Nanostructure of a Catalyst Ink Investigated by Contrast-Variation Small-Angle Neutron Scattering

One of the major aims when investigating catalyst inks for polymer electrolyte fuel cell applications is to understand the relationship between their macroscale physical properties and nanostructure. Catalyst inks are complex colloidal dispersions consisting of catalyst particles, ionomers, and water–alcohol mixtures. Some of the ionomer is adsorbed onto the surface of the catalyst particles. Due to the electrostatic repulsion observed between the particles covered with the ionomer, the ink possesses high dispersion stability. Meanwhile, the role of the remaining ionomer in the dispersion is unclear to date. To clarify the effect of the non-adsorbed ionomer on the physical properties of a catalyst ink, we have established a method to elucidate the amount of non-adsorbed ionomer in the catalyst ink using contrast-variation small-angle neutron scattering (CV-SANS). CV-SANS measurements revealed that the volume fraction of the non-adsorbed ionomer in catalyst ink monotonically increases upon increasing the weight ratio of ionomer/carbon. It was found that the non-adsorbed ionomer in the catalyst ink acts as a typical ionomer dispersion, which can be described by the Huggins equation at high shear viscosity. Our findings may serve as useful guidelines for the preparation of catalyst inks, which is a crucial step for the construction of rationally designed membrane electrode assemblies used for polymer electrolyte fuel cell applications.