Spatial Confinement Engineering in a Bioinspired Capillary Electrode for Efficient Uranium Recovery From Seawater

The electrochemical extraction of critical elements from complex aqueous environments is fundamentally limited by sluggish ion transport and low utilization efficiency in conventional porous electrodes, where reactions are confined to the surface. Inspired by the hierarchical network of blood capillaries, we designed an electrode incorporating carbon dots within amidoxime-functionalized g-C₃N₄ nanotubes (g-C₃N₄/CD-AO), forming an electroactive nanochannel network. The confined nanochannels enrich UO₂ ²⁺ via steric and coordination effects, while embedded carbon dots enable rapid charge transport throughout the volume. Consequently, the extraction mode shifts from surface adsorption to continuous volumetric filling of the nanotubes, as directly visualized by TEM. In a uranium extraction cell (UEC) with natural seawater, this cathode achieves 2.86 mg g^-1 within 8 h and 8.12 mg g^-1 within just 7 days. This work demonstrates that engineering spatial confinement coupled with integrated conduction pathway provides new design strategy for next-generation electrodes in resource recovery and environmental remediation.