In situ characterization of the formation of a mixed conducting phase on the surface of yttria-stabilized zirconia near Pt electrodes

The electrochemical reactions of solid oxide fuel cells occur in the region where gas-phase species, electrode, and electrolyte coincide. When the electrode is an ionic insulator and the electrolyte is an electronic insulator, this triple phase boundary is assumed to have atomic dimensions. Here we use photoemission electron microscopy to show that the reduced surface of the electrolyte yttria-stabilized zirconia (YSZ) has a sharp electronic metalinsulator boundary near Pt negative electrodes. The electronic conductivity of the reduced YSZ allows for oxygen reduction, allowing the reduced YSZ to behave as an extended triple phase boundary. This extended triple phase boundary can be many microns in size, depending on oxygen pressure, temperature, applied voltage, and time.