Tatsuya OGUCHI
Relation of the Cathodic Reaction to the Oxygen Nonstoichiometry in the Lanthanum Nickel Iron Oxide for the Cathode Material of the Solid Oxide Fuel Cell
Kazunori SATO
La(Ni0.6Fe0.4)O3-¦Ä(LNF) was found to be promising as a new cathode material for solid oxide fuel cells (SOFC) due to its high electronic conduction. However, nonstoichiometry of LNF is not well understood in relation to the performance of cathodic reduction of oxygen, which determines the overall cell performance particularly at the low operation temperature below about 800 degrees Celsius. In the present study, oxygen deficiency in LNF has been investigated in terms of the chemical analysis by iodimetry, lattice constant determination, and the cathodic overvoltage. LNF samples exposed to various partial oxygen pressures were analyzed. The ratio of Fe2+ and Fe3+ was constant under an inert gas (Ar) atmosphere at least up to 24 h at 850 degrees Celsius. However, prolonged treatment for LNF under the Ar atmosphere at this temperature resulted in a decomposition of the LNF phase. Phase stability of LNF was observed at 800 degrees Celsius at an oxygen pressure as low as 2 vol% (about 2.0 x 103 Pa) under the cell operation conditions. However, the LNF cathode exposed at a low oxygen pressure showed a higher cathodic overvoltage than that exposed at a high oxygen pressure about 20 vol% (about 2.0 x 103 Pa). This behavior seems to be caused by a low surface exchange reaction rate constant in LNF.
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