(Invited) Synchrotron X-Ray Techniques for Structure-Functionality of Ionomers: Current Status and New Directions

For the desired clean energy transition, hydrogen technologies are sought to play a critical role from electrolyzers to produce clean hydrogen from water to fuel cells to decarbonize heavy-duty transportation. Centerpiece to the successful operation of these technologies is the durable performance of the membrane-electrode assembly systems, which consists of an ion-conducting membrane between the electrodes where electrochemical reactions take place. Ion-conducting polymers, ionomers, play a key function in these systems as the separator solid-electrolyte, which requires typically a well-hydrated, phase-separated morphology that can block reactant species while transporting water and ions (protons). In addition, ionomers are also present in the electrodes as a nanometer thick film with a dual function of conducting medium and catalyst binder. In such heterogeneous structures, ionomer forms many interfaces with catalyst particles (e.g., Platinum in fuel cells) and interactions with the other species thereby making it challenging, yet critically important, to characterize their multi-scale structural features. What emerges from these distinct roles of ionomers in fuel cells and electrolyzers is the need for characterizing their structure in relation to the expected properties and desired performance for the operation. For example, for membrane, while structure-property relationships have been commonly investigated via x-ray scattering techniques, with almost three decade of work on interpretation of the so-called ionomer peak associated with the water nano-domains. However, the need for understanding and mitigating membrane degradation requires an expansion to structure-property-durability relationship. Similarly, for catalyst ionomers, their confined structure interfacing metal substrates have been idealized and examined by grazing-incidence x-ray scattering or reflectometry techniques. Yet, understanding the formation of catalyst layers from the mix of catalyst inks and ionomers require a shift to characterize their structure-processing-property relationships. This presentation will provide an overview of the current state of research on ionomer characterization with synchrotron x-rays and the ongoing an expansion of modalities that aim to image not only ionomer structure, but also their evolution under various applied stimulus via in-situ and operando x-ray techniques. Selected examples will be highlighted as case studies, demonstrating, first, how small/wide-angle x-ray scattering (SAXS/WAXS) can be repurposed with various environmental controls to understand structural changes due to degradation as well as the presence of cationic particles added to mitigate degradation. We will also show how energy-tunable x-rays at targeted absorption energies can be used to enhance the scattering signal and the acquired structural information. Then, we will show how grazing-incidence (GI) SAXS/WAXS can be employed to track the real-time evolution of the ionomer film nanostructure and crystallinity on a substrate during casting from a solution. These examples will be discussed together to provide a broader summary the current status and the future directions of both ionomer research for fuel cells and electrolyzer and the required characterization needs using synchrotron x-rays.