Doping magnesium hydroxide with sodium nitrate: a new approach to tune the dehydration reactivity of heat-storage materials.

Thermochemical energy storage (TES) provides a challenging approach for improving the efficiency of various energy systems. Magnesium hydroxide, Mg(OH)2, is known as a suitable material for TES at temperature T>300 °C. In this work, the thermal decomposition of Mg(OH)2 in the absence and presence of sodium nitrate (NaNO3) is investigated to adapt this material for TES at T<300 °C. The most notable observations described for the doped Mg(OH)2 are (1) a significant reduction of the decomposition temperature Td that allows tuning the dehydration reactivity by varying the NaNO3 content. The Td decrease by 25 °C is revealed at a salt content Y≤2.0 wt %. The maximum Td depression of some 50 °C is observed at Y=15-20 wt %; (2) the NaNO3-doped Mg(OH)2 decomposes considerably faster under conditions typical for closed TES cycles (at T>300 °C in vapor atmosphere) than a pure Mg(OH)2; (3) the morphology of the dehydration product (MgO) dramatically changes. Differential scanning calorimetry, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and vibrational spectroscopy (IR and Raman) are used to study the observed effects and to elucidate possible ways the NaNO3 influences the Mg(OH)2 dehydration and morphology of the dehydration product. The mechanism involving a chemical interaction between the salt and the hydroxide accompanied by nitrate embedding into brucite layers is discussed.