**3.2 TENGs harvesting energy from vibration**

Vibration, as a type of common mechanical phenomena, ubiquitously exists in ambient environment in a variety of forms and wide range of scales. Therefore, vibration can be regarded as a sustainable source of power for driving small electronics if it can be effectively collected. Contributing to the distinctive working mechanism, TENG has been proposed recently and proved a promising approach for scavenging mechanical energy from vibration, especially in the low-frequency range. To date, a variety of device and machine-based TENGs have been applied to convert mechanical energy induced from vibration into electric energy.

Chen et al. presented a harmonic-resonator-based TENG as a sustainable power source and an active vibration sensor [60]. The harmonic-resonator-based TENG, held a multilayer structure consisting of aluminum with nanoporous surface as contact electrode and nanowire-modified PTFE as frictional layer, is the first TENG that can harness random and tiny ambient vibration. It can effectively respond to vibration frequencies ranging from 2 to 200 Hz with a considerably wide working bandwidth of 13.4 Hz.

The above-mentioned harmonic resonator-based TENG with a simple structure design can only scavenge vibration energy from a single direction. In practice, vibrations in living environments generally display multiple motion directions. With this in mind, a three-dimensional TENG (3D-TENG) was designed for harvesting random vibration energy from multiple directions [61]. The 3D-TENG has a multilayer structure with circular acrylic as supporting substrates, as shown schematically in **Figure 5a**. The cylindroid core of the 3D-TENG lies at the center of the acrylic substrate with a bottom diameter of 3 cm. On the top of the core, an iron mass is mobile and suspended by three identical springs with an included angle of 120° between each other. The designed structural symmetry ensures that the whole system has a constant resonant frequency in arbitrary in-plane directions. A layer of PTFE film as one contact surface is adhered onto the bottom side of

#### **Figure 5.**

*3D triboelectric nanogenerator: (a) schematic of a 3D-TENG, (b) SEM image of nanopores on an aluminum electrode, and (c) a photograph of the fabricated 3D-TENG [61].*

**73**

m3

*Small-Scale Energy Harvesting from Environment by Triboelectric Nanogenerators*

range with a maximum output power density of 2.76 W/m2

72 V, an Isc of up to 32 μA, and a peak power density of 0.4 W/m2

eters, and a micro-meteorological instrument.

After that, an elastic multiunit TENG was also realized to efficiently harvest low-frequency vibration energy over a wide frequency range [65]. The obtained TENG can provide a maximum instantaneous output power density of 102 W/

 at as low as 7 Hz and maintain its stable current outputs over a wide frequency range (from 5 to 25 Hz). Besides, it can act as an active vibration sensor to monitor the running status of equipment. Moreover, by combining the TENG with a power management unit to form a self-charging power unit, the vibration energy harvesting from ambient environment, such as an operating machine and running bicycle, can sustain power electronics such as thermometers, humidity sensors, speedom-

For improving the lower output current, a multi-layered stacked TENG was reported as a cost-effective, simple, and robust approach for harvesting ambient vibration energy [66]. The 3D-TENG has a multilayered structure with acrylic as

circular iron mass with a deposited copper thin film as the back electrode. Attached to the bottom acrylic substrate, an aluminum thin film with nanopore modification plays dual roles as a contact electrode and the other contact surface. The scanning electron microscopy (SEM) images of aluminum nanopores can be observed in **Figure 5b**. A photograph of the real 3D-TENG device is shown in **Figure 5c**. Owing to the conical-shaped spring structure, the 3D-TENG can operate in a hybridization mode combining with the vertical contact-separation mode and the in-plane sliding mode, which is beneficial to harvest random vibrational energy in multiple direc-

For better sensitivity response to external disturbance, a suspended 3D spiral structure was integrated with a TENG for energy harvesting and sensor applications [62]. Operating in the vertical contact-separation mode, the desired TENG with unstable mechanical structure can balance itself when be oscillated, which makes it a superior choice for vibration energy harvesting and vibration detection. The newly designed TENG has a wide working bandwidth of 30 Hz in low-frequency

Beyond that, a spherical three-dimensional TENG (3D-TENG) with a single electrode, consisting of an outer transparent shell and an inner polyfluoroalkoxy (PFA) ball, was designed for scavenging ambient vibration energy in full space [63]. By working at a hybridization of both the contact-separation mode and the sliding mode, the 3D-TENG can deliver a maximal output voltage of 57 V, a maximal output current of 2.3 μA, and a corresponding output power of 128 μW on a load of 100 MΩ, which can be used to directly drive tens of green light-emitting diodes. Moreover, the TENG is utilized to design the self-powered acceleration sensor with

Besides multiple motion directions, ambient vibrations generally exhibit a wide spectrum of frequency distribution. To solve this problem, a TENG with a wavystructured Cu-Kapton-Cu sandwiched between two flat nanostructured PTFE films was designed for broadband vibration energy harvesting [64]. The core of the wavy structure is composed of a set of metal rods (with a diameter of 1/4 in.), as shown in **Figure 6a**. PTFE films are processed with inductively coupled plasma (ICP) etching to produce the nanostructures shown in **Figure 6b**, which would largely enhance contact electrification. The device structure is schematically shown in **Figure 6c**, accompanied by a magnified schematic in **Figure 6d** and a picture of a real device in **Figure 6e**. This structure design allows the TENG to be self-restorable after impact without the use of extra springs and converts direct impact into lateral sliding. Based on the wavy structure, the TENG can harvest vibrational energy from 5 to 500 Hz, and the generator's resonance frequency was determined to be ∼100 Hz at a broad full width at half-maximum of over 100 Hz, producing a Voc of up to

on a load of 6 MΩ.

.

*DOI: http://dx.doi.org/10.5772/intechopen.83703*

tions over a wide bandwidth.

a detection sensitivity of 15.56 V/g.

#### *Small-Scale Energy Harvesting from Environment by Triboelectric Nanogenerators DOI: http://dx.doi.org/10.5772/intechopen.83703*

circular iron mass with a deposited copper thin film as the back electrode. Attached to the bottom acrylic substrate, an aluminum thin film with nanopore modification plays dual roles as a contact electrode and the other contact surface. The scanning electron microscopy (SEM) images of aluminum nanopores can be observed in **Figure 5b**. A photograph of the real 3D-TENG device is shown in **Figure 5c**. Owing to the conical-shaped spring structure, the 3D-TENG can operate in a hybridization mode combining with the vertical contact-separation mode and the in-plane sliding mode, which is beneficial to harvest random vibrational energy in multiple directions over a wide bandwidth.

For better sensitivity response to external disturbance, a suspended 3D spiral structure was integrated with a TENG for energy harvesting and sensor applications [62]. Operating in the vertical contact-separation mode, the desired TENG with unstable mechanical structure can balance itself when be oscillated, which makes it a superior choice for vibration energy harvesting and vibration detection. The newly designed TENG has a wide working bandwidth of 30 Hz in low-frequency range with a maximum output power density of 2.76 W/m<sup>2</sup> on a load of 6 MΩ.

Beyond that, a spherical three-dimensional TENG (3D-TENG) with a single electrode, consisting of an outer transparent shell and an inner polyfluoroalkoxy (PFA) ball, was designed for scavenging ambient vibration energy in full space [63]. By working at a hybridization of both the contact-separation mode and the sliding mode, the 3D-TENG can deliver a maximal output voltage of 57 V, a maximal output current of 2.3 μA, and a corresponding output power of 128 μW on a load of 100 MΩ, which can be used to directly drive tens of green light-emitting diodes. Moreover, the TENG is utilized to design the self-powered acceleration sensor with a detection sensitivity of 15.56 V/g.

Besides multiple motion directions, ambient vibrations generally exhibit a wide spectrum of frequency distribution. To solve this problem, a TENG with a wavystructured Cu-Kapton-Cu sandwiched between two flat nanostructured PTFE films was designed for broadband vibration energy harvesting [64]. The core of the wavy structure is composed of a set of metal rods (with a diameter of 1/4 in.), as shown in **Figure 6a**. PTFE films are processed with inductively coupled plasma (ICP) etching to produce the nanostructures shown in **Figure 6b**, which would largely enhance contact electrification. The device structure is schematically shown in **Figure 6c**, accompanied by a magnified schematic in **Figure 6d** and a picture of a real device in **Figure 6e**. This structure design allows the TENG to be self-restorable after impact without the use of extra springs and converts direct impact into lateral sliding. Based on the wavy structure, the TENG can harvest vibrational energy from 5 to 500 Hz, and the generator's resonance frequency was determined to be ∼100 Hz at a broad full width at half-maximum of over 100 Hz, producing a Voc of up to 72 V, an Isc of up to 32 μA, and a peak power density of 0.4 W/m2 .

After that, an elastic multiunit TENG was also realized to efficiently harvest low-frequency vibration energy over a wide frequency range [65]. The obtained TENG can provide a maximum instantaneous output power density of 102 W/ m3 at as low as 7 Hz and maintain its stable current outputs over a wide frequency range (from 5 to 25 Hz). Besides, it can act as an active vibration sensor to monitor the running status of equipment. Moreover, by combining the TENG with a power management unit to form a self-charging power unit, the vibration energy harvesting from ambient environment, such as an operating machine and running bicycle, can sustain power electronics such as thermometers, humidity sensors, speedometers, and a micro-meteorological instrument.

For improving the lower output current, a multi-layered stacked TENG was reported as a cost-effective, simple, and robust approach for harvesting ambient vibration energy [66]. The 3D-TENG has a multilayered structure with acrylic as

*A Guide to Small-Scale Energy Harvesting Techniques*

**3.2 TENGs harvesting energy from vibration**

mode (**Figure 4e**).

bandwidth of 13.4 Hz.

the mechanical energy from people's walking motion when it is bonded to human legs (**Figure 4d**). An excellent stability and maxmiun energy conversion efficiency of 85% are realized at a matched load resistance of 88 MU under the noncontact

Vibration, as a type of common mechanical phenomena, ubiquitously exists in ambient environment in a variety of forms and wide range of scales. Therefore, vibration can be regarded as a sustainable source of power for driving small electronics if it can be effectively collected. Contributing to the distinctive working mechanism, TENG has been proposed recently and proved a promising approach for scavenging mechanical energy from vibration, especially in the low-frequency range. To date, a variety of device and machine-based TENGs have been applied to

Chen et al. presented a harmonic-resonator-based TENG as a sustainable power source and an active vibration sensor [60]. The harmonic-resonator-based TENG, held a multilayer structure consisting of aluminum with nanoporous surface as contact electrode and nanowire-modified PTFE as frictional layer, is the first TENG that can harness random and tiny ambient vibration. It can effectively respond to vibration frequencies ranging from 2 to 200 Hz with a considerably wide working

The above-mentioned harmonic resonator-based TENG with a simple structure

design can only scavenge vibration energy from a single direction. In practice, vibrations in living environments generally display multiple motion directions. With this in mind, a three-dimensional TENG (3D-TENG) was designed for harvesting random vibration energy from multiple directions [61]. The 3D-TENG has a multilayer structure with circular acrylic as supporting substrates, as shown schematically in **Figure 5a**. The cylindroid core of the 3D-TENG lies at the center of the acrylic substrate with a bottom diameter of 3 cm. On the top of the core, an iron mass is mobile and suspended by three identical springs with an included angle of 120° between each other. The designed structural symmetry ensures that the whole system has a constant resonant frequency in arbitrary in-plane directions. A layer of PTFE film as one contact surface is adhered onto the bottom side of

*3D triboelectric nanogenerator: (a) schematic of a 3D-TENG, (b) SEM image of nanopores on an aluminum* 

*electrode, and (c) a photograph of the fabricated 3D-TENG [61].*

convert mechanical energy induced from vibration into electric energy.

**72**

**Figure 5.**

#### **Figure 6.**

*(a) Schematic of the method to fabricate wavy Kapton films. (b) SEM image of the ICP-processed PTFE film surface. (c) Schematic of the device structure. (d) Magnified schematic of the device, showing that the wavy core is in periodical contact with the nanostructures on the PTFE films. (e) Photograph of an as-fabricated TENG device before packaging [64].*

supporting substrates, as schematically shown in **Figure 7a**. A photograph of an asfabricated TENG and SEM image of the PTFE nanowires is shown in **Figure 7b**-**c**. With superior synchronization, the 3D-TENG produces a short-circuit current as high as 1.14 mA and an Voc up to 303 V with a remarkable peak power density of 104.6 W/m<sup>2</sup> . As a direct power source, it is capable of simultaneously lighting up 20 spot lights as well as a white G16 globe light.

To reduce the direct friction between triboelectric layers, a liquid-metal-based TENG (LM-TENG) was developed for high-efficiency vibration energy harvesting [67]. Owing to an intimate contact between the liquid metal and the polymer dielectric layer, the direct friction between triboelectric layers for energy loss is effectively reduced, resulting in high effective contact, shape adaptability, and low friction coefficient with solid. Therefore, the LM-TENG exhibits an output charge density of 430 μC/m<sup>2</sup> , which is four to five times higher than that in the case if the electrode is solid film.

On the other hand, soft electrode can effectively increase the contact intimacy between the triboelectric layers [68]. Xu et al. reported a novel soft and robust TENG made of a silicone rubber-spring helical structure with nanocompositebased elastomeric electrodes for harvesting arbitrary directional vibration energy and self-powered vibration sensing [69]. The schematic diagram and a photo of the S-TENG are shown in **Figure 8a,c**, respectively. As displayed, the TENG exhibits a helical structure based on the integration of elastomer and spring. A mixing well silicone rubber and carbon nanofiber, which can be stretched up to the strain of 133%, serves as the elastomeric electrode (**Figure 8b**). The working mechanisms of the S-TENG under vertical and horizontal vibration are shown in **Figure 8d,e**, respectively. Under external vertical vibration excitation, the distance between a helical structure's adjacent surfaces changes, forming a contact-separation mode TENG. Under horizontal vibration excitation, the S-TENG's helical structure's adjacent surfaces can contact on one side and separate on the other side, also forming a contact-separation mode TENG. Under the resonant states of the S-TENG, its peak power density is found to be 240 and 45 mW/m<sup>2</sup> with an external load of

**75**

**Figure 8.**

**Figure 7.**

*vibration excitation [69].*

*Small-Scale Energy Harvesting from Environment by Triboelectric Nanogenerators*

*(a) The device schematic of the S-TENG. Note that the gray silicone rubber layer containing a spring forms a base on which other layers can be built, and the black silicone rubber layer along with the electrode layer forms a contact-separation pair. Both top and bottom electrodes are made of carbon nanofiber-mixed silicone rubber. (b) SEM image of the carbon nanofiber for preparing the elastomeric electrode. (c) Photo of the as-prepared S-TENG. Working mechanisms of the S-TENG under (d) vertical vibration excitation and (e) horizontal* 

*Three-dimensional triboelectric nanogenerator. (a) Schematic of a 3D-TENG. (b) SEM image of nanopores on* 

*aluminum electrode. (c) A photograph of the as-fabricated 3D-TENG [66].*

*DOI: http://dx.doi.org/10.5772/intechopen.83703*

*Small-Scale Energy Harvesting from Environment by Triboelectric Nanogenerators DOI: http://dx.doi.org/10.5772/intechopen.83703*

#### **Figure 7.**

*A Guide to Small-Scale Energy Harvesting Techniques*

supporting substrates, as schematically shown in **Figure 7a**. A photograph of an asfabricated TENG and SEM image of the PTFE nanowires is shown in **Figure 7b**-**c**. With superior synchronization, the 3D-TENG produces a short-circuit current as high as 1.14 mA and an Voc up to 303 V with a remarkable peak power density of

*(a) Schematic of the method to fabricate wavy Kapton films. (b) SEM image of the ICP-processed PTFE film surface. (c) Schematic of the device structure. (d) Magnified schematic of the device, showing that the wavy core is in periodical contact with the nanostructures on the PTFE films. (e) Photograph of an as-fabricated* 

To reduce the direct friction between triboelectric layers, a liquid-metal-based TENG (LM-TENG) was developed for high-efficiency vibration energy harvesting [67]. Owing to an intimate contact between the liquid metal and the polymer dielectric layer, the direct friction between triboelectric layers for energy loss is effectively reduced, resulting in high effective contact, shape adaptability, and low friction coefficient with solid. Therefore, the LM-TENG exhibits an output charge

On the other hand, soft electrode can effectively increase the contact intimacy between the triboelectric layers [68]. Xu et al. reported a novel soft and robust TENG made of a silicone rubber-spring helical structure with nanocompositebased elastomeric electrodes for harvesting arbitrary directional vibration energy and self-powered vibration sensing [69]. The schematic diagram and a photo of the S-TENG are shown in **Figure 8a,c**, respectively. As displayed, the TENG exhibits a helical structure based on the integration of elastomer and spring. A mixing well silicone rubber and carbon nanofiber, which can be stretched up to the strain of 133%, serves as the elastomeric electrode (**Figure 8b**). The working mechanisms of the S-TENG under vertical and horizontal vibration are shown in **Figure 8d,e**, respectively. Under external vertical vibration excitation, the distance between a helical structure's adjacent surfaces changes, forming a contact-separation mode TENG. Under horizontal vibration excitation, the S-TENG's helical structure's adjacent surfaces can contact on one side and separate on the other side, also forming a contact-separation mode TENG. Under the resonant states of the S-TENG,

. As a direct power source, it is capable of simultaneously lighting up 20

, which is four to five times higher than that in the case if the

with an external load of

**74**

104.6 W/m<sup>2</sup>

**Figure 6.**

density of 430 μC/m<sup>2</sup>

*TENG device before packaging [64].*

electrode is solid film.

spot lights as well as a white G16 globe light.

its peak power density is found to be 240 and 45 mW/m<sup>2</sup>

*Three-dimensional triboelectric nanogenerator. (a) Schematic of a 3D-TENG. (b) SEM image of nanopores on aluminum electrode. (c) A photograph of the as-fabricated 3D-TENG [66].*

#### **Figure 8.**

*(a) The device schematic of the S-TENG. Note that the gray silicone rubber layer containing a spring forms a base on which other layers can be built, and the black silicone rubber layer along with the electrode layer forms a contact-separation pair. Both top and bottom electrodes are made of carbon nanofiber-mixed silicone rubber. (b) SEM image of the carbon nanofiber for preparing the elastomeric electrode. (c) Photo of the as-prepared S-TENG. Working mechanisms of the S-TENG under (d) vertical vibration excitation and (e) horizontal vibration excitation [69].*

10 MΩ and an acceleration amplitude of 23 m/s2 . Additionally, the dependence of the S-TENG's output signal on the ambient excitation can be used as a prime selfpowered active vibration sensor that can be applied to monitor the acceleration and frequency of the ambient excitation.

## **3.3 TENGs harvesting energy from water**

Water energy deriving from rainwater, ocean waves, and waterfalls has been regarded as an alternative renewable energy resource source without polluting the environment. Energy harvesting from water has been further reinforced due to the abundant reserves and little dependence on environmental conditions. Through decades of exploration, a variety of wave energy converting devices and machines based on TENG has been invented to harvesting energy from water.

Liquid-solid-mode TENGs for harvesting liquid-wave energy have drawn much attention for the features of relatively stable output and durability [70–72]. For the liquid–solid-mode TENG, contact separation is the main representative strategies applied to scavenge water energy [73, 74]. A hydrophobic surface on water-solid TENGs is beneficial for inducing separation at the interface of liquid and solid [75]. Based on this, Zhu et al. reported a liquid-solid electrification-enabled TENG based on a FEP thin film for harvesting energy from a variety of water motions [76]. Owing to the modification of aligned nanowires, the thin film with a property of hydrophobicity can increase the contact area at the liquid-solid interface, leading to enhanced surface charging density and thus electric output at an efficiency of 7.7%. Due to the creation of continuous contact separation between water and the solid surface, a cylindrical water TENG was designed by using a hydrophilic surface along with the hydrophobic surface to control the water flow inside a packaged system for enhanced electrostatic induction [77].

Generally, an effective way of integrating a number of electrodes together to make them area scalable is helpful for promoting output power density. On the other hand, the electric power is highly affected by nanostructures at the solid/liquid interface. According to this, a flexible thin-film TENG was reported for harvesting kinetic wave energy [78]. Because of the integration method that use an array of surface-mounted bridge rectifiers to connect multiple parallel electrode together, the induced current between any pair of electrodes can be constructively added up, leading to a significant enhancement in output power and realizing area-scalable integration of electrode arrays. However, the thin-film TENG is only applicable to regular water waves that interact with the TENG through a linear water level. For improving the adaptive means of harvesting water energy, a networked integrated TENG was fabricated for harvesting energy from interfacing interactions with water waves of various types [79]. Additionally, interdigital electrode-based TENGs were designed in the contact-sliding mode for the harvesting of triboelectric energy from water [80], resulting in a higher output performance than those of one- and two-electrode-based TENGs.

Beside liquid-solid-mode TENGs, other structure TENGs were designed for harvesting water energy generating by flowing water, such as multi-layered disk structure [81], floating buoy structure [82], radial-arrayed rotary structure [10], and so on. Although many water-based TENGs have been fabricated, there is a lack of effort in realizing TENG harvesting water energy directly on the fabric/ textile, due to the poor water resistance of the fabrics related to their intrinsic hydrophilicity that can be ascribed to their abundant hydrophilic groups, and the strong adsorption capacity because of their large specific surface area [83]. For realizing the practical wearable device harvesting energy from water flow, Xiong et al. reported a wearable fabric-based WTEG with additional self-cleaning and

**77**

**Figure 9.**

*Small-Scale Energy Harvesting from Environment by Triboelectric Nanogenerators*

antifouling performance for the first time [83]. This is realized with the preparation of hydrophobic cellulose oleoyl ester nanoparticles by a nontoxic esterification method and nanoprecipitation technology based on the microcrystalline cellulose. In this study, PET fabric-based WTEG can generate the output power density of

There are two parts to water wave energy including the electrostatic energy from the contact electrification between water and surrounding media and the mechanical impact energy. For simultaneously scavenging both the energy from water, some works have been well done. For example, Su et al. presented an all-in-one hybridized TENG based on the conjunction of liquid-solid interfacial electrification enabled TENG and impact-TENG for harvesting water wave energy and as a self-powered distress signal emitter [84]; Lin et al. designed a fully integrated TENG for harvesting water energy and as a self-powered ethanol nanosensor, which contained a water-TENG unit to collect the electrostatic energy of water and a contact-TENG unit to collect the mechanical/kinetic energy of water [85]; Cheng et al. developed a water wheel hybridized TENG, composed of a water-TENG part and a disk-TENG part, for simultaneously harvesting the two types of energies from the tap water flowing from a household faucet [86]. Based on a unique structure design, the hybridized TENGs are shown to be suitable for harvesting multiple

During a working process, the acting surfaces of the above mentioned TENGs will be exposed to ambient atmosphere, which will limit their applications in some cases. The interface electrification was seriously affected by humidity, causing a quick decline of the surface charge density [87]. In order to improve the performance of TENGs under harsh conditions with the presence of water, fully enclosed or packaged TENGs should be developed for tolerating the environment. So far, different designs were developed based on packaged TENG such as wavy-shaped models [88], fully packaged contact-separation configurations [89–91], and rolling spherical structure [92]. Wang et al. designed a freestanding, fully enclosed TENG that encloses a rolling ball inside a rocking spherical shell for harvesting low-frequency water wave energy [93]. An image of the fabricated TENG floating on water is shown in **Figure 9a**. **Figure 9b** shows the schematic diagram of the freestanding structured design that consists of one rolling ball and two stationary electrodes. To enhance the electric output of the TENG, nanowire arrays are fabricated on the surface of the Kapton film (**Figure 9c**) that provides a large contact area to generate more triboelectric charges on the surface. Through the optimization of materials and structural parameters, a spherical TENG of 6 cm in diameter actuated by water waves can provide a peak current of 1 μA over a wide load range from a short-circuit condition to 10 GΩ, with an instantaneous output power of up to 10 mW. This rolling-structured TENG is extremely lightweight, has a simple structure, and is capable of rocking

*Device structure, basic operations of the freestanding-triboelectric-layer-based nanogenerator (RF-TENG) with a rolling Nylon ball enclosed. (a) Photograph of a rocking nanogenerator floating on water. (b) Schematic diagrams of freestanding-structured design. (c) SEM image of nanorod structure on the Kapton surface [93].*

*DOI: http://dx.doi.org/10.5772/intechopen.83703*

at a load resistance of 100 MΩ.

0.14 W/m2

types of energies from water.

## *Small-Scale Energy Harvesting from Environment by Triboelectric Nanogenerators DOI: http://dx.doi.org/10.5772/intechopen.83703*

antifouling performance for the first time [83]. This is realized with the preparation of hydrophobic cellulose oleoyl ester nanoparticles by a nontoxic esterification method and nanoprecipitation technology based on the microcrystalline cellulose. In this study, PET fabric-based WTEG can generate the output power density of 0.14 W/m2 at a load resistance of 100 MΩ.

There are two parts to water wave energy including the electrostatic energy from the contact electrification between water and surrounding media and the mechanical impact energy. For simultaneously scavenging both the energy from water, some works have been well done. For example, Su et al. presented an all-in-one hybridized TENG based on the conjunction of liquid-solid interfacial electrification enabled TENG and impact-TENG for harvesting water wave energy and as a self-powered distress signal emitter [84]; Lin et al. designed a fully integrated TENG for harvesting water energy and as a self-powered ethanol nanosensor, which contained a water-TENG unit to collect the electrostatic energy of water and a contact-TENG unit to collect the mechanical/kinetic energy of water [85]; Cheng et al. developed a water wheel hybridized TENG, composed of a water-TENG part and a disk-TENG part, for simultaneously harvesting the two types of energies from the tap water flowing from a household faucet [86]. Based on a unique structure design, the hybridized TENGs are shown to be suitable for harvesting multiple types of energies from water.

During a working process, the acting surfaces of the above mentioned TENGs will be exposed to ambient atmosphere, which will limit their applications in some cases. The interface electrification was seriously affected by humidity, causing a quick decline of the surface charge density [87]. In order to improve the performance of TENGs under harsh conditions with the presence of water, fully enclosed or packaged TENGs should be developed for tolerating the environment. So far, different designs were developed based on packaged TENG such as wavy-shaped models [88], fully packaged contact-separation configurations [89–91], and rolling spherical structure [92]. Wang et al. designed a freestanding, fully enclosed TENG that encloses a rolling ball inside a rocking spherical shell for harvesting low-frequency water wave energy [93]. An image of the fabricated TENG floating on water is shown in **Figure 9a**. **Figure 9b** shows the schematic diagram of the freestanding structured design that consists of one rolling ball and two stationary electrodes. To enhance the electric output of the TENG, nanowire arrays are fabricated on the surface of the Kapton film (**Figure 9c**) that provides a large contact area to generate more triboelectric charges on the surface. Through the optimization of materials and structural parameters, a spherical TENG of 6 cm in diameter actuated by water waves can provide a peak current of 1 μA over a wide load range from a short-circuit condition to 10 GΩ, with an instantaneous output power of up to 10 mW. This rolling-structured TENG is extremely lightweight, has a simple structure, and is capable of rocking

#### **Figure 9.**

*Device structure, basic operations of the freestanding-triboelectric-layer-based nanogenerator (RF-TENG) with a rolling Nylon ball enclosed. (a) Photograph of a rocking nanogenerator floating on water. (b) Schematic diagrams of freestanding-structured design. (c) SEM image of nanorod structure on the Kapton surface [93].*

*A Guide to Small-Scale Energy Harvesting Techniques*

10 MΩ and an acceleration amplitude of 23 m/s2

frequency of the ambient excitation.

**3.3 TENGs harvesting energy from water**

system for enhanced electrostatic induction [77].

two-electrode-based TENGs.

the S-TENG's output signal on the ambient excitation can be used as a prime selfpowered active vibration sensor that can be applied to monitor the acceleration and

Water energy deriving from rainwater, ocean waves, and waterfalls has been regarded as an alternative renewable energy resource source without polluting the environment. Energy harvesting from water has been further reinforced due to the abundant reserves and little dependence on environmental conditions. Through decades of exploration, a variety of wave energy converting devices and machines

Liquid-solid-mode TENGs for harvesting liquid-wave energy have drawn much attention for the features of relatively stable output and durability [70–72]. For the liquid–solid-mode TENG, contact separation is the main representative strategies applied to scavenge water energy [73, 74]. A hydrophobic surface on water-solid TENGs is beneficial for inducing separation at the interface of liquid and solid [75]. Based on this, Zhu et al. reported a liquid-solid electrification-enabled TENG based on a FEP thin film for harvesting energy from a variety of water motions [76]. Owing to the modification of aligned nanowires, the thin film with a property of hydrophobicity can increase the contact area at the liquid-solid interface, leading to enhanced surface charging density and thus electric output at an efficiency of 7.7%. Due to the creation of continuous contact separation between water and the solid surface, a cylindrical water TENG was designed by using a hydrophilic surface along with the hydrophobic surface to control the water flow inside a packaged

Generally, an effective way of integrating a number of electrodes together to make them area scalable is helpful for promoting output power density. On the other hand, the electric power is highly affected by nanostructures at the solid/liquid interface. According to this, a flexible thin-film TENG was reported for harvesting kinetic wave energy [78]. Because of the integration method that use an array of surface-mounted bridge rectifiers to connect multiple parallel electrode together, the induced current between any pair of electrodes can be constructively added up, leading to a significant enhancement in output power and realizing area-scalable integration of electrode arrays. However, the thin-film TENG is only applicable to regular water waves that interact with the TENG through a linear water level. For improving the adaptive means of harvesting water energy, a networked integrated TENG was fabricated for harvesting energy from interfacing interactions with water waves of various types [79]. Additionally, interdigital electrode-based TENGs were designed in the contact-sliding mode for the harvesting of triboelectric energy from water [80], resulting in a higher output performance than those of one- and

Beside liquid-solid-mode TENGs, other structure TENGs were designed for harvesting water energy generating by flowing water, such as multi-layered disk structure [81], floating buoy structure [82], radial-arrayed rotary structure [10], and so on. Although many water-based TENGs have been fabricated, there is a lack of effort in realizing TENG harvesting water energy directly on the fabric/ textile, due to the poor water resistance of the fabrics related to their intrinsic hydrophilicity that can be ascribed to their abundant hydrophilic groups, and the strong adsorption capacity because of their large specific surface area [83]. For realizing the practical wearable device harvesting energy from water flow, Xiong et al. reported a wearable fabric-based WTEG with additional self-cleaning and

based on TENG has been invented to harvesting energy from water.

. Additionally, the dependence of

**76**

on or in water to harvest wave energy. Additionally, rolling spherical TENGs and coupled TENG networks have been demonstrated to harness the water wave energy because of the advantages of light-weight, small-resistance under the water wave motions, and easy to be integrated [94, 95].

For enhancing the output current and enlarging the practical applications of packaged TENG, introducing a spring structure into the TENG can store the kinetic energy from water impact and later convert into electric power via residual vibrations [96]. Combining the advantages of spring structure and integrated multilayered structure, Xiao et al. demonstrated a kind of spherical TENG with spring-assisted multilayered structure for harvesting water wave energy [97]. The introduction of spring structure enhances the output performance of the spherical TENG by transforming low-frequency water wave motions into high-frequency vibrations, while the multilayered structure increases the space utilization, leading to a higher output of a spherical unit. The structure of spherical TENG designed with spring-assisted multilayered structure floating on water surface is schematically shown in **Figure 10a**. **Figure 10b** displays a photograph of asfabricated spherical TENG device, and the inset shows the photograph of the device in the water waves. The working principle of each TENG unit is demonstrated in **Figure 10c**. The periodic movement of the mass block under the triggering of water waves, which leads to the contact and separation between two surfaces of the top aluminum foil and FEP film, produces periodic electric output signals. Owing to its unique structure, the output current of one spherical TENG unit can reach 120 μA, which is two orders of magnitude larger than that of previous rolling spherical TENG, and a maximum output power up to 7.96 mW is realized as triggered by the water waves.

#### **Figure 10.**

*(a) Schematic diagram of the spherical TENG with spring-assisted multilayered structure floating on water, and schematic representation enlarged structure for the zigzag multilayered TENG with five basic units. (b) Photographs of the as-fabricated TENG device. (c) Working principle of each TENG unit of the spherical TENG [97].*

**79**

**Figure 11.**

*Small-Scale Energy Harvesting from Environment by Triboelectric Nanogenerators*

are the two main methods for preparing wind-driven TENG.

Wind energy can be a renewable energy sources for energy harvesting on account of widespread and absolute abundance. The practical application of traditional wind power in our daily life is largely limited by the extra-large volume, high cost of installation, noise and geographical environment. In this regard, TENG is one of the most alternative wind energy conversion strategies on accord of its small scale, low cost, simple fabrication routes, and portability [98]. In order to harvest wind energy, flutter-driven structure [99, 100] and rotational structure [101, 102]

Flutter-driven structure TENG for harvesting wind energy was realized by Yang et al. for the first time [103]. As displayed in **Figure 11**, the TNEG is composed of two layers of Al foils and a FEP film laying in midair of a cuboid acrylic tube. The Al foils act as both triboelectric surfaces and electrodes, respectively. The FEP film is fixed one side, leaving the other side freestanding. The FEP film will vibrate periodically to contact the two Al foils inducing from wind, resulting in an output signal in an external circuit. Output voltage and current about 100 V and 1.6 μA are achieved, and a corresponding output power of 0.16 mW is realized under a loading

Although single-side fixed-based TENG exhibits good performance for scavenging wind energy, the stability of output performance is a challenge because of the arbitrary fluttering of the FEP film. For solving the problem, an elasto-aerodynamics-driven TENG, consisting of a Kapton film with two Cu electrodes fixed on two ends in an acrylic fluid channel, was reported for scavenging air-flow energy [104], where the flutter effect of Cu electrodes was induced to contact two triboelectric materials of the PTFE films and the Kapton film to realize the output performance

Based on flutter-driven structure, many other efforts have been made to enhance the performance of TENG through optimizing the structure or the morphologies of material surface design. A lightweight and freestanding flag-type woven TENG, consisting of conductive belts of Ni-coated polyester textiles and Kapton filmsandwiched Cu belts, was designed for scavenging high-altitude wind energy from arbitrary directions [105]. When wind fluttering is applied in each woven unit, wind energy converts into electrical energy induced by the interlaced interactions between the Kapton film and a conductive cloth under wind-introduced fluttering of the flag. Besides, a flutter-driven TENG, consisting of a flag and a counter plate arranged in parallel with interwoven microstructure, was fabricated for harvesting wind energy based on contact electrification caused by the self-sustained oscillation of flags [106]. As shown in **Figure 12**, a flexible flag and a rigid plate are arranged in face to face in order to prepare a wind-driven energy-harvesting system using fluttering behavior. Owing to the design, interaction between them can lead to a rapid

*(a and b) The structure and photograph of the first reported flutter-driven mode WD-TENG [103].*

*DOI: http://dx.doi.org/10.5772/intechopen.83703*

**3.4 TENGs harvesting energy from wind**

resistance of 100 MΩ.

of the device.
