Effect of Oxygen Nonstoichiometry on a La[sub 1−x]A[sub x]MnO[sub 3+δ] Cathode under a Polarized State

Abstract

Lanthanum manganite, the air electrode material of solid oxide fuel cells, is well known for its complicated nonstoichiometry, which depends upon oxygen partial pressure and temperature. This nonstoichiometry has a relationship with electrode reactivity or the exchange current density because it affects the oxygen activation step. The electrode reactivity is usually studied in terms of its exchange current density, in other words, under a nonpolarized ͑equilibrium͒ state. This paper aims to elucidate the effect of oxygen nonstoichiometry on cathode reactivity under polarized states. The complex impedance was measured on an Ar-O 2 /porous La 0.9 Sr 0.1 MnO 3ϩ␦ /yttria-stabilized zirconia electrode under steady-state polarization in the oxygen partial pressure range of 10 2 -10 5 Pa and a temperature range of 873-1273 K. The results suggest that oxygen nonstoichiometry affects the cathode reactivity both under polarized and nonpolarized states through the exchange current density.

Model for Solid Electrolyte Gas Electrode Reaction Kinetics; Key Concepts, Basic Model Construction, Extension of Models, New Experimental Techniques for Model Confirmation, and Future Prospects

“…99 Under the condition that the nonstoichiometry variation in La 1¹x Sr x MnO 3+D is negligibly small, we find that Q E is proportional to P(O 2 ) 1/2 . 22,44,45,66,70 With the increase in oxygen content, Q E becomes smaller than the value expected from the P(O 2 ) 1/2 relationship. 44,45,66 Similarly, ac impedance measurements under polarization revealed that Q E varies with not only the gas phase P(O 2 ) but also by the electrode potential determined by the chemical overpotential defined by Eq.…”

“…(11). 45,70 The lattice parameter decreases with the increase in oxygen content, irrespective of defect species. 100,101 It is considered that the lattice parameter decrease is related to the decrease in the surface diffusivity of O ad .…”

Evaluation of strontium substituted lanthanum manganite-based solid oxide fuel cell cathodes using cone-shaped electrodes and electrochemical impedance spectroscopy

“…This finding was later confirmed by Hansen et al [4] at a somewhat lower temperature of 500 °C. In contrast, Yasamoto et al [5] found that the activity of LSM10 was much higher than the activity of LSM40, using porous planer electrodes. In a later publication, Hansen [6] showed that at temperatures of 400 °C or less, a less clear dependence on the composition existed.…”

Exchange Current Density of Solid Oxide Fuel Cell Electrodes

“…These results suggest that higher electrochemical activity of cathode materials becomes essential with decreasing operating temperature. Following empirical equation related to the cathode reaction was obtained by the fitting procedure: [4] Anodic Exchange Current Density vs. Operating Conditions…”

“…Concerning the cathode reactions, the values of __ and ___ have been reported in the literature (1)(2)(3)(4)(5). These values are summarized in Table II.…”