Thermodynamic Analyses of Structural and Functional Changes of Cathode Materials of Solid Oxide Fuel Cells Involving the Change of Oxygen Content

Abstract

Solid oxide fuel cell, SOFC, is expected as high-temperature electrochemical devise with the high energy conversion efficiency. In this study, new SOFC cathode materials were explored and their characters were investigated. The perovskite-type (or -related) oxides including 3d transition metal (Mn, Fe, Co, Ni) are usually utilized as SOFC cathode materials. The change of the physical properties such as structural phase transition and thermal expansion of these oxides are more complex than that of standard oxide materials, because oxygen contents of these oxides change by temperature and oxygen partial pressures, P(O₂). In this article, the investigation of the functional changes of LaNi_0.6Fe_0.4O_3-δ with oxygen nonstoichiometry at high temperature were reviewed. The temperature and P(O₂) dependence of the oxygen nonstoichiometry and electrical conductivity of LaNi_0.6Fe_0.4O_3-δare smaller than that of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ. This is caused by the small change of average valence of Ni and Fe ions for the change of temperature and P(O₂) owing to continuous hole exchange between Ni and Fe ions in LaNi_0.6Fe_0.4O_3-δ. Small change of oxygen nonstoichiometry of LaNi_0.6Fe_0.4O_3-δ was also confirmed by other evaluation procedure such as dilatometry. P(O₂) dependence of the total linear thermal expansion of LaNi_0.6Fe_0.4O_3-δ was also small compared to La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ.