Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks

We present a rheological investigation of fuel cell catalyst inks. The effects of ink parameters, which include carbon black-support structure, Pt presence on carbon support (Pt-carbon), and ionomer (Nafion) concentration, on the ink microstructure of catalyst inks were studied using rheometry in combination with ultrasmall-angle X-ray scattering (USAXS) and dynamic light scattering (DLS). Dispersions of a high-surface-area carbon (HSC), or Ketjen black type, demonstrated a higher viscosity than Vulcan XC-72 carbon due to both a higher internal porosity and a more agglomerated structure that increased the effective particle volume fraction of the inks. The presence of Pt catalyst on both the carbon supports reduced the viscosity through electrostatic stabilization. For carbon-only dispersions (without Pt), the addition of ionomer up to a critical concentration decreased the viscosity due to electrosteric stabilization of carbon agglomerates. However, with Pt-carbon dispersions, the addition of ionomer showed contrasting behavior between Vulcan and HSC supports. In the Pt-Vulcan dispersions, the effect of ionomer addition on the rheology was qualitatively similar to Vulcan dispersions without Pt. The Pt-HSC dispersions showed an increased viscosity with ionomer addition and a strong shear-thinning nature, indicating that Nafion likely flocculated the Pt-HSC aggregates. These results were verified using DLS and USAXS. Further, the observations of the effect of ionomer:carbon ratio and a comparison between carbons of different surface areas provided insights on the microstructure of the catalyst ink corresponding to the optimized I/C ratio for fuel cell performance reported in the literature.