Impact of Nonadsorbed Ionomer on Viscosity of Catalyst Inks for Polymer Electrolyte Fuel Cells

In a catalyst ink for polymer electrolyte fuel cells (PEFCs), some of the ionomer is adsorbed on the carbon-supported Pt catalyst (Pt/C), which enhances the ink stability, while the rest is dispersed in the solvent as a nonadsorbed ionomer. To clarify the effect of nonadsorbed ionomer on the ink viscosity, the amount of nonadsorbed ionomer in the catalyst ink was evaluated by small-angle neutron scattering (CV-SANS). At high-shear viscosity, the nonadsorbed ionomer was found to act as ionomer solution described by the Huggins equation. Moreover, comparison with the CV-SANS results revealed that the conventional filtration method overestimates the amount of nonadsorbed ionomer in a catalyst ink.