Solvent effects on the rheology of fuel cell catalyst ink and the adsorption of ionomers on the particles

The solvent affects the morphology of ionomers, agglomeration structure, adsorption of ionomers on carbon or catalyst particles, and rheology in the proton exchange membrane fuel cell (PEMFC) ink, which are all related to the microstructure of catalyst inks. We investigated the link between the morphology of the ionomers or the fractal dimension (df) of graphitized carbon (GC) dispersion and the microstructure of the catalyst ink by solvent to change the morphology of the ionomers or df of GC dispersion. The nuclear magnetic resonance results showed relatively low mobility of the ionomer in ethylene glycol (EG), resulting in a weak interaction between the ionomer and EG. GC in water, EG, and n-propanol (nPA) form transition gels with a network structure by a diffusion-limited aggregation mechanism, with df of 1.76 ± 0.03, 1.95 ± 0.04, and 2.01 ± 0.06, respectively. Moreover, the addition of ionomers reduces the viscosity of inks due to the introduction of steric hindrance and an increase in electrostatic repulsion, transforming the ink from shear-thinning to Newtonian fluids. Further, the interaction between ionomer and particles in GC and graphitized-carbon-supported platinum (Pt/GC) inks is solvent-dependent, and nPA is more favorable than water and EG for ionomer adsorption on GC. These findings provide valuable insight into the internal interactions and microstructure of PEMFC catalyst inks.