In-situ scanning photoelectron microscopy study of operating (La,Sr)FeO3-based NOx-sensing surfaces

Scanning photoelectron microscopy and impedance spectroscopy were used to study simultaneously the impact of NO gas on surface chemistry and the electrochemical response of electrolyte-supported (La1 - xSrx) FeO3-delta (LSF) sensing layers in electrochemical NOx sensors, during high-temperature operation. Electrochemical model cells were fabricated by screen printing macro- and micro-patterned thin films on zirconia electrolyte. Cell bias, temperature and NO/O-2 gas ratio were varied during the experiments. LSF/3YSZ sensing layers remained stable over a wide temperature-bias range in an oxygen-containing environment. LSF compositions with low strontium content (x = 0.2) did not undergo any changes in their surface chemistry, while compositions with higher strontium level (x = 0.4) showed partial reduction of surface iron at low oxygen chemical potential. Upon exposure to NO gas at low oxygen partial pressure, NO was observed to dissociate on the LSF surface and form chemisorbed O- and N-adatoms with strong interactions to surface iron or zirconium. Concentration and stability of these ad-complexes increased with the NO/O-2 ratio, cathodic bias and decreasing temperature. At low NO level in the gas, NO was observed to interact preferentially with iron atoms at the ISF surface and the surface reaction was reversible. At high NO/O-2 ratios and low temperatures, irreversible formation of surface ad-complexes was observed, with reduced iron and zirconium, and caused even new phase formation. Simultaneous assessment of surface chemistry and cell impedance demonstrated that modifications in surface chemistry directly affected the oxygen exchange rate in the cell. In the presence of NO, the sensing oxide resistance decreased for zero or small bias. For large bias, no additional decrease in cell resistance was observed beyond the corresponding intrinsic cell performance in oxygen. Under these conditions, again, the particular cell response depended on the strontium substitution level in LSF, favoring lower redox-stability and thus a more complicated cell response to NO gas for high strontium level. (C) 2012 Elsevier B.V. All rights reserved.