Boosting the Durability of Triboelectric Nanogenerators: A Critical Review and Prospect

Triboelectric nanogenerators (TENGs) have attracted great interests in the development of sustainable energies and intelligent society. However, a big challenge for TENGs in practical applications is the unavoidable external mechanical abrasion and/or contaminant adsorption on the triboelectric materials, which leads to the significant decrease of the durability of TENGs and is urgently needed to be addressed. There are already a series of interesting progresses on the topic of the TENGs’ durability. In this study, reviewing the durability of TENGs via both the advanced materials/structure designing and the novel surface/interface engineering is focused upon, which includes choosing basic TENG materials, improving composites performance, optimizing structures, and designing triboelectric surfaces and interfaces. To get a better understanding of the durability of TENGs in published studies, the quantifiable levels of service life are also summarized including operation cycles, time, friction coefficient, and wear loss of triboelectric materials, where the boosting mechanisms are also discussed and summarized. Finally, the challenges as well as key strategies toward high durable TENGs are presented.


Introduction
The rapid growth in global energy consumption is leading to the overuse of fossil fuels, which greatly exacerbates the world's energy and environmental problems. Thus, green and renewable energy plays a defining role for the world sustainable development, which is the priority trend of the entire human society. [1] It is an attractive way to harvest environment energy from solar, thermal, mechanical, and biochemical systems. However, solar cells have less efficiency as light intensity is lower than 1 sun, and the electricity generating is not available at night. [2] The power densities of the thermoelectric and biochemical devices are too low (around 10 Wm −2 ) for many efficient alternatives for powering IoT sensors. Traditional sensors are usually battery-powered, and cannot manage wireless data transmission. The problem of the sustainability of such sensors arises due to the limited battery life and the impracticality of replacing them. The self-powered feature of TENGs provides a desirable solution. For example, TENG based cardiac pacemaker is very close to commercialization! It can extend device operation time inside the body, which will reduce the necessity for high-risk repeated surgery. [12] Besides using as sensor function, the electric energy harvested by the TENGs can be further used to boost power management circuits, facilitate signal processing techniques in the sensor network, and to transmit data wirelessly across devices. [13] However, the mechanical damage is a big challenge preventing TENGs' practical applications. TENGs have four working modes, that is, single electrode mode, contact-separation mode, lateral sliding mode, and freestanding triboelectric layer mode. For single electrode and contact-separation modes, TENGs are subjected to constant external tapping, bending, and stretching, which will cause mechanical damages, such as cracks, leading to eventually rupture. In case of lateral sliding and freestanding triboelectric modes, TENGs suffer from significant friction, wear, adhesion of material during sliding. For example, the triboelectric surface of TENGs is prone to being worn out by friction forces. [14] The worn particles are transferred to the surface of the opposite material resulting in output degradation, life loss, and the safety hazard. Achieving the high durability performance is an important task that urgently needs to be addressed, which is considered as the precondition realizing the extensive and large-scale applications of TENGs. The mechanical strength, plasticity, and thermal stability of materials are important factors that need to be considered at first. [15] Polymeric materials such as polytetrafluoroethylene (PTFE), polyimides (PI), and polyvinyl chloride (PVC) are widely used in TENGs. There are also extensive studies on the uses of nature materials such as silk, cellulose, chitin, etc. However, these TENG materials still inevitably suffer mechanical wear and show a much lower robustness in some harsh operation environments, for example, massive dust, heavy moisture, and high temperature. For example, when the operating temperature is higher than 200 °C, polymeric materials will be decomposed or deformed significantly. [16] In addition to choosing basis materials, composites have the capability of enhancing the mechanical performance of TENG materials. Via adding anti-wear nanoparticles into triboelectric materials, TENGs exhibit good wear-resistance performance. New technologies, that is, selfhealing materials, look promising for eliminating the problems from wear and damages caused by mechanical stress. At the same time, the durability of TENGs can be enhanced via structure optimizing. To address the issue of contaminants adsorption and wear, hydrophobic/oleophobic surfaces and lubricated interface are the key strategies not only for boosting their durability but also for enhancing the signal output. [17] Some previous works have shown good strategies and results on improving the durability of TENGs via both advanced material/structure designing and novel surface/interface engineering. In this work, we will give a deep review on the work of improving the durability of TENGs. The boosting strategies include choosing basis materials, optimizing composites performance, and designing surface/interface properties (Figure 1). The mechanisms of the promising strategies for addressing the durability challenges of TENGs are also discussed and summarized (Figure 2). The basis and composited materials with high mechanical strength can enhance the Young modulus, hardness, and tensile strength of TENGs, which can also provide low friction coefficient and high wear resistance properties. Cracks and wear scratches of TENGs can be smartly treated via developing self-healing materials, which can not only heal the damages but also can recover the triboelectrification properties of TENGs after the damages. There is almost no wear and no adhesion when noncontact or contact/noncontact transition working structures are designed and applied. The coating with flexible textures, tight chemical bonds, low friction, etc. are always designed to increase the durability of TENGs. Utilizing lubricants to lubricate the triboelectric interfaces will not only avoid the air breaking and moisture contaminant (resulting in a higher signal output), but also will decrease the friction and wear of TENGs (resulting in a high durability). Accordingly, the representative quantifiable levels of service life are summarized, including cycles, time, fiction coefficient, and wear loss. Finally, the challenges and outlook of the future studies toward highly durable TENGs are presented.
such as PTFE, fluorinated ethylene propylene (FEP), nylon, and rubber, etc., because of their good mechanical performance and significant electron-trapping capability. In 2013, Wang et al. [18] developed a PTFE-based TENG consisting of several layers of units (Figure 3a,b). Each layer incorporated an PTFE thin film as triboelectric material and an Al foil as the contact electrode. Only a slight decline (7%) was observed for the maximum current output after a total of 100 000 cycles (Figure 3c). At the same year, they designed an integrated rhombic gridding TENG, in which a layer of PTFE nanowire array adhered on the other side of a polyethylene terephthalate (PET) substrate. [19] The PTFE-based TENG also exhibited significant robustness with only about 5% drop for the current output after more than 100 000 cycles of vibration. Li et al. [20] fabricated a rolling friction contact-separation mode TENG by utilizing a cylindrical PTFE surface with the conjunction of rolling contact electrification and electrostatic induction (Figure 3d,e). There was no obvious normalized current decay after 1008 000 rotation cycles indicating the satisfactory mechanical durability (Figure 3f). Apart from choosing the PTFE/metal pairs, a PTFE-based TENG using a nylon as a contact-separate counterpart was reported by Yang et al. [21] A negligible loading-unloading signal change was experimentally observed over 40 000 cycles indicating its performance stability and durability.
In 2015, Niu et al. [22] chose an FEP film with a higher hardness value than that of PTFE as the triboelectric materials together with an Al foil used in a multilayered TENG (Figure 3g). In the durability test, the normalized transferred charge did not show obvious degradation even after about 180 000 cycles (Figure 3h). Humidity and moisture had also been proven to have some influence on the TENG's performance. In 2019, Zhang et al. [23] fabricated an FEP-based multilayered TENG (Figure 3i,j). It was found that the multilayered TENG had excellent performance for a long operation time, in which the open-circuit voltage and short-circuit transferred charge were well maintained after 24 000 cycles ( Figure 3k). Rubber is a type of copolymer elastomer, which has the characteristics of good shock absorption, low permeability, good aging resistance, and high physical strength. Zhang et al. [24] found the rubber-based TENG shown possible approaches to optimize the output power as well as its potential practical applications as self-powered motion sensors in 2021. After 800 cycles of constant force testing, the output voltage remained almost unchanged.

Lateral Sliding and Freestanding Modes TENGs
For TENGs working with the lateral sliding and freestanding triboelectric-layer modes, there is a specific requirement for the triboelectric materials, that is, low friction coefficient and high wear resistance. It is because the friction of interfacial rough asperities from triboelectric surfaces will result in big materials wear and will lead to the failure of TENGs. Wang et al. [25] provided a set of rules for selecting the polymer materials for TENGs based on friction coefficient. It was found the average friction coefficient of most dielectric films was <0.4, and the PTFE film presented the smallest friction coefficient of ≈0.17 under different applied loads (2-10 N). This is the reason that PTFE is attractive in the lateral sliding and freestanding triboelectric-layer modes. As shown in Figure 4, in 2018, Zhu et al. [26] proposed a sliding TENG, in which a paper and a PTFE tape served as the triboelectric pairs and carbon ink was used as the conductive electrode (Figure 4a-c). The results showed that the output voltage was approximately linearly proportional to the applied loads (5-10 N) and the output performance of the TENG was stable within 2000 cycles under the pressure of about 1.8 kPa (Figure 4d). In 2019, Choi et al. [27] used diamondlike carbon (DLC) as the counterpart triboelectric material. The DLC coating exhibited the low friction coefficient against PTFE at an average value of 0.18. Kapton-PTFE pair exhibited the friction coefficient of 0.3 and was constantly rising at a applied load of 9.8 N. Due to the low friction feature of DLC-PTFE pair, it demonstrated an outstanding durability for 108 000 cycles.
FEP not only has similar features to PTFE but also has good processing properties, high toughness, and excellent creep resistance, and is widely used in many cases. [28] In 2018, Liao et al. [29] proposed an FEP-based TENG for simultaneously detecting wind speed and direction (Figure 4e-g). The FEP film had the characteristics of excellent chemical compatibility, electrical reliability, and mechanical durability (stable working for more than one month). In 2019, Kim et al. [30] introduced butylated melamine formaldehyde (BMF) as a promising triboelectric material due to its good mechanical durability and highly triboelectric property (Figure 4h). Compared with PTFE, the average Young modulus (2.98 GPa) of BMF was about six times higher than that of PTFE (0.5 GPa). It was also found the hardness of BMF was placed at 5H compared to the 7B of PTFE using a pencil hardness tester. The wear rate of BMF was about four times lower than that of PTFE (Figure 4i). The TENG prepared from BMF had a durable performance with 27 500 sliding cycles (Figure 4j).
In case of freestanding mode, in 2014, Wang et al. [31] prepared a freestanding triboelectric-layer TENG using FEP as the dielectric material. They continuously ran the TENG for 20 000 cycles with no obvious decay of the generated short-circuit charge density. They further designed a grating-structured FEP-based triboelectric-layer TENG, which showed supreme stability during the continuous 80 000 cycles operating. [32] However, FEP film showed inferior stability for sliding mode. Because its surface layer is easily stripped off during the stability test, which would jam the gap and make the air breakdown hard to occur in direct-current TENGs. [25] Due to the low toxicity and skin-friendly properties of rubber, in 2015, Yi et al. [33] introduced a stretchable rubber-based TENG with a piece of stretchable rubber and an Al film (Figure 4k-i). An alternating output current can be produced by periodically stretching and releasing of the rubber. There was no significant change in electrical output after stretching and releasing the rubber for about 5000 cycles.

Introduction of Choosing Natural Triboelectric Materials for Boosting TENGs' Durability
Most of the synthetic polymers have low degradation abilities, high recycling difficulties, and poor remodeling properties. After the service, synthetic polymer materials may cause serious environment pollution problems due to their extremely low recycling possibilities and low biodegradation capabilities. It is very essential to find the materials for TENGs which are low cost, biodegradable and easy processing for large-scale applications. Renewable, sustainable, and biomimetic materials are, therefore, deserved for fabricating TENGs. Using sustainable and eco-friendly biomass is also the most important task nowadays in view of the environment, health, and safety. The biomass used in TENGs can be divided into two types according to their sources, that is, virginal-natural materials (woods, cottons, leaves, furs, etc.) and extractive-natural materials (silk, cellulose, chitin, etc.). The basis triboelectric materials with long cross-linked molecule chains can enhance the Young modulus, hardness, and tensile strength of TENGs, which also show low friction forces because of their low surface potential. [34] At the same time, biomimetic materials are rich in active groups such as hydroxyl, acetamide, amino and carboxyl groups that are recognizable active sites for chemical reaction without additional agents for chemical modification (Figure 5). [35] The active chemical groups generate strong hydrogen bonds, which can enhance the mechanical properties of triboelectric materials. Basis materials can be directly assembled or integrated with active materials to produce high-performance films/coatings for various types of TENGs.

Virginal-Natural Materials Based TENGs
Natural woods are the most abundant resources on earth, which can be converted into a high-performance triboelectric materials with excellent mechanical properties. In 2019, Luo et al. [36] reported a wood-based TENG using a ping-pong table with excellent durable performance. It was found that the transferred charge density of the wood-based TENG only had little decay in continuous operation of 20 000 cycles confirming superior durability stability as shown in Figure 6a-d.
In 2021, Nie et al. [37] obtained a conductive wood which was used as the dielectric electrification layer. The open-circuit voltage of the TENG was nearly unchanged after 20 000 cycles of periodic contact-separation operations, meaning the good durability of the wood-based TENG. In 2019, Dudem et al. [38] prepared a cotton textile as a positive triboelectric material. After over 5000 cycles measurements, the amplitudes of the output voltage were almost stable without any obvious degradation. In 2021, Graham et al. [39] also proposed a TENG made from cotton, which was utilized to form a positively charged triboelectric film without any further processing or chemical treatment (Figure 6e). There was little decrease of the output voltage for more than 9000 cycles under a 4 N compression force and a 5 Hz frequency (Figure 6f). In 2018, Cao et al. [40] presented a leaf-based TENG with excellent environmental friendliness by using natural leaves as the triboelectric material.
It was commonly believed that the leaves were not mechanically strong and would be fragile after water evaporation or continuous mechanical contacts. But the prepared TENG had good durability for several days during the 10 000 cycles tests in the laboratory environment. In 2019, Xia et al. [41] proposed a TENG made from a tea-leaf triboelectric layer on a conductive Al film ( Figure 6g). The leaf-based TENG was reliable when used as mechanical vibration exciter for 10 000 cycles (Figure 6h). In 2021, Saqib et al. [42] used natural seagrass leaves as the tribopositive material, presenting an extraordinary working stability up to 72 000 cycles. Animal furs can also serve as the low friction materials for reducing the material wear and improving the durability of TENGs. At the same year, Jiang et al. [43] reported a fur-based TENG with introduction of soft and dense rabbit furs for the reduction of the friction and wear of TENGs. It was observed that after continuous triggering of 400 000 cycles, the output current only had a slight decrease of about 5%, which was believed that the rabbit furs provided a soft contact surface. Human hair exhibits a profound triboelectrification effect and is a natural regenerative substance. In 2021, Chakraborty et al. [44] reported a robust hair-based TENG containing a valley featuring of human hairs in contact with Fe 2 O 3 /PDMS nanocomposite. The output voltage and current of the TENG had no statistically relevant variations over 1200 cycles.

Nature-Extractive Materials Based TENGs
Li et al. developed various fully bioresorbable materials-based TENGs using five nature-extractive materials (silk, cellulose, chitin, rice paper, egg white). A "triboelectric series" of these materials was conducted by the pairwise combinations test, which greatly promoted the development of natural materials for TENGs and other triboelectric devices. [45] Silk, as one of the earliest animal fibers used by human, is composed mainly of fibroin, produced by certain insect larvae to form cocoons. It has excellent toughness (6 × 10 4 -16 × 10 4 J kg −1 ) and tensile strength (0.5-1.3 GPa). In 2016, Oh et al. [46] reported a silk fibroin nanofiber network-based TENG prepared by a simple electrospinning method (Figure 7a,b). An over 25 000 cycles test was conducted to confirm the durability of the silk-based TENG. In 2018, Tao et al. [47] engineered the genetic sequence of recombinant spider silk proteins to customize the triboelectric performance and the mechanical strength of TENG devices. The recombinant TENG exhibited extraordinary highpower output and durable performance (36 000 cycles). In 2019, Gogurla et al. [48] utilized a nanostructured silk protein and silver nanowires buried in the silk nanostructure to achieve an efficient, flexible, and durable TENG for biomechanical energy harvesting and motion sensing.
Cellulose, the most abundant natural polymer on Earth, can be directly obtained from various sources such as cotton, cereal, wood, fiber, etc. Cellulose-based TENGs are the most representative results of biomass-based TENGs. They usually lose electrons in a positive state when they contact with most other polymers generating electricity with a very high charge density. [49] In 2016, Cai et al. [50] reported a cellulose nanofibril TENG, which exhibited a comparable performance to the reported TENGs built on synthetic polymers. Chemical modification of cellulose surface has been accounted as the most effective strategy to improve the efficiency of cellulose-based TENGs, which can improve the surface polarity, surface roughness and electronic orientation. In this regard, Roy et al. [51] introduced a biodegradable naturally compound allicin to modify the surface of cellulose nanofibers. After allicin modification, the cellulose nanofibers film became mechanically robust and generated stable output signals after been stored under open atmosphere even for 90 days. In 2020, Gong et al. [52] demonstrated the feasibility of using sustainable cellulose papers and nitrocellulose membranes to fabricate high-output and mechanical durable TENGs. No obvious difference in output voltage was observed during the 10 000 press-release cycles. The output voltage was also maintained well after storage in ambient air (≈40% relative humidity, RH) for 75 days. In 2020, Nie et al. [53] prepared an amino silane-modified cellulose nanofibril. The performance of this modified cellulose-based TENG showed outstanding output stability when the environmental humidity was even up to 70%.
Apart from cellulose, extensive research has been made to study using lignin in TENGs, because lignin also has strong capability to lose electrons. Bae et al. [54] proposed a peanut shell powder-based TENG with high output performance by utilizing peanut outer shells, which was composed of cellulose (34-45%) and lignin (27-33%). The stability and durability of the TENG was investigated by stepping motor for 6000 stepping cycles. Limited by its insolubility and non-melting properties, lignin is hard to be directly utilized to the fabrication of TENGs. To address this issue, Zhou et al. [55] used polyamide to modify the lignin in 2020. Compared with the TENGs prepared by traditional materials, the modified lignin-based TENG showed outstanding performance, which was an extremely competitive candidate for eco-friendly energy-conversion devices. Results showed that the prepared TENG had superior durability and stability (200 000 cycles).
Chitosan is an abundant natural biopolymer from marine crustacean shells, which enables exciting opportunities for cost-efficient and biodegradable TENG applications in related fields. In 2018, Wu et al. [56] developed a biodegradable and flexible TENG based on chitosan for the first time. It was found that the chitosan-based film also maintained good mechanical stability during more than 18 000 cycles test.
Lin et al. [57] developed a chitosan-based TENG as shown in Figure 7c,d. As the relative humidity changed from 20% to 80%, the output characteristics of the TENG remained unchanged. In the same year, Long et al. [58] reported a TENG based on carboxymethyl chitosan and carboxymethyl cellulose sodium. The TENG showed big green advantages and could be dissolved quickly when it encountered water, which will not cause environment pollution. To develop a highly efficient TENG, in 2021, Vittayakorn et al. [59] proposed a durable chitosan composite by incorporating protein-based compounds as fillers. This biomass-based TENG had long-term stability within 9 weeks.
Gelatin, one of the most abundant natural biomacromolecules, is widely used in food, pharmaceutical, cosmetic, and photographic applications due to its excellent attributes such as low cost, superior biocompatibility, and excellent degradability. Fish gelatin is rich in amino acid residues with electron donor groups, which could endow a strong ability to lose electrons during friction. In 2020, Huang et al. [60] introduced a flexible, eco-friendly, and multifunctional fish gelatin-based TENG, which was composed of fish gelatin and PTFE/PDMS composite. The output voltage of the TENG remained stable after 10 000 cycles. However, these gelatin materials exhibited very low power outputs compared to those with non-biodegradable triboelectric materials. To improve the performance, Luo et al. [61] reported a high-power density TENG using biodegradable materials: electro-spun poly(lactic) acid, nanostructured gelatin films as the triboelectric materials ( Figure 7e). The TENG could generate an output voltage up to 900 V and a peak power density over 5 W m −2 , higher than or comparable to those made from synthetic polymer materials.
Starch is also one of the most abundant natural polymers. The large number of OH groups in starch may offer a proper matrix to dissolve cations and ions for obtaining higher performance. In 2019, Vela et al. [62] found starch electrolyte films showed an inalterable electrical performance after 5000 cycles of activity despite cracks generation after fatigue, However, the strong hydrophilic groups (OH) in starch hinders its use in the development of liquid-repellent TENGs. In 2021, Khandelwal et al. [63] enhanced the hydrophobicity of starch by introducing an edible filler (laver). The starch/laver composite had a contact angle of 107°, which was far higher than that of pristine starch. It was not degraded over 30 days in phosphate-buffered saline.

Introduction of Using Composited Triboelectric Materials for Boosting TENGs' Durability
As the requirements of TENGs are complex in practical applications, there are needs for special properties of materials that cannot be found in commercial or nature materials. In this regard, researchers have tried to produce composites to obtain the desired properties by taking the advantage of fillers and matrix. [64] Nanomaterials have shown the great potential to increase the durability of TENG by improving the mechanical properties and tribological properties of triboelectric materials. The types of triboelectric nanocomposites can be classified based on the dimension of fillers, that is, 0D, 1D, and 2D nanomaterials ( Table 1). There are several things need to be taken into consideration for using the nanomaterials as fillers. 1) Nanomaterials need to be dispersed uniformly in the matrix. For example, 0D BaTiO 3 and 2D MXene can be dispersed uniformly in the triboelectric materials due to chemical bonds such as hydrogen bonds between the nanomaterials and triboelectric matrix (Figure 8a,b).
2) Nanomaterials should have good mechanical properties. 0D SiO 2 , BaTiO 3 , 1D carbon nanotube, etc., with high hardness, high toughness, and tensile strength, are effective to boost the durability of TENGs.
3) The concentration of nanomaterials in the composites needs to be optimized. e.g., high amount doped MXene tends to agglomerate together due to the weak van der Waals force. These agglomerated grains are the potential split areas that are easily propagated to cracks under external force resulting in the break of the composites (Figure 8c). 4) Nanomaterials should provide good tribological performance. For example, hard nanomaterials, e.g., SiO 2 , can improve surface hardness of TENG, which is beneficial for lowering down the wear. While using self-lubricating nanomaterials, like some typical 2D graphene, MoS 2 , and MXene flakes, can reduce frictional force. Because these self-lubricating nanomaterials have layered microstructures and weak van der Waals forces bonding among the interlayers, which is beneficial for the formation of easy-to-shear lamellas resulting in lower friction and better protection of triboelectric surfaces (Figure 8d).

The Durability of 0D Nanomaterials Filled Triboelectric Materials Based TENGs
As shown in Figure 9a-c, in 2015, Baik et al. [68] developed a facile and scalable synthesis of mesoporous films impregnated with Au nanoparticles as effective dielectrics for enhancing TENGs' performance. The durability of the developed TENG was boosted from 500 cycles to 4000 cycles at a fixed Au content of 0.28 wt%. In 2017, Lim et al. [69] reported a durable TENG using Au nanoparticle-embedded PDMS matrix to enhance the mechanical robustness and reliability. The mechanical flexibility and stretchability of the TENG are greatly improved enabling to achieve the outstanding output stability during 10 000 cycles of repeated pushing and stretching tests. In 2018, Chen et al. [70] developed an Au-based nanocomposite using Au nanoparticles impregnated PTFE (Figure 9d,e). The TENG based on the Au/PTFE nanocomposite showed a superior tolerance to water even after 3000 cycles. In comparison, the pure PTFE became dim, whereas the Au/PTFE-based TENG remained bright even after washing. Hu et al. [71] prepared a porous PDMS by filling PDMS with Ag nanoparticles and by constructing an internal cellular structure (Figure 9f-h). The stability/durability of the Ag-composited TENG was demonstrated over 2500 cycles indicating good stability during long-term operation. Park et al. [72] made a mechanically robust composite via introducing Fe 3 O 4 nanoparticles in polyvinylidene fluorides (PVDF) by an electrospinning method. The mechanical strength of the Fe 3 O 4 /PVDF composite material was enhanced by a dispersion strengthening mechanism (Figure 6i-k). Harnchana et al. [73] found that by adding 0.2 wt% TiO 2 nanoparticles in a cement, the mechanical strength of the cement could be improved by approximately 1.25 times of that of ordinary Portland cement. The TENG fabricated from this type of cement had an output retention of about 84% over 5000 cycles. Yin et al. [74] introduced BaTiO 3 nanoparticle into a bacterial cellulose to fabricate a TENG by vacuum filtration. The peak value of the surface Young modulus of the composite film (13.5% BaTiO 3 particles) was about 398 MPa, which was much higher than that of pure material (274 MPa). The stress-strain curves of the composites showed good stability and excellent flexibility during over 20 000 compression cycles. Du et al. [75] introduced the poly (tert-butyl acrylate)-modified BaTiO 3 into PVDF matrix. Because of the excellent mechanical properties of the nanocomposite film, the TENG exhibited good stability and durability. The generated current did not appear to change through 21 000 cycles. Huang et al. [65] found the BaTiO 3 /cellulose triboelectric composite showed excellent mechanical strengths. No obvious decline in the TENG performance was observed after 5000 operation cycles.
As known, PTFE, PI, FEP etc. are popular triboelectric materials, but they have poor wear resistance properties. [76] With the recent availability of nanomaterials, composites made with such fillers have significantly less abrasion, which provide a new family of non-abrasive and wear-resistant materials. For example, the wear resistance of PTFE was improved by nearly two orders of magnitude via filling with 0D ZnO nanoparticles at the concentration of roughly 15% by volume. [77] To overcome the wear of TENGs materials, Wen et al. [16] fabricated a wearresistant nanocomposite as the triboelectric layer via adding 0D Al nanoparticles in PTFE films ( Table 1). The results showed that the PTFE nanocomposite had good wear resistance (10 −5 mm 3 Nm −1 ), excellent high-temperature tolerance (temperature range of −30 to 550 °C) and high hardness (60 HRM). Thereby, it had the capable of being used in autonomous vehicles and industrial brakes. To reduce the wear abrasion of TENGs, Chou et al. [78] introduced 0D SiO 2 nanoparticles into a silicone rubber for TENGs. The added SiO 2 enlarged the dielectric constant of the silicone rubber and increased the contact area between the friction layers due to the microstructures of composite silicone rubbers. The prepared TENG continuously operated at 150 rpm for approximately 10 h with an unchanged signal output. The surface of the silicone rubber was free of wear, which proved the durability and stability of the TENG.
www.afm-journal.de www.advancedsciencenews.com possessed good flexibility, with the elastic limit of 850 kPa for a 50% strain and elastic deformation, where stress was increased almost linearly with strain ( Figure 10b,c). The TENG device exhibited excellent mechanical robustness over a wide range of mechanical pressures during 10 000 cycles (Figure 10d,e). In 2020, Su et al. [81] developed a flexible TENG device based on silk/CNT nanocomposites via electrospinning and electrospray. The silk/CNT nanocomposite (0.99 wt%) had a higher Young's modulus (16.67 MPa) than that of pure silk material (11.85 MPa). The durability of the TENG was tested by using a vibration platform. After 756 000 cycles of beating with an external force, the output voltage of the TENG had a slight decrease of 16.7%. Because of the enhanced surface charge potential and the mechanical stability of Ag, the Ag nanowire-PVDF nanocomposite was utilized for boosting high-output and mechanical-durable TENGs. Thermal treatment for a short time of 30 s created an extensive fused network among the nanofibers without destroying the fibrous structure. The thermal welding process dramatically enhanced the TENG operational stability and there was no output change for the composited TENG after repeated operations over 16 000 cycles. [82] In addition to pure 1D nanomaterials, hybrid 1D nanomaterials filled triboelectric materials can also enhance the durability of TENGs. In 2021, Qi et al. [83] developed a doublenetwork hydrogel using hybrid 1D hydroxypropyl celluloses/αlipoic acid monomers (6 wt%) as building materials. The reinforcement from the networks significantly enhanced the tensile stress of hydrogels (11.85 MPa) compared with the nontreated hydrogels (0.56 MPa). And the prepared TENG showed good stability (1000 cycles). Han et al. [84] demonstrated a fully stretchable TENG by employing an oxidized CNT embedded with Ag in a PDMS substrate (Figure 10f,g). This Ag-CNT/PDMS-based TENG showed a good mechanical flexibility and stretchability over 10 000 cycles of stretching tests. It was also found there was not significant performance degradation under periodic and round-trip sliding ( Figure 10h).

The Durability of 2D Nanomaterials Filled Triboelectric Materials Based TENGs
Apart from 1D nanomaterials, 2D nanomaterials also have attracted interests to boost the durability of TENGs. Graphite nanosheets with excellent thermal and electrical conductivity have already been widely applied to diverse composites and constructs. As shown in Figure 11a, Roy et al. [85] introduced 2D graphite filled into PDMS matrix, which was achieved by a scalable fabrication process of roller bar-assisted printing. The graphite/PDMS TENG exhibited good durability with robust electrical output even after 15 000 loading/unloading cycles ( Figure 11b). Kim et al. [86] reported a chemically modified graphene oxide nanosheets aimed to create robust ethylene vinyl acetate (EVA) nanocomposites. It was found that after adding 1.2 wt% modified graphene oxide, the tensile strength, Young's modulus, and storage modulus of EVA composites were increased by 80%, 50%, and 24%, respectively. Moreover, the  [65] Copyright 2020, Wiley-VCH GmbH. b) Schematic illustration of the uniform dispersion of MXene in elastic silicone rubber via hydrogen bonding, c) illustration of the influence of doped MXene on the mechanical performance. Reproduced with permission. [66] Copyright 2020, Wiley-VCH GmbH. d) Schematic illustration of the anti-wear mechanism of MoS 2 /PI composites. Reproduced with permission. [67] Copyright 2021, Elsevier Ltd.
Hu et al. [87] prepared graphene sheets embedded in PDMS by repeated spin-coating technique ( Figure 11c). The morphologies on the surface of PDMS composite film were also smooth after working for one week indicting the great stability of the composite (Figure 11d). Yang et al. [88] designed a self-powered pressure sensor by integrating a reduced graphene oxide/PI composite foam as a pressure sensitive element. The device could achieve durable external load sensing with a range from 0 to 30 N.
An emerging 2D nanomaterial, MXene, exhibits outstanding metallic electrical conductivity, large electronegative surface, and good mechanical strength. Thus, MXene becomes a promising alternative of electronegative material for TENGs to provide enhanced performance and durability. [89] Ping et al. [90] proposed a MXene/PVA-composite nanofiber used as a negative TENG layer via electrospinning manufacturing technique. The durability and flexibility of the TENG was detected for more than 124 000 cycles. The electrical output performance maintained stable even the TENG was deformed arbitrary. Cai et al. [91] prepared a TENG, in which MXene was added into a wrinkled PDMS. This TENG had good flexibility, which could be bent more than 90°. After 10 000 cycles continuous testing, the output performance had no obvious degradation, and the wrinkles had no visible damages. Zhang et al. [92]   . a) Schematic diagram, b) micro morphologies, c) mechanical performances of the fabrication process for the Au-based nanocomposite TENG. Reproduced with permission. [68] Copyright 2015, Royal Society of Chemistry. Schematic diagram and digital images of d) the Au/PTFE nanocomposite films by spin-coating, e) the stability and durability test of the TENG's output for 3000 cycles by water washing 4 times. Reproduced with permission. [70] Copyright 2018, Springer Nature. Schematic process of f) fabricating the Ag/PDMS nanocomposite film and TENG, g) micro morphologies, and h) durability of the TENG. Reproduced with permission. [71] Copyright 2017, Tsinghua University Press and Springer-Verlag GmbH Germany. Fabrication process of i) the Fe 3 O 4 /PVDF nanocomposite by the electrospinning method, j,k) mechanical performances of the nanocomposite TENG. Reproduced with permission. [72] Copyright 2018, The American Chemical Society.
www.afm-journal.de www.advancedsciencenews.com were added into 2 mL pure PTFE aqueous emulsion. The MXene/PTFE nanocomposite showed a strain rate of 90% and a tensile strength of 48 MPa, which were 450% and 50% larger than those of the pure PTFE film. After the 100 000 cycles test, the fabricated TENG showed no distinct damages ( Figure 11f). Salauddin et al. [93] introduced an MXene/Ecoflex nanocomposite as a promising triboelectric material. The prepared MXene/Ecoflex nanocomposite showed outstanding flexibility from being twisted, rolled, and crumpled and exhibited highly stable output performance over 86 400 cycles without any measurable degradation. Taking the advantages of 1D and 2D materials, Ma et al. [66] demonstrated a 2D MXene-based composite enhanced by 1D cellulose nanofibers as a dispersant and interlocking agent ( Figure 11g). It was seen that the TENG possessed a stable electrical output capability after 6000 cycles of repeated contact-separation motion and exhibited no obvious change in output voltage under a 50% strain during 1000 cycles (Figure 11h).
2D graphene also provides a low friction force because of its weak interlayer bonding. [94] Koratkar et al. [95] reported a four orders of magnitude reduction in the steady state wear rate of PTFE by filling graphene. The wear rate of unfilled PTFE was measured to be ≈0.4 × 10 −3 mm 3 (Nm) −1 which was reduced to ≈10 −7 mm 3 (Nm) −1 by the incorporation of 10 wt% of graphene nanosheets. MoS 2 with layered microstructures and weak van der Waals forces bonding among the interlayers, leads to the formation of easy-to-shear lamellas and lower friction. [96] Chueh et al. [97] reported a stable, high-performance, long-life TENG made from MoS 2 /PVC composite as the triboelectric material. The friction coefficient of the MoS 2 /PVC composite was 0.29, which was 19.4% lower than that of pure PVC. The composite also had narrower wear scars (21-29% reduction in wear width) and fewer wear debris leading to effective improvement of the TENGs' lifespan. Adding 2.5 wt% of MoS 2 can enhance the output voltage and current to the highest value and exhibited the excellent stability of the output current over a continuous working time of ≈15 h.
MXene not only exhibits good conductivity and good mechanical strength, but also has an excellent wear resistance performance. Rosenkranz et al. [98] found MXene demonstrated a sixfold friction reduction and an ultralow wear rate (4 × 10 −9 mm 3 (Nm) −1 ) over 100 000 sliding cycles, which outperforms state-of-the-art 2D nanomaterials by at least 200%. At the same time, MXene has strong triboelectric ability, similar to PTFE, which is attributed to the electric negative functional groups of OH and F. Thus, MXene has the capability Figure 10. Schematic of the CNT-based nanocomposite, SEM images, and digital photograph of a) the 3D structured pressure TENG sensor, b,c) mechanical performances of the nanocomposite film, d,e) durability test of the TENG. Reproduced with permission. [80] Copyright 2018, Elsevier Ltd. Schematic of the fabrication process for f) the Ag/SWCNT-based TENG, schematic and SEM image of g) the Ag/SWCNT composite current collector and a stretchable PVA-based electrode, h) durability test of the TENG. Reproduced with permission. [84] Copyright 2021, Elsevier Ltd.
www.afm-journal.de www.advancedsciencenews.com of effectively capturing electron. Gao et al. [92] introduced an MXene/PTFE composite as a durable triboelectric material via a spray coating method. The obtained composite film showed a great enhancement in tensile property up to 450% and the wear volume was reduced more than 80%. It was found that there was no significant output-signal attenuation during 100 000 cycles test. There had no distinct variance of surface morphologies after the test, which identified the excellent stability of the MXene/PTFE based TENG.

Introduction of Developing Self-Healing Triboelectric Materials for Boosting the TENGs' Durability
Because of constant external mechanical stresses, such as tapping, bending, and stretching, the durability of TENGs is severely degraded by frequent and unavoidable mechanical  Figure 11. Schematic process of a) fabricating the graphite/PDMS TENG, b) durability of TENGs based on the composite over 15 000 cycles. Reproduced with permission. [85] Copyright 2021, Elsevier Ltd. c) Schematic diagram and d) durability test of the graphene/PDMS TENG. Reproduced with permission. [87] Copyright 2017, Elsevier Ltd. e) Illustration and f) durability test of the MXene/PTFE TENG film. Reproduced with permission. [92] Copyright 2021, Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature. Schematic diagram of the charges distribution of the TENG caused by g) the contact triboelectrification effect of the MXene/cellulose, h) stability and robustness measurement of the TENG at a frequency of 2 Hz. Reproduced with permission. [66] Copyright 2020, Wiley-VCH GmbH.
damages. Especially, polymers are susceptible to damages in the form of deep cracks, which are difficult to repair. The cracking induced by friction or mechanical fatigue will also lead to mechanical degradation of materials. [99] Advances in next-generation soft electronic devices rely on the development of highly deformable, flexible, and printable energy generators to power these electronics. [100] Here traditional synthetic polymers cannot meet the requirement for materials with highly flexible, stretchable and transparent features, while hydro-, oleo-, ionic-gels show great advantages. In this regard, developing self-healing materials can effectively improve the durability of these highly deformable triboelectric materials. Thus, the development of self-healing materials has emerged as a particularly promising way to effectively increase the durability of TENGs (Table 1). [101] There are two types of self-healing materials: extrinsic self-healing materials and intrinsic self-healing materials (Figure 12a,b). Extrinsic self-healing materials are based on the release of monomers or catalysts stored inside capsules or vessels and present within the matrix, which are immediately released after damages. The most reported approach of extrinsic self-healing is embedding healing agents in microcapsules, which is able to protect the materials substrates and subsurfaces (Figure 12c,d). Intrinsic self-healing materials contain reversible noncovalent bonds (e.g., hydrogen bond, metalligand bond, and crystallization) and covalent bonds (e.g., imine bond, borate bond, and disulfide bond), which allow the restoration of the materials after damages. For example, the functionality of the hydroxyl groups and isocyanates (NCO groups) can be used to prepare self-healing materials via the presence of H-bonding between NH groups and CO groups (Figure 12e). [100] The proportion of Fe/pyridinedicarboxamide coordination in materials can result in tunable self-healing abilities (Figure 12f). Compared to reversible non-covalent bonds, reversible covalent bonds have a higher bond energy for creating strong bonding between molecules. Dynamic covalent imine bonds can be introduced into the PDMS networks as reversible healing sites when bis(amine)-terminated PDMS reacts with 1,3,5-triformylbenzene by a Schiff base reaction (Figure 12g). [103] Reversible borate bonds have shown great versatilities and capabilities as dynamic crosslinks for the design of self-healing and reprocessable polymer networks. [104] Dynamic disulfide bonds can also be introduced into the epoxidized natural rubber/polylactide blend via the epoxy-acid reaction between epoxidized natural rubber and 2,2-dithiodibenzoic acid to prepare self-healing materials (Figure 12h). [105]

The Durability of Extrinsic Self-Healing Triboelectric Materials Based TENGs
Extrinsic self-healing makes use of a pre-embedded healing agent in materials, which is easily realized because no structural modification of the matrix molecules is required. [107] In 2010, inspired by plants maintaining extrinsic self-healing superhydrophobic function, Li et al. [108] fabricated porous polymer coatings that were rigidly flexible and had micro-and nano-scaled hierarchical structures, in which fluoroalkyl silane was capsuled as healing agents. Similarly, Zhang et al. [109] reported a simple spray-coating approach for the fabrication of durable and self-healing superamphiphobic coatings repellent to both cool and hot liquids by the combination of palygorskite, 1H,1H,2H,2H-perfluorodecyltriethoxysilane and tetraethoxysilane. Once the coating was damaged, the perfluorodecyltriethoxysilane molecules preserved in the coating spontaneously migrated to the surface of the coating via thermal motion, then the damaged areas with hydrophilic groups were buried inside the coating with lower the surface energy. The damaged coating was self-healed under room conditions for 24 h. Figure 12. Schematic illustration of a) self-healing through exhaustion of healing agents and b) intrinsically self-healing systems with reversible chemical bonds. Reproduced with permission. [106] Copyright 2017, Wiley-VCH GmbH. c) A microencapsulated healing agent embedded in a material matrix containing healing agent, Reproduced with permission. [99] Copyright 2001, Springer Nature. and d) embedded in a material subsurface. Reproduced with permission. [106] Copyright 2017, Wiley-VCH GmbH. Intrinsically self-healing materials with reversible chemical bonds: e) hydrogen bond, f) coordination bond, g) amine bond, h) borate bond, and i) disulfide bond. Reproduced with permission. [105] Copyright 2022, Elsevier Ltd.
www.afm-journal.de www.advancedsciencenews.com Taking the advantage of extrinsic self-healing characteristic, Xu et al. [110] constructed a superhydrophobic and self-healing TENG based on normal microarc oxidation coating, which was in situ formed on the pure Al substrate by microarc oxidation. The hydrophobic/superhydrophobic surface was further achieved by direct fluorination treatment. The inorganic coating as the dielectric layer, exhibited better aging, wear, and corrosion resistance than other polymer materials. The micro-and nano-porous structures of the coating could be used as storage tanks for self-healing agents, which endowed the coatings with self-healing properties and could cope well with the damages caused by aging, wear, and external force in the long-term process. The signal output of this Al-based selfhealing TENG had remained relatively stable after 10 800 cycles of continuous operation. Wang et al. [111] reported a hydrophobic self-healing TENG, which was composed of SiO 2 nanoparticles and poly(vinylidenefluoride-co-hexafluoropropylene) or perfluorodecyltrichlorosilane. When the damaged hydrophobic surface was heated for several minutes or placed at room temperature for several hours, perfluorodecyltrichlorosilane molecules would migrate to the damaged hydrophobic surface to lower the surface free energy and to recover the surface hydrophobicity. After working for 18 000 cycles, the output voltage showed no noticeable fluctuation indicating its longterm stability for application as practical water droplet energy harvesting.

Hydrogen-Bonded Triboelectric Materials Based TENGs
In principle, intrinsic self-healing materials can give an unlimited healing cycle. Intrinsic self-healing TENGs based on hydrogen bond are of considerable interests. The hydrogen bond, as an important non-covalent, is existed in the interaction between functional groups of carboxyl, amide, urea pyrimidine, hydroxyl, etc. Lee et al. [112] used slime-based ionic conductors as transparent current-collecting layers instead of metallic electrodes, thus significantly enhanced the electrodes' energy generation, stretchability, transparency, and self-healing characteristics. The layers could endure a uniaxial strain up to 700% and provide an autonomously self-healing performance even after 300 times of complete bifurcation. The stability of the produced TENG was evaluated by measuring the voltage output for 1000 continuous cycles with an interval of 15 days. The device showed stable performance even after 15 days, indicating the good durability of the self-healing TENG. In 2022, Sun et al. [113] constructed a self-healing TENG by applying a linear silicone-modified polyurethane (PU) coating as a triboelectric layer. When the triboelectric layer experienced abrasion, the broken silicone-modified polymer chains would gradually be cross-linked again through hydrogen bonding to achieve the self-healing effect. The results showed that there was no degradation in performance between the origin and healed TENG for about 15 000 cycles.
Filling nanomaterials into a TENG matrix not only can significantly improve the mechanical and tribological properties, but also will boost the self-healing performance as a physical cross-linking agent. Long et al. [114] prepared 0D polydopamine nanoparticles and introduced them into a polyacrylamide hydrogel. The polydopamine-hydrogel exhibited super-stretchability of about 6000%. It could self-heal in about 10 min after being cut down. More importantly, when it was used as the electrode in flexible TENGs, stable and excellent electrical output performance could be maintained even after being seriously stretched. Lee et al. [100] developed a highly conductive and healable composite based on thermoplastic elastomer with liquid metal and 0D Ag nanoparticles. The elastomer is used both as the matrix for the conductor and as the triboelectric layer. The elastomer TENG showed a stretchability of 2500% and it recovered its energy-harvesting performance after extreme mechanical damage due to the supramolecular hydrogen bonding of the thermoplastic elastomer and the stretchability of Ag nanoparticles. The TENG showed no obvious degradation in performance after 50 000 continuous cycles of mechanical impact. Guan et al. [115] filled 1D MWCNTs in a self-healing hydrogel TENG as shown in Figure 13a. The electrical outputs remained stable even with 200% strain since the MWCNTs dispersed evenly in the matrix and played the role of conductive fillers. Yan et al. [116] found that flexible composite films consisting of n-isopropylacrylamide and 2-methoxyethyl acrylate and 2D graphene nanosheets exhibited effective self-healing effect (reaching 96%) at room temperature because of the reversible hydrogen bonds. The filling of 2D MXene nanosheets promoted the crosslinking of the PVA hydrogel and improved the stretchability of the composite hydrogel. This composited TENG can be stretched up to 200% of the original length. [117] Upon stretching/releasing cycles with a maximum strain of 300% for 200 cycles, the resistance of the films showed a variation less than 5%, which represented to the relatively high electrical property recovery after experiencing repetitive long-term cycles. Kong et al. [118] also reported a highly efficient TENG assembled by modified MXene nanocomposite elastomers. After five breaking-healing cycles, the healed TENG processed an excellent healing capability with the elongation at break of 806% with a self-healing efficiency of 85.3%.

Coordination-and Crystallization-Bonded Triboelectric Materials Based TENGs
Coordination interaction from metal-ligand bonds, one of the dynamic interactions, is adopted to form strong reversible cross-links to improve the interfacial interaction. For example, Liu et al. [119] incorporated a Zn-PDMS elastomer for triboelectric layer with excellent viscoelastic behaviors, superstretchability (>5200%), self-healing ability, and transparency (Figure 13b-d). Even after several cutting-healing cycles, the output performance still maintained consistent. Wen et al. [120] achieved the forming coordination bond between Zn 2+ and COOH of acrylic acid in TENGs. The TENG exhibited the ability to restore a certain degree of stretchability due to the reconstruction of this dynamic coordination bond. The TENG signal output after 2 months was basically the same as the original one. The 8000 cycles of continuous contact and separation test indicated that the TENG also had excellent durability. www.afm-journal.de www.advancedsciencenews.com Another non-covalent interaction, for example, crystallization, can be used for some thermally self-healing materials. Cooling down the material when it is above its transition temperature (Tt) (either the melting temperature or the glass transition temperature) will recover its permanent shape. This unique shape programmable property makes them a promising candidate in self-healing TENGs. Liu et al. [121] prepared a semicrystalline polymer in a chemically cross-linked elastomer, that is, the polymer chains were immersed in the network elastomer used as the triboelectric material. When the polymer was deformed at a temperature above the melting temperature of semicrystalline, the small crystals were melted and deformed with the elastomer network. Upon cooling down without releasing the load, the semicrystalline chains reformed a physically cross-linked network via small crystals, which could lock the deformed network (Figure 13e). When 20 wt% of semicrystalline was filled, the TENG material showed fracture strain as high as 900%, while that of the control sample was about 450% (Figure 13f). The TENG was able to remain 97% of its initial performance after 21 000 cycles (Figure 13g). Luo et al. [122] fabricated a self-healing TENG with a one-step repairable friction layer and conductive layer by combining healable polycaprolactone with flexible Ag nanowires. The polycaprolactone films contained many long chain molecules and polymer crystals which consisted of entangled and curly molecular chains. Upon raising the temperature beyond the melting point of polycaprolactone, the polymer crystals will stretch and flow to the damaged region together with the Ag nanowire. The broken molecular chains on both sides of the cut will reconnect. Once reducing the temperature below the melting point, the long chains can curl and entangle, which will result in the recrystallization formation. This designed TENG can work for nearly 130 000 cycles without obvious decrease of the signal output and with little/no surface fretting.

The Durability of Covalent Bonded Self-Healing Triboelectric Materials Based TENGs
Chen et al. [123] introduced reversible dynamic imine bonds in the polymer networks of PDMS (Figure 14a,b), which helped the prepared TENG achieving 100% healing efficiency. They  Society of Chemistry. [115] b) Synthetic route of the cross-linked elastomer Zn-PDMS, c) photograph of gelation behaviors with the addition of various Zn 2+ salts and photographs at different strains (0%, 3000%, and 5200%), d) transferred charge density of the TENG with intact structure and surface breakage under different bending angle of finger. Reproduced with permission. [119] Copyright 2020, Elsevier Ltd. Schematics showing e) the mechanism of self-healing performance, f) tensile stress-strain curves, and g) durability test of the TENG. Reproduced with permission. [121] Copyright 2018, Wiley-VCH GmbH & Co. KGaA.
www.afm-journal.de www.advancedsciencenews.com further proposed a PVA-based hydrogel consisting of modified CNTs on the surface of the imine-bond PDMS. Owing to the repairable networks of the dynamic imine bonds, the prepared device could recover its original state after mechanical damage within 10 min at room temperature and the CNTs in the matrix could enhance the mechanical strength of hydrogel. [124] Sun et al. [103] designed a self-healing PDMS using dynamic imine bonds filled with Ag nanowires to repair the mechanical damage. The designed TENG could recover the electricity generation (100% healing efficiency) even after accidental cutting.
Disulfide bonds have higher bond energy, which can promote the formation of strong bonds between molecules and improve the healing efficiency at relatively low temperature. Zhang et al. [125] used bis(4-hydroxyphenyl)-disulfide as the chain extender in PU substrates, which could effectively enhance the elasticity, toughness, and self-healing functions of PU (Figure 14c,d). The fabricated PU reached 1.11 µm min −1 in a cut-through sample and recovered more than 93% of virgin mechanical properties in 6 h at room temperature. After 1000 times cyclic stretching, the tensile strength hardly changed, which proved its excellent fatigue resistance. Zhang et al. [126] achieved a self-healable, flexible TENG by combining an elastomer with a 1D Ag nanowire conducting network (Figure 14e). Due to the dynamic disulfide exchange reaction in the elastomer, the device possessed a series of fascinating features like excellent elasticity/stretchability, outstanding formability/ deformability, and fast scratch or break recovery (Figure 14f,g). Better recovery was obtained using longer healing time with nearly 100% recovery after healing at 65 °C for 4 h. Higher healing temperature also contributed to better healing. With a healing time of 30 min, the healing efficiency is 45% at 35 °C, which increases to nearly 100% at 95 °C.
Reversible borate-oxide bond has great potential for enhancing the durability of TENGs because trigonal planar boronic esters undergo reversible depolymerization due to the hydrolytic cleavage of boronic ester bond in their main chains. Pan et al. [115] introduced water-active dynamic borate bond into the hydrogel, in which the active 0D polydopamine and 1D CNT were also embedded. The TENG achieved high stretchability (400%), mechanical (93%), and electrical (100%) healing efficiency, respectively, and also exhibited very stable outputs over 500 cycles. Figure 14. Chemical structures of a) dynamic imine bonds and b) demonstration of healing process. Reproduced with permission. [123] Copyright 2021, Elsevier Ltd. c) Chemical structure and network structure of bis(4-hydroxyphenyl)-disulfide PU, d) schematic of an elongated PU film with water droplets unable to infiltrate and the crack could be self-healed driven by dynamic disulfide bonds. Reproduced with permission. [125] Copyright 2020, The American Chemical Society. Schematic illustration of e) self-healing process by disulfide metathesis, f) optical microscope images of the notched and self-healed BS-PU-3 film showing a gradual disappearance of the scar during self-healing at room temperature for 300 min, g) open-circuit voltages of original and repaired TENG. Reproduced with permission. [126] Copyright 2018, Wiley-VCH GmbH.

Multiple Non-Covalent Bonded Triboelectric Materials Based TENGs
The durability of TENGs can be further improved by taking the advantage of multiple non-covalent reversible bonds in triboelectric materials. Jiang et al. [127] incorporated hydrogen bonds and dynamic metal-ligand coordination into PDMS which can simultaneously heal the fracture and abrasion at room temperature. The TENG possessed ultrahigh stretchability (10 000%), remarkable self-healing property (100% efficiency), and long-term durability (180 days). In 2021, we presented a PU-based self-healing TENG with good mechanical self-healing and electric breakdown self-healing performance. The excellent self-healing properties mainly comes from a combination of intermolecular hydrogen bonding and dynamic coordination bonds. [128] After several times cycle mechanical electrical breakdown followed by self-healing, the developed triboelectric material can be almost 100% restored to the original state. The prepared TENG showed an excellent stability (no output performance degrading even after 3000 cycles). Qi et al. [83] designed a self-healing ionogel via using hydroxypropyl cellulose chains with the double-network structure with molten α-lipoic acid poly(disulfides) chains (15 wt%) by multiple hydrogen bonding interactions. The self-healing ionogel had a higher value of ultimate tensile strain of 800 kPa, while that of pure ionogel was about 100 kPa. Sun et al. [129] fabricated an ultra-durable skinslike TENG by impregnating ionic liquids into a mechanically robust PU network. The PU network was composed of flexible poly (ethylene glycol) with hydrogen bonds and crystallized poly(ε-caprolactone). The prepared TENG showed a highly reproducible electrical response over 10 000 uninterrupted strain cycles. The performance of the TENG stored in open air for 200 days was almost the same as that of the freshly prepared one.
Lai et al. [130] designed a robust TENG, in which both the triboelectric layer and the electrode were intrinsically and autonomously self-healable under ambient conditions. It consisted of a hydrogen-bonded ionic gel as the electrode and a metal-coordinated polymer as the triboelectrically charged layer. Even after 500 cutting and healing cycles or under extreme 900% strain or after being exposed to a 1 N contact force >1000 times, the prepared TENG showed no obvious performance degradation.
Most of the reported works just healed the fracture of TENGs. It is required for TENGs to repair the abrasion which is caused by uneven surface or continuous friction. Therefore, Jiang et al. [127] developed a polymer-based composite via adding organic polyamide and zinc chloride in the mixture of PDMS and PU. It was found that the composite TENG got many deep surface furrows due to the wear abrasion caused by continuous friction from the uneven surface, which would cause malfunction. Fortunately, the rough surface caused by wear abrasion became a little smoother after healing for 30 min. After 2 h self-healing, the rough surface became almost as smooth as the original sample due to the incorporation of hydrogen bonds and dynamic metal-ligand coordination. In contrast, the abrasion of the commercial PDMS cannot be repaired after wear.

Noncovalent and Covalent Bonded Triboelectric Materials Based TENGs
Combination of non-covalent and covalent bonds can effectively address the material damaging issues of TENGs. Qi et al. [83] obtained reversible double networks through high density interactions of hydrogen bonds and disulfide bonds. The energy-harvesting performance of the TENG was found to be stable after long-term use, being stretched, being recycled, and even after mechanical damages. The mechanical and electrical performance of broken ionogels could be nearly recovered to 92.2% and 93.7% after exposure to the temperature of 100 °C for 1 h. Liu et al. [131] developed an ultrathin self-healing skin-like TENG that exhibited excellent mechanical and durable performance induced by strong hydrogen bonds and disulfide bonds. The TENG maintained the identical electrical output performance after 45 000 bending tests and over 146 000 operational cycles. Hao et al. [132] developed a healable and shape-memory TENG via introducing poly (1,4-butylene adipate) segments and disulfide bonds. The original sample showed a tensile stress of about 10.5 MPa and a tensile strain of 910% at breaking point. When the sample was broken, these parameters sharply declined to about only 2.7 MPa and 25%. The stress and strain can recover to around 9.5 MPa and 880%, respectively after healing at 90 °C for 1 h. The developed TENG showed great durability with no obvious performance degradation after continuous operation for 14 400 cycles.
Yang et al. [124] introduced repairable networks of dynamic imine bonds in the charged layer and the borate bonds in electrodes with the assist of sodium borate and CNTs. The tensile strain of the self-healing TENG reached 388% with a self-healing efficiency of 86%. The output signal of the TENG remained almost the same over 500 cycles. For pursuing self-healing TENG at broad working temperatures, Khan et al. [133] designed a reversible multi-bonded triboelectric material including hydrogen bond, coordinate bond, and disulfide bond. An gel electrode was prepared by mixing poly (lipoic acid) with supramolecular crosslinkers of Fe 3+ and phytic acid, followed by adding the conductive polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. The ultimate tensile strain of the self-healing electrode was about 120 kPa, while that of the pure sample was only about 40 kPa. These gel networks were constructed by supramolecular interactions of reversible physical bonds. After 24 h of healing, the TENG could still be stretchable up to more than 4700%. The resulting TENG maintained excellent performance even after 5000 cyclic operations. Moreover, the energy-harvesting capability was verified to be applicable from −40 to 80 °C. [133]

Introduction of Optimizing Triboelectric Structures for Boosting TENGs' Durability
The durability is limited by the relatively large frictional resistive force between triboelectric surfaces during the operation of the TENGs. Optimizing the triboelectric structures including fabricating rolling-motion and noncontact structures has been www.afm-journal.de www.advancedsciencenews.com also investigated on enhancing the output performance and robustness of TENGs. In comparison to sliding motion, rolling motion not only is likely to consume less mechanical energy but also presents high robustness with minimized wear of materials. There are three contact modes in terms of tribology, that is, point contact (point-to-face contact), line contact (line-to-line or line-to-face contacts), and face contact (face-to-face contact), can be utilized for the design. For example, electrification from line contact between cylinder-shaped steel rods and planar thin films of FEP was employed for converting the kinetic energy of rolling rods into electric power. The rolling motion of the steel rods between the FEP thin films introduced triboelectric charges on both surfaces and led to the change of potential difference between each pair of electrodes on the back of the FEP layer, which drove the electrons flow through the external circle. The TENG with rolling motion showed little degradation, while most of other samples enduring sliding friction were destroyed (Figure 15a-c). Direct physical contact between two triboelectric materials results in unavoidable materials wear. The charges flow can be induced by relative motion between the per-charged triboelectric layers without direct contact. [135] One big issue for the totally noncontact TENG is that the initial-contact charges can only remain on the surfaces for some hours resulting in the decrease of electric output with time. To address this issue, it is ideal to set TENG work in the non-contact mode, which can also automatically shifts into contact mode intermittently for charge replenishing to retain the electrical stability, that is, noncontact/contact-transited structures (Figure 15d).

The Durability of Rolling-Motion Triboelectric Structures Based TENGs
In 2014, Zhang et al. [138] demonstrated a TENG consisting of one inner ball and an outer transparent sphere with a pointto-face contact mode. One Al sheet was adhered to the inner surface of the outer sphere and was used as the electrode. The fabricated TENG could scavenge the vibration energy in full space by point-to-face contact from ball on plate state. The fabricated TENG can be utilized as a stable and reliable selfpowered sensor to detect the vibration acceleration. In 2016, Lee et al. [139] demonstrated and optimized parameters for a wind-rolling TENG to harvest a wide range of winds based on the point-to-face mode. No fractures or critical damages were found even the spheres were deformed due to the high contact pressure ( Table 2) [139] Rolling motion: small, flat and light in weight 56 days Free-standing 2018 [140] Rolling motion: Excellent supporting performance at different loads 9 h Free-standing 2020 [141] Rolling motion: surface with little degradation (Sliding motion: surface worn and destroyed) 1000 cycles Free-standing 2015 [136] Rolling motion: no obvious wear after 3 days Free-standing 2018 [143] Rolling motion: normalized wear rate: 6.418 (Rolling motion: normalized wear rate: 649) 4 100 000 cycles (34 500 cycles) Free-standing 2022 [144] Noncontact/contact transition: no obvious wear after 50 h sliding 50 h Free-standing 2014 [145] Structure (Noncontact/ contact) Noncontact/contact transition: no surface wear (Contact structure: severe wear) 120 000 cycles (6000 cycles) Free-standing 2015 [147] Noncontact/contact transition: negligible wear (Contact structure: worn out) 24 h (1 h) Free-standing 2020 [137] Noncontact/contact transition: insignificant wear (Contact structure: strong mechanical abrasion)
In case of line contact, Lin et al. [136] designed a rolling electrification TENG by applying a line contact from rods on the triboelectric surface to improve the energy conversion efficiency and device durability in 2015. Owing to the low friction feature of the rolling motion, the rolling-motion TENG had little degradation, while other samples enduring sliding friction were destroyed after 1000 cycles at 0.25 N. In 2018, Yang et al. [142] provided a large roller TENG device based on line-to-line contact motion mode fabricated by a PTFE cylinder as negative materials and a middle copper strip uniformly acted as electrodes (Figure 16b). With the rolling friction and gear transmission structure, the TENG device was more durable and more facile to be installed on the rotation objects. No obvious normalized current decay was found after 1 008 000 rotation cycles (at a fixed rotation speed of 700 r min −1 for 1 day). In 2019, Wang et al. [143] designed a full-packaged rolling hybrid TENG by integrating silicone rubber-magnet rods and electrodes. The rolling friction resulted in almost no damages to the triboelectric surface and there was no wear or no obvious normalized current decay during 3 days continuous working.
Utilizing the static friction of the rolling balls or rods is an effective strategy to address the mechanical durability of TENGs, but the point and line contacts seriously reduces the output performance because of their low triboelectric contact areas. In 2022, Hu et al. [144] provided a high output and ultra-durable TENG based on face-to-face contact of rolling belts (Figure 16c,d). Due to no sliding friction, the frictioninduced heat of the TENG is negligible compared with that of sliding TENG (Figure 16e). For the rolling TENG, the material abrasion is 6.42 µg g −1 (<1% of traditional sliding TENG) every 10 000 cycles. The rolling TENG remained nearly 98.5% of its output performance after more than 4 100 000 cycles (Figure 16f), while the durability of sliding TENG was only about 34 500 cycles.

000 s (6000 s)
Lateral sliding 2021 [196] Lubricating interface: uniform and slight scratches; (Dry interface: obvious scratches) 500 000 cycles (5000 cycles) Lateral sliding 2020 [194] Lubricating interface: friction torque: 0.0275 Nm (Dry interface: friction torque: 0.0348 Nm) 432 000 cycles Contactseparating 2021 [197] a) Note: The data in brackets are from the compared materials without treatment. www.afm-journal.de www.advancedsciencenews.com as a part of TENG and the other part of TENG was composed of a printing paper together with a metal electrode film. The prepared TENG avoided the direct physical contact and in principle its service life is unlimited. At the same time, the noncontact energy harvesting of a human daily motion would also be tried to be achieved by the proposed TENG.
One problem for the total non-contact TENG is that the charge on the surface will degrade with time. For surface charge replenishment, in 2015, Li et al. [147] showed a rational designing for achieving a significant improvement of the long-term stability through automatic transition between contact and noncontact working states (Figure 17a-c). The TENG could work in the noncontact state with minimum surfaces wear and could be transited into contact state intermittently driven by winds to maintain high triboelectric charge density. After 120 000 cycles, nearly 95% of the maximum output signals was preserved. As a comparison, a TENG operating in the contact state could only maintain 60% of the original value. Moreover, the rubbing surface of the compared TENG was almost completely wiped out due to constant wear after only 24 000 cycles.
In 2020, Lin et al. [148] integrated a radial-arrayed TENG with a transmission mechanism, which successfully maximized the sustainable operation with less triboelectric materials wear and damage (Figure 17d,e). After running for 500 000 cycles, there was only 1.8% degradation. In the same year, Chen et al. [137] also reported a wide-frequency and ultra-robust rotational TENG Figure 16. a) Structural design of the ball-to-surface contact rolling-motion TENG. Reproduced with permission. [141] Copyright 2020, Elsevier Ltd. b) Structural design of a rolling-motion TENG via line-to-line contact. Reproduced with permission. [142] Copyright 2018, Elsevier Ltd. c) Basic structure of the face-to-face TENG, the inset: the whole device image comparing with a button battery. Schematic of charge distribution (Stage 1-3) under d) short-circuit condition, e) the temperature variation of the triboelectric layer surface for two mode TENGs after working continuously for 300 s, f) long-term durability of the roll motion TENG (RB TENG) compared with sliding TENG (FS TENG). Reproduced with permission. [144] Copyright 2022, Wiley-VCH GmbH.
composed of a built-in traction rope structure and capable of transforming from contact mode to noncontact mode automatically (Figure 17f). Mechanical wear of the TENG was negligible compared to that of contact-structural TENG, which was severely worn out. The prepared TENG also exhibited excellent electrical stability, which could maintain 90% electric output after over 24 h of continuous operation, while the noncontact mode TENG suffered severe wear and only retained 30% charge output and the contact mode TENG retained 2% charge output, respectively (Figure 17g). In 2022, Liu et al. [149] proposed an ultra-robust and high-performance rotational TENG enabled by automatic transitioning (contact mode at low speed and noncontact at high speed). There was a strong mechanical abrasion on the surface of nylon and FEP for normal contact mode TENG, while the mechanical wear was insignificant for the developed automatic transitioning TENG. It displayed excellent stability, maintaining 94% electrical output after 72 000 cycles, which was much higher than that of the normal contact TENG (30%).
The wear still exists in the above noncontact/contact transition TENGs because of the period of surfaces charge replenishment. Thus, partial flexible-and soft-contact materials are designed to reduce abrasion and to enhance the device durability of the noncontact TENG. Chen et al. [150] introduced animal furs as the contact materials. It found that the transferred charge of the fur-brush automatic TENG exhibited only 5.6% attenuation after continuous operation for 300 000 cycles maintaining high output performance. Lin et al. [151] introduced a spring and flexible dielectric fluff to the novel pendulum-like structural design replenished in soft contact period under the intermittent mechanical excitation. A dual-mode and frequency multiplied TENG with ultrahigh durability and efficiency was achieved with negligible change of output performance after a total of 2 000 000 cycles. They also reported a swingstructured noncontact/contact TENGs. [152] The output voltage and current amplitudes exhibited negligible changes after a total of 1 000 000 cycles exhibiting excellent robustness, stability, and durability. Similarly, Jiang et al. [153] reported a robust Figure 17. a) Schematic of the basic structure of the wind-driven TENG, b) force diagram of the rotational part in the wind-driven TENG, c) theoretical relationship between short-circuit charge transfer and wind speed for the wind-driven TENG. Reproduced with permission. [147] Copyright 2013, American Chemical Society. Schematic illustration of d) the TENG in water and photograph of e) the fabricated TENG, a rotator, and a stator. Reproduced with permission. [148] Copyright 2019, Elsevier Ltd. Hierarchical structure diagram of f) the rotational TENG, g) long-term durability of contact mode, autotransition mode, and non-contact mode TENG. Reproduced with permission. [137] Copyright 2020, WILEY-VCH GmbH & Co. KGaA, Weinheim.
swing-structured TENG. The design of the air gap and flexible dielectric brushes enabled minimized wear damage and sustainable triboelectric charges leading to enhanced robustness and durability. The TENG performance was controlled by external triggering conditions with an undiminished performance after continuous triggering for 400 000 cycles.

Introduction of Designing Triboelectric Surfaces for Boosting TENGs' Durability
The tribo-surface charge density dominates the TENGs' performance, which is dependent on the contact tightness between triboelectric materials. [151] The friction-induced heat and material abrasion on contact surfaces are the bottlenecks for achieving high stability and durability. Therefore, it is greatly desirable to develop approaches to achieve better engineering contact surfaces to improve the durability of TENGs. Texturing, coating, and functioning are the main effectively methods for boosting the durability of TENGs in terms of designing triboelectric surfaces. Textile-like triboelectric surfaces have gained tremendous attentions due to their excellent flexibility, high sustainability, and light weight. This means that the robustness of flexible wearable TENGs can be improved by fabricating nanostructured surfaces. At the same time, these micro texturing spaces can store wear debris when wear occurs during sliding process. Severe abrasive wear thus will be avoided. [154] In addition, microbrush-faced nanostructures from microfibers or animal furs can also be introduced for enhancing the durability of TEGNs due to their good mechanical properties and flexibility (Figure 18a). [155] It should be noticed here that the texture with proper size (hundreds of nanometers) can ensure retained orientation and morphology even after numerous contacts. For overly large size textures, strain of texture generated at the root is likely to exceed the elastic limit of the polymer material resulting in permanent deformation, which will be no longer able to enhance the durability of TENGs. [156] Fabricating protective coatings with high hardness, low friction coefficient and high wear resistance is one of the most effective strategies to reduce the friction and wear of rubbing surfaces. For example, diamond-like carbon (DLC), containing both sp 2 and sp 3 bonding [157] has its distinctive mechanical and physical properties (Figure 18b). MXenes are believed to be one of the state-of-the-art 2D nanomaterials regarding their tribological performance (Figure 18c). [98] These protective coatings will consequently enable and facilitate a tribolayer formation maintaining low friction and ultrahigh wear resistance.
Grafting proper chemical molecules or groups via surface functionalization can provide the strong covalent bonds to the triboelectric surfaces resulting in tight coating forming on the surface, which can maintain stability of the TENGs even under applied stresses over long period time (Figure 18d,e). [158] Figure 18. a) Schematic illustration of a textured surface based TENG regularly aligned fabrics interlocking the lateral side of the microfibers. Reproduced with permission. [155] Copyright 2021, Wiley-VCH GmbH. b) Atomic structure of DLC. [157] c) Schematic illustration of the anti-wear mechanism of MXenes. Reproduced with permission. [98] Copyright 2021, The American Chemical Society. Schematic representation of d) the surface functionalization and e) the prepared contact pairs of the surface functionalized TENG. Reproduced with permission. [158] Copyright 2015, The American Chemical Society. f) Schematic illustration of a designed self-cleanable surface. Reproduced with permission. [17] Copyright 2022, Springer Nature. g) Schematic illustration of a designed slippery surface. Reproduced with permission. [159] Copyright 2011, Springer Nature.
In addition, self-cleanable surface can be designed via surface functionalization, which will benefit the fast recovery of the contaminated surfaces caused by water, oil, and other contaminants (Figure 18f). [17] Inspired by the nepenthes pitcher plant, slippery liquid infused porous surface (SLIPS) was first introduced in 2011 (Figure 18g). [159] SLIPS could be fabricated by trapping a layer of oils at the surface of a porous medium, thus achieving superhydrophobicity. The SLIPS performance is determined by the type and volume of injected oils. Thus, the liquid with proper viscosity and dielectric constant, as well as the porous substrate, can eliminate the unwanted wetting transition and increase the effective contact area, which are beneficial for enhancing the durability of TENGs. [160]

The Durability of Textured Triboelectric Surfaces Based TENGs
In 2012, Wang et al. [156] designed a simple polymer nanopattern on a PI film. The elastic property ensured retained orientation and morphology of the TENG even after 1000 numerous contacts. Kim et al. [161] reported a fully flexible and foldable nanopatterned textile for wearable electronics (Figure 19a). The voltage and current output increased linearly with the number of stacks. Interestingly, very high voltage and current outputs with an average value of 170 V and 120 µA, respectively, were obtained from the four-layer-stacked TENG under a normal compressive force of 100 N. The results showed that there was no significant difference in the output voltage over 12 000 cycles Figure 19. Schematic illustration of a) a texturing TENG with nanopattern structure, b) mechanical durability test of the TENG after 12 000 cycles. Reproduced with permission. [161] Copyright 2015, The American Chemical Society. Fabrication of c) a crumpled Au-based TENG, d) mechanical endurance test of the fabricated TENG in 6 months. Reproduced with permission. [162] Copyright 2018, Elsevier Ltd. Schematics of e) the difference in the degree of the change of the morphology of the micro/nanoscale structures, f) stability of the output voltage during 6000 operation cycles of the TENG. Reproduced with permission. [163] Copyright 2020, Elseveir Ltd.
for the stacked TENG confirming the excellent mechanical durability (Figure 19b). In addition to the dielectric materials, the nanopatterns are also fabricated on the metal electrode (such as Al and Cu electrodes), which exhibits no significant output degradation even after extremely high numbers of operation cycles. Xu et al. [162] reported a flexible TENG based on crumpled Au films (Figure 19c). By introducing crumpled morphology onto the Au film (crumple degree 300%), a great improvement in output performance was achieved, paving the way toward practical applications. Importantly, no obvious degradation was observed after 6 months storage with a vertical force of 20 N at a frequency of 4 Hz in regular environment (Figure 19d). Choi et al. [163] developed a robust TENG by employing a cold-rolled metal layer with micro/nanoscale structure (Figure 19e). The developed TENG with 75% cold-rolled specimens exhibited no significant output degradation even after extremely large numbers of operation cycles (1 500 000 cycles) (Figure 19f), but the electric output of the compared TENG decreased obviously after 500 000 (as-received surface with 0% cold-rolled specimen). The textile-like nanostructure can also be achieved by adding textured interfacial materials. For example, Lee et al. [164] prepared a 2D crumpled MoS 2 film on the SiO 2 contact surface using pulsed laser-directed thermolysis. The surface-crumpled MoS 2 -based TENG generated 40% more power than a flat MoS 2 one and did not show serious degradation or defect on the triboelectric surface after 10 000 cycles. Due to the flexibility, high sustainability, and comfort of animal furs, the fur brush/PTFE contact surfaces can maintain tight contact and low friction state. There was nearly no damage on the PTFE surface rubbed by rabbit furs for 300 000 cycles, while obvious damages were found for the TENG without fur brush after 13 000 cycles. The surface topography became more delicate and flat owing to the high density and softness of fur-brush nanostructure. [150] Li et al. [64] proposed a polyester fur-reinforced rotary TENG. After 100 000 cycles testing, the surface of the PTFE had many scratches and showed a severe abrasion. However, the mechanical abrasion of the fur-reinforced rotary TENG after testing was nearly negligible. The soft contact layer, as the charge pump and charge transmitter, could make the TENG maintain 100% electrical output and the mechanical abrasion was nearly negligible. The durability of the TENG was boosted from 10 000 cycles (PTFE-based) to 100 000 cycles (fur-based).

The Durability of Coated Triboelectric Surfaces Based TENGs
Wang et al. [165] developed an oleic acid-enhanced polystyrene (PS) coating on the PI triboelectric layer by doping oleic acid in PS coatings. It was found that the addition of oleic acid could reduce the wear volume by about 90% compared to that of pure PS. The PS composite coating had a very low friction coefficient and wear volume (reduced by 90%) ensuring the solidsolid friction electrification with high efficiency and durability. Choi et al. [27] deposited a DLC coating on the electrode of a contact-separation TENG using plasma-based ion implantation technique. This DLC/polymer-based TENG produced a peak current of 3.5 µA and a power density of up to 57 mW m −2 , which was better than other conventional dielectric pairs like Al/PTFE (3.4 µA), PI/PTFE (2.9 µA), and PI/Al (3 µA). The coating exhibited a low friction coefficient against PTFE at an average value of 0.18. Kapton/PTFE pair exhibited a much higher friction coefficient of 0.3. In the durability assessment, the uncoated pair exhibited a highly unstable output behavior. The damage was observed on the surface at around 75 min due to severe wear resulting in a sudden drop in the output current. The DLC/polymer-based TENG exhibited good durability by producing a more stable output current compared with other parts during the 3 h operation at an applied force of 9.8 N. Wang et al. [166] devised a new DLC-based TENG with a macroscale superlubricity that can withstand high-contact stress (a Hertz contact stress of 1.37 GPa) with ultralow friction coefficient (<0.01) and ultralow wear rate (Figure 20a-c). After the DLC film undergone a friction process of 1, 2, and 3 h, the wear rate did not drastically change indicating long-term operating life at a applied force of 10 N. Tremmel et al. [167] found the DLC coating performed superior performance compared to MoS 2 . The DLC film also withstood long-term tests without notable signs of wear after 9000 cycles during 1 Hz and 2.3 N. It is also found that MXenes outperformed the other coatings due to the good electron gain ability of functional oxygen and fluorine groups (Figure 20d).

Modified Triboelectric Surfaces Based TENGs
Engineering the triboelectric materials surface chemical environment by appropriate modification is also one of the most approaches for TENGs' durability. Shin et al. [158] manipulated the triboelectric polyethylene terephthalate (PET) by modifying the surface with positively charged amino groups and negatively charged fluorocarbon groups. The modified surfaces exhibited superior stability owing to the tight chemical bonds. No reduction of voltage or current output was observed during the periodical measurement of TENGs for over 4 weeks. Zhu et al. [168] presented a TENG with excellent triboelectric and mechanical properties by developing an effective, general, straightforward, and area-scalable modification approach via inductive-coupled plasma etching. It was found that the modified fluorination and enhanced surface roughness played a joint role in promoting the electric output and durability. After 72 000 cycles of continuous test, the output voltage only reduced by around 2%.
By taking the advantage of texture and functionalization, Zhang et al. [169] fabricated a flowchart of the surface-textured PDMS film followed by deposited with a chemical fluorocarbon modified polymeric layer by the plasma treatment. The output performance of the TENG after the fluorocarbon plasma treatment was very stable and almost kept constant during the observation time of 8 weeks. By using the plasma treatment method, Zhang et al. also prepared a fluorocarbon layer on the wrinkle structure of PDMS. The as-modified TENG had a great output ability and enhanced durability.
Li et al. [170] proposed a surface modification method using low-energy ion irradiation for tuning the chemical structures and functional groups of triboelectric polymers at the Figure 20. Schematic illustration of a) the preparation and b) friction of the hydrogen-containing DLC film, c) output performance and its corresponding friction coefficient of the TENG. Reproduced with permission. [166] Copyright 2022, Cell Press. d) Triboelectric properties considering voltage, current, and charge versus time for PTFE, DLC, MoS 2 , and MXene coatings. Reproduced with permission. [167] Copyright 2022, Elsevier Ltd. molecular level (Figure 21a,b). The low-energy ion irradiation brought negligible change to both the surface roughness and the mechanical durability of the target polymer.
Liu et al. [171] fabricated a nanowire array on the triboelectric surface and modified with fluoroalkyl silane (Figure 21c). The nanowire appeared little wear after a 37 000 cycles durability test (Figure 21d,e). It meant that the fluoroalkyl silane modified TENG could work for a long time without the output performance having a significant decrease.
Kim et al. [161] reported a fully flexible, foldable metal-coated nanopatterned TENG with high power-generating performance and mechanical robustness. Due to the chemically and physically damage-free coated textile, there were no significant differences in the output voltages measured over 12 000 cycles. Cai et al. [172] found cellulose nanofibers could be also introduced to achieve a nanopattern film. Via modifying nitro and methyl, the functionalized-nanofibers film generated a long-term stability. And the prepared TENG had no output signal decay after 500 000 cycles. Nie et al. [173] further provided an Ag-coated cellulose-based triboelectric material via the surface amino modification. The TENG signal output was stable after 10 000 cycles with open-circuit voltage maintained at approximately 120 V.
Lee et al. [174] presented a mechanically robust TENG via surface-textured glass fabric reinforcement by the surface modification of siloxane (Figure 21f). The TENG exhibited superior thermal and mechanical stability, compared to other polymer materials, achieving high durability during 100 000 cycles of TENGs operation (Figure 21g).

Self-Cleanable Triboelectric Surfaces Based Solid-Solid TENGs
Except for mechanical damages, some other factors from the environment, such as dust, humidity, acid/alkali, oil, and other contaminants, etc. will also lead to damages or degradations of TENGs. For example, TENGs in wearable/attachable electronics directly expose to the ambient environment. They cannot prevent the contamination of surfaces by dusts and particulates resulting in a decrease of electrical output performance. In this case, a concept of self-cleanable TENG is proposed to benefit the fast recovery of the contaminated surfaces, [175] that is, like the self-clean surface of lotus leaves in nature.
Sun et al. [176] obtained a stably self-cleanable and transparent TENG, which functioned as a mean of real-time communication between human-machine interface (Figure 22a). Functionalization with (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trichlorosilane (HDFS) ensured that the contact surface very Figure 21. Schematic diagram of a) ion irradiation function and b) a TENG electrification process. Reproduced with permission. [170] Copyright 2020, Royal Society of Chemistry. Schematic depiction of c) the fabrication process of nanowires prepared by hot processing technique and post surface function with fluorinated compounds, d) SEM images of the original functional nanowire and e) the nanowire working after 37 000 cycles. Reproduced with permission. [171] Copyright 2016, Elsevier Ltd. Synthetic process of the epoxy-functionalized siloxane resin and the fabrication process of g) a textured glass-fabric reinforced surface, h) stability of the TENG during 100 000 cycles. Reproduced with permission. [174] Copyright 2020, Wiley-VCH GmbH. Figure 22. Fabrication process of a) self-cleanable, transparent, and attachable ionic communicators, b) self-cleanability of the TENG contaminated by activated charcoal powders. Reproduced with permission. [176] Copyright 2018, Springer Nature. c) Fabrication procedures of a porous flexible triboelectric layer, d) optical images of self-cleaning performances. Reproduced with permission. [177] Copyright 2021, Elsevier Ltd. Schematic structure of e) a bioinspired sweat-resistant wearable TENG, inset: SEM images of the interlayer structure between the triboelectric layer, the electrode, and the protection layer, f) the stability performance of the TENG. Reproduced with permission. [180] Copyright 2022, Elsevier Ltd. hydrophobic, which made the water droplets could remove dust contaminants by rolling down (Figure 22b). In high humidity environment, the formation of a water film on triboelectric interfaces may deteriorate charge transfer or induce charge dissipation, which significantly limits their applications. Qu et al. [177] designed a humidity-resistant and stretchable TENG consisting of a porous flexible layer and a waterproof flexible conductive fabric (Figure 22c). The resulted TENG possessed great superhydrophobicity and stretchability (Figure 22d) and showed a stable output under a high relative humidity of 80%. Kim et al. [178] fabricated a PDMS interlayer with a 3D hierarchical pattern by using a particle lithography method. Benefitting from the 3D hierarchical PDMS, the TENG retained its initial electrical output at high humidity and recovered quickly. Long et al. [179] provided a novel approach for TENGs working in harsh acid/alkali environments by using a super-hydrophobic sintered polyvinyl alcohol-polytetrafluoroethylene membrane as a triboelectric surface. The TENG had significantly acid/ alkali resistance even after 72 h of soaking the membrane in a strong acid solution followed by a strong alkali solution. After 9000 cycles, there was no obvious signal weakening or missing. In addition, the counterpart-contacted material was an oiladsorbed paper with a large surface area, which could absorb oil by a capillary phenomenon. Thus, the TENG had huge application prospects in harsh environments.
Regular exercise plays an important role in remedying body suboptimal health status and releasing daily stress. During exercise, if the body sweats in the vicinity of the TENG, the regional humidity will increase significantly and sweat composing various salts may even flow and penetrate the triboelectric interface causing unrecoverable and permanent performance degradation of TENGs. Pei et al. [180] proposed a bioinspired sweat-resistant wearable TENG for movement monitoring during exercise. The TENG was consisted of two superhydrophobic and self-cleaning triboelectric layers, which featured the hierarchical micro/nanostructures replicated from lotus leaves (Figure 22e). The TENG demonstrated excellent humidity-resistance with only 11% output reduction as the relative humidity increased from 10% to 80%, which was further verified under extreme harsh conditions including surface contamination and ultra-humid water spraying. After 10 000 working cycles under such a large load (260 N), the TENG remained almost the same as those observed before testing indicating the excellent long-term stability (Figure 22f).

Self-Cleanable Triboelectric Surfaces Based Liquid-Solid TENGs
Solid-solid TENGs are highly influenced by several environmental factors. At the same time, the continuous collision and friction between two solid surfaces with getting wear causing an adverse effect on the long-term mechanical durability. In addition, it is very difficult to make a proper full contact between two solid surfaces leading to weaker signal generating. [181] In this regard, TENGs based on contact electrification between a liquid and a solid, that is, liquid-solid TENGs, [182] can resolve the issues mentioned above, which thus, mitigates the material abrasion and improves the durability of TENGs.
However, during the contact and separation process, liquid may not roll off the TENG and remains on the contact surface resulting in lower output performance and decreased service life (Figure 23a).
In 2021, we, for the first time, studied the internal relationship between signal output ability of TENGs and the adsorbing behavior of liquid through quartz crystal microbalance with dissipation (Figure 23b,c). [183] The liquid with larger adsorption mass could carry more contaminants to remain on the surface of TENGs, which led to the quick saturation of screen effect and the decrease of signal output capacity. For example, compared with rapeseed oil, paraffin oil was more difficult to be adsorbed on the TENG surface, so that the screen effect was much weaker, and the output signal could maintain a high value. It is expected that the liquid-solid TENGs associated with superhydrophobic/superoleophobic surfaces can provide high levels of immunity against adsorbing contaminants. High surface roughness and low surface energy both are characteristics of hydrophobic or superhydrophobic materials. Choi et al. [184] fabricated a liquid-solid TENG surface with a micro-bowl PDMS, which was very hydrophobic (average measured contact angle was over 150°) showing high durability.
The micro-bowl structure was derived from the packaging materials, and the property of these materials was difficult to be reshaped, which limited the design of novel devices. Herein, Ping et al. [185] adopted a mold-to-mold method to replicate the natural superhydrophobic structure of lotus leaves to the surface of a flexible TENG. The innovative application of lotus leaflike bionic structure enhanced the self-cleaning capability, flexibility, and electrical output performance of TENGs. The coulomb efficiency of the TENG was about 99% after 5000 cycles. Liu et al. [186] prepared a hydrophobic surface via a fluorinecontained acrylic resin coating without any reconstructed microstructure. When the mass ratio between the fluorinemodified acrylate resin and the polyisocyanates curing agent was 5:1, the output current of the TENG reached the maximum value and the TENG remained in its original operating state after 33 000 cycles. After the test, the contact angle of the surface was 97°, while that of the compared sample was only 41°.
According to the synergetic effect of microstructure and low surface energy, Nie et al. [187] proposed an elastic superhydrophobic cellulose paper created by spray-coating nano-fumed silica dispersed in a thermoplastic elastomer solution, followed by treatment with triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane (Figure 23d). 1 mL of triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane was dissolved in 50 mL of ethanol and left at room temperature for 1 h for the complete the hydrolysis of PFOTES. The sprayed filter paper was soaked in the PFOTES hydrolysate for 6 h. The superhydrophobic cellulose paper exhibited good robustness, and it could stay dry and clean after knife scratching and water drop rinsing.
Different from water, oils often give rise to adsorption and fouling problems, which will cause a lot of issues in different applications, and it may even be detrimental for human life. For example, face masks for capturing PM2.5 or preventing COVID-19 have low efficiency in the surroundings containing oil vapor due to adsorption. [188] The damage caused by oil stains from human-electronic interfaces has always been a thorny issue, which not only affects the aesthetics of materials but also corrodes and affects the performance of electronic circuits. In 2022, to address the issue of oil contaminants, we developed self-cleanable oil-solid TENGs (O-TENGs) via designing the solid surface's wetting properties (Figure 23e). [17] For fabricating the O-TENG, SiO 2 nanoparticle was first dissolved in deionized water. The ratio of SiO 2 :water is 1:30 g. After 10 min stirring, fluorocarbon surfactant (18 g) was dissolved in the SiO 2 solution under stirring for 1 h. After spraying the solution on the electrode and drying under room conditions, 1H, 1H, 2H, 2H-Perfluorodecyltriethoxysilane (20 mg) was deposited from air at 80 °C for 30 min. The received O-TENG achieved super oleophobic properties and had an oil contact angle (CA) of 167°. In contrast, the PI and PTFE surfaces were oleophilic with oil contact angles of 53° and 58°, respectively. The designed O-TENG could generate an excellent electricity, which was an order of magnitude higher than other O-TENGs made from PTFE and PI. It had a significant durability (30 000 cycles) and generated good electricity (with a charge density of 9.1 µC m −2 and a power density of 1.23 mW m −2 ), which was an order of magnitude higher than other O-TENGs made from PTFE and PI. The designed O-TENG could power a digital thermometer for self-powered sensor applications (Figure 23f,g), which was also the first time to harvest energy from oil wave for self-powered applications

Slippery Triboelectric Surfaces Based Liquid-Solid TENGs
Currently for the liquid-solid TENG, the solid phase is designed to be superhydrophobic/superoleophobic so that liquid droplets can be repelled in a timely manner to clean contact sites for enhancing the durability of TENGs as summarized above. However, under mechanical stretching, bending and abrasion, the self-clean structures will be easily damaged. A liquid-slippery surface allows for timely shedding of liquid from contacting surfaces, especially in environments involving freezing temperature and high humidity, which can prevent the degradation of the physiochemical properties of triboelectric materials. Wang et al. [189] reported a lubricantimpregnated SLIPS-based TENG via placing a perfluoropolyether (PFPE) on the PTFE membrane (Figure 24a). It was found that the PTFE membrane could hinder the formation of ice crystals (Figure 24b,c). Owing to the flowability of the lubricant, the surface could recover after a scratch without weakening the electricity output performance (Figure 24d,e). Thus, the TENG exhibited optical transparency, configurability, selfclean, long-lifetime, and power generation stability in a wide range of working environments, while the properties of the non-slippery surface were very poor (Figure 24f,g). To study the liquid type and the layer thickness influencing the output performance of SLIPS-based TENGs, Liu et al. [190] studied on the role of the lubricant on SLIPS properties and SLIPS-TENG performance to provide theoretical support to the fabrication and optimization of SLIPS-based TENG. It was found that the PFPE had a superior film-forming stability. Differently, both mineral oil and silicone oil would result in negative potentials indicating that there was an attraction between PTFE and air, which meant the resulting film was not stable for film thicknesses. The contact angle of PFPE slippery surface was 121° and that of the silicone oil slippery surface was 60°. The electric output of the PFPE SLIPS-TENG remained stable for more than half an hour at −5 °C, while other oils SLIPS-TENG and original PTFE-based TENG showed obvious degradation after 8 min operation. A small volume of PFPE per surface area (4 µL cm −2 of porous PTFE) was sufficient for efficient energy conversion. They also afforded the TENGs device that worked normally and stably in harsh environments under freezing temperature or high humidity. Wang et al. [191] introduced PFPE between ferrofluid/PTFE tube interface, which solved the important issue of motion resistance between the liquid and solid contacting surface. The ferrofluid droplet sticked over bare PTFE surface with a titled angle of 25°. In contrast, the droplet could easily slide off the slippery lubricant oilinfused layer with a titled angle of around 5° indicating that the slippery lubricant oil-infused layer would facilitate the sliding of ferrofluid for its low attraction between PFPE and ferrofluid ( Figure 24h). The electric signals of the PFPE-based TENG were monitored in clean water and polluted water with dye and oil under the same water level falling velocity. The short-circuit current curves presented the same character, meaning the TENG can avoid the effect of environment to the electric signal.

Introduction of Lubricating Triboelectric Interfaces for Boosting TENGs' Durability
In most cases, the contact surfaces of TENGs work under dry friction condition, and thus the contact surfaces suffer from high friction resistance resulting in serious materials wear during the operation processes. Under dry condition, the triboelectric materials, such as PTFE, PI, Nylon, and FEP, are susceptible to failure by the thermal softening induced by the frictional heat generated inside the TENG contact interfaces, which will cause wear resulting in a low durability of TENGs. It is believed that the wear of materials is resulted from materials scuffing. Common terminology includes "gross damage," "solid-phase welding," and "adhesive wear." [192] There is a huge difference between dry friction and lubricating cases. For the lubricating case, lubricating fluids have the capability of separating the two parts of friction surfaces completely. As a result, the two friction surfaces do not contact with each other when objects move relative to each other, the friction only occurs between the fluid molecules. So, under proper lubricating conditions, wear is very small or almost no wear. In case of some contact occurring at micro asperities, the friction surfaces also can be separated by a very thin fluid film, that is, boundary lubrication film. [193] Liquid lubrication has the following advantages for TENGs. 1) Liquid lubrication can provide super wear-resistive TENGs, while the unlubricated TENGs will fail within a relative short period due to the wear. The formation of transferred film from one triboelectric material to the counterpart is the main reason causing the decrease of the electric outputs of TENG, for which liquid lubrication can obviously increase the electric outputs by preventing the formation of transfer film (Figure 25a,b). [14] 2) At the same time, liquid lubricant can significantly suppress interfacial breakdown due to its higher breakdown field strength requirement and lower electrostatic field strength in the microgap between triboelectric layer and electrode (Figure 25c). The electrostatic field strength in microgap between triboelectric electrode and dielectric film can be increased in one order of magnitude when liquid lubricant replaces the air. [194] 3) The relative permittivity is another key factor that determines the electric output of TENGs (Figure 25d). [14] The electrical performance drops with the increase of permittivity. For the liquids with low permittivity, such as heptane, squalene and paraffin oil mainly consisting nonpolar hydrocarbon components, can prevent the charge transfer resulting in higher electrical performance and longer durability.
Liquids with high permittivity can also be polarized by the electric field and donate more electrons. Due to the flow ability of liquid, the distribution of polarized liquid will be changed and lead to the neutralization of charges. This is why the electric output and durability of TENGs decreases under the lubrication of rapeseed oil, water and alcohol. The viscosity has a great influence on the electric performance of lubricated TENGs. It is because low viscosity is more helpful for the liquid to form a thin liquid film at the interface (Figure 25e).

Figure 24.
Fabrication of a) a SLIPS-based TENG, b) a droplet sliding on a PTFE-based TENG and c) on the SLIPS-based TENG, d) the restored behavior of the SLIPS-based TENG after scratches, e) comparison of the measured short-circuit current before and after the creation of scratches, f) enhanced stability in the electricity generation at low temperature g) compared with PTFE-TENG. Reproduced with permission. [189] Copyright 2019, Oxford University Press. h) The schematic of the ferrofluid/PTFE TENG and the relationship between short-circuit current peak value and moving velocity of ferrofluid driven by magnet. Reproduced with permission. [191] Copyright 2020, Elsevier Ltd.

The Durability of Lubricated Triboelectric Interfaces Based TENGs
In the beginning of 2020, for the first time, we introduced liquid lubrication to increase the anti-wear property of TENGs (Figure 26a). [14] For illustrating the effects of adding liquid lubrication to TENGs, a series of liquids, that is, squalane, paraffin oil, PAO10, olive oil, rapeseed oil, ionic liquid, water, etc., were employed. It was surprising to find that proper liquid lubrication could not only provide a super wear resistive TENG, but also could increase the electric signal output. It could be found that the lubricant effectively reduced the friction force. Almost no wear was found under lubrication conditions, whereas the wear depth of the triboelectric surfaces was about 30 µm for dry friction case. Under dry sliding condition, the voltage output of TENG was initially increased with the increase of applied loads (up to 15 N). A big voltage signal drop was found under the load of 20 N, which was due to the damages of PI and Al film (Figure 26b). With the lubrication of squalane, the voltage output of TENG is 2.4-3.5 times higher than that of dry cases. In addition, squalane, paraffin oil, and PAO10-lubricated TENGs showed improved voltage and current output, while squalane gave the best improvement. Other polar liquids, that is, Pluriol A 500 PE, PEG 200, [Emim][NTf2] ionic liquid, and water, showed ignorable electric outputs. The open-circuit voltage and short-circuit current of the squalane-lubricated TENG were more than three times of the unlubricated TENG (Figure 26c,d). The electric output decreased obviously for the slide-mode TENG under dry contact after 3600 cycles, while the service life of TENGs could be greatly improved through liquid lubrication and there was no detected wear even after 36 000 cycles of operation (Figure 26e,f). The mechanism has been discussed in Section 7.1. Feng et al. [195] further confirmed that carbon and hydrogen based alkanes, such as PAOs and squalane, could effectively increase the wear resistance and the triboelectric output of triboelectric materials at an applied load of 0.2-5 N. Based on the strategy of liquid lubrication, Li et al. [196] fabricated a hexadecane-lubricated TENG by adding a liquid film of hexadecane onto the TENG surface. The friction coefficient and wear depth were reduced by 73% and 70%, respectively. The service lifetime of the TENG increased from 6000 s (dry friction) to 64 000 s (lubrication) at the applied load of 10 N.
Accordingly, interface liquid lubrication could be proposed as a universal strategy to improve the durability of TENGs. Wang et al. [194] further found that liquid lubricant significantly suppressed interfacial break-down due to its higher breakdown field strength requirement and lower electrostatic field strength in the micro gap between triboelectric layer and electrode. The lubricated TENG showed more uniform and slight scratches compared to dry friction TENG. The lubricated TENG had high electric output and exhibited superior output durability after 500 000 operation cycles at the applied load of 10 N, while the dry friction TENG can maintain performance for only 500 cycles (Figure 26g-i). Kim et al. [197] studied the generation mechanism of mineral oil-lubricated TENG and identified that the air breakdown could be effectively blocked owing to the large Debye length of such lubricants. Taking the advantage Figure 25. Schematic illustration for the interfacial behaviors of TENGs under a) dry and b) liquid-lubricated conditions. Reproduced with permission. [14] Copyright 2020, Elsevier Ltd. c) The schematic of interfacial breakdown in air and avoiding interface. Reproduced with permission. [194] Copyright 2020, Wiley-VCH GmbH. The key factors of liquids affecting the outputs of lubricated TENGs: d) the relative permittivity and e) dynamic viscosity. Inset of (d): the potential polarization of liquid lubricant and liquid flow at the interface. Reproduced with permission. [14] Copyright 2020, Elsevier Ltd.  [14] Copyright 2020, Elsevier Ltd. g) Transferred charge, h) short-circuit current, and i) long-term durability of a sliding TENG in air and via interface liquid lubrication. Reproduced with permission. [194] Copyright 2020, Elsevier Ltd. of low friction of rolling motion, they introduced a lubricantsubmerged rolling TENG, and it could significantly reduce the friction of the TENG device. The acquired input torque in the air and in the liquid lubricant was 0.0348 and 0.0275 N m, respectively. And the electrode surface of the TENG with a rolling motion after 72 h of operation appeared nearly no wear damage. This result indicated that the TENG could remain its output for 432 000 cycles.
Although the liquid lubricants could suppress the air breakdown on the dielectric surface, the obtained electrical current was as low as other conventional TENGs. To overcome the current limitations of TENGs, Lee et al. [198] developed a dielectric liquid-based self-operating switch TENG, which could control the field emission on the electrode surface through the movement of the dielectric liquid. The movement of the dielectric material plate regulated the air breakdown by acting as a selfoperating switch, which could enhance the current output of the TENG. Our previous experiments indicate that formulated lubricants (with traditional lubricating additives) will cause big problems of TENGs resulting in much lower or no signal output. It means new formulation strategies need to be developed for simultaneously enhancing the durability and electric signal output performance of TENGs.

Prospect and Outlook
The durability of TENGs is determined by the multifaceted roles of triboelectric materials and surface/interface properties, whose understanding is urgent required for the future development of highly durable TENGs. In this manuscript, we have provided an overview of the durability study progress and practical strategies for improving TENGs' durability. Different materials including synthetic and natural materials can be utilized for preparing durable TENGs, which have been scientifically discussed. Preparing composite materials with mechanical, wear-resistance, and self-healing advantages is an effective way to improve TENGs' durability. Optimizing structure characteristics can also enhance the output performance and service life. Surface engineering and interface lubricating will greatly decrease the sliding frictional loss and increase the wear resistance/durability of TENGs. Nevertheless, there are still challenges and issues for progressing and enhancing the durability of TENGs, which are suggested as follows.
1. Lack of an in-depth study of the anti-wear matching pairs of triboelectric materials for TENGs. Tribology takes place at surfaces/interfaces that are quite complex entities. Different from the electrification series, friction and wear are directly related to the counterpart roughness, matching hardness, crystal comparability, of which the information is important, but very deficient. There is no report on the natural materials matching in terms of tribology, which also limits the development of green triboelectric materials. 2. Introducing nanomaterials in the substrate and physical/ chemical functionalization effectively enhance the durability of TENGs materials. But the contact or sliding motions inevitable result in the surface scratch and wear. Thus, developing a superlow or even zero-wear triboelectrification pair is a great challenge, which is the future development direction. 3. It is well-known that non-contact mode exhibits super robust advantage over contact ones. TENGs with partial noncontact and contact mode further exhibit high durability. The contact period, however, easily damage the materials due to mechanical abrasion, similar to (2) above. In this regard, noncontact/ contact structure will have a significant advantage if the contact part is fabricated via a rolling contact mode such as utilizing the static friction of the rolling rod/ball instead of sliding friction. In addition, although the TENGs with partial noncontact and contact mode exhibit high durability, the complex-fabricating structure makes it difficult to be applied in practice, especially in the application of wearable devices. 4. Lubrication is the most important method to reduce friction and wear. It is also found lubricant oils can reduce air discharge during triboelectrification and improve the electrical performance and wear resistance. Accordingly, it is very promising to study base oils with green, non-polar, low viscosity, and low dielectric constant properties. The oils' performance can also be improved by adding functional additives with the functions of electrical breakdown resistance, low conductivity, low dielectric constant, and high lubrication properties. Smart microcapsules contained these oils can be further prepared to be embedded into triboelectric matrixes. When the matrixes experience are worn-out, the released oils will fill in the micro uneven triboelectric surface and lubricate the interface. The wear-exposed microcapsules will also exhibit the ability of collecting debris leading to a smoother contact surface for increasing the durability of TENGs. 5. The contaminant adsorption is the biggest challenge limiting the service life of liquid-solid TENGs. Endowing TENGs' surfaces with self-cleanability performance will address this issue. Recently, oil-solid TENG rise attractively because oils are everywhere behaving as the "blood" of industry. Oil contaminants adsorbed on the contact surface is much more difficult to address than that of water contaminant due to oils' low surface energy and high stickiness. There is great research prospect for oil-solid TENGs such as via designing durable oleophobic/superoleophobic surfaces.
Altogether, the durability of TENGs is the most crucial challenge for their practical applications. This review presents a comprehensive progress of published works on enhanced durability, and proposes key strategies that have high potential to achieve high durability of TENGs.