Temporal and thermal evolutions of surface Sr-segregation in pristine and atomic layer deposition modified La0.6Sr0.4CoO3−δ epitaxial films

Abstract

The bulk-to-surface Sr segregation can seriously compromise the stability of oxygen electrocatalysis in La_1−xSr_xCoO_3−δ and limit its practical applications such as in solid oxide fuel cells. Here we show via in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) that the surface Sr-segregation is a kinetically fast process and the equilibrium surface Sr-concentration follows Arrhenius law from 250 to 520 °C at a fixed p_O2 = 1 × 10⁻³ atm. We also show that application of a nanoscaled, atomic layer deposition (ALD) derived ZrO₂ overcoat can effectively suppress the Sr-segregation by reducing the surface concentration of oxygen vacancies. Electrochemical impedance spectroscopy (EIS) study further confirms that the ALD-ZrO₂-coated LSCo epitaxial film exhibits a much lower and more stable polarization resistance than the uncoated one at 550 °C for >300 hours, suggesting that Sr-segregation is the source of the higher resistance.