Relationship between output voltage of water droplet-based electricity nanogenerator and electrolyte concentration

Droplet-based electricity generators (DEGs) that convert mechanical energy from falling droplets into electricity based on the coupling effect of triboelectrification and electrostatic induction are candidates for sustainable energy harvesters because of their low production costs and simple fabrication. The DEG is composed of an electrode and friction layer which is easily charged and generates electricity when the droplet flows on the surface of friction layer. Although environmental water is used to harvest energy, it contains impurities, organic substances, and electrolytes that may influence the electrical output of the DEG. Herein, the DEG output voltage and current were measured for different electrolyte species and concentrations, and a relationship between them was established. The output voltage was enhanced by increasing the electrolyte concentration until a certain concentration was reached, thereafter, at higher concentrations, the output decreased exponentially. This behavior implies that the charge on friction layer is neutralized by cations in the residue solution, and the output is decreased. Using rainwater, river water, and seawater collected from the environment, the DEG outputs were compared. Rainwater was found to be the most suitable water resource for energy harvesting because it had the highest voltage of the three types of water. This study provides guidelines for the use of DEGs in environments with various electrolyte concentrations.