Surface Decorations on Mixed Ionic and Electronic Conductors: Effects on Surface Potential, Defects, and the Oxygen Exchange Kinetics

The oxygen exchange kinetics of epitaxial Pr0.1Ce0.9O2-delta electrodes was modifiedbydecoration with submonolayer amounts of different basic (SrO, CaO)and acidic (SnO2, TiO2) binary oxides. The oxygenexchange reaction (OER) rate and the total conductivity were measuredby in situ PLD impedance spectroscopy (i-PLD), which allows to directly track changes of electrochemicalproperties after each deposited pulse of surface decoration. The surfacechemistry of the electrodes was investigated by near-ambient pressureXPS measurements (NAP-XPS) at elevated temperatures and by low-energyion scattering (LEIS). While a significant alteration of the OER ratewas observed after decoration with binary oxides, the pO(2) dependence of the surface exchange resistance and its activationenergy were not affected, suggesting that surface decorations do notalter the fundamental OER mechanism. Furthermore, the total conductivityof the thin films does not change upon decoration, indicating thatdefect concentration changes are limited to the surface layer. Thisis confirmed by NAP-XPS measurements which find only minor changesof the Pr-oxidation state upon decoration. NAP-XPS was further employedto investigate changes of the surface potential step on decoratedsurfaces. From a mechanistic point of view, our results indicate acorrelation between the surface potential and the altered oxygen exchangeactivity. Oxidic decorations induce a surface charge which dependson their acidity (acidic oxides lead to a negative surface charge),affecting surface defect concentrations, any existing surface potentialstep, potentially adsorption dynamics, and consequently also the OERkinetics.

Automated summary

The oxygen exchange kinetics of Pr0.1Ce0.9O2-delta electrodes were modified by decorating with submonolayer amounts of binary oxides. The OER rate and total conductivity were measured using i-PLD and changes in electrochemical properties were observed after each pulse of surface decoration. The surface chemistry of the electrodes was investigated using NAP-XPS and LEIS. The results showed that the OER rate was significantly altered by the surface decorations, but the fundamental OER mechanism was not affected.