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<p>The desalination of brackish water provides water to tens of millions of people around the world, but
current technologies deplete much needed nutrients from the water, which is detrimental to both public
health and agriculture. A selective method for brackish water desalination, which retains the needed nutrients,
is electrodialysis (ED) using monovalent-selective cation exchange membranes (MVS-CEMs). However, due to
the trade-off between membrane selectivity and resistance, most MVS-CEMs demonstrate either high transport
resistance or low selectivity, which increase energy consumption and hinder the use of such membranes for
brackish water desalination by ED. Here, we used molecular layer deposition (MLD) to uniformly coat CEMs with
ultrathin layers of alucone. The positive surface charge of the alucone instills monovalent selectivity in the CEM.
Using MLD enabled us to precisely control and minimize the selective layer thickness, while the flexibility and
nanoporosity of the alucone prevent cracking and delamination. Under conditions simulating brackish water
desalination, this compound provides monovalent selectivity with negligible added resistance—the smallest
reported resistance for a monovalent-selective layer, to date—thereby alleviating the selectivity–resistance
trade-off. Addressing the water–energy nexus, we show that using these membranes in ED will cut at least half
of the energy required for selective brackish water desalination with current MVS-CEMs.
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