Resolving Anisotropic Interfacial Phenomena Using, In Situ, Grazing Transmission Ultra-Small Angle X-Ray Scattering Together with Electrochemical Impedance Spectroscopy

Abstract

Inherent to many electrochemical systems is phase formation at or near the electrode surface. Resolving these phases, in-situ, is challenging with the presence of the necessary electrolyte between the working and counter electrodes. The high X-ray flux available at the Advanced Photon Source (APS) allows these phases to be resolved, in-situ, using small angle X-ray scattering (SAXS) in a grazing transmission geometry shown below. Using this geometry, dimensions both parallel and perpendicular to the surface can be resolved, in-situ, concurrently with an electrochemical technique such as electrochemical impedance spectroscopy (EIS). In this work we show how combined SAXS and EIS is used to resolve unique nanoparticle-solvent interactions that occur in a deep eutectic solvent. Using Ultra-small angle X-ray scattering (USAXS), together with EIS, we were able to resolve ~ 100 nm sized layers that reside above Pd and Ag nanoparticle ensembles on a glassy carbon surface. This study was also performed on the high-energy pinhole SAXS setup, where the distance between the opposing layers was also resolved. While USAXS/SAXS provides information about the size, shape and structure of these phases, the EIS provides their electrochemical response for a comprehensive understanding of interfacial chemistry. These results show how combined USAXS/SAXS and EIS can be used as complementary techniques to resolve otherwise elusive electroactive phases at the interface of electrochemical systems. This technique is accessible through the general user program at the APS. Some general considerations of the experimental setup are presented, including: signal to noise, geometry and cell capabilities. An overview of beamline capabilities is also presented and consists of USAXS/pinSAXS, USAXS, wide angle X-ray scattering (WAXS), USAXS imaging and the high-energy pinhole SAXS setup. With these capabilities, the 15-ID-D beamline offers the potential for a wide variety of in-situ electrochemical experiments towards an understanding of elusive interfacial phenomena.