Microstructure and Rheology of Ionomer Dispersions and Catalyst Inks for Low-Temperature Polymer Electrolyte Membrane Water Electrolyzers: Effect of Ionomer Equivalent Weight and Dispersion Media Composition

Abstract

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 -- composed of catalyst particles and ionomer, a binder for the catalyst and a proton conducting medium -- where the electrochemical reactions occur. The microstructure of the catalyst layer is well-known to play a key role in MEA performance by affecting critical properties such as catalyst utilization, proton conductivity, and gas transport. The evolution of the structure of the catalyst layer is strongly affected by the fabrication process, which is commonly fabricated by solution processing an ink. The microstructure and rheological properties of the inks play an important role in the evolution of final structure of the catalyst layer by affecting the processing behavior during the fabrication. In this talk, the effects of ionomer equivalent weight (EW) and dispersion media composition on the microstructure and rheological behavior of the catalyst inks will be presented. The effects on the structure and rheology of neat ionomer dispersions (with no catalyst) will also be discussed, both to gain sights into the catalyst-ionomer interactions and the rheological behavior, and their significance in membrane fabrication. The ink consists of unsupported iridium oxide (IrO2) catalyst particles and 3M ionomer dispersed in a mixture isopropanol and water. A combination of rheology and ultra small-angle X-ray scattering (USAXS) techniques were primarily used to characterize the microstructure. Preliminary findings show that ionomer stabilizes the agglomerated structure of catalyst dispersions. Reducing the ionomer EW and the alcohol fraction in the dispersion media, for any given EW, decreases the agglomerated structure of the catalyst as well as of the ionomer dispersions (with no catalyst) suggesting dominant electrostatic repulsions between the particles/ionomer at these conditions.This work was authored in part by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding was provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Fuel Cell Technologies Office. This work was created, in part, by UChicago Argonne, LLC, Operator of Argonne National Laboratory, Argonne, U.S. Department of Energy Office of Science laboratory, operated under Contract No. DE-AC02- 06CH11357. This research used the resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory, also under Contract No. DE-AC02-06CH11357. Funding provided as a part of H2 from the Next-generation of Electrolyzers of Water (H2NEW) funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.