Impedance studies of oxygen exchange on dense thin film electrodes of La_0.5Sr_0.5CoO_3-δ

Solid-state electrochemical cells with dense oriented thin film electrodes of La_0.5Sr_0.5CoO_3-δ (LSCO) were prepared on (100) surfaces of single-crystal disks of yttria-stabilized zirconia (YSZ) by the pulsed laser deposition technique. Oxygen exchange at the electrodes was studied with alternating current impedance spectroscopy under various temperature and oxygen partial pressure conditions. Three distinctive features were observed in the impedance spectra from high to low frequency corresponding to contributions from the ionic conduction of the YSZ electrolyte, ionic transfer at the LSCO/YSZ interface, and the oxygen exchange on the LSCO electrode surface. An equivalent circuit model of the electrode process is used to fit the impedance data. The time constant for the oxygen surface exchange was derived from the impedance simulation. The surface chemical exchange coefficients k_chem, were calculated from the time constants as a function of temperature and pO₂·k_chem is 7×10^-4 cm/s at T = 700 °C and pO₂ = 1 atm. The activation energy at pO₂ = 1 atm is ≈1.1 eV. The interfacial conductivity data were also derived from the impedance simulations as a function of temperature and pO₂. The activation energy for the interfacial transport at pO₂ = 1 atm is ≈1.6 eV.