Heterogeneous electrocatalysis in porous cathodes of solid oxide fuel cells
Author(s)Poizeau, S.; Bertei, A.; Qi, C.; Mohanram, A.; Pietras, J.D.; Fu, Yeqing; Bazant, Martin Z; ... Show more Show less
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A general physics-based model is developed for heterogeneous electrocatalysis in porous electrodes and used to predict and interpret the impedance of solid oxide fuel cells. This model describes the coupled processes of oxygen gas dissociative adsorption and surface diffusion of the oxygen intermediate to the triple phase boundary, where charge transfer occurs. The model accurately captures the Gerischer-like frequency dependence and the oxygen partial pressure dependence of the impedance of symmetric cathode cells. Digital image analysis of the microstructure of the cathode functional layer in four different cells directly confirms the predicted connection between geometrical properties and the impedance response. As in classical catalysis, the electrocatalytic activity is controlled by an effective Thiele modulus, which is the ratio of the surface diffusion length (mean distance from an adsorption site to the triple phase boundary) to the surface boundary layer length (square root of surface diffusivity divided by the adsorption rate constant). The Thiele modulus must be larger than one in order to maintain high surface coverage of reaction intermediates, but care must be taken in order to guarantee a sufficient triple phase boundary density. The model also predicts the Sabatier volcano plot with the maximum catalytic activity corresponding to the proper equilibrium surface fraction of adsorbed oxygen adatoms. These results provide basic principles and simple analytical tools to optimize porous microstructures for efficient electrocatalysis.
DepartmentMassachusetts Institute of Technology. Department of Chemical Engineering; Massachusetts Institute of Technology. Department of Mathematics
Fu, Y.; Poizeau, S.; Bertei, A.; Qi, C.; Mohanram, A.; Pietras, J.D. and Bazant, M.Z. “Heterogeneous Electrocatalysis in Porous Cathodes of Solid Oxide Fuel Cells.” Electrochimica Acta 159 (March 2015): 71–80. © 2015 Elsevier Ltd