Mixed-conducting perovskite-type electrodes which are used as cathodes in solid oxide fuel cells (SOFCs) exhibit pronounced performance improvement after cathodic polarization. The current in situ study addresses the mechanism of this activation process which is still unknown. We chose the new perovskite-type material La_0.75Sr_0.25Cr_0.5Mn_0.5O_3±δ which is a potential candidate for use in symmetrical solid oxide fuel cells (SFCs). We prepared La_0.75Sr_0.25Cr_0.5Mn_0.5O_3±δ thin film model electrodes on YSZ (111) single crystals by pulsed laser deposition (PLD). Impedance spectroscopy (EIS) measurements show that the kinetics of these electrodes can be drastically improved by applying a cathodic potential. To understand the origin of the enhanced electrocatalytic activity the surfaces of operating LSCrM electrodes were studied in situ (at low pressure) with spatially resolving X-ray photoelectron spectroscopy (μ-ESCA, SPEM) and quasi static secondary ion mass spectrometry (ToF-SIMS) after applying different electrical potentials in the SIMS chamber. We observed that the electrode surfaces which were annealed at 600 °C are enriched significantly in strontium. Subsequent cathodic polarization decreases the strontium surface concentration while anodic polarization increases the strontium accumulation at the electrode surface. We propose a mechanism based on the reversible incorporation of a passivating SrO surface phase into the LSCrM lattice to explain the observed activation/deactivation process.

Due to the short electron mean free path in dense media samples in liquid or ambient pressure environment cannot be easily characterized by photoelectron spectroscopy. We overcome this limitation by employing electron transparent graphene membranes to separate the sample environment from the vacuum conditions in the electron spectrometer.