Recent advances in high charge density triboelectric nanogenerators

Triboelectric materials with high charge density are the building-block for the commercial application of triboelectric nanogenerators (TENGs). Unstable dynamic processes influence the change of the charge density on the surface and inside of triboelectric materials. The charge density of triboelectric materials depends on the surface and the internal charge transfer processes. The focus of this review is on recent advances in high charge density triboelectric materials and advances in the fabrication of TENGs. We summarize the existing strategies for achieving high charge density in triboelectric materials as well as their fundamental properties. We then review current optimization methods for regulating dynamic charge transfer processes to increase the output charge density: first, increasing charge injection and limiting charge dissipation to achieve a high average surface charge density, and second, regulating the internal charge transfer process and storing charge in triboelectric materials to increase the output charge density. Finally, we present the challenges and prospects in developing high-performance triboelectric materials.


Introduction
The growing demand for distributed energy harvesters, particularly in the artificial intelligence (AI) and the Internet Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. of things (IoT) fields, has increased interest in triboelectric nanogenerators (TENGs), which can harvest irregular and low-frequency mechanical energy to power distributed sensors [1][2][3][4][5][6][7][8][9][10][11][12].Based on the expanded Maxwell's equations and basic selection rules for triboelectric materials, various materials and structures for TENGs have been developed [1,2,5,[13][14][15].The output of TENGs has been significantly improved, paving the way for potential commercial applications [16][17][18][19].Since the charge density (D Charge ) of triboelectric materials determines the output performance of TENGs [20][21][22], researchers have developed various optimization methods to improve TENG performance, such as material design synthesis of triboelectric polymers, physical/chemical surface modification, material modification, intermediate layer engineering, etc [15,[23][24][25][26][27].Polytetrafluoroethylene (PTFE) is the most commonly used triboelectric material in TENG research due to its good charge storage ability [15].There needs a guideline for designing and fabricating high D Charge based on various triboelectric materials.
As energy harvesters, the key to improving the output power of TENGs is to increase their surface D Charge [12].D Charge can be increased to hundreds of µC m −2 through material selection [1,2,14,15], ion injection [17, [28][29][30], environmental control [31][32][33], physical/chemical surface modification [25,[34][35][36], charge excitation/pumping [37][38][39], etc [40][41][42].For instance, in ambient atmosphere, the air ion injection method was used to increase D Charge of TENGs from 50 to 240 µC m −2 [28].An increase in surface D Charge can lead to air breakdown, releasing some charges and thus limiting D Charge of TENGs.D Charge of TENGs improved significantly at lower atmospheric pressure [28].In a high vacuum environment, D Charge of TENGs increased to 1.003 mC m −2 for the first time [16].This approach is limited by high requirements for device packaging.Ultrathin dielectric films are another strategy to improve D Charge [27].Zhang et al obtained D Charge of 1090 µC m −2 using an ultrathin dielectric film in the ambient atmosphere [43].As the thickness of the dielectric continues to decrease, it becomes difficult for TENGs to achieve higher D Charge due to insufficient contact.In addition, external circuit optimizations, including charge pumping [38,44], charge excitation [27,39] and electrode structure design [2,45], have been employed to overcome the limitations of triboelectrification and significantly increase D Charge of TENGs, reaching a milestone of 8.80 mC m −2 [2].D Charge of TENGs needs to overcome the limitation caused by the dielectric breakdown of triboelectric materials.
In this review, we present recent advances in the design and manufacturing of high D Charge triboelectric materials.First, the selection and structural design of triboelectric materials with high D Charge are discussed.Second, improving D Charge by regulating the dynamic charge transfer on the surface and within the triboelectric material is discussed.Finally, we discuss future challenges and prospects for high energy density TENG materials.

Development of high charge density TENG
In this section, we present the development of high D Charge TENGs.According to different calculation methods of D Charge , TENG can be divided into two types: first type is the alternating-current output (AC-TENG), and second type is the direct-current output (DC-TENG).
Over the past decade, numerous AC-TENGs with high D Charge have been developed (figure 1(a)).D Charge is determined by the output charge per unit area.In 2014, an AC-TENG using the ion-injection method was reported [28] with D Charge of 0.24 mC m −2 .In 2016, an AC-TENG with fragmental structural design and soft materials was fabricated [41].Due to the increased contact intimacy, D Charge of this AC-TENG reached 0.25 mC m −2 .In 2017, D Charge improved by high vacuum was reported [16].This AC-TENG achieved D Charge of 1.003 mC m −2 .In 2018, a novel technique for increasing D Charge emerged, named the charge pump technique [44].Under ambient conditions, this technique further increased D Charge to 1.02 mC m −2 .External and self-charge excitation was reported [39].By using ultrathin dielectric layers (5 µm Kapton film) and voltage-multiplying circuit (VMC), D Charge reached 1.25 mC m −2 .In 2022, there is D Charge of 5.35 mC m −2 with multiple electrode pairs [46].Furthermore, a 3D fractal structure TENG was unveiled in 2023 [47], which enabled D Charge to reach 8 mC m −2 .
Subsequently, various DC-TENGs with high D Charge based on electrostatic breakdown were developed (figure 1(b)).D Charge depends on the logarithm of the electrodes.In 2019, an electrostatic-breakdown-based DC-TENG was reported [48].This DC-TENG had D Charge of 0.43 mC m −2 .In 2020, an enhanced air breakdown approach was reported to improve D Charge of DC-TENG [49].By enhancing air breakdown under low pressure (300 Pa) and high temperature (473 K), D Charge was increased to 0.645 mC m −2 .By integrating 50 DC units, D Charge reaches 5.4 mC m −2 [45].In 2021, selection rules for triboelectric materials were proposed [2].By increasing the number of DC units to 50 and selecting PVC as the triboelectric material, D Charge of the microstructure-designed DC-TENG (MDC-TENG) reaches 8.80 mC m −2 .In 2024, a rolling mode dielectric-dielectric DC-TENG was reported with D Charge of 10.06 mC m −2 [50].The following sections discuss these two high D Charge TENGs and present advanced manufacturing methods suitable for large-scale commercial use.

High charge density AC-TENG
In this section, the charge transfer processes on the surface and within triboelectric materials are reviewed.In addition, we discuss the two main types of charge storage position: charge storage on the surface of triboelectric material and charge storage within triboelectric material.Finally, we provide a table demonstrating methods, materials, and schemes for increasing D Charge of AC-TENG.
D Charge depends on the process of carrier accumulation and decay.Specifically, there are the dynamic balance for carrier accumulation and decay processes.In general, higher surface D Charge is achieved by injecting more charge into the triboelectric material.However, during the carrier decay process, the density gradually decreases.Zhang et al analytically derived  increases from 12.5 µm to 15 µm, its surface D Charge increases to a maximum value but then decreases as the thickness continues to increase beyond 15 µm.The surface D Charge of FEP is lower than the theoretically calculated value when its thickness is 12.5 µm.Due to the incomplete contact between the thin triboelectric material and the electrode, charge transfer is reduced during the triboelectrification process.This could result in slower charge accumulation and lower surface D Charge .Thin triboelectric materials can achieve high and stable surface D Charge in the atmosphere when a dynamic balance is established between carrier accumulation and decay.
The charge transfer process between the surface and the interior of the material is divided into three parts: the charge generation process, the charge storage and decay process [65][66][67][68].This is most useful for describing the dynamic equilibrium behavior of D Charge and is crucial for dealing with the dynamic balance between triboelectric charge generation and decay.The amount of charge injection per cycle (σ i ), the transfer time and the decay coefficient are important factors in determining the maximum D Charge [69].By increasing σ i , providing more charged storage sites, and suppressing charge decay, D Charge of TENG can reach a higher equilibrium state.Improving D Charge by increasing σ i has been extensively studied, such as material selection [70, 71], structure design [41], surface modification [30,34], charge pumping [40,72,73], and charge excitation [24,27,39,67].

The selection rules of triboelectric materials for AC-TENG
TENG requires materials with high D Charge and high energy density.This section recalls the selection rules of high D Charge triboelectric materials used in AC-TENG.
Liu et al have quantified the maximum D Charge of triboelectric materials in TENGs [1].D Charge was evaluated against over 40 dielectric materials in vacuum.By choosing polyvinyl chloride and copper as triboelectric materials, triboelectric charge densities of up to 1250 µC m −2 were achieved.Electrostatic-induced charges in the external circuit are not same to the charge generated (figure 3(a)).The induced D Charge measured under atmosphere conditions is smaller than the real triboelectric D Charge .The impact of the breakdown effect on D Charge and energy density is shown in figure 3(b).D Charge is influenced by material, geometry, motion and environment.Under atmospheric conditions, it is still very difficult to determine the true triboelectric D Charge from the induced Triboelectric D Charge measurements under atmospheric conditions are influenced by air breakdown [24,31,32,43,74].It is complex to reveal the intrinsic properties and triboelectric behavior.This inaccuracy in measuring the triboelectric D Charge between solid-solid pairs continues to hinder both the widespread adoption of TENGs and a deeper understanding of contact electrification [1].The development of high-performance TENGs requires a standardized rule to evaluate the maximum D Charge for different materials.Energy density is a standardized parameter that is not influenced by test factors [12].A promising approach to maximizing energy density is to first achieve V oc and then release charges in the short circuit [20,75].With this method, the energy density can reach its maximum value.It could also improve our understanding of contact electrification in relation to the intrinsic properties of materials.

Charge storage on the surface of triboelectric material
In electret-based TENGs, charges are stored on the material surface under external periodic mechanical triggering.Consequently, numerous strategies to increase surface D Charge have been proposed.Among them, TENG based on charge transfer process has been widely studied.
Zhao et al proposed an ultrafast charge self-injection strategy [76].VMC is used to induce ionization of air and enable charge injection on the surface of triboelectric materials.7 µm polyimide (PI) film takes only 22 s to saturate the surface D Charge and reach D Charge of 1480 µC m −2 .The schematic structural diagram of a TENG that utilizes charge self-injection technology is shown in figure 4(a).During air breakdown, positive charges are injected downward into the triboelectric material, while negative charges travel to the top electrode.After charge injection, the power density and D Charge experienced a significant increase of 1024-fold and 895-fold, respectively.The discharge mitigation (DM) strategy can reduce the discharge intensity (figure 4(b)).The charge retained on the triboelectric material approaches the maximum value.D Charge of the triboelectric material decreases as the charge injection time increases.DM strategies can be used to maximize charge retention.By connecting an external capacitor, the gap voltage of the contact-separation TENG can be reduced.D Charge remains stable by applying the DM strategy.Implementing the DM strategy can increase D Charge by 179% compared to scenarios where the DM strategy is not used.
With the development of control technology for charge transfer process, AC-TENGs with charge densities exceeding the theoretical limit of Paschen's law have been reported.Guo et al designed a high D Charge TENG using a charge reversal process, exceeding the air breakdown limit [77].Figure 4(c) shows the structure of the TENG with a PTFE friction layer.Charge excitation is limited by air breakdown effects [38,69,78].The air breakdown effect limits further increase in surface D Charge [43,79].Self-charge excitation strategies generate high voltages using VMC.This leads to surface charge reversal in triboelectric materials due to electrostatic breakdown effects.Effective use of reversed-polarity charges can result in higher surface D Charge that exceeds the theoretical threshold of Paschen's law.
Many strategies used to increase surface D Charge require deeply understanding of charge transfer process, including material selection [80], ion injection [22,28,43], material modification [24,81], and charge excitation [38,39,67].Liu et al achieved a triboelectric D Charge of 1250 µC m −2 at high vacuum [1].Theoretically, the surface D Charge of dielectric materials can be improved by directly injecting charges into the surface [28,43].Ion injection technology can increase the surface D Charge to 200 µC m −2 -500 µC m −2 [17, 28,30,82].In addition, the surface charge stability after ion injection is poor [43].Researching a simple and effective charge injection technology for TENG is of great importance.Currently, charge-excitation TENGs utilize ultrathin and surface-modified polymer materials to increase D Charge [27,65,67].Wang et al provides a new way to successfully obtain a high D Charge of 1310 µC m −2 on thick inorganic materials [83].Furthermore, investigating methods to effectively maintain the surface D Charge is crucial for achieving high-performance TENGs.

Charge storage within triboelectric material
Due to their high surface D Charge and long charge decay time, electret materials are widely used.However, factors such as material wear and environmental influences can inevitably modulate the stability of charge storage of the material and thus reduce D Charge .To solve this problem, storing charges in triboelectric materials has been identified as a promising solution.AC-TENG exploits the structural or charge-migration characteristics of materials to modulate charge storage positions and realize high D Charge .

AC-TENG based on heterogeneous polymers.
Through the structural design of the material, AC-TENG specifically adjusts the charge storage positions within the material by regulating the dynamic charge transfer process.As is known, the operation of AC-TENGs involves two steps: the negative and positive charge separation through triboelectrification and electrostatic induction.Current and voltage of TENG depend on D Charge .The dynamic charge transfer process on the surface and inside of triboelectric materials can be further divided into three sub-processes: charge injection caused by friction or charge excitation, charge accumulation, and charge decay.The factors influencing this process and the extent of their impact are still unknown.Solving these issues could significantly improve the performance of TENGs.
Within the material, the charge transfer process and its relationship to the composite structure were investigated by Cui et al [66].D Charge of TENG is increased by 11.2 times by using multilayer structural designs to regulate the dynamic charge transfer process.The model of triboelectric charge generation, transfer and loss process is shown in figure 5(a).The multilayer structure design effectively increases D Charge (figure 5(b)).D Charge increases 7 times when a PS charge storage layer is added between the poly (vinylidene fluoride) (PVDF) friction layer and the electrode.Moreover, D Charge can be increased by 11.2 times with a composite structure including the friction, charge transport and storage layers.Tests were conducted to investigate the charge accumulation and decay processes for PVDF TENG (red) and PVDF-PS TENG (black).The decay time for the PVDF TENG is 22 min.However, with the PS charge storage layer, the decay time increases up to 44 h.The decay time serves as a characteristic measure of the charge storage ability of various dielectric materials in TENGs.
Recent theoretical study on the structural design of triboelectric materials suggests that higher energy density can be achieved by using ultrathin charge storage layer arrays at the same thickness [85].Triboelectric materials can increase capacity and achieve higher breakdown voltages by connecting ultra-thin charge storage layers in series.Furthermore, modification of polymers, such as forming pores and filling nanoparticles, can increase the surface D Charge by increasing their capacity [86-89].Xia et al prepared multilayer polydimethylsiloxane (PDMS) filled by aligned graphene sheets [84].PDMS filled with aligned graphene sheets has greater capacity and can withstand higher voltages, allowing it to store more electrical energy.The fabrication process of the multilayer PDMS filled with oriented graphene sheets is shown in figure 5(c).A multilayer PDMS was fabricated using repeated spin-coating technology.The multilayer composite films were produced by spin-coating 2-11 times.By connecting the micro-capacitors in series, the capacity of the triboelectric materials can be increased and thus the output power of TENGs can be significantly increased.

Triboelectric materials with strong charge-migration
characteristics.In addition to structural design, the chargemigration characteristics of the triboelectric material can also be used to modulate its internal dynamic charge transfer process.As shown in figure 6, TENG uses thick triboelectric materials with strong charge-migration characteristics to achieve high power output while maintaining excellent durability.
Fu et al innovatively converted the dielectric surface effect into a dielectric volume effect [90].This transformation significantly improved the average power densities and durability of TENGs.Under ambient conditions and using 1 mm polyurethane (PU) foam, the TENG achieved maximum average power densities of 20.7 W m −2 Hz −1 .This performance was due to charge migration within the porous structure of the PU foam.This research not only identifies the physical mechanisms of thick dielectric films used in TENGs, but also proposes a novel approach to simultaneously improve the performance and durability of TENGs.The AC-TENG based on the dielectric volume effect is shown in figure 6(a).TENGs that utilize the dielectric volume effect can maintain excellent durability while achieving high output charge.In contrast to the dielectric surface effect, the triboelectric charge in these devices is not completely confined to the material surface.Instead, some of the charge can be transferred within the triboelectric material.The structural scheme of the TENG with the dielectric volume effect is shown in figure 6  We provide a summarized table to demonstrate the methods, materials, and schemes for AC-TENG to improve D Charge , as shown in table 1.

High charge density DC-TENG
A typical DC-TENG consists of the charge collected electrode (CCE), the friction electrode (FE), and the friction layer (figure 7(a)).According to the different properties of the two triboelectric materials, DC-TENG is divided into metal-dielectric DC-TENG and dielectric-dielectric DC-TENG [108].In this section, several high D Charge DC-TENGs are presented.The triboelectrification effect combined with electrostatic breakdown can produce high D Charge and average power density.In addition, the mechanism and structure of the device are also discussed.At the end, a table summarizing the methods, materials, and schemes for increasing D Charge of DC-TENG is provided.
Zhao et al proposed a strategy to significantly improve D Charge of DC-TENG through a patterned electrode structure [45].By integrating 50 electrode units, the effective surface D Charge of MDC-TENG can be increased to 5.4 mC m −2 .The limitation factor of the MDC-TENG has been described as: D Charge of DC-TENG is significantly improved by integrating more electrode units in the same area of the slider.The DC-TENG has rationally patterned electrodes, including multiple charge collection electrodes and FEs, as shown in figure 7(a).Each FE maintains a small distance from the charge collection electrodes, with a very narrow gap between the triboelectric material and the charge collection electrode.Each approximately 250 µm wide FE is flanked by an approximately 100 µm wide charge collection electrode.When the FE contacts the triboelectric material, electrons are transferred from the FE to the triboelectric material due to the triboelectrification effect.This results in a DC output due to the air breakdown that occurs between the charge collection electrode and the triboelectric material.Factors such as electrode length, sliding distance, and the number of electrode units affect the output performance of the device (figure 7 Researchers have investigated a variety of strategies for enhancement of electrostatic breakdown, including gas pressure (P) [32,49], gas species [32], temperature [49], humidity [111], and structural design [110].According to Paschen's law, the breakdown voltage (V b ) is as follows: where, B is a constant related to the excitation energy and the ionization energy, d is the gap distance, γ se is the secondary electron emission coefficient, and A is the saturation ionization constant in the gas.B, A and γ se are related to the gas species.
With different P and different gas species, V b is different [32,49].Liu et al reported that electrostatic breakdown is enhanced by both low pressure and high temperature [49] (figure 8(a)).The output charge initially increases and then decreases as air pressure decreases, reaching a maximum at about 300 Pa.The output charge shows an exponential increase with temperature.The output power density of DC-TENG at 473 K is 500 times larger than the value at 293 K. Yi et al studied the effects of P and gas species on the electrostatic breakdown [32] as shown in figure 8(b).Under varying P, D Charge and voltage are higher in oxygen than in air and nitrogen.The output power of DC-TENG is higher in an oxygen environment than in air.Liu et al reported that the electrostatic breakdown enhances significantly at high humidity [111] (figure 8(c)).The breakdown voltage decreases as the humidity increases from 30% to 90%.When the electrode unit is increased to 20, D Charge reaches 2.97 mC m −2 .
In addition, the electrostatic breakdown of the metal dielectric DC-TENG is also influenced by the structural design of the dielectric material.Chen et al reported a metal dielectric DC-TENG [110], which consists of a dielectric material with a double-layer structure.In particular, the electric field inside the triboelectric layer and the CCE can be increased by adding a layer of charged electrets under the triboelectric layer.

Dielectric-dielectric DC-TENG
Compared to a metal-dielectric type, a dielectric-dielectric DC-TENG shows high D Charge and average power density.Shan et al proposed a dielectric-dielectric DC-TENG by triboelectrification and corona discharge [93] (figure 9(a)).In this DC-TENG design, the commonly used metal triboelectric material on the metal-dielectric DC-TENG slider is replaced with dielectric material, and two CCEs are placed at both ends of the slider.The two triboelectric materials maintain good contact and the gap between the CCE and the triboelectric material serves as the discharge path.Different from the traditional metal-dielectric DC-TENG, the dielectric-dielectric DC-TENG achieves bi-directional and dual-channel output by using CCEs on both sides of the slider.
Dielectric-dielectric materials have strong triboelectrification capabilities that allow them to generate large amounts of triboelectric charges [8,93].The accumulation of these charges creates a strong electric field.However, this field can lead to air breakdown and thus loss of charge.Li et al proposed a rolling mode dielectric-dielectric DC-TENG [50] (figure 9 Textile-based DC-TENG can provide reliable power supply for flexible wearable electronic devices [112,113].Shan et al developed a double mode TENG (DM-TENG) by flexible fibers [114].By combining electrostatic induction TENG (EI-TENG) and corona discharge TENG (CD-TENG), the DM-TENG achieved a maximum power density of 9.2 W m −2 .Furthermore, the DM-TENG achieves an output charge of 20 µC in just 4.8 s, which is 6.3 s and 2.2 s faster than a single EI-TENG and CD-TENG, respectively.Compared to a single EI-TENG or CD-TENG, the DM-TENG achieves a higher output charge.
We provide a summarized table to demonstrate the methods, materials, and schemes for DC-TENG to improve D Charge , as shown in table 2.

Advances in fabrication of TENG
Although researchers have made a breakthrough in D Charge , the increasing demand for industrial production and commercialization of TENGs still requires advanced manufacturing  methods.Therefore, some novel manufacturing methods have recently been disclosed.Figure 10(a) shows a monolithic fabrication process based on laser-induced graphene (LIG) [116], which one of the commonly most methods in recent years [117][118][119][120]. Taking TENGs that collect droplet energy as an example, the metal electrodes are prone to corrosion, which limits the reliability of the device.One of these measures is the use of LIG to fabricate flexible superhydrophobic electrodes with excellent chemical and physical stability [116].A similar fabrication process has been extended to create ordered micronano structures on material surfaces (figure 10(b)) [121].The aim is to produce high-quality micro/nanostructures [122,123].A TENG was designed as a micro-nano dual-scale structure.The micro/nano structure increases the effective contact area of two contacted surfaces.The TENG surface D Charge increases through a strategy of increasing the contact area [121,124].In addition, the printing technology is ideal for mass production of TENGs due to its simplicity, high production efficiency and low cost [125].Figure 10(c) shows that a textile-based TENG with enhanced output power was fabricated using a screen printing method to print silver electrodes on the surface of PVC fabric [126].Figure 10(d) shows an example that can be used to fabricate TENGs for large-scale production using a spray coating process [127].Figure 10(e) shows the electrospinning process [128], which is a cost-effective coating method.Figure 10(f) shows a TENG with three-dimensional structure fabricated using an extrusion-based printing process [129].Specifically, there is a micro/nano 3D structure TENG fabricated by printing ink on two respective substrates.Furthermore, the integration of a large number of components can pose a significant challenge for the industrial-scale manufacturing of TENGs. Figure 10(g) shows the integration of fiber-based electronic components using standard weaving techniques [130].Specifically, a textile electronics system was developed by automating the weaving process.We demonstrate these examples using advanced manufacturing methods.The manufacturing methods of highperformance TENG still face many challenges (figure 11).A key consideration is developing manufacturing processes for ultra-large-scale systems.For blue energy harvesting applications, a power network can consist of millions of spherical TENGs [131].In order to ensure the robustness and durability of the systems in practical applications, standardized manufacturing processes for ultra-large-scale systems should be developed.In addition, ultra-small micro-nano manufacturing and operation in extreme environments are also required for applications.

Summary and perspectives
This review highlights the significant progress made in high D Charge TENGs in recent years.Currently, high D Charge TENGs include high D Charge AC-TENG and high D Charge DC-TENG.With the growing demand for distributed energy sources, the need for TENGs with high D Charge becomes more pressing.Consequently, exploring new mechanisms, materials and devices is important to advance the development of high D Charge TENGs.
Mechanisms: one of the main challenges of TENGs with high D Charge is to efficiently collect more charges from the surface and interior of the triboelectric material during the charge transfer process.Various strategies are discussed above, including increasing charge injection, reducing charge loss, and integrating more output channels.Developing innovative mechanisms to increase D Charge by dynamically regulating charge accumulation and decay time will be crucial.
Materials: although there are many options, triboelectric materials with high initial D Charge are in demand, and meanwhile the operating frequency of practical application needs to be considered.Researchers should determine the optimal operating frequency to fully utilize the triboelectric charge during the charge accumulation period.This approach can solve the problem of electrostatic dissipation of triboelectric materials during the decay period while maintaining high D Charge .
Device: the output performance and durability are invariably modulated by factors such as the materials used, the structure and the working environment.Consequently, standardized integration and packaging technologies should be developed to ensure the robustness of TENG in practical applications.Furthermore, new manufacturing processes are required to accommodate diverse applications, such as the manufacture of ultra-large-scale systems for large-scale blue energy collection.
In summary, increasing the output of TENGs is always a goal.D Charge of TENGs has been continuously increased by various methods.In addition, the manufacturing process plays a crucial role in the durability and stability of the device.As D Charge increases and the manufacturing process advances, TENG will provide a more reliable power source for AI and IoT applications.

Figure 2 .
Figure 2. The factors limiting surface charge density.Reprinted from [43], © 2019 Elsevier Ltd.All rights reserved.(a) A schematic diagram illustrating the charge decay and accumulation process of TENGs.It also shows the maximum surface charge density of FEP with different thickness.(b) The comparison of surface charge density of TENG before and after ion injection.(c) The surface charge density of TENG achieved via triboelectrification.

Figure 3 .
Figure 3.The suppression of air breakdown for achieving high charge density and energy density.Reproduced from [1].CC BY 4.0.(a) The working mechanism and charge decay process of CS-TENG.(b) The triboelectric charge density with or without the breakdown effect.(c) The quantified maximum charge density and energy density of triboelectric materials in CS-TENG.

Figure 4 .
Figure 4.The charge excitation strategy for achieving high charge density.(a) The schematic structure of the CS-TENG, which achieves high charge density and power density after charge injection.[76] John Wiley & Sons.© 2023 Wiley-VCH GmbH.(b) The strategy for mitigating discharge.(c) The use of reversed-polarity charges to achieve higher surface charge densities, exceeding the theoretical threshold of Paschen's law.Reproduced from [77], with permission from the Royal Society of Chemistry.

Figure 5 .
Figure 5.The design of a multilayer composite structure to store more charge.(a) A schematic diagram illustrating the charge transfer process of triboelectric materials in CS-TENG.Reprinted with permission from [66].Copyright (2016) American Chemical Society.(b) The design of a multilayer composite structure for achieving high charge density and storing more charges.(c) The fabrication processes of multilayer composite films and the output performance of CS-TENG, based on the multilayer composite film.Reprinted from [84], © 2016 Elsevier Ltd.All rights reserved.
(b).The dielectric volume effect effectively increases the transferred charge by 514% compared to the dielectric surface effect.Furthermore, the output energy of the dielectric volume effect based TENG enhances in each cycle by adding an external load.The output energy increases to 1.2 mJ at the load of 1200 MΩ.The dielectric volume effect can be used to sliding and rotary TENGs (figure 6(c)).The dielectric volume effect-based rotary-mode TENG reaches its maximum average power density of 20.7 W m −2 Hz −1 when the external load is 70 MΩ.At a speed of 60 rpm, it can light up 7552 green light-emitting diodes (LEDs).In addition, the rotary-mode TENG based on the dielectric volume effect has high durability.The short-circuit current of the rotary-mode TENG gradually increases with the number of cycles.After 200 000 cycles, the PU foam surface shows no wear and the transferred charge slowly increases with the number of cycles.Scanning electron microscope images show that the surface of the PU foam surface becomes flat after 200 000 cycles.There are cost-effectiveness, ease of manufacturing and versatility in material selection of TENG [91-96].In traditional TENGs, electrostatic charges are completely confined to the dielectric film surface.When D Charge reaches a threshold value, air breakdown occurs on the film surface.This phenomenon represents a significant limitation to improving D Charge of TENGs[16].For excellent output performance, TENG requires the use of micron-sized films as triboelectric materials [72].However, the use of thin triboelectric materials significantly compromises the durability of TENGs, a crucial factor for their commercial applications[97, 98].Maintaining high durability while ensuring high average power densities remains a major challenge [99-103].

Figure 6 .
Figure 6.dielectric volume effect for achieving high output energy and excellent durability [90].John Wiley & Sons.© 2023 GmbH.(a) The working mechanism of the dielectric volume effect.(b) The structure and output performance of the AC-TENG, based on the dielectric volume effect.(c) The output performance and durability of the rotary-mode TENG based on the dielectric volume effect.
(b)).The charge of the TENG increases from 0.025 µC to 2.6 µC as the electrode length, sliding distance, and number of electrode units increase.D Charge of DC-TENGs experiences an increase from 0.5 mC m −2 to 5.2 mC m −2 .The short-circuit current and current density increase rapidly with increasing electrode length, sliding distance and number of electrode units.D Charge is affected by the electrode distance between the FE and the charge-collecting electrode, as well as the gap distance between the charge-collecting electrode and the triboelectric material (figure7(c)).D Charge gradually increases as the distance between electrodes decreases.With the gap distance increasing, D Charge increases from 0.49 mC m −2 to 2.0 mC m −2 .D Charge increases with the number of DC-TENG units.When the number of units is 50, D Charge of MDC-TENG is 5.4 mC m −2 .D Charge of MDC-TENG can reach ∼8.80 mC m −2 by selecting appropriate triboelectric materials [2].D Charge of DC-TENG with 11 triboelectric materials is shown in figure 7(d).DC-TENG can achieve high D Charge when PVC is used as a triboelectric material.

7 .
The design of the structure and selection of materials for achieving high charge density.(a) The structural design of the DC-TENG.Reproduced from [45].CC BY 4.0.(b) The working mechanism and output performance of the DC-TENG.(c) The structural optimization and output performance of the DC-TENG.(d) The selection of materials for the DC-TENG to achieve high charge density.Reproduced from [2].CC BY 4.0.
(b)).Li et al by integrating 36 PTFE-electrode pairs on the stator, achieved D Charge of 10.06 mC m −2 .This was achieved by rationally arranging CCEs for efficient charge capture.Cu electrodes serve as CCE.CCE is attached to both ends of each PTFE film.Compared with a single PTFEelectrode pair, the integration of multiple PTFE-electrode pairs can reduce charge loss and improve output performance.

Figure 11 .
Figure 11.Summary of challenges in manufacturing TENGs on an industrial scale.Images for blue energy and charge transfer.Reproduced from [131], with permission from Springer Nature.Image for vacuum reprinted from [32], © 2021 Elsevier Ltd.All rights reserved.
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Table 1 .
Methods, materials, and schemes of AC-TENG to improve charge density.

Table 2 .
Methods, materials, and schemes of DC-TENG to improve charge density.
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