Towards an atomic scale understanding of the early-stage deterioration mechanism of LSCF

Abstract

The early-stage degradation behaviour of a porous lanthanum strontium cobalt ferrite (LSCF) cathode in a solid oxide fuel cell is investigated at a low temperature of 600 °C by operating four cells under different conditions: in dry air and at open circuit voltage (Dry-OCV), in dry air and under bias (Dry-bias), in wet air and at OCV (Wet-OCV), and in wet air and under bias (Wet-bias). Compared with Dry-OCV, either H₂O or bias shows a negative effect on the electrochemical and transport characteristics, resulting in the most severe degradation in Wet-bias. The mechanism is explained in terms of LSCF deterioration, which is intensively studied on a micro- to atomic scale. No surface segregation is observed in the as-prepared cathode; however, sulfur is found to be incorporated into the lattice of the LSCF(110) surface. After operation, nano-segregation occurs in all the cathodes. SrSO₄ particles form in all the cathodes while Sr(OH)₂ flakes precipitate under wet conditions. The size and distribution of segregations vary with the conditions. For example, some of the SrSO₄ particles in Dry-bias grow into bar-like ones (up to 280 nm) compared with smaller ones (36 nm) in Dry-OCV indicating that the kinetics of Sr diffusion and O vacancy formation is accelerated by bias. Their distribution is limited in the cathode surface layer (CSL) in Dry-OCV, Dry-bias and Wet-OCV, while it extends to the cathode/electrolyte interface in Wet-bias, which is caused by the decrease of SO₂ adsorption/dissociation kinetics in the CSL in the presence of competitive H₂O species (for SrSO₄) and the enhancement of H₂O mass transport driven by the O₂ concentration gradient under bias (for Sr(OH)₂). With the formation of segregation, other deteriorations such as sulfur incorporation into the surface, Sr-deficiency in the subsurface, and La–Co-rich regions near the surface occur and evolve in LSCF grains. All these changes lead to the deactivation of the surface O exchange, which is believed to be the dominant reason for the performance degradation.