Researchers in China discovered the effect by dragging small droplets of salty water across a graphene surface. The salt in this case wasn’t ordinary sodium chloride, but copper chloride. As the salty droplet moved across the surface, it drew electrons out of the graphene on its leading edge; the electrons then returned to the graphene on the trailing edge. The faster the researchers pulled the drops across the surface, the greater the electric potential.
Yogi Patel, writing for Ars Technica:
The scientists then scaled this technology up to demonstrate that you can harvest electricity from it. They used a droplet made of copper chloride and placed it on a graphene surface. The surface was tilted to one side and the droplet was allowed to flow from one end to the other under gravity, resulting in the generation of a measurable voltage—approximately 30mV.
That’s a pretty small amount of electricity, but for a proof of concept it’s impressive. Graphene’s nanoscale dimensions—it’s only one atom thick—could allow for some unusual applications of what is, for all intents and purposes, hydropower.