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

Abstract

Low-temperature polymer electrolyte membrane water electrolyzers (PEMWE) are an attractive clean energy technology to produce hydrogen (H2) as 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 porosity, connectivity, etc. 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 interactions between the ink components (catalyst-ionomer-solvent), and the ionomer concentration in the ink, which dictates the ink microstructure and the processing behavior during coating and drying, play a critical role in the formation of the CL microstructure. In this talk, an investigation of catalyst-ionomer interactions, 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, ultra-small/small angle x-ray scattering, dynamic light scattering, and electrophoretic mobility were used to understand inter-particle and particle-ionomer interactions and characterize microstructure. Our results show that iridium catalyst dispersions in the absence of ionomer are agglomerated. When ionomer is added to the dispersion it adsorbs to the surface of IrO2 and provides stabilization through both electrostatic and steric mechanisms. As the ionomer content increases, we see increasing interparticle interactions. The stability and microstructure evolution as a function of ionomer concentration in the ink will be discussed. As well as their relation to device performance.