Defect-driven water migration in MgCl2 tetra- and hexahydrates

Hendrik Pieter Huinink (Corresponding author), Stefano Sansotta, Dirk Zahn

Onderzoeksoutput: Bijdrage aan tijdschriftTijdschriftartikelAcademicpeer review

Uittreksel

This molecular dynamics simulations study elucidates how water diffusion in MgCl2·nH2O (n=4 and 6) is facilitated by defects and dopants. Both the impact of a single vacancy (one water molecule was removed) and an interstitial (one water molecule was added) on long range water motion has been investigated. Spontaneous vacancy-interstitial pair defect formation was not observed, which is in line with the predicted high energy costs for defect formation: 150 and 200 kJ/mol for the tetrahydrate and hexahydrate, respectively. The vacancy defects did not show long range mobility, which we relate to the strong bonding of water in the coordination shell of the Mg-ion that prevents water molecules from shifting to neighbouring magnesium ions. On the contrary, the addition of an extra water molecule, facilitates long range motion, which was found as a sequence of hopping events. The interstitial motion is anisotropic in both tetrahydrate and hexahydrate crystals, as interstitials preferentially reside at locations of unfavourable Cl-Cl interactions. Strikingly, Mg ↔ Ca substitutional defects neither increase the mobility of vacancies nor facilitates interstitial motion in the MgCl2 lattice. While Ca dopants slightly facilitate vacancy formation, it also stabilizes interstitial water molecules by incorporating these molecules in its coordination shell. As a consequence, the interstitial becomes trapped and loses its mobility. Therefore, Ca dopants will not increase the hydration/dehydration kinetics of MgCl2 hydrates and cannot be used to boost the power output of MgCl2-based heat storage devices.

TaalEngels
Pagina's221-228
Aantal pagina's8
TijdschriftJournal of Solid State Chemistry
Volume277
DOI's
StatusGepubliceerd - 1 sep 2019

Vingerafdruk

interstitials
Defects
Water
Vacancies
defects
water
Molecules
molecules
Doping (additives)
Ions
heat storage
Heat storage
acceleration (physics)
Hydrates
Dehydration
Crystal lattices
dehydration
Hydration
hydrates
Magnesium

Citeer dit

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abstract = "This molecular dynamics simulations study elucidates how water diffusion in MgCl2·nH2O (n=4 and 6) is facilitated by defects and dopants. Both the impact of a single vacancy (one water molecule was removed) and an interstitial (one water molecule was added) on long range water motion has been investigated. Spontaneous vacancy-interstitial pair defect formation was not observed, which is in line with the predicted high energy costs for defect formation: 150 and 200 kJ/mol for the tetrahydrate and hexahydrate, respectively. The vacancy defects did not show long range mobility, which we relate to the strong bonding of water in the coordination shell of the Mg-ion that prevents water molecules from shifting to neighbouring magnesium ions. On the contrary, the addition of an extra water molecule, facilitates long range motion, which was found as a sequence of hopping events. The interstitial motion is anisotropic in both tetrahydrate and hexahydrate crystals, as interstitials preferentially reside at locations of unfavourable Cl-Cl interactions. Strikingly, Mg ↔ Ca substitutional defects neither increase the mobility of vacancies nor facilitates interstitial motion in the MgCl2 lattice. While Ca dopants slightly facilitate vacancy formation, it also stabilizes interstitial water molecules by incorporating these molecules in its coordination shell. As a consequence, the interstitial becomes trapped and loses its mobility. Therefore, Ca dopants will not increase the hydration/dehydration kinetics of MgCl2 hydrates and cannot be used to boost the power output of MgCl2-based heat storage devices.",
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Defect-driven water migration in MgCl2 tetra- and hexahydrates. / Huinink, Hendrik Pieter (Corresponding author); Sansotta, Stefano; Zahn, Dirk.

In: Journal of Solid State Chemistry, Vol. 277, 01.09.2019, blz. 221-228.

Onderzoeksoutput: Bijdrage aan tijdschriftTijdschriftartikelAcademicpeer review

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