Maximilian Krödel

CaO-based sorbents are cost-efficient materials for high-temperature CO₂ capture, yet they rapidly deactivate over carbonation-regeneration cycles due to sintering, hindering their utilization at the industrial scale. Morphological stabilizers such as Al₂O₃ or SiO₂ (e.g., introduced via impregnation) can improve sintering resistance, but the sorbents still deactivate through the formation of mixed oxide phases and phase segregation, rendering the stabilization inefficient. Here, we introduce a strategy to mitigate these deactivation mechanisms by applying (Al,Si)O_x overcoats via atomic layer deposition onto CaCO₃ nanoparticles and benchmark the CO₂ uptake of the resulting sorbent after 10 carbonation-regeneration cycles against sorbents with optimized overcoats of only alumina/silica (+25%) and unstabilized CaCO₃ nanoparticles (+55%). ²⁷Al and ²⁹Si NMR studies reveal that the improved CO₂ uptake and structural stability of sorbents with (Al,Si)O_x overcoats is linked to the formation of glassy calcium aluminosilicate phases (Ca,Al,Si)O_x that prevent sintering and phase segregation, probably due to a slower self-diffusion of cations in the glassy phases, reducing in turn the formation of CO₂ capture-inactive Ca-containing mixed oxides. This strategy provides a roadmap for the design of more efficient CaO-based sorbents using glassy stabilizers.

To advance CaO‐based CO₂ sorbents it is crucial to understand how their structural parameters control the cyclic CO₂ uptake. Here, CaO‐based sorbents with varying ratios of Na₂CO₃:CaCO₃ are synthesized via mechanochemical activation of a mixture of Na₂CO₃ and CaCO₃ to investigate the effect of sodium species on the structure, morphology, carbonation rate and cyclic CO₂ uptake of the CO₂ sorbents. The addition of Na₂CO₃ in the range of 0.1–0.2 mol% improves the CO₂ uptake by up to 80% after 10 cycles when compared to ball‐milled bare CaCO₃, while for Na₂CO₃ loadings >0.3 mol% the cyclic CO₂ uptake decreases by more than 40%. Energy dispersive X‐ray spectroscopy (EDX), transmission electron microscopy, X‐ray absorption spectroscopy (XAS), and ²³Na MAS NMR, reveal that in sorbents with Na₂CO₃ contents 100 nm, and enhance the diffusion of CO₂ through CaCO₃. For Na₂CO₃ contents >0.3 mol%, the accelerated deactivation of the sorbents via sintering is related to the formation of crystalline Na₂Ca(CO₃)₂ and the high mobility of Na.

Yolk(CaO)–shell(ZrO ₂ )-structured sorbents yield superior materials for high-temperature CO ₂ capture by mitigating deactivation via sintering and mixed phase (CaZrO ₃ ) formation.

Carbon dioxide capture and storage technologies are short to mid‐term solutions to reduce anthropogenic CO₂ emissions. CaO‐based sorbents have emerged as a viable class of cost‐efficient CO₂ sorbents for high temperature applications. Yet, CaO‐based sorbents are prone to deactivation over repeated CO₂ capture and regeneration cycles. Various strategies have been proposed to improve their cyclic stability and rate of CO₂ uptake including the addition of promoters and stabilizers (e. g., alkali metal salts and metal oxides), as well as nano‐structuring approaches. However, our fundamental understanding of the underlying mechanisms through which promoters or stabilizers affect the performance of the sorbents is limited. With the recent application of advanced characterization techniques, new insight into the structural and morphological changes that occur during CO₂ uptake and regeneration has been obtained. This review summarizes recent advances that have improved our mechanistic understanding of CaO‐based CO₂ sorbents with and without the addition of stabilizers and/or promoters, with a specific emphasis on the application of advanced characterization techniques.