Electrodeposition of UO₂ at Mild Temperatures in Phosphonium Ionic Liquids: A Combined Theoretical and Experimental Study

Dry reprocessing of spent nuclear fuel traditionally relies on high-temperature molten salts with high energy consumption and corrosion. Ionic liquids offer a promising low-temperature alternative due to their low melting points and wide electrochemical windows. Here, we use the room-temperature phosphonium ionic liquid tributylmethylphosphonium chloride ([P₁₄₄₄]Cl) to electrodeposit and recover uranium as UO₂. Quantum chemical calculations reveal that the [P₁₄₄₄]⁺ cation has a uniformly low surface electrostatic potential and weak P–Cl binding energy, explaining its liquid state at ambient temperature. Molecular dynamics simulations show that uranyl mainly forms UO₂Cl₄^2– complexes. Electrochemical measurements indicate that the U(VI) → U(IV) reduction is an irreversible, diffusion-controlled two-electron process. Potentiostatic deposition at −1.3 to −2.0 V yields a dense amorphous UO₂ layer; after annealing, XRD confirms conversion to crystalline UO₂. These findings demonstrate the feasibility of [P₁₄₄₄]Cl for low-temperature, low-corrosion uranium recovery and provide insights into uranyl speciation and reduction in ionic liquid media.