The role of MgO supported sodium sulfate molten salt for calcium looping thermochemical energy storage

Calcium looping (CaL) is considered as a promising process for thermochemical energy storage systems. A key challenge to CaL is the multicycle stability of calcium-based materials for long-term usage. Here, we present an MgO-supported molten salt strategy for calcium-based materials. The main novelty of this study lies in the formation of (Na0.8CaO0.1)2SO4 under a micro-mechanism of competitive wetting-induced structural development in the supported molten salt, which ensures an active interface for the reaction of CaO with CO2 during cycling. The microstructural development of the composite is characterized by X-ray diffraction, scanning electron microscopy and micro-computed tomography. The effect of the microstructure on the thermochemical reactions during cycling is investigated by thermal analysis. The kinetics and underlying mechanisms are discussed. In particular, the use of the supported molten salt boosts the cyclic stability of the composite during CaL, which contributes most of the thermal energy in the storage/release process. For the thermal storage stage, the averaged energy density of the composite with the supported molten salt surpasses the values of CaO after nine cycles and maintains a value of ∼ 1380 J/g. For the thermal release stage, the averaged thermal energy density of the composite with the supported molten salt surpasses that of CaO after five cycles and maintains a value of ∼ 969 J/g. The supported molten salt therefore supplies stable thermal energy with cycling and paves the way for the design of composites for high-temperature thermal energy storage systems.