Effective catalyst layer diffusivity

Divisek et al. presented a similar two-phase, two-dimensional model of DMFC. Two-phase flow and capillary effects in backing layers were considered using a quantitatively different but qualitatively similar function of capillary pressure vs liquid saturation. In practice, this capillary pressure function must be experimentally obtained for realistic DMFC backing materials in a methanol solution. Note that methanol in the anode solution significantly alters the interfacial tension characteristics. In addition, Divisek et al. developed detailed, multistep reaction models for both ORR and methanol oxidation as well as used the Stefan—Maxwell formulation for gas diffusion. Murgia et al. described a one-dimensional, two-phase, multicomponent steady-state model based on phenomenological transport equations for the catalyst layer, diffusion layer, and polymer membrane for a liquid-feed DMFC. 

COMMENTS We could also have added the catalyst layer diffusion resistance. This model, while serving as a useful qualitative tool, is not precise, simply because we have no idea of the thickness of any layer of liquid water of ionomer locally in the electrode structures. Also, some local flooding simply turns off the current in this location by these effects, but this only means areas which are flooding have reduced performance. Other areas in the fuel ceU may not be flooded, so that the net effect of the flooding is actually to reduce the electrochemicaUy active surface area, which is an approach taken by modely including flooding, discussed later in this chapter. 

The obvious question then arises as to whether the effective double layer exists before current or potential application. Both XPS and STM have shown that this is indeed the case due to thermal diffusion during electrode deposition at elevated temperatures. It is important to remember that most solid electrolytes, including YSZ and (3"-Al2C)3, are non-stoichiometric compounds. The non-stoichiometry, 8, is usually small (< 10 4)85 and temperature dependent, but nevertheless sufficiently large to provide enough ions to form an effective double-layer on both electrodes without any significant change in the solid electrolyte non-stoichiometry. This open-circuit effective double layer must, however, be relatively sparse in most circumstances. The effective double layer on the catalyst-electrode becomes dense only upon anodic potential application in the case of anionic conductors and cathodic potential application in the case of cationic conductors. 

The simplest way to treat the catalyst layers is to assume that they exist only at the interface of the diffusion media with the membrane. Thus, they are infinitely thin, and their structure can be ignored. This approach is used in complete fuel-cell models where the emphasis of the model is not on the catalyst-layer effects but on perhaps the membrane, the water balance, or multidimensional effects. There are different ways to treat the catalyst layer as an interface. 

A more sophisticated and more common treatment of the catalyst layers still models them as interfaces but incorporates kinetic expressions at the interfaces. Hence, it differs from the above approach in not using an overall polarization equation with the results, but using kinetic expressions directly in the simulations at the membrane/diffusion medium interfaces. This allows for the models to account for multidimensional effects, where the current density or potential changes 16,24,46-48,51,52,54,56,60-62,66,80,82,87,107,125 although... 

The earliest models of fuel-cell catalyst layers are microscopic, single-pore models, because these models are amenable to analytic solutions. The original models were done for phosphoric-acid fuel cells. In these systems, the catalyst layer contains Teflon-coated pores for gas diffusion, with the rest of the electrode being flooded with the liquid electrolyte. The single-pore models, like all microscopic models, require a somewhat detailed microstructure of the layers. Hence, effective values for such parameters as diffusivity and conductivity are not used, since they involve averaging over the microstructure. 

The deposition of an inert porous diflfusion barrier on top of the catalyst layer can significantly hinder the rate of mass transfer of reactants to the catalyst surface, at the same time affecting only negligibly the rate of heat transfer to the gas phase. The effect is equivalent to that observed with fuels, like higher hydrocarbons, whose mass diffusi vity in air is considerably lower than thermal diffusivity of the fuel-air mixture (Lewis number >1). Such unbalancing of heat and mass transfer rates results in a significant decrease in the catalyst wall temperature. 

In the laboratory experiments, DOC monolith samples (length 7.5 cm, diameter 1.4 cm) with rather thin catalyst layer coating ( 25 pm) were employed to minimize the internal diffusion effects. The samples were placed into a thermostat to suppress the formation of temperature-gradients along the channels. In the course of each experiment, the temperature of the inlet gas and the monolith sample was increased at a constant rate of /min within the range of 300-800 K. The exhaust gases at the inlet of the converter were simulated by synthetic gas mixtures with defined compositions and flow rates (cf. individual figure captions all gas mixtures contained 6% C02 and 6% H20). 

Since this depth is significantly smaller than the dimensions of the piece of catalyst, and at the same time is much greater than the diameter of individual pores, the phenomenon can be represented schematically by introducing the effective coefficient of diffusion through the porous substance (which depends on the number of the pores and their diameters), and by examining a layer of the catalyst of indefinite depth with a flat surface. 

Two impedance arcs, which correspond to two relaxation times (i.e., charge transfer plus mass transfer) often occur when the cell is operated at high current densities or overpotentials. The medium-frequency feature (kinetic arc) reflects the combination of an effective charge-transfer resistance associated with the ORR and a double-layer capacitance within the catalyst layer, and the low-fiequency arc (mass transfer arc), which mainly reflects the mass-transport limitations in the gas phase within the backing and the catalyst layer. Due to its appearance at low frequencies, it is often attributed to a hindrance by finite diffusion. However, other effects, such as constant dispersion due to inhomogeneities in the electrode surface and the adsorption, can also contribute to this second arc, complicating the analysis. Normally, the lower-frequency loop can be eliminated if the fuel cell cathode is operated on pure oxygen, as stated above [18],... 

In Song et al. s same work [5], the effect that Nafion content in the catalyst layer had upon electrode performance was also investigated, following their work on the optimization of PTFE content in the gas diffusion layer. The optimization of Nafion content was done by comparing the performance of electrodes with different Nafion content in the catalyst layer while keeping other parameters of the electrode at their optimal values. Figures 6.8 and 6.9 show the polarization curves and impedance spectra of fuel cells with electrodes made of catalyst layers containing various amounts ofNafion . 

If the catalyst layer is very ihin. intraparticle diffusion is negligible, thus giving Ihe Irue kinetic. . The cxternal rpass-lransfcr effects should, however, be minimized by proper mixing. 

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