Investigation of the Microstructure and Rheology of Iridium Oxide Catalyst Inks for Low-Temperature Polymer Electrolyte Membrane Water Electrolyzers

Abstract

We present an investigation of the structure and rheological behavior of catalyst inks for low-temperature polymer electrolyte membrane water electrolyzers. The ink consists of iridium oxide (IrO 2 ) catalyst particles and a Nafion ionomer dispersed in a mixture of 1-propanol and water. The effects of ionomer concentration and catalyst concentration on the microstructure of the catalyst ink were studied. Studies on dilute inks (0.1 wt % IrO 2 ) using zeta potential and dynamic light scattering measurements indicated a strong adsorption of the ionomer onto the catalyst particles which resulted in an increase in the ζ-potential and the z-average diameter. Steady-shear and dynamic-oscillatory-shear rheological measurements of concentrated IrO 2 dispersions (35 wt % IrO 2 ) indicated that the particles are strongly agglomerated in the absence of the ionomer. The addition of even a small amount of the ionomer (2.4 wt % with respect to total solids) caused the rheology to transition from shear thinning to Newtonian because of the reduction in agglomerated structure due to stabilization of the aggregates by the ionomer, consistent with the behavior of dilute inks. At intermediate ionomer loadings, between 2.4 and 9 wt %, the viscosity increased with increasing ionomer wt %, though remained Newtonian, predominantly due to the increasing ionomer volume fraction in the ink. For ionomer loadings greater than 9 wt %, the particles were found to be flocculated, likely induced by a dispersed ionomer. The flocculated inks exhibited strong shear-thinning and gel-like behaviors in steady-shear and oscillatoryshear rheology. The onset of flocculation was found to be sensitive to the catalyst concentration, where below 35 wt % of IrO 2 , flocculation was not observed. The rheological observations were further verified by ultra-small-angle X-ray scattering.

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

“…An I/C of ∼0.35, where the maximum stability of the catalyst in the inks was observed for all DM H 2 O % with 5 wt % catalyst concentrations, is a guideline for optimizing I/C in the inks with different catalyst concentrations. The particle agglomeration degree varies with the particle concentration (generally increases with the concentration); thus, the minimum I/C that maximizes catalyst stability may be expected to be higher or lower than 0.35 I/C depending on the catalyst concentration relative to 5 wt %.…”

“…From extensive research on PGM CLs, many studies have demonstrated that the ink microstructure is dependent on the particle surface chemistry and formulation parameters such as DM composition − and ionomer concentration. − Catalyst inks are generally found to be agglomerated in the absence of an ionomer . The addition of an ionomer was found to reduce the agglomerated structure (i.e., stabilize the particles against agglomeration) of most catalyst/carbon inks (Pt-Vulcan, iridium oxide, Vulcan, high-surface area carbon), with an exception of Pt on high surface carbon. , Beyond the catalyst and ionomer, the DM type and composition (i.e., water/alcohol ratio) have also been found to affect the structure, performance, and stability of the resulting electrode. − ,, …”

“…22 The addition of an ionomer was found to reduce the agglomerated structure (i.e., stabilize the particles against agglomeration) of most catalyst/carbon inks (Pt-Vulcan, iridium oxide, Vulcan, high-surface area carbon), with an exception of Pt on high surface carbon. 21,22 Beyond the catalyst and ionomer, the DM type and composition (i.e., water/alcohol ratio) have also been found to affect the structure, performance, and stability of the resulting electrode. [16][17][18]23,24 Because of the recent improvements in PGM-free catalyst activity and stability, and more recent focus on CL development, the understanding of the interactions of PGM-free ink constituents and the resulting effects on the CL microstructure is in its infancy versus that for Pt-based inks.…”

One-Pot Synthesis of a Three-Dimensional Au-Decorated Cellulose Nanocomposite as a Surface-Enhanced Raman Scattering Sensor for Selective Detection and in Situ Monitoring

“…As shown in Figure B, in the low-frequency region, G ′ < G ″ represented the viscous fluid behavior. With the content of synthesized Au increasing, the difference between G ′ and G ″ was reduced, demonstrating the occurrence of the aggregation and interwinding of cellulose chains in the solutions . With the rise of the angular frequency, both G ′ and G ″ increased and intersected with each other at a certain frequency, demonstrating that the gelation occurred.…”

“…With the content of synthesized Au increasing, the difference between G′ and G″ was reduced, demonstrating the occurrence of the aggregation and interwinding of cellulose chains in the solutions. 40 With the rise of the angular frequency, both G′ and G″ increased and intersected with each other at a certain frequency, demonstrating that the gelation occurred. Subsequently, the frequency continued to increase, G′ > G″, by complying with the elastic behavior.…”