High‐efficiency Ion Transport in Ultrathin Three‐dimensional Covalent Organic Framework Nanofluidics

High‐efficiency ion transport is essential for both biological and non‐biological processes, including the regulation of cell homeostasis, energy conversion, and mass transfer in chemical industry. Nanofluidic channels are considered ideal platforms for delicate control of ion transport in their unique nanoconfinement, yet currently reported one‐ and two‐dimensional nanofluidics are subjected to elevated transport resistance due to discontinuous and random channels. Here, we engineer ultrathin, three‐dimensional covalent organic framework (3D‐COF) nanofluidics featuring continuously interpenetrated pathways and well‐ordered pore arrangements, demonstrating superior ion conductance. The energy barrier for ion transport across 3D‐COF nanofluidics is exceptionally low, suggesting ultrafast and low‐resistance ion movements. Theoretical calculations indicate that 3D‐COF nanofluidics facilitate group adsorption to anions, leading to high energy barriers for anion mobility, thus enhancing ion selectivity and high‐throughput cation transport. In osmotic energy applications, 3D‐COF nanofluidics achieve a power density of 217.7 W m‐2 with artificial seawater and river water, potentially scalable to 1238.2 W m‐2 under a 500‐fold salinity gradient. The proposed 3D‐COF nanofluidics offer new avenues for desalination and ion/molecular separation.