Temporal and thermal evolutions of surface Srsegregation in pristine and atomic layer deposition modified La0.6Sr0.4CoO3-d epitaxial films

The bulk-to-surface Sr segregation can seriously compromise the stability of oxygen electrocatalysis in La1-xSrxCoO3- 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 degrees C at a fixed p(O2) = 1 x 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 degrees C for >300 hours, suggesting that Sr-segregation is the source of the higher resistance.