Durability of La0.20Sr0.25Ca0.45TiO3-based SOFC anodes: identifying sources of degradation in Ni and Pt/ceria co-impregnated fuel electrode microstructures

Abstract

Solid oxide fuel cells (SOFC) comprising LSM-YSZ/LSM composite cathodes, 6ScSZ electrolytes and La_0.20Sr_0.25Ca_0.45TiO₃ (LSCT_A⁻) anode ‘backbone’ microstructures were prepared using thick-film ceramic processing techniques. Activation and decoration of the LSCT_A⁻ anode ‘backbone’ with electrocatalytic coatings of cerium-based oxides and metallic Ni or Pt particles was achieved using the technique of catalyst co-impregnation. SOFC containing Ni/CGO, Ni/CeO₂ and Pt/CGO impregnated LSCT_A anodes were tested up to ∼1000 hours by the Swiss SOFC manufacturer: HEXIS, under realistic operating conditions, including 15 redox, thermo and thermoredox cycles. The voltage degradation rates observed over the entire test period for the SOFC containing the Ni/CGO, Ni/CeO₂ and Pt/CGO impregnated LSCT_A⁻ anodes were 14.9%, 7.7% and 13.4%, respectively. Post-mortem microscopic analyses indicated that CeO₂ formed ubiquitous coatings upon the LSCT_A⁻ anode microstructure, allowing retention of a high population density of metallic (Ni) particles, whilst CGO formed ‘islands’ upon the microstructure and some agglomerates within the pores, leading to more facile agglomeration of metallic (Ni and Pt) nanoparticles. Correlation of the post-mortem microscopy with AC impedance analysis revealed that the agglomeration of metallic catalyst resulted in an increase in the high-frequency anode polarisation resistance, whilst agglomeration of the ceria-based component directly resulted in the development of a low-frequency process that may be attributed to combined contributions from gas conversion and chemical capacitance.