Mechanistic insight into electricity generation from moving ionic droplets on graphene

Abstract

Recent experiments have demonstrated that moving water droplets on polymer-supported graphene can generate electric voltages in graphene. Here, we perform a multi-scale analysis on the mechanism of the generated voltages on the basis of an interplay among the substrate, graphene and ionic water. We find that the attraction of ions in water by substrate dipoles drives charge redistribution in graphene, forming an electric triple layer (ETL) at the water/graphene/substrate interface, made of an ion layer fixed on graphene, an image charge layer in graphene and a counterion layer in water. As a droplet moves on graphene, dynamic formation of the ETL at its front end drives a flow of charge in graphene. Using Langmuir adsorption theory combined with ab initio calculations, we determine the ion concentration in the ETL and estimate the amount of charge that each ion can draw in graphene. Then, the electric current in graphene is formulated in terms of ion concentration, droplet velocity, graphene thickness and density of substrate dipoles, which well reproduces experimentally measured currents in graphene. These results underscore the importance of tailoring substrate dipoles in optimizing the performance of devices for water energy harvesting and promoting practical applications.