Alkali Metal Nitrates Promoted MgO Composites with High CO₂ Uptake for Thermochemical Energy Storage

Thermochemical energy storage (TCES) by means of reversible chemical reactions represents a promising strategy for harnessing renewable energy and recovering industrial waste heat. In this work, alkali metal nitrate promoted MgO composites were synthesized, and the MgO–CO2 working pair was suggested as an alternative candidate for harvesting intermediate-temperature industrial waste heat. However, it remains unclear how operating parameters would affect the heat storage density, and the long-term cycling stability of the alkali metal nitrate promoted MgO composites under harsh conditions close to practical application scenarios needs to be clarified. The objectives of this work are to expound the effects of operating parameters on TCES performance and to evaluate the long-term cycling stability of the alkali metal nitrate promoted MgO composites. With increasing NaNO3 content, the heat storage density of NaNO3–MgO increases dramatically first and then exhibits no significant change. With rising carbonation temperature, the heat storage density of the 10NaNO3–MgO composite increases first and then declines marginally, while it increases with the increasing CO2 partial pressure. Doping alkali metal nitrate eutectic mixtures adversely affects the heat storage density. 10NaNO3–MgO exhibits a high heat storage density of 2291 kJ/kg at 340 °C in 100% CO2. The 10NaNO3–MgO and 10(K–Na)NO3–MgO composites suffer a significant decline in heat storage density within 10 consecutive cycles, while 10(Li–Na)NO3–MgO retains good stability with a slight decrease of heat storage density from 1073 to 770 kJ/kg in 50% CO2. The desired 10(Li–Na)NO3–MgO is suggested as a promising material for TCES application.