A Holistic Approach for Elucidating Local Structure, Dynamics, and Speciation in Molten Salts with High Structural Disorder

Abstract

To examine ion solvation, exchange, and speciation for minority components in molten salts (MS) typically found as corrosion products, we propose a multimodal approach combining extended X-ray absorption fine structure (EXAFS) spectroscopy, optical spectroscopy, ab initio molecular dynamics (AIMD) simulations, and rate theory of ion exchange. Going beyond conventional EXAFS analysis, our method can accurately quantify populations of different coordination states of ions with highly disordered coordination environments via linear combination fitting of the EXAFS spectra of these coordination states computed from AIMD to the experimental EXAFS spectrum. In a case study of dilute Ni(II) dissolved in the ZnCl 2 +KCl melts, our method reveals heterogeneous distributions of coordination states of Ni(II) that are sensitive to variations in temperature and melt composition. These results are fully explained by the difference in the chloride exchange dynamics at varied temperatures and melt compositions. This insight will enable a better understanding and control of ion solubility and transport in MS.

Solving the structure of “single-atom” catalysts using machine learning – assisted XANES analysis

“…The first challenge could be solved if spectra that correspond to unique structures could be isolated. To solve the first part, several approaches have been proposed, ranging from forward modelling 38,39 to spectral deconvolution methods, such as principal component analysis (PCA), 40 linear combination analysis, 41 and multivariate curve resolution-alternating least square (MCR-ALS). 40 The latter approaches are particularly attractive if a series of spectra are available, such as when the sample is exposed to reaction conditions, where the speciation of different structures of catalytically active metal complexes changes with the reaction.…”

Chemistry Dissolved in Ionic Liquids. A Theoretical Perspective

“…They point out the importance of AIMD when studying such systems because the consideration of electronic interactions and spin polarization is necessary. They give a warning that the right prediction of the local dynamical coordination environment and chemical speciation of the complex solvent mixtures is necessary to allow high-quality predictions. , …”

“…In general, it is found that the excess electron can delocalize over cations or anions depending on the nature of the ions involved. 130 Roy and Gill et al 131,132 studied d-block metal ions (Ni 2+ , Co 2+ , and Cr 3+ ) in molten salts by a combined AIMD and experimental extended X-ray absorption fine structure (EXAFS) approach. They point out the importance of AIMD when studying such systems because the consideration of electronic interactions and spin polarization is necessary.…”

First-principles molecular dynamics simulations of UCln–MgCl2 (n = 3, 4) molten salts

“…3,5 A thorough understanding of the atomic-level interactions, structures, and dynamics of molten salts is beneficial to gather accurate knowledge of these properties and to predict them from first principles. 3,4,[6][7][8][9][10][11] Conventional thermal-spectrum MSRs use graphite to slow neutrons down and moderate temperature, and employ fluoride salts, such as LiF-BeF 2 -ZrF 4 , for their high stability, low vapor pressure, and low neutron-capture cross section. 5 A new generation of fast-spectrum MSRs uses fast neutrons (without the need of a moderator) to sustain the fission chain reaction but requires more enriched fissile materials as the fuel, thereby necessitating the molten salts for lower viscosity and higher actinide solubility and, hence, chloride-based salts.…”

“…3,5 A thorough understanding of the atomic-level interactions, structures, and dynamics of molten salts is beneficial to gather accurate knowledge of these properties and to predict them from first principles. 3,4,6–11…”