www.afm-journal.de © 2020 Wiley-VCH GmbH 2004714 (1 of 33) PROGRESS REPORT Enhancement of Triboelectric Charge Density by Chemical Functionalization Yanhua Liu, Jilong Mo, Qiu Fu, Yanxu Lu, Ni Zhang, Shuangfei Wang,* and Shuangxi Nie* A triboelectric nanogenerator (TENG) can convert energy in the surrounding environment to electricity. Therefore, in recent years, research related to TENGs has signifcantly increased owing to its simple and low-cost manufac- turing process, high portability, and high efciency. The principle of the TENG lies in the coupling efect of contact electrifcation and electrostatic induction. Its output performance is directly proportional to the square of the surface charge density, which is related to friction materials. To increase the output power of a TENG and continuously provide electricity for other electronic equipment, many scholars have conducted detailed studies on the triboelec- tric properties of materials. Particularly, there has been research interest in the chemical functionalization of TENGs due to their unique advantages, such as high triboelectric charge density, durability, stability, and self-cleaning properties. This Progress Report highlights the research progress in chemical modifcation methods for improving the charge density of TENGs, and classi- fes their modifcation methods according to their mechanisms. The efects of chemical reaction, surface chemical treatment, and chemical substance doping on the output performance of TENGs are systematically elaborated. Further- more, the applications of chemically modifed TENG in self-powered sensors and emerging felds, including wearable electronic devices, human-machine interfaces, and implantable electronic devices, are introduced. Lastly, the chal- lenges faced in the future developments of chemical modifcation methods are discussed, thereby guiding researchers to the use of chemical modifcation methods for the improvement of charge density for further exploration. DOI: 10.1002/adfm.202004714 Y. H. Liu, J. L. Mo, Q. Fu, Y. X. Lu, N. Zhang, Prof. S. F. Wang, Dr. S. X. Nie School of Light Industry and Food Engineering Guangxi University Nanning 530004, P. R. China E-mail: wangsf@gxu.edu.cn; nieshuangxi@gxu.edu.cn Y. H. Liu, J. L. Mo, Q. Fu, Y. X. Lu, N. Zhang, Prof. S. F. Wang, Dr. S. X. Nie Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control Nanning 530004, P. R. China The ORCID identifcation number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202004714. devices provide great convenience. Thus, providing safe, convenient, and sustain- able energy devices has become among urgent concerns. [2] Small electronic devices mainly rely on external power sources, such as batteries, for their normal opera- tion, prompting several issues. Limited bat- tery life requires its frequent replacement, resulting to battery wastes that are difcult to degrade and are contributors to huge environmental pollution when improperly handled. [3] In 2012, Wang and co-workers [4] developed a triboelectric nanogenerator (TENG) that can convert mechanical energy into electricity through contact electrifcation and electrostatic induction. This device did not only efectively har- ness ambient mechanical energy for power generation, [5] but also exhibits great advantages in self-powered sensors. [3,6] According to diferent working principles, TENG can be divided into four modes: contact separation (CS) mode, lateral sliding (LS) mode, single electrode (SE) mode and freestanding triboelectric-layer (FT) mode. [7] When two materials with diferent frictional polarities (tendency to gain or lose electrons) are in contact, their surfaces generate positive and negative static charges due to contact electrifca- tion. When they are separated, positive and negative charges are also separated, building an induced electric potential diference between the upper and lower levels of the material. If a load is either connected between the two electrodes or in a short-circuit state, this induced potential diference drives electrons to fow in it via an external circuit. [8] Theoretical simulation methods dem- onstrate that the output power of TENG is directly proportional to the square of the triboelectric charge density. [9] Therefore, it is important to increase the charge density on the friction surface to improve the performance of TENG. In recent years, there have been numerous researches in increasing the charge density of materials with the main focus of creating friction between materials and retaining gener- ated charges. Among these, physical modifcation is the most common method. The contact efciency of friction materials is improved by directly introducing micro/nano structures such as nanowires and nanorods, into triboelectric materials to change its physical morphology and increase its surface roughness. [10] Liquid friction material is used to replace the solid friction layer, converting solid–solid friction force to solid–liquid 1. Introduction Nowadays, the development and utilization of clean and renew- able energy could achieve sustainable human development by addressing major global problems of insufcient fossil fuel reserves and environmental crisis. [1] With the advent of the era of miniaturization, light weight, and portability, small electronic Adv. Funct. Mater. 2020, 30, 2004714