Catalyst layer operation effective properties

In the section Structure Eormation in Catalyst Layers and Effective Properties aspects related to the self-organization phenomena in CL inks will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Thereafter, catalyst layer performance models that involve parameters related to structure, processes, and operating conditions will be presented. 

This section provides a comprehensive overview of recent efforts in physical theory, molecular modeling, and performance modeling of CLs in PEFCs. Our major focus will be on state-of-the-art CLs that contain Pt nanoparticle electrocatalysts, a porous carbonaceous substrate, and an embedded network of interconnected ionomer domains as the main constituents. The section starts with a general discussion of structure and processes in catalyst layers and how they transpire in the evaluation of performance. Thereafter, aspects related to self-organization phenomena in catalyst layer inks during fabrication will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Finally, physical models of catalyst layer operation will be reviewed that relate structure, processes, and operating conditions to performance. 

At macroscopic level, the overall relations between structure and performance are strongly affected by the formation of liquid water. Solution of such a model that accounts for these effects provides full relations among structure, properties, and performance, which in turn allow predicting architectures of materials and operating conditions that optimize fuel cell operation. For stationary operation at the macroscopic device level, one can establish material balance equations on the basis of fundamental conservation laws. The general ingredients of a so-called "macrohomogeneous model" of catalyst layer operation include ... 

The effect of impurities on fuel cells, often referred to as fuel cell contamination, has been identified as one of the most important issues in fuel cell operation and applications. Studies have shown that the component most affected by contamination is the MEA [3]. Three major effects of contamination on the MEA have been identified [3,4] (1) the kinetic effect, which involves poisoning of the catalysts or a decrease in catalytic activity (2) the conductivity effect, reflected in an increase in the solid electrolyte resistance and (3) the mass transfer effect, caused by changes in catalyst layer structure, interface properties, and hydrophobicity, hindering the mass transfer of hydrogen and/or oxygen. 

The two-step strategy in the physical modeling of catalyst layer operation is depicted in Figure 3.5. The first step relates structure to the physical properties of the layer, considered as an effective medium. The second step relates these effective properties to electrochemical performance. Relations between structure and performance are complicated by the formation of liquid water, affecting effective properties and performance. Solutions for such a model provide relations between structure, properties, and performance. These relations allow predictions of architectures of materials and operating conditions that optimize catalyst layer and fuel cell operation to be made. 

At this point, it is important to realize that the ultimate optimization target of electrode design is not Pt utilization, which is a static statistical property of a catalyst layer, but more importantly the effectiveness factor, which includes as well the effects of non-uniform reaction rate distributions due to mass transport phenomena at finite current densities in the operating fuel cell. In simple ID... 

The objective of catalyst layer modeling is to establish relations between fabrication procedures and conditions microstructure effective properties of transport and reaction and performance (Eikerling et al., 2007a). The foregoing sections illustrated that structure and function of catalyst layers evolve over a wide range of scales. The modeling of structure and operation of CLs is, therefore, a multiscale problem. 

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