Degradation of High-Performance Oxygen Electrodes for Solid Oxide Cells
Authors: Lu Matthew Yunching , Zhang Shan-Lin , Park Beom-Kyeong , Chart Yvonne Antonia , Barnett Scott A 
Published: February 2020

Oxygen electrodes represent a significant resistive component in solid oxide cells (SOCs) and can degrade during operation in fuel cell and electrolysis modes. This paper compares the performance and stability of two oxygen electrode materials: La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Sr(Ti0.3Fe0.7-xCox)O3-δ (STFC). Life testing is carried out in reversing-current operation, in which the cells work part time in fuel cell mode and part time in electrolysis mode.
LSCF has become widely popular as the oxygen electrode in solid oxide cells due to its relatively low polarization resistance, and reduced tendency to delaminate during electrolysis operation when compared with LSM-YSZ oxygen electrodes. However, considerable degradation is often observed in LSCF electrodes due to strontium segregation, especially at relatively high current densities and temperatures. Prolonged electrolysis operation at high current densities leads to severe degradation due to the high overpotentials experienced during these extreme operation conditions. Nevertheless, periodically reversing the current direction between fuel and electrolysis modes has been shown to alleviate some of the associated degradation and improve cell stability. By applying reversing current densities under a wide range of operating conditions, we examine the narrow stability window for LSCF oxygen electrodes and are able to highlight the influence temperature and current density on electrode degradation.
Here we present a life testing study over a range of temperatures and reversing current densities. Although the results at lower temperatures (650 – 700 °C) show an increase in segregation-induced degradation with increasing current density, at higher temperature (750 °C) stability is improved and the electrode is completely stable at the highest current density of 1.5 A·cm-2. This unexpected result is explained by the good electrochemical activity of LSCF at the higher temperature and a very rapid development of a stable surface segregated Sr layer.
Results on the new mixed conducting Sr-containing perovskite electrode, STFC, which exhibits better low temperature performance and much improved stability over that of both LSM and LSCF, are also discussed. Using inductively coupled plasma-optical emission spectroscopy, we measure the amount of surface Sr species and find a higher quantity of Sr on STFC, as expected with the entirety of the A-site being composed of Sr in STFC and only 40% in LSCF. Despite the large quantity of surface Sr species, STFC exhibits lower and more stable polarization resistance. This improvement in electrochemical performance is explained by an order of magnitude increase in the solid state oxygen diffusion and oxygen surface exchange coefficients at 700 °C compared to that of LSCF, as determined via a combination of electrochemical impedance spectroscopy (EIS) and 3D tomographic characterization.
Oxygen electrode delamination is monitored by tracking the ohmic resistance during operando EIS, and identified after the life tests using scanning electron microscopy. Both LSCF and STFC are found to delaminate at sufficiently low operating temperatures and high current densities, conditions that give rise to high electrode overpotentials. Figure 1 shows an example of LSCF delamination at the electrode/electrolyte interface.

Figure Caption

Figure 1 - La0.6Sr0.4Co0.2Fe0.8O3-δ porous electrode delaminated from a dense Ce0.9Gd0.1O2-δ electrolyte operated at 750 °C at 2 A·cm-2 reversing current density.

Figure 1