  Graphene-based electric power generation that converts mechanical energy of
flow of ionic droplets over the device surface into electricity has emerged as
promising candidate for a blue-energy network. Yet the lack of a microscopic
understanding of the underlying mechanism has prevented ability to optimize and
control the performance of such devices. This requires information on
interfacial structure and charging behavior at the molecular level. Here, we
use sum-frequency vibrational spectroscopy (SFVS) to probe the interfaces of
devices composed of aqueous solution, graphene and supporting polymer
substrate. We discover that the surface dipole layer of the polymer is
responsible for ion attraction toward and adsorption at the graphene surface
that leads to electricity generation in graphene. Graphene itself does not
attract ions and only acts as a conducting sheet for the induced carrier
transport. Replacing the polymer by an organic ferroelectric substrate could
enhance the efficiency and allow switching of the electricity generation. Our
microscopic understanding of the electricity generation process paves the way
for the rational design of scalable and more efficient droplet-motion-based
energy transducer devices.
