263147 Modeling Charged-Defect Transport in Proton-Conducting Ceramic Membranes

Monday, October 29, 2012: 4:55 PM
402 (Convention Center )
Robert J. Kee and Huayang Zhu, Mechanical Engineering, Colorado School of Mines, Golden, CO

Certain doped perovskite ceramics, such as yttrium- and/or cerium-doped barium zirconates (BCZY), can transport protons through their bulk.  Thus, they are potentially valuable for application as separation membranes and in membrane reactors.   The most natural thought is that these materials are hydrogen-selective permeable membranes.  However, depending upon the gas compositions across the membrane, and possibly electrode polarization, the membranes can effectively transport H2, H2O, and O2.  

Figure 1 illustrates a shell and tube configuration in whicn a thin (order 10 microns) BCZY membrane is supported on a porous-ceramic tube.  At elevated temperatures around 600 ūC and above, the BCYZ materials are mixed ionic-electronic conductors (MIEC).  As such, these materials enable multicomponent defect transport of protons, oxygen vacancies, electrons and electron holes.  The subject of this presentation is the development of a model that can predict the radial electrochemical defect transport through the membrane.

Figure 1.  Shell and tube membrane configuration.

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Figure 1. Shell and tube membrane configuration.

The model is based upon formulating and solving defect-conservation equations where the defect fluxes are represented via the Nernst-Planck equations.   At the membrane surfaces the defect reactions are assumed to be in equilibrium with the gas phase, thus establishing boundary conditions for the transport equations.   Internal electric-potential gradients are formed within the membrane to maintain electroneutrality.  Thus the defect fluxes depend upon both concentration gradients and the local electric fields.   The membrane performance depends upon physical parameters including defect mobilities and equilibrium constants for the gas-surface defect reactions.

The steady-state system of governing equations forms an ordinary-differential-equation boundary-value problem (BVP) that is solved computationally using finite-volume discretization.  The mathematical structure of the BVP is somewhat unusual because of the requirement to determine the internal electric fields. 


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