Studies of ionic transport and oxygen exchange on oxide materials for electrochemical gas sensors

Abstract

Theory and examples for the application of electron blocking microelectrodes to the study of ion transport and electrode processes in mixed electronic and ionic conductors are given for a couple of mixed conducting oxidic perovskites. Emphasis was laid on the analysis of steady state current–voltage and cyclic voltammetric curves. Yttria-stabilized zirconia (YSZ) was used as the oxygen ion conducting microcontact material. The measurements were performed in the temperature range between 500 and 750 °C both on encapsulated and non-encapsulated oxide surfaces in order to detect the relative importance of surface oxygen exchange as compared to bulk oxygen ion transport and changes of stoichiometry. Gd 0.8Sr 0.2CoO 3− δ and La 0.8Sr 0.2FeO 3− δ proved to be typical mixed conductors with medium oxygen ion conductivities. Both show increasing ion conductivities for decreasing oxygen activities indicating a typical vacancy transport mechanism of oxygen ions. At 700 °C in air, we obtained for the oxygen ion conductivity of Gd 0.8Sr 0.2CoO 3− δ a value of 2×10 −4 S cm −1 with very weak dependence on the oxygen partial pressure and for La 0.8Sr 0.2FeO 3− δ , a value of 1.2×10 −5 S cm −1. The oxygen ion conductivity of La 0.8Sr 0.2FeO 3− δ was proportional to p O 2 −0.22 in the investigated range of temperatures and oxygen partial pressures between 10 and 10 −4 bar. In addition to that, a series of manganites was investigated with the composition A 0.7 E 0.3MnO 3 (A=Gd, Y, Pr and E=Ca, Sr). The presence of slow diffusion coupled redox processes could be detected in all manganites at low and high oxygen activities versus air. The oxygen ion conductivity was considerably lower than 10 −5 S cm −1 in all investigated manganites. Nevertheless, the comparison of results for glass encapsulated and non-encapsulated microcontacts showed that Pr 0.7Ca 0.3MnO 3 and in particular Pr 0.7Sr 0.3MnO 3 are quite active catalysts for the oxygen electrode reaction at the three-phase boundary air/zirconia/manganite, whereas Y 0.7Sr 0.3MnO 3 does not catalyze the oxygen electrode reaction significantly at the surface.