Surface oxygen vacancy and oxygen permeation flux limits of perovskite ion transport membranes
KAUST Grant NumberKUS-L1-010-01
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Abstract© 2015 Elsevier B.V. The mechanisms and quantitative models for how oxygen is separated from air using ion transport membranes (ITMs) are not well understood, largely due to the experimental complexity for determining surface exchange reactions at extreme temperatures (>800°C). This is especially true when fuels are present at the permeate surface. For both inert and reactive (fuels) operations, solid-state oxygen surface vacancies (δ) are ultimately responsible for driving the oxygen flux, JO2. In the inert case, the value of δ at either surface is a function of the local PO2 and temperature, whilst the magnitude of δ dictates both the JO2 and the inherent stability of the material. In this study values of δ are presented based on experimental measurements under inert (CO2) sweep: using a permeation flux model and local PO2 measurements, collected by means of a local gas-sampling probe in our large-scale reactor, we can determine δ directly. The ITM assessed was La0.9Ca0.1FeO3-δ (LCF); the relative resistances to JO2 were quantified using the pre-defined permeation flux model and local PO2 values. Across a temperature range from 825°C to 1056°C, δ was found to vary from 0.007 to 0.029 (<1%), safely within material stability limits, whilst the permeate surface exchange resistance dominates. An inert JO2 limit was identified owing to a maximum sweep surface δ, δmaxinert. The physical presence of δmaxinert is attributed to a rate limiting step shift from desorption to associative electron transfer steps on the sweep surface as PO2 is reduced. Permeate surface exchange limitations under non-reactive conditions suggest that reactive (fuel) operation is necessary to accelerate surface chemistry for future work, to reduce flux resistance and push δpast δmaxinert in a stable manner.
CitationHunt A, Dimitrakopoulos G, Ghoniem AF (2015) Surface oxygen vacancy and oxygen permeation flux limits of perovskite ion transport membranes. Journal of Membrane Science 489: 248–257. Available: http://dx.doi.org/10.1016/j.memsci.2015.03.095.
SponsorsThe authors would like to thank the King Fahd University of Petroleum and Minerals (KFUPM) in Dhahran, Saudi Arabia, for partially funding the research reported in this paper through the Center of Clean Water and Clean Energy at the Massachusetts Institute of Technology and KFUPM under project number R2-CE-08. This work is also supported through funding from the King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia under project number KUS-L1-010-01. A special thanks is also extended to Air Products and Chemicals, Inc. (APCI) for their guidance in this field and for the sharing of knowledge regarding LCF membranes; additionally the cooperative efforts with Ceramatec to produce the LCF membranes for the experimental reactor in this work are gratefully acknowledged.
JournalJournal of Membrane Science
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