Oxygen Reduction Kinetics of Lanthanum Manganite (LSM) Model Cathodes: Partial Pressure Dependence and Rate-Limiting Steps

Sr-doped LaMnO3 (LSM) thin film microelectrodes prepared by pulsed laser deposition and photolithography on YSZ (yttria stabilised zirconia) electrolytes are investigated by impedance spectroscopy. Oxygen partial pressure (p(O2)), microelectrode diameter and microelectrode thickness are varied in order to achieve a mechanistic interpretation of the impedance spectra. By means of an equivalent circuit based on a continuum model a quantitative analysis of the spectra was possible and allowed a separation of the resistances associated with oxygen exchange at the surface of the LSM electrodes and oxide ion transport through the thin film electrodes. At low oxygen partial pressures (mbar range) the surface-related resistance is rate Limiting. On increasing p(O2) the transport resistance becomes larger while the surface resistance decreases and a change in the rate-limiting step can be concluded for high p(O2). Capacitances are interpreted in terms of processes at interfaces and stoichiometry changes in LSM (chemical capacitance). The p(O2) dependence of the ionic conductivity and of the chemical diffusion coefficient of LSM is determined.