  Nanoporous carbon composite membranes, comprising a layer of porous carbon
fiber structures with an average channel width of 30-60 nm grown on a porous
ceramic substrate, are found to exhibit robust desalination effect with high
freshwater flux. In three different membrane processes of vacuum membrane
distillation, reverse osmosis and forward osmosis, the carbon composite
membrane showed 100% salt rejection with 3.5 to 20 times higher freshwater flux
compared to existing polymeric membranes. Thermal accounting experiments found
that at least 80% of the freshwater pass through the carbon composite membrane
with no phase change. Molecular dynamics simulations revealed a unique salt
rejection mechanism. When seawater is interfaced with either vapor or the
surface of carbon, one to three interfacial atomic layers contain no salt ions.
Below the liquid entry pressure, the salt solution is stopped at the openings
to the porous channels and forms a meniscus, while the surface layer of
freshwater can feed the surface diffusion flux that is fast-transported on the
surfaces of the carbon fibers, driven by the chemical potential gradient. As
the surface-transported water does not involve a phase change, hence that
component involves no energy expenditure in the form of latent heat.
