Robust Working Mechanism of Water Droplet-Driven Triboelectric Nanogenerator: Triboelectric Output versus Dynamic Motion of Water Droplet

Harvesting energy from water flow based on liquid–solid contact electrification has been receiving intensive attention recently. However, due to the high deformability of water, challenges remain on exploring the dynamic motion of water in relation to the electrical output characteristic of water-based triboelectric nanogenerator (TENG). Herein, by measuring the electrical output of a water droplet-driven TENG (Wd-TENG) along with observing the dynamics of the droplets by a high-speed camera, a robust working mechanism of the Wd-TENG is proposed, considering droplet spread, rebound, slide, and coalescence on the solid surface during the triboelectric cycle. It is demonstrated that the spread–rebound motion of droplet upon impact on the triboelectric layer surface results in one positive–negative current peak and the detachment of the droplet from the surface leads to another negative current peak. Droplet coalescence is also observed and found to cause larger negative peak when the merged droplet leaves the surface. The proposed mechanism is sufficiently proved by the results under different droplet dripping rates, sizes, surface wetting properties, and types of liquid. These findings provide an extended understanding of the liquid–solid contact electrification, which is significant in both fundamental and applied science of liquid–solid TENG.