals or lubricant monitoring sensors.
In some cases, the output performances of TENG devices have been observed to drop beyond a certain input frequency, although logically, output energy should increase with elevating input frequency.For example, Han et al. [68] reported the maximum output of wave-driven floated C-TENG at 1 Hz, while it drops when input was increased.Xiao et al. [63] also reported a similar behavior where increasing input frequency from 0.5 to 2.0 Hz generated maximum output only at the optimum (1 Hz) frequency.Similarly, Xu et al. [70] reported the generation of a short circuit current of 8.3 μA for 1 Hz input, while a dramatic reduction was noticed in the case of 2 Hz resulting in only 2.2 μA.The successive triggering waves causing displacement before TENG elements return to starting positions may cause such phenomena.Research is needed to understand this type of behavior better as the ocean waves are unpredictable and valuable energy might be lost when frequency surpasses optimum input.
Another important aspect for TENG-based marine devices is the surrounding environment and conditions they are expected to operate in.Typically, marine environment is quite harsh and challenging most of the time because of the chaotic nature of the waves.It is important to consider how the unpredictable movements of the waves and air could potentially impact the performing devices.On this note, the material selection is quite significant.Longer lasting durability, good encapsulation ability to prevent water penetration, and anti-corrosion are among the major considerations during material selection. [234]Ocean condition is largely dependent on weather conditions.Therefore, TENG devices have been reported to perform inconsistently when the weather is rough. [66,235]Excessive rotations and movement might compromise and limit TENGs harvesting performance, especially, with aging. [66]Although some solutions have been offered in the form of added accessories, [236] the ultimate product becomes heavy, nonflexible, and bulky.Therefore, studies can be directed from material selection and design perspective to integrate various value-added retrofits while keeping the final device lightweight, flexible, and within the size limit, specially, in harsh and rough weather conditions.On the contrary, various studies have pointed out that TENGs rather perform poorly in placid marine environments despite their much better output generation ability in harsher sea waves. [66]This is associated with the lower input frequency in an imperturbable ocean, which reveals more architectural schemes are to be explored to ensure optimum performance.Performance consistency is particularly important because the use of TENG-based marine sensors has been identified as a significant advancement in weather forecasting.To supply accurate weather predictions and meteorological data, it is necessary to constantly generate signals under different ocean conditions.This means that consistent energy generation, efficient information processing, and reduced processing time are essential for improving the system's effectiveness.Future studies could concentrate on improving the TENG's performance in ever-changing ocean environments, both in rough and tranquil situations. [7]n order to achieve commercial success, TENGs must be capable of supporting industrial-scale networks.This is crucial for gaining recognition in the industry.Many bottlenecks, such as complex structure, materials selection, maintenance, and complicated circuitry still exist and impede TENGs potential at the industrial level.Therefore, it is pivotal to discover viable solutions for these bottlenecks for TENG-based self-power systems to revolutionize their commercialization. [237,238]Being one of the primary sources of energy, the wave amplitude of water in the ocean depends on its surroundings factors like the flow direction of offshore wind, speed of wind, intensity, frequency of ship movement, etc. [133,239] Thus, the correlation between ocean wave-related factors and the contact area of the energy-scavenging TENGs still needs further research to generate industrial-scale energy capable of supporting the whole marine monitoring system. [56]nother area worth mentioning is the type of electricity TENGs produce.In existing methods, it is necessary to convert alternating current (AC) to direct current (DC), resulting in considerable energy loss. [32]Hence, upcoming researchers must find a way to redirect TENG-powered devices to produce and circulate DC to enhance their efficiency.
From a specific application perspective, a lot of challenges can be addressed.Different working environments and application scenarios demand different features to be prioritized.For example, one hurdle with TENG-based sensors for monitoring fish behavior is their potential antibacterial properties when attached to marine species. [115]In this regard, the selection of sustainable materials and the simple structured TENG-based sensors with wireless communication facilities are expected to be explored to lessen the adverse effects on aquatic species and provide useful information on the surroundings. [93,132]In leakage monitoring scenario, the TENG-powered sensors need to be more durable and continuous in energy generation for powering the sensors. [112]The tracking system of ships using selfpowered TENG sensors require them to be more sensitive following the ship's movement, even with the lower frequency of wave amplitudes. [125]Furthermore, research should focus on the self-healing and self-maintenance of TENG structures, using sustainable materials and optimized electrical connections for better performance.
TENG-based devices have successfully been incorporated into numerous marine equipment as either an energy harvesting component or a self-powered sensor.However, Additional research is necessary to attain a performance level that is deemed satisfactory.Some recent studies suggested that marine buoys tend to lose their durability and efficiency within a short span.So, it is important for TENG-based buoys to have a sturdy structure and be highly energy efficient.This ensures optimal performance and longevity. [109,240]Adding to that, It is essential to consider marine information regarding microorganisms and sea life to ensure that the devices are functioning optimally and safely. [176]In the case of marine IoTs, most of the devices perform better when put close to each other. [32,40]The placement of devices at a distance has caused disconnection, leading to security concerns and a lack of information flow. [241,242]In modern days, data security has become a major factor. [243]Therefore, much more robust, secured, and well-programmed TENG devices with stronger connectivity are expected to be enlightened in the TENG devices going forward.
Most of the studies concerning TENGs for marine energy harvesting are about sea surface waves.However, major potential and kinetic energy also exists underneath the sea surface. [64]urthermore, TENG-based harvesting of other forms of energy around marine atmospheres, like wind, ocean thermal energy, salinity, etc., has not been explored as much as wave energy.Further research should focus on investigating explicit applications for harnessing energy from raindrops and seawater, as well as developing self-powered navigation systems and improving marine search, monitoring, and rescue efforts.
In summary, this review article discusses three main aspects of TENG in the context of its application in the marine environment.These include the potential use of TENG to harvest blue energy from multiple marine surroundings, monitor various marine phenomena, and develop wireless and self-sustaining marine equipment for diverse purposes.Device design features, dielectric material selection, motion frequency, and optimum sur-face contact impact the output performance.Since the nature of marine elements (wave, tide, vibration, air, etc.) is random and unpredictable, the optimum performance can only be achieved through optimizing the harvester design and selection of appropriate materials.Consequently, the prime driver for TENG's structural design and material selection is the "working mechanism" based on which the device is fabricated.For wave and tidal energy harvesting, all four major working mechanisms can be incorporated individually or combinedly (hybrid).However, CS and FS mechanisms are more frequent than others.The reason being, simpler structural design, durability, higher contact area between triboelectric layers and electrodes, etc.Among the four major principles, the rolling structures usually utilize FS since the ball rolling inside can be subjected to better contact with the inner structure wall.For multi-layer or stacked structures, CS is better for larger surface contact and avoids wearing off the constituents.Rotational disk structures usually employ sliding mechanisms, but these are a limitation of material durability.In the case of vibration harnessing, spring or elastomerassisted TENGs have been found to work better with harmonious vibration, while contact-driven point, planer, or curved structures have been used for non-harmonious vibration.Attachment of these additional parts provides TENG with the ability to store energies, ceaseless oscillation motion, and exploit the best of the complicated non-uniform and non-linear dynamic behavior of the TENGs.In the case of air energy harvesting, rotating structures (disk-wheel, rotor-stator, etc.) have been adopted, as the linear motion of any fluid like air can be converted to rotary motion to achieve better energy harvesting.
Modern IoT-integrated TENGs being developed are mostly driven by self-generated electricity.The scope for ocean monitoring is humongous as it provides vital information on the present condition of important factors and using those many upcoming incidences can be foretold.TENG-integrated self-powered devices can monitor specific variables to forecast weather, water level, and marine pollution conditions.Thus, they can provide important information for ships, water vessels, and any coastal disasters in advance.
Further, the type of TENG-integrated marine equipment developed to carry out these tasks has been overviewed.While most of the equipment has been working swiftly, more research is needed to address (i) making the TENG structure simpler and more flexible to adapt to any adverse conditions, (ii) integrating the TENG data acquisition system with global internet to have a more effective distribution of information regarding various parameters monitoring and forecasting, (iii) how to improve the life-span of such devices based on resilient and durable material selection and packaging, (iv) data security.

Figure 1 .
Figure 1.The main aspects of applications in a marine environment.

Figure 2 .
Figure 2. Sources of renewable energy in the marine environment.

Figure 4 .
Figure 4.The recent trend in research focusing TENG-related marine applications.a) Web of science.b) Scopus.

Figure 5 .
Figure 5. Rolling structures for ocean energy harvesting.a) Energy harvesting mechanism by contact electrification of a TENG spherical rolling ball method.Reproduced with permission.[55]Copyright 2015, Wiley-VCH GmbH.b) Fabrication information and working mechanism of rolling sphere TENG.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/.[56]Copyright 2022, The Authors, published by MDPI.c) Working mechanism of sea-snake shaped rolling ball TENG.Reproduced with permission.[29]Copyright 2018, Elsevier.

Figure 6 .
Figure 6.Cylindrical and hybrid structures for ocean energy harvesting.a) Schematics and working mechanism of a frequency multiplying C-TENG in the ocean.Reproduced with permission.[57]Copyright 2022, Elsevier.b, i-vi) Working mechanism of TENG-EMG-SC hybrid power unit.Reproduced with permission.[59]Copyright 2017, Elsevier.c) Working principle of the hybridized EMG and TENG.Reproduced with permission.[18]Copyright 2016, Wiley-VCH GmbH.d) Schematics of freestanding contact electrification of an MT-TENG to harvest marine energyReproduced under the term of CC-BY licensehttps://creativecommons.org/licenses/by-nc-nd/4.0/.[58]Copyright 2022, The Authors, published by MDPI.

Figure 7 .
Figure 7. Enhanced TENG structures for ocean energy harvesting.a) Structure and working principle of stacked pendulum structured TENG.Reproduced with permission.[60]Copyright 2019, Elsevier.b) Schematics and working mechanism of a spring-assisted TENG device.Reproduced with permission.[61]Copyright 2017, Elsevier.c) Schematics and working mechanism of a matryoshka doll inspired HS-TENG.Reproduced with permission.[62]Copyright 2019, Elsevier.

Figure 8 .
Figure 8. TENG-based vibration and wind energy harvesting mechanisms.a) Schematics of design and energy harvesting of a TENG for harvesting general vibration energy.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/.[81]Copyright 2021, The Authors, published by MDPI.b) Schematics of energy harvesting mechanism induced by both vertical and horizontal vibrations.Reproduced with permission.[88]Copyright 2018, Wiley-VCH GmbH.c) Working principle of cotton-FEP TENG for harnessing energy from wind blow and water flow.Reproduced with permission.[89]Copyright 2022, Elsevier.

Figure 10 .
Figure10.Application scenario for ocean wave amplitude monitoring.a) 3D schematics of wave height monitoring experimental setup and TENG integrated buoy.Reproduced with permission.[102]Copyright 2022, Elsevier.b) Schematic design of FL-TENG and application in the marine environment.reproducedwith permission.[19]Copyright 2022, Elsevier.c) Design of unidirectional and anisotropic TENG integrated floating buoy.Reproduced with permission.[12]Copyright 2021, Elsevier.

Figure 11 .
Figure 11.Application scenario for ocean wave oscillation monitoring.a) TENG-driven ocean water fluctuation monitoring operation.Reproduced with permission.[104]Copyright 2022, Elsevier.b) The output of Gyro-structured TENG-EMG in the different surface oscillation of water.Reproduced with permission.[50]Copyright 2020, Elsevier.

Figure 13 .
Figure13.The application scenario for leakage monitoring.a) Full setup and working process of TENG-based ammonia-leakage monitoring self-powered device.Reproduced with permission.[111]Copyright 2022, Elsevier.b) Schematical model and voltage signal generation of BM-TES for bubble motionbased leakage and blockage detection.Reproduced with permission.[112]Copyright 2022, Springer Nature.

Figure 15 .
Figure 15.Application scenario of ship draft detection.a) Conceptual design and dynamic analysis of EC-TENG driven water level monitoring for ship draft detection.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/.[66]Copyright 2022, The Authors, published by Springer Nature.b) Conceptual working mechanism of bubble-like TENG for ship draft detection via water level monitoring.Reproduced with permission.[119]Copyright 2022, Elsevier.

Figure 16 .
Figure 16.Application scenario of ship tilt monitoring and attitude sensing.a) Conceptual design, working mechanism, and application of box-shaped TENG for ship attitude detecting.Reproduced with permission.[123]Copyright 2023, Elsevier.b) Self-powered DcCc-TENG driven ship tilt sensor with an alarm system.Reproduced with permission.[124]Copyright 2022, Elsevier.c) Schematics of TENG driven tilt sensor and its application in ship attitude monitoring.Reproduced with permission.[125]Copyright 2021, Elsevier.

Figure 17 .
Figure 17.Application scenario of vibration and position monitoring.a) Schematics and working modes of trapezoidal cantilever-TENG for lower frequency vibration detection.Reproduced with permission.[127]Copyright 2022, American Chemical Society.b) Schematics and application of TENG-EMG hybrid device for position tracking by sensing wave vibration.Reproduced with permission.[44]Copyright 2020, Elsevier.

Figure 18 .
Figure 18.Application scenario of oil condition monitoring.a) Illustration of oil detection mechanism of FO-TENG and contact angle with paraffin oil and deionized water.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/.[130]Copyright 2022, The Authors, published by Springer Nature.b) Illustration of MT-TENG unit integrated with floating pipeline in the ocean for oil contamination sensing.Reproduced under the term of CC-BY license https://creativecommons.org/licenses/by-nc-nd/4.0/.[58]Copyright 2022, The Authors, published by MDPI.

Figure 19 .
Figure 19.TENG-based marine equipment for different application areas.

Figure 22 .
Figure 22.TENG-integrated marine IoTs.a) Schematics architecture of AC/DC-TENG.Reproduced with permission.[177]Copyright 2020, American Chemical Society.b) Application of high-performance flag-type (HF) TENG.Reproduced under the term of CC-BY license https://creativecommons.org/ licenses/by-nc-nd/4.0/.[183]Copyright 2022, The Authors, published by MDPI.c) Rotary electrodeless TENG structure with collectors.Reproduced with permission.[184]Copyright 2018, Wiley-VCH GmbH.d) Design of a wind tunnel and architecture of a flutter-driven TENG with surface characteristics of a flexible flag.Reproduced with permission.[185]Copyright 2014, Springer Nature.e) A blue energy network integrated with TENG.Reproduced with permission.[142]Copyright 2017, Springer Nature.f) TENG integrated wind sensing system Reproduced with permission.[186]Copyright 2018, American Chemical Society.

Figure 25 .
Figure 25.TENG-integrated distress signal emitter.a) RF-TENG in ocean wave-powered LED light signal emission.Reproduced with permission.[55]Copyright 2015, Wiley-VCH GmbH b) Schematics of providing power for a lighthouse model via S-TENGs.Reproduced with permission.[25]Copyright 2021, American Chemical Society.c) Distress signal LED bulbs driven by the hybrid TENG embedded on a life jacket.Reproduced with permission.[215]Copyright 2014, Elsevier.d) TENG network-based self-powered ocean emergency SOS emitter (10 cm scale bar).Reproduced with permission.[218]Copyright 2014, American Chemical Society.

Figure 26 .
Figure 26.Future application prospects for TENG-based devices from the perspective of marine applications.

Table 1 .
Some recent works concerning energy harvesting from marine waves by incorporating TENG-based harvesters.

Table 2 .
Recent studies concerning TENG-based devices for monitoring various marine elements.

Table 3 .
Recent studies encompass TENGs for self-powered marine equipment development.