Pumping Charges for Ultrahigh-Performance Triboelectric Nanogenerators at Ambient Conditions
Authors: Xu Liang , Bu Tianzhao , Yang Xiaodan , Zhang Chi , Wang Zhong Lin 
Published: February 2020

The development of wearable electronics, implantable devices and internet of things put forward an ever-growing demand on distributed power supply, providing an opportunity for technologies that can harvest energy locally from environment. As a promising energy harvesting technology, triboelectric nanogenerators (TENGs) show merits of easy fabrication, low cost, abundant choice of structures and materials compared with other energy harvesting technologies. The working principle of the TENG is based on the conjugation of triboelectrification of dielectric materials and electrostatic induction and its theoretical fundamental can be derived from the Maxwell’s displacement current.

The practical application of TENGs imposes a challenging requirement to improve the power density of the device, which is highly related to the charge density because it has a quadratic dependence on the charge density. The enhancement of the charge density is restricted mainly by two issues for normal TENG devices under certain intensity of contact or rubbing. The first one is the triboelectrification capability of tribomaterial pairs with certain surface topographies, and the other is the discharge caused by air breakdown. In previous studies, great efforts have been conducted to improve the charge density, based on material selection, structure optimization, surface modification or environment control. The corona charging is a relative facile method that is widely adopted, which can inject charges into dielectric films to enhance the charge density from under 100 μC m-2 to about 240 μC m-2. However, the injected charges are not stable and the dielectric materials are restricted to electrets. High vacuum environment can suppress air breakdown, which can greatly enhance the charge density to 660 μC m-2 for Cu versus polytetrafluoroethylene (PTFE), and 1003 μC m-2 with an extra ferroelectric barium titanate (BT) layer. Still, this method is restricted by high requirements of device packaging.

To develop a facile and universal method to enhance the charge density of TENG devices, an integrated self-charge-pumping TENG (SCP-TENG) device with features of floating layer structure and charge pumping is proposed in this work. Derived from normal TENGs, the proposed device adopts a floating layer to store and bind charges for electrostatic induction, while a TENG based charge pump is designed to continuously pump charges into the floating layer at the same time. The SCP-TENG can operate like normal TENGs, while accumulating bound charges with high efficiency to ultrahigh effective surface density of 1020 μC m-2 in ambient conditions, which has potentials to be further elevated in the future. More importantly, this method is rather facile and robust compared to other charge density enhancing approaches, providing an important strategy to achieve ultrahigh-performance for TENG devices, which is crucial for various practical applications ranging from wearable electronics to blue energy.