Microstructural control by freeze-casting of CaO architectures for improved and stable thermochemical energy storage performance

Abstract

This study investigates the development of porous calcium-based monoliths via freeze-casting (FC) as a novel approach for thermochemical energy storage, particularly within the Calcium Looping (CaL) process. The freeze-casting technique enabled the fabrication of scaffolds with controlled porosity using polyvinyl alcohol (PVA) as a binder. Experimental results demonstrated that freeze-cast monoliths exhibited superior multicycle performance under various carbonation and calcination conditions. The FC-CaCO3 monolith achieved the highest residual conversion of 68.1 % under mild vacuum calcination conditions (780 °C, 0.1 bar CO2), significantly surpassing other configurations. Tests conducted in an inert atmosphere also yielded favorable results, with a conversion of 56.1 %, outperforming equivalent raw powder samples. The enhanced performance is attributed to improved CO2 interaction with the porous structure, mitigating sintering effects and preserving active surface area. Morphological observations by X-ray tomography and SEM confirmed limited particle sintering after multiple cycles, maintaining a reactive surface that supported consistent conversion rates. The pore size distribution of the material evolves upon cycling resulting in an increased microporosity, while the pore network maintains a low tortuosity (τ ~ 1.5–2.0). The addition of dopants such as ZrO2 and SiO2 did not enhance performance, as the monoliths' inherent structure provided sufficient stability. These findings highlight freeze-casting as a promising method for creating advanced porous materials suitable for energy storage applications.