Reactive force field development for magnesium chloride hydrates and its application for seasonal heat storage

Abstract

MgCl₂ hydrates are considered as high-potential candidates for seasonal heat storage materials. These materials have high storage capacity and fast dehydration kinetics. However, as a side reaction to dehydration, hydrolysis may occur. Hydrolysis is an irreversible reaction, which produces HCl gas thus affecting the durability of heat storage systems. In this study, we present the parameterization of a reactive force field (ReaxFF) for MgCl₂ hydrates to study the dehydration and hydrolysis kinetics of MgCl₂·H₂O and MgCl₂·2H₂O. The ReaxFF parameters have been derived by training against quantum mechanics data obtained from Density Functional Theory (DFT) calculations consisting of bond dissociation curves, angle bending curves, reaction enthalpies, and equation of state. A single-parameter search algorithm in combination with a Metropolis Monte Carlo algorithm is successfully used for this ReaxFF parameterization. The newly developed force field is validated by examining the elastic properties of MgCl₂ hydrates and the proton transfer reaction barrier, which is important for the hydrolysis reaction. The bulk moduli of MgCl₂·H₂O and MgCl₂·2H₂O obtained from ReaxFF are in close agreement with the bulk moduli obtained from DFT. A barrier of 20.24 kcal mol⁻¹ for the proton transfer in MgCl₂·2H₂O is obtained, which is in good agreement with the barrier (19.55 kcal mol⁻¹) obtained from DFT. Molecular dynamics simulations using the newly developed ReaxFF on 2D-periodic slabs of MgCl₂·H₂O and MgCl₂·2H₂O show that the dehydration rate increases more rapidly with temperature in MgCl₂·H₂O than in MgCl₂·2H₂O, in the temperature range 300–500 K. The onset temperature of HCl formation, a crucial design parameter in seasonal heat storage systems, is observed at 340 K for MgCl₂·H₂O, which is in agreement with experiments. The HCl formation is not observed for MgCl₂·2H₂O. The diffusion coefficient of H₂O through MgCl₂·H₂O is lower than through MgCl₂·2H₂O, and can become a rate-limiting step. The diffusion coefficient increases with temperature and follows the Arrhenius law both for MgCl₂·H₂O and MgCl₂·2H₂O. These results indicate the validity of the ReaxFF approach for studying MgCl₂ hydrates and provide important atomistic-scale insight of reaction kinetics and H₂O transport in these materials.