(404h) Investigation of the Microstructure and Rheology of Iridium Oxide Catalyst Inks for Low-Temperature Proton Exchange Membrane Water Electrolyzers

Low-temperature polymer electrolyte membrane water electrolyzers (PEMWE) are an attractive clean energy technology to produce hydrogen (H2) which is an energy carrier for several applications such as transportation and grid-scale energy storage and distribution (as supported by the US Department of Energy’s H2@Scale initiative). A critical component of PEMWE membrane electrode assemblies (MEA) is the catalyst layer (CL), where the electrochemical reactions occur. The microstructure of the CL plays a key role in MEA performance by affecting catalyst utilization, proton conductivity and gas transport. The CL microstructure is formed by the catalyst particles and an ionomer - which acts both as a binder for catalyst particles and as a proton conducting medium. The evolution of CL microstructure is primarily controlled by the nature of fabrication of CL, which is commonly solution processing of an ink dispersion. In this process, an ink is formulated at a specified catalyst and ionomer ratio, dispersed in water-alcohol solvent, and then coated (via. spray-coating, roll-coating or brush-painting) onto a substrate (polymer electrolyte membrane or decal) and dried to form the CL. The interactions between the ink components (catalyst-ionomer-solvent), and the ionomer concentration in the ink, which dictate the ink microstructure and the processing behavior during coating and drying, play a critical role in the evolution of final CL microstructure. In this talk, an investigation of catalyst-ionomer interactions, and the influence of alcohol-water solvent mixture on microstructure and rheological behavior of the catalyst inks will be presented. The ink consists of iridium oxide (IrO₂) catalyst particles and Nafion ionomer dispersed in a mixture of 1-propanol and water. A combination of rheology, dynamic light scattering, electrophoretic mobility and ultra small-angle X-ray scattering (USAXS) techniques will be used to understand inter-particle/particle-ionomer interactions and characterize microstructure. Our initial findings suggest that iridium catalyst dispersions in the absence of ionomer are agglomerated. The addition of ionomer stabilizes the particles through adsorption via the electrosteric mechanism. The stability and microstructure evolution as a function of ionomer concentration in the ink will be discussed.