Ca/Co-based composites with improved cyclic stability and optical absorption for advanced thermochemical energy storage systems

Abstract

Thermochemical energy storage (TCES) is a promising technology to overcome solar intermittency and volatility. However, weak solar absorption, poor cyclic stability for calcium carbonates, and cost issues for metal oxides hinder the applicability of these materials for thermochemical energy storage. Herein, an advanced, affordable, and effective TCES system is proposed, and the corresponding calcium carbonate-metal oxide synergistic material is designed with improved solar absorption and cyclic stability. The optimum molar ratio is Ca:Co:Mn:Mg = 100:20:7.5:5 for the synergistic material fabricated by the modified sol–gel method, where Ca element plays the primary role in heat storage, Co element serves as the primary optical performance enhancer and secondary heat storage component, and Mn/Mg elements act as the secondary performance modifiers. The solar absorptivity is boosted to ∼79.3%, and the heat storage density is 1295 kJ/kg after 30 high-temperature cycles, which are 6 times and 2.2 times higher than those of commercial calcium carbonate, respectively. Homogeneous mixing of modifiers at the microscale and stable, well-developed pore structure are critical factors for cyclic stability enhancement. The change of electronic structure on the Fermi level caused by Co/Mn ion doping is the key to optical absorption improvement, and the promoting amplitude exhibits wavelength dependence. This work expands the limited list of currently available TCES materials and the comprehensive description of optical properties offers guidance for future implementation of TCES materials with high solar absorption in specific wavelengths.