Operating Mechanism and Activation Phenomena of Solid Oxide Fuel Cell Cathodes

La_0.8Sr_0.2MnO_3-d (LSM) is one of the most commonly utilized cathodes for SOFC systems. Although well studied, a number of questions relating to the electrochemical performance of LSM-based electrodes remain unanswered. For example, it has been observed that the performance of LSM-based electrodes improves dramatically upon the first application of current. This activation behavior has been attributed to a number of mechanisms including restructuring or reduction of the LSM phase and surface segregation of Sr. In addition, it is suggested that a bulk pathway for oxygen ion transport becomes available due to reduction of the LSM phase under cathodic polarization.

In this study, we have fabricated LSM film electrodes of thickness 100, 450, 650 and 700 nm, on yttria-stabilized zirconia (YSZ) electrolyte pellets by ultrasonic spray pyrolysis (USP). Electrodes were symmetrically deposited on either side of the YSZ pellets and the individual electrode impedance spectra measured at 973K in laboratory air using a reference electrode placed on the free electrolyte surface.

As reported for composite LSM/YSZ electrodes, all thicknesses of the pure LSM film electrodes showed a strong initial activation upon application of cathodic current. Surface doping of the electrode with SrO or MnO₂ had little influence on this activation behavior. In contrast, no activation was observed upon surface doping with La₂O₃ suggesting that segregation of La may play an important role in this activation phenomenon and that it is a surface phenomenon.

All of the electrodes showed a strong, non-linear decrease in polarization resistance accompanied by a linear decrease in ohmic resistance with increasing cathodic current. These changes are attributed to the introduction of a bulk ion transport path in the LSM film and increase in the oxygen exchange rate due to the formation of oxygen vacancies by reduction of LSM under cathodic polarization. A minimum polarization resistance of 0.17 Ω.cm² was measured for the La-surface doped 650 nm thick electrode at 250 mA/cm².