Investigating the effect of the precursor on magnesium hydroxide for thermochemical energy storage

Abstract

Thermal energy storage is an important aspect of a clean future and the Mg(OH)2/MgO reversible reaction is a promising option for this. However, its poor mass/heat transfer and reactivity over many cycles has limited its implementation. Improvements reported often enhance one aspect, but at the expense of another. The precursor used to precipitate Mg(OH)2 has been shown to affect the physical properties of the precipitated material, but has not been investigated in the energy storage sphere. It has the potential to improve the cycling capability and thermal properties without the downsides of previous improvements. Mg(OH)2 precipitated from Mg(CH3COO)2, Mg(NO3)2, MgCl2 and MgSO4 with NH4OH, as well as industrial Mg(OH)2 was investigated by DSC, TGA, XRD, SEM, BET, Raman, LIBS, kinetic analysis and cycling. The acetate and chloride derived Mg(OH)2 initially dehydrated at significantly lower temperatures (296 oC and 319 oC) compared to the industrial material (343 oC) with comparable energy stored. The activation energy, determined by isoconversional Starink methodology and non-isothermal data also differed significantly, with acetate derived Mg(OH)2 showing Ea = 75.40 kJ mol-1 via a nucleation and growth mechanism and industrial material an Ea = 151.36 kJ mol-1 via a contracting area mechanism. When cycled through multiple dehydration and rehydration reactions using a novel steam and water approach the acetate derived material showed a 97% average rehydration compared to 87% for the industrial material over 5 cycles. This equates to 693 kJ kg-1 of extra energy stored due to the morphology differences in the materials, with no negative effects unlike previously reported improvements. It could also be combined with these to reduce their drawbacks and create a more efficient and productive thermal energy storage system.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Martin (Supervisor)