(Invited) Modeling Transport Processes in Perovskite-Based Mixed Ionic-Electronic Conducting Membranes

Monday, 2 October 2017: 15:00
National Harbor 7 (Gaylord National Resort and Convention Center)
R. J. Kee (Mechanical Engineering, Colorado School of Mines) and H. Zhu (Colorado School of Mines)
Although many mixed ionic-electronic conductors (MIEC) share crystallographically similar doped perovskite structures, wide ranges of particular materials offer a diverse range of practically useful applications. Broadly speaking, the MIEC materials may be dominantly oxygen-ion conductors or proton conductors. The present paper is primarily concerned with transport processes that are relevant for membrane applications. Such applications include fuel cells, electrolyzers, gas separations, membrane reactors, and electrochemical compressors. The presentation uses illustrative examples.

In practice gases must react at the membrane surfaces to incorporate or release ions or other charge-carrying defects. The mobile charge carrying defects can include oxygen vacancies, protons, and a variety of small polarons. The defect transport may be represented in terms of Nernst-Planck fluxes. An internal electrostatic-potential field must be developed maintain charge-neutrality within the membrane. The Nernst-Planck flux depends on gradients of defect concentrations, electrostatic potential, and hydrostatic stress.

Depending on the material and the application, the membrane may be polarized by applying an electric potential from an external circuit. Ideally, for most membrane applications, the defect transport is entirely ionic. However, electronic defects (typically small polarons) can effectively result in electrical “leakage” that frustrates polarization. Applications such fuel cells, electrolyzers, and electrochemical compressors require membrane polarization. Depending on the material’s effective electrical conductivity polarization may not be practical.

Local defect concentrations cause crystal-lattice-scale strain, which results in macroscopic stress through the membrane and its mechanical support structure. Such chemo-mechanical coupling can cause membrane distortion and potentially damaging high stresses.

Modeling membrane performance requires incorporating the Nernst-Planck fluxes into conservation equations. The rate of change in defect concentrations is balanced by the divergence of the Nernst-Planck fluxes. The local electrostatic fields may be determined in the context of a Poisson equation that effectively produces local electroneutrality. The resulting nonlinear, coupled, system of partial differential equations, generally called the Nernst-Planck-Poisson equations, may be solved computationally.