*Review* **Advances of High-Performance Triboelectric Nanogenerators for Blue Energy Harvesting**

**Huamei Wang 1,2, Liang Xu 1,2,\* and Zhonglin Wang 1,2,3,4,\***


**Abstract:** The ocean is an enormous source of blue energy, whose exploitation is greatly beneficial for dealing with energy challenges for human beings. As a new approach for harvesting ocean blue energy, triboelectric nanogenerators (TENGs) show superiorities in many aspects over traditional technologies. Here, recent advances of TENGs for harvesting blue energy are reviewed, mainly focusing on advanced designs of TENG units for enhancing the performance, through which the response of the TENG unit to slow water agitations and the output power of the device are largely improved. Networking strategy and power management are also briefly discussed. As a promising clean energy technology, blue energy harvesting based on TENGs is expected to make great contributions for achieving carbon neutrality and developing self-powered marine systems.

**Keywords:** triboelectric nanogenerator; network; blue energy; wave energy; energy harvesting

### **1. Introduction**

Covering over 70% of the earth's surface, ocean plays a crucial role for lives on the planet and can be regarded as an enormous source of blue energy, whose exploitation is greatly beneficial for dealing with energy challenges for human beings [1–3]. With extreme climate conditions taking place more frequently nowadays, the world feels the urge to take immediate action to alleviate climate deterioration caused by global warming [4–6]. Carbon neutrality is thus put forward as a goal to reach balance between emitting and absorbing carbon in the atmosphere [7]. One of the most effective methods is to develop and expand the use of clean energy that generates power without carbon emission, such as the enormous blue energy [8]. Meanwhile, with the increasing activities in ocean, equipment deployed in the far ocean is facing problems regarding an in situ and sustainable power supply, where blue energy is an ideal source for developing new power solutions for such applications, allowing self-powered marine systems and platforms, though the harvesting scale can be much smaller [9–11].

The ocean blue energy is typically in five forms: wave energy, tidal energy, current energy, thermal energy, and osmotic energy, among which the wave energy is promising for its wide distribution, easy accessibility, and large reserves. The wave energy around the coastline is estimated to be more than 2 TW (1 TW = 10<sup>12</sup> W) globally [12]. However, the present development of wave energy harvesting is challenged by its feature as a type of high-entropy energy, which refers to the chaotic, irregular waves with multiple amplitudes and constantly changing directions that are randomly distributed in the sea [13,14]. Most significantly, wave energy is typically distributed in a low-frequency regime, yet the most common and classic method of blue energy harvesting at status quo, the electromagnetic generator (EMG), performs rather poorly in low-frequency energy harvesting, which relies on propellers or other complex mechanical structures to drive bulky and heavy magnets

**Citation:** Wang, H.; Xu, L.; Wang, Z. Advances of High-Performance Triboelectric Nanogenerators for Blue Energy Harvesting. *Nanoenergy Adv.* **2021**, *1*, 32–57. https://doi.org/ 10.3390/nanoenergyadv1010003

Academic Editor: Joao Oliveira Ventura

Received: 8 July 2021 Accepted: 22 August 2021 Published: 26 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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and metal coils in order to transform mechanical energy into electricity [8]. Thus, it usually has high cost and low reliability.

First proposed by Wang in 2012 [15], the triboelectric nanogenerator (TENG, also called Wang generator) derived from Maxwell's displacement current shows great prospect as a new technology to convert mechanical energy into electricity [16], based on the triboelectrification effect and electrostatic induction [17–20]. TENGs present superiorities including light weight, cost-effectiveness, easy fabrication, and versatile material choices [21–25]. The concept of harvesting blue energy using the TENG and its network was first brought out in 2014 [20]. As a new form of blue energy harvester, the TENG surpasses the EMG in that it intrinsically displays higher effectiveness under low frequency, owing to the unique feature of its output characteristics [22,26–28]. Moreover, adopting the distributed architecture of light-weighted TENG networks can make it more suitable for collecting wave energy of high entropy compared with EMGs, which are oversized in volume and mass [29–33].

Gaining attention, the material selection of TENGs, aimed at fulfilling their requirements and maximizing output, is governed by the mechanism. In triboelectrification process, surface charge density, a fundamental parameter regarding the performance of TENGs, relates to the electron affinity difference of triboelectric material pairs. As a result, polymers prevail as the dielectric layer since their various functional groups enhance charge transfer and capturing, as manifested in the triboelectric series, and they also possess the merits of excellent flexibility, scalability, and low cost [34] (Figure 1a). On top of that, as deduced from the Maxwell's displacement current, other key factors contributing to output include dielectric permittivity, surface morphology, and robustness [35]. In addition, the rapid development of TENGs requires additional material properties that suit devices of various modes under conditioned application scenarios, such as a low friction coefficient for sliding mode TENGs, and high electrostatic breakdown strength and chemical stability for high-voltage TENG devices.

In this paper, recent advances of TENGs for harvesting wave energy are reviewed. The review mainly focuses on advanced designs of TENG units for enhancing the performance of wave energy harvesting, including rolling ball structure, multilayer structure, grating structure, pendulum structure, mass-spring structure, spacing structure, water-solid contact structure, and charge pumping strategy. Through such designs, the response of the TENG unit to slow wave agitations and the output power of the device are largely improved. Networking strategy and power management are also briefly discussed. Finally, further development challenges and the prospect of blue energy harvesting by the TENG are discussed.

**Figure 1.** Triboelectric series and four fundamental working modes of TENGs. (**a**) Quantified triboelectric series. Reprinted with permission from ref. [34], Copyright 2019, Springer Nature. (**b**) Vertical contact-separation mode. (**c**) Lateral sliding mode. (**d**) Single-electrode mode. (**e**) Freestanding triboelectric-layer mode.

#### **2. TENG Systems for Blue Energy Harvesting**

*2.1. Fundamental Working Modes of TENGs*

TENGs generate electricity by the coupling of triboelectrification and electrostatic induction, which are classified into the four fundamental working modes [19]: vertical contact-separation mode, lateral sliding mode, single-electrode mode, and freestanding triboelectric-layer mode (Figure 1b–e).

As shown in Figure 1b, in vertical contact-separation mode, the two dielectric surfaces are oppositely charged after physical contact due to triboelectrification. When the two surfaces are vertically separated with a gap in between, a potential drop is produced between the two electrodes attached to the backsides of the two dielectric layers due to the separation of positive and negative static charges, which drives current to flow between the electrically connected electrodes to balance the electrostatic field. When the gap vanishes, the potential drop due to static charges exists no more and so the induced free charges flow backwards. In this way, an alternate current (AC) output is generated in the external circuit under periodic contact and separation movement of the TENG [36].

For the lateral sliding mode shown in Figure 1c, the two dielectric surfaces are charged through triboelectrification during initial sliding motion. Under the full alignment of the two surfaces, no potential difference by static charges is created across the electrodes, since surface static charges of opposite signs completely compensate each other. When a relative displacement paralleled to the interface is introduced under lateral sliding, the mismatched area of the dielectric layers leads to bare static charges and so a potential difference appears between electrodes. The sliding back and forth of the TENG hence results in periodical changes of potential that drive free electrons to flow between the electrodes [37].

In single-electrode mode TENG (Figure 1d), the moving dielectric layer no longer has to be bound to electrodes or electrically connected by wires. The only electrode essential to the mode is one electrically connected to the ground, which can be regarded as another electrode. The surface of the dielectric layer is first charged under full contact with the electrode. When they start to move apart from each other, the induced charges in the electrode decrease in order to balance the electric potential, through charge exchange with the ground. Then, when the dielectric layer moves back to contact again, electrons flow in the opposite direction to re-establish an electrostatic equilibrium until the two surfaces fully overlap. Although charge transfer is not effective as a result of electrostatic screening effect, the triboelectric layer can move freely without any restrains [38].

The freestanding triboelectric-layer mode TENG can also work with the moving dielectric layer disconnected to electrodes, yet without any screening effect (Figure 1e). When the dielectric layer charged by triboelectrification approaches a pair of electrodes asymmetrically, a potential difference is induced across the two electrodes, causing electrons to flow between them to balance the local potential distribution. Under the back-and-forth movement of the dielectric layer, electrons oscillate across the paired electrodes, generating an AC current output [39].

#### *2.2. TENG Systems for Harvesting Blue Energy*

For effectively harvesting distributed wave energy, the TENG is conceived to be organized in networks, which can have hierarchical structure of modules [29]. The network structure also enables the device to be applied in different scales of harvesting, ranging from self-powered systems to large-scale clean energy (Figure 2a). In the development of TENGs for blue energy, efforts are mainly focusing on four aspects: TENG unit design, networking strategy, power management, and application system (Figure 2b). The primary and most significant part is the fundamental design of the TENG unit, which relies on continuous improvements focusing on the structure, principle, and material to enhance its energy harvesting performance and to meet demands raised by various ocean environments, both on the water surface and beneath it. Networking strategy is then adopted to add outputs of single TENG units and expand them in a reliable way. It decides the connection pattern of massive TENG units and the coupling effect between TENG units, which could further enhance the performance. Before finally supplying electricity power to the application system, power management is required to manipulate the TENG output for a better match with appliances and improve the power efficiency with circuit approaches. This review mainly emphasizes methods to advance the design of TENG units, which is the most challenging part, and networking strategy and power management are also discussed.

**Figure 2.** Schematics of blue energy harvesting based on TENGs. (**a**) Schematic diagram of the TENG network for harvesting wave energy. Reprinted with permission from ref. [40], Copyright 2021, IOP publishing, Ltd. (**b**) Schematic diagram of major aspects for blue energy harvesting based on TENGs, including TENG unit design, networking strategy, power management, and application system. Reprinted with permission from ref. [41], Copyright 2019, Elsevier. Reprinted with permission from ref. [42], Copyright 2019, Elsevier. Reprinted with permission from ref. [43], Copyright 2017, Elsevier. Reprinted with permission from ref. [44], Copyright 2015, American Chemical Society. Reprinted with permission from ref. [45], Copyright 2018, Elsevier. Reprinted with permission from ref. [46], Copyright 2019, Elsevier. Reprinted with permission from ref. [47], Copyright 2018, Elsevier. Reprinted with permission from ref. [48], Copyright 2020, Elsevier. Reprinted with permission from ref. [49], Copyright 2017, Elsevier. Reprinted with permission from ref. [50], Copyright 2020, Springer Nature. Reprinted with permission from ref. [51], Copyright 2014, American Chemical Society. Reprinted with permission from ref. [52], Copyright 2020, Springer Nature. Reprinted with permission from ref. [11], Copyright 2019, Elsevier. Reprinted with permission from ref. [29], Copyright 2017, Springer Nature.

#### **3. Advanced Designs of TENG Units for Blue Energy**

#### *3.1. Rolling Ball Structure*

The rolling ball structure is the most fundamental and typical structure for blue energy harvesting based on TENGs [29,53,54]. The structure basically features a rolling ball inside a rocking spherical shell, which is simple and capable of rocking in water. Due to the small rolling friction, it can be sensitive to tiny wave agitations. Moreover, the structure naturally has a low frequency related to the size of the ball and shell, which can be tuned to reach resonance with water wave motion. Since the rolling ball normally functions as a freestanding triboelectric layer and no wire is connected to the ball, TENG units based on such a structure are durable. Wang et al. reported the rolling-ball-structured TENG based on the freestanding triboelectric-layer mode for the first time, which consists of one rolling nylon ball and two stationary electrodes covered by Kapton films [53] (Figure 3a). Wave vibration drives the ball to roll back and forth between electrodes, producing alternate current in the external circuit connecting the electrodes. However, due to the limited contact area and charge density, the output charge and current are relatively low, reaching only 24 nC.

Advancement on the structure includes the evolution of material and structure of the ball and shell components. As proposed by Xu et al., the UV-treated silicon rubber is adopted as the material for the rolling ball, since its softness can contribute to enhancing the real contact area and durability of the TENG units [54] (Figure 3b). The UV treatment enables Si−O−Si chain scission and oxidative conversion of radical groups in rubber, altering the electron affinity and strengthening triboelectrification with silicone rubber dielectric layer. In addition, microstructures are created on the dielectric surface by mixing polyformaldehyde particles into the rubber to make the surface less sticky, lowering the

damping force, thus resulting in a high charge output of the TENG unit even under small agitations (Figure 3c). The charge output of single unit is improved to 72.6 nC.

For further enhancing the contact of the ball, Cheng et al. designed a TENG consisting of a soft liquid/silicone rolling ball and a hollow acrylic sphere attaching two Cu electrodes [55]. The silicone rubber is shaped in a hollow sphere for sufficient softness while containing water inside to ensure full contact (Figure 3d). In this way, the structure of triboelectric rubber can expand the contact area and generate charges as high as 500 nC, more than 10 times that of the conventional structure of polytetrafluoroethylene (PTFE) dielectric layer and hard core (Figure 3e). Systematically speaking, the energy harvesting performance of the soft-contact spherical TENG is largely affected by the contact area, which relates to many factors, including sphere hardness, fill rate, acceleration speed, and bulk density. A theoretical model based on contact electrification and mainly concerning the sphere hardness was quantitatively established by Guan et al. [56]. The result indicates that the dynamic contact of sphere is affected by core mass and fill rate, which is optimized at 83%.

**Figure 3.** Design and output of TENGs based on rolling ball structure. (**a**) Schematic diagram of TENG with the first rolling ball structure based on freestanding mode. Reprinted with permission from ref. [53], Copyright 2015, Wiley. (**b**) Photo of TENG adopting UV-treated rubber rolling ball and shell. (**c**) Charge output of a single TENG unit before and under UV treatment and POM doping. (**b**,**c**) Reprinted with permission from ref. [54], Copyright 2018, American Chemical Society. (**d**) Working principle of TENG based on soft liquid/silicone rolling ball structure. (**e**) Charge output of TENG with soft liquid/silicone rolling ball structure. (**d**,**e**) Reprinted with permission from ref. [55], Copyright 2019, Elsevier. (**f**) Schematic diagram of TENG with 3D electrode structure intercalated by FEP pellets. (**g**) Current and power output of 3D-electrode-structured TENG in water waves. (**f**,**g**) Reprinted with permission from ref. [42], Copyright 2019, Elsevier. (**h**) Schematic diagram of TENG structure featuring nest-assembling multiple shells. Reprinted with permission from ref. [57], Copyright 2019, Elsevier. (**i**) Schematic diagram of hybrid TENG containing power management, EMG module, and TENG module. (**j**) Charging curve to a supercapacitor by the hybrid TENG in water. (**i**,**j**) Reprinted with permission from ref. [58], Copyright 2019, American Chemical Society.

Another improvement approach concerning the ball structure is adopting multiple balls. Yang et al. featured TENG units with a 3D electrode structure intercalated by fluorinated ethylene propylene (FEP) pellets in internal channels as the tribo-material [42] (Figure 3f). A pair of 3D-structured electrodes are made by electrode plates of connected copper layers on the substrates, largely enhancing the surface area of the electrodes and improving the utilization of the internal space of the shell. The device can output under different modes of external excitation as controlled by inertial forces and gravity. For harmonic translational mode, the increase in displacement extent and frequency leads to rapid rising of the transferred charges and current, obtaining a high charge output of 520 nC. Under impact translational mode, the structure of multiple rolling balls widens the current peak, boosting the peak power density and average power density to 32.6 W m−<sup>3</sup> and 8.69 W m−3, respectively. Meanwhile, an average power density of 2.05 W m−<sup>3</sup> can be achieved in real water waves agitated at 1.45 Hz (Figure 3g). Moreover, the amount of FEP pellets influences charge output due to the cancel-out of electrostatic induction by FEP pellets in the two 3D electrodes when more than half of the device volume is filled. Considering the evolution of the shell structure, a similar TENG structure reported by Pang et al. is characterized by nest-assembling multiple shells with decreasing sizes, with PTFE balls filled into the gap space between neighboring shells [57] (Figure 3h). On the interior of each shell, gaped Cu electrodes are painted with good adhesion, conductivity, and durability. The PTFE balls can move freely on electrode surfaces. Such unique structure design also improves the utilization of limited device volume and elevates the contact area between balls and electrodes.

Combining the TENG with the EMG by making the rolling ball magnetic is another enhancement method, which was achieved by Wu et al. The packaging hollow shell of the hybridized device is divided into four zones by acrylic disks, which are set for power management, EMG module, and TENG module, respectively [58] (Figure 3i). On one hand, the magnetic sphere drives the EMG module via electromagnetic induction, as two coils are embedded on both sides of the magnet; on the other hand, it motivates the mover by magnetic attractive force, since the mover is centered by a magnetic cylinder. Then, the mover can drive the TENG module through its interaction with PTFE friction layers adhered to Tai Chi-shaped electrodes. Eventually, such design can collect wave energy from any orientation. It was also placed on a buoy to test in lake Lanier. During 162 s, the electricity stored in a supercapacitor reached 1.64 mJ (Figure 3j).

#### *3.2. Multilayer Structure*

Considering that triboelectrification is a phenomenon happening at the surface, to improve the output, enhancing the contact area in a certain space with a multilayer structure should be an effective method. The strategy is realized in many different types of TENGs, and the output charge and power are greatly improved. The major challenge lies in the structure design to drive the complex structure by external agitations, which requires delicate structure designs.

An integrated multilayered TENG based on soft membrane was proposed by Xu et al. The structure includes the inner oscillator and the outer shell, which are connected by elastic bands to form a spring-levitated oscillator structure [43] (Figure 4a). The oscillator is based on air-driven soft membranes, which shape into air-pocket-like structures to split the air chamber into upper and lower parts. Accordingly, the electrodes of TENGs attached to the membranes form upper and lower multilayered TENG arrays. When the outer shell moves up and down as pushed by waves, the inner oscillator goes down and up relative to the shell and compresses the air chambers alternately. Differences in air pressure reshape membranes, making the upper and lower TENGs contact and separate periodically. Thus, the agitations from water waves can effectively drive the multilayered TENGs via the airdriven mechanism. The device with a series of integrated TENGs working simultaneously and effectively can produce high transferred charges of 15 μC per cycle (Figure 4b).

**Figure 4.** Design and output of TENGs based on multilayer structure. (**a**) Schematic diagrams of integrated multilayered TENG based on soft membrane and spring levitation. (**b**) Charge output of integrated soft membrane TENG per cycle. (**a**,**b**) Reprinted with permission from ref. [43], Copyright 2017, Elsevier. (**c**) Schematic diagram of TENG with tower-like structure containing multiple units in one tubular block. (**d**) Charge output of tower-like TENG collecting wave energy in different directions. (**c**,**d**) Reprinted with permission from ref. [59], Copyright 2019, American Chemical Society. (**e**) Schematic diagram of TENG with nodding duck structure. (**f**) Current and power density of nodding duck-structured TENG under water waves. (**e**,**f**) Reprinted with permission from ref. [60], Copyright 2021, American Chemical Society. (**g**) Photo and schematic figure of TENG with zigzag multilayered structure. Reprinted with permission from ref. [61], Copyright 2018, Wiley. (**h**) Schematic diagram of oblate TENG. Reprinted with permission from ref. [62], Copyright 2019, Wiley. (**i**) Schematic diagram of floating buoy-based TENG structure. Reprinted with permission from ref. [47], Copyright 2018, Elsevier.

Similarly, Xu et al. reported a tower-like TENG structure consisting of multiple parallel connected units in one tubular block [59] (Figure 4c). In the unit, PTFE balls roll on 3Dprinted arc surface shell, which is attached with two stationary electrodes coated by melt adhesive reticulation nylon film, forming a freestanding mode TENG. Only one rectifier is needed in a block since all PTFE balls move in the same phase under agitations. The structure is also capable of collecting water wave energy in arbitrary directions (Figure 4d).

Liu et al. reported a nodding duck-structured TENG that consists of external nodding duck blocks installed with shafts and bearings, and internal multitrack TENG units [60] (Figure 4e). Each unit is made up of a pair of Cu electrodes, free-rolling nylon balls, and dielectric layers placed on arc-shaped tracks. Together, they form a multilayer-structured TENGs of three layers that share electrodes and dielectric films, maximizing the utilization of internal space. A multiple-track structure was also introduced to avoid arbitrary movements of balls in different TENG layers. Upon triggering of external waves, nodding duck shell rotates around the shaft, leading balls to roll back and forth on the tracks. Under the

combination of gravity, supporting force, and damping motion, an AC output is generated with a maximum peak power density of 4 W m−<sup>3</sup> by one block at 800 MΩ under waves excited at 0.21 Hz (Figure 4f).

A spherical TENG that consists of a zigzag multilayered structure with spring assistance was proposed by Xiao [61] (Figure 4g). The multilayered structure is fabricated by deforming a thin Kapton film at evenly distanced intervals to serve as the substrate for five TENG units on both sides.

Liu et al. introduced an oblate spherical TENG structure, which is divided into the upper part for rough sea energy harvesting and the lower part for tranquil ocean energy collection [62] (Figure 4h). The upper part adopts a multilayer structure, which is composed of a pie iron and three arched units. The units are made up of two joint spring steel plates, one coated with FEP film. The upper part operates as the pie iron presses FEP film to contact the plates. The lower part is composed of a radial patterned FEP film with copper electrodes underneath, while another same-structured PET film is suspended above it. The lower part works as the external excitation triggers the iron shot to push FEP films into contact (shot on them) or separation (shot off them). Such structure also enhances self-stabilization and costs less, since the utilization of spring plates with good elasticity makes extra supporting structures for the units unnecessary.

Kim et al. reported a floating buoy-based TENG that contains a power generation unit and a height-adjustable support, which connects the unit to a floating buoy [47] (Figure 4i). There are four arrays of cylinder TENGs in the unit, each rotationally stacked upward for every 45◦, forming a multilayered structure for energy harvesting in arbitrary directions. For each freestanding mode TENG, a solid PTFE bar oscillates inside a tube, with the outer surface of the tube being attached by four Cu electrodes. The height-adjustable support changes the force applied on the unit according to the alteration of the water wave amplitude. The acrylic protection layer can tolerate harsh environmental conditions.

#### *3.3. Grating Structure*

The grating structure is regarded as a high-performance TENG design that has superior output. It can convert low-frequency agitations to high-frequency electrical output through a periodic grating electrode pattern. However, large friction at the interface makes it difficult to be agitated by slow water waves.

Bai et al. proposed a radial-grating-disk-structured TENG, where every TENG unit is made of facing sides of a stator disk and a rotator disk, patterned with radial-gratingstructured electrodes [46] (Figure 5a). The acrylic surface as one tribo-surface is frosted to reduce the friction force for easier agitation and better durability of the rotation device. In this way, the device can be successfully driven by the slow water waves, and the average power density of the device reaches 7.3 W m−<sup>3</sup> at a wave frequency of 0.58 Hz (Figure 5b). Its capability of driving a self-powered total dissolved solids testing system is demonstrated (Figure 5c), indicating that the high-power wave energy harvester can be applied for in situ, real-time mapping of water quality.

#### *3.4. Pendulum Structure*

The pendulum structure features swinging mass blocks, which temporarily transform and store kinetic energy as potential energy to drive TENGs to output electricity. The structure inherently features with high sensitivity under mechanical excitations, and improves the energy conversion efficiency since pendulums can convert impact agitations to continuous swinging in its inherent frequency and can elongate the operation time of TENGs if the friction is small enough.

In this previously discussed work, Bai et al. set the TENG units of rotators and stators in a tandem with two swing mass blocks on both sides, together encapsulated in a waterproof acrylic shell [46] (Figure 5a). While stators are fixed on the shell, rotators are linked with swinging mass blocks by shafts; therefore, when the eccentric blocks are agitated by waves, rotators can rotate synchronously. The tandem structure allows unit expansion, which scales up the total output and reduces the matched resistance through simple parallel connection of units. As the number of units expends, the peak current increases from 8 μA to 120 μA, and the charge output raises from 0.2 μC to 3.4 μC, under a harmonic reciprocating rotation. When interacting with waves, the nature of the TENG in frequency response resulted by various configurations was revealed. An additional mass block is capable of adjusting frequency characteristics of the device, since changing block mass alters the resonant frequency of the TENG.

**Figure 5.** Design and output of TENGs based on grating structure and pendulum structure. (**a**) Schematic diagrams of radial-grating-disk-structured TENG based on pendulum. (**b**) Current and power output of grating-structured TENG agitated by water waves. (**c**) Demonstration of grating-structured TENG driving a self-powered total dissolved solids testing system. (**a**–**c**) Reprinted with permission from ref. [46], Copyright 2019, Elsevier. (**d**) Schematic diagram of TENG adopting a pendulum triboelectric layer and thin stripes. (**e**) Damping output of the pendulum TENG. (**f**) Power output curve of the pendulum TENG. (**d**–**f**) Reprinted with permission from ref. [41], Copyright 2019, Elsevier. (**g**) Schematic diagram of cylindrical TENG with a bearing-based swing. Reprinted with permission from ref. [63], Copyright 2020, Wiley. (**h**) Schematic diagram of hybrid TENG adopting rotational pendulum structure. Reprinted with permission from ref. [64], Copyright 2019, Elsevier. (**i**) Schematic diagram of hybrid TENG with chaotic pendulum structure. (**j**) Voltage and power output of TENG module in the hybrid device. (**i**,**j**) Reprinted with permission from ref. [65], Copyright 2020, Elsevier. (**k**) Working principle of active resonance TENG device. Reprinted with permission from ref. [66], Copyright 2021, Elsevier.

Other different types of pendulum structures are utilized as well to suit diverse designs of TENGs. Lin et al. reported a freestanding mode TENG that is mainly composed of an electrode layer, a pendulum triboelectric layer, and thin stripes [41] (Figure 5d). The pendulum is a copper layer capable of free oscillation with high sensitivity to excitations, since it is connected to the outer shell by a cotton thread. The electrode layers are an internal circular one and an external ring one, covered by PTFE films. Thin strips on the outer edge of acrylic shell act by electrification with the pendulum layer. Such design can be easily agitated to swinging and output electricity continuously (Figure 5e,f).

Jiang et al. proposed a TENG structure that consists of a bearing-based swing component and a cylindrical acrylic shell, whose internal surface is attached by equally sized Cu electrodes and PTFE stripes [63] (Figure 5g). The swing component is made of acrylic blocks with upper hollow and lower solid structures, the top and bottom of which are

both adhered by arc-shaped acrylic strip. Swing motion of the block is realized by adding copper blocks to the bottom to lower the gravity center.

Apart from swinging pendulum devices, Hou et al. demonstrated a hybrid TENG adopting the rotational pendulum structure, which consists of a pendulum rotor, TENG blades, coils, and a cylindrical-tube-shaped frame [64] (Figure 5h). The pendulum rotor is made up of magnets twined by a copper ring, which are installed on the center shaft with ceramic bearings. The TENG blades connected in parallel are sandwiched flexible structure of FEP/Cu/FEP, with one side freestanding and the other side fixed on the frame. The blades and the Cu ring form the TENG part. The EMG is composed of coils that curl around the shaft and are embedded in the bottom, and the magnets of pendulum rotor, which are just above the coils. As the pendulum rotor rotates clockwise and anticlockwise around the shaft, a contact-separation mode TENG forms as the copper ring touches blades, and the EMG works under the periodic movement of magnets passing coils. The bearing support on the magnet rotor enables rotation even under tiny external excitations.

Another hybridized device reported by Chen et al. adopts a chaotic pendulum structure for the virtue of low working frequency [65] (Figure 5i). The chaotic pendulum consists of a major pendulum that swings back and forth under oscillation, and an inner pendulum that moves in a different chaotic manner following the major one. Both the TENG and the EMG are driven by the chaotic inner pendulum. The major pendulum of a weighted magnetic ball is used to enhance the oscillation frequency of the inner pendulum. The TENG is made of PTFE films and Au electrodes fixed on the inner swing. The EMG consists of the inner pendulum, which is composed of three magnetic balls evenly spaced on a rotating shaft, and three coils attached to the inside. When the main swing is pushed by water excitations, the freestanding mode TENG works based on the chaotic movement of the inner swing to send PTFE films moving back and forth on the electrodes, converting mechanical energy into electricity. The EMG part generates magnetic flux change of copper coils, as the magnetic balls begin moving under gravity and external magnetic incentive condition. The maximum output of the TENG reaches 15.21 μW and the EMG is up to 1.23 mW under an external excitation frequency of 2.5 Hz (Figure 5j).

To collect wave energy in all directions, Zhang et al. designed an active resonance TENG device that mainly integrates a simple pendulum, a floating tumbler, and flexible ring TENG [66] (Figure 5k). The tumbler intercepts wave energy, which is then captured by the pendulum, and finally transformed into electricity through TENGs. In the initial floating state, TENGs with flexible ring multilayered structure remain separate. The wave propagation makes the device lean aside; the pendulum thus brings the TENGs into full contact. A similar process proceeds as waves leave the system, generating two rounds of contact–separation output in one wave agitation. The pendulum with a circle-tableshaped core and the tumbler composed of a hemispherical float and a steel ball can create resonance effect by themselves, regardless of the external excitation frequency. Together with their damped motion, the energy harvesting efficiency is also enhanced. Moreover, the omnidirectional structural design ensures effective collection of omnidirectional water wave energy by the device.

#### *3.5. Mass-Spring Structure*

The mass-spring structure can produce oscillation under external agitations, which can be tuned to the frequency of the water waves to achieve resonance. It normally vibrates along the vertical direction with the effect of gravity. The structure shares the energy transfer mechanism of the pendulum structure, along with its merits.

Hu et al. reported a TENG built on a suspended 3D structure within a cube, which is a spiral with a seismic mass at the bottom that behaves like a spring [67] (Figure 6a). The top of the spiral is fixed to the inner top surface of the cube. One of TENG plates is attached to the bottom of the spiral, and the other plate is at bottom of the cube. Initially, the TENG plates slightly contact because the spiral is released. When the external disturbance pushes the cube's bottom to displace, the seismic mass vibrates with it and so the spiral begins

oscillation. TENG plates thus separate as a result of spiral contraction, forming a vertical contact-separation mode TENG. A maximum output power density of 2.76 W m−<sup>3</sup> on a load of 6 MΩ can be achieved.

**Figure 6.** Design and output of TENGs based on mass-spring structure and spacing structure. (**a**) Schematic diagram of TENG based on suspended 3D structure. Reprinted with permission from ref. [67], Copyright 2013, American Chemical Society. (**b**) Schematic diagram of TENG device adopting the principle of auto-winding mechanical watch. (**c**) Constant output of the TENG device under low frequencies and varying working conditions. (**b**,**c**) Reprinted with permission from ref. [68], Copyright 2021, Elsevier. (**d**) Charge output of cylindrical TENG with a bearing-based swing during cycling test. Reprinted with permission from ref. [63], Copyright 2020, Wiley.

The previously discussed multilayered TENG structure raised by Xu et al. contains a spring-levitated oscillator, where the inner oscillator and the outer shell are connected by elastic bands [43] (Figure 4a). The shell acts as a mounting base that also protects the oscillator. Energy transfer is achieved by the oscillator, whose elastic bands provide elastic support for the oscillator both horizontally and perpendicularly, making it stable under movement. The device can oscillate several times under single agitation.

The work by Xiao et al. mentioned beforehand also contains multilayered TENG units that rely on the structure for energy conversion [61] (Figure 4g). Four flexible springs are to support the mass block, and four rigid springs are attached to the mass's top to exert resilience force to press the TENG units upon wave excitation. Four shafts are also fixed for the mass and springs to move along with.

He et al. developed a mechanical regulator to drive TENGs with steady AC output, using the principle of auto-winding mechanical watch, which is to store random energy by transformation into potential energy and then release it at a designed pace [68] (Figure 6b). The structure involves more complex mechanisms by springs. The device consists of three parts: energy harvest and storage module, energy controllable release module, and energy conversion module. The first module contains input shaft, input gear train, and flat spiral spring, which transforms environmental energy into elastic potential energy. The second module is composed of escapement-spring-leaf and transmission gear train. The

escapement-spring-leaf is an integration of hairspring system and escapement, in order to reserve their kinetic merits and simplify the transmission chain to reduce energy loss. The last module is made up of output gear train, one-way shaft system, flywheel, and TENG units. When the system works, the spiral spring is first compressed by wave movements, driving the rotor of TENGs to rotate under the control of the escapement-spring-leaf. With such design, constant output is realized under even a low frequency of 0.5 Hz and random working stimulations (Figure 6c).

#### *3.6. Spacing Structure*

The spacing structure introduces an air gap between the triboelectric layer and electrodes, which reduces friction and surface wear, thus enhancing the robustness and durability of the TENG device, and the triboelectric charges can be replenished by soft structures and intermittent contact.

The feature was first realized in the work by Lin et al. that has been discussed in the pendulum structure section. In the device, the pendulum triboelectric layer and electrode layer are not allowed to contact intimately [41] (Figure 5d). A freestanding gap is reserved to reduce material abrasion and enhance oscillation degree. Hence, the TENG device boosts superior durability for long time operation, with one trigger lasting for more than 120 s. PTFE soft structures around the electrodes allow a supplement of charges on the pendulum triboelectric layer.

In the work by Jiang et al., an air gap also exists between the interfaces of triboelectrification, namely, the surfaces of arc-shaped acrylic strips and Cu electrodes, as have been introduced in the pendulum structure section [63] (Figure 5g). Although the electrodes contact with PTFE brushes in the swing process for charge-replenishing purpose, the friction resistance is low. Moreover, the swing motion of disks relative to the central steel shaft is smoothed by bearings with lubrication oil. Apart from that, animal furs can also be adopted as low-friction materials to further reduce wearing and enhance charge output. As a result, the performance remains steady after 400,000 cycles (Figure 6d).

#### *3.7. Water-Solid Contact Structure*

The water-solid contact (L-S contact) structure involves two components: electrodes covered by dielectric films and liquid. The working mechanism includes two steps: firstly, charges are generated on the dielectric surface through L-S contact electrification; secondly, electrostatic induction works through the asymmetric screening of triboelectric charges by liquid. Here, liquid can be regarded as the freestanding layer, and so TENGs based on L-S contact are mainly in freestanding mode. Essentially, L-S contact TENGs can only operate under the condition that they are constantly shifting between the states of being submerged in liquid and emerging out of liquid. Surface charge density is enhanced since contact area is increased at the liquid-solid interface. Moreover, the structure includes no extra mechanical component. The friction at the interface is also suppressed.

The first work for wave energy harvesting based on the mechanism was proposed by Zhu et al. [51]. The specific working process of the TENG is described in Figure 7a. After L-S contact electrification, the FEP film is distributed with negative triboelectric charges. The partial submerge of electrode A by water wave induces an interfacial electrical double layer, formed by positive charges in water like hydroxonium to screen the negative surface charges on FEP. Accordingly, the potential difference between electrode A and B rises until B starts to dive into water too. When the whole device is submerged, triboelectric charges are completely screened by ions in water and no electrostatic induction exists any more (Figure 7b). In addition, the FEP film surface is intentionally patterned with hydrophobic nanowires to create nanoscale roughness. The modified surface can not only realize water infiltration into aligned nanowires to enlarge contact area, but also ensures immediate water repellency as water leaves the surface.

**Figure 7.** Design and output of TENGs based on L-S contact structure. (**a**) Working principle of the first reported L-S contact TENG. (**b**) Charge output of the L-S contact TENG. (**a**,**b**) Reprinted with permission from ref. [51], Copyright 2014, American Chemical Society. (**c**) Schematic diagram of TENG with surface-mounted bridge rectifier arrays. Reprinted with permission from ref. [44], Copyright 2015, American Chemical Society. (**d**) Schematic diagram of TENG with two-dimensional networked structure. (**e**) Power output of TENG with two-dimensional networked structure. (**d**,**e**) Reprinted with permission from ref. [69], Copyright 2018, American Chemical Society. (**f**) Schematic diagram of buoy-like TENG. (**g**) Working principle and output of the outer TENG of buoy-like device under up-down movement. (**h**) Working principle and output of the inner TENG of buoy-like device under shaking or rotation movement. (**f**–**h**) Reprinted with permission from ref. [70], Copyright 2018, Wiley. (**i**) Working principle of U-tube TENG. (**j**) Output of eleven types of liquids with different polarities, dielectric constants, and contact angles. (**i**,**j**) Reprinted with permission from ref. [71], Copyright 2018, Springer Nature.

Further optimization focuses on the electrode structure, the film nanostructure, and liquid type. An integration approach was proposed by Zhao et al. that mounts bridge rectifier arrays on the TENG substrate surface to connect parallel electrodes together [44] (Figure 7c). Paralleled arrays of strip-shaped Cu electrodes are sputtered on Kapton substrate. The bridge rectifiers rectify currents between any pair of neighboring electrodes, by connecting pairs of adjacent electrodes through input pins, and linking all the negative output pins and positive output pins respectively to form comb-shaped joint cathode and insulated anode. In this way, the output can be added up to form pulsed direct current (DC) between the anode and cathode. The outermost electrification PTFE layer also protects and insulates all the conductive parts from water, with PTFE nanoparticles of 200 nm evenly distributed on its surface.

Zhao et al. further reported a TENG featuring a two-dimensional networked structure that yields high and stable output regardless of the wave type [69] (Figure 7d). The characteristic is realized through a rectifying chip based on p-n junction that is connected to both ends of electrode units, where the chip acts as a gate that only allows induced current from the anode to cathode. The electrode units in strip shape form a two-dimensional array on the Kapton substrate, and the two chips at the opposite sides of every electrode unit are in serial connection. A stable short circuit current of 13.5 μA and electric power of 1.03 mW are produced (Figure 7e).

As an intriguing structure design, the buoy-like TENG device proposed by Li et al. involves an outer TENG whose bottom electrode is on the outside, and an inner TENG whose bottom electrode is inside the buoy [70] (Figure 7f). As PTFE film rubs water, charges are induced on the surface, leading to a potential difference between the bottom electrode and the electrode on the backside of PTFE. The outer TENG works under up-down wave movements, while the parallel-connected inner TENGs function under shaking or rotation movements (Figure 7g,h). Consequently, the device can collect energy from different types of wave excitations.

The work constructed by Pan et al. aimed to study the influence of liquid properties on TENG output, by a U-tube TENG that includes a FEP U-tube, Cu electrodes, and liquid solution [71] (Figure 7i). The liquid flows on the coarsely nanostructured inner surface of the U-tube. As the U-tube rolls rightward or leftward and drives the liquid to overlap the FEP U-tube, currents are generated on the Cu electrodes wrapped outside the tube. It is concluded that liquid with higher polarity, dielectric constant, and contact angle yields higher output, and pure water, which is the eleventh specimen, showed the best experimental result in eleven types of liquids (Figure 7j).

#### *3.8. Charge Pumping Strategy*

As is known, the TENG is based on triboelectrification to generate static charges, and electrostatic induction to drive free charge transfer. However, the limited surface charge density by triboelectrification greatly hinders the output enhancement of TENGs [21]. Compared to other methods, developing new mechanisms on charge accumulation can address the issue fundamentally under ambient conditions.

The charge pumping strategy is shown to effectively enhance the charge density [45]. The utilization of the strategy requires a floating layer that can accumulate and bind large amounts of charges for electrostatic induction, and a charge pump that can pump charges into the floating layer simultaneously (Figure 8a). The realization of such mechanism involves a pump TENG, which is a normal TENG with a rectifier to make charge flow unidirectionally, and a main TENG (Figure 8b). The floating layer of M2 is achieved owing to the insulation from electrodes M1 and M3 of the main TENG by two dielectric layers, D1 and D2. Therefore, M2 cannot exchange charges with the main TENG electrodes and can only accept electron flow of one direction from the rectifier. The charge pumping process from the pump TENG to the main TENG is similar to charging a capacitor, and can last until the dielectric layers break down electrically. The pumped charges inside the floating layer are like static charges in normal TENGs, thus achieving large output through electrostatic induction. The energy collected by pump TENG adds up through charge accumulation in the main TENG, achieving an ultrahigh effective surface charge density of 1020 μC m−<sup>2</sup> in ambient conditions, which does not rely on intensive rubbing or contact, improving energy efficiency (Figure 8c).

Another similar method reported by Liu et al. utilizes only one TENG combined with a self-voltage-multiplying circuit [72] (Figure 8d). Charge amplification is realized through altering the connection pattern of external capacitors accompanying the motion of the TENG. A voltage-multiplying circuit (VMC) module is adopted that can automatically switch the capacitors between parallel and serial connection during the contact and separation process of the TENG, owing to the unidirectional property of diodes. The doubled charge output from the VMC can flow back to the TENG in each cycle, thus greatly

enhancing the charge density. A high output is achieved with a Zener diode for voltage stabilization (Figure 8e).

**Figure 8.** Design and output of TENGs based on charge pumping strategy. (**a**) Schematic diagram on mechanism of charge pumping. (**b**) Working principle of TENG based on charge pumping. (**c**) Ultrahigh surface charge density of charge-pumping TENG compared with other works. (**a**–**c**) Reprinted with permission from ref. [45], Copyright 2018, Elsevier. (**d**) Schematic diagram of TENG combining VMC. (**e**) Output of TENG with VMC. (**d**,**e**) Reprinted with permission from ref. [72], Copyright 2019, Springer Nature. (**f**) Schematic diagram of rotation and sliding TENG combined with charge pumping. (**g**) Average power output of rotation and sliding TENG based on charge pumping. (**f**,**g**) Reprinted with permission from ref. [73], Copyright 2020, Wiley. (**h**) Schematic diagram of TENG based on charge shuttling. (**i**) High effective charge density of TENG based on charge shuttling. (**j**) Demonstration of TENG device based on charge shuttling for wave energy harvesting. (**h**–**j**) Reprinted with permission from ref. [52], Copyright 2020, Springer Nature.

The work reported by Bai et al. develops a successful combination of charge pumping mechanism with rotation and sliding TENGs [73] (Figure 8f). The device is composed of a pump TENG and a main TENG, each with two disks as the rotator and the stator. A novel synchronous rotation structure is designed to allow direct injection of bound charges from the pump TENG to the main TENG. Specifically, an inversion structure allows the electrode disk of pump TENG to rotate, which is assembled with the main TENG by shafts to form the rotator, while the output electrodes of the main TENG are part of the stator. In this way, the electrodes of the pump TENG and storing bound charge layer can rotate in a synchronous mode. After the pump TENG injects charges into the storing electrodes of the main TENG, opposite charges are induced in the underlying output electrodes. Then, with the rotational sliding of storing electrodes, induced charges move following the motion of the bound charges, generating ultrahigh charge density and an average power density of 1.66 kW m−<sup>3</sup> under a low drive frequency of 2 Hz (Figure 8g). Moreover, the decoupling of surface charge generation and friction enables lubricant to be applied to the interfaces to enhance durability. Ultrafast accumulation of bound charges is achieved in the rotation TENG that also possesses good expandability to charge multiple main TENGs by one pump TENG.

Wang et al. proposed a fundamentally different mechanism of charge shuttling to generate output on TENGs, which is not based on the electrostatic induction of static charges but on the shuttle of corralled charges in a TENG and a buffer capacitor [52] (Figure 8h). The corralled mirror charge carriers are accumulated through charge pumping strategy, and shuttle due to the interaction of two quasi-symmetrical conduction domains, which doubles the output. The device based on charge shuttling is composed of a pump TENG, a main TENG, and a buffer capacitor. The electrodes of the main TENG and the buffer capacitor form two conduction domains respectively presenting a quasi-symmetrical structure with Q<sup>+</sup> side and Q<sup>−</sup> side. The pump TENG injects charges into domains through a rectifier. Upon contact and separation of the main TENG, its capacitance changes while that of the buffer capacitor remains, inducing a voltage difference between them to drive charges to shuttle in a quasi-symmetrical way, generating electricity on two loads. Consequently, the charge output is doubled by the two shuttling mirror charge carriers, achieving a total effective charge density of 1.5 mC m−<sup>2</sup> with a Zener diode for stabilization (Figure 8i). The mechanism was successfully applied in an integrated device for wave energy harvesting. The peak current of the device reached about 1.3 mA under wave agitations of 0.625 Hz. The maximum peak power was 126.67 mW at 300 kΩ, corresponding to a volume power density of 30.24 W m−<sup>3</sup> (Figure 8j).

#### **4. Networking Strategy and Power Management**

#### *4.1. Networking Designs*

The networking design of TENG units for blue energy harvesting involves mechanical connection and electrical networking. The ideal mechanical connecting method should be highly resilient, anti-fatigue, mechanically robust, and resistant to corrosion, while the electrical networking demands maximized output, avoidance of the disturbing electrostatic induction, and sufficient protection from the interference and corrosion by sea water. Meanwhile, the interconnection of TENG units is expected to introduce a coupling effect to enhance the total harvesting efficiency.

The coupled TENG network for wave energy harvesting was first realized by Xu et al. [54]. The work investigated the coupling design in details. Three connection methods were proposed and tested, of which the flexible connection shows higher efficiency and better performance (Figure 9a,b). The output of the linked unit is 10 times greater than that without linkage due to the coupling between TENG units. The network of 16 units is rectified and electrically connected for self-powered sensing by wave agitations (Figure 9c).

In order to enhance the robustness of mechanical connection, another intriguing way is to construct a network structure that can heal by itself after being broken by strong waves, as put forward by Yang et al. [42]. The idea was accomplished by the design of a self-adaptive magnetic joint (SAM-joint), which is composed of a rotatable spherical magnet and a limit block mounted around the TENG device shell (Figure 9d). The SAMjoint functions by self-adaptive mechanism of the pole and anisotropic restriction on the degree of freedom, enabling the network structure with characteristics of self-assembly, selfhealing, and facile reconfiguration, which greatly improve the autonomy and robustness of the TENG network (Figure 9e).

Liu et al. put forward a study concerning four types of electrical networking topology [74]. The effect of wire resistance and output phase asynchrony of different units on the network output were analyzed and concluded to be crucial (Figure 9f).

Liu et al. designed a special type of plane-like power cable for TENGs to meet the demands raised by both electrical networking and mechanical linkage, which consists of spring steel tapes and three polymer films on the outside [48] (Figure 9g). Firstly, the plane shape that limits the entanglement among interconnected units, along with the highly resilient steel tape as the structural skeleton and the hydrophobic PTFE film, fulfills requirements for mechanical connection. Secondly, the conducting wires of TENGs are sandwiched in nearby steel tapes to avoid water screening effect for higher electrical output. Moreover, the power cable structure has an additional merit of

generating electricity by the cable itself as a TENG device. Because as the cable contacts and separates with water, the steel tapes act as electrodes, which are covered by PTFE films, forming a L-S contact mode TENG.

**Figure 9.** Networking design and output of TENGs. (**a**) Schematic diagram of three connection methods of coupled TENG network for wave energy harvesting. (**b**) Current output of coupled TENG network by three connection methods. (**c**) Schematic diagram of TENG network rectification and demonstration of coupled TENG network for self-powered sensing by wave agitations. (**a**–**c**) Reprinted with permission from ref. [54], Copyright 2018, American Chemical Society. (**d**) Schematic diagram of SAM-joint mechanism. (**e**) Demonstration of self-healing of TENG network by SAM-joint. (**d**,**e**) Reprinted with permission from ref. [42], Copyright 2019, Elsevier. (**f**) Schematic diagram of electrical networking topology of TENG network. Reprinted with permission from ref. [74], Copyright 2020, Elsevier. (**g**) Schematic diagram of plane-like power cable and perspective. Reprinted with permission from ref. [48], Copyright 2020, Elsevier.

#### *4.2. Power Management*

TENGs are generally characterized to have high internal impedance, which does not match the impedance of most general electronics. Direct powering the electronics by TENGs will result in low efficiency. Meanwhile, a switch strategy is required to maximize the power generated in a single cycle [21]. Moreover, for the network with multiple TENG units, there are new challenges to maximize the total output of modules or the whole network by power management.

The power management module (PMM) proposed by Xi et al. consists of two components: a tribotronic energy extractor to enhance the energy transfer efficiency from the TENG device to the circuit system, and a DC-DC buck converter to generate DC output on the load [49] (Figure 10a). The core goal of power management is to optimize and maximize the TENG output power on the loads through step-down flow, which is achieved by the PMM in a universal, efficient, and autonomous way because it is compact in size and applicable for various forms of TENGs with a self-management mechanism.

Xi et al. then utilized the PMM to support an application system based on wave energy [11] (Figure 10b). The harvested energy by TENGs passing through the PMM can drive a microprogrammed control unit (MCU), several microsensors, and a transmitter due to the enhanced energy transfer efficiency. The MCU based on an intelligent monitoring mechanism can deploy electricity for each sensor by varied priority and data transmission cycle, accomplishing a sustainable and autonomous wireless sensing for acceleration, magnetic intensity, and temperature.

**Figure 10.** Power management and output of TENGs. (**a**) Circuit diagram of PMM including tribotronic energy extractor and DC-DC buck converter. Reprinted with permission from ref. [49], Copyright 2017, Elsevier. (**b**) Demonstration of utilizing PMM to support MCU and sensors. Reprinted with permission from ref. [11], Copyright 2019, Elsevier. (**c**) Demonstration of combining PMM with a hexagonal network of TENGs to power wireless transmitter. Reprinted with permission from ref. [75], Copyright 2019, Wiley. (**d**) Schematic diagram comparing SCC with FSCC. (**e**) Charge output of TENG adopting power management based on FSCC. (**f**) Current and power output of TENG adopting power management based on FSCC. (**d**–**f**) Reprinted with permission from ref. [50], Copyright 2020, Springer Nature. (**g**) Schematic structure of air-membrane TENG device with contact switches. (**h**) Schematic diagram of the switch circuit. (**i**) Pulse current of the device with switches. Inlet: single current pulse. (**g**–**i**) Reprinted with permission from ref. [43], Copyright 2017, Elsevier.

Liang et al. proposed a hexagonal network of TENGs combined with the above PMM, successfully powering a wireless transmitter to send signals every 10 s under wave agitations, which further verified the application of the PMM in wave energy harvesting [75] (Figure 10c).

In addition, the transformer of switched-capacitor-convertor (SCC) is also a good candidate for power management since it is magnet-free, light-weight, and easy for integration (Figure 10d). Liu et al. put forward an SCC that adopts fractal design (FSCC), which optimizes the topology to decrease the impedance and lower the switch loss of transistors by integration on printing circuit boards [50]. FSCC also shows high step-down ratio and electrostatic voltage applicability. By coupling TENGs with FSCC, the power management

system can boost the charge output 67 times (Figure 10e), and reaches a high power density of 954 W m−<sup>2</sup> in pulse mode at 1 Hz (Figure 10f).

Apart from the above module, a switch circuit can also intensify the current and power output, as put forward in the mentioned multilayered air-membrane TENG by Xu et al. [47]. The switches are formed by attaching Al foils on air chambers and their corresponding contact spots on the outer shell, and are closed when matching foils contact. As the inner oscillator reaches its highest or lowest position and creates a voltage owing to the charge separation, switches are closed and a pulse current driven by the voltage appears in the external circuit (Figure 10g,h). Such switch circuit results in swift charge transfer that no longer depends on mechanical contact-separation speed of electrodes, rising the peak current to 1.77 A and the instantaneous power to 313 W, at a load of 100 Ω (Figure 10i), which further enhances previous experimental results by Cheng et al. [74]. Apart from that, the possible energy output per cycle is also increased, the principle of which was thoroughly discussed by Zi et al. [21].

#### **5. Summary and Perspectives**

In this paper, major aspects in the construction of TENG networks for blue energy harvesting are reviewed. The design of TENG unit for performance improvement is mainly discussed, from structure advancement to mechanism alteration, including rolling ball structure, multilayer structure, grating structure, pendulum structure, mass-spring structure, spacing structure, water-solid contact structure, and charge pumping strategy. Different kinds of structure designs are born with advantages to cater to specific needs, including the naturally low frequency of rolling ball structure to match with slow wave agitations, the superior output density of multilayer and grating structures, the high sensitivity and elongated operation time under mechanical excitations to improve the energy conversion efficiency through pendulum and mass-spring structures, and the outstanding robustness and durability by spacing structure. Principle innovation such as charge pumping is significant as it can bring large promotion to the system. Networking strategy and power management are also briefly discussed. As a promising clean energy technology, blue energy harvesting based on TENGs is expected to make great contributions for achieving carbon neutrality and developing self-powered marine systems. Revealed as a type of mechanical energy harvester more suitable for low-frequency excitations, the key to the commercialization of TENGs lies in the combination of high power density and robustness. The following aspects are suggested to be focused upon in future investigations:

(1) The design of TENG units is still quite crucial for further enhancing the power density, especially in a real ocean environment, which has much more complex wave conditions than in the lab, and the design of the device can be further validated and optimized based on current devices [76]. A detailed comparison on typical devices is shown in Table 1. In general, devices based on multilayer structure and grating structure intrinsically output with greater power density. Making the components soft can expand contact area, which enhances triboelectrification. Mechanism innovations regarding charge pumping strategy achieve ultrahigh charge density. Rolling ball, pendulum, and massspring structures can make blue energy harvesting more adaptive to changing directions and broad frequency of waves, with better durability. To reach higher output of TENGs from the material aspect, polymers can be improved in dielectric permittivity, electrostatic breakdown strength, stability, contact status, and mechanical robustness, through surface morphology and molecular functionalization as well as bulk composition modification [77].

(2) The durability of the TENG with friction or contact interfaces should be further examined and optimized. For long-term operation at sea, the device should achieve high reliability.

(3) The networking strategy is important for organizing and coupling TENG units together to reach higher efficiency as an integrated system. It is less investigated in the past for its complexity, which should be emphasized with further development of blue energy.



*Nanoenergy Adv.* **2021**, *1*

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**Table 1.** *Cont.*

(4) The interaction of devices with water motion is crucial for power-take-off (PTO) performance of the system, which should be theoretically investigated and optimized based on fluid-structure interaction dynamics.

(5) Adaptability to the ocean environment involves packaging of the device, antifouling, and anticorrosion, which can ensure that the function of the network is not damaged in the severe ocean environment. The complete encapsulation of the device by waterproof materials can protect the circuit and core device from water.

(6) Hybrid harvesting that can harness various other energy forms at sea, such as wind, rain drops, and sun light, can further improve the utilization efficiency of certain ocean area [78].

(7) Environmentally friendly designs and degradable materials are also highly required to reduce the environment risks of such systems in ocean [79].

(8) Power management that optimizes at the module level or network level is highly required, which extends the present work to larger systems. It is expected to greatly improve the total efficiency of the whole system.

**Author Contributions:** Conceptualization, L.X. and Z.W.; writing—original draft preparation, H.W.; writing—review and editing, L.X. and Z.W.; supervision, L.X. and Z.W.; project administration, L.X. and Z.W.; funding acquisition, Z.W. and L.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R & D Project from Minister of Science and Technology, China (No. 2016YFA0202704), the Key Research Program of Frontier Sciences, CAS (ZDBS-LY-DQC025), National Natural Science Foundation of China (No. 51605033, 51735001), and Youth Innovation Promotion Association, CAS (No. 2019170).

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

#### **References**


## *Review* **Ferroelectric Materials Based Coupled Nanogenerators**

**Jabir Zamir Minhas 1,2, Md Al Mahadi Hasan 1,2 and Ya Yang 1,2,3,\***


**Abstract:** Innovations in nanogenerator technology foster pervading self-power devices for human use, environmental surveillance, energy transfiguration, intelligent energy storage systems, and wireless networks. Energy harvesting from ubiquitous ambient mechanical, thermal, and solar energies by nanogenerators is the hotspot of the modern electronics research era. Ferroelectric materials, which show spontaneous polarization, are reversible when exposed to the external electric field, and are responsive to external stimuli of strain, heat, and light are promising for modeling nanogenerators. This review demonstrates ferroelectric material-based nanogenerators, practicing the discrete and coupled pyroelectric, piezoelectric, triboelectric, and ferroelectric photovoltaic effects. Their working mechanisms and way of optimizing their performances, exercising the conjunction of effects in a standalone device, and multi-effects coupled nanogenerators are greatly versatile and reliable and encourage resolution in the energy crisis. Additionally, the expectancy of productive lines of future ensuing and propitious application domains are listed.

**Keywords:** ferroelectric materials; nanogenerators; piezoelectricity; triboelectricity; pyroelectricity; bulk ferroelectric photovoltaic effect (BPVE); harvesting; coupled effects

#### **1. Introduction**

With the advancement in modern mobile electronics, wireless communication systems, and the internet of things (IoT), researchers became more concerned with the miniaturization and multi-functionality of devices, e.g., low power, flexibility, etc. Mobile devices are handy and carry information that is immediately accessible and transmittable wirelessly. The devices are small enough to be worn or fastened with objects of everyday use, such as goggles, clothes, wristwatches, and many others. These minuscule devices need small operational powers, usually in micro-watt or milli-watt, for which the use of the traditional chemical batteries is a miser and are the cause of environmental threaten, too; hence, small-scale sustainable power solutions are in need. The make-up to this innovates the maturing of energy harvesting units, which can efficiently harvest energy, such as mechanical, heat, and light energy, to electrical energy and reuse sources of energies from the ambient atmosphere [1–4]. In 2006, Z. L. Wang and J. Song developed nanogenerators (NGs) by transforming nanoscale mechanical energy into electricity using arrays of piezoelectric ZnO nanowire and marked energy harvesters to jump into a new span [5]. The basic theory of nanogenerators was established based on the concept of the use of the Maxwell displacement current as a driving force to convert environmental energies into electrical energy signals. Therefore, NGs work by current generations and employ the concept of internal polarization of the material [6]. Out of thirty-two crystal structures, twenty-one crystal classes do not possess central symmetry and are non-centrosymmetric, twenty of which show polarization as the reaction of mechanical stress forces are piezoelectric and, among these, twenty (1, 2, m, 222, mm2, 4, −4, 422, 4 mm, −42 m, 3, 32, 3 m, 6, −6, 622 m

**Citation:** Minhas, J.Z.; Hasan, M.A.M.; Yang, Y. Ferroelectric Materials Based Coupled Nanogenerators. *Nanoenergy Adv.* **2021**, *1*, 131–180. https://doi.org/ 10.3390/nanoenergyadv1020007

Academic Editor: Joao Oliveira Ventura

Received: 9 October 2021 Accepted: 19 November 2021 Published: 25 November 2021

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6 mm, −62 m, 23, −43 m). Among these twenty piezoelectrics, ten show spontaneous polarization variations, induced by temperature changes are pyroelectric (1, 2, m, mm2, 3, 3 m, 4, 4 mm, 6, 6 mm). Their polarization turned out to be reversed with the implementation of the electric field; additionally, the particular materials are ferroelectrics [7,8]. Ferroelectrics are functional dielectric materials that have shown spectacular properties of spontaneous polarization at zero electric fields. The spontaneous polarization (Ps) can be switched to reversed direction when a large poling electric field (in units of kV/cm to MV/cm) is applied. Figure 1a(i) demonstrates the typical ferroelectric P-E curve [9], showing both the states are steady and thermodynamically stable. Thus, ferroelectric materials have been of great research interest for almost over a century [10,11]. In conventional ferroelectric (FE) materials, also known as proper ferroelectrics, the spontaneous polarization (Ps) is responsible for the change in structure (i.e., structural phase transition) breaking the crystal symmetry at curie temperature (Tc). So, the order parameter of phase transition in proper ferroelectrics is spontaneous polarization. Ferroelectrics have posted a wide range of applications contributing to frequency filters, pressure and temperature sensors, actuators, hydrophones, oscillators, and many others [12–18]. Since all the ferroelectrics are piezoelectric and pyroelectric, they speculate the phenomenon of piezoelectricity, pyroelectricity, and photovoltaic ferroelectric effects. These effects are the result of responses from external stimuli, such as mechanical, thermal, and solar energies, respectively [19–22]. The piezoelectric coefficient of semiconductor piezoelectric ZnO is ~12 pC/N, which is quite low, in contrast to the ferroelectric materials with perovskite structure, e.g., BaTiO3 and Pb(Zr, Ti)O3, which exhibit high piezoelectric coefficients of 100 pC/N and 200 pC/N, respectively [23]. These properties of ferroelectrics promote their use in the fabrication of nanogenerators for energy scavenging and harvesting from low frequency natural and artificial energy sources from the environment [24,25]. Along with energy storage units, self-powered systems can also be integrated by nanogenerators for powering functional devices. Many nanogenerators have been fashioned after, including piezoelectric NGs (PENG), pyroelectric NGs (PyENG), triboelectric NGs (TENG), and photovoltaics (PVC) [26–29]. Piezoelectric and triboelectric NGs transform mechanical stress forces and/or energies from wind/airflow, water waves, human motions, biomechanical energies, etc., to electrical energy, accompanied by changing polarization degrees in ferroelectrics [30–35]. Pyroelectric NGs works by temperature variations from industrial heat wastes, radiations from the sun, etc. [36–41]. The photovoltaic effect in ferroelectrics demonstrated that the photocurrent is not limited by the bandgap of material; rather, it is associated with material polarity and separation of light-induced photo carriers [42–45]. At present, hybrid nanogenerators and multi-effect coupled NGs are of practical interest, developed by integrating the above mentioned effects into a single structure and, hence, contributing to the maximization of energies for high electrical power outputs, depending on the strength of external stimuli [6,46–50]. The ever first one-structured coupled nanogenerator, based on piezo–tribo–pyro–photovoltaic effects, was proposed by professor Ya Yang and his co-workers in 2015 [51]. Ferroelectric materials possess permanent dipole moments in the electric field, which increases their polarization density and facilitates their use in wearable, flexible electronics. The conventional semiconductor materials do not own this property and so ferroelectrics gather more attention [52]. The most active ferroelectric materials encapsulated in nanogenerator application are inorganic ferroelectric ceramics and organic polymers; the best of them include BaTiO3 (BTO) [53], PbZr1-xTixO3 (PZT) [54], Na0.5Bi0.5TiO3 (NBT) [55], KNaNbO3 (KNN) [56], BiFeO3 (BFO) [57], Pb(Mg1/3Nb2/3)O3- PbTiO3 (PMN-PT) [58], PVDF [59–61], and P(VDF-TrFE) [62]. In comparison with inorganic ferroelectrics, very few organic ferroelectrics exist. For example, single-component polar organic molecules, such as CDA, DNP, CT complexes, and polymers, such as PVDF, nylon-11, and organic-inorganic composites, such as HdabecoReO4, TGS, TSCC, are excellent for applications in piezoelectric and triboelectric nanogenerators [63]. Other reports on semiconductor ferroelectric have also been found. Ferroelectric polymers are highly flexible and have found applications in wearable and foldable devices. Ferroelectric ceramics

are hard and are often complexed with other ferroelectric/non-ferroelectric material and stacked to multilayered architecture, in order to improve their output features with various structural morphologies and, e.g., (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3-x BiHoO3/PDMS [64], PDMS/PZT [65], and CNTs-PMNT/PDMS [66]. In this peer review paper, we briefly talk about the basic properties of ferroelectric materials, together with related phenomena for energy conversion, i.e., piezoelectricity, pyroelectricity, triboelectricity, and ferroelectric photovoltaic effects, as well as operating conditions of various types of ferroelectric material-based NGs. Mechanism of ferroelectrics-based hybrid and multi-effect coupled NGs, with their structure-related performances and power conversion efficiencies, are also discussed. Some recent applications, including self-powered micro/nano-systems, multifunctional sensors, and wearable flexible devices, are discussed. Furthermore, the advantages of ferroelectric-based NGs and future prospects are devised.

**Figure 1.** Hysteresis curve and typical ferroelectric crystals structures. (**a**) Hysteresis curve for ferroelectric materials with important factors of coercive field (Ec), remanent (PR), and spontaneous polarization (Ps); (**i**) PZT crystal, representing the

two stable positions of Zr4+ or Ti4+ ions; (**ii**) PVDF polymer chain with the two orientations for polarization; (**iii**) hafnium oxide with switching O2- oxygen ions; (**iv**) O2- ion switching in fluoride structure of orthorhombic hafnium oxide (HfO2). (**b**) Domain wall movement. Reprinted with permission from reference [67], Copyright 2017, AIP Publishing. (**c**) Transition temperature dependence of ferroelectrics. Reprinted with permission from reference [68], Copyright 2016, Springer Nature.

#### **2. Ferroelectric Materials**

#### *2.1. Fortunes of Ferroelectric Materials*

Ferroelectric materials in their ferroelectric phase exhibit net dipole moment, i.e., the centers of charges (positive and negative) do not coincide, resulting in robust polarization lasting permanently below a particular temperature, called the curie temperature (Tc), in the non-poled state. Before the time of the second world war, from 1920–1943, ferroelectric materials remained of academic and theoretical interest for small applications zones. In the 1950s, BaTiO3 (BTO) discovered that strong ferroelectric behavior evoked the electronic ceramic industry and became an interesting candidate for piezoelectric transducers. By the 1960s, researchers undertook ferroelectric thin films and realized practicing non-volatile memories, but quality maintenance, restricted their practical applications, up until the 1980s; later in the 1990s, ferroelectrics, alongside microsensors, found widespread applications in radio frequency and microwave devices. In 1994, the bypass ferroelectric capacitors installed in digital mobile phones provided a breakthrough in the leading microdigital industry [69–71]. For the past two decades, ferroelectric materials subsisted a great application tempt in energy harvesting.

#### *2.2. Crystal Structures*

Joesph Valasek has discovered the phenomenon of ferroelectricity in Rochelle salt (NaKC4H4O6.4H2O) orthorhombic crystal structure first. Up until now, more than seven hundred ferroelectric materials have been demonstrated, inclusive of oxides, polymers, ceramics, and liquid crystals, as well. Typical ferroelectric materials have either ABO3 perovskite or hydrogen-bonded potassium dihydrate phosphate KH2PO4 (KDP) structures, but others also exist, such as GeTe, SrAlF5, SbSI, SbSeI, etc. [72,73]. Perovskite ferroelectric materials are with crystal structure isomorphous to the mineral perovskite calcium titanium oxide CaTiO3, general formula ABO3, whereupon A and B stand as cation, and O is the oxygen ion. The ionic radius of cation A and B is particularly around 1.2 Å to 1.6 Å and 0.6 Å to 0.7 Å, respectively. Oxygen atoms/ions are positioned at the face center and A ion at the cube corner, forming octahedral surrounding B ions and staying at the body center. Structure stability requires that the valencies of ions be balanced. Moreover, the structural distortions, such as octahedral distortion or tilting, associated with low space group symmetries, cause relief of B-site electronic instabilities, leading to high values of ferroelectric polarization, piezoelectric coefficients, unusual photovoltaic behaviors, and dielectric constants [7]. Figure 1a(ii) represent perovskite structure-based inorganic ferroelectric materials, such as lead zirconate titanate PZT (Pb(TixZr1−x)O3), barium titanate BTO (BaTiO3), and layered strontium bismuth tantalite SBT (SrBi2Ta2O9); in all of them, the central cation switches its position between the stable states [9]. The literature revealed the existence of morphotropic phase boundary (MBP) in PZT, with compositional variations that benefited piezo ceramics a lot [74–77]. Furthermore, the nature of lead (Pb) is weighty and detrimental to humans and the environment. Therefore, lead-free materials (e.g., BiFeO3 (BFO), KNaNbO3 (KNN), etc.) which are non-toxic, environmentally friendly, and biocompatible, are of more research interest [78,79]. Some ferroelectrics, with more complex perovskite structures, have also been observed with cations of different valencies but fixed molar concentrations. For example, PbMg1/3Nb2O3 (PMN), PbSc1/2Ta1/2O3 (PST), and Bi1/2Na1/2TiO3 (BNT) [80,81]. Ferroelectric hafnium oxide (HfO2) is also of great interest and becoming a spotlight for the memories industry [9].

Ferroelectrics with ilmenite structure are similar to perovskites, with distinction in the A cation, which is small enough to fill ABO3 structure coordinated site. Oxygen ions are packed closely in a hexagonal layer, with cations A and B located between the layers at octahedral sites LiNbO3 and LiTaO3, which are two important uniaxial ferroelectric materials of this class [82–84].

Polyvinylidene fluoride (PVDF) with CH2CF2 monomer was observed to be the first ferroelectric polymer, without any particular curie temperature, as melting occurs first. PVDF exists in four different polymorphs (i.e., the *α*−, *β*−, *γ*−, and *δ*−phases), depending on the configuration of carbon–carbon links. Fluorine atoms are electronegative and make the C–F bond polar, resulting in the molecule having a net dipole moment orthogonal to molecular length carbon backbone chain. Though the molecules of polymer arrange in the unit cell and, hence, the dipoles balance each other in the *α*-phase; so, the PVDF in *α*-phase is not ferroelectric, but its field application produces a net dipole moment on unit cell and put it in *δ*-phase. Annealing of these systems at high temperatures crystalizes them in the *γ*-phase, which also has a net dipole moment perpendicular to the carbon backbone and is polar. Orthorhombic *β*-phase can be achieved by stretch or draw in previously derived phases and is an all-trans configuration with high spontaneous polarization and a dielectric constant [85]. So, electric poling makes *β*-PVDF-phase a strong piezoelectric and pyroelectric phase, when compared with other polymorphs. *β*-PVDF-phase can be directly obtained by the copolymerization of vinylidene fluoride with trifluroethylene TrFE (~10 to 46% by wt.); the resultant P(VDF-TrFE) material has the same activity rate as pure PVDF but exhibits high remnant polarization [82,86,87]. Figure 1a(iii,iv) show the switching behavior of polar polymer chains in organic ferroelectric polyvinylidene fluoride (PVDF) polymerized, with tetrafluoroethylene (TrFE) and O−<sup>2</sup> ion switching in fluoride structure of orthorhombic hafnium oxide (HfO2), respectively [9]. Hexafluoropropylene HFP is another important copolymer material for the PVDF matrix. Other polymers that exhibit ferroelectric behavior include odd-numbered nylons, but these are only weakly piezoelectric. These polymeric ferroelectrics are useful in flexible electronics, especially in wearable devices.

Another class of ferroelectrics comprises of improper ferroelectrics, for those in which the order parameter is not the polarization; rather, the spontaneous polarization arises as to the by-product of phase transition, as a secondary effect. The temperature dependence of the permittivity does not follow Curie–Wiess law; additionally, the phase transition is not suppressed by the electric fields in improper ferroelectrics [88]. The dielectric constant of improper ferroelectric remains low, usually near the phase transition temperature, and is conducive to the large pyroelectric figure of merits. Examples of improper ferroelectric involve iron-iodine-boracite [FeB7O13I (TMO)], dicalcium-lead-propionate [Ca2Pb(CH3CH2COO)6 (DLP)], [Hdabeco]ClO4, etc., [89]. A subset of ferroelectric materials forms ferroelastic materials, which may show spontaneous strain and possibly exhibit two or more stable orientation states under zero electric field or mechanical stress. Pb3(PO)4 is a particular example of ferroelastic materials [68].

Anti-ferroelectric (AFE) structures are mainly characterized by the phase transition from the low symmetry state, usually from the low-temperature phase to the high symmetry state (usually high-temperature phase). Contrary to ferroelectrics, the anti-ferroelectrics do not have permanent electric polarization. Therefore, an anti-ferroelectric crystal lattice can be considered to be made up of two interpenetrating sublattices with equal but opposite polarizations. However, high spontaneous polarization (Ps) appears as the transition from the anti-ferroelectric phase to the ferroelectric phase. They exhibit low dielectrics losses and coercive fields. A typical anti-ferroelectric material with a perovskite structure is PbZrO3, with Sr doping energy density of 14.5 J/cm3, which had been observed at 900 kV/cm by Hao et al. [90]. Anti-ferroelectric Zr doped HfO2 had shown the largest energy density of 46 J/cm3 at 4.5 MV/cm [91]. Ferroelectric relaxors have also been demonstrated in the past few years; relaxors belong to disordered crystals when nonequilibrium ions are added into normal ferroelectric materials or heated above transition temperatures and de-poled under critical electric fields. They exhibit broad and suppressed dielectric peaks but higher susceptibilities. The ferroelectric (SrxBa1−x)Nb2O6 (SBN) contains five Sr or Ba ions; so, one

of the A-site remains unfilled, therefore, varying the Sr/Ba concentration to an increased ratio, they transform from normal ferroelectrics to relaxor ferroelectric, as reported for x = 0.75 in reference [92]. The 0.67 Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 composite ferroelectric relaxor shows two phase transitions at 34 ◦C (from rhombohedral to tetragonal) and at 144 ◦C (from tetragonal to cubic) [93]. PVDF shows the relaxor behavior with improved dielectric properties and piezoelectric responses by carrying temperature treatment and further copolymerization to TrFE-CFE [94,95].

#### *2.3. Ferroelectricity and Hysteresis*

Ferroelectricity is the material property of exhibiting spontaneous polarization (Ps). Ferroelectric materials have high dielectric constants, and field removal does not bring polarization back to its original direction spontaneously, which signifies that the polarization is persistent. This characteristic behavior of ferroelectric materials is defined by a schematic, non-linear curve observed between the electric field (E) and polarization (P), known typically as hysteresis, displayed in Figure 1a(i) [9]. Field escalations align polarizations in distinct dipolar regions of a ferroelectric crystal, reaching a saturation value of spontaneous polarization (Ps) and setting off the field to zero gives remanent polarization (P+), which is slightly smaller than Ps and the result of charge displacement. Negative field value reduces the polarization to zero at the coercive field (Ec−), and further increases cause reverse saturation polarization (−Ps) and remanent polarization (P−) on returning to zero field value. Polarization, again, reaches zero at Ec+, towards a positive applied field and back to Ps. An important feature of ferroelectrics is that remanent polarization (2PR = |P+| + |P−|) persists, even after externally applied E is abolished; this makes them suitable for application in non-volatile random-access memories. BaTiO3 shows a remanent polarization and coercive field of 2.55 μC/cm<sup>2</sup> and 3.14 kV/cm, respectively [96]. NBT (Na0.5Bi0.5TiO3) has a remanent polarization of 38 μC/cm2 and coercive field of 70 kV/cm only [97]. However, doping of ST (SrTiO3) to NBT reduces the value of remanent polarization and coercive field to 0.007 μC/cm<sup>2</sup> and 0.15 kV/mm [98]. During the process of poling for polarization reorientations in the same direction, the bound charges restrain at the opposite faces of polarization moment, i.e., positive at the negative end and vice versa. The bound charges establish a depolarization field and side of the material, in order to maintain charge neutrality, while the screening for free charge accumulates on the surface, compensating the bound charge keeping the surface neutral [97,98].

#### *2.4. Ferroelectric Domains and Phase Transition*

In the ferroelectric materials, certain regions, called domains, exist and are separated by interfaces known as domain walls. Each region containing polarization mixtures has a different polarization direction for other regions, inferring that the virgin ferroelectric material does not show a particular net polarization direction. When ferroelectric materials are poled, their domains begin to set in the field direction and show a net polarization. This signifies that polarization switching is quantum-mechanically functionalized by domain wall motion. Figure 1b shows the domain wall motions and corresponding polarization switching in BTO ceramics [67]. Two unique domains are labeled as 180◦ domains, with polarizations aligned antiparallel to each other and 90◦ domains with polarizations aligned perpendicular to each other. The motion of the 180◦ domain walls gives rise to dielectric and piezoelectric response, whilst for 90◦ to the only dielectric response. The fact is important for ferroelectric material's use as energy harvesters and has been observed in many perovskite structure ferroelectrics. The symmetry of the crystal determines the favorable domain formation; hence, ferroelectric materials need to be non-centrosymmetric. The polar ferroelectric behavior exists below a particular temperature, called the critical temperature or curie point (Tc), above which nonpolar highly symmetric para-electric phase transition occurs, reflecting the optical mode softening at the BZ center [99]. BaTiO3 is cubic in its para-electric phase and tetragonal in its ferroelectric phase at Tc 120 ◦C. At 5 ◦C, a second-order phase transition from tetragonal to orthorhombic occurs for BTO [7],

the transition/curie temperature for KDP (~−150 ◦C) and Rochelle salt (~24 ◦C) for PVDF ~150 ◦C [100–102]. In PbTiO3 tetragonal ferroelectric phase occurs below 490 ◦C, assisted by cation displacement in <100>. LiNbO3 and LiTaO3 are two important uniaxial ferroelectric materials, comprising of high curie temperatures at 1200 ◦C and 620 ◦C, respectively, and containing only 180◦ domains [7]. Studies show that for PZT ferroelectric, the piezoelectric response is related to the volume snippet of 180◦ domain walls. Pyroelectric coefficients can be tuned at phase transitions ascribed with temperature-controlled domain wall motions, as observed in PZT films subjected to tensile strains [103–106]. Huan et al. reported the grain size effect of BTO and found that by reducing grain size, the width of the 90◦ domain diminished, leading to enhanced piezoelectric properties of BTO [107]. However, the materials with complexed domain wall structures show different properties from single domain wall ferroelectrics. Multiferroic BiFeO3 (BFO) is ferroelectric at Tc = 1103 K with a rhombohedral structure and possesses 71◦, 109◦, and 180◦ domain walls. Seidel et al. reported the observation of room-temperature electronic conductivity at 109◦ and 180◦ ferroelectric domain walls in insulating BeFiO3, proving domain walls as discrete functional entities with potential in nanoelectronic applications [108]. Ghara et al. demonstrated the formation of highly conductive domain walls in multiferroic GaV4S8, which can be annihilated magnetically by driving the system to a single domain state [109]. Werner et al. reported the stable, metal-like conductivity of charged domain walls in lithium niobate crystals, with orders 13 higher than the bulk, high stability for the temperatures ≤70 ◦C, and promoted advanced LN-semiconductor optoelectronic devices [110].

Lattice defects can also pin the motion of domain walls, as observed oxygen vacancies and dipole defects lower the polarization values in KBNNO pyro and piezo response. The defects can couple with polarization and beget anisotropic lattice deformations, complementing the curie point, as shown in Figure 1c. Hence, ferroelectric material properties can be tailored by controlled poling, defects, and engineering at a small scale, with nonferroelectric materials producing composite multilayer structures [68].

#### **3. Energy Harvesting Ferroelectric Materials for Nanogenerators (NGs)**

Ferroelectric materials can efficiently harvest energy, with variations of the internal dipole moments or potentials, induced by surrounding ambient energies. These available ambient energies are being utilized individually and by coupling with physical phenomena related to mechanical, thermal, and solar energy to generate electricity. In this section, we will discuss the basic energy conversion phenomenon related to ferroelectric materials and the corresponding energy harvesting nanogenerators (NGs).

#### *3.1. Piezoelectric Effect and Piezoelectric Nanogenerators*

The piezoelectric effect is interwined to the causation of electric charges in a material; when subjected to mechanical stress or strain forces, the center of cations and anions move apart in an asymmetric manner, the material becomes electrically polarized and builds piezoelectric potential. The polarity of the developed charge depends on the stress direction, i.e., either the stress is compressive or tensile. Whilst in converse piezoelectric effect, the applied electric field develops mechanical strain in the material, and field direction determines either the material expands or contracts. P. Cure and J. Curie were the first to observe the phenomenon of piezoelectricity (in 1880) in quartz and Rochelle salt [111]. The dielectric polarization (*P*) is defined by a linear relation for direct piezoelectric effect:

$$P\_i = d\_{i\bar{j}} \ X\_{\bar{j}} \tag{1}$$

where in *d* and *X* represent the piezoelectric coefficient or constant and the applied stress, respectively. The piezoelectric coefficient *d* evinces piezoelectric material performance and is anisotropic. The subscripts *i* and *j* indicate the directions of dielectric displacement and applied stress, respectively [112]. Ferroelectric materials (such as PZT, KNN, and PMN) with morphotropic (MPB) or polymorphic phase boundary (PPB) relate to the rotation of polarization and indicate the presence of multiple phases within the material, show

exceptional piezoelectric response, and piezoelectric coefficient [113,114] BTO; PVDF has also been studied widely in piezoelectric harvesters of mechanical energies, known as piezoelectric nanogenerators (PENG). The operational mechanism of ferroelectric PENG is based on the volume density model; that is, compressing the ferroelectric dipole density increases over the reduced material thickness and stretching material dipole density declines over increased material thickness [85,115]. Consequently, ferroelectric material polarization changes as dipole density vary and results in the generation of the piezoelectric signal. Figure 2a(I) portrays a poled piezoelectric generator (PENG) in a stress-free situation; polarized to value Ps, the charge will sit on the surface to establish charge balance. Subjecting the material to compressive stress, as in Figure 2a(II), the polarization level decreases, and the surface charge becomes free to flow generating electric current signal across the load R. Figure 2a(III) depicts that, when removing stress or applying tensile stress, the material polarization level increases, generating current in the opposite direction to keep charge balance. Typically, the current signal, generated by PENGs, is AC; therefore, the rectification of the output signal is needed. For energy harvesting by PENGs, the selection of load resistance R must lead to an optimum value of piezo potential and current for power generation, which is possible by impedance matching of R and energy material. Furthermore, the infinite value of R leads to an open-circuit device condition, which is suitable to develop sensitive voltage sensors, and the zero value of R leads to a short-circuiting device [116]. Perovskite ceramic ferroelectrics, e.g., BTO and KNN, have usually high piezoelectric coefficients; hence, ferroelectric ceramics-based PENGs show high-output power performances. Guo et al. estimated the piezoelectric coefficient (d33) of ~30 pC/V and dielectric constant of (ε) ~340 for 140 nm thick BTO film, deposited on Pt/TiOx/SiO2/Si, with the LiNbO3 buffer layer [117]. Huan et al. accounted for the grain size effects on the piezoelectric coefficient of BTO, they found that reducing the grain size to 1 μm gives a maximum d33 of 519 pC/V, the dielectric constant of (ε) ~6079, and electromechanical coupling factor (Kp) of 39.5% [107]. Du et al. derived the piezoelectric features of (K0.5Na0.5)NbO3 (KNN) with the perfect perovskite phase, orthorhombic symmetry, piezoelectric coefficient (d33) of 120 pC/N, electromechanical coupling factor (Kp) of 0.40, and curie temperature of 400 ◦C [118]. Matsubara et al. showed that, for KNN, there was an estimated ~180 pC/N piezoelectric coefficient (d33), with an electromechanical coupling factor (Kp) ~0.39 and curie temperature of 420 ◦C [119]. As the ceramic materials make it difficult to achieve flexibility, they should be deposited with smaller dimensions, i.e., as thin films or nanoparticles. Chen et al. studied lead-free Ba0.9Ca0.1Ti0.90Sn0.10O3-xLa2O3 for x = 0.03 mol%, and they got an optimized d33 = 496 pC/N and Kp = 41.7, with coexisting orthorhombic and tetragonal phase [120]. Ferroelectric polymer composites (e.g., ZnSnO3:PDMS) can easily be employed in PENGs and has the advantage of low-cost, largearea manufacturing and large mechanical durability, even under high-stress conditions. Ferroelectric polymers, such as PVDF and P(VDF-TrFE), with piezoelectric coefficients 18 pC/N and −21 pC/N with dielectric constants of 8.4 and 10 are promising for total flexible PENGs, they are even foldable, stretchable, and twistable [94,121].

**Figure 2.** Various effects in ferroelectrics. (**a**) Piezoelectric ferroelectric nanogenerators. Reprinted with permission from reference [116], Copyright 2020, IScience. (**b**) The pyroelectric effect in SbSeI nanowires *ϕ*1, *ϕ* 2, and *ϕ* <sup>3</sup> represent the degrees of dipole oscillations. Reprinted with permission from reference [122], Copyright 2019, Elsevier. (**c**) Triboelectric effect in contact-separation mode. Reprinted with permission from reference [116], Copyright 2020, Elsevier. (**d**) Ferroelectric photovoltaic effect in SbSI. Reprinted with permission from reference [29], Copyright 2019, MDPI.

#### *3.2. Pyroelectric Effect and Pyroelectric Nanogenerators*

The pyroelectric effect is associated with the variations in spontaneous polarization with temporal temperature changes and, hence, the generation of electric current. Thermal energy, generated by nature, mechanical frictions, and machines, can efficiently be altered into electric energy, owing to the pyroelectric effect. Pyroelectric coefficient p is defined as:

$$\mathbf{p} = \frac{\partial \mathbf{P}\_s}{\partial \mathbf{T}} \tag{2}$$

where p represents the efficiency of pyroelectric material, and pyroelectrics have high pyroelectric coefficients. Ps is the spontaneous polarization of pyroelectric, and T is the temperature [123]. Pyroelectric nanogenerators include three main layers: the metallic top layer, acting as the top electrode, is designed to collect heat efficiently; the middle layer is the pyroelectric material layer, facilitating heat conversion into electricity by variations in internal polarization; and the third metal layer is the bottom electrode [124,125]. The pyroelectric materials have a unique polar axis. Figure 2b shows SbSeI ferroelectric-based PyENG [122]; when the material temperature is fixed, the electric dipole oscillates randomly around aligning axes, *ϕ*1, reaching their equilibrium, as shown in Figure 2b(I). No current was observed since Ps remained the same. Thermal fluctuations produce polarization change, which heads to the separation of bound charges and their accretion at the electrodes. Increasing temperature creates net dipole moments, by enhanced dipole oscillation (*ϕ*<sup>2</sup> > *ϕ*1) and spontaneous polarization, decreases by increased thermal agitations. This leads to the depletion of bound charge, so free charge redistributes itself to compensate for the bound charge and, consequently, pyroelectric current flows (Figure 2b(II)). Cooling the sample reverts the current direction and increases spontaneous polarization as the dipole oscillation reduces (*ϕ*<sup>3</sup> < *ϕ*<sup>1</sup> < *ϕ*2) (Figure 2b(III)). The pyroelectric effect lasts only until temperature fluctuation remains. The output power of PyENGs can be significantly improved by enhancing the absorption of thermal energy with structural modifications, increasing the pyroelectric coefficient by material alteration, and strengthening spontaneous polarization carried out by thermal expansion [122]. Pyroelectric materials exhibit piezoelectricity, too; so, accordingly, the temperature variations encourage stress production and piezo polarization, i.e., secondary pyroelectric effect. Therefore, the bulk pyroelectric coefficient is the additive of both the primary and secondary coefficients [126]. However, in the hybrid systems, with polar inclusions embedded into polar matrixes, it is possible to control the individual component polarization, so the pyroelectric and piezoelectric responses can be compensated. For example, Ploss et al. reported that 27% volume of lead titanate (PT) embedded in PVDF-TrFE matrix, followed by parallel or antiparallel poling inclusions, can cause piezoelectric-compensated pyroelectric material or pyroelectric-compensated piezoelectric material to be constructed, respectively [127]. Similar trends were observed by Meirzadeh et al. for α-glycine crystals doped with minute amounts of different L-amino acids [128].

Inorganic ferroelectric PyNGs, e.g., PZT, show low-output power, due to a small pyroelectric constant, ~−80 nC/cm2K; polymer PVDF ferroelectric is found to be most favorable and shows a high coefficient, ~200 μC/cm2K [126,129]. For BTO, Song et al. found the pyroelectric coefficient to be dependent on temperature, from 16 nC/cm2K to 57 nC/cm2K for 299–310 K and 45.2 nC/cm2K for 310–324 K for light-induced pyroelectric effect [130].

#### *3.3. Triboelectric Effect and Triboelectric Nanogenerators*

The triboelectric effect is delineated as the exchange of charge among two different materials in contact with each other. Triboelectric nanogenerators TENGs work by coupling triboelectrification (static polarized charge production on the material surface) and electrostatic induction [131–133]. Electrostatic induction drives the harvesting of ambient mechanical energy to electrical energy by variations in potential, which are incited by impulsively agitated material layer separation (particularly by displacement). Periodic mechanical triggering of the triboelectric layer generates AC output. TENGs works by four different mechanisms, taking account of the change in the polarization direction of two triboelectric material surfaces/layers and the configuration of electrodes, which are insulated

carefully from each other. The four distinct modes are (i) vertical contact-separation (CS) mode, prompted by series of contacting and separating the triboelectric layers attached to the electrodes. The mode shows high-output peak current and figure of merit (FOM), which can further be enhanced by a stacked multilayer structure [134]. Figure 2c is a schematic of CS-mode TENG [116]. In Figure 2c(I), two layers are pressed by external force, in order to create contact and generation of charges at the interface of dielectric triboelectric layers A and B. Releasing outside force (Figure 2c(II)) leads to charge separation among the layers, establishing the potential difference that drives free charges in the electrode for potential balance setting current across the load R. When the charge is balanced, the current disappears, see Figure 2c(III). Again, pressing layers to make contact results in the flow of free charge, accumulated on the surface of the electrode, back into the circuit, resulting in current, but reversed, direction. The electric potential (V) of TENG is defined as:

$$N = \frac{\sigma\_{tr}}{\varepsilon\_0 \varepsilon\_r} d\tag{3}$$

In the above equation, *σtr* is the triboelectric charge density, *d* is the gap distance betwixt two triboelectric material layers, *ε*<sup>0</sup> is the permittivity of free space, and *ε<sup>r</sup>* is the relative permittivity of triboelectric material. Clearly, the potential produced is a function of layer gap and depends on layer movement, so, the current. (ii) Lateral-sliding (LS) mode [116] is triggered by sliding the tribo-layers, studies revealed that longer sliding distances lead to lower FOM, but charge densities are high. The output performance of LS mode TENGs can be improved by structure grating, which can also be altered to either rotational or cylindrical gratings [135]. (iii) In the single-electrode (SL) mode, the back electrode is removed, which offers a very small amount of charge transfer and voltage [136]. (iv) In the freestanding triboelectric layer (FL) mode, the triboelectric layers are free from electrodes and can locomote with or without contacting with static electrodes, resulting in high-output power and energy conversion efficiency records [137]. Introducing the ferroelectric material layer to TENG speculates the increased surface charge density, e.g., the barium titanate ferroelectric ceramic layer, on polytetrafluoroethylene (PTFE) TENG [138]. Liu et al. obtained a high piezoelectric and pyroelectric coefficient of 150 pC/N and 29.7 nC/cm2K for BTO polarized disc by wind-driven TENG at speeds of 14 m/s, which resulted in the high-output voltage of 1000 V in <10 ms [40]. The literature revealed the improved output performance of PVDF-TrFE, based TENG, due to well-oriented polarization. TENGs offer broad material availability and high efficiency, yet low operation frequency [139].

#### *3.4. Ferroelectric Photovoltaic Effect and Photovoltaic Cells*

The ferroelectric photovoltaic effect refers to the photovoltaic response, i.e., photovoltage and/or photocurrent, observed in ferroelectric materials when exposed to light. The photovoltaic phenomenon in ferroelectrics is significantly dissimilar to the conventional photovoltaic effect (PVE) observed in semiconductor pn-junction. The built-in electric field, across the depletion region of the pn-junction, separates the generated photo-carriers, thereby defining that the open-circuit voltage for particular photovoltaic effects in semiconductors has a bandgap constraint. However, in the ferroelectric photovoltaic effect, the built-in field is because of remnant polarization (PR) and goes throughout the ferroelectric material, and there is no bandgap restriction for charge separation. Therefore, the photopotential generated in ferroelectrics is much greater to abnormal value than their band gaps. This phenomenon is the so-called abnormal photovoltaic effect or bulk photovoltaic effect (BPVE) and had been known in barium titanate and lithium niobate for decades. However, its clear explanation is still a mystery and the photovoltage integrated can be visualized as the photovoltage of domains [140,141]. The bulk ferroelectric photovoltaic effect has been observed in bulk ferroelectric materials, such as BFO (BiFeO3), with large photovoltage stipulating the internal bias field presence. The thin films of BTO (BaTiO3) evidenced that the direction of photocurrent and photovoltage can be rolled over by reverting polarization, proving the polarization as a dominant feature in the bulk photovoltaic effect [140,142,143]. Figure 2d(I) represents the ferroelectric photovoltaic effect mechanism in poled SbSI nanowires with Pt metal contacts [29]. The SbSI nanowires proposed the bulk photovoltaic ferroelectric (BPVE) mechanism, shining light with energy greater than the bandgap of SbSI ferroelectric, resulting in the absorption of light photons, and promoting the photo-generation of electron and holes as excess carriers. Poling the SbSI ferroelectric caused band bending and, thus, the internal electric field, due to the spontaneous polarization of nanowires (Figure 2d(II)). The internal field determines spatial separation charge carriers in ferroelectric photovoltaic devices and these excess carriers contribute to the photovoltaic output current [29].

#### *3.5. Coupled Effects NGs*

Recently, hybridized nanogenerators NGs have also been reported by integrating individual piezoelectric, pyroelectric, triboelectric, and PVC harvesters, in series or parallel, for the realization of self-powering of devices with higher consumption powers by utilizing multi-energies. The hybrid stacked structures enhance the overall output power but restrict the interaction among various effects in the device, as different electrodes export electrical energy. Furthermore, it is discouraging for reducing device size and heavy productions [6,46,56,144]. Therefore, it is greatly aspired to deploy multifunctional energy harvesting materials for establishing the coupled nanogenerators, based on a single structure. Ferroelectric materials are multifunctional, thereby defining that the piezoelectric, pyroelectric, and photoelectric properties concur, also a common feature among piezoelectricity, pyroelectricity, triboelectricity, and the ferroelectric photovoltaic effect is that they all work by having an influence on the polarization of ferroelectric materials. This similarity indicates the potential to couple these effects, in order to magnify the charge quantity and electric power from various energy resources simultaneously and promote their use in multi-effect coupled NGs, whenever all the mechanical, thermal, and solar energies are either available individually or simultaneously. The multi-effect coupled NGs are composed of multilayers, with only one electrode pair that is highly versatile, reliable, simply smaller, and low cost. The multi-effects coexist and interact to produce electrical output; their performance hangs on the strength and type of external stimuli. Device design determines the performance of multi-effect coupled nanogenerators. By now, a sandwich layer layout with a function material embedded in two electrodes, planer structure consisting of two parts, functional material layer, coplanar electrodes realized by laser etching or masking, and heterojunction design (which commonly intervene in PVCs) have been contrived [6,47,145].

#### **4. Device Structure and Performances of Ferroelectric NGs**

Exceptional output device performances are always in search of better design and tailoring material properties.

#### *4.1. Device Design and Output Power Optimization in Piezoelectric PENGs*

The piezoelectric ceramic perovskites show high piezoelectric constants and electromechanical response because of polarization rotations. The ferroelectric, ceramic-based PENGs with active layers of lead zirconate titanate (PZT) [146,147], barium titanate (BTO) mboxciteB148-nanoenergyadv-1435434,B149-nanoenergyadv-1435434,B150-nanoenergyadv-1435434, lead magnesium niobate titanate (PMN-PT) [151–154], sodium bismuth titanate (NBT) [155–159], and sodium potassium niobate (KNN) [160,161] have been reported with high-output performances. Ferroelectric thin film PENGs have also been reported on flexible plastic substrates with metal electrodes and by transferring techniques [146,154]. However, ceramic PENGs have limitations of large-area devices fabrication; additionally, ceramics are frangible in nature and cannot withstand very strong strain-producing forces. To get out of these problems, reports on a polymer matrix supported by ferroelectric powder composites have been found, which fortified a large area of manufacturing

devices, reduced cost, sustain high-stress forces, and enhanced mechanical resilience, but are not admissible in low magnitude and frequency input forces [146,153]. This time, highly durable and elastic polymer ferroelectric-based PENGs were proposed, with high mechanical permanence and optimistic output performances [62,162–164]. Semimetallic/conductive layers or electrodes are deposited over the ferroelectric active layers to get charge separation without poling; graphene is a potential conductive layer, due to high thermal and electrical properties over the large surface area. Additionally, surface treatments or doping can make graphene either p-type or n-type, which can further align the polarized charges efficiently. Polyvinylidene fluoride (PVDF) and its copolymers are rosy for polymeric ferroelectric-based PENGs. PVDF exhibit a smaller dielectric constant but have extremely high parameters of voltage (g33 = 28.26) and piezoelectric constants (d33 = 16.2 pC/N) [63,163]. The d33 value can be further enhanced by introducing ceramic fillers with relatively larger permittivity, e.g., PZT, PLZT (PZT:La), PTO, BTO, and PMN-PT into the PVDF matrix. The quantity of ceramic powder fillers is most important, and it controls the harvesting properties of PENGs; the positive and negative piezocoefficients of ceramic and polymeric ferroelectrics may rule out each other's effects, thereby reducing the output performances, so tedious measurements must be taken into account [165,166]. Yaqoob et al. [167] fabricated a novel tri-layer PVDF-BTO/n-Gr/PVDF-BTO piezoelectric NG, with 60 μm thickness. The PVDF/BTO layers stacked on both sides of the N-graphene layer deposited over Si substrate. Figure 3a demonstrates the making process of tri-layer PENG. The working mechanism of tri-layer PENG is illustrated in Figure 3b, escribing that between n-Gr and PVDF-BTO, the layer charge barriers are created, due to the accumulation of opposite charges near the interfaces. The lower n-Gr/PVDF-BTO layer acts as a blocker and restricts recombination and sustains the charge on the upper layer, and dipole alignment happens on both sides. Extreme bending of PENG shows high flexibility. FESEM studies confirmed that BTO nanoparticles were dispersed uniformly in the PVDF matrix and graphene over PVD-BTO film in the *β*-tetragonal phase. The high peak-to-peak opencircuit voltage of 1.5 V and 10 V was observed in un-poled and poled PENG under the force of 2 N by human finger tapping. Figure 3c shows the maximum short-circuit current output of 2.5 μA and instantaneous powers of tri-layer PENG was found saturated at 10 MΩ load resistance, with a maximum value of 5.8 μW at 1 MΩ. PENG was found to be highly stable, even under continuous pressing and releasing at 2 N for 1000 s, and is attributed with N-graphene, suggesting graphene-enhanced PENG performance and realizing that the tri-layer structure is efficient for mechanical energy harvesting and perspective to use as a pressure sensor and power source of self-powered devices. Hanani et al. [33] investigated a bio-flexible PENG with potential in self-powered biomedical devices, as well as lead-free, bio-ceramic Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT), functionalized with polydopamine that is embedded with polylactic acid. BCZT@PDA/PLA is sandwiched between two Cu-foils and encapsulated with Kapton tape to keep it water- and dust-proof and secure, as of repeated mechanical excitations, see Figure 3d. PLA has a high piezoelectric coefficient d13 of 10 pC/N, without poling or being self-poled. The BCZT nanoparticle's surface was functionalized via core-shell saturation (Figure 3e), with a PDA layer to promote interaction among the two. Under gentle finger tapping, the mechanical performances, open-circuit voltage, and short-circuit current were quantified to be 14.40 V and 0.55 μA, respectively; however, the increased imparting pressures caused Voc to rise. Young modulus of 2.1 GPa endures mechanical robustness, and the high-output power density of 7.54 mW/cm3 is capable of driving the 1 μF capacitor, with an energy storage of 3.92 μJ, only in 115 s, under gentle finger tapping and longer stability at 23 Hz for 14,000 cycles by sewing machine, which showed no performance degradation for 4800 cycles, even after one year.

**Figure 3.** Device structure and performances of piezoelectric ferroelectric nanogenerators (PENGs). (**a**) Fabrication steps for tri-layer n-Gr/BTO-PVDF on BTO-PVDF/Si PENG. (**b**) The working mechanism of tri-layer PENG, along with an optical image of the extreme bending situation. (**c**) Short-circuit current and instantaneous power output of tri-layer PENG. (**a**–**c**) Reprinted with permission from reference [167], Copyrights 2017, Elsevier. (**d**) Schematic of Bio-flexible BCZT-based nanogenerator. (**e**) Illustration of the core-shell structuration of BCZT nanoparticles, by PDA, along with microimages of BCZT and BCZT@PDA. (**d**,**e**) Reprinted with permission from reference [33], Copyrights 2021, Elsevier.

Xiaohu et al. [168] exposited BiFeO3-PDMS composite (~100 nm)-based flexible PENG on polyethylene terephthalate (PET) substrate and Al-foil electrodes (PET/Al/BFO-PDMS/Al/PET), see Figure 4a. BiFeO3 nanoparticles (NPs) had a rhombohedral structure, with 180◦ domain switching of permanent polarization. The PENGs showed the maximum output, open-circuit voltage of 3 V for 40% BFO NPs in PVDS matrix and a short-circuit current of 250 nA, under hand pressing, can be seen in Figure 4b. Simulated piezo potential distribution is indicated on the right side of the voltage-time graph, as a response to the compressive stress of 10 KPa. The fabricated BFO PENG can light commercial LEDs. Lee et al. [169] reported a novel fiber-type piezoelectric NG, based on PTO (PbTiO3) nanotubes, grown radially on Ti metal fiber; the PTO/Ti core-shell fiber was embedded

in the PDMS cylindrical templates, with inter-wire spacing formed and thin, insulating PDMS. Poling (via convex and concave bending of the PENG) the output voltage and current of 623 mV and 1.0 nA/cm2 was estimated. When a second core-shell was added in the back of the first (Figure 4c), poled in radial direction because of radial alignment of PTO domains, the output voltage estimated in- and out-of-plane bending were ±320 and ±390 mV (for 0◦ and 180◦) and ±370 and ±450 mV (for 90◦ and 270◦), respectively. Under natural wind, blowing the output voltage of 69.74 ± 10.97 mV with 0–360◦, bending was observed. The idea captivates applications in portable and flexible devices. Yan et al. [170] reported a PDMS-based, flexible PENG, with BTO nanofibers in three different alignment modes (random, horizontal, and vertical) and tetragonal structure, as shown in Figure 4d, poled for 12 h at 120 ◦C and an electric field of 5 kV/mm. Random (BTNF-R), horizontal (BTNF-H), and vertical (BTNF-V) alignment of BTO nanofiber PENGs showed output voltages of 0.56 V, 1.48 V, and 2.67 V and currents 57.78 nA, 103.33 nA, and 261, 40 nA, respectively, under a poled situation and periodic mechanical compressive pressure of 0.002 MPa, with a harvesting power of ~0.1841 μW in vertical mode (BTNF-V). This pointed out that the vertically lined BTO nanofibers were finest in dielectric and piezoelectric response and used to light commercial LED and charge a 1 μF capacitor to 0.46 V in just 34 s (shown in Figure 4e), stating wonderful alignment for use as harvester units or storage entities for wireless networks. Jung et al. [171] presented a PENG based on NaNbO3 nanowires, composited with the PDMS matrix and Au/Cr coated films (PS/Au-Cr-coated kapton/NaNbO3-PDMS/Au/Cr-coated Kapton; PS is the supporting polyester film, the strain neutral line lies near the vicinity of PS film). Estimated output voltage Voc of 3.2 V, short-circuit current Isc of 72 nA, and power density of 0.6 mW/cm3, under the compressive strain produced by vibrations of 0.33 Hz. Production of NaNbO3, at relatively low temperature and domain control (by poling) make them perfect nanogenerator members. Park et al. [150] devised MIM structure-based Au/BTO/Pt PENG with a ribbon structure over a plastic substrate, using PDMS stamp, the Kapton film, and poled for 15 h at 140 ◦C, with a field application of 100 kV/cm. Poling caused d33 to increase from 40 pm/V to 105 pm/V; periodic bending (by finger) resulted in an output voltage and current values of ~1.0 V and ~26 nA and the power density of 7 mW/cm3. In comparison to inorganic ferroelectrics, very few organic ferroelectrics exist. For example, single-component polar organic molecules, such as CDA, DNP, CT complexes, polymers (such as PVDF and nylon-11), and organic-inorganic composites (such as HdabecoReO4, TGS, and TSCC), are excellent for applications in piezoelectric and triboelectric nanogenerators. Isakov et al. [172] reported a piezoelectric nanogenerator, based on organic ferroelectric nanofibers of hybrid 1,4 diazabicyclo[2.2.2]octance perrhenate (dabacoHReO4), with an output voltage of 100 mV under moderate strain values. Ferroelectric diphenylalanine peptides are also becoming promising for piezoelectric energy harvesting and are biocompatible. Zelenovskii et al. [173] reported the fabrication of the 2D layered biomolecular crystals of diphenylalanine, with a piezoelectric constant d33 of 20 pm/V and voltage coefficient g33 of 0.75 Vm/N, which is suitable for microfluidic device applications. Nguyen et al. [174] estimated strong piezoelectricity in diphenylalanine (FF) peptide nanotubes, with a piezoelectric constant d33 of 17.9 pm/V, open-circuit output voltage of 1.4 V, and power density of 3.3 nW/cm2. In a report by Lee et al. [175], peptide-based piezoelectric energy harvesters, at a force of 42 N, can produce an output voltage of 2.8 V, current of 27.4 nA, and power of 8.2 nW, which is sufficient to power multiple liquid-crystal display panels. Table 1 gives a quick insight into various piezoelectric nanogenerators (PENG), along with their output characteristics and working conditions, made with various structural variations and morphologies, indicating the high-output characteristics.

**Figure 4.** Device structure and performances of piezoelectric ferroelectric nanogenerators (PENGs). (**a**) Lead-free flexible BFO-PDMS PENG. (**b**) Time-dependent voltage and COSMOL simulated piezo potential for 0–40% BFO nanoparticle concentration PENGs. (**a**,**b**) Reprinted with permission from reference [168], Copyrights 2016, ACS Publications. (**c**) PTO/Ti fiber PENG in arbitrary bending direction with PDMS polymer matrix. The bottom line shows the double PTO fiber PENGs, along with two possible bending directions (in- and out-plane) and corresponding output voltages. Reprinted with permission from reference [169], Copyrights 2017, John Wiley and Sons. (**d**) Schematic of BTO-based PENG nanogenerator in three different alignment modes. (**e**) Rectified voltage-current signals and voltage-time graphs for different capacitors. (**d**,**e**) Reprinted with permission from reference [170], Copyrights 2016, ACS Publications.


**Table 1.** Piezoelectric nanogenerators, based on ferroelectric materials, for energy harvesting.

#### *4.2. Device Design and Output Power Optimization in Pyroelectric PyENGs*

Low-grade heat wastes coming from the industry of which maximum portion is hard to recuperate, a way to rescue these heat wastes is harvesting them into electric energy by temperature fluctuations either by Seebeck effect or by pyroelectricity. Thermoelectric nanogenerators require a steady gradient temperature that is infrequent in nature, otherwise, by pyroelectric nanogenerators, PyENG, lined with temporal temperature sways. The material and structure designs are crucial for the improvement in the output of PyENGs because the pyroelectric coefficients have small values. Improving these various approaches has already been reported, including the control over crystallinity for im-proving pyroelectric coefficient [185], introducing strain coupling effects [186], or polymer modifications [187]. Kim et al. [185] used high dipole moment solvents for improving the crystallinity and pyroelectric coefficient of P(VDF-TrFE); the solvents they used were THF (tetahydrofuran), MEK (methylethylketone), DFM (dimethylformamide), and DMSO (dimethylsulfoxide). High crystallinity and improved pyroelectric constant were observed and figured to be associated with the long-chain lengths of P(VDF-TrFE); they exhibited high molecular weight and 1.4 times higher output voltage and current. Hence, pyroelectric coefficient enhancement, by utilizing high dipole moment solvents, is a promising way to improve output PENG performance. Ghosh et al. [187] presented improved pyroelectricity of PVDF PyENG by introducing the Er+3 via formation of self-polarized β-phase and porous flower-like structures, as well as enhanced thermal sensitivity, triggered by IR irradiations. Under a temperature change rate of 1 K/s, the estimated output current was 13 nA. The 4.7 μF capacitor was easily charged by Er-PVDF PyENG, which operates low-power electronic devices with great ease. Mistewicz et al. [122] demonstrated low-temperature heat waste by SbSeI-based PyENG, with a pyroelectric coefficient of 44(5) nCK−1/cm2 at 327 K. The output voltage, current, and power of 12 mV, 11 nA, and 0.59(4) μW/m<sup>2</sup> were generated by PyENG for temperature fluctuations of 324 K to 334 K. Yang et al. [188] reported KNbO3 single-crystal nanowire (~150 nm), along a [011] direction composite

with PDMS (KNbO3-PDMS in 3:7 and pyroelectric constant 0.8 nC/cm2K) PyENG, embedded with Ag and ITO electrodes, Figure 5a. Positive poling caused the dipoles in the KNbO3 nanowires to align from top to bottom and reverse, i.e., bottom to top, for negative poling. Increased temperature from RT resulted in output voltage and current peaks to be positive, which go opposite for lowing the temperature (see Figure 5b). At a temperature change of about 40 K, the observed output voltage and current peaks were 10 mV and 120 pA, and the output current is directly related to temperature variations (dT/dt > 0). They also deployed the generator for harvesting the solar energy, as heat is induced by the sun, showing potential for self-powered widgets. Lee et al. [186] designed an innovatively stretchable pyroelectric nanogenerator (SPNG), by coupling thermally induced piezoelectric and pyroelectric effects, using P(VDF-TrFE) and PDMS; the piezoelectric effect was generated by thermal energy, only by accomplishing the different thermal expansions (122 × <sup>10</sup><sup>−</sup>6/K for P(VDF-TrFE) and 310 × <sup>10</sup><sup>−</sup>6/K for PDMS) and generating a compressive strain in P(VDF-TrFE) by PDMS. Figure 5c shows a schematic of stretchable PyENG, Ag/AgNWs/P(VDF-TrFE)/Au/PDMS, along with an optical micrograph of 500 μm × 700 μm P(VDF-TrFE) in β-phase and 200 μm × 200 μm PDMS. The temperature sensitivity of stretched nanogenerator was analyzed by comparing it with a normal P(VDF-TrFE)/Ni/SiO2/Si nanogenerator, and negative peak signals were reported for temperature elevation from RT (right part of Figure 5c). The sensitive, stretchable pyroelectric NG, under extremely low temperature variations (ΔT) of 0.64 K, to higher variations 18.5 K, showcased highly stable output voltage performance of 8 mV to 2.48 V under extreme stretch circumstances over a normal nanogenerator 2 mV to 0.54 V. Almost 15% stretching, caused only the PDMS layer to expand by 30 μm, and temperature variation of 22 K from RT caused an output voltage and current density of 2.5 V and 570 nA/cm2, respectively. Yang et al. [37] reported a PZT film-based pyroelectric nanogenerator (PyENG) with Ni, a top and bottom electrode connected, and Cu-wires (fixed with Ag paste) for electrical measurements and encased with Kapton tape. For temperature changes of 45 K, at the rate of 0.2 K/s, the output open-circuit voltage, short-circuit current, and short-circuit current density of PENG reached 22 V, 430 nA, and 171 nA/cm2, respectively; the corresponding maximum power density of 0.215 mW/cm3 was estimated, and a single electrical pulse can power a liquid crystal display (LCD) for more than the 60 s. The proposed PyENG showed potential applications as wireless sensors, drivable by a rechargeable Li-ion battery with a voltage of 2.8 V, see Figure 6a. The pyropotential calculation of PZT film showed distributions from −200 V to 200 V for temperature variations from 295 K to 340 K. With the measured electric potential of 22 V, much smaller than the theoretical value, they showed increased output current, as the surface area of PyENG was doubled than original. Xue et al. [27] proposed the self-powered wearable pyroelectric nanogenerator, based on Al-coated PVDF film, exercised in an N95 respirator for the sustainable harvesting and monitoring of human breathing energy, see Figure 6b. They observed that the open-circuit output voltage Voc of ~42 V is generated by the temperature oscillations rate, 13 ◦C/s, encouraged by humans breathing exhausted gas, at an ambient temperature of 5 ◦C with short-circuit current of 2.5 μA and the maximum power output of 8.31 μW. The power produced can drive a liquid crystal display (LCD) and/or LED directly. They employ the fabricated PyENG as self-power sensors for human health monitoring and temperature. Gao et al. [189] proposed a P(VDF-TrFE)-based pyroelectric nanogenerator, driven by temperature fluctuations, induced up to 23 ◦C/s by water vapors during the process of evaporation and condensation. As can be seen in Figure 6c, the oscillation airflow environment was created by a fan with an air speed of 1–2 m/s. The PyENG was capable of generating a high-output voltage and current density of 145 V and 0.12 μA, with a power density of 1.47 mW/cm<sup>3</sup> and 4.12 μW/cm<sup>3</sup> by volume and area. The water-operated PyENG could run a low-power digital watch, blue LED light, and charge a capacitor of 2.2 μF, giving a new map to harvest energy from the wastewater of industries. Yang et al. [36] prepared a self-powered temperature sensor, based on a pyroelectric nanogenerator, using a single micro/nanowire of PZT on a thin glass substrate, packaged by PDMS. Employing this device to touch the heat

source the voltage was found to vary linearly with temperature increments, as well as the response and reset time of 0.9 s and 3 s. The minimum constraint on detecting temperature variations at RT was 4 K, and the device was as sensitive to detecting the temperature of the fingertip; large temperature fluctuations could light up the LCD. Some other pyroelectric nanogenerators, PyENG, have been listed in Table 2.

**Figure 5.** Device structure and performances of pyroelectric ferroelectric nanogenerators (PyENGs). (**a**) Schematic of KNbO3-PDMS composite pyroelectric nanogenerator, with Ag ad ITO electrodes; the bottom line is the SEM image of KNbO3 in the bent state. (**b**) Cyclic changes in temperature, with corresponding differential curves of open-circuit voltage Voc and short-circuit current Isc. (**a**,**b**) Reprinted with permission from reference [188], Copyrights 2012, John Wiley and Sons. (**c**) Schematic of highly stretchable P(VDF-TrFE) pyroelectric nanogenerator (SPNG); the bottom line left image indicates the top view of SPNG, along with photo image. The right hand image indicates the measured output characteristics for both stretchable and normal pyroelectric nanogenerators. Reprinted with permission from reference [186], Copyrights 2015, John Wiley and Sons.

**Figure 6.** Device structure and performances of pyroelectric ferroelectric nanogenerators (PyENGs). (**a**) Photograph of wireless temperature sensor, based on PZT PyENG, along with temperature signals from a wireless sensor in off- and on-state, temperature signal data, as a function of time, was recorded for half an hour (for every one minute) and enhanced the output current profile of as-deposited PyENG for doubled surface area. Reprinted with reference from [37], Copyrights 2012, ACS Publishers. (**b**) Physical schematic image of the wearable PyENG with PVDF film, driven by human respiration, along with states of (I) inspiration and (II) expiration. Reprinted with permission from reference [27], Copyrights 2017, Elsevier. (**c**) Powering digital watch from P(VDF-TrFE)-based PyENG, powered by hot water vapors. Reprinted with permission from reference [189], Copyrights 2016, Elsevier.


**Table 2.** Pyroelectric nanogenerators, based on various ferroelectric materials.

#### *4.3. Device Design and Output Power Optimization in Triboelectric Nanogenerators TENG*

The electrical outputs generated from TENG are proportional to the intensity of the mechanical input strengths. Triboelectric nanogenerators can be supported by ferroelectrics because of their switchable intrinsic polarization. It increases the triboelectric charging behavior to enhance their output characteristics by coupling residual dielectric and surface polarization. Organic ferroelectrics, such as P(VDF-TrFE), show controllable ferroelectric polarization, leading to elevated surface charge density and improved TENG performance. High dielectric, inorganic ferroelectric ceramics and polymer composites of TENG need precise control of the nanoparticles' dispersion into the dispersive media. The precision control avoids the agglomeration that leads to the degradation of device performances. Park et al. [196] reported TENG with multilayer nanocomposites, comprised of alternate layers of organic and inorganic ferroelectric materials P(VDF-TrFE) and BTO nanoparticles with Al and Cu electrodes (Figure 7a). The organic soft layer P(VDF-TrFE) significantly transfers the vertically applied stress to the BTO inorganic layer. It exhibits a dielectric constant of 17.06 at 10 kHz in a multilayer structure, which yields superior interfacial polarization. The multilayer structure facilitates less leakage current, instigated by the P(VDF-TrFE) barrier between BTO layers. Poling at 30 MV/m further induced the surface potential of −2.85 V. As a result, more increased output characteristics of current, voltage, and dielectric constants, compared to only P(VDF-TrFE) and a single layer of BTO dispersed in P(VDF-TrFE) (Figure 7b,c). Multilayered TENG had a pressure sensitivity of 0.94 V/kPa with an output power density of 29.4 μW/cm2 and an output current density of 1.77 μAcm−<sup>2</sup> and voltage of 44.5 V, with active application in healthcare monitoring devices. Fang et al. [197] reported TENG with self-assembled nanospheres of polystyrene with PVDF porous film. Its unique structure showed an outstanding high-output voltage of 220 V being able to directly pole single crystal of 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (PMN-PT). They realized the application of TENG in ferroelectric FET, with PMN-PT as a gate material, posting the potential of TENG in a self-powered memory system. Ferroelectric ceramics, polymerized with non-ferroelectric materials, have a great impact on the output performance enhancement of TENGs. Sahu et al. [64] demonstrated the increase in output performance of TENG by introducing a ferroelectric material layer of (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3x BiHoO3 nanoparticles (BCZTBH for x = 0.02 to 0.1) into non-ferroelectric PDMS negative triboelectric layer. They correlated the piezoelectric properties of BCZTBH and integrated a multistacked piezo/tribo hybrid nanogenerator. They also concluded that mechanical excitation gives a higher output voltage of 300 V, a current of 6.6 μA, and a power density of 157 mW/m2. Further, they integrated the multi-stacked device to charge a capacitor,

in order to monitor the biomedical energy released from the body in various yoga poses. The module, as IR receivers and transmitters, were installed into a self-powered, wireless IR sensor communication system for high reliability and efficiency checks. The literature revealed that the annealing of poly(vinylidene fluoride-co-trifluroethylene) leads to crystal orientation instability and improper polarization axis match with the substrate. It prohibits its merits and obstructs its applications to TENGs. So, crystal orientation tailoring is in need. To treat the problem, researchers have worked to introduce a graphene layer [198], employed self-assembled monolayer-modified Au-substrates [198,199], and organo-silicate lamella [200], lattice matching with molecularly ordered PTFE substrates via epitaxy [201]. All these epitaxial fabrication and graphene layer insertions yielded fabulous results by controlling the orientation of the crystal polarization axis by recrystallization after melting. However, during TENG operation, insurance of adequate charge density on the surface is a big problem that leads to inferior performance of TENG. To get out of this problem, the right angle alignment of the b-axis of PVDF-TrFE with the substrate is proposed. Eom et al. [139] reported on the TENG that was based on epitaxially grown PVDF-TrFE over chitin film (Figure 7d), the crystallographic overlap between two enabled tailoring the PVDF-TrFE crystal orientation with polarization axis (b-axis) orthogonal to chitin (c-axis) substrate. The observed remanent polarization (~4.2 μC/cm2) was quite high, corresponding to a sweep voltage of ±80 V and coercive field ~60 MV/m higher than its theoretical value of 50 Mv/m, attributed with the series connection of PVDF-TrFE and chitin film. Chitin film showed severe distortions in humid circumstances, which can cause the short-circuiting of TENG. To avoid penetration of water molecules into chitin they deposited PVDF-TrFE film on two sides of chitin substrate by spin coating. Figure 7e depicts PVDF-TrFE/chitin hybrid layer TENG in vertical contact separation mode with an Al electrode. The 200 ◦C annealed hybrid film structure showed 11.7 and 4.9 times higher output current when compared with freestanding chitin film and non-epitaxial PVDF-TrFE/chitin hybrid film with a maximum power density of ~418 nW/cm<sup>2</sup> at 100 MΩ (Figure 7f). They fabricated triboelectric sensors of epitaxial PVDF-TrFE/chitin films. The allowed monitoring of subtle pressures suggested that tailoring the crystal orientation of PVDF-TrFE is encouraging for developing high-performance TENGs.

Since ferroelectrics have large remanent polarization. They can trap the spatial charge on electrodes for longer times and limit the TENG application in energy harvesting and sensing. To avoid this, exploitation of PLZT, a flexible anti-ferroelectric ceramic with large polarization differences (ΔP=Pr − Pmax), was carried out by Zhang et al. [202]. They proposed that Pb0.94La0.04Zr0.98Ti0.02O3, with zero remanent polarization (Pr) and large maximum polarization (Pmax), can enable all the dielectric polarization-induced charges, when used as an inner layer in TENG. A conducting fabric was attached with a natural rubber mat and PVDF/PZLT on LNO/Ni, as electrodes behave as a tribo-positive and -negative layer, as shown in Figure 8a. PLZT–TENG showed a record high <sup>Δ</sup>P = 1.065 × <sup>10</sup><sup>5</sup> nC/cm2 at 1300 kV/cm, much higher than PZT-TENG (PbZr0.52Ti0.48O3), shown in Figure 8b,c. PLZT–TENG delivered a high-output voltage of 456 V, with a high current density of 11.6 μA/cm2 and charge transfer density of 13.5 nC/cm2 at the impulse force of 5 N. Their study revealed that PLZT-based TENG exhibits excellent mechanical and polarization stability and can serve as self-powered sensors for joint motion detection, along with biomedical energy harvesting. Their study has paved a path to upgraded and optimized TENGs. Singh et al. [203] determined improved triboelectrification, by incorporating ZnO nanorods to PVDF polymer and PTFE and fastening to a moveable wooden frame with double-sided foam tape. They reported that the fabricated ZnO/PDMS TENG showed 21% and 60% increased voltage (~119 V) and current (1.6 mA), as compared to PVDF/PTMS TENG, with output voltage and current 98 V and 1mA, as well as power output 10.6 mW/cm<sup>2</sup> and 6.4 mW/cm<sup>2</sup> at 150 MΩ. They proposed that the increase in triboelectric energy, observed by them in ferroelectric polymer TENGs, is because of *β*-phase of PVDF enhancement. The surface roughness of films, hydrophobicity, and decreased work function is due to ZnO nanorod incorporations.

**Figure 7.** Device structure and performances of triboelectric ferroelectric nanogenerators (TENGs). (**a**) Schematic illustration of PVDF-TrFE/BTO composite TENG with single PVDF-TrFE, single PVDF-TrFE/BTO, and multilayer PVDF-TrFE/BTO layers. (**b**) Plots for output current density, voltage, and dielectric constant of three TENGs at vertical pressure of 98 kPa and 2 Hz. (**c**) Simulated potential in single and multilayered PVD F-TrFE/BTO composites. (**a**–**c**) Reprinted with permission from reference [196], Copyrights 2020, ACS Publisher. (**d**,**e**) Schematics of chitin-based PVDF-TrFE TENG. (**f**) Current and power plots of chitin /PVDF-TrFE hybrid film TENG. (**d**–**f**) Reprinted with permission from reference [139], Copyrights 2020, ACS Publications.

**Figure 8.** Device structure and performances of triboelectric ferroelectric nanogenerators (TENGs). (**a**,**b**) Schematic illustration of TENG with PLZT inner dielectric layer on flexible electrodes. (**c**) Maximum polarization, remanent polarization, and polarization difference for PLZT TENG and PZT TENG. (**a**–**c**) Reprinted with permission from reference [202], Copyrights 2021, Elsevier. (**d**) Structure of MoS2, composited with PVDF-TrFE and nylon-11; right is an optical image of single PVDF-TrFE; PVDF-TrFE composited with MoS2 and with nylon-11 and nylon-11. MoS2, bottom line. (**e**); Voltage, current, and power conversion dependence, as for various load resistances and LED lighting. (**d**,**e**) Reprinted with permission from reference [204], Copyrights 2019, ACS Publications.

The continuous reiteration of TENG contact and separation is inevitable and leads to the life-shortening of the device. To overcome this wearing problem in TENGs, serval attempts had already been made, inclusive of which are the use of shape memory polymer as a contact material to heal damaged surfaces by heat application, proposed by Lee et al. [205]. The atomic layer deposition (ALD) technique for the in-filtering of inorganic materials from polymers was reported by Yu et al. [206]. Application of permanent magnet isolating outside vibrations was introduced by Huang et al. [207]. All the ways are innovative, but a much more facile way to avoid wearing issues, compatible with TENGs, is in need. Kim et al. [204] reported Nylon-11- and PVDF-TrFE-based TENGs, composited with bulk MoS2, as shown in Figure 8d. Both composite ferroelectric layers, along with increased

polarization, led to an increased surface charge density and produced high-output voltage, current, and power density of 145 V, 350 μA/cm2, and 50 nW/cm2. The highest power density was reported at 10 MΩ, and the repeated output voltage measures for 12,000 cycles at 6.5 Hz operating frequency gave same voltage results. Additionally, can light LEDs without charge storage (see Figure 8e). Kim et al. [208] have reported a much simpler way to enhance TENG durability, by employing the concept of reduced contact number cycles in ferroelectric, PVD-based TENG, with ITO as the electrode and PDMS as an elastomer. They showed that the up-sided polarization (58 pm/V) of PVDF led to extremely fast charge accumulation and larger TE-outputs, when compared with a downside (36 pm/V) and non-polarized (1.1 pm/V) PVDF under contact state. Up-side polarized TENG showed as Li-ion battery chargeable, even by 40% reduced contact cycles. Figure 9a,c depicts convention TENG vs. PVDF ferroelectric TENG; SEM and power curves are depicted in Figure 9b,d. Continuous contact caused surface deterioration at a pushing cycle of 1.7 × 106 for vibration of 1 Hz. The power is reduced to 0.18 <sup>μ</sup>W from 85 <sup>μ</sup>W at a load resistance of 10 MΩ and mechanical force of 35 N for conventional TENG. In PVDF-based TENG, random contacts were performed, which significantly lowered surface wear and mitigated power after longer usage. The surface of PVDF remained clean and flat, even after the pushing cycle of 1.7 × 106, with the power loss from 1.23 mW to 0.87 mW, only a bit smaller. The results lasted for many load resistances and forces, even in non-contact mode (Figure 9e), realizing that stable power can be generated from random contacts, with output peak-to-peak values of voltage, current, and charge (410 V, 25 μA, and 60 nC, respectively). The deposited TENG show potential in wireless temperature sensor from water waves. Park et al. [209] developed the triboelectric nanogenerator, which relied on a cyclo-diphenylaladine (cyclo-FF) nanowire array with high-output performance voltage and a current of 350 V and 10 μA, respectively, which is sufficient to light up 100 LEDs.

Some more triboelectric nanogenerators have been listed in Table 3.


**Table 3.** Triboelectric nanogenerators, based on various ferroelectric materials.

**Figure 9.** (**a**,**c**) Schematic of conventional (ITO/PDMS) and ferroelectric TENG (ITO/PDMS/PVDF) with magnified ITO, PDMS, and PVDF surfaces in contact (**b**,**d**). Pushing morphology-dependent power generation of conventional and ferroelectric TENG. (**e**) Power generation and decay for both types, with contact and noncontact conditions. (**a**–**e**) Reprinted with permission from reference [208], Copyrights 2019, John Wiley and Sons.

#### *4.4. Device Design and Output Power Optimization in Ferroelectric Photovoltaics (PVC)*

Ferroelectric photovoltaic devices are marvelous for harvesting solar energy. For this scientist's endeavor to tailor the properties of photovoltaic devices, a prime way is the implication of ferroelectric-based materials with narrow bandgap, designed with micro or nanostructure domains and unifying both conventional and ferroelectric photovoltaic effects. Inorganic ferroelectric oxides exhibit relatively wide band gaps, Eg, ranging from, e.g., 2.2–2.7 eV for BiFeO3 [214,215], 2.48 eV for LiNbO3 [216], 3.3 eV for KNbO3 [217], and 1.5–3 eV for (KNO)x(BNNO)(1−x) [218], etc., and are susceptible to the high absorption of UV-light. Maximum solar spectrum light absorption happens around 1.5 eV and gives small photocurrents. The bulk photovoltaic effect (BPVE) in ferroelectric materials assists in parting the photogenerated charge carriers (e–h pairs), without requisition of conventional pn-junction photovoltaics. The past studies were only restricted to theoretical studies, which showed low-power conversion efficiency PCE of perovskite ferroelectrics (~10−4, much smaller than conventional semiconductor PV materials), assisted by low conductivity (~10−<sup>9</sup> S/m) and leading to a high recombination rate of photo-generated carriers [219–221]. However, the recent reports showed that the intrinsic electric field present in non-centrosymmetric ferroelectrics, due to spontaneous polarization, Ps, has supported the power conversion efficiency (PCE) to transcend beyond the Schokley–Queisser limit [222]. This has ruled into an emerged era of solar energy harvesting, assisted by optimization of bandgap engineering, carried by chemical ordering and structural modifications in thicknesses of the layer material and/or electrode and domain interfaces [223]. He et al. [224] have presented a first-principle study to investigate the effect of charge ordering in Bi-based ferroelectric perovskite oxides (ABiO3) by the combination of A-site cations (Ca, Cd, Zn, and Mg) with Bi3+ or Bi5+ charge disproportions. They revealed that the cationic size, along with chemical featuring, shifted the valance band edge towards the high energy side, reducing band gap to ~2 eV, which ushered strong visible light absorption, smaller effective masses, and a large polarization field, alongside producing a dispersive Bi 6s state in the near vicinity of the Fermi level. Han et al. [225] determined a photocurrent effect in (1−x)KNbO3-xBaFeO3−<sup>δ</sup> (KN-BF, where x = 0.0 to 0.1) ferroelectric semiconductor ceramics with bandgap (Eg) ~1.82 eV at x = 0.1, ~43% less than pure KN, Eg = 3.22 eV, and which coexisted with cubic and tetragonal phase. The activation energies for KN-BF; x = 0.07 domains and domain walls were 0.26 eV and 0.52 eV, attributed to the ionization conduction mechanism of oxygen and remanent polarization of 3.50 μC/cm2 for pure KN. They analyzed that the third states of Fe are important in KN-BF for band reduction, under light illumination of 1.5AM, for x = 0.07, demonstrated Jsc and Voc, for positive voltage 1.92 nA/cm<sup>2</sup> and 0.31 V, while under dark 11.39 nA/cm<sup>2</sup> and 0.217 V, respectively. Their study promoted ferroelectric semiconductor ceramics as a potential member of photovoltaic devices. Alkathy et al. [226] reported bandgap tuning by doping of Ba0.92Bi0.04Na0.04Ti0.96M0.04O3 (BNBT-M); (M = Ni, Fe, Co), all the samples crystallized in tetragonal structure with the estimated Eg and corresponding power conversion efficiency PCE for BNBT, BNBT-Fe, BNBT-Ni, and BNBT-Co were 2.95, 0.468521; 2.38, 0.387803; 2.24, 0.368166; and 2.54, 0.409616. Their study revealed a guide to find perovskite ferroelectric oxide for visible light PV applications. Jimenez et al. [227] reported stability of bandgap of BiFe1−xCrxO3 (BFCO), with high Cr concentration over La0.7Sr0.3MnO3 (LMSO) coated SrTiO3 substrate by high-pressure processing (HP) technique. For PV response, the Pt/BFCO/LSMO/STO device structure was employed, with BFCO as an absorber layer. Additionally, HP 12.5-BFCO/SiTrO3 showed an energy band gap (Eg) of 2.57 eV and remanent polarization Pr of 40 μC/cm2, see Figure 10a. The non-polarized illuminated device showed Voc and Jsc at RT to be 0.10 V and 6 × <sup>10</sup>−<sup>3</sup> mA/cm2, with a power output of 0.6 μW/cm2, unveiling their potential in solar energy harvesting and polarization, depicted with slightly higher values, 0.16 V, 40 × <sup>10</sup>−<sup>3</sup> mA/cm2 and 6.4 <sup>μ</sup>W/cm2 (Figure 10b), attributed to reduced recombination rates and ohmic resistances, due to ferroelectric domains. The grain boundaries also play an important role in photocurrent and PV performance, as they can lead to the increased area of minority carrier collection, due to the development of built-in potential on the grain boundaries (GB) [228], which can serve as recombination centers and reduce the photocurrent densities [229]. Mengwei et al. [230] reported that hexagonal h-YMnO3 (YMO) films over Si substrates with PV device structure Au/h-YMO/Au; h-YMO, as an absorber layer, poled to ±700 V for 100 s at RT and led to

the production of a high short-circuit current density of ~3.92 mA/cm<sup>2</sup> and open-circuit voltage −0.41 V, under the illumination of 1 sun (or 100 mW/cm2). The Jsc and Voc reached 3.16 mA/cm2 under 65 mW/cm<sup>2</sup> intensity and 4.01 mA/cm<sup>2</sup> under 140 mW/cm<sup>2</sup> intensity, see Figure 10c. This is accomplished by the magnification of the impulsive force, relying on polarization and grain boundary effects for the separation of photogenerated charge carriers. Their study suggested downward band bending, induced by the potential difference and assisted by grain and interfaces between grain boundaries, behaving as a conductive path for effective transport of carriers. The bandgap, Eg, was found at 1.55 eV, and another absorption edge at 1.35 eV suggested the absorption in near IR region. Tu et al. [231] reported a Bi0.93Ni0.07FeO3 (BNFO)-based PV device, with a field-modulated PV effect, carried by the pn-junction, in conjunction with a polarization-modulated Schottky barrier, under irradiation of 405 nm, with Eg ~ 3.95 eV and ITO/BNFO/Au heterostructure, see Figure 10d. The currents density and voltages are taken at an irradiation intensity of 1000 W/m2, under 405 nm, with electric poling at 1 kV/cm ~8 A/m2 and 0.9 V, with PCE ~1% and EQE ~11%. The photo-response of as-grown devices, under 532 nm and 633 nm irradiation, showed a weak response, mainly attributed to the transitions with intermediated defect levels. Nan et al. [43] investigated the temperature dependence of photocurrents in BTO ceramics and found it enhanced, by approximately 121.9% to 179.6% at 80 K to 240 K, assisted with ferro-pyro-phototronic effect, convincing the activation of shallow traps and polarization, as compared to RT, which was useful for detecting 405 nm light at low temperatures. Jia et al. [215] realized a temperature-modulated, self-powered UV photodetector, based on the ITO/BFO/Ag device, for spotting light of 365 nm with intensity 271.4 mW/cm<sup>2</sup> and observed increased photocurrent ~60% when the bottom of the device was subjected to temperature variations (heating) of around 42.5 ◦C. The amplified current is manifested with the change in carrier concentration, convinced by the temperature effect and electron shift in the BFO layer, instigated by the thermo-phototronic effect. Mai et al. [232] reported heterostructure, designed for the conversion of IR light into electricity, by employing BFO as a ferroelectric layer and upconversion layer of YOT (Y2O3:Yb). The structure revealed the convertible and stable PV-effect, under the illumination of 980 nm laser light, with the energy much smaller than the BFO Eg, to induce a photovoltaic effect in BFO itself. YOT mutates the incident to produce photovoltage Voc ~ 0.4 V and photocurrent Isc ~25 nA. This kind of heterojunction structure lined a pathway to bulk photovoltaic effects in ferroelectrics, by expanding the absorption of the solar spectrum. Table 4 discusses the output performances of some other ferroelectric photovoltaic (FPV) devices.

**Figure 10.** (**a**) The electrical configuration of the BFCO/LSMO/STO device for ferroelectric polarization and photoresponse, with and without illumination. (**b**) Short-circuit photocurrent density J-V curve at room temperature and non-polarized film (**i**); current density measurement J-V curve at room temperature and polarized film, with and without illumination (**ii**). (**a**,**b**) Reprinted with permission from reference [227], Copyrights 2021, Elsevier. (**c**) J-V characteristic, with inset showing a schematic of Au/h-YMO/Au device with photocurrent and photovoltage curve, as a function of light intensity. Reprinted with permission from reference [230], Copyrights 2021, Elsevier. (**d**) Schematics of a ITO/BFNO/Au photovoltaic device, optical transmission of ITO film, and top view of test disk. J-V curves of poled and unpoled under dark and illumination of 405 nm, 532 nm, and 633 nm. Reprinted with permission from reference [231], Copyrights 2019, Elsevier.


#### *4.5. Device Design and Output Power Optimization of Coupled Effect Nanogenerators*

The atmosphere is rich in energies, whether natural or artificial; therefore, the energy harvesters of light, heat, and mechanical energy can coexist as hybrid- and multi-effectcoupled nanogenerators, stemming into continuous and intensified production of electricity. The different ferroelectric, material-based coupled energy nanogenerators have been reported to date, including the meld of photovoltaic and mechanical energy, photovoltaic and thermal energy, mechanical and thermal energy, and some comprising of all of these collectively and synchronously. It is believed that the coupled nanogenerators can be proved highly nifty for harvesting multi-energies from the environment. Ferroelectric materials-based nanogenerators have been reported with high-output characteristics, but their coupling is still a daring challenge. Zhao et al. [239] have fabricated a flexible nanogenerator, employing the ferro-pyro-phototronic effect, cited by the coupling of photovoltaic and pyroelectric effects in ferroelectric BaTiO3 (BTO), with device structure ITO/BTO/Ag and Kapton membrane, in order to achieve flexibility, Figure 11a. A ferrophoto-phototronic effect makes use of a temperature fluctuation-incited pyroelectric field, in order to attune the photoexcited charge carriers for outstripping the photocurrent in the wake of coupling between ferroelectric material, pyroelectricity, and photoexcitation. The device was poled at 2.3 kV/mm for 30 min and responded well to detecting light and temperature variations, applied simultaneously with the individual detection. The photocurrent and current plateaus were found to increase with increasing light intensity (7.78 mW/cm<sup>2</sup> to 127.6 mW/cm2), from 5.4 nA to 43.2 nA and 5 nA to 40 nA at room temperature, owing to the ferro-pyro-phototronic effect. Heating at ΔT = 22.4 K, the lower coupled current plateaus were observed from 4.46 nA to 35 nA and illuminated under same light intensities; however, the coupled photocurrent showed similar increasing behavior from 29.8 nA to 69.7 nA. Cooling (ΔT = −14.5 K) the device led to a reduction of photocurrent (from −19.2 nA to −12 nA) but enhanced current plateaus (from 5.9 nA to 44.9 nA). Their results showed that the coupled photocurrent peak and photocurrent peak enhancement ratio, under the state of heating + light, were 451.9% and 61.3%, while the photocurrent plateaus and photocurrent plateau enhancement ratio, generated under

cooling + light, were 17.2% and 11.1% sharper than light generated via the photovoltaic effect alone, at a light intensity of 127.6 mW/cm2, see Figure 11b. For heating + light, the currents from heating and light are in one direction, superimpose, and are attributed with the higher temperature difference, while reduced current plateau might be associated with the reduced polarization of BTO, due to large thermal agitations. The reverse is true for cooling + light. Similar increasing trends for voltage peak/voltage, peak enhancement ratio values were 391.5%/125% for heating + light, and plateau/voltage plateau enhancement ratios were observed to be 50.9%/92.8% for cooling + light, respectively. Therefore, the fabricated device boost in current and voltage plateau was carried by the device under simultaneous light + cooling applications, whilst photocurrent and photovoltage were enhanced under the condition of light + heating. They developed 16 photo-pyro sensing matrices (4 × 4) (shown in Figure 11c) and achieved a photodetector system for light illumination detection of 405 nm and temperature variations, demonstrating the possible application in environmental and artificial intelligent sensors. Zhao et al. [240] reported photovoltaic pyroelectric-coupled NG with a planner structure, based on radially polarized BTO, as a functional layer and coplanar laser-engraved ITO electrode (see Figure 11d). The pyro-phototronic effect enabled BTO to exhibit a high pyroelectric constant of 16 nC/cm2K and showed the dramatically increased photocurrent; it can perform fast sensing of 365 nm ultraviolet illuminated light with a 0.5 s response time. Heating temperature modulations led to the current enhancement of almost >30 times, under 0.6 mW/cm2 intense light, as compared to only light illuminated current; however, the photocurrent enhancement ratios were found to decrease as the light intensity increased. Under the cooling + light condition, the currents found were larger than that of heating + light condition; this is because of elevated pyroelectric coefficient. The current plateau dependence on the ferro-pyrophototronic effect can be visualized in Figure 11e. The currents were measured individually, under the illumination of 81.8 mW/cm2, 1.25 K/s heating, and simultaneous heating-light. The heating + light showed that photocurrent is not the superposition of individual currents produced, and coupling light and heating caused a decreased current plateau, rather than individual light illuminated. The current plateau increased by ~23%, by simultaneous cooling and light shining on the sample, indicating the coupling enhancement "1 + 1 > 2" of photocurrent plateau, associated with the band bending, induced at the BTO/ITO interface. The estimated peak currents under illumination, pyroelectric heating, and cooling were 50 nA, 29.3 nA, and 36 nA, respectively, while the total peak currents of illumination, along with heating and cooling, estimated were 71 nA (<50 nA + 29.3 nA) and 0.2 nA (<50 nA − 36 nA), respectively. Hence, the coupled current's relation with individual currents is not additive. Zhang et al. [241] have reported the ferroelectric-metal-semiconductor BiFeO3/20sAu/ZnO heterostructure as intriguing, coupled pyroelectric and photovoltaic effects. They analyzed that photocurrent density, induced by light illumination of 405 nm, was 1.4 times that of BFO/ZnO structure associated with the introduction of Au nanoparticles that tuned the energy band alignments and by light-induced temperature oscillations, across the heterojunction interface, resulting in improved photoelectric performances. They additionally responded deliberately to a wide range of solar spectrum wavelengths, 360 nm to 1060 nm, by coupled pyroelectric–photovoltaic effects. The BTO/20sAu/ZnO heterojunction produced short-circuit current densities of 191.9 μA/cm2 and 90.8 μA/cm<sup>2</sup> under light illumination of 405 nm and 360 nm with 60 mW/Cm2 and 26 mW/cm2 intensities. For wavelengths of 532 nm and 635 nm, with intensities 150 mW/cm2 and 650 mW/cm2, the heterojunction current densities in short-circuiting were 3.7 μA/cm2 and 2.8 μA/cm2, while the photoelectric response of simple BTO/ZnO was negligible. So, Au nanoparticle induction led to the broad range of light detection by the device. Ji et al. [145] reported piezo–pyro–photoelectric coupled effect in BTO with coupling enhancement of 1 + 1 > 2 for charge quality, the maximum output power obtained for pyroelectric, photovoltaic, and pyroelectric–photovoltaic units were 117 nW, 11 nW, and 189 nW, respectively. The total photocurrent Ipyro + photo + piezo estimated was ~0.67 μA, with a charge transfer of 67.74 μC. Wang et al. [242] proposed a one structure-based coupled nanogenerator to

scavenge mechanical and thermal energies, via the tribo–piezo–pyroelectric effect, using PVDF nanowires as pyro–piezoelectric active material and PVDF–PDMS composite as a triboelectric layer. Their hybrid nanogenerator showed improved charging performance, rather than individual TENG, PENG, and PyENG. The triboelectric–piezoelectric (TiPENG) unit showed the problem of lower voltage, while the pyroelectric unit had to face the low current performance; so, individual units were unable to power the light bulbs connected in series. However, the synergetic effect of hybridized nanogenerator with maximum output energy supply lightened the bulbs much more efficiently under the state of both units working simultaneously. Song et al. [243] developed Ag/BTO/Ag sensor arrays, based on a pyro–piezoelectric conjunction nanogenerator and realized that, with the advantage of 1 + 1 = 2, evidencing the simultaneous detection of both temperatures and pressure-assisted with zero loss and interference of overlying multi-functionality of device. They observed the intensified device voltage, with rising temperature and/or pressure, as well as a sensitivity of ~0.048 V/◦C and /or 0.004 k/Pa, respectively. They proposed a potential application era of such devices in intelligent compliant structure, smart sensors, and human–machine interactions, as well.

Ji et al. [46] reported a multi-effect coupled NG by integrating pyro–piezo–tribo–photo effects simultaneously in multifunctional BTO ceramic with only one electrode pair, piezo– tribo TPiENG, which harvests mechanical energy by strain-induced deformations in BTO and between FEP and nylon film, as well as PDMS as a protective layer for sustained confrontment of ITO and Ag electrodes (see Figure 12a). The individual pyroelectric nanogenerator (PyENG) produced current and voltage peaks of 15.2 nA and 1.5 V, at a heating rate of 0.34 K/s for the temperature change of 302 K to 307 K, cooling the device back to 302 K. Similar current and voltage peaks (but in the opposite direction) and maximum output power of 7.8 nW at 150 MΩ were observed. Individual PVC outputs were examined under light illumination of LED with a wavelength 405 nm, stable current of 10.1 nA, voltage of 0.6 V, and maximum power of 7.6 nW, obtained at 25 MΩ load resistance. To quantify individual TPiENG, an airflow of 15 m/s was blown from an air blower at 12 cm from a multi-coupled NG. An AC output current of 3.5 μA was obtained, followed by the oscillation of nylon film. A baseline of voltage was swinging, so it was not easy to detect voltage. The multi-effect coupled NG under the conditions of the heating rate 0.98 K/s, a 405 nm light illumination by LED, and an airflow speed ~15 m/s, which proved to be a compatible power source with a current, voltage, and platform voltage of 1.5 μA, 7 V, and 6 V, respectively. The charging capability of the 0.33 μF capacitor to 1.1 V in 10 s, only as compared to individual units of PyENG, PVC, and TiPENG, are shown in Figure 12b. Zhang et al. [51] prepared a multifunctional-based PZT multi-effect coupled nanogenerator by integrating PyENG, PVC, and TPiENG units. The one device structure had three parts, with PZT being a piezo–pyro–photoelectric active material and flexible nylon vibrating film, which, with FEP, introduce triboelectrification and apply strains to PZT, Ag, and ITO/AgNws/PDMS, as bottom and top electrodes, see Figure 12c. The investigated performance of coupled nanogenerator, with different combinations of individual units with PyENG+PVC, PVC+TPiENG, PyENG+TPiENG, and PyENG + PVC + TPiENG, are shown in Figure 12d. The output of PyENG+PVC acquired the peak values of current, voltage, and platform to 1 μA, 91 V, and 200 nA; all other coupled nanogenerators, comprised of TPiENG, showed similar AC output current peaks, greater than 5 μA. However, the individual TPiENG, under wind flow, showed a higher output current than PyENG and PVC but output voltages are smaller. The coupled nanogenerator gathered the fringe benefits of piezo–pyro–tribo–photoelectric modules and aroused high electric peak current, voltage, and platform voltage of 5 μA, 80 V, and 50 V. The coupled nanogenerator showed outstanding charging response by charging 10 μF capacitors only in 90 s up to 5.1 V. Singh et al. [244] has reported piezo–tribo based HNG with combined piezoelectric and triboelectric effect in ZnO-PVDF film coupled with PTFE (polytetrafluoroethylene) as shown in Figure 12e, the determined that incorporating ZnO nanorod in ferroelectric PVDF polymer uplifted the triboelectric and piezoelectric

response of PVDF and given the maximum output instantaneous power of 24.5 μW/cm2, while combined current and voltage of 0.46 μA and 78 V, without subjecting any surface treatment and poling.

**Figure 11.** Schematic of ferro–pyro–phototronic effect coupled nanogenerator. (**a**) Schematic illustration of Kapton/ITO/BTO/Ag/light fabricated device. (**b**) Detector current performance I1, I2, and I3, under different light intensities and conditions of "light", "light + heating", "light + cooling", current peak and current peak enhanced ratios, and current plateau and current plateau enhanced ratios. (**c**) The flexible photodetector 4 × 4 array, consisting of 16 BTO units. (**a**–**c**) Reprinted with permission from reference [239], Copyrights 2020, Elsevier. (**d**) Laser engraved ITO electrode at the surface of BTO coupled ferro–pyro–phototronic NG device. (**e**) Current measurements of ITO/BTO/ITO device, under illumination, heating, cooling, heating + illumination, and cooling + illumination. (**d**,**e**) Reprinted with permission from reference [240], Copyrights 2018, Elsevier.

**Figure 12.** Multi-effects coupled nanogenerators. (**a**) Schematic of coupled nanogenerator with piezo–tribo–pyro– photoelectric effect, along with photo image of a coupled device, with BTO as a functional layer. (**b**) Rectified output current performances of discrete TPiENG, PyENG, PVC, and voltage measurement of capacitor charging for individual and coupled nanogenerator, comprising of TPiENG + PyENG+PVC. (**a**,**b**) Reprinted with permission from reference [46], Copyrights 2018, John Wiley and Sons. (**c**) Schematic diagram of a one-structure, multi-effects coupled nanogenerator, with PZT as a functional layer. (**d**) Output characteristics current and voltages of coupled nanogenerator in different combinations. (**c**,**d**) Reprinted with permission from reference [51], Copyrights, 2017, John Wiley and Sons. (**e**) Schematic of hybrid piezo–tribo nanogenerator based on ZnO-PVDF semiconductor–ferroelectric material. (**f**) Output electric power and voltage, as a function of the resistance of the ZnO-PVDF hybrid nanogenerator. (**e**,**f**) Reprinted with permission from reference [244], Copyrights 2018, Elsevier.

Ji et al. [245] demonstrated the electricity induction by employing the photovoltaic– pyroelectric–piezoelectric coupled effect for BTO-based sensors, utilized to spotlight intensities, temperature differential, and vibrational frequency, individually and all-together, with

high susceptivity. The nanogenerator device for couple sensing is shown in Figure 13a. The BTO ceramic slice is interlocked between the ITO top and Ag bottom electrode. One end of TO slice was fixed on a commercial TE module for providing temperature variations and masses on another end. The right-hand photograph is the optical image of the fabricated device. The individual examinations for sensing light, temperature, and vibration marked out the sensitivity of ~0.17 nA/(mW/cm2), 4.80 nA/K, and 1.75 nA/K for cooling and heating condition, 21.02 nA/Hz, and −14.07 nA/Hz for low (≤17 Hz) and high (>17 Hz) vibrations, respectively, without any external power supply. The device sensitivity for monitoring light-temperature, temperature-vibrations, and light-temperature-vibrations coupled effects were found to be dependent on variations in the spontaneous polarization of BTO. The light + temperature + vibration current (Iphoto + Ipyro + Ipiezo), as a real-time function, observed at a constant light intensity of ~211 mW/cm<sup>2</sup> and constant temperature change of 12.9 K, the total current was an AC-type piezoelectric current, as in Figure 13b(i). The total current, as a function of different vibration conditions under various light intensities and at constant temperature change of 12.9 K, can be seen in Figure 13b(ii). The total current increased under low-frequency vibrations (≤17 Hz) but started to decrease as the frequency vibrations enhanced above 17 Hz, as a response to increasing light intensities from 20–211 mW/cm2. Additionally, the corresponding sensitivities K and K of low and high vibration fall to a minimum of 15.33 and −8.57 nAcm2/mW, respectively, at 211 mW/cm2. The trend is attributed to the dwindling of spontaneous polarization, convinced by thermionic and photo-thermionic effects. Their study revealed motivation into commercial and industrial applications of multifunctional sensors. Zhao et al. [246] designed a multi-effects coupled nanogenerator and implicated it in self-powered, multifunctional coupled sensor applications. The schematic of the device consisted of BTO ceramic wafers, with transparent ITO and Ag electrodes deposited on the upper and lower surfaces, as well as a total of the BTO array, encapsulated in a PDMS matrix to achieve flexibility, see Figure 13c. Fabricated device characteristics were conducted for the light of 405 nm, with intensities 8.39–83.2 mW/cm2, temperature fluctuations of 9.1 K to 35.1 K, and pressure variations of 7.6 kPa to 33.7 kPa. The coupled NG gave an improved current of ~387.3% under simultaneous light gleaming (83.2 mW/cm2) and pressure (7.1 kPa), as well as for a temperature difference of −19.5 K, the current enhancement of ~375% was observed, rather than only light. They employed the fabricated coupled NG, as the realization of the multi-functional coupled sensor, at the benefit of external power, see Figure 13d. The coupled nanogenerator showed powerful coupling enhancement with the detection sensitivity of 0.42 nA/mWcm−2, 1.43 nA/kPa for light and pressure detection at a constant temperature change of −19.5 K, and 8.85 nA/K for temperature sensing at light irradiance of 83.2 mW/cm2. Their study opened a way to sense electronic skin evolutions. Shi et al. [247] demonstrated a flexible one structure hybrid nanogenerator, based on coupled piezoelectricity and triboelectricity, using cellulose/BaTiO3 aerogel paper PDMS nanocomposites. The PENG unit, under mechanical impact force, separately showed maximum voltage and power of 15.5 V and 11.8 μW. The PENG unit was coupled to a single electrode TENG. By adding the electrode in the PENG system, the polarization direction of PENG positively couples with triboelectricity and enhances the output voltage and power to ~48 V and 85 μW, at the mechanical impact of 80 kPa. The study revealed an easy coupling approach for the enhancement of device performance and practical applications. Ma et al. [248] also demonstrated sandwich layer structure NG, based on coupled pyroelectric and photovoltaic effects. The BTO functional material was embedded between ITO and Ag electrodes. BTO acts as a pyroelectric and photovoltaic material, deposited on Ag film, with a thickness of ~0.3 mm. The Schottky contact was formed at the interface of ITO/BTO, which created the built-in potential. The photo-pyroelectric coupled effect in ITO/BTO/Ag film was induced by 405 nm light illumination, with the intensity of 111.1 mW/cm<sup>2</sup> by more than 260%. The coupled current of 79 nA was generated with a maximum power output of 7.1 nW for 20 MΩ. The incident light irradiation cause BTO to absorb photons, promoting electrons to hop from valence to conduction band, creating

free excitons, which further divide by the built-in and depolarized fields generating a photocurrent. The photons absorbed interact with the lattice phonons of BTO, producing heat that reduces the polarization of BTO, resulting in a pyroelectric current signal. The pyroelectric current signals are mediated through light irradiance, so the NG performance is promoted via the pyro-photoelectric coupled effect. They postulated that coupled photovoltaic and pyroelectric effects have potential sensing/detecting the near UV-light; for this purpose, they fabricated a photodetector system composed of 3 × 3 matrix arrays of an ITO/BTO/Ag device and utilized it to detect dispersion of 405 nm light intensity at zero cost of external power. The light information was determined by scrutinizing the electric signals, which were self-powered and showed a high photoconductive gain, responsivity, and specific detectivity of the photodetector. Their toil efforts gave a novel plan to attain fast light spotting by self-powered sensors, based on coupled photovoltaic–pyroelectric effects in ferroelectric BTO. The studies above let us make the statement that coupling the various effects in a single device structure increased the effectiveness of energy harvesting from ambient temperature by utilizing ferroelectric materials. Qi et al. [249] devised a pyroelectric–photovoltaic coupled nanogenerator, employing BiFeO3 as a functional ferroelectric material layer and ITO and Ag electrodes, owing to a narrow bandgap of 2.67 eV BFO, which can actively absorb light of even wavelengths less than 465 nm. Additionally, owing to the non-centrosymmetric nature, the light-induced temperature in BFO lattice can instigate pyroelectricity, and light-induced electric signals match the pyroelectric signal and coupled nanogenerator exhibits high-output performance; additionally, the fabricated nanogenerator can be used as a self-powered photodetector. Under the illumination of 450 nm light, the ITO/BFO/Ag coupled NG produced an output current and voltage of 8.8 nA and 0.13 V. The photoconductive gain, responsivity, and detectivity of coupled NG was 9.7 times higher at 0.86 mW/cm2, compared to the utilization of photovoltaic effect. Until now, many other coupled nanogenerators, based on hybrid and one structure, have been fabricated and more are in progress. Coupled effect devices are highly innovative, as they are rich in power generation capability.

**Figure 13.** Schematic of coupled nanogenerators for self-powered multifunctional sensing. (**a**) Design structure optical image of the fabricated, BTO-based sensor for coupled sensing. (**b**) Sensing performance under light + heating + vibration condition, a real-time output current, at a constant light intensity of 211 mW/cm<sup>2</sup> and temperature change of 12.9 K (**i**); current, as a function of vibration frequency under different light intensities, at a constant temperature change of 12.9 K (**ii**). (**a**,**b**) Reprinted with permission from reference [245], Copyrights 2019, John Wiley and Sons. (**c**) Schematic of coupled nanogenerator for human body sensing. (**d**) Performance characteristics of multi-effect coupled nanogenerator. (**c**,**d**) Reprinted with permission from reference [246], Copyrights 2020, Elsevier.

#### **5. Applications**

The technology of nanogenerators has promoted plenty of advanced modern devices in self-powered micro/nanosystems, comprising of sensors like pressure sensors, tactile sensors, infrared sensors, actuators, flexible and wearable devices for energy harvesting, medical implantations (such as biosensors), health care monitoring through humancomputer interfaces, electrochemical systems (such as supercapacitors and batteries), as

a power source for many tiny energy harvesting devices, and many others. The nanogenerators can drive small power/self-powered electronic devices, such as a lead-free BTO/PDMS-based PENG, which can charge 1.0, 2.2, and 4.7 μF capacitors to 1.26, 1.68, and 2.58 V in only 55, 170, and 527 s [168]. All-fiber wearable nanogenerators, based on nanofiber, nonwoven fabric, employing the NaNbO3 nanoparticles embedded with PVDF and covered with PDMS, successfully harvested the low-frequency mechanical energy of human activity and motion into electricity [250]. Wang et al. [251] prepared a piezoelectric pressure sensor, based on cellular fluorocarbon with a high sensitivity of 7380 pC/N, in the pressure regime of <1 kPa and with a very fast response time of 5 ms, to check physiological motions of the body by attaching it to human skin, e.g., facial muscle contraction, wrist motion, eye blinking, and breathing; their current signals are shown in Figure 14a. The breathing frequency was recorded at ~21 times/minute, which is the normal range of a healthy 36-year-old man. Lee et al. employed a stretched PyENG to charge two capacitors of ~3.3 μF, along with a rectification circuit, and can light up their red, yellow, and green commercial LEDs simultaneously, as well as the LCD, see Figure 14b [186]. Zhang et al. used a PLZT–TENG self-powered wearable sensor as a harvester of biomechanical energies, in order to sense elbow joint motions for different angles of ~130◦, 110◦, and 70◦ at a humidity rate of ~20% and action time of Δt = 300 ms. Under dynamics of ultrafast response times of 7 ms, the PLZT–TENG sensor is found extremely suitable for AMP test, as estimated for volunteer AMP was 345, lighting up 119 LEDs by gentle hand taping and recording the footsteps of a running man (Figure 14c) [202]. A multilayer triboelectric wearable biomedical and acoustic sensor (TESs) for biomedical interaction interface for diagnosing pulse pressure and human breathing rates, the acoustic sensors were capable of detecting the high-frequency of Fourier transform signal (STFT) sound sources (shown in Figure 14d), which were proposed by Park et al. [196]. Sahu et al. proposed a hybrid, multi-stacked (MS-HG) BCZT-BHO TENG self-powered IR communication module (see Figure 15a(i,ii)) to analyze the calorie burned from yoga poses of the body, consisting of a receiver and transmitter, voltage regulator circuit, two capacitors, and VGA camera. In order to see IR light, the remote of the LED transmitter was powered via the hybrid system, and IR LED with red LED was lightened by the solar cell, so pressing the remote button glowing light indicated the established communication among the IR LED receiver and transmitter. The low powered appliances, used in this way, were lightened by capacitor charging through hybrid TENG [64]. Sahu et al. established a piezo–tribo hybrid structure (P-DMHS) that showed potential biomechanical energy harvesting by finger tapping and hand and leg motion. The generated currents were proportional to the weight of the body part (see Figure 15b), which can power a 50 μF capacitor, light commercial LEDs, and power wristwatch (Figure 15c). P-DMHS also was demonstrated as an impact sensor and recorded impact by ball dropping through certain heights from 10 cm to 60 cm, with a device sensitivity of 2.101 V/m and maximum impact of 370 mJ, shown in Figure 15d inset. For the impact sensitivity, they further launched the device on a bicycle helmet and found a maximum impact of 810.7 mJ, stating the sensitivity of the impact sensor is as much that they can respond to an impact of <1 J. This study evokes interest in battery-free sensors to sense impacts/shocks in helmets, cars, and other protective equipment [252]. You et al. [253] developed a hybrid piezoelectric–pyroelectric nanogenerator, using non-woven nanofiber membranes of PVDF, with versatile applications in self-powered electronic textiles. By implanting the NG insole of the shoe, they observed the maximum output current of 2.83 nA and 78.55 nA, while walking and running. Coupled nanogenerators have great potential to be used in photodetectors, e.g., a BTO based pyro–photoelectric nanogenerator can efficiently detect light of 405 nm, with a responsivity of 10–7 A/W [45]. A pyro–photoelectric nanogenerator that is based on BFO can detect 450 nm light of intensity 65 mW/cm<sup>2</sup> [215]. Coupled nanogenerators have shown marvelous applications as multifunctional sensors, too, e.g., an Ag/BTO/Ag piezo–pyroelectric coupled nanogenerator has marked to sense pressure and temperature simultaneously [51]. A multifunctional sensor to detect light intensity, pressure, and the temperature has also been constructed, with light

and pressure detection sensitivities of 0.42 nA/mWcm2 and 1.43 nA/kPa, respectively, via an ITO/BTO/Ag structured coupled photo–piezo–pyroelectric nanogenerator [246]. The multi-effect coupled nanogenerators have great potential in the Internet of Things, as well as many other industrial and commercial applications.

**Figure 14.** Application of NGs. (**a**) Short-circuit current and image positions of piezoelectric pressure sensor, attached on the skin to check wrist motion (**i**), facial muscle contraction (**ii**), eye blink (**iii**), and breathing (**iv**). Reprinted with permission from reference [251], Copyright 2017, Elsevier. (**b**) Capacitor charging circuit and curve through pyroelectric NG; a photo image of turning red, yellow, and green LEDs off and on, as well as a photo image of turning an LCD off and on. Reprinted with permission from reference [186], Copyright 2015, John Wiley and Sons. (**c**) Photograph of a PLZT–TENG biomechanical energy harvester to light 119 LEDs (**i**) and as a self-powered wearable sensor to detect elbow joint motions at different angles (**ii**). Reprinted with permission from reference [202], Copyright 2021, Elsevier. (**d**) Acoustic sound wave sensor with three configurations of poled, unpoled P(VDF-TrFE), and poled P(VDF-TrFE)/BTO, as a function of sound frequency, alongside the time-dependent sound waveforms and short-time STFT signal. Reprinted with permission from reference [196], Copyright 2020, ACS Publications.

**Figure 15.** (**a**) Multi-stacked TENG, as a self-powered sensor, body activity counter for various yoga configurations, from count calorie burnt (**i**) to wireless IR communication module (**ii**). Reprinted with permission from reference [64], Copyrights 2020, Royal Society of Chemistry. (**b**) Biomechanical energy harvesting from hybrid piezo–tribo P-DHMG; voltage and current response of human body motion finger tapping, as well as hand and leg motion. (**c**) As a response to compressive force lightening commercial LEDs, powering of the UV LED wristwatch for sports purposes, with inset profile of capacitor charging. (**d**) Self-powered impact sensor, voltage response, as a function of time for various ball impacts, sensitivity as a function of distance, digital photograph of the helmet, with installed impact sensor, as well as voltage amplitudes and electrical properties. (**b**–**d**) Reprinted with permission from reference [252], Copyrights 2018, Elsevier.

#### **6. Conclusions and Future Prospect**

In the present era, power consumption has reached its peak, while energy generation and storage technology is at high risk to meet the power consumption demand. So, there is a need to improve the harvesting of units, while the most tedious part is selecting the appropriate energy materials. Ferroelectric materials have emerged and proved that their properties are promising for harvesting ambient energies and auxiliary power systems in upcoming small technology. In this review, we pictorially visualized the energy harvesting mechanisms. Their output functionalities are mainly the current progressions by various ferroelectric-based individuals and coupled nanogenerators. The detailed analysis helped us to realize the different enhanced nanogenerator features, as a function of material nature organic polymers or inorganic perovskite oxide ceramics, compositions, and morphology control, by doping and device geometry tailored to nanowires, thin films, multilayers, etc. Detailed review let us embrace that all ferroelectric NGs are hooked with spontaneous polarization connected to the configuration of domain walls, as determined by the crystal microstructure and electric poling for energy conversions. High piezoelectric potential and surface potential are produced by strong ferroelectric polarization. The outputs can be

optimized by strengthening the switching polarization property of ferroelectric materials and hybridizing and coupling multi-effects in one device, resulting in superimposing of electrical signals. The synergy of various effects imitates the control over production, separation, movement, and recombination of carriers and, hence, strongly influences the energy harvesting and versatility of coupled nanogenerators, in order to make their integration to diverse electric arrays. Emerging wearable and self-powered devices need to be flexible, so the active materials are polymers, such as PVDF or copolymerized PVDF materials with TrFE, nylon, etc., and polymer-ceramic-embedded materials. The nanogenerators are intelligent, flexible, simple, and low cost, with smart applications in self-powered devices, multifunctional sensors and detectors, and wearable and portable devices, with the potential for surveilling the physiological activities, motion detection, tactile sensors for human-environment interactions, examination of respiratory tracks, and photovoltaic response to photodetectors. Multifunctional sensors have also been processed by utilizing multi-effect coupling. Even with a lot of advantages of ferroelectric-based nanogenerators, there is still a long route to go for practical and commercial applications, as a comprehensive revealing of the interactions of various coupled effects is needed for charge modulations to better output performances and promote multi-energies harvesting.

**Author Contributions:** Y.Y. planned and supervised the whole process. J.Z.M. has surveyed the literature, prepared the outline, and wrote the manuscript. Y.Y. and M.A.M.H. revised the outline. All authors revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was sponsored by the ANSO Scholarship for International Students, National Key R&D Project from Minister of Science and Technology in China (No. 2016YFA0202701), the University of Chinese Academy of Sciences (Grant No. Y8540XX2D2), and the National Natural Science Foundation of China (No. 52072041).

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

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