Nanogenerators for smart cities in the era of 5G and Internet of Things

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

Triboelectric Nanogenerator: Structure, Mechanism, and Applications.

Stretchable piezoelectric energy harvesters and self-powered sensors for wearable and implantable devices

Breath-based human–machine interaction system using triboelectric nanogenerator

The era of artificial intelligence and internet of things is rapidly developed by recent advances in wearable electronics. Gait reveals sensory information in daily life containing personal information, regarding identification and healthcare. Current wearable electronics of gait analysis are mainly limited by high fabrication cost, operation energy consumption, or inferior analysis methods, which barely involve machine learning or implement nonoptimal models that require massive datasets for training. Herein, we developed low-cost triboelectric intelligent socks for harvesting waste energy from low-frequency body motions to transmit wireless sensory data. The sock equipped with self-powered functionality also can be used as wearable sensors to deliver information, regarding the identity, health status, and activity of the users. To further address the issue of ineffective analysis methods, an optimized deep learning model with an end-to-end structure on the socks signals for the gait analysis is proposed, which produces a 93.54% identification accuracy of 13 participants and detects five different human activities with 96.67% accuracy. Toward practical application, we map the physical signals collected through the socks in the virtual space to establish a digital human system for sports monitoring, healthcare, identification, and future smart home applications.

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Deep learning-enabled triboelectric smart socks for IoT-based gait analysis and VR applications

Surface charge density is the key factor for developing high performance triboelectric nanogenerators (TENG). The previously invented charge excitation TENG provides a most efficient way to achieve maximum charge output of a TENG device. Herein, criteria to quantitatively evaluate the contact efficiency and air breakdown model on charge excitation TENG are established to enhance and evaluate charge density. The theoretical results are further verified by systematic experiments. A high average charge density up to 2.38 mC m−2 is achieved using the 4 μm PEI film and homemade carbon/silicone gel electrode in ambient atmosphere with 5% relative humidity. This work also reveals the actual charge density (over 4.0 mC m−2) in a TENG electrode based on quantified surface micro-contact efficiency and provides a prospective technical approach to improve the charge density, which could push the output performance of TENG to a new horizon. Surface charge density is a key factor for developing high performance triboelectric nanogenerators. Herein, authors establish criteria to quantitatively evaluate the contact efficiency and air breakdown model on charge excitation triboelectric nanogenerators to maximize output charge density.

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Quantifying contact status and the air-breakdown model of charge-excitation triboelectric nanogenerators to maximize charge density.

Owing to the advantages of integration and being magnet-free and light-weight, the switched-capacitor-convertor plays an increasing role compared to traditional transformer in some specific power supply systems. However, the high output impedance and switching loss largely reduces its power efficiency, due to imperfect topology and transistors. Herein, we propose a fractal-design based switched-capacitor-convertors with characteristics including high conversion efficiency, minimum output impedance, and electrostatic voltage applicability. As a double-function output power management system for triboelectric nanogenerators, it delivers over 67 times charge boosting and 954 W m−2 power density in pulse mode, and achieves over 94% total energy transfer efficiency in constant mode. The establishment of the fractal-design switched-capacitor-convertors provides significant guidance for the development of power management toward multi-functional output for numerous applications. The successful demonstration in triboelectric nanogenerators also declares its great potential in electric vehicles, DC micro-grids etc. Power management systems are urgently needed for maximally utilizing the energy generated from triboelectric energy devices. Here, the authors report a fractal-designed switched-capacitor-convertor as a high efficiency power management strategy for triboelectric nanogenerators.

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

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

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

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. High reliability is a huge challenge for sliding mode triboelectric nanogenerator (TENG). Here the authors develop a floating self-excited sliding TENG achieving both high durability and output for sustainable micro mechanical energy harvesting.

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

https://www.cell.com/joule/pdf/S2542-4351(20)30622-X.pdf

Wencong He

Papers

Quantifying contact status and the air-breakdown model of charge-excitation triboelectric nanogenerators to maximize charge density.

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

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

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

Ultrahigh Electricity Generation from Low-Frequency Mechanical Energy by Efficient Energy Management