Advanced design of triboelectric nanogenerators for future eco-smart cities

Abstract

Eco-smart cities follow the ecological principles, utilize smart information technologies (Internet of Things, artificial intelligence, cloud computing) to build an efficient, harmonious, resilient, and sustainable habitable environment in the form of informatization. Triboelectric nanogenerators (TENGs) offer the benefits of being self-powered, affordable, extremely customizable, and multi-scenario applications. The researches depict that TENGs are well positioned to support the digitization, intellectualization and sustainable urban services, since they have been repeatedly demonstrated as renewable power providers and self-powered sensors. In this assessment, the most recent applications of TENGs technology in eco-smart cities over the past two years in various categories are investigated, including renewable energy supply (water, wind, solar and raindrop energy, etc.), human–machine interaction, intelligent healthcare, intelligent transportation, intelligent agriculture, intelligent industry and intelligent environmental protection. There is additional sketch of the distinctions in TENG materials, architectures, working modes, and contact modes for serving diverse living usage scenarios of the eco-smart cities. This review will promote and popularize the utilization of TENG in smart ecological cities, as well as provide instruction of its construction for future smart cities and eco-cities.

1 Introduction

Cities represent complex, interconnected systems where the integration of "ecology and wisdom" serves as the cornerstone for crafting future high-quality urban environments. "Wisdom" underscores the efficient flow of information, emphasizing innovative approaches to address diverse urban needs, while "ecology" stresses the symbiotic relationship between humanity and nature throughout the urban development process. The concept of eco-smart cities emerges as a synthesis of eco-city and smart city paradigms, striving for both sustainable urban expansion and technological modernization [1, 2]. The concept has been embraced by many countries around the world. For instance, in the United States, Telosa City plans to integrate renewable energy, intelligent transportation systems, and digital infrastructure to create a livable, environmentally friendly, and sustainable city. Japan's Woven City aims to achieve high automation and intelligence through artificial intelligence, robotics, and autonomous driving. Similarly, in Germany's Siemens City 2.0 project, smart grids and Internet of Things (IoT) technology are utilized to improve energy efficiency and optimize urban resource utilization. Additionally, Singapore's Tengah City endeavors to seamlessly integrate with the natural environment through smart buildings and energy-saving measures [3, 4].

The objective of eco-smart cities is twofold: firstly, to embed ecological principles within urban planning and design, leveraging natural systems and environmental resources to deliver urban services sustainably. Secondly, eco-smart cities harness advanced information technologies across various sectors to create a seamlessly connected urban fabric [5, 6]. Notably, these cities prioritize the development of green energy sources to achieve ecological energy supply and promote sustainable production and lifestyles [7]. Furthermore, eco-smart cities leverage a plethora of data collected through IoT devices, cameras, GPS, etc. This data undergoes intelligent analysis via artificial intelligence and cloud computing, facilitating efficient interactions with essential urban systems such as power supply, water management, transportation, healthcare, agriculture, and industry. Through the utilization of information and communication technologies (ICT) platforms, eco-smart cities foster enhanced engagement with citizens [8, 9]. Consequently, the development of eco-smart cities is envisaged to contribute to the coordinated, sustainable, and healthy advancement of urban economies, societies, environments, and resources. By addressing challenges such as urban congestion and pollution, eco-smart cities aspire to ameliorate the symptoms of "big city diseases" while enhancing the overall well-being of their inhabitants [10].

Briefly, the creation of eco-smart cities requires clean energy technologies and cutting-edge sensor technologies, both of which TENGs have shown remarkable promise [11, 12]. On the one hand, TENG is a fierce rival in the pursuit of renewable energy. Collecting ocean energy include water-tube-based TENG, spherical TENG based on eccentric structure, monodirectional continuous spinning TENG, and cylindrical TENG with unidirectional rotation [13, 14]. TENG has also been validated to be utilized to gather broad-band wind energy such as breeze, strong wind, as well as wind energy produced by high-speed trains [15]. TENG can additionally be employed as a hybrid energy collection tool to capture both solar and raindrop energy, water and wind energy [16]. Notably, TENG may be availed to efficiently harvest and exploit low-frequency mechanical energy that is ubiquitous in the environment, such as vibration energy from motors, bridges, and railroads as well as micro biomechanical energy like heartbeat, breathing, muscle action [17, 18].

TENG, on the other hand, is regarded as an extraordinary technology for building an Internet of Things-era sensor network. TENGs have applications in smart home for wireless control of household appliances, electronic password locks and anti-theft authentication, etc [19, 20]. In human–machine interaction systems, TENG can be employed for human motion recognition, touch recognition, speech recognition, etc [21]. In particular, TENG plays an outstanding role in intelligent medical domains such as human shoulder and waist rehabilitation, nerve and bone repair, asthma prevention and alarm, and subcutaneous antibacterial, etc [22]. Also in the intelligent transportation fields, the TENGs have shined in wireless traffic control system, violation and speeding monitoring, traffic facility monitoring and intelligent driving, etc [23]. Even in smart agriculture fields, there have been reports that TENGs being utilized in wireless smart farm monitoring systems, crop growing environment and health monitoring, etc [24]. For smart industry, TENGs have been involved in industrial cooling water system, mechanical condition and equipment frequency monitoring, etc [25]. In terms of the deteriorating urban environment, TENG has also excelled in domains of environmental preservation such as wastewater purification and air filtration [26]. Hence, it is believed that TENGs can be utilized as one of the fundamental technologies to actualize eco-smart cities to address the current energy and sensing challenges and deal with the impending intelligent Internet of Things.

In recent years, several notable review articles have delved into the application of TENGs in contexts relevant to our study. For instance, Wang et al. explored the 'Multidiscipline Applications of Triboelectric Nanogenerators for the Intelligent Era of Internet of Things,' providing a comprehensive overview of TENG applications across various IoT scenarios such as smart agriculture, industries, cities, emergency monitoring, and ML-assisted AI applications. Additionally, Lee et al. introduced the 'Triboelectric Nanogenerator Enabled Wearable Sensors and Electronics for Sustainable Internet of Things Integrated Green Earth,' emphasizing TENG utilization in wearable sensors and electronics for applications in healthcare, environmental monitoring, transportation, and smart homes within the IoT framework. Moreover, Chen et al. summarized 'Nanogenerators for smart cities in the era of 5G and Internet of Things,' evaluating cities across crucial sectors including water and wind-based energy sources, intelligent transportation, smart vehicles, human–machine interface, and smart healthcare utilizing piezoelectric NGs (PENGs) and TENGs. In contrast to existing reviews, our study offers a distinct perspective by focusing specifically on the advanced design aspects of TENGs tailored for future eco-smart cities. While previous reviews may have covered broader applications of TENGs in smart cities, renewable energy systems, or TENG sensors for IoT, our review concentrates solely on TENGs, excluding PENGs from consideration, and examines only the latest advancements within the past two years. Significantly, our exploration encompasses various facets of TENG applications in smart ecological cities, including renewable energy sources such as water, wind, and hybrid energy from solar and raindrops, as well as aspects of eco-smart home, human–machine interaction, healthcare, transportation, agriculture, industry, and environmental protection. By underscoring the innovative aspects of our review, we aim to offer valuable insights into the development of sustainable urban environments and pave the way for the widespread adoption of TENGs in future eco-smart city initiatives.

2 TENGs and eco-smart cities

2.1 Trends of publications

Ecological and intelligent urban construction has received a growing attention in recent years. Purely ecological or smart cities do have some restrictions, thus it is required to investigate a new smart urban construction mode that is compliant with both social development law as well as human sustainable development. The core concept of an eco-smart city is to adhere to the scientific ecological outlook on development, and to satisfy the needs of residents for ongoing improvement through the development and use of intelligent technology, information processing technology, and the Internet, which integrate all levels, all fields, and all systems of urban construction into a stable and mutually supportive network framework (Fig. 1).

Fig. 1

Eco-smart city construction with stable and mutually supportive network framework

In order to focus on research hotspots and trends in the field of ecological cities and smart cities, we conducted a comprehensive analysis of publications on ecological cities and smart cities by scanning pertinent fields in the database of Web of Science, with data collected from 2012 to 2022. As illustrated by the broken line in Fig. 2a, the total number of ecological city publications has increased dramatically in the past decade, from 988 in 2012 to 4,453 in 2022, indicating that people's concept of ecology has gradually strengthened. In the case of smart cities, the total number of publications climbed significantly from 1,041 in 2012 to 6,624 in 2018, and the number of publications in 2018–2022 fluctuated relatively steadily and still maintained at a high level, among which 7,455 were published in 2022, demonstrating that people have widely realized the concept of smart cities. In addition, such a large number of publications of eco-cities and smart cities suggest that they have emerged as an important direction of urban development.

Fig. 2

Publication analysis. a Smart city. b Ecological city. c, d Triboelectric nanogenerator

Urban construction includes innovative technologies and resource management in household, medical, transportation, energy, agriculture, enterprise, environment, etc. Based on the premise, we selected 8 keywords "home", "healthcare", "human", "transportation", "agriculture", "energy", "industry", "environmental protection", respectively together with smart city and eco-city for field search. There is slightly gap between the line chart and the bar chart of the total search volume of these eight subcategories. For instance, in 2022, the overall number of the eight subcategories under eco-city is 3,741, close to the total number of 4,453. In addition, the eight subcategories under smart cities add up to 5,865 articles, also approaching their total of 7,455 articles. This implies that the study of eco-cities or smart cities can practically be covered by these 8 fields. Additionally, it can be inferred from the data that the number of environmental protection disciplines with more publications in eco-cities is the least in smart cities. Furthermore, the energy sector has received the most attention in smart cities, and the number of its publications in eco-cities also accounts for a large proportion, authenticateing that energy has a significant impact on urban construction.

As shown in Fig. 2c, since the TENG was originally invented by Zhonglin Wang and his team in 2012 [27], there had been a rapidly rise in both the number of publications (from 4 in 2012 to 1,327 in 2022) and the citations (from 18 in 2012 to 66,098 in 2022). It is worth noting that TENGs have been widely applied in smart home (Fig. 3a), human–machine interaction (Fig. 3b), smart healthcare (Fig. 3c), smart transportation (Fig. 3d), smart agriculture (Fig. 3e), smart industry (Fig. 3f), environmental protection (Fig. 3g), smart energy (water energy in Fig. 3h, wind energy in Fig. 3i, solar and raindrop energy in Fig. 3j) fields. The corresponding number of papers was 95, 156, 296, 69, 81, 584, 56, 4774, with a total number of 6111 papers. And TENG, as a potent clean energy harvesting device, accounts for the largest number of relevant publications in the energy field (Fig. 2d). All in all, the domains of TENGs described above are highly oriented towards smart cities and eco-cities (Fig. 3).

Fig. 3

Triboelectric nanogenerator for eco-smart cities. a Smart home [28]. Copyright 2022, American Chemical Society. b Human–computer interaction [29, 30]. Copyright 2023, Elsevier. Copyright 2022, Science. c Smart healthcare [31]. Copyright 2022, Elsevier. d Smart transportation [32]. Copyright 2021, Elsevier. e Smart agriculture [33]. Copyright 2022, Nature. f Smart industry [34]. Copyright 2021, American Chemical Society. g Smart environmental protection [35] Copyright 2023, Elsevier. h Water energy. [36] Copyright 2021, American Chemical Society. i Wind energy. [15] Copyright 2021, American Chemical Society. j Solar and raindrop energy. [16] Copyright 2022, American Chemical Society

2.2 TENGs working principle

TENGs operate based on the principles of the triboelectric effect and electrostatic induction. There are currently five types of TENG operating modes based on structure differences: contact-separation mode (Fig. 4a), horizontal sliding mode (Fig. 4b), freestanding mode (Fig. 4c) and single electrode mode (Fig. 4d) for AC-TENG, air breakdown mode for DC-TENG (Fig. 4e) [37]. The working mechanism of contact separation mode, which is most frequently adopted in TENGs, is then illustrated. The electrode material layer, friction material layer and conductive wire make up the main body of the TENG, and the electrostatic induction effect and friction electrification are the two primary operating principles. There is a small space between the two materials in their initial state and no electric charge is created or introduced (I). The two dielectrics come into contact when an external pressure or bending force is applied, and a charge transfer takes place, resulting in the formation of a triboelectric charge (II). The two surfaces are separated when the deformation force is released. The presence of the air layer in the middle prevents the positive and negative static charges on the two surfaces from being entirely neutralized, creating a potential difference. The external circuit will produce an instantaneous current (III) in order to balance this potential by inducing the opposing electrical charge on the back electrode plate with the aid of electrostatic induction. Equilibrium will be reached when the two surfaces are completely separated (IV). Re-applying pressure causes the triboelectric charge's potential to continuously reduce (V), and the induced charge flows in the opposite direction through the external circuit until the two dielectrics re-contact and the current returns to zero. Such periodic AC signals will be continuously generated on condition that mechanical deformations are applied on a regular basis [14, 38].

Fig. 4

Working mode of TENGs. a Contact separation mode, b Contact sliding mode, c Freestanding mode and d Single electrode mode of AC-TENG. e Air breakdown mode of DC-TENG

The output current mechanism of TENG fundamentally relies on Maxwell’s displacement current, which is distinct from the traditional electric current generated by the movement of charges. Maxwell's displacement current is defined as the partial derivative of the electrical displacement flux with respect to time. Mathematically, it can be expressed as:

$$J_{\mathrm D}=\frac{\partial_D}{\partial_t}=\varepsilon_0\frac{\partial_E}{\partial_t}+\frac{\partial P}{\partial_t}$$

(1)

where D represents the displacement field; ε₀ is the permittivity in vacuum; E is the electric field; and P denotes the polarization field.

The corresponding displacement current density within a TENG could be illustrated as:

$$J_{\mathrm D}=\frac{\partial_Z}{\partial_{\mathrm t}}=\frac{\partial\sigma_I\left(\mathrm z,\;\mathrm t\right)}{\partial_t}$$

(2)

where σ_I(z,t) represents the charge density accumulated within the electrode.

The parallel-plate capacitor model is applicable to TENGs featuring planar configurations. This model operates based on two fundamental assumptions: firstly, that charges distribute uniformly across the dielectric surface, and secondly, that only a perpendicular electric field component exists within the dielectric, disregarding any parallel components. The V–Q–x relationship, pivotal to the parallel-plate capacitor model, elucidates the correlation between the output voltage (V), the transferred charge (Q) between the two electrodes, and the separation distance (x). This relationship can be expressed as:

$$V=-\frac1{C\left(x\right)}Q+V_{OC}\left(x\right)$$

(3)

Here, V denotes the potential difference between the electrodes, V_oc(x) represents the potential difference contributed by polarized charges, and − Q/C(x) signifies the potential difference contributed by transferred charges.

In addressing non-planar TENG configurations, the Distance-Dependent Electric Field (DDEF) model is introduced. Unlike the parallel-plate capacitor model, the DDEF model considers spatial fluctuations in the electric field, rendering it suitable for simulating TENGs with restricted dimensions. This model computes the electric field produced by charged surfaces by integrating charged microelements along these surfaces, resulting in a distance-dependent electric field. Consequently, the DDEF model retains theoretical applicability even for non-planar surfaces, offering enhanced flexibility in the design and advancement of TENG devices.

In general, the DDEF model serves as a versatile simulation approach applicable to both planar and non-planar TENG devices. For planar configurations, comprising a rectangular plate with dimensions (L × W) and a surface charge density (σ), the electric field at the midpoint along the z axis can be determined using Gauss's law, expressed as:

$${E_{\text{z}}} = \smallint {\text{d}}{E_Z} = \frac{\sigma }{\pi \varepsilon }{\text{arctan}}\left( {\frac{{{L \mathord{\left/ {\vphantom {L W}} \right. \kern-0pt} W}}}{{2\left( {{{\text{z}} \mathord{\left/ {\vphantom {{\text{z}} W}} \right. \kern-0pt} W}} \right)\sqrt {4{{\left( {{{\text{z}} \mathord{\left/ {\vphantom {{\text{z}} W}} \right. \kern-0pt} W}} \right)}^2} + {{\left( {{L \mathord{\left/ {\vphantom {L W}} \right. \kern-0pt} W}} \right)}^2} + 1} }}} \right) = \frac{\sigma }{\pi \varepsilon }{\text{f}}\left( {\text{z}} \right)$$

(4)

where z represents the distance to the surface and ε denotes the permittivity of the medium. Magnitude of E_z decreases as z increase.

For non-planar surfaces, the DDEF can also be computed utilizing a similar methodology. Consideration of an arc curved surface with a diameter (w) and a length (L) serves as a typical example. The overall electric field at the midpoint of the convex surface along the z axis is represented as:

$${E_{{\text{z}},\quad{{\text{convex}}}}} = \frac{\sigma L}{{\pi {\varepsilon_0}}}\int\limits_0^{{{\text{w}} \mathord{\left/ {\vphantom {{\text{w}} 2}} \right. \kern-0pt} 2}} {\frac{{\left( {{\text{z}} + \frac{{\text{w}}}{2} - \sqrt {\frac{{{{\text{w}}^2}}}{4} - {{\text{x}}^2}} } \right)}}{{R_1^2\sqrt {4R_1^2 + {L^2}} }}} {\text{dx}}$$

(5)

where

$$R_1^2 = {{\text{x}}^2} + {\left( {z + {w \mathord{\left/ {\vphantom {w {2 + \sqrt {{{w^2} \mathord{\left/ {\vphantom {{w^2} {4 - {x^2}}}} \right. \kern-0pt} {4 - {x^2}}}} }}} \right. \kern-0pt} {2 - \sqrt {{{w^2} \mathord{\left/ {\vphantom {{w^2} {4 - {x^2}}}} \right. \kern-0pt} {4 - {x^2}}}} }}} \right)^2}$$

(6)

Similarly, the overall electric field at the midpoint of the concave surface along the z axis is expressed as:

$${E_{{\text{z}},\quad{{\text{concave}}}}} = \frac{\sigma L}{{\pi {\varepsilon_0}}}\int\limits_0^{{{\text{w}} \mathord{\left/ {\vphantom {{\text{w}} 2}} \right. \kern-0pt} 2}} {\frac{{\left( {{\text{z}} - \frac{{\text{w}}}{2} + \sqrt {\frac{{{{\text{w}}^2}}}{4} - {{\text{x}}^2}} } \right)}}{{R_2^2\sqrt {4R_2^2 + {L^2}} }}} {\text{dx}}$$

(7)

where,

$$R_2^2 = {{\text{x}}^2} + {\left( {z - {w \mathord{\left/ {\vphantom {w {2 + \sqrt {{{w^2} \mathord{\left/ {\vphantom {{w^2} {4 - {x^2}}}} \right. \kern-0pt} {4 - {x^2}}}} }}} \right. \kern-0pt} {2 + \sqrt {{{w^2} \mathord{\left/ {\vphantom {{w^2} {4 - {x^2}}}} \right. \kern-0pt} {4 - {x^2}}}} }}} \right)^2}$$

(8)

In summary, when a TENG device exhibits a planar configuration and the dielectric thickness is significantly smaller than its planar size, the parallel-plate capacitor model adequately describes its electrical behavior. However, in cases where the TENG device features a non-planar configuration, the applicability of the parallel-plate capacitor model diminishes. Instead, the DDEF model is introduced for non-planar TENGs, offering increased accuracy through the incorporation of spatial electric field variations [39, 40].

3 TENGs applications in Eco-smart cities

3.1 Eco-smart home

Smart homes are becoming increasingly crucial in enhancing people's quality of life and health as information technology and artificial intelligence technology permeate every area of our existence. Power supply and sensing equipments are used as two essential components in the construction of smart home systems. TENGs can be made to harness the significant amount of mechanical energy found in a normal home and utilize it to provide a sustainable power source for a variety of home appliances. So far, several sensing technologies based on diverse detecting mechanisms, such as heat sensitivity, optics, resistance, and capacitance, have been widely used in smart homes. However, traditional sensors are not immune to certain drawbacks, such as limited battery life, high cost, and environmental contamination [41]. Triboelectric sensing technology with its low cost, long-term stability and high-sensitivity may therefore hold the promise to lower the threshold of smart home popularization.

3.1.1 Outdoor monitoring and energy harvesting

Rainy days are a common occurrence in the weather. The TENG can be installed on sizable rooftops to collect raindrop energy as a source of household energy. Meanwhile, the TENG's electrical impulses can be employed as sensor signals for weather monitoring. Cao et al. [42] built a self-driven window closing system based on liquid–solid TENG (LS-TENG) using fluorinated ethylene propylene (FEP) film (Fig. 5a). The MCU recognized the electrical signal generated by the raindrop touching the LS-TENG and sent a Bluetooth command signal to control the working motor driving the doors and windows to close. Furthermore, the entire system also had a self-driven voice reminder function, reminding those present to gather their garments when it rains, etc. The convenience this automatic window closing device will improve our daily lives. Also, Oh et al. [43] described a ideal smart roof that had a self-cleaning feature and could generate green energy from wind and rain in overcast conditions (Fig. 5b). YbNTiO₂@C, ytterbium and nitrogen co-doped mesoporous titania-carbon, could decompose dirt molecules, with a self-cleaning decomposition rate of up to 96.40% of methylene blue within one minute under sunlight. It is anticipated that this stable, self-cleaning green roof would eventually serve as an energy collector for smart homes.

Fig. 5

Recent developed devices based on TENG for eco-smart home. a Rain droplet sensor based on a liquid–solid triboelectric nanogenerator [42]. Copyright 2022, Elsevier. b Self-cleanable smart roof operated with heavy rain and strong wind [43]. Copyright 2021, Elsevier. c Intelligent electronic passworded locker (IEPL) [20]. Copyright 2023, Springer. d Smart table security systems for data/material theft protection [19]. Copyright 2022, Wiley. e Fire alarm and escape route guidance [44]. Copyright 2022, Elsevier. f Smart home combining IS-fabrics and IoT techniques [28]. Copyright 2022, American Chemical Society. g Wood-based triboelectric self-powered sensor (WTSS) for controlling smart home appliances [41]. Copyright 2022, American Chemical Society. h Artificial Intelligence of Toilet (AI-Toilet) using triboelectric pressure sensors and image sensor for integrated health monitoring system (IHMS) [45]. Copyright 2021, Elsevier. i Yoga instruction in a digital-twin smart home [46]. Copyright 2023, Wiley. j Energy-harvesting wooden floor [47]. Copyright 2021, Elsevier. k Self-heating TENG floor and thermal images of the surface [48]. Copyright 2022, The Royal Society of Chemistry

3.1.2 Home security

The electronic passworded locker is a common security tool used to safeguard people's belongings and personal space. Wang et al. [20] presented an intelligent electronic passworded locker (IEPL) based on distinct and tailored security obstacles, with the aid of a single electrode mode TENG composed of Kapton and PVC (Fig. 5c), which could precisely extract and identify users' password-entry behaviors and habits using integrated deep learning. This dependable and unrepeatable IEPL offers a solid remedy for raising the level of security at home in general. For more thorough home protection, Park et al. [19] suggested a double-side-contact- based TENG (DSC-TENG) that was made from commercially available nylon-coated fabric for microstructures, human skin for positive contact, MXene/silicone nanocomposite for charge generation. Fabricated DSC-TENGs demonstrated their usefulness in controlling an electric fan and light bulb wirelessly in a smart house. Additionally, a smart table security system was proposed based on DSC-TENG that used an Arduino board to sound an alert and turn on an LED and buzzer when an intruder or assailant touched the smart table (Fig. 5d). These examples point to a new direction for self-powered systems in door anti-theft protection and smart home security. Fire is one of the most frequent hazards that endanger the personal safety and property inside a building. Wang et al. [44] reported a robust and flame-resistant wood-based triboelectric nanogenerator (FW-TENG). Using PTFE as the free-moving object, negative triboelectric charges were produced on the PTFE film when it came into contact with the flame-resistant wood film, while positive ones were produced on the wood film. The FW-TENG embedded on the floor was successfully used to drive the indicator lights, which would be helpful in displaying the escape direction for people under fire scenarios, by harnessing the energy from human walking and running (Fig. 5e). This work broadens the usage of self-powered systems to the construction of safe and intelligent house.

3.1.3 Household sensor

For a more comfortable home environment, Ling et al. [28] manufactured a sheath-core triboelectric nanogenerator (SC-TENG) yarn with a sheath layer composed of many electrospinnable materials such as PVDF, PAN, and PU. The SC-TENG was woven into the IntelliSense fabric (IS-fabric) exhibiting a certain ability to sense and discriminate the instantaneous mechanical impulses produced by different materials. An intelligent carpet was created by combing the IS-fabric with the Internal-of-Things (IoT) techniques which is widely desired in intelligent homes. The triboelectric signal acquisition terminal and the infrared emission module were also wirelessly linked. The resulting system allowed for remote control of various appliances, i.e., zapping the television channel and controlling the sound volume (Fig. 5f). Given the widespread usage of wood materials in home construction, the flexible wood-based triboelectric self-powered sensor (WTSS) can be highly integrated with wooden household facilities. Wang et al. [41] installed WTSS to the furniture's surface, and the charge was transferred at the contact interface between the PTFE and the wood layer film during the approaching process. The WTSS allowed for remote control of home appliances by collecting mechanical energy from human activity (Fig. 5g). To increase home security by detecting personal features, the WTSS could also be installed on a wooden door as a self-powered access control system. Additionally, the WTSS array could be inserted into a wooden floor to monitor gait and home safety. This research broadens the application area of self-powered wood-based electronics in smart homes.

Smart toilet can provide a practical platform for the long-term analysis of person’s health. Lee et al. [45] demonstrated an artificial intelligence of toilet (AI-toilet) based on a triboelectric pressure sensor array (Fig. 5h). The textile-based TENG (T-TENG) sensor contained four functional layers: a nitrile thin film, a silicone rubber film, and two conductive textiles for charge collection attached to the back of the two contact electrification layers. The device was incorporated a camera sensor to analyze the simulated urine by comparing it with urine chart and classify the types and quantities of items using deep learning. All information, including two-factor user identification and the duration of sitting using pressure sensor array, and data from the urinalysis and stool analysis were all automatically transferred to a cloud system and then displayed in the users' mobile devices for better tracking of their health status. As a result, a smart toilet can offer valuable clinical information in a smart home.

Virtual reality (VR), augmented reality (AR), and physical reality are all combined in the idea of the metaverse. Incorporating digital twin in metaverse would hasten the development of smart houses in the era of the Internet of Things (IoT). Lee et al. [46] created a reliable triboelectric mat monitoring device called the information-mat (InfoMat) that was crammed with sensory data. This mat was a single-electrode TENG made of PVC as the supporting layer and PET as the positive triboelectric layer. The real home setting was projected into the digital realm and visualized in virtual reality to create a digital-twin smart home. Time-domain and multi-modality deep learning (DL) analyses were used to examine interactive signals that were extracted from the InfoMat. Two digital-twin smart home enabled applications were developed, including the accumulated and distinct skipping counting of two users concurrently, as well as the tailored two-way interaction (VR space-real space) yoga guidance (Fig. 5i). The digital-twin smart home foresees the bright future of the metaverse toward broader and more immersive global connectivity, including online commerce, remote meetings, and remote education, et al.

3.1.4 Indoor electricity and heat production

A significant issue for smart houses is achieving a full system of self-supplied electrical and heat energy. Panzarasa et al. [47] used zeolitic imidazolate framework-8 (ZIF-8) particles and PDMS to surface modify the wood, increasing the wood's ability to lose (ZIF-8@wood) or gain (PDMS@wood) electrons in periodic "contact-separation" to generate electricity. The functionalized wood TENG (FW-TENG) could turn on household lights when a person walked on a wooden board (Fig. 5j). And the FW TENG could power smart electrochromic windows to control sunlight transmission and reduce lighting energy consumption. In addition to being able to provide power instantly, the FW-TENG also functioned as a power source by storing the generated electricity in a capacitor. Renewable wood could help to design the next generation of sustainable power supplies for smart buildings. During the winter months, the floor and furniture can be automatically heated to make smart homes self-sufficient. To achieve such a system, Guo et al. [48] developed a self-heating wooden floor with self-heating and energy storage capabilities. Wooden boards served as the packaging material, and FEP and Kapton films were employed as negative and positive friction electrodes respectively. As a result of integrating TENG flooring with Joule heating components to create self-heating flooring and furnishing, a completely smart home may produce not only heat but also use less carbon-based energy (Fig. 5k). The self-heating TENG makes it feasible to achieve full intelligence in the future construction sector.

Table 1 summarizes the previous two years' worth of study on the usage of TENGs as a smart home energy and sensing technology. Currently, TENGs are engaged in home security, home appliance control, energy supply and intelligent sensing systems. TENGs' applicability in home video, lighting control, and central control management systems can then be expanded. Importantly, the sensor's precision is vital for smart home, due to the accuracy can most naturally reflect the sensor's functionality and user experience. Home scene is more individualized, fragmented, and random, therefore it places great demands on the sensor's accuracy in order to adapt to varied home situations. Secondly, a significant number of sensors with varied functions that are dispersed around the house supposed to be integrated into a home network, which presents a challenge for multiple device compatibility. Whole-house intelligence is an unavoidable trend in smart home development. The ecological platform for smart homes will be gradually unified with the underlying system to enable numerous device interconnection and create whole-house intelligence.

Table 1 Recent developed devices based on TENG for smart home

3.2 Human–machine interaction

Human–machine interaction (HMI) plays a crucial role in the development of eco-smart cities by enabling seamless interaction between humans and machines, thanks to the integration of advanced technologies such as artificial intelligence, Internet of Things, and automation systems. This integration facilitates efficient management and control of various urban systems, allowing for the collection and analysis of vast amounts of data from sensors, devices, and infrastructure in eco-smart cities. In the domain of energy management, for example, HMI enables residents to access real-time energy consumption data and adjust their usage patterns accordingly. Smart grids, supported by HMI, can efficiently monitor and manage energy distribution, minimizing wastage and promoting renewable energy integration. Furthermore, HMI enhances the quality of life in eco-smart cities by providing citizens with personalized services and information through interactive platforms and applications. This empowers individuals to make informed choices and actively participate in sustainable practices. As science and technology evolve, HMI continues to be enriched and upgraded, resulting in subversive innovation in the meta-universe era. Interaction modalities, scenarios, and experiences are elevated to new heights, with a tendency from isolation to integration observed in current interaction modes. Touch, voice, gesture, and facial recognition are extensively explored and fused to adapt to the needs of varied scenarios, further enhancing the role of HMI in shaping the future of eco-smart cities.

3.2.1 Facial senses

Facial senses can transmit HMI signals through the eyes, facial expressions, speech, and even breathing. Hu et al. [54] prepared a multifunctional triple-network conducting hydrogel, showing excellent flexibility, elasticity and stretchable mechanical properties. When the MPP-hydrogel was attached to the human body, it could be used to sense tiny and low-frequency human motion, such as identifying facial expressions (Fig. 6a). This work demonstrates a promising flexible electronic skin for HMI applications. Mu et al. [55] grew triboelectric film on the Cu electrode and attached it to an insulated double eyelid strap. The normal blink rate was about 15 times per minute. After a long period of driving, a driver's eyelids progressively entered the fatigue zone and blinked at an exceptionally fast or slow pace Real-time monitoring of the output waveform's flicker frequency allows for the evaluation of the driver's level of exhaustion, which has a broad application prospect in the field of self-powered fatigue driving (Fig. 6b). Mao et al. [56] described a breathing-driven TENG as an HMI sensor to assist intensive care unit (ICU) or plant human patients in achieving linguistic expressiveness (Fig. 6c). Chen et al. [57] developed a waterproof acoustic sensor (WAS) as a wearable translation interface to communicate with machines. WAS has a significant broadband response range of 0.1–20 kHz, which covers almost the entire human audible range. It has been demonstrated to be employed in the high-fidelity auditory platform, speech verification, and wireless control of smart cars (Fig. 6d).

Fig. 6

Recent developed devices based on TENG for human-interface interaction. a Facial expression recognition [54]. Copyright 2022, Wiley. b Eye recognition [55]. Copyright 2022, Elsevier. c Breathe recognition [56]. Copyright 2022, Springer. d Speech recognition [57]. Copyright 2022, Wiley. e Tapping recognition [58]. Copyright 2021, Springer. f Handwriting recognition [59]. Copyright 2022, Springer. g Tactile recognition [29]. Copyright 2023, Elsevier. h Tactile recognition [30]. Copyright 2022, Science. i Gesture recognition [60]. Copyright 2022, Wiley. j Texture recognition [61]. Copyright 2022, Elsevier. k Physical exercise recognition [62]. Copyright 2022, Elsevier. l Foot motion recognition [63]. Copyright 2022, American Chemical Society

3.2.2 Hand recognition

Most existing wearable HMI devices use low frequency (1–10 Hz) touch or hand movements (e.g., tapping, bending, and quivering) to transmit simple commands to the machine. Wang et al. [58] built a self-powered wearable keyboard by integrating a large-area F-TENG sensor arrays, which could track and record electrophysiological signals, as well as recognize individual typing characteristics through Haar wavelet (Fig. 6e). Such functions have broad application prospects in information encryption and network security. Lee et al. [59] prepared a biodegradable conductive carboxymethyl chitosan-silk fibroin (CSF) film for a wearable electronic human–machine interface for calligraphy practice and correction (Fig. 6f). The CSF-TENG HMI is designed to track writing steps in time and obtain the accuracy of letters, which is conducive to data processing and analysis of HMI systems. Tactile sensation plays an important role in virtual reality and augmented reality system. Wang et al. [29] manufactured an integrated device composed of two different working modes of laser-induced graphene (LIG) based TENGs, simultaneously achieving its accurate wireless control and sensitive tactile mode recognition (Fig. 6g). Shi et al. [30] developed a virtual electrical tactile device based on TENG and suspended electrode array, which realized a self-actuated, skin-integrated, safe and painless virtual electrical tactile (ET) system. This TENG-based ET system can work in many areas, including virtual tactile displays, intelligent protective clothing, braille instruction and even nervous stimulation (Fig. 6h). Yuan et al. [60] proposed a novel micro-pyramid-patterned double-network ionic organohydrogels that could detect very slight human physiological motions. The sensor possessed outstanding performance and stability in extreme environments (− 20 ~ 60℃, 90% RH and − 0.1 MPa). As a flexible switch, it could control the electrical equipment and manipulator by monitoring human finger gestures (Fig. 6i). Sensing geometric features and material properties simultaneously using a single tactile sensing mechanism remains a challenge due to the bottleneck of signal decoupling. Ding et al. [61] deployed a MTSensing system on a robotic hand that could real-time decoup signals into macro/micro features through signal processing pipeline, realizing material and texture recognition of common objects with an accuracy of 99.07% and 99.32%, respectively (Fig. 6j).

3.2.3 Exercise recognition

Electronic textiles have evolved into a multifunctional flexible electronic platform. Qin et al. [62] created a waterproof breathable fabric TENG (CSYF TENG) based on nano/micro core-sheathing yarn for an energy-harvesting element, self-powered humidity sensing system. They could also be exploited as self-powered pressure sensors to detect tiny forces and as flexible fabric-based keyboards for human–machine interfaces (Fig. 6k). Chen et al. [63] proposed an in-shoe sensor pad (ISSP) system attached to the lining of the vamp. The system could realize almost all known functions of traditional smart shoes, including step counting and human–machine interaction (Fig. 6l). More importantly, the ISSP could sense and recognize the static and athletic comfort of the shoe, providing a reference for the athlete's training and personalized design of the shoe.

Table 2 summarizes the main advances of TENGs in the HMI field over the last two years. HMI aims to obtain a secure, efficient and pleasant experience from human and machine interaction. When the devices can be perceived and communicated, they become a systematic whole rather than isolated devices. The emergence of intelligent ecosystems necessitates consideration, definition and optimization of the symbiotic circumstances between humans and technologies such as TENG sensor. Secondly, as a cognitive subject, it is an extremely challenge for the machine to comprehend human's natural interaction behaviors and intentions, as well as to perform accurate feedback. At present, natural perception technologies such as TENG still have a lot of room for improvement in terms of accuracy and real-time performance. Finally, physiological and psychological changes in persons will influence the status of interaction at any time. The core of HMI design is gradually evolving towards the direction of humanization and scenization, which propels human civilization forward.

Table 2 Recent developed devices based on TENG for human-interface interaction

3.3 Eco-smart healthcare

Medical issues such as low efficiency in the medical system, poor quality medical services, and difficulty accessing affordable medical treatment have become significant societal concerns. To address these challenges, renewable energy technologies like TENG offer promising solutions. TENG can harness small amounts of energy generated by patient movements or physiological activities in medical environments to power sensors and monitoring devices. This reduces reliance on traditional batteries, lowers the frequency of equipment battery replacements, and mitigates environmental pollution from discarded batteries. Additionally, integrating smart medical devices with TENG technology enables real-time monitoring and data collection of patients' physiological parameters, empowering healthcare professionals to promptly diagnose conditions and implement effective treatment strategies. These advancements not only improve medical efficiency and reduce resource wastage but also foster the sustainable development of medical services. The progression of smart medicine holds the potential to alleviate the issues of fragmentation and information isolation present in traditional medical systems. Leveraging advanced technologies such as the Internet of Things, blockchain, and big data facilitates seamless interaction between patients, medical staff, medical institutions, and equipment, thereby significantly enhancing overall medical efficiency and patient outcomes.

3.3.1 Health monitoring

TENGs can monitor body posture, ECG, heart rate, sleep and other health signs, which enables early detection of disease risk, from "disease treatment" to "disease prevention". Li et al. [70] manufactured a non-invasive, pressure-sensitive and comfortable smart pillow based on a flexible and breathable TENG (FB-TENG) sensor array. This provides a viable sensing device for sleep monitoring (Fig. 7a), and can be extended to real-time monitoring of certain diseases in the future, such as brain diseases and cervical spondylosis. Cao et al. [71] produced a triboelectric patch using common skin-friendly materials PTFE and nylon fabrics. This patch can not only provide power for potential wearable applications, but it can also be integrated to to monitor eye movements during sleep (Fig. 7b). Zhang et al. [72] proposed a TENG-based wearable, skin-friendly neck motion sensor that supports deep learning (DL). The DL model based on the convolutional neural network could identify 11 types of neck movements, including 8 bending directions, 2 twisting directions and 1 stationary state (Fig. 7c), with an average recognition accuracy of 92.63%. The developed neck motion detector has broad application prospects in neck monitoring, control and rehabilitation. Lin et al. [73] developed a new generation of self-powered flexible wear-resistant sensor based on solid–liquid TENG. The triboelectric (TE) layers were composed of highly elastic liquid metal and unique hydrophobic biomimetic shark skin-like microstructure. This sensor provides an on-demand user-friendly real-time gait monitoring system that is critical to the diagnosis and rehabilitation of neuromuscular diseases (Fig. 7d). Yu et al. [74] constructed a self-switched TENG (SS-TENG) based on breathing motion. The device could provide steady electrical energy through smooth breathing movements to deliver preventive medicine with a transdermal system. The SS-TENG switched to pulse mode for intense and rapid breathing movements, and the output could be enhanced by orders of magnitude, thus endowing it high sensitivity and resolution for emergency alerts of acute asthma attacks (Fig. 7e). Alavi et al. [75] created a new generation of self-aware interbody fusion implants. The implant was self-sensing, self-powered and mechanically tunable, utilizing an internally generated voltage signal to diagnose the bone healing process (Fig. 7f).

Fig. 7

Recent developed devices based on TENG for smart healthcare. a Smart Pillow [70]. Copyright 2022, American Chemical Society. b Eye Movement Monitoring [71]. Copyright 2022, American Chemical Society. c Neck motion detection [72]. Copyright 2022, American Chemical Society. d Gait analysis and rehabilitation monitoring [73]. Copyright 2022, Elsevier. e Alarm of asthma [74]. Copyright 2023, Elsevier. f Bone healing detection [75]. Copyright 2022, Wiley. g Waist rehabilitation [76]. Copyright 2022, Wiley. h Shoulder rehabilitation [77]. Copyright 2022, Elsevier. i Intelligent plaster [78]. Copyright 2023, Elsevier. j Electroacupuncture treatment for spinal cord injury [79]. Copyright 2022, Elsevier. k Sciatic nerve restoration [80]. Copyright 2022, Wiley. l Wound sealing and healing [81]. Copyright 2023, Wiley. m Bone repairment [82]. Copyright 2022, Wiley. n Integrated cartilage therapy [83]. Copyright 2023, Elsevier. o Drug delivery system [84]. Copyright 2022, American Chemical Society. p Subcutaneous Antibacterial [85]. Copyright 2023, Wiley. q Cardiac pacemaker system [86]. Copyright 2021, Nature. r Powering transient electronics [87]. Copyright 2022, Science. s Cardiopulmonary resuscitation training [31]. Copyright 2022, Elsevier. t Medical nursing HMI system [88]. Copyright 2022, Elsevier

3.3.2 Rehabilitation

Rehabilitation treatment of patients with diseases such as shoulders and waists usually adopt passive support and active mobile exoskeleton suits. In contrast, passive devices have numerous advantages, including portability, affordability, and no need for an external power source. Lee et al. [76] reported a textile-based triboelectric sensing system for waist rehabilitation. Four TENGs were sewn into a fabric belt to endow real-time robot operation and virtual games to enhance the immersion of waist training (Fig. 7g). Two TENGs were bulit in insole to record training sessions and can identify users and select rehabilitation plans for privacy. Park et al. [77] designed an origami based gravity support device integrated with TENGs to support shoulder rehabilitation. The author conducted a clinical pilot study on three stroke patients, which demonstrated the effectiveness of origami TENG on shoulder rehabilitation (Fig. 7h). Li et al. [78] developed a wearable liquid metal based TENG (LM-TENG), which exhibited excellent energy harvesting capacity, electrothermal performance and favorable mechanical strength. Based on this, an intelligent plaster device with anti-impact, thermophysiotherapy and motion detection performance is assembled (Fig. 7i), which is of great significance for the rehabilitation of human fracture.

3.3.3 Nervous and bone repair

Electroacupuncture, as a special electrical stimulation therapy, has been widely applied in biomedical field. It is noteworthy that the electrical signal of triboelectric nanogenerator (TENG) can be directly used for electrical stimulation.Wu et al. [79] proposed a traditional Chinese medicine electroacupuncture therapy, which showed that TENG-driven electroacupuncture enhanced the survival of abdominal horn neurons and inhibited astrocyte activation at the injured site (Fig. 7j). Feng et al. [80] developed an implantable sciatic nerve stimulation system, in which electrical signals generated by the device can stimulate the injured sciatic nerve through the cannula electrode, effectively promoting sciatic nerve regeneration (Fig. 7k). Kim et al. [81] designed a bioadhesive TENG (BA-TENG) for immediate and stable wound sealing as a first aid, as well as ultrasound-driven accelerated wound healing (Fig. 7l). In vitro studies confirmed that these effects were attributable to cell migration and proliferation accelerated by electric fields, and BA-TENG could also be used for nerve stimulation and regeneration.

Elderly bone marrow mesenchymal stem cells have limited osteogenic potential and repair ability, so real-time bone repair is a challenging medical problem for elderly patients. Pan et al. [82] adopted triboelectric stimulation technology to promote bone defect repair and regeneration through mechanical induction of piezoelectric ceramics (Fig. 7m). To reduce pain and treatment costs for patients with cartilage defects, Liu et al. [83] integrated a tissue battery for smart cartilage therapy. The sensor based on the contact-separation TENG had a high sensitivity (52.5 V MPa⁻¹) over the pressure range of joint motion (0–1.8 MPa), enabling the tissue battery to detect the status of cartilage repair in real-time. In addition, the tissue battery could convert mechanical energy into electricity to stimulate chondrocyte proliferation in the scaffold (Fig. 7n), thus shortening the cartilage repair period.

3.3.4 Drug and treatment

Targeted drug delivery systems (DDS), as a precise targeted chemotherapy method, can deliver drugs to the site of action with minimal side effects. Li et al. [84] built an DDS for the in-situ treatment of hepatocellular carcinoma based on implantable TENG (iTENG) and red blood cell (RBC). The implanted self-powered DDS have remarkable therapeutic effect on hepatocellular carcinoma and are expected to be applied in clinical medicine (Fig. 7o). Microorganisms often cause surgical site infections that significantly increase morbidity and mortality. Kim et al. [85] proposed an implantable, biodegradable, and vibrant TENG (IBV-TENG). A current of ≈22 µA and a voltage of ≈4 V produced by IBV-TENG confirmed ≈99% inactivation of E. coli and ≈100% inactivation of Staphylococcus aureus under in vitro ultrasound (Fig. 7p). To reduce the need for high-risk repetition surgery, it is necessary to prolong the operation time of implantable devices in vivo. Kim et al. [86] continued to develop a high performance commercially coin battery-sized inertia-driven TENG (I-TENG) based on body motion and gravity. A self-charging cardiac pacemaker system was successfully constructed by using I-TENG for battery charging (Fig. 7q). Kim's group [87] also reported an ultrasound-mediated implantable transient TENG to generate electricity for transient electronic devices (Fig. 7r). Interestingly, Sun et al. [31] developed a wearable cardiopulmonary resuscitation (CPR) training system based on flexible and stretchable fibrous TENGs (FTENGs). It can assess the quality of chest compressions, including compressions rate and depth (Fig. 7s), and is expected to contribute medical assistance training widely. Jiang et al. [88] reported on a hydrogel-based TENG self-powered medical care HMI system attached to a patient's finger, which successfully transmitted distress call requirements such as "hungry", "help", and "thirsty" care messages by simply gently bending the finger during diagnosis (Fig. 7t). This work is expected to develop into an alternative strategy in the healthcare HMI field, and can be extended to areas such as brain-computer interface.

We've compiled a list of TENG's past two-year achievements in the field of smart medicine in Table 3. Our medical field is progressing towards the direction of big data-based smart medicine. For implantable devices, the biological safety still needs to be verified by long-term rigorous research, and host foreign body reaction can also impair the function of implants. Natural biological materials that minimize immunological rejection should be produced in the future. Furthermore, there are variations in the working mechanism between small animals and humans after all, and more clinically pertinent studies from small animals to humans are required. Indeed, research in materials science and micro and nano manufacturing technology should be strengthened to develop the TENG with its compact size and high energy density, which is the basis for ensuring its long-term stability as a power management unit. When employed as a self-powered sensor, TENG's inherent signal stability serves as the cornerstone for rendering accurate medical care.

Table 3 Recent developed devices based on TENG for smart healthcare

3.4 Eco-smart transportation

The rapid development of the Internet of Things and advanced material nanotechnology have set off a revolution in the intelligent traffic field. With the continuous improvement of people's requirements for safety and comfort, the key to the future development of intelligent transportation lies in the traffic driving safety, wireless signal transmission, self-driven energy supply and environmental protection of vehicle operation. There are a lot of idle mechanical energy in highway traffic and rail transit that TENG can capture. Smart driving and components of cars and trains necessitate higher sensitivity sensors, which TENGs can also fulfill. It is believed that TENG will thrive in the field of intelligent transportation and benefit mankind through the continuous crossover and integration of the two realms.

3.4.1 Traffic safety and environmental protection

The three-dimensional acceleration sensor (3D AS), which detects the collision position and force of the vehicle to safeguard passenger safety, has been demonstrated to be a component of the vehicle safety restraint system. Wang et al. [32] described a lightweight 3D AS built on a liquid metal triboelectric nanogenerator (LM-TENG). With a sensitivity of 800 mV/g and detection ranges of 0 ~ 50 m/s² in vertical direction and 0 ~ 100 m/s² in horizontal direction, the produced sensor has a potential use in vehicle restraint systems (Fig. 8a). The violation of traffic regulations on zebra crossing is easy to cause traffic accidents. Kim et al. [89] developed an intrusion detection system (IDS) as the application of paint based TENG (PBT) that successfully detected parking line violations (Fig. 8b), which is expected to apply in the next generation traffic system. As the main cause of accidents, overspeed on the road needs to be monitored to improve driving safety. Ding et al. [90] proposed a self-powered overspeed wake-up alarm system (SOWAS) based on TENGs. It was composed of an energy harvesting TENG (E-TENG), an energy management module (EMM), an overspeed sensing TENG (S-TENG), a power switch module (PSM) and a wireless transceiver module (WTM). The developed SOWAS could work sustainably in an unattended traffic environment, realizing intelligent monitoring of overspeed and alarm signal transmission (Fig. 8c). Structural health monitoring is very important to the safe operation of bridge, railway and other transportation facilities. Wu et al. [91] constructed a turnout monitoring system and a bridge health monitoring system based on a capsule-shaped TENG (CS-TENG). The CS-TENG could detect tiny changes in external forces and accurately and sensitively identify the state of traffic facilities, alerting employees to bridge and other road infrastructure concerns in a timely manner (Fig. 8d). This work illustrates a direction for the application of TENG in the field of health monitoring of transportation facilities.

Fig. 8

Recent developed devices based on TENG for smart transportation. a Vehicle restraint system [32]. Copyright 2021, Elsevier. b Traffic norms violation dectection [89]. Copyright 2021, Elsevier. c Overspeed wake-up alarm system [90]. Copyright 2023, Elsevier. d Health monitoring of traffic facilities [91]. Copyright 2022, American Chemical Society. e Lubricating oil condition monitoring [92]. Copyright 2021, American Chemical Society. f PM capture from vehicle exhaust [93]. Copyright 2022, Elsevier. g Flow sensing for automobile [94]. Copyright 2023, Elsevier. h Conditionally automated driving [95]. Copyright 2023, Wiley. i Driver’s steering action detection [96]. Copyright 2021, Elsevier. j Intelligent fatigue warning [97]. Copyright 2021, American Chemical Society. k Driver status monitoring [98]. Copyright 2023, Elsevier. l Smart traffic monitoring and management system [99]. Copyright 2022, Elsevier. m Wireless traffic monitoring system [100]. Copyright 2021, Elsevier. n Intelligent transportation system [101]. Copyright 2022, Wiley. o Freight train monitoring [102]. Copyright 2021, Elsevier. p Freight train synergistic maintenance [103]. Copyright 2023, Elsevier. q Propelling boats moving forward, micro aircrafts flying [104]. Copyright 2022, Elsevier

For vehicle fuselages, intelligent monitoring lubricants are crucial to the development of intelligent vehicles, as unexpected and fatal failures of key dynamic components in machines occur every day. Shi et al. [92] proposed a TENG sensor for monitoring lubricating oils via the contact electrification process of oil − solid contact (O − S TENG), and also realized real-time monitoring of formulated engine oil in real engine fuel tanks. This research provides a online strategy for intelligent diagnosis of engine lubricants (Fig. 8e). The tail gas emission of vehicle fuselage constitute the main cause of environmental pollution such as greenhouse effect. Yang et al. [93] developed a self-powered ceramic porous brick triboelectric filter (CPB-TEF) for efficient in situ collection of PM particles from automobile exhaust (Fig. 8f). After simple cleaning, the device could be further incorporated into the exhaust pipe for sustainable application. Automobile flow sensors are essential for improving fuel efficiency, ensuring the engine air–fuel ratio and reducing environmental pollution. Cheng et al. [94] proposed a shaftless turbine intake flow sensor (STIFS) for automotive applications consisting of a ball-bearing TENG (BB-TENG) and a magnetic field modulation type magnetic gear electromagnetic generator (MG-EMG). This discovery demonstrates an effective strategy for fully self-powered and real-time wireless monitoring of airflow, which further facilitates the use of triboelectric sensors in vehicles (Fig. 8g).

3.4.2 Intelligent driving

Autonomous driving is regarded as the future trend of transportation, which can reduce accidents and improve traffic efficiency. Wang et al. [95] proposed a real-time non-driving behavior recognition system (RNBRS) that integrated triboelectric sensors operating in single-electrode mode and a deep learning model. The RNBRS endows vehicles with conditional automation to dynamically adjust the takeover time budget based on current driver behavior, hence enhancing the safety and stability of the takeover (Fig. 8h). Early recognition of a driver's driving actions on the steering wheel by sensors is a complimentary approach to Intelligent Driver Assistance Systems (IDAS) that can assist drivers in making rapid and safe driving operational decisions. Sun et al. [96] studied the method of detecting driver's turning behavior by using TENGs as sensors, and designed and trained a machine learning algorithm to detect driver's turning action, offering a new avenue for IDAS (Fig. 8i). Identifying driver fatigue is crucial to avoiding traffic accidents. Cheng et al. [97] designed a TENGs based intelligent system consisting of a signal processing unit and a self-powered steering-wheel angle sensor (SSAS), which could provide real-time driver status monitoring and fatigue warning (Fig. 8j). Then, Chen et al. [98] developed a triboelectric sensor-based driver status monitoring system for biological signal monitoring. The sensors were attached to the driver's neck and seat belt and were used to monitor neck activity and breathing state to determine driver's fatigue and concentration levels. The proposed system is beneficial to improving road traffic safety and has potential application in intelligent transportation (Fig. 8k).

3.4.3 Wireless traffic system

Traffic intelligence and information is based on big data acquisition, which has to be realized by the perception network composed of traffic sensor nodes. Tang et al. [99] developed a self-powered triboelectric sensor (CN-STS) made of electrospun composite nanofibers for intelligent traffic monitoring and management. With the assistance of the Internet of Things platform, functions such as overlapping and speeding vehicle capture, traffic flow management and license plate recognition were realized through the CN-STS array with compensation circuits (Fig. 8l), opening up new potential for improving the safety and ease of road traffic. Luo et al. [100] proposed a wireless multivariable traffic monitoring system based on a fully automatic TENG, which could directly convert the energy generated by pedestrian trampling and electric motorcycle rolling into oscillating signals with encoded sensing information and wirelessly transmit them to a receiver through magnetic resonance coupling. The results show that the system can instantly identify passing pedestrians and illegal motorcycles, and monitor the flow of people and motorcycles on the sidewalk, as well as check the speed and direction of the motorcycles on non-motorized lanes (Fig. 8m). The TENG, as a viable distributed power energy harvesting device, needs to address difficulties with long-term wear and durability. Jiang et al. [101] constructed a double charge supplement TENG (DCS-TENG), which had the advantages of low wear and high output performance. It was used to build a self-powered intelligent transportation system that continuously powered the vehicle's radar and accelerometer to demonstrate the DCS TENG's significant electrical load capability (Fig. 8n).

3.4.4 Railway freight, ships and aircraft

Railway freight is an important way for land transportation and an important part of global economic development. The lack of electrical pipelines between cargo trains makes it difficult to supply power to the equipment that monitors the train cars. Peng et al. [102] designed a amultiple mode TENG (MM-TENG) fabricated from multiple layers of waveform contact separation components and multiple layers of floating sliding components, aiming to collect environmental mechanical energy in freight train car joints to power sensors that monitor train status (Fig. 8o). This in situ energy harvesting method can be used for self-powered monitoring systems in a variety of fields.The unstable operation of freight trains seriously affects the safety of freight transport. Zhang et al. [103] proposed a breakthrough energy self-consistent system (ESCS) based on rolling bearing triboelectric nanosensor (RB-TENS) and LSTM deep learning model. The detection module enhanced and extracted the characteristic electrical signals collected by RB-TENS, and then trained and learned the running characteristics of freight trains by combining the LSTM deep learning model, so as to effectively detect the oscillation instability of freight trains (Fig. 8p).

Electroaerodynamics (EAD) force, generated by the collision of moving ions with neutral molecules in air or other gases in a high intensity electric field, has been shown to be an alternative method with great propulsion potential without combustion emissions. By taking advantage of the TENG's natural advantages in high voltage output, Mai et al. [104] combined different forms of EAD thrusters and TENG with appropriate circuits to form a complete propulsion system. The propulsion system has been successfully used to propel ships forward and micro-aircraft to fly, showing great application potential in the field of water transport and aviation (Fig. 8q).

Table 4 lists the variations of TENG materials, working modes, contact modes and power densities in various smart transportation applications during the last two years. At present, the majority of TENG's intelligent transportation reports focus on terrestrial transportation, but further research on water and air transportation is needed. In addition, it remains challenge to commercialize autonomous driving technologies on a large scale. Significantly, contemporary research on intelligent transportation places more emphasis on "intelligent networking and collaborative optimization" to improve traffic safety and efficiency, with less consideration on ecology and environmental conservation. In the future, it is strived to promote the construction of "intelligent green, safe and efficient, autonomous and unmanned" transportation systems, and achieve the transportation vision of "zero death, zero emission, and carbon neutrality" through the development and integration of material technology, nanotechnology, and electronic technology.

Table 4 Recent developed devices based on TENG for smart transportation

3.5 Eco-smart agriculture

The latest "Global Food Crisis report" was recently released by the World Food Program and the Food and Agriculture Organization of the United Nations. According to the report's findings, the situation of global food crisis and severe food insecurity has intensified. The production and development mode of traditional agriculture cannot keep up with the advancement of modern civilization. Sensors with various functionalities are gradually being used in agricultural advances around the world with the aim of improving food yield and quality. Farmers can monitor their crops and make adjustments to the environment using the information provided by agricultural sensors, including plant growth information and physiological information detection sensors, soil smart sensors applied to soil temperature and humidity, pH, etc., meteorological smart sensors applied to environmental temperature and humidity, as well as oxygen concentration, carbon dioxide concentration and illuminance. TENG's battery-free and high-precision features make it a viable research direction in the realm of agricultural sensing.

3.5.1 Farm environmental monitoring

TENG is an effective strategy for collecting irregular and low-frequency mechanical energy, which can be well obtained from various sources such as vibration, airflow and rotation. Park et al. [105] reported a contactless mode triggering-based ultra-robust rotary hybridized nanogenerator (CMTUr-HNG) for efficient capture of airflow and water flow in natural environments. The hybrid electromagnetic-TENG exhibits high output performance in a wide range of rotating motion (50–1000 rpm), and has been successfully applied to water quality monitoring systems and self-powered wireless intelligent farm monitoring systems (Fig. 9a). Then, Du et al. [106] developed a pulsed TENG of corn husk composite film (CH-Pulsed TENG) for multi-channel wireless agricultural sensing system driven by wind energy. CH-Pulsed TENG can simultaneously collect and transmit four different signals, including humidity, temperature, soil moisture and light intensity, with a wireless transmission distance of up to 1.7 km (Fig. 9b). The system can also recycle waste crops, which has potential applications in developing smart and green agriculture. Real-time monitoring of wind direction and wind speed is of great significance to agricultural production and crop management in order to adjust agricultural production in time. Ying et al. [107] introduced a transparent and degradable hydroxyethyl cellulose film for the preparation of a slit effection-based TENG, which could accurately monitor wind direction and speed in the range of 0.5 to 10 m/s, providing a reliable basis for intelligent agriculture (Fig. 9c). Low efficiency when collecting low speed wind energy is an issue for the wind energy collection device. Jiang et al. [108] designed and manufactured an improved segmented structure of a soft contact rotating TENG (SCR-TENG) for collecting low-speed wind energy, and the initial wind speed of a 36-grid TENG was close to 1 m s⁻¹. On the basis of optimized SCR-TENG, a smart farm was built for insect trapping, night indication, ambient temperature/humidity monitoring and soil moisture detection (Fig. 9d). Cheng et al.¹⁰⁹ devised a wind energy acquisition and sensing device based on electromagnetic-triboelectric hybrid generator (ES-ETHG), which could accurately measure wind speed and wind level within 3 − 15 m/s, and accurately detect wind direction within 2 s. In addition, a self-powered distributed weather sensing system based on ES-ETHG is developed, which has reference value for the design and development of loT nodes in smart agriculture (Fig. 9e). In view of the limited average power density and durability of TENG, Gao et al.¹¹⁰ designed a soft-contact flower-bud array cotton-assisted TENG (SFC-TENG) with a dual-rotor–stator structure to collect wind energy. The assembled TENG devices achieved excellent output performance of 2500 V, 85 µA and 80 mW, and could run stably for 320,000 cycles. These remarkable characteristics confirm that the SFC-TENG is feasible to power smart agriculture in temperature and humidity monitoring, pH detection and night lighting (Fig. 9f).

Fig. 9

Recent developed devices based on TENG for smart agriculture. a A wireless smart-farm monitoring system [105]. Copyright 2021, Elsevier. b Agricultural sensing system with 1.7 km signal transmission [106]. Copyright 2022, Elsevier. c Wind vector monitoring in agricultural environment [107]. Copyright 2022, Elsevier. d Smart farm: night indication, mosquito trapping, etc [108]. Copyright 2022, Wiley. e Weather sensing system for smart agriculture [109]. Copyright 2021, American Chemical Society. f Real-time monitoring of crop growth environment [110]. Copyright 2022, Elsevier. g Temperature and humidity monitoring system [111]. Copyright 2022, Elsevier. h Intelligent greenhouse environmental management [112]. Copyright 2021, American Chemical Society. i Real-time monitoring of the health status of plants [113]. Copyright 2021, American Chemical Society. j A humidity sensor to monitor plant leaf humidity [114]. Copyright 2023, Elsevier. k Agricultural sensors and automatic irrigation system [115]. Copyright 2022, Wiley. l Automatic irrigation, water level warning, etc [116]. Copyright 2023, Elsevier. m Controllable pesticide release system [117]. Copyright 2022, Elsevier. n Electrical stimulation system for enhancing crop production [33]. Copyright 2022, Nature

In addition to mechanical and wind energy, solar energy is one of the most widely distributed environmental energy sources on open farms. Zheng et al. [111] designed a hybrid energy harvesting device (HEHD) for capturing wind and solar energy by combining two TENGs, two electromagnetic generators (EMG) and two solar cells (SC). Combining the self-developed mobile phone APP with HEHD, a self-powered temperature and humidity monitoring system and a wireless passive infrared security alarm system outside the farm were constructed (Fig. 9g). This discovery serves as a significant guide for the design of multi-functional energy collection equipment, and offers fresh insights for the development of TENG-based intelligent agriculture. Establishing a sustainable energy supply system is essential for the temperature regulation, ventilation, monitoring and other functions of intelligent greenhouse environment. Although many greenhouses already use a variety of photovoltaic devices to collect solar energy, they are not always suitable for mid-to-high latitudes or areas with sufficient rainfall, especially in marine or tropical climates where it rains year-round. Ping et al. [112] proposed a fluorinated superhydrophobic greenhouse film as a negative triboelectric layer material for constructing a TENG (RDE-TENG) that obtained raindrop energy. RDE-TENG can be employed as a reliable power source for temperature and humidity sensors in greenhouses (Fig. 9h), assisting to restore environmental parameters to a range conducive to plant growth, which is crucial for directing agricultural production.

3.5.2 Crop monitoring

It is necessary to accurately and timely monitoring of plant health. However, limited efforts have been made to develop breathable and self-attached TENG that have good compatibility with plant leaves and high sensitivity to subtle vibrations/stimuli. Lee et al. [113] obtained a thin, waterproof and breathable TENG (WB-TENG) by using a film of nanofiber-micron sphere composite structure, which could adhere firmly to the surface of plant leaves without affecting the physiological activities of plants such as respiration. The device can convert wind energy and raindrops into electricity in agricultural environments and realize self-powered soil moisture sensors, which is expected to establish a sustainable intelligent interface between plants and the environment (Fig. 9i). Wang et al. [114] proposed a living plant leaf-based TENG (LPL-TENG), which was composed of GO-PVSQ/PVA composite membranes and living leaves. LPL-TENG could respond linearly to the humidity of plant leaves with the sensitivity of roughly 3.0 V/% RH(Fig. 9j), which had little effect on plant growth. In addition, the LPL-TENG could also play the function of high wind speed alarm and perform remote control. This work contributes new perspectives for the advancement of smart agriculture.

3.5.3 Agricultural production assistance

From the perspective of long-term eco-friendliness and sustainability, renewable resources from agricultural environments are ideal for smart agriculture in the future. Ping et al. [115] produced a soft contact, low damping and agricultural debris based TENG (AD-TENG) (including waste plant fibers and PVC greenhouse film) to capture low frequency and low speed water flow energy. The system realizes the recycling of agricultural waste and can power agricultural sensors. Significantly, an automatic irrigation system has been built to speed up the automation of smart agriculture (Fig. 9k). Meanwhile, Jiang et al. [116] constructed a fur-brush TENG (FB-TENG) based on natural animal fur. Since soft fur has little friction and wear, FB-TENG shows strong robustness and durability. By collecting wind and water flow energy, FB-TENG enables a self-powered multi-functional management system for automatic irrigation, wireless water level warning and weather monitoring (Fig. 9l).

The misuse of fertilizers and pesticides also places a huge burden on the environment and human health. Xi et al. [117] developed an inverted pendulum-typed multilayer TENG (IPM-TENG) for harvesting water wave energy in weak wave environment. The optimized IPM-TENG could be utilized as a console for the "on-demand" release of pesticides, promoting the development of "precision agriculture" (Fig. 9m). In addition, there is a need to develop new yield enhancement techniques to help traditional agriculture reduce its dependence on pesticides while maximizing sustainable growth in agricultural yields. Wang et al. [33] created a self-powered electrical stimulation system (SESS) for enhancing agricultural production based on the all-weather TENG (AW-TENG) integrated by a raindrop-driven TENG (R-TENG) and a bearing and hair structured TENG (BH-TENG) (Fig. 9n). AW-TENG could be directly used to stimulate plant growth and significantly increase pea germination rate and yield by about 26.3% and 17.9% respectively. This work demonstrates an innovative concept of a self-powered electrical stimulation system for optimizing crop growth.

The differences in TENG materials, working modes, contact modes, and power densities in various applications of smart agriculture over the past two years are summarized in Table 5. Smart agriculture is a high-level agricultural production form with resource allocation optimization and early warning prediction. Because of the complexities of the agricultural production environment, TENG-based multiple sensor units have significant promise of meeting the demand of multi-objective monitoring. And, the supporting intelligent algorithm is the key to realize the intelligent agricultural sensor. More importantly, there is still room for improvement and optimization in the TENG sensor's accuracy, integration, and stress resistance. TENG based-animal and plant ontology sensors are basically blank, and animal and plant growth models have not been built. In the future, it is necessary to further promote the cross-integration of agronomy, nanomaterials, microelectronics, communication and other disciplines to promote the development of intelligent agriculture.

Table 5 Recent developed devices based on TENG for smart agriculture

3.6 Eco-smart industry

Traditional industrial production has traditionally relied on Machine-to-Machine (M2M) communication, facilitating interaction between equipment. However, the advent of the Industrial Internet of Things (IIoT) presents a paradigm shift, aiming to establish connectivity not only between machines but also between machines and humans, and all items with the network. This facilitates efficient identification, management, and control, ushering in the era of intelligent factories. The vision of intelligent factories is to combine automation and digitalization to create highly efficient, energy-saving, environmentally friendly, comfortable, and human-centric production environments. In the pursuit of energy conservation and emissions reduction within industrial settings, TENG technology offers promising solutions. TENG can be integrated into smart industrial equipment to harvest energy from vibrations, pressure, and other environmental sources. This harvested energy can power smart industrial equipment, reducing reliance on traditional power grids, decreasing energy consumption during production, lowering carbon emissions, and advancing energy conservation and emission reduction objectives. Furthermore, the integration of TENG technology enables intelligent monitoring and management of production processes, optimizing resource utilization efficiency, minimizing environmental pollution, and fostering the sustainable development of industrial production. Through this integration, smart factories can achieve greater efficiency, reduce environmental impact, and contribute to the advancement of sustainable industrial practices.

Chemical plants and workshops may leak or splash corrosive solutions, such as acid and alkali, or burn, which may seriously endanger the health and safety of operators. Wang et al. [118] demonstrated an intelligent chemical protection suit based on fabric TENG (F-TENG), which had good air permeability, chemical protection, detectability and intelligence (Fig. 10a). Equipped with a biokinetic energy capture and self-powered safety monitoring system, the smart kit features four features: acid and alkali resistance, self-supplied electrochemical leak detection, operator vital signs monitoring and real-time remote alarm. The intelligent protective clothing will benefit workers in laboratories, chemical plants, special workshops and emergency sites. Oily wastewater produced in various industrial processes seriously threatens the environment ecology and public health, and the separation of oil–water mixtures requires high energy consumption for its reuse. In the machinery industry, water in lubricating oil can lead to the failure and wear of machinery and equipment, which greatly accelerates its corrosion and wear. Wang et al. [119] designed an electrospun nylon and PVDF nanofibers-based TENG (ENTENG) with an asymmetric AC electric field for efficient W/O emulsion separation (Fig. 10b). Output voltages up to 2800 V were achieved by adding double polyimide(PI) films as the transition layer for charge storage, which could meet the requirements of various emulsion separations. During the operation of the TENG used for W/O separation, there was almost no breakdown due to the high voltage and low current safety characteristics of the TENG. This study clearly demonstrates the enormous potential for the treatment of oily wastewater in actual industries. Conventional wind power technology relies on turbines and the principle of electromagnetic induction. Chen et al. [34] developed a turbine vent TENG (TV-TENG) that could directly convert irregular wind into electrical energy. The turbine vent had the advantages of both waterproofing and ventilation, and it could also work in harsh rainy days. Moreover, a self-powered on-site industrial monitoring system was integrated to improve the simplicity and ease of temperature monitoring and safety warning in the industrial environment (Fig. 10c).

Fig. 10

Recent developed devices based on TENG for smart industry. a Smart anti-chemical protective suits [118]. Copyright 2021, Wiley. b Water/oil emulsion separation [119]. Copyright 2021, Elsevier. c On-Site Industrial Monitoring System [34]. Copyright 2021, American Chemical Society. d Smart conveyor roller system [120]. Copyright 2022, Elsevier. e Wireless frequency monitoring system [121]. Copyright 2022, Elsevier. f Vibration sensor for conveyor belt [122]. Copyright 2022, Elsevier

The automation and digitalization of factory product transportation has attracted wide attention with the advent of Industry 4.0. Choi et al. [120] proposed an integrated IoTE conveyor system based on triboelectric sensor (TES) and electromagnetic generator (EMG) (Fig. 10d). A drum with TES could generate a frictional electrical signal, which contained information about the material, size, weight and moving speed of the product being delivered. Product information was transmitted wirelessly to the user's smartphone through Bluetooth module and displayed visually in real time. The proposed IoTE delivery system can operate completely independently without the need for additional power supply and is therefore expected to be a cornerstone for the development of net-zero smart factories. Xiao et al. [123] innovatively engineered a multi-dimensional tactile perception system using machine learning technology to facilitate smart grasping. Their system, which integrates eight arrays of triboelectric sensors onto a soft gripper, effectively grasps and identifies a variety of 18 objects with an impressive accuracy rate of 96.3%, leveraging force signals. This development represents a significant advancement in intelligent sorting within smart factory applications, addressing the need for enhanced efficiency and energy conservation in industrial processes. Vibration frequency monitoring is an important means for the safety detection and fault detection of mechanical equipment. Zhang et al. [121] reported a vibration-energy driven autonomous wireless frequency monitoring system (AWFMS) based on two TENGs (Fig. 10e), one for a broadband vibration energy harvesting element (P-TENG) and the other for an active vibration-frequency sensing (S-TENG). In the wide range of 6–20 Hz, the P-TENG's output voltage could be converted to a stable DC voltage of 2.5 V via a power management module and drives a microcontroller unit (MCU). The output signal of S-TENG was collected and processed by MCU for frequency calculation and wireless data transmission. The AWFMS had good frequency monitoring accuracy with a maximum relative standard deviation of 0.09 and stability at 36,000 cycles, demonstrating its potential of intelligent adaptive in industrial broadband vibration environments. Vibration sensors rely on mechanical vibrations of machinery to monitor the operational stability of machinery to prevent mechanical failure and reduce loss of life and property. Li et al. [122] proposed a self-powered mechanical vibration sensor based on the coupling of a TENG for active sensing and electromagnetic generator (EMG) for energy harvesting. The cooperation of TENG and EMG modules could realize the real-time monitoring of small mechanical vibration and the transmission of alarm signals (Fig. 10f). This multi-mechanism coupled self-powered vibration sensor can be applied in many scenarios, such as conveyor belts and freight elevators in smart factories.

Table 6 outlines the various of TENG materials, working modes, contact modes and power densities in different applications of smart industry during the last two years. The establishment of smart factories requires efficient use of energy, recycling and reuse of resources, as well as the integration of smart equipment and sensors. And priority should be given to robots instead of humans in risky and contaminated links. TENG's construction in the area of intelligent factory is currently in its initial stages. Attractively, TENGs are capable of capturing lost and omnipresent mechanical energy in the manufacturing. The self-powered, highly sensitive qualities of triboelectric sensors to mechanical disturbances also make them perfect for factories that require low energy consumption and great precision. In the future, TENG-based robotics and HMI are also anticipated to be applied to the smart factory sector.

Table 6 Recent developed devices based on TENG for smart industry

3.7 Eco-smart environmental protection

"Smart environmental protection" refers to the emergence and dissemination of "digital environmental protection". It fully utilizes the existing environmental information platform to embed sensors and equipment into a variety of environmental monitoring objects by utilizing the Internet + , big data, Internet of Things, sensor network, cloud computing, satellite remote sensing, virtual reality, and other new generation information technologies. The integration of the human social environment and the environmental business system enables more sophisticated and dynamic "intelligent" environmental management decisions.

3.7.1 Wastewater Purification

With the development of global economy, the widespread use of pesticides has led to serious pollution of pesticides in water environment. Nie et al. [125] adopted a spherical TENG (S-TENG) to establish self-powered photoelectric catalysis under pulsed DC electric field in order to improve the removal rate of pesticide Atrazine (ATZ) (Fig. 11a). The results showed that the frictional pulse direct current could increase the photogenic free radicals during the photocatalytic degradation process. This study provides a feasible method for improving the degradation of pollutants in wastewater. Organic dyes in industrial effluents are toxic, persistent and carcinogenic, affecting not only water quality but also the ecological balance by impeding sunlight penetration and reducing dissolved oxygen. Dong et al. [126] combined a self-powered TENG with a three-dimensional graphene oxide photocatalyst doped with carbon dots-TiO₂ sheets (3DGA@CDs-TNs) to construct a unique collaborative photoelectric hybrid catalytic system (Fig. 11b). This hybrid system could significantly improve the photocatalytic degradation efficiency of direct blue 5B (DB) and brilliant green (BG). It could effectively remove pollutants from wastewater without consuming any electrocatalysts. This work serves as an example for the creation of efficient, sustainable and ecologically friendly photocatalytic systems that harness mechanical and solar energy. Antibiotics are one of the most important drugs in medicine, and the release of heavily used antibiotics into the environment causes water pollution. Nie et al. [127] developed a self-powered electro-photocatalytic system driven by a crowned TENG (C-TENG) to improve the efficiency of photocatalytic degradation of antibiotics (Fig. 11c). The designed C-TENG not only collected water wave energy from multiple angles which wasted in the treatment of existing antibiotic wastewater, but also enhanced photocatalytic degradation of tetracycline through self-powered external electric field. This system realizes the simultaneous absorption and degradation of tetracycline, which has important guiding significance for future antibiotic degradation research. Electrocoagulation (EC) has become one of the most advanced water purification technologies. Kim et al. [128] proposed a self-powered EC system for a water-wheel hybrid generator (W-HG) based on a disk-type TENG (D-TENG) and an electromagnetic generator (EMG) (Fig. 11d). The D-TENG and EMG were combined with the rotating water wheel to demonstrate their ability to power the EC purification system. The power management integrated circuit (PMIC) was used with the D-TENG for the high current input of the EC module. Finally, W-HG showed an excellent removal rate of 95% algal wastewater within 18 h. This work is expected to facilitate subsequent research into the fusion of electrical engineering and desalination. Every year, countries around the world produce about 350 million metric tons of plastic waste. Cho et al. [129] developed a high-performance TENG-driven self-powered electrophoresis system based on three-dimensional porous pyramid polydimethylsiloxane to remove micro/nano particles composed of various materials, such as plastics, heavy metal composites, metal oxides, and ceramics (Fig. 11e). Since TENG typically produces very high voltages, it is suitable for creating a strong electric field that can be used for electrophoresis without complex circuits, and is relatively safe for aquatic organisms due to its low output current. This electrophoretic removal system can be applied to personal water purifiers and water treatment plants.

Fig. 11

Recent developed devices based on TENG for smart environmental protection. a Mitigating refractory organic pollutants in wastewater [125]. Copyright 2022, Elsevier. b Wastewater Purification System [126]. Copyright 2021, Springer. c Antibiotic degradation [127]. Copyright 2022, Elsevier. d Purification of dye wastewater and microalgae wastewater [128]. Copyright 2022, Elsevier. e Removing toxic micro/nano particles in water [129]. Copyright 2022, Elsevier. f Self-priming air filtration system [35]. Copyright 2023, Elsevier. g Air purification and quality monitoring [26]. Copyright 2023, Wiley

3.7.2 Air purification

The severity of air pollution is rising, which has a significant influence on human activity and survival. One of the major air pollutants that endanger human life and health is dust particle matter (PM). Long et al. [35] introduced a triboelectric air filtration system (TAFS) based on TENGs and considerably increased the effectiveness of filtration for low efficiency and low resistance particle matter (Fig. 11f). With a 99% removal rate, PM_2.5 was removed in a restricted environment loaded with cigarettes for 30 min using a self-priming air filtration system made up of an air pump and TENG. For the purpose of removing particle pollution from outside dusty construction sites, dust factories, dusty or hazy weather, self-powered TAFS can be employed. The work also provides a novel strategy for automatic air purification. Recently, Guo et al. [26] proposed a self-powered air purification and quality monitoring system based on a stackable triboelectric-electromagnetic hybrid nanogenerator to take full advantage of the high voltage output of the TENGs and the high current output of the electromagnetic generators (EMGs) (Fig. 11g). Driven by airflow, a hybrid nanogenerator was designed to release negative ions and continuously power an air quality detector. The strategy for the proposed system contribute to the achievement of environmentally clean and carbon–neutral goals.

Table 7 displays the variations in TENG materials, operating modes, contact modes, and power densities in diverse smart industry applications during the preceding two years. Intelligent environmental protection should be in conjunction with Internet technologies and environmental data. Challenges to environmental protection zones include land pollution, water pollution and air pollution, and the scope covered by TENG are relatively limited currently. Future development of multi-source ecological environment monitoring at the technical level as well as enhancement of the pollution monitoring and information release system can both benefit from TENG's self-powered sensing advantages. Additionally, a dynamic monitoring network and real-time pollution treatment technology reliant on the resources and environment load-carrying capability can be formed by leveraging TENG's advantages of efficient collection of environmental energy and re-power supply.

Table 7 Recent developed devices based on TENG for smart environmental protection

3.8 Eco-smart energy

Energy is the primary driving force behind human civilization's growth and a critical contributor in worldwide technological improvement. The usage of energy alters our lives every day, but it also presents us with new obstacles and increased demands.The fossil fuels that humans rely on for a long period has been the source of climatic and environmental change. The energy exhaustion also confronts mankind due to the irreversibility of coal and oil combustion. Nanoenergy, as an emerging research field, refers to the efficient conversion, storage and utilization of high entropy energy using nanomaterials and nanodevices. TENG is a type of energy harvesting device that can transform mechanical energy with low frequency and low amplitude into electric energy. It can harvest small amounts of random ambient energy such as hand touch, human walking, wheel rotation, and machine roar, as well as massive amounts of clean energy such as wind, raindrop, and sea waves.

3.8.1 Water energy harvesting

Wave and tidal energy are two types of clean, sustainable blue energy that are copious found in the ocean. If large-scale commercial utilization of marine energy is implemented, it will be a new type of green energy that can significantly meet human energy needs while also lower carbon dioxide emissions and cause dramatic changes in the global energy pattern. Kinetic energy resources such as waves, currents, lapping water, and water up and down from rivers, rainfall, and ocean waves can be effectively recovered by combining the four main modes of TENGs. These hydro-TENGs may convert the water's erratic velocity into a constant and steady stream of electricity if they were netted into the ocean or river.

Wave energy devices based on electromagnetic generators have been extensively studied. However, these devices have low energy harvesting efficiency for widely distributed and irregular micro waves. Wang et al. [130] designed a TENG device that could effectively extract mechanical energy from random chaotic environments, and the dual symmetry breakings were introduced in the design (Fig. 12a). The chiral network structure and the one-way bearing structure with ratchet effect were chiral breakings, which converted the irregular wave excitation into the reciprocating swing of the shell, and further transformed into one-way rotation of the internal rotor. At the same time, the inertial wheel was introduced to realize energy caching. It could accumulate mechanical energy from low-frequency instantaneous excitation, realize the effective superposition of multiple excitation energies and form a continuous monodirectional rotation, which was gradually converted into electric energy. This work provides a basis for the design of energy harvesting devices that can better adapt to random chaos driving forces and extract energy from them efficiently. In addition, Wang et al. [131] designed and manufactured another cylindrical wave-driven linkage TENG (WLM-TENG) with unidirectional rotation to effectively capture water wave energy (Fig. 12b). Also, Wang et al. [135] prepared a barycenter self-adapting TENG (BSA-TENG), which could more easily harvest the abundant high entropy energy (HEE) in the ocean randomly, depending on the device's clockwise deflection under gravity and continuous periodic unidirectional rotation motion under water surface fluctuations (Fig. 12f). When operating at a frequency of < 1 Hz, the unit could provide peak power of 0.1 mW at a 500 MΩ load resistance. Liu et al. [140] proposed a fully symmetrical elliptic cylinder structured TENG (EC-TENG), which exhibited excellent self-stability and ultra-high sensitivity to wave excitation at sea (Fig. 12l). Most importantly, EC-TENG had unique anti-overturning ability. TENG, which enables wave energy harvesting in rough and calm sea conditions at the same time, contributes to more efficient all-weather blue energy harvesting.

Fig. 12

Recent developed devices based on TENG for smart water energy harvesting. a Monodirectional continuous spinning TENG [130]. Copyright 2023, Royal Society of Chemistry. b Cylindrical TENG with unidirectional rotation [131]. Copyright 2023, Wiley. c Gyroscope-Structured TENG [132]. Copyright 2022, American Chemical Society. d Spherical TENG based on eccentric structure [133]. Copyright 2022, Wiley. e Water-Tube-Based TENG [134]. Copyright 2021, Wiley. f Barycenter self-adapting TENG [135]. Copyright 2022, Wiley. g TENG with a seesaw structure [136]. Copyright 2022, Elsevier. h Bifilar-pendulum-assisted multilayer-structured TENG [137]. Copyright 2021, Wiley. i Spherical TENG with Dense Point Contacts [138]. Copyright 2021, American Chemical Society. j Nodding duck structure multi-track directional TENG [36]. Copyright 2021, American Chemical Society. k 0.5 m TENG with multi-arch shape [139]. Copyright 2022, Wiley. l Fully symmetrical TENG with an elliptic cylindrical structure [140]. Copyright 2022, Springer. m Seaweed-like TENG [141]. Copyright 2021, American Chemical Society. n Active resonance TENG [142]. Copyright 2021, Elsevier. o Electrochemical conversion system driven by TENG [143]. Copyright 2022, Wiley. p Buoy-type ocean monitoring system and q Intelligent life jacket [144]. Copyright 2022, Wiley. r Sea surface wireless positioning. Copyright 2022, Royal Society of Chemistry [145]. s Self-powered sensing and underwater power supply [146]. Copyright 2022, Wiley. t Environmental monitoring and protection [147]. Copyright 2023, Wiley. u Intelligent forest monitoring and Internet of Things in remote areas [148]. Copyright 2022, Wiley

Due to the randomness of wave direction, the multi-direction capture technology of ocean energy has a certain bottleneck. At present, the multi-sphere structure adopted by most schemes can achieve multi-direction capture, but the real-time contact area of the generating unit is limited. Cheng et al. [132] published the study of a gyroscope-structured TENG (GS-TENG) for multi-direction ocean energy capture, which increased the generating area of the generating unit effectively, and realized the effective capture of multi-direction wave energy (Fig. 12c). During the underwater experiment for more than 30 consecutive days, the DC current attenuation of the output of the GS-TENG is about 8%, showing good reliability. Li et al. [133] designed a spherical eccentric structured TENG (Se-TENG) to collect omnidirectional low-frequency water wave energy (Fig. 12d). The regular dodecahedron structure of Se-TENG improved the space utilization, enabling twelve independent eccentric structured TENG (E-TENG) units to be integrated in the spherical shell and operate in twelve different directions to capture the energy generated by water wave movement in different directions. Pan et al. [138] developed a spherical TENG with dense point contact to obtain water wave and vibration energy (Fig. 12i). The use of polyacrylate pellets enlarged the point contact frequency to improve contact efficiency, and provided appropriate mechanical space to increase volume power density. Therefore, spherical TENG provides an effective method for large-scale wave energy acquisition [149].

Zi et al. [134] developed a water-tube based TENG (WT-TENG), and WT-TENG was suitable for collecting energy in a variety of motion modes based on the mobility of water, including rotation, rocking, seesaw, horizontal linear mode, etc. Besides, each WT-TENG unit could be pieced together like building blocks into a larger power generation unit. The team lowered the box containing 34 WT-TENG units to the surface of the ocean to capture wave energy, generating enough energy at peak to power 150 LED light bulbs (Fig. 12e). Zhang et al. [136] reported a new type of seesaw swing structured TENG (SS-TENG) based on the principle of equal arm bar, which was used to collect wave energy in two directions (Fig. 12g). When there was no weight difference between the two sides of the bar, the seesaw structure could collect low-frequency ocean energy in the vertical direction. In the case of a weight difference between the two sides of the transverse bar, the manufactured SS-TENG could collect wave energy in the horizontal direction based on the inertia difference caused by the weight difference. Therefore, the SS-TENG can obtain wave energy of higher or lower frequencies without being restricted by its structure. Wang et al. [137] designed a new type of wave energy collection device by innovatively integrating a large number of bifilar-pendulum-assisted multi-layer structured TENG (BM-TENG) modules on the ship platform (Fig. 12h). The bifilar pendulum and multi-layer structure of the wedge-shaped pendulum cone enhanced the spatial utilization of TENG. Soft contact and thin dielectric materials achieved high charge density. Combined with these structural advantages, the maximum power density of the BM-TENG was up to 200 W m⁻³, and its output performance of the BM-TENG was improved by 1–2 orders of magnitude compared with previous TENGs to collect wave energy.

Recently, other excellent TENG structures for blue energy harvesting have been reported. For example, Tang et al. [36] constructed a nodding duck structure multi-track freestanding TENG (NDM-FTENG) for low-frequency wave energy collection (Fig. 12j). Liu et al. [139] advanced the TENG shell to 0.5 m and adopted a unique multi-arch structure to solve the difficulty of fully obtaining contact with a wide range of friction layers (Fig. 12k). This work is a first step towards a near-meter-scale shell for large-scale wave energy harvesting. Xu et al. [141] proposed a kind of seaweed-like TENG (S-TENG), whose flexible structure enabled it to convert wave energy into electricity in real time on, in and under coastal water (Fig. 12m). Wang et al. [142] introduced a active resonance TENG (AR-TENG) for omnidirectional water wave energy collection, which could effectively harvest water wave energy of various frequencies through resonance effect, and demonstrated outstanding experimental results in water wave testing (Fig. 12n). Jiang et al. [150] fabricated an innovative liquid–solid TENG featuring an asymmetric configuration. Distinguished by its utilization of opposing dynamic electric-double-layers and a novel operational scheme, this TENG represents a significant departure from conventional fully enclosed and liquid–solid TENG design. These are distinctive and intriguing structures, each with its own unique properties for improving the efficiency of water wave energy harvesting [151].

TENG-based blue energy also show its excellent use value in marine exploration, deep-sea operations, sea conditions monitoring, coastal defense, ship's anticorrosive, water purification, and early warning, etc [139].. Jiang et al. [143] proposed a self-powered electrochemical system (SPECS) based on TENGs, which converted blue energy represented by ocean wave energy into green hydrogen fuel through electrochemical transformation (Fig. 12o). Jeong et al. [144] developed a all-recyclable TENG (AR-TENG), which confirmed that it can power buoy-type ocean monitoring system (Fig. 12p) and intelligent life jacket (Fig. 12q). Tang et al. [145] developed a self-powered seesaw-structured spherical triboelectric-electromagnetic hybrid nanogenerator (SSTE-HNG) for wide-band wave energy acquisition, which could be used for real-time wireless sea surface positioning (Fig. 12r). Duan et al. [146] proposed a structured triboelectric surface (STS) that could be attached to various surfaces or independently applied underwater for self-powered sensing and underwater power supply, showing great potential in deep-sea environment applications (Fig. 12s). Wu et al. [147] studied a a multi-layer stacked TENG (MLS-TENG) based on rotation-to-translation mechanism. In addition, a metal corrosion protection system and a self-powered wireless temperature and humidity monitoring system were successfully established with the assistance of the MLS-TENG (Fig. 12t). Hu et al. [148] studied a charge excitation and mode adjustable TENG (CEMA-TENG) to collect water flow energy as a power source for intelligent forest monitoring, expanding the application of self-powered system of Internet of Things in remote areas (Fig. 12u).

Table 8 summarizes the differences of TENG materials, working mode, contact mode and power density in different applications of water energy harvesting in the past two years. The ocean, which covers about 70% of the planet, is the largest energy storage system with the advantages of renewable energy and a wide distribution. More importantly, large-scale offshore power station building can lessen wave erosion along the coast and won't occupy our limited land resources. In addition, it can supply sufficient electricity for Marine resource development and Marine monitoring in situ. It is challenging to efficiently collect wave energy due to its low frequency and unpredictable nature. To boost the power density of the entire TENG device and realize effective wave energy collection, it is necessary to further achieve multi-directional collection, improve spatial utilization, and increase the effective contact area. The successful application of ocean energy would bring about profound changes in the world energy landscape and affect all aspects of the economy and society.

Table 8 Recent developed devices based on TENG for smart water energy harvesting

3.8.2 Wind energy harvesting

Worldwide wind energy growth in 2021 was 14%, or 227 TWh. The total reached 1,814 TWh, with wind accounting for 6.6% of global electricity in 2021, up from 3.5% in 2015 when the Paris Agreement was signed. Wind energy is of great significance as a reliable source of energy production and supply, in particular for coastal islands, remote mountainous areas with difficult access, sparsely populated grasslands and pastures, as well as rural and border areas far from or inaccessible to the power grid. With the advantages of vast reserves, nearly limitless, wide distribution and mitigation of greenhouse effect, wind energy can be exploited as as a substantial energy source. However, only a limited amount of wind energy can be properly converted into electricity due to technical restrictions. The efficient conversion of wind energy into electricity is an intriguing area of TENG technology research and development that has recently attracted extensive attention.

Moderate winds of less than 3 m s⁻¹, which is about the average annual surface wind speed for most parts of the Earth. Wang et al. [152] developed a triboelectric − electromagnetic hybrid nanogenerator (FTEHG) based on electromagnetic generators (EMGs) and TENGs, which could efficiently collect all levels of wind energy in the environment, such as breeze, moderate and high wind speed. The wind speed response range of FTEHG was extended to 1.55–15.0 m s⁻¹, which had been successfully implemented for reliable and continuous power supply for electronic devices and wind speed sensing (Fig. 13a). High-speed trains create enormous amounts of wind energy that would otherwise be squandered due to the challenges in collecting them. Wang et al. [153] also designed a double-layer elastic rotation TENG (ER-TENG) with low friction and high output performance, which could be installed on both sides of the railway track according to a fixed gap and collect wind energy generated by high-speed railway vehicles when they pass through to power the large-scale signal and sensor network of the railway section (Fig. 13b). Typical TENG exhibits low heat resistance, low moisture resistance and low durability. To this end, Choi et al. [154] reasonably designed and created an efficient polyimide friction surface with a customizable non-compact packed microbody array (md PI) for the assembly of the TENG (md TENG). The md-TENG had excellent durability with over 16,000 contact-separation cycles, demonstrating the ability to capture windmill energy and vibration from the engine even in hot air and high humidity environments (Fig. 13c). Additionally, Wang et al. [158] suggested a W-TENG that converted offshore wind energy into electric energy, which could effectively reduce the electricity cost (Fig. 13h). The novel NiCoP-MOF electrode enhanced the performance and stability of electrolytic hydrogen production. In addition to being able to directly use natural saltwater as an electrolyte to alleviate the water shortage bottleneck, it also boost the maximum hydrogen production efficiency by 2.3 times over the previous self-powered water electrolysis based on nanogenerator. The creation of electrolytic seawater hydrogen on a massive scale is powerfully explored in this work.

Fig. 13

Recent developed devices based on TENG for smart wind energy harvesting. a Broadband wind energy harvesting [152]. Copyright 2021, American Chemical Society. b Wind energy harvesting from high-speed trains [153]. Copyright 2021, American Chemical Society. c Wind energy harvesting from engine even [154]. Copyright 2022, Elsevier. d A breeze-wind-driven autonomous wireless anemometer [15]. Copyright 2021, American Chemical Society. e Self-powered wireless sensing [155]. Copyright 2022, Elsevier. f Intelligent fire detection systems [156]. Copyright 2021, Wiley. g Detecting breeze vibration on electric transmission lines [157]. Copyright 2022, Elsevier. h Seawater Electrolysis for Hydrogen Production [158]. Copyright 2022, American Chemical Society

The monitoring of wind speed information is of utmost importance to the survey of wind energy resources and weather forecast. Zhang et al. [15] designed an autonomous wireless anemometer driven by breeze energy based on planetary rolling TENG (PRTENG), which simultaneously realized wind energy capture and wind speed sensing. The anemometer could be started under the wind speed of below 2 m/s profitting by the planetary rolling friction. When the wind speed was 5 m/s, the anemometer could transmit the monitored wind speed data to the surrounding 10 m every 2 min (Fig. 13d). Dai et al. [155] presented a novel vortex-induced vibration (VIV) based TENG that enabled efficient energy harvesting from wind at low speeds of less than 3 m/s, potentially powering wireless sensor networks (WSNs) deployed in remote areas (Fig. 13e). Considering that the wind on the earth surface has the characteristics of directional randomness and low velocity flow, Xi et al. [156] illustrated a flow-induced vibration effect based TENG (F-TENG), which effectively continuously converted low-speed wind energy (1.8–4.3 m/s) in random directions into electrical energy, achieving self powered wind monitoring for extended periods of time. Based on these environmental monitoring data, basic data could be provided for intelligent fire detection system (SIFDS) (Fig. 13f). The electromechanical conversion model and the novel structures with fatigue resistance reported in this work provide some ideas for the large-scale capture and utilization of breeze energy. Zheng et al. [157] proposed a wind monitoring TENG (WM-TENG) that could measure wind speeds ranging from 1.7 to 6.7 m/s. The accumulated duration of breeze vibration of transmission lines is recorded and analyzed by WM-TENG, which is expected to be applied in long-distance power networks to solve the power supply challenges of grid sensors (Fig. 13g).

The characteristics of TENG materials, working modes, contact modes and power densities in diverse applications of wind energy harvesting in the past two years are described in Table 9. The International Energy Agency (IEA) estimates that the capacity of wind energy worldwide has grown by almost 300 percent in the last decade and is anticipated to continue to increase by more than 200 percent by 2030. Offshore wind, which currently accounts for nearly 5% of global installed capacity, will be the main driver of future growth. Compared with land wind power, the construction environment and cost of marine wind power are more complicated and expensive. The low cost and simple structure of the TENG installation will promote the large-scale installation of offshore wind power and reduce the overall installation cost. Additionally, TENG, acting as a sensor, can be used to create an intelligent control system by optimizing the functioning of wind power collection equipment. In general, the future development of wind power will be concentrated in enhancing efficiency, innovative technology, reasonable layout, and cost reduction. The continuous upgrading of wind power technology and further advancement of energy storage technology will contribute to attain more efficient and reliable generation of wind power.

Table 9 Recent developed devices based on TENG for smart wind energy harvesting

3.8.3 Solar energy, hybrid energy, and environmental energy harvesting

Global solar capacity expanded by 23% (188 TWh) to 1,023 TWh in 2021, accounting for 3.7% of total global electricity generation. TENGs have gained popularity in recent years for their ability to capture solar energy, as well as other renewable energy sources, or, more enticingly, as hybrid energy systems that harvest energy from multiple sources [171].

Typically, the output performance of solar cells is severely reduced in rainy days, and potential mechanical damage to surface solar cells may also inhibit their output capacity. Liu et al. [172] integrated a liquid–solid transparent self-healing TENG (SH-TENG) on the surface of a solar cell to propose a hybrid energy system for simultaneously collecting solar energy and raindrop energy (Fig. 14a). The SH-TENG showed excellent output performance in rainy days, with peak short-circuit currents of 0.8 µA and 6 V, respectively. The self-healing TENG with its elastic texture can act as a protective layer where it can significantly curb the risk of damage to solar cells. Overall, the hybrid energy system combines the advantages of the SH-TENG's high voltage with the solar cell's high current signal, while extending the working hours without affecting the solar cell's operation. Due to the irregular nature of natural rainfall in terms of frequency, volume, location and density, designing an efficient rainfall TENG (R-TENG) array faces huge challenges. Jiang et al. [16] developed an R-TENG array panel with simple structure, large area and high transparency, which could realize all weather irregular raindrops and solar energy collection (Fig. 14b). By systematically optimizing the size, structure and array distribution density, R-TENG array could effectively avoid overlapping of multiple raindrops on the spatial and temporal scales, thus reducing the probability of electrical cancellation of adjacent raindrops. On rainy days, the average power density of R-TENG (40.80 mW m⁻²) is higher than that of solar cells (37.03 mW m⁻²), proving the feasibility and effectiveness of the integration of raindrop TENG and solar cells. In addition, a self-powered wireless environmental monitoring system powered by an R-TENG array integrated with solar panels was constructed to monitor ambient light intensity and weather conditions in real time. Park et al. [173] reported an all-aerosol-sprayed transparent TENG (ST-TENG), which showed excellent light transmittivity of 81%. By utilizing the ST-TENG's full aerosol spray process and high light transmission rate, a hybrid energy collector that generates electricity from both solar energy and rainwater was directly manufactured on a commercial solar panel (Fig. 14c). Pan et al. [174] systematically optimized the bionic surface structure and instant switch design to develop a high-performance multi-layer instantaneous TENG (I-TENG) with a high energy conversion efficiency of 24.89%. In addition, the I-TENG device could capture the kinetic energy of the raindrop repeatedly during the multi-step falling process. Finally, modern buildings equipped with the hybrid system composed of I-TENG and polymer solar cell (PSC) was illustrated to alternately capture solar energy and raindrop energy (Fig. 14d). The paper provides strategies for the design and manufacture of efficient devices that will accelerate the commercialization of the TENG.

Fig. 14

Recent developed devices based on TENG for smart hybrid energy harvesting. Solar and rain energy: a [172] Copyright 2021, Elsevier. b [16] Copyright 2023, Wiley. c [173] Copyright 2022, Elsevier. d [174] Copyright 2022, Wiley. Mechanical and Solar Energy: e [175] Copyright 2022, Wiley. Impact energy: f [176] Copyright 2022, Elsevier. Rotational energy: g [177] Copyright 2022, Elsevier. Wind and Water energy: h [178] Copyright 2022, Springer. i [179] Copyright 2022, Elsevier. j [180] Copyright 2022, Elsevier

Direct current TENGs (DC-TENGs) can directly convert mechanical energy into direct current energy, and have a wide range of applications in micro and nano power supply. Yang et al. [175] proposed a rolling-mode DC-TENG structure based on frictional photovoltaic effect by constructing dynamic Al/CsPbBr₃ Schottky junction using the all-inorganic perovskite structure CsPbBr₃, which could simultaneously collect mechanical energy and solar energy (Fig. 14e). Under illumination condition, the output voltage and current density reached 3.69 V and 11.46 A m⁻², respectively. Most TENGs are created for low-frequency motion due to their lower energy conversion efficiency in high-frequency motion such as impacts. Kyung et al.¹⁷⁶ introduced a contactless bilateral oscillating TENG (CBO-TENG), which could produce a lasting 13-s output following a single impact. In addition, the CBO-TENG was successfully demonstrated for the high-voltage application of a new self-cleaning solar panel, which utilized the ubiquitous roadside pounding of human feet (Fig. 14f). After 12 impacts, the system effectively removed 79.2% of dust from the surface of the test plate. In a nutshell, the work offered a method for capturing energy from impacts and practical uses for self-cleaning solar panels. There are various forms of energy widely distributed in nature, among which rotational energy is extremely abundant, such as being obtained from airflow, sea waves, human motion and mechanical vibration. Zheng et al.¹⁷⁷ elaborated a snap-through TENG with buckled bistable mechanism and magnetic coupling to obtain rotational energy (ST-RTENG) (Fig. 14g). The curved bistable beam generates convex and concave clasps under magnetic excitation, which could obviously increase the contact force and area of the functional materials of the TENG, thus evidently improving the electrical output. The suggested rotary energy collector can be used for wind energy acquisition in field environments, self-powered condition monitoring of pressure pipelines and so on.

The TENG based on triboelectrification and air breakdown coupling can achieve large DC output. Inspired by the primary cell and its DC signal output characteristics, Hu et al. [178] proposed a novel primary cell structured TENG (PC-TENG) based on contact electrification and electrostatic induction. It has a variety of modes of operation, including freestanding mode, contact-separation and rotary modes, which is promising for wind and water hybrid energy harvesting components (Fig. 14h). Compared with other traditional triboelectric materials, cotton is an easy to obtain green, environmental protection, low-carbon, natural renewable resources, and cotton has a strong ability to lose electrons which makes it very suitable as a frictional layer. Cao et al. [179] demonstrated a cotton-assembled TENG (C-TENG) with a turntable structure, which, once stimulated by the environment (wind or water flow), would rotate to promote the relative motion between the cotton sheet and the FEP film, thereby generating electrical energy (Fig. 14i). Meanwhile, Cao et al. [180] reported another pulsed cylindrical TENG based on PTFE particles, which can convert low-frequency mechanical energy collected from wind and water into electrical energy in a low-cost and efficient manner, particularly it can also be utilized to drive seawater electrolytic hydrogen production (Fig. 14j).

Table 10 illustrates the variations in TENG materials, working modes, contact modes, and power densities in solar energy, hybrid energy, and environmental energy harvesting applications over the last two years. The results demonstrate that simultaneous collection of several energy sources can efficiently produce high output power. There is currently no substantially established method for collecting these multiple energy sources simultaneously and maximizing efficiency. Microenergy harvesting for more than two input sources is challenging, particularly those with extremely varying internal resistance, such TENGs and micro-solar panels. In addition, it is crucial to disclose the interactions between various energies for improving the effectiveness of energy harvesting in composite devices through material management and structural optimization. The most promising replacement for low-power electronic product batteries in the future is to overcome the energy collector's inherent simplicity and select a hybrid energy collection apparatus with complementary qualities.

Table 10 Recent developed devices based on TENG for solar energy, hybrid energy, and environmental energy harvesting

4 Summary and perspectives

Leveraging TENG technology, energy is harvested from distributed resources available within or surrounding urban environments, including water sources, wind energy, solar power, raindrop energy, vibrational energy, and human motion energy. The collection of these energy sources serves not only for power generation but also directly as various physical or biochemical sensors, enabling real-time monitoring and data collection of urban environments and vital health information of residents. To ensure the smooth operation of eco-smart cities, power management modules are employed to convert the alternating current signals from TENG into direct current signals and store them in energy storage systems such as batteries or capacitors. Robust power supply management, long-term data collection, IoT infrastructure development, and maintenance modules are designed to ensure optimal operation of IoT devices. These modules are not only limited to managing TENG devices but also expected to expand to larger, more distributed networks or extensive electronic devices, further enhancing the energy efficiency and intelligence of cities. To manage such a vast and complex network, IoT devices must be tightly connected within information network infrastructure to monitor, protect, and potentially manipulate all IoT devices using TENG technology in smart cities. This close connection and information exchange provide accurate data support for city managers to timely respond to various needs and challenges in urban areas [9]. An integrated perspective on the comprehensive framework of self-powered eco-smart cities as illustrated in Fig. 15, this design paradigm is not only applicable to traditional fields such as households, healthcare, transportation, industry, agriculture, and environmental protection but also holds broad application prospects in emerging areas such as human–machine interaction. This innovative design concept will provide strong support for the sustainable development and intelligence process of future cities, promoting the healthy development of the socio-economic environment.

Fig. 15

Schematic represents the ideal paradigm of sustainable TENG devices in eco-smart cities. a Energy harvesting module leveraging TENG technology. Water energy [140] Copyright 2022, Springer. Wind energy [152] Copyright 2021, American Chemical Society. Solar and rain energy [173] Copyright 2022, Elsevier. Environmental mechanical energy [177] Copyright 2022, Elsevier. Human motion energy [62] Copyright 2022, Elsevier. b Power supply management, IoT data collection and maintenance modules. c Overarching smart city design, emphasizing the interconnectedness of various components and their integration into the urban landscape

Energy is the fundamental driving force for the progress of human civilization and a decisive factor in worldwide technological change. Entering the era of big data and artificial intelligence, the self-power supply of small devices in the Internet of Things era, as well as sensor networks are also active fields for human exploration. Based on this foundation, the role of nanotechnology with TENGs as the core in the future eco-smart cities represents the following prospects:

• 1. Blue Energy: The ocean covers about 70% of the Earth and is the largest energy reservoir. If the ocean can be properly exploited and utilized, it will be a new form of green energy. According to estimates, the annual generation of global ocean wave energy can reach 80,000 TWh, far exceeding the current global annual demand of 16,000 TWh. Building a floating wave energy harvesting device by utilizing TENG's advantages of portability, low cost and low frequency efficiency will be one of the best designs for large-scale ocean wave energy harvesting. So far, there have been numerous reported structures used by TENG to collect blue energy, including bearing, folding, water-tube, double-line pendulum, cylindrical and spherical structures, etc., which use their distinctive characteristics to achieve multi-direction collection of ocean waves or to enhance the contact area to improve the efficiency of water wave energy collection. Future successful deployment of mature TENG-based Marine energy harvesting devices would fundamentally alter the global energy landscape and have an impact on all facets of the economy and society.

• 2. Distributed Hybrid Micro-nano Energy. The information era has ushered in the downsizing of a variety of electrical devices. The future advanced energy system will be a complicated system of multi-energy simultaneous collecting as well as an intelligent distributed energy system. Billions of "small energy" with mobility, variation, and uncertainty can be created to build a diversified small-scale smart energy system that is directly targeted to consumers and supplied on demand. It has been demonstrated that TENGs can collect distributed energy, such as mechanical energy from stepping on a household floor, wind energy generated by a high-speed train, and random falling raindrop energy. TENGs also have the potential to simultaneously absorb hybrid energy sources such as offshore "wind + water energy", rooftop "solar + raindrop energy" and roadside "solar + mechanical energy". Complement collection can provide a more efficient, reliable and sustainable energy supply. It is conceivable that clean energy may eventually replace fossil fuels, change the existing energy structure and achieve carbon neutrality through extensive, massive and random energy collection by TENG and other high-tech technologies in cities.

• 3. Unified Healthcare System. Unified healthcare system, data collection and sharing, analysis and research practices will usher in a new era of solutions to modern health problems. The Internet of Things (IoT) in healthcare enables remote patient data collection through sensors like TENG, realizing personalized treatment and medication as well as preventive measures through real-time data collection. In addition, TENG-based wearable sensing devices are being studied in full swing, as TENG is certificated to be successful in harvesting energy from biomechanical movement, and can also be utilized for human health monitoring, rehabilitation, nerve and bone repair, drug delivery, disease diagnosis, etc. The integration of wearables with IoT healthcare could aid in the creation of smart healthcare networks and even the introduction of TENG-based HMI assistance in communicating, diagnosing and treating patients. Future unified medical care system will allow the interaction between patients with healthcare personnel, medical institutions and medical equipment to serve patients and safeguard public health in a faster and more efficient manner.

• 4. Ecological Intelligent Environment Monitoring. Ecological intelligent environment monitoring system belongs to the category of intelligent environment, and is also the concrete implementation of the Internet of Things. It fully utilizes existing environmental information platforms to embed sensors and equipment into a variety of environmental monitoring objects, with an emphasis on air, soil and water. TENG has currently been verified for monitoring meteorological data such as ambient temperature and humidity, wind speed and direction, air purification and quality monitoring, CO₂ concentration monitoring, soil pH monitoring, water pollutant monitoring, automobile exhaust detection system, etc. In the future, benefiting from the self-powered sensing advantages of TENG, the development of multi-source ecological environment monitoring at the technical level and the enhancement of pollution monitoring and information dissemination system can form dynamic monitoring networks and real-time pollution treatment technologies dependent on resource and environment carrying capacity, as well as responding to extreme and sudden natural disasters as well as ecological restoration.

• 5. Eco-smart Urban Agriculture. Eco-smart urban agriculture is a high-level agricultural production form that anticipates, forewarns and optimizes resource allocation. Multiple sensor units are usually required for multi-objective monitoring due to the complex agricultural production environment. Eco-smart urban agriculture systems are envisioned as a promising approach to urban agriculture that allows residents to control most elements of production through a range of innovative technologies by planting vertically stacked areas inside and outside houses. Advanced sensor technologies such as TENG have been proven to be useful for monitoring light intensity, temperature and humidity, soil moisture and nutrients, plant health monitoring and growth assistance, as well as quantitative release of pesticides, on which a customized and intelligent "precision agriculture" can be developed. In addition to boosting productivity, this also lowers the food transportation costs and its associated environmental impacts. It is desired that eco-smart urban agriculture will be fully integrated into the smart city ecosystem in the future to achieve a more optimized, automated and data-driven agricultural production mode.

• 6. Intelligent Green Transportation. Intelligent green transportation lies in the safety of traffic driving, wireless signal transmission, self-driven energy supply and environmental protection of vehicle operation. Real-time collection and processing of traffic information is critical for implementing informationization and intelligent management of urban roads, vehicles and parking lots, etc. TENGs are increasingly being applied in traffic safety guidance, vehicle environmental protection, wireless traffic system control and unmanned driving. Based on the advancement and integration of material technology, nanotechnology and electronic technology, efforts are being made to promote the construction of the future "intelligent green, safe and efficient, autonomous and unmanned" transportation system, as well as the transportation vision of "zero death, zero emission, carbon neutral", so as to effectively coordinate the service capacity of transportation system in urban areas.

• 7. Low-carbon Smart Factory. Building a smart factory necessitates low-carbon manufacturing and maximization of energy efficiency. TENGs can capture a huge quantity of wasted and ubiquitous mechanical energy in factories, reducing consumption and pollution, and realizing the energy cycle chain and recovery chain, thus improving the benefits while achieving the ecological economy. In addition, the Industrial Internet of Things, which aims to actualize things with things, things with people, and things with the Internet of things, offers a new breakthrough for smart factories by benefiting identification, management, and control.. The TENG's high sensitivity to mechanical disturbances also makes it an excellent sensing choice for factories that require both low energy economy and high precision. Meanwhile, TENG-based robots and HMI are projected to be deployed in the future smart factory sector. The vision of smart factory is to establish a productive, energy-saving, green, comfortable and humanized factory.

• 8. Ecological Smart House. Smart home refers to a residential platform that utilizes network communication technology, security prevention technology, automatic control technology, audio and video technology to integrate facilities relevant to home life. Constructing an efficient management system for residential facilities and family schedule affairs, improve home safety, convenience, comfort and artistry, and achieve an environmentally friendly and energy-saving living environment. On the one hand, energy support is necessary for both the machine's operation and electrical behavior in humans. TENGs have been demonstrated to be capable of being installed on the roof or window as a “solar + raindrop” energy collection device, or the smart floor to collect the mechanical energy from human movement to power the house. On the other hand, the ultimate realization of the smart home requires the integration of a large number of sensors distributed around the house with diverse functions to form a family network that monitors and administers the house environment. TENGs have also demonstrated value as electronic password lock, security alarm system, and for identifying and controlling various domestic appliances. The future smart home is an eco-system where products can be interconnected and logically linked to realize intelligent driving of the whole house.

The integration of TENGs into eco-smart cities presents numerous opportunities and benefits. However, several challenges need to be addressed to ensure its successful implementation and widespread adoption into various sectors, including:

• 1. Material Selection and Performance Optimization: While TENGs offer the advantage of being highly customizable, selecting appropriate materials with desirable triboelectric properties and optimizing their performance remains a challenge. Different environmental conditions and usage scenarios may require specific material compositions and structures. To meet the practical requirements of urban applications, further improvement of TENG's energy density and output power is needed to support a wider range of application areas.

• 2. Environmental Adaptability: TENGs should demonstrate robustness and reliability to withstand the rigors of urban environments, including temperature fluctuations, humidity levels, and exposure to pollutants, et al. Ensuring the durability and reliability of TENG devices through improved materials, manufacturing processes, and protective coatings is critical for their long-term performance and maintenance.

• 3. Eco-friendliness and Sustainability: Many TENG designs utilize materials such as Kapton, PVC, and PTFE, which indeed pose challenges to its eco-friendliness. TENG technology must be evaluated for its environmental impact, considering aspects such as material recyclability, biodegradability, and overall sustainability. Efforts should be directed towards developing alternative materials and optimizing fabrication processes to minimize environmental harm. Implementing efficient recycling and disposal strategies for TENG components can enhance its eco-smart credentials.

• 4. Interoperability and Scalability: The integration of TENG technology into existing infrastructure presents significant challenges, particularly in scaling up production to meet the demands of eco-smart cities. Achieving compatibility with other renewable energy sources and seamless integration into smart grids necessitates meticulous planning and engineering. This integration must account for the intricate network of existing smart infrastructure, heavily reliant on technologies such as the Internet of Things (IoT), artificial intelligence (AI), and cloud computing. Ensuring seamless interoperability with these systems is imperative for the effective communication of TENG devices with other smart technologies and sensors, thus fostering holistic urban sustainability and efficiency.

• 5. Standardization and Regulation: Establishing industry standards and regulations for TENG technology is crucial to ensure safety, interoperability, and compliance with environmental guidelines. Addressing regulatory barriers and standardizing testing protocols can facilitate the wider adoption of TENG applications.

With continued performance advancements and design innovation, TENGs hold great potential for enabling sustainable and energy-efficient systems across various domains, contributing to a greener and smarter future.