Performance enhancement mechanisms of calcium-based thermochemical energy storage compounds: insights from first-principles and experimental investigations

Abstract

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.