General Framework of Performance Modeling

A fuel cell electrode can be seen as a highly dispersed interface between Pt nanoparticles and electrolyte (ionomer or water). Random composition, phase-segregated morphology, pore size distribution, and wetting properties of pores determine the complex spatial distributions of (i) potentials, (ii) concentrations of reactants and water, and (iii) local rates of electrocatalytic processes that evolve under operation. A subtle electrode theory has to establish the links between these distributions in order to determine catalyst utilization, voltage efficiency (or power density), and water handling capability. 

Agglomeration and bimodal PSDs are vital in view of the conflicting structural requirements for transport and reaction. Structural control during fabrication should thus focus not only on composition but also on effects of agglomeration, porous morphology, and wetting properties of pores. 

Accounting for the peculiar role of water in physical models of CCL operation requires knowledge of the following two aspects in theory and experiment. 

Local equilibrium of water in CCLs. How does the loeal water eontent in CCLs depend on materials morphology and operating eonditions By whieh mechanisms does it attain local equilibrium The approaeh pursued in [50, 51] proposes that capillary forces at the liquid-gas interfaces in pores equilibrate the local water content in CCLs. This approach neglects surface film formation or droplet formation in pores of CLs. Ex situ diagnostics, probing porous structures and water sorption characteristics, are needed to relate equilibrated water distributions to structure and conditions in CCLs. 

Processes involving water transport and transformation. What are the relevant mechanisms and transport coefficients of water fluxes (diffusion, convection, hydraulic permeation, electroosmotic drag) What are relevant mechanisms and rates of phase changes (between liquid water, water vapor, surface water, water in membrane) These mechanisms and relevant parameters are amenable to evaluation by ex situ diagnostics. The statistical theory of random composite media [136-138] and percolation theory [44-46] provides various tools for assessing effective parameters of transport and interfacial processes, as discussed in Section 8.4. 

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