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. K 3 PO 4 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 K 2 SO 4 under a 50 K temperature difference, establishing nonionic COF nanochannels as a charge‐screening‐resistant platform for high‐salinity ionic thermoelectrics.