Layer Structure and Operation

There is more to catalyst layer operation than electrocatalysis, a lot more The design of fuel cell electrodes with high performance, long lifetime, and low cost is about embedding the catalyst, usually the most expensive and least stable material in the cell, into a porous composite host medium. It turns out that material selection and structural design of the host medium is as important as that of the catalyst material itself. 

The objective of catalyst layer design is twofold from a materials scientist s perspective, the objective is to maximize the electrochemically active surface area (ECSA) per unit volume of the catalytic medium Secsa, by (i) catalyst dispersion in nanoparticle form or as an atomistically thin film and (ii) optimization of access to the catalyst surface for electroactive species consumed in surface reactions. From a fuel cell developers point of view, the objective is to optimize pivotal performance metrics like voltage efficiency, energy density, and power density (or specific power) under given cost constraints and lifetime requirements. These performance objectives are achievable by integration of a highly active and sufficiently stable catalyst into a structurally well-designed layer. 

This chapter provides a systematic account of the pertinent challenges and approaches in catalyst layer design. The hierarchy of structural effects and physical phenomena discussed includes materials design for high surface area and accessibility, statistical utilization of Pt evaluated on a per-atom basis, transport properties of charged species and neutral reactants in composite media with nano- to meso-porosity, local reaction conditions at internal interfaces in partially electrolyte-filled porous media, and global performance evaluated in terms of response functions for electrochemical performance and water handling. 

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