Atomistic study of segregation and diffusion of yttrium and calcium cations near electrolyte surfaces in solid oxide fuel cells

Segregation behaviors of yttrium (Y) and calcium (Ca) cation dopant species near zirconia (1 1 1) surfaces have been investigated utilizing atomistic simulations. The results reveal that surface segregation of both species can be achieved when the density of cation and oxygen vacancies near the surface is relatively high compared to the bulk. Comparing Y with Ca, the local equilibrium concentration at the surface of Ca is estimated to be lower than that of Y; however, Ca is kinetically much easier to diffuse from bulk to the surface than Y. The analysis of in-plane cation and oxygen diffusion at Y- or Ca-rich surfaces shows that the cation diffusivity is enhanced with increasing Ca-concentration at the surface, whereas oxygen diffusivity is inhibited. This tendency is more remarkable than that of the Y-rich surface, suggesting that Ca segregation has a greater negative impact than Y segregation on electrolyte performance in solid oxide fuel cells.