Phase decomposition of La2NiO4+δ under Cr-and Si-poisoning conditions

Poisoning of the air electrode by impurities released from stack-components like interconnects and sealing materials is still regarded a severe issue limiting the life time of SOFC-stacks. Recently, the mixed ionic-electronic conductor La2NiO4+δ (LNO) has received much attention as a potential cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) due to its high catalytic activity for the oxygen exchange reaction. LNO has been considered to be more chromium tolerant because it does not contain alkaline earth elements like Sr and Ba which are known to segregate from the bulk towards the surface forming insulating or catalytically inactive secondary phases with Cr.
In the present work, the long-term stability of LNO in dry and humid Cr- and Si-containing atmospheres was investigated at 800°C using the dc-conductivity relaxation method. Dense samples of LNO were exposed to dry and humid Cr- and Si-containing atmospheres while monitoring the degradation process via the chemical surface exchange coefficient (kchem) of oxygen for a total duration of 3500 h. To determine chemical as well as morphological changes extensive post-test analyses using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (SEM-EDXS), analytical scanning transmission electron microscopy (STEM) with EDXS and electron energy loss spectroscopy (EELS) as well as high resolution
transmission electron microscopy (HRTEM) were applied. In dry atmospheres (pO2 = 0.10 bar) no degradation was observed in the presence of a Cr- and a Si-source over a period of 1300 h. Humidification of the test gas (pO2 = 0.1 bar, 30-60% relative humidity), however, resulted in a significant decrease of kchem. After the degradation experiment, XPS depth profiles, SEM-EDXS and STEM confirm the presence of an approximately 1.5 μm thick layer of Cr- and Si-containing compounds on the LNO surface. The main Si- and Crcontaining
phases were identified by means of HRTEM selected area diffraction.