Multimodal Resonant x-Ray Scattering: Elucidating Electrochemical Membranes
Authors: Cordova Isvar A. , Kusoglu Ahmet , Su Gregory M. , Kilcoyne David , Feng Jun , Wang Cheng 
Published: February 2020

The development of functionally complex mesoscale (nm - µm) materials requires comparable evolution in the analytical instruments and techniques in order to understand the physical and chemical structure-property relationships underlying their performance. Resonant X-Ray Scattering (ReXS) is a powerful technique due to its ability to statistically-significant sub-nm morphological and chemical sensitivity over broad lengthscales, but it has so far been underutilized in many of the scientific communities whose progress demands spatiochemically-sensitive in-situ or operando characterization techniques. In this presentation, we will show how our group has developed and applied a versatile in-situ cell to harness the potential of ReXS while maintaining compatibility with a variety of synchrotron x-ray techniques and using an environment that is also amenable to other advanced lab-scale techniques such as transmission electron microscopy (TEM).

First, we will go over the basic physics of why ReXS measurements can be especially beneficial for in-situ characterization. Then, we will demonstrate the adaptability and multimodality of our cell by spotlighting both spectroscopic and scattering experiments conducted across different “soft” (i.e. 200 eV – 1.5 keV) and “tender” (i.e. 1.5 keV – 5 keV) x-ray systems at different synchrotrons. In particular, we will highlight the cell’s fuel-cell like capabilities (i.e. heating, electrochemical, and gas/liquid flow) by focusing on recent studies conducted on a variety of proton exchange membranes (PEM) materials spanning a wide range of thicknesses, and which have proven to be difficult to characterize with other techniques. Advanced characterization of PEMs is critical, since they are are still poorly understood cost and performance-limiting components that are used across fuel cells[1], electrolyzers,[2] and redox-flow batteries.[3] Perfluorinated sulfonic acid (PFSA) ionomers are the dominant class of PEMs used across these aforementioned applications, where an electrically neutral chemically inert polytetrafluoroethylene (PTFE) backbone is tethered with SO3
- terminated side-chains that rearrange into a phase-separated morphology where their hydrophilic sulfonate nano-domains endow PEMs with their crucial proton conductivity. Therefore, results from resonant soft x-ray scattering (RSoXS) and absorption spectroscopy conducted at the oxygen and fluorine K-edges will be used to show the response of nanometer-thick PFSA films (such as those relevant for water desalination, fuel cell catalysts, and more) to hydration and interactions with an organic solvent.

Finally, the ability of tender resonant x-ray scattering (T-ReXS) near the sulfur K-edge to improve scattering contrast for nanostructural studies of PFSAs spanning a wide range of industrially relevant thicknesses (i.e., 2-50 micrometers). In addition, the unique ability to measure the hydrophilic domains of a variety of cation exchanged/contaminated (i.e. Co, Ce, Na, etc) samples and impact beam damage on PFSA membranes and their resonant response will be discussed with the audience in attendance.