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

Sunilkumar Khandavalli, Michael Ulsh, Scott Mauger, Jae Park, Nancy Kariuki, Deborah Myers, Sarah Zaccarine, Svitlana Pylypenko

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38 Scopus Citations

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 (IrO2) 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 % IrO2) 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 IrO2 dispersions (35 wt % IrO2) 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 oscillatory-shear rheology. The onset of flocculation was found to be sensitive to the catalyst concentration, where below 35 wt % of IrO2, flocculation was not observed. The rheological observations were further verified by ultra-small-Angle X-ray scattering.

Original languageAmerican English
Pages (from-to)45068-45079
Number of pages12
JournalACS Applied Materials and Interfaces
Volume11
Issue number48
DOIs
StatePublished - 4 Dec 2019

Bibliographical note

Publisher Copyright:
Copyright © 2019 American Chemical Society.

NREL Publication Number

  • NREL/JA-5900-74843

Keywords

  • catalyst inks
  • catalyst layer
  • ionomer
  • iridium oxide
  • low-Temperature polymer electrolyte membrane exchange water electrolysis
  • rheology

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