Atomistic Simulations of Molten Salt Double Layers

Imagine you had a bottle of table salt, NaCl in its solid form, and heated it in a furnace until it became a liquid. This high-temperature ionic liquid is referred to as a molten salt. Molten salts are being widely explored as coolants and fuels for next generation nuclear reactors because molten salts can self-regulate the reactor and do not require high pressures to operate [3]. However, many of these reactors operate at temperatures above 700℃, and the corrosive nature of molten salts with impurities at these high temperatures poses a challenge. The structure and mechanical properties of structural metallic materials can degrade in extreme conditions that combine irradiation, corrosion, and high temperatures, posing safety risks [3]. One of the main goals of the Fundamental Understanding of Transport Under Reactor Extremes (FUTURE) Energy Frontier Research Center (EFRC) is to study the fundamental mechanisms of corrosion of metals in contact with molten salts and how irradiation alters these corrosion mechanisms. Considering specifically fluoride salts, which are attractive for nuclear applications due to their chemical stability [4], scientists in FUTURE computationally simulated interfaces between molten fluoride salts and a metal through molecular-scale models of the interactions and dynamics. These interfaces relate to the reactor in places where molten salt comes into contact with metal. 

For NaCl in its molten form, the elements become ionic—Na becomes positively charged Na⁺ cations, while Cl becomes negatively charged Cl^- anions. Similarly, in fluoride salts, which have shown promising outlook for fluoride-salt-cooled high-temperature reactors (FHRs) [4], F^- fluorine ions are the anions, and the salts contain a mixture of cations, such as Li⁺, Na⁺, K⁺ and/or Be²⁺. One of the more widely studied fluoride salts is Li₂BeF₄, also known as FLiBe, which has been the subject of various experimental and computational works. FLiBe is considered to be a strongly associated salt due to the presence of higher charged Be²⁺ ions and the salt’s ability to maintain stable complexes in the molten phase for longer amounts of time [1]. Another salt of interest involves Li⁺, Na⁺, K⁺, and F^- (FLiNaK), which is a weakly associated salt because the cations do not form long-lasting complexes.

Research Findings

Scientists in FUTURE utilized a sophisticated modeling technique that involves molecular dynamics to simulate the electrochemical behavior of molten fluoride salts sandwiched between two electrodes under a constant potential at extremely high temperature conditions [2]. The scientists found that the molecular structure of the salt would change as a function of distance away from the metal electrodes, leading to different properties at the interface than in the bulk salt. Due to the charges at the metal electrodes that result from an applied voltage, the cations or anions redistribute to screen the electric field and thus lower the free energy of the system. As the distance from the electrode increases, the degree of ion redistribution decreases, resulting in properties that are characteristic of a bulk salt. Contrastingly, as the distance from the electrode decreases, this electrostatic screening perturbs the local composition and density of the salt, as well as the preferred arrangement of the neighboring ions. Specifically, the ions near the interface are observed to become more strongly structured with bonds between the anions and cations, adopting preferred alignment with the metal.

This study suggests that different molecular ordering and salt compositions between the solid–liquid interface and the bulk molten salt can impact chemical reactions related to corrosion at solid–liquid interfaces. Be²⁺ and F^- ions come together and form BeF₄^2- associates. When these associates chain together, a Be²⁺ ion can share F^- ions with neighboring associates, forming oligomers. In particular, the interactions between the Be²⁺ atoms that share F^- atoms are stronger near the interface compared with the bulk salt. Furthermore, the oligomers are more lined up at the interface and more randomly oriented in the bulk, suggesting that the molecular orientations of the salt are impacted by an interface. The different structures of the salt molecules between the interface and the bulk, as well as different salt compositions, could impact the rate of movement of corroding ions across the interface, and thus alter the chemical reactions at a metal surface in contact with molten salt. 

The findings of the computer simulations demonstrate how electrochemical effects play a role in changing the interactions of salt molecules, providing a framework for future experiments and computer models to determine corrosion mechanisms at interfaces between molten salt and metal, as well as the role of irradiation, with the goal of elucidating the impact of irradiation on these mechanisms. Synthesizing computational and experimental research on these interfaces will help scientists in FUTURE better understand how corrosion of metals can be mitigated at extreme temperatures and environments, contributing to the development of safer reactors. Ultimately, understanding how the atomistic structures of molten salt can evolve at high-temperature and in contact with metal could lead to better material selections in extreme environments.