Role of multi-species transport in proton-conducting perovskite permeation membranes, The

Abstract

Proton conducting ceramics have been of great scientific interest since their initial development over 30 years ago. In that time, the ABO3 perovskite family of materials has shown particularly significant promise, with doped barium zirconates arguably being the subject of the most intense research. One of the most interesting and yet also scientifically underappreciated abilities of many of these ceramics is they can exhibit multi-species conduction behavior--specifically they can exhibit simultaneous protonic and oxygen vacancy conduction in addition to electronic (electron or hole) conductivity. This allows for uses that do not require electrodes or a driving force other than chemical potential gradients, such as gas permeation and reactor membranes. The studies in this dissertation probe and clarify the implications arising from multi-species transport in these materials. Importantly, this dissertation also underscores the difficulties associated with assessing multi-species transport using standard electrochemical measurement techniques such as conductivity. To combat this difficulty, the modeling and experimental studies presented in this dissertation make use of permeation conditions to probe multi-species transport in greater detail and clarity than is possible using conductivity studies. This research explicitly examines the multi-species transport behavior, not only when two charge-carrying defects are major contributors, but also when at least a third defect has a significant contribution. This addition of a third carrier introduces behaviors that are not present in either single or two carrier systems, such as "uphill" transport. This uphill transport, the diffusion of a species up its chemical potential gradient, can occur for individual defects, the gas components on either side of the membrane, or both. A numerical model was developed to assist in the investigation of membrane performance under a wide range of possible experimental conditions. Along with improved handling of multi-species transport, the new model incorporates newly developed algorithms to calculate the impact of gas conditions on the final gas-phase species that will appear on either side of a permeation membrane. Using this model, contour maps of the relevant performance properties were created to quickly and intuitively discover patterns and identify regions of experimental conditions that contain interesting behavior.