Electrode agglomerate models

The rest of the comparisons were done for the cathode. The results all showed that the agglomerate model fits the data better than the porous-electrode model. However, it should be noted that the porous-electrode model used was usually a thin-film model and so was not very robust. Furthermore, the agglomerate model has more parameters that can be used to fit experimental data. Finally, some of the agglomerate models compared were actually embedded models that account for both length scales, and therefore, they normally agree better with the experimental data. 

The other approach is more complicated and requires a deeper knowledge of the agglomerate structure or yields more fitting parameters. In this approach, the porous-electrode equations are used, but now the effectiveness factor and the agglomerate model equations are incorporated. Hence, eq 64 is used to get the transfer current in each volume element. The gas composition and the overpotential... 

The membrane electrode assembly (MEA), which consists of three components (two gas diffusion electrodes with a proton exchange membrane in between), is the most important component of the PEMFC. The MEA exerts the largest influence on the performance of a fuel cell, and the properties of each of its parts in turn play significant roles in that performance. Although all the components in the MEA are important, the gas diffusion electrode attracts more attention because of its complexity and functions. In AC impedance spectra, the proton exchange membrane usually exhibits resistance characteristics the features of these spectra reflect the properties of the gas diffusion electrode. In order to better understand the behaviour of a gas diffusion electrode, we introduce the thin-film/flooded agglomerate model, which has been successfully applied by many researchers to... 

The characteristics of a gas diffusion electrode can also be illustrated by the thin-film/flooded agglomerate model. Paganin et al. [4] summarized the parameters that often appear in the impedance spectra of H2/02 and H2/air fuel cells ... 

Springer TE, Raistrick ID (1989) Electrical impedance of a pore wall for the flooded-agglomerate model of porous gas-dilFusion electrodes. J Electrochem Soc 136 1594-603... 

Prins-Jansen, J.A. Hemmes, K. de Wit, J.H.W. An extensive treatment of the agglomerate model for porous electrodes in molten carbonate fuel cells—I. Qualitative analysis of the steady-state model. Electrochim. Acta 1997, 42 (23-24), 3585-3600. 

Four categories of electrode models can be identified from Table 28.3 the spatially lumped model, the thin-film model, the agglomerate model, and the volume-averaged model. Schemes of the basic concepts of these four model categories are depicted in Figure 28.4. Each of the schemes shows an electrode pore, with the gas channels located at the top and the liquid electrolyte, depicted in gray, at the bottom. In some models, electrolyte is also present in the pore. The reaction zones are indicated by a black face (spot, line, or grid structure). Fluxes of mass and ions are indicated by arrows. 

The fourth type of electrode models [39, 46] is based on the volume-averaging approach for porous media [51-55]. As indicated in Figure 28.4, it is similar to the agglomerate model, but no macropores are considered. Hence the main direction of mass transport in these micropores is along the depth of the electrode. 

As noted throughout, while the general equations hold, there are many nuances that can go into the expressions. For example, one can consider agglomerate models, coupling of the equations at the macro and mesoscales with phenomena and models at the nano and particle scales, etc. It is clear that reaction within a porous electrode is complex and nonlinear, making mathematical simulation of it very useful. 

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... 

Similar agglomerate approaches were adopted by Iczkowski and Cutlip [30] and by Bjbmbom [31], Those works already identified the doubling of the apparent Tafel slope as a universal signature of the interplay of mass transport limitations and interfacial electrochemical kinetics. Flooded agglomerate models have been employed since then to analyze sources of irreversible voltage losses, optimum electrode thickness, and effectiveness of catalyst utilization. Moreover, it was... 

The catalyst particles are found to exist as agglomerates in the catalyst layer of PEMFC electrodes, and the simulation of cell performance is often conducted by using the agglomerate model shown in Fig. 16.3 [83-86]. The agglomerate with the radius of ragg is filled with... 

In the active parts of a gas diffusion electrode, the pore electrolyte is contained in the small pores between the catalyst particles and the carbon support particles. The larger pores are then filled with gas. Gas has to diffuse through a thin film of electrolyte and in the small pores that contain the pore electrolyte. This introduces an additional mass transfer resistance that can be described with an agglomerate model. Such a model is described with a diffusion-reaction equation for the gaseous species dissolved in the pore electrolyte with the charge transfer reactions as source or sink. The solution to this reaction diffusion model is used to calculate new reaction terms that replace the reaction terms in the balance of charge, material, mass, and energy. These models can often be solved analytically for steady-state if the reactions are of... 

In the solution of electrolyte potential equation with electrode active layer as the boundary, constant ionic current densities and at the anode and cathode active layer boundaries are specified as boundary conditions on the basis of an agglomerate model as... 

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