Theoretical Insights into the Adsorption-Electrochemical Reduction Mechanism of Uranyl on Fe-N₄-C Catalysts

Adsorption-electrochemistry coupling strategies have attracted considerable interest due to their high efficiency for uranium extraction, in which solid electrocatalysts are widely adopted in diverse uranium extraction systems. Experimental limitations in resolving atomic-scale details hinder the elucidation of the uranium adsorption-electrochemical reduction mechanisms at the solid-liquid interface. Moreover, the complex uranyl coordination chemistry and strong coupling between the electronic structure and interfacial electrostatics have rendered relevant theoretical studies relatively scarce. To address this gap, we take the typical uranium extraction system over Fe-N-C-based electrocatalysts as the model and employ the density functional theory (DFT) to investigate the adsorption-electrochemical reduction mechanism of uranyl species. Our calculations reveal a more favorable end-on adsorption mode via stronger Fe-O bonding to uranyl axial oxygen than to the coordinated water in the side-on configuration. The critical role of proton binding to uranyl axial oxygen in U(VI) electroreduction is further demonstrated. Among the three proposed electron-transfer pathways, the bridgewater-assisted internal proton transfer (B-IPT) is identified as the most advantageous for promoting the reduction, outperforming both the direct-internal (D-IPT) and external (EPT) pathways. These results provide important insights into adsorption-electrochemical uranium extraction, advancing the fundamental understanding of the coordination chemistry of uranium.