In this study, five different mixed conducting cathode materials were grown as dense thin films by pulsed laser deposition (PLD) and characterized via in-situ impedance spectroscopy directly after growth inside...

Temperature and p(O2) dependences of the oxygen exchange kinetics of SOFC cathode materials were correlated with point defect concentrations. Defect chemical contributions were identified as a major part of effectively measured activation energies.

La 0.6 Sr 0.4 FeO 3−δ (LSF64) thin films are prepared by pulsed laser deposition (PLD) on yttria stabilized zirconia single crystals (YSZ) and characterized by electrochemical impedance spectroscopy (EIS) measurements before and after decoration with platinum nanoparticles. The platinum on the surface of LSF64 strongly accelerates the oxygen surface exchange kinetics. Especially at low oxygen partial pressures, the area-specific resistance (ASR) decreases by almost two orders of magnitude (e.g. in 0.25 mbar pO 2 from 125 Ωcm 2 to ca. 2 Ωcm 2 at 600°C). While the pure LSF64 films exhibit severe degradation of the polarization resistance, Pt decorated films degrade much slower and show less scatter between individual samples. Surprisingly, faster oxygen incorporation (=lower polarization resistance) results for lower oxygen partial pressures, which indicates a severe mechanism change compared to undecorated LSF64 surfaces. The obtained results thus also reveal valuable information on the ratedetermining step of oxygen exchange on LSF64 surfaces with and without platinum. On undecorated LSF64 surfaces oxygen dissociation is suggested to be rate limiting, while the Pt particles on LSF64 enable fast oxygen dissociation. Consequently, on Ptdecorated LSF64 electrodes a kind of job sharing mechanism results, with oxygen dissociation taking place on Pt and oxide ion formation and incorporation proceeding on the oxide.

The oxygen exchange kinetics of epitaxial Pr 0.1 Ce 0.9 O 2−δ electrodes was modified by decoration with submonolayer amounts of different basic (SrO, CaO) and acidic (SnO 2 , TiO 2 ) binary oxides. The oxygen exchange reaction (OER) rate and the total conductivity were measured by in situ PLD impedance spectroscopy (i-PLD), which allows to directly track changes of electrochemical properties after each deposited pulse of surface decoration. The surface chemistry of the electrodes was investigated by near-ambient pressure XPS measurements (NAP-XPS) at elevated temperatures and by low-energy ion scattering (LEIS). While a significant alteration of the OER rate was observed after decoration with binary oxides, the pO 2 dependence of the surface exchange resistance and its activation energy were not affected, suggesting that surface decorations do not alter the fundamental OER mechanism. Furthermore, the total conductivity of the thin films does not change upon decoration, indicating that defect concentration changes are limited to the surface layer. This is confirmed by NAP-XPS measurements which find only minor changes of the Pr-oxidation state upon decoration. NAP-XPS was further employed to investigate changes of the surface potential step on decorated surfaces. From a mechanistic point of view, our results indicate a correlation between the surface potential and the altered oxygen exchange activity. Oxidic decorations induce a surface charge which depends on their acidity (acidic oxides lead to a negative surface charge), affecting surface defect concentrations, any existing surface potential step, potentially adsorption dynamics, and consequently also the OER kinetics.

Minimizing the overpotential at the air electrode of solid oxide fuel cells (SOFC) is one of the key challenges regarding a broad applicability of this technology. Next to novel materials...