This paper examines the multi-species (simultaneous proton, oxygen vacancy, and electron/electron hole) transport behavior of proton conducting oxides. Multi-species transport behavior broadens both the realm of applications and also the potential challenges for the technical use of these perovskite materials in devices such as fuel cells, separation membranes, and membrane reactors. In order to better understand the interplay between the various transporting species and their impact on macroscale conduction and permeation processes, a new conceptualization of multi-species transport is proposed. A general theoretical description of multi-species transport is presented in a graphical manner that is similar to a ternary phase diagram. When combined with data gathered from water isotope permeation experiments, we show that it is possible to specify the relative contributions from each mobile defect species in a yttria-doped barium zirconate multi-species perovskite transport membrane system under a variety of experimental conditions. This approach provides a unique method to obtain and analyze transport behavior on the basis of transference numbers, thereby providing information which is often difficult to determine by other means.
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