Recent progress towards smart transportation systems using triboelectric nanogenerators

The field of transportation plays a crucial role in the development of society. It is vital to establish a smart transportation system to increase the convenience and security of human life. The incorporation of artificial intelligence and the internet of things into the traffic system has facilitated the emergence of innovative technologies like autonomous vehicles or unmanned aerial vehicles, which contribute to the reduction of traffic accidents and the liberation of human driving time. However, this improvement involves the use of multiple sensor devices that need external power sources. As a result, pollution occurs, as do increases in manufacturing costs. Therefore, the quest to develop sustainable energy remains a formidable obstacle. Triboelectric nanogenerators (TENGs) have emerged as a possible solution for addressing this problem owing to their exceptional performance and simple design. This article explores the use of TENG-based self-power sensors and their potential applications in the field of transportation. Furthermore, the data collected for this study might aid readers in enhancing their comprehension of the benefits linked to the use of these technologies to promote their creative ability.


Introduction
Mobilities transport goods and people from one area or country to another via road, sea, or air.Therefore, transport is among life's most critical aspects [1].A system with advanced transportation and infrastructure will foster economic growth and enhance the quality of life [2].Despite this progress, there are still many issues that need to be resolved, such as traffic accidents and pollution from vehicle emissions [3][4][5].Traffic accidents are the 8 th leading cause of mortality worldwide and will rise to 7 th in 2030 if current trends continue, according to World Health Organization (WHO) estimates [6].Consequently, the development of a smart transportation system (STS) is critical for the safety and convenience of human life [6][7][8].Currently, the use of artificial intelligence and the internet of things (IoT) in transportation has brought significant advancements [9][10][11].These are autonomous automobiles, unmanned aerial vehicles (UAVs), and monitoring systems that have decreased traffic accidents, boosted vehicle safety, and cut driving time [12][13][14].This advancement has resulted in an increase in the number of IoT-related sensors exceeding 50 billion in quantity; projections indicate that this number will surpass 200 billion by 2025 [15,16].Despite their notable efficiency, wireless sensor systems still necessitate the supply of external power to operate [17,18].This leads to a capacity limit, inconvenient charging or replacement, and pollution of the environment [19,20].Consequently, the constraints of external power sources may be advantageously addressed by means of self-powered sensors.
The self-powered efficiency of triboelectrification nanogenerators (TENGs) has garnered considerable interest in recent years [21][22][23].In 2012, Professor Zhonglin Wang's team developed TENGs by combining triboelectrification with electrostatic induction.When two materials with differing charge affinities contact and separate, the electrons transfer between the two electrodes via the external resistor, producing an alternating current (AC) [24][25][26].This allows TENG to convert mechanical energy from its surroundings into electrical energy [27].TENG has become the most efficient energy harvesting technology because of its simple method, low cost, and high conversion efficiency [28][29][30].It has been shown to produce an output area power density of up to 500 W m −2 and a total energy conversion efficiency of up to 85% [31,32].TENG in STS can harvest energy from wheel rotation, vibration of moving equipment, wind energy, and so on to power sensors or function as a sensor [33,34].Furthermore, since TENG materials and designs are so varied, using TENG technology is still a potential chance for many future applications [35,36].This article outlines the advancements and successes of TENG for mobility in STS.Simultaneously, offer insights and obstacles regarding the advancement of TENG, including energy conversion efficiency and reliability.

TENG operating principles in mobility applications
Utilizing triboelectrification and electrostatic induction effects, TENGs operate in four different modes (figure 1): • Vertical contact-separation mode (CS): CS has a layered structure with tribo-pair materials facing each other.
When they make contact, electrons exchange, resulting in oppositely charged surfaces.When an external mechanical force separates them, a gap is formed between the two materials, and a potential difference is established.When two electrodes are connected to a load, free electrons from one electrode flow to the other to balance the voltage drop caused by triboelectricity.When the two tribo-material layers reestablish contact, the electron flow comes back, and the electric potential disappears.Repetition of this procedure generates AC output [37,38].Because CS are simple structures that are straightforward to construct, they are employed in a wide range of applications.CS tribo-pairs are frequently designed with an arch-shaped structure, spacer structure, or spring-assisted structure due to the requirement for space between their surfaces [39].• Lateral sliding mode (LS): The electricity generation mechanism of LS is like that of CS; however, LS involves horizontal sliding between two materials [40].Because of its large contact surface, LS has a high energy conversion efficiency.A plain-sliding structure, a linear-grating structure, or a case-encapsulated structure are common LS structures [41].• Single-electrode mode (SE): For electrons to move through CS and LS, two electrodes must be connected to the load to create a closed circuit.This arrangement is unsuitable for converting energy from freely moving objects such as walking or running vehicles.This difficulty is solved by the SE structure, which has just one ground electrode.When the friction layer contacts the electrode, an induced current is produced.The current flows back to the ground when the friction material separates from the electrode [42].• Free-standing triboelectric-layer mode (FS): A pair of symmetrical electrodes are positioned beneath the dielectric layer to construct the FS.A potential difference results from the movement of the dielectric layer across the two electrodes.To achieve potential equilibrium, electrons will move between the two electrodes [43].This procedure results in the generation of an electric current.FS is well suited to free rotation energy collection applications such as wheel rotation and wind energy.
Different modes can be chosen based on the type of energy to be harvested and the goal of each application, which will be discussed in detail in the section below.

Key parameters and strategies for the efficiency of TENG devices
TENG is a breakthrough technological advancement that collects and transforms mechanical energy into electrical energy.The use of TENG technology in transportation has proven to be very beneficial, particularly in the implementation of self-powered sensor systems.Nevertheless, the reliability and energy conversion efficacy of TENG devices remain formidable challenges that need improvement.

Energy conversion efficiency of TENG devices
Since TENGs function as energy conversion devices, energy conversion efficiency is a significant determinant of the effectiveness of TENGs in practical applications.High energy conversion efficiency refers to the efficient transformation of mechanical energy into electrical energy.The four operational modes of TENG are all founded on the principles of contact electrification and electrostatic induction.Therefore, the intrinsic capacitance of TENG devices serves as a defining characteristic of their energy conversion efficiency.Capacitance denotes the capacity of the TENG device to retain electrical charge [44,45].Consequently, the conversion efficacy can be improved by increasing the charge density [46,47].Tribo-pairs are the primary component of all TENG devices; they have a direct impact on the TENG's output performance via the surface charge density characteristic [48].The material's ability to gain and lose electrons determines this characteristic.Materials with a high electron affinity tend to gain electrons through friction, while strongly electropositive materials lose electrons [49].Positive materials are those that donate electrons, while negative materials are those that receive electrons.For the tribo-pair of TENGs to produce a higher output, their polarization must differ significantly [50,51].Most negative friction materials are polymers, such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polydimethylsiloxane (PDMS) or fluorinated ethylene propylene (FEP) since fluorine is the most electronegative element in their chemical structure [52][53][54][55][56][57].Additionally, certain materials will be described in the following section.Because of their ability to give electrons, metals such as aluminum (Al), copper (Cu), nickel (Ni), and palladium (Pd) are often selected as electropositive materials [58][59][60].Additionally, nylon, polyurethane (PU), and polyvinyl alcohol (PVA) are often used due to their exceptional electro-positivity and resistance to abrasion [61,62].Currently, the commonly used materials are non-modified polymers, which have limited properties.Conversion efficiency is increased further by enhancing the material's intrinsic characteristics.Many recent studies have demonstrated that mixing polymers with nanoparticles increases permittivity and, thus, charge density [63,64].Another great method for improving charge transfer features is to graft functional groups to the polymer backbone that can gain or lose electrons by chemical modification (NH 2 , OH, F, Cl, etc) [65,66].
The contact area of the tribo-pair also improves the electrical output efficiency of TENGs devices [67,68].Typically, the output efficiency increases as the contact area increases.As a result, numerous studies have focused on increasing contact area, such as enhancing surface roughness by producing nanostructured surfaces using processes such as lithography, etching [69,70], water-assisted oxidation [71,72], or electrospinning nanofibers [73,74].Sandpaper with a naturally rough surface may also serve as a template for friction materials [75,76].In addition, the large surface area and high porosity of the porous structure, when applied to friction materials, aid in increasing charge accumulation abilities and contact area.There are several ways to produce porous materials, including the sacrificial template technique, electrostatic spinning, and aerogel [77][78][79].Additionally, material thickness is a significant factor; typically, materials possessing a controlled thickness of microscales or less contribute to the establishment of reliable and consistent performance [80,81].
In addition to enhancing charge density, minimizing charge loss also plays a crucial function.Electrodes are also one of the main structural components of TENG devices.Electrode materials must possess high conductivity; therefore, inexpensive, and low-resistance metals, including Al and Cu, are frequently selected for this purpose [82,83].Although gold (Au) and silver (Ag) are the most conductive metals, they are also expensive.Non-metals with excellent electrical conductivity, flexibility, and mechanical strength, such as carbon nanotubes and composite materials like carbon black-silicon rubber, are also employed [84].Moreover, due to their AC output, TENG devices are not suitable for direct charging capacitors or sensor devices.A rectifier is used to convert AC to direct current (DC), which results in a charge loss.Hence, the development of DC-TENG devices represents a prospective approach to enhancing output performance [85,86].

Reliability of TENG devices
For TENGs to be utilized effectively in practical applications, their reliability must be thoroughly evaluated.If TENG devices have high robustness and reliability, they will provide consistent output over long periods of time and in a variety of operating situations.Various factors, such as temperature, mechanical force, dust, humidity, contamination, and other variables, might impact the TENG equipment in the transport system, leading to a decrease in its durability.Furthermore, TENG devices operate based on the friction mechanism, necessitating the friction materials to endure lengthy and direct contact.Prolonged periods of use will lead to reduced durability and reliability, thereby diminishing the device's performance.How to enhance reliability and robustness is still a difficult topic that must be addressed.In general, this parameter may be improved by combining two factors: tribomaterial selection and TENG structure optimization.
Optimizing TENG structure: The efficiency of the encapsulation method in safeguarding TENG against harsh environments has been demonstrated [87,88].In addition, the LS and FS modes of operation often exhibit high friction frequencies, leading to a decrease in durability.Therefore, there have been a lot of studies focused on enhancing durability in these two modes.Efficient approaches include automatic transitions between contact and noncontact working modes, lubrication, rolling structures, and soft contact structures.
• Automatic transition between contact and noncontact working modes: Static charges are produced on the tribomaterial and electrode layers upon contact because of triboelectrification.The charges can persist for an extended period on the surface of the friction layer.Hence, the TENG can effectively operate without physical friction for a certain duration.The absence of physical contact during operation greatly minimizes the wear on the material surface, hence improving the durability and stability of the TENG.However, these charges gradually diminish, necessitating their conversion to contact mode to generate supplementary friction charges.Hence, establishing an automated switching mechanism between contact and non-contact modes can significantly improve the reliability of TENG and provide steady performance [89,90].• Rolling structure: A group of rolling rods or rolling balls is inserted between two layers of friction material in this design.Consequently, the tribomaterials do not contact each other but rather the rolling materials.
Compared with traditional constructions, durability is greatly enhanced because of the much lower rolling friction coefficient in contrast to the planar sliding friction coefficient [91,92].• Soft contact structure: Due to the significantly reduced physical frictional resistance between liquid and solid, surface wear is decreased, leading to enhanced material durability [93,94].• Adding lubrication: Most TENGs function in conditions of dry friction.This condition may cause scratches on the material's surface as well as crucial corrosion, reducing its durability.A lot of research has shown that adding a lubricant layer between two tribo-layers can significantly increase the durability and stability of the TENG.This phenomenon occurs due to the ability of the lubricating fluid to create a barrier between the surfaces of the two layers of friction material.Therefore, the two friction surfaces are not in direct contact with each other but rather only in contact with the lubricant.Consequently, there is a substantial enhancement in durability.On the other hand, the lubricant effectively inhibits air breakdown, leading to less charge loss and improved performance.Widely used lubricants include squalane, paraffin oil, ionic liquid, olive oil, PAO10, rapeseed oil, and others [95,96].

TENGs for mobility
4.1.TENGs for non-motored mobility 4.1.1.Wheelchairs According to WHO statistics, more than one billion people worldwide have impairments such as hearing, vision, speech, physical, and so on [97].People with disabilities may face a range of hazards, challenges, and difficulties due to their health problems.Therefore, assistive devices for persons with disabilities are a practical support tool that helps individuals with impairments operate more easily in their daily activities.Among them, people with disabilities or weak legs frequently find it challenging to participate in traffic.
Although electric wheelchairs are useful mobility aids, they encounter some limitations [98].For instance, wheelchair users find it inconvenient to use public transportation because of the limited space and difficulty getting on and off the vehicle [99].Furthermore, sitting in the wrong position or losing balance in a wheelchair for a long time can lead to health issues such as spine deformities, pressure sores, or other concerns when engaging in road traffic [100].As a result, developing smart wheelchairs with embedded sensors that detect inappropriate sitting posture or warn of crashes to assist individuals with impairments in traveling small distances without relying on public transit is a viable alternative.However, these sensor systems necessitate external power for operation, hence increasing the size and cost of the wheelchair.Additionally, they must be frequently replaced or charged, resulting in inconvenience for the user.Hence, the development of self-sustaining electronic assistive devices for individuals with disabilities is an important task.TENG has advantages such as low cost, lightweight, and small size, allowing it to be used as a self-powered sensor system or a sensor power supply device [101,102].In 2022, Zhu's research team developed a portable and low-cost triboelectric nanogenerator (PL-TENG) that utilizes a TENG-based sensor to control the body balance of those with disabilities who use wheelchairs.They examined how well PL-TENG worked for wheelchair table tennis, a sport where body balance is crucial [101].In this study, due to their flexibility and heat resistance, PU sheets and Kapton sheets were chosen as friction materials, thereby rendering PL-TENG resistant to sweat and body temperature.PL-TENG also had foam glue between the two tribo-layers and copper foil as the conducting electrode (figure 2).With this structure, PL-TENG operated in CS mode to collect biomechanical signals and turn them into electric signals.PL-TENG could be easily installed on the left (PL-TENG-L) and right (PL-TENG-R) wheelchair seats using adhesive tape, as shown in figure 2. As the athlete's center of gravity changed in response to body movement, distinct electrical signals were produced under the pressure variation on each side.Therefore, the PL-TENG could transmit real-time data on an athlete's equilibrium and posture to assist them in maintaining excellent body balance.The signals were wirelessly sent to a computer using an analog-to-digital acquisition module and a Bluetooth module.As a result, coaches can check the data on the computer and provide players with advice.Similarly, Graham et al developed a triboelectric energy harvester and sensor (TEHS) installed on wheelchair seats and backrests to harvest energy from body movements and transform it into electrical signals for a people movement monitoring system [102].The unique part of this study is the friction material pair, which is made up of PVA and polycaprolactone (PCL) (both biocompatible polymers).In addition, these polymers were electrospun microfibers, which increased the ratio of surface area to volume, thereby enhancing electrical performance.TEHS was coated with an arch-shaped polyethylene terephthalate (PET) substrate and worked in CS mode.Different electrical signals were created and recorded as the volunteers sat, leaned, or moved.Indeed, TEHS served as a self-powered sensor to monitor the postures of wheelchair users.The energy conversion abilities and durability of PL-TENG and TEHS are also shown in table 1.These investigations demonstrated that TENGs, with their simple design, small size, and effectiveness, hold great potential as a wheelchair support device for individuals with impairments.It may significantly enhance their safety in daily activities.

Bicycles
In recent years, many countries around the world have encouraged people to ride bicycles in urban traffic for short distances [137].Bicycles have been considered a means that can effectively address societal issues such as reducing traffic congestion, protecting the environment, and promoting public health.Particularly since the COVID-19 outbreak, the tendency to take public transportation has shifted, and the usage of personal vehicles has expanded dramatically [138].Bicycles are frequently utilized in several aspects of life due to their compact size, lack of requirement for large parking spaces, and safety.However, there are still many problems that need to be overcome.For example, cycling at night has numerous disadvantages, such as reduced visibility, increased risk of collision with other vehicles, or difficulty detecting insufficient tire pressure.This has stimulated interest in enhancing bicycle safety.Technological advancements that convert conventional bicycles into smart vehicles are among those that attract attention.Especially, using TENG devices to increase the safety of bicycles in traffic is a practical solution.The TENG devices can collect bicycle rotation energy and convert it to electricity.This electrical energy can be used to power light-emitting diodes (LEDs), serve as a warning signal to keep people safe while riding in the dark, or serve as a self-powered speedometer or tire pressure sensor [103][104][105][106][107]. For example, Yang et al created a three-dimensional TENG (3D-TENG) in 2014 to harvest the rotational energy of bicycle wheels [103].The friction material layers (FLs) in this design were PTFE film and aluminum thin film.An electrochemical method was used to modify the aluminum surface to a nanopore surface, which improved output performance.The 3D structure of the external frame consisted of a circular acrylic substrate serving as a support and an iron block anchored to the substrate with three identical spring wires, creating a 120 • angle.This structure permitted 3D-TENG to operate in both CS and LS modes.When 3D-TENG was connected to bicycle wheels, the rotational energy of the wheels drove the vibration of the 3D-TENG and converted it to electricity, which was then used to power 30 LEDs.
In 2015, Chen research group developed an automated TENG converter (AT-TENG) with excellent output efficiency [104].AT-TENG's structure consisted of a functioning part and a rotating part.The functional part was protected by an acrylic substrate containing two friction materials: an Al nanoporous film that operated as both a positive friction layer and an electrode, and a negative friction layer of PTFE nanowire arrays film with a thickness of 25 µm, which was deposited with a copper electrode on one side.The PTFE membrane was attached to the substrate's bottom, while the aluminum membrane was on top.As seen in figure 3, one end of the friction layer was fixed while the other was free.The rotation part was located below the function part.A fascinating feature of this configuration is the utilization of a dual-poled magnet assembly fixed to the upper section of both the operational component and the rotating component.Due to the complete substrate coverage and the rotating part's use of magnets, AT-TENG was able to operate in harsh conditions, which made it superior to conventional TENG.Furthermore, AT-TENG worked in two modes with this structure.When rotating at normal speed, the AT-TENG had a CS mechanism based on the push of the pair magnet; however, when rotating at high speed, the Al layer vibrated around its equilibrium position at the frequency of the rotation, changing the mechanism of the AT-TENG to a noncontact free-standing mode due to the joining effect of centrifugal force and gravitation.When AT-TENG was integrated into the bicycle wheel, the rotor rotated according to the rotation of the wheel; thus, AT-TENG harvested rotational energy and converted it into electricity to light 104 LEDs.
In addition, another research group employed a dual-mode rotary TENG (DMR-TENG) to collect the kinetic energy generated during the operation of bicycle disc brakes, both during riding and braking maneuvers (Zhou et al 2021) [105].Figure 4 depicts the DMR-TENG construction, which consists of a rotator and a stator.The rotator was a steel plate with a brake disc structure; it was both a positive tribo-material and a brake disc.The stator part had PTFE film as a negative tribo-material, a copper radial electrode, a sponge layer, and an acrylic layer (figure 4).Because steel is the original material of the brake disc, it can maintain the original braking performance and mechanical characteristics.The sponge and acrylic functioned as protective layers; the sponge also brought the flexible PTFE closer to the steel.DMR-TENG's construction allowed it to be simply mounted to a bicycle disc brake, collect kinetic energy, and convert it to electricity.The DMR-TENG was capable of lighting 156 LEDs and acting as a power source for charging a 300 µF capacitor in a speedometer.Son's research team successfully created a triboelectric bicycle tire (TBT) in 2022.An aluminum film (50 mm × 25 mm) was put between the tire thread and the inner tube, separated from the inner tube by a PTFE layer (70 mm × 50 mm) [107].TBT was operated in SE or FS mode, depending on the PTFE and Al layer designs.As the wheel turned, electrons were transferred between the electrode layer and the tire thread, creating an electric current.This current could power nine LEDs through a rectifier circuit when the bike was ridden on asphalt or concrete roads.Furthermore, different tire pressures result in varying contact areas between tire threads and the road surface, resulting in different electrical outputs.This phenomenon enabled the TBT to function as a self-powered bicycle tire pressure sensor.Specifically, using commercial bicycle components, the TBT system had a fully packed construction and could create exceptionally stable electrical output for 120 000 cycles.The application of TENG on bicycles is still being explored by many research groups, as shown in table 1.The investigations demonstrate that TENG has practical utility in smart bicycle devices.

TENGs for motored mobility 4.2.1. Cars
Because of their speed, comfort, and convenience, automobiles have become the predominant means of urban transportation.In addition, after the COVID-19 pandemic, private automobiles became more popular than public ones [144].This results in apprehensions regarding the escalation of issues including traffic congestion, environmental pollution, and auto accidents.Car accidents are frequently the result of vehicle damage and negligent driving, and according to statistics, they are among the primary causes of mortality in the United States [3].Consequently, there has been a strong emphasis on enhancing safety.Thanks to the application of IoT and AI, a series of sensor systems have been strategically installed in many locations on the car to increase safety, such as navigation systems, driver behavior monitoring sensors, detection sensors for damage to the vehicle, and many other sensors [145,146].Notably, the development of autonomous vehicles represents a technological breakthrough in IoT and AI, with the potential to usher in numerous remarkable developments in the field of transportation.Self-driving vehicles release driving time and decrease human-caused mishaps, including distracted driving, drinking before driving, and exhaustion, which are among the top causes of traffic accidents.Through the integration of sensors and Wi-Fi systems, autonomous vehicles can establish real-time connections to detect potential risks, automatically modify their speed and distance from other vehicles to prevent accidents, and simultaneously choose the most efficient and secure route [147][148][149].The advantages demonstrate that autonomous cars are a very efficient solution for enhancing the safety of the traffic system.Studies have predicted that autonomous vehicles have the potential to decrease road fatalities by as much as 90% [150].In addition, this technology also solves traffic congestion and is environmentally friendly since it reduces fuel usage, thereby decreasing harmful carbon dioxide emissions.Conversely, self-driving vehicle systems are also humane since they may provide convenient mobility for those with disabilities.Despite the notable efficiency of these technologies, the task of finding sustainable energy sources for sensor devices continues to be formidable.TENG technology has demonstrated its revolutionary potential in the field of energy harvesting, particularly in the development of self-powered sensor systems for automotive applications.This technology possesses distinct benefits and unique properties that make it highly suitable for this purpose.There has been a growing proliferation of research groups proposing the utilization of TENG in the development of autonomous sensing systems capable of harnessing energy for application in automobiles [108][109][110][111][112][113][114][115][116][117].
The TENGs can convert the mechanical energy generated by the car into electrical energy.As an illustration, the operation of automobiles on roadways generates oscillations within the engine.Numerous research groups have studied the use of vibration energy, leading to the development of TENG devices made specifically for harvesting vibration energy [108][109][110].For example, Wen's research team created a harsh environment TENG (heTENG) that could operate as a self-powered smart braking (SP-SB) device in 2018 [108].The distinctive structure of the heTENG consists of an Al-ball-filled PTFE circular grid frame (rolling structure).When vibrated, the Al balls may collide straight up and down between the two PTFE membranes.The Cu electrode was utilized on one side of the heTENG, while Q235 steel was used on the other, as seen in figure 5.A hybridizing micro-nanocomposite material was employed as the substrate to encapsulate heTENG to increase wear resistance and endure high temperatures.The heTENG comprised two fundamental components: the heTENG-I, which functioned in FS mode, and the heTENG-II, which operated in SE mode with a grounded electrode on the lower portion.The heTENG converted both slide and vibration energy, achieving a high output performance of 221 V, 27.9A µcm −2 , and 33.4 µC cm −2 at a high engine speed of 4000 rpm and a frequency of roughly 1 Hz.The heTENG also had strong wear resistance (dynamic friction coefficient around 0.69 µm at low friction around 8.1 N at room temperature), excellent high-temperature tolerance (from −30 • C to 550 • C), and high hardness.Wen et al created a SP-SB device from heTENG to show how it works.With the SP-SB device's designed structure (illustrated in figure 5), it was possible to detect when the brake pads required replacement without the need for an engineering expert.The SP-SB worked as a driving safety early warning system for cars, detecting overweight, eccentric load, and tire pressure.
Based on the combined triboelectric and chemical resistive effects, Shen et al developed TENG as a self-powered source for the emission detector of a vehicle in 2018 [109].The device was made up of three independent parts: the TENG was the vibration energy harvester, the exhaust detector was a resistive kind of gas sensor, and the warning signal was a commercial LED.The TENG was made up of an Al foil that served as the positive triboelectric material and electrode, and a PTFE film that served as the negative triboelectric material.PTFE nanowires (diameter 100 nm and length 1 µm) were manufactured in addition to the core TENG component to increase roughness and surface charge density.Two acrylic substrates with four springs protected the device (figure 6).Finally, a heavy iron block was put on top of the TENG to ensure that the upper and lower parts of the system are in direct contact, and the bottom was an insulating plate for working at high temperatures.The NO 2 sensor included WO 3 nanorods that connected in parallel to two electrodes of the TENG and many commercial LEDs.This device could be easily installed on an automobile engine to collect lost vibration energy.The TENGs device operated in CS mode based on mobile vibration and NO 2 gas flow, and the sensor detected NO 2 concentration to send out a warning signal.By adjusting the number of LEDs illuminated by the TENG, the concentration sensitivity of NO 2 gas could be modified.
Xu's research team also suggested a TENG (S-TENG) based on a soft and powerful spring for collecting vibration energy [110].S-TENG possessed a distinctive spiral structure along the spring wire.The spring wire coating was an elastic electrode composed of silicon rubber and carbon nanofibers with diameters ranging from 100 to 200 nm.This layer was resistant to deformation up to 133%.An extra layer of silicon rubber covered the bottom electrode.The top part of the S-TENG was free to move, while the bottom part was fixed to the base.As a result, S-TENG vibrated vertically and horizontally and worked in the CS mode of the elastic electrode layer and silicon rubber layer.When mounted on the top of a model automobile, the TENG collected the shaker energy and powered over 20 LED lights, or it charged a temperature and humidity sensor capacitor in 300 s.By adjusting the spring's hardness and the thickness of the silicon rubber layer, this design permitted the device to be utilized at various resonant frequencies.
The utilization of the motion energy of the car steering wheel has also been harnessed.In 2022, Li's research team created a self-powered dual-type pulse signal angle sensor (DSA-TENG) that directly converted motion variables into AC and DC electrical signals in various directions of motion [111].DSA-TENG had a stator/rotor structure, as shown in figure 7. The rotor consisted of a FL, which was a FEP film layer with a thickness of 150 µm, and a periodic grading structure that was created using a laser cutting machine.This FEP layer was adhered to a foam (thickness of 0.5 mm) and circular acrylic substrate.The friction efficiency of the two contact layers was improved by this substrate layer.The stator was a thin copper foil (thickness of 30 µm) connected to the base of a circular substrate with a fan-shaped interspace structure (the angle between the fan blades was 45 • ); the copper foil worked as a friction electrode (FE).Two identical copper foils were additionally bonded to both sides of a fan-shaped hole, with one side grounded to act as a DC filtering electrode (DCFE) and the other as a charge-collecting electrode (CCE).The activity of DSA-TENG was a combination of the operations of each DS-TENG unit in the DSA-TENG structure (as indicated in the picture).When the rotor part rotated clockwise (the FE is in front of the CCE), the triboelectrification effect occurred first, followed by air breakdown, which generated the DC current.When the rotor part turned counterclockwise (the CCE is in front of the FE), it generated AC signals without a DCFE via triboelectrification and electrostatic induction.DSA-TENG was installed on the automobile steering wheel in an innovative design to operate as a real-time monitoring sensor in rotation angle and direction.The DSA-TENG sensor signal was sent to the computer for analysis, with the pulse peak type (DC or AC) representing rotation direction, the pulse peak number representing rotation angle, and the pulse peak width representing rotation speed.As a result, the DSA-TENG sensor provided real-time monitoring and steering wheel information feedback.This could be applied to automated vehicle systems that track the trajectory of the vehicle in real time to ensure driving safety.
The spinning wheel also generates robust steady energy.The capability of TENG to effectively gather and convert rotational energy into electrical energy has been demonstrated [112][113][114].In the year 2019, the research team of He successfully designed and created a hybrid electromagnetic generator TENG (EMG-TENG) device with the purpose of collecting rotational energy [112].The device was comprised of a cylindrical framework made of acrylic material, referred to as the stator component, and a central shaft that was connected to four acrylic blades capable of rotational movement around the shaft, known as the rotator component.The rotator was affixed with a layer of PTFE film, which acted as a negative friction layer, and was accompanied by sponge padding.On the other hand, the stator component was embedded with an Al foil electrode (figure 8).The structure of the EMG consisted of four pairs of cylindrical magnets and cores that were uniformly installed on an acrylic substrate.When this design was incorporated into the wheel, the rotor exhibited rotational movement in alignment with the wheel's revolution, thereby generating an electric current.This hybrid device had the capability to provide electricity to several sorts of sensors, including tire pressure sensors, self-powered counters, and timers for potential speed detection.Furthermore, numerous accomplishments are gathered from other reports, as depicted in table 1.

Trains
The utilization and popularity of high-speed rail networks are increasing because of enhanced convenience, improved time management, and economic benefits.As a result, monitoring the railway to guarantee safety is a crucial objective.Currently, the sensors used in the railway system remain operational.Nevertheless, the power supply capabilities of these sensor systems remain limited.Self-powering devices that harvest and convert a variety of energy sources into electricity have been growing in recent years.For instance, solar cells harvest solar energy, whereas wind turbines capture wind energy and convert it into electrical power.However, these devices are weather-dependent and costly [151,152].Recent research has demonstrated that vibrational energy possesses numerous advantages.Vibrations generated by the train's interaction with the rails are harnessed and transformed into electrical energy.This electricity is suitable for sensor network systems that operate wirelessly [153,154].The present vibration energy harvesting techniques that are widely used include electromagnetic, piezoelectric, triboelectric, and hydraulic [155][156][157][158]. Vibration energy harvesting has frequently employed electromagnetic techniques due to their numerous benefits, including high output power, high energy density, and compactness.Piezoelectric technology is popular for collecting vibration energy because of its tiny and light construction, high sensitivity, and stability.However, the railway system's vibration energies are discontinuous, low-frequency, and of small amplitude.Consequently, the effectiveness of these energy-harvesting technologies is limited.According to many studies, TENG technology has been shown to be extremely sensitive to vibrations at frequencies of a few hertz [158,159].TENGs could be used to collect energy from vibrations in railways.Various types of TENGs have been developed for integration with train systems [118-121, 139, 160].
Trains possess heavy cargo loads and operate at high velocities, resulting in the generation of significant levels of vibrational energy during their traversal along the railway.TENG devices are capable of harnessing and transforming this vibrational energy into electrical energy.In 2017, Zhao et al developed a self-powered triboelectric nano vibration accelerometer known as TEVA [160].This device was specifically designed for application in rail tracks, with the aim of harnessing the low-frequency vibration energy generated by the passing of high-speed trains (below 10 Hz).The TEVA device had a simple spring structure (provided the device recovery force), and a poly (methyl methacrylate) (PMMA) substrate covered it.PMMA provided device protection due to its notable attributes such as high hardness, lightweight, convenient processability, and cost-effectiveness.The tribo-pair consisted of an Al nanolayer film and a Kapton nanostructure, with thin Al film serving as the electrode.The TEVA exhibited sensitivity to vibration acceleration ranging from 1.07 m s −2 to 1.25 m s −2 .This sensitivity enabled the generation of electric current to charge a lithium battery (1.5 V to 3.1 V) and self-power the accelerometer or the wireless sensor network nodes.Therefore, the self-powered TEVA presents a solution to the issue of powering the accelerometer, and the accelerometer location will not be limited.On the other hand, the timely collection of the vibration signal from equipment on trains facilitates a seamless execution of the diagnostic process.Table 1 demonstrates that this energy source is also utilized by different other research groups.
High-speed trains produce a substantial quantity of wind energy that is often dispersed.TENG can capture and transform the energy that is now wasted into useful electrical energy.In 2021, Zhang et al created elastic rotation TENGs (ER-TENGs) that had less friction and more output to collect wind energy from high-speed rail vehicles [118].As seen in figure 9, ER-TENG had a dual stator/rotor construction.The rotator included a circular acrylic board with 12 negative tribo-membranes of PTFE arranged in a fan shape, connected to one side, and an additional 12 PTFE membranes attached to the other side.An inner layer of Kapton was added to the PTFE layer to improve mechanical contact and decrease the driving force.The stator component included two grooved copper panels attached to both sides of the rotor, functioning as positive layers and electrodes.An acrylic covering protected the ER-TENG, ensuring its durability even in harsh conditions.The ER-TENG was strategically installed at intervals along both sides of the railway track to collect the wind energy generated by a high-speed train passing over a railway and power the traffic lights and sensor networks.As a result, the ER-TENG is a self-powered system that significantly reduces high-speed train operating costs.
The high-speed train wheels' rotational energy may serve as a potential source for TENGs.In 2021, Jin et al developed a free-fixed TENG (FF-TENG) for monitoring the wheel train [119].The FF-TENG had a stator/rotor structure and functioned in FS mode.The key idea of this design was to use magnets to attach the stator to the bogie.As a result, the stator did not make direct contact with the bogie, reducing the damage to the wheel.In addition to stabilizing the stator, the magnetic force increased the stator-rotor contact area, which increased the output.The stator consisted of a blade-shaped PU (electropositive layer) linked to one side of a circular acrylic sheet and two sets of magnets attached to the other side.Similarly, two more sets of magnets are attached to the bogie to help stabilize the stator section.The rotator had a PTFE electronegative layer, a Cu-blade-shaped electrode, and an acrylic cylindrical shell, as shown in figure 10.The rotation of the rotator induced friction between the PU and PTFE, which generated energy.The FF-TENG triggered 140 commercial LEDs and provided electricity to a wheel monitoring system.The energy conversion efficiency and reliability of the investigations, as well as several additional studies, are summarized in table 1.According to the research, TENG can be utilized in the construction of a modern and safe rail system.

TENGs for ships-blue energy
Wireless sensor systems are very handy and easy to install in road systems.However, there are still several issues when installing in large oceanic areas because of limited energy resources.Although there have been a variety of wireless warning systems used, such as radio navigation aids, they frequently run on standard batteries, which have a short lifespan and pose environmental risks.Therefore, the quest for clean and sustainable energy sources has garnered global attention.Ocean energy, also referred to as blue energy, has garnered considerable interest in recent times.The ocean, which comprises 70% of the earth's surface, is a boundless source of energy, particularly wave energy [161][162][163].Due to its high density, widespread distribution, and environmental friendliness, wave energy can be collected and converted into useful electrical energy.However, since ocean waves often move at random and low frequencies, developing highly efficient exploitation technology remains difficult.TENG has emerged as a promising technology due to its cost-effectiveness, simplicity, and ability to harness irregular and low-frequency mechanical energy.Several TENG structures, including rolling structures, liquid-solid structures, and rotary disks, have been suggested for wave energy harvesting [122][123][124][125].
Using a liquid-solid structure, Li et al established a network of high-performance TENGs based on liquid-solid interface contact electrification (LS TENG) in 2018 [122].Herein, the buoy-like LS-TENG can collect low-frequency blue energy from the ocean.TENG is a liquid-solid-contact buoy with three concentric cylinders of varying diameters (4 cm, 7.7 cm, and 11 cm).The cylinders were composed of many layers, with an acrylic substrate layer as the innermost layer, a middle electrode layer (Cu, Ag, and Al) on both sides of the acrylic layer, and a polymer negative friction layer on the outermost layer, as shown in figure 11 (PTFE, FEP, FET, and PDMS).The LS-TENG was completed when the water and bottom electrodes were inserted within the concentric cylinder.When the buoy LS-TENG shook, the water touched the polymer within, generating an output current.The LS-TENG captured energy from several movements, including vertical oscillation, shaking, and rotation.Additionally, the energy generated by a liquid-solid contact LS-TENG was shown to be 48.7 times higher than that of a solid-solid contact TENG, demonstrating a substantial increase in friction.The group of 18 LS-TENGs successfully charged a capacitor (10 mF) in a wireless SOS system from 0 to 5 V in around 13 min.By combining several LS TENGs, a substantial amount of blue energy can be collected from the ocean.Furthermore, it is anticipated that LS-TENG will exhibit stability for 10 years.In 2023, Liang's research group also created an asymmetric liquid-solid TENG (DE-TENG) [123].DE-TENG was composed of two rectangular acrylic panels (70 mm × 65 mm × 2 mm) aligned with a 2 mm gap.The package included two identical PP and FEP films (64 mm × 64 mm × 100 µm).Magneton sputtering was used to create Cu electrode layers (60 mm × 60 mm) on these film surfaces.PP is more electropositive than water, but FEP is more electronegative than water; hence, DE-TENG has an asymmetric structure.When immersed in water, the water waves fill the gap between the two membrane surfaces, forming two electroactive layers facing each other and converting the kinetic energy of the water into electricity.One thousand DE-TENGs can be interconnected to harvest the energy of ocean waves.DE-TENG arrays provided electricity to signal lights, digital thermometers, and water quality indicators.
Hybrid TENG/EMG and pendulum structures have also been effectively designed by many groups [124][125][126][127][128][129][130][131].Ren et al showed a self-powered route avoidance warning system for ocean navigation (HW-NG) in 2021 using hybrid TENG/EMG and pendulum structures [124].The TENG had a multilayer structure with six pairs of CS-mode TENGs connected by springs, as indicated in figure 12. Polyimide was selected as the TENG's backbone because of its high mechanical strength and flexibility.The Cu film acted as the positive friction layer and an electrode on the frame, while the FEP with a nanostructured surface worked as the negative friction layer.A foam base was sandwiched between the layers.EMG is made up of a wobbling magnet block and four Cu coils arranged in an arc on a PMMA substrate.EMG and TENG could communicate through the pendulum bar.TENG and EMG were both protected by a cubic-shaped PMMA shell.As a result, the HW-NG successfully collected wave energy via pendulum bar vibration and operated as a power source for long-distance (1.5 km) wireless transmission.Thus, connecting hundreds of thousands of HW-NG nodes in seas near islands or coral reefs would result in a self-powered navigational warning system that provides vessel safety in all-weather situations.Table 1 summarizes the presentations of this structure by the research groups of Liu et al [126] and Demircioglu et al [125].
Swing structure has also been exploited effectively in some studies [127,128].Typically, Jiang et al fabricated a TENG with a powerful rotating structure in 2020 [127].The cylindrical SS-TENG was made from a cylindrical acrylic shell that housed 6 copper electrodes and 4 PTFE (negative layer) stripes.The core of the cylinder had a spinning component comprised of three acrylic blocks affixed to a steel shaft using three bearings (figure 13).An arc-shaped acrylic strip (positive layer) was connected to the protruding sections of three acrylic blocks.A space existed between the copper electrode and the acrylic strips.In addition, several copper blocks are inserted at the bottom of the acrylic block to lower the center of gravity and facilitate the rotation process.Lubricant oil was also added to the connecting bearings to decrease friction.The SS-TENG device efficiently collected blue energy and transferred it to electricity, which powered environmental monitoring systems.
The emphasis has also been placed on using eco-friendly materials [129,130].Ahn et al developed an all-recyclable, mechanochemically robust TENG (AR-TENG) in 2022 to harvest wave energy as well [129].AR-TENG has a straightforward design, with PMMA acting as the substrate or negative layer and noble metal (Pd) functioning as the positive layer and electrode (figure 14).PMMA was chosen for its formability, chemical resistance, weather resistance, UV resistance, abrasion resistance, high track and arc resistance, mechanical strength, and low cost, whereas Pd was chosen for its chemical stability and maximum triboelectric effect when combined with PMMA.Both PMMA and Pd contain nanohole pattern structures that improve AR-TENG's energy harvesting ability.PMMA and Pd are readily recycled in acetone, a green solvent.AR-TENG has spring structure and is protected by an acrylic base frame.AR-TENG can operate in contact-separation mode with this arrangement.AR-TENG has outstanding output power density and better mechanochemical stability following immersion in saltwater or 1000 000 cycling tests.This AR-TENG's practical applicability is proven by its usage to power a buoy-type ocean monitoring system and an intelligent life jacket.
The research achievements demonstrate that TENG is an innovative technology for harvesting blue energy.This offers potential for the development of sustainable energy sources that can power STS on land and at sea.

TENGs for unmanned aerial vehicles (UAVs)
UAVs are an excellent practical IoT application.UAVs have been employed in a variety of applications ranging from military to civilian, including detecting and tracking objects, exploring dangerous locations, transporting goods, and monitoring infrastructure [164,165].When UAVs are incorporated into road traffic monitoring systems, it attracts a lot of interest.UAVs can assist in automating this system by collecting road traffic conditions to minimize congestion, identifying vehicles with high speeds, and giving a good perspective on traffic incidents [166][167][168][169].As a result, using a self-powered TENG on UAVs will help them become a potential tool for the future, opening many more practical applications [132,133,140].
Tao's research team created a TENG with a honeycomb form (h-TENG) in 2021 [132].The h-TENG includes a negative friction thin film layer of FEP with nanostructured pillars, a transparent PET substrate layer with electrode silver nanowires (AgNWs) to support cycling pressing and releasing operations, and a transparent PET substrate layer with electrode AgNWs to sustain cycling pressing and releasing operations (figure 15).The considerable roughness of the AgNW and FEP surfaces assists in charge capture, improving h-TENG performance.The h-TENG is divided into a parallel-connected array of arch-shaped deformable capacitors by the porous honeycomb structure.Each unit will be capable of bending and compressing.As a result, overall capacitance changes are greatly magnified, making h-TENG very susceptible to external perturbations.The h-TENG can operate for 75 000 cycles under mechanical shaker.The h-TENG devices are small, flexible, transparent, and lightweight due to their outstanding resilience and self-rebounding qualities.With this unique characteristic, h-TENG may be incorporated into UAVs to collect and convert kinetic energy from morphing wing motions into electricity.As a result, h-TENG may be utilized as a self-powered source for UAVs.
Gyroscopes are now frequently used as sensors in applications that require the measurement of angle, tilt, and angular velocity, such as monitoring the flying posture of a UAV and taking photos.Gyroscopes, although effective, still need an external power source.To address this issue, Xie et al developed a freestanding-mode rotary disc-shaped TENG (FRD-TENG) in 2021 for a self-powered gyroscope angle sensor (SGAS) [133].FRD-TENG had a stator/rotor structure, as seen in figure 16.The rotor consisted of 30 radial copper electrodes (each at a 6 • with the other), and the stator included two circular interdigital electrodes and negative tribolayer PTFE.The FRD-TENG has been connected in series with a resistive rotary potentiometer to form the SGAS.The results demonstrated that various electrical signals were received at different rotation angles, implying that SGAS might be employed as a very sensitive real-time rotation angle sensor.In addition, SAGS activated the LEDs of the alert system.As a result, Xie et al have effectively developed a self-powered rotation angle sensor system with broad applicability in areas such as UAV navigation or automatic driving.
In 2022, Yao et al created an arc-shaped TENG, known as AW-TENG, which serves as a wind speed sensor [140].The AW-TENG structure included a negative tribolayer of PTFE, a copper electrode, and a 3D-printed PLA arc-frame shell.The arc design of AW-TENG helped reduce air resistance.AW-TENG achieved a voltage frequency of 50-200 Hz within a wind speed range of 6-20 ms −1 .The sensitivity was 9.88 Hz ms −1 , and the goodness of fit R 2 was 0.998.When integrated into the UAV and operated in CS mode, AW-TENG gathered wind speed signals and communicated them wirelessly to the ground station, triggering a response from the UAV.Thus, AW-TENG can function as a self-powered wind speed monitoring system for UAVs.
Briefly, TENGs offer a renewable energy solution for sensor systems in UAVs, enhancing the safety and in-real-time precision of intelligent transportation systems.

TENGs for infrastructures
The presence of a synchronized and contemporary infrastructure system is indicative of a nation that has achieved a high level of development.This system plays a crucial role in guaranteeing the safety of traffic.As a result, most countries prioritize the building of modern infrastructure [170,171].Furthermore, the inescapable degradation of infrastructure systems resulting from prolonged operation in severe conditions and substantial vehicle loads is also a cause for concern [172].The utilization of TENG in infrastructure systems has yielded several noteworthy accomplishments, encompassing the development of self-sustaining traffic light systems for roadways and tunnels, as well as self-powered monitoring systems for bridges and other civil infrastructure [118,[134][135][136][141][142][143].
In this application, TENG devices have the capability to capture wind energy produced by moving vehicles.For instance, Zhang et al created a rotating disk-based hybridized nanogenerator of EMG and TENG in 2016 [134].This study used the stator/rotor TENG structure with the above rotor of four intercrossing acrylic blades mounted on a circular shaft and grated negative tribolayer (PTFE) (a buffer PU layer was between PTFE and blades).The below stator was positive tribolayer of grated Al foil.These two tribolayers' surfaces featured a nanostructure to increase the contact area.The EMG included four magnets on one side (rotor) and four symmetrical coils on the other (stator), as shown in figure 17.The device has been installed in the tunnel for operational efficiency.The movement of vehicles generated airflow, resulting in the rotation of the rotor.This lead the EMG-TENG to generate electricity, which then powered wireless traffic volume sensors.Consequently, EMG-TENG is an efficient tool for real-time monitoring of traffic flow, which aids in the reduction of traffic accidents.Similarly, in 2023, Su et al also used the stator/rotor design (AG-TENG), but with structural upgrades to increase the power output [135].The upper stator of the AG-TENG consisted of a copper foil with a grating structure with a gap of 4 mm, mounted on a PCB board serving as an electrode.Additionally, three polyester furs acted as a ternary dielectric material.The rotor was a grating structure composed of PTFE and nylon film, as shown in figure.When AG-TENG was in FE mode, PTFE first touched the polyester fur, receiving electrons from it, and then nylon contacted the fur, transferring electrons to it.Therefore, the fur could both donate its own electrons and receive more electrons from nylon to transfer them to PTFE.This technique enhanced the quantity of electrons transferred, hence enhancing the output (a single AT-TENG yielded a power density of 0.2 W m −2 at a wind speed of 3 ms −1 ).In addition, AG-TENG enabled the transfer of electrons between opposite-polarity tribomaterials without contact.This feature reduced the wear and tear on the system.It was worth noting that AG-TENG can operate continuously for 80 h, performing 1440 000 cycles without any stability issues.Moreover, the traditional design of the stator/rotor, with the rotor positioned above the stator, presents some issues, such as electrostatic attraction and gravitational forces that result in the rotor being in closer proximity to the stator.This closeness leads to increased levels of friction and wear.To address this issue, AG-TENG can be employed by positioning the rotor beneath the stator and utilizing the gravitational force of the rotor to counterbalance the gravitational electromechanical force.AG-TENG also included an anti-glare panel system.When implemented in highway systems, the utilization of large-scale AG-TENG technology enabled the collection of wind energy from moving vehicles to supply power to LED lights, signal lights, and low-power IoT devices along the highways.Additionally, it served to reduce the likelihood of traffic accidents by obstructing the intense light that approaching cars emit.
The mechanical excitation of the infrastructure system may potentially be used as an energy source for the TENGs.Zhang's research team successfully incorporated TENG into the reinforcing steel bars (FRBs) that comprise the structural components of civil infrastructure systems (TENG-SEs) in 2021 [136].The five layers of FRB are illustrated in figure 18.The innermost layer was composed of Nylon 66 with 10% carbon fiber reinforcement.The conductor layer was composed of PLA with carbon black.These two interconnected layers were commonly referred to as the 'core' .The dielectric PU layer and the PLA with carbon black layer were the next two layers, which were referred to as 'cloth' overall.The outermost layer was PU, as shown in figure 18. Due to the cloth layer's curved structure, there was a gap between the core and the cloth.This enabled the operation of TENG-SE in CS mode.With this design, the rebar worked as both a normal way to strengthen concrete structures and as a TENG that could capture mechanical excitations and detect problems in civil infrastructure systems.In 2022, Xu's research team implemented TENG into a self-powered health monitoring system (CS-TENG) based on the fundamental structure of CS mode [141].In this design, a copper foil layer serves as the positive material and electrode (Copper 1), while a PTFE layer (240 µm) serves as the negative material layer and is attached to the copper foil electrode (Copper 2).Melting and heating PP straw creates a cavity-like structure to cover the device (figure 19).CS-TENG's structure is simple but very effective; it has a 50 000 cycles operating life, is accurate to within 1 mm, and can detect pressure fluctuations of 0.86 V kPa −1 at ambient pressure.CS-TENG has the capability to effectively identify and monitor the structural problems associated with bridges and road infrastructure in real-time.

Perspective and conclusion
The overview of TENG-based STS has been summarized from various ambient energy harvesting sources such as rotation, wind energy, and rotation of mechanical components, which provides huge benefits to our daily life applications.Besides, the rapid development of the IoT, AI, and 5G or 6G technologies brings us to the era of smart cities.In the past few years, the data collected in our daily lives for serving in safety transportation, weather forecasting, monitoring, and other services in the city has made huge progress.As a result of that, there is a relatively strong connection between STS, AI, IoT, and so on that can be developed toward safer monitoring and more rapid service for smart cities.To do this, the lower power consumption with high-sensitivity sensors and the light weight, eco-friendly material selection, flexibility, etc of TENG, resulting in the low cost and miniaturization of sensors for STS, a part of smart cities, are still a big challenge.Finally, the applications of TENG-based STS need to be realized in industrial production soon if current challenges and future perspectives are considered.
• Current challenges are about the wireless connection to the smart grid, the efficiency of TENG in STS, the durability of TENG due to the harsh environment, its integration to avert the safety effects on transportation vehicles, and the selection of materials for environmental friendliness.These factors are truly crucial to developing a sustainable TENG-based STS.In addition, due to the rapid development of STS powered by TENG, recycling materials and technology should be considered for long-term operation and an ecofriendly solution.• Future perspectives in our view are about the automatic communication wirelessly among transportations (vehicles, ships, airplanes, UAVs, motorized and non-motorized mobility), infrastructure, and other electronic devices for real-time monitoring, data analysis and sharing, etc.Moreover, the combination with other energy harvesting technologies to establish a hybrid system for sufficient energy is also a need.These mentioned requirements really play an important role for emerging autonomous vehicles such as UAVs, unmanned vehicles and drones, self-driving cars, etc.

Figure 1 .
Figure 1.Illustration of four working modes of TENG.

Figure 2 .
Figure 2. Demonstration of TENGs for wheelchairs.Structure, working mechanism, and application of PL-TENG for controlling the body's balance.Reproduced from [101].CC BY 4.0.

Figure 3 .
Figure 3. Demonstration of TENGs for collecting rotation energy from bicycle wheels.Design of AT-TENG for self-powered source speedometer and warning signal.Reprinted with permission from [104].Copyright (2015) American Chemical Society.

Figure 4 .
Figure 4. Demonstration of TENGs for bicycle disc brakes.Design of DMR-TENG for harvesting kinetic energy of disc brake.Reproduced from [105].CC BY 4.0.© 2021 The Authors.Advanced Energy and Sustainability Research published by Wiley-VCH GmbH.

Figure 7 .
Figure 7. Demonstration of TENGs for harvesting rotation energy of car steering wheel.Design and working mechanism of DSA-TENG for self-powered sensor on car steering-wheel [111].John Wiley & Sons.© 2022 Wiley-VCH GmbH.

Figure 8 .
Figure 8. Demonstration of TENGs for harvesting rotation energy of wheel cars.Design and working mechanism of hybrid TENG/EMG for self-powered sensor on car steering-wheel.Reproduced from [112].CC BY 4.0.© 2019 The Authors.Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd.

Figure 9 .
Figure 9. Demonstration of TENGs for harvesting wind energy from high-speed train.Design and working mechanism of ER-TENG for self-powered traffic signal and sensor network.Reprinted with permission from [118].Copyright (2021) American Chemical Society.

Figure 10 .
Figure 10.Demonstration of TENGs for harvesting energy from train wheel.Design and working mechanism of FF-TENG for self-powered source [119].John Wiley & Sons.© 2021 Wiley-VCH GmbH.

Figure 15 .
Figure 15.Demonstration of TENGs for harvesting morphing wing energy from UAVs.Design and working mechanism of h-TENG.Reproduced from [132].CC BY 4.0.

Figure 17 .
Figure 17.Demonstration of TENGs for harvesting wind energy at the tunnel.(a) Design and working mechanism of EMG-TENG for self-powered traffic signals.Reprinted with permission from [134].Copyright (2016) American Chemical Society.

Figure 19 .
Figure 19.Demonstration of TENGs for traffic facilities.Design and working mechanism of CS-TENG for self-powered health monitoring of traffic facilities.Reprinted with permission from [141].Copyright (2022) American Chemical Society.

Table 1 .
Summary of various TENG techniques for transportation.