Challenges for the Structural Design

For the catalyst layers, the foremost objective is to obtain the highest reactivity or transfer current density with respect to the desired electrochemical reactions with a minimum amount of the catalyst (DOE target for 2010 0.29 g Pt per kW). This requires a huge electrocatalytically active surface area, small kinetic barriers to bulk transport and interfacial transfer of protons, electrons and reactant gases, and proper handling of product water and waste heat. 

Gas diffusion is a much more effective mechanism of reactant supply and water removal. Yet, CLs with sufficient gas porosity, usually in the range Yp - 30% -60%, have to be made much thicker, 10 pm - 20 pm. At such thicknesses, proton diffusion in liquid water is not sufficient for providing uniform reaction conditions. Porous gas diffusion electrodes are therefore impregnated with proton-conducting ionomer, usually Nafion [1-2, 4]. Resulting CLs are random composite media of carbon/Pt, ionomer, and a complex pore space. 

The typical agglomerated structure provides a bimodal, bi-functional Pore Size Distribution (PSD), with primary pores between Pt/C particles inside of agglomerates (with pore radii r -l-lOnm) and secondary pores between 

---