Origin and Tunability of Unusually Large Surface Capacitance in Doped Cerium Oxide Studied by Ambient-Pressure X-Ray Photoelectron Spectroscopy.

The volumetric redox (chemical) capacitance of the surface of CeO2-δ films is quantified in situ to be 100-fold larger than the bulk values under catalytically relevant conditions. Sm addition slightly lowers the surface oxygen nonstoichiometry, but effects a 10-fold enhancement in surface chemical capacitance by mitigating defect interactions, highlighting the importance of differential nonstoichiometry for catalysis.