Structural evolution of a catalyst ink for fuel cells during the drying process investigated by CV-SANS

Polymer electrode fuel cells are fabricated by drying a 'catalyst ink', that is, a mixture of carbon, ionomers, Pt and water, and it is of particular importance to elucidate the structural evolution of the catalyst ink during its drying process. The structural evolution of catalyst ink was investigated by contrast-variation small-angle neutron scattering. The scattering functions of the catalyst ink at various concentrations were successfully decomposed to the corresponding partial structure factors S-CC(q), S-PP(q) and S-CP(q) for all ink concentrations, where the subscripts C and P denote the carbon and polymer, respectively, and q is the magnitude of the scattering vector. The cross term S-CP(q) indicates that the microscopic structure of the catalyst ink retains the carbon agglomerates surrounded by an ionomer shell during the drying process. S-CC(q) and S-PP(q) indicate that a catalyst ink is formed with carbon agglomerates of core radius of similar to 42 nm and an ionomer shell of thickness >48 nm. Both the size of the carbon agglomerates and the thickness of the ionomer shells decrease with increasing ink concentration, which is attributed to the exclusion of solvent molecules from the carbon and ionomer agglomerates during the drying process.