Structure Formation in Catalyst Layers and Effective Properties

STRUCTURE FORMATION IN CATALYST LAYERS AND EFFECTIVE PROPERTIES 

FIGURE 3.32 A schematic illustration of the catalyst layer formation process by mixing of Pt/C particles and ionomer in a solvent. 

Improvements in structure versus dynamics relationships of CLs demand a high level of control over parameters and processes involved in the fabrication process. For good references on ink formation and CL fabrication, the reader is referred to 

Wang et al. (2004) and Zhang (2008). Systematic modifications at the fabrication stage include (i) the selection of materials (e.g., carbon or metal-oxide-based support materials, Pt or Pt-alloy catalyst materials, perfluorinated or alternative ionomer materials), (ii) size dispersion of catalyst and support particles, (iii) gravimetric composition of the ink (amounts of carbon, ionomer, and Pt) and solvent properties, (iv) thickness of the CL, and (v) fabrication conditions (temperature, solvent evaporation rate, pressure, and tempering procedures). 

These parameters and conditions determine complex interactions between Pt nanoparticles, carbon support, ionomer molecules, and solvent, which control the catalyst layer formation process. Self-organization of ionomer and carbon/Pt in the colloidal ink leads to the formation of phase-segregated and agglomerated morphologies. The choice of a dispersion medium determines whether ionomer exists in solubilized, colloidal, or precipitated form. This influences the microstructure and pore size distribution of the CL (Uchida et al., 1996). It is believed that mixing of ionomer with dispersed Pt/C catalysts in the ink suspension, prior to deposition to form a CL, enhances the interfacial area of Pt with water in pores and with Nation ionomer. 

---