Secondary metal ion-induced electrochemical reduction of U(VI) to U(IV) solids

Recent studies have shown that aqueous U(VI) ions can be transformed into U(VI) precipitates through electrocatalytic redox reactions for uranium recovery. However, there have been no reports of U(IV) solids, such as UO₂, using electrochemical methods under ambient conditions since low-valence states of uranium are typically oxidized to U(VI) by O₂ or H₂O₂. Here we developed a secondary metal ion-induced strategy for electrocatalytic production of U(IV) solids from U(VI) solutions using a catalyst consisting of atomically dispersed gallium on hollow nitrogen-doped carbon capsules (Ga-N_x-C). This method relies on the presence of secondary metal ions, e.g., alkaline earth metals, transition metals, lanthanide metals, and actinide metals, which promote the generation of UO₂ or bimetallic U(IV)-containing oxides through a two-electron transfer process. No U(IV) solid products were generated in the presence of alkali metal ions. Mechanistic studies revealed that the strong binding affinity between U(IV) and alkaline earth metals (Ca²⁺/Mg²⁺/Sr²⁺/Ba²⁺), transition metals (Ni²⁺/Zn²⁺/Pb²⁺/Fe³⁺, etc.) and lanthanide/actinide metals (Ce⁴⁺/Eu³⁺/Th⁴⁺/La³⁺) suppressed re-oxidation of U(IV) to U(VI), leading to the generation of U(IV)O₂ and M_x(M = Ce, Eu, Th, La)U(IV)_yO₂. This work provides fundamental insights into the electrochemical behavior of uranium in aqueous media, whilst guiding uranyl capture from nuclear waste and contaminated water.