Catalyst layer impedance modeling, 402

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. 

Kulikovsky, A. A. 2012g. A physical model for the catalyst layer impedance. 

Figure 5.30. Schematic of the catalyst layer geometry and its composition, exhibiting the different functional parts, a A sketch of the layer, used to construct a continuous model, b A one-dimensional transmission-line equivalent circuit where the elementary unit with protonic resistivity Rp, the charge transfer resistivity Rch and the double-layer capacitance Cj are highlighted [34], (Reprinted from Journal of Electroanalytical Chemistry, 475, Eikerling M, Komyshev AA. Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells, 107-23, 1999, with permission from Elsevier.)...

Fuel cell performance is affected by MEA composition, including catalyst loading, PTFE content in the gas diffusion layer, and Nafion content in the catalyst layer and membrane, each of which affects the performance in different ways, yielding distinct characteristics in the electrochemical impedance spectra. Even different fabrication methods may influence a cell s performance and electrochemical impedance spectra. With the help of the model described above, impedance spectra can provide us with a useful tool to probe structure-performance relationships and thereby optimize MEA structure and fabrication methods. 

Basic Relations In order to obtain the complex impedance response of the catalyst layer, the general model, outlined in Sect. 8.2.3.2, has to be amended by the process of double-layer charging in Eq. (45) or (46),... 

Markovic et al. [17] review the data on platinum particles and suggest that the data in dilute sulfuric acid is consistent with Kinoshita s model and further suggest that essentially all of the reactivity can be attributed to the (100) surface. They go on to suggest that this difference in reactivity between the crystal faces is due to structure sensitivity of anion adsorption that impedes the reaction. They point out that in PEM systems, where anion adsorption by the sulfonic acid groups is unlikely, there might be considerably less of a particle-size effect. Still, most PEM catalyst layers employ platinum particles on the order of 3nm, roughly the same size as the maximum in mass activity identified by Kinoshita. 

Figure 8.15. Plot of cell potential vs. fuel cell current density, (/o), indicating the effect of liquid water accumulation in the CCL on performance (soUd hne). The interplay of liquid water accumulation in pores and impeded oxygen transport causes the transition from the ideally wetted state to the fully saturated state (dotted tines), as indicated [51]. (Reprinted from Electrochimica Acta, 53.13, Liu J, Eikerting M. Model of cathode catalyst layers for polymer electrolyte fuel cells The role of porous structure and water accumulation, 4435— 46, 2008, with permission from Elsevier.)...

Yang SC (2000) Modeling and simulation of steady-state polarization and impedance response of phosphoric acid fuel-cell cathodes with catalyst-layer microstructure consideration. J Electrochem Soc 147 71-77... 

Below, the model for DMFC cathode impedance is presented, assuming the electrochemical mechanism of MOR on the cathode side (Kulikovsky, 2012b). In this section, the nonstationary version of the DMFC cathode performance model (the section Cathode Catalyst Layer in a DMFC ) is used to calculate the cathode impedance. As discussed in the section Cathode Catalyst Layer in a DMFC, the model takes into account spatial distribution of the MOR and ORR, through the cathode thickness. It is shown below that the spatial separation of MOR and ORR, discussed in the section Cathode Catalyst Layer in a DMFC, leads to the formation of a separate semicircle in the impedance spectrum. 

Chan, K. and Eikerling, M. 2012. Impedance model of oxygen reduction in water-flooded pores of ionomer-free PEEC catalyst layers. 159, B155—B164. 

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