Evolution of cathode-interlayer interfaces and its effect on long-term degradation

Abstract

The degradation of electrochemical performance of solid oxide fuel cells (SOFCs) with long-term durability testing is generally evaluated in terms of increases in polarization resistances with time. Practical SOFCs normally employ an interlayer between cathode and electrolyte to improve the stability with long-term operation. To elucidate the relationship between the cathode-interlayer interfacial properties and degradation, various types of model interfaces based on La0.6Sr0.4CoO3-δ (LSC) thin film cathodes prepared on gadolinia-doped ceria (GDC) interlayers having different microstructures are investigated. Equivalent circuit analysis of the electrochemical impedance spectra reveals an increase of the interfacial resistance associated with degradation of the LSC/GDC interface with operation time. The physical origin of the interfacial resistance is attributed to the formation of yttrium-doped SrZrO3 (SZY) at the interface of LSC and GDC, a behavior which appears exacerbated for a nanocolumnar GDC interlayer. Investigation of model interfaces created with SZY inserted between LSC and GDC reveals large polarization resistances mostly dominated by the interfacial resistance, consistent with oxide ion blocking effects observed at interfaces. The role of the GDC interlayer microstructure on the long-term stability of LSC and GDC interfaces and cell performance is discussed.