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...

Multicomponent gas transport is investigated with unprecedented precision by AC impedance analysis of porous YSZ anodesupported solid oxide fuel cells. A fuel gas mixture of H 2 -H 2 O-N 2 is fed to the anode, and impedance data are measured across the range of hydrogen partial pressure (10-100%) for open circuit conditions at three temperatures (800 • C, 850 • C and 900 • C) and for 300 mA applied current at 800 • C. For the first time, analytical formulae for the diffusion resistance (R b ) of three standard models of multicomponent gas transport (Fick, Stefan-Maxwell, and Dusty Gas) are derived and tested against the impedance data. The tortuosity is the only fitting parameter since all the diffusion coefficients are known. Only the Dusty Gas Model leads to a remarkable data collapse for over twenty experimental conditions, using a constant tortuosity consistent with permeability measurements and the Bruggeman relation. These results establish the accuracy of the Dusty Gas Model for multicomponent gas diffusion in porous media and confirm the efficacy of electrochemical impedance analysis to precisely determine transport mechanisms. The Solid Oxide Fuel Cell (SOFC) is currently the highesttemperature fuel cell in development and can be operated over a wide temperature range from 600• C-1000• C allowing a number of fuels to be used. To operate at such high temperatures, the electrolyte is a thin, nonporous solid ceramic membrane that is conductive to charge carrier, O 2− ions. The operating efficiency in generating electricity is among the highest of the fuel cells at about 60%.1 Furthermore, the high operating temperature allows cogeneration of high-pressure steam that can be used in many applications. Combining a hightemperature SOFC with a turbine into a hybrid fuel cell further increases the overall efficiency of generating electricity with a potential of an efficiency of more than 70%.1 Therefore, it is a very promising alternative energy source that could potentially be used for home heating or large scale electricity production in the future.Solid oxide fuel cell consists of a porous cathode, an electrolyte, a porous anode and interconnects. Two different types have been explored in the development of SOFC, the electrolyte supported cell and the electrode supported cell. In the former, electrolyte is the thickest and serves as the mechanical support for the whole cell. However, due to the high Ohmic resistance of the relatively thick electrolyte layer, the electrolyte supported design has been gradually replaced by the new electrode supported cells, in which one of the porous electrodes is the supporting structure. Moreover, since cathode supported cell usually gives higher resistance, and is much harder to fabricate due to the mismatched thermal expansion coefficient of cathode support and functional layer, the anode supported cell (ASC) is the most widely accepted design in current SOFC research.The solid oxide fuel cell is operated with fuel and oxidant being continuously fed from two sides of the ce...

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