Department of Materials Science and EngineeringMost recently, a newly designed energy generating device named the triboelectric nanogenerator (TENG) was reported and various types of TENGs have been demonstrated, proven as a highly efficient, simple, robust and cost-effective technique for efficiently converting various mechanical energies around us to electricity. In principle, the electrical energy is generated as two different materials are brought into contact with each other, in conjunction with the triboelectric electrification and electrostatic induction. The sources of mechanical energy such as winds and moving things are available anywhere and anytime in our surroundings, thus, TENG will be appropriate as a power-supply unit for portable devices although the energy is not big as expected. Since the first demonstration of the TENG on 2012, various TENG structures and new functional materials brought about the significant increase of the instantaneous power density up to several tens of mW/cm2. However, further enhancement is required for faster commercialization, which may be possible by developing functional triboelectric materials, such as dielectrics. First, we set out to design and synthesize Polyvinylidene fluoride (PVDF) graft copolymers to incorporate poly(tert-butyl acrylate) (PtBA) through an atom-transfer radical polymerization (ATRP) technique as an efficient dielectric to enhance the output performance of the TENGs. This increase in the dielectric constant significantly increased the density of the charges that can be accumulated on the copolymer during physical contact. The markedly enhanced output performance is quite stable and reliable in harsh mechanical environments due to the high flexibility of the films. Second, we demonstrate high-output TENGs based on polyimide (PI)-based polymers by introducing functionalities (e.g., electron-withdrawing and electron-donating groups) into the backbone. The TENG based on 6FDA-APS PI, possessing the most negative electrostatic potential and the low-lying lowest unoccupied molecular orbital level, produces the highest effective charge density in practical working conditions without the ion injection process. This may be ascribed to the excellent charge-retention characteristics as well as the enhanced charge transfer capability. This article provides a comprehensive review of effective dielectrics used so far in TENG, as well as the fundamental issues regarding the materials. Finally, we show some strategies for obtaining the properties that the materials should have as effective dielectrics.clos
