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.     

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.     

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.     

Design of Mechanical Frequency Regulator for Predictable Uniform Power from Triboelectric Nanogenerators

Mechanical energy scavengers convert irregular input mechanical energy into irregular electrical output. There is a need to enable uniform and predictable electric output from energy scavengers regardless of the variability in the mechanical input. So, in this work, a mechanical frequency regulator is proposed that fixes the input forces and input frequency acting on a triboelectric nanogenerator, thus enabling predictable electric output. The irregular low frequency mechanical input energy is first stored in a spiral spring following which the energy is released at the desired frequency by means of an appropriate design of gear train, cam, and flywheel. By regulating the nanogenerator output at 50 Hz, a standard power transformer can be optimally driven to increase the output current to 6.5 mA and reduce its voltage to 17 V. This output is highly compatible for powering wireless node sensors as is demonstrated in this work.     

Sustainable Energy Source for Wearable Electronics Based on Multilayer Elastomeric Triboelectric Nanogenerators

Wearable electronics have attracted a wide range of attention with various functions due to the development of semiconductor industry and information technology. This work focuses on a triboelectric nanogenerator‐based self‐charging power system as a continuous energy source for wearable electronics. The triboelectric nanogenerator has a multilayer elastomeric structure with closely stacked arches as basic functional units. Owing to material and structural innovations, this triboelectric nanogenerator performs outstanding electric output with the maximum volume charge density ≈0.055 C m^?3 and practical properties for energy harvesting from body motions. Utilizing the triboelectric nanogenerator as outsole to harvest energy from walking or jogging, a pair of shoes is fabricated with the maximum equivalent charge current of each shoe being around 16.2 µA and specific fitness functions realized on each shoe separately without complex connections.     

Suppressing Lithium Dendrite Growth via Sinusoidal Ripple Current Produced by Triboelectric Nanogenerators

Lithium metal as an ultimate anode material of future rechargeable batteries may furnish the highest energy density for its pairing cathode, although preventing the growth of lithium dendrites in liquid electrolytes is a major challenge. This work reports that stable lithium metal anodes can be achieved by charging with high‐frequency sinusoidal ripple current generated by rotating triboelectric nanogenerators (R‐TENGs). Compared with constant DC current charging, sinusoidal ripple current charging by R‐TENG improves the uniformity of lithium deposition during cycling test. Consequently, symmetric Li/Li cells exhibit lower overpotential and better cycling stability. In addition, full cells assembled with lithium metal anodes and LiFePO₄ cathodes show considerably improved capacity retention when charged by R‐TENG's sinusoidal ripple current (99.5%) compared to constant current (78.7%) after 200 cycles. The charging strategy device in this work provides a promising direction toward improving the cycle life of Li metal batteries. In addition, the combination of R‐TENGs with Li metal batteries offers an encouraging solution for achieving stable energy supply in self‐powered systems.     

A Universal Strategy for Improving the Energy Transmission Efficiency and Load Power of Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) have recently been invented as a potential energy technology for harvesting low‐frequency mechanical energy. The load power acquired from a TENG is far less than the maximum output power of the TENG for the large internal impedance and impedance mismatch, and this difference results in an extremely low energy transmission efficiency. Here, a universal strategy is proposed for improving the energy transmission efficiency and load power through dielectric material design, including a reduction in the effective thickness and the directional alignment of the electric dipole. This strategy reduces the internal impedances of TENGs with different modes and results in the improvement of energy transmission efficiency and load power. According to this strategy, the internal impedance of an as‐fabricated TENG is reduced from 16 to 1.3 MΩ, and the energy transmission efficiency is enhanced from 22.46% to 99.5%. Moreover, the load power under 1 MΩ resistance is improved from 0.014 to 0.251 µW, an increase of 18 times. The strategy not only opens a universal and new road to power management, but also paves the way for the industrial applications of TENGs.     

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.     

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.