Structure of Molten Alkali Chlorides at Charged Interfaces and the Prediction and Interpretation of Their X-ray Reflectivity

WV Karunaratne and S Sharma and BM Ocko and CJ Margulis, JOURNAL OF PHYSICAL CHEMISTRY C, 125, 25227-25242 (2021).

DOI: 10.1021/acs.jpcc.1c07135

The fundamental properties of molten salts have been the subject of research that spans a century. Yet, in the past few years, there has been an unprecedented surge in interest for these systems in the bulk and under confinement by walls and interfaces including under applied potentials. This is driven by the prospect of exciting and very practical energy technologies, including those in the solar and nuclear fields. This article sets to answer two simple but fundamental questions. How does the liquid structure of alkali chlorides change at a real interface when it is charged? Also, how would such changes on the liquid side of the interface be detected in X-ray reflectivity experiments? We use an interface mimicking conductive diamond, which because of its lattice spacing, is an excellent choice for reflectivity experiments. The reason for our interest in X-ray reflectivity is that, as opposed to electrochemical measurements alone, this is likely the only technique in which atomic level information at the liquid side of the interface can be gained under the extreme temperature environments of molten salts. As it will become apparent, the interpretation of reflectivity results in terms of atomic positions is complex when multiple species with different X-ray contrasts on the liquid side are considered. A theoretical scheme termed "the peaks and antipeaks analysis of reflectivity" originally introduced in our prior work (J. Phys. Chem. C 2019, 123 (8), 4914-4925) is expanded to interpret the structural changes at the interface as a function of applied electrical bias.

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