State of the Art in Theory and Modeling Multiple Scales

At the nanoparticle level, the specifie exehange eurrent density, 

Equation 8.2 is sensitive to size, surface morphology, and the surfaee eleetronic structure of Pt nanoparticles as well as to the properties of the substrate [8, 71-75]. A better understanding of the relationships between partiele size and aetivity, including those effects, is critical in view of the design of highly performing catalyst systems [68, 76-78]. Obviously, a reduction in particle size improves the 

At the mesoscopic scale, interactions between molecular components control the self-organization phenomena between molecular components that lead to random phase-segregation during fabrication of CLs [7]. Mesoscale simulations can describe the morphology of heterogeneous materials and rationalize their effective properties beyond length- and time-scale limitations of atomistic simulations. A recently introduced computational method allows evaluation of the key faetors during fabrication of CLs. These simulations rationalize structural 

4 Structure Formation of Catalyst Layers and Effective Properties 

In this section, we discuss theoretical and computational studies that provide insights into structural correlations and dynamical behavior of species in CLs. Structural complexity is an inherent trait of CLs. Advanced fabrication aims to improve Pt utilization by enhancing the interfacial area of Pt with water in pores and with Nafion ionomer [12, 94—95], A practical way to achieve this is by mixing ionomer with dispersed Pt/C catalysts in the ink suspension prior to deposition to form a CL. The solubility of the ionomer depends upon the choice of a dispersion medium. This influences the microstructure and pore size distribution of the CL [95]. Self-organization of ionomer and carbon/Pt in the colloidal ink leads to the formation of phase-segregated agglomerated morphologies. 

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