Quantifying Dielectric Material Charge Trapping and De-Trapping Ability Via Ultra-Fast Charge Self-Injection Technique.

Recently, utilizing the air breakdown effect in the charge excitation strategy proves as an efficient charge injection technique to increase the surface charge density of dielectric polymers for triboelectric nanogenerators (TENGs). However, quantitative characterization of the ability of dielectric polymers to trap reverse charges and the effect on the startup time of secondary self-charge excitation (SSCE) are essential for extensive applications. Here, an ultra-fast charge self-injection technique based on a self-charge excitation strategy is proposed, and a standard method to quantify the charge trapping and de-trapping abilities of 23 traditional tribo-materials is introduced. Further, the relationship among the distribution of dielectric intrinsic deep, shallow trap states, and transportation of trapped charges is systematically analyzed in this article. It shows that the de-trapping rate of charges directly determines the reactivation and failure of SSCE. Last, independent of TENG contact efficiency, an ultra-high charge density of 2.67 mC m-2 and an ultra-fast startup time of SSCE are obtained using a 15 µm poly(vinylidene fluoride-trifluoroethylene) film, breaking the historical record for material modification. As a standard for material selection, this work quantifies the charge trapping and de-trapping ability of the triboelectric dielectric series and provides insights for understanding the charge transport in dielectrics.

Boosting Output Performance of Sliding Mode Triboelectric Nanogenerator by Shielding Layer and Shrouded-Tribo-Area Optimized Ternary Electrification Layered Architecture.

Sliding mode triboelectric nanogenerator (S-TENG) is effective for low-frequency mechanical energy harvesting owing to their more efficient mechanical energy extraction capability and easy packaging. Ternary electrification layered (TEL) architecture is proven useful for improving the output performance of S-TENG. However, the bottleneck of electric output is the air breakdown on the interface of tribo-layers, which seriously restricts its further improvement. Herein, a strategy is adopted by designing a shielding layer to prevent air breakdown on the central surface of tribo-layers. And the negative effects of air breakdown on the edge of sliding layer are averted by increasing the shrouded area of tribo-layers on slider. Output charge of this shielding-layer and shrouded-tribo-area optimized ternary electrification layered triboelectric nanogenerator (SS-TEL-TENG) achieves 3.59-fold enhancement of traditional S-TENG and 1.76-fold enhancement of TEL-TENG. Furthermore, even at a very low speed of 30 rpm, output charge, current, and average power of the rotation-type SS-TEL-TENG reach 4.15 µC, 74.9 µA, and 25.4 mW (2.05 W m-2 Hz-1 ), respectively. With such high-power output, 4248 LEDs can be lighted brightly by SS-TEL-TENG directly. The high-performance SS-TEL-TENG demonstrated in this work will have great applications for powering ubiquitous sensor network in the Internet of Things (IoT).

Conversion of Dielectric Surface Effect into Volume Effect for High Output Energy.

Improving the output energy and durability of triboelectric nanogenerators (TENGs) remains a considerable challenge for their practical applications. Owing to the interface effect of triboelectrification and electrostatic induction, thinner films with higher dielectric constants yield a higher output; however, they are not durable for practical applications. Herein, the dielectric surface effect is changed into a volume effect by adopting a millimeter-thick dielectric film with an inner porous network structure so that charges can hop in the surface state of the network. Charge migration inside the dielectric film is the key factor affecting the output of the triboelectric nanogenerator (TENG) with a thick film, based on which each working stage follows the energy-maximization principle in the voltage-charge plot. The maximum peak and average power densities of the TENG with polyurethane foam film in 1 mm thickness reach 40.9 and 20.7 W m-2  Hz-1 , respectively, under environmental conditions, and the output charge density is 5.14 times that of TENGs with a poly(tetrafluoroethylene) film of the same thickness. Superdurability is achieved in the rotary-mode TENG after 200 000 operation cycles. This study identifies the physical mechanism of the thick dielectric film used in TENGs and provides a new approach to promote the output and durability of TENGs.

Harvesting Environment Mechanical Energy by Direct Current Triboelectric Nanogenerators.

As hundreds of millions of distributed devices appear in every corner of our lives for information collection and transmission in big data era, the biggest challenge is the energy supply for these devices and the signal transmission of sensors. Triboelectric nanogenerator (TENG) as a new energy technology meets the increasing demand of today's distributed energy supply due to its ability to convert the ambient mechanical energy into electric energy. Meanwhile, TENG can also be used as a sensing system. Direct current triboelectric nanogenerator (DC-TENG) can directly supply power to electronic devices without additional rectification. It has been one of the most important developments of TENG in recent years. Herein, we review recent progress in the novel structure designs, working mechanism and corresponding method to improve the output performance for DC-TENGs from the aspect of mechanical rectifier, tribovoltaic effect, phase control, mechanical delay switch and air-discharge. The basic theory of each mode, key merits and potential development are discussed in detail. At last, we provide a guideline for future challenges of DC-TENGs, and a strategy for improving the output performance for commercial applications.

Large Harvested Energy by Self-Excited Liquid Suspension Triboelectric Nanogenerator with Optimized Charge Transportation Behavior.

To enhance the durability of triboelectric nanogenerator (TENG), liquid lubrication has been used to reduce mechanical abrasion. However, as the charge transportation behavior in dielectric liquid is not clearly understood, the output energy is still low although some improvements have been reported. Herein, the charge transportation behaviors in dielectric liquid by self-excited liquid suspension triboelectric nanogenerator (LS-TENG) are systematically investigated. The important role of solid-liquid triboelectrification effect, charge-liquid transmission and dissipation effect, and the homogeneous dielectric induction effect in promoting its output performance is found. The LS-TENG with a dual dielectric tribolayer has advantages of slight driving force and long lifetime for harvesting micro energy. The output of LS-TENG remains almost constant for more than 234 k operating cycles. A high charge density of 704 µC m-2 is obtained, 2.7 times as much as that of the current highest record in non-contact TENG. Additionally, the rotary LS-TENG lights up 4200 LEDs and continuously powers a variety of wireless sensors by harvesting wind energy at low wind speed. This work provides an important insight toward the charge transportation mechanism in dielectric liquid, and a prospective strategy for achieving highly robust TENG in micro energy harvesting for practical applications.

Matching Mechanism of Charge Excitation Circuit for Boosting Performance of a Rotary Triboelectric Nanogenerator.

The triboelectric nanogenerator (TENG) as an ideal low-frequency mechanical energy harvester has received extensive attention, while low output charge density limits its application. A charge excitation strategy is one of the techniques to effectively improve the surface charge density of the TENG. However, there is little in-depth research on the matching factors between the TENG and excitation circuit. Herein, a soft-contact charge excitation rotary TENG (SCER-TENG) is developed to explore the matching mechanism of different charge excitation strategies. The total output power transferred by the voltage-multiplying circuit (VMC) is 2.13 times that of the full-wave bridge rectifier, which effectively improves the output performance of the SCER-TENG. Moreover, through the established capacitor model and the theoretically calculated maximum output charge of the SCER-TENG with VMC and Zener diodes (VMC-Z), it is found that the output of the Main TENG is mainly affected by capacitors and Zener diodes. The theories have been verified by experiments. After optimization, the output charge of the Main TENG with VMC-Z (1.54 µC) is 3850% higher than that without excitation (0.04 µC). The SCER-TENG successfully harvests low-speed (2.5 m s-1) wind energy to form a self-powered system. This work has crucial instructive implications for using charge excitation strategies to improve the performance of the rotary TENG.

Ultrahigh Performance Triboelectric Nanogenerator Enabled by Charge Transmission in Interfacial Lubrication and Potential Decentralization Design.

Triboelectric nanogenerator (TENG) is a promising strategy for harvesting low frequency mechanical energy. However, the bottlenecks of limited electric output by air/dielectric breakdown and poor durability by material abrasion seriously restrict its further improvement. Herein, we propose a liquid lubrication promoted sliding mode TENG to address both issues. Liquid lubrication greatly reduces interface material abrasion, and its high breakdown strength and charge transmission effect further enhance device charge density. Besides, the potential decentralization design by the voltage balance bar effectively suppresses the dielectric breakdown. In this way, the average power density up to 87.26 W·m-2·Hz-1, energy conversion efficiency of 48%, and retention output of 90% after 500,000 operation cycles are achieved, which is the highest average power density and durability currently. Finally, a cell phone is charged to turn on by a palm-sized TENG device at 2 Hz within 25 s. This work has a significance for the commercialization of TENG-based self-powered systems.

High Output Performance and Ultra-Durable DC Output for Triboelectric Nanogenerator Inspired by Primary Cell.

Triboelectric nanogenerator (TENG) is regarded as an effective strategy to convert environment mechanical energy into electricity to meet the distributed energy demand of large number of sensors in the Internet of Things (IoTs). Although TENG based on the coupling of triboelectrification and air-breakdown achieves a large direct current (DC) output, material abrasion is a bottleneck for its applications. Here, inspired by primary cell and its DC signal output characteristics, we propose a novel primary cell structure TENG (PC-TENG) based on contact electrification and electrostatic induction, which has multiple working modes, including contact separation mode, freestanding mode and rotation mode. The PC-TENG produces DC output and operates at low surface contact force. It has an ideal effective charge density (1.02 mC m-2). Meanwhile, the PC-TENG shows a superior durability with 99% initial output after 100,000 operating cycles. Due to its excellent output performance and durability, a variety of commercial electronic devices are powered by PC-TENG via harvesting wind energy. This work offers a facile and ideal scheme for enhancing the electrical output performance of DC-TENG at low surface contact force and shows a great potential for the energy harvesting applications in IoTs.

Anti-Overturning Fully Symmetrical Triboelectric Nanogenerator Based on an Elliptic Cylindrical Structure for All-Weather Blue Energy Harvesting.

Triboelectric nanogenerators (TENGs) have shown promising potential for large-scale blue energy harvesting. However, the lack of reasonable designs has largely hindered TENG from harvesting energy from both rough and tranquil seas. Herein, a fully symmetrical triboelectric nanogenerator based on an elliptical cylindrical structure (EC-TENG) is proposed for all-weather blue energy harvesting. The novel elliptical cylindrical shell provides a unique self-stability, high sensitivity to wave triggering, and most importantly, an anti-overturning capability for the EC-TENG. Moreover, benefiting from its internal symmetrical design, the EC-TENG can produce energy normally, even if it was overturned under a rude oscillation in the rough seas, which distinguishes this work from previous reported TENGs. The working mechanism and output performance are systematically studied. The as-fabricated EC-TENG is capable of lighting 400 light-emitting diodes and driving small electronics. More than that, an automatic monitoring system powered by the EC-TENG can also monitor the water level in real-time and provide an alarm if necessary. This work presents an innovative and reliable approach toward all-weather wave energy harvesting in actual marine environments.

Deep Learning Enabled Neck Motion Detection Using a Triboelectric Nanogenerator.

The state of neck motion reflects cervical health. To detect the motion state of the human neck is of important significance to healthcare intelligence. A practical neck motion detector should be wearable, flexible, power efficient, and low cost. Here, we report such a neck motion detector comprising a self-powered triboelectric sensor group and a deep learning block. Four flexible and stretchable silicon rubber based triboelectric sensors are integrated on a neck collar. With different neck motions, these four sensors lead-out voltage signals with different amplitudes and/or directions. Thus, the combination of these four signals can represent one motion state. Significantly, a carbon-doped silicon rubber layer is attached between the neck collar and the sensors to shield the external electric field (i.e., electrical changes at the skin surface) for a far more robust identification. Furthermore, a deep learning model based on the convolutional neural network is designed to recognize 11 classes of neck motion including eight directions of bending, two directions of twisting, and one resting state with an average recognition accuracy of 92.63%. This developed neck motion detector has promising applications in neck monitoring, rehabilitation, and control.

Achieving Remarkable Charge Density via Self-Polarization of Polar High-k Material in a Charge-Excitation Triboelectric Nanogenerator.

Boosting output charge density is top priority for achieving high-performance triboelectric nanogenerators (TENGs). The charge-excitation strategy is demonstrated to be a superior approach to acquire high output charge density. Meanwhile, the molecular charge behaviors in the dielectric under a strong electric field from high charge density bring new physics that are worth exploring. Here, a rapid self-polarization effect of a polar dielectric material by the superhigh electric field in a charge-excitation TENG is reported, by which the permittivity of the polar dielectric material realizes self-increase to a saturation, and thus enhances the output charge density. Consequently, an ultrahigh charge density of 3.53 mC m-2 is obtained with 7 µm homemade lead zirconate titanate-poly(vinylidene fluoride) composite film in the atmosphere with 5% relative humidity, which is the highest charge density for TENGs with high durability currently. This work provides new guidance for dielectric material optimization under charge excitation to boost the output performance of TENGs toward practical 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.

Enhanced electrochemical performance in an aluminium doped δ-MnO2 supercapacitor cathode: experimental and theoretical investigations.

One of the biggest challenges faced by the layered manganese oxide MnO2 used as a supercapacitor cathode is the capacity fading caused by chemo-mechanical degradation and/or structural transformation occurring in the discharging/charging process. Here, based on systematical experimental measurements and theoretical calculations, we show that both the stability and rate performance of the δ-MnO2 supercapacitor cathode can be significantly enhanced by Al doping. Compared with pure δ-MnO2, Al doping (δ-Al0.06MnO2) clearly improves the specific capacitance (7% enhancement at 0.1 mA cm-2) and cycling stability (12% enhancement after 5000 cycles) simultaneously. These improvements can be attributed to the enhanced electronic transport and formation of more active sites, which are introduced by Al doping. Additionally, our calculations demonstrate that the doped systems (Al atoms located at Mn or O sites) show smaller surface energies than that of pure δ-MnO2, which hinders side reactions or structure transformations and leads to a better cycling lifetime. Our work gives a comprehensive understanding of the impacts on the performance of δ-MnO2 introduced by Al doping, and provides a feasible scheme to study the electrochemical mechanism of metal-doped δ-MnO2.

High performance floating self-excited sliding triboelectric nanogenerator for micro mechanical energy harvesting.

Non-contact triboelectric nanogenerator (TENG) enabled for both high conversion efficiency and durability is appropriate to harvest random micro energy owing to the advantage of low driving force. However, the low output (<10 µC m-2) of non-contact TENG caused by the drastic charge decay limits its application. Here, we propose a floating self-excited sliding TENG (FSS-TENG) by a self-excited amplification between rotator and stator to achieve self-increased charge density, and the air breakdown model of non-contact TENG is given for a maximum charge density. The charge density up to 71.53 µC m-2 is achieved, 5.46 times as that of the traditional floating TENG. Besides, the high output enables it to continuously power small electronics at 3 m s-1 weak wind. This work provides an effective strategy to address the low output of floating sliding TENG, and can be easily adapted to capture the varied micro mechanical energies anywhere.

Zn induced NiCo composites modified by carbon materials as a battery-type electrode material for high-performance supercapacitors.

The development of simple preparation and excellent capacity performance electrode materials is the key to energy conversion and storage for supercapacitors. Based on the growth mechanism of crystal, Zn induced NiCo nanosheets and nanoneedles composite structure deposed on Ni foam (ZNC) are successfully attained by a facile one-step method, the growth mechanism of the composite structure is further discussed. Because of its unique composites structure and additional modification of carbon, the carbon modified ZNC (ZNC@C) delivers better energy storage ability (2280 mC cm-2at 2 mA cm-2) compare to ZNC. An asymmetric supercapacitor (ASC) is assembled by ZNC@C as the positive electrode and carbonized popcorn as the negative electrode. The ASC exhibits good energy storage performance. Zn also positively affects the adsorption energy to enhance the capacitance property based on Density Functional theory calculation. The simple method for the composite structure by tuning the kinetics behaver of the crystal can provide a new strategy in synthesizing the materials, and the material with a unique structure and high performance will have potential applications in the field of energy storage.

A Mobile and Self-Powered Micro-Flow Pump Based on Triboelectricity Driven Electroosmosis.

Electroosmotic pumps have been widely used in microfluidic systems. However, traditional high-voltage (HV)-sources are bulky in size and induce numerous accessional reactions, which largely reduce the system's portability and efficiency. Herein, a motion-controlled, highly efficient micro-flow pump based on triboelectricity driven electroosmosis is reported. Utilizing the triboelectric nanogenerator (TENG), a strong electric field can be formed between two electrodes in the microfluidic channel with an electric double layer, thus driving the controllable electroosmotic flow by biomechanical movements. The performance and operation mechanism of this triboelectric electroosmotic pump (TEOP) is systematically studied and analyzed using a basic free-standing mode TENG. The TEOP produces ≈600 nL min-1 micro-flow with a Joule heat down to 1.76 J cm-3 nL-1 compared with ≈50 nL min-1 and 8.12 J cm-3 nL-1 for an HV-source. The advantages of economy, efficiency, portability, and safety render the TEOP a more conducive option to achieve wider applications in motion-activated micro/nanofluidic transportation and manipulation.

Magnetic Array Assisted Triboelectric Nanogenerator Sensor for Real-Time Gesture Interaction.

In human-machine interaction, robotic hands are useful in many scenarios. To operate robotic hands via gestures instead of handles will greatly improve the convenience and intuition of human-machine interaction. Here, we present a magnetic array assisted sliding triboelectric sensor for achieving a real-time gesture interaction between a human hand and robotic hand. With a finger's traction movement of flexion or extension, the sensor can induce positive/negative pulse signals. Through counting the pulses in unit time, the degree, speed, and direction of finger motion can be judged in real-time. The magnetic array plays an important role in generating the quantifiable pulses. The designed two parts of magnetic array can transform sliding motion into contact-separation and constrain the sliding pathway, respectively, thus improve the durability, low speed signal amplitude, and stability of the system. This direct quantization approach and optimization of wearable gesture sensor provide a new strategy for achieving a natural, intuitive, and real-time human-robotic interaction.

Bionic Ultra-Sensitive Self-Powered Electromechanical Sensor for Muscle-Triggered Communication Application.

The past few decades have witnessed the tremendous progress of human-machine interface (HMI) in communication, education, and manufacturing fields. However, due to signal acquisition devices' limitations, the research on HMI related to communication aid applications for the disabled is progressing slowly. Here, inspired by frogs' croaking behavior, a bionic triboelectric nanogenerator (TENG)-based ultra-sensitive self-powered electromechanical sensor for muscle-triggered communication HMI application is developed. The sensor possesses a high sensitivity (54.6 mV mm-1 ), a high-intensity signal (± 700 mV), and a wide sensing range (0-5 mm). The signal intensity is 206 times higher than that of traditional biopotential electromyography methods. By leveraging machine learning algorithms and Morse code, the safe, accurate (96.3%), and stable communication aid HMI applications are achieved. The authors' bionic TENG-based electromechanical sensor provides a valuable toolkit for HMI applications of the disabled, and it brings new insights into the interdisciplinary cross-integration between TENG technology and bionics.

Wearable triboelectric sensors for biomedical monitoring and human-machine interface.

A growing advocacy of healthy and quality life makes wearable electronics spring up. Triboelectric nanogenerator (TENG) has developed as an energy harvesting technology and as an advanced sensor technology in wearable electronics. The triboelectric sensor (TS) is sensitive to the mechanical motion and driven by the motion itself. Therefore, TS is capable of monitoring certain vital signs and kinds of movements of human body. Based on these monitoring, novel human-machine interfaces (HMIs) can be established. In this review, a comprehensive overview of some key progresses in this field over last 5 years are presented. Several main aspects of biomedical monitoring based on TSs are classified: pulse/cardiac/micro-motion, respiration/airflow/vibration, and pressure/tactile/body movement. The major types of HMIs taking these biomedical monitoring as basis are introduced accordingly: eye movement, voice/auditory, gesture/joint movement, and touch/tactile based HMIs. Finally, the current limitations and future trends are put forward for biomedical monitoring and HMIs based on TSs.

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.