Analysis of Performance Losses and Degradation Mechanism in Porous La_2-X NiTiO_6-δ:YSZ Electrodes

The electrode performance and degradation of 1:1 La_2-xNiTiO_6-δ:YSZ composites (x = 0, 0.2) has been investigated to evaluate their potential use as SOFC cathode materials by combining electrochemical impedance spectroscopy in symmetrical cell configuration under ambient air at 1173 K, XRD, electron microscopy and image processing studies. The polarisation resistance values increase notably, i.e., 0.035 and 0.058 Ωcm² h^-1 for x = 0 and 0.2 samples, respectively, after 300 h under these demanding conditions. Comparing the XRD patterns of the initial samples and after long-term exposure to high temperature, the perovskite structure is retained, although La₂Zr₂O₇ and NiO appear as secondary phases accompanied by peak broadening, suggesting amorphization or reduction of the crystalline domains. SEM and TEM studies confirm the ex-solution of NiO with time in both phases and also prove these phases are prone to disorder. From these results, degradation in La_2-xNiTiO_6-δ:YSZ electrodes is due to the formation of La₂Zr₂O₇ at the electrode-electrolyte interface and the ex-solution of NiO, which in turn results in the progressive structural amorphization of La₁₈NiTiO_6-δ phases. Both secondary phases constitute a non-conductive physical barrier that would hinder the ionic diffusion at the La_2-xNiTiO_6-δ:YSZ interface and oxygen access to surface active area.