  Current approaches for electric power generation from nanoscale conducting or
semi-conducting layers in contact with moving aqueous droplets are promising as
they show efficiencies of around 30 percent, yet, even the most successful ones
pose challenges regarding fabrication and scaling. Here, we report stable,
all-inorganic single-element structures synthesized in a single step that
generate electrical current when alternating salinity gradients flow along its
surface in a liquid flow cell. 10 nm to 30 nm thin nanolayers of iron,
vanadium, or nickel produce several tens of mV and several microA cm^-2 at
aqueous flow velocities of just a few cm s^-1. The principle of operation is
strongly sensitive to charge-carrier motion in the thermal oxide nano-overlayer
that forms spontaneously in air and then self terminates. Indeed, experiments
suggest a role for intra-oxide electron transfer for Fe, V, and Ni nanolayers,
as their thermal oxides contain several metal oxidation states, whereas
controls using Al or Cr nanolayers, which self-terminate with oxides that are
redox inactive under the experimental conditions, exhibit dramatically
diminished performance. The nanolayers are shown to generate electrical current
in various modes of application with moving liquids, including sliding liquid
droplets, salinity gradients in a flowing liquid, and in the oscillatory motion
of a liquid without a salinity gradient.
