Fig. 1: Hypothesis of field emission in triboelectric nanogenerator.

a Working mechanism of TENGs based on contact electrification and electrostatic induction. b Schematic diagram shows the charges flowing process in a TENG. In ideal condition, the output charges of TENGs is equal to the triboelectric charges if there are no charge dissipations. In general conditions, partial triboelectric charges are often released by breakdown effects, and then the output charges are equal to the residual surface charges. c–e The output charge density of polyimide (6 μm) and copper under vacuum conditions. c The cyclic output charge density at 1 Hz; d the charge density at the separating process after contacting several minutes; e the cyclic output charge density at 1 Hz again. f The represented charge density in contact electrification tested by TENGs^{5,7,8,12,13,24,25,26,27}. g Electron transfer at the contact state. At the separate state, the distance between atom A and atom B is x, which is larger than the distance a where two atoms are at equilibrium position. When x < a, the electronic clouds of two atoms are strongly overlapped, and electrons transfer occurs at this state. h Balanced charge density in TENGs considering both charge generation in the contact state and charge dissipations in the separate state. i Possible charge dissipations in TENGs, including air breakdown, field emission, leakage current, and dielectric breakdown. The surface charge density (SCD) on dielectric material is denoted as σ_D; the transferred charge density in the external circuit is denoted as σ_T. Source data are provided as a Source data file.