Nanogenerators for energy harvesting: a holistic review of mechanisms, materials, and emerging applications

1.1 Methodology

A systematic literature analysis was done to ensure comprehensive coverage of recent advances in nanogenerators. Scientific databases used to collect relevant studies in the field include Scopus and Web of Science. Studies published between 2020 and 2025 were considered to include the latest material innovations, fabrication techniques and applications. Keywords used for searching included nanogenerator, piezoelectric, triboelectric, hybrid nanogenerator, self-powered systems, wearable, and energy harvesting. Boolean combinations were used to broaden the search and capture emerging interdisciplinary work. Only peer-reviewed articles that presented original research, theoretical approaches, or reviews on nanogenerators were considered. Papers focusing only on macroscale harvesters, non-energy-related devices or papers lacking sufficient technical detail were excluded. The data collected was then synthesized to identify key advancements, common challenges and emerging directions across various nanogenerator types.

2.1 Piezoelectric nanogenerators

Piezoelectric nanogenerators (PENGs) use a mechanical structure to transfer motion or vibrations to a piezoelectric material, and the deformation of the material generates an electric charge. Figure 2 illustrates the working principle of a PENG. The mechanical deformation leads to polarization changes and charge separation and generates electrical output during the pressing and releasing cycles. Depending on the application, these structures are commonly designed as cantilevers, beams, or membranes [25], [26], [27]. Their primary function is to transform kinetic energy into mechanical strain within the piezoelectric layer. The resulting charge is collected by the thin electrodes on the surface. Commonly used electrode materials are platinum [28], silver [29], aluminum [30], and copper [31]. The device is enclosed in a protective casing. To enhance insulation and structural stability, a polymer layer such as PET is often used. The generated electrical output can either be used directly or passed through conditioning circuits for further processing. This efficient setup enables mechanical energy to be used to power various electronic devices and sensors [5].

Figure 2:

Working mechanism of a piezoelectric nanogenerator under mechanical deformation. (a) Initial state with randomly oriented dipoles, (b) aligned dipoles after mechanical poling, (c) compression generates positive and negative charges on opposite surfaces, (d) release leads to charge redistribution, (e) full cycle of deformation and relaxation results in alternating current generation (adapted from Ref [32] copyrights© 2022, The Royal Society of Chemistry).

PENGs are solid-state devices with low maintenance requirements and long operational lifetimes. They can produce varying voltage levels, but the current output is usually low. These devices are suitable for both micro-scale applications, like MEMS, and larger energy harvesting systems. The energy conversion performance depends on the frequency of the mechanical vibrations [5]. The piezoelectric materials used in a PENG determine the efficiency and durability of the device. These materials are generally classified into four main types: ceramics, single crystals, polymers, and organic-inorganic hybrid composites [33]. A wide range of substances, including paper and hydrophobic materials, has been utilized in various piezoelectric applications. PENG performance can be enhanced by using materials with higher d₃₃​ values, along with optimizing fabrication techniques. Common methods for developing PENGs are hydrothermal synthesis, electrospinning, solution casting, dip coating, chemical vapor deposition (CVD) and sol-gel processes [34], [35], [36], [37], [38]. Piezoelectric materials can deteriorate over time or under harsh environmental conditions.

Beyond the structural operation shown in Figure 2, the working principle of a PENG is governed by the electromechanical coupling of piezoelectric materials. When mechanical stress σ is applied, the crystal lattice undergoes non-centrosymmetric distortion, producing a change in electric displacement D according to D = d₃₃σ+ε ^T E, where d₃₃ is the piezoelectric charge coefficient and ε ^T is the dielectric permittivity under constant stress. The generated charge flows to the electrodes to maintain electrostatic equilibrium, producing an open-circuit voltage V _OC ≈ g₃₃σt, where g₃₃ = d₃₃/(ε) is the piezoelectric voltage coefficient and the material thickness. At the atomic scale, this process results from the displacement of positive and negative charge centers in the unit cell and the reorientation of switchable ferroelectric domains. Since the induced current is proportional to the rate of strain I ∝ d 33   d ε dt . PENGs generate larger currents at higher vibration frequencies but saturate under quasi-static loads. Their equivalent electrical model is a voltage source connected in series with a capacitor, which is why they have high internal impedance and generate high voltage but low current [39]. These considerations are key to understanding material selection, device geometry and frequency-matched design for optimal PENG performance.

The concept of PENGs was first introduced by Wang in 2006, using zinc oxide (ZnO) nanowires to convert mechanical motion into electrical output [40]. Since then, PENG efficiency has improved significantly. This progress is mainly due to advances in piezoelectric materials, fabrication techniques and device design [41]. Table 2 provides a comparative overview of recent PENG materials, their synthesis methods, device fabrication, characterization methods, and performance metrics. Some PENGs can generate voltages higher than 100 V but have limited use due to low power output. For instance, PMMA/PDMS/organic nanowires (ONW)–Au–ZnO have demonstrated outputs of 170 mV [49], and PDMS/P(VDF-TrFE) achieved 150 V with a power density of 8.75 μW/cm² [48]. Xu et al. [59] demonstrated self-powered nanowire devices using integrated ZnO nanowire arrays. This produced a peak voltage of 1.26 V at 0.19 % strain through lateral integration and a power density of 2.7 mW/cm³ via vertical stacking, sufficient to operate nanowire-based pH and UV sensors.

Lead zirconate titanate (PZT) thin films are widely used in PENGs, but their fabrication is complex, and they contain high levels of lead (Pb), which is a toxic element. This toxicity limits their use in flexible, wearable and implantable medical technologies. Recent research has shifted toward developing environmentally friendly, cost-effective and lead-free alternatives, such as polyvinylidene fluoride (PVDF), zinc oxide (ZnO), barium titanate (BaTiO₃), aluminum nitride (AlN), and complex perovskite ceramics like KNN-NTK [60], 61]. Table 2 also shows a strong trend towards lead-free alternatives. For example, PVDF/cellulose acetate (CA) composites have shown output voltages of 7.5 V [31], and ZnO nanowires produced up to 80 nJ of energy [42]. Similarly, PDMS-BaTiO₃ achieved 13 V [43] and silicone rubber/samarium/titanium doped BiFeO₃ (BFO) composites achieved 16 V [45]. PVDF and BaTiO₃ have shown strong potential for integration into electronic systems. Bio-flexible piezoelectric nanogenerators (BF-PENGs) have exhibited excellent performance, particularly in the development of self-powered medical devices. BF-PENGs like PLA achieved 14.4 V [46]. Flexible biocompatible composites such as KNN-PDMS have demonstrated a V _OC = 14.19 V [53]. Hydrogel-based PENGs have recently gained attention for their exceptional stretchability and flexibility [62], combining strong piezoelectric performance with conductive hydrogel structures. Natural polymer hydrogels are well-suited to green electronics due to their biocompatibility, elasticity, biodegradability, and non-toxic nature, making them ideal for sustainable energy harvesting applications.

Doping with rare-earth (RE) ions can enhance piezoelectric properties by reducing screening effects. The rare-earth group includes 17 elements, 15 lanthanides plus scandium and yttrium. Among these, samarium (Sm) and europium (Eu) dopants have demonstrated the highest piezoelectric coefficients [63]. Table 2 also illustrates the use of various advanced materials and dopants, such as P(VDF-TrFE)/AlN/ZnO composite films with V _OC = 23 V [58] and PVDF/MoS₂ @ZnO composites with V _OC = 6.22 V [56]. RE doping can also enhance thermal and electric field stability and provide photoluminescent properties. Integrated synthesis and characterization approaches are advancing high-performance piezoelectric materials.

PENG performance depends on dipole alignment, domain mobility, and electromechanical coupling. The piezoelectric coefficient (d₃₃) can be improved through domain engineering in ceramics (e.g., PZT, BaTiO₃, KNN) by controlling domain orientation via poling, grain texturing, or epitaxial growth to maximize non-180° domain wall motion [64]. In polymer-based PENGs, such as PVDF and its copolymers, performance can be enhanced by inducing the β-phase through mechanical stretching, electrical poling, or the use of conductive fillers. Crystalline porous materials, including MOFs and COFs, have emerged as nanogenerator fillers due to their high surface area, tunable porosity, and chemical stability, leading to improved interfacial interactions and performance [65]. Dopant engineering (e.g., Mn²⁺, La³⁺ in PZT; Nb⁵⁺ in KNN) and artificial piezoelectric metamaterial design further improve d₃₃ and g₃₃ tunability beyond intrinsic limits [66]. Fatigue behavior is linked to polarization degradation under cyclic stress, which remains critical for reliability. Ceramics experience cracking and domain pinning, while polymers face dipole relaxation. Flexible composite structures, strain buffering interfaces, and self-healing polymers have recently improved endurance, highlighting a shift toward dynamic domain and interface engineering for durable, high-performance PENGs [67].

Ceramic systems such as BaTiO₃, PMN, PT, and PZT-based devices have relatively higher power densities due to their strong electromechanical coupling. But they are brittle and have limited flexibility. Polymer-based systems, such as PVDF composites and PDMS, generate more modest energy output. They are suitable for wearable applications due to better flexibility and mechanical stability. Hybrid and nanostructured architectures such as MoS₂@ZnO, PVDF-LiNbO₃, and GaN/ZnO nanowires demonstrate how interfaces and filler induced polarization can improve performance by enhancing β-phase content, dielectric constant, and local electric fields. Overall, materials that improve dipole alignment, interfacial polarization, and flexibility exhibit the most balanced and scalable PENG performance, emphasizing the role of composite design and microstructural control.

2.2 Triboelectric nanogenerators

In 2012, Zhong Lin Wang and his team introduced the triboelectric nanogenerator (TENG) [68]. TENGs utilize the triboelectric effect combined with electrostatic induction to convert low-level mechanical energy into electrical power [69], 70]. There are four basic operational modes. In the single-electrode mode, only one electrode is attached to the triboelectric material, enabling a simple and flexible device architecture. This mode is suitable for self-powered sensing in complex or constrained environments where direct electrode pairing is impractical [71]. In contact-separation mode, the repeated contact and separation of two materials generates alternating charge flow. This is ideal for integrating into wearable or vibrating systems [72]. In plane-sliding mode, the lateral movement between two surfaces induces charge transfer. This is commonly used for sensing motion such as breathing or environmental waves [73]. The freestanding mode involves a dielectric layer moving independently between two electrode generating current from potential differences. This is used for applications in transport systems, environmental monitoring, and smart farming [74]. Figure 3 illustrates the four basic operational modes of TENGs.

Figure 3:

Working principles of four fundamental triboelectric nanogenerator (TENG) modes: (a) vertical contact-separation mode, (b) lateral-sliding mode, (c) single-electrode mode, and (d) freestanding triboelectric-layer mode (adapted from Ref [77] copyrights© 2018, WILEY-VCH Verlag GmbH & co. KGaA).

Electrostatic and triboelectric generators are compact, lightweight, produce high voltages and are used for portable and confined spaces. They are useful in dynamic environments involving irregular or fluctuating mechanical motion [75]. Due to lower power density than electromagnetic generators, they have limited use in high-power applications and require voltage conversion [76]. TENGs are affected by environmental conditions like humidity, temperature, and air pressure, which can impact their stability and performance.

TENGs are developed using various synthesis methods, including the sol-gel process, modified Hummer’s method, and a one-pot electrospinning approach [78], [79], [80]. Spin-coating techniques are used to fabricate both the electrode and triboelectric layer. Multiple cleaning processes are applied before assembly to ensure surface purity. Triboelectric materials and electrodes are trimmed to the specific dimensions required for the TENG device after fabrication. The triboelectric layers are then connected to their corresponding electrodes and arranged to face each other with a spacer separating them. Copper wires are finally attached to the electrodes and act as electrical terminals, as shown in Figure 4.

Figure 4:

Schematic structure of a typical triboelectric nanogenerator (adapted from Ref [81] copyrights© 2020, Elsevier Ltd.).

Beyond the four basic TENG modes, electricity generation is through contact electrification and electrostatic induction. When two materials with different electron affinities come into contact, electrons transfer across the interface, creating an interfacial charge density (σ _s) whose magnitude depends on the triboelectric potential, surface chemistry, and micro/nanostructures. After separation, these charges cannot recombine due to electrical insulation, creating a potential difference that drives free electrons through the external circuit to balance the electrostatic field. The open-circuit voltage can be approximated as V _OC ≈ σ _s d/ε ₀, where dis the separation distance, revealing why voltage increases with spacing. The short-circuit current is determined by the rate of change of capacitance, I = d Q d t = σ s d A d t , or for sliding structures, by the rate of relative displacement between electrodes. TENGs thus behave as current sources at high frequency (where d C d t is large) but as voltage sources at high separation, explaining their characteristic high voltage–low current outputs [82].

TENGs face scalability and long-term stability challenges [83]. Humidity is a major challenge, as absorbed moisture forms conductive layers that increase charge leakage and reduce output stability. Dielectric breakdown can occur under repetitive contact or high voltage operation, especially in thin film architectures, leading to charge loss and material degradation. Surface recombination and environmental neutralization cause charge decay, reducing the durability of triboelectric charges. In oxide-based TENGs, surface states and ambient adsorption significantly limit charge retention and transfer efficiency [84]. Hydrophobic surface treatments, multilayer dielectric stacking, and elastomeric encapsulation are used to improve charge stability under humid conditions. Such material and structural optimizations are critical for achieving reliable large-area and wearable TENG devices.

TENGs often face challenges in energy use and storage because unpredictable mechanical inputs lead to inconsistent output amplitude and frequency [85], 86]. Their high internal impedance can lead to poor compatibility with electronic devices or storage systems, reducing overall efficiency. Conventional energy management approaches are insufficient, making improved energy management circuits essential. Among the four main application areas of TENGs, self-powered sensors are crucial for advancing microelectronic networks and expanding battery-free technologies [87]. Blue energy, particularly marine energy harvesting using triboelectric effects in liquid environments, shows strong potential for sustainable development [88]. TENGs harvest low-frequency mechanical energy in micro-nano systems to enable self-powered sensing and biosystems. [89]. They are also used in devices that require high-voltage input [77].

Four main methods are used to enhance TENG performance. Energy management uses circuit designs that are easy to implement and integrate, but increase system complexity and size. Structural optimization allows designs to be tailored for specific applications or durability, while increasing complexity. Surface microstructuring offers a compact method of improvement but requires specialized fabrication tools, raising production costs. Material modification is another important approach used to improve friction layer properties and increase output, but its complexity must be evaluated to ensure real performance improvements [86]. Numerous studies have explored TENG materials, their synthesis and production methods to optimize performance. For instance, MXene, a 2D material, has been utilized through MILD etching and spray coating to achieve high V _OC ranging from ∼500 V to ∼650 V with an instantaneous peak power of 0.5–0.65 mW [90]. Researchers have also used cellulose nanofiber combined with Ag nanolayers, prepared via CNF homogenization and AgNW solution preparation, then processed through vacuum filtration and hot pressing/annealing, yielding V _OC = 21 V and I _SC = 2.5 µA [91].

Flexible composites and polymers are also widely studied. For example, PTFE/Ecoflex elastomers are formed by PTFE particle embedding and Ecoflex casting. This is followed by screen coating and thermal curing, and produces V _OC = 1.33 V and I _SC = 60.29 nA [92]. In another example, Nylon polymer nanofibers are synthesized via electrospinning and layer-by-layer deposition and achieve V _OC = 200 V and I _SC = 30 µA [93]. Natural polymers like chitosan have been transformed into TENGs through dissolving, film drying, and neutralization, yielding V _OC ∼77 V and I _SC ∼13 µA [94]. Similarly, a graphene oxide/carboxymethyl cellulose nanocomposite, derived from a modified Hummers’ method and sonication/blending, followed by casting and drying, produced V _OC = 97 V and I _SC = 1.2 µA [95].

Other innovations include laser Kapton-PDMS polymer-polymer layered structures [96], Polyurethane (PU) with Al electrodes via layer-by-layer casting and UV curing (V _OC ∼500 V and I _SC ∼2 µA) [97], and ITO-coated PET assembled layer-by-layer (V _OC = 224.1 V and I _SC = 79.9 µA) [98]. Furthermore, complex hybrid materials such as ZnO-AgNWs/PDMS (synthesized by electrospinning and hydrothermal synthesis, and processed via drop coating) [99], MoS₂/PVDF-HFP prepared by hydrothermal synthesis and electrospinning [100], and paper-based TENGs utilizing oxidation, cationic modification, freeze drying, and compression molding [16] have shown significant output performance, demonstrating the vast potential of material modification in enhancing TENG capabilities. Table 3 provides various TENG materials explored in recent studies, synthesis approaches, device fabrication techniques, characterization methods, and performance metrics.

The studies summarized in Table 3 collectively highlight how material chemistry, surface functionalization, and micro/nanostructuring drive performance improvements in TENGs. Device efficiency is determined by the combined effects of dielectric properties, charge affinity, mechanical flexibility, and interfacial engineering, not just output values. For example, 2D materials such as MXenes use electronegative surface groups, natural polymers rely on molecular dipoles and hydrogen bonding, and hybrid composites like ZnO–AgNWs/PDMS depend on combined piezoelectric and triboelectric effects. These qualitative trends demonstrate that material selection and structural tuning are equally, if not more, important than the absolute voltage or current values reported.

2.3 Pyroelectric nanogenerators

The pyroelectric effect was first identified during studies on how temperature changes influence the polarization intensity of materials. Research into pyroelectric materials and their practical applications has become a key area of focus with advancements in laser and infrared technologies [104]. This effect arises when temperature fluctuations over time lead to variations in a material’s surface charge density. In piezoelectric materials, the effect stems from intrinsic lattice asymmetry, but in pyroelectric materials, it requires self-polarization to generate an electric field within their crystal structure [105]. As the external temperature changes, the positions of electric dipole centers shift within the material. This results in a redistribution of bound surface charges. Pyroelectric materials can be categorized into single crystals such as LiNbO₃ and ZnO, ceramic metal oxides such as PZT, organic polymers such as PVDF, and various composites [106]. For example, ZnO synthesized via hydrothermal methods and seed-assisted growth produced a V _OC ≈ 5.8 mV [107]. In another example, KNbO₃ nanowires prepared through hydrothermal and spin coating techniques, followed by electric poling, achieved I _SC = 120 pA and V _OC = 10 mV [108].

Pyroelectric materials such as barium titanate and tourmaline exhibit spontaneous polarization along their polar axis due to aligned dipole moments, resulting in surface charge generation under temperature variation [109], 110]. These materials exhibit spontaneous polarization due to the alignment of dipole moments in the direction normal to the surface. This polarization, defined as the net dipole moment per unit volume at room temperature without any external electric field, enables the material to attract charged particles like electrons or ions. When placed between two conductive plates of a capacitor (as shown in Figure 5), the material induces charge accumulation on the electrodes until its surface charge is balanced. It will be discharged when this charged capacitor is connected to an external circuit. Current stops flowing when the system stabilizes and the temperature remains unchanged. In pyroelectric materials, a rise in temperature leads to a reduction in the net dipole moment and a decrease in spontaneous polarization; a drop in temperature has the opposite effect. As illustrated in Figure 5(c–d), temperature variations modify the amount of bound charge on the material. This change induces a movement of free charges to balance the altered bound charge and generates current in the external circuit known as pyroelectric current.

Figure 5:

Schematic illustration of the working mechanism of a pyroelectric nanogenerator (PENG). (a) Initial state with aligned dipoles and spontaneous polarization (Ps); (b) static condition with no temperature change (Dt/Dt = 0), resulting in no current flow; (c) increasing temperature (Dt/Dt > 0) causes reorientation of dipoles and outward current generation; (d) Decreasing temperature (Dt/Dt < 0) reverses dipole orientation and induces inward current flow. The cyclic variation in temperature leads to alternating current output due to the pyroelectric effect (adapted from Ref [111] copyright© 2018, MDPI).

Lead-based ceramic PZT and BaTiO₃-doped PMNT composites are widely studied due to their strong pyroelectric coefficients [11]. Ferroelectric oxide thin films like BiFeO₃ are studied for their high d₃₃ values [112]. Organic and conductive polymer materials such as PVDF-TrFE are also key areas of research, offering flexibility and demonstrating promising power and energy densities [113].

The operation of a pyroelectric nanogenerator (PyNG) is governed by the temperature-dependent behavior of spontaneous polarization in ferroelectric materials. Unlike piezoelectricity, which responds to mechanical stress, pyroelectricity arises exclusively from the time derivative of temperature. The governing constitutive relation is I = p A d T d t , where p is the pyroelectric coefficient, A is the electrode area, and d T d t is the rate of temperature change. A temperature increase reduces the magnitude of spontaneous polarization P _s , while cooling enhances it, producing a displacement current as free charges flow to screen the changing bound charge. The open-circuit voltage can be approximated as V O C ≈ p t ε d T d t , where t is the film thickness and ε is dielectric permittivity, revealing why thinner materials produce higher output. Because pyroelectricity depends on d T d t rather than absolute temperature, PyNGs act as current sources driven by thermal transients, not static thermal gradients [114]. Their equivalent electrical behavior resembles a current source in parallel with a capacitor and resistor, leading to strong frequency dependence: faster and periodic thermal fluctuations yield higher output, while steady temperatures produce no current. At the atomic scale, the effect is governed by thermally induced reorientation of off-center ions (e.g., Ti in BaTiO₃, Nb in KNbO₃), which modify the dipole moment of the unit cell. These principles explain why pyroelectric devices are most effective in environments with cyclic heating, such as IR radiation, pulsating heat sources, or periodically modulated thermal fields, and why thermal diffusivity, heat capacity, and interface engineering are critical for maximizing PyNG performance. Table 4 summarizes various PyNG materials with their synthesis, device fabrication, characterization methods, and performance metrics. Overall, polymer-based composites currently show the highest pyroelectric outputs due to interfacial polarization effects, while oxide and nanowire-based PyNGs offer superior thermal stability and intrinsic polarization but lower current densities. This distinction is critical when selecting materials for wearable, flexible, or high-temperature applications.

2.4 Thermoelectric nanogenerators

The thermoelectric effect is a phenomena where charge carriers, electrons or holes, move from a region of higher temperature to a region of lower temperature under the influence of a thermal gradient and thus generates electric current or voltage. This converts heat into electrical energy and vice versa. The three primary effects involved in this conversion are the Seebeck effect, Peltier effect, and Thomson effect [115], [116], [117]. The Seebeck effect (Figure 6(a)), discovered by Thomas Johann Seebeck in 1821, is the direct conversion of a temperature difference into an electric voltage. When a temperature gradient exists across dissimilar conductors or semiconductors, the charge carriers redistribute and induce a thermoelectric potential. The strength of this behavior is characterized by the Seebeck coefficient (S). It is defined as the voltage generated per unit temperature difference [118], 119].

Figure 6:

Schematic of thermoelectric effects (a) Seebeck effect: voltage from temperature gradient (b) Peltier effect: heat transfer via current at junctions (c) Thomson effect: heat absorption/emission in a single material with current and temperature gradient (adapted from Ref [120] copyrights© 2024, Elsevier B.V.).

The Peltier effect (Figure 6(b)), reported by Jean Charles Athanase Peltier in 1834, is the reverse of the Seebeck effect. When an electric current passes through a circuit of two different conductors, the heat is absorbed or released at the junction, resulting in heating or cooling of that region. This occurs because charge carriers carry thermal energy, which is exchanged as they move between materials with different energy levels. The Peltier coefficient (π) quantifies this heat transfer per unit of electric current [121], 122]. The Thomson effect (Figure 6(c)), discovered by William Thomson, is observed when a single conductor with a temperature gradient carries an electric current. As charge carriers move through this gradient, they absorb or release heat depending on the direction of both the current and the temperature gradient. This is given by the Thomson coefficient (τ) and is usually smaller in amplitude [123], 124].

In terms of materials, studies include 2D materials like MXene composites, synthesized via chemical etching and produced through vacuum filtration for low-power applications [125]. Another example is the carbon nanotubes that are developed using electrostatic spraying and fiber-based TEG assembly [126]. Materials like Bi₂Te₃, an N-type semiconductor, and Silicon Nanowires are also studied for their high ZT values, which is a measure of thermoelectric efficiency. They are fabricated mainly through solvothermal synthesis and metal-assisted chemical etching, respectively [127], 128]. PEDOT:PSS, a conductive polymer, and SnSe/PEDOT:PSS composite film, often produced via wet-spinning or ultrasonication dispersion, are explored for their high electrical conductivity and potential in thermoelectric devices [129], 130]. Tellurium nanowires have also been integrated into thermoelectric hydrogel composites through hydrothermal synthesis for flexible applications [131].

At the material level, the operation of a thermoelectric nanogenerator is driven by the diffusion of charge carriers along a temperature gradient. When one side of a thermoelectric material is heated, electrons or holes gain higher kinetic energy and drift toward the cooler side, creating a built-in electric field and a measurable thermovoltage. The magnitude of this response is defined by the Seebeck coefficient, which is strongly influenced by carrier concentration, band structure, and chemical composition. Heat and charge transport are tightly coupled, meaning that electrical conductivity and thermal conductivity compete, i.e., materials with high electrical conductivity but low lattice thermal conductivity (e.g., Bi₂Te₃, SnSe, PEDOT:PSS composites) are preferred for high thermoelectric performance [132], 133]. At the nanoscale, phonon scattering at grain boundaries, defects, and interfaces reduces heat conduction while preserving charge transport, which is why nanostructuring is a widely used strategy to enhance efficiency. In practical terms, TEGs behave like steady DC sources with low voltage and current output, so their performance is highly dependent on achieving sufficient temperature gradients and optimizing material architecture for efficient heat flow. Table 5 provides various thermoelectric nanogenerator materials, synthesis, device fabrication techniques, characterization methods, and performance metrics.

2.5 Electromagnetic generators

The electromagnetic generators (EMGs) operate on the principle of electromagnetic induction, first discovered by Michael Faraday [135]. Electromagnetic induction occurs when a change in magnetic flux within a closed circuit induces an electromotive force (EMF), leading to the flow of electric current. Different EMG designs have been developed by changing how magnets and coils move relative to each other to improve energy harvesting under different types of mechanical motion (Figure 7). This relationship is mathematically described by the equation:

where E is the induced EMF, ΔΦ is the change in magnetic flux over time, and n is the number of turns in the coil. To amplify the induced voltage, coils with multiple turns are commonly used.

Figure 7:

Schematic illustrations of common EMG configurations: (i) perpendicular relative motion of a magnet over a coil, (ii) parallel relative motion, (iii) vertical motion of a magnet inside a coil, and (iv) horizontal movement of a coil between a pair of magnets (adapted from Ref [136] copyrights© 2020, Elsevier Ltd.).

Stable and continuous output of EMG makes them suitable for large-scale power generation. They are highly efficient in converting mechanical energy into electricity, resulting in minimal energy losses. It has a large size and weight due to magnetic materials and coil windings, making it unsuitable for compact or lightweight applications like portable electronics or wearables. Their manufacturing is also complex due to these components [137], 138]. EMGs use copper coils and strong permanent magnets to produce electricity. Neodymium-iron-boron (NdFeB) magnets are commonly used because they create strong magnetic fields and improve efficiency [139]. Miniaturized EMGs have been made using MEMS-compatible methods, where planar coils are fabricated on silicon substrates together with integrated magnetic components [140].

An EMG works based on Faraday’s law. When mechanical motion changes the magnetic field around a coil, it induces an electric voltage. At the microscopic scale, this process arises from the Lorentz force acting on free electrons in the conductor. As the coil or magnet moves, charge carriers experience a force q(v × B) that drives them along the wire and generates current. The induced voltage scales with the rate of flux change (dΦ/dt), meaning that EMGs naturally favor high-frequency, large-amplitude motions. Their equivalent circuit resembles a low-impedance AC power source, in contrast to the high-impedance behavior of TENGs and PENGs. However, when miniaturized, the magnetic flux linkage drops sharply because magnetic field strength and effective area decrease, causing substantial losses in output [141]. Due to this limitation, hybrid systems are used, where EMGs harvest energy from high-frequency motion. Piezoelectric or triboelectric parts are added to collect energy from low-frequency movements.

Recent work has led to flexible EMGs that use soft structures and elastomer films. These designs allow the devices to bend and stretch easily. As a result, they can harvest energy efficiently from low-frequency motion in wearable applications [142]. Traditional EMGs work very efficiently at large sizes, but their performance drops when they are made smaller. This is because the magnetic flux becomes weaker, and bulky magnets are difficult to integrate into miniaturized devices. These constraints have motivated significant research interest in hybrid nanogenerator systems that combine electromagnetic induction with other mechanisms. For instance, several studies have demonstrated hybrid EMG-PENG configurations where electromagnetic coils are integrated with PVDF or ZnO nanowire arrays. This enables simultaneous harvesting of low-frequency vibrations through piezoelectric effects and high-frequency motions through electromagnetic induction [143]. EMG-TENG hybrid systems have also been developed using rotating magnetic parts that activate both triboelectric and electromagnetic generation at the same time. This allows the device to harvest energy over a wider range of motion and frequencies [22]. These hybrid approaches help overcome the main limitations of traditional EMGs. They allow different energy harvesting methods to work together. This opens new possibilities for collecting energy from multiple types of motion and environments.

2.6 Photovoltaic nanogenerators

Solar cells (SCs) convert sunlight into electricity using the photovoltaic effect. This process occurs in a semiconductor material featuring two oppositely doped regions separated by a depletion layer. When solar radiation is absorbed, it generates electron-hole pairs within the semiconductor [144]. These charge carriers are driven apart by the internal electric field at the junction, and when the two regions are connected through an external circuit, the electrons flow, producing an electric current. As illustrated in Figure 8, a photovoltaic cell consists of N-type and P-type semiconductor layers that generate electric current when exposed to sunlight through the photovoltaic effect.

Figure 8:

Basic structure of a photovoltaic cell showing N-type and P-type layers generating electricity from sunlight via the photovoltaic effect (adapted from Ref [136] copyrights© 2020, Elsevier Ltd.).

Among the various types of SCs available, those most used in photovoltaic-nanogenerator hybrid cells (PV–NG HCs) include silicon-based cells, dye-sensitized solar cells (DSSCs), quantum dot-sensitized solar cells, and polymer-based bulk heterojunction cells [145]. The first reported PV–NG hybrid device integrated a ZnO nanowire–based PENG with a DSSC constructed on aligned ZnO nanowire arrays, enabling simultaneous harvesting of mechanical and solar energy within a shared nanowire architecture [146]. Since this pioneering work, advancements in solar cell technologies have led to the development of PV–NG HCs incorporating a wider range of novel SC types, as documented in subsequent research.

The operation of a photovoltaic cell is governed by photogeneration and carrier transport processes within the semiconductor. When incident photons with energy greater than the bandgap are absorbed, they create electron–hole pairs, which are immediately separated by the built-in electric field of the p–n junction or heterojunction interface. The resulting photocurrent density (Jph) depends on the absorption coefficient, minority carrier diffusion length, and recombination dynamics within the depletion region. Materials with high carrier mobility and long diffusion lengths, such as crystalline silicone, perovskites, or organic bulk heterojunctions, facilitate efficient charge extraction. However, carrier recombination at grain boundaries, defects, or interfaces reduces V _OC and fill factor (FF), making interface engineering and surface passivation essential [147], 148].

Recent studies show that different materials are being used to improve the performance of SCs. For example, monocrystalline silicon with blended TiO₂–Al₂O₃ antireflective coatings applied via RF sputtering demonstrated a power conversion efficiency of 20.16 % [149]. For thin-film solar cells, amorphous silicon (a-Si) deposited by plasma-enhanced chemical vapor deposition (PECVD) has achieved efficiencies of 10.22 % in single-junction devices and 12.69 % in tandem cell configurations [150]. Hybrid organic–inorganic perovskite materials such as CH₃NH₃PbI₃, typically fabricated using solution-based methods like spin, blade or slot-die coating, have shown high efficiencies. Rigid perovskite solar cells have reached 19.91 %, while flexible devices have achieved 17.4 % [151]. DSSCs, based on solution-based dye adsorption, have achieved an efficiency of 14.96 % [152]. Nanostructured oxides such as hydrothermally synthesized ZnO nanorods have been used in DSSCs, achieving efficiencies of around 2.08 % [19]. Graphene-TiO₂ quantum dot composites, produced using electrochemical growth and applied in perovskite solar cells, improve charge separation and help achieve efficiencies of about 15.1 % [18].

PV-NG hybrid cells are generally classified into rigid and flexible types, depending on their application. Rigid HCs are primarily designed for large-scale and stationary applications like traditional silicon-based solar panel systems. In these systems, the solar cell functions as the main energy source, while the nanogenerator adds extra energy to improve the total output. In contrast, flexible HCs prioritize portability, adaptability and ambient energy harvesting. These systems are often used to power small, portable devices like IoT nodes and sensors, allowing them to work independently without external power [153], 154]. Nano-engineered antireflective coatings can enhance photovoltaic efficiency but do not constitute photovoltaic nanogenerators because the nano layers remain optically passive [155].

Based on this classification, hybrid cells using amorphous silicon (α-Si) and other thin-film solar technologies are typically flexible, and those based on monocrystalline or polycrystalline silicon are considered rigid. Fiber-based dye-sensitized solar cells (FDSSCs) and polymer-based bulk heterojunction solar cells are also classified as flexible devices. Table 6 provides an overview of diverse solar cell materials in recent studies, their synthesis approaches, device fabrication techniques, characterization methods, and performance metrics.

2.7 Hybrid nanogenerators

Hybrid nanogenerators (HNGs) combine different energy harvesting mechanisms into one microsystem. They can generate electricity from multiple sources, either separately or at the same time [41]. Nanogenerators have improved in energy conversion efficiency and power density, but still struggle to provide continuous and sufficient energy for electronic devices. To overcome these challenges, HNGs have been developed by combining two or more energy harvesting mechanisms within a single system. This captures different types of ambient energy simultaneously and enhances the overall energy output. In recent years, substantial research has been devoted to optimize the design and materials of HNGs to maximize their performance for various energy harvesting applications [143]. HNGs achieve performance improvements not just by combining multiple energy harvesting mechanisms, but through coupling effects that create complementary energy pathways.

Piezo-triboelectric HNGs have been developed using PDMS combined with graphene quantum dot (GQD) and PVDF with titanium dioxide (TiO₂) nanoparticles via spin coating, demonstrating combined outputs (V _OC = 14.3 V; I _SC = 5.1 μA for piezo and V _OC = 105 V; I _SC = 18.1 μA for tribo) [156]. In PENG-TENG hybrids, mechanical deformation creates electrical charges through the piezoelectric effect. At the same time, contact and separation between surfaces generate charges by the triboelectric effect. When these two processes work together, the charge density increases, leading to higher voltage and more stable output under irregular motion. In another example, piezo-pyroelectric HNG with flexible Pb(Zr0.52Ti0.48)O₃ (PZT) films fabricated through spin coating achieved a piezoelectric coefficient of 140 pC/N and pyroelectric coefficient 50 nC/cm² K [157]. In pyro-piezo hybrid systems, temperature changes alter the pyroelectric polarization and reduce internal charge screening. This makes it easier for mechanical stress to switch piezoelectric domains and improves the electrical output. Research also extends to triboelectric-electromagnetic HNGs, which have successfully integrated components like carbonized cotton-derived graphite-like textile through multilayer mold casting to produce outputs from both mechanisms (e.g., 186.5 V from triboelectricity and ±9 V from EM waste) [158]. Thermo-tribo hybrids benefit from enhanced surface charge retention, as elevated temperature can increase polymer segment mobility and thus triboelectric charge transfer. More complex, multi-modal systems like piezo-tribo-pyroelectric HNGs are developed using materials such as reduced graphene oxide (rGO) on PVDF fibers via electrospinning, producing promising outputs [159]. In another study, Pyro-Piezoelectric HNG was developed using titanium carbide-MXene electrodes with NiSnO₃/FeSnO₃-PVA-KOH gel electrolytes from recycled plastic, achieving V _OC = 700 mV from combined force, bending, and heat, alongside high energy density [17]. Furthermore, PV-Triboelectric HNGs with CsPbBr₃ perovskite doped with CaF₂ and PDMS/MWCNTs on flexible substrates were fabricated using spin-coating, reaching I _SC = 9.7 µA [160].

However, synergistic gains are accompanied by internal losses, such as impedance mismatch between high-voltage/low-current TENGs and low-voltage/higher-current PENGs, which causes charge leakage or inefficient rectification. Simultaneous operation of multiple units may introduce parasitic capacitances, and material interfaces may suffer from polarization cancellation or dielectric heating. Therefore, effective HNG design requires impedance matching, optimized multilayer architectures, and diode/capacitor-based stage management to minimize internal losses while leveraging complementary frequency responses and charge generation mechanisms [161]. HNGs include a wide array of material and structural combinations tailored for multi-modal energy harvesting. Numerous other hybrid configurations integrating combinations such as piezoelectric-electromagnetic, triboelectric-thermoelectric, or photoelectric-pyroelectric mechanisms are also being actively explored and reported in recent literature [9], 143], 145]. Table 7 provides a comparative overview of hybrid nanogenerators explored in recent studies.

3.1 Functional 2D materials and nanocomposites

MXenes are a family of two-dimensional transition-metal carbides and nitrides (M_n+1X_nT_x). It was first introduced as highly conductive 2D materials with tunable surface chemistry. Now MXenes are studied extensively for their use in energy storage and energy harvesting technologies [162]. They are synthesized by selectively etching “A” layers (e.g., aluminium) from MAX phases; these materials combine metallic conductivity (∼20,000 S/cm) with exceptional mechanical resilience (Young’s modulus ∼502 GPa) and surface functionality. Their noncentrosymmetric atomic arrangement, particularly in oxygen-terminated variants, facilitates intrinsic piezoelectricity. MXenes are ideal triboelectric layers, and their electron-withdrawing capacity is better than polymers like PTFE. MXenes do not need extra conductive coatings. Their natural electrical conductivity allows charges to move easily, reducing energy loss during charge transfer. The nanoscale gaps between MXene flakes enhance pressure-responsive behaviour and enable precise mechanical sensing and durability in dynamic environments [163]. MXenes are used in applications ranging from wearable textiles to large-scale environmental energy harvesters. TENGs enhanced with MXenes demonstrate 50 % higher output than standard designs, capturing energy from the ambient sources like wind, raindrops, and biomechanical motion.

Main challenges of energy harvesters based on MXenes are synthesis scalability and environmental stability. Their production often involves hazardous etchants such as hydrofluoric acid, which further complicates its large scale adoption [164]. Susceptibility to oxidation in humid conditions degrades its performance over time. Spin coating and filtration fabrication methods can be used to make them cost-effective and to achieve uniform layering. For instance, spray-deposited MXene films suit large area applications but lack precise thickness control, and chemical crosslinking, though rapid, risks material instability. Mechanical adaptability for wearable systems can be enhanced using innovations such as solvent-free spinning and modular fiber designs. Advanced methods like 3D printing can create complex structures, but they are often limited by slow production speeds and high costs. MXenes are integrated into breathable fabrics or corrosion-resistant coatings. Research focuses on hybrid composites (e.g., MXene-polymer blends) and surface engineering to overcome these limitations, paving the way for self-powered devices with high energy density, flexibility, and environmental resilience [165], [166], [167], [168]. Figure 9(a) shows a clear increase in research on MXene-based nanogenerators. Most studies focus on TENGs and PENGs. The rising number of publications and citations highlights the growing interest in using MXenes for mechanical energy harvesting.

Figure 9:

Functional 2D materials and nanocomposites for energy harvesting applications. (a) Recent advancements in Mxene-based mechanical energy harvesters, highlighting publication trends and key developments in triboelectric (TENGs), piezoelectric (PENGs), and overall Mxene nanogenerator research (adapted from Ref [169] copyrights© 2024, Elsevier Ltd.). (b) Overview of printable graphene-based materials (adapted from Ref [170] copyrights© 2024, Elsevier B.V.). (c) Schematic structure of a quantum dot-sensitized solar cell (adapted from Ref [171] copyrights© 2025, Elsevier Ltd.).

Despite their high conductivity and tunable dielectric properties, MXenes face long-term stability challenges in nanogenerator applications. Their high surface activity enables strong triboelectric and piezoelectric coupling but makes them prone to oxidation in humid or oxygen-rich environments. This decreases conductivity, weakens surface charge retention and reduces overall output performance [172]. The synthesis process also raises sustainability concerns, particularly when HF or other corrosive etchants are used, creating challenges for large-scale and environmentally safe production [173]. The biocompatibility and potential cytotoxicity of delaminated MXene flakes must also be considered when developing wearable or biomedical nanogenerators. To slow degradation and to enhance long term reliability, recent studies emphasize surface passivation, antioxidant coatings, hybrid polymer MXene composites, and encapsulation strategies [174]. MXene-based nanogenerators require careful materials engineering to ensure stability, safety, and environmental resilience over extended operational lifetimes.

Graphene-based aerogels show strong potential for energy storage systems. They are synthesized using advanced 3D printing methods. Aerogels have hierarchical pores, high electrical conductivity, and good mechanical strength. Techniques such as direct ink writing (DIW) allow precise control of their structure and improve ion transport and charge storage in supercapacitors. For example, 3D-printed graphene micro lattices with integrated pseudocapacitive materials achieve high areal capacitance and rapid charge-discharge rates, while composite aerogels incorporating metals or conductive polymers enhance energy density and cycling stability [175]. Layered graphene architectures and nitrogen-doped frameworks further improve electrochemical performance and offer scalable solutions for next-generation energy storage technologies.

Aerogels are not widely used because they are expensive to produce and difficult to scale up. Current research focuses on improving their flow behavior and post-processing while maintaining structural stability and performance. Hybrid approaches combining graphene with MXenes or magnetic nanomaterials demonstrate enhanced electromagnetic shielding and catalytic efficiency. Future advances in additive manufacturing aim to improve printing resolution and material compatibility. This will enable multifunctional aerogels for flexible electronics and grid-scale energy storage, supporting more sustainable renewable energy systems. [176], [177], [178], [179]. As shown in Figure 9(b), printable graphene-based materials have excellent mechanical properties and can be fabricated using 3D printing methods such as DIW and Digital Light Processing (DLP). These techniques enable their use in energy storage, sensing, and catalytic applications.

Quantum dots (QDs) have revolutionized nanotechnology since their Nobel Prize-winning discovery. QDs give control over optoelectronic properties through quantum confinement. There are challenges such as toxicity (e.g., cadmium-based QDs), environmental instability and high production costs that limit its widespread adoption [180], 181]. Research focuses on non-toxic alternatives like InP, silicon, and perovskite QDs and also on scalable synthesis techniques such as colloidal synthesis [182], hot injection [183], and sol-gel methods [184]. Surface passivation and hybrid nanostructures are used to improve stability and increase durability for real-world applications. As shown in Figure 9(c), quantum dot-sensitized solar cells (QDSSCs) utilize a multilayer structure where quantum dots are deposited onto a TiO₂ scaffold to absorb sunlight and transfer photoinduced electrons through a buffer and electrode system. Efforts to integrate machine learning and eco-friendly fabrication processes further underscore the push toward sustainable, high-performance QD technologies.

In energy harvesting, quantum dots can improve solar technology by allowing tunable bandgaps and multi-exciton generation, which may enable efficiencies beyond traditional silicon solar cells. Beyond photovoltaics, hybrid QD-metal systems have also shown enhanced photocatalytic performance under visible light, supporting applications in solar-driven chemical reactions and environmental remediation [185]. QDs are increasingly explored in electrochemical energy systems, where their nanoscale dimensions and surface tunability can improve ion transport, interfacial stability, and charge storage behaviour.

Metal halide perovskites have emerged as high-performance active layers due to their strong polarization, defect-tunable dielectric properties, and ease of film formation. Their limitations are instability under humidity, UV exposure, and elevated temperatures and undergoing phase transitions or ionic migration that degrade output performance over time [186]. Lead-based perovskites also raise toxicity concerns due to possible Pb²⁺ leaching, particularly in wearable or environmental applications [187]. While lead-free perovskites (e.g., tin or bismuth-based) show promise, they remain prone to oxidation and moisture-driven degradation [188].

3.2 Eco-friendly and bio-derived materials

Green macroalgae are sustainable biomass materials that can be converted into nitrogen-doped porous carbon for energy storage using carbonization and chemical activation. Such structures serve as high surface area hosts for lithium sulfur batteries, delivering initial discharge capacities as high as 1,617 mAh g⁻¹ [189]. Their biomass is converted into porous carbon materials using pyrolysis and activation.

These materials are used as efficient electrodes in energy storage devices. Algae-derived chlorophyll and xanthophyll pigments are used as natural dyes in dye-sensitized solar cells. Using them together improves the cell’s photovoltaic performance [190]. Cell-free biohybrid systems made from self-organised photosystem I nanoparticles, combined with electron mediators and metal catalysts, can produce hydrogen using light. These systems have shown efficient hydrogen generation of about 5.5 µmol H₂ h⁻¹ mg⁻¹ chlorophyll and remain stable during long-term operation [191]. Microbial fuel cells (MFCs) utilize algae for oxygen supply and organic waste degradation, generating power densities up to 1,926 mW/m² while sequestering CO₂ [192]. Green algae show strong potential for energy storage, energy conversion and biofuel applications. Their main limitations are efficiency, scalability, and cost.

Chitosan is a biodegradable polymer from crustacean shells that is used in TENGs for wearable electronics. Modified chitosan composites increase triboelectric output and flexibility, making them suitable for self-powered sensors [193], [194], [195]. Similarly, cellulose-based materials from algae or plants are used to make flexible separators for lithium-ion batteries. These separators offer good thermal stability and ionic conductivity similar to commercial products. Innovative composites like chitosan-silk fibroin and cellulose-nanoparticle hybrids show how sustainability can be combined with good performance. For example, tea-leaf-reinforced polystyrene composites in TENGs achieve power densities of 61.25 W/m², demonstrating durability over 110 days [196].

Biodegradable tribo and piezo materials such as cellulose, chitosan, starch, PLA and other biopolymers offer low toxicity and excellent environmental compatibility. Their long-term reliability is limited by moisture absorption, hydrolysis, and microbial degradation. This leads to softening of the material, lower surface charge density and weaker mechanical strength. Many biodegradable materials also exhibit lower dielectric constants and reduced fatigue resistance under repeated mechanical cycling [197], [198], [199]. These materials are suitable for transient or implantable devices, but their durability needs improvement. Techniques such as crosslinking, hydrophobic coatings, or adding stable fillers can help ensure reliable long-term performance.

3.3 Recycled and waste materials

Recent advances have demonstrated that biodegradable and waste-derived materials can function as highly effective triboelectric layers. Agricultural residues and biomass-based films, such as radish leaf, banana leaf, nopal powder, and corn husk, have suitable surface roughness, natural functional groups, and good electron-donating properties. This makes them suitable as tribopositive layers for contact electrification. For instance, radish leaf-based TENG fabricated from recycled and waste materials has been demonstrated as a stable, low-cost self-powered system capable of converting finger motions into electrical signals for wireless IoT-enabled communication applications [200]. Similarly, banana leaf and EVA foam-based triboelectric devices demonstrate stable performance under mechanical vibration, with the EVA foam providing a porous tribonegative interface and the banana leaf contributing a textured biopolymeric surface, together supporting sustained power generation for smart home security systems [201].

Waste-based materials have also emerged as effective alternatives to synthetic polymers. Spent coffee grounds have been employed as biodegradable triboelectric layers in TENGs, achieving a peak output power density of 75.48 mW m⁻² with stable operation over tens of thousands of cycles. These coffee-based nanogenerators have been demonstrated as power sources for thermohygrometers, digital timers, LED arrays, and emergency alarm systems, highlighting the potential of food waste for sustainable energy harvesting [202]. Studies have also reported fully degradable negative triboelectric layers by co-processing coffee grounds with polycaprolactone, achieving 67–127 % of the triboelectric charge density of PTFE-based systems while enabling fully biodegradable and enclosed TENG architectures [203].

Agricultural waste can also be combined with recycled plastics to enhance circular economy benefits. Elvira Hernández et al. [204] have demonstrated that waste corn husk paired with recycled polystyrene forms a high-performance triboelectric pair capable of delivering 670.5 mW/m² and powering 472 LEDs, as well as small electronic devices. The lignocellulosic microstructure of the corn husk, featuring micron-scale protrusions and roughness, promotes high charge transfer, while the recycled polystyrene provides mechanical stability and strong electron-accepting behavior. A similar design strategy is observed in banana leaf and EVA foam-based systems, where natural polymers are aligned with recycled synthetics to maximize durability, charge polarization, and environmental adaptability.

4.1 Flexible and stretchable devices

Recent advancements in flexible and stretchable nanogenerators have revolutionized healthcare technologies and the Internet of Things (IoT) by enabling self-powered, wearable, and implantable systems [205], [206], [207]. The study emphasizes the use of TENGs and PENGs mixed with elastomeric dielectrics such as polydimethylsiloxane (PDMS), Ecoflex, and thermoplastic polyurethane (TPU). These materials are engineered for enhancing mechanical adaptability. For example, Cu²⁺ polyurethane elastomers made with polycaprolactone segments show high flexibility, self-healing ability and stable electrical output even during degradation. This makes them suitable for on-skin and implantable energy harvesting and sensing applications [208]. In a study, a novel stretchable TENG was developed using nitrogen-doped graphene sheets covalently bonded with nickel-based metal organic polyhedra (NG@MOP), enhancing mechanical durability and conductivity [209]. When embedded in a polyurethane matrix, the NG@MOP composite film with 5 wt% content can stretch up to 500 % and produce a high output of 417 V and 10.8 µA. This combination makes it well-suited for wearable electronic skin applications. The device also showed high strain sensitivity and could reliably detect subtle body movements, highlighting its potential for real-time motion tracking and health monitoring.

For PENGs, piezoelectric composites like lead-free perovskite Cs₃Bi₂Br₉ embedded in PVDF-HFP and SEBS matrices achieve high mechanoelectrical conversion (400 V output) while maintaining flexibility [188]. Structural design improvements, such as using droplet-shaped ceramic fillers inside elastomers, help reduce stress buildup. This allows the device to maintain stable performance even after 5,000 stretching cycles at 60 % strain [210]. These designs are important for harvesting energy from biomechanical motions, such as joint movement and breathing, to power wearable sensors and implantable devices. Many flexible TENG and PENG-based wearables are already used as sensing nodes in healthcare IoT networks. For example, Munirathinam and Chandrasekhar [211] developed a self-powered SGAS-TENG wrist-wear device that integrates directly with an ESP8266 Wi-Fi module and the Blynk IoT platform for real-time wireless alerts. Recent bibliometric analyses highlight the growing role of nanogenerators as power sources for wearable healthcare IoT systems. A 2025 bibliometric study by Suryani et al. [212] analyzed 1,575 NG-related publications and identified TENG and PENG-based harvesters integrated at the wrist, fingers, chest, and feet as the dominant configurations reported for continuous physiological monitoring. Nanogenerator-based intelligent interfaces are also rapidly emerging, such as the electret nanofiber triboelectric sensor for noncontact 3D gesture recognition in VR systems [213].

Practical applications span diagnostics, therapeutics, and wireless telemedicine. Implantable NGs, such as bioresorbable triboelectric sensors (BTS) made from poly (lactic acid)-chitosan, detect vascular occlusions by monitoring pressure changes [214], while ultrathin PZT membranes on wrists enable noninvasive blood pressure monitoring. Self-powered therapies include TENG-based cardiac pacemakers that treat arrhythmias using heart motion and electrogenerative dressings that accelerate wound healing via endogenous electric field modulation. Wireless communication modules powered by NGs transmit physiological data (e.g., ECG signals) to smartphones, exemplified by a chest-mounted TENG transmitting a 71 BPM heart rate. Figure 10 provides a summary of recent wearable nanogenerator systems and sensors developed for human motion monitoring and rehabilitation applications. Challenges persist in biocompatibility, signal noise reduction, and scalable fabrication. Future efforts should focus on advanced encapsulation and AI-based analytics to enable clinical and IoT-ready nanogenerator systems for personalized healthcare and telemedicine.

Figure 10:

Flexible and stretchable devices for self-powered sensing and motion monitoring. (a) Schematic of the Rehab-TENG used for playing a motivational game to aid recovery (adapted from Ref [215] copyrights© 2020, Elsevier Ltd). (b) Schematic structure of the BSRW-TENG and SEM images of the interlayer structure between the triboelectric layer, the Au electrode, and the PDMS protection layer (adapted from Ref [216] copyrights© 2022, Elsevier Ltd). (c) Photo of the BSRW-TENG attached to the knee joint for leg curl monitoring. Leg curl monitoring results before and after sweating (adapted from Ref [216] copyrights© 2022, Elsevier Ltd). (d) The application of tactile hydrogel sensors to detect human motion (adapted from Ref [217] copyrights© 2022 The authors. Advanced science published by Wiley-VCH GmbH. (e). Working principle and the electrical output under different stretching strain of the HFSSs. (a) Structural illustrations of the HFSSs at different states: (i) Side view without deformation; (ii) detached windings under tension; (iii) stretched to maximum. (b) Working principle of the HFSSs; the states i, ii, and iii correspond to i, ii, and iii of (a). (c) Simulation results of the electric potential model using the COMSOL software. (Adapted from Ref [218] copyrights© 2022, American Chemical Society).

4.2 Implantable and transient systems

Implantable medical electronic devices (IMEs) are increasingly used to support organ function, monitor health, and deliver therapy inside the body. Recently, natural body motions such as breathing, heartbeat, and muscle activity have attracted attention as potential energy sources for harvesting. PENGs are used because they are compact and avoid bulky designs. They can be made ultra-thin, lightweight, flexible, and able to conform to curved body surfaces. Degradable piezoelectric biomaterials are increasingly used in transient IMEs because they are biocompatible and can generate their own power [219]. Medical applications include energy harvesting, sensing, and treatments. Yang et al. [220] demonstrated a wafer-scale heterostructured piezoelectric bio-organic thin film in which γ-glycine crystals self-assemble and align between poly (vinyl alcohol) (PVA) layers, yielding biodegradable, water-stable films with a macroscopic piezoelectric response (d₃₃ ≈ 5.3 pC/ N) suitable for transient implantable electromechanical devices. Das et al. [221] reported biodegradable piezoelectric PLLA nanofiber scaffolds activated by ultrasound. The generated electrical stimulation enhanced osteogenesis in vitro. Bone formation was observed in mouse calvarial defects.

Challenges persist in optimizing degradable piezoelectric biomaterials for clinical use. A critical issue is the modulus mismatch between rigid small-molecule crystals (e.g., β-glycine at gigapascal scales) and soft tissues (e.g., skin at kilopascal levels). This shows the importance of hybrid designs like glycine-PCL composites (d₃₃ = 19 pC/N) [222]. Additionally, their longitudinal piezoelectric performance is generally lower than that of inorganic materials such as PZT. Many biomaterials show shear piezoelectric effects (for example, FF nanotubes with d₁₃ ≈ 60 pm V⁻¹), which makes device design and integration more challenging. Balancing high crystallinity for strong piezoelectric performance with amorphous regions for mechanical flexibility makes material design more challenging. Degradation kinetics also pose stability concerns. For example, β-glycine transitions to non-piezoelectric phases in air, while PLLA’s slow hydrolysis requires enzymatic acceleration. Future work will focus on improving dipole alignment using electric field polarization to boost performance. These materials are also being studied for tumor therapy, where degradable systems may enable targeted drug delivery or piezoelectric-induced cell death, extending their use beyond regenerative and diagnostic applications. Figure 11 shows an overview of recent implantable nanogenerator technologies for biomedical applications, including cardiac monitoring, neural stimulation, and hearing enhancement.

Figure 11:

Implantable and transient systems for biomedical energy harvesting and stimulation. (a) Macroscopic images of a T2ENG located in the subdermal Dorsal region ((i) Sutured; (ii) opened) demonstration of a Sprague-Dawley mouse (adapted from Ref [2] copyrights© 2018, WILEY-VCH Verlag GmbH & Co. KGaA). (b) Advanced piezo array for retinal stimulation (F-URSP) (adapted from Ref [223] copyrights© 2022. (c) An implantable middle ear hearing device with a piezoelectric transducer attached to the Incus body (adapted from Ref [224] copyrights© 2019, MDPI).

4.3 Large-scale integration and industrial applications

Recent advances in TENG technology have demonstrated remarkable progress in large-scale integration across diverse applications. A dual-functional TENG/photocatalytic system was developed by integrating cobalt coordination polymers (Co-CP) into cellulose acetate (CA) films, where the 12 % Co-CP@CA composite achieved optimal power output [225]. Scalable TENG floors were constructed using these composites to harvest human walking energy, powering blue LEDs for self-driven photocatalytic oxidation reactions. This work demonstrates the feasibility of large-scale, bifunctional energy harvesting flooring systems that simultaneously capture mechanical energy and enable photocatalytic applications. Beyond conventional indoor IoT applications, nanogenerators are now playing a central role in next-generation smart agriculture systems by enabling fully self-powered, maintenance-free sensor networks. Wang et al. [226] developed a hybrid wind-solar energy harvesting device (HEHD) that integrates triboelectric generators, electromagnetic generators, and photovoltaic cells into a single platform, achieving simultaneous harvesting of wind and sunlight for powering distributed temperature-humidity sensors and NB-IoT-based farm security systems. Complementing these hybrid harvesters, Lan et al. [227] introduced a breathable, waterproof TENG (WB-TENG) made from PVDF-HFP nanofibers embedded with fluorinated CNT microspheres, capable of conformally attaching to plant leaves without disturbing physiological activity. This device efficiently scavenges energy from wind and raindrops and can continuously power wireless plant health monitoring nodes. Together, these advances show that nanogenerators can power battery-free IoT systems in agriculture. They enable continuous operation of sensors for precision irrigation, local climate monitoring and early detection of plant stress without relying on conventional batteries. Building on this concept of scalable energy harvesting, researchers have extended TENG technology to marine environments. The One Meter TENG (OM-TENG) overcomes the limitations of conventional TENGs in marine environments by employing a meter-scale lantern structure with stacked silicon-manganese steel sheets, achieving enhanced charge transfer (60.82 µC/cycle) and optimized internal space utilization [228]. Its segmented design allows multiple TENG units to work together in sync. As a result, it delivers strong wave energy harvesting performance (584 V, 444 µA) and remains stable under real-world operating conditions. This scalable design can reliably power high-wattage LED arrays and wireless systems. It represents a major step forward for large-scale blue energy harvesting technologies.

A scalable woven TENG textile was created using core-shell yarns made of wool/polyester with copper electrodes. The multilayer single-electrode/contact-separation design produced 18.5 V and 3.7 µA and a power density of 51 mW/m². The structure improves durability, washability and sensitivity to motion while remaining comfortable to wear. It is also compatible with standard industrial textile manufacturing methods [229]. This approach bridges laboratory-scale TENGs and commercial smart garments, enabling applications such as athletic monitoring while allowing seamless integration into everyday clothing. NG-powered nodes are used in smart city infrastructures for structural health monitoring, traffic flow detection, water leakage sensing and environmental pollution tracking. These autonomous sensor units harvest energy from ambient vibrations, footsteps and wind, enabling maintenance-free deployment across bridges, tunnels and urban walkways [230]. Figure 12 demonstrates examples of advanced TENG architectures for chemical catalysis, large-scale blue energy harvesting, and motion-based power generation.

Figure 12:

Large-scale integration and industrial applications. (a) The oxidation of sulphides driven by Quadruple-M−TENG device (adapted from Ref [225] copyrights© 2024, Elsevier B.V.), (b) Schematics of working principle and structure of WPc-PWc TENG, working mechanism and elements of WPc-PWc TENG under tapping motion (adapted from Ref [229] copyrights© 2024, Sage Publications).

4.4 Robotics and autonomous systems

Energy harvesting technologies are actively studied for application in autonomous robotic systems. For example, solar energy harvesting is used in unmanned aerial vehicles (UAVs), where weight, endurance and environmental adaptability remain critical constraints. AtlantikSolar and RoboRaven 3 are examples of real-world implementations showcasing ultra-light, high-efficiency photovoltaic integration for long-duration flight [231], 232]. For drones, flexible perovskite and organic solar cells are promising because they are lightweight and can easily conform to curved surfaces. Solar power is not always available, so it often needs support from batteries or supercapacitors. Mechanical energy harvesting using piezoelectric and triboelectric materials can help recharge onboard systems during motion. Dynamic soaring concepts offer energy-free propulsion inspired by nature by exploiting atmospheric wind gradients, although they require advanced real-time sensing and control systems [233]. Self-powered sensors enable tactile perception, force feedback, and object recognition in autonomous robotic systems. Hybrid triboelectric-piezoresistive skins, robotic grasp sensors, and neuromorphic droplet-based e-skins have been demonstrated. This supports advanced human-robot interaction and adaptive control in intelligent robotic systems [234].

In the broader context of autonomous mobile robots (AMRs), similar energy harvesting approaches are being tailored to ground-based and soft robotic systems. TENGs with their capacity to scavenge low-frequency mechanical energy have been integrated into robotic skins, smart textiles, and sensory actuator systems [235]. They are useful in wearable robotics and soft actuators as they can act as both energy harvesters and active sensors. Flexible solar cells (e.g., perovskite and fiber-shaped organic cells) and thermoelectric generators harvesting waste heat further support onboard power in robotics. Hybrid systems combined with compact storage technologies such as photo batteries, redox flow systems, or supercapacitors, enhance autonomy, making robots capable of prolonged operation in constrained or remote environments [236].

For drones, energy harvesting can enhance operational autonomy by supplying power to onboard sensors and other low-power electronic components. Recent studies show that piezoelectric transducers can harvest energy from vibrations and airflow when integrated into UAV structures. This helps reduce dependence on onboard batteries [237]. In the future, solar and mechanical energy harvesting can be combined. Lightweight, multifunctional materials will help reduce payload weight. Better power management circuits will improve energy use. Bio-inspired designs, such as wing-based harvesters, can increase energy capture. Together, these advances can enable long-endurance, self-powered UAVs for environmental monitoring and disaster response.

TENGs have made it possible to power autonomous soft robots. For example, a very lightweight flapping-wing micro-aerial robot (160 mg) was able to take off using high voltage dielectric elastomer actuation driven by a TENG [238]. The system integrates an in-plane charge pump TENG (ICP-TENG) comprising PFA (perfluoroalkoxy alkane) and PA (polyamide) triboelectric layers with aluminum electrodes, which delivers 1,640 V and 117 mW peak power. This outperforms conventional designs by 280 % in voltage and 920 % in energy. The MAV’s rolled Elastosil P7670 dielectric elastomer actuator (DEA), embedded with carbon nanotube electrodes, is driven by the TENG via a voltage-doubler circuit, achieving 400 Hz flapping at 1,540 V. Key results include a lift-to-weight ratio of 1.75 and stable operation over 72,000 contact cycles. This highlights the synergy between TENGs and robust soft actuators for resilient microsystems. Figure 13 highlights key advancements in energy harvesting for robotics and drones, including major milestones in solar and mechanical energy harvesting for UAVs, the integration of nanogenerators in robotic systems, and the use of wearable solar cells as conformable power sources for robotic surfaces.

Figure 13:

Energy harvesting strategies for robotics and autonomous systems. (a) The milestone in solar and mechanical energy harvesting for UAVs (adapted from Ref [239] copyrights© 2023, Elsevier Ltd.). (b) Wearable solar cells for robots. Bendable commercial thin-film solar cells could be the “cloth” or “skin” of a robot. Bendable, stretchable, or wearable PSCs, OSCs, and DSSCs are being further studied, such as I) a bendable PSC (adapted from Ref [236] copyrights© 2022, Wiley-VCH GmbH).

While nanogenerators are used in wearables, IoT devices, and soft robotics, several practical challenges remain. Wearable TENG systems face several limitations. As highlighted in studies of skin-contact TENGs, performance is highly sensitive to environmental variability, including humidity, temperature fluctuations, sweat, and mechanical vibrations, all of which can suppress charge density and reduce output stability [240]. Additionally, biocompatibility and long-term skin safety remain critical concerns, since many triboelectric layers and electrodes may cause irritation or degrade under continuous contact. Durability under repeated deformation, including stretching, bending, and torsional loads typical of wearable applications, can induce mechanical fatigue, interfacial delamination, and gradual output degradation. Many devices lack robust encapsulation to protect against moisture, oils, and abrasion, and they do not integrate energy management circuits that are essential for rectifying, stabilizing, storing, and conditioning the irregular AC outputs generated during human motion. Together, these issues illustrate that while wearable nanogenerators show promising sensing and energy harvesting capabilities, substantial engineering improvements are needed before large-scale, long-term, real-world applications [241].

For large-scale IoT integration, nanogenerators still face several practical limitations. First, long-term durability is a major concern. Continuous vibrations, outdoor conditions, and repeated mechanical loading can wear TENG surfaces and cause microcracks in piezoelectric films, which lowers reliability in distributed sensor networks [230]. Second, most TENGs generate high voltages but very low currents with intermittent output, creating a mismatch with the stable, low-voltage power requirements of IoT nodes. This requires compact energy management modules, including rectifiers, impedance matching circuits, and small energy storage units to provide stable power for sensors and wireless transmitters. Third, environmental sustainability is a concern. Many triboelectric polymers are non-degradable, and traditional piezoelectric ceramics may contain toxic elements, leading to waste management challenges. Therefore, eco-friendly materials and long-life encapsulation strategies will be essential for future NG-powered IoT infrastructures. In soft robotics and human-machine interfaces, repeated deformation can induce mechanical fatigue or interfacial failure, making robust packaging and strain-tolerant architectures essential for maintaining sensing accuracy [242]. Solving these challenges requires better material stability, effective encapsulation and improved power management circuits. This is essential to move nanogenerators from laboratory prototypes to reliable real-world systems.

5.1 Current limitations and research priorities

5.2 Emerging opportunities and strategic directions

5.3 Roadmap for commercialisation

To accelerate adoption, future efforts should focus on:

Industry partnerships: Must collaborate with electronics manufacturers for embedded nanogenerator solutions.

Regulatory standards: Safety and performance guidelines for medical/consumer applications must be established.

Lifecycle analysis: Assessing environmental impact from production to disposal.

After discussing commercialization potential, it is also important to clearly identify the technical challenges that still limit large-scale deployment of nanogenerators. First, persistent impedance mismatch between the inherently high-voltage/low-current output of NGs, especially TENGs and the low voltage requirements of practical electronics restricts seamless integration. Low-loss impedance-matching circuits, soft magnetic micro transformers, and adaptive power conditioning modules optimized for irregular mechanical inputs can be a solution for this. Second, environmental instability remains a major constraint. Polymer-based TENGs exhibit humidity-induced charge decay, MXenes oxidize rapidly in air, and emerging perovskites degrade under moisture and UV exposure. Advances in encapsulation layers, hydrophobic surface engineering and the development of intrinsically stable, eco-friendly alternatives are required to ensure long-term reliability. Third, the low current density of most NGs limits their ability to drive communication units, sensors, and edge computing elements in IoT systems. Solutions include multilayer stacking, high dielectric nanocomposites, interfacial polarization engineering, and hybridization with piezoelectric or electromagnetic modules to boost charge generation. Addressing these specific challenges through coordinated materials innovation and system-level optimisation is essential for transitioning nanogenerators from laboratory prototypes to robust, commercially viable self-powered technologies.