Nanofluidic reverse electrodialysis (RED) captures a portion of that energy as electrical work by allowing salt ions — but not water — to cross between fresh and salt water through ion-selective nanochannels.
Because each channel only lets one charge sign pass, the two reservoirs become electrically charged. Electrodes at the ends of a stack convert this ionic current into an electronic current that powers a load.
A RED stack consists of alternating cation- and anion-exchange membranes separating compartments of salt water and fresh water. Two electrodes terminate the stack.
Each membrane develops a Donnan (membrane) potential because only one ion sign crosses. The potentials of all CEM/AEM pairs add in series along the stack.
≈ 80–150 mV per cell pair under realistic river/sea conditions.
Inside the stack, the current is ionic. To deliver power to a load, electrodes convert it to electronic current via a reversible redox couple. A common choice is the iron(II/III) hexacyanoferrate couple recirculating in both end-compartments:
Cathode (reduction, gains e⁻):Because the couple is recycled, no net electrolysis or reagent consumption occurs — the only consumed input is the salinity gradient itself.
Traditional polymer membranes have fixed ionic groups (e.g. –SO₃⁻ for CEM, –NR₄⁺ for AEM) that exclude co-ions chemically.
In nanofluidic RED the membrane is a sheet of nanopores (graphene oxide, MoS₂, BN nanotubes, anodised alumina). Selectivity comes from the geometry of the pore and its surface charge — see the Nanochannel tab.
Reported lab power densities in nanofluidic devices reach 10²–10³ W·m⁻², compared with ~3 W·m⁻² for commercial polymer RED.
The pore wall in this view carries a fixed negative surface charge (deprotonated –OH or –COO⁻ groups, for example).
Ions of opposite sign — the counterions (Na⁺) — are attracted toward the wall and form a thin layer called the electric double layer (EDL).
Ions of the same sign — the co-ions (Cl⁻) — are repelled and excluded.
λ_D is the distance over which the surface charge is screened by the surrounding ions. For aqueous NaCl at 25 °C:
λ_D ≈ 0.304 / √c nm (c in mol·L⁻¹)So a 5–20 nm pore behaves very differently against fresh vs salt solutions — and that's exactly the regime we want for a salinity-gradient generator.
If the pore is much larger than λ_D (right of slider), the EDLs are thin films on the wall, the channel interior is essentially neutral electrolyte, and both ions pass freely. There is no selectivity, and no useful voltage.
RED extracts energy from mixing two solutions. Reverse osmosis (RO) consumes energy to do the opposite — separating pure water from a salt solution against the osmotic gradient.
Both processes use a membrane, but:
To force water through against osmotic pressure π, the applied pressure P must satisfy P > π. For sea water:
π_seawater ≈ 27 bar (at 25 °C)The slider lets you raise the piston pressure and watch the membrane flux.
The polyamide film is dense — there are no continuous pores at all. Water dissolves into the film and diffuses across (the solution-diffusion mechanism). Hydrated Na⁺ and Cl⁻ ions can't fit through the polymer matrix.
Notice: RED's theoretical maximum (~0.8 kWh·m⁻³ mixed) and RO's theoretical minimum (~1 kWh·m⁻³ separated) sit on either side of the same Gibbs free energy of mixing. They are thermodynamic opposites.