  High-performance osmotic energy conversion requires both large ionic
throughput and high ionic selectivity, which can be significantly promoted by
exterior surface charges simultaneously, especially for short nanopores. Here,
we investigate the enhancement of ionic diffusion by charged exterior surfaces
under various conditions and explore corresponding effective charged areas.
From simulations, ionic diffusion is promoted more significantly by exterior
surface charges through nanopores with a shorter length, wider diameter, and
larger surface charge density, or under higher salt gradients. Effective widths
of the charged ring regions near nanopores are reversely proportional to the
pore length and linearly dependent on the pore diameter, salt gradient, and
surface charge density. Due to the important role of effective charged areas in
the propagation of ionic diffusion through single nanopores to cases with
porous membranes, our results may provide useful guidance to the design and
fabrication of porous membranes for practical high-performance osmotic energy
harvesting.
