Charge transfer at the Magnéli-phase Ti₄O₇ surface with a self-passivated oxide layer

Magnéli-phase Ti₄O₇ is widely utilized as an electrode material or conductive support in electrochemical systems due to its outstanding conductivity and stability. However, this material typically exhibits sluggish electrode kinetics during electrochemical tests. Herein, we reveal that a self-passivated oxide layer forms on the Ti₄O₇ surface, severely impeding the electrode process. A pristine Ti₄O₇ electrode initially demonstrates significant electrochemical reactivity, which gradually diminishes as the surface becomes passivated via TiO₂ formation in both acidic and alkaline solutions. Using a kinetic-thickness model, the TiO₂ layer thickness is estimated to be ∼3 nm in alkaline solutions and ∼ 12 nm in acidic solutions.