Interfacial reaction and electrochemical properties of dense (La,Sr) CoO_3-δ cathode on YSZ (1 0 0)

Reaction between La_0.6Sr_0.4CoO₃ electrode and yttria stabilized zirconia electrolyte (YSZ) was investigated at 973 K to estimate its effect on performance of a solid oxide fuel cell at reduced operation temperatures. Since La_0.6Sr_0.4CoO₃ is a good mixed conductor of electron and oxide ion, oxygen incorporation through the bulk diffusion is fairly fast if it is compared to the surface diffusion and reaction at triple phase boundaries (TPB). In order to simplify the oxygen reaction pathway, a dense electrode film of La_0.6Sr _0.4CoO₃ was fabricated on an YSZ single crystal by pulsed laser deposition. Electrochemical measurements and secondary ion mass spectrometry (SIMS) analysis were performed before and after a long-term operation test for 3800 h at 973 K. Electrochemical impedance analysis enabled us to distinguish the contribution of the electrode/electrolyte interface resistance from the total electrochemical resistance. The interface resistance was almost independent of oxygen partial pressure, and increased with time according to the parabolic rate law. After the long-term test, Sr rich layer was found between the electrode and the electrolyte. The parabolic rate constant, k_p, was obtained from the time dependence of the resistance and the thickness of the reaction layer. It was about 10^-17 to 10 ^-18 cm² s^-1 at 973 K, on the extrapolated line from the literature data measured at higher temperatures.