Sr Surface Segregation on La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-δ) Porous Solid Oxide Fuel Cell Cathodes

摘要

La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-δ) (LSCF) symmetric-electrode cells with Gd_(0.1)Ce_(0.9)O_(1.95) (GDC) electrolytes were aged in ambient air at temperatures ranging from 550 to 800°C for times up to 1400 h, without current/polarization. Electrochemical impedance spectroscopy measurements, taken periodically during the tests at a testing temperature of 700°C for cells aged ≥ 700°C, showed a continuous increase in polarization resistance. Focused ion beam-scanning electron microscopy (FIB-SEM) 3D tomography showed that the LSCF electrode did not coarsen measurably even at the highest ageing temperature, 800°C, ruling out LSCF microstructural changes as the mechanism behind the resistance increase. On the other hand, Sr surface segregation, determined by chemical etching with inductively coupled plasma-optical emission spectrometry (ICP-OES) detection, was found to increase with increasing ageing temperature > 650°C. The effect of Sr surface segregation on the oxygen surface exchange and diffusion processes is quantified by comparing the measured electrochemical and microstructural data.