Diffusive transport of water in Magnesium chloride dihydrate (cas 19098-17-0) under various external conditions for long term heat storage: A ReaxFF-MD study

Add time:07/24/2019    Source:sciencedirect.com

Pure Magnesium chloride hydrates are exceptional materials for long term thermochemical heat storage. In a heat storage cycle, the material is charged by dehydration, taking up heat, and discharged by hydration, generating heat. On charging, the H2O molecules released by the dehydration reaction have to diffuse through the solid material. Gas–solid interactions play an important role in mass and heat transport throughout the volume of the storage materials. Under certain experimental conditions, the mass transport may become a rate-determining step. In order to simulate the complex reaction coupled diffusion process, a reliable MgCl2–H2O reactive forcefield (ReaxFF) is parameterized using a single parameter search algorithm. The parameters of ReaxFF are trained against a set of data obtained from density functional theory (DFT). ReaxFF-MD simulations are carried out on a 2D periodic slab of MgCl2⋅2H2O to simulate various possible operating conditions of temperature, external water vapor pressure, incomplete dehydration layers and various vacancy defects during a charging–discharging cycle. The diffusion coefficient of H2O through the 2D periodic slab of dihydrate is observed as 1.24±0.37×10−10  m2/sec at 300 K. The diffusivity increases with temperature and follows the Arrhenius equation. External water molecules impede the dehydration and promote the diffusion of water. The vacancies of MgCl2⋅2H2O molecules are characterized using vibrational density of states obtained from ReaxFF-MD simulations. The vacancy concentrations for MgCl2⋅2H2O molecules have been varied from 1.38% to 4.16%, which result in a diffusivity enhancement from 32.9% to 107.7% when compared with perfect slab. The incomplete hydration at the surface and the mid-layer increases diffusivity by 76.7% and 75.0%.

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