Impurity features in Ni-YSZ-H₂-H₂O electrodes

The impurity content of previously electrochemically characterized patterned Ni anodes for SOFC (solid oxide fuel cells) has been analyzed with a combination of SEM (scanning electron microscopy), AFM (atomic force microscopy), XPS (X-ray photoelectron spectroscopy) and TOF-SIMS (time-of-flight secondary ion mass spectrometry). This analysis yields comprehensive information on composition and lateral distribution of impurity species as well as the size of impurity features. Small impurity striations are found at the triple phase boundary (TPB) as well as on the former electrode-electrolyte interface and the impurity features were found to be influenced by the electrode configuration and the initial behavior of the Ni electrode during thermal exposure (creep or shrinkage). Furthermore, the electrochemical performance (the line specific resistance LSR) was compared to data reported for Ni point anodes. Good agreement was obtained for data with comparable impurity features. Additionally, an order of magnitude estimation of the effect of SiO₂ content on surface coverage with an impurity film is performed for different electrode designs (point, patterned and cermet anode) and shows different preconditions for model anodes and nano- or microstructured cermet anodes. With typical impurity levels, a complete coverage of the electrolyte surface with SiO₂ seems almost inevitable in the case of model anodes.