Operando Resonant Soft X-Ray Scattering As a Spatio-Chemical Characterization Technique for Electrochemistry
Authors: Cordova Isvar A. , Wang Cheng , Weber Adam Z , Segalman Rachel A. , Brady Michael A. , Su Gregory M. 
Published: February 2020

The development of complex mesoscale (nm - µm) materials used for electrochemical applications requires comparable progress in the analytical instruments and techniques in order to understand the physical and chemical structure-property relationships underlying their performance. Conventional “hard” X-ray (i.e. > 10 keV) scattering has received considerable attention due to the fact that it is a high-resolution nondestructive structural probe that can interrogate a statistically significant 3-dimensional sample area. The non-resonant nature of this scattering process limits its applicability to materials that possess significantly different electron densities. Unfortunately, the performance of many electrochemical materials hinges on subtle heterogeneities that do not possess a high electron density contrast such as interfacial nanostructures, impurities, and chemical composition gradients. To help address this challenge, resonant soft X-ray scattering (RSoXS) uses tunable “soft” X-rays (100 - 2000 eV) to dramatically enhance the scattering cross sections from heterogeneous materials when the X-ray photon energy is judiciously chosen to coincide with favored transitions near a material’s absorption edges. The RSoXS results in Fig. 1a show an example of how we used the resonance-enhanced scattering signals at selected photon energies to isolate the scattering contribution from different polymers in a phase separated block copolymer in order to unambiguously define the complex morphology of a triblock copolymer sample with both chemical and nm-scale spatial sensitivity.1

In this presentation, we reveal how operando RSoXS can be a powerful reciprocal space probe for mesoscale electrochemistry due to its chemical sensitivity, large accessible size scale, and polarization control.2, 3 We will convey how this technique can be applied under operando conditions to study pores, surfaces,4 and buried interfaces5 of low-Z element materials6 including many transition metals; the practical considerations of conducting such experiments will also be discussed. We will explain how the intrinsic combination of scattering and spectroscopy allows us to monitor spatio-chemical changes at a specific location by detecting the change in intensity at a specific scattering vector,
q
, for X-ray energies that are both ON and OFF resonance with the species of interest (Fig. 1b).

As an example of the utility of RSoXS to electrochemical applications, we present recent results on Nafion, a perfluorinated sulfonic acid (PFSA) membrane material that is considered to be a critical cost and performance-limiting component in many devices including fuel cells, electrolyzers, and redox-flow batteries. Recent RSoXS results acquired with a wet sample cell interrogated the Nafion films’ partially orientated molecules inside ionomer domains. Using polarized X-rays with a photon energy tuned to the fluorine absorption edge (~690 eV), we observed a surprisingly strong scattering anisotropy that indicated preferred local crystalline grain orientation at the interface between different phases, an effect which is not visible when the X-ray photon energy is off-resonance with the fluorinated ionomers (Fig. 1c). These results enable us to develop a full electron density map that helps us understand why the pore structure of Nafion works so well, but may also yield insights into whether the development of porous separators as alternatives to PFSAs require pore sizes that are comparable to the hydrophilic channels in PFSAs (e.g., ≤ 3 nm).7 We will then expand on how combining such operando RSoXS data with electrochemical analytical methods could uncover important dynamic structure-property relationships underlying the interplay of various factors such as migration and electro-osmosis, chemical/physical stability, water uptake, permeability, etc.8-10

References

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Figure 1