Scavenging Wind Energy by Triboelectric Nanogenerators

To meet future needs for clean and sustainable energy, tremendous progress has been achieved in development for scavenging wind energy. The most classical approach is to use the electromagnetic effect based wind turbine with a diameter of larger than 50 m and a weight of larger than 50 ton, and each of them could cost more than $0.5 M, which can only be installed in remote areas. Alternatively, triboelectric nanogenerators based on coupling of contact‐electrification and electrostatic induction effects have been utilized to scavenge wind energy, which takes the advantages of high voltage, low cost, and small size. Here, the development of a wind‐driven triboelectric nanogenerator by focusing on triboelectric materials optimization, structure improvement, and hybridization with other types of energy harvesting techniques is reviewed. Moreover, the major applications are summarized and the challenges that are needed to be addressed and development direction for scavenging wind energy in future are highlighted.     

Environmentally Friendly Hydrogel‐Based Triboelectric Nanogenerators for Versatile Energy Harvesting and Self‐Powered Sensors

Triboelectric nanogenerators (TENGs), as a promising energy harvesting technology, have been rapidly developed in recent years. However, the research based on fully flexible and environmentally friendly TENGs is still limited. Herein, for the first time, a hydrogel‐based triboelectric nanogenerator (Hydrogel‐TENG) with high flexibility, recyclability, and environmental friendliness simultaneously has been demonstrated. The standard Hydrogel‐TENG can generate a maximum output power of 2 mW at a load resistance of 10 MΩ. The tube‐shaped Hydrogel‐TENG can harvest mechanical energy from various human motions, including bending, twisting, and stretching. Furthermore, the system can serve as self‐powered sensors to detect the human motions. Additionally, the utilized Polyvinyl Alcohol hydrogel employed in this study is recyclable to benefit for fabricating the renewable TENG. The open‐circuit voltage of renewed hydrogel‐TENG can reach up to 92% of the pristine output voltage. This research will pave a potential approach for the development of flexible energy sources and self‐powered motion sensors in environmentally friendly way.     

Tribovoltaic Effect on Metal–Semiconductor Interface for Direct‐Current Low‐Impedance Triboelectric Nanogenerators

The triboelectric nanogenerator (TENG) is a new energy technology that is enabled by coupled contact electrification and electrostatic induction. The conventional TENGs are usually based on organic polymer insulator materials, which have the limitations and disadvantages of high impedance and alternating output current. Here, a tribovoltaic effect based metal–semiconductor direct‐current triboelectric nanogenerator (MSDC‐TENG) is reported. The tribovoltaic effect is facilitated by direct voltage and current by rubbing a metal/semiconductor on another semiconductor. The frictional energy released by the forming atomic bonds excites nonequilibrium carriers, which are directionally separated to form a current under the built‐in electric field. The continuous average open‐circuit voltage (10–20 mV), short‐circuit direct‐current output (10–20 µA), and low impedance characteristic (0.55–5 kΩ) of the MSDC‐TENG can be observed during relative sliding of the metal and silicon. The working parameters are systematically studied for electric output and impedance characteristics. The results reveal that faster velocity, larger pressure, and smaller area can improve the maximum power density. The internal resistance is mainly determined by the velocity and the electrical resistance of semiconductor. This work not only expands the material candidates of TENGs from organic polymers to semiconductors, but also demonstrates a tribovoltaic effect based electric energy conversion mechanism.     

Multilayered‐Electrode‐Based Triboelectric Nanogenerators with Managed Output Voltage and Multifold Enhanced Charge Transport

The open‐circuit voltage of a triboelectric nanogenerator (TENG) increases with the tribo‐charge density and the separated distance between two tribo‐surfaces, which can reach several thousand volts and is much higher than the working voltage required by most electrical devices and energy storage units. Therefore, improving the effective efficiency of TENGs requires reducing the output voltage and enhancing the transferred charges. Here, a multilayered‐electrode‐based TENG (ME‐TENG) is developed in which the output voltage can be managed by controlling the charge flow in a process of multiple (N) steps, which results in N times lower voltage but N times higher total charge transport. The ME‐TENG is demonstrated to work in various modes, including multichannel, single‐channel, and double‐tribo‐surface structures. The effects of insulator layer thickness and total layer number on the output voltage are simulated by the finite element method. The output voltage can be modulated from 14 to 102 V by changing the insulator layer number between two adjacent working electrodes, based on which the 8‐bit logic representations of the characters in the ACSII code table are demonstrated. The ME‐TENG provides a novel method to manage the output power and has potential applications in self‐powered sensors array and human–machine interfacing with logic communications.     

An Ultra‐Mechanosensitive Visco‐Poroelastic Polymer Ion Pump for Continuous Self‐Powering Kinematic Triboelectric Nanogenerators

A mechanosensitive, visco‐poroelastic polymer ion pump that can rapidly establish a dense electrical double layer via mechanical pressure, thereby significantly enhancing output performance of an ionic triboelectric nanogenerator (iTENG), is described. A working mechanism of an iTENG using a highly mechanosensitive, visco‐poroelastic ion pump is suggested and the optimal characteristics of the polymer ion pump are reported by investigating optical, mechanical, electrical, and electrochemical properties. Surprisingly, the pressure sensitivity of the iTENG reaches 23.3 V kPa^?1, which is tens of times the record value. To achieve controlled high‐frequency pulses from an iTENG, kinematic systems using a gear train and a cam are integrated with a single grounded iTENG, which produces a maximum of 600 V and 22 mA (≈2.2 W cm^?2) at an input frequency of 1.67 Hz; after power transforming, those values are converted to 1.42 V and 225 mA. A capacitor of 1 mF can be fully charged to 2 V in only 60 s, making it possible to continuously operate a wireless‐communicating self‐powered humidity sensor. Also, due to the high transparency and deformability of the polymer ion pump, a self‐powered transparent tactile sensor is successfully assembled using a 5 × 5 iTENG array.     

Hybrid All‐in‐One Power Source Based on High‐Performance Spherical Triboelectric Nanogenerators for Harvesting Environmental Energy

With the development of the Internet of Things (IoTs), widely distributed electronics in the environment require effective in situ energy harvesting technologies, which is made challenging by the unstable supply and severe conditions in some environments. In this work, a hybrid all‐in‐one power source (AoPS) is demonstrated for widely adaptive environmental energy harvesting. With a novel structure, the AoPS hybridizes high‐performance spherical triboelectric nanogenerators (TENGs) with solar cells, enabling the harvesting of most typical environmental energies from wind, rain drops, and sun light, for complementary supply. The spherical TENG units with a packaged structure can work robustly to collect energy from fluid. Nearly continuous direct current and a high average power of 5.63 mW can be obtained by four TENG units, which is further complemented by solar cells. Typical application scenarios are also demonstrated, achieving self‐powered soil moisture control, forest fire prevention and pipeline monitoring. The work realizes the concept of an environmental power source that can be deployed in the environment with high adaptability to make use of all kinds of surrounding energies for powering electronics in all‐weather conditions, providing a reliable foundation for the era of the IoTs.     

High‐Performance Triboelectric Nanogenerators Based on Solid Polymer Electrolytes with Asymmetric Pairing of Ions

In general, various kinds of surface modifications are utilized to enhance the power output performance of triboelectric nanogenerators (TENGs), but they typically have limited stability. Here, a new strategy of adding electrolytes with asymmetric ion pairing to polymer friction layers of TENGs is introduced in order to enhance their triboelectric property. Indeed, Kelvin probe force microscopy (KPFM) measurements show that an addition of phosphoric acid (H₃PO₄ ), an electrolyte with more cations than anions, to polyvinyl alcohol (PVA) can make it one of the most negative triboelectric materials; whereas, an addition of calcium chloride (CaCl₂ ), an electrolyte with more anions than cations, to PVA can make it one of the most positive triboelectric materials. Furthermore, the TENGs based on such solid polymer electrolytes (SPEs) produce significantly higher power output than typical metal‐polymer TENGs. Due to these unique features, SPEs are a promising triboelectric material for realizing high‐performance TENGs for self‐powered small electronics.     

Quantifying Energy Harvested from Contact‐Mode Hybrid Nanogenerators with Cascaded Piezoelectric and Triboelectric Units

This paper presents an investigation of novel contact‐mode hybrid nanogenerators comprising cascaded piezoelectric and triboelectric units. For the first time, a theoretical analysis of the contact‐mode hybrid generator is presented to describe the relationships among transfer charges, voltage, current, and average output power in terms of material properties, hybrid generator structural parameters, harvesting, and operational conditions. New hybrid generators with much enhanced piezoelectricity are fabricated via a simple, room‐temperature, cost‐effective route by using nonwoven fabrics made from electrospun Polyvinyledenedifluoride‐trifluoroethylene (PVDF‐TrFE)/Ag nanowire nanofibers and Polydimethylsiloxane (PDMS) composites with graphite nanoparticles. The results provide a powerful tool for synthesis and selection of materials, design and optimization of the configuration, and operation of such kind of hybrid generators as well as determination of the value of external capacitor.     

A Fully Verified Theoretical Analysis of Contact‐Mode Triboelectric Nanogenerators as a Wearable Power Source

Harvesting mechanical energy from human activities by triboelectric nanogenerators (TENGs) is an effective approach for sustainable, maintenance‐free, and green power source for wireless, portable, and wearable electronics. A theoretical model for contact‐mode triboelectric nanogenerators based on the principles of charge conservation and zero loop‐voltage is illustrated. Explicit expressions for the output current, voltage, and power are presented for the TENGs with an external load of resistance. Experimental verification is conducted by using a laboratory‐fabricated contact‐mode TENG made from conducting fabric electrodes and polydimethylsiloxane/graphene oxide composite as the dielectric layer. Excellent agreements of the output voltage, current, and power are demonstrated between the theoretical and experimental results, without any adjustable parameters. The effects of the moving speed on output voltage, current, and power are illustrated in three cases, that is, the motion with constant speed, the sinusoidal motion cycles, and the real walking cycles by human subject. The fully verified theoretical model is a very powerful tool to guide the design of the device structure and selection of materials, and optimization of performance with respect to the application conditions of TENGs.     

Boost the Performance of Triboelectric Nanogenerators through Circuit Oscillation

Triboelectric nanogenerator (TENG) which harvests ubiquitous ambient mechanical energy is a promising power source that can meet the distributed energy demand in the internet of things, wearable electronics, etc. However, the available output of TENG is severely limited by the saturated polarized charge density, small intrinsic capacitance, and large matching impedance. Herein, an effective power management strategy is proposed that flips the free charges on the conductive layer through a controlled LC oscillating circuit composed of the diode, switch, inductor, and the intrinsic capacitor of TENG. In this way, the equivalent charge density reaches a level higher than the saturated polarized charge density. The simulation and experiments show that the limit of energy output can be exceeded under arbitrary load resistance, especially for the low‐impedance common electronics. It is believed that such a general, low‐cost, and highly effective strategy can further broaden the applications of TENG devices across the fields and probably be a new performance evaluation standard for TENG.     

Boosting Power‐Generating Performance of Triboelectric Nanogenerators via Artificial Control of Ferroelectric Polarization and Dielectric Properties

Low output current represents a critical challenge that has interrupted the use of triboelectric nanogenerators (TNGs) in a wide range of applications as sustainable power sources. Many approaches (e.g., operation at high frequency, parallel stacks of individual devices, and hybridization with other energy harvesters) remain limited in solving the challenge of low output current from TNGs. Here, a nanocomposite material system having a superior surface charge density as a triboelectric active material is reported. The nanocomposite material consists of a high dielectric ceramic material, barium titanate, showing great charge‐trapping capability, together with a ferroelectric copolymer matrix, Poly(vinylidenefluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)), with electrically manipulated polarization with strong triboelectric charge transfer characteristics. Based on a contact potential difference study showing that poled P(VDF‐TrFE) has 18 times higher charge attracting properties, a fraction between two components is optimized. Boosting power‐generating performance is achieved for 1130 V of output voltage and 1.5 mA of output current with this ferroelectric composite‐based TNG, under 6 kgf of pushing force at 5 Hz. An enormously faster charging property than traditional polymer film‐based TNGs is demonstrated in this study. Finally, the charging of a self‐powering smartwatch with a charging management circuit system with no external power sources is demonstrated successfully.     

Charge Pumping Strategy for Rotation and Sliding Type Triboelectric Nanogenerators

Triboelectric nanogenerator (TENG) is an emerging approach for harvesting energy from the living environment. But its performance is limited by the maximum density of surface charges created by contact electrification. Here, by rationally designing a synchronous rotation structure, a charge pumping strategy is realized for the first time in a rotary sliding TENGs, which is demonstrated to enhance the charge density by a factor of 9, setting up a record for rotary TENGs. The average power is boosted by more than 15 times compared with normal TENGs, achieving an ultrahigh average power density of 1.66 kW m^?3, under a low drive frequency of 2 Hz. Moreover, the charge pumping mechanism enables decoupling of bound charge generation and the severity of interfacial friction in the main TENG, allowing surface lubricants to be applied for suppressing abrasion and lowering heat generation. The adaptability of the strategy to rotation and sliding type TENGs in low‐frequency agitations provides a breakthrough to the bottleneck of power output for mechanical energy harvesting, and should have a great impact on high‐power TENG design and practical applications in various fields.     

High Permittivity CaCu3Ti4O12 Particle‐Induced Internal Polarization Amplification for High Performance Triboelectric Nanogenerators

Here, a composite material based on the butylated melamine formaldehyde (BMF) and high permittivity CaCu₃Ti₄O₁₂ (CCTO) particles as a triboelectric dielectric material for stable high output triboelectric nanogenerators (TENGs) is proposed. CCTO particles, which have the high permittivity of 7500, can potentially result in the formation of strong internal polarization into the dielectric material under the electric field from triboelectric charges. As a consequence, the charge induction on the bottom electrode is enhanced thereby increasing the triboelectric output performance. A rotation‐type freestanding mode TENG based on BMF–CCTO 1 wt% composite material demonstrates high performance power output of a root‐mean‐square voltage and current density with 268 V and 25.8 mA m^?2, respectively. The strategy of incorporating the high permittivity CCTO particles can be universally applied to any triboelectric polymer matrix in order to enhance the output performance of TENGs.