Catalyst layer conventional design

In this chapter, we will mainly address the vital topics in theoretical membrane research. Specifically, we will consider aqueous-based proton conductors. Our discussion of efforts in catalyst layer modeling will be relatively brief. Several detailed accounts of the state of the art in catalyst layer research have appeared recently. We will only recapitulate the major guidelines of catalyst layer design and performance optimization and discuss in some detail the role of the ionomer as a proton-supplying network in catalyst layers with a conventional design. 

FIG U RE 3.17 The self-consistency problem in Pt electrocatalysis. The metal phase potential determines oxidation state and charging properties at the catalyst surface. These properties in turn determine the local reaction conditions at the Hehnholtz or reaction plane. At this point, structural design and transport properties of the catalyst layer come into play (as illustrated for conventional and ultrathin catalyst layers). Newly developed methods in the emerging field of first-principles electrochemistry attempt to find self-consistent solutions for this conpled problem. 

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. 

The most common electrode design currently employed is the thin-film design, characterized by the thin Nafion film that binds carbon-supported catalyst particles. The thin Nafion layer provides the necessary proton transport in the catalyst layer. However, this is a significant improvement over the PTFE-bound catalyst layer, which requires the less effective impregnation of Nafion . Sputter deposited catalyst layers have been shown to provide some of the lowest catalyst loadings, as well as the thinnest layers. The short conduction distance of the thin sputtered layer dissipates the requirement of a proton-conducting medium, which can simplify production. The performance of the state of the art sputtered layer is only slightly lower than that of the present thin-film convention [125]. 

A novel design route expanding on such a two-phase concept has already been followed by the company 3M for some time. The benefits in terms of catalyst utilization and performance enhancement are evident from that work [147]. The activity per total mass of Pt of the 3M layers is larger by about a factor 6 than the mass activity of conventional three-phase CCLs. This affirms the awful inactivity of Pt in conventional CCLs. 

The supremacy of such layers in terms of catalyst utiliza tion is demonstrated by recent MEA development of the company 3 M [96]. In this design, nanos-tructured films of oriented crystalline organic whiskers form the substrate for a sputter-deposited Pt film. The activity per total mass of Pt of these ultrathin films is about a factor six greater than the mass activity of conventional three-phase CCLs, which affirms the much better catalyst utilization. 

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