(1 of 10) 1600017 wileyonlinelibrary.com Adv. Mater. Technol. 2016, 1, 1600017 and freestanding mode, [15][16][17][18] the energy conversion effi ciency and device durability may be limited by the frictional resistive force between triboelectric surfaces, especially for the sliding-friction-based TENG. [19][20][21] Compared to in-plane sliding friction, rolling friction not only consumes less mechanical energy but also results in a higher robustness with minimized wearing of the materials. [ 21,22 ] In a previous work, a rolling triboelectric nanogenerator (RTENG) based on the rolling electrifi cation between cylindrical steel rods and planar thin fi lms of fl uorinated ethylene propylene (FEP) was reported, and a high energy conversion effi ciency of 55% was demonstrated, with a smaller surface wearing due to the low friction coeffi cient of the rolling movement. [ 21 ] Despite the unique merit of RTENGs, so far a systematic theoretical model to provide an in-depth understanding of their working principle is still lacking, and the issue of how to optimize their output performance remains to be studied.Recently, a standard method to quantitatively evaluate the performance of TENGs was proposed by Wang's group. [ 23 ] The performance fi gure-of-merit for TENGs was defi ned, consisting of a structural fi gure-of-merit (FOM S ), related to the structural design, and a material fi gure-of-merit (FOM M ), related to the square of the surface charge density. Hence, the performance FOM is directly proportional to the highest possible average output power and associated with the highest achievable energy conversion effi ciency, regardless of the mode and size of the TENG. Standards and evaluation methods would facilitate the comparison of various TENGs and lay the foundation for further applications of TENG technology. In the above-mentioned report, the structural FOM for various working modes was calculated using analytical formulae and fi nite-element simulations and it was found that the freestanding confi guration possesses the highest FOM S because of the lower capacitance between the electrodes. Besides the effect of the working mode, it should be possible for a certain working mode to explore the impact of structural design on the structural FOM, which was not included in the previous work, such as the electrode-gap effect for freestanding TENGs.In the present work, a theoretical model of rolling-frictionbased TENGs in a rolling, freestanding mode was constructed. The fi nite-element method (FEM) and theoretical equations were applied to reveal the fundamental physics and dynamic output characteristics of rolling TENGs for both
Figures-of-Merit for Rolling-Friction-Based Triboelectric NanogeneratorsTao Jiang , Wei Tang , Xiangyu Chen , Chang Bao Han , Long Lin , Yunlong Zi , and Zhong Lin Wang * Triboelectric nanogenerators (TENGs) have emerged as a highly effective and easily scalable technology for harvesting mechanical energy. Recently, standards and fi gure-of-merits have been defi ned to quantify the pe...