Performance Enhancement Mechanisms of Calcium-Based Thermochemical Energy Storage Compounds: Insights from First Principles and Experimental Investigation

Calcium-based thermochemical energy storage (TCES) provides a realizable solution to address the challenges of intermittence and volatility in the large-scale utilization of clean energy. Although modified CaCO3/CaO systems have shown promise for stable cyclic performances, the modification mechanism of different additives remains unclear, and a unified theoretical framework for selecting modified elements is needed. This work systematically explores the effects of 34 metal elements on the cyclic stability, heat release capacity, and thermal conductivity of a CaCO3/CaO TCES material using density functional theory (DFT) calculations and experimental tests. The surface energy of the modified CaO surfaces is proportional to the atomic radius for non-transition metal atoms or the ionization energy for transition metal atoms. Trends in Ca4O4 adsorption energy are generally opposite to those of surface energy. Both low surface energy and high Ca4O4 adsorption energy indicate surface stability. Furthermore, the heat release capacity of the modified CaO surfaces is investigated by studying the CO2 adsorption process. Al, Zr, and Ce modifications enhance the stability and affinity for CO2 of the surfaces, but competition with the original CaO for CO2 adsorption may reduce heat release. Additionally, phonon calculations indicate metal substitution can hinder heat transfer by strengthening the coupling between optical and acoustic branches. Finally, the microstructure, cyclic stability, heat release capacity, and thermal conductivity are studied by experiments, and the reliability of the computational predictions is verified. This work uncovers the modification rules of different metal elements on the TCES performances of CaCO3/CaO pairs at the atomic scale. The findings provide a theoretical reference for material modification, and can serve as a basis for higher-scale study to offer more reliable guidance for material modification and facilitate future applications. This work establishes a structure-performance relationship between the atomic parameters of metals and the performance of modified calcium-based TCES materials.