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.

First-Principles Study of Chemical Mixtures of CaCl2 and MgCl2 Hydrates for Optimized Seasonal Heat Storage

“…59 The proton transfer is an important step in the hydrolysis of chloride-based hydrates. 34 To quantify the comparative hydrolysis resistance among MgCl 2 and CaCl 2 hydrates, we have calculated the enthalpy change in proton removal for CaCl 2 ·2H 2 O and MgCl 2 ·2H 2 O. The enthalpy change is obtained by gradually increasing the O–H interatomic distance.…”

Thermal Boundary Characteristics of Homo-/Heterogeneous Interfaces

“…ReaxFF MD is known to capture the physical and chemical phenomena under various conditions [18,26]. We have chosen various force fields for Pt/Ni and Si/SiO 2 systems, which can mimic their material properties.…”

Study on the sorption mechanism of middle‐low temperature sorption thermal storage materials from the microscale simulation: A review

“…37 ReaxFF, proposed by van Duin et al 21 in 2001 and continuously improved by Senftle et al, 38 is widely applied in the study of STSMs. 36,37,[39][40][41][42] It describes the interaction between atoms in the molecule system by BO. According to the electronegativity equilibrium theory (EEM), it renews the charge distribution around the atoms in the model.…”

“…They found the increase the concentration of HCl could inhibit the occurrence of proton transfer and thus inhibit the hydrolysis reaction, which was consistent with the conclusion of le Chatelier's principle. Since then, Pathak et al 40 studied the hydrolysis reaction of MgCl 2 Á 2H 2 O. The DFT simulation results showed that the barrier of the proton transfer was 19.55 kcal Á mol -1 .…”

“…It may decrease the cycle stability of materials. Pathak et al 40,42,74,76 have done a lot of research using DFT and ReaxFF MD. Their results proved that mixing different adsorbents could improve not only the heat and mass transfer but also sorption properties.…”