Protein‐Like Salting‐Out Aggregation Toward Polymeric Cation Nanochannels for Precise Ion Transport

ABSTRACT Selective ion transport is crucial to harvest sustainable osmotic energy, dominating conversion efficiency. In nature, ultrahigh cation/anion selectivity is achieved by biological ion channels, based on ∼3 Å “selectivity filter.” Artificial polymeric nanochannels that feature microphase‐separated or microporous structures promise fast ion transport, however finely tuning nanochannel size to ∼3 Å, to enhance ion selectivity remains challenging. Here, we present a breakthrough in the construction of precisely‐defined cation channels of ∼3.2 Å, via salting‐out aggregation of sulfonated poly(ether ether ketone) (SPEEK). In a dimethyl sulfoxide solution of SPEEK, the addition of organic salts affects the solvation shells of SPEEK to induce polymer chain assembly, whose surface potential increases from −3.76 to −4.40 mV. Subsequently, cation nanochannel membranes (CNMs) form spontaneously at room temperature, and residual salt solutions are recycled directly for the next fabrication. Used in osmotic energy generators, optimal CNMs exhibit far higher Na + (1.9 Å)/Cl − (3.6 Å) selectivity (t + ) of 0.97 than common SPEEK membranes, with ion conductivity improving. Then, both high power density of 5.31 W m −2 and energy conversion efficiency of 44.2% can be achieved, over most reported membranes (<40%) by complicated design and fabrication. This work provides an economically feasible method for fabricating highly selective nanochannel membranes.