Surface-limited ionic transport in perovskites Sr0.97(Ti,Fe,Mg)O3-δ

Oxygen permeation and faradaic efficiency measurements of perovskite solid solutions Sr0.97Ti1-x-yFexMgyO3-delta (x = 0.20-0.40; y = 0-0.10) at 973-1223 K showed that the oxygen transport at membrane thicknesses below 2 mm is limited by both bulk ionic conductivity and the surface exchange kinetics. Incorporation of either iron or magnesium into the B sublattice of strontium titanate results in greater p-type electronic and oxygen ionic conductivities. For Sr-0.97(Ti,Fe)O3-delta solid solutions, the role of the surface exchange as the permeation-determining factor decreases with reducing temperature. In contrast, the limiting effect of the interphase exchange on oxygen transport through Sr0.97Ti0.70Fe0.20Mg0.10O3-delta membranes is observed to be significant within the studied temperature range, suggesting that doping with magnesium leads to higher ionic conductivity and lower surface exchange rates in comparison with Sr0.97Ti0.60Fe0.40O3-delta perovskite which exhibit similar permeation fluxes. The ion transference numbers of the solid solutions in air, estimated from the oxygen permeation and faradaic efficiency results, do not exceed 0.14. TGA/DTA results demonstrated the stability of the perovskite phases in CO2-containing atmospheres at temperatures above 770 K. The effect of the surface exchange limitations on the faradaic efficiency results is analysed.