Oxygen surface exchange kinetics of La 1− x Sr x CoO 3– δ thin-films decorated with binary oxides: links between acidity, strontium doping, and reaction kinetics
Résumé
Recent studies showed that binary oxide modifications can induce substantial changes in the oxygen exchange kinetics at the surface of Pr0.1Ce0.9O2-delta electrodes. The effect of these oxides was linked to their Smith acidity. In this work, these observations of oxides at the surface of mixed conducting oxides are extended to a metal-like conducting oxide La1-xSrxCoO3-delta. The samples of La1-xSrxCoO3-delta with various Sr contents deposited by PLD were investigated in situ during the deposition process by impedance spectroscopy, before and after decoration with SrO, CaO and SnO2 at different temperatures and pO(2). Considering experimental observations from the literature, the kinetics of decorated surfaces follow the Smith acidity of the binary oxide, which confirms that this scale is a good descriptor for sorting impurities. No notable effects of the Sr content on the decoration impact were measured and the effect of impurities was compared between La1-xSrxCoO3-delta and Pr0.1Ce0.9O2-delta from a previous study. Basic impurities added at the surface of La1-xSrxCoO3-delta electrodes showed less influence on the reaction kinetics than on Pr0.1Ce0.9O2-delta, but a stronger impact was found with acidic oxides. This effect is supposed to originate from the acidity difference that controls a major part of the oxygen exchange kinetics. These results underline that the outermost surface is decisive for solid oxide cell electrode materials and that the relative insensitivity of the kinetics to bulk properties might lead to a high flexibility in terms of electrode design and material combination.
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