1. Introduction
The evolution of energy streaks across the sky has been lighting up the dawn of technological development. The price of chemical energy has gradually deviated from the ideal acceptable range, energy usage has increased sharply every day, and collecting environmental energy has been favored by researchers. It is also extremely difficult to build and maintain a certain amount of power plants in countries and regions with high construction costs, high site selection requirements, large maintenance and labor needs, backward living standards, or complex geographical environments [
1,
2,
3,
4]. Entering the 21st century, the Internet of Things has achieved milestone development. Whether it is medical health, daily life peripherals, or even industrial production and transportation, there are a large number of sensors, and the power gap is only increasing. The distinctive feature of this type of power demand is that it is not physically located and spreads across various spaces. Assuming battery power is used, the pollution to the environment will be immeasurable. Frequent replacement of batteries is likely to result in uncontrollable front-end and post-production costs and maintenance costs. In order to solve these problems, a large number of green energy supply devices have been developed, such as solar photoelectric converters [
5,
6], wind turbines [
7], thermoelectric generators [
8,
9], fuel cells, etc. Solar converters require a large initial investment, occupy a large area, and have slow cost recovery. Wind turbines are similar to the former, but more attention should be paid to safety and noise issues. Throughout human history, the triboelectric effect has been seen as a detrimental occurrence in science and technology. Uncontrolled high-voltage static electricity consumes energy and increases the danger of electromagnetic interference and electronic component failure. It must be minimized, if not completely protected. There are several motion modes, including heave, rotation, and vibration, while gathering fluid kinetic energy. These energies have high entropy, low frequency, and chaos.
In the long river of human exploration of science and technology, the frictional electrification effect has long been regarded as a negative phenomenon. Uncontrolled high-voltage static electricity not only carries the risk of electromagnetic interference and breakdown of electronic components but also wastes energy, which needs to be reduced or even shielded as much as possible. In terms of fluid kinetic energy collection, there are motion modes such as vibration, oscillation, and rotation, which are characterized by disorder, low frequency, and high entropy. If these unnecessary high entropy fluid energies are collected using Faraday electromagnetic induction generators (EMGs) based on the law of electromagnetic induction, some EMGs need to operate under certain high-frequency motion, and some also need to exhibit certain motion trends, such as the certainty of rotation direction. Relatively speaking, the high entropy characteristics of the fluid capacity collection can lead to a significant waste of inexhaustible low-frequency vibration energy, including wave energy and wind energy. In 2012, Wang Zhonglin’s team proposed a conversion device that utilizes the electrostatic effect, triboelectric nanogenerators [
10]. In terms of cost, since the human energy revolution entered the era of electrification, most of the sources of electricity obtained by humans have come from electromagnetic induction generators (EMGs). EMG is equipped with a large number of iron cores and copper coils inside, which is costly. Unlike EMG, nanogenerators use a large amount of polymer materials, which are easy to obtain and have low manufacturing costs. In the operation of multiple single-unit parallel networking, multiple nanofriction generators can be assembled into a large-scale power storage network through specific control and energy storage circuits. The light weight, the fluid energy harvesting response efficiency in low-frequency, low amplitude, and high entropy vibrations are much better than that of electromagnetic induction generators, making nanogenerators an ideal energy harvesting component for the new generation.
Under low-cycle conditions, the energy conversion efficiency of nanogenerators is much higher than that of electromagnetic induction generators suitable for high-cycle cycles. An energy conversion device converts mechanical energy into thermal energy or other energy when mechanical energy with a low circulating frequency is expressed in the form of pulses. For example, shock absorbers convert mechanical energy into thermal energy by setting damping. The development of clean energy and the conversion of the remaining mechanical energy into other energy can not only extend the service life of the equipment and reduce the maintenance cost but also improve the energy utilization rate by using energy reserves. The theoretical core of nanogenerators is Maxwell’s equations. Maxwell’s equations are as follows. The first term is the induced current caused by changes in the electric field, which provides a prerequisite for wireless communication. The second term is the displacement current caused by the polarization field generated by the electrostatic charge carried on the surface of the medium, which becomes the theoretical basis of the triboelectric nanogenerator [
11,
12,
13].
where
D is the displacement field,
E is the electric field,
t is the time,
ε0 is the vacuum permittivity, and
Ps is the current caused by the polarization field caused by the charge on the surface of the dielectric.
At present, electrodes and triboelectric layers are required components of all commercially available triboelectric nanogenerator materials. Using the vertical contact-separation mode as an example, charge communication between the nanogenerator and the external circuit load is facilitated by two metal electrodes with low internal resistance located on the outermost side. Two metal electrode pieces are encased in two pieces of a certain substance. The triboelectric series contains these two materials.
Figure 1 displays the triboelectric sequence of widely used triboelectric materials. Generally, in order to increase the power density of power generation, it is necessary to look for materials near both ends of the triboelectric sequence, aiming to find materials that tend to be “positive” and tend to be “negative”. The “tendency” here indicates that the attraction of materials to electrons is different, with materials that tend to be “positive” being more likely to lose electrons, and materials that tend to be “negative” are more likely to gain electrons [
14,
15].
Solid–solid coupling nanogenerators are now the subject of further in-depth investigation.
Figure 2 depicts the single electrode mode, independent layer mode, vertical contact-separation mode, and horizontal sliding mode of solid–solid coupling nanogenerators. According to various purposes, numerous fundamental modes may be employed [
16,
17]. According to the triboelectric sequence, polyimide (Kapton) tends to gain electrons from polymethacrylic acid (PMMA). As the friction layers approach or recede from one another, the air gap in the friction layer continues to diminish, causing a charge to move and a potential difference to form. A significant component of current research endeavors involves horizontal slip nanogenerators. To finish the power production process, two triboelectric material pieces with electrodes connected on the rear slide parallel to the joint surface may be made to engage in friction or there may be a gap between the friction layers that results in relative displacement [
18].
Figure 2c depicts the single-electrode mode nanogenerator. As a zero-potential reference point, it has a reference electrode. When propelled by an external force, the moveable electrode produces a potential difference or current, much like the vertical contact-separation or sliding mode. The electrified friction layer is liberated from the confines of the wire, which is a significant benefit [
19]. The structure of the independent-layer modal nanogenerator is shown in
Figure 2d, which is similar to the horizontal slip mode, and the friction layer can be designed to be in contact and non-contact state.
3. Optimization of Fluid Energy Harvesting Devices
Since the introduction of triboelectric nanogenerators in 2012, the pace of development has been impressive. Compared with solenoid valve click EMG, triboelectric nanogenerators exhibit natural adaptability in low-frequency energy harvesting [
76]. After more than a decade of development, the materials used in nanogenerators are safer and more stable and have shown unprecedented biocompatibility, such as when placing them in the human body for cardiovascular sensing and detection [
77,
78], with high energy density, low space occupancy [
79], extremely tolerant environment [
80], simple preparation process [
81], and low cost. However, triboelectric nanogenerators still need to be improved, and their application occasions and frequencies are still restricted. For example, the power output and energy density of the energy harvesting device are not satisfactory, and the internal structure of the nanogenerator is worn. Obviously, improving the performance of nanogenerators has a positive effect on their use scenarios, use frequency, and commercialization. Recently, research strategies for improving triboelectric nanogenerators have been proposed.
3.1. Material and Structural Optimization
The application environment of fluid energy collection devices based on triboelectric nanogenerators is often in deserts or oceans [
82], which means that the frequency of manual maintenance cannot be too high, and high-frequency maintenance will bring high labor costs. The presence of mechanical wear may significantly reduce the service life of an energy harvesting device, so choosing better materials is the lifeblood of an energy harvesting device. Better materials need to be more wear-resistant, referring to the triboelectric sequence [
83], to be able to produce long-lasting, high-energy output power. The stability and corrosion resistance of inorganic materials are due to organic materials. In 2023, Zhao et al. studied the contact polarization effect between metal electrodes and inorganic materials and found that the electron transfer mechanism is the basis of the charge transfer mechanism for contact charging. The work function of the dielectric layer and atomic species on the metal has an important influence on the charge transfer process when contacting the metal. The friction layer is made of inorganic single crystals, which greatly expands the application of inorganic materials in fluid energy harvesting devices [
84]. Han et al. discovered that rabbit hair has the function of reducing frictional resistance, and introduced rabbit hair to the improvement of fluid energy collection devices to design SCR-TENG. When rabbit fur and FEP work together, rabbit fur not only plays a role in reducing friction but also provides more frictional charges and enhances the output power of the fluid energy collection device [
85]. The existence of friction pairs that play a non-positive role will definitely shorten the life of the mechanical structure. The friction material is made into a brush shape, which greatly reduces the wear between materials. A dual-mode and frequency-doubled TENG with ultra-high durability and efficiency through elastic connection and soft contact design is used for ultra-low frequency mechanical energy harvesting. By introducing springs and flexible dielectric fluff into a novel pendulum structural design, the TENG’s surface tribocharge is replenished in a soft contact mode under intermittent mechanical excitation, while the robustness and durability are enhanced in a non-contact operating mode, as shown in
Figure 12a,b [
86,
87].
Similarly, the use of liquid lubrication effectively reduces the wear between friction media. He et al. proposed a liquid lubrication-facilitated TENG, namely LP-TENG. The reasonable design of the voltage balance rod can obtain an energy output density of 87.26 W/m
2 under the low-frequency disturbance of 2 Hz by using the space charge accumulation effect. This energy output density makes the output power of LP-TENG comparable to that of solar panels, as shown in
Figure 12c [
88]. The use of pendulum structures, flutter structures, etc., is all prepared to reduce friction pairs. However, another way to reduce frictional losses is to adopt a structural model of solid–liquid contact and solid–gas contact. At present, the most studied mode is solid–liquid contact, and its general structure is to use a solid container of a specific shape, such as an optional FEP, which encapsulates special liquid and metal electrodes according to the topological shape of the container.
Pan et al. had a Voc of 81.7 V and an Isc of 0.26 μA for pure water-based U-tube TENG at 0.5 Hz shaking. The improved sandwich-like U-tube TENG exhibits excellent output performance with a Voc of 350 V, an Isc of 1.75 μA, and a power density of 2.04 W/m
3, as shown in
Figure 12d [
89]. Wang et al. made a TENG tilt sensor. A ring-shaped PTFE tube is used, a certain amount of water is filled inside, and when a certain inclination angle is given, the specific charged contact mechanism between PTFE and water forms the movement of charge, generating electric potential and current. It is proven that there is a proportional relationship between the output voltage and current and the tilt angle of the TENG tilt sensor. This packaged self-powered sensor device is particularly suitable for the operating environment of ocean ships, as shown in
Figure 12e [
90].
3.2. Expending Hybrid Energy Harvesting Device
The power generated by wave energy fluid energy harvesting devices (WEHs) depends on the fluid disturbance in the current working environment, which means there is a strong correlation between the disturbance and the output power. Under weak disturbances, the output power is naturally insufficient, and the volume of a single TENG is limited. The contact area of the frictional layer is limited, which makes it difficult to meet people’s demand for electricity for a long time due to the transfer of charges. At present, the output power of a single TENG is limited, and most of the output power is concentrated in the
order of magnitude. Parallel multi-group TENG is a solution that has been practiced for a long time. After networking, TENG effectively concentrates the energy of multiple individual devices through reasonable design of control circuits, achieving high-capacity output capacity. However, fundamentally, improving individual TENG is the fundamental guarantee for solving the problem of high-capacity TENG networking. A feasible solution to improve the efficiency of a single TENG is to integrate TENG with ENG to form a hybrid energy harvesting system. The working frequency of electromagnetic generator EMG is higher, and compared to TENG, it has the characteristics of high current and low voltage. TENG itself has a higher output voltage, but a lower current. The two work in different working environments and can complement each other’s shortcomings [
91,
92]. Sun et al. studied a liquid-solid electromagnetic combined water kinetic energy harvesting nanogenerator (TTENG). In addition to the traditional structure mentioned above, an innovative approach is to add a permanent magnet and an induction coil to combine TENG with EMG, resulting in consistent current direction and significantly enhancing the output level of TENG, as shown in
Figure 13a,b [
93]. Zhao et al. designed a composite wave energy harvesting element (WEC) consisting of a mechanical transmission mechanism, a floating ball, TENG, and an electromagnetic induction generator. In this system, the power generation system is designed in a cylindrical shape. The side of the cylinder is equipped with permanent magnets and coils, and it is an electromagnetic induction generator RD-EMG. The cylindrical surface of the cylinder is equipped with MBC-TENG, and the friction layer material is selected as PTFE and nylon brush. The floating ball receives wave disturbance in the water, and its motion is transmitted to the floating ball. The speed is then increased through gear racks and a carefully proportioned variable speed gear mechanism, and the rotational kinetic energy is transmitted to MBC-TENG and RD-EMG, as shown in
Figure 13c,d [
94]. It has been proven that based on the difference in working frequency, ENGs and TENGs working together can greatly improve the efficiency of energy harvesting equipment.
3.3. Development of High-Power TENG
At present, a large number of studies have focused on the output electrical performance of TENG, extending the service life, and reducing the threshold of external excitation energy, and the results of their research are impressive. However, as mentioned above, due to some current limitations, the electrical output of TENG is still some way from practical use. Therefore, combining existing advanced technologies, such as semiconductor technology, into TENG will greatly increase the output power of TENG, broaden the research prospects of TENG, attract more researchers to join TENG research, and create a virtuous circle in the field of TENG and its interdisciplinary fields such as triboelectronics [
95].
Combined with transistor technology, Wu et al. innovatively proposed a high-performance and high-power OCT-TENG. As shown in
Figure 14, the core of OCT-TENG is the use of reverse charge enhancement and transistor-like designs to increase the charge output and reduce the output impedance. The OCT-TENG consists of a stator substrate and a slider. The stator contains a coplanar friction surface with opposite charge polarization (fluorinated ethylene propylene, abbreviated as FEP, and polycarbonate, abbreviated as PC, both 100 μm thick) and four electrodes (E1, E2, EL, and ER). EL and ER are two floating electrodes placed on the left and right sides of the stator substrate. E1 and E2 are two sheet electrodes (Cu) placed below the FEP and PC membranes. The slide contains the sheet electrode E3 below the FEP film (10 μm). The entire device resembles a pair of complementary transistors and consists of two transistor-like components with the same structure and opposite friction surfaces. E1 and E2 can be thought of as the “source” (S) of each “transistor”, and E3 can be thought of as a dynamic “drain” (D). EL and ER are the “gates” (G). When E3 comes into contact with EL or ER, the transistor’s “gate” is switched to “on”, so that charge can flow between the “source” and “drain” [
96].
4. Summary and Outlook
Fluid energy is a vast, mostly untapped source of power for the human species. The formation of wave energy from wind energy and currents is not only common in our everyday lives but also contains large reserves. With the advent of the information age, the demand for electricity as an energy supply undoubtedly increased. The gap between the demand and supply of energy is becoming larger and deeper. The proliferation of sensors and data-transfer infrastructures will lead to steadily rising energy demands. This indicates that we need to think about and be accountable for the preservation of the natural environment [
97]. Second, sensors are becoming more pervasive in the Internet of Things age. Distributed sensor networks, also known as intelligent sensing networks, are set up in the manner and location determined by human needs. As a result, there will be a growing need for decentralized sources of energy. New energy sources such as various batteries and solar energy may not necessarily meet such large capacity needs. The age of big data and the Internet of Things is here, and the triboelectric nanogenerator is a revolutionary technology that enables the use of a decentralized energy supply for components. Simultaneously, it marks a new turning point in the evolution of the electrification and information age and hastens the advent of the micro-electromechanical era. The self-powered capability of nanogenerators may significantly increase their range of applications, including intelligent sensing systems, the collection of wave and wind energy, biological and medical health and monitoring, pollution prevention, and control, etc. This system includes a nanogenerator, a converter, and a storage device. For the nanogenerator system, the energy receiver is the first point of energy input. The collection of wind energy is more likely to produce high-frequency oscillations than either water kinetic energy or biological energy. There are primarily flutter structures and rotating structures because films are often employed to enhance the friction frequency of the friction layer. It is important to capture instantaneous kinetic energy, transform it into potential energy, and then transfer the energy via a transducer since the chance of capturing water kinetic energy is large at low frequencies. Vibration energy is undeniably a vast repository of treasure, despite the fact that its effective collection is currently impossible. The existing energy difficulties may be solved in part by gathering enough vibration energy with a suitable energy conversion rate and reserve [
98].
The decentralized energy supply additionally presents prospects for the advancement of real-time sensor network monitoring. For example, in desert hinterlands or vast oceans, the era of electrification means that only turbine wind turbines and solar converters are used without a stable self-energy supply. Conversely, nanogenerators provide a less expensive and less prone to failure alternative for the power supply of warning systems installed on buoys and climate monitoring devices in the desert. This article starts with the nanogenerator material that requires two or more friction layers with high electronegativity to create the induced electromotive force acquired by contact and electrostatic induction in response to a mechanical movement. This section introduces the triboelectric sequence that is necessary for nanogenerators. Generally speaking, choosing friction materials necessitates choosing two or more materials in the sequence in which it is simple to gain electrons and easy to lose electrons in order to offer the highest feasible energy conversion rate. Second, the fundamental theory behind the nanogenerator—the second term of Maxwell’s displacement current that has been “ignored” for a long time is briefly presented and its working principle is examined. This paper introduces the four basic modes that are currently commonly used in nanogenerators: single electrode type, independent layer type, vertical contact separation type, and horizontal sliding type. To present the research development of nanogenerators in the area of fluid energy collection, energy collection can be separated into wind energy and water flow wave energy based on the collected fluid medium and the structural design of the generators. The advancements in fluid energy harvesting-based nanogenerator research highlight some issues and provide remedies from earlier studies.
At the moment, the investigation of a significant number of nanogenerators is plagued by a number of flaws that need to be fixed. Firstly, the coupling structure of gas flow and coupling devices or wave and coupling devices is the primary focus of current energy harvesting research in nanogenerators. Though a lot of progress has been achieved in the mechanical structure, research on semiconductors and triboelectric materials is still insufficient. Especially, after multiple wear and tear, the mechanical tensile strength and mechanical strength may suffer from attenuation [
99]. Second, although one of the development trends is to work with the network to create a large-scale network for nanogenerator power generation, the energy reserve and power management systems of nanogenerators have not yet reached a very mature state following the networking. This is because the power generation power of nanogenerators is currently relatively low. The coordination of the power generation network and the circuit must be studied when TENG constructs a power generation network or adopts an excitation circuit. Not only would improper use of excitation and power management circuits fail to optimize the efficiency of each TENG, but it may also result in a decrease in the total power output. As a result, research on TENG network excitation and power management circuits is also ongoing. Thirdly, in order to attract more researchers to the topic, equations or theories pertaining to the dynamics of nanogenerators should be substantially simplified. Ultimately, the development of nanogenerators requires dependability. In the TENG network that collects ocean wave energy, the transmission line should have sufficient wind and wave resistance, the link between the transmission line and the TENG should have fatigue tolerance, and the power attenuation of long-distance transmission should be carefully checked. The development of nanogenerators is motivated by the fact that they are maintenance-free after a specific design life, and if they are not dependable enough and need human maintenance, the value of nanogenerators will not be completely embodied. Thus, in the realm of semiconductors, the design of ordinary semiconductor devices might be compared to the design of nanogenerators for energy harvesting.
Transistor principles inspired the fabrication of OCT-TENG, which has triboelectric enhancement with reverse charge enhancement. The porosity and pore depth of the functional rubber film can be controlled. Using transfer printing to manufacture micropores, periodic porous morphology can be achieved on the surface of rubber molds while maintaining the typical mechanical strength and super stretchability of rubber. Coupled 3D printing is also a new research hotspot. The coupled 3D printing transfer printing technology developed by researchers can produce double-layer frictional PDMS films, which can provide better charge transfer efficiency [
100]. A breakthrough in nanogenerator friction layer material allows designers to choose materials with low maintenance costs, low raw material costs, excellent manufacturability, environmental friendliness, and high power generation performance, greatly expanding nanogenerator use scenarios. The concept of TENG networking to produce electricity and create a power-generating array was put out by Wang et al. in 2017 [
101,
102]. The use cases of TENG may be increased by arranging the grid in a sensible way [
103]. Thirdly, an essential solution to the energy storage issues of nanogenerator power-generating networks may be the development of high-performance asymmetric supercapacitors (ASCs), hybrid supercapacitors (HSCs), and hybrid energy storage devices (BSHs) comprising supercapacitors and lithium-ion batteries. Supercapacitors are low-energy-density devices with high power densities. Lithium batteries, on the other hand, have a high energy density but a low power density; the two together will effectively address the technical issues with energy release and storage [
104,
105,
106,
107,
108,
109,
110]. Eventually, the key to repairing mechanical damage is to develop materials that are highly wear-resistant, self-lubricating, and self-healing. However, the best way to address wear and friction is to develop solid–liquid TENG that does not have an interaction structure or mutual corrosion.