Rheological Characterization of Interparticle Interactions and Polymer Entanglements in Fuel Cell Catalyst Dispersions

Abstract

Rheometry is a powerful tool to characterize the structure and interparticle interactions in colloidal dispersions, like those used to cast fuel cell catalyst layers. The rheological properties of colloidal dispersions are dependent on many factors including concentration of the dispersed particles, polymer entanglements, particle porosity, and aggregation. There are many publications examining the rheological properties of Nafion dispersions or carbon black dispersions, however, there is little published research examining the rheological properties of fuel cell catalyst inks containing both Nafion and carbon-black-supported catalysts.1-4 This may stem from the fact that the dispersions often used for lab-scale fabrication of fuel cell electrodes tend to be dilute, and thus the rheological properties are dominated by the dispersion medium (solvent), not the dispersed catalyst and ionomer. However, for roll-to-roll production of electrodes, the inks are an order of magnitude more concentrated, and the rheological properties of these inks are significantly influenced by the properties of and interactions between the catalyst and ionomer. In this work we used steady-shear and oscillatory rheometry to quantify the influences of carbon type, presence of catalyst particles, and ionomer-to-carbon ratio on the rheological properties of catalyst inks. We found that, for neat carbon black dispersions, the rheological properties were non-Newtonian and highly dependent on the pore volume of the particles. The addition of Pt to the particles or Nafion to the dispersion decreased the viscosity. The decrease in viscosity with the addition of Nafion was due to Nafion acting as a surfactant and stabilizing the carbon black particles, consistent with studies of aggregation kinetics.5 Interestingly, we found that in dispersions with Nafion, when neat carbon black was replaced with Pt-on-carbon, the change in rheological properties was dependent on carbon type: we observed an increase in viscosity for high-surface carbon but a decrease for Vulcan XC-72 (Figure 1). This suggests a difference in interparticle interactions and polymer entanglements that is dependent on carbon type and Pt location (i.e. on the surface or inside carbon pores), which may have implications for fuel cell performance. We explored these differences using oscillatory shear rheometry, which probes the viscoelastic properties of the dispersions that are dependent on interparticle interactions, including polymer entanglements. Figure 1 Steady-shear rheology measurements of the relative viscosity (viscosity of dispersion/viscosity of dispersion media) of carbon black/Nafion dispersions with and without Pt for Vulcan XC-72 (left) and high surface carbon (right). Acknowledgements: This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Fuel Cell Technology Office, program manager Nancy Garland. 1. Moore, R. B. & Martin, C. R. Chemical and morphological properties of solution-cast perfluorosulfonate ionomers. Macromolecules 21, 1334–1339 (1988). 2. Aldebert, P., Gebel, G., Loppinet, B. & Nakamura, N. Polyelectrolyte effect in perfluorosulfonated ionomer solutions. Polymer 36, 431–434 (1995). 3. Gibaud, T., Frelat, D. & Manneville, S. Heterogeneous yielding dynamics in a colloidal gel. Soft Matter 6, 3482 (2010). 4. Negi, A. S. & Osuji, C. O. New insights on fumed colloidal rheology—shear thickening and vorticity-aligned structures in flocculating dispersions. Rheol. Acta 48, 871–881 (2008). 5. Shukla, S., Bhattacharjee, S. & Secanell, M. Rationalizing Catalyst Inks for PEMFC Electrodes Based on Colloidal Interactions. ECS Trans. 58, 1409–1428 (2013). Figure 1