Effects of ionomer and dispersion methods on rheological behavior of proton exchange membrane fuel cell catalyst layer ink

The catalyst layers of proton exchange membrane fuel cells are usually fabricated by (1) mixing catalyzed carbon black, ionomer solution, and solvents to form an ink, (2) coating this ink by spraying or transferring from decal to a membrane or gas diffusion layer, and (3) drying. During the mixing stage, dispersion methods, such as ball milling and sonication, are employed to homogenize the ink and prevent agglomeration of solid ingredients. The present study focused on the first step of fabricating the catalyst layer and conducting a rheological investigation on catalyst inks. The viscosity and thixotropy of inks that were prepared by ball milling and sonication, respectively, were measured, and some major factors affecting the change in the viscosity were analyzed. The catalyst layer inks exhibited pseudoplastic fluid behavior, which was clearly non-Newtonian, and their viscosity decreased with stress rate. The results indicated that the ionomer solution plays a decisive role in the dispersion of carbon-supported catalysts. Hence, homogeneous ink cannot be formed without the ionomer solution. The viscosity of the ink was dependent on the solid content ratio and the dispersion method used, while the dispersion time and amplitude were less critical. Ball milling and sonication methods result in inks with different rheological characteristics. Rheological data indicate that the thixotropic recovery of the ink prepared by ball milling is faster in comparison to that prepared by sonication.