On the performance of freestanding rolling mode triboelectric nanogenerators from rotational excitations for smart tires

In this study, we propose a novel freestanding rolling mode triboelectric nanogenerator (FR-TENG) for harvesting energy generated by tire rotation, located within the tire to achieve optimal energy capture. First, to investigate the operating principles of the FR-TENG, the finite element method (FEM) is conducted to predict the potential distribution of spherical rolling elements. Additionally, we employ an atomic-scale electron cloud potential well model to describe the electron transfer mechanism. Second, kinematic simulation is employed to examine the motion state of rolling elements of varying sizes and shapes at different rotation speeds. The results demonstrate that the cylindrical and spherical rolling elements exhibit more stable motion performance compared to the elliptical cylindrical rolling element. Third, the effect of rolling element shapes and sizes on the output performance of the FR-TENG is investigated through systematic experiments. At a rotation speed of 120 rpm, the maximum power of the sphere (diameter 20 mm) reaches 5.33 & mu;W with a load resistance of 50 M & omega;. The maximum volume power density of the FR-TENG reaches 3.29 W/m3 when the rolling element is a cylinder (width 22 mm). Regarding charging ability, the cylindrical rolling element outperforms the spherical rolling element by achieving a higher charging voltage. The FR-TENG can charge a 22 & mu;F capacitor to 4.4 V and continuously drive a watch for 20 s. This study provides a comprehensive outlook on the design and configuration of freestanding triboelectric-layer mode TENGs, which can aid in the development of energy recovery systems for vehicle tire rotation.