Impedance spectroscopic study on well-defined (La,Sr)(Co,Fe)O 3− δ model electrodes

Abstract

Geometrically well-defined, dense thin film microelectrodes of the mixed conducting solid oxide fuel cell cathode material La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ have been prepared by pulsed laser deposition and standard photolithographic techniques on yttria-stabilised zirconia substrates. The electrochemical properties of these model electrodes were investigated by impedance spectroscopy as a function of temperature and dc bias. It is shown that an equivalent circuit derived rigorously in Ref. [J. Jamnik and J. Maier, Phys. Chem. Chem. Phys. 3, 1668–1678 (2001).] for a cell with a mixed conducting electrode provides an appropriate description for this experimental system, enabling a correct interpretation of the measured impedance data. Under zero or small dc bias, the electrochemical resistance is dominated by the oxygen exchange reaction at the surface of the electrode, with minor contributions from the electrode/electrolyte interface and the ohmic resistance of the electrolyte. The main capacitive process is associated with oxygen stoichiometry changes in the bulk of the electrode (chemical capacitance), while an additional electrode/electrolyte interfacial capacitance is also present. The temperature and dc bias dependencies of these processes are discussed in terms of defect chemistry.