Surface Exchange Coefficients of Composite Cathode Materials Using In Situ Isothermal Isotope Exchange

Abstract

Isothermal isotope exchange (IIE) was used to evaluate the surface kinetics of the oxygen reduction reaction on composite cathode materials. With IIE, powder materials can be tested to ensure surface exchange behavior is isolated. Macroscopic diffusion coefficients (D*) and surface exchange coefficients (k*) were extracted from 50:50 wt % composite cathode powders (La 0.6 Sr 0.4 ) (Co 0.2 Fe 0.8 ) O 3-δ /(Ce 0.9 Gd 0.1 ) O2-δ (LSCF/GDC), (La 0.8 Sr 0.2 ) Mn O 3 / (Y 2 O 3 )0.08 (Zr O 2 )0.92 (LSM/YSZ), and (La 0.8 Sr 0.2 ) Mn O 3 / (Ce 0.9 Gd 0.1 ) O2-δ (LSM/GDC) using IIE, and characteristic thicknesses (L c ) were calculated from the ratio of D* / k*. Mixed ionic and electronic conductor LSCF/GDC was shown to have the highest D* values between 500 and 850°C and k* values above 825°C. Adding an ionically conducting phase to LSM significantly improved the magnitude of k*, which below 825°C were highest for LSM/YSZ of the three composites. In addition, LSM/YSZ and LSM/GDC exhibited an apparent negative activation energy, which can be explained by a precursor-mediated mechanism for dissociative adsorption. Sample thicknesses (particle size) were shown to be well below characteristic thicknesses (L c ), ensuring that samples were in a surface exchange controlled regime and validating the accuracy of the extracted k* values. © 2011 The Electrochemical Society.