On the Origin of Ion Selectivity in Ultrathin Nanopores: Insights for Membrane‐Scale Osmotic Energy Conversion

Abstract

AbstractNanopores in ultrathin or atomically thin membranes attract broad interest because the infinitesimal pore depth allows selective transport of ions and molecules with ultimate permeability. Toward large‐scale osmotic energy conversion, great challenges remain in extrapolating the promising single‐pore demonstration to really powerful macroscopic applications. Herein, the origin of the selective ion transport in ultrathin nanopores is systematically investigated. Based on a precise Poisson and Nernst–Planck model calculation, it is found that the generation of net diffusion current and membrane potential stems from the charge separation within the electric double layer on the outer membrane surface, rather than that on the inner pore wall. To keep the charge selectivity of the entire membrane, a critical surface charged area surrounding each pore orifice is therefore highly demanded. Otherwise, at high pore density, the membrane selectivity and the overall power density would fall down instead, which explains the giant gap between the actual experimental achievements and the single‐pore estimation. To maximize the power generation, smaller nanopores (pore diameter ≈1–2 nm) are appropriate for large‐scale osmotic energy conversion. With a porosity of ≈10%, the total power density approaches more than 200 W m‐2, anticipating a substantial advance toward high‐performance large‐scale nanofluidic power sources.