Switched-capacitor-convertors based on fractal design for output power management of triboelectric nanogenerator

A review of material design for high performance triboelectric nanogenerators: performance improvement based on charge generation and charge loss

As a type of innovative device, triboelectric nanogenerators (TENGs) are capable of converting mechanical energy into electrical energy through the triboelectric effect. Based on the working mechanism, the output performance of TENGs heavily relies on the triboelectric materials used. The modification of triboelectric materials is the most efficient way to improve the output performance of TENGs. Herein, this review focuses on the recent progress in triboelectric material design for high-performance TENGs. First, the basic theory of TENGs is introduced. Second, the relationship between the triboelectric materials and the output performance of TENGs is summarized in detail based on a theoretical model of the triboelectric charge dynamic equilibrium. Furthermore, the relevant strategies are analyzed in detail. Finally, challenges and shortcomings of the triboelectric materials for high-performance TENGs are discussed. This review provides a basis for the research status and future development of triboelectric materials.

Facet Engineering Boosts Interfacial Compatibility of Inorganic-Polymer Composites

The interfacial compatibility between inorganic particles and polymer is crucial for ensuring high performance of composites. Current efforts to improve interfacial compatibility preferentially rely on organic modification of inorganic particles, leading to their complex process, high costs, and short lifespans due to aging and decomposition of organic modifiers. However, the fabrication of inorganic particles free from organic modification that is highly compatible in polymer still remains a great challenge. Herein, a novel facet-engineered inorganic particle that exhibit high compatibility with widely used polymer interface without organic modification is reported. Theoretical calculations and experimental results show that (020) and (102) facets of inorganic particles modulate local coordination environment of Ca atoms, which in turn regulate d-orbital electron density of Ca atoms and electron transfer paths at interfaces between polymer and inorganic particles. This difference alters the molecular diffusion, orientation of molecular chains on surface of inorganic particles, further modulating interfacial compatibility of composites. Surprisingly, the facet-engineered inorganic particles show exceptional mechanical properties, especially for tensile strain at break, which increases by 395%, far superior to state-of-the-art composites counterparts. Thus, the method can offer a more benign approach to the general production of high-performance and low-cost polymer-inorganic composites for diverse potential applications.

Synchronous Switching Strategy to Enhance the Real-Time Powering and Charging Performance of Triboelectric Nanogenerator

Triboelectric nanogenerators (TENGs) are of great significance as sustainable power sources that harvest energy from the human body and environment. Nevertheless, due to TENG's impedance-dependent output voltage characteristics, in traditional strategy (TS), real-timely powering a sensor with TENG has a poor sensing on/off ratio (or response), and directly charging a capacitor with TENG shows a low charging efficiency. This degraded real-time powering and charging performance of TENG compared to a commercial constant voltage source is a huge challenge of the TENG field for a long time. This work proposes a synchronous switching strategy (SSS) for TENG to real-timely power sensors or charge capacitors without degrading its performance. Compared with TS, this new strategy enables sensors to have 5-7 times sensing on/off ratio enhancement when using TENG as a power source, reaching the powering ability of a commercial constant voltage source, it makes the powering performance of TENG stable under different driving frequency, improving the powering robustness of TENG. In addition, compared with TS, SSS can also enhance the charging efficiency of TENG in every charging cycle by up to 2.4 times when charging capacitors. This work contributes to real-timely powering or charging the distributed, mobile and wireless electronics using TENG.

Efficient energy conversion mechanism and energy storage strategy for triboelectric nanogenerators

Energy management strategy is the essential approach for achieving high energy utilization efficiency of triboelectric nanogenerators (TENGs) due to their ultra-high intrinsic impedance. However, the proven management efficiency in practical applications remains low, and the output regulation functionality is still lacking. Herein, we propose a detailed energy transfer and extraction mechanism addressing voltage and charge losses caused by the crucial switches in energy management circuits. The energy conversion efficiency is increased by 8.5 times through synergistical optimization of TENG and switch configurations. Furthermore, a TENG-based power supply with energy storage and regularization functions is realized through system circuit design, demonstrating the stable powering electronic devices under irregular mechanical stimuli. A rotating TENG that only works for 21 s can make a hygrothermograph work stably for 417 s. Even under hand driving, various types of TENGs can consistently provide stable power to electronic devices such as calculators and mini-game consoles. This work provides an in-depth energy transfer and conversion mechanism between TENGs and energy management circuits, and also addresses the technical challenge in converting unstable mechanical energy into stable and usable electricity in the TENG field.

Effective Charging of Commercial Lithium Cell by Triboelectric Nanogenerator with Ultrahigh Voltage Energy Management

It is an increasingly mature application solution that triboelectric nanogenerator (TENG) supplies power to electronic devices through its power management system (PMS). However, the previous PMS is able to manage a limited voltage magnitude and the energy storage elements are limited to capacitors. This work proposes an ultrahigh voltage PMS (UV-PMS) to realize the charging of commercial lithium cells (LCs) by TENG. The design of UV-PMS enables energy management of TENGs with ultrahigh open-circuit voltages up to 3500 V and boosts the peak charging current from 30.9 µA to 2.77 mA, an increase of 89.64 times. With the introduction of UV-PMS, the effective charging capacity of LC charged by a TENG at a working frequency of 1.5 Hz for 1 h comes to 429.7 µAh, making a 75.3 times enhancement compared to charging by TENG directly. The maximum charging power comes to 1.56 mW. The energy storage efficiency is above 97% and the overall charge efficiency can be maintained at 81.2%. This work provides a reliable strategy for TENG to store energy in LC, and has promising applications in energy storage, LC's life, and self-powered systems.

Triboelectric Nanogenerators: State of the Art

The triboelectric nanogenerator (TENG), as a novel energy harvesting technology, has garnered widespread attention. As a relatively young field in nanogenerator research, investigations into various aspects of the TENG are still ongoing. This review summarizes the development and dissemination of the fundamental principles of triboelectricity generation. It outlines the evolution of triboelectricity principles, ranging from the fabrication of the first TENG to the selection of triboelectric materials and the confirmation of the electron cloud overlapping model. Furthermore, recent advancements in TENG application scenarios are discussed from four perspectives, along with the research progress in performance optimization through three primary approaches, highlighting their respective strengths and limitations. Finally, the paper addresses the major challenges hindering the practical application and widespread adoption of TENGs, while also providing insights into future developments. With continued research on the TENG, it is expected that these challenges can be overcome, paving the way for its extensive utilization in various real-world scenarios.

Ultrafast Metal-Free Microsupercapacitor Arrays Directly Store Instantaneous High-Voltage Electricity from Mechanical Energy Harvesters

Harvesting renewable mechanical energy is envisioned as a promising and sustainable way for power generation. Many recent mechanical energy harvesters are able to produce instantaneous (pulsed) electricity with a high peak voltage of over 100 V. However, directly storing such irregular high-voltage pulse electricity remains a great challenge. The use of extra power management components can boost storage efficiency but increase system complexity. Here utilizing the conducting polymer PEDOT:PSS, high-rate metal-free micro-supercapacitor (MSC) arrays are successfully fabricated for direct high-efficiency storage of high-voltage pulse electricity. Within an area of 2.4 × 3.4 cm2 on various paper substrates, large-scale MSC arrays (comprising up to 100 cells) can be printed to deliver a working voltage window of 160 V at an ultrahigh scan rate up to 30 V s-1. The ultrahigh rate capability enables the MSC arrays to quickly capture and efficiently store the high-voltage (≈150 V) pulse electricity produced by a droplet-based electricity generator at a high efficiency of 62%, significantly higher than that (<2%) of the batteries or capacitors demonstrated in the literature. Moreover, the compact and metal-free features make these MSC arrays excellent candidates for sustainable high-performance energy storage in self-charging power systems.

Electrostatic generator enhancements for powering IoT nodes via efficient energy management

Electrostatic generators show great potential for powering widely distributed electronic devices in Internet of Things (IoT) applications. However, a critical issue limiting such generators is their high impedance mismatch when coupled to electronics, which results in very low energy utilization efficiency. Here, we present a high-performance energy management unit (EMU) based on a spark-switch tube and a buck converter with an RF inductor. By optimizing the elements and parameters of the EMU, a maximum direct current output power of 79.2 mW m-2 rps-1 was reached for a rotary electret generator with the EMU, achieving 1.2 times greater power output than without the EMU. Furthermore, the maximum power of the contact-separated triboelectric nanogenerator with an EMU is 1.5 times that without the EMU. This excellent performance is attributed to the various optimizations, including utilizing an ultralow-loss spark-switch tube with a proper breakdown voltage, adding a matched input capacitor to enhance available charge, and incorporating an RF inductor to facilitate the high-speed energy transfer process. Based on this extremely efficient EMU, a compact self-powered wireless temperature sensor node was demonstrated to acquire and transmit data every 3.5 s under a slight wind speed of 0.5 m/s. This work greatly promotes the utilization of electrostatic nanogenerators in practical applications, particularly in IoT nodes.

Self-Powered Sensing in Wearable Electronics─A Paradigm Shift Technology

With the advancements in materials science and micro/nanoengineering, the field of wearable electronics has experienced a rapid growth and significantly impacted and transformed various aspects of daily human life. These devices enable individuals to conveniently access health assessments without visiting hospitals and provide continuous, detailed monitoring to create comprehensive health data sets for physicians to analyze and diagnose. Nonetheless, several challenges continue to hinder the practical application of wearable electronics, such as skin compliance, biocompatibility, stability, and power supply. In this review, we address the power supply issue and examine recent innovative self-powered technologies for wearable electronics. Specifically, we explore self-powered sensors and self-powered systems, the two primary strategies employed in this field. The former emphasizes the integration of nanogenerator devices as sensing units, thereby reducing overall system power consumption, while the latter focuses on utilizing nanogenerator devices as power sources to drive the entire sensing system. Finally, we present the future challenges and perspectives for self-powered wearable electronics.

A droplet friction/solar-thermal hybrid power generation device for energy harvesting in both rainy and sunny weathers

Photovoltaic device is highly dependent on the weather, which is completely ineffective on rainy days. Therefore, it is very significant to design an all-weather power generation system that can utilize a variety of natural energy. This work develops a water droplet friction power generation (WDFG)/solar-thermal power generation (STG) hybrid system. The WDFG consists of two metal electrodes and a candle soot/polymer composite film, which also can be regarded as a capacitor. Thus, the capacitor coupled power generation (C-WDFG) device can achieve a sustainable and stable direct-current (DC) output under continuous dripping without external conversion circuits. A single device can produce an open-circuit voltage of ca.0.52 V and a short-circuit current of ca.0.06 mA, which can be further scaled up through series or parallel connection to drive commercial electronics. Moreover, we demonstrate that the C-WDFG is highly compatible with the thermoelectric device. The excellent photothermal performance of soot/polymer composite film can efficiently convert solar into heat, which is then converted to electricity by the thermoelectric device. Therefore, this C-WDFG/STG hybrid system can work in both rainy and sunny days.

Self-Powered TENG with High Humidity Sensitivity from PVA Film Modified by LiCl and MXene

Triboelectric nanogenerators (TENGs) are promising for a variety of applications that require a reliable output performance and stability. In this work, by utilizing the synergistic effect of lithium chloride (LiCl) and MXene, poly(vinyl alcohol) (PVA) based composite films with humidity-sensitive properties were prepared and employed as a friction layer to achieve self-powered TENGs with enhanced output performance under high humidity. The composite material demonstrates exceptional and stable output performance in the humidity range of 30-95% while exhibiting a strong linear correlation with increasing relative humidity (RH). At 95% RH, its short-circuit current increases up to 31.91 μA, which is three times the output of the TENG fabricated by PVA and PTFE (P-TENG). The rich hydroxyl group in PVA, the strong hygroscopicity of LiCl, and the microcapacitor network provided by MXene nanosheets significantly improve the water absorption capacity and surface roughness of the composite material, resulting in an excellent triboelectric output of TENG. Short-circuit current of the TENG in a wide range of RH (from 50% to 98%) responds very sensitively to humidity fluctuations in the environment and superior adsorption-desorption performance as humidity decreases. Furthermore, TENG regarded as a power supply in high humidity conditions was realized and it can light up 240 LEDs instantaneously with the transfer charge density of TENG reaching 194.37 μC m-2. This technology presents an effective method for stable energy harvesting and self-powered sensing in fog, the ocean, and other high-humidity environments.

Recent Advances in Triboelectric Nanogenerators: From Technological Progress to Commercial Applications

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

Universal Energy Solution for Triboelectric Sensors Toward the 5G Era and Internet of Things

The launching of 5G technology provides excellent opportunity for the prosperous development of Internet of Things (IoT) devices and intelligent wireless sensor nodes. However, deploying of tremendous wireless sensor nodes network presents a great challenge to sustainable power supply and self-powered active sensing. Triboelectric nanogenerator (TENG) has shown great capability for powering wireless sensors and work as self-powered sensors since its discovery in 2012. Nevertheless, its inherent property of large internal impedance and pulsed "high-voltage and low-current" output characteristic seriously limit its direct application as stable power supply. Herein, a generic triboelectric sensor module (TSM) is developed toward managing the high output of TENG into signals that can be directly utilized by commercial electronics. Finally, an IoT-based smart switching system is realized by integrating the TSM with a typical vertical contact-separation mode TENG and microcontroller, which is able to monitor the real-time appliance status and location information. Such design of a universal energy solution for triboelectric sensors is applicable for managing and normalizing the wide output range generated from various working modes of TENGs and suitable for facile integration with IoT platform, representing a significant step toward scaling up TENG applications in future smart sensing.

An Advanced Strategy to Enhance TENG Output: Reducing Triboelectric Charge Decay

The Internet of Things (IoT) is poised to accelerate the construction of smart cities. However, it requires more than 30 billion sensors to realize the IoT vision, posing great challenges and opportunities for industries of self-powered sensors. Triboelectric nanogenerator (TENG), an emerging new technology, is capable of easily converting energy from surrounding environment into electricity, thus TENG has tremendous application potential in self-powered IoT sensors. At present, TENG encounters a bottleneck to boost output for large-scale commercial use if just by promoting triboelectric charge generation, because the output is decided by the triboelectric charge dynamic equilibrium between generation and decay. To break this bottleneck, the strategy of reducing triboelectric charge decay to enhance TENG output is focused. First, multiple mechanisms of triboelectric charge decay are summarized in detail with basic theoretical principles for future research. Furthermore, recent advances in reducing triboelectric charge decay are thoroughly reviewed and outlined in three aspects: inhibition and application of air breakdown, simultaneous inhibition of air breakdown and triboelectric charge drift/diffusion, and inhibition of triboelectric charge drift/diffusion. Finally, challenges and future research focus are proposed. This review provides reference and guidance for enhancing TENG output.

Metallic glass-based triboelectric nanogenerators

Surface wear is a major hindrance in the solid/solid interface of triboelectric nanogenerators (TENG), severely affecting their output performance and stability. To reduce the mechanical input and surface wear, solid/liquid-interface alternatives have been investigated; however, charge generation capability is still lower than that in previously reported solid/solid-interface TENGs. Thus, achieving triboelectric interface with high surface charge generation capability and low surface wear remains a technological challenge. Here, we employ metallic glass as one triboelectric interface and show it can enhance the triboelectrification efficiency by up to 339.2%, with improved output performance. Through mechanical and electrical characterizations, we show that metallic glass presents a lower friction coefficient and better wear resistance, as compared with copper. Attributed to their low atomic density and the absence of grain boundaries, all samples show a higher triboelectrification efficiency than copper. Additionally, the devices demonstrate excellent humidity resistance. Under different gas pressures, we also show that metallic glass-based triboelectric nanogenerators can approach the theoretical limit of charge generation, exceeding that of Cu-based TENG by 35.2%. A peak power density of 15 MW·m-2 is achieved. In short, this work demonstrates a humidity- and wear-resistant metallic glass-based TENG with high triboelectrification efficiency.

Fractal structured charge-excitation triboelectric nanogenerators for powering portable electronic devices

Effective power management on the outputs of triboelectric nanogenerators (TENGs) is critical for their practical applications due to the large impedance and unbalanced load matching. Recently proposed voltage multiplying circuits for external-charge excitation and self-charge excitation are usually unstable and require reversal for device restarting and common switched-capacitor-converters usually cause large switching losses. In this work, we fabricated a fractal structured charge-excitation circuit for TENGs using diodes and capacitors. The fractal switched-capacitor-converter coupled with voltage regulator diodes can greatly boost the output charge and current of the TENG without reverse starting. The managed output performance of the TENG can be controlled by the electronic component parameters and external operating frequency. Through the component and condition optimization, the fractal structured charge-excitation TENG (FSC-TENG) can achieve nearly 5.8 times charge boosting and almost 16.8 times power boosting in the pulsed mode. Furthermore, the FSC-TENG successfully drove a hygrothermograph and was integrated into a yoga mat for harvesting human-body motion energy to power an electronic watch and a pedometer. The FSC-TENG with good charge accumulation properties and stability is a promising candidate for practical self-powered applications.

Thin, soft, 3D printing enabled crosstalk minimized triboelectric nanogenerator arrays for tactile sensing

With the requirements of self-powering sensors in flexible electronics, wearable triboelectric nanogenerators (TENGs) have attracted great attention due to their advantages of excellent electrical outputs and low-cost processing routes. The crosstalk effect between adjacent sensing units in TENGs significantly limits the pixel density of sensor arrays. Here, we present a skin-integrated, flexible TENG sensor array with 100 sensing units in an overall size of 7.5 cm × 7.5 cm that can be processed in a simple, low-cost, and scalable way enabled by 3D printing. All the sensing units show good sensitivity of 0.11 V/kPa with a wide range of pressure detection from 10 to 65 kPa, which allows to accurately distinguish various tactile formats from gentle touching (as low as 2 kPa) to hard pressuring. The 3D printing patterned substrate allows to cast triboelectric layers of polydimethylsiloxane in an independent sensing manner for each unit, which greatly suppresses the cross talk arising from adjacent sensing units, where the maximum crosstalk output is only 10.8%. The excellent uniformity and reproducibility of the sensor array offer precise pressure mapping for complicated pattern loadings, which demonstrates its potential in tactile sensing and human-machine interfaces.

Toward a New Era of Sustainable Energy: Advanced Triboelectric Nanogenerator for Harvesting High Entropy Energy

Widely distributed across the environment, irregular micro-nano mechanical high entropy energy (HEE) is a new promising recoverable energy, in which the development of matched harvesting technology is imperative to fit in with the requirements of booming sustainable energy in the new era. The triboelectric nanogenerator (TENG) is a very efficient technology for harvesting micro-nano HEE, especially when converting irregular, low-frequency, weak mechanical energy into electricity. Here, the latest advancements are comprehensively reviewed in using TENGs for sustainable energy, sensing, and other applications. The fundamental theory and overwhelming superiority of TENG is systematically analyzed as a sustainable energy with four representative domains: micro-nano distributed power sources, self-powered sensing systems, direct high-voltage power sources, and large-scale blue energy. The review is concluded with a discussion of the challenges of leveraging TENGs for sustainable energy engineering. The striving directions of TENG technologies are proposed with a concentration on basic research and commercialization for the new ear of 5G and Internet of Things.

Triboelectric nanogenerators as wearable power sources and self-powered sensors

Smart wearable technologies are augmenting human bodies beyond our biological capabilities in communication, healthcare and recreation. Energy supply and information acquisition are essential for wearable electronics, whereas the increasing demands in multifunction are raising the requirements for energy and sensor devices. The triboelectric nanogenerator (TENG), proven to be able to convert various mechanical energies into electricity, can fulfill either of these two functions and therefore has drawn extensive attention and research efforts worldwide. The everyday life of a human body produces considerable mechanical energies and, in the meantime, the human body communicates mainly through mechanical signals, such as sound, body gestures and muscle movements. Therefore, the TENG has been intensively studied to serve as either wearable sources or wearable self-powered sensors. Herein, the recent finding on the fundamental understanding of TENGs is revisited briefly, followed by a summary of recent advancements in TENG-based wearable power sources and self-powered sensors. The challenges and prospects of this area are given as well.

Switchless Oscillating Charge Pump-Based Triboelectric Nanogenerator and an Additional Electromagnetic Generator for Harvesting Vertical Vibration Energy

According to energy crisis and increasing number of small electronics, energy harvesting is one of the most promising technologies for scavenging several types of wasted energy. Especially, with regard to vibrational energy harvesting, triboelectric nanogenerators and electromagnetic generators stand out due to their working mechanisms. Here, an oscillating charge pump-based hybrid generator with two triboelectric nanogenerators and an electromagnetic generator using a switching-free characteristic was fabricated with material optimization. To enhance the electrical output of the triboelectric nanogenerator, a concept of charge pumping with simply connecting an additional charge pump device and the nanostructure formation process on the contact metal layer were adopted. The electrical outputs were characterized as an output current of 9.17 μA with a current density of 83.2 mA m-3 in the triboelectric nanogenerator. The power density of the triboelectric nanogenerator reached 4.45 W m-3 with a 40 MΩ resistor. The electromagnetic generator showed an output current density of 0.91 A m-2. Moreover, with the cuboid structure, the device can sense the collision force with different angular displacements from 5 to 30°. The proposed hybrid generator with a simple structure can be applied to sensing applications with the one-point-fixed state.

Semiconductor Contact-Electrification-Dominated Tribovoltaic Effect for Ultrahigh Power Generation

The semiconductor direct-current triboelectric nanogenerator (SDC-TENG) based on the tribovoltaic effect is promising for developing a new semiconductor energy technology with high power density. Here, the first SDC-TENG built using gallium nitride (GaN) and bismuth telluride (Bi₂ Te₃ ) for ultrahigh-power generation is reported. During the friction process, an additional interfacial electric field is formed by continuous contact electrification (CE), and abundant electron-hole pairs are excited and move directionally to form a junction current that is always internally from Bi₂ Te₃ to GaN, regardless of the semiconductor type. The peak open-circuit voltage can reach up to 40 V and the power density is 11.85 W m^-2 (average value is 9.23 W m^-2 ), which is approximately 200 times higher than that of previous centimeter-level SDC-TENGs. Moreover, compared to traditional polymer TENGs under the same conditions, the average power density is remarkably improved by over 40 times. This study provides the first evidence of CE on the tribovoltaic effect and sets the normalized power density record for TENGs, which demonstrates a great potential of the tribovoltaic effect for energy harvesting and sensing.

Advances in High-Performance Autonomous Energy and Self-Powered Sensing Textiles with Novel 3D Fabric Structures

The seamless integration of emerging triboelectric nanogenerator (TENG) technology with traditional wearable textile materials has given birth to the next-generation smart textiles, i.e., textile TENGs, which will play a vital role in the era of Internet of Things and artificial intelligences. However, low output power and inferior sensing ability have largely limited the development of textile TENGs. Among various approaches to improve the output and sensing performance, such as material modification, structural design, and environmental management, a 3D fabric structural scheme is a facile, efficient, controllable, and scalable strategy to increase the effective contact area for contact electrification of textile TENGs without cumbersome material processing and service area restrictions. Herein, the recent advances of the current reported textile TENGs with 3D fabric structures are comprehensively summarized and systematically analyzed in order to clarify their superiorities over 1D fiber and 2D fabric structures in terms of power output and pressure sensing. The forward-looking integration abilities of the 3D fabrics are also discussed at the end. It is believed that the overview and analysis of textile TENGs with distinctive 3D fabric structures will contribute to the development and realization of high-power output micro/nanowearable power sources and high-quality self-powered wearable sensors.

Recent Progress of Switching Power Management for Triboelectric Nanogenerators

Based on the coupling effect of contact electrification and electrostatic induction, the triboelectric nanogenerator (TENG) as an emerging energy technology can effectively harvest mechanical energy from the ambient environment. However, due to its inherent property of large impedance, the TENG shows high voltage, low current and limited output power, which cannot satisfy the stable power supply requirements of conventional electronics. As the interface unit between the TENG and load devices, the power management circuit can perform significant functions of voltage and impedance conversion for efficient energy supply and storage. Here, a review of the recent progress of switching power management for TENGs is introduced. Firstly, the fundamentals of the TENG are briefly introduced. Secondly, according to the switch types, the existing power management methods are summarized and divided into four categories: travel switch, voltage trigger switch, transistor switch of discrete components and integrated circuit switch. The switch structure and power management principle of each type are reviewed in detail. Finally, the advantages and drawbacks of various switching power management circuits for TENGs are systematically summarized, and the challenges and development of further research are prospected.

Filling the gap between topological insulator nanomaterials and triboelectric nanogenerators

Reliable energy modules and higher-sensitivity, higher-density, lower-powered sensing systems are constantly required to develop wearable electronics and the Internet of Things technology. As an emerging technology, triboelectric nanogenerators have been potentially guiding the landscape of sustainable power units and energy-efficient sensors. However, the existing triboelectric series is primarily populated by polymers and rubbers, limiting triboelectric sensing plasticity to some extent owing to their stiff surface electronic structures. To enrich the current triboelectric group, we explore the triboelectric properties of the topological insulator nanofilm by Kelvin probe force microscopy and reveal its relatively positive electrification charging performance. Both the larger surface potential difference and the conductive surface states of the nanofilms synergistically improve the charge transfer behavior between the selected triboelectric media, endowing the topological insulator-based triboelectric nanogenerator with considerable output performance. Besides serving as a wearable power source, the ultra-compact device array demonstrates innovative system-level sensing capabilities, including precise monitoring of dynamic objects and real-time signal control at the human-machine interface. This work fills the blank between topological quantum matters and triboelectric nanogenerators and, more importantly, exploits the significant potential of topological insulator nanofilms for self-powered flexible/wearable electronics and scalable sensing technologies.

Wearable Triboelectric Sensors Enabled Gait Analysis and Waist Motion Capture for IoT-Based Smart Healthcare Applications

Gait and waist motions always contain massive personnel information and it is feasible to extract these data via wearable electronics for identification and healthcare based on the Internet of Things (IoT). There also remains a demand to develop a cost-effective human-machine interface to enhance the immersion during the long-term rehabilitation. Meanwhile, triboelectric nanogenerator (TENG) revealing its merits in both wearable electronics and IoT tends to be a possible solution. Herein, the authors present wearable TENG-based devices for gait analysis and waist motion capture to enhance the intelligence and performance of the lower-limb and waist rehabilitation. Four triboelectric sensors are equidistantly sewed onto a fabric belt to recognize the waist motion, enabling the real-time robotic manipulation and virtual game for immersion-enhanced waist training. The insole equipped with two TENG sensors is designed for walking status detection and a 98.4% identification accuracy for five different humans aiming at rehabilitation plan selection is achieved by leveraging machine learning technology to further analyze the signals. Through a lower-limb rehabilitation robot, the authors demonstrate that the sensory system performs well in user recognition, motion monitoring, as well as robot and gaming-aided training, showing its potential in IoT-based smart healthcare applications.

An Ultrarobust and High-Performance Rotational Hydrodynamic Triboelectric Nanogenerator Enabled by Automatic Mode Switching and Charge Excitation

The triboelectric nanogenerator (TENG) is an emerging technology for ambient mechanical energy harvesting, which provides a possibility to realize wild environment monitoring by self-powered sensing systems. However, TENGs are limited in some practical applications as a result of their low output performance (low charge density) and mechanical durability (material abrasion). Herein, an ultrarobust and high-performance rotational TENG enabled by automatic mode switching (contact mode at low speed and noncontact at high speed) and charge excitation is proposed. It displays excellent stability, maintaining 94% electrical output after 72 000 cycles, much higher than that of the normal contact-mode TENG (30%). Due to its high electrical stability and large electrical output, this TENG powers 944 green light-emitting diodes to brightness in series. Furthermore, by harvesting water-flow energy, various commercial capacitors can be charged quickly, and a self-powered fire alarm and self-powered temperature and humidity detection are realized. This work provides an ideal scheme for enhancing the mechanical durability, broadening the range of working frequency, and improving the electrical output of TENGs. In addition, the high-performance hydrodynamic TENG demonstrated in this work will have great applications for Internet of Things in remote areas.

A Single-Droplet Electricity Generator Achieves an Ultrahigh Output Over 100 V Without Pre-Charging

The working principle of the triboelectric nanogenerator (TENG), contact electrification and electrostatic induction, has been used to harvest raindrop energy in recent years. However, the existing research is mainly concentrated on solid-liquid electrification, and adopts traditional electrostatic induction (TEI) for output. As a result, the efficiency of droplet electricity generators (DEGs) is severely constrained. Therefore, previous studies deem that the DEG output is limited by interfacial effects. This study reveals that this view is inappropriate and, in reality, the output strategy is the key bottleneck restricting the DEG performance. Here, a switch effect based on an electric-double-layer capacitor (EDLC) is introduced, and an equivalent circuit model is established to understand its working mechanism. Without pre-charging, a single droplet can generate high voltage over 100 V and the output is directly improved by two-orders of magnitude compared with TEI, which is precisely utilizing the interfacial effect. This work provides insightful perspective and lays solid foundation for DEG applications in large scale.

Flexible triboelectric nanogenerator based on polyester conductive cloth for biomechanical energy harvesting and self-powered sensors

As a new nanotechnology of mechanical energy harvesting and self-powered sensing, triboelectric nanogenerators (TENGs) have been explored as a new path of using various low-frequency disordered mechanical energies in the surrounding environment to provide power and/or sensing. However, the research of TENGs that provide full flexibility and environmental friendliness is still limited. Herein, a flexible single-electrode TENG (S-TENG) based on polyester conductive cloth as the working electrode is developed to harvest human motion energy for powering light emitting diodes (LEDs) and portable electronics. The flat conductive cloth was wrapped in a flexible elastomer. Defatted cowhide was firstly selected as a friction positive charge material for TENGs. When the size of the fabricated S-TENG is 40 × 100 mm², high output performance has been achieved and it can generate an open-circuit voltage of 534 V and a power density of 230 mW m^-2 at an operation frequency of 3.0 Hz. After integrating with a rectifier, the S-TENG can power 240 LEDs, charge various capacitors, and drive an electronic watch or a calculator. Moreover, the S-TENG can harvest the biomechanical energy of wrist movement, hand tapping, and human walking. Meanwhile, the S-TENG as a self-powered sensor can be employed to monitor subtle signals of human physiological activities, such as finger motion, facial masseter activity, and diaphragmatic breathing. Additionally, the S-TENG can be attached to clothes (such as wool coats, polyamide sweaters) to harvest the energy of cuff movement. Therefore, this work provides new insights for clean power sources of skin-mounted electronics and promotes the development of a sustainable energy supply for wearable and portable electronics.

Large-Scale Lever-Based Triboelectric Nanogenerator for Sensing Lateral Vibration and Wrist or Finger Bending for Controlling Shooting Game

With advances in internet of things technology and fossil fuel depletion, energy harvesting has emerged rapidly as a means of supplying small electronics with electricity. As a method of enhancing the electrical output of the triboelectric nanogenerator, specialized for harvesting mechanical energy, structural modification to amplify the input force is receiving attention due to the limited input energy level. In this research, a lever structure was employed for delivering the amplified input force to a triboelectric nanogenerator. With structural optimization of a 2.5 cm : 5 cm distance ratio of the first and second parts using two lever structures, the highest electrical outputs were achieved: a V_OC of 51.03 V, current density of 3.34 mA m⁻², and power density of 73.5 mW m⁻² at 12 MΩ in the second part. As applications of this triboelectric generator, a vertical vibration sensor and a wearable reloading trigger in a gun shooting game were demonstrated. The possibility for a wearable finger bending sensor with low-level input was checked using a minimized device. Enhanced low-detection limit with amplified input force from the structural advantage of this lever-based triboelectric nanogenerator device can expand its applicability to the mechanical trigger for wearable electronics.

Achieving ultrahigh instantaneous power density of 10 MW/m2 by leveraging the opposite-charge-enhanced transistor-like triboelectric nanogenerator (OCT-TENG)

Converting various types of ambient mechanical energy into electricity, triboelectric nanogenerator (TENG) has attracted worldwide attention. Despite its ability to reach high open-circuit voltage up to thousands of volts, the power output of TENG is usually meager due to the high output impedance and low charge transfer. Here, leveraging the opposite-charge-enhancement effect and the transistor-like device design, we circumvent these limitations and develop a TENG that is capable of delivering instantaneous power density over 10 MW/m² at a low frequency of ~ 1 Hz, far beyond that of the previous reports. With such high-power output, 180 W commercial lamps can be lighted by a TENG device. A vehicle bulb containing LEDs rated 30 W is also wirelessly powered and able to illuminate objects further than 0.9 meters away. Our results not only set a record of the high-power output of TENG but also pave the avenues for using TENG to power the broad practical electrical appliances.

TENG suffers from two fundamental limitations: high output impedance and low charge transfer. Herein, these limitations are circumvented by leveraging the opposite-charge-enhancement effect and transistor-like device design, thereby achieving the instantaneous power density over 10 MW/m² at the low frequency of ~ 1 Hz.

Ultrathin Stretchable Triboelectric Nanogenerators Improved by Postcharging Electrode Material

Sustainable ultrathin stretchable power sources have emerged with the development of wearable electronics. They obtain energy from living organisms and the environment to drive these wearable electronics. Here, an ultrathin stretchable and triboelectric nanogenerator (TENG) improved by chargeable carbon black (CB)/thermoplastic polyurethane (TPU) composite material (CT-TENG) is proposed for mechanical energy harvesting and physiological signal sensing. The CB/TPU composite can act as both a stretchable electrode and a triboelectric layer due to the coexistence of conductive CB and dielectric TPU. The CT-TENG demonstrates good stretchability (≈646%), ultrathin thickness (≈50 μm), and a lightweight (≈62 mg). The triboelectric electrode material can be improved by postcharging treatment. With the corona charging process, the output performance of the CT-TENG was improved eightfold and reached 41 V. Moreover, the CT-TENG with a self-powered sensing capability can inspect the amplitude and frequency of different physiological movements. Consequently, the CT-TENG is promising in promoting the development of electronic skins, wearable systems of self-powered sensors, human-machine interactions, soft robotics, and artificial intelligence applications.

Triboelectric Nanogenerator-Based Sensor Systems for Chemical or Biological Detection

The rapid advances in the Internet of things and wearable devices have created a massive platform for sensor systems that detect chemical or biological agents. The accelerated development of these devices in recent years has simultaneously aggravated the power supply problems. Triboelectric nanogenerators (TENGs) represent a thriving renewable energy technology with the potential to revolutionize this field. In this review, the significance of TENG-based sensor systems in chemical or biological detection from the perspective of the development of power supply for biochemical sensors is discussed. Further, a range of TENGs are classified according to their roles as power supplies and/or self-powered active sensors. The TENG powered sensor systems are further discussed on the basis of their framework and applications. The working principles and structures of different TENG-based self-powered active sensors are presented, along with the classification of the sensors based on these factors. In addition, some representative applications are introduced, and the corresponding challenges are discussed. Finally, some perspectives for the future innovations of TENG-based sensor systems for chemical/biological detection are discussed.

Nodding Duck Structure Multi-track Directional Freestanding Triboelectric Nanogenerator toward Low-Frequency Ocean Wave Energy Harvesting

Development of ocean energy conversion technique is a strategic requirement to optimize the energy structure and expand the "blue economic" space. Triboelectric nanogenerator (TENG) provides a potential approach for efficiently capturing wave energy with its unique advantages. Herein, a nodding duck structure multi-track freestanding triboelectric-layer nanogenerator (NDM-FTENG) is developed for ocean wave energy harvesting at a low-frequency range. Configuration parameters including track numbers, connection approach, oscillation frequency, and swing amplitude on electrical output performances of NDM-FTENG are systematically investigated and optimized; the maximum instantaneous power density of 4 W/m³ is obtained by one NDM-FTENG block with 320 LEDs lighted up simultaneously. Overall, NDM-FTENG is proved to be an efficient device for driving small electronics with excellent stability and durability in a real water wave environment, and the power potential can be further magnified by combining more NDM-FTENG devices in parallel to form a network toward large-scale blue energy harvesting.

A universal managing circuit with stabilized voltage for maintaining safe operation of self-powered electronics system

Harvesting mechanical energy via a triboelectric nanogenerator (TENG) is a promising strategy for solving energy problems. However, it is necessary to develop an effective and safe energy managing circuit for preventing high voltage breaking electronic devices. Here, a universal managing circuit is developed to optimize TENG's output performance, which for the first time allows the TENG to safely power various sensor systems with a safe and stable voltage. Based on the circuit, TENG's output can be transformed into a stable voltage with tunable amplitude, while an enhanced short-circuit current of 94 mA with an energy loss lower than 5% is achieved. For demonstrations, three different types of TENGs, respectively, targeting at ocean energy, wind energy, and walking energy have been prepared to reveal the capability of the circuit. This study offers a strategy to greatly enhance the output performance of TENGs to provide useful guidance for constructing self-powered and distributed sensor systems.

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UMC is designed for a TENG to maintain stable voltage with a lower resistance

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UMC provides a short-circuit current of 94 mA with an energy loss lower than 5%

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UMC can completely avoid the breakdown of electronic devices due to TENG's high voltage

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Three self-powered sensor systems have been successfully established

Boosting the Power and Lowering the Impedance of Triboelectric Nanogenerators through Manipulating the Permittivity for Wearable Energy Harvesting

Triboelectric nanogenerators (TENGs), which hold great promise for sustainably powering wearable electronics by harvesting distributed mechanical energy, are still severely limited by their unsatisfactory power density, small capacitance, and high internal impedance. Herein, a materials optimization strategy is proposed to achieve a high performance of TENGs and to lower the matching impedance simultaneously. A permittivity-tunable electret composite film, i.e., a thermoplastic polyurethane (TPU) matrix with polyethylene glycol (PEG) additives and polytetrafluoroethylene (PTFE) nanoparticle inclusions, is employed as the triboelectric layer. Through optimizing the dielectric constant of the composite, the injected charge density and internal capacitance of the TENG are significantly enhanced, thus synergistically boosting the output power and reducing the impedance of the TENG. The optimal output power reaches 16.8 mW at an external resistance of 200 kΩ, showing a 17.3 times enhancement in output power and a 90% decline in matching impedance. This work demonstrates a significant progress toward the materials optimization of a triboelectric generator for its practical commercialization.

Recent Advances in Self-Powered Electrochemical Systems

Electrochemistry, one of the most important research and production technology, has been widely applicated in various fields. However, the requirement of external power source is a major challenge to its development. To solve this issue, developing self-powered electrochemical system (SPES) that can work by collecting energy from the environment is highly desired. The invention of triboelectric nanogenerator (TENG), which can transform mechanical energy into electricity, is a promising approach to build SPES by integrating with electrochemistry. In this view, the latest representative achievements of SPES based on TENG are comprehensively reviewed. By harvesting various mechanical energy, five SPESs are built, including electrochemical pollutants treatment, electrochemical synthesis, electrochemical sensor, electrochromic reaction, and anticorrosion system, according to the application domain. Additionally, the perspective for promoting the development of SPES is discussed.

Triboelectric Nanogenerator: Structure, Mechanism, and Applications

With the rapid development of the Internet of Things (IoT), the number of sensors utilized for the IoT is expected to exceed 200 billion by 2025. Thus, sustainable energy supplies without the recharging and replacement of the charge storage device have become increasingly important. Among various energy harvesters, the triboelectric nanogenerator (TENG) has attracted considerable attention due to its high instantaneous output power, broad selection of available materials, eco-friendly and inexpensive fabrication process, and various working modes customized for target applications. The TENG harvests electrical energy from wasted mechanical energy in the ambient environment. Three types of operational modes based on contact-separation, sliding, and freestanding are reviewed for two different configurations with a double-electrode and a single-electrode structure in the TENGs. Various charge transfer mechanisms to explain the operational principles of TENGs during triboelectrification are also reviewed for electron, ion, and material transfers. Thereafter, diverse methodologies to enhance the output power considering the energy harvesting efficiency and energy transferring efficiency are surveyed. Moreover, approaches involving not only energy harvesting by a TENG but also energy storage by a charge storage device are also reviewed. Finally, a variety of applications with TENGs are introduced. This review can help to advance TENGs for use in self-powered sensors, energy harvesters, and other systems. It can also contribute to assisting with more comprehensive and rational designs of TENGs for various applications.

Triboelectric Nanogenerators and Hybridized Systems for Enabling Next-Generation IoT Applications

In the past few years, triboelectric nanogenerator-based (TENG-based) hybrid generators and systems have experienced a widespread and flourishing development, ranging among almost every aspect of our lives, e.g., from industry to consumer, outdoor to indoor, and wearable to implantable applications. Although TENG technology has been extensively investigated for mechanical energy harvesting, most developed TENGs still have limitations of small output current, unstable power generation, and low energy utilization rate of multisource energies. To harvest the ubiquitous/coexisted energy forms including mechanical, thermal, and solar energy simultaneously, a promising direction is to integrate TENG with other transducing mechanisms, e.g., electromagnetic generator, piezoelectric nanogenerator, pyroelectric nanogenerator, thermoelectric generator, and solar cell, forming the hybrid generator for synergetic single-source and multisource energy harvesting. The resultant TENG-based hybrid generators utilizing integrated transducing mechanisms are able to compensate for the shortcomings of each mechanism and overcome the above limitations, toward achieving a maximum, reliable, and stable output generation. Hence, in this review, we systematically introduce the key technologies of the TENG-based hybrid generators and hybridized systems, in the aspects of operation principles, structure designs, optimization strategies, power management, and system integration. The recent progress of TENG-based hybrid generators and hybridized systems for the outdoor, indoor, wearable, and implantable applications is also provided. Lastly, we discuss our perspectives on the future development trend of hybrid generators and hybridized systems in environmental monitoring, human activity sensation, human-machine interaction, smart home, healthcare, wearables, implants, robotics, Internet of things (IoT), and many other fields.

Network Topology Optimization of Triboelectric Nanogenerators for Effectively Harvesting Ocean Wave Energy

The emerging triboelectric nanogenerator (TENG) network shows great potential in harvesting the ocean wave energy, which can help to achieve large-scale clean wave power generation. However, due to the lack of an effective networking strategy and theoretical guidance, the practicability of the TENG network is heavily restricted. In this paper, based on the typical spherical TENG, we investigated the networking design of TENGs. Four fundamental forms of electrical networking topology are proposed for large-scale TENG networks, and the influences of cable resistance and output phase asynchrony of each unit to the network output were systematically investigated. The research results show that the forms of electrical networking topology can produce an important influence on the output power of large-scale TENG networks. This is the first strategy analysis for the TENG network, which provides a theoretical basis and a universal method for the optimization design of large-scale power networks.

Self-charging power system for distributed energy: beyond the energy storage unit

Power devices for the smart sensor networks of Internet of things (IoT) are required with minimum or even no maintenance due to their enormous quantities and widespread distribution. Self-charging power systems (SCPSs) refer to integrated energy devices with simultaneous energy harvesting, power management and effective energy storage capabilities, which may need no extra battery recharging and can sustainably drive sensors. Herein, we focus on the progress made in the field of nanogenerator-based SCPSs, which harvest mechanical energy using the Maxwell displacement current arising from the variation in the surface-polarized-charge-induced electrical field. Prototypes of different nanogenerator-based SCPSs will be overviewed. Finally, challenges and prospects in this field will be discussed.

Triboelectric nanogenerators based on elastic electrodes

New technologies such as the Internet of Things and big data have become the strategic focus of national development in the world. Triboelectric nanogenerators are one of the important technologies to solve the problem of distributed energy supply of wireless sensor networks. Since the invention of the triboelectric nanogenerator in 2012, it has attracted extensive attention due to its light weight, low cost, high flexibility, and the diversity of its function. Different from the common rigid inelastic electrode, the elastic electrode is deformable, flexible, and stretchable, which is significant for some specific triboelectric nanogenerators to expand their function. In this review, the latest achievements and research studies of triboelectric nanogenerators based on elastic electrodes are summarized. In addition, the basic classifications, fabrication processes, material selections, structural designs, and working mechanisms regarding the elastic electrode are comprehensively and systematically reviewed. Finally, the future perspectives and remaining challenges of this field are discussed.

Boosting output performance of sliding mode triboelectric nanogenerator by charge space-accumulation effect

The sliding mode triboelectric nanogenerator (S-TENG) is an effective technology for in-plane low-frequency mechanical energy harvesting. However, as surface modification of tribo-materials and charge excitation strategies are not well applicable for this mode, output performance promotion of S-TENG has no breakthrough recently. Herein, we propose a new strategy by designing shielding layer and alternative blank-tribo-area enabled charge space-accumulation (CSA) for enormously improving the charge density of S-TENG. It is found that the shielding layer prevents the air breakdown on the interface of tribo-layers effectively and the blank-tribo-area with charge dissipation on its surface of tribo-material promotes charge accumulation. The charge space-accumulation mechanism is analyzed theoretically and verified by experiments. The charge density of CSA-S-TENG achieves a 2.3 fold enhancement (1.63 mC m-2) of normal S-TENG in ambient conditions. This work provides a deep understanding of the working mechanism of S-TENG and an effective strategy for promoting its output performance.