The insertion of a transition metal enriched domain between the cathode and yttria-stabilized zirconia (YSZ) electrolyte improves the obtainable current density and power output of a solid oxide fuel cell (SOFC). By electrochemical impedance spectroscopy, it is demonstrated that an optimized interface develops during the first cell operation under load. This seems to be a crucial step in the fabrication, formation and function of these interfaces. The spectra also clearly reveal that electrocatalysis is involved in the reduction mechanism of oxygen at the cathode.