<
p>
The performance of solid oxides fuel cells (SOFCs) with four different Mn excess of lanthanum strontium manganite (LSM) -based cathodes were examined under different temperatures (1,000 �C, 900 �C), current densities (0, 380, and 760 mA cm<
sup>
-2<
/sup>
), and cathode atmospheres (p<
sub>
O<
sub>
2<
/sub>
<
/sub>
)=0.1,0.15,0.21) for durations ranging from 58 to 1,008 h. Each yttria-stabilized zirconia (YSZ) electrolyte-supported ?button? cell had a porous Ni/YSZ composite anode and a porous LSM/YSZ composite cathode. The cells? output voltage versus time were recorded and electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) measurements were performed every 24 hours. The total area specific resistance (ASR) was calculated from these measurements. The values of ASR from both EIS and LSV were comparable between each other but lower than that from durability test. Distribution of relaxation times (DRT) analysis was performed to investigate the electrochemical processes and their corresponding relaxation frequencies as well as their attributions to the total ASR. The series resistance and parallel resistance that obtained from the equivalent circuit fit of the Nyquist plot were higher at low temperature and low p<
sub>
O<
sub>
2<
/sub>
<
/sub>
regardless of LSM compositions. The area of the peaks derived from DRT analysis increased with time during individual tests.<
/p>
<
p>
The microstructures of the tested cells were examined using scanning electron microscopy and energy-dispersive x-ray spectroscopy (SEM/EDS). Manganese oxide particles were observed near the cathode-electrolyte interface after prolonged (1,008 h) testing in air of a cell with LSM of 11% manganese excess, and after a short test (58 h) under low oxygen (p<
sub>
O<
sub>
2<
/sub>
<
/sub>
= 0.10) in a cell with LSM of 5% manganese excess. However, the role of MnOx in cell degradation is still unknown.<
/p>