Ultrathin Magnesium‐Ion Selective COF Membranes for Efficient Osmotic Power and Iontronic Logic Control

Magnesium ions (Mg 2+ ), vital for biological signaling and energy regulation, remain challenging to selectively and efficiently transport using artificial membranes, which typically lack molecular recognition and confined ion channels. Here, we report a β‐ketoenamine covalent organic framework (TAPA‐TFP COF) membrane that achieves near‐ideal Mg 2+ selectivity (t + = 0.98) and ultrafast transport through ordered 13.6 Å nanochannels. Structural and molecular dynamics analyses reveal that the precisely arranged O/N coordination sites within the porous nanochannels partially strip the Mg 2+ hydration shell, while C = O···H─O─H hydrogen‐bond networks lower migration barriers, enabling high permeability. The ultrathin, large‐area, and defect‐free membrane delivers record osmotic power and current densities of 636.64 W m −2 and 9213.3 A m −2 under a 500‐fold salinity gradient. Beyond efficient power generation, the Mg 2+ ‐selective nanochannels enable iontronic transistors, logic operations, and multimodal signal transduction, establishing a molecularly engineered porous platform that integrates ion‐specific transport with energy conversion and information processing.