  High-performance osmotic energy conversion (OEC) requires both high ionic
selectivity and permeability in nanopores. Here, through systematical
explorations of influences from individual charged nanopore surfaces on the
performance of OEC, we find that the charged exterior surface on the
low-concentration side (surfaceL) is essential to achieve high-performance
osmotic power generation, which can significantly improve the ionic selectivity
and permeability simultaneously. Detailed investigation of ionic transport
indicates that electric double layers near charged surfaces provide high-speed
passages for counterions. The charged surfaceL enhances cation diffusion
through enlarging the effective diffusive area, and inhibits anion transport by
electrostatic repulsion. Different areas of charged exterior surfaces have been
considered to mimic membranes with different porosities in practical
applications. Through adjusting the width of the charged ring region on the
surfaceL, electric power in single nanopores increases from 0.3 to 3.4 pW with
a plateau at the width of ~200 nm. The power density increases from 4200 to
4900 W/m2 and then decreases monotonously that reaches the commercial benchmark
at the charged width of ~480 nm. While, energy conversion efficiency can be
promoted from 4% to 26%. Our results provide useful guide in the design of
nanoporous membranes for high-performance osmotic energy harvesting.
