Active Reaction Sites and Oxygen Reduction Kinetics on La1-xSrxMnO3+δ(x=0.1-0.4)/YSZ (Yttria-Stabilized Zirconia) Electrodes for Solid Oxide Fuel Cells

Abstract

Active reaction sites and electrochemical O2 reduction kinetics on (x=0.1-0.4)/YSZ (yttria-stabilized zirconia) electrodes are investigated in the temperature range of 700-900 ℃ at -0.21 atm. Results of the steady-state polarization measurements, which are formulated into the Butler-Volmer formalism to extract transfer coefficient values, lead us to conclude that the two-electron charge transfer step to atomically adsorbed oxygen is rate-limiting. The same conclusion is drawn from the -dependent ac impedance measurements, where the exponent m in the relationship of (exchange current density) ∝ is analyzed. Chemical analysis is performed on the quenched Mn perovskites to estimate their oxygen stoichiometry factors (δ) at the operating temperature (700-900 ℃). Here, the observed δ turns out to become smaller as both the Sr-doping contents (x) and the measured temperature increase. A comparison between the 8 values and cathodic activity of Mn perovskites reveals that the cathodic transfer coefficients for oxygen reduction reaction are inversely proportional to δ whereas the anodic ones show the opposite trend, reflecting that the surface oxygen vacancies on Mn perovskites actively participate in the reduction reaction. Among the samples of x= 0.1-0.4, the manganite with x=0.4 exhibits the smallest 8 value (even negative), and consistently this electrode shows the highest and the best cathodic activity for the oxygen reduction reaction.