  Reverse electrodialysis is a promising method to harvest the osmotic energy
stored between seawater and freshwater, but it has been a long-standing
challenge to fabricate permselective membranes with the power density
surpassing the industry benchmark of 5.0 W $m^{-2}$ for half a century. Herein,
we report a vertically-oriented graphene oxide membranes (V-GOMs) with the
combination of high ion selectivity and ultrafast ion permeation, whose
permeation is three orders of magnitude higher than the extensively studied
horizontally stacked GOMs (H-GOMs). By mixing artificial seawater and river
water, we obtained an unprecedented high output power density of 10.6 W
$m^{-2}$, outperforming all existing materials. Molecular dynamics (MD)
simulations reveal the mechanism of the ultrafast transport in V-GOMs results
from the quick entering of ions and the large accessible area as well as the
apparent short diffusion paths in V-GOMs. These results will facilitate the
practical application of osmotic energy and bring innovative design strategy
for various systems involving ultrafast transport, such as filtration and
catalysis.
