Recent Progress in Blue Energy Harvesting Based on Triboelectric Nanogenerators

: This paper reviews and summarizes recent progress in blue energy harvesting based on a triboelectric nanogenerator (TENG). This review covers TENG-based blue energy harvesters (BEHs) with different inertial units in spherical structures, derivative spherical structures, buoy structures, and liquid–solid contact structures. These research works have paved the way for TENG-based BEHs working under low-frequency waves and harvesting wave energy efﬁciently. The TENG-based BEH unit design and networking strategy are also discussed, along with highlighted research works. The advantages and disadvantages of different TENG structures with other inertial units are explored and discussed. Meanwhile, power management strategies are also mentioned in this paper. Thus, as a promising blue energy harvesting technology, the TENG is expected to signiﬁcantly contribute to developing low-cost, lightweight, and high-performance BEHs supporting more frequent marine activities.


Introduction
The ocean, covering over 70% of the earth's surface, is the most significant carbon sink and acts as a vital buffer against the impacts of climate change.Moreover, it is one of the most widely available renewable energy sources.It has attracted extensive attention from energy harvesting studies like extracting electricity from waves, tides, ocean currents, temperature differences, and salinity gradient energy [1][2][3][4].Waves are the most powerful energy carriers in these renewable energy sources, given by the advantages of large energy reserves, high density, and wide distribution [5,6].It is estimated that the wave power resource is at 2.11 ± 0.05 TW globally, of which 4.6% is extractable with wave energy converter (WEC) configuration [7].Notably, the power from the ocean is collectively known as "Blue energy", and corresponding energy converters are also called "Blue energy harvesters" [8].Conventional blue energy harvesters (BEHs) are designed with stiff structures like oscillating water columns (OWCs), oscillating bodies (OBs), or over-topping devices.The pivotal components, a power take-off (PTO) system based on electromagnetic generators (EMGs), are installed inside to transmit the wave excitation from the oscillating body to the electric generator.These BEHs are challenged with cost, weight, efficiency, and reliability issues.
With the increasing human activities in the ocean and the development of Marine IoT, power-intensive demands are evolving into power distribution demands.Furthermore, a large-scale BEH-based power plant is less cost-efficient for the increasing number of distributed Marine IoT applications.Given the challenges mentioned above, Wang's group has proposed the entropy theory of distributed energy and a solution of harvesting random energy like irregular ocean waves to supply remaining distributed electronics away from ordered power plants [9][10][11].The triboelectric nanogenerator (TENG), invented by Wang's group in 2012, presents the merits of being lightweight and having a high power density, cost effectiveness, easy fabrication, and versatile material choices [12][13][14][15].Meanwhile, it has attracted global attention from blue energy harvesting studies [16][17][18].The TENG is a novel blue energy harvesting technology that shows excellent advantages over the EMG under low-frequency motion (<5 Hz) [19,20].It can generate electricity based on triboelectrification and electrostatic induction coupling, either from wave-driven solidsolid interactions or wave-involved liquid-solid interactions [15].Thus, the growth studies of the BEH based on the TENG are blowout-type and promote corresponding technological progress into commercial applications [21,22].
In this paper, recent progress in blue energy harvesting based on the TENG is reviewed and summarized.This review covers TENG-based BEHs with different inertial units, derivative spherical structures, buoy structures, and liquid-solid contact structures.The TENG-based BEH unit design and networking strategy are also discussed, along with highlighted research works.These research works have paved the way for TENG-based BEHs working under low-frequency waves and harvesting wave energy efficiently.Lastly, existing problems, potential solutions, and further research directions are discussed under commercial application considerations.

History and Development of Applying TENG in Blue Energy Harvesting
Since invented by Wang's group in 2012, the TENG has become an energetic part of Wang's science tree [23,24].Maxwell's displacement current is the driving force for the TENG's conversion of blue energy into electric power, like the "root" of the science tree [15,25].As the surfaces of the two dielectric materials are in contact and charged, this results in equal charge densities.The subsequent separation of these friction materials triggers the phenomenon of electrostatic induction.This leads to a localized distribution of opposite charges on the two surfaces, creating an electric field.This electrostatic induction activates the charge separation, resulting in a potential difference between the two materials.By connecting the negative dielectric material to the electrodes, electrons are repelled and flow to the electrodes, resulting in the formation of alternating current on the outside.Eventually, the system reaches a new state of equilibrium.The different states keep going back and forth, creating a cycle.Maxwell's displacement current serves as the driving force behind the TENG's efficient conversion of mechanical energy into electric energy, and this basic physics mechanism is applied to explore source quantities and field quantities based on mathematical-physical models [26][27][28].The TENG has four fundamental working modes for application in BEHs, including the contact-separation mode [29][30][31][32], sliding mode [33,34], single-electrode mode [35,36], and freestanding triboelectric-layer mode [37,38].These working modes have been detailed and quantitatively analyzed based on the capacitor models and discussed with surface charge density influenced by the triboelectric effect [39][40][41][42][43]. As illustrated in Figure 1, typical BEHs are encapsulated in spherical shells to restrain adverse effects from the ocean environment and respond to all-direction ocean waves, as highlighted in the "blue energy dream".As shown in Figure 1a, millions of spherical balls of based TENG units are connected as fishing nets and move with waves [44,47].These BEHs use cheap conventional materials and convert the water wave energy into electricity by rolling a dielectric ball inside a spherical shell.The concept of the "blue energy dream" has been demonstrated by connecting 400 nanogenerators over 4 square meters.The generated power can be applied to produce hydrogen fuel [48], power navigation systems As illustrated in Figure 1, typical BEHs are encapsulated in spherical shells to restrain adverse effects from the ocean environment and respond to all-direction ocean waves, as highlighted in the "blue energy dream".As shown in Figure 1a, millions of spherical balls of based TENG units are connected as fishing nets and move with waves [44,47].These BEHs use cheap conventional materials and convert the water wave energy into electricity by rolling a dielectric ball inside a spherical shell.The concept of the "blue energy dream" has been demonstrated by connecting 400 nanogenerators over 4 square meters.The generated power can be applied to produce hydrogen fuel [48], power navigation systems [49], or remove pollutants [50].
Specific to individual devices, the first TENG-based BEH unit, as shown in Figure 1b  As shown in Figure 2, spherical structural BEHs have been improved by introducing different ball inertial units.In Figure 2a, Cheng et al. reported a soft-contact spherical TENG (SS-TENG)-based BEH using a flexible water/silicone ball that significantly increased the contact area.Constructed soft contact between the soft ball and two copper electrodes resulted in a 10-fold increase in the maximum output charge compared to conventional PTFE-based hard contact during the rolling process.Another soft ball taking the softness and friction force into account was applied in harvesting waves, resulting in a 2- As shown in Figure 2, spherical structural BEHs have been improved by introducing different ball inertial units.In Figure 2a, Cheng et al. reported a soft-contact spherical TENG (SS-TENG)-based BEH using a flexible water/silicone ball that significantly increased the contact area.Constructed soft contact between the soft ball and two copper electrodes resulted in a 10-fold increase in the maximum output charge compared to conventional PTFE-based hard contact during the rolling process.Another soft ball taking the softness and friction force into account was applied in harvesting waves, resulting in a 2-fold increase of the maximum output charge under the same comparison.In Figure 2b, Xu et al. also used soft balls to enhance the contact area, in which the silicone rubber ball was given UV treatment and the inner silicone rubber surface of the spherical shell was roughened with a polyformaldehyde (POM) particle [52].Attributed to achieving a less sticky intersurface after treatments, soft balls could smoothly roll with slow water waves and effectively generate electricity signals.
Moreover, Xu et al. developed a coupling design in TENG networks with string connection, which proved that the unit's charge output had increased 10.8 times and the network's charging performance was much better than the rigid connection for extra internal degrees of freedom.In Figure 2c, Yang et al. improved the spherical structural BEHs by applying multiple ball inertial units [53].Different from a single ball rolling on the inner shell surface, a group of 3D electrode plates was fabricated for Fluorinated ethylene propylene (FEP) pellet units freely moving inside the spherical shell.Rotatable self-adaptive magnetic joints (SAM-joints) mounted on the outside of the shells were utilized to form the network's nonrigid connection and ensured self-healing capability in extreme ocean environments.A network of 4 × 9 arrays, including 18 TENG units, generated a peak power of 34.6 mW and an average power density of 2.05 W m −3 in the wave experiment.
As for multiple ball inertial units for spherical BEHs, Yuan et al. reported another spherical BEH made of polyacrylate (PA) balls and a group of thin FEP film-based electrode plates, which achieved a volume power density of 6.9 W m −3 at 0.8 Hz [55].Pang et al. proposed a matryoshka-inspired hierarchically structured TENG by nest-assembling multiple shells with decreasing sizes, in which PTFE balls were rolling on inner shell surfaces under wave excitation [56].As illustrated in Figure 2d, Pang et al. reported a hybrid nanogenerator (TEHG)-based BEH consisting of a soft ball-based TENG (SB-TENG) and an EMG [54].Therefore, the SB-TENG was improved by using the Ecoflex B component as the filled liquid in silicone rubber balls.The EMG was composed of a sliding magnet and a fixed copper coil; thus, it could effectively respond to wave excitation.In contrast, Wu et al. applied a rolling magnetic sphere-based EMG and a sliding copper-coated acrylic plate-based TENG as inertial units [57].
As mentioned above, ball inertial units are crucial for TENG-based BEHs responding to outside wave excitation.Researchers have explored strategies for improving such BEHs' outputs by introducing soft contact, material modification, and flexible connection in BEHs' networks.
Wu et al. applied a rolling magnetic sphere-based EMG and a sliding copper-coated acrylic plate-based TENG as inertial units [57].
As mentioned above, ball inertial units are crucial for TENG-based BEHs responding to outside wave excitation.Researchers have explored strategies for improving such BEHs outputs by introducing soft contact, material modification, and flexible connection in BEHs' networks.Figure 3 summarizes a spherical structural BEH based on polydirectional inertial units.Lin et al. proposed a super-robust and frequency-multiplied pendulum-inspired TENG (P-TENG) for efficient wave energy harvesting [58].Illustrated in Figure 3a, the P-TENG featured an acrylic rod with one end fixed to a copper-coated cambered acrylic disk serving as the triboelectric layer.The other end was connected to the outer acrylic spherical shell using a nylon rope, enabling free oscillation under wave excitation.Additional identical PTFE stripes adhered inside the spherical shell were applied as the charge pump to replenish charges for electrostatic induction, which essentially enhanced the P-TENG-based BEH's robustness and durability.Zheng et al. introduced a swing magneticbased pendulum to build a hybridized water wave energy harvester (H-WWEH) [62].The pendulum/swing inertia units usually request BEHs to have a base position, which prevent spherical structural BEHs from working effectively.Liang et al. fabricated six spring-assisted multilayered TENGs that interacted with six copper balls in a spherical shell to collect multidirectional wave energy, as shown in Figure 3b [59].The output performance of the device was found to be controlled by the orientation angle between the triggering direction and middle plane, which indicated that the proposed BEH achieved wave energy conversion no matter which angle the spherical structural BEH presented in water waves.In 2022, as shown in Figure 3c, Qu et al. reported a spherical eccentric structured TENG (Se-TENG)-based BEH [60].Twelve TENG units were symmetrically distributed in different directions between the interior regular dodecahedron and spherical shell, and each unit had an FEP film-coated eccentric rotary plate and a stationary part with four aluminum foil electrodes.The results illustrated that TENG units subjected to horizontal wave forces resulted in a higher electrical output than other units, and this BEH could continuously harvest wave energy as these units were connected separately to the rectifier bridge and then connected in parallel.An AC-DC buck circuit was applied as a power management module (PMM) by Liang et al. and Qu et al., in which a thermometer was successfully powered.Other similar multiple polydirectional designs with multiple inertial TENG units were developed based on five origami-inspired TENGs [63], four spring-assisted swing TENGs [64], four spiral TENGs [65], and four flower-like TENGs [66].
Compared with the polydirectional inertial design via increasing units, Gao et al. proposed a gyroscope-structured TENG (GS-TENG)-based BEH harvesting multidirectional ocean wave energy [61].As shown in Figure 3d, the TENG part was designed as inner and outer generation units oriented perpendicular to each other.Two units operated interdependently with an eccentric ball rotation at the center under different directional wave excitations.Four identical GS-TENG-based BEHs were networked and successfully drove a thermometer in a large water tank under a simulated ocean environment.
In Figure 4, recent derivative spherical structural BEHs are highlighted.Wang et al. reported a high-performance TENG based on charge shuttling (CS-TENG) and encapsulated it in a spherical shell responding to wave excitation, as shown in Figure 4a [67].The CS-TENG consisted of four main TENGs (L1, L2, R1, R2) and two pump TENGs (left pump, right pump) fabricated on an acrylic slider and stator.Thus, this slider could act as an inertial unit moving with waves and dive one pump TENG to complete charge injection when two main TENGs are in contact.Wang et al. also adopted a zigzag networking strategy using elastic rods, which imposed torques and effectively agitated each spherical device.The above inertial unit design achieved high-performance blue energy harvesting by introducing an advanced method to enhance TENG output and considering the response mechanism in the spherical structural BEH.Liu et al. proposed an oblate spheroidal TENG (OS-TENG) for all-weather blue energy harvesting [68].As illustrated in Figure 4b, the OS-TENG was derived from a spherical shell design but was smaller.Inside the device, the upper part worked under vertical pie iron movements, while the lower part operated with a rolling ball.Based on two inertial unit designs, the OS-TENG-based BEH could work under rough and tranquil ocean environments.
(OS-TENG) for all-weather blue energy harvesting [68].As illustrated in Figure 4b, the OS-TENG was derived from a spherical shell design but was smaller.Inside the device, the upper part worked under vertical pie iron movements, while the lower part operated with a rolling ball.Based on two inertial unit designs, the OS-TENG-based BEH could work under rough and tranquil ocean environments.In Figure 4c, Zhang et al. reported an inverted pendulum-type multilayer TENG (IPM-TENG), which was composed of a floating body with four independent multilayered paper-based TENG units sealed inside a spherical shell, an assist body, and a connecting rod that linked the floating body and the assist body [69].On the one hand, the TENG units were equipped with the nuggets as inertia units to drive TENGs working under wave excitation.On the other hand, the assisting body helped the BEH generate a larger amplitude of reciprocating motion and a larger average output charge than a single spherical design.The assist body design has also been reported by Zhang et   In Figure 4c, Zhang et al. reported an inverted pendulum-type multilayer TENG (IPM-TENG), which was composed of a floating body with four independent multilayered paper-based TENG units sealed inside a spherical shell, an assist body, and a connecting rod that linked the floating body and the assist body [69].On the one hand, the TENG units were equipped with the nuggets as inertia units to drive TENGs working under wave excitation.On the other hand, the assisting body helped the BEH generate a larger amplitude of reciprocating motion and a larger average output charge than a single spherical design.The assist body design has also been reported by Zhang et  Analyzed through the above progresses, spherical structural TENG-based BEHs are fundamental units of the network in the "blue energy dream" and channels through which wave excitation is effectively transmitted into reciprocating mechanical motions inside.Inertial units play a crucial role in the transmitting progress, including balls rolling, pendulums swinging, and sliders sliding.A suitable inertial unit design can achieve high-performance TENGs' output and multidirectional responses in the changeable ocean environment.

Buoy Structural TENG Applied in Blue Energy Harvesting
While the above research has predominantly focused on using spherical structures in blue energy harvesting devices, the buoy offers an alternative paradigm for applying TENGs in BEH development [18,22].The buoy structures have been applied in EMG-based BEHs and support wave energy installed capacity [72][73][74].
As shown in Figure 4d Analyzed through the above progresses, spherical structural TENG-based BEHs are fundamental units of the network in the "blue energy dream" and channels through which wave excitation is effectively transmitted into reciprocating mechanical motions inside.Inertial units play a crucial role in the transmitting progress, including balls rolling, pendulums swinging, and sliders sliding.A suitable inertial unit design can achieve highperformance TENGs' output and multidirectional responses in the changeable ocean environment.

Buoy Structural TENG Applied in Blue Energy Harvesting
While the above research has predominantly focused on using spherical structures in blue energy harvesting devices, the buoy offers an alternative paradigm for applying TENGs in BEH development [18,22].The buoy structures have been applied in EMGbased BEHs and support wave energy installed capacity [72][73][74].As shown in Figure 5a, Zhang et al. devised an active resonance TENG (AR-TENG) to enable the operational versatility and triggering of the TENG in diverse wave conditions [15].The AR-TENG equipped a pendulum as an inertial unit with a circle table-shaped pendulum cone that interacted with a flexible ring multilayer-structure TENG device in a cylindrical shell.A floating tumbler structure with a hemispherical float and a steel ball mounted on the bottom were applied in response to wave excitation.Thus, the pendulum and tumbler structure can produce a resonance effect without coupling with external driving frequency so that the AR-TENG attains omnidirectional freedom, facilitating comprehensive, full-spectrum water wave collection from all directions.Wang et al. designed a self-powered triboelectric coral-like sensor-integrated buoy, which equipped the bionic coral wave sensor (BCWS) in a buoyancy tray interacting with outside waves [78].The BCWS can avoid capsizing in the undulating ocean waves by being linked with an anchor chain.
Following further optimization, as depicted in Figure 5b, the Arctic-TENG was designed and applied in a self-powered Arctic satellite communication system [75].The Arctic-TENG used an inertial unit of a cylindrical stator-rotor, in which the rotor rotated as the deviation angle increased due to wave excitation.Jung et al. reported a frequencymultiplied cylindrical TENG (FMC-TENG) that applied a cylindrical buoy and utilized magnets to store potential energy and instantly release it to produce high-frequency kinetic energy [79].Incorporating soft rabbit hair for stator-rotor isolation mitigates charge dissipation and was also reported by Feng et al., in which the cylindrical buoy structure was applied to support a swing structure-based hybrid nanogenerator made of a soft-contact cylindrical TENG (SCC-TENG) and EMG [80].
As depicted in Figure 5c, Chen et al. reported a chaotic pendulum triboelectricelectromagnetic hybridized nanogenerator collecting low-frequency chaotic vibrations induced by outside wave excitation [76].When the inner pendulum was motivated by water oscillation, the TENG unit operated with a rotation plate, and the EMG worked with synchronized moving magnetic balls.Thus, the self-powered wireless sensing node distant transmission was realized, and the data transmission capability exceeded 300 m.Likely, Ahn et al. introduced an all-recyclable TENG (AR-TENG) for sustainable ocean monitoring systems monitoring the condition of seawater and the activated life jacket regularly transmitting an emergency signal to land through wireless communication [81].In Figure 5d, Liu et al. proposed a novel hybridized BEH aiming at all-weather IoT applications, which integrated an interdigital electrode-based TENG (I-TENG), a switchbased TENG (S-TENG), and an EMG [77].The hybridized BEH was propelled by the rolling motion of a 30 mm magnet and a lightweight, adaptable circular enclosure following the magnet's rotation that could maximize the contact area.Thus, this BEH establishes an all-weather IoT platform with a solar cell panel and a Bluetooth sensor module.The ambient humidity and temperature information can be detected and sent to the user end by the Bluetooth sensor module under various conditions with or without daylight and water waves.Zhang et al. used multiple PTFE balls moving inside the buoy structure BEH and found its peak power reached 61.20 mW when the wave height was 6 cm [82].Furthermore, Zhu et al. proposed a highly integrated triboelectric-electromagnetic wave energy collector (TEWEH) that was applied to roll permanent magnet PTFE balls [83].The TEWEH had been demonstrated as a self-powered marine buoy deployed near Dalian Bay and transmitted measurements to the receiver.

Liquid-Solid-Based TENG Applied in Blue Energy Harvesting
The liquid-solid contact TENG (LS-TENG) has been proven to solve the wear problem that existed in the solid-solid contact TENG (SS-TENG), providing a new solution for harvesting blue energy [84][85][86][87].As shown in Figure 6a, Liu et al. proposed a thin-film blue energy harvester based on the triboelectric mechanism of liquid-solid contact [88].The device was designed with a novel external Ū electrode TENG, including a bar electrode (B electrode) and a U-shape electrode (U electrode).Thus, the shielding effect from water is hugely minimized, and the outputs are effectively improved.Meanwhile, several IoT applications like wave level warning, continuous powering, and wireless signal transmission were demonstrated, indicating significant application prospects.As shown in Figure 6b, thin film-type BEHs based on the LS-TENG were also reported by Gu et al., in which a bulk effect was built with 3D electrodes above the dielectric layer and a closed-looped system was achieved with enhanced instantaneous output power density [89].
The liquid-solid contact TENG (LS-TENG) has been proven to solve the wear problem that existed in the solid-solid contact TENG (SS-TENG), providing a new solution for harvesting blue energy [84][85][86][87].As shown in Figure 6a, Liu et al. proposed a thin-film blue energy harvester based on the triboelectric mechanism of liquid-solid contact [88].The device was designed with a novel external Ū electrode TENG, including a bar electrode (B electrode) and a U-shape electrode (U electrode).Thus, the shielding effect from water is hugely minimized, and the outputs are effectively improved.Meanwhile, several IoT applications like wave level warning, continuous powering, and wireless signal transmission were demonstrated, indicating significant application prospects.As shown in Figure 6b, thin film-type BEHs based on the LS-TENG were also reported by Gu et al., in which a bulk effect was built with 3D electrodes above the dielectric layer and a closed-looped system was achieved with enhanced instantaneous output power density [89].A novel liquid-solid TENG array was proposed by Sun et al. in Figure 6c [90].A PTFE film-based electrode completely sealed two open terminals of the PET tubes, and water was filled into the container.Thus, the water could periodically oscillate back and forth in the sealed tube driven by wave excitation.Zhang et al. also reported a similar liquid-solid tubular TENG based on the contact electrification between PTFE and water; the water inside the tube acted as an inertia unit and interacted with outside wave excitation [91].Similarly, Wu et al. invented a water/FEP tube-based TENG for harvesting wave energy and body motion energy [92].In Figure 6d, Wei et al. designed a simple structure of an all-weather droplet-based TENG (DB-TENG) [38].The DB-TENG simply used a grounded water droplet sliding on an FEP film with two symmetrical copper electrodes.It was verified that the DB-TENG-based BEHs could work stably under extreme conditions like high concentrations of salt solutions, strong acids, or alkalis.
Li et al. applied multiple LS-TENGs in a buoy structural BEH with oscillated water inside.They powered a radio frequency emitter to form a self-powered wireless SOS system for ocean emergencies [93].In contrast, Liang et   A novel liquid-solid TENG array was proposed by Sun et al. in Figure 6c [90].A PTFE film-based electrode completely sealed two open terminals of the PET tubes, and water was filled into the container.Thus, the water could periodically oscillate back and forth in the sealed tube driven by wave excitation.Zhang et al. also reported a similar liquid-solid tubular TENG based on the contact electrification between PTFE and water; the water inside the tube acted as an inertia unit and interacted with outside wave excitation [91].Similarly, Wu et al. invented a water/FEP tube-based TENG for harvesting wave energy and body motion energy [92].In Figure 6d, Wei et al. designed a simple structure of an all-weather droplet-based TENG (DB-TENG) [38].The DB-TENG simply used a grounded water droplet sliding on an FEP film with two symmetrical copper electrodes.It was verified that the DB-TENG-based BEHs could work stably under extreme conditions like high concentrations of salt solutions, strong acids, or alkalis.
Li et al. applied multiple LS-TENGs in a buoy structural BEH with oscillated water inside.They powered a radio frequency emitter to form a self-powered wireless SOS system for ocean emergencies [93].In contrast, Liang et al. designed dynamic electric double-layer TENG (DE-TENG) arrays for blue energy harvesting by dynamic wave motions between DE-TENG electrodes [94].Wu et al. developed a hybrid spherical TENG (S-TENG) with solid-solid and solid-liquid contact modes that worked for blue energy harvesting [95].The S-TENG was demonstrated as a low-cost and environmentally friendly self-powered approach for electrochemical cathodic protection in marine environment.

Performance Comparison and Power Management
In view of the four different structures of blue energy harvesting devices mentioned above, we summarize the key parameter settings and the performance output performance of the TENGs with different structures, as shown in Table 1.Moreover, there have also been significant improvements in output power, robustness, efficiency, and practicality in the last decade, as shown in Figure 7 [32,37,80,87,93,[96][97][98][99][100][101][102][103][104][105].It can be categorized as a solid-solid TENG-based BEH unit, a liquid-solid TENG-based BEH unit, and TENG-based BEH networks.It is found that the multilayered TENG is an efficient strategy to increase outputs and promote the expansion of the size [66,96,99,104].Soft contact is achieved by introducing flexible materials like rabbit hairs or the liquid-solid interface [80,105].Networking is also an important method to cope with the complex ocean environment, and BEH units work synergistically under wave excitation [32,93,100,102].It is important to note that since the TENG-based BEHs often operate in high humidity environments, packaging is critical to ensure that the device operates properly while maintaining a high output.The commonly used external packaging of the device is either an acrylic enclosure or a 3D-printed enclosure based on a PLA material.This form of encapsulation is lightweight, hermetically sealed, highly stable, and most importantly well suited to a variety of design needs based on 3D printing technology, and most of the spherical and buoy structures mentioned in this review utilize this strategy.For liquidsolid contact applications, sealing tapes and over-molded membranes can also be used for waterproofing.Due to long-term exposure to the marine environment, heat-resistant, corrosion-resistant, and non-polluting materials are required to avoid short-circuiting the electrodes by seawater.During operation, TENGs may be disturbed by marine organisms, which are also being disturbed by the TENG at the same time.Therefore, it is necessary to set up certain physical barriers around the TENG equipment to prevent marine organisms from coming into contact with the equipment.
In fact, the output of the TENG is usually characterized by high impedance and low current levels.Its direct application to commonly used electronic devices will result in matching failures and high output is not guaranteed.The process of extracting energy from the environment is subject to fluctuating environmental conditions, leading to instability and time-varying output power.To cope with this situation, the conventional practice is to introduce advanced power management strategies to enhance the stability of the system output.The essence of these problems is the matching of the resistive load and intrinsic capacitance of the external device and the TENG.When the internal resistance of the TENG is much greater than the resistance of the external load, the capacitive imped- It is important to note that since the TENG-based BEHs often operate in high humidity environments, packaging is critical to ensure that the device operates properly while maintaining a high output.The commonly used external packaging of the device is either an acrylic enclosure or a 3D-printed enclosure based on a PLA material.This form of encapsulation is lightweight, hermetically sealed, highly stable, and most importantly well suited to a variety of design needs based on 3D printing technology, and most of the spherical and buoy structures mentioned in this review utilize this strategy.For liquidsolid contact applications, sealing tapes and over-molded membranes can also be used for waterproofing.Due to long-term exposure to the marine environment, heat-resistant, corrosion-resistant, and non-polluting materials are required to avoid short-circuiting the electrodes by seawater.During operation, TENGs may be disturbed by marine organisms, which are also being disturbed by the TENG at the same time.Therefore, it is necessary to set up certain physical barriers around the TENG equipment to prevent marine organisms from coming into contact with the equipment.
In fact, the output of the TENG is usually characterized by high impedance and low current levels.Its direct application to commonly used electronic devices will result in matching failures and high output is not guaranteed.The process of extracting energy from the environment is subject to fluctuating environmental conditions, leading to instability and time-varying output power.To cope with this situation, the conventional practice is to introduce advanced power management strategies to enhance the stability of the system output.The essence of these problems is the matching of the resistive load and intrinsic capacitance of the external device and the TENG.When the internal resistance of the TENG is much greater than the resistance of the external load, the capacitive impedance of the TENG itself manifests itself as the total circuit impedance.And when the internal resistance of the TENG is less than that of the external load, the external resistor carries most of the voltage.Therefore, the situation in which the TENG can obtain the maximum output power occurs when the internal and external load resistances are roughly equal [107].
Currently, the mainstream power management strategies can be categorized into four types: electromagnetic conversion, rectification, capacitive conversion, and DC conversion [108].Xu et al. proposed a half-wave rectification method, which solves the problem of the dissipation of the voltage output by realizing charge replenishment through a parallel circuit of the load, diode, and excitation source [109].Zhu et al. designed an electromagnetic transformer, and the inclusion of voltage regulators and stabilizers greatly improves energy conversion efficiency [110].According to the given rotation speed in this paper, the electrical power output can reach 1.5 W, and the efficiency of the conversion to electrical energy can reach 24%.Xi et al. have utilized the PMM strategy applied to blue energy harvesting to achieve a rational distribution of power across the load by means of a voltage reducer and friction electron energy extractor [111].Mechanical switches can also be configured for power management.Qin et al. designed a rectifier travel switch where the load electronics are connected to the electrodes only at a specific time at the end of the cycle.Energy will be stored sequentially into the inductor, capacitor in the process, and at the end to realize the AC/DC conversion; it has been confirmed that this design can achieve nearly 48% energy storage efficiency [112].In conclusion, for the high impedance characteristics of the TENG, how to realize impedance matching through the power management circuit, so as to further improve the output efficiency, is still a topic to be solved.
At present, underwater sensors deployed in the marine environment currently play a pivotal role.In order to extend the battery life of these devices, low-power designs are imperative.It is worth noting that underwater sensors usually operate with power in the range of microwatts to milliwatts.For example, the Arctic-TENG in Table 1, with a peak power density of 21.4 W/m 3 , generates a total energy of 8.59 kJ per year and can transmit 540 bytes of data per day for extended operation [75].Zhao's team achieved a major milestone in 2018 by developing a networked TENG capable of generating 1.03 MW of stabilized power at a wave height of 12 cm.These are good breakthroughs [87].
However, there is still considerable room for improvement in applications and devices that require high power output.The key to this improvement lies in power management design.In the future, with overall improvements in construction, materials, packaging, and power management, it is expected that the TENG will power even more demanding devices.

Summary and Perspectives
This paper reviews and summarizes recent progress in applying different triboelectric nanogenerators (TENGs) in blue energy harvesting.The critical contributions of TENGs towards the "blue energy dream" are highlighted, and the corresponding "blue energy harvester (BEH)" is verified by conclusive evidence either in experiments in the water tank or a realistic wave environment.Different inertial units, including rolling balls, pendulum units, polydirectional designs, and assisted bodies, conclude the history and development of TENG-based BEHs.Other considerable TENG-based BEHs applying buoy structures are surveyed to emphasize integrating new TENG technology with conventional BEH structures.Moreover, liquid-solid mode TENG-based BEHs are shared, indicating that the TENG has created more possibilities for developing BEHs.Finally, this paper also summarizes the performance of these structures and common power management strategies for reference under different requirements.
Predictably, with the current fast-growing studies in TENG's technology and fundamental theory, TENG-based BEHs will be promisingly applied in the ocean environment.Firstly, the TENG facilitates the hybridization of blue energy harvesting technologies, potentially reducing costs and increasing efficiency.Secondly, networks of TENG-based BEHs emerged in harvesting wave energy in a three-dimensional ocean environment by equipping lightweight BEHs overseas and anchoring BEHs underwater.Also, TENG-based BEHs will play an essential role in the global carbon cycle in consideration of reducing the fossil energy requirements of increasing ocean activities.In 2024, Jiao et al. developed a self-powered and self-sensing blue carbon ecosystem with a fur-TENG-based BEH [113].This progress achieved a remarkable increase in both the average net biomass accumulation and linear growth rate in seaweed cultivation.Convincingly, by applying TENG-based BEHs, sensor devices that do not require high power levels can achieve long-term operation without the need for frequent battery replacement.Promising application directions in the future include but are not limited to wireless alarm systems, marine environment monitoring, etc.In the future, within the field of marine life monitoring, corresponding monitoring systems built using TENG-based BEHs can be used to track and study the behavior of marine organisms and protect them for a long time.It will be of great benefit to improve the marine ecological environment and develop the marine economy.
, was proposed in 2013 by Yang et al. [45].A plastic ball wrapped in polytetrafluoroethylene (PTFE) film and a spherical shell attached with polyamide (PA) film constitute a contactseparation-mode TENG.Outside, wave excitation will drive the ball at the center to contact the inner wall of this spherical structural BEH.The link cable ensures the contact-separation process by restricting the ball's movements.To release the ball's kinetic energy, Zhang et al. applied the single-electrode mode to build a BEH, in which a polyfluoroalkoxy (PFA) ball was rolling on the Al electrode attached to the inner wall of the spherical shell.Furthermore, Wang et al. utilized a rolling-structured BEH based on the freestanding triboelectric-layer mode [46].Specifically, in Figure 1c, outside wave excitation will indirectly drive the inside nylon ball to roll above two Kapton film-based electrodes on the spherical shell's inner wall, thus effectively generating electric signals.The rolling ball is a critical inertial unit for TENG-based BEHs converting wave excitation into the TENG's working cycles.This freestanding rolling structural TENG design ensures charge transfer efficiency under actual water waves and helps the construction of BEH networks in the "blue energy dream".Nanoenergy Adv.2024, 4, FOR PEER REVIEW 4

Figure 2 .
Figure 2. Spherical structural blue energy harvester based on ball inertial units.(a) Largely enhanced blue energy harvester with liquid-silicone core-shell unit (reprinted with permission [51], copyright 2018, Elsevier).(b) Coupled blue energy harvester networks based on silicone rubber balls (reprinted with permission [52], copyright 2018, American Chemical Society).(c) Self-assembly blue energy harvester network with multiple pellet units (reprinted with permission [53], copyright 2019, Elsevier).(d) Hybrid blue energy harvester is based on numerous soft balls (reprinted with permission [54], copyright 2022, John Wiley and Sons).

Figure 2 .
Figure 2. Spherical structural blue energy harvester based on ball inertial units.(a) Largely enhanced blue energy harvester with liquid-silicone core-shell unit (reprinted with permission [51], copyright 2018, Elsevier).(b) Coupled blue energy harvester networks based on silicone rubber balls (reprinted with permission [52], copyright 2018, American Chemical Society).(c) Self-assembly blue energy harvester network with multiple pellet units (reprinted with permission [53], copyright 2019, Elsevier).(d) Hybrid blue energy harvester is based on numerous soft balls (reprinted with permission [54], copyright 2022, John Wiley and Sons).
al. to support the 360 degrees of movement of the inertial ball in a fully symmetric TENG-based BEH [71].
al. to support the 360 degrees of movement of the inertial ball in a fully symmetric TENG-based BEH [71].As shown in Figure 4d, Wang et al. introduced a bio-inspired butterfly wing-assisted multidirectional TENG (BBW-TENG)-based BEH [70].The BBW-TENG had a spherical shell with five pairs of bionic blades fixed outside and 28 TENG units based on PTFE balls rolling on internal arc tracks.The bionic blades possessed the characteristic of drag amplification, which facilitated the BBW-TENG-based BEH's response sensitively to the multidirectional underwater wave excitation and presented excellent durability without diminished electrical performance.
, Wang et al. introduced a bio-inspired butterfly wing-assisted multidirectional TENG (BBW-TENG)-based BEH [70].The BBW-TENG had a spherical shell with five pairs of bionic blades fixed outside and 28 TENG units based on PTFE balls rolling on internal arc tracks.The bionic blades possessed the characteristic of drag amplification, which facilitated the BBW-TENG-based BEH's response sensitively to the multidirectional underwater wave excitation and presented excellent durability without diminished electrical performance.
al. designed dynamic electric doublelayer TENG (DE-TENG) arrays for blue energy harvesting by dynamic wave motions between DE-TENG electrodes [94].Wu et al. developed a hybrid spherical TENG (S-TENG) with solid-solid and solid-liquid contact modes that worked for blue energy harvesting

Table 1 .
Comparisons of typical blue energy harvesters.