Catalyst Layer Performance

The transport properties of water-filled nanopores inside of agglomerates and the properties of the ionomer film at the agglomerate surface define local reaction conditions at the mesoscopic scale. These local conditions, which involve distributions of electrolyte phase potential, proton density (or pH), and oxygen concentration, determine the kinetic regime, under which interfacial electrocatalytic processes must be considered. Combining this information, a local reaction current can be found, which represents the source term to be used in performance modeling of the cathode catalyst layer. 

This chapter is devoted entirely to performance models of conventional catalyst layers (type I electrodes), which rely on reactant supply by gas diffusion. It introduces the general modeling framework and employs it to discuss the basic principles of catalyst layer operation. Structure-based models of CCL rationalize distinct regimes of performance, which are discernible in polarization curves. If provided with basic input data on structure and properties, catalyst layer models reproduce PEFC polarization curves. Consistency between model predictions and experimental data will be evaluated. Beyond polarization curves, performance models provide detailed maps or shapes of reaction rate distributions. In this way, the model-based analysis allows vital conclusions about an optimal design of catalyst layers with maximal catalyst utilization and minimal transport losses to be drawn. 

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The properties and composition of the CL in PEM fuel cells play a key role in determining the electrochemical reaction rate and power output of the system. Other factors, such as the preparation and treatment methods, can also affect catalyst layer performance. Therefore, optimization of the catalyst layer with respect to all these factors is a major goal in fuel cell development. For example, an optimal catalyst layer design is required to improve catalyst... 

Equations (6.59)-(6.61) represent a highly simplified scheme for evaluating various catalyst layer designs. Refinements of this crude framework for evaluating catalyst layer performance should address all transport limitations, account for water accumulation, and include two- and three-dimensional effects. 

The case of fast oxygen transport through the catalyst layer is of practical interest, since nowadays thin catalyst layers (5-10 pm) are usually used. One may expect the oxygen concentration to vary only moderately across such a thin layer. The catalyst layer performance is then governed by Eqs. (43) and (46), with... 

In the case of negligible proton transport limitations, that is, r](x) = rjo, but possible oxygen diffusion limitations the catalyst layer performance is determined by... 

In spite of the widely recognized importance of an advanced catalyst layer design, detailed structural data for catalyst layers are still scarce in the open literature on fuel cells [116, 117]. In one of the rare experimental studies, Uchida et al. showed the effect of the variation of the PFSI (and PTFE) content on catalyst layer performance [101]. An attempt to rationalize the experimentally observed composition dependence theoretically was first undertaken in Ref. 17. The prerequisites for an adequate theoretical study... 

Tab. 3 Calculated parameters, which characterize the catalyst layer performance in the limit of vanishing oxygen diffusion limitations for various compositions at jo = 0.01 A cm-2. Reference parameters are specified in the caption of Fig. 16...

Modeling of PEMFC Catalyst Layer Performance and Degradation... 

So far, we have focused on the formal description of current generation in the catalyst layer and discussed major effects of structure and composition on exchange current density and catalyst utilization. In the remainder of this chapter, we will explore in detail, how electrocatalytic activity interferes with other processes at the catalyst surface (e.g. surface diffusion) and transport in the bulk phases. The key measure of catalyst layer performance is the current density that could be extracted from a cell for a given cell potential. This links the spatially varying concentrations and reaction rates with the global performance, rated in terms of power density and fuel cell efficiency. 

A. A. Kulikovsky. Quasi three-dimensional modeling of PEM fuel cell Comparison of the catalyst layers performance. Fuel Cells, 1(2) 162-169, 2001. 

These voltage losses are most pronounced for low proton conductivity, that is, small a. The ohmic contribution (a a ) results from a simple fact if there is a layer close to the membrane with no oxygen, the protons must pass this layer (without any reaction) to reach another layer, close to the backing layer, where there is oxygen to react with. Actually, at current densities jo > / the catalyst layer performs as if it had one flexible boundary at x = 1 — Sgff /1. While the boundary on one side, X = 1, remains fixed, the boundary at x = 1 — /1 moves away from the membrane... 

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