Ab Initio Metadynamics Study of U(III) Oxidation on Carbon Surfaces

The low cathode collection efficiency (CCE) in molten-salt electroreduction of uranium originates from persistent U4+/U3+ redox cycling. However, the underlying mechanism has remained inaccessible to experiments. Using first-principles molecular dynamics with enhanced sampling, we show that interfacial environments fundamentally alter uranium transport and reactivity. In the bulk melt, U4+ exhibits a compact U–Cl solvation structure and diffuses more than twice as fast as U3+. Near a carbon interface, however, both U3+ and U4+ experience strongly suppressed mobility, while their local solvation structures converge toward a UCl4-like configuration, indicating a decoupling between solvation structure and transport. Free-energy calculations further show that UCl3 oxidation follows a single desorption–coordination reaction coordinate pathway, in which an interfacially activated Cl detaches from the surface (barrier ∼ 1.0 eV) and binds strongly to U3+, providing ∼ −2.3 eV stabilization. This identifies a robust interfacial pathway by which reactive Cl species drive undesired U3+ → U4+ oxidation, offering molecular insight into the origins of low CCE.