Triboelectric nanogenerator—Progress and perspectives

Abstract

In the modern society, the rapid development of the Internet-of-Things (IoT), smart city, and big data brings urgent needs of enormous quantity of devices widely distributed in every corner of the world, forcing the world to adopt a new form of energy solution that needs to be pervasive, sustainable, and clean. Most of current distributed devices are powered by batteries. However, the batteries have limited lifetime, and thus the additional maintenances and replacements are needed. To provide a more sustainable solution, or at least extend the lifetime of the batterypowered systems, energy harvesting technology has been intensively studied in recent years. Nanogenerator is an emerging technology that coverts small-scale mechanical/ thermal energy into electricity. In them, the triboelectric nanogenerator (TENG) is a typical one that can generate electricity from mechanical energy with high performance. On the other hand, energy crisis is becoming a huge challenge in the world. Due to disadvantages of the high pollution and the highly volatile price, people are actively looking for alternative renewable energy resources to replace the traditional fossil energy. In renewable energy sources, mechanical energy is ubiquitous available in the ambient environment in various forms in droplets, water flow, wind flow, vibration, body motion, and so forth, thus providing a huge space for TENG to harvest mechanical energy for power generation. Especially, TENG is very suitable to harvest low-frequency energy, such as that in the ocean waves, which are rich but were not well utilized in the world. As predicted, the TENG technology may lead the blue energy revolution in the future. The charge in the TENG originates from the triboelectric effect, or contact electrification (CE), which describes the charge generation when two surfaces are in contact or sliding. The mechanism of the CE, especially how the charge was transferred between surfaces, puzzled people for over 2000 years. The electron transfer mechanism between surfaces is supported by scientists due to evidence of chemical reactions in surfaces and charge dissipation behaviors. The material/ion transfer mechanism is observed in some CE pairs, which may also contribute to the charge transfer. To further understand the CE mechanism through electron transfer, the electron-cloud-potential-well model was proposed, which may also be used to explain other effects such as the electroluminescence effect during CE. To build up the high-performance TENG devices, the material choices are quite important to enhance the surface charge generation, as well as to reduce the friction and wear. The material choices should follow the general principle based on the triboelectric series, which have been quantified in recent years. Polymer nanowires were demonstrated to enhance the triboelectric charge generation. To investigate how to enhance the surface charge density, the new material systems, such as perovskite materials, were studied. To allow the TENG to be compatible with current trends of wearable and implantable TENGs, flexible and biocompatible materials were developed, such as paper-based, textile-based, degradable, and skin-like materials. In the material/device preparation, emerging technologies such as 3D or 4D printing have been used. To promote the output performance of the TENG, multiple methods have been demonstrated. The droplet TENG based on triboelectrification on the liquid–solid interface has been reported for years, while related studies in recent 2 years demonstrated the high-output performance. Further studies on the liquid–solid interface based TENG toward practical applications attract broad interest and are being intensively conducted in the field of TENG. The direct-current (DC) TENG is another newly developed design which can generate DC power output instead of alternating current (AC) output, which has shown the potential to deliver high power density. The improved design for DC-TENG is still required to fit the demands of practical applications. Portable, wearable, and implantable electronics have become a hot research topic in the past a few decades, due to the increasing demands on the wearable and implantable devices, with the demonstrated applications in biosensors, eskin, e-paper, health monitoring, and therapy. These emerging technologies set the solid foundation for the IoT, human machine interface, virtual reality, and so forth. TENG technology can provide a potential power source for these applications, while the device design and materials need further development. Developing portable, wearable, and DOI: 10.1002/eom2.12129