Modeling distributed charge-transfer processes in SOFC membrane electrode assemblies

A model is developed to represent chemistry and transport in porous mixed ionic-and-electronic conducting composite electrode structures in solid oxide fuel cells (SOFC). The model considers the coupled behavior of a full membrane electrode assembly (MEA, i. e., cathode, electrolyte, and anode), which is an important advance compared to earlier models that consider only a single electrode structure. Within each electrode the model represents parallel conduction of electrons and ions, as well as porous-media, chemically reacting gas transport. The model predicts electric-potential distributions in both phases. Charge-transfer chemistry is handled via a modified Butler-Volmer formalism, which depends on the local electric-potential difference between phases. Heterogeneous chemistry (e. g., reforming or partial oxidation) is handled via a detailed surface-reaction mechanism. For typical composite-electrode structures the charge-transfer region extends about 10-20 mu m from the dense electrolyte. The results show cell performance depends upon particle sizes within the porous electrodes. Smaller particles generally improve cell performance as a result of expanded three-phase-boundary length. However, smaller particle sizes impede gas transport. Cell performance can be optimized as a function of functional-layer thickness and particle sizes. A distributed charge-transfer formulation is especially important in advanced thin-film electrode structures (e.g., segmented-in-series architectures) where the entire MEA is only a few tens of micrometers thick. The model is formulated as continuum differential equations, which are solved computationally on a discrete mesh network. The paper illustrates the model with examples comparing alternative MEA structures. (c) 2008 The Electrochemical Society.