Electricity generation from microdroplet condensation and evaporation on monolayer graphene

Graphene has attracted growing interest as a platform for directly converting interfacial charge dynamics into electrical energy. While previous studies have demonstrated voltage generation from forced droplet motion on graphene, the electricity produced during spontaneous droplet condensation and evaporation remains largely unexplored. Furthermore, how microscale three-phase contact line dynamics influences variations in electricity generation has yet to be elucidated. Here, we explore an electricity generation mechanism driven by the spontaneous formation and disappearance of liquid–solid interfaces during microdroplet condensation and evaporation on monolayer graphene. Using in situ environmental scanning electron microscopy, we directly visualized the dynamic phase-change processes of water microdroplets on graphene and simultaneously measured the corresponding transient voltage signals in real time. Our measurements of advancing and receding contact angles reveal strong pinning effects that prevent the contact line from moving freely and significantly reduce voltage generation. We reveal an electricity generation mechanism driven by instantaneous charge redistribution during electric double layer (EDL) formation at the onset of condensation and EDL collapse during evaporation. This process occurs independently of the directional droplet movement, with measured peak voltages reaching 15 μV during condensation and 80 μV during evaporation. The observed voltage variation directly correlates with dynamic changes in the three-phase contact line. Through coupled analysis of microscale phase-change dynamics and transient electrical measurements, we reveal microscopic electricity generation mechanisms that fundamentally differ from macroscale directional droplet motion. Our work offers new guidelines for developing ultra-thin, high-efficiency hydrovoltaic devices that harness energy from spontaneous phase-change processes.