Catalyst layer agglomeration

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

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. 

A schematic of a typical fuel-cell catalyst layer is shown in Figure 9, where the electrochemical reactions occur at the two-phase interface between the electrocatalyst (in the electronically conducting phase) and the electrolyte (i.e., membrane). Although a three-phase interface between gas, electrolyte, and electrocatalyst has been proposed as the reaction site, it is now not believed to be as plausible as the two-phase interface, with the gas species dissolved in the electrolyte. This idea is backed up by various experimental evidence, such as microscopy, and a detailed description is beyond the scope of this review. Experimental evidence also supports the picture in Figure 9 of an agglomerate-type structure where the electrocatalyst is supported on a carbon clump and is covered by a thin layer of membrane. Sometimes a layer of liquid water is assumed to exist on top of the membrane layer, and this is discussed in section 4.4.6. Figure 9 is an idealized picture, and... 

If external mass-transfer limitations can be neglected, then the surface concentration in eq 58 (via eq 13) can be set equal to the bulk concentration, which is assumed uniform throughout the catalyst layer in the simple agglomerate models. Otherwise, the surface concentration is unknown and must be... 

Perry et al. [24] and Jaouen et al. [25] have provided useful diagnostic criteria. They concluded that cathodes controlled by either Tafel kinetics and oxygen diffusion in the agglomerate regions, or by Tafel kinetics and proton transport in the catalyst layer could result in double Tafel slopes. If the cathode was controlled by Tafel kinetics, oxygen diffusion, and proton transport all together, quadruple Tafel slopes would appear. 

Figure 3.49. Slice of a PEM cell showing gas diffusion layer (A), catalyst layer (B) and membrane layer (C), at a magnification factor of 200 (a). Tunnelling electron microscope pictures of catalyst layer at a magnification factor of 500 (b), 18 400 (c) and in (d) 485 500. (From N. Siegel, M. EUis, D. Nelson, M.v.Spakovsky (2003). Single domain PEMFC model based on agglomerate catalyst geometry. J. Power Sources 115, 81-89. Used with permission from Elsevier.)...

Figure 2.1. Structure and composition of catalyst layers at three different scales At the nanoparticle level, anode and cathode processes are depicted, including possible anode poisoning by CO. At the agglomerate level, ionomer functions as binder and proton-conducting medium are indicated, and points with distinct electrochemical environments are shown (double- and triple-phase boundary). At the macroscopic scale, the interpenetrating percolating phases of ionomer, gas pores, and solid Pt/Carbon are shown, and the bimodal porous structure is indicated.

Spherical and planar model geometries of agglomerates will be considered. The planar geometry leads to important conclusions about the performance of ultrathin catalyst layers. 

The length scale i a 50 — 100 nm determines the effectiveness of catalyst utilization for spherical agglomerates. Analogous relations apply for ultrathin planar catalyst layers with similar thickness, L 100 — 200 nm. We consider layers that consist of Pt, water-filled pores and potentially an electronically conducting substrate. With these assumptions, we can put/(dfptc, XfXptc = 1 and g Sr) = 1. The volumetric exchange current density is, thus. 

The nonuniform distribution of protons and potential in water-filled agglomerates and ultrathin catalyst layers is predominantly an electrostatic effect. It is determined by the Debye length. Ad- Resulting reaction rate distributions and effectiveness factors depend on the characteristic sizes of agglomerates (i a) or ultrathin CCLs ( L) and on the transfer coefficient a. 

As a major conclusion, primary pores inside agglomerates and ultrathin catalyst layers should be hydrophilic (maximum wetting). Under such conditions effectiveness of catalyst utilization can approach 100%. Moreover, the microscopic mechanism of the electrochemical reaction, represented by the transfer coefficient a, is essential for the effectiveness of catalyst utilization. 

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