Carbonyl-Enriched Nonionic Covalent Organic Framework Nanochannels for High-Salinity Ionic Thermoelectrics

High-salinity electrolytes coupled with low-grade heat offer an attractive opportunity for ionic thermoelectric conversion, yet conventional fixed-charge membranes rapidly lose ion permselectivity in concentrated media because Donnan exclusion is strongly screened. Here, we report an isoreticular series of nonionic covalent organic framework membranes with comparable one-dimensional nanochannels but systematically varied pore-wall oxygen chemistry. Hydroxyl-substitution-regulated hydrazone-to-β-ketoenamine tautomerism generates carbonyl-enriched BthTb-3OH nanochannels, which preferentially partition and transport cations while suppressing anion migration without relying on covalently anchored charges. Across diverse electrolytes, BthTb-3OH exhibits broadly cation-favored transport, with particularly strong cation/anion mobility contrasts in sulfate- and phosphate-containing media. Experiments and molecular dynamics simulations further reveal that the carbonyl-rich channels couple high K⁺ permselectivity with rapid K⁺ conduction under concentrated-salt conditions. K₃PO₄ preconditioning reorganizes the confined transport landscape through retained phosphate/K⁺ species within the carbonyl-rich nanochannels, facilitating K⁺ transport and amplifying thermodiffusive cation/anion contrast in sulfate media. Consequently, the conditioned membrane delivers a peak power density of 224.2 W m^-2 in saturated K₂SO₄ under a 50 K temperature difference, establishing nonionic COF nanochannels as a charge-screening-resistant platform for high-salinity ionic thermoelectrics.