  It is generally assumed that oxygen potential in a thin oxide electrolyte
follows a linear distribution between electrodes. Jacobsen and Mogensen have
shown, however, that this is not the case for thin zirconia membranes in solid
oxide electrochemical cells. Here we demonstrate that there is a ubiquitous
oxygen potential transition rooted in the p-type/n-type transition of
electronic conductivity inside mixed conducting oxides, and that the transition
is extremely sensitive to electrode potential and current density. It is also
remarkably sensitive to the conductivity ratio of electrons and holes, as well
as their association with lattice oxygens and vacancies, which tends to
increase the oxygen flow. Direct evidence of a sharp oxygen potential
transition has been found in an equally sharp grain size transition in
electrically loaded zirconia. More broadly speaking, the oxygen potential
transition is akin to a first-order phase transition. Therefore, it will suffer
interface instability, especially in high-current-density devices. These
findings provide new opportunities to understand several disparate observations
in the literature, from microstructural degradation and stress distribution in
solid oxide fuel/electrolyzer cells, to field-assisted sintering, to conducting
filaments in resistance memory, to dendrite formation in electrochemical cells.
