Friction nanogenerator (TENG) is an emerging platform technology to realize electromechanical energy conversion, which has great application potential in artificial intelligence, Internet of Things and high-entropy energy. In the past 10 years, through extensive efforts around the world, TENG's related research has made great progress. The representative progress of TENG in basic research and technological innovation was reviewed. The latest strategies and methods to improve the output performance of TENG are discussed; This paper summarizes the research progress of TENG application in different fields, and looks forward to the challenges and opportunities of TENG research.
overview
Triboelectric nanogenerators (TENG) have attracted a lot of attention since they were first reported in 2012. By coupling contact electrocution and electrostatic induction effects, TENG can convert various types of mechanical energy into electrical energy, including water wave energy, wind energy, vibration energy, body movement energy, etc. Compared with traditional technology, TENG has better performance for low-frequency (<5Hz) mechanical energy collection; At the same time, TENG facilitates reasonable structural design and material selection according to different application scenarios. Since contact electrification is a universal effect, TENG can be implemented with dielectric, semiconductor or electrolyte materials in solid, liquid or even gas phases to suit different application needs. Potential applications of TENG include: (1) as a power device for electronic devices, effectively collecting environmental mechanical energy, or by integrating with energy storage devices, realizing a power system that can be self-charged, and by realizing a power system that is self-sufficient in energy, TENG is expected to solve the bottleneck problem of long-term power supply difficulties and high maintenance costs for electronic devices; (2) As an electromechanical transducer mechanism, TENG can convert various weak mechanical signals (force, deformation, motion, etc.) into electrical signals to realize self-driven or active sensing technology; (3) TENG has the characteristics of high output voltage, and recent research can even achieve voltage output up to 10kV, so TENG can be applied as an economical, safe and reliable high-voltage power supply; (4) TENG can effectively collect low-frequency water wave energy, collect and utilize water wave energy that exists all the time in the natural marine environment, realize marine blue energy, and help achieve the major strategic goal of "dual carbon". In addition, the extensive research of TENG has also promoted the progress of related basic scientific research, such as the physical model of contact start-up, the new physical effect of friction voltaics, the contact electrocatalytic effect, etc., and these discoveries will inevitably further promote the research of new application technologies of TENG.
In recent years, TENG has developed into a frontier field of extensive research at home and abroad, and is a field with significant multidisciplinary cross-innovation characteristics. Using the Web of Science database as the data source, statistics found that from 2012 to 2022, the number of research papers related to TENG increased year by year (Figure 1). As of the end of 2022, more than 1,500 institutions in 83 countries and regions have been engaged in TENG research.
Figure 1 Statistics of TENG-related SCI papers published from 2012 to 2022
Progress in TENG basic research
TENG converts mechanical energy into electrical energy usually involves two processes: one is to generate a static charge on the surface when two objects are in contact, which is called the contact energizing process; The second is that when the two charged objects move relative to each other, the induced current is generated in the external circuit, that is, the electrostatic induction process. Although this simple physical image can understand how TENG works, in fact, the comprehensive theoretical description may be much more complicated, and TENG research has also promoted the development of related basic science, and also led to the discovery of some new physical effects.
01 Electron transfer model for contact energizing
The phenomenon of frictional electrification has been discovered more than 2600 years ago, but the scientific explanation of the energizing process is still incomplete. Since this effect exists in almost all solid, liquid or gas phase materials, whether the contact surface electricity generation is generated by ions, electrons, or the transfer of material substances at the interface has not been uniformly accepted for a long time physical description or model. The systematic research reported by Wang et al. supports that electron transfer is the main mechanism of the interfacial charged process of solid, liquid, or gas. Representative examples of experiments that strongly support this conclusion are as follows. At the metal-dielectric interface, Zhou et al. have shown that the polarity of the charge generated on the surface of the dielectric material can be adjusted by a bias voltage applied to the metal tip. The effective Fermi energy level of the metal can be adjusted to a higher empty surface energy level than the dielectric material by the bias voltage, and the surface of the dielectric material will accept electrons from the metal and be negatively charged; Conversely, when the effective Fermi level of the metal is adjusted to a level lower than the empty surface state of the dielectric material, the surface of the dielectric material will be positively charged. Lin et al. also showed through KPFM research that the contact power initiation law of the two materials at different temperatures is also consistent with the electron transfer mechanism. Another strong evidence is that the charge bound to the surface state of the dielectric material is dissipated at high temperature, and the dissipation law is in line with the principle of electron-thermion emission, so it is proved that the electron-transfer-dominated electron-based energization process on the surface of the dielectric material. Based on this experimental evidence, Xu et al. propose a universal contact power-on model (Figure 2(a)). For many materials with well-defined molecular or electronic structures, the electrogenesis phenomenon can be explained by energy band diagrams. However, there is also a large amount of material that is difficult to describe with an energy band diagram. Therefore, the electroatomic interaction potential simplification model can be used to explain the electrogenesis process. When the 2 surfaces touch, the 2 atoms of the 2 materials must be close enough to start electron transfer, and the distance must be within the repulsion zone of the potential well, supported by KPFM research and quantum mechanical modeling. After separation, the transferred electrons will be trapped in the surface or defect state in the form of electrostatic charges.
02Electric double layer model of liquid-solid interface energization
At the liquid-solid interface, the frictional charge generated by the sliding of water droplets on the surface of the dielectric material has also been shown to be mainly contributed by electron transfer, since most of the charge on the surface of the dielectric material is also dissipated according to the electron-thermionic emission law, and only a small part of the residual "viscous" charge can be attributed to ion adsorption. This discovery led to a new understanding of the formation of liquid-solid interface electric double layer (EDL), and Lin et al. proposed a two-step model. As shown in Figure 2(b), the model considers both electron transfer and ion adsorption. In step 1, the liquid comes into contact with the solid surface, and some molecules and ions in the solution (including H2O, cations, anions, etc.) will hit the solid surface due to thermal movement and pressure from the liquid. During impact, electrons will be transferred between solid and liquid atoms due to the overlapping electron clouds of atoms, while ionization reactions may also occur. Broken molecules that break away from the solid surface become free-migrating ions in the liquid, so electrons and ions will be produced on the surface at the same time. In step 2, liquid molecules close to the solid surface break away from the interface due to liquid flow or turbulence. After separation, if the energy fluctuations of the electrons are below the energy barrier, most of the electrons transferred to the surface will remain on the surface. The oppositely charged ions in the liquid are then attracted by electrostatic interactions and migrate to the charged surface, forming an electric double layer.
Figure 2 Progress of TENG basic research
03Contact electrocatalysis
Wang et al. found that the electron exchange process at the interface between water and dielectric materials can induce the degradation of methyl orange in aqueous solution. This progress not only contributes to the understanding of the contact initiation mechanism dominated by electron transfer, but also promises the development of a promising new catalytic technology, namely contact electrocatalysis. As shown in Figure 2(c), contact electrocatalysis (CEC) uses surface polarized electrons induced by contact initiation to accelerate chemical reactions. Ultrasound-induced cavitation bubbles can not only produce multiple contact separations, but also promote electron transfer by reducing the energy barrier generated by various active substances. During contact priming, electrons transferred between the original dielectric powder and water can be used to directly catalyze the reaction without the use of conventional catalysts. Catalytic efficiency can be further improved by introducing micro-nanostructures on dielectric powders to increase contact surface area or by chemical modification as a means of increasing surface charge density. Contact electrocatalysis is a versatile technique that is effective against a variety of dielectric materials such as PTFE, nylon, and rubber. This catalytic principle greatly expands the selection of catalytic materials, and also makes catalytic processes and devices simpler and less costly.
04 Interface spectrum of contact energizing
Li et al. have shown that light emission is detected during contact electrification between the surfaces of 2 dielectric materials. This result not only strongly supports the idea that electron transfer dominates contact initiation, but also finds that light emission carries the fingerprint information of the electron energy of interface atoms, so it is expected to develop a new spectral detection technology, namely contact electrogenetic interface spectroscopy (CEIPES). As shown in Figure 2(d), photon emission proves the transfer of electrons from one atom in one material to another at the interface during contact priming. Three possible physical processes are used to understand photon emission caused by the transferred electron charge in contact priming: (1) contact initiation induces electrons to transition to a lower energy level in an atom by emitting photons; (2) transfer to the excited state of another atom through energy resonance, and then transition to the lower energy level of that atom; (3) Transition to a lower energy level in another atom, and then transition from a photon to a lower energy level. When atoms from different materials are close to each other, energy resonance transfers to produce CEIPES. This paves the way for spectroscopy corresponding to the electrokinetic phase of the interface and is expected to have a profound impact on understanding the interactions between solids, liquids, and gases.
05 Frictional volt effect
Zhang et al. found that when semiconductor materials are used as friction materials, direct current output can be generated, which makes the new physical effect of friction volts discovered. As shown in Figure 2(e), the frictional volt effect refers to the effect of mechanical energy at the sliding interface when the metal and semiconductor or semiconductor and semiconductor slide relative to each other, and the electron-hole pair is then separated by the built-in electric field, and the external electric work is done externally, and direct current is output in the external circuit. The frictional volt effect can be compared to the photovoltaic effect, except that the energy excitation source is mechanical energy, while photovoltaic is a photon energy excitation electron-hole pair. The friction volt effect has been found to exist in the Schottky interface of metal-semiconductor relative slip, the relative sliding interface of P-N semiconductor, and even the sliding interface between liquid and semiconductor. For the physical description of electron-hole pairs excited by mechanical energy, it is claimed that it is caused by local thermal effects caused by friction, but usually the phonon energy of thermal effects is low and insufficient to excite electrons; It is claimed that when the surface is in contact, the continuous bonding-breaking process between atoms, analogous to the mechanochemical process, releases the bonding energy between atoms to excite electrons. Meng et al. have found that direct current can be generated even in contact separation motion mode, which largely eliminates the possibility of thermal effects on the one hand, and avoids mechanical wear effects on the other hand, improving the durability of friction volt devices. In addition, when low-doped GaN is used as a friction volt material, it is found that the output voltage can be as high as hundreds of volts, indicating that the interfacial electric field caused by the electrostatic charge accumulated on the surface of GaN also participates in the separation process of electron-hole pairs. The interfacial electric field is much higher than the built-in electric field caused by diffusion, which greatly increases the output voltage. In short, the frictional volt effect is a new physical effect, which still requires more in-depth mechanism and application research.
06 Kinetic Maxwell's equations
The second process of TENG power generation is the generation of an induced current by a live dielectric under mechanical motion. As shown in Figure 3, the basic principle of this induced current is the Maxwell displacement current, which is not a conduction current generated by the free charge transfer of movement, but is caused by a time-varying electric field. Contact initiation makes the surface of the dielectric material electrostatically charged, and further, the mechanical movement of the charged surface causes the electric field of these electrostatic charges to change with time, resulting in a displacement current. This displacement current is different from the dielectric polarization (P) caused by an externally applied electric field (E), but is related to the polarization (Ps) of the surface electrostatic charge. P depends on the applied electric field, but Ps is driven by mechanical motion. Therefore, in order to clearly show its physical significance and calculate the electrical output of the TENG, Wang et al. added the polarization term Ps to the displacement vector, and Maxwell's displacement current (JD) can be rewritten as
where D is the electrical displacement vector, ε is the permittivity, and t is the time.
is the second term of the newly added Maxwell displacement current caused by mechanical motion or external strain fields
Corresponds to the theoretical basis of TENG in applications such as sensing and energy harvesting. Further, for dielectric systems moving at variable speeds (e.g., non-inertial systems), Maxwell's equations are extended as follows to describe their electromagnetic behavior
where D' is the electric displacement vector in the relative stationary medium system; ρf is the free charge density; E is the electric field strength; B is the magnetic induction intensity; D is the electric displacement vector; H is the magnetic field strength; ρfv represents the local current generated by the free charge moving at a certain speed v, v(r,t) is the motion speed of the medium, which is a function of time and space, vr is the velocity of the charge relative to the circuit; Jf is the external applied oscillation current;
is the induced displacement current.
This system of equations is the general Maxwell's equations for a slowly moving medium driven by mechanical motion at any velocity field, known as the kinetic Maxwell's equations. These equations describe the coupling between the mechanical, electrical, and magnetic properties of a system moving with acceleration and are based on the assumption that: (1) velocity varies with time and is much less than the speed of light (ν<<c）； (2) The approximation ignores relativistic effects. Due to mechanical motion or shape volume can vary at any speed field. That is, these equations are derived for non-inertial systems, not for media moving at uniform speeds in inertial systems. Since the mechanical energy of the input is coupled with electricity and magnetism, it does not satisfy Lorentz covariance. However, from an engineering application point of view, these equations apply not only to charged dielectrics in solid/soft matter form, but also to liquid or fluid charged dielectrics.
Figure 3 Kinetic Maxwell's equations
High-performance friction nanogenerator
The rapid research and development of high-performance TENG has greatly promoted the development of this research field, attracting more researchers to devote themselves to the research of micro-nano energy and sensing, forming a virtuous circle. The power density of TENG is proportional to the square of the charge density, so increasing the surface charge density of the material can greatly increase the output power density of the TENG. Considering the main charge generation and charge dissipation processes in TENG, Wang et al. proposed the maximum charge density limitation equation of TENG, that is, the maximum surface charge density (σmax) of TENG is limited by the minimum charge density corresponding to σtriboelectrification, air breakdown and dielectric breakdown
The improvement of TENG charge density mainly revolves around how to break through the factors with the lowest charge density limit in the above formula.
01Progress in material innovation
The triboelectric sequence is one of the tools commonly used to describe the advantages and disadvantages of the triboelectric initiation performance of materials, but the traditional triboelectric sequence is ordered by the charge polarity of the frictional initiation between the two pairs of the material, and does not consider the amount of charge transferred by the triboelectric current, which has certain limitations when applied to the material selection of TENG. Zou et al. report a quantified triboelectric sequence based on a controlled experimental platform and environment, which can provide a good reference for material selection for TENG because both polarity and quantity of transferred charges are considered in this sequence. On this basis, Liu et al. combined with TENG technology and vacuum environment reported a new triboelectric sequence, which can reflect the inherent properties of materials to a certain extent while avoiding air breakdown and environmental factors as much as possible, and help to understand the maximum frictional charge density of various materials (Figure 4(a)).
Figure 4 Energizing materials and charge excitation strategies of TENG
In addition, it is a simple and direct method to effectively improve the triboelectric properties of any material by physically modifying the preparation of micro-nano structures on the surface of friction materials, or introducing functional groups that are easy to gain and lose electrons on the surface of friction materials through chemical modification. Due to the effective adjustment of surface functional group composition, crystallinity and permittivity, the ethylene fluoride propylene copolymer (FEP) film prepared by this method can maintain a high charge density of 510μC·m-2. The team achieved high output charge densities of 525 μC·m-2 and 1237 μC·m-2 in contact separation mode TENG (CS-TENG) and sliding mode TENG (S-TENG) by using radical ion transfer to compensate for charge dissipation in the FEP membrane after corona polarization (Figure 4(b)). Therefore, in the actual device preparation, a number of performance indicators including electrical performance of the material should be comprehensively considered. First of all, two materials with as large differences in electrical initiation performance as possible in the triboelectric sequence should be selected; Secondly, when selecting the surface modification scheme, it is necessary to comprehensively consider the wear resistance robustness, surface hydrophilic performance, electric starting capacity, dielectric constant, thickness and other parameter indicators, dielectric materials with high dielectric constant and thin thickness are also conducive to obtaining high electrical output, and surface hydrophobicity is conducive to reducing the influence of environmental humidity; Finally, if versatility such as high elasticity and light transmission are required, specific functional dielectric polymers can also be designed to achieve this.
02Charge pump excitation
In order to break through the limitation of the charge density generated by frictional start-up of the output performance of TENG, Xu et al. and Cheng et al. reported in 2018 a method for using a charge pumping strategy to increase the charge accumulation rate in TENG (Figure 4(c)), that is, using a TENG as a high-voltage power supply (pump TENG) in parallel with another variable capacitor (main TENG), during its movement, the pump TENG uses its own generated high voltage output to provide a steady stream of charge for the main TENG. The main TENG, as a parallel plate capacitor, uses the capacitance to change the capacitance to output alternating current during its contact separation process. Taking advantage of the high voltage output of TENG and the large capacitance value of the parallel plate capacitor, the charge-pumping strategy was used to achieve an output charge density of up to 1020μC·m-2 under atmospheric conditions. Liu et al. propose a strategy for self-excitation circuits to further increase the charge accumulation rate of TENG (Figure 4(d)). The self-excitation circuit can change the series and parallel connection mode of the capacitor during the TENG contact separation process, and the output charge increases exponentially, and this strategy successfully increases the charge density of the 5μm Kapton film to 1.25mC·m-2. The effective surface charge density of 4μm polyimide (PI) films can be increased to 2.38mC·m-2 by further improving the TENG contact efficiency by carbon/silica electrode. By adjusting the thickness and dielectric constant of the dielectric film, Li et al. and Wu et al. used 9μm polyvinylidene trifluoroethylene (P(VDF-TrFE)) film and 7μm lead zirconate titanate-polyvinylidene fluoride (PZT-PVDF) film to achieve high output charge densities of 2.2mC·m-2 and 3.53mC·m-2, respectively. Wu et al. achieved a 100% actual output efficiency by disabling the charge trap effect by adjusting the carrier trap state of the dielectric polymer film, and the output charge density reached 4.13 mC·m-2 (Figure 4(e)).
03Inhibition/use of air breakdown
The charge excitation strategy can greatly improve the charge accumulation rate of TENG and break through the limitation of frictional power generation on the output performance of TENG, but its charge output is still limited by air breakdown. On the one hand, ultra-thin and ultra-high dielectric constant films can be used, high vacuum environment can be used, atmosphere and air pressure can be adjusted, and interface liquid lubricants can be added to inhibit air breakdown to a certain extent, so as to achieve ultra-high output charge density; On the other hand, air breakdown can also be used to generate electricity.
Liu et al. fabricated a novel DC friction nanogenerator (DC-TENG) based on triboelectric and electrostatic breakdown effects (Figure 5(a)), which uses the strong electric field formed between the charge collection electrode and the friction medium layer to ionize the air, thereby generating a DC output in an external circuit. The charge output of DC-TENG can be further improved by optimizing the frictional start-up process with a two-dielectric layer, or by optimizing the electrostatic breakdown process by adjusting environmental factors such as temperature, atmosphere, and air pressure. Zhao et al. used a microstructured electrode design to increase the output charge density of DC-TENG to 5.4mC·m-2 (Figure 5(b)), and then increased the output charge density to 8.8mC·m-2 through material selection. Dai et al. found that DC-TENG has harmful breakdown, which limits its performance improvement. Zhang et al. revealed 3 discharge domains in DC-TENG, constructed a "bucket model" to explain the connection between the 3 breakdown domains, and increased the output power of DC-TENG by 1 order of magnitude over a wide load range by suppressing the breakdown of the 2nd domain (Figure 5(c)). In addition, Shan et al. designed a dual-dielectric breakdown TENG, in which corona discharge occurs between the two electrodes and the PA and PTFE when the slider periodically slides left and right, resulting in a signal output in the external loop (Figure 5(d)). Through the coupling of frictional starting, electrostatic induction and electrostatic discharge, Zeng et al. proposed another discharge TENG, which uses the back electrode to generate output through electrostatic induction through air gap discharge between two friction medium layers (Figure 5(e)).
Figure 5 DC TENG using the air breakdown effect
04Power Management
The power management circuit can convert the high voltage and low current output of TENG into the low voltage and constant current output required by electronic devices, improve the energy conversion efficiency between the electrical energy output by TENG and the electrical energy obtained by electronic devices, and provide continuous and stable energy supply for electronic devices. In general, the power management circuit of TENG needs to consider 2 problems: (1) how to get more energy from TENG; (2) Design an efficient step-down circuit for the energy characteristics of TENG.
Due to the high output energy and high current frequency, the turntable TENG can achieve high energy conversion efficiency by selecting a transformer that matches the impedance. Most of the TENG due to low operating frequency, small output current, matching impedance is very large, commonly used transformers are difficult to effectively manage the output energy of TENG, so the use of inductor, capacitor and freewheeling diode composed of LC circuit as a TENG buck circuit is a good choice. How to achieve a higher energy output cycle of TENG and design a buck circuit to match it has been the focus of researchers in recent years. Taking CSTENG as an example, its energy output cycle has significant asymmetry, and the voltage of the separate process is significantly higher than the voltage of the contact process. Depending on the half-wave circuit connection, CS-TENG can generate high-voltage low-charge (HV-LQ) energy output cycles and high-charge low-voltage (LV-HQ) energy output cycles. Wang et al. realized an energy output cycle with HV-LQ characteristics by using CS-TENG, which has a large area and large separation distance (Figure 6(a)), and optimized the air switch and inductor design to achieve 110mJ·m-2· Output energy density of Hz-1. Gao et al. proposed that the inherent capacitance formed by CS-TENG at contact can be used to simplify the design of power management circuits, while reducing the cost of repeated trial and error, and realize the energy output cycle with LV-HQ characteristics by using a half-wave circuit to suppress the air breakdown phenomenon generated during CS-TENG separation (Figure 6(b)), and obtain an LC circuit consisting of a film capacitor with low leakage current and a magnetic shielded inductor with small magnetic leakage magnetism, and obtain 100mJ·m-2· Output energy density of Hz-1. In addition, switches in power management circuits are also the focus of research in this field, and different operating modes and energy characteristics will make TENG have different requirements for switches. TENG with high voltage characteristics often choose air switches, mechanical switches with fixed motion modes, and SCR (silicon controlled rectifier) thyristors that can achieve passive control can meet the power management needs of most TENG with low energy density. In short, the design of TENG high-efficiency power management circuit will greatly promote TENG to a wide range of practical applications.
Figure 6 The power management policy of TENG
Technological innovation and application progress
Based on the unique advantages of TENG, the research and innovation of related technologies of TENG have made continuous progress in recent years, especially in micro-nano energy, self-driving sensing, blue energy and high-voltage power supply.
01 Micro-nano energy
Teng has the unique advantages of small size, light weight, easy miniaturization, and high flexibility, which can convert mechanical energy widely present in living organisms and the environment into electrical energy, thereby providing continuous power for micro-small electronic devices. At present, the application of TENG in micro-nano energy mainly includes the energy supply of implantable electronic devices, the energy supply of flexible wearable electronic devices, and the energy supply of environmental Internet of Things electronic devices. TENG combines packaging materials and technologies to convert the energy of muscle movement into electrical energy, realize implantable ENG with stable performance, and successfully drive electronic devices such as pacemakers. Ouyang et al. proposed an implantable symbiotic pacemaker based on TENG, which is mainly composed of implantable TENG, power management module and pacemaker 3 units, by collecting the energy of the heartbeat, providing electric energy for the pacemaker itself, realizing that the electrical energy generated by a single heartbeat can drive the pacemaker to work once, and successfully realizes energy collection, storage and use in large mammals. Hinchet et al. proposed an ultrasonic energy harvesting technology based on TENG, which provides the possibility to solve the reliable power transmission of implantable medical electronic devices. Teng based on flexible and deformable materials is shape-adaptive and provides a solution for energy supply of wearable electronics. Wang et al. prepared a silicone-based TENG and placed it in a shoe, which can collect human movement energy to continuously supply electronic devices. In addition, Liu et al. designed DC-TENG, which uses friction initiation combined with electrostatic breakdown effect, which can power electronic watches without the need for rectifier bridges and energy storage units, and provides new ideas for efficient collection of mechanical energy. Massive sensors are the cornerstone of the development of the Internet of Things, and their widespread distribution poses new challenges to the demand for electrical energy. TENG efficiently captures all forms of mechanical energy in the environment to provide solutions for the energy supply of environmental IoT electronics. For example, real-time environmental detection and protection, self-driving electrochemical heavy metal treatment and organic pollutant degradation are realized by collecting environmental wind energy. In addition, the use of rabbit hair, metal glass and other materials further improves the output performance and durability of TENG as a micro-nano energy source, and is expected to realize long-term self-supply of electronic devices. In the future, the self-driving of electronic devices based on TENG can get rid of the constraints of power supply, which is an important technical support for the development of micro-integration, wireless mobility and intelligent functions of the Internet of Things.
02Self-driving sensing
The self-powered sensor based on TENG can obtain the motion state of the object and the surrounding environment information without an external power supply and convert it into an electrical signal to realize active sensing, and the sensor based on TENG has the advantages of low cost, multi-form, easy installation and high precision, and shows potential application prospects in vector sensors, wearable devices, biomedical, human-computer interaction, chemical and environmental monitoring, intelligent transportation and smart cities, intelligent robots and other research fields. Vector sensors are TENG-based mechanical motion to achieve signal monitoring, such as linear or rotary motion monitoring, multi-dimensional motion and acceleration monitoring, pressure sensor and water level monitoring, etc., which is of great significance to promote the development of triboelectric sensors in physical quantity monitoring. Wearable sensors based on TENG can be directly worn or integrated into the user's clothing to be prepared into wearable devices such as smart helmets, smart bracelets, smart shoes, and electronic skins to capture the energy in human movement and realize the monitoring of human body information such as walking, running, gestures, touch, and perception. Zu et al. can realize real-time monitoring of head impact by 3D printing a flexible curved sensing array composed of multi-angle TENG (Figure 7(a)). In the biomedical field, TENG can not only be placed on the cochlea or skin surface to monitor low-frequency pulses or slight joint and muscle movements, but can also be implanted in the body as an active sensor to monitor physiological and pathological features such as heart rate. In human-computer interaction applications, by integrating TENG on fingertips, glasses, keyboards, etc., multi-point tactile trajectory mapping, wireless home appliance remote control, intelligent security system establishment and other functions have been realized. In addition, TENG can realize tire condition monitoring, driver behavior monitoring and analysis, intelligent control, edge ball judgment and intelligent analysis of landing point distribution, and promote its application in smart transportation and smart city. The application of TENG in the field of robotics mainly focuses on self-driving sensing and high-voltage drive, for example, designing stretchable triboelectric-optical intelligent skin to realize multi-dimensional tactile and gesture sensing of robot hands; Integrate friction skin into soft robotic hands to improve the ability of soft robots to perform various active perception and interaction tasks; Build an electronic hearing system for external hearing aids in intelligent robot applications. Using the coupling of TENG and piezoelectric technology, Xu et al. developed a new stall sensing system that self-drives lightweight in situ monitoring the degree of airflow separation on the airfoil, which can realize the digitization, visualization and array perception of stall monitoring (Figure 7(b)). In terms of industrial transformation, a number of start-ups have achieved application transformation in areas such as self-driving smart shoes, intelligent safety protection keyboards, and medical rehabilitation monitoring.
Figure 7 Application of TENG as an autonomous sensor in smart helmets (a) and aircraft stall warning (b).
03 Blue energy
Blue energy technology uses encapsulated TENG units to form a power generation network, which is distributed on the surface and underwater, converting marine energy into electricity. At present, TENG has made some progress in wave energy collection. In terms of device design, it is mainly divided into closed waterproof structure and liquid-solid interface friction structure. For example, Pang et al. adding rabbit hair as a flexible friction layer to the oscillating structure can reduce the energy conversion efficiency to 23.6% while reducing the resistance (Figure 8(a)). Qiu et al., an asymmetric structure designed by Qiu and other teng network units inspired by the Brown electrode, can convert irregular wave excitation into unidirectional rotation of internal movers, and efficiently extract irregular water wave energy by using the inertial wheel to buffer the energy from each transient excitation (Figure 8(b)). Zhang et al. use a monopendulum structure to collect water wave energy in all directions and frequency changes through resonance effects (Figure 8(c)). After various structural optimizations, an ultra-high power density of 200W·m-3 was finally achieved in terms of marine energy collection, and a power density of 34.7W·m-3 was generated under the drive of low-frequency water waves. In order to collect a large range of ocean blue energy, a network of TENG can be constructed, for example, Li et al. based on a three-dimensional electrode-spherical structure power generation unit, built a demonstration network of 18 units, with an average power density of 2.05W·m-3, which can be used for self-driving sensing and wireless signal transmission, on this basis, through the design of three-dimensional chiral network, there is a chiral connection between unbalanced units, which gives network flexibility, superelasticity in water and absorption behavior (Figure 8(d)). In the solid-liquid TENG side, Xu et al. collect energy from the impact water droplet through liquid-solid contact, and a 100μL water droplet hits the device surface from a height of 15 cm can generate a voltage of 140V and a current of 200μA (Figure 8(e)). Using the opposite dynamic electric double layer design TENG, Liang, et al. achieved a current output of 60μA and a voltage output of 60V under irregular water waves (Figure 8(f)). In order to maximize the comprehensive use of various energy forms or energy technologies and improve energy conversion efficiency, TENG can also be combined with other energy collection methods such as solar cells and wind turbines to collect composite energy, and design matching power management circuits, and finally realize the wide application of TENG in marine environment self-driving systems, for example, the self-powered marine beacon lamp based on TENG has entered the field sea trial stage.
Figure 8 Application of TENG in harvesting blue energy
04 High voltage power supply
The TENG has the output characteristics of high voltage and low current, so it is a new high-voltage power supply with unprecedented portability and safety. In addition, the outstanding advantages of small size and low cost also make it have great potential in the field of high-voltage power supply. At present, the high voltage using TENG has successfully realized automotive exhaust gas filtration treatment, indoor and outdoor air purification, and high-voltage process drives (such as electrospinning, electron field emission and microplasma, etc.). Guo et al. recently reported that the ultra-high voltage generated by TENG can cause local ionization of air molecules at the carbon brazing fiber electrode, thereby generating negative air ions to purify the air. Using a palm-sized device, 1×1013 negative air ions can be generated in a single slide, rapidly reducing particulate matter (PM2.5) from 999 μg·m-3 to close to zero. Triboelectric air purification system has successfully achieved industrial transformation because of its unique advantages such as zero consumables, low power consumption and high purification efficiency. In addition, Li et al. realize electrical stimulation of crops through the high voltage of TENG to shorten their germination cycle and accelerate the growth of plant seedlings. Driven by a breeze, the hair configuration of TENG can continuously output a stable high pressure of ~3kV, which significantly increases the germination rate and yield of peas by about 26.3% and 17.9%. The research results demonstrate an innovative concept of an auto-driven electrical stimulation system for crop growth, which has great application potential in the field of agriculture in the future.
prospect
Both TENG Basic and Applied Research have stimulated widespread interest and continuous innovation worldwide. TENG's research field has developed tremendously in just 10 years, and it is expected that in the next 10 years, its development will far exceed expectations, especially considering that TENG has great application potential in clean energy, smart electronics, sensors, medical electronics, Internet of Things and other fields. In future research, the following areas may need to be given special attention.
1) TENG basic research is essential. The study of basic theory is the prerequisite for the development of new materials, new devices and new applications of TENG. How to characterize charge transfer at the interface of microscopic tables has been a key scientific question for a long time, and it is also a fundamental research challenge in the field of TENG. Recently, the field of TENG research has developed some new methods to study this basic problem, for example, using a variety of external field control methods (thermal, magnetic, optical, etc.) to observe the surface charge transfer phenomenon, and quantitatively analyzing and demonstrating the electron transfer model at the interface of solid-liquid, solid-solid, solid-gas, etc. In the future, more direct detection methods still need to be developed to obtain strong experimental evidence. TENG basic research has led to a deeper understanding of a series of fundamental problems or led to the discovery of some new physical effects, such as electron transfer model for contact start-up, contact electric double layer model, contact interface spectroscopy, new friction volt effect, new effect of contact electrocatalysis, etc. On the one hand, these basic problems still need to be studied in more depth to realize the whole chain research from basic theory to practical application, for example, on the basis of the research on the electronic excitation mechanism of friction volt effect, the performance of friction volt materials and devices is improved through band engineering, and the engineering problems such as the practicality and durability of friction volt devices are further solved; On the other hand, the field of TENG is also looking forward to more new breakthroughs in basic theory, such as the coupling of mechanical-electrical-magnetic in non-inertial dielectric systems and potential basic application research.
2) High-performance TENG is key. High-performance TENG requires joint research in three aspects: materials, devices, and circuits. For TENG based on electrostatic induction effect, increasing the electrostatic charge density is fundamental to improving electromechanical conversion. Through the modification of dielectric polymer materials, the potential energy trap site that traps the electrostatic charge can be increased and the stability of the electrostatic charge can be improved. Charge pumping has also proven to be an important method for increasing electrostatic charge density; Circuit management is the key to conditioning the TENG output and making efficient use of the TENG electrical output. As mentioned earlier, the surface charge density of triboelectric materials is limited by physical limits such as electrostatic breakdown, and how to break through the limits to obtain higher charge density and electrical output power is a future challenge. In addition, in addition to the electrostatic induction effect, the rational use of the discharge effect is also an effective way to improve the output of TENG. How to prepare high-performance triboelectric materials and devices on a large scale is also a key engineering issue that needs to be deepened in the field.
3) TENG's applied research should make more breakthroughs. TENG's application fields are very wide, and it has great potential in various fields as micro-nano energy devices, self-driving sensor devices, high-voltage power supplies and blue energy. However, the focus of research will vary depending on the application area. As a micro-nano energy device, improving output power is the core, and the miniaturization, flexibility, multi-function, and implantability of the device need to be designed in specific application scenarios. As a self-driving sensor device, the sensitivity and repeatability of sensing performance are the key, considering the unique electromechanical transducer mechanism of TENG, it is necessary to explore the application requirements that traditional force-sensitive sensor devices cannot reach; As a high-voltage power supply, TENG has achieved practical applications in electrostatic precipitation and air purification, and for high-voltage electronic devices driven by TENG, it is still necessary to explore suitable application scenarios that solve market or technical pain points; As a blue energy, the potential impact of TENG is huge, and important breakthroughs have been made in the key technology of self-energy supply navigation beacon based on TENG, which has laid an important foundation for the practical application of TENG technology in the field of stable energy supply of marine Internet of Things nodes, and the future focus is on engineering or large-scale application verification or demonstration, as well as the composite of multi-energy systems. Although TENG has made some breakthroughs in industrial transformation, in the future, it is necessary to focus on the advantages of competing technologies in various application fields in order to differentiate competition and find application breakthroughs. In short, the application research of TENG has made great progress, and it can be expected to achieve application breakthroughs in many fields in the future.
The authors of this article: Pu Xiong, Wang Jie, Wang Zhonglin
About author:Pu Xiong, researcher at Beijing Institute of Nanoenergy and Systems, Chinese Academy of Sciences, with research interests in flexible energy and flexible electronic materials; Wang Zhonglin (corresponding author), Beijing Institute of Nanoenergy and Systems, Chinese Academy of Sciences, academician and researcher, research direction of nanoenergy.
The original article was published in the 19th issue of Science and Technology Review in 2023, welcome to subscribe to view.