Empowering high-performance triboelectric nanogenerators: advanced materials strategies

Abstract

The rise of artificial intelligence (AI) and the Internet of Things (IoT) has heightened the urgency for advanced research in energy harvesting technologies. Triboelectric nanogenerators (TENGs) present a promising approach for converting high-entropy mechanical energy into electrical energy, particularly excelling in powering miniaturized and distributed electronic devices. Triboelectric materials are the fundamental component of high-performance TENGs. This review provides an in-depth analysis of the mechanisms and performance limitations of alternating-current (AC) and direct-current (DC) TENGs in detail. Furthermore, advanced materials-related strategies for high-performance AC-TENGs and DC-TENGs are thoroughly reviewed. Finally, it discusses emerging challenges and highlights opportunities for future research, offering valuable guidelines for the development of high-performance TENG technologies.

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1 Introduction

With the rapid development of science and technology, a smart digital world is being built with the help of the Internet of Things (IoT) and artificial intelligence (AI). Numerous distributed sensors and devices connected via the IoT as well as computing power are a treasure trove for the future digital world, which has brought about a huge increase in demand for electricity.^1,2 Furthermore, serious environmental problems resulting from conventional energy sources, such as coal and oil, highlight an urgent need for new, environmentally friendly and renewable energy power technologies. Renewable energy sources, including solar energy and mechanical energy, which are abundant and ubiquitous in nature, have attracted significant research interest.^3–5 Among these technologies, energy harvesting technology, which converts waste mechanical energy into sustainable electrical energy, has emerged as a promising solution, leading to the development of diverse energy harvesting devices targeting vibration,^6–8 wind,^9–11 wave energy,^12–14etc. Notably, piezoelectric nanogenerators (PENGs), triboelectric nanogenerators (TENGs), and electromagnetic generators (EMGs) featuring moving parts have been invented for mechanical energy harvesting.^15–20 Among these, TENGs, first invented by Wang et al. in 2012, operate by the coupling of the contact-electrification effect and electrostatic induction.²¹ TENGs have gained widespread attention due to their simple structure, diverse material compatibility, portability, and multifunctionality.^22,23 While their energy harvesting density may not match conventional electromagnetic generators, TENGs excel in efficiency and decentralized energy generation at low frequency, making them particularly suitable for modern applications.²⁴

To meet diverse application needs, TENGs have evolved into two major categories based on their output characteristics: alternating-current TENGs (AC-TENGs) and direct-current TENGs (DC-TENGs).²⁵ AC-TENGs, based on triboelectrification and electrostatic induction, exhibit alternating current output, making them particularly advantageous for self-powered sensing, tactile/sound sensing and human–computer interaction.^26,27 Moreover, AC-TENGs can achieve high charge density through charge excitation technology. However, their AC output requires efficient rectification technology when charging electronic devices or energy storage systems. In contrast, DC-TENGs operate on triboelectrification and electrostatic breakdown, providing direct/constant current output suitable for directly charging electronic devices or storage systems. Over the past decade, significant advancements in high-performance TENGs have been achieved through material synthesis and optimization, structural design, environmental control, etc.^28–30 Among these, optimizing triboelectric materials, whether for DC-TENGs or AC-TENGs, is essential for enhancing TENGs’ performance and advancing their applications, as the triboelectrification effect between materials significantly influences both the charge density and the output power of TENGs.³¹

Researchers have devoted considerable efforts to developing triboelectric materials for achieving high-performance TENGs. Some strategies, such as material selection,³⁰ surface modification,³² materials synthesis,³³ limiting air breakdown through high specific capacitance materials³⁴ and dielectric enhancement materials,³⁵ have improved charge density to 9.23 mC m⁻² for single-unit AC-TENGs and 10.06 mC m⁻² for DC-TENGs.^36,37 In addition, to achieve specific functions such as environmental sustainability and flexible wearability, various eco-friendly and flexible materials, such as cellulose, hydrogels, and organogels, have been widely studied.^38–43 Notably, AC-TENGs and DC-TENGs have different requirements for triboelectric materials due to their distinct working mechanisms (Fig. 1). Previous reviews have concentrated on various aspects of TENGs, including fundamental working principles, the progress of triboelectric materials, structural design and applications, providing valuable insights into the development and optimization of TENG technology.^44–46 However, according to the unique working mechanisms and application demands of these two TENG types, it is important to identify and address their respective performance-limiting factors to further enhance output performance. Moreover, a systematic summary of existing triboelectric material strategies for AC-TENGs and DC-TENGs is essential for guiding future research and innovation.

Fig. 1 An overview diagram of high-performance TENGs with materials-related strategies.

In this review, a comprehensive review of materials-related strategies for high-performance AC-TENGs and DC-TENGs is presented, highlighting key advancements, challenges, and potential directions for optimization and improvement. The review systematically discusses their working mechanisms, performance limitations, and materials-based strategies. The first section provides an overview of the fundamental principles of TENGs. The second section explores performance constraints of AC-TENGs and DC-TENGs. The third section focuses on materials-related strategies for AC-TENGs, emphasizing enhancements in intrinsic triboelectrification and the suppression of air breakdown under charge excitation conditions. The fourth section highlights materials-related strategies for DC-TENGs, aiming to improve triboelectrification and electrostatic breakdown effects (Fig. 1). The fifth section presents applications of TENGs in energy harvesting and intelligent sensing. Finally, this review concludes by discussing the challenges and opportunities in developing high-performance TENGs, offering insights and guidelines for future research directions.

2 Basic principles of TENGs

2.1 The mechanism of triboelectrification

Triboelectrification is the basic principle of TENGs, applicable to both AC-TENGs and DC-TENGs. When different materials come into contact or rub against each other, charges are transferred from the surface of one material to another and materials become oppositely charged. The material properties play a decisive role in the polarity and magnitude of the transferred charges. Triboelectrification can occur between interfaces of various materials, such as solid–solid, liquid–solid, liquid–liquid, gas–solid, and gas–liquid interfaces^47–55 and lead to the tribocatalysis phenomenon.^56,57 Among them, TENGs based on solid–solid interfaces, including metal–dielectric, two different dielectrics and the same dielectric, are the most common devices for harvesting mechanical energy.

As one of the most important principles of TENGs, scientific understanding of charge generation during interfacial contact for two triboelectric materials is important for achieving high-performance TENGs. Using Kelvin probe force microscopy (KPFM), a nano-scale study of interfaces can be conducted.^58,59 Initially, the charge transfer of triboelectric materials containing ionic polymers or nonionic polymers in high humidity environments was believed to be caused by a conventional ion transfer model.^60–62 Later, as for the metal–dielectric interface between titanium (Ti) and silicon dioxide (SiO₂) or aluminum oxide (Al₂O₃), excluding the influence of nonionic polymers and the influence of water in the environment, the discharge performance of the surface electrostatic charges at high temperature was found to be consistent with the thermionic emission equation of electrons, which reveals that electron transfer is the dominant process between two inorganic solids.⁶³ The electron transfer mechanism at the metal–dielectric interface can be explained using an energy band diagram (Fig. 2a–i). In the energy band diagram, the metal is characterized by its Fermi level (E_F), below which all the states are occupied and above which all the states are empty (assuming that the temperature is 0 K). The dielectric surface is characterized by using its conduction band and valence band. At the same time, it is necessary to consider the presence of surface/defect states in the band gap due to the breaking of symmetry at the surface. Some of the surface states in the bandgap (E_g) with energy below E_F could be filled up by the electrons transferred from the metal to the dielectric. Thus, when the metal contacts the dielectric material, electrons with high kinetic energy transfer from the metal to the unoccupied surface states in the dielectric material, resulting in negative charges on the dielectric surface. A similar situation may occur between different dielectrics (Fig. 2a-ii).⁴⁷ Specifically, due to the different energy structures of dielectric A and dielectric B, the occupied surface states of dielectric A can have higher energy than the unoccupied states of dielectric B. When two dielectrics come into contact, electrons transfer from dielectric A to dielectric B. These electrons remain on dielectric B after separation, leaving dielectric A positively charged and dielectric B negatively charged. These two charge transfer frameworks also apply to scenarios where dielectrics are replaced by liquids, with a similar electron transfer mechanism driving contact electrification.⁶⁵

Fig. 2 The mechanisms of triboelectrification at solid–solid interfaces. (a) Charge transfer mechanisms of three different interfaces.^47,63,64 Copyright 2019, Elsevier. Copyright 2018, Wiley-VCH. Copyright 2019, American Chemical Society. (b) Interatomic interaction potential and the force between two atoms in (i) the equilibrium region and (ii) the repulsive region.⁴⁷ Copyright 2019, Elsevier. (c) Electron-cloud-potential-well model for explaining the charge transfer process between any two solid materials.⁶³ Copyright 2018, Wiley-VCH.

In addition, triboelectrification can also occur between interfaces of two chemically identical materials. Such a situation cannot be explained using either of the models proposed for dissimilar materials but is related to the curvature of materials (Fig. 2a-iii). A theoretical study has shown that altered surface curvature can change surface energy,⁶⁴ meaning that the presence of a curved surface breaks the symmetry of the two sides due to the stretched or compressed surface molecules, thus shifting the energy levels of surface states. Finally, this results in the electron transfer between two chemically identical surface upon physically contact. Positive curvature surfaces generally acquire negative charges, while negative curvature surfaces tend to gain positive charges.⁶⁴

The surface state model, originating from semiconductor band theory, is insufficient for explaining triboelectrification involving polymers and non-crystalline materials. To address this issue, an electron-cloud-well model⁶³ based on fundamental electron cloud interaction is proposed to explain all types of general materials. KPFM experiments⁶⁶ reveal that triboelectrification occurs only when two materials come within a distance shorter than the bonding length, in the repulsive force region of atomic interaction potential. The detailed charge transfer process is shown in Fig. 2b and c:

(1) Pre-contact phase: electron clouds of two materials remain separated without overlapping. This is the attractive force region. The potential well binds the electrons tightly in specific orbitals and stops them from freely escaping, which is the case for nonconducting materials.

(2) Contact phase: as atoms of the two materials approach and make contact, their electron clouds overlap, forming ionic or covalent bonds. External compression can further shorten the bonding length. In this case, the initial single potential wells become an asymmetric double-well potential, and the energy barrier between the two is lowered because of strong electron cloud overlap, resulting in electron transition from one material to the other.

(3) Post-separation phase: after separation, the transferred electrons remain as static charges on the material's surface.

This model provides a comprehensive explanation for triboelectrification across various triboelectric materials, offering insights at the atomic level.

2.2 The working mechanism of AC-TENGs

Electron transfer at the interface between two triboelectric materials generates surface triboelectric charges. These charges induce a Maxwell displacement current, serving as the theoretical basis of the TENG.⁶⁷ According to Maxwell's equations for a mechano-driven media system,⁶⁸ the displacement current density (J_D) is defined asD is the displacement field, t represents time, ε₀ is the vacuum permittivity, E is the electric field strength, P is the polarization owing to an external field, and P_s is the mechano-driven polarization arising from the surface electrostatic charges. The first term on the right-hand side of the equation is the induced current due to the field created by the free charges. The second term is the medium polarization induced current caused by the external electric field. The third term is the mechano-driven polarization current caused by the surface bound electrostatic charges and the time variation in boundary shapes as well as the relative movement of multiple objects. This term is necessary for TENG theory. For TENGs, this polarization field is generated by the triboelectrification effect. The internal circuit of the TENGs is governed by the displacement current, while the external circuit is governed by the conduction current. The internal and external circuits are connected to the electrodes to form a complete circuit. Therefore, the displacement current is the inside driving force of the TENG. It is the physical core that enables electromechanical energy conversion, while the conduction current is the external manifestation of the displacement current.

TENGs mainly include four basic operating modes (Fig. 3a): contact-separation (CS) mode,⁷¹ sliding (SL) mode,⁷² single-electrode (SE) mode,⁷³ and freestanding (FS) mode.⁷⁴ These modes serve as the foundational mechanisms for various TENGs. Although triboelectric devices operating in different modes share the same fundamental energy harvesting mechanisms, triboelectrification and electrostatic induction, they exhibit distinct working characteristics.⁶⁹ Consequently, they differ in intrinsic output characteristics, such as open-circuit voltage (V_oc), short-circuit transferred charge (Q_sc), and inherent capacitance (C). More importantly, the output of TENGs is closely related to the surface charge density of triboelectric materials.

Fig. 3 (a) The theory model of a conductor-to-dielectric contact-separation mode TENG, sliding mode TENG, single-electrode mode TENG and freestanding mode TENG.⁶⁹ Copyright 2015, Elsevier. (b) The schematic diagram and (c) working mechanism and constant current output of a DC-TENG based on electrostatic breakdown.⁷⁰ Copyright 2019, AAAS.

For CS-TENGs, the periodic contact and separation of the two triboelectric layers drive the induced electrons to flow back and forth between the electrodes, producing an AC output in the external circuit.^75,76 Specifically, at full contact, the two triboelectric layers acquire opposite charges due to contact electrification. During separation, the increasing potential difference between the two electrodes drives free electrons to flow from the high-potential electrode to the low-potential electrode through the external circuit, continuing until the separation reaches its maximum distance. Conversely, during the contact process, the potential difference between the electrodes gradually decreases, causing the free electrons to flow back to the original electrode. Thus, a TENG can be seen as a variable capacitor.

Assuming that the electrodes can be seen as infinitely large because the area size (S) of the metals is several orders of magnitude larger than their separation distance, the V–Q–x relationship can be derived based on electrodynamics, as shown below:⁷¹

where σ is surface charge density, x(t) is the separation distance related to time (t), S represents an effective contact area, and d₀ is the effective dielectric thickness, which can be expressed as , meaning the summation of all the thicknesses of the dielectric (d_i) between the two metal electrodes divided by its relative effective thickness (ε_ri). It is evident that the surface charge density of triboelectric materials plays a crucial role in determining the output performance of CS-TENGs.

Additionally, in the SL mode, relative displacement by sliding back and forth leads to a potential difference between the two electrodes, driving alternating electron flow in the external circuit.⁷⁷ The SE mode utilizes charge induction between a moving insulator and a grounded metal to generate electricity during contact and separation cycles.⁷³ The FS mode relies on asymmetric charge distribution caused by a moving object interacting with symmetric electrodes, producing AC current through periodic sliding.⁷⁸ Similarly, the surface charge density of triboelectric materials directly affects the output of TENGs.

2.3 The working mechanism of DC-TENGs based on the electrostatic breakdown effect

DC-TENGs are emerging based on different mechanisms, including triboelectrification and electrostatic breakdown effects,^70,79,80 the tribovoltaic effect,^81–84 and phase-shift design.^85–88 In this review, we mainly discuss DC-TENGs based on electrostatic breakdown, which have some unique advantages, such as high output power, efficient use of the surface triboelectric charges, suitability for low frequency mechanical movement and a simple structure compared with other kinds of DC-TENGs. Compared with AC-TENGs, DC-TENGs can not only drive commercial electronics directly without a rectifier unit, but also avoid electromagnetic interference and electrostatic breakdown limits.^89,90

A typical DC-TENG structure includes a triboelectric layer (TL), a frictional electrode (FE), and a charge collecting electrode (CCE), as illustrated in Fig. 3b.⁷⁰ In this configuration, the FE is in direct contact with the triboelectric layer to initiate contact electrification, while the CCE, positioned nearby but without touching the triboelectric layer, collects charges generated by electrostatic breakdown.

Using polytetrafluoroethylene (PTFE), an electret, as an example of a triboelectric layer (Fig. 3c), the device operates as follows:

(1) Initial state: the FE is aligned on the left side of the PTFE film, making contact and causing positive charges to accumulate on the FE and negative charges on the PTFE through the triboelectric effect (Fig. 3c(i)). PTFE can retain these negative charges on its surface for prolonged periods.

(2) Motion state: when the FE slides forward under an external force, a strong electrostatic field builds up between the CCE and the negatively charged PTFE. If this field exceeds the air breakdown threshold (3 kV mm⁻¹), air breakdown occurs, creating an ionized channel in the gap (Fig. 3c(ii)). This results in a continuous DC output current between the CCE and FE as long as the FE moves forward along with the PTFE.

(3) End of cycle: when the FE reaches the right side of the PTFE, the DC output ceases. During the backward motion, no DC output is generated in the external circuit because the residual surface charges on the PTFE are insufficient to trigger air breakdown (Fig. 3c(iii)).

Obviously, the triboelectric properties of triboelectric materials play a critical role in the output performance of DC-TENGs.

2.4 The figure of merit of AC/DC-TENGs

To evaluate the TENG's performance, the structure figure-of-merit (FOM_s) and the performance figure-of-merit (FOM_p) are proposed. They can be defined as:³¹where ε₀ is the vacuum dielectric permittivity, E_m is the maximum energy output, σ is the surface charge density, A is the triboelectrification area, x_max is the maximum displacement, Q_sc,max (Q_sc = σ × A) is the maximum short-circuit transferred charges and V_oc,max and are the maximum open-circuit voltage and the maximum achievable absolute voltage, respectively. According to the cycle for maximized energy output (CMEO), the maximum energy output can be calculated as⁹¹

E_m = 0.25 × Q_sc² × C_T (9)

C_T is the reciprocal of the absolute value of the V–Q curve slope. Obviously, a high surface charge density (σ) plays a critical role in any high-performance TENG.

In addition, Gao et al. proposed a new figure-of-merit, coulombic efficiency (η(V), the charge utilization efficiency of TENGs under a fixed load or voltage), to correctly evaluate TENGs' performance considering the issue of electrostatic breakdown.⁹¹ Coulombic efficiency can be calculated as

where α is a constant, relating to the TENG's inherent capacitor, the TENG's parasitic capacitor, and electrostatic breakdown. Even though different TENGs have the same Q_sc and V_oc, their maximum output energy is different. Under these conditions, the maximum energy output can be calculated as⁹¹

E_{m(η(Voc)≤50%} = 50% × Q_sc × V_η=50% (11)

E_{m(η(Voc)≥50%} = η_Voc × Q_sc × V_oc (12)

This comprehensive evaluation highlights the critical role of surface charge density of triboelectric materials for optimizing TENGs’ performance.

3 Performance limitations of AC/DC-TENGs

In practical applications, achieving high power density in both AC-TENGs and DC-TENGs is essential. As discussed in Section 2, the surface charge density of triboelectric materials is critical to the output of both AC-TENGs and DC-TENGs. Furthermore, the power density is proportional to the square of the surface charge density.³¹ However, the limiting factors for surface charge density differ slightly between AC-TENGs and DC-TENGs, as outlined below.

3.1 Limiting factors of surface charge density in AC-TENGs

The surface charge density of AC-TENGs (σ_AC-TENG) is described by the following equation:⁹²

σ_AC-TENG = φ × min(σ_{triboelectrification},σ_{air breakdown},σ_{dielectric breakdown}) (13)

where σ_{triboelectrification} is the charge density generated by triboelectrification between triboelectric layers, σ_{air breakdown} is the maximum surface charge density before air breakdown occurs between the triboelectric layers, σ_{dielectric breakdown} is the maximum charge density before the dielectric triboelectric layer experiences breakdown, and φ is the contact efficiency between the triboelectric layers. Their detailed influence is shown as follows:

(1) σ_{triboelectrification}: as the basic principle of AC-TENGs (as discussed in Section 2.1), σ_{triboelectrification} is determined by the physical and chemical properties of the triboelectric materials, as well as the contact efficiency between the triboelectric layers. Factors such as surface roughness, material work functions, and device mode significantly influence the triboelectric performance of the materials. For instance, to improve the σ_{triboelectrification}, some strategies, such as surface modification, material selection/optimization, improving charge retention capability, device structure design, etc., can enhance the triboelectrification properties between triboelectric layers.^93–96

(2) σ_{air breakdown}: as the surface charge density of the triboelectric material increases, the voltage between the triboelectric layers rises accordingly. Once this voltage surpasses the air breakdown strength, a portion of the surface charges leak due to air breakdown, and a certain amount of reverse charge may be deposited on the surface of the triboelectric material.^97,98 This phenomenon imposes a limitation on further increase in charge density within the TENG. With charge excitation technology overcoming the limitations of triboelectrification, air breakdown has emerged as the most critical factor constraining further performance improvements in TENGs. To suppress the air breakdown, various methods have been proposed, including high-vacuum conditions, ultrathin friction films and high-pressure environments to overcome the limitation of σ_{air breakdown}.^30,99,100

(3) σ_{dielectric breakdown}: if a TENG surpasses the air breakdown limit, any further increase in surface charge density may cause dielectric breakdown of the triboelectric material, thereby constraining the TENG's output. Currently, the issues of air breakdown and leakage properties of dielectric triboelectric materials remain unresolved; thus, σ_{dielectric breakdown} serves as the last limiting factor that needs to be addressed. When there is a dielectric breakdown problem, to solve it, selecting or synthesizing triboelectric materials with higher breakdown strength and enhancing the breakdown strength of existing materials are viable strategies to further improve the charge density of the TENG.

(4) φ: contact efficiency not only affects the amount of charge generated per cycle due to contact electrification. It will also affect the intrinsic capacitance of the TENG, which will directly affect the maximum charge storage capability of the TENG. To maximize TENG output, optimizing contact efficiency between triboelectric materials is essential. Strategies such as selecting an appropriate film thickness, adjusting surface roughness, increasing applied pressure, and other methods are effective for enhancing contact efficiency.

3.2 Limiting factors of surface charge density in DC-TENGs

The surface charge density of DC-TENGs (σ_DC-TENG) depends on both triboelectrification and electrostatic breakdown, expressed as:⁸⁹

σ_DC-TENG = k × min(σ_{triboelectrification},σ_{electrostatic breakdown}) (14)

where σ_{electrostatic breakdown} is the collected charge density from the electrostatic breakdown process between the triboelectric surface and the CCE and k is the factor related to the electrode structure, which is equal to the number of DC-TENG units.

(1) σ_{triboelectrification}: the electrostatic breakdown originates from the electrostatic field generated by triboelectrification charges between the triboelectric layers. Therefore, σ_{triboelectrification} is a critical factor for DC-TENG performance. Similarly, many strategies employed to enhance the triboelectrification of AC-TENGs can also be applied to DC-TENGs.

(2) σ_{electrostatic breakdown}: as the underlying mechanism of DC-TENGs, electrostatic breakdown at the gap between the triboelectric surface and the CCE is important for DC-TENGs. Generally, a higher σ_{electrostatic breakdown} results in more charges on the surface of the triboelectric layer, leading to a higher electric field in the gap and higher σ_{electrostatic breakdown}. Therefore, σ_{triboelectrification} is a key limiting factor for σ_{electrostatic breakdown}. In addition to triboelectric charges, the gap distance between the triboelectric surface and the CCE influences the strength of the electric field. A smaller gap distance allows more charges to undergo breakdown, leading to higher σ_{electrostatic breakdown}. Furthermore, after the electric field between the triboelectric layer and the CCE is established by the triboelectric charges on the surface of triboelectric layers, the breakdown medium will affect the extent of electrostatic breakdown. According to Paschen's law, different gases and pressures exhibit different breakdown strengths. Hence, the gap distance and breakdown medium, including gas species and pressure, are also important limiting factors for σ_{electrostatic breakdown}.

(3) k: the design of the electrode structure significantly influences the number of electrostatic breakdowns occurring during a sliding process, thereby impacting the final output performance. For instance, Zhao et al.⁸⁹ presented a microstructure-designed DC-TENG where a single large-scale FE was replaced with patterned micrometer-sized FEs, interlaced with micrometer-sized CCEs between two adjacent FEs. This configuration enables multiple contact electrification and electrostatic breakdowns within a single sliding process. Finally, the charge density at k = 50 was about 13 times higher than that at k = 5. Obviously, the higher k means a higher charge density for DC-TENGs. The optimal k needs to be determined according to the specific device design and manufacturing conditions.

4 Strategies for optimizing triboelectric materials in AC-TENGs

4.1 Enhancing intrinsic triboelectrification

4.1.1 Material selection – triboelectric materials series. The selection strategy for appropriate triboelectric materials is crucial for enhancing the performance of TENGs and other electrostatic applications, often relying on the triboelectric series. To characterize the triboelectrification of existing materials and enable researchers to use existing triboelectric materials easily, the triboelectric series was first published by Johan Carl Wilcke in 1757.^101,102 The triboelectric series ranks materials based on their inherent tendency to gain or lose electrons, highlighting their natural charge-transfer properties. As research has progressed, the triboelectric series has evolved from being primarily empirical to a more quantitative framework,¹⁰³ with versions even being established under controlled conditions that exclude environmental interference (humidity and gas).³⁰ With the triboelectric series serving as a guide to identify optimal material pairs, the charges achieved by triboelectrification can be maximized, thereby enhancing TENGs’ performance.

A wide range of materials have been studied for their triboelectrification and established the triboelectric series quantitatively, including organic polymer films,^103–105 inorganic non-metallic materials,¹⁰⁶ liquids¹⁰⁷ and some two-dimensional materials.^108,109 Compared with other kinds of triboelectric materials, organic polymers show higher charge density (Fig. 4), making them the most widely used option. Representative materials include polytetrafluoroethylene (PTFE),^98,110–113 polyvinyl chloride (PVC),⁹¹ fluorinated ethylene propylene (FEP),^91,98 polyimide (PI),^{98,112,114,115} and polyethylene (PET).⁹⁸ Among them, PTFE shows higher electronegativity due to its perfluorocarbon chain structure. Moreover, PTFE's low friction coefficient and excellent hydrophobicity enable stable high output even in complex environments.

Fig. 4 Triboelectric series of (a) organic polymers,³⁰ Copyright 2022, Springer Nature, (b) inorganic non-metallic materials,¹⁰⁶ Copyright 2020, Springer Nature, (c) liquids,¹⁰⁷ Copyright 2023, Wiley-VCH and (d) two-dimensional (2D) materials,^108,109 Copyright 2024, Wiley-VCH. Copyright 2018, Wiley-VCH.

Other materials, such as polyamide (PA),¹¹⁶ polydimethylsiloxane (PDMS),¹¹⁷ polyvinylidene difluoride (PVDF),¹¹⁸ polyurethane (PU),^119,120 and various natural materials (wood, cellulose, cotton, and silk),^121–123 are also utilized. Each material possesses unique characteristics and should be selected based on specific requirements. For example, PDMS is widely utilized in the medical field due to its exceptional biocompatibility and natural materials are particularly valued for their biodegradability and environmental friendliness. Moreover, hydrogels and organogels with a three-dimensional network structure, offering exceptional flexibility, tunable conductivity and mechanical properties, have been recognized as promising triboelectric materials for wearable flexible TENGs.^40,124 In TENGs, conductive hydrogels and organogels are primarily used as electrode materials, enhancing triboelectric performance and enabling applications in flexible, stretchable sensing devices.^{42,43,125,126} However, despite significant progress, the triboelectric performance of existing materials still fails to meet the requirements of ongoing development, necessitating the continuous advancement of materials optimization and synthesis.

4.1.2 Materials optimization and synthesis. To achieve high surface charge density, it is essential to maximize the contact area to enhance contact efficiency. One effective approach is to create micro/nano-patterns on the surface of a triboelectric material. This can be realized using techniques such as etching, template molding, lithography, and other fabrication methods. Three types of patterned arrays (line, cube, and pyramid) were first fabricated on polymers by the photolithography method to improve contact efficiency.¹²⁷ However, the photolithography method is hindered by challenges such as high cost and a complex preparation process. To address these issues, Luo et al. proposed a cost-effective and facile electrospray-etching method for fabricating micro/nano-pores on silk fibroin films (Fig. 5a).¹²⁸ Quasi-periodically distributed pores were generated by the patterned surface electric field on the etched silk fibroin film, as shown in Fig. 5b. The hierarchically structured silk fibroin film, incorporating SiO₂ nanospheres, exhibited a maximum output voltage 2.6 times higher than the planar device, due to the rougher surface and the charge retention capability of the SiO₂ electrets (Fig. 5c). Additionally, a novel diamond-structured fabric-based triboelectric nanogenerator is developed using an easy, economical, and scalable weaving method, with no chemical modifications, shown in Fig. 5d.³² The surface interactions were enhanced for greater charge generation, while the strengthened mechanical engagement promoted more efficient charge transfer. An adhesive contact model considering textures was also established, revealing the correlation between the contact area and electrical performance of TENGs with textured surfaces and providing the theoretical and experimental basis for texture design of TENGs (Fig. 5e–h).¹²⁹ Subsequently, other surface patterns have been designed to maximize the contact area, such as nanowires/nanopillars, rings, dots, holes, and spongy/porous structures.^130–134

Fig. 5 (a) The fabrication process of micro/nano-pores on silk fibroin by an electrospray-etching method. (b) SEM images of a hierarchically porous silk fibroin film. (c) Output voltages of devices with different silk fibroin films.¹²⁸ Copyright 2020, Elsevier. (d) Structural diagram of a TENG with diamond structure fabric woven through a weaving machine.³² Copyright 2024, Wiley-VCH. (e) (i) Schematic diagrams of the contact and state of a vertical-contact-mode TENG; (ii) and (iii): the adhesive contact and separation between the electrode and textured layer, respectively; (iv): a unit block of the pyramid textured surface. (f) Schematic representation of the contact image formation. (g) Variations of the contact area with applied force. (h) The comparison between experimental results and numerical results of open-circuit voltage with applied force.¹²⁹ Copyright 2019, Elsevier.

In addition, surface chemical modification is also an effective way to improve the surface charge density, which combines some appropriate functional groups on the surface to induce easy electron loss or gain. In 2014, Zhang et al. used a single-step fluorocarbon plasma treatment to cover C₄F₈ molecules on PDMS's surface.¹³⁵ To improve both effectiveness and long-term stability, a surface modification method using low-energy ion irradiation with controllable energy and fluence has been proposed to tailor the chemical bonds and functional groups of triboelectric polymers at the molecular level.¹³⁶ For example, Kapton was irradiated with He ions with 50 keV and 10¹⁶ ions cm⁻² (Fig. 6a). The Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectra revealed the generation of a strong electron-donating group, attributed to the conjugation effect between the benzene ring and the irradiation-induced –NHCOR functional group (Fig. 6b). This modification accounts for the significant charge density observed in the modified Kapton film (KAPTON1E16), as shown in Fig. 6c. Furthermore, many functional groups, such as CO₃⁻, NO₂⁻, NO₃⁻, O₂⁻, O₃⁻, poly-L-lysine, 1H,1H,2H,2H-perfluorooctyl-chlorosilane (FOTS), 3-aminopropyltriethoxysilane (APTES), 3-glyci-doxypropyltriethoxysilane (GPTES), and trichloro(3,3,3-trifluoropropyl)silane (TFPS), were introduced onto the triboelectric surface to improve the triboelectric performance.^138–140 Moreover, Li et al. investigated the impact of functional groups on the charge enhancement of polymers in polymer–polymer and polymer–liquid triboelectric interactions.⁹³ The results show that the strength of the electron-withdrawing ability of each functional group follows the order CH₃ < H < OH < Cl < F (Fig. 6d and e). Fluorinated groups, particularly –CF₃, and unsaturated groups like –CF=CF₂ show strong electron-withdrawing abilities, enhancing surface charge density, especially in PTFE and FEP, which provide guidance for material optimization and synthesis. It is evident that while surface patterns and chemical modification are effective for performance optimization, their complex preparation processes drive up production costs and present challenges for large-scale manufacturing.

Fig. 6 (a) Schematic diagram of ion irradiation simulation. (b) ATR-FTIR spectra and the molecular structure change of the Kapton film before and after ion irradiation. (c) Charge density of different irradiated Kapton films in contact with FEP.¹³⁶ Copyright 2020, Elsevier. (d) Testing method for polymers and liquids; alternative polymer films with the main carbon chain and different side chains. (e) The amounts of transferred charges between polymers/liquids and Al.⁹³ Copyright 2020, Wiley-VCH. (f) Synthetic scheme of fluorinated sulphur copolymers. (g) The image of the fluorinated sulphur film. (h) Elemental analysis of sulphur copolymer and surface fluorinated sulphur copolymer films. (i) Peak-to-peak voltage and current of sulphur copolymers, surface fluorinated sulphur copolymers and PTFE.¹³⁷ Copyright 2019, Elsevier.

Rational molecular design, based on the varying electron-withdrawing abilities of different functional groups, is key to enhancing triboelectric performance and this approach offers better cost control and is well-suited for large-scale production. Lee et al. designed high-performance triboelectric materials by rationally selecting molecular structures based on electron affinity and electron valency from the periodic table.¹³⁷ Unlike conventional methods using triboelectric carbon-based polymers (e.g., PTFE, PVDF, with an electron affinity of −123 kJ mol⁻¹ and a valency of 4), they employed inorganic polymers with a sulphur backbone (electron affinity of −200 kJ mol⁻¹ and valency of 6), specifically fluorinated sulphur copolymers (Fig. 6f–h). The increased electron affinity of the sulphur backbone and hypervalency resulted in a 6-fold increase in voltage output and a 3-fold increase in current output compared to fluorinated carbon-based polymer films such as PTFE (Fig. 6i). On the other hand, copolymerization is an effective strategy to tailor and enhance triboelectric properties by combining the advantages of two or more polymers. This approach not only can adjust molecular structures and functional groups but also modify crystallization behaviors. For example, PVDF graft copolymers were designed by incorporating poly(tert-butylacrylate) (PtBA) through the atom-transfer radical polymerization (ATRP) technique (Fig. 7a).¹⁴¹ Grafting PtBA onto the PVDF backbone suppressed the formation of β phases, resulting in copolymers primarily composed of α phases (Fig. 7b). As the grafting ratio increased to 18%, the dielectric permittivity significantly improved from 8.6 to 16.5 due to the increased net dipole moment supported by KPFM (Fig. 7c and d). The TENG fabricated with this graft copolymer achieved a current density of 18.9 μA cm⁻² (Fig. 7e). After polarization, the current density can be further improved to 25 μA cm⁻² (Fig. 7f). In addition, the repeated rheological forging method was also used to increase the density of functional group composition, as well as crystallinity and dielectric properties of FEP, successfully achieving a high surface charge density of 352 μC m⁻² (Fig. 7g–j).³³

Fig. 7 (a) Synthetic scheme of PVDF-Gn. (b) XRD pattern, (c) dielectric constant, and (d) KPFM surface potential distribution of PVDF-Gn films. (e) Current density generated by the PVDF-based TENGs. (f) Current densities for PVDF-G18-based TENGs in different poling electric field directions.¹⁴¹ Copyright 2017, Springer AAAS. (g) Illustration of the repeated rheological forging technique. (h) Fourier transform infrared (ATR-FTIR), and (i) X-ray diffraction (XRD) pattern of FEP films. (j) Saturated charge density comparison in the previous studies of TENGs.³³ Copyright 2022, Springer Nature.

4.2 Suppressing air breakdown

When working in the air environment, the TENG output is constrained by air breakdown. Through both theoretical and experimental analyses, researchers have identified the key factors influencing air breakdown and have proposed innovative solutions, such as air conditions control, materials optimization, and reducing reverse charge deposition.^{30,34,100,142} These methods significantly limited air breakdown and promoted the development of high-performance TENGs.

4.2.1 The limitation of air breakdown under charge excitation technology. To achieve a high charge density, the balance between charge accumulation and decay of the TENG is mainly regulated. Limited by the triboelectrification and charge storage ability of conventional triboelectric materials, the charge density of CS-TENGs is only about 1.25 mC m⁻² under vacuum conditions even when excluding the effect of the environment.³⁰ The introduction of charge excitation technology successfully broke the limitation of triboelectrification of triboelectric materials, making the output performance of TENGs reach a new level.^143–146

Although charge excitation technology has broken through the limitations of the electrification properties of triboelectric materials and achieved breakthrough performance, the problems of charge decay and air breakdown have not been solved. When the electric field of the air exceeds the critical electric field for air breakdown, bound electrons are removed from the gas molecules by impact ionization and positively charged ions are formed. Directional migration occurs under the action of an electric field, showing an electric spark or arc phenomenon.^147,148 In 2014, the phenomenon of TENG maximum surface charge density limited by air breakdown was discovered and confirmed by Wang et al.¹³⁸ Numerous studies have revealed that air breakdown is unavoidable when TENGs are operated in air, and the maximum surface charge density of TENGs is limited by the air breakdown.^92,149,150 The application of charge excitation technology promotes the increase of charge density, and this increased surface charge density will increase the voltage of the air gap, thus making the air breakdown more serious. Consequently, in this part, we mainly summarize the suppression of air breakdown through triboelectric material strategies to achieve the optimization of TENGs' performance.

4.2.2 The relationship between the dielectric properties of the triboelectric layer and air breakdown. To suppress air breakdown using triboelectric materials, understanding the relationship between their dielectric properties and air breakdown is essential. Some studies have shown that the voltage of the air gap (V_gap) is related to the dielectric properties and thickness of triboelectric layers in TENGs. For example, in a CS-TENG, this voltage forms between the top electrode and dielectric surface. Based on the parallel-plate model, the voltage under short-circuit conditions can be derived and expressed as⁹²where σ is the triboelectric surface charge density on the electrode. d and ε_r are the thickness and relative permittivity of the dielectric, respectively. ε₀ and x represent the vacuum permittivity (ε₀ = 8.85 × 10⁻¹² F m⁻¹) and the distance between the top electrode and dielectric.

In addition, according to Paschen's law, the breakdown voltage (V_b) between two parallel plates can be determined empirically using

where P is the barometric pressure. x is the gap distance between the two plates. A and B are the constants determined by the atmosphere, including atmospheric composition, relative humidity, temperature, etc. A is inversely proportional to relative humidity and directly proportional to the maximum ionization. For common ambient conditions, A = 2.87 × 10⁷ V atm⁻¹ m⁻¹ and B = 12.6.

Obviously, air breakdown can be avoided when V_b is larger than V_gap. In this case, the maximum charge density for different dielectrics can be obtained as

Consequently, the suppression of air breakdown can be achieved by using a triboelectric material with high dielectric permittivity and a thin thickness. To unify these two parameters, the specific capacitance (ε₀ε_r/d) was defined. This offers an insightful strategy for achieving high-performance TENGs.

4.2.3 Performance optimization and suppression of air breakdown in AC-TENGs. Numerous studies have been dedicated to suppressing the air breakdown through improving the specific capacitance of triboelectric materials. Table 1 summarizes various triboelectric materials used in charge excitation TENGs, highlighting the correlation between high specific capacitance and improved charge density and power density.^{34,36,120,142,144,146,151–153} This evidence demonstrates the effectiveness of enhancing specific capacitance in optimizing TENG performance. In general, it is more challenging to enhance the dielectric permittivity of a triboelectric material than to reduce its thickness. There are two main ways to increase the dielectric permittivity of triboelectric materials: one is to develop a new type of triboelectric polymer with a high dielectric permittivity and the other is composite films.

Table 1 Triboelectric materials employed in charge excitation TENGs in previous studies

Materials	Dielectric permittivity (1 kHz)	Thickness (μm)	Specific capacitance (F m^−2)	Charge density (mC m^−2)	Power density (W m^−2 Hz^−1)	Ref.
a Peak power density. b Average power density.
PU	4	1000	3.54 × 10^−8	0.25	40.9^a	120
PI	3.8	9	3.74 × 10^−6	0.81	9.55^a	144
PP	2.4	5	4.25 × 10^−6	1.02	39.6^a	146
PI-PVDF	6.3	7	7.97 × 10^−6	2.2	30.7^a	151
P(VDF-TrFE)	7.5	9	7.38 × 10^−6	2.2	16^a	152
PZT/PVDF	23	7	2.91 × 10^−5	3.53	35.6^a	142
BOPP	3.3	2	1.46 × 10^−5	4.24	127^a	153
P(VDF-TrFE-CFE)	70	8	7.74 × 10^−5	8.6	0.77^b	34
BTO/P(VDF-TrFE-CFE)	52	8	5.72 × 10^−5	8.85	0.85^b	36
PEI/P(VDF-TrFE-CFE)	50	8	5.5 × 10^−5	9.23	0.92^b	36

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In the past, the commonly used triboelectric polymers with high dielectric permittivity were mainly PVDF and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), and their dielectric permittivity is all less than 12 at 1 kHz. Even if the thickness of P(VDF-TrFE) is controlled at 9 μm (specific capacitance: 7.38 × 10⁻⁶, 1 kHz), the charge density can only be increased to 2.2 mC m⁻² in SCE-TENGs (Fig. 8a–c).¹⁵² It was not until 2024 that Liu et al. first proposed a triboelectric polymer with high dielectric permittivity, poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)), and increased its specific capacitance to 7.74 × 10⁻⁵ (1 kHz).³⁴ By combining with external charge excitation technology, they achieved a charge density of 8.6 mC m⁻², nearly doubling the previously reported optimal value (Fig. 8d–i). This fully validates the effective suppression of air breakdown by triboelectric materials with high specific capacitance.

Fig. 8 (a) Structural illustration of a SCE-TENG. (b) Relative permittivity of three different dielectric layers. (c) Surface charge density and output current density of the SCE-TENG.¹⁵² Copyright 2021, Wiley-VCH. (d) Maximum theoretical charge density of P(VDF-TrFE-CFE) films with different thicknesses. (e) Frequency dependence of dielectric permittivity and cross-sectional and surface SEM images of the P(VDF-TrFE-CFE) film. (f) The structure of an ECE-TENG. (g) Charge density of an ECE-TENG with a 1 cm² external TENG and a 1 cm² main TENG, an ECE-TENG with a 4 cm² external TENG and a 1 cm² main TENG, and an ECE-TENG with a 10 cm² external TENG and a 1 cm² main TENG. (h) V–Q curve of the ECE-TENG under a load of 2 MΩ. (i) Comparison of the charge density of different TENGs.³⁴ Copyright 2024, Royal Society of Chemistry. (j) 3D diagram of the PZT/PVDF composite film. (k) The structure of the TENG. (l) Relative permittivity before and after self-polarization. (m) The self-polarization effect. (n) Output charge density comparison in the developmental stage of the TENG.¹⁴² Copyright 2022, Wiley-VCH.

In addition, compositing polymers with inorganic materials possessing higher dielectric permittivity is an effective way to increase specific capacitance. For example, a composite film with high relative permittivity (23 at 1 kHz) and thin thickness (7 μm) was successfully synthesized by coupling lead zirconate titanate (PZT) and PVDF (Fig. 8j–l).¹⁴² The specific capacitance increases from 2.78 × 10⁻⁵ to 2.91 × 10⁻⁵. By coupling the self-polarization effect (Fig. 8m), a high charge density of 3.53 mC m⁻² was achieved at 5% relative humidity (Fig. 8n). In addition to PZT, other commonly available inorganic materials with high dielectric permittivity, such as barium titanate (BTO), calcium copper titanate (CCTO), can also be used. Also, Liu et al. showed that the specific capacitance of the BTO/P(VDF-TrFE-CFE) composite can be improved to 5.72 × 10⁻⁵ by introducing high-polarity interfaces. Based on the effective suppression of air breakdown and charge loss, the charge density of the ECE-TENG was improved from 8.31 mC m⁻² to 8.85 mC m⁻².³⁶ By further suppressing charge loss through chain stacking design, the charge density and power density were increased to 9.23 mC m⁻² and 0.92 W m⁻² Hz⁻¹ in the ECE-TENG with the PEI/P(VDF-TrFE-CFE) composite.

Compared to current polymers with high dielectric permittivity, inorganic ceramics offer significantly higher dielectric permittivity (>1000). However, their inherent brittleness makes them prone to fracture at the micrometer scale during operation. Despite their advantages in dielectric properties, the fabrication of inorganic ceramics with high specific capacitance remains challenging due to thickness limitations. Additionally, the high dielectric permittivity of inorganic ceramics also leads to greater charge loss in electric fields.¹⁵⁴ These factors collectively restrict their application in enhancing the performance of TENGs. Looking ahead, high-dielectric polymers are expected to remain a primary focus of research.

5 Strategies for optimizing triboelectric materials in DC-TENGs

As discussed in Section 3.2, the optimization strategies for DC-TENGs can be categorized into two main approaches: increasing triboelectrification (σ_{triboelectrification}) and enhancing electrostatic breakdown (σ_{electrostatic breakdown}). Some methods, such as material selection and modification,^155,156 contact efficiency,¹⁵⁷ liquid lubrication¹⁵⁸ and environmental control,^159–161 were used for increasing triboelectrification. In addition, enhancing electrostatic breakdown can be achieved through structural design^89,110,162 and environmental control.^159–161 In this section, we provide a summary of key material advancements for enhancing both triboelectrification and electrostatic breakdown in DC-TENGs.

5.1 Materials selection in DC-TENGs

In DC-TENGs, the output performance is closely tied to the triboelectrification properties of the triboelectric layers. Higher triboelectrification between them contributes directly to an increase in DC output. Among them, it is essential to select appropriate triboelectric materials for obtaining a high-performance DC-TENG. Triboelectric materials for high-performance DC-TENGs should consider the following characteristics:

(1) Surface charge density: as the power density of TENGs is proportional to the square of the surface charge density,³¹ achieving a high charge density is the most critical factor in selecting triboelectric materials for high-performance TENGs.

(2) Friction coefficient: since DC-TENGs operate in sliding mode, the durability of the device heavily depends on the triboelectric material's wear resistance. Therefore, it is crucial for the triboelectric material to possess an appropriate friction coefficient to ensure reliable performance over time.

(3) Leakage current: the surface charge density achieved between the triboelectric material pair represents the equilibrium state between triboelectrification and charge decay processes.¹⁵² Charge decay is influenced by air breakdown and the triboelectric layers’ leakage current. Consequently, the leakage current characteristics of triboelectric materials is a critical consideration of material selection in certain structured DC-TENGs.

(4) Polarization: due to the internal polarization effect in triboelectric materials, the triboelectric charges near the surface become bound, which makes it harder for these charges to participate in the air breakdown process.¹⁵⁵ Therefore, selecting triboelectric materials with appropriate polarization is crucial for maximizing the availability of surface charges.

(5) Charge utilization ratio (η): the DC-TENG based on electrostatic breakdown collects the charges caused by the air breakdown between the CCE and triboelectric layer, rather than the surface charges on the triboelectric layer generated by the triboelectric effect. Generally, more surface charges create a stronger electric field in the gap between the CCE and triboelectric layer, enhancing the air breakdown effect and improving the charge utilization ratio. Therefore, the charge utilization ratio, which represents the ratio of collected charges to surface charge density, is also a critical factor for an excellent triboelectric material.

(6) Stability: the stability of triboelectric materials, with consistent voltage and current output, is crucial for ensuring the repeatability and accuracy of TENG performance over time.

To select appropriate triboelectric materials for obtaining a high performance TENG in practical applications. Zhao et al. first provided a comprehensive material selection model for selecting triboelectric materials for DC-TENGs based on five performance evaluation indices, including surface charge density, friction coefficient, polarization, utilization rate of charges, and stability (Fig. 9a).¹⁵⁵ They took PVC, FEP, PTFE, polyetheretherketone (PEEK), PVDF, and PI as representative materials. As shown in Fig. 9b, PVC, FEP, and PTFE films present a high charge density, high DC output and high utilization rate (60%) of contact electrification charges in the air breakdown process. Meanwhile, PVC excels in charge density, PTFE offers superior friction and the FEP film shows inferior stability for DC-TENGs. In contrast, PEEK and PI films show moderate properties, and PVDF has low efficiency and stability. With the help of a comprehensive selection strategy, the effective surface charge density of DC-TENGs can be increased by about 60% via utilizing PVC films compared to DC-TENGs with PTFE films (Fig. 9c).

Fig. 9 (a) Selection rules of triboelectric materials for DC-TENGs. (b) Comprehensive selection rules of triboelectric materials when considering the normalized effective surface charge density of the AC-TENG , the effective surface charge density of the DC-TENG , the ratio of effective charges to triboelectrification charges (η*), the reciprocal of the friction coefficient (1/μ*) and stability (ρ*). (c) The relationship between the surface charge density and effective surface charge density of DC-TENGs with different triboelectric materials.¹⁵⁵ Copyright 2021, Springer Nature. (d) The diagram of the output of the DC-TENG improved by the dielectric enhancement effect. Comparison of output charge (e) and open-circuit voltage (f) between a normal DC-TENG and a dielectric enhanced DC-TENG with a dielectric enhancement layer. (g) Working mechanism of the dielectric enhanced DC-TENG.³⁵ Copyright 2022, Elsevier. (h) The schematic diagram of a BDC-TENG and output charge of various negatively charged materials with positively charged nylon.¹⁶³ Copyright 2022, Wiley-VCH. (i) The schematic diagram of an ICC-TENG and output current of the ICC-TENG with different tribomaterial pairs.³⁷ Copyright 2024, Royal Society of Chemistry. (j) The schematic diagram of the working mechanism of a CTC-TENG.¹⁶⁴ Copyright 2024, Royal Society of Chemistry.

5.2 Triboelectrification optimization based on dielectric enhancement materials

The use of dielectric enhancement materials is an effective strategy to improve the output performance of DC-TENGs due to the improvement of triboelectrification. This concept was first introduced by Cui et al. in 2022 (Fig. 9d).³⁵ Specifically, when PTFE comes into contact with nitrile, it exhibits a higher charge and voltage compared to its interaction with Cu, PE, or Kapton (Fig. 9e and f). This improvement is not only attributed to the increased triboelectrification but also the leakage properties of the dielectric enhancement layer, which allow breakdown electrons to pass through the nitrile film and into the dielectric layer as leakage current, thereby enabling continuous DC output (Fig. 9g). Finally, the dielectric enhancement effect boosts the output performance of DC-TENGs by a factor of 2.6.

Similarly, Shan et al. presented a novel bidirectional and double-channel TENG (BDC-TENG) design that replaces conventional metal layers with dielectric materials (Fig. 9h).¹⁶³ Nylon was used as the triboelectric layer due to its strong electropositive properties. The performance of different electronegative triboelectric materials (Kapton, PE, and FEP) was evaluated. The findings show that the pair of nylon and PTFE, recognized as one of the most effective positive and negative triboelectric material pairs, yields the highest output performance. Additionally, Li et al. introduced an innovative integrated constant current TENG (ICC-TENG), addressing the challenge of charge loss caused by air breakdown between tribo-materials (Fig. 9i).³⁷ They also investigated the impact of tribo-material selection on the triboelectrification process in the ICC-TENG. The output current was tested using various tribo-materials for the belt (foam, nylon, and polyester) and the stator (PTFE, FEP, PVC, PVDF, and PET). The results revealed that the optimal output performance is achieved when foam is used as the belt's tribo-material and PTFE as the stator's tribo-material. Moreover, a triboelectric material pair, foam and PTFE, was used in the newly designed charge target collection TENG (CTC-TENG) due to their excellent triboelectrification properties (Fig. 9j).¹⁶⁴

To select an appropriate triboelectric material pair (TMP) for improving DC-TENGs’ performance based on dielectric enhancement materials, Cui et al. proposed a universal strategy by simultaneously considering five parameters, including the friction coefficient, surface charge density, leakage current, breakdown charge density and breakdown efficiency.¹⁵⁶ Different TMPs are evaluated, including thermoplastic urethanes (TPU)–ethylene-tetra-fluoro-ethylene (ETFE), polyamide (PA)–ETFE, TPU–PTFE, PA–PTFE, nitrile–PTFE, TPU–polyfluoroalkoxy (PFA), and PA–PFA. Among them, the TPU–ETFE pair presents better comprehensive performance based on the selection strategy, showing excellent normalized charge density and a breakdown efficiency of 0.3. Meanwhile, it exhibits appropriate friction performance.

By targeting material properties such as surface charge density, wear resistance, leakage current, and polarization, DC-TENG performance can be effectively improved. Advanced material selection models and TMP optimization strategies pave the way for developing high-performance triboelectric materials, enhancing both triboelectrification and electrostatic breakdown control.

6 Applications of TENGs

As devices that convert mechanical energy into electrical energy, TENGs can capture various forms of mechanical energy from the environment to power electronic devices.^{12,37,153,165–167} To better illustrate the advantages of TENGs, a comparison with PENGs and EMGs is provided in Table 2. Compared to PENGs and EMGs, TENGs offer several advantages, including a wide selection of materials, simple structure, low weight, and high efficiency at low frequency, among others.^67,168,169 They exhibit considerable potential for applications in high-entropy energy harvesting and intelligent sensing. For example, Ren et al.¹² designed an energy regularization TENG (ER-TENG) (Fig. 10a), which temporarily stores wave energy in coil springs via a pendulum and one-way bearings, converting it into controlled rotational motion. This motion drives the TENG for stable output through a gear set and centrifugal speed limiter. By optimizing wear resistance and electro-mechanical conversion efficiency with ternary dielectric materials (PTFE, PA, and polyethylene (PE)) and multi-layer units, the rectified ER-TENG is capable of directly lighting 944 LEDs (Fig. 10b). Additionally, the potential of the ER-TENG for smart ocean systems is further demonstrated by its ability to power devices such as an anemometer and a nuclear radiation detector (Fig. 10c and d).

Table 2 Comparison of TENGs, PENGs and EMGs^67,168,169

	TENGs	PENGs	EMGs
Mechanism	Triboelectrification, electrostatic induction	Piezoelectric effect	Electromagnetic induction
Output characteristics	Ultra-high voltage (hundreds of V–kV)	High voltage (<hundreds of V)	Wide voltage range (mV–kV)
	Low current (<mA)	Low current (<mA)	High current (>mA)
Pros	Wide material selection	Small volume	High power
	Simple structure	Easy to integrate	High efficiency at high frequency
	Low weight	High humidity resistance	High durability
	Low cost	High flexibility	Easy to scale up
	High efficiency at low frequency		Mature technology
	High flexibility
Cons	High matched impedance	High matched impedance	Low output for small devices
	Humidity sensitive	Limited material selection	Heavy
	Low durability		High cost
Applications	High entropy energy harvesting	Vibration energy harvesting	Large-scale power generation (wind/hydroelectric power station)
	Intelligent sensing

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Fig. 10 (a) Schematic diagram of an ER-TENG's internal structure. (b) 944 LEDs powered by the ER-TENG in water. (c) An anemometer and (d) a nuclear radiation detector powered by the ER-TENG with PMC.¹² Copyright 2024, Elsevier. (e) Schematic diagram of an umbrella with Zn-Car_TPHG. (f) Six LEDs powered by Zn-Car_TPHG under squirting water. (g) The voltage and current signals of Zn-Car_TPHGs at different parts of the sole.¹⁶⁷ Copyright 2025, Wiley-VCH. (h) Schematic diagram of the triboelectric sensor and pressure sensor. (i) Schematic diagram of tactile perception on five fingers. (j) The object shape and temperature recognition by a robot hand with triboelectric sensors.¹⁷⁰ Copyright 2025, Springer Nature. (k) Triboelectric stethoscope for cardiac sound sensing and its structure. (l) The matrix for the five triggers.¹⁷¹ Copyright 2024, Wiley-VCH.

In addition to wave energy, TENGs also show significant potential for harvesting raindrop energy and human mechanical energy. A sandwich-like double-electrode flexible tribo-piezoelectric hybrid nanogenerator (TPHG) structure is created by integrating Zn–Car particles (a metal–organic framework synthesized via a solvothermal method using zinc(II) and carnosine) between patterned and smooth PDMS layers (Fig. 10e).¹⁶⁷ This structure combines both triboelectric and piezoelectric effects to enhance output. Zn-Car_TPHG is well suited for outdoor surfaces to capture raindrop energy. After 12 hours of charging a 1 μF capacitor with Zn-Car_TPHG, six LEDs were successfully illuminated (Fig. 10f). Furthermore, Zn-Car_TPHG was attached to the foot to monitor gait and movement conditions based on the magnitude and waveform of the voltage and current signals generated at different locations (Fig. 10g).

With the development of artificial intelligence, the demand for sensors to collect information has grown significantly. Various sensors based on TENGs have been extensively studied.^26,170–173 For example, to enhance robot intelligence, tactile sensors that enable robots to interact with their surroundings in a friendly manner are essential. Liu et al.¹⁷⁰ developed a triboelectric tactile sensor that responds to pressure and temperature beyond the human tactile perception range in extreme environments (Fig. 10h). This is attributed to stable cellulosic materials at high temperatures, combined with an asymmetric structure that allows dual signal output, enabling multiple responses under complex conditions. The sensors were then integrated into the five fingertips of a robotic hand, forming an intelligent tactile system capable of identifying the shape and temperature of objects in extreme environments, with an average recognition accuracy of around 94% (Fig. 10i and j). Additionally, a high-performance stethoscope based on a triboelectric acoustic sensor (TAS) was developed for cardiac sound sensing and disease diagnosis (Fig. 10k), using FEP as the triboelectric material.¹⁷¹ The TAS demonstrates a sensitivity of 1215 mV Pa⁻¹ and a signal-to-noise ratio (SNR) of 56 dB under low-intensity acoustic stimuli. A point-of-care testing interface was also designed to intelligently identify and diagnose five heart conditions with 97% accuracy (Fig. 10l). These advancements highlight the promising future of intelligent sensing based on TENGs in the smart era.

7 Conclusion and outlook

In this review, recent advancements in materials-related strategies for AC-TENGs and DC-TENGs are summarized. The working mechanisms, performance limitations, and materials-based strategies are carefully reviewed to serve as a guide for future research and innovation in AC-TENGs and DC-TENGs. For high-performance AC-TENGs, triboelectric materials with high triboelectrification, high specific capacitance, low leakage current, and excellent charge stability are essential. Similarly, for high-performance DC-TENGs, triboelectric materials with high triboelectrification, low polarization, an appropriate friction coefficient, and excellent charge stability are required. Despite these advancements, several challenges (Fig. 11) must be addressed to further enhance the performance of TENGs and bring them closer to practical applications. These challenges include:

Fig. 11 Triboelectric materials to be further developed.

(1) TENGs based on existing triboelectric materials still fall short of meeting the demands of practical applications. It is essential to develop new triboelectric materials with higher triboelectricity, charge storage capacity, and stability for high-performance TENGs. This effort is the most direct way to enhance output power and enable TENGs for various energy harvesting and storage applications.

(2) When a TENG operates at room temperature, two primary charge dissipation phenomena occur: electron emission caused by the electric field and the dissipation of surface triboelectric charges into the material's interior. As the surface charge density of the triboelectric material increases, the negative impact of these phenomena becomes more and more obvious and significant. On the other hand, when the TENG is exposed to higher temperatures, the thermal electron emission phenomenon will severely degrade the performance of the TENG. In future research, suppressing charge dissipation caused by the electric field and thermal field in triboelectric materials, aiming to further improve the TENG's output performance, is a very important research topic.

(3) Humidity causes polar water molecules to absorb onto triboelectric surfaces, leading to charge dissipation and reduced TENG output. For example, a TENG with charge excitation achieves a charge density of 6.36 mC m⁻² under 85% humidity,³⁶ but this still falls short of practical requirements. To enable the widespread practical application of TENGs, it is crucial to address the issue of reduced output in environments such as marine environments, high-humidity environments, rainy conditions, and exposure to sweat, among others. Packaging offers an effective solution. However, designing materials that maintain high charge density under humid conditions through innovative material designs remains both a significant challenge and an exciting opportunity.

(4) Wear resistance refers to the ability of a material to withstand wear and tear caused by friction, mechanical stress, and surface degradation over time. In TENGs, wear resistance is a critical parameter because it directly impacts the longevity and durability of the materials and the consistency of output performance after multiple friction cycles. TENGs operate based on the triboelectric effect, which involves repeated mechanical movement, such as contact-separation or sliding. Over time, this movement can lead to surface degradation, such as abrasion or erosion. This will affect the triboelectrification ability, roughness and charge retention of the friction material, leading to decreased output performance of TENGs. At present, common triboelectric polymers with excellent charge density generally have poor wear resistance due to their low Young's modulus (such as FEP, PVDF, PVC, etc.). These materials are more prone to surface wear, especially when subjected to repetitive mechanical movement, as seen in practical applications like insoles, ground energy harvesters, and road power generation devices, which limits the practical application potential of TENGs. Consequently, in future studies, it is of great significance to further explore the relationship between the molecular structure and wear resistance of triboelectric materials and then achieve higher wear resistance while improving charge density, which will promote the further development of TENGs.

(5) Charge excitation technology has significantly enhanced the performance of TENGs. However, the increased surface charge density strengthens the electric field across the air gap, intensifying the corona discharge process. This not only results in the deposition of reverse charges on the triboelectric material surfaces, suppressing the TENG's output, but also generates high-energy emissions, such as light and heat, which heighten the risk of thermal breakdown of triboelectric materials. Additionally, the elevated surface charge density subjects triboelectric materials to greater electric field stress, further increasing the possibility of electrical breakdown. To achieve further advancements in TENG performance, enhancing the breakdown resistance and robustness of triboelectric materials is imperative.

(6) In order to realize the industrial application of TENGs, the large-scale and low-cost production of triboelectric materials is an important problem that must be addressed. Taking the excellent performance of the P(VDF-TrFE-CFE) material as an example, its high raw material cost significantly increases the TENG's production expenses. In addition, some of the best-performing triboelectric materials require complex and stringent processing conditions, as well as special treatment methods. For example, the processes such as annealing treatments and nanoparticle dispersion are not conducive to reducing the cost of TENGs or achieving large-scale production. Therefore, it is urgent to start with raw material and preparation processes to solve the cost challenges associated with large-scale manufacturing.

(7) In the context of environmental protection and sustainable development, the application of biodegradable materials across various fields has become increasingly important. There has been a notable rise in interest towards sustainable TENGs based on natural materials, biopolymers, biodegradable materials and even waste materials from production and daily life. Among these, cellulose-based TENGs stand out due to their biodegradability, cost-effectiveness, and abundant natural availability. Currently, the highest charge density achieved by cellulose-based TENGs is 0.533 mC m⁻².¹⁷⁴ However, a significant challenge remains in striking an optimal balance between high output and environmental sustainability. Moreover, while biopolymers offer advantages in terms of biocompatibility and degradability, they often encounter issues such as declining mechanical performance, rapid biodegradation, and heightened environmental sensitivity in long-term applications. These challenges necessitate ongoing efforts to enhance the stability and durability of the materials while maintaining an optimal balance between high output and environmental sustainability.

Although these challenges remain to be addressed, TENGs still show significant promise in the fields of high-entropy energy harvesting and sensor technology. We hope that this review offers valuable insights for future research and innovation in high-performance TENGs, paving the way for advanced energy solutions for IoT sensors and devices within self-sustaining systems.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Author contributions

Xiaoru Liu: conceptualization and writing – original draft; Zhihao Zhao: conceptualization, data curation, and writing – review & editing; Jie Wang: conceptualization and writing – review & editing.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (52302214, U21A20147), the National Key R&D Project from the Ministry of Science and Technology, China (2021YFA1201602) and the Beijing Natural Science Foundation (2252061).

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