Dynamic Acidic Adsorbates Govern Fast Oxygen Incorporation in Ceria through Interfacial Electrostatics

Abstract The oxygen reduction reaction (ORR) is a key heterogeneous process underpinning energy conversion, catalytic, and sensing technologies. However, reported ORR kinetics at oxide surfaces often vary widely because surface chemistry evolves under operating conditions. Ceria-based fluorites have been proposed as protective layers to stabilize perovskite electrodes, but ORR kinetics are typically considered sluggish and reported values have large variability. Herein, we demonstrate that cathodic polarization triggers an unexpected activation in Gd-doped ceria (CGO) dense thin films at 500 °C, enabling oxygen incorporation rates that surpass state-of-the-art materials such as La0.5Sr0.5FeO3−δ and approach the performance of La0.6Sr0.4CoO3−δ. Operando near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), supported by ex situ surface-sensitive tip-enhanced Raman spectroscopy (TERS), reveals that CGO surface activation coincides with a reversible, overpotential-dependent modulation of sulfate adsorbates. Integration of these measurements with nondilute space-charge modeling suggests that charged adsorbates dynamically tune the near-surface electrostatic potential and electron concentration, thereby controlling oxygen incorporation kinetics. These results support adsorbate-induced electrostatic effects as a key mechanism in ceria and show that ORR kinetics are dynamically set by surface chemistry under operation, rather than by intrinsic bulk defect chemistry alone.