The Effect of Dispersion-Medium Composition and Ionomer Concentration on the Microstructure and Rheology of Fe-N-C Platinum Group Metal-Free Catalyst Inks for Polymer Electrolyte Membrane Fuel Cells.

We present an investigation of the microstructure and rheological behavior of catalyst inks consisting of Fe-N-C platinum-group-metal-free catalysts and perfluorosulfonic acid (PFSA) ionomer in a dispersion medium (DM) of water and 1-propanol (nPA). The effects of ionomer to catalyst (I/C) ratio and weight percentage of water (H2O%) in the DM on ink microstructure were studied. Steady-shear and dynamic-oscillatory-shear rheology, in combination with synchrotron X-ray scattering, were utilized to understand interparticle interactions and the level of agglomeration of the inks. In the absence of ionomer, the inks were significantly agglomerated, approaching a gel-like microstructure for catalyst concentrations as low as 2 wt%. The effect of H2O% in the DM on particle agglomeration was found to vary with particle concentration. In concentrated inks (≥ 2 wt% catalyst), increasing H2O% was found to increase agglomeration due to the hydrophobic nature of the catalysts. In dilute inks ( 0.35) interesting differences were observed between nPA-rich inks (H2O% ≤ 50%) and H2O-rich (82% H2O) inks. The nPA-rich inks remained predominantly stable - ink viscosity only weakly increased with I/C and Newtonian behavior was maintained for I/C up to 0.9. In contrast, the H2O-rich inks exhibited a significant increase in viscoelasticity with increasing I/C suggesting flocculation of the catalyst by the ionomer. These differences suggest that the nature of the interactions between ionomer and catalyst are highly dependent on the H2O% in the DM.