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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 2 adsorbed oxygen adatoms. These results provide basic principles and simple analytical tools to optimize porous microstructures for efficient electrocatalysis.Keywords: electrochemical impedance spectroscopy (EIS); Gerischer element; porous electrode; oxygen reduction reaction (ORR); strontium-doped lanthanum manganite (LSM). 1) IntroductionMany important electrochemical reactions require electrocatalysts that accelerate the kinetics, while remaining unaltered. For Faradaic reactions at electrodes, electrocatalysis is heterogeneous since charge transfer occurs at the interface between electrode and electrolyte phases. Heterogeneous reactions typically involve multistep processes of consecutive and parallel elementary reaction steps, such as adsorption of chemical species on the electrode surface, surface or bulk diffusion of intermediate species, and charge transfer reactions at the electrode-electrolyte interface. An electrocatalyst accelerates the global reaction rate by lowering the barrier for the slowest elementary step in the reaction mechanism, and it is important to recognize that this might not necessarily be a charge transfer step.The oxygen reduction reaction (ORR) in different types of fuel cell cathodes typically requires an electrocatalyst. For room-temperature proton-exchange-membrane fuel cells (PEMFC), platinum-based electrocatalysts are used and have been described by models that emphasize the charge transfer step [1,2]. Here, we focus on high-temperature solid oxide fuel cells (SOFC) with conducting ceramic electrocatalysts and also consider the intermediate steps of dissociative surface adsorption and...