Advancements in Energy Harvesting: Piezoelectric, Triboelectric, Pyroelectric, and Magnetoelectric Technologies for Self-powered Sensor Systems

2.1.1 Mechanism of Piezoelectric Effects

Mechanical energy harvesting is facilitated through the use of Piezoelectric Nanogenerators (PENGs). For a better understanding of piezoelectric effects, the working mechanism for the PENG is described in Fig. 1(a). The piezoelectric layer, when polarized, contains dipoles arranged in a specific direction, which enhances its piezoelectric performance. Initially, when the material is in a stress-free state (Fig. 1(a-I)), charges accumulate on the surface to maintain electrical balance. When compressive stress is applied (Fig. 1(a-II)), the polarization decreases, causing the surface charges to move and generate an electric current. Conversely, removing stress or applying tensile stress (Fig. 1(a-III)) increases the polarization, leading to a current flow in the opposite direction to restore balance [14].

Fig. 1

(I) (a) Working mechanism of PENGs [4] (Adapted from Ref. 4 with permission), (b) Schematic of capacitor charging process with an equivalent circuit diagram and several capacitor charging performances, [18] (Adapted from Ref. 18 with permission, Copyright 2022, Wiley), (c) Schematic of the all-fiber PPNG comprising of (i) a large area PVDF–MWCNT nanofiber mat (8 × 7 cm). (ii) FESEM images of the PVDF–MWCNT nanofiber and (iii) micro-fiber based conducting electrode. (d) The excellent flexibility and conformability of PVDF–MWCNT electrospun nanofibers is shown by wrapping them on a human finger. Energy harvesting characterization of nanogenerators. (e) Pressure dependent output open circuit voltage (V_oc) of the PPGN, [3] (Adapted from Ref. 3 with permission, Copyright 2022, RSC), (f) Schematic of the electrospinning setup where a needle is connected to the positive terminal of the high-voltage supply and the plate collector is grounded. (g) Digital photograph of the electrospun nanofiber mat. (h) Open-circuit output voltage of C-PNG under a compressive pressure of 22 kPa [19] (Adapted from Ref. 19 with permission), (II) The concept of hetero-layer structure PENG (HT-PENG). (a) The schematic of layer-by-layer hetero-structure PENG with interspace metal sheet. b) Cross-sectional SEM image of the HT-PENG. (c) open circuit voltage (Voc) and (d) short-circuit current (Isc) of the HT-PENGs. (e) The V_oc and (f) I_sc of the optimized 6 layers HT-PENG [6] (Adapted from Ref. 6 with permission)

2.1.2 Piezoelectric Materials and Its Progress

Ferroelectric materials are promising for energy harvesting through piezoelectricity. Various materials, including ceramics, polymers, and piezoelectret polymers, are used in PENG fabrication. The piezoelectric ceramics exhibit micro/nanostructures of wurtzite and perovskite. Examples of piezoelectric ceramics with wurtzite micro/nanostructures include ZnO NW, CdS, ZnS, and group III nitrides (InN, GaN, AlN). Examples of piezoelectric ceramics with perovskite micro/nanostructures include PZT, PMN-PT, BaTiO₃, ZnSnO₃, and K, NaNbO₃. The performance of piezoelectric ceramics is superior due to their high piezoelectric coupling coefficient (d₃₃), but they are brittle in nature and some contain lead, which is highly toxic to the environment. Therefore, their use in flexible sensors or certain biomedical applications is limited. In contrast, the ferroelectric polymer such as synthetic (e.g. PVDF and its copolymers) and natural (e.g. silk, cellulose, and collagen) are have gained intense interest in energy harvesting and sensor applications due to their exceptional mechanical flexibility, ease of fabrication, environment friendly, and biocompatibility [6,15]. In addition to that, ferroelectret polymers, like cellular PP foams, possess piezoelectret properties as a result of charge accumulation within their voids. To achieve high piezoelectric performance while preserving excellent device flexibility, a specific number of piezoelectric ceramics is incorporated into the polymer matrix [9,15]. The mechanism of charge generation in ferroelectret polymers differs from that of ferroelectric polymers [16,17]. Extensive research has been conducted to enhance the performance of piezoelectric nanogenerators (PENGs), focusing on parameters such as open circuit output voltage, short circuit current, power density, and energy conversion efficiency [2,6,15,16]. For example, Ghosh et al. proposed platinum (Pt) nanoparticles incorporated P(VDF-HFP) composite film for flexible ferroelectretic nanogenerator (FTNG) [18]. The as prepared FTNG generated an 18 V open circuit voltage with an energy conversion efficiency of 0.2%. The schematic of the capacitor charging process with an equivalent circuit diagram and several capacitor charging performances is illustrated in Fig. 1(I-b). Mahanty et al. proposed an all-fiber pyro-and piezoelectric nanogenerator (PPNG) by incorporating MWCNT into PVDF nanofiber [3]. The schematic of the fabricated device is shown in Fig. 1(I-c) with excellent flexibility in Fig. 1(I-d). The fabricated PPNG demonstrated improved performance with an open-circuit voltage of approximately 35 V, an instantaneous power density of around 34 μW·cm⁻², and an energy conversion efficiency of 19.3%. The pressure dependent output voltage of PPNG is shown in Fig. 1(I-e). The fabricated device was able to detect human physiological signals wirelessly through a mobile device, enabling remote healthcare monitoring. Furthermore, Roy et al. adopted a scalable approach by incorporating a [Cd(II)-μ-I₄] two-dimensional (2D) metal-organic framework (MOF) bridged by naphthylamine into PVDF composite nanofibers mat [19]. The schematic of electrospinning setup and fabricated nanofiber mat is shown in Figs. 1 II-f and 1II-g respectively. The fabricate device generated high electrical output (open circuit output voltage ~22 V (Fig. 1(I-h)), and power density ~24 μW·cm⁻²) under periodic imparting. The composite nanofibers showed a superior electroactive β-phase content of 98% with a high piezoelectric coefficient of 41 pC N⁻¹. However, all these devices are composed of a single layer as the piezoelectric active layer. In order to improve the performance of the PENG layer-by-layer structure was adopted further [20–24]. Several studies have demonstrated that homo-layered structure PENGs can significantly improve output performance. To overcome the limitations of homo-layered PENGs, a hetero-structured PENG (HT-PENG) was created, showing enhanced voltage and current output. The schematic of the layer-by-layer hetero-structure PENG with interspace metal sheet and cross-sectional SEM images of the as-prepared HT-PENG are shown in Figs. 1(II-a) and 1(II-b), respectively. The open-circuit voltage and short-circuit current of the HT-PENG are shown in Figs. 1(II-c) and 1(II-d), respectively. Furthermore, the open-circuit voltage and short-circuit current of the optimized 6th layer HT-PENG are shown in Figs. 1(II-e) and 1(II-f), respectively. The optimized six-layered HT-PENG achieved an open circuit output voltage of 350 V, a short circuit current of 6 μA, and a power output of 3.62 W·m⁻² [6].

2.2.1 Mechanism of Triboelectric Effects

In addition to PENGs, mechanical energy can be efficiently converted into electrical energy using Triboelectric Nanogenerators (TENGs), which operate in four distinct modes: vertical contact separation (VCS), linear sliding mode (LSM), single-electrode mode (SEM), and free-standing mode (FSM), as depicted in Fig. 2(a) [14,25]. TENGs function based on the principles of the parallel-plate capacitor model and the dependence of the electric field on the distance to optimally match electrical loads [14]. Table 2 presents a detailed overview of the structural characteristics, advantages, disadvantages, and potential applications of each operational mode [14]. Fig. 2(b) demonstrates a TENG operating in the VCS mode. Initially (Fig. 2(b-I)), when the layers of the device are compressed together, triboelectric charges are generated. As these layers are released and separate (Fig. 2(b-II)), a potential difference is created between the charged layers. This difference drives the movement of free charges in the electrodes to balance the potential, thereby generating a current flow when the device is connected to a load resistance (R). Once the surface charge balances, the current flow ceases (Fig. 2(b-III)). Re-compressing the films allows the previously accumulated charges to flow back into the circuit, producing a current flow in the opposite direction.

Fig. 2

Fundamental modes of triboelectric nanogenerators and triboelectric effects for energy harvesting, [14] (Adapted from Ref. 14 with permission). (a) Four fundamental modes of operation of TENGs, (b) triboelectric effect in contact-separation mode for energy harvesting [14] (Adapted from Ref. 14 with permission). (c) Schematic illustration of electrospinning of Nylon 11 followed by PLL surface modification. (d), (e) Output performances (V_oc, and I_sc), [31] (Adapted from Ref. 31 with permission). (f) The microporous hierarchical composite TENG, dielectric permittivity of TPU and its different composite films, and the output voltage of TPU and its composite based TENGs, [32] (Adapted from Ref. 32 with permission, Copyright © 2022, American Chemical Society). (g) The schematic representation of the fabrication process of LC-MXene/ZiF-67, (h) variation of power and power density with load resistance from CM-TENG, [34] (Adapted from Ref. 34 with permission).

Table 2

Comparison of different working modes of TENGs

2.2.2 Triboelectric Materials and Its Progress

TENGs generate static electricity through contact electrification. When two materials come into contact, charges are exchanged between them, resulting in one material becoming positively charged and the other becoming negatively charged [26–28]. Tremendous research work has been carried out in order to improve the electrical performance of TENGs for energy harvesting and sensor applications [25,29,30]. However, there is still room for further improvement in terms of energy conversion efficiency, open circuit output voltage, short circuit current, power density, and sensitivity for self-powered wearable sensors in human-machine interaction. The charge transfer mechanism occurring on the surface of the two tribo materials during physical contact stands as a pivotal factor in optimizing the output performance of a TENG. To increase the surface charge density, many methods have been adopted, including surface modifications (e.g., physical and chemical), material modifications (e.g., dielectric constant and mechanical properties) or surface and bulk modifications, and the layer-by-layer self-assembly approach has also been adopted [29]. For example, Prasad et al. investigated the self-powered triboelectric sensors (TES) through a straightforward poly-L-lysine (PLL) post-surface-modification method. This approach aimed to enhance the positive polarity of Nylon 11 electrospun sheets [31]. The Nylon 11 fabrication and post-surface-modification using PLL is shown in Fig. 2(c). The as fabricated TENG exhibited superior electrical output performance (such as, output voltage~270 V, short circuit current~7.2 μA, and power density~2 W·m⁻²) under 8.5 N imparting force (Figs. 2d and 2e). Ghosh et al. suggested modifying materials by creating a polyurethane polymer composite film infused with ferroelectric barium-titanate-coupled (BTO-coupled) 2D MXene (Ti₃C₂T_x) nanosheets [32]. MXene enhances the dielectric constant, while coupling with the ferroelectricity of BTO reduces dielectric loss, improving the nanogenerator’s output performance. The fabricated device uses quantum-mechanical calculations to convert biomechanical energy into electricity, producing 260 V open-circuit voltage, 160 mA·m⁻² short-circuit current, and 6.65 W·m⁻² power output, ideal for charging various consumer electronics. Fig. 2(f) illustrates the microporous hierarchical composite TENG, the dielectric permittivity of TPU and its various composite films, and the output voltage of TPU and its composite-based TENGs respectively. Interlayer engineering is a promising method for enhancing triboelectric performance, in addition to surface and bulk modifications. Feng et al. found that using a polyimide (PI) layer as a charge storage layer between a PVDF tribo-material and a Cu electrode enhanced the triboelectric performance significantly [33]. The PI layer stored negative charges, leading to a 9-fold improvement in triboelectric output compared to a PVDF monolayer-based TENG. The TENG with a PI transition layer reaches a peak output voltage of 1,010 V and a short-circuit current of 65 μA. Salauddin et al. developed a composite material called LC-MXene/ZiF-67 with a silicone friction layer [34]. The LC-MXene/ZiF-67 intermediate layer has abundant charge trapping sites and a porous structure, leading to improved charge trapping properties. The voltage and current densities of CM-TENG are 13.4 and 14.5 times higher than a single layer. The schematic illustration of the fabrication process of LC-MXene/ZiF-67 is shown in Fig. 2(g). The as prepared CM-TENG shown superior power density of 65W·m⁻² under a matching load impedance of 1.3 MΩ as shown in Fig. 2(h).

2.3.1 Mechanism of Pyroelectric Effects

In addition to environmental mechanical energy harvesters, pyroelectric nanogenerators (PyNGs) convert environmental waste heat into electrical energy through pyroelectric effects [35]. PyNG technology integrated into wearable electronics can efficiently harness wasted heat from the environment. It can also be integrated into masks, which have become essential due to the Covid-19 pandemic, a global health crisis. Numerous research studies have been conducted to refine the design of PyNGs using pyroelectric materials [3,4,35–38]. Pyroelectric materials display inherent spontaneous polarization (P_s) even in the absence of an electric field. The pyroelectric effect is observed when P_s undergoes temporary changes in response to variations in temperature within the material [39]. When the temperature increases, the surface charges bound to the surface decrease due to thermal vibration, leading to a decrease in P_s. This generates an electrical potential across the pyroelectric material if it is under an open circuit condition. In the case of a short circuit condition, electrical current flows through the external circuit. The equation governing the pyroelectric current output is given as Eq. (1).

Here, “i” represents current, “Q” signifies pyroelectric charge, “p” is the pyroelectric coefficient, “A” is the surface area of pyroelectric materials, and “t” signifies time. Various methods are used to accurately measure the pyroelectric coefficient (p), including static and dynamic techniques [40]. Static methods include charge compensation, hysteresis measurement, and X-ray techniques. Dynamic methods involve temperature ramping and optical techniques. The practical assessment of the pyroelectric coefficient of PyNG can be attained by measuring the output current and relevant physical parameters. This coefficient is estimated using the following Eq. (2):

Fig. 3(a) depicts the energy harvesting system utilizing the pyroelectric effect. The working mechanism of PyNG is detailed in Figs. 3(b)–3(d), based on temperature-induced changes in spontaneous polarization (P_s) [37,41]. Polymers with crystalline structures, where molecular chains align, can exhibit (P_s) due to the alignment of polarized covalent bonds. Similarly, ceramics with ionic bonding can also show P_s due to polarization within their crystal lattice [42]. Ideally, internal dipoles should align in one direction, but wiggling atoms disrupt this. When polarization and wiggling angle (θ) remain constant at a constant temperature, no current flows through the external circuit (Fig. 3(b)). As the temperature rises (dT/dt > 0), thermal energy causes dipole alignment to change, with the dipoles wiggling around their respective pole axes (Fig. 3(c)). Higher temperatures result in larger wiggling angles for diverse polarizations, leading to a decrease in the intensity of P_s. The decrease in induced surface charges on pyroelectric material is a result of the rising temperature, prompting released surface charges to traverse through the external circuit. Upon returning to the initial state, the dipole alignment reverts, inducing reverse current flows in the external circuit. Conversely, a temperature decrease (dT/dt < 0) leads to internal dipoles with reduced angles (θ2 < θ), owing to diminished thermal energy. This, in turn, amplifies the intensity of P_s significantly, as illustrated in Fig. 3(d). The surface charges on pyroelectric material increase when temperature changes, causing them to flow through the external circuit. When the temperature returns to normal, the dipole alignment of the material is restored, leading to a decrease in surface charges and a reverse current flow through the external circuit. This process generates electrical energy through the pyroelectric effect. Fig. 3(e) shows the temperature-dependent P_s change in pyroelectric materials [43].

Fig. 3

(a) Pyroelectric energy harvesting system schematic. Schematic diagrams showing the working principle of the PyNG under different temperature conditions. (b) constant temperature, (c) heated, and (d) cooled conditions [37] (Adapted from Ref. 37 with permission), (e) A graph depicting the temperature dependence of internal spontaneous polarization and pyroelectric coefficient of the material [37] (Adapted from Ref. 37 with permission). (f) Schematic diagram showing the structure of a single PZT microwire PyNG [51] (Adapted from Ref. 51 with permission), (g) The optical image when a self-powered sensor was attached on a metallic body, (h) the optical image of a LCD screen in normal condition, (i) optical image when the sensor is in contact with a hot-plate, (j) image of LCD screen, which is connected to the PyNG in panel h [51] (Adapted from Ref. 51 with permission). (k) Schematic of the experimental setup for characterizing PyNG, (l) schematic showing the multilayered Au/LDH/Al device structure, (m) short circuit output current of ZnAl-LDH, and MgAl-LDH PyNGs with temperature change [59] (Adapted from Ref. 59 with permission)

2.3.2 Pyroelectric Materials and Its Progress

Pyroelectric materials have shown great potential in sensors, energy harvesting devices, and even in medical applications. The ability of pyroelectric materials to convert temperature changes into electrical signals makes them valuable in a wide range of technologies. Researchers continue to explore new ways to enhance the performance and versatility of pyroelectric materials for future applications [35,43,44]. Selection of pyroelectric materials from a wide variety of materials, including both ferroelectric and non-ferroelectric materials, is a significant area of research. All ferroelectric materials exhibit pyroelectric properties, and all pyroelectric materials demonstrate piezoelectric behavior, but the reverse is not necessarily the case [43,45,46]. It can be observed that triglycine sulphide (TGS) is extremely potential materials for pyroelectric energy harvesters. TGS, with the chemical formula (NH₂CH₂COOH)₃H₂SO₄, consists of crystals derived from the glycine group (NH₂CH₂COOH) that are polar and demonstrate exceptionally high pyroelectric figures of merit (FOM) [42,47]. TGS (Triglycine Sulfate) faces limited interest for energy harvesting applications primarily because of its low Curie temperature of 49°C, water solubility, hygroscopic nature, and relatively low mechanical strength [48]. Lead magnesium niobate-lead titanate (PMN-PT) and lead zirconate titanate (PZT) are popular ceramics known for their simple fabrication process and excellent piezoelectric properties. These materials have garnered attention for their potential in pyroelectric energy harvesting applications [49–51]. Yang et al. showcased the initial use of a PyNG as a self-powered sensor to detect temperature variations with a single PZT micro/nanowire. The sensor’s response time and reset time were approximately 0.9 and 3 seconds, respectively [51]. The schematic of a single PZT microwire PyNG is shown in Fig. 3(f). Figs. 3(g)–3(j) show the self-power sensor demonstrations. The sensor is attached to a metallic body as shown in Fig. 3(g) with an LCD screen in Fig. 3(h). When there is no change in temperature in the sensor, the LCD is off, whereas the LCD is on and showing a number on the screen as shown in Fig. 3(j) while the sensor is under heat treatment at a temperature of 473 K as shown in Fig. 3(j). Lead-free ferroelectric materials such as bismuth sodium titanate-barium titanate (BNT-BT) and Mn:BNT-BT offer high pyroelectric coefficients and figure of merit, making them appealing alternatives to lead-based materials [52,53]. Additionally, Ba_0.65Sr_0.35TiO₃ (BST) thin films are commonly used to enhance pyroelectric energy harvesting [54]. Besides these, lithium tantalate (LiTaO₃, LTO), lithium niobate (LiNbO₃, LNO), and potassium sodium niobate-based materials have gained popularity over lead-based pyroelectric materials due to their low dielectric loss [43], high Curie temperature [55], and excellent pyroelectric coefficient [56], respectively. Besides ferroelectric materials, certain non-ferroelectric materials are also crucial in the design of a PyNG. These materials include wurtzite-based compounds like AlN, GaN, CdS, and ZnO [43]. However, their pyroelectric coefficients are lower than those of ferroelectric materials, but they have higher thermal conductivities [43].

The materials mentioned above are usually ceramic-like, resulting in high density, stiffness, and brittleness. If mechanical flexibility and toughness are desired, a polymeric pyroelectric material like polyvinylidene fluoride (PVDF) and its copolymers (e.g. polyvinylidene-difluoride trifluoro-ethane; P(VDFTrFE)) can be used for pyroelectric energy harvesting [3,9,57,58]. Mahanty et al. proposed an all-fiber pyro- and piezo-electric nanogenerator (PPNG) that was demonstrated using MWCNT-doped PVDF composite nanofiber [3]. The PVDF-MWCNT composite nanofiber possesses fifteenth times higher pyroelectric coefficient (~60 nC·m⁻²·K⁻¹) compare to that of pure PVDF nanofibers (~4 nC·m⁻²·K⁻¹). In another study, Sultana et al. proposed a composite electrospun nanofiber made of methylammonium lead iodide (CH₃NH₃PbI₃) and PVDF to harvest thermal energy [57]. The prepared PyNG showed a fast response time of 1.14 s, a reset time of 1.25 s, a voltage of 41.78 mV, and a pyroelectric coefficient of approximately 44 pC ·m⁻²·K⁻¹. Recently, there has been a growing demand for layered double hydroxide (LDH)-based pyroelectric nanogenerators (PyNGs) due to their unique properties, including high gradients of composition and energy levels, size effects, and the ease of fabrication and tuning [59]. Prestopino et al. demonstrated experiments where they fabricated several LDH-based PyNGs. Remarkably, they observed that without the need for poling treatment, the pyroelectric coefficient can range from positive, up to 150 μC·m⁻²·K⁻¹ for PyNGs made of ZnAl-LDH, to negative, down to −160 μC·m⁻²·K⁻¹ for those composed of MgAl-LDH [59].

2.4.1 Mechanism of Magnetoelectric Effects

In addition to mechanical and heat energy converters, magnetoelectric (ME) energy harvesters stand out as a notable energy harvesting technology. These devices convert magnetic energy into electrical energy by leveraging mechanical strain/stress-mediated magnetoelectric (ME) coupling effect. Multiferroic composites, which exhibit both ferromagnetism and ferroelectricity, have attracted considerable interest for their magnetoelectric (ME) couplings, making them promising candidates for energy harvesting applications. The magnetoelectric (ME) effect emerges from the coupling between magnetic and polar sublattices within single-phase materials. It is uncommon to find long-range ordered magnetic moments and electric dipoles coexisting in the same phase, leading to the discovery of only a few single-phase compounds exhibiting the ME effect. Obtaining significant magnetoelectric (ME) coupling in single-phase compounds at room temperature has proven to be challenging due to the concurrent transitions from ferroelectric to paraelectric states and from ferro/ferri/antiferromagnetic to paramagnetic states [60–62]. However, these constraints can be surpassed through the integration of ferroelectric and ferromagnetic materials within ME composites, capitalizing on the remarkable properties inherent in these phases. Research has shown that multiphase ME composites can achieve a significantly improved ME response [63] compared to single-phase ME materials [64] at ambient temperature. Layered ME composites, which combine piezoelectric and magnetostrictive materials in an elastic structure, have been extensively researched due to their simple manufacturing process and adaptable design. Various factors such as strain, spin, or charge carrier exchange among the constituent phases are responsible for the interplay of magneto-electricity within the composite materials. While the mechanism of strain-mediated coupling is comprehensively understood, the exploration of the other two mechanisms remains ongoing [10,65,66]. In composites, strain-mediated coupling occurs due to the elastic interaction between piezoelectric and magnetostrictive components, as shown in Fig. 4(a) [63,67]. The detailed working mechanism is illustrated below. The ME effect encompasses the process wherein the imposition of a magnetic field induces mechanical strain within the magnetic layer, a phenomenon referred to as magnetostriction. Subsequently, this strain propagates to the piezoelectric layer, obtaining an electric displacement via the piezoelectric effect, as illustrated in Fig. 4(a). The ME response is quantified using the ME voltage coefficient αME=δEacδHac, which indicates the ratio of induced E_ac to applied H_ac (expressed in V·cm⁻¹·Oe⁻¹). This coefficient measures the efficiency of energy conversion between the applied H_ac and induced E_ac under a bias field H_dc. Fig. 4(b) illustrates the magneto-elasto-electric coupling process in the (1-1) composite, along with its corresponding longitudinal magnetization and transverse polarization (L-T) mode ME-coupling equivalent-circuit expression [63].

Fig. 4

(a) Schematic representation of the direct magnetoelectric effect in a 2–2 configuration ferroelectric (FE)/ferromagnetic (FM) composite system (the numbers 2–2 refer to the connectivity of the phases in free space). Here the applied magnetic field (H_ac/H_dc) generates strain (ɛ) in the magnetic layer via the magnetostriction effect and this strain is transferred to the piezoelectric layer resulting in electric displacement or dielectric polarization (P) through the piezoelectric effect [67] (Adapted from Ref. 67 with permission), (b) magnetoelastoelectric coupling process and its L-T mode equivalent-circuit expression in which the magnetostriction and piezoelectric effects in the magnetic and piezoelectric phases, and elastic strain coupling between the two phases are clearly illustrated, [63] (Adapted from Ref. 63 with permission). (c) Schematic of the experimental setup, [81] (Adapted from Ref. 81 with permission). (d) Schematic and digital photograph of the as-fabricated device along with the harvesting process (pink circles indicate the positions of the MENG device). (e) Harvested voltage output from the household device. (f) Cole−Cole plot of the impedance. Upper inset: Frequency-dependent impedance plot of the PS10-based nanogenerator. Lower inset: RT dc conductivity plots of all film samples as a function of the volume percentage of the filler NPs (the dotted red line corresponds the onset of the percolation threshold) [81] (Adapted from Ref. 81 with permission). (g) Schematic representation of the magneto-mechano-electric (MME) mechanism of TrFE/NFO 0–3 nanocomposites to harvest stray magnetic field from an electric power cable [10] (Adapted from Ref. 10 with permission). (h) (i) Schematic of the device architecture and (ii) magnetic field energy harvesting using MMENG, (iii) digital photograph of the original device (with the TrFE/NFO1 composite), which is (iv) flexible and (v) rollable; (i) harvested output voltages from TrFE/NFO-based MMENGs; (j) voltage output from the flat TrFE/NFO2-based MMENG at several distances from the power cable of electric kettle; (k) peak output voltage and evaluated power density as a function of load resistance [10] (Adapted from Ref. 10 with permission)

2.4.2 Magnetoelectric Materials and Its Progress

In the past decade, there has been significant research on magnetoelectric (ME) composites [9,68]. The ME effect involves converting magnetic energy into electric energy and vice versa. This transducer responds to changes in electric polarization due to a magnetic field (direct ME effect) or changes in magnetization due to an electric field (converse ME effect). These transducers consist of magnetostrictive and piezoelectric layers, which can function as sensors, actuators, or energy harvesters. These devices efficiently transform AC magnetic fields into vibration and electric charge, rendering them ideal for harvesting energy from diverse sources. This includes but is not limited to mobile base stations, Wi-Fi routers, satellite communications, radio and TV transmitters, and power distribution lines [69]. According to Ampere’s circuital law, current-carrying conductors connected to house hold appliances can generate a low-amplitude magnetic field with a low frequency. The ME effect has indeed been observed in single-phase materials like Cr₂O₃ and BiFeO₃, though typically it is rather weak at room temperature [70]. Van Suchtelen [71] demonstrated that piezoelectric-piezomagnetic composites could manifest a more dominant ME effect. The constitutive relations for these composites are typically expressed using the following equations [72],

where B, and H are the magnetic induction and magnetic field intensity, σ, μ, and β are the elastic compliance, magnetoelastic constant, magnetic permeability, and ME susceptibility, respectively.

Tremendous research has been conducted in the field of magnetoelectric energy scavenging/harvesting technology using a variety of materials and composites [10,73–84]. Wu et al. developed a rotational parameter sensor using a FeNi/PZT magnetostrictive/piezoelectric laminated composite (MPLC) and a multi-pole magnetic ring [85]. Tang et al. developed a self-biased magnetoelectric (ME) charge coupling within a two-phase laminate stack termed Ni/PZT. This stack incorporates Nickel foils and a Pb (Zr,Ti)O₃ (PZT) plate boasting high capacitance [74]. Experimental findings reveal that the Ni/PZT-stack with n = 0.4 showcases a remarkable zero-biased resonant ME charge coefficient (α_Q,r) of 47.52 nC· Oe⁻¹, notably surpassing the values reported for other ME laminates in prior research. Additionally, Ghosh et al. developed an increased magneto-electric (ME) effect at room temperature (RT) and above in a nanocomposite of LaYFe₂O₆/P(VDF-HFP) [81]. At room temperature, this nanocomposite exhibits a first-order ME coupling coefficient of approximately 2.92 mV·cm⁻¹·Oe⁻¹ and a second-order ME coupling coefficient of around 0.051 μV·cm⁻¹·Oe⁻². The experimental setup is illustrated in Fig. 4(c). Fig. 4(d) shows the schematic and digital photograph of the fabricated device, as well as the harvesting process. The harvested output voltage from the household device and the Cole-Cole plot of the impedance are depicted in Figs. 4(e) and 4(f), respectively. Recently, Ghosh et al. reported an enhanced ME effect in spin-photon coupled single-phase La_1-x Sm_xYFe₂O₆ (0 ≤ x ≤ 1) [84]. The composition with x = 0.75 exhibited the most effective polarization and magnetoelectric response. It demonstrated a first-order ME coupling coefficient approximately 31% higher and a second-order ME coupling coefficient approximately 1 order higher than the original x = 0 sample. The spin-reorientation transition significantly impacts the magnetic and magnetoelectric properties, making it a valuable tool for adjusting these properties. With advancements in inorganic materials, polymer-based ME materials have achieved similar levels of ME coefficients, and their usage as energy harvesters is rising owing to their excellent flexibility, lightweight, and biocompatibility [10,86,87]. To date, many research works have been carried out using polymer-based materials to improve magnetoelectric energy harvesting [10,73,77,88–90]. Kwon et al. showed a bilayer composite sample consisting of Metglas and PVDF with epoxy bonding in their study [89]. DC and AC magnetic fields were produced using a pair of Helmholtz coils. The fields were adjusted from 0 to 3 kA·m⁻¹ (~37.7 Oe), and the AC frequency was varied from 0 to 200 Hz. The magnetic field intensity was monitored using a Hall sensor to control the field level. Silva et al. proposed a three-layer (piezoelectric+epoxy+magnetostrictive) (PVDF/epoxy/Vitrovac) ME structure [73]. Experimental results demonstrate that the magnetostrictive effect response improves with the increase of the PVDF thickness. The highest response of 53 V·cm⁻¹·Oe⁻¹ was achieved with a 110 μm thick PVDF/M-Bond epoxy/Vitrovac laminate. This excellent ME response makes it a prominent material for use in sensors, actuators, memories, and energy harvesting devices. Additional research by Martins et al. explored the dispersion of cobalt ferrite (CoFe₂O₄) nanoparticles in a poly (vinylidene fluoride)-trifluoroethylene (P(VDF-TrFE)) matrix [79]. The study investigated the impact of nanoparticle dispersion on the piezoelectric, magnetic, and magnetoelectric characteristics of the nanocomposite. Two distinct dispersion techniques were employed in sample preparation: ultrasound and citric acid nanoparticle surfactation. No significant distinctions were found in the ferroelectric, piezoelectric, magnetic, and magnetoelectric properties between samples fabricated with or without surfactants, thereby streamlining the process for large-scale production. Ghosh et al. introduced a versatile and flexible magneto-mechano-electric nanogenerator (MMENG) that can be rolled up [10]. This study showcased the fabrication of a fully rollable MMENG employing P(VDF-TrFE)/nickel ferrite (NiFe₂O₄/NFO) 0–3-type magnetoelectric nanocomposites. This device is designed for wireless Internet of Things (IoT) sensors to detect and harness environmental magnetic noise even without a direct current magnetic field. The working principles of MMENG with TrFE/NFO 0–3 nanocomposites is illustrated in Fig. 4(g). When subjected to an AC magnetic field (Fig. 4(g-i)), the NFO nanoparticles in the MMENG undergo magnetostriction, causing them to elongate or contract in sync with the magnetic field frequency (Fig. 4(g-ii)). This strain is then transferred to the P (VDF-TrFE) polymer chain through strong interfacial interactions, resulting in stress application (Fig. 4(g-iii)) that generates electric charges due to polarization alignment. Subsequently, the generated voltage is delivered across an external electrical load via the direct piezoelectric effect (mechano-electric coupling). The harvested electrical power can be utilized to power consumer electronics (Fig. 4(g-iv)). Fig. 4(g-i) shows the fabricated MMENG. Fig. 4(h-ii) illustrates the demonstration of magnetic field energy harvesting from the stray magnetic field surrounding the power cable of an electric kettle (1 kW, 50/60 Hz). Additionally, Fig. 4(h-iii) presents a digital photograph of the MMENG. The MMENG is known for its flexibility, as depicted in Fig. 4(h-iv), and rollability, as showcased in Fig. 4(h-v). Fig. 4(i) illustrates the MMENG producing an AC voltage output with a sinusoidal waveform. The MMENG using TrFE/NFO1 yielded a peak-to-peak output voltage (V_pp) of approximately 640 mV, while TrFE/NFO2 produced a V_pp of around 1.4 V. Increasing the distance between the MMENG and power cable from 0.5 to 2 mm resulted in a decrease in voltage output from approximately 1.4 to 0.4 V as the AC magnetic field strength decreased from 1.7 to 0.4 mT (Fig. 4(j)). The power output of the MMENG was assessed by capturing peak output voltages across various external load resistances (R_L), ranging from 0.25 MΩ to 1 TΩ, during the boiling of water in an electric kettle (Fig. 4(k)). The peak power density reached 0.05 μW·cm⁻³, across R_L~100MΩ.