Triboelectric Nanogenerators for Energy Harvesting in Ocean: A Review on Application and Hybridization

: With recent advancements in technology, energy storage for gadgets and sensors has become a challenging task. Among several alternatives, the triboelectric nanogenerators (TENG) have been recognized as one of the most reliable methods to cure conventional battery innovation’s inadequacies. A TENG transfers mechanical energy from the surrounding environment into power. Natural energy resources can empower TENGs to create a clean and conveyed energy network, which can ﬁnally facilitate the development of different remote gadgets. In this review paper, TENGs targeting various environmental energy resources are systematically summarized. First, a brief introduction is given to the ocean waves’ principles, as well as the conventional energy harvesting devices. Next, different TENG systems are discussed in details. Furthermore, hybridization of TENGs with other energy innovations such as solar cells, electromagnetic generators, piezoelectric nanogenerators and magnetic intensity are investigated as an efﬁcient technique to improve their performance. Advantages and disadvantages of different TENG structures are explored. A high level overview is provided on the connection of TENGs with structural health monitoring, artiﬁcial intelligence and the path forward.


Introduction
Ocean energy is one of the most powerful energy resources in the environment. Waves and tides have a considerable amount of mechanical energy that can be harvested and used toward technological development [1][2][3][4][5][6][7][8]. However, harvesting the ocean's energy is a challenging task because the difference of ions inside the water may damage electronic devices [9]. During the last decade, triboelectric nanogenerators (TENG) have played a remarkable role in ocean energy storage development with numerous benefits [7,8,10]. For example, wind farms have usually been built according to electromagnetism systems and a turbine structure produces environmental noise which is classified as an ecological problem [11][12][13][14][15][16][17][18][19]. There are several limitations associated with wind farms: They need to be operated under high wind speed, their equipment is very large and the installation price is typically high [20][21][22]. On the other hand, the TENG can resolve some of the mentioned issues by performing well with lightweight equipment and a low vibration system [23], which allows its application in various situations [24][25][26][27][28][29][30]. TENGs have multiple benefits in the context of hybrid energy collection, for instance, the electromagnetism compound with TENG to produce energy based on the ocean waves [31,32]. The principles gional climate condition and the strategies needed to control them, these devices interact to generate more energy and reduce costs [112][113][114][115][116][117]. Many companies have invested in this technology and have set up a wave farm in different dimensions over the years [118][119][120][121][122].
Wave characteristics such as wavelengths and velocity at the water surface are fundamental features because they analyze how the waves break on the shores. Several countries including the United States, the United kingdom, Portugal and Australia operate wave farms and kinds of power take-off systems, including elastomeric hose pump, hydraulic ram, pimp-to-shore, hydroelectric turbine, air turbine and linear electrical generator.

Characteristics of Ocean Wave
The wave energy flux formula is formulated in deep waters where the depth of the water is more than half the wavelength. This equation represents the wave power in terms of the wave energy period, T and the significant wave height, H m0 , [123]: where P is the wave energy flux per unit of wave-crest length. ρ and g are water density and gravitational acceleration, respectively. On the other hand, the total potential of an ocean wave can be presented as [124]: where A is the wave amplitude. The average energy flux or wave power, P w , is calculated by multiplying the energy term, E, by wave propagation speed, V g = L 2T . In this formula, T is the wave period and L is the wavelength [125].
The dispersion relationship which describes the connection between the wave period and the wavelength, L = gT 2 2π , is combined with Equation (3) and results in [125]: or P w = ρg 2 TH 2 32π (4) where H is the wave height. Wave energy generation is an emerging commercial technology, in comparison with the other renewable energy sources. Since the 1890s, many countries located next to the oceans have been trying to harness this high-potential energy source [126]. In general, the energy harvesting devices from ocean waves can be classified according to their location (including shoreline, nearshore and offshore) and the power take-off system (including hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine and linear electrical generator) [127]. Similar to other renewable energy sources, the energy from the ocean waves is a significant and endless energy source. While it has been used in many countries like China, the United States, Scotland and Australia, there are several disadvantages associated with this type of energy. Table 1 summarizes the pros and cons of this energy source.

Advantage Disadvantage
Renewable: Unlike fossil fuels, which we see running out every day, the energy from waves is renewable and vast [128] Not applicable everywhere: Just like most natural resources, it is locationspecific. Thus, this type of energy is only useful for countries with access to the ocean and sea [128].
Environment Friendly: These days, energy production is a significant problem in the world. On the other hand, pollution from energy production is another concern that human beings pay special attention to because pollution from the production of fossil fuels causes global warming. In contrast, the energy from the waves is environmentally friendly [128].
Danger to the marine ecosystem: This type of energy poses a threat to aquatic habitats. Some of these devices fixed to the ocean floor, which can damage habitats and sometimes cause sea creatures to collide with turbine blades or even get electrocuted [129].
Easy access: The advantage of easy access arises for nations with borders along the coast with high wave intensity [128] Disruption of ship traffic: Energyefficient devices from the ocean waves located near shores and in the direction of the wave, a transit point for cargo ships and cruise ships. These devices make it a bit difficult to get around.
Technology growth in this area: : Many devices have been designed and implemented to extract energy in this [128].

Poor performance in stormy weather:
One of the problems with wave energy extraction devices is poor performance in rough weather, which can even cause severe damage.

Predictable:
One of the essential advantages of this type of energy is calculating the amount of energy production and its predictability.
Noise pollution: Another downside of these devices is the noise, which can significantly reduce real estate value for areas near the coast [128].
Less dependence on fossil fuels: It reduces dependence on fossil fuels. It can help reduce pollution around the globe by reducing the dependence on fossilgenerated energy [128].

Dependence on wind:
This wave category is driven by wind, so when the wind is not stable, it is not possible to extract significant energy.
Earth protection: Unlike fossil fuels that require deep drilling for extraction, there is no need to damage the earth to extract this type of energy [130] Figure 1a illustrates the absorption point device which works by floating on the water's surface. It is maintained by the cables connected to the seafloor. Generally, the perfect point absorber has the same features as a useful wave-maker. The system of absorption points is ocean-level floating structures whose structural physics is designed to have slightly horizontal dimensions relative to their vertical dimensions. The technology has been hailed by researchers working to obtain energy from ocean waves. When the vessel is excited by the waves (point absorption), it works so that the current moves relative to the fixed reference point. They use linear generators. At the top of this device, the degree of freedom glass considered to create the best necessary performance and also this device use the movement in the heave axis [131][132][133][134]. The wave energy converter(WEC) system has three main parts: Buoy, power take-off and heave plate. The buoy part consists of components placed in a series and a vertical direction, the most important hydraulic cylinder components and a spring. To prevent the device from drowning, the empty volume inside the device is filled with urethane foam. With this method, if water enters the system through the cracks in the device, it will not cause the system to drown. Another innovative feature of this device is that a urethane-shaped ring is attached to the device's outer wall, increasing buoyancy. The power take-off device uses a spring to maintain linear stress. The WEC's hydraulic system also uses four low-pressure valves to control the movement between the inlet and outlet of the end of the cylinder, which is installed at the outlet and before the flow limiter. The primary purpose of this placement is to control the pressure as a function of the current. The heave plate comprises a steel rod fastened to a steel plate. Cast press weights slide over the steel bar, allowing straightforward modification of the common heave plate mass. A parabolic bowl with a center hole slides over the center shaft and clamps onto the barbell weights. The reason for the heave plate is to supply a counter constrain to the buoy.  Figure 1b Shows the surface attenuators that are used to convert the energy from ocean waves. They typically move parallel to the ocean waves' direction or parallel to the movement of the ocean waves. In this system, two long pieces are usually connected. The attenuators rely on the flexibility of the joints to produce the power. Examples include surge, sway and heave. One of the most popular and used types of attenuators is WEC Pelamis [134]. Figure 1c demonstrates the oscillating wave surge converter (OWSCs) whose energy is produced using the movement of ocean waves. The structure of this device is made of a central arm or flap, such that, when ocean waves are created, these waves stimulate the designed arm. The flaps are located in a device connected to a generator and its operation is such that it acts as an arm for a large lever and rotates the generator, which generates electricity or, in other words, converts wave energy into electricity. It is possible. These devices are designed to be used frequently in shallow waters [134][135][136]. Figure 1d shows the air chamber located at the top of the water surface in the device configuration. In this system when a wave travels across the device, the water level rises and falls with standard performance. The system is designed so that the top of the compartment is a turbine as well as a duct. The primary function of the channel is to enter and exit the air. When the water level in the chamber drops, the pressure in the chamber also decreases, which results in a vacuum. In this case, the air comes from outside the device compartment to the inside. This process occurs back and forth. When the water level rises, excess air escapes from the designed turbine. This water movement puts pressure on the turbine blades, causing them to move and generate electricity [137][138][139][140]. Figure 1e presents an overtopping device, also called a stabilizer, that uses the ocean waves to generate energy. The device's function is to simulate the same wave motion seen on the shores and, through that simulation, the electricity generated. The Wave Dragon Project is an example of a conceptual design of an overflow hydraulic power plant. The mechanism of this device's operation is such that the two sloping obstacles are concentrated towards the center of the device. This is to optimize energy harvesting. As a result, spilled water moves the turbine to the center, which is a low-pressure turbine. After that, the spilled water is temporarily stored in a reservoir and returned to the ocean. The system acts as a floating marine power plant not connected to the shore. These types of devices are mostly located near the shores. In order to be most efficient, it adjusts its surface height to the height of the waves that occur in the ocean [141,142]. Figure 1f illustrates submerged pressure differential. This system produces energy based on the differential pressure of immersion in the ocean's depths. It is made up of flexible but amplified membranes to extract energy from ocean waves. The device generally uses the pressure difference under the wave at different locations to create a pressure difference in a liquid system that rises from closed power. This pressure difference directs the turbine, which results in the production of electrical energy. There are two different systems, one located near the coast and on the seabed that relies on pressure fluctuations. The other model is similar to point absorption but immersed in water, which floats back and forth in a wave motion, moving a linear generator to convert energy [143][144][145][146][147][148][149][150][151].
In summary, Table 2 compares and contrasts the advantages and disadvantages of the six above-discussed modern technologies for energy harvesting from the ocean waves.

Physics of Triboelectric Nanogenerators
According to Figure 2, several models have been proposed for the TENGs. The first category is the formal physical model, which is implemented according to the classical electromagnetic theory [152][153][154][155][156][157][158][159][160][161][162]. It should be noted that the 3D mathematical models and the distance-dependent electric field model are created according to the quasi-electrostatic model [141,[154][155][156]. In the second category, an equivalent electrical circuit model is notable. The circuit includes the CA model as well as the Norton equivalent circuit model [153][154][155][156].  It should be noted that both models are linked to each other. As it turns out, φ AB is a potential drop for the TENG system which appears in the left side of the equation. Moreover, V = ∂Q ∂t × Z presents the voltage across the external load (appearing on the right side). According to Kirchhoff's voltage law, the potential difference between two TENG electrodes is equal to the load resistance voltage. The final product is the transportation equation. The physics of TENGs is determination by the variation of potential, φ, electric field, E, polarization of the dielectric material, P and the Maxwell's displacement current, I D . The circuit models determine the outputs from the external circuit, e.g., variation of voltage, V, current, I, power P and extracted electrical energy, E [142].
According to Figure 2, Maxwell's equations, known as Wang's term, are added by the term P S [157]. It should be noted that Wang's term is not the result of moderate polarization due to the P electric field. Wang's term derives from the existence of electrostatic surface charges: The corresponding displacement current density, J D , is given by where ε 0 and ε are permittivity of free space (vacuum) and permittivity of the material (or medium), respectively. These two terms are connected as ε ≡ ε 0 (1 + γ e ), where γ e presents the electric susceptibility of the medium. Knowing that P = (ε − ε 0 )E, the volume charge density (Equation (7)) and the density of current density (Equation (8)) are defined by Satisfying the charge conversion and continuation equation [157]: As a result, Maxwell's equations are rewritten as [157]: It is noteworthy that the self-consistent equations mentioned above describe the relationships between electromagnetic fields and charges as well as the current distribution in TENGs [157], where: (ε∂E/∂t): Well-known contribution to Maxwell's displacement current.
(ε∂P s /∂t): Displacement current due to the presence of surface charges The equation mentioned in this section is very important. Because this equation is a link between the internal circuit and the external circuit. It is also noteworthy that, by calculating the surface integral J D , the displacement current I D is obtained [142,157,163].
The following results are based on the fact that Q is a free charge of the electrode [142]: • The displacement current is the internal driving force in TENGs. While conducting current, the received current is on the load. • Ideally, the conduction current is equal to the displacement current. • The conduction current and the displacement current form a complete loop in the TENG electrodes (where they are connected). Moreover, using formal physical and equivalent electrical circuit models, the TENG outputs are fully predictable [157].
In general, there are four main modes for TENG, each of them with unique features and benefits, see Figure 3, i.e., freestanding triboelectric layer mode, single electrode mode, lateral sliding mode and contact separation mode. The general basis of the TENG function, in all modes, is the transfer of electrostatic charges to the electrodes. All TENG modes, except single electrode mode, use two electrodes. When a displacement is applied to one of the TENG layers, the state is out of electrostatic mode and a potential difference occurs. The current from the external charge is driven by such a potential difference to balance the electrostatic state. It should be mentioned that moving in the contrary direction of the TENG layer will induce an inverse potential difference between the electrodes. Hence, by having a reciprocating motion, an AC output can be received from TENG [164][165][166]. The contact separation mode consists of two electrodes, which are located behind the TENG-layers. In this case, a potential difference occurs when the contact and separation processes takes place. The output voltage can be measured using a voltmeter by connecting it to one electrode and the other end to the other electrode. Then, periodically contact and disconnect operation and the output voltage can observed [167][168][169].
The lateral sliding mode consists of two electrodes, which, like the contact separation mode and freestanding triboelectric layer mode, are located behind the TENG layers. The TENG-layers' relative slip creates the lateral sliding mode, the state removed from the electrostatic state and a potential difference. In this case, using a simple voltmeter, the resulting voltage can be measured. It is enough to connect one end of the voltmeter to one electrode and the other end of the voltmeter to another electrode. With the reciprocating slip of the TENG layer, the voltage created by the potential difference can be seen [169]. One of the most significant drawbacks of the lateral sliding mode and contact separation mode is that both electrodes must have an output wire, limiting their applications. A solution to solve this problem is to provide a single electrode mode. In this case, only one electrode is used. When the TENG layer comes in contact with the electrode, the condition goes out of the electrostatic state and the potential difference causes an electric current [168][169][170][171][172][173].
In this case, known as freestanding triboelectric layer mode, the TENG layer is self-moving without connecting to the electrode and the two electrodes are spaced apart. When the TENG layer slips from the first electrode to the second electrode, it causes a potential difference. In this case, to display the output voltage, it is enough to use a simple voltmeter [174]. Connect one end of the voltmeter to one electrode and the other end of the voltmeter to the other electrode. Then, by moving the TENG layer back and forth from one electrode to another, the voltage can be display by the voltmeter [175]. Figure 4a demonstrates the trends of publications about the energy harvesting system base on the TENG. Once the TENG technology was discovered around 2012, many researchers turned to work in this field and the number of published articles has increased significantly. This can be considered a turning point in the field of mechanical energy harvesting [176]. Figure 4b shows various applications of TENGs. After a decade of working on nanogenerators, researchers have demonstrated many applications and potentials in this field. TENGs are not only useful in the production of hydropower and wind energy; they can also be used in medical, civil engineering (such as structural health monitoring (SHM) systems [177] and self-power sensors as an energy source for some structures like a bridge) and other fields to protect the environment and reduce fossil fuel production [176] (Note: Various images are adopted from [176,177]). Various images are adopted from [176,177] and photo by authors)

Spherical Triboelectric Nanogenerator Networks
Figure 5a illustrates blue energy, including solar panels, wind turbines and TENGs. This design provides a concept to combine all three sources of energy as networks. However, the main problem is transporting the voltage and power to a port to be used in the city. On the other hand, this structure must be fixed to islands or underwater mountains to prevent its movement and protect marine life [104]. Figure 5b illustrates the split ball-shell structured TENG principle with silicone rubber balls and outer shells. By rotating, silicone rubber balls from one layer to another can transfer electrons and generate a high and efficient voltage from ocean waves. This design is a network that can be located on the surface of the water [178]. Figure 5e also supports this concept with different materials and different connections [19]. Figure 5c shows spherical TENG networks with different operations. In this design, when the ocean waves come from any direction, the pendulum can rotate freely and transfer electrons between layers to harvest energy [179][180][181]. Figure 5d illustrates another concept of spherical TENG networks with a different structure. The principle of this design is based on the movement of polyacrylate balls between several layers of spherical TENG [182]. The main advantage of the all-spherical design is that it can work in any ocean wave direction. (b) Design and working principle of the split ball-shell structured TENG with silicone rubber balls and outer shells [178].
(c) Principle of the ES-TENG networks for collecting energy from ocean waves [179]. (d) Theory of the spherical TENG with dense point contacts [182]. (e) Freestanding-triboelectric-layer nanogenerator's design and working principles (RF-TENG) with a rolling nylon ball enclosed [19]. Note: Various images are adopted from [19,104,178,179,182].

Spring-Assisted Triboelectric Nanogenerator
TENG without help from other components such as spring and magnet are more productive for harvesting energy. Nevertheless, ocean waves have a lower frequency, which may cause some difficulties in generating a very restricted electrical energy. Some gadget TENGs use springs to collect unstable dynamic energy that perform poorly concerning high-frequency oscillations and enable energy transformation efficiency. Figure 6a presents a conceptual framework of tandem TENG with a cascade impact structure (CIT-TENG) for generating energy from different layers of TENG (TENG-1, TENG-2, TENG-3 and TENG-4) with various frequencies for each TENG layer. The tribo-layer used in this design is PTEF and the electrode is aluminum. In this design, having a small vibration, all TENG layers move with different frequencies and generate energy based on the contact mode of TENGs [183]. Figure 6b presents a TENG built on a suspended 3D spiral structure that is assisted by mass and spring. In this method, the frequency can be increased by changing the mass and spring values to develop output voltage from ocean waves. Contact separation mode is also used in this structure. A triboelectric layer is Kapton and the electrode is aluminum [23]. Figure 6c illustrates the working principle of a spring-assisted TENG based on the vertical contact separation mode. In this method, the spring is used to increase the high-frequency oscillations and generate more dynamic energy. In this structure, the triboelectric layer is PTFE and the electrode is copper [184][185][186].  (b) Principle of TENG built on a suspended 3D spiral structure [23]. (c) Principal and fabrication of spring-assisted TENG device with process for generating negative charges on the surface of PTFE [184]. Note: Various images are adopted from [23,183,184].

Liquid-Solid Interfacing Triboelectric Nanogenerator
TENGs based on liquid-solid contact have numerous benefits such as increasing output voltage from ocean wave energy, improving contact area between the FEP film and water and having a simple structure. Figure 7a shows the conceptual framework of a liquid-solid electrification-enabled generator (LSEG). In this design, the structure will be located in front of the wave and when the wave is contacted, water will come up and water will cross from one layer to another to transfer the electron [27,[187][188][189][190][191]. Figure 7b presents the structure of a novel wave sensor based on a liquid-solid interfacing triboelectric nanogenerator (WS-TENG), which is useful for structural health monitoring of marine equipment. The triboelectric layer used in this design is PTEF and the electrode is copper. The principle of this method is based on the water which can come up and the water will cross from one layer to another for electron exchange [192]. As shown in Figure 7c, this structure can harvest energy by the network of the liquid-solid-contact buoy TENGs from ocean waves. The innovation of this design includes two external and internal layers of TENG, which can generate energy from the shaking and rotating movement of waves [26]. Figure 7d illustrates the mechanism of droplet-based TENGs for wave energy harvesting (DB-TENG) for marine vehicles. The principle of this design is also based on the water. The triboelectric layer used in this structure is FEP and the electrode is copper [193,194].  [192]. (c) Structural design of the blue energy harvested by the network of the liquid-solid-contact buoy TENGs [26]. (d) Working mechanism of droplet-based TENG for wave energy harvesting (DB-TENG) [193]. Note: Various images are adopted from [26,27,192,193].

Hybridization of TENG with Other Energy Harvester Systems
So far, the concept of TENG, its applications, mechanism and different modes have been discussed. However, it is always challenging to improve the efficiency of TENGs by combining them with other energy-generating systems. The resulting hybrid system will benefit from advantages of both systems. In this section, the TENG is hybridized with solar cells, electromagnetic systems and some others. Figure 8a describes a high output piezo/triboelectric hybrid generator. This hybrid generator merges triboelectric output voltage and a high piezoelectric output current, which generates a peak voltage of ∼370 V, a current density of ∼12 µA cm −2 and the average power density ∼4.44 mW cm −2 . The amount of power strongly turned on 600 LED bulbs by applying a mechanical force of 0.2 N and it is able to charge a 10 µF capacitor to 10 V during 25 s [195].  [195]. (b) Structure design of the wave-shaped hybrid piezoelectric and TENG based on P(VDF-TrFE) nanofibers [196]. (c) Schematic illustration and characterizations of the PTNG devices based on ZnO nanoflakes/polydimethylsiloxane composite films [197]. (d) Structural configuration of the generator for low-frequency and broad-bandwidth energy harvesting [198]. Note: Various images are adopted from [195][196][197][198].

Hybridized Piezoelectric and TENG
In Figure 8b, a wave-shaped hybrid piezoelectric and TENG is illustrated founded on P(VDF-TrFE) nanofibers. In this technique, a piezoelectric P(VDF-TrFE) nanofiber is sandwiched between two wave-shaped Kapton films to form a three-layer pattern. This structure produces piezoelectric triboelectric outputs concurrently in one press and release cycle. Within well-organized experimental validation and situational analysis, the threelayer fabrication can produce noticeable output performance for both parts. Meanwhile, triggered with 4 Hz external force, the piezoelectric part produces a peak output and current of 96 V and 3.8 µA, which is ∼2 times higher than its first output. While the execution of triboelectric parts increases 8 V and 16 V with the support of piezoelectric potential [196]. Figure 8c presents ultrahigh output piezoelectric and triboelectric association nanogenerators based on ZnO nanoflakes/polydimethylsiloxane composite films. There is a hybrid nanogenerator (NG) utilizing both piezoelectric and triboelectric effects influenced from ZnO nanoflakes (NFs)/polydimethylsiloxane (PDMS) composite films by a facile, costeffective fabrication design. This hybrid NG showed high piezoelectric output current owing to the improved surface piezoelectricity of the ZnO NFs, including high triboelectric output voltage owing to the notable triboelectrification of Au-PDMS contact, generating a voltage of ∼470 V, a current density of ∼60 µA. cm −2 and an average power density of ∼28.2 mW. cm −2 . The hybrid NGs with a space of 3 × 3 cm 2 immediately turned on 180 green light-emitting diodes within cyclic control compression [197].
In Figure 8d, a hybrid piezoelectric-triboelectric generator is depicted for low-frequency and broad-bandwidth energy harvesting. The generator is created using a piezoelectric energy harvester (PEH) patch, a TENG patch, a spring-mass method and an amplitude limiter. The spring-mass method takes energy from applied excitation and utilizes forces to the piezoelectric component and the triboelectric films. The novel amplitude limiter is deliberately organized inside the system, obtaining the effect of favorable frequency up-conversion increasing the voltage responses considerably. Moreover, the limiter creates hardening nonlinearity and dynamic bifurcation triggering super harmonic resonance due to resonation concerning the generator at a cycle of approximately 3 Hz. The implemented PEH adopts the extreme compressive performance mode and employs a truss mechanism to amplify the impact forces effectively. The open-circuit voltages following excitation of 1.0 g at resonance are 58.4 V and 60 V from PEH and TENG, respectively. The hybridized generator produces the highest power of 19.6 mW from two sources by matched impedances. The working bandwidths of the PEH and the TENG are increased to 5.39 Hz and 7.25 Hz, respectively. Meanwhile, the applications to charge capacitors, the high saturation voltage and the approximately short charging time validate the effectiveness of the power management technique. Besides, the generator is useful to efficiently scavenge energy from human body actions and charge a capacitor of 4.7 µF to 7.6 V in approximately 50 s. It shows a high potential of functional usages in wearable mechanisms [198]. Figure 9a describes integrating a TENG with a silicon solar cell by an electrode for harvesting energy from sunlight and raindrops. A heterojunction silicon (Si) solar cell is combined with a TENG using a mutual electrode of a poly (3.4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) film. The solar cell, printed PEDOT:PSS is applied to decrease light reflection, which begins to improve short-circuit current density. A single-electrode-setup water-drop TENG on the solar cell is made with merging imprinted polydimethylsiloxane (PDMS) as a triboelectric material merged with a PEDOT:PSS layer as an electrode. The expanding contact section among the imprinted PDMS and water drops considerably increases the voltage output of the TENG with a peak short-circuit current of ∼33.0 nA and a peak open-circuit voltage of ∼2.14 V, sequentially [67]. Figure 9b illustrates biomimetic anti-reflective TENGs for concurrent harvesting of solar and raindrop energies. A moth's eye mimicking TENG (MM-TENG) can perform the role of equivalent energy harvester to a standard solar cell due to its higher specular transmittance (maximum 91% for visible light). At the first time, strongly examine the visible effect of the MM-TENG on a solar cell by considering solar-weighted transmittance (SWT). The 0.01% developed SWT in the MM-TENG enhances the fill part and power transformation efficiency of the solar cell by 0.5% and 0.17%, respectively, compared with a traditional protective glass plate that constantly utilized a solar panel. Moreover, in addition to such prominent high transmittance, the self-cleaning property of the MM-TENG enables the long-term production of the solar panel. Besides, this design summarizes a unique electric circuit for effective control in a hybrid energy harvester through intermittently transferring the preserved electrical energy output of the MM-TENG [121]. Figure 9. Various structures of integrating TENG with solar cells: (a) Schematic of a working mechanism of integrating a silicon solar cell with a TENG via a mutual electrode for harvesting energy from sunlight and raindrops [67]. (b) Structure design of the biomimetic anti-reflective TENG for concurrent harvesting of solar and raindrop energies [121]. (c) Schematic diagram of the hybrid energy system with the SH-TENG and solar cell [199]. (d) Structural configuration of the selfcleaning hybrid energy harvester to generate power from raindrop and sunlight [105]. Note: Various images are adopted from [67,105,121,199]. Figure 9c shows a hybrid energy system based on solar cells and a self-healing/selfcleaning TENG. This self-healable and transparent TENG presents an outstanding output execution on a rainy day with a peak short-circuit current of 0.8 µA and a peak open-circuit voltage of 6 V, separately. At the same time, it can work very strongly beside the solar cell on sunny days. Moreover, the self-healing TENG with elastic composition can function as a protection film for the solar cell, where the of the solar cell's risk of breakinfg can be significantly overcome [199]. Figure 9d shows a self-cleaning hybrid energy harvester to generate power from raindrops and sunlight. The transparent TENG made with superhydrophobic PDMS and ITO/PEN substrate. The amount of voltage current was 7 V and 128 nA, separately.

Hybridized Solar Cells and TENG
The highest output power is 0.27 µW. Additionally, output components of water TENG are studied with several solutions such as natural rainwater and flowing water to investigate the sensible potential of water TENG in harvesting actual raindrop energy [105]. Figure 10a demonstrates a self-powered and self-functional tracking system-based triboelectric-electromagnetic hybridized blue energy harvesting module. A sophisticated designed rotating gyro structure, including a triboelectric-electromagnetic working principle, strongly established a battery-less tracking method. The unique gyro rolling mode solution of the TENG features its sensitivity, multiple directions and robustness. Eventually, the triboelectric-electromagnetic hybridized module was completely confirmed autonomously within the Huanghai Sea project [200]. Figure 10b shows a hybridized triboelectric-electromagnetic water wave energy harvester (WWEH) based on a magnetic sphere. A freely rolling magnetic sphere senses the water movement to drive the friction object sliding on a solid surface for TENG back and forth. Simultaneously, two coils convert the motion of the magnetic sphere into electricity based on the electromagnetic induction effect. Harvesting the blue energy from any place, the electrodes of the TENG are determined as the Tai Chi form, the effectiveness of which is analyzed and described. According to experimental results, the two friction films and two coils are given in parallel and series connection. A paper-based super-capacitor of ∼1 mF is fabricated to save the generated energy. The WWEH is located on a buoy to examine in Lake Lanier. The super-capacitor can be charged to 1.84 V and the electric energy storage approximately 1.64 mJ during 162 s [201]. Figure 10c presents the complementary electromagnetic-triboelectric active sensor (ETAS) for detecting multiple mechanical triggerings. This work displays a combined ETAS for simultaneous detection of various mechanical triggering signals. The high-grade combination of a contact-separation mode TENG and an electromagnetic generator (EMG) recognizes the complements of their incomparable benefits. The logical consideration of EMG and TENG analysis are presented to explain the relation of output and the external mechanical signals. Corresponding to the experimental outcomes, the output voltage of the TENG part recognizes the magnitude of the external triggering force with a sensibility of around 2.01 VN −1 , while the output current of the EMG part is more suitable to showing that the triggering speed and the sensation is approximately 4.3 mA s/m. Furthermore, both the TENG and EMG parts display high stability after 20,000 cycles of force loadingunloading [202]. Figure 10d demonstrates an ultra-low-friction triboelectric-electromagnetic hybrid nanogenerator for generating energy as a self-powered sensor. A freestanding mode TENG and a rotating EMG are combined to recognize unique merits. The principal is based on contact between soft and flexible triboelectric elements in the TENG outputs with low friction force. The impressions of the model and the dimensions of the dielectric material on the performance of the TENG are regularly considered in hypotheses concerning experiments. According to the results, voltage output is improved by the rotation velocity, which is very different from a standard rotary TENG and is due to the contact area's increase. The optimized TENG has a maximal load voltage of 65 V and maximal load power per unit mass of 438.9 mW/kg under a velocity rotation of 1000 rpm. The EMG has a maximal load voltage of 7 V and a maximal load power density of 181 mW/kg. This illustrates that the hybrid NG can power various sensors such as humidity and temperature by converting wind energy within electric energy during wind speeds of 5.7 m/s. In contrast, it can be utilized as a self-powered wind velocity sensor to recognize wind rates as low as 3.5 m/s [203]. principle of a self-powered and self-functional tracking system based on a triboelectric-electromagnetic hybridized sort [200]. (b) Structure and illustration of the working principle of the hybridized triboelectric-electromagnetic water wave energy harvester (WWEH) [201]. (c) Structure design of the hybridized EMG-TENG active sensor [202]. (d) Working principle of an ultra-low-friction triboelectric-electromagnetic hybrid nanogenerator for rotation energy harvesting and self-powered wind speed sensor [203]. (e) Structural diagram of the pendulum hybrid generator used in a hydrophone-based system with the scanning electron microscopy (SEM) image of the silicone film [204]. (f) Schematic diagrams of the FSHG internal structure and SEM image of surface microstructure with the charge generation process in TENG modules [205]. Note: Various images are adopted from [200][201][202][203][204][205]. Figure 10e illustrates a pendulum hybrid generator for water wave energy harvesting and hydrophone-based wireless sensing. The aimed pendulum fabrication can harvest irregular water wave energy from random directions sensitively. Combining a freely rolling mode TENG and a magnetic sphere-based EMG produces complementary benefits and harvests wave energy in a wide frequency range. The hybrid generator is shown to drive 177 LEDs and power electronic equipment. At a wave driving rate of 1.4 Hz, the output power of the EMG and TENG is 6.7 mW and 8.01 µW, sequentially. A capacitor in this design can be charged to 26 V by the hybridized generator in 200 s by 1.8 Hz [204]. Figure 10f shows a 3D full-space triboelectric-electromagnetic hybrid nanogenerator toward high-efficient mechanical energy harvesting in a vibration method. The output voltage of the field test demonstrated the performance of TENG and EMGs, which can be influenced by the direction of external vibration and excitation frequency. The result of output voltage of TENG and EMGs develops as the excitation frequency rises. The outcome explains that the maximum output power of TENG is 18 mW at an external loading resistance of 200 MΩ and the maximum output power of EMG is 640 mW at an external loading resistance of 1000 Ω. The FSHG displays a fast-charging capacity for capacitor and the capability to power hundreds of LEDs. After saving energy within the capacitor, the DC signal can power a humidity/temperature sensor [205]. Figure 11a demonstrates a self-powered multi-functional motion sensor enabled by a magnetic-regulated TENG. Typically, a self-powered multi-functional movement sensor (MFMS) is aimed in this design to detect the motion parameters such as area, velocity and acceleration of linear and rotary movements concurrently. The MFMS is made of a TENG module, a magnetic regulation module and an acrylic shell. The mode of TENG used in this structure is free standing with a polytetrafluorethylene (PTFE) plate and six copper electrodes. The working mechanism of the MFMS design is based on the sliding of the MD on the PTFE plate for electron exchange between layers. The precisely designed six copper electrodes can detect eight directions of action with the acceleration and determine the rotational velocity and direction.

Hybridized TENG with Magnetic Intensity
Furthermore, the magnetic regulation module is utilized here by fixing a magnetic cylinder (MC) in the shell, right below the center of the PTFE plate. Because of the magnetic attraction utilized by the MC in this design, the MD will automatically return to the center to prepare for the next discovery round, making the proposed sensor much more suitable for practice [206]. Figure 11b illustrates wind energy and blue energy harvesting according to the magnetic-assisted noncontact TENG. An innovative approach to wind and blue energy harvesting based on a magnetic-assisted noncontact TENG has been described. With the compound of the magnetic responsive composite with the TENG design, the wind and water forces could be transformed toward the contact-separation movement between Al/Ni electrode and PDMS film. The influence of the related parameters (contact-separation frequency, wind speed and humidity) on the performances of the fabricated TENG has been regularly examined. The results explain the strong potential of magnetic-assisted noncontact TENG for wind and blue energy harvesting purposes [207]. Figure 11c shows a floating oscillator-embedded triboelectric generator (FO-TEG) for versatile mechanical energy harvesting. The FO-TEG is appropriate for impulse excitation and sinusoidal vibration, which are in the natural ecosystem. For the impulse excitation, the created current sustains and moderately decays by the residual oscillation of the floating oscillator. Resonance oscillation maximizes the output energy for sinusoidal vibration. The acting frequency range can be optimized via a high degree of freedom to satisfy different application requirements. Furthermore, the highest resistance to ambient humidity is empirically illustrated, which stems from the inherently packaged fabrication of FO-TEG. The prototype design presents a peak-to-peak open-circuit voltage of 157 V and an instantaneous short-circuit current of 4.6 µA, within sub-10 Hz of operating frequency [208].  [206]. (b) Demonstration and schematic illustration of magnetic-assisted noncontact TENG for harvesting energy from wind and water flow (four devices arranged with perpendicular angle) [207]. (c) Schematic illustration of the FO-TEG structure. The FO-TEG contains a tube part with a mobile oscillator part floating inside, suspended by magnetic repulsive forces. Vertical nanowire-like structures are formed on the sidewalls of the PTFE charging layer [208]. Note: Various images are adopted from [206][207][208]. Figure 12a demonstrates a magnetic switch structured TENG for continuous and regular wind energy harvesting. A magnetic switch structured triboelectric nanogenerator (MS-TENG) is developed, consisting of transmission gears, energy modulation modules and a creation unit. Meanwhile, wind falls intermittently on the wind scoop; the energy collected and released through the energy modulation do not depend on wind velocity. However, the magnetic force of the magnets allows the wind energy to be changed into continuous and normal electric energy. The experimental outcomes show that the MS-TENG can succeed as a power supply, generating output characteristics of 410 V, 18 µA, 155 nC and peak power of 4.82 mW, satisfactory to power 500 LEDs in series [209].  [210]. (c) Schematic illustrating the structure of assembled and digital photo of MR-TENG [211]. Note: Various images are adopted from [209][210][211]. Figure 12b explains a multi-functional sensor according to a translational rotary TENG. A cylindrical self-powered multi-functional sensor (MS) with a translational-rotary magnetic mechanism aims to recognize acceleration, force and rotational parameters. The MS can convert a translational movement into a swing motion or a multi-cycle rotational movement of a low damping magnetic cylinder around a friction layer, therefore driving the TENG to produce voltage output. To augment the TENG's output performance, an electrode material with a small work function, low resistance and suitable surface topography is the best choice. Based on the fabrication characteristic of the translational rotary magnetic mechanism, the MS can simply respond to a low striking and can measure the rotational parameters without the need for coaxial installation. According to the MS, some applications are installed [210]. Figure 12c shows a novel TENG relying on magnetically induced retractable spring steel tapes (MR-TENG) for efficient energy harvesting of large amplitude periodic motion.
An ingenious design utilizes a new material. The tape-like fundamental design ensures that the contact/separate direction of the friction layers is straight concerning the force direction, breaking the amplitude limitation of prior nanogenerators with vertical contact/separate movement. Conjunction of flexible spring steel tapes makes the structure of design enable portability, therefore widening its application. The outcomes present that the maximum short-circuit current, open-circuit voltage and instantaneous power are 21 µA, 342 V and 1.8 mW, sequentially [211]. Figure 13 illustrates the advantages and disadvantages of various design strategies for TENGs, including spherical TENG networks, spring-assisted TENGs, liquid-solid interfacing TENGs, hybridized piezoelectrics and TENGs, hybridized solar cells and TENGs, hybridized TENGs with electromagneticm and finally hybridized TENGs with magnetic intensity.

Spherical TENG Networks
Spring-assisted TENG It can harvest all kinds of mechanical energy, especially at low frequencies in the ocean waves.
Generating high output voltage based on the increasing frequency.

Spring fatigue and its impact on longterm performance
The effect of magnetic field on the life of marine organisms.
The effect of magnetic field on the life of marine organisms.

Applications, Challenges and Future Trends of TENGs
The quest to find new (and also renewable) energy resources has always been a key point in modern history. Traditional sources like coal and oil are associated with a lot of pollution, as well as climate change. Therefore, the majority of recent efforts have been concentrated on clean energy resources. In the last decade, one of the most significant human discoveries in this field is the triboelectric nanogenerator. As discussed in Figure 4b, TENG has a broad domain of applications in human life, from medical sciences to engineering and technology.
Maybe two of the most interesting applications for TENG can be found in the civil engineering field. The first one is associated with the SHM of large bridges. For such infrastructure, not all parts are easily accessible by the technicians to install the sensors for damage detection purposes. These sensors require a continuous source of energy to be able to detect the vibrations, analyze them and transmit data to the center. Since the installation and maintenance of the batteries for these sensors are very difficult, an alternative can be to use a TENG as an energy source. On the other hand, TENGs can be used even in a larger scale to supply the required energy for smart cities.  Figure 14 shows the challenges and future trends related to TENGs. In January 2012, professor Zhong Lin Wang introduced the principle of another type of TENG, which can also harvest ambient mechanical energy by combining contact-electrification and the electrostatic phenomenon. The TENG concept has been expanded to biodegradable TENGs, wireless TENGs and blue energy and more recently in smart cities. TENGs also have a bright future in robotics and self-powered sensing technologies when they are combined with artificial intelligence (AI). TENGs have been proposed in ubiquitous computation, which presents the potential of applying them to increase an entire sensor network. Table 3 presents the performance output comparison of various TENG devices to collect ocean wave energy such as spherical TENG networks, spring-assisted TENG, liquidsolid interfacing TENG, hybridized piezoelectrics and TENGs, hybridized solar cell sand TENGs, electromagnetic hybridized TENGs and hybridized TENGs with magnetic intensity. Most of the electrodes used in TENG are Al and Cu and also most of the triboelectric layers are PTFE.

Summary
This paper reviews and summarizes the concept of triboelectric nanogenerators (TENGs) for energy harvesting purposes from ocean waves. This review covers the TENGs from the fabrication of the design to the working principle, the recent improvements and various applications for ocean waves energy. The TENG technology that aims at numerous environmental energies is extensively utilized in the energy storage unit, which can be useful for self-power sensors. Furthermore, conjunction nanogenerators of a TENG and various types of power sources have been discussed for ocean as an innovative technology approach. Through this study, the emerging TENG technology has displayed a high potential in several fields from medical science to engineering.
It is expected that, with the current growth in TENG technology, as well as its hybridization with other energy sources, more efficient applications could be identified in the near future. This study provided a condensed (yet comprehensive) review of the current improvements in TENGs and the relevant technologies. First, we present the characteristics of ocean wave energy, followed by the underpinning principles of TENGs. Next, the function of hybridization of TENGs with another type of energy harvester and the various structural designs of TENGs as energy harvesters have been discussed. In addition, future improvements in TENG technology including a self-powered sort combined with sensors and actuators have been studied. Finally, the expansion of TENGs has been summarized and future challenges and opportunities have been briefly explained. Low motion frequency is the most important characteristic of ocean energy, so it can be considered an energy harvester application based on the TENG. Figure 15 shows a summary of the cloud words used in this review paper highlighting the most frequent words.

Conflicts of Interest:
The authors declare no conflict of interest.