Abstract
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.
Highlights
Because of the potential operational safety advantages associated with low vapor pressures even at high temperatures,[1−6] molten salts are seeing a major resurgence in interest[7] for varied technology applications
Excellent benchmarks of force fieldbased simulations against transport properties measured in the bulk, are available.[25−28] In particular, the fact that, as demonstrated by Wang et al.,[26] fixed charge force fields the so-called rigid ion models are reasonably good for the alkali chlorides studied here is very advantageous
The key dif ference between the partition philosophy we propose and that seen in the literature is that we are not focused on the effect of real space zones in the reciprocal space signal but instead, the ef fect in reciprocal space of species derived f rom their f ull real space positional range; we believe that both approaches should be useful and complementary
Summary
Because of the potential operational safety advantages associated with low vapor pressures even at high temperatures,[1−6] molten salts are seeing a major resurgence in interest[7] for varied technology applications. V. Karunaratne.17) The choice of the alkali chlorides for our interfacial studies at the conductive diamond interface (modeled as diamond in this study) was in part made based on the fact that their bulk properties are well understood from modern studies, including our own recent work on LiCl, NaCl, and KCl using synchrotron X-ray measurements, simulations, and rate theory.[24] excellent benchmarks of force fieldbased simulations against transport properties measured in the bulk, are available.[25−28] In particular, the fact that, as demonstrated by Wang et al.,[26] fixed charge force fields the so-called rigid ion models are reasonably good for the alkali chlorides studied here is very advantageous. This is because obtaining atomic or electronic density profiles of the quality needed to derive useful information for X-ray reflectivity at and Received: August 11, 2021 Revised: October 8, 2021 Published: November 5, 2021
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