Catalyst layer flooded agglomerate model

Electrode Kinetic and Mass Transfer for Fuel Cell Reactions For the reaction occurring inside a porous three-dimensional catalyst layer, a thin-film flooded agglomerate model has been developed [149, 150] to describe the potential-current behavior as a function of reaction kinetics and reactant diffusion. For simplicity, if the kinetic parameters, such as flie exchange current density and diffusion limiting current density, can be defined as apparent parameters, the corresponding Butler-Volmer and mass diffusion relationships can be obtained [134]. For an H2/air (O2) fuel cell, considering bofli the electrode kinetic and the mass transfer, the i-rj relationships of the fuel cell electrode reactions within flie catalyst layer can be expressed as Equations 1.130 and 1.131, respectively, based on Equation 1.122. The i-rj relationship of the catalyzed cathode reaction wifliin the catalyst layer is... 

More complicated expressions than those above can be used in the 0-D models, but these usually stem from a more complicated analysis. For example, the equation used by Ticianelli and co-workers comes from analysis of the catalyst layer as a flooded agglomerate. In the same fashion, eq 21 can be embedded and used to describe the polarization behavior within a much more complicated model. For example, the models of Springer et al. " and Weber and Newman " use a similar expression to eq 21, but they use a complicated 1-D model to determine the parameters such as ium and R. Another example is the model of Newman,who uses eq 22 and takes into account reactant-gas depletion down the gas channels by, in essence, having a limiting current density that depends on the hydrogen utilization. All of these types of models, which use a single equation to describe the polarization behavior within a more complicated model, are discussed in the context of the more complicated model. 

In addition to the equivalent circuit method, the impedance results can also be analyzed using mathematical models based on physicochemical theories. Guo and White developed a steady-state impedance model for the ORR at the PEM fuel cell cathode [15]. They assumed that the electrode consists of flooded ionomer-coated spherical agglomerates surrounded by gas pores. Stefan-Maxwell equations were used to describe the multiphase transport occurring in both the GDL and the catalyst layer. The model predicted a high-frequency loop due to the charge transfer process and a low-frequency loop due to the combined effect of the gas-phase transport resistance and the charge transfer resistance when the cathode is at high current densities. 

Gerteisen et al. [28] have summarized the PEMFC modeling development and introduced a ID, two-phase, transient model including GDL, catalyst layer, and membrane, under the assumption that GDL, catalyst layer, and membrane are spatially resolved in ID with an agglomerate approach for the structure of catalyst layer. The faults of water flooding and membrane dehydration are anbedded by the assumption that the saturation due to the continuous capillary pressure and immobile saturation due to the mixed wettability of the GDL structure are discontinued. In order to allow dehydration of the ionomer on the anode side, the water content is not a constant but follows the Cauchy boundary condition. The model is validated by voltammetry experiments, and the simnlated cnrrent responses are compared with the measured ones from chronoamperometric experiments. 

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