Enhancing electrode kinetics of SOFC cathode materials by structural or compositional surface modification

State-of the art solid oxide fuel cells (SOFCs) efficiency is limited by the oxygen reduction reaction (ORR) at the cathode. Considering degradation, start-up time, and costs, a key challenge in SOFC research is to reduce the operation temperature to an intermediate range (IT = 500-800 °C), while simultaneously ensuring fast ORR kinetics. Lanthanum strontium cobaltite (La _1-xSr _xCoO _3-δ with 0 < x < 1, LSC ₁₁₃) is a good candidate for IT-SOFC cathodes. We demonstrate that surface modification of LSC ₁₁₃ (x = 0.2 and 0.4) can strongly influence the ORR kinetics, and is thus a promising method to accelerate the ORR of SOFC cathodes: a Sr-modified LSC ₁₁₃ surface showed an enhanced oxygen surface exchange coefficient, k ^q, by 1 order of magnitude compared to unmodified epitaxial LSC ₁₁₃ films, and 1-3 orders of magnitude improvement were obtained for heterostructured LSC ₁₁₃/(La _0.5Sr _0.5) ₂CoO _4±δ surfaces. Aiming for strategies to improve cathode performance by understanding basic electrode processes, we study structurally, compositionally, and geometrically well-defined thin film microelectrode model systems prepared with pulsed laser deposition (PLD) and photolithography. Electrochemical impedance spectroscopy (EIS) is carried out at about 500°C and different oxygen partial pressures (p(O ₂) = 1 atm - 10 ^-3 atm). The sample characterization includes scanning electron and atomic force microscopy (SEM/AFM: surface morphology/roughness, film thickness), high resolution X-ray diffraction (XRD: phase purity, texture, strain), high resolution scanning transmission electron microscopy (STEM: atomic structure), (in situ)-X-ray photoelectron spectroscopy (XPS: surface composition), and time-of-flight secondary ion mass spectrometry (Tof-SIMS: compositional depth profiling). Possible reasons for the higher surface exchange coefficients of surface-modified LSC ₁₁₃ films will be discussed.