Recent Progress of Nanogenerators for Green Energy Harvesting: Performance, Applications, and Challenges

Natural sources of green energy include sunshine, water, biomass, geothermal heat, and wind. These energies are alternate forms of electrical energy that do not rely on fossil fuels. Green energy is environmentally benign, as it avoids the generation of greenhouse gases and pollutants. Various systems and equipment have been utilized to gather natural energy. However, most technologies need a huge amount of infrastructure and expensive equipment in order to power electronic gadgets, smart sensors, and wearable devices. Nanogenerators have recently emerged as an alternative technique for collecting energy from both natural and artificial sources, with significant benefits such as light weight, low-cost production, simple operation, easy signal processing, and low-cost materials. These nanogenerators might power electronic components and wearable devices used in a variety of applications such as telecommunications, the medical sector, the military and automotive industries, and internet of things (IoT) devices. We describe new research on the performance of nanogenerators employing several green energy acquisition processes such as piezoelectric, electromagnetic, thermoelectric, and triboelectric. Furthermore, the materials, applications, challenges, and future prospects of several nanogenerators are discussed.


Introduction
The internet of things (IoT) gadgets, smart sensors, internet of medical things (IoMT) for healthcare systems, and consumer electronics devices have seen significant expansion in recent years. These devices often employ traditional batteries, which have drawbacks owing to their huge size, finite lifetime, and harmful components that contaminate the environment [1][2][3]. This issue with traditional batteries may restrict the efficiency of future IoT gadgets, smart sensors, and wearable devices. Thus, new eco-friendly alternative technologies to power these gadgets are current and future research challenges. Recent studies [4][5][6][7][8] have described nanogenerators capable of harvesting green energy by several transduction methods such as the piezoelectric, triboelectric, electromagnetic, and thermoelectric effects. The nanogenerators can harvest green energy from natural and artificial 2.1. 1

. Piezoelectric Nanogenerators
The piezoelectric nanogenerators (PENGs) use the piezoelectric effect to capture green energy from ocean water waves, wind, biomechanical movements, and environmental mechanical vibrations. The output voltage of this type of nanogenerator is affected by mechanical deformations and the parameters of its piezoelectric layer. Mechanical vibrations in the environment can induce varied deformations in the piezoelectric nano-generators that generate the AC output voltage. A piezoelectric layer, a substrate, and two electrodes make up these nanogenerators. PENGs feature a basic structural design, easy performance, a simple construction method, high stability, and a low cost [36][37][38][39][40][41][42][43][44].

Electromagnetic Nanogenerators
Electromagnetic generators (EMGs) employ magnetic materials and coils to function according to the Faraday law. These generators may convert the kinetic energy of flowing water into electricity [45]. This wave flow is utilized to vary the location of the magnet material relative to the coil, resulting in a changing magnetic field that induces a voltage in the coil. However, as compared to triboelectric nanogenerators, these generators can have a larger volume and weight. Furthermore, EMGs require support structures that let them float on the water's surface [46]. The performance of electromagnetic nanogenerators is determined by the rate of change of the magnetic flux. EMGs can be made to function at frequencies comparable to those of ocean waves to scavenge energy from them. Ocean waves move randomly at low frequencies of roughly 1 Hz [47]. The EMGs' performance is limited by their low frequency. Due to wind sources and environmental mechanical vibrations, which may function at higher frequencies, EMGs are ideal for scavenging green energy.
TENGs may be configured to function in four basic modes ( Figure 1): vertical contactseparation (CS), lateral sliding (LS), single-electrode (SE), and freestanding triboelectriclayer (FSTL). TENGs usually require two triboelectric surfaces and two electrodes. Electron attraction between two triboelectric surfaces creates an electrostatic charge transfer from one surface to another in these operational modes. The displacement of the triboelectric layers changes their initial electrostatic state, resulting in an electric potential difference between the layers. The potential difference drives the current through the external load to balance the electrostatic state. The movement of the triboelectric layer in the opposite direction will generate a difference in the current flow. TENGs can therefore have alternating current (AC) voltages between their two output electrodes, depending on the triboelectric material type, operating mechanism, and green energy source.

Piezotronic Nanogenerators
The piezotronic nanogenerators harvest low-frequency vibration/friction energy into electricity by using the linked piezoelectric and semiconducting capabilities of nanowires/ nanobelts, as well as the influence of a Schottky barrier at the metal-semiconductor [73,74].
These nanogenerators might be incorporated into textile strands to recycle energy generated by human movement. Thus, the piezoelectronic nanogenerator is a potentially useful technology for harvesting/recycling energy from the environment to power self-powered nanodevices that may be operated wirelessly and remotely. This technique will enable selfpowered wireless nanosystems and nanodevices to have a sustained energy supply [75]. Reprinted with permission from [72]. Copyright 2014, Royal Society of Chemistry.

Piezotronic Nanogenerators
The piezotronic nanogenerators harvest low-frequency vibration/friction energy into electricity by using the linked piezoelectric and semiconducting capabilities of nanowires/nanobelts, as well as the influence of a Schottky barrier at the metal-semiconductor [73,74]. These nanogenerators might be incorporated into textile strands to recycle energy generated by human movement. Thus, the piezoelectronic nanogenerator is a potentially useful technology for harvesting/recycling energy from the environment to power selfpowered nanodevices that may be operated wirelessly and remotely. This technique will enable self-powered wireless nanosystems and nanodevices to have a sustained energy supply [75].

Thermal Energy
Thermoelectric and pyroelectric nanogenerators can transform thermal energy from the environment into electrical energy to power electronic devices.

Thermoelectric and Pyroelectric Nanogenerators
Another sort of green energy that may be obtained from the environment is thermal energy. This energy may be transformed into electric energy and used to power lowpower electronic devices employing thermoelectric nanogenerators (TEGs) [76]. TEGs produce electricity by using the Seebeck effect to scavenge thermal energy caused by temperature differences between two thermoelectric (TE) materials ( Figure 2). This temperature differential causes charge carriers to migrate from a high-temperature TE material to a low-temperature TE material [77,78]. A TEG's voltage output is proportional to the temperature gradient. TEGs, on the other hand, need significant temperature gradients across TE materials. TEGs are classified into two types: rigid thermoelectric nanogenerators and flexible thermoelectric nanogenerators, with the latter depending on their deformation properties. Stretchable, compressible, collapsible, lightweight, tiny in volume, affordable, and simple are advantages of TEGs [79][80][81]. Flexible TEGs have the potential to be employed in waste heat recovery [82][83][84], portable electronics [85][86][87], and human health Reprinted with permission from [72]. Copyright ©2014, Royal Society of Chemistry.

Thermal Energy
Thermoelectric and pyroelectric nanogenerators can transform thermal energy from the environment into electrical energy to power electronic devices.

Thermoelectric and Pyroelectric Nanogenerators
Another sort of green energy that may be obtained from the environment is thermal energy. This energy may be transformed into electric energy and used to power low-power electronic devices employing thermoelectric nanogenerators (TEGs) [76]. TEGs produce electricity by using the Seebeck effect to scavenge thermal energy caused by temperature differences between two thermoelectric (TE) materials ( Figure 2). This temperature differential causes charge carriers to migrate from a high-temperature TE material to a lowtemperature TE material [77,78]. A TEG's voltage output is proportional to the temperature gradient. TEGs, on the other hand, need significant temperature gradients across TE materials. TEGs are classified into two types: rigid thermoelectric nanogenerators and flexible thermoelectric nanogenerators, with the latter depending on their deformation properties. Stretchable, compressible, collapsible, lightweight, tiny in volume, affordable, and simple are advantages of TEGs [79][80][81]. Flexible TEGs have the potential to be employed in waste heat recovery [82][83][84], portable electronics [85][86][87], and human health monitoring due to their properties [88][89][90].
Pyroelectric nanogenerators (PyENGs) use the variation in spontaneous polarization inside pyroelectric materials to transform heat energy into electric energy. This is generated by oscillations of electric dipoles caused by a change in time-dependent temperature [91,92]. The creation of electric current through materials having a non-center symmetrical crystalline structure when subjected to a time-dependent temperature gradient is referred to as the pyroelectric effect [93,94].  [78]. This nanogenerator has potential application for harvesting green energy from human body heat. FESEM images with magnifications of (a) 500 μm and (b) 250 μm rGOcoated fabric and (c) 200 μm of rGO-PEDPT:PSS coated fabric. (d-f) Color mapping of SEM images' infrared rendering. Reprinted with permission from [78]. Copyright 2021, Royal Society of Chemistry.
Pyroelectric nanogenerators (PyENGs) use the variation in spontaneous polarization inside pyroelectric materials to transform heat energy into electric energy. This is generated by oscillations of electric dipoles caused by a change in time-dependent temperature [91,92]. The creation of electric current through materials having a non-center symmetrical crystalline structure when subjected to a time-dependent temperature gradient is referred to as the pyroelectric effect [93,94].
Pyroelectric nanogenerators have been identified as the energy collectors of the future, with the potential to be a viable energy technology for scavenging thermal energy in everyday life [94]. Thus, PyENGs and TEGs may have significant uses in powering future intelligent electronic sensors and IoT-connected wearable devices. More investigations on inorganic and organic materials, structure, performance, and reliability are required for the development of these nanogenerators.
Pyroelectric nanogenerators have been identified as the energy collectors of the future, with the potential to be a viable energy technology for scavenging thermal energy in everyday life [94]. Thus, PyENGs and TEGs may have significant uses in powering future intelligent electronic sensors and IoT-connected wearable devices. More investigations on inorganic and organic materials, structure, performance, and reliability are required for the development of these nanogenerators.

Performance and Applications
Hu et al. [155] designed an eco-friendly fabric-based TENG for converting biomechanical energy into electric energy, which can then be utilized to drive self-powered gadgets and wearable electronic sensors. This energy may be acquired by everyday human movements including leaping, jogging, walking, arms lifting, arms bending, and leg lifting. This TENG is made up of cellulose-based conductive macrofibers with key properties such as being super-strong, biodegradable, and washable. As illustrated in Figure  4, these microfibers were created by wet-stretching and wet-twisting bacterial cellulose (BC) hydro-gel with polypyrrole (PPy) and carbon nanotubes (CNTs). The microfibers were woven into a nylon fabric to generate the cellulose-based/nylon macrofiber. In this scenario, nylon serves as a positive triboelectric material, and a silver thin membrane is attached to a PDMS thin membrane to form a PDMS/silver film. As a result, the TENG features a cellulose-based/nylon macrofiber fabric that acts as a friction layer/electrode and a PDMS/silver layer that acts as a second friction film/electrode. The proposed microfibers demonstrated great tensile strength (449 MPa), strong electrical conductivity (5.32 Scm −1 ), and good stability. The highest open-circuit voltage of the TENG is 170 V, the short-circuit current is 0.8 μA, and the output power is 352 μW. ( Figure 5). Furthermore, these TENG may function as self-powered devices for tracking human body motions (Figure 6).

Performance and Applications
Hu et al. [155] designed an eco-friendly fabric-based TENG for converting biomechanical energy into electric energy, which can then be utilized to drive self-powered gadgets and wearable electronic sensors. This energy may be acquired by everyday human movements including leaping, jogging, walking, arms lifting, arms bending, and leg lifting. This TENG is made up of cellulose-based conductive macrofibers with key properties such as being super-strong, biodegradable, and washable. As illustrated in Figure 4, these microfibers were created by wet-stretching and wet-twisting bacterial cellulose (BC) hydro-gel with polypyrrole (PPy) and carbon nanotubes (CNTs). The microfibers were woven into a nylon fabric to generate the cellulose-based/nylon macrofiber. In this scenario, nylon serves as a positive triboelectric material, and a silver thin membrane is attached to a PDMS thin membrane to form a PDMS/silver film. As a result, the TENG features a cellulosebased/nylon macrofiber fabric that acts as a friction layer/electrode and a PDMS/silver layer that acts as a second friction film/electrode. The proposed microfibers demonstrated great tensile strength (449 MPa), strong electrical conductivity (5.32 Scm −1 ), and good stability. The highest open-circuit voltage of the TENG is 170 V, the short-circuit current is 0.8 µA, and the output power is 352 µW. ( Figure 5). Furthermore, these TENG may function as self-powered devices for tracking human body motions ( Figure 6).
Zhao et al. [156] produced a triboelectric-electromagnetic hybrid nanogenerator (TEHG) that can gather wind energy while also powering electronic gadgets. This nanogenerator is made up of a TENG that operates in the sliding independent triboelectric-layer mode and an EMG that operates in the rotating mode. Figure 7 depicts the structure and materials of the TEHG, which is made up of a rotor and a stator. The stator has a cylindrical shell that is sealed, while the rotor has a disk and a projecting cylinder. This cylinder features an inside cylinder that can accommodate wind cups to convert environmental wind energy into mechanical energy. The cylindrical magnets of the EMG are positioned in ten cylindrical grooves on the upper surface of the rotor disk. The completed TEHG structure has an outside diameter of 80 mm and a height of 20 mm. The TENG employs PTFE and nylon as triboelectric layers that are in touch with one another. The PTFE functions as a 0.3 mm thick negative friction layer, while the nylon acts as a positive friction substance. Six aluminum electrodes are joined as interdigital electrodes on the nylon layer. Furthermore, the bottom of the shell has nine grooves for installing the copper coils of the EMG. These coils are wired in series to boost the output signal. The rotation of the TEHG structure caused by the wind source causes surface charge transfer between the two triboelectric layers. During the TEHG rotation process, an alternating current with a changing direction is produced. Figure 8 displays the TEHG's output open-circuit voltage and short-circuit current readings at various rotation speeds. Peak-to-peak voltage and peak-to-peak current of the TENG grow from 106 V to 190 V and 2.27 µA to 14.6 µA, respectively, with rotation speeds ranging from 100 rpm to 900 rpm (14 m/s of wind speed). The output response of the EMG is determined by the relative rotation of the magnet and coil based on electromagnetic induction (Faraday's law). The peak-to-peak voltage and peak-to-peak current rise from 5 V to 38 V and 3.3 mA to 20 mA, respectively, as the rotation speed increases from 100 rpm (5 m/s of wind speed) to 900 rpm. The TENG has a maximum average output power of 0.33 mW at an ideal load resistance of 12 MΩ. The EMG, on the other hand, has a maximum average output power of 32.87 mW and a maximum load resistance of 1.25 KΩ. The TEHG was evaluated for its ability to provide energy to wireless sensor network nodes. For this, Zhao et al. designed a circuit that incorporates a test device for measuring voltage changes and a cell phone for receiving data from the node (Figure 9). The TEHG was used to light up 200 LEDs in tandem and power an ambient humidity and temperature sensor at a rotation speed of 400 rpm (9 m/s of wind speed).
OR PEER REVIEW 7 of 33    [155]. Images of (b) BC macrofibers, (c) BC/CNT macrofibers, and (d) BC/CNT/PPy macrofibers. Reprinted with permission from [155]. Copyright 2022, Springer Nature. (f) Results of the instantaneous power in relation to external load resistance, measurements of the output voltage of the fabric-based TENG considering (g) several impact forces at 1 Hz, (h) relative humidity variations, (i) before and after washing, (j) contact-separation mode with a frequency of 1 Hz during 100 s, and (k) mechanical strains with repetition of 100 cycles. Reprinted with permission from [155]. Copyright 2022, Springer Nature. Zhao et al. [156] produced a triboelectric-electromagnetic hybrid nanogenerator (TEHG) that can gather wind energy while also powering electronic gadgets. This nanogenerator is made up of a TENG that operates in the sliding independent triboelectriclayer mode and an EMG that operates in the rotating mode. Figure 7 depicts the structure and materials of the TEHG, which is made up of a rotor and a stator. The stator has a cylindrical shell that is sealed, while the rotor has a disk and a projecting cylinder. This cylinder features an inside cylinder that can accommodate wind cups to convert environmental wind energy into mechanical energy. The cylindrical magnets of the EMG are positioned in ten cylindrical grooves on the upper surface of the rotor disk. The completed TEHG structure has an outside diameter of 80 mm and a height of 20 mm. The TENG employs PTFE and nylon as triboelectric layers that are in touch with one another. The PTFE functions as a 0.3 mm thick negative friction layer, while the nylon acts as a positive friction substance. Six aluminum electrodes are joined as interdigital electrodes on the nylon layer. Furthermore, the bottom of the shell has nine grooves for installing the copper coils of the EMG. These coils are wired in series to boost the output signal. The rotation of the TEHG structure caused by the wind source causes surface charge transfer between the two triboelectric layers. During the TEHG rotation process, an alternating current with a changing direction is produced. Figure 8 displays the TEHG's output open-circuit voltage and short-circuit current readings at various rotation speeds. Peak-to-peak voltage and peak-to-peak current of the TENG grow from 106 V to 190 V and 2.27 μA to 14.6 μA, respectively, with rotation speeds ranging from 100 rpm to 900 rpm (14 m/s of wind speed). The output response of the EMG is determined by the relative rotation of the magnet and coil based on electromagnetic induction (Faraday's law). The peak-to-peak voltage and peak-to-peak current rise from 5 V to 38 V and 3.3 mA to 20 mA, respectively, as the rotation speed increases from 100 rpm (5 m/s of wind speed) to 900 rpm. The TENG has a maximum average output power of 0.33 mW at an ideal load resistance of 12 MΩ. The EMG, on the other hand, has a maximum average output power of 32.87 mW and a maximum load resistance of 1.25 KΩ. The TEHG was evaluated for its ability to provide energy to wireless sensor network nodes. For this, Zhao et al. designed a circuit that incorporates a test device for measuring voltage changes and a cell phone for receiving data from the node (Figure 9). The TEHG was used to light up 200 LEDs in tandem and power an ambient humidity and temperature sensor at a rotation speed of 400 rpm (9 m/s of wind speed).   Wu et al. [157] introduced a hybrid energy cell ( Figure 10) that combines a TENG, an electrochemical cell (EC), and eight amorphous silicon-based solar cells (SCs) to gather wind, chemical, and solar energies from the environment simultaneously or independently. This hybrid energy cell might power low-power electronic devices such as wind speed sensors and temperature sensors. The key benefit of this technology is its capacity to scavenge three separate energy sources at the same time, which improves the usage of energy from the environment. A polytetrafluoroethylene (PTFE) film and an Al film are bonded to two acrylic tubes to form the TENG. The periodic contact/separation between the PTFE film and the Al film can generate charge transfer between the Al electrode and the ground by utilizing the coupling between the triboelectric effect and the electrostatic effect ( Figure 11). First, both the Al and PTFE films are in an aligned position, where the two surfaces are completely in touch with one other. The two films have opposing charge polarities and are entirely balanced in this configuration, resulting in no Wu et al. [157] introduced a hybrid energy cell (Figure 10) that combines a TENG, an electrochemical cell (EC), and eight amorphous silicon-based solar cells (SCs) to gather wind, chemical, and solar energies from the environment simultaneously or independently. This hybrid energy cell might power low-power electronic devices such as wind speed sensors and temperature sensors. The key benefit of this technology is its capacity to scavenge three separate energy sources at the same time, which improves the usage of energy from the environment. A polytetrafluoroethylene (PTFE) film and an Al film are bonded to two acrylic tubes to form the TENG. The periodic contact/separation between the PTFE film and the Al film can generate charge transfer between the Al electrode and the ground by utilizing the coupling between the triboelectric effect and the electrostatic effect ( Figure 11). First, both the Al and PTFE films are in an aligned position, where the two surfaces are completely in touch with one other. The two films have opposing charge polarities and are entirely balanced in this configuration, resulting in no electron flow across from Al film to PTFE film. This sequence is completed, and the mismatch between the two films is obtained. Since the relative rotation of both films continues, the PTFE film travels back to touch the Al film, creating electrons that flow from the Al electrode to the ground. This electrostatic induction action can lead the TENG's output signals to increase, indicating that the charges are entirely balanced. An alternating electric shape output is obtained during a TENG operation cycle. Figure 12 depicts the manufactured TENG's output signals. This TENG has an open-circuit voltage approaching 90 V, a shortcircuit current density close to 0.5 mA/m 2 , and a maximum power density of 16 mW/m 2 , allowing it to power up 20 blue light-emitting diodes directly (LEDs). For charging a capacitor, the hybrid energy cell outperformed separate energy units significantly. The hybrid device's gathered energy can be stored in a Li-ion battery as a controlled power module for powering electronic equipment. Increasing the surface roughness and effective surface area of the triboelectric material induces a higher triboelectric charge density and improves TENG output performance. Nanomaterials 2022, 12, x FOR PEER REVIEW 12 of 33 electron flow across from Al film to PTFE film. This sequence is completed, and the mismatch between the two films is obtained. Since the relative rotation of both films continues, the PTFE film travels back to touch the Al film, creating electrons that flow from the Al electrode to the ground. This electrostatic induction action can lead the TENG's output signals to increase, indicating that the charges are entirely balanced. An alternating electric shape output is obtained during a TENG operation cycle. Figure 12 depicts the manufactured TENG's output signals. This TENG has an open-circuit voltage approaching 90 V, a short-circuit current density close to 0.5 mA/m 2 , and a maximum power density of 16 mW/m 2 , allowing it to power up 20 blue light-emitting diodes directly (LEDs). For charging a capacitor, the hybrid energy cell outperformed separate energy units significantly.
The hybrid device's gathered energy can be stored in a Li-ion battery as a controlled power module for powering electronic equipment. Increasing the surface roughness and effective surface area of the triboelectric material induces a higher triboelectric charge density and improves TENG output performance.  The collagen fibrils of vegetables, fruits, and plants may be the responsible constituents of these natural materials' piezoelectricity. The piezoelectricity in collagen fibrils is caused by intermolecular hydrogen bonding, which results in a uniaxial orientation of the molecular dipoles [158]. Tomato peels (TPs), for example, include 16 different amino acids [159] and non-centrosymmetric properties due to their low symmetrical orthorhombic and monoclinic space groups, which might contribute to the piezoelectric effect [159]. Furthermore, the TPs feature structures with significant porosity, which causes additional displacement owing to applied external stresses, boosting the TPs' piezoelectricity [160,161]. Furthermore, the hydroxyl groups in TPs' lutein and zeaxanthin contribute to their piezoelectricity. Hydrogen bonding occurs in the hydroxyl group due to the extremely electropositive hydrogen and electronegative oxygen atoms [160]. According to the findings of these studies, TPs can be employed to generate piezoelectric energy. Nanomaterials 2022, 12, x FOR PEER REVIEW 13 of 33 Figure 11. Stages of the operation principle of the TENG used in the hybrid energy cell fabricated by Wu et al. [157]. Reprinted with permission from [157]. Copyright 2014, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
The collagen fibrils of vegetables, fruits, and plants may be the responsible constituents of these natural materials' piezoelectricity. The piezoelectricity in collagen fibrils is caused by intermolecular hydrogen bonding, which results in a uniaxial orientation of the molecular dipoles [158]. Tomato peels (TPs), for example, include 16 different amino acids [159] and non-centrosymmetric properties due to their low symmetrical orthorhombic and mono-clinic space groups, which might contribute to the piezoelectric effect [159]. Furthermore, the TPs feature structures with significant porosity, which causes additional displacement owing to applied external stresses, boosting the TPs' piezoelectricity [160,161]. Furthermore, the hydroxyl groups in TPs' lutein and zeaxanthin contribute to their piezoelectricity. Hydrogen bonding occurs in the hydroxyl group due to the extremely electropositive hydrogen and electronegative oxygen atoms [160]. According to the findings of these studies, TPs can be employed to generate piezoelectric energy.
Saqib et al. [160] examined the triboelectric and piezoelectric action of tomato peel (TP) in order to build a hybrid nanogenerator (TP-TPENG) with bio-organic nature materials for collecting green energy with potential applications in pollution-free and self-powered devices. The tomato's very porous structure boosts the TP-output TPENG's responsiveness. The open-circuit voltage, short-circuit current, and highest instantaneous power of a TP-based piezoelectric nanogenerator (TP-PENG) are 24.5 V, 2.5 μA, and 19.5 μW, respectively. The open-circuit voltage, short-circuit current, and highest instantaneous power of the TP-based triboelectric nanogenerator (TP-TENG), on the other hand, are 135 V, 81 μA, and 3750 μW, respectively. The combination of triboelectric and piezoelectric effects resulted in an enhanced TP-TPENG output response with a rectifier circuit. The rectified open circuit voltage, short circuit current, and maximum instantaneous power of this TP-TPENG are 150 V, 84 μA, and 5400 μW, respectively. Thus, TPs may be used to create unique non-toxic and eco-friendly hybrid nanogenerators based on their piezoelectric and triboelectric capabilities. This hybrid nanogenerator powered 141 commercial LEDs while also charging a 10 μF capacitor. Figure 13 depicts the primary components and materials of the hybrid nanogenerator. The TPs' hydroxyl and carbonyl groups contribute to their piezoelectric and triboelectric characteristics. Furthermore, TPs offer high flexibility and robustness. Figure 14 depicts the electrical output behavior of the TP-based nanogenerator when the piezoelectric and triboelectric effects are taken into account. The electrical output performance of the TP-TENG is superior to that of the TP-PENG. On the other hand, the combined responsiveness of TP's piezoelectric and triboelectric capabilities allows for a TP-TPENG with more superior electric output response than both TP- Figure 11. Stages of the operation principle of the TENG used in the hybrid energy cell fabricated by Wu et al. [157]. Reprinted with permission from [157]. Copyright ©2014, Tsinghua University Press and Springer-Verlag Berlin Heidelberg. PENG and TP-TENG. The alternating output signal of the hybrid nanogenerator is rectified using two rectifier circuits. In addition, the hybrid nanogenerator was used to charge four different capacitors (0.22 μF, 10 μF, 50 μF, and 100 μF). The three nanogenerators were used to power several commercial LEDs. In addition, TP-TENG and TP-TPENG with rectifier circuits were used to power many commercial stopwatches ( Figure 15).  Saqib et al. [160] examined the triboelectric and piezoelectric action of tomato peel (TP) in order to build a hybrid nanogenerator (TP-TPENG) with bio-organic nature materials for collecting green energy with potential applications in pollution-free and self-powered devices. The tomato's very porous structure boosts the TP-output TPENG's responsiveness. The open-circuit voltage, short-circuit current, and highest instantaneous power of a TPbased piezoelectric nanogenerator (TP-PENG) are 24.5 V, 2.5 µA, and 19.5 µW, respectively. The open-circuit voltage, short-circuit current, and highest instantaneous power of the TP-based triboelectric nanogenerator (TP-TENG), on the other hand, are 135 V, 81 µA, and 3750 µW, respectively. The combination of triboelectric and piezoelectric effects resulted in an enhanced TP-TPENG output response with a rectifier circuit. The rectified open circuit voltage, short circuit current, and maximum instantaneous power of this TP-TPENG are 150 V, 84 µA, and 5400 µW, respectively. Thus, TPs may be used to create unique nontoxic and eco-friendly hybrid nanogenerators based on their piezoelectric and triboelectric capabilities. This hybrid nanogenerator powered 141 commercial LEDs while also charging a 10 µF capacitor. Figure 13 depicts the primary components and materials of the hybrid nanogenerator. The TPs' hydroxyl and carbonyl groups contribute to their piezoelectric and triboelectric characteristics. Furthermore, TPs offer high flexibility and robustness. Figure 14 depicts the electrical output behavior of the TP-based nanogenerator when the piezoelectric and triboelectric effects are taken into account. The electrical output performance of the TP-TENG is superior to that of the TP-PENG. On the other hand, the combined responsiveness of TP's piezoelectric and triboelectric capabilities allows for a TP-TPENG with more superior electric output response than both TP-PENG and TP-TENG. The alternating output signal of the hybrid nanogenerator is rectified using two rectifier circuits. In addition, the hybrid nanogenerator was used to charge four different capacitors (0.22 µF, 10 µF, 50 µF, and 100 µF). The three nanogenerators were used to power several commercial LEDs. In addition, TP-TENG and TP-TPENG with rectifier circuits were used to power many commercial stopwatches ( Figure 15).
Gokana et al. [91] developed a pyroelectric nanogenerator (PyNG) capable of producing electric energy from waste heat in the environment. As illustrated in Figure 16, this PyNG is made using a screen-printed serpentine electrode (SRE) that has been modified with cesium tungsten bronze (Cs 0.33 WO 3 ). Furthermore, Cs 0.33 WO 3 was applied to both the electrode and the PVDF sheets. With a load resistance of 20 ΩM, the PyNG with 7 wt% Cs 0.33 WO 3 can reach a temperature of 121 • C and electrical output voltage, current, and power density of 4.36 V, 214 nA, and 23.38 µW/m 2 , respectively. A liquid crystal display (LCD) and four LEDs were powered by the proposed PyNG. This PyNG is an alternate source for capturing solar energy and powering low-power electrical gadgets. Figure 16 depicts the various materials and components utilized in the fabrication of the PyNG, as well as the temperature values of its electrodes determined by IR thermographic imaging. The thermoelectric conversion behavior of the PyNG was evaluated by measuring its thermal and electrical output responses during heating and cooling at 8 mHz switching frequency. Under near-infrared (NIR) radiation, the PVDF PyNG with 7 wt% Cs 0.33 WO 3 quickly registered 75 • C and reverted to 29 • C after 60 s of radiation removal. Using the same experimental conditions, PVDF PyNG without Cs 0.33 WO 3 reached 58 • C and reverted to 29 • C. As a result, the rate of temperature changes of PVDF/Cs 0.33 WO 3 PyNG was increased by over 27% when compared to pure PVDF PyNG. Furthermore, as compared to the pure PVDF PyNG, the electrical output voltage and current of the PVDF/ Cs 0.33 WO 3 PyNG increased by around 26% and 16%, respectively ( Figure 17). The PyNG's better performance might be attributed to its high photothermal conversion behavior and NIR light absorption. When NIR light is shone on the improved PyNG with a 10 µF capacitor, the PyNG may activate four LEDs and display an LCD, as illustrated in Figure 18.    Gokana et al. [91] developed a pyroelectric nanogenerator (PyNG) capable of producing electric energy from waste heat in the environment. As illustrated in Figure 16, this PyNG is made using a screen-printed serpentine electrode (SRE) that has been modified with cesium tungsten bronze (Cs0.33WO3). Furthermore, Cs0.33WO3 was applied to both the electrode and the PVDF sheets. With a load resistance of 20 ΩM, the PyNG with 7 wt% Cs0.33WO3 can reach a temperature of 121 °C and electrical output voltage, current, and power density of 4.36 V, 214 nA, and 23.38 μW/m 2 , respectively. A liquid crystal display (LCD) and four LEDs were powered by the proposed PyNG. This PyNG is an alternate source for capturing solar energy and powering low-power electrical gadgets. Figure  16depicts the various materials and components utilized in the fabrication of the PyNG, as well as the temperature values of its electrodes determined by IR thermographic imaging. The thermoelectric conversion behavior of the PyNG was evaluated by measuring its thermal and electrical output responses during heating and cooling at 8 mHz switching frequency. Under near-infrared (NIR) radiation, the PVDF PyNG with 7 wt% Cs0.33WO3 quickly registered 75 °C and reverted to 29 °C after 60 s of radiation removal. Using the same experimental conditions, PVDF PyNG without Cs0.33WO3 reached 58 °C and reverted to 29 °C. As a result, the rate of temperature changes of PVDF/Cs0.33WO3 PyNG was increased by over 27% when compared to pure PVDF PyNG. Furthermore, as compared to the pure PVDF PyNG, the electrical output voltage and current of the PVDF/ Cs0.33WO3 PyNG increased by around 26% and 16%, respectively ( Figure 17). The PyNG's better performance might be attributed to its high photothermal conversion behavior and NIR light absorption. When NIR light is shone on the improved PyNG with a 10 μF capacitor, the PyNG may activate four LEDs and display an LCD, as illustrated in Figure 18.      Table 1 reported the comparison of the main characteristics of various nanogenerator types, including the transduction mechanism, energy source, materials, advantages, and weaknesses. For a single transduction mechanism, the triboelectric nanogenerators have important characteristics such as high electrical output performance, good stability, sim-  Table 1 reported the comparison of the main characteristics of various nanogenerator types, including the transduction mechanism, energy source, materials, advantages, and weaknesses. For a single transduction mechanism, the triboelectric nanogenerators have important characteristics such as high electrical output performance, good stability, simple structure, and cost-efficient fabrication. In addition, this nanogenerator can be designed to convert different energy sources (e.g., biomechanical, water waves, environmental vibrations, and wind) into electrical energy. On the other hand, the hybrid nanogenerators present an enhanced electrical output performance in comparison with nanogenerators based on a single transduction mechanism. The hybrid nanogenerators that include the triboelectric effect have the best operation parameters such as compact and simple structure, good flexibility, stability, low-cost manufacturing, and high electrical output performance.

Challenges and Perspectives
This section discusses the main difficulties and prospects for nanogenerators in terms of design, materials, output performance, reliability, and prospective applications.

Design
The nanogenerator's design phase is critical to achieving the greatest performance for certain applications. This stage of design must take into account the various requirements and working circumstances of the prospective application of the nanogenerators. Thus, nanogenerator designers should investigate the required electric power, size and weight limitations, working time, and environmental conditions (relative humidity, temperature, pressure, radiation, wind, vibrations, dust, and so on), green energy acquisition mechanisms, materials more suitable for nanogenerator electromechanical behavior, fabrication process, packaging type, minimum electronic components, and so on. Analytical and numerical modeling may be utilized in the design of nanogenerators to find the best operation principle, materials, and electromechanical configuration that allow for the safety and reliability of the nanogenerators under various working conditions. Furthermore, numerical simulation tools such as ANSYS, COMSOL, NASTRAN, ABAQUS, and MATLAB may be used to evaluate the performance of nanogenerators. As a result, the designers may create bidimensional or tridimensional models of the nanogenerators, complete with their major components, materials, and operating conditions. However, due to errors in the selection of the materials' characteristics, load values and analysis types, boundary conditions, and mesh size, the findings of the analytical and numerical simulation models of the nanogenerators might have a large error percentage in comparison to experimental results. To reduce this error percentage, designers should evaluate the important elements that determine the performance of the nanogenerators, such as genuine values of material characteristics, the right selection of load and boundary conditions, and the mesh quality and type of the suggested models.

Materials
The appropriate material selection for nanogenerators is critical for improving their performance and reliability for each prospective application. Each nanogenerator application necessitates unique performance characteristics in order to maximize the green energy collecting process from various natural sources and under varying climatic conditions. Thus, the design of a specific nanogenerator requires superior materials to meet the electrical signal requirements and performance stability for future applications. Nanogenerators for biomechanical applications, for example, may need stretchable, lightweight, and flexible materials to improve their output electrical responsiveness and mechanical behavior [162]. These materials must have the structural strength to minimize wear and mechanical failures in this application, as well as low density to reduce the weight of the nanogenerators. Furthermore, the operation of nanogenerators might be harmed by environmental dampness. The application of hydrophobic materials or materials to nanogenerator packing can be used to solve the humidity problem. For instance, nanogenerators for blue energy harvesting are built with specific packaging made of low-density materials that are resistant to corrosion and solar radiation. Nanogenerators that operate under mechanical vibrations, on the other hand, should be designed with materials that have adequate structural strength to reduce structural failures caused by cracks, wear, fatigue, or fracture. Recent nanogenerator research [39,63,64,160,[163][164][165][166][167][168][169] has focused on organic or waste materials from the environment, such as tomato, chitin, eggshell, fish swim bladder, spider silk, peanut shell, sunflower husks, rice paper, garbage soda cans, silk fibroin, coconut husk, and so on. However, measuring the piezoelectric and triboelectric properties of these materials is extremely difficult. Another significant constraint is the analytical modeling of the piezoelectric and triboelectric response of organic materials. More research is needed to determine the piezoelectric and triboelectric effects of organic or waste materials employed in nanogenerators for green energy harvesting.

Energy Storage and Electrical Interfaces
The electrical output performance of nanogenerators can be impacted by changes in ambient circumstances and green energy stability, which can vary over time and exhibit erratic behavior. Due to these circumstances, the output electrical signals of nanogenerators might exhibit erratic behavior. In addition, most electronic equipment must be powered by DC voltage and current. The nanogenerators require rectifier circuits to convert their AC output electrical impulses into DC signals in order to power these devices. These DC signals must also be stored in capacitors or batteries in order to power electronic devices with controlled electrical signals. The development of effective energy storage devices is an intriguing research problem for nanogenerators.
Another important challenge of the nanogenerators is the development of electrical interfaces that achieve high efficiency with minimum power consumption [170][171][172]. For instance, these electrical interfaces could be self-powered and consider cold-start circuit architectures [173,174]. To reduce the size of the nanogenerators, the electrical interfaces should have a small footprint. It could be obtained using the Application-Specific Integrated Circuit (ASIC) implementation. Moreover, the electrical interfaces could be adaptive in order to maximize the harvested power, considering low-power maximum power point tracking (MPPT) algorithms [174][175][176][177].

Fabrication
To create nanogenerators for commercial uses, large-scale manufacture should be enabled. Alternatives to this difficulty include no-complex manufacturing procedures and new infrastructure with flexible phases for new nanogenerator designs and use. Furthermore, a low-cost production technique with few processing steps is critical for the market feasibility of nanogenerators. Another possibility to reduce nanogenerator manufacture costs is to re-use inorganic or organic materials from trash [63]. Furthermore, future nanogenerator manufacturing processes may contain biodegradable and environmentally benign materials.

Reliability
Future studies will focus on the stability and reliability of the electromechanical behavior of nanogenerators. For commercial applications, nanogenerators must provide output electrical signals that are stable throughout time. During the life of a nanogenerator, its electrical and structural components may have performance issues owing to abrasion, mechanical impact, crack development, fatigue, humidity, radiation, high temperature, environmental pollution, and other factors. To extend the life of nanogenerators, basic structural layouts with the fewest number of electrical and mechanical components should be considered. Electrical or mechanical failures of some of these components can affect the operation of nanogenerators with complicated structural designs that involve multiple components. To decrease the possibility of electrical and mechanical failures in the various nanogenerator components, the use of robust materials and appropriate packaging might increase the service life of nanogenerators.

Conclusions
The most recent advances in nanogenerators for green energy harvesting via various transduction processes were discussed. Triboelectric, piezoelectric, electromagnetic, and thermoelectric effects were all explored in these processes. The principles of operation and materials of several nanogenerators are reviewed. In addition, the behavior of the output electrical signals (voltage, current, and power) of multiple nanogenerators was considered, taking into account the combination of green energy acquisition processes. It was stated that nanogenerators were used to power several commercial electronic products. This review also discussed the problems and perspectives of nanogenerators in design, materials, energy storage, fabrication, and reliability.