Investigation of the Colloidal Behaviour of Carbon Black and Perfluorosulfonated Ionomer in Various Solvents
Catalyst Ink , Ionomer , Carbon Black , Aggregation
This thesis investigates the interactions between ionomer and carbon black in solution as influenced by different ink parameters. The motivation for this work is to collect necessary data that would lead toward the development of an improved microstructure model of catalyst layer of polymer electrolyte membrane fuel cells. Ionomer aggregation behaviour in solution was investigated at different ionomer concentration, solvent type and solvent composition. Ionomer was found to exist in three forms in solution. Extended molecule chains having a hydrodynamic diameter between 20nm to 30nm. Primary aggregates formed by the hydrophobic interaction of fluorocarbon backbone are around 300nm in diameter. Secondary aggregates formed from electrostatic interaction of side chain ion pairs usually having diameter above 1000nm. Aggregation behaviour of ionomer in different solvents and solvent mixtures at different mixing ratios was also investigated. 1-propanol and 1-propanol/water mixture were found to be the best solvent and solvent mixture among the solvents examined. Carbon black (CB) particle size distribution in solution was measured under different conditions. A maximum 2hr sonication time is suggested for dispersing CB in solution. Size distribution of CB particles increases with concentration. Water content in the co-solvent mixture influences the particle size distribution of CB. It is suggested that water content should be kept under 50wt% to prevent aggregation of CB particles. Ionomer is found to adsorb on platinum/carbon (Pt/CB) surface strongly when mixed. The adsorption process comprises a primary adsorption stage and a secondary adsorption stage. At low ionomer concentration, the primary adsorption was found to follow a Langmuir isotherm with maximum surface coverage around 2.66 x 10-3 g ionomer/m2 Pt/CB. The Pt/CB particle size change due to ionomer adsorption was measured. A maximum size increase around 40nm was found. The size change combined with ionomer molecule geometry estimation implies that only part of the ionomer molecule adsorbed on Pt/CB surface.