Calcium doped Magnesium Oxide as improved Material for Thermochemical Energy Storage

Thermochemical energy storage (TCES) is considered as auspicious method for sustainable energy management, allowing for decreased energy consumption by recycling of waste heat. Materials for a technological implementation of... [ view full abstract ]

Thermochemical energy storage (TCES) is considered as auspicious method for sustainable energy management, allowing for decreased energy consumption by recycling of waste heat.

Materials for a technological implementation of TCES processes should be cheap, highly cycle stable, show fast reaction kinetics and be completely reversible. Mg(OH)₂ undergoes calcination and partially reversible hydration and is available in large quantities.¹ With calcination temperatures around 400°C it is also compatible with many industrial waste heat sources.² So far low cycle stability and incomplete hydration of MgO impede application, so the target of this study was to increase the performance of the material conserving low calcination temperatures.

Compared to MgO, the hydration of CaO is considerably faster and more exothermic. The reason for this varying behaviour is due to the kinetic barrier of the water dissociation on the oxide surface. This barrier is significantly smaller for CaO favouring the water dissociation, leading to rapid Ca(OH)₂ formation.

The approach for this study was to dope MgO with Ca²⁺ to enhance the lattice parameters and utilize the favoured water dissociation behaviour on the surface. By calcination of a mixed Mg_1-xCa_x(OH)₂ precipitated from a solution of calcium and magnesium chlorides, the immiscibility of MgO and CaO could be circumvented.³ Both the fundamental nature of mixed calcium-magnesium oxide-hydroxide systems as well as their rehydration kinetics were investigated while utilizing the lower reaction temperatures of magnesium-based materials.

The resulting mixed oxides revealed a much higher rehydration rate compared to pure MgO. Mg_0.9Ca_0.1O was found to show the highest reactivity with full rehydration conversion within 80min and remarkably enhanced cycle stability. DFT calculations and X-ray diffraction reveals that this improved behavior results from enhanced lattice parameters and not from physical mixture of MgO and CaO.

[1] Y. Kato, Y. Sasaki, et al., Magnesium oxide/water chemical heat pump to enhance energy utilization of a cogeneration system. Energy, 2005. 30 (11-12): p. 2144-2155.

[2] D. Müller, C. Knoll, et al., In-situ rehydration of MgO obtained by low temperature calcination ChemSusChem, 2017. submitted

[3] D. Müller, C. Knoll, et al., Calcium Doping Facilitates Water Dissociation in Magnesium Oxide. Advanced Sustainable Systems, 2018. 10.1002/adsu.201700096